REVIEW

EnergyChem

www.journals.elsevier.com/energychem

Metal-organic frameworks for catalysis: State of the art, challenges, and opportunities

Dandan Li, a , b Hai-Qun Xu, a , c Long Jiao, a and Hai-Long Jiang

∗ , a , d , e

a Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Soft Matter Chemistry, Collaborative Innovation Center of Suzhou Nano Science and Technology, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, PR China b Institutes of Physics Science and Information Technology, Anhui University, Hefei 230601, PR China c School of Food Science and Biotechnology, Zhejiang Gongshang University, Hangzhou 310038, PR China d State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China e Fujian Institute of Innovation, Chinese Academy of Sciences, PR China ∗Correspondence: jianglab@ustc.edu.cn (H.-L. J.)

ABSTRACT: Metal-organic frameworks (MOFs), also known as porous coordination polymers (PCPs), are a unique class of porous crystalline materials that are constructed by metal ions/clusters and organic ligands. The intriguing, numerous and tailorable structures as well as permanent porosity of MOFs make them very promising for a variety of potential applications, especially in

catalysis. In this review, we systematically summarize the recent progress of MOF-based materials (including pristine MOFs, MOF

composites, and MOF derivatives) for heterogeneous catalysis, photocatalysis and electrocatalysis, according to the category of active site origin. We clearly indicate the significant strengths (and also weaknesses) of the MOF-based materials, in reference to traditional catalysts, in catalytic studies. The challenges and opportunities in regard to the MOF-based materials for catalysis have also been

critically discussed.

KEYWORDS: Metal-organic frameworks, MOF-based materials, Porous materials, Heterogeneous catalysis, Photocatalysis, Electrocatalysis

A

Received: January 4, 2019 Revised: April 23, 2019 Accepted: April 23, 2019 Published: 24 April 2019

INTRODUCTION

s a relatively new class of crystalline porous materials,metal-organic frameworks (MOFs), also known as porous

coordination polymers (PCPs), which are assembled by metalions/clusters and organic ligands, have attracted significantattention recently ( ∼20 years). 1–3 Due to their intriguing andenormous structures, high crystallinity, permanent porosity,ultrahigh surface area, adjustable pore size and environment,tunable functionality and designable framework topology, MOFshave shown huge potential applications in various fields, in-cluding gas sorption and separation, catalysis, luminescence,biomedicine, proton conductivity, sensor, and so on. 1–11 TheMOF-related research has become one of the hottest topics inchemistry and materials science in the past decade. Amongst allMOF applications, heterogeneous catalysis was firstly exploredin 1994

12 as one of the earliest demonstrations and presentsa continuously growing research interest due to the chemicalmutability, bespoke pore structures and large, readily accessibleinternal surface areas; the original heterogeneous catalysis hasbeen gradually extended to different types of catalytic reactions( Scheme 1 ), especially photocatalysis and electrocatalysis. 5,13–34

Compared with the traditional porous materials (such aszeolites, clays or mesoporous silica), MOFs as catalysts hold theirparticular advantages, mostly as follows: 1) the structural diver-sit y and tunabilit y of all MOF components (nodes, linkers andpores) makes it feasible to develop MOF-based catalysts by im-mobilizing various catalytic sites into a single MOF; 2) the well-

© 2019 Elsevier Ltd. All rights reserved. 1

defined structures offer a great opportunity to understand thereaction at the molecular level; 3) the mutable pore environments( hydrophi lic and hydrophobic) favor the recognition and trans-portation of substrates and products; and 4) the synergistic catal-ysis can be demonstrated via the various catalytic sites and collab-orative microenvironment. With all these advantages, MOFs havebeen successfully applied in a variety of catalytic reactions.

For catalytic applications, active sites are definitelyindispensable and their origin is of great importance ( Scheme 2 ).The catalytic active sites of pristine MOFs basically originatefrom two places: the coordinatively unsaturated metal sites(CUSs, also called open metal sites) as Lewis acid sites and thedangling acid/base sites on the organic linkers. Occasionally,functional molecular catalysts (metalloporphyrins, Schiff-base complexes, etc.) behaving as building units have beenassembled into MOFs for related catalytic reactions. Therefore,the limited types of active sites involved in pristine MOFsgreatly restrict the applicable scope of reactions and performanceimprovement. Fortunately, the structural tailorability of MOFsgreatly extends these weaknesses and enriches the origin of activesites. Both metal clusters and organic linkers are modifiable in pre-introduction or post-synthesis ways to graft different functional

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Scheme 1. The timeline of metal–organic frameworks for catalysis.

Scheme 2. Catalytically active site origin in MOF-based materials: (A) sites on metal clusters and linkers in pristine MOFs; (B) grafted sites in MOFs by modification; (C) encapsulated sites in MOF composites; (D) generated sites in MOF derivatives upon thermal/chemical conversion.

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roups and/or active sites for catalysis. More importantly,he pore space engineering of MOFs greatly extends theirpportunities towards catalysis. As porous materials, MOFs areble to encapsulate diverse catalytically active species, such asetal-based nanoparticles (NPs), molecular catalysts, enzymes,

tc., into their pores or structural-defect spaces for efficientatalysis. In addition, they have been intensively employeds templates/precursors for the synthesis of derived porousaterials, including porous carbons, metal-based compounds

nd their composites, upon thermal/chemical conversion. TheseOF derivatives, possessing large surface area, highly dispersed

ctive sites, excellent stability and tunable structures/composites,re very promising candidates for catalytic applications.

In this review, we discuss the particular advantages of MOF-ased materials, including MOFs, MOF composites and MOFerivatives, for catalysis (mainly, heterogeneous catalysis, pho-

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ocatalysis, and electrocatalysis) at first. Centered on the originf diverse active sites in MOF-based materials, an overview on

he recent progress of MOF-based materials for various catalyticpplications (mainly, heterogeneous catalysis, photocatalysis, andlectrocatalysis) wi l l be summarized. Finally, the challenges andpportunities of MOF-based catalysts for further development

owards advanced catalysis wi l l also be critically presented.

TYPES OF CATALYTIC REACTIONS

eterogeneous catalysis or economic and environmental reasons, heterogeneousatalysis plays an increasingly imperative role in chemicalanufacturing due to the ease of catalyst separation and the

eduction in waste. Recent reports have demonstrated that MOFsre a class of very promising heterogeneous catalysts for organicransformations. 35–39 Due to their unique components andtructures, MOF catalysts possess the following strengths: First,

OFs as a unique platform combine the advantages of homoge-eous catalysts (easily accessed sites, high activity and selectivity)ith atomically precise catalytic sites and heterogeneous catalysts

ease of separation from reaction products and recyclability).herefore, in a sense, MOFs can be viewed as a bridge toomogeneous and heterogeneous catalysts. Second, they haveighly uniform and tunable pore structure and size, which are ofital importance for size-selective catalysis: reactant or productsith specific shape/size that are transportable. Moreover, MOFs

re typically microporous, featuring with pore sizes ranging from (nonporous) to 9.8 nm, which can fil l up the gap between zeolitemainly microporous, < 2 nm) and silica (mainly mesoporous,–50 nm) and are suitable for most of the industrially importanteaction substrates. Third, the high porosity and surface areaavors the adsorption and enrichment of substrates, whichenefits the contact and interaction between catalytic sites and

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reactants and, thus, improves the catalytic efficiency. Fourth, thepore space provides special catalytic microenvironments, suchas chiral environment, appropriate hydrophilic and hydrophobicpore nature, electron-deficient or electron-rich circumstance,which would greatly affect the interaction and reactivity ofreactant molecules. Fifth, as mentioned above, the structuraltunability of MOFs makes it possible to graft desired activesites onto metal nodes or organic linkers by the multivariateapproach or post-synthetic modification. On the other hand,they are able to encapsulate a variety of additional active speciesinto their pore space for synergistically enhanced catalysis. 24

Sixth, MOF-derived porous catalysts exhibit great potential inthe creation of diverse active sites, showing large surface area andhigh stability, and are highly desired for heterogeneous catalysis.Finally, the well-defined and tailorable structures of MOFs greatlyfacilitate the understanding the underlying catalytic mechanismand relationship between structure and catalytic performance,which is of great importance in fundamental catalysis. Therecent significant progress in the development of MOF-basedmaterials for heterogeneous catalysis wi l l be summarized in detailbelow.

Photocatalysis Photocatalysis is one of the most important methods to addressthe essential energy and environment issues via the conversionof inexhaustible solar energy into clean fuel and/or chemicals. 40

MOFs have been demonstrated to be ideal candidates forphotocatalysis due to the following considerations: 41–47 1) Thehigh porosity of MOFs provides the free diffusion of substratesand products, and greatly suppresses the recombined volume ofphotoexcited electron-hole (e-h) due to the shortened transportdistance of charge carriers. 2) The high crystallinity avoidsthe structural defects that are usually recombination centers ofcharge carriers. 3) Both metal ions/clusters and organic linkers inMOFs can be designed and adjusted rationally as light-harvestingcenters w ith w ide spectrum absorption. 4) The metal clusters inMOFs, resembling inorganic semiconductors, provide uniformdistributed catalytic active sites. 5) The high surface areas andporous structures of MOFs ensure the possibility of introducingphotosensitizers or cocatalysts, such as polyoxometalates(POMs), metal NPs, metal complexes, semiconductors, etc.leading to the spatial separation of charge carriers and, thus,achieving an enhanced photocatalytic efficiency. 6) The excellentCO 2 adsorption capacity of MOFs might cause high CO 2 concentration around the active sites, which helps to acceleratethe CO 2 photoreduction reaction. 44,48 7) The MOF-derivedporous materials, such as metal oxides and metal sulfides, withporous structure and large surface area that are favorable to theaccessibility of active sites, possess remarkably higher photocat-alytic performance than the corresponding bulk counterparts. Inaddition, MOF-based photocatalysts also possess some commonadvantages that have been aforementioned for heterogeneouscatalysts such as the understanding of structure-propertyrelationship. In the following sections, a wide range of the currentphotocatalytic applications, including water splitting, CO 2 re-duction and organic transformations, over MOF-based materialsis presented.

Electrocatalysis With the urgent demand for renewable energy storage andconversion, MOFs are emerging materials for electrocatalysis,mainly including oxygen reduction reaction (ORR),

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electrochemical water splitting (OER/HER), electrochemicalCO 2 reduction, electrochemical N 2 reduction, etc. Similar toMOFs for heterogeneous catalysis and photocatalysis, the largesurface area, tunable pore sizes, designable and tailorablestructures and homogeneous dispersion of active centersin MOFs are highly desired for electrocatalysis. However,given the extreme conditions required in electrocatalysis(strong acidic/alkaline solutions), MOFs with poor conduc-tivit y/stabilit y have been less studied. 49,50 In sharp contrast,MOF-derived materials, such as doped porous carbons, metaloxides/carbides/nitrides/phosphides, and their composites,etc., which are readily obtained by chemical/thermal conversion,have been intensively explored for electrocatalysis in recent years.On one hand, these MOF-derived materials can inherit thestrengths of pristine MOFs to a certain degree, including highsurface area, tunable porosity and uniform doping/depositionwith heteroatoms, metal/metal oxides, etc. On the other hand,the obtained porous carbon composites are usually doped withheteroatoms which, in many cases, are beneficial to improvethe degree of graphitization and conductivity, much higherthan that of MOFs, which wi l l result in better performance inelectrocatalysis. On account of these advantages, MOF-basednanocomposites for electrocatalysis have received explosiveattention in recent years. 51–53 The following sections wi l lsummarize representative progress in fabricating MOF-basedmaterials for electrochemical applications.

ACTIVE SITES AT METAL NODES/CLUSTERS

The presence of metal nodes/clusters in MOFs makes themsuitable as catalysts. During the last few years, many efforts havebeen devoted to incorporate active metal nodes/clusters withinMOFs with high catalytic activities ( Table 1 ). Herein, the rela-tionship between catalytic performance and metal nodes/clustersin MOFs has been well addressed.

Heterogeneous catalysis The metal nodes/clusters in MOFs are usually coordinated byorganic ligands or solvent molecules, the latter of which can beeasily removed without disturbing their whole framework due tothe weak coordinated interaction. The resultant exposed metalsites of metal nodes/clusters in MOFs are able to provide emptyorbitals and interact with substrates or other molecules with lonepair electrons. In this case, the MOFs can be Lewis acid catalystsor grafted with suitable functional active molecules for specializedand sophisticated applications.

Coordinatively unsaturated metal sites as catalytic sites Lewis acid sites. The presence of coordinatively unsaturatedmetal sites (CUSs) of metal nodes/clusters in MOFs give riseto Lewis acid sites which can be exemplified by HKUST-154,55 and MOF-74. 56 Additionally, recent investigations show thatHKUST-1 loses its Lewis acid activity very rapidly under humidconditions due to water re-occupation of the open Cu sites and/orframework instability, whereas surface hydrophobic modificationis able to inhibit the water attack and prevent its decreased activityto a certain extent. 57

Besides Cu and Mg, other metals in MOFs that can alsogenerate unsaturated metal centers include Mn, Cr, Ni, Co, Zr,Fe, Zn, etc. 58–61 Thereinto, MIL-101(Cr) (Cr 3 X(H 2 O) 2 O(1,4-bdc) 3 ; X = F, OH; bdc = benzene-1,4-dicarboxylate), developedby Férey et al., 62 is an ideally stable MOF for applications in

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Table 1. Catalogue of known catalytic MOFs with active metal nodes/clusters.

MOF Metal nodes/clusters Active sites Reactions Ref.

Mn 3 [(Mn 4 Cl) 3 (BTT) 8 (CH 3 OH) 10 ] 2 Mn(II) Mn(II) Cyanosilylation of aromatic

aldehydes and Mukaiyama-aldol reaction

58

MIL-101 Cr(III) Cr(III) Lewis acid sites 62

Hf-NU-1000 Hf(IV) Hf(IV) CO 2 cycloaddition 65

USTC-253 Al(III) Al(III) and sulfone groups CO 2 cycloaddition 68

[Cu(mipt)-(H 2 O)](H 2 O) 2 Cu(II) Cu(II) CO oxidation 70

MIL-100(Fe) Fe(III) Fe(III) Friedel–Crafts benzylation 73

NU-1000 Zr(IV) Ni ions (modified Zr 6 cluster)

hydrogenation 74

UiO-68 Zr(IV) Co- or Fe-(modified Zr 6 cluster)

Site-selective sp 3 C

–H

functionalization reactions 78

Ce-BTC Ce(III) Li ions (modified Ce 6 cluster)

Hydroboration and hydrophosphination reactions

79

MOF-808 Zr(IV) -SO 4 and Zr(IV) Acid-catalyzed reactions 80

Cu-I-bpy Cu 2 I 2 Cu 2 I 2 Photocatalytic H 2 evolution 91

PCN-415 Ti 8 Zr 2 Ti 8 Zr 2 Photocatalytic H 2 evolution 92

Ru-TBP and Ru-TBP-Zn Ru 2 Ru 2 Photocatalytic H 2 evolution 93

NU-1000 Zr 6 NiS x modified Zr 6 Photocatalytic H 2 evolution 94

MIL-101(Fe) and MIL-101-NH 2 (Fe)

Fe–O cluster Fe–O cluster Photocatalytic water oxidation 95

MIL-101(Fe) Fe–O cluster Fe–O cluster Photocatalytic water oxidation 96

Cd-TBAPy Cd–O cluster Cd–O cluster Photocatalytic water oxidation 97

NH 2 -MIL-125(Ti) Ti–O cluster Ti–O cluster Photocatalytic CO 2 reduction 98

NH 2 -UiO-66(Zr) Zr–O cluster Zr–O cluster Photocatalytic CO 2 reduction 99

MIL-101(Fe), MIL-53(Fe), MIL-88B(Fe)

Fe–O cluster Fe–O cluster Photocatalytic CO 2 reduction 100

PCN-222 Zr–O cluster Zr–O cluster Photocatalytic CO 2 reduction 101

NH 2 -UiO-66(Zr/Ti) Zr/Ti–O cluster Zr/Ti–O cluster Photocatalytic CO 2 reduction 103

UiO-66-NH 2 Zr–O cluster Zr–O cluster Photocatalytic selective aerobic oxygenation

104

NH 2 −MIL-125 Ti–O cluster Ti–O cluster Photocatalytic amines oxidation 105

PCN-222 Zr–O cluster Zr–O cluster Photocatalytic oxidative coupling of amines

108

Cu-bipy-BTC Cu(II) Cu(II) Electrocatalytic ORR

109

Ni 3 (HITP) 2 NiN 4 NiN 4 Electrocatalytic ORR

110

NENU-500 POM POM Electrocatalytic HER

112

H 3 [Ni III 3 (tht) 2 ] Ni–S cluster Ni–S cluster Electrocatalytic HER

113

Fe(BTC) Fe–O cluster Fe–O cluster Electrocatalytic OER

116

NiCo-UMOFNs Ni/Co–O cluster Ni/Co Electrocatalytic OER

129

HKUST-1 Cu(II) Cu(II) Electrocatalytic CO 2 reduction 133

[Cu 2 (ade) 2 (CH 3 COO) 2 ] Cu(II) Cu(II) Electrocatalytic CO 2 reduction 134

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eterogeneous catalysis attributing to its Lewis acid activity.he water molecules coordinated to chromium centers can be

asily removed in vacuum at an elevated temperature, leavingehind open trimeric chromium(III) clusters accessible forotential reactants and offering a significantly higher catalyticctivity in the cyanosilylation of benzaldehyde than HKUST- due to the greater Lewis acidity of Cr(III) over Cu(II). 63

ecently, Farha, Hupp and coworkers reported a carefully chosenOF (NU-1000) w ith Lew is-acidic Zr IV ions as the active

ites that are extraordinarily effective for the degradation oferve agents and their simulants ( Fig. 1 (a)). 64 In another work,

he same group prepared a polyoxohafnium-cluster-based MOFsostructural with Zr-based NU-1000, Hf-NU-1000, with high-ensity Lewis acidic sites. 65 This demonstrates excellent catalyticerformance for the quantitative chemical fixation of CO 2 intoyclic carbonates under ambient conditions.

The catalytic activ ity of MOFs w ith Lew is acid sites can bereatly enhanced by defect engineering. It has been demonstrated

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hat the use of monocarboxylate modulators (e.g. benzoic, acetic,r trifluoroacetic acid, TFA) in the synthesis of UiO-66 facilitatesot only the control of the crystallite size 66 but also of the

ormation of defects. Vermoortele et al. showed how the incor-oration of TFA opens up more Lewis-acidic CUSs on the Zr 6 lusters. 67 The TFA can be removed from the already defectiveramework by thermal treatment, thereby further increasing theumber of Zr-CUSs ( Fig. 1 (b)). Jiang and coworkers synthesized sulfone-functionalized MOF, denoted as USTC-253. 68 Thentroduction of TFA during the synthesis of USTC-253 leads to aefect-containing USTC-253-TFA with polar sulfone groups andxposed metal centers, increasing CO 2 uptake and improvingatalytic activity in the cycloaddition of CO 2 and epoxiden reference to USTC-253. Recently, Chen et al. prepared aotal of 16 chiral porous MOFs from a single phosphono-

carboxylate ligand of 1,1

′ -biphenol and 16 different metalons, allowing for systematic tuning of Lewis acidity, catalyticctivity and enantioselectivity. 69 The series of MOF materials

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Fig. 1. (a) Molecular representations of the NU-1000 node and linker (left), MOF topology (two views, center), and the dehydration of the NU-1000 node (right). Adapted with permission. 64 Copyright 2015, Nature Publishing Group. (b) Removal of the coordinated trifluoroacetic acid by thermal activation. Reproduced with permission. 67 Copyright 2013, American Chemical Society.

are able to catalyze asymmetric allylboration, propargylation,Friedel–Crafts alkylation and sulfoxidation with good to highenantioselectivity. In contrast, the homogeneous catalyst systemscannot catalyze these reactions enantioselectively.

Additionally, MOFs with Lewis acid sites can also be usedas catalysts for gas-phase reactions. Xu and coworkers presenteda microporous metal-organic polymers, Cu(mipt) (mipt = 5-methylisophthalate), containing open Cu(II) sites on channelwal ls, for excel lent and stable catalysis towards the CO oxidiza-tion. 70

Brønsted acid sites. Brønsted acidity can usually be found inMOFs with ligands functionalized with uncoordinated hydroxyl,oxalic acid, sulfuric acid groups, etc. Hydroxyl-based Brønstedacidity in MOF systems is inspired by acidified zeolites inwhich hydroxyl groups bridge silicon and aluminum atoms.MOFs with hydroxyl groups are also well know n, w here thehydroxyl groups are found to bridge two or more metal ionsin the secondary building unit (SBU). For example, Ravonet al. reported that the MOF-69C, consisting of chains ofZnO 2 (OH) 2 tetrahedron and ZnO 4 (OH) 2 octahedron andwith μ3 -OH as bridging species, was shaped selectively for thealkylation of large polycyclic aromatics, such as biphenylene. 71

In another work, the MIL-53(Ga) [Ga( μ2 -OH)(BDC)], withbridging hydroxyl groups, exhibited high activity towards acid-catalyzed Friedel-Craft alkylation of aromatics. 72 Férey andco-workers reported the catalytic activity of MIL-100(Fe, Cr) forFriedel–Crafts benzylation. 73 Despite their identical structures,MIL-100(Fe) showed much higher catalytic activity thanMIL-100(Cr).

Metal nodes modified with active species Many efforts have been made to functionalize the metalnodes/clusters of MOFs with active transition metals (metaloxides), inorganic acidic sites, and molecular catalysts to affordMOF-based catalysts with high performance.

It is a promising approach to construct bimetallic clustersin a MOF by means of metal-oxo nodes/clusters as structural

5

supports and other transition metals as active sites. Farha, Huppand coworkers installed Ni ions uniformly and precisely on thenode of NU-1000, using atomic layer deposition (ALD) in aMOF (AIM), denoted as Ni-AIM, which is one of the best avail-able hydrogenation catalysts based on earth-abundant elements( Fig. 2 (a)). 74 Besides the AIM method, the same group depositedMo(VI) oxide on the Zr 6 nodes of NU-1000 via solvothermaldeposition in MOFs (SIM) ( Fig. 2 (b)), which exhibited higheractivity for the epoxidation of cyclohexene than that of Mo(VI)oxide powder and was comparable to that of a zirconia-supportedanalogue (Mo–ZrO 2 ) prepared in a similar fashion. 75 In addition,a highly electrophilic single-site d

0 Zr-benzyl catalytic centerwas embedded on the nodes of Hf-NU-1000, resulting in theformation of Hf-NU-1000-ZrBn, a promising single-componentcatalyst for ethylene and stereoregular 1-hexene polymeriza-tion. 76 Recently, copper oxide clusters have been deposited onthe nodes of NU-1000 by AIM, which is active for oxidation ofmethane to methanol under mild reaction conditions. 77

Owing to the high stability of Zr-MOFs and the ease offunctionalizing Zr-clusters with other transition metal ions, Linand coworkers reported Co- or Fe-functionalized UiO materialsthrough the deprotonation of Zr-clusters with n-BuLi followed byreactions with Co(II) and Fe(II) salts. 78 The obtained UiO-CoCland UiO-FeBr were highly active and reusable single-site solidcatalysts for site-selective sp

3 C

–H functionalization reactions,such as undirected benzy lic C

–H bory lation, sily lation andamination. In another work, the same group reported the directtransformation of Ce IV

6 ( μ3 -O) 4 ( μ3 -OH) 4 (OH) 6 (OH 2 ) 6 nodein a new Ce-BTC (BTC = trimesic acid) MOF into Ce III

6 ( μ3 -O) 4 ( μ3 -OLi) 4 (H) 6 (THF) 6 Li 6 node within the MOF, whichcould effectively catalyze hydroboration and hydrophosphinationreactions. 79

The metal nodes/clusters can also be modified employinginorganic acidic sites. Jiang et al. reported superacidity in thesulfated MOF-808, MOF-808-2.5SO 4 , with a stronger Brønstedacidity, which was obtained by treating it with aqueous sulfuricacid ( Fig. 2 (c)). 80 The MOF-808-2.5SO 4 is catalytically activefor various acid-catalyzed reactions including Friedel–Crafts

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Fig. 2. Schematic illustration showing (a) the synthesis of Ni-AIM using ALD technique and b) the synthesized Mo-SIM by SIM technique for the epoxidation of cyclohexene. (c) The structural transformation from MOF-808 to MOF-808-2.5SO 4 . (d) Schematic diagram showing the NU- 1000 supported dihydride iridium pincer complex. (a) Adapted with permission. 74 Copyright 2016, American Chemical Society; (b) Reproduced with permission. 75 Copyright 2016, American Chemical Society; (c) Adapted with permission. 80 Copyright 2014, American Chemical Society; (d) Reproduced with permission. 84 Copyright 2016, Royal Society of Chemistry.

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cylation, esterification, and isomerization, as well as for theonversion of methylcyclopentane.

An alternative method to obtain MOF catalysts is to mod-fy the metal nodes/clusters with molecular catalysts. Hwangt al. presented a method for the coordination of ethylenedi-mine (ED) and diethylenetriamine (DETA) to CUSs in MIL-01(Cr). 81 The resulting amine-grafted MIL-101(Cr) exhibits remarkably high activity for the Knoevenagel condensation.ater, Banerjee et al. attached L-proline-based chiral ligands to

he open metal coordination sites of MIL-101 by post-syntheticodification. 82 The resulting chiral materials show much higher

nantioselectivity in asymmetric aldol reactions than the chiraligands themselves. In another work, Fischer and coworkers re-orted the post-synthetic functionalization of 1,1

′ -ferrocenediyl-imethylsilane ( 1 ) to the bridging OH-group between two metalenters of the secondary building units of MIL-53(Al). 83 The as-

synthesized 1 0:25 @MIL-53(Al) is active for liquid-phase benzenexidation to phenol as a test reaction for redox activity. In 2016,imoldi et al. immobilised iridium pincer complex in NU-1000y solvent-assisted ligand incorporation, the resultant Ir-pincerodified NU-1000 is an active catalyst for the condensed phase

ydrogenation of a liquid alkene and operated as an efficienteterogeneous catalyst under flow conditions ( Fig. 2 (d)). 84

hotocatalysis revious studies have revealed that the metal nodes/clusters ofOFs can be regarded as isolated semiconductor quantum dots

nd distributed regularly within the networks of MOFs. 85,86 It isell known that a quantum-sized nanoparticle possesses a high

urface energy due to the coordinatively unsaturated metal site onts surface. There are a large number of coordinatively unsaturated

etal sites in MOFs. These open metal sites are electron-deficiententers that can interact with electron-rich reagents. Such uniqueharacter is beneficial for driving the photocatalytic application.n this way, the nodes of MOFs containing open metal sites can

6

lso be utilized as promising matrix for designing photocatalysty doping foreigner metal ions and active species. MOF catalystsith catalytic sites on metal nodes for photocatalytic water

plitting, CO 2 reduction, and organic transformations wi l l beiscussed below.

hotocatalytic water splitting

s early as 2010, García and coworkers reported that MOF- undergoes charge separation (electrons and holes) decaying

n the microsecond time scale upon light excitation ( Fig. 3 ). 85

roposing the semiconductor-like behavior of MOFs. Takingdvantage of the semiconductor-like properties, MOFs exhibitromising properties for photocatalytic hydrogen production byater splitting. In this regard, since the first porous MOF [Ru 2 (p-DC) 2 ] n was reported for photo-reducing water to hydrogenolecules, 87 metal-node engineering has been successfully de-

eloped for different MOFs, such as Zr-MOF, Ti-MOF, and Cu-OF, etc., 88–90 to promote the photocatalytic performance. Recently, Yuan et al. developed a stepwise route consisting of

re-assembled [Ti 8 Zr 2 O 12 (COO) 16 ] clusters and the followingigand-exchange pathway for the construction of a family ofsoreticular photoactive frameworks. 91 These resulting MOFsxhibit high porosity, excellent chemical stability, tunable visible-ight photo-response, and good activity towards photocatalyticydrogen evolution reactions. Shi et al. reported a Cu 2 I 2 -basedOF, which exhibits efficient photocatalytic hydrogen produc-

ion, where Cu 2 I 2 clusters serve as photoelectron generatorsroviding redox reaction sites for hydrogen evolution. 92 Linnd co-workers reported the design of two new MOFs, Ru-BP and Ru-TBPZn, based on Ru 2 secondary building unitsnd porphyrin-based tetracarboxylate ligands. 93 The proximityf Ru 2 SBUs to porphyrin ligands ( ∼1.1 nm) facilitates theultielectron transfer from excited porphyrins to Ru 2 SBUs for

fficient visible-light-driven hydrogen evolution reactions in neu-ral water ( Fig. 4 (a)). In addition to pristine MOFs, MOFs with

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Fig. 3. Photophysical processes that occur after the irradiation of the MOF-5. Reproduced with permission. 85 Copyright 2010, Wiley-VCH.

Fig. 4. (a) Photoexicted porphyrin ligands inject electrons to adjacent Ru2 SBUs to reduce protons to hydrogen. Adapted with permission. 93 Copyright 2018, American Chemical Society. (b) Idealized representation of NU-1000 after imparting NiS x functionality via AIM for photocatalytic reaction. Adapted with permission. 94 Copyright 2016, American Chemical Society. (c) Schematic mechanism of photocatalytic water oxidation catalyzed by Fe- based MOFs. Reproduced with permission. 95 Copyright 2016, Wiley-VCH. (d) The proposed mechanism for visible-light-driven photocatalytic H 2 and O 2 evolution over Cd-TBAPy. Reproduced with permission. 97 Copyright 2018, Wiley-VCH.

modified metal clusters also can be applied for photocatalytic H 2 generation. Peters et al. reported atomic layer deposition (ALD)in MOFs (AIM) as an approach to install cluster-scale nickelsulfide functionality into NU-1000 ( Fig. 4 (b)). 94 The resultingNiS-AIM exhibits excellent activity as a heterogeneous hydrogenevolution catalyst in pH = 7 aqueous buffer solution.

In comparison to hydrogen production, much less exampleshave been reported for photocatalytic water oxidation. The Fe-based MOFs were found to be promising candidates for photocat-alytic water oxidation under visible light irradiation ( Fig. 4 (c)). 95

Among those, Fe-MIL-101 presents the best efficiency forphotocatalytic water oxidation. 96 It is assumed that the finelydispersed iron-oxo clusters embedded as nodes of the porousframework contribute importantly to the efficient catalysis ascompared to bulk hematite ( α-Fe 2 O 3 ). Most recently, Xiao et al.

7

developed a cadmium-based MOF, Cd-TBAPy, with a 2D layeredframework featuring dual functions of both water reduction andoxidation under visible-light irradiation ( Fig. 4 (d)). 97 The wateroxidation over this MOF were examined by using AgNO 3 aselectron scavengers. The deposition of Ag NPs on the catalystscaused by the Ag + reduction during the catalysis may be anunfavorable factor for the recyclability of the catalyst.

Photocatalytic CO 2 reduction

MOFs containing redox-active metal clusters can be promisingmaterials for photocatalytic CO 2 reduction. In 2012, Li andco-workers reported that NH 2 -MIL-125(Ti), a Ti-containingMOF material, was able to photocatalytically reduce CO 2 togive HCOO

− under light irradiation. 98 ESR results reveal thatthe photo-generated Ti 3 + moiety via the ligand-to-metal charge

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Fig. 5. (a) Proposed mechanism for the photocatalytic CO 2 reduction over NH 2 -MIL-125(Ti) under visible light irradiation. Adapted with permission. 98 Copyright 2012, Wiley-VCH. (b) Dual excitation pathways over amino-functionalized Fe-based MOFs. Adapted with permission. 100

Copyright 2014, American Chemical Society. (c) Mechanisms underlying the photoexcited dynamics involved in H 2 TCPP (left) and PCN-222 (right) for photocatalytic CO 2 reduction. Reproduced with permission. 101 Copyright 2015, American Chemical Society. (d) Proposed enhanced mechanism

for the photocatalytic reactions over NH 2 -UiO-66(Zr/Ti). Reproduced with permission. 103 Copyright 2015, Royal Society of Chemistry.

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ransfer (LMCT) is responsible for the formation of HCOO

− Fig. 5 (a)). Later, the same group investigated photocatalyticO 2 reduction over amine-functionalized NH 2 -UiO-66(Zr). Itas found that CO 2 can be reduced to HCOO

− over NH 2 -UiO-6(Zr) under visible light irradiation, in which Zr 3 + intermediateas generated based on the LMCT process. The photocatalyticO 2 reduction activity over NH 2 -UiO-66(Zr) is even higher

han that over NH 2 -MIL-125(Ti). 99 Direct excitation on theetal clusters on MOFs would be also desired for photocatalyticO 2 reduction. Li and coworkers reported a series of earth-

bundant Fe-containing MOFs (MIL-101(Fe), MIL-53(Fe),IL-88B(Fe)) for catalytic CO 2 reduction to give formate

nder visible light irradiation. 100 The direct excitation of thee-oxo clusters in these MOFs induces the electron transfer from

2 − to Fe 3 + to form Fe 2 + , contributing to a part of photocatalyticO 2 reduction activity ( Fig. 5 (b)). MOFs with strong light absorption in the visible region wi l l be

ighly desired for photocatalysis. Therefore, to efficiently harvestisible light, Jiang, Zhang and coworkers judiciously selected porphyrin-involved MOF (PCN-222) that can selectivelyapture and further photoreduce CO 2 with high efficiency underisible-light irradiation. 101 The ultrafast transient absorption andhotoluminescence spectroscopy demonstrates the existencef an extremely long-lived electron trap state in PCN-222,hich substantially suppresses the detrimental electron-ole recombination, thereby boosting the efficiency of theO 2 photoreduction ( Fig. 5 (c)). Later, Chen et al. reported microporous robust Zr-MOF, NNU-28, constructed by visible light-responsive anthracene-based organic ligand,hich shows efficient CO 2 photoreduction under visible

ight. 102

Post-synthetic metal exchange for MOFs has been appliedor improving photocatalytic efficiency. For example, partialubstitution of Zr in NH 2 -UiO-66(Zr) by a Ti moiety via a post-

synthetic metal exchange method leads to the mixed metal NH 2 -iO-66(Zr/Ti) with enhanced photocatalytic performance forO 2 reduction under visible light irradiation ( Fig. 5 (d)). 103

8

hotocatalytic organic transformations mine-functionalized aromatic ligands are widely used to

reate visible-light photoactive MOFs. In these MOFs, metalodes can act as catalytic sites with electrons transferring from

he organic ligands upon visible-light excitation. Long et al.sed UiO-66-NH 2 as a visible-light photocatalyst for selectiveerobic oxidation of alcohols, olefins and cyclic alkanes withigh efficiency and high selectivity. 104 Upon light irradiation,

he photogenerated electrons reduce O 2 molecules to O 2 −˙,

hile the photogenerated holes oxidize the organic reactiveubstrates to carbonium ions. The superoxide radicals furthereact with carbocations to form the final products ( Fig. 6 (a)).n another related work, Sun et al. used NH 2 -MIL-125 for theerobic selective oxidation of amines to imines under visible lightrradiations. 105 The photogenerated Ti 3 + and O 2

−˙formed viahe reaction between Ti 4 + and O 2 were proposed to be involvedn this transformation process based on the experimental results.dditionally, NH 2 -MIL-125 was post-synthetically functional-

zed with dye-like molecular fragments. 106 The resulting materialxhibits improved light absorption over a wide range of theisible spectrum and shows enhanced photocatalytic activity forxidizing benzyl alcohol into benzaldehyde under visible light l lumination. Given that the coupling of photocatalysis basedn the Fe-oxo clusters with activation of H 2 O 2 over Fe-basedOFs is feasible, the Fe-based MOFs would be promising

hotocatalysts for benzene hydroxylation to produce phenol. Lind coworkers employed two Fe-based MOFs, MIL-100(Fe) and

IL-68(Fe), to hydroxylate benzene to phenol with H 2 O 2 as anxidant under visible light irradiation. 107 An optimized benzeneonversion of 30.6% was achieved with a H 2 O 2 to benzeneatio of 3:4 with MIL-100(Fe) as catalyst for 24 h reaction time.ecently, Jiang and coworkers employed PCN-222 to catalyze

elective aerobic oxidative coupling of amines under visible lightrradiation. 108 The PCN-222 exhibits excellent photocatalyticctivity, selectivity and recyclability, far superior to its organicounterpart. In addition to

1 O 2 generated by the porphyrin

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Fig. 6. (a) Photocatalytic mechanism of UiO-66-NH 2 . Reproduced with permission. 104 Copyright 2012, Royal Society of Chemistry. (b) Proposed charge transfer process for the photocatalytic oxidative coupling of benzylamine over PCN-222. Reproduced with permission. 108 Copyright 2018, Royal Society of Chemistry.

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ligand based on energy transfer, the synergistic effect betweenoxygen centered active sites in Zr-oxo clusters and porphyrin π -cation radicals generated from charge transfer-induced electronsand holes greatly contributes to the reaction ( Fig. 6 (b)).

Electrocatalysis The particular structural features make MOFs promising for elec-trocatalysis. Unfortunately, the development of electrocatalysisby using pure MOFs is usually restricted by their poor con-ductivity and stability, especially under extremely acidic/alkalineconditions.

ORR

ORR is the key process in fuel cells and metal–air batteries. MOF-based catalysts present great strengths for ORR, though this issti l l in its infancy. The first demonstration on the application ofpristine MOFs as the electrocatalysts, a water-stable Cu-bipy-

TC (bipy = 2,2

′ -bipyridine), for the reduction of oxygen wasreported by Mao et al. 109 It exhibits good and stable electrocat-alytic activity in pH 6.0 phosphate buffer. The rotating ring-diskelectrode (RRDE) voltammetry reveals an electron number of3.8, which is close to the theoretical value for the four-electron re-duction of O 2 to H 2 O. Recently, using a highly electron conduct-ing 2D MOF [Ni 3 (hitp) 2 ] (hitp = tha), Miner et al. has achievedthe lowest ORR overpotential of MOFs so far. 110 [Ni 3 (hitp) 2 ]is constructed by square planar NiN 4 units ( Fig. 7 (a)), grownsolvothermally as a thin film on the GC electrode and displaysan E onset of 0.82 V vs. RHE in an O 2 -saturated 0.1 M KOHsolution.

Water splitting

Electrolysis of water is considered to be a clean and efficientmethod for producing H 2 ; some MOFs have been successfullyused as hydrogen evolution reaction (HER) catalysts. As an early-stage example, Nohra et al. synthesized a series of electroactivepolyoxometalate-based MOFs (POMOFs) catalysts for HER. 111

After that, Qin et al. reported a novel polyoxometalate (POM)-based MOFs named NENU-500 as electrocatalysts towardsHER. 112 Due to its good stabilit y, porosit y and availabilityof active sites, NENU-500 is highly active for HER underacidic conditions, with a Tafel slope of 96 mV dec −1 and anoverpotential of 237 mV at a current density of 10 mA cm

−2 .Furthermore, the high HER activity can be maintained after

9

2000 CV cycles. Recently, 2D MOFs possessing active sitessimilar to transition-metal dichalcogenides have also been foundto be highly efficient HER catalysts. 113–115 Dong et al. reporteda 2D MOF H 3 [Ni III

3 (tht) 2 ], the corresponding nanosheets wereprepared by a Langmuir–Blodgett approach with a thickness ofapproximately 0.7 nm assigned as single-layer ( Fig. 7 (b)). 113 Thenanosheet samples were horizontally transferred onto the GC-

DE to serve as the working electrode. HER experiments wereevaluated in the N 2 -saturated 0.5 M H 2 SO 4 solution at 1600 rpm;the H 3 [Ni III

3 (tht) 2 ] electrode showed a η10 of 333 mV and a TSof 80.5 mV dec −1 .

Oxygen evolution is necessary in half reaction for waterelectrolysis. The electrochemical reactivity of Fe(BTC) wasinvestigated by Babu et al., 116 which can be activated cathodicallyto act as an oxygen evolution electrocatalyst. Later, variousresearch works reveal that non-precious metals, especially Co,play a dominant role in MOF-based OER catalysis studies. 117–125

In 2014, Wang et al. studied the electrocatalytic OERperformance of [Co(bim) 2 ] (Co-ZIF-9) in a wide pH range. 126

Recently, Zhang, Li and coworkers reported a material [Co 2 ( μ-OH) 2 (bbta)] (MAF-X27-OH, H 2 bbta = 1 H ,5 H -benzo-(1,2-d:4,5-d

′ )bistriazole) obtained by post-synthetic ion exchangeof [Co 2 ( μ-Cl) 2 (btta)] (MAF-X27-Cl) for oxygen evolutionreaction (OER) ( Fig. 7 (c)), featuring an overpotential of 292 mVat 10.0 mA cm

−2 in 1.0 M KOH solution that was found tobe much better than most reported inorganic catalysts. 127 Inanother work, Zhang’s group successfully immobilized thepaddle-wheel type cobalt carboxylate cluster in a unique Fe(III)dicarboxylate framework by using a modular synthetic methodinvolving two-step, single-crystal to single-crystal, post-syntheticmodification and achieved exceptionally high electrocatalyticoxygen evolution activity. 128 In addition, Tang and coworkersreported ultrathin NiCo bimetal-organic framework nanosheets(NiCo-UMOFNs) as promising electrocatalysts for OERunder alkaline conditions. 129 The NiCo-UMOFNs catalystexhibited high electrocatalytic activity. The authors found thatthe coordinated unsaturated metal atoms are the dominatingactive centers, and the coupling effect between Ni and Cometals was found to be crucial for tuning the electrocatalyticactivity. Duan et al. presented a generic approach to fabricateultrathin nanosheet array of MOFs on different substrates. 130

Moreover, they fabricated a nickel-iron-based MOF array, whichdemonstrated superior electrocatalytic performance towardsOER with a small overpotential of 240 mV at 10 mA cm

−2 ,

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Fig. 7. (a) Framework structure of [Ni 3 (hitp) 2 ] for ORR. Adapted with permission. 110 Copyright 2016, Nature Publishing Group. (b) Synthesis of nanosheets of a 2D MOF H 3 [Ni III

3 (tht) 2 ] by using the Langmuir–Blodgett method for HER. Reproduced with permission. 113 Copyright 2015, Wiley- VCH. (c) Framework and pore surface structures of MAF-X27-OH for OER. Adapted with permission. 127 Copyright 2016, American Chemical Society. (d) Structural details of Cu-based metal-organic porous materials for CO 2 reduction. Reproduced with permission. 134 Copyright 2017, Wiley-VCH.

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nd robust operation for 20,000 s without detectablectivity decay.

O 2 reduction

O 2 electroreduction is a meaningful way for energy storagend conversion; MOFs have also been demonstrated as suitablelectrocatalysts for CO 2 reduction, although related examplesre sti l l limited. There were some early works related to Cu-ased MOFs as electrocatalysts for CO 2 reduction, such ashe well-know n ex ample HKUST-1. 131–133 Hinogami et al.eported a copper rubeanate MOF (CR-MOF) was synthesizedor electrochemical reduction of CO 2 . 132 The onset potentialor CO 2 reduction of a CR-MOF electrode is about 0.2 V

ore positive than that observed on a Cu metal electroden an aqueous electrolyte solution. Moreover, formic acid islmost the only reduction product on the CR-MOF electrode,hereas the Cu metal electrode generates a range of products.

n another work, Kumar et al. reported that a HKUST-1 filmmmobilized onto glass carbon in 0.1 M KCl showed well-defined

u(II)/Cu(I) and Cu(I)/Cu(0) reversible redox responses. 133

he electrochemically generated Cu(I) acts as Lewis acid to formn adduct with carbon dioxide in-situ and is further transformednto oxalic acid in N, N-dimethylformamide containingetrabutylammonium tetrafluoroborate with saturated CO 2 .

Recently, Albo et al. showed that [Cu 2 (ade) 2 (CH 3 COO) 2 ], 3D framework based on paddle-wheel like dicopper unitsimilar to HKUST-1, can act as an electrocatalyst for CO 2 eduction to methanol and ethanol in CO 2 -saturated bicarbon-te ( Fig. 7 (d)). 134 In another case, Kang et al. showed thatZn 3 (btc) 2 (H 2 O) 3 ] is isostructural with HKUST-1 and canct as an electrocatalyst for CO 2 reduction to form CH 4 andO. 135 It was found that the morphology of the Zn-MOFs

10

ad significant effect on the electrochemical reaction and thatmidazolium-based ionic liquids (ILs) with fluorine were moreffective electrolytes.

It is worthy to note that, the active sites coming from metalodes/clusters in MOFs often locate in the corners of theore, which are not easily accessible to substrate molecules –

f not very small. Furthermore, the coordination sites of metalodes/clusters are not always available for bonding with reactantolecules, leading to a relative low catalytic efficiency. Therefore,

t is necessary and also full of opportunity to address thishallenge by different strategies, such as the fabrication of MOFsith hierarchical pores and/or more open metal nodes, for

nhanced catalysis.

ACTIVE SITES AT ORGANIC LINKERS

he availability of various organic linkers makes it possible toonstruct MOFs that contain active sites for catalysis. In thisection, the synthetic strategies and progress of stable MOFatalysts possessing catalytic sites within organic linkers areiscussed ( Table 2 ).

eterogeneous catalysis unctional groups on organic linkers can be tailored on a

arge degree for heterogeneous catalysis. There are a variety ofunctional groups that serve as active sites for catalysis, such asulfonic acid group, amino, amide, pyridyl, sulfoxy, bipyridyl,tc. In addition, some molecular catalysts (metalloporphyrins,alens, chiral molecules, etc.) can act as organic linkers in theonstruction of MOFs. Alternatively, these functional speciesan also be introduced into organic linkers by the functionalizedodification approach.

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Table 2. List of MOF catalysts with catalytic sites immobilized in organic linkers.

MOF MOF linker Active sites Reactions Ref.

UiO-66 H 2 BDC-COOH H 2 BDC-COOH Brønsted acidic sites 138

MIL-101-SO 3 H BDC-SO 3 H BDC-SO 3 H Alcoholysis of epoxides 141

Fe-MIL-101-NH 2 and Al-MIL-101-NH 2

BDC-NH2 BDC-NH 2 Knoevenagel condensation reaction

163

PCN-222(Fe) TCPP Fe-TCPP Oxidation reactions 174

bpy-UiO bpy Ir-/Pd- functionalized bpy C

–H borylation of arenes 187

MOF-253 bpydc Pt-bpydc Photocatalytic H 2 evolution 191

Al-TCPP TCPP Pt-TCPP Photocatalytic H 2 evolution 195

UiO-67 dcppy/dcbpy Cp ∗Ir-functionalized dcppy/dcbpy

Photocatalytic water oxidation 196

Zr-carboxylate MOFs, 1 and 2 Cp ∗Ir(L)Cl complex Cp ∗Ir(L)Cl complex Photocatalytic water oxidation 197

UiO-67 bpydc Cp ∗Rh(bpydc)Cl 2 Photocatalytic CO 2 reduction 201

MOF-253-Ru(CO) 2 Cl 2 bpydc- Ru(CO) 2 Cl 2 bpydc- Ru(CO) 2 Cl 2 Photocatalytic CO 2 reduction 202

MOF-525 TCPP Co-TCPP Photocatalytic CO 2 reduction 205

UiO-67 bpydc Ru(bpy) 3 -bpydc Photocatalytic aerobic oxidation

206

Zn-Sn-TPyP MOF Sn(IV)-porphyrin Sn(IV)-porphyrin Photocatalytic oxygenation of phenols and sulfides

209

Zn-PYI1 and Zn-PYI2 L- or D-pyrrolidin-2-ylimidazole (PYI)

PYI Photocatalytic α-alkylation reaction

218

Co-Al-PMOF Co-TCPP Co-TCPP Electrocatalytic ORR

220

G-dye-FeP Fe-porphyrin Fe-porphyrin Electrocatalytic ORR

222

Hf 12 -CoDBP Co-porphyrin Co-porphyrin Electrocatalytic HER

223

UiO-67 bpydc Ru(tpy)-bpydc Electrocatalytic OER

224

Al 2 (OH) 2 TCPP-Co Co-TCPP Co-TCPP Electrocatalytic CO 2 reduction 226

Fe-MOF-525 Fe-TCPP Fe-TCPP Electrocatalytic CO 2 reduction 227

Organic linkers with acid or base sites Vermoortele et al. showed that the presence of functional groupson the aromatic linkers can strongly influence the intrinsiccatalytic activity of the metal nodes through inductive effects. 136

In addition to metal nodes, the catalytic activity of MOFs canalso reside on the functional groups at the organic linkers that cantypically introduce activity as acid and basic catalysts. 38,137

Brønsted acidic groups can be dangled on the organic linkers.For example, a free carboxylic acid group was dangled in thepores of UiO-66 [Zr 6 O 4 (OH) 4 (BDC) 6 ] by using H 2 BDC-COOH instead of H 2 BDC for the preparation of UiO-66. 138

Later, the same MOF was reported to be synthesized from anenvironmentally friendly synthesis route in aqueous solutionusing ZrCl 4 , H 2 BDC-COOH, and H 3 BTC as modulator. 139

MOFs with stronger Brønsted acids were made by functionalizedwith sulfonic acids. Kitagawa and coworkers prepared -SO 3 Nagroup (partially protonated) functionalized MIL-101(Cr) bysolvothermal reaction of chromium(VI) oxide, monosodium2-sulfoterephthalic acid, and hydrochloric acid in water. 140 Witha strong Brønsted acid site on its pore surface, the resultingmaterial shows high catalytic activity for the cellulose hydrolysisreaction. After that, Jiang and coworkers prepared ∼100%sulfonic acid functionalized MIL-101(Cr), MIL-101-SO 3 H, bya hydrothermal process followed by a facile postsynthetic HCltreatment strategy. 141 The obtained MIL-101-SO 3 H showedexceptionally high activity, excellent selectivity and recyclabilitytowards the alcoholysis of epoxides under ambient conditions. Inanother work, a series of homochirally porous lamellar lanthanidebisphosphonates were prepared, which possess Brønsted acidicsites from the protonated phosphonates and Lewis acidity fromthe open lanthanide sites. 142 These acidic MOFs are able to

11

behave as enantioselective acid catalysts for cyanosilylationof benzaldehyde and ring opening of meso-carboxylicanhydrides.

Besides acid sites, base sites can also be precisely grafted ontoorganic linkers by basically using basic N-containing ligands asbuilding blocks. 143–146 On one hand, N-containing groups can beintroduced, instead of their unmodified linker counterparts, forconstructing MOFs. For example, IRMOF-1 (commonly knownas MOF-5) is constructed from the ligand BDC, whereas thereplacement of BDC with NH 2 -BDC yields IRMOF-3, showingbasic properties. 147 On the other hand, partial substitutionof original ligands with functional N-containing groups isan effective strategy. For instance, the synthesis using mixedligands consisting of both BDC and NH 2 -BDC leads to theformation of MIXMOFs. 104,105,148–162 For the direct synthesisof basic MOFs, the most frequently used N-containing ligandis NH 2 -BDC. In one of the examples i l lustrating the activityof NH 2 -functionalized MOFs, the Knoevenagel condensationreaction between benzaldehyde and malononitrile was reportedusing Fe-MIL-101-NH 2 and Al-MIL-101-NH 2 , observing90% yield of the expected condensation product at 80 °C. 163

Other N-containing ligands have been employed to constructMOF basic catalysts as well. For instance, Kim and coworkerssynthesized a homochiral MOF, known as D-POST-1. 164

The presence of the pyridyl groups exposed in the channelsendows D-POST-1 with unique opportunities for catalysis of theenantioselective transesterification of racemic 2,4-dinitrophenylacetate. Kitagawa’s group constructed a three-dimensional (3D)porous coordination polymer (PCP) functionalized with amidegroups based on tridentate ligand ( Fig. 8 (a)). 161 The materialselectively accommodated and activated guests in its channels

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Fig. 8. (a) The Knoevenagel condensation reaction over a 3D PCP

functionalized with amide groups. Reproduced with permission. 161

Copyright 2007, American Chemical Society. (b) Peroxidase-like oxi- dation reaction of pyrogallol catalysed by PCN-222(Fe). Adapted with permission. 174 Copyright 2012, Wiley-VCH.

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ecause of the active amide groups. As a result, the Knoevenagelondensation reaction was selectively promoted in good yield.

Moreover, acid and base sites can be installed within oneOF for heterogeneous catalysis. Vermoortele et al. reportediO-66(NH 2 ) can catalyze the highly selective synthesis of

asminaldehyde. 151 The close proximity of Zr Lewis acid sitesnd basic amino groups inside the cages suppress byproductormation and accelerate the cross-aldol reaction. In anotherork, Kim and coworkers reported NH 2 -MIL-101(Al), which

cts as a site-isolated Lewis acid-Brønsted base catalyst, catalyze tandem Meinwald rearrangement-Knoevenagel condensationeaction with remarkable substrate selectivity. 165

etalloporphyrins as catalytic sites etalloporphyrins are the active sites in monooxygenases

hat can efficiently oxidize a variety of organic moleculesnder mild conditions, 166 which are ideal catalytic struts toonstruct MOFs. 167–169 In 2009, Hupp, Nguyen, and co-workersuccessfully incorporated metalloporphyrins into a MOFhrough a mixed-ligand strategy, affording a pi l lar-layered ZnPO-

OF, with large pores and fully accessible active sites, whichromoted the acyl-transfer reaction with about 2400-fold ratenhancement relative to the reaction in the absence of catalyst. 170

ater, they synthesized an extended fami ly of metal loporphyrin-ased MOFs (Al 3 + , Zn

2 + , Pd

2 + , Mn

3 + , and Fe 3 + complexes)hat feature large channels and readily accessible active sites. 171

mong them, the Mn-containing MOF is catalytically active forhe oxidation of alkenes and alkanes.

In 2012, Zhou and coworkers also reported a porous metal-et alloporphyrin frame work, PCN -222(Fe) (also referred to asOF-545

172 or MMPF-6), 173 containing 1D open channels with diameter of up to 3.7 nm, exhibiting excellent peroxidase-likeatalytic activity ( Fig. 8 (b)). 174 Subsequently, Zhou’s group

12

eveloped a series of highly stable Zr-porphyrinic MOFs, such asCN-223, PCN-224, PCN-225, etc., 175–177 which are promisingandidates for catalysis. PCN-224(Co) was demonstrated to be reusable heterogeneous catalyst for the CO 2 /epoxide couplingeaction. 177 Recently, Wang et al. reported a stable Ir(III)-orphyrin Hf-MOF that promotes the carbenoid insertion reac-

ion into Si–H bonds, featuring high chemoselectivity towardsrimary silanes among primary, secondary and tertiary silanes. 178

etallosalens as catalytic sites alens, the condensed products of aryl aldehydes and diamines,re a class of universal and excellent ligands to chelate variousetal ions for the synthesis of different metallosalen catalysts. 179

etallosalens are excellent synthons for synthesising porousOF catalysts, which have been realized for heterogeneous cat-

lytic applications towards alkene epoxidation, cyclopropanationnd hydrolytic kinetic resolution of epoxides with interesting size- shape- and enantio-selectivities. 180–183

In 2010, Lin and co-workers prepared a family of isoreticularhiral MOFs by directly introducing chiral Mn-Salen struts intohe frameworks, which catalyze the asymmetric epoxidation of variety of unfunctionalized olefins w ith high activ ities and upo 92% ee. 181 Later, the same group developed Ru(salen)-based

OFs using redox active ruthenium/salen-derived dicarboxylateridging ligands. 182 The resulting MOFs are highly active for

he asymmetric cyclopropanation of substituted alkenes withery high diastereo- and enantioselectivities. More recently, Cuind co-workers constructed chiral porous MOFs by introducingo(Salen) units. 183 The Co-Salen-based MOF was used toromote the hydrolytic kinetic resolution (HKR) of epoxidesith an ee value up to 99.5%. The MOF structure makeso(Salen) units in a highly-dense and close to each other, leading

o improved catalytic activity and enantioselectivity in HKRompared with its homogeneous analogues, especially at lowatalyst/substrate ratios.

unctionalized modification for organic linkers rganic linkers are readily modifiable with organic functional

roups and molecular catalysts by post-synthetic ways foratalysis. Bonnefoy et al. anchored enantiopure peptide insidehe cavities of Al-MIL-101-NH 2 , In-MIL-68-NH 2 and Zr-iO-66-NH 2 , which were used as catalysts for the asymmetric

ldol reaction. 184 Hupp, Farha and coworkers incorporatedn acidic hydrogen-bond-donating squaramide moiety into aixed-strut UiO-67 derivative that behaves as an efficient, and

obust heterogeneous catalyst for Friedel–Crafts reactions. 185

y preventing detrimental self-association, the active state ofhe catalyst can be maintained, thus promoting activity. Inddition, Ma and coworkers successfully inserted carbon dioxidento the aryl C

–H bonds of the backbone of UiO-67(dcppy)dcppy = 2-phenylpyridine-5,4

′ -dicarboxylic acid) to generateree carboxylate groups. 186 The resultant UiO-67(dcppy)-

COOH acts as a solid-state Brønsted acid catalyst to efficientlyatalyze the methanolysis of an epoxide.

Lin and coworkers developed bpy-UiO-Ir and bpy-UiO-d catalysts via post-synthetic metalation strategy based on aiO-type MOF bearing the bipyridyl moiety. The resulting

atalysts not only show enhanced (up to at least 1250 times)atalytic activities but also greatly improved stability comparedo their homogeneous analogues. 187 Additionally, the sameroup synthesized a chiral Zr-MOF based on a BINAP-derivedicarboxylate linker (BINAP = 2,2

′ -bis(diphenylphosphino)-,10-binaphthyl), which was post-synthetic metalated with Ru

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Fig. 9. (a) Schematic showing BINAP-MOF catalysts for the addition of aryl and alkyl groups to α, β-unsaturated ketones and hydrogenation of substituted alkene and carbonyl compounds. Adapted with permission. 188 Copyright 2014, American Chemical Society. (b) Schematic illustration of the reaction of partial dihydroxyl groups with Li ions for asymmetric cyanation of aldehydes. Reproduced with permission. 190 Copyright 2014, American Chemical Society.

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and Rh complexes to afford highly enantioselective catalysts forimportant organic transformations, including the addition of aryland alkyl groups to α, β-unsaturated ketones and hydrogenationof substituted alkene and carbonyl compounds ( Fig. 9 (a)). 188

Cohen and coworkers synthesized highly robust UiO-66bearing isolated monocatecholato metal sites on the strut oforganic linkers via the post-synthetic exchange (PSE) route. 189

Furthermore, metalation of these catecholato groups resultedin the formation of Fe- and Cr-monocatecholato species. TheCr-metalated UiO-66 exhibited high activity and was completelyrecyclable and reusable for the oxidation of alcohols to ketones.In another work, Cui and coworkers reported a homochiralbiphenol-based MOF decorated with chiral dihydroxyl groups. 190

After the partial reaction of the dihydroxyl groups with Liions, it was used to catalyze the asymmetric cyanation ofaldehydes with up to > 99% ee, exhibiting enhanced activity andenantioselectivity in comparison to homogeneous counterpart,especially at a low catalyst/substrate ratio ( Fig. 9 (b)).

Photocatalysis The incorporation of rigid photocatalyst as linkers into MOFscaffolds opens a new avenue for photocatalysis. The homo-geneous photocatalyst can drive photocatalysis more efficientlyafter being incorporated or embedded into MOFs. MOF catalystswith active sites at organic linkers for photocatalysis havegarnered increased attention.

Photocatalytic water splitting

Recently, Xu and coworkers have synthesized a new MOF-253-t material through immobilizing a platinum complex into the

framework of MOF-253 using a post-synthesis modificationstrategy. 191 The modified MOF served both as a photosensitizerand a photocatalytic H 2 evolution catalyst. The photocatalyticactivity of MOF-253-Pt was approximately five times higherthan that of the corresponding complex. In a later publication,

13

a molecular proton reduction catalyst [FeFe](dcbdt)(CO) 6 (dcbdt = 1,4-dicarboxylbenzene-2,3-dithiolate) with astructure similar to [FeFe]-hydrogenase active moiety has beenincorporated into the framework of UiO-66 by the post-syntheticexchange strategy. 192 The resulting UiO-66-[FeFe](dcbdt)-(CO) 6 exhibited high efficiency for photochemical hydrogenevolution, exceeding that of the homogeneous referencesystem. In addition, other molecular catalysts (Ru complex,Ir complex, metalloporphyrin, etc.) can also be employed asbuilding units to fabricate MOFs for hydrogen production. 193–195

Jiang and coworkers have developed a facile synthetic strategyto atomically dispersed Pt into a highly stable aluminum-based porphyrinic MOF (Al-TCPP). 195 Given that the singlePt atoms maximized the atom utilization, the resulting Al-

CPP-0.1Pt catalyst exhibited commendable visible-lightphotocatalytic efficiency in hydrogen production. The achievedTOF value is about 30 times than that of Pt NPs stabilized byAl-TCPP.

The Ir complexes have widely been developed for photocat-alytic water oxidation. Lin and coworkers incorporated threetypes of Ir complexes with dicarboxylic acid functionalities intothe framework of UiO-67 for water oxidation using ceriumammonium nitrate as an oxidant ( Fig. 10 (a)). 196 The obtainedIr-doped MOFs have good reusability and stability. However,they show the lower catalytic performance in oxygen evolutionreaction compared to their homogeneous counterparts, whichmight be attributed to the difficult transport of the cerium (IV)oxidant throughout the small pores of the MOF, given thedecreased porosity of the UiO-67 furnished by the Ir complexes.Thus, the diffusion limitation caused by the MOFs with limitedpore sizes cannot be ignored in certain examples. Subsequently,Lin and coworkers constructed two highly porous and stableZr-MOFs using a longer Ir-based bridging ligand and studiedtheir water oxidation activities. 197 These MOFs provide aninteresting platform to study water oxidation pathways owing tothe elimination of multimolecular degradation pathways.

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Fig. 10. (a) Synthesis of doped UiO-67. Reproduced with permission. 196 Copyright 2011, American Chemical Society. (b) Photochemical reduction of CO 2 with homogeneous Ru II catalyst (1) and PCP-Ru II composite (3). Adapted with permission. 48 Copyright 2016, Wiley-VCH.

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hotocatalytic CO 2 reduction

omogeneous metal complexes containing R u, R e, Ir, etc., as wells some porphyrin-based complexes have long been examined forhotocatalytic CO 2 reduction. 198 Their use as linkers to constructOFs can produce photocatalytic active MOFs with isolated

atalytic sites, which not only show comparable or even improvederformance as compared with their homogeneous counterpartsut also possess the advantage of being heterogeneous catalyticystems for the ease of recycling.

The first heterogenization of molecular complexes onto MOFsor CO 2 photoreduction was reported by Lin and coworkersn 2011. 196 By successfully incorporating Re I (CO) 3 (dcbpy)Cldcbpy = (2,2-bipyridine)-5,5-dicarboxylic acid, H 2 L) into UiO-7(dcppy), they obtained a Re complex-doped MOF heteroge-eous photocatalyst that is active for CO 2 reduction under visible

ight. Later, Ryu et al. reported the simultaneous incorporation of Re complex and -NH 2 functionalized BPDC-(NH 2 ) 2 into UiO-7, which led to the formation of Re-MOF-NH 2 with improvedhotocatalytic activity for CO 2 reduction to produce CO. 199

ecently, by coating Re-containing UiO-67 onto Ag nanocubes,hoi et al. reported the construction of Ag ⊂Re n -MOF with

nhanced photocatalytic activity for CO 2 reduction to formO, which is due to the cooperation of the spatially confinedhotoactive Re centers and the intensified near surface electricelds at the surface of Ag nanocubes. 200

In addition to the Re containing photoactive complex,hotoactive Rh and Ru containing homogeneous catalysts canlso be doped into MOFs for photocatalytic CO 2 reduction.or instance, Chambers et al. reported that Cp

∗Rh(bpydc)Cl 2 as incorporated into UiO-67 via ligand exchange, showinghotocatalytic activity for CO 2 reduction to form HCOO

− inhe presence of a photosensitizer. 201 Kitagawa group reported aorous coordination polymer (PCP)-based catalyst, PCP-Ru

II

omposite, bearing a Ru

II -CO complex as a metallolinker forO 2 reduction. 48 Owing to a synergistic effect between the CO 2

nrichment and catalytically active sites within the PCP-Ru

II

omposite, the catalytic activity of photoreduction CO 2 overCP-Ru

II was comparable to the corresponding homogeneousu

II catalyst and even maintained under a 5% CO 2 /Ar gasixture CO 2 reduction ( Fig. 10 (b)). In addition to the ligand exchange, the direct construction of

upported photoactive metal complexes using the uncoordinatedites (such as bipyridine N atoms) from the MOF linkers isppealing. Li and co-workers prepared a MOF-253 supported Ru-

14

complex (MOF-253-Ru(CO) 2 Cl 2 ) via the direct coordination ofRu(CO) 3 Cl 2 ] 2 with the 2,2-bipyridine units in MOF-253. 202

he photocatalytic CO 2 reduction over MOF-253-Ru(CO) 2 Cl 2 howed that both HCOO

− and CO were produced as the maineduction products. Similarly, the post-synthetic metalation of robust Zr-based MOF with open bpy metal-chelating linkersffords isolated Mn(bpy)(CO) 3 Br moiety in the MOF, which isctive for photocatalytic CO 2 reduction to form formate. 203

In addition, the porphyrin macrocycle in the porphyrin-OFs can be metalated with catalytic active metal sites to

romote their photocatalytic performance. For example, Liu et al.eported that a Cu-based porphyrin-MOF (S Cu ) prepared by the

etallation of a porphyrin-MOF based on TCPP (S p ) exhibitsetter performance in both CO 2 capture and photocatalytic CO 2 onversion to methanol than porphyrin-MOFs without Cu(II)enter. 204

Recently, Ye and coworkers realized atomic dispersion of activeites in a Zr-porphyrinic MOF (MOF-525), thus generating pho-ocatalysts with higher activity in CO 2 reduction. 205 Mechanisticnvestigation revealed that the presence of single Co atoms inhe porphyrin center of MOF greatly boosted the electron-holeeparation efficiency in porphyrin units. The resultant MOF-525-

Co easily uptakes and subsequently reduces CO 2 into CO andH 4 with dramatically improved activity and CH 4 selectivity.

hotocatalytic organic transformations rganic ligands in photoactive MOFs can work as both light

ntennae and catalytic sites. Benefiting from well understoodolecular photochemistry, various photocatalytic ligands can be

asily incorporated into MOFs. In 2011, Lin and coworkers incorporated Ir and Ru complexes

nto a highly stable Zr-MOF (UiO-67) via a mix-and-match syn-hetic strategy. 196 These resulting UiO-67 derivatives were usedor photocatalytic organic transformations (aza-Henry reaction,erobic amine coupling, and aerobic oxidation of thioanisole),xhibiting good yields and reusability even after three catalyticuns. After that, Cohen and coworkers incorporated Ir and Ruomplexes into UiO-67 via post-synthetic modification. 206,207

he resulting UiO-67-Ru(bpy) 3 showed efficient and recyclableatalytic activity for the photocatalytic aerobic oxidation ofrylboronic acids. Meanwhile, the resulting UiO-67-Ir MOFsere used as heterogeneous photocatalysts for trifluoroethylation

eactions of styrene. Surprisingly, the MOF catalysts had betterelectivity for formation of the desired hydroxytrifluoroethyl

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Fig. 11. (a) Schematic illustration showing the indium metalation of porphyrin macrocycles in UNLPF-10 for photocatalytic selective oxygenation of sulfides. Adapted with permission. 212 Copyright 2014, American Chemical Society. (b) Schematic representation of Zn- PYI1 for photocatalytic α-alkylation reaction upon light excitation. Reproduced with permission. 218 Copyright 2012, American Chemical Society.

Fig. 12. (a) Schematic illustration showing the ORR performance of Co-Al-PMOF. Adapted with permission. 220 Copyright 2017, Royal Society of Chemistry. (b) Schematic illustration of MOF-525 containing Fe-porphyrins for electrochemical CO 2 reduction. Reproduced with permission. 227 Copyright 2015, American Chemical Society.

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products compared to the homogeneous catalysts, which is likelydue to confined space within the MOF pore structure.

In addition to Ru and Ir polypyridyl complexes, porphyrinsare another important ligands used to build photocatalytic MOFsand act as light harvesters and/or photoredox centers. 208–213 Wuand coworkers have synthesized a Sn(IV)-porphyrin-based MOF(Zn-Sn-TPyP MOF) from pre-metalated Sn-porphyrin buildingblocks. 209 The resulting Zn-Sn-TPyP MOF showed excellentproduct yields for the oxygenation of phenols and sulfides, whichare superior to its corresponding counterparts. Later, Zhangand coworkers constructed a new anionic indium porphyrinframework, denoted as UNLPF-10. 212 The metalation of por-phyrin macrocycles was tuned by varying the indium/ligand ratioduring synthesis. The resulting UNLPF-10 exhibited excellentphotocatalytic activity towards the selective oxygenation ofsulfides that could be tunable by indium metalation ( Fig. 11 (a)).Besides metal complexes and metalated porphyrins, other typesof organic molecules in organic linkers of MOFs also play animportant role for photocatalytic organic transformations. 214–217

Combining enantioselective catalysts into MOFs can affordMOF-based asymmetric photocatalysts for asymmetric catalyticconversions. In 2012, Duan and coworkers incorporated thestereoselective organocatalyst L- or D-pyrrolidin-2-ylimidazole(PYI) and triphenylamine photoredox group into a MOF,developing two enantiomeric MOFs, Zn-PYI1 and Zn-PYI2( Fig. 11 (b)). 218 These two MOFs were successfully appliedto prompt the light-driven α-alkylation reaction with excellentcatalytic efficiency and high enantioselectivity. The enantiose-lectivity is superior to that of simply mixing the correspondingMOFs with the chiral adduct.

Electrocatalysis ORR

Transition metal porphyrin complexes are highly efficientmolecular catalysts for ORR. 219 Several metalloporphyrin-based MOFs have been examined for their ORR catalysis

15

performances. 220,221 For example, Lions et al. investigatedthe ORR performance of Co-Al-PMOF ( Fig. 12 (a)), 220 theporphyrin metalated analogue of [Al 2 (OH) 2 (H 2 tcpp)] (H 2 -Al-

MOF), in an O 2 -saturated 0.1 M H 2 SO 4 . Loh and coworkerssynthesized a hybrid MOF termed (G-dye-FeP)n by addingpy ridine-f unctionalized graphene (reduced graphene oxide,r-GO) sheets to the metalloporphyrin MOF for ORR. 222 Theirstudies reveal that functionalized r-GO sheets can influence thecrystallization process of MOF and improve the electrocatalyticproperties of the composite. The composite possess a muchhigher selectivity for ORR compared to Pt catalyst and showfacile 4-electron ORR pathway that is useful for direct methanolfuel-cell operation.

Water splitting

Lin and coworkers synthesized Co-porphyrin MOFs supportedon carbon nanotubes (CNTs) for electrocatalytic proton reduc-tion. 223 Because the covalent attachment of MOF nanoplates toconductive CNTs improves electron transfer from the electrodeto Co-porphyrin active sites, the MOF/CNT hybrid is highlyactive for HER in acidic media with an onset potential of 315 mVand TOFs of over 17.7 s −1 .

Molecular complexes based on precious metals have attractedmuch attention as OER catalysts that inspire MOFs containingsuch precious metal ions applied as potential OER catalysts. 224,225

For example, Ott and coworkers used [Ru(tpy)(dcbpy)(H 2 O)]to partially replace the bpdc 2 − ligand in UiO-67 to obtainRu-UiO-67 as an OER catalyst. 224 Morris and coworkers alsoreported UiO-67 thin films doped with a Ru water oxidationcatalyst (WOC) for electrochemical water oxidation. 225 The Ru-

iO-67 remains the high catalytic activity of the parent Rucomplex and exhibits chemical stabilities thanks to the UiO-67framework stabilzation.

CO 2 reduction

Similar to the behavior of the small molecular complexes, themain product of Co and Fe-porphyrin based MOF for CO 2

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Fig. 13. (a) Schematic presentation of heterogenization of single-site transition-metal catalysts in ZJU-28 via cation exchange. Reproduced with permission. 235 Copyright 2013, American Chemical Society. (b) Schematic illustration showing catalysis in MOFs using aperture- opening encapsulation. Reproduced with permission. 237 Copyright 2018, American Chemical Society.

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eduction is usually CO with high selectivity. Yaghi, Yang andoworkers employed [Al 2 (OH) 2 (M(tcpp))] (M = Zn, Cu, Co)s electrocatalysts for reducing CO 2 to CO. 226 Thereinto, theobalt-porphyrin MOF (Al 2 (OH) 2 TCPP-Co) showed a selectiv-ty of 76% to CO and high turnover number (TON = 1400).n situ spectroelectrochemical measurements showed that the

ajority of catalytic centers in this MOF were Co(I) reducedrom Co(II) during catalysis. In another case, Hupp, Farha andoworkers demonstrated the use of a thin film of Fe-porphyrin-ased MOF-525 (Fe- MOF-525) on a conductive electrode forlectrochemical reduction of CO 2 ( Fig. 12 (b)). 227 Due to theigh effective surface coverage of catalytic sites and the well-efined nanoscale porosity of the MOF, the chemical productsf the reduction, obtained with ∼100% Faradaic efficiency, wereixtures of CO and H 2 , constituting a potential feedstock for

ischer–Tropsch synthesis of hydrocarbons. Besides metal-por phyrin, the [M( bpy)(CO) 3 X] (M = Mn,

e, X = halide) complexes are well-known to be good ho-ogeneous electrocatalysts for the reduction of CO 2 to COith high TOF and selectivity. Recently, Ye et al. incorporated molecular catalyst, ReL(CO) 3 Cl (L = 2,2

′ -bipyridine-5,5

′ -icarboxylic acid), into surface-grafted MOF (SURMOF) thinlms that exhibiting an extremely higher faradaic efficiency than

ree Re complexes as well as high selectivity for electrocatalyzedO 2 reduction to CO. 228

Up to now, a number of active sites have been successfullymmobilized onto organic linkers to modify the pore surfaces of

OFs, leading to remarkable catalytic efficiency. This is a rationalnd straightforward method to implant the catalytic active siteso MOFs, which facilitates the accessibility of substrates to theatalytic centers immobilizing on the pore surfaces of MOFs.otably, this modification not only endows MOF catalysts with

nteresting size and shape-selective properties, but also makeshiral catalysis feasible over MOFs.

ENCAPSULATION OF CATALYTIC CENTERS IN

OFS

ue to the tunability of pore features (including size, shape andicroenvironment), MOFs are suitable host matrices for the

ncapsulation of diverse catalytic centers (metal complexes, metalPs, polyoxometalates, etc.) into the pores, and the resulting

uest@MOFs can serve as a multifunctional platform for theirynergistic effect in catalytic reactions ( Table 3 ).

eterogeneous catalysis olecular catalysts he immobilization of molecular catalysts into the poresf MOFs is a promising approach to isolate and protect

he active units. Alkordi et al. synthesized (indium-midazoledicarboxylate)-based rho-ZMOF with large cavitiesor hosting large catalytically active molecules, specificallyorphyrins. 229 The resultant Mn-porphyrin encapsulated rho-MOF was highly efficient on the oxidation of cyclohexane toyclohexanol/cyclohexanone. Similarly, Larsen et al. developed Fe4SP@HKUST-1 platform by encapsulating Fe III -tetrakis(4-

sulphonatophenyl) porphyrin (Fe4SP) into the pore spacef HKUST-1 for mimicking heme enzymes. 230 The catalyticfficiency of Fe4SP@HKUST-1 is comparable to that oficroperoxidase-11 (MP-11) enzyme and homogeneousolecular Fe4SP.

16

The mesoporous structure and superior stability of MIL-101ake it an ideal platform for the encapsulation of molecular

atalysts. 231–234 In 2013, a chiral Mn(III)salen complex wasntrapped in MIL-101 to obtain an enantioselective catalyst

n-salen@MIL-101(Al) 231 showing excellent recyclability andhe same selectivity as that of the homogeneous complex.ecently, Chołuj et al. incorporated Ru–alkylidene complexesearing ammonium-tagged NHC ligands inside MIL-101(Al)-H 2 �HCl, 234 the resulting materials were used as heterogeneous

atalysts in olefin metathesis reactions. Besides the encapsulation of molecular catalysts in MOFs

hrough ligand design and in situ entrapment, the encapsulationf molecular catalysts into the pores by ion interactions waslso explored. Genna et al. reported that NH 2 Me 2 + cations inhe pore of anionic ZJU-28 can be partially exchanged with variety of cationic complexes of Pd, Fe, Ir, Rh, and Ru Fig. 13 (a)). 235 The activity of Ru-MOF catalyst is comparableo its homogeneous counterpart for hydrogenation of 1-octene to -octane. In another work, Liu et al. synthesized two chiral porous

OFs functionalized with carboxylic acid groups as a host forncapsulation of an enantiopure organic amine catalyst throughcid-base interactions. 236

The catalyst-loaded MOFs have been demonstrated toe efficient and recyclable heterogeneous catalysts for thesymmetric direct aldol reactions with markedly improvedatalytic performance in relative to the homogeneous catalysts.ecently, an aperture-opening process resulting from dissociative

inker exchange in UiO-66 for encapsulation of the rutheniumomplex was developed ( Fig. 13 (b)). 237 The resultingRu]@UiO-66 was a very active catalyst, which exhibitedreater recyclability, slower bimolecular deactivation events andesistance to poisoning for the hydrogenation of CO 2 to formateompared to its homogeneous counterpart. In addition to actings a host for guests that have already been formed, an MOF can

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Table 3. List of encapsulation of catalytic centers in MOFs for catalysis.

MOF Encapsulated object Active sites Reactions Ref.

Fe4SP@HKUST-1 Fe4SP Fe4SP Selectively Encapsulated Heme 230

Mn-salen@MIL-101(Al) Mn-salen Mn-salen Enantioselective epoxidation of dihydronaphthalene 231

[Ru]@UiO-66 ( tBu PNP)Ru(CO)-HCl ( tBu PNP)Ru(CO)-HCl Hydrogenation of CO 2 to formate 237

Au@ZIF-8 Au NPs Au NPs CO oxidation 239

POMs@MIL-101 POMs POMs Oxidative reactions 254

polyILs@MIL-101 polyILs polyILs CO 2 c ycloaddition w ith epoxides 266

Pt@UiO-66-NH 2 Pt NPs Pt NPs Photocatalytic H 2 evolution 271

Pt@MOF/Au Pt NPs, Au NPs Pt NPs/Au NPs Photocatalytic H 2 evolution 272

UCNPs-Pt@MOF/Au UCNPs, Pt NPs, Au NPs UCNPs, Pt NPs, Au NPs Photocatalytic H 2 evolution 273

MoNi@MIL-101 MoNi NPs MoNi NPs Photocatalytic H 2 evolution 276

[CoII(TPA)Cl][Cl]@MIL-125-NH 2 [CoII(TPA)Cl][Cl] [CoII(TPA)Cl][Cl] Photocatalytic H 2 evolution 277

P 2 W 18 Co 4 @MOF-545 P 2 W 18 Co 4 P 2 W 18 Co 4 Photocatalytic water oxidation 282

M/NH 2 -MIL-125(Ti) (M = Pt and Au) Pt, Au Pt, Au Photocatalytic CO 2 reduction 283

Zn 2 GeO 4 /ZIF-8 Zn 2 GeO 4 Zn 2 GeO 4 Photocatalytic CO 2 reduction 287

Ru(bpy) 3 @PCN-99 Ru(bpy) 3 2 + Ru(bpy) 3 2 + Photocatalytic oxidative hydroxylation 299

Pd nanocubes@ZIF-8 Pd nanocubes Pd nanocubes Photocatalytic hydrogenation 303

CdS-UiO-66-NH 2 CdS CdS Photocatalytic alcohols oxidation 305

CdS-MIL-100(Fe) CdS CdS Photocatalytic alcohols oxidation 306

Co-salen@MIL-100(Cr) Co-salen Co-salen Electrocatalytic ORR

308

ε-MnO 2 / MOF(Fe) ε-MnO 2 ε-MnO 2 Electrocatalytic ORR

311

[H 6 CoW 12 O 40 ]@ZIF-8 H 6 CoW 12 O 40 H 6 CoW 12 O 40 Electrocatalytic OER

313

Cu/NU-1000/FTO Cu NPs Cu NPs Electrocatalytic CO 2 reduction 314

serve as a nanoreactor for the assembly of functional speciesthat are larger than the MOF w indows w ithin its pores. Ma andcoworkers developed a metal-action-directed de novo assemblystrategy to encapsulate the functionalized guest molecules into aMOF. 238 The resultant Co-Pc@bio-MOF-1 efficiently catalyzedthe styrene epoxidation reaction, the activity of which wassuperior to the homogeneous Co-Pc counterpart.

Metal nanoparticles MOFs are promising platforms for solid heterogeneous catalystsby embedding noble metal NPs into the pores which wouldrestrict the migration and aggregation of MNPs. Here somerepresentative examples with stable MOFs as supports areintroduced. The readers are suggested to refer to the existingreviews specifically dealing with this topic for an exhaustivecoverage of the literature in this area. 24

Jiang et al. reported Au NPs deposited onto ZIF-8 by grindingthe pre-treated ZIF-8 and desired quantitative volatile organogoldcomplex in an agate mortar in air at room temperature, followedby H 2 reduction. 239 The obtained Au@ZIF-8 was employed asan active catalyst in the gas-phase CO oxidation and exhibitedconsiderable activity. Subsequently, Aijaz et al. rationallyimmobilized ultrafine Pt NPs inside the pores of MIL-101(Cr),without aggregation of Pt NPs on the external surfaces of theframework, by using a “double solvents” method. 240 The as-synthesized Pt@MIL-101(Cr) composite was employed forcatalytic reactions in all three phases: liquid-phase ammoniaborane hydrolysis, solid-phase ammonia borane thermaldehydrogenation, and gas-phase CO oxidation.

Yuan et al. used MIL-101(Cr) as the support for palla-dium NPs to catalyse a water-mediated coupling system. 241

The resulting Pd/MIL-101(Cr) was applied as a heteroge-neous catalyst for water-mediated Suzuki-Miyaura and Ull-mann coupling reactions of aryl chlorides with negligible metalleaching and high catalytic activity over a number of cycles.In another case, palladium supported amine-functionalized

17

MIL-101, Pd/MIL-101(Al)-NH 2 242 shows selective hydrogena-

tion of biomass-based 5-hydroxy methylf urf ural (HMF) to 2,5-dihydroxymethyl-tetrahydrofuran. The Pd@MIL-88B-NH 2 (Fe)exhibits highly selective heterogeneous procedures for C

–Hactivation/halogenation reactions. 243 Most recently, Tang andcoworkers reported sandwiching Pt NPs between an inner coreand an outer shell comprising an MOF with metal nodes of Fe 3 +(MIL-101(Fe)), Cr 3 + (MIL-101(Cr)) or both, which results instable catalysts that convert a range of α, β-unsaturated aldehy-des with high efficiency and significantly enhanced selectivitytowards unsaturated alcohols. 244 In addition, UiO-67 was chosenas the host matrix for the encapsulation of metal NPs becauseof its high physicochemical stability and tunable functionalitiesof ligands with 2,2-bipyridine moieties to serve as anchor sitesfor the metal centers. Li and coworkers developed a method toencapsulate palladium precursors through ligand design prior toUiO-67 assembly, achieving uniformly distributed Pd(II) insidethe pores of MOFs. The prepared Pd(II)-in-UiO-67 was treatedunder H 2 at 250 °C for 4 h to afford Pd

0 -in-UiO-67, whichshowed excellent shape-selectivity in hydrogenation of olefin andhigh catalytic efficiencies in aerobic oxidation of alcohols andnitrobenzene reduction. 245 The DMF solution of Pd

0 -in-UiO-67was bubbled with H 2 for 1 h at room temperature, AgNO 3 solu-tion was added under stirring, leading to the growth of Ag on thesurface of the embedded Pd NPs. Therefore, Pd@Ag core-shellNPs on MOFs were formed by a seed mediated growth strategy( Fig. 14 (a)), which can be ascribed to the presence of activatedphysiosorbed H atoms on embedded Pd NPs as reducing agentsto selectively direct the deposition of A g onto Pd while minimiz -ing Ag self-nucleation. 246 The as-synthesized Pd@Ag core–shellNPs showed a higher selectivity in the partial hydrogenation ofphenylacetylene than their monometallic counterparts, resultingfrom the surface dilution and electron modification of the surfacePd sites by Ag deposition. In addition, Pd@Ag NPs exhibitedunparalleled high stability and recyclability in the catalyticreactions, which can be attributed to the nano-confinement

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Fig. 14. Schematic illustration of (a) the fabrication of Pd@Ag core-shell NPs encapsulated in the MOF pore and (b) the synthesis of HP-MOFs with adjustable porosity. (a) Adapted with permission. 246 Copyright 2016, Royal Society of Chemistry; (b) Reproduced with permission. 263 Copyright 2017, Wiley-VCH.

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ffect and the strong metal–support interaction provided byhe MOF.

In addition, other metal NPs immobilized into stable MOFsere also studied. 247–251 For example, the UiO-bpy MOF wassed to anchor ultrasmall Cu/ZnOx NPs in order to prevent

he agglomeration of Cu NPs and phase separation betweenu and ZnOx in MOF-cavity-confined Cu/ZnOx NPs. 251 The

esultant Cu/ZnOx@MOF catalysts exhibit high activity and00% selectivity for CO 2 hydrogenation to methanol. Veryecently, metal NP-encapsulated MOFs with controllable spatialocalization were realized through employing a metal oxide asoth the support to load metal NPs and the sacrificial template torow MOFs. 252,253 The resultant metal NPs@MOF compositesxhibit not only selectivity contributed by MOF cavities, but alsonhanced activity resulting from the spatial regulation of metalPs as close as possible to the MOF surface.

olyoxometalates (POMs) olyoxometalates (POMs) offer numerous advantages asatalysts making them economically and environmentallyttractive. Numerous efforts have been made using MOFs tommobilize POMs to stabilize POMs and optimize the catalyticerformance. By using titanium, cobalt mono-substitutedhosphotungstates and the Keggin anion [PW 12 O 40 ] 3 −,OMs@MIL-101 was prepared and used as a catalyst forxidative reactions with O 2 and H 2 O 2 . 254,255 The inclusion of 3 [PW 12 O 40 ] in [Cu 3 (btc) 2 ] to prepare active heterogeneous

atalysts has been investigated as well. 256–259 In 2013, Hatton andoworkers prepared phosphotungstic acid (H 3 PW 12 O 40 , PTA)unctionalized MOFs and employed them as a sorbent in thehemical conversion of acetaldehyde vapor. 260

Encasing POMs logically requires the pore openings of MOFsmaller than the POMs radius. Jie et al. reported that a MOF withore sizes closely matching a POM guest self-assembles around

his guest to make a material, CuPW 11 O 39 @MOF-199, whichossesses significantly enhanced catalytic activity than either theOM or MOF component alone. 261 By a related approach, Balula,unha-Silva and coworkers reported encapsulation of the POM

18

W 11 Fe into the NH 2 -MIL-101(Fe) to yield PW 11 Fe@NH 2 -IL-101(Fe), which has been employed for the ring-opening

f styrene oxide with amines. 262 The very small pore sizes ofost MOFs sometimes restrict the immobilization of POMsith large size. In this sense, Jiang and coworkers devel-ped a versatile modulator-induced defect-formation strategy tochieve the controllable synthesis of hierarchically porous MOFsHP-MOFs) with high stability for encapsulating large activeOM (HP W, H 3 P W 12 O 40 ·nH 2 O). 263 The resultant HPW-

mpregnated HPUiO-66 (HPW/HP-UiO-66) gave significantlyigher activity than HPW-impregnated UiO-66 (HPW/UiO-66)ecause a higher content of active HPW was loaded into HP-UiO-6 ( Fig. 14 (b)). Recently, Ahn et al. encapsulated a high weight

oading of phosphotungstic acid (PTA, H 3 PW 12 O 40 ) within NU-000 and demonstrated that the POM and MOF structures re-ained intact following the benchmark strong acid-catalyzed re-

ction of oxylene isomerization/disproportionation at 523 K. 264

ther active species n addition to the above discussed stuffs, many other functionaluests such as metal oxide, 251 metal hydroxide, 265 ionic

iquids (ILs), 266 and biomacromolecules 267 have also beenomposited with MOFs for catalysis. Jiang and coworkersonfined imidazolium-based poly(ionic liquid)s (denoted asolyILs) into MIL-101 via in situ polymerization. 266 Theesultant polyILs@MIL-101 exhibited high activity towards CO 2 ycloaddition w ith epoxides at sub-atmospheric pressure in thebsence of any cocatalyt due to its good CO 2 capture capability.hou and coworkers reported a water-stable MOF, PCN-333(Al)

hat can encapsulate three enzymes with different sizes, namelyorseradish peroxidase (HRP), cytochrome c (Cyt c) andicroperoxidase-11 (MP-11). 267 These immobilized enzymes

xhibited higher catalytic performance than free enzymes.

hotocatalysis n addition to the metal nodes and the organic linkers, the porepace in MOFs affords great opportunities for photocatalysis.uitable photocatalysts can be introduced into the pores of

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Fig. 15. (a) Schematic illustration showing the synthesis of Pt@MIL-125/Au. Adapted with permission. 272 Copyright 2018, Wiley-VCH. (b) One-pot synthesis of the POM@MOF system for photocatalytic hydrogen production. Reproduced with permission. 278 Copyright 2015, American Chemical Society.

P

MOFs as guests and exhibit favorable properties resulting fromthe synergistic effect between the host and guest. Meanwhile, themicroenvironment of MOF pores might be finely adjusted forenergy and electron transfer.

Photocatalytic water splitting

Given the efficiency of charge separation plays a vital importancerole in photocatalysis, various cocatalysts, such as noble metalNPs, metal complexes and polyoxometalates (POMs), etc.,have been integrated into MOFs, exhibiting excellent catalyticperformance for hydrogen production. 268–279

The incorporation of precious metal NPs into MOFs wasfound to dramatically enhance the photocatalytic activity.Jiang, Zhang and coworkers investigated Pt NPs incorporatedinside or supported on a representative UiO-66-NH 2 , denotedas Pt@UiO-66-NH 2 and Pt/UiO-66-NH 2 , respectively, forphotocatalytic hydrogen production. As a result of the shorterpath of electron transfer from the MOF to internal Pt, Pt@UiO-66-NH 2 exhibited higher photocatalytic hydrogen productionactivity than Pt/UiO-66-NH 2 . 271 Later, they exploited a novelPt@MOF/Au catalyst composed of two types of metal-MOFinterfaces ( Fig. 15 (a)), Pt@MOF and MOF/Au, offering excep-tionally high photocatalytic H 2 production rate through watersplitting under visible-light irradiation. 272 The phenomenon wasattributed to the integration of the surface plasmon resonanceexcitation of Au nanorods and the Pt-MOF Schottky junction,which can extend the light-absorption range and greatly acceler-ate the charge transfer, respectively. In their subsequent work, thecore–shell structured up-conversion nanoparticles (UCNPs)-

t@MOF/Au composites were rationally fabricated to achieve abroadband spectral response from the UV to near-infrared (NIR)region for photocatalytic hydrogen production. 273

The limited reserves and high cost of precious metal NPshinder their large-scare application. Recently, earth-abundantmetal NPs have been encapsulated within MOFs for photo-catalysis. For example, Lu and co-workers incorporated Ni NPsinto MOF-5 with a high dispersion, the obtained Ni@MOF-5 composite showed high photocatalytic activity for hydrogenproduction comparable to Pt@MOF-5. 275 Nonprecious metalalloys can also be introduced into MOF for photocatalytichydrogen production. Lu and co-workers encapsulated MoNialloy NPs into MIL-101 (MoNi@MIL-101) by the double

19

solvents method for photocatalytic hydrogen production undervisible light irradiation. 276

The strategy to improve the photocatalytic performance ofMOFs is not limited to the combination with metal NPs, theimmobilization of other active species in MOFs has also beendeveloped. Jiang and coworkers have rationally encapsulated aCo(II) molecular photcatalys inside the cages of MIL-125-NH 2 for visible-light-driven H 2 production. 277 Recently, Lin andcoworkers developed a simple and effective charge-assistedself-assembly process to encapsulate a noble metal-free POMinside the cavities of a MOF ( Fig. 15 (b)). 278 The resultantPOM@MOF enabled fast multielectron injection from thephotoactive framework to the POMs upon photoexcitation,leading to efficient visible-light-driven hydrogen production.

Recently, a series of water oxidation catalysts, such as POMs,molecular catalysts and cobalt oxide have been encapsulated intothe pores of MOFs for photocatalytic water oxidation. 232,280–282

Das and co-workers incorporated a molecular water oxidationcatalyst (MnTD) into the pores of MIL-101(Cr) for oxygengeneration. 232 The resulting composite exhibited slightly loweractivity but much enhanced stability compared to the homoge-neous catalyst. In a later publication, a sandwich-type POM wasimmobilized in the hexagonal channels of the Zr(IV) porphyrinicMOF-545 hybrid framework. 282 This work achieved the stableincorporation of the photosensitizer and catalysts within a solidMOF. The resulting material exhibited a high photocatalyticactivity and good stability for visible-light-driven water oxidation.

Photocatalytic CO 2 reduction

MOF-based composite photocatalysts with high photocatalyticCO 2 reduction activity have been designed and synthesizedby the incorporation of MOFs with semiconductors, metals,or photosensitizers. 200,283–295 Li and coworkers synthesized M-doped NH 2 -MIL-125(Ti) (M = Pt and Au) for the CO 2 pho-toreduction reaction, 283 in which H 2 and HCOO

− were detectedover M-doped NH 2 -MIL-125(Ti), while only HCOO

− was de-tected over pure NH 2 -MIL-125(Ti), indicating that the electron-trapping effect of noble metals formed and thus changed the cat-alytic selectivity. Choi et al. obtained Ag-nanocubes–MOF core-shell composite Ag ⊂Re 3 -MOF to conduct the photocatalyticCO 2 reduction reaction, exhibiting a sevenfold enhancement inthe CO 2 -to-CO conversion activity compared to pure Re 3 -MOFdue to the fact that the photoactive Re metal sites in the shell

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ere confined spatial ly into a local ly strong electromagnetic fieldaused by the plasmon resonance of the Ag nanocubes. 200

The incorporation of semiconductor bearing superiorhotocatalytic CO 2 reduction activity into MOFs is an effectiveethod to obtain photocatalysts that can inherit the advantages

f both materials. Especially, the direct contact between theman further improve the electron-transfer efficiency for enhancedhotocatalytic CO 2 reduction activity. Liu et al. synthesizedn 2 GeO 4 /ZIF-8 hybrid nanorods by growing ZIF-8 NPs onn 2 GeO 4 nanorods. 287 The Zn 2 GeO 4 /ZIF-8 nanocomposite

nherited high CO 2 adsorption capacity of ZIF-8 and highrystallinity of Zn 2 GeO 4 nanorods, exhibiting high activity inhe photocatalytic conversion of CO 2 into liquid CH 3 OH fuel.iong and coworkers synthesized a HKUST-1@TiO 2 core-shell

tructure and examined their photocatalytic performance of CO 2 hotoreduction. 288 As the photogenerated electrons could beffectively transferred from the semiconductor to the MOF, theKUST-1@TiO 2 exhibited sound performance in terms of both

ctivity and selectivity. Similarly, CPO-27-Mg/TiO 2 compositeas successfully obtained via a hydrothermal method. 290 ThePO-27-Mg/TiO 2 composite showed enhanced photocatalyticerformance for CO 2 reduction because of its high adsorptionapacity towards CO 2 and the existence of open alkaline metalites. Additionally, incorporating nanosized carbon nitrideanosheets with UiO-66 via a facile electrostatic self-assemblyethod also could improve photocatalytic activity for the CO 2

onversion. 291

Additionally, a physical mixing of MOFs with metal-complexhotosensitizers is also a feasible strategy. Wang et al. conducted

he photocatalytic conversion of CO 2 into CO and H 2 in theresence of the [Ru(2,2-bpy) 3 ]Cl 2 ·6H 2 O photosensitizer ando-ZIF-9 cocatalyst, 294 indicating that the addition of Co-ZIF-9ad an enhancement effect on CO 2 photoreduction. Recently,ome novel MOF materials such as Co-ZIF-67

295 and Co 6 -MOF96 were synthesized to combine with [Ru(2,2-bpy) 3 ]Cl 2 ·6H 2 Ond exhibited high photocatalytic efficiencies in the conversionf CO 2 into CO and H 2 compared to their correspondingu-complex photosensitizer.

hotocatalytic organic transformation

he porous structure of MOFs allows for photoredox species toe encapsulated for a variety of organic transformations. 297,298

hou and coworkers reported the encapsulation of Ru(bpy) 3 2 +

nto an anionic MOF (PCN-99) through ion exchange. 299 Theesulting Ru(bpy) 3 @PCN-99 showed relatively high efficiencyor photocatalytic oxidative hydroxy lation of ary lboronic acids.ome fascinating studies based on encapsulated photocatalysisere reported using polyoxometalates (POMs) trapped inOF pores. Duan and coworkers immobilized decatungstate

or light-driven acceleration of β- or γ -site C

–H alkylation ofliphatic nitriles by incorporating photosensitizing [W 10 O 32 ] 4 −nions within the pores of copper-based MOF. 300 The resultingecatungstate-based MOF exhibited remarkable photocatalyticctivities for the β- or γ -site C

–H alkylation of aliphaticitriles under mild conditions, with good size selectivity andecyclability.

Loading metal NPs into MOFs can enhance charge separationy driving photoexcited electrons from the MOF to the NPs,

hus improving photocatalytic organic transformations. 301–304

iang and coworkers rationally fabricated Pd nanocubes@ZIF-8omposite by encapsulating Pd nanocubes into ZIF-8 for hydro-enation of olefins ( Fig. 16 ). 303 This composite exhibited highctivity and selectivity at room temperature under 1 atm H 2 and

20

ight irradiation due to the surface-plasmon-driven photothermalffect of the Pd NCs and the specific properties of the MOF,hich accelerated the reaction by H 2 enrichment, acted as a

molecular sieve” for olefins with specific sizes and stabilized thed cores. By integrating Pt nanocrystals with porphyrinic MOFs,CN-224(M), they later designed and synthesized Pt/PCN-24(M) composites. 304 The resultant composites exhibitedxcellent catalytic performance towards the selective oxidationf aromatic alcohols to corresponding aldehydes by 1 atm O 2 atmbient temperature under light irradiation, based on synergetichotothermal effect and singlet oxygen production.

In addition to metal NPs, the combination of semiconductorsith MOFs has been studied for photocatalytic organic transfor-ations. For example, CdS-UiO-66-NH 2 nanocomposites were

abricated by photo-deposition CdS on the surface of UiO-66-H 2 . 305 The CdS-UiO-66-NH 2 nanocomposites showed good

onversion and high selectivity for photocatalytic oxidation oflcohols to the corresponding aldehydes under mild conditions.imilarly, CdS-MIL-100(Fe) nanocomposites were also reported

o be efficient photocatalysts for selective oxidation of benzyllcohol to benzaldehyde. 306 Such a photocatalytic reaction haslso been demonstrated by decorating mesoporous HKUST-1ith amorphous TiO 2 layers. 307 Incorporation of amorphousiO 2 within the mesoporous MOF could successfully develop aew type of photocatalyst system for selective aerobic oxidationf benzylic alcohols with > 93% selectivity and 89% conversionfter 15 h light irradiation.

lectrocatalysis RR

s porous frameworks, MOFs can embed ORR active moleculesnside their pores to combine the advantages of the heteroge-eous nature of the MOF host and the catalytic activity of theolecular guest. Miao et al. investigated the ORR performance

f a host-guest inclusion material Co-salen@MIL-100(Cr) 308 bysing the ‘‘ship-in-a-bottle” synthetic strategy. MOFs can alsoomposite with conventional ORR catalysts such as graphenend metal sulfide to achieve improved electrocatalytic perfor-ances. 222,309,310 For example, Wang et al. integrated ε-MnO 2

nd a MOF(Fe) support to synthesize ε-MnO 2 / MOF(Fe)omposite. 311 In the composite, ε-MnO 2 was in the form ofanorods, where each had one end protruding, and the other endrmly anchored onto the MOF(Fe) matrix. Due to its uniquetructure, the composite exhibited much better ORR activity andtability than that of ε-MnO 2 in alkaline electrolyte.

ater splitting

ecently, Zhang et al. synthesized two POM-encapsulatedetal-organic nanotube (MONT) framework crystallineaterials, HUST-200 and HUST-201, with exceptional chemical

tabilities. 312 Remarkably, HUST-200 was a highly efficientlectrocatalyst for HER in acidic aqueous medium, displaying low overpotential of 131 mV (catalytic current density isqual to 10 mA cm

−2 ). Mukhopadhyay et al. incorporated theeggin anion [CoW 12 O 40 ] 6 − into the well-defined void spacef ZIF-8 for electrocatalytic oxygen evolution. 313 The resultingH 6 CoW 12 O 40 ]@ZIF-8 showed good OER activ ity w ith aOF of 10.8 mol O 2 ·(mol Co) −1 s −1 and retained its initial

ctivity even after 1000 catalytic cycles under neutral conditions.ontrolled experiments eliminated the chances of formation andarticipation of CoO x during electrocatalytic water oxidation.

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Fig. 16. Self-assembly of Pd nanocubes@ZIF-8 for hydrogenation of olefins. Adapted with permission. 303 Copyright 2016, Wiley-VCH.

CO 2 reduction

Farha, Hupp and coworkers embedded copper NPs into athin film of NU-1000 grown on the fluorine-doped tin oxide(FTO) surface. 314 The obtained composite exhibited promisingelectrocatalytic activity for CO 2 reduction, with 31% faradaicefficiency at an applied potential of −0.82 V vs. RHE and formateas the major product.

The permanent pore space in MOFs offers the pre-requisiteof encapsulating the guest catalytic centers in a defined en-vironment, which prevents catalytic site leaching by diverseinteractions. However, upon guest encapsulation, the porositysignificantly decreases in MOFs and the diffusion limitation in thecomposite might not be ignored during the reaction. There hasbeen very few report on this specific aspect. It is highly desired toencapsulate active centers by highly porous MOFs that are ableto almost maintain the porosity of MOFs to ensure the transportand diffusion of substrates and products.

MOF DERIVATIVES

In the recent 10 years, MOF derivatives haves been intensivelystudied for catalysis, 26 since the first seminal work on MOFpyrolysis was reported in 2008 by Xu and coworkers 315 Thereports on MOF derivatives present a trend of explosive increase( Table 4 ), especially towards energy and catalysis applications,due to their particular strengths.

Heterogeneous catalysis Abundant reports have demonstrated that diverse MOF-derivedporous materials, such as porous carbons, metal/metal ox-ide/metal sulfide/metal phosphide nanostructures and theircomposites, are very promising for various organic reactions,including oxidation reactions, reduction reactions and otherreactions.

Oxidation reactions In recent years, many MOF-derived porous catalysts have beendeveloped for plentiful oxidation reactions, involving oxidationof alcohols, cyclohexane and toluene, oxidative coupling ofamines and aerobic epoxidation of styrene. 316–325 Jiang andcoworkers exploited a Co-based MOF, ZIF-67, as a self-templateto generate Co NPs with surface-oxidized CoO species uniformlyincorporated in N-doped porous carbon by one-step pyroly-

21

sis, Co-CoO@N-doped porous carbon nanocomposites, whichexhibited excellent catalytic activity, selectivity and magneticrecyclability towards the direct oxidation of alcohols to estersunder mild conditions, with 1 bar of O 2 as an environmentallyfriendly oxidant ( Fig. 17 (a)). 319 In addition, other MOF-derivedporous catalysts, such as Co@C-N, 320,325 Fe 3 O 4 @C, 321 andFe-Co/C, 322 have also been developed for different oxidationreactions by simple pyrolysis of respective Fe, Co-based MOFs.Not limited to metal NPs, highly dispersed clusters can also bestabilized by MOF-derived porous carbons. Li, Wang and co-workers successfully prepared atomically dispersed Ru 3 clustersstabilized by N species (Ru 3 /CN) via pyrolysis of the composite,where well-defined Ru 3 (CO) 12 was separated as a precursorby suitable molecular-scale cages of ZIF-8 ( Fig. 17 (b)). 324

Importantly, the Ru 3 /CN catalyst exhibited excellent catalyticactivity towards the oxidation of 2-aminobenzaldehyde with100% conversion, 100% selectivity and an unexpectedly highTOF (4320 h

−1 ), which are superior to those of Ru single atomand small-sized Ru particle catalysts.

Besides liquid-phase oxidation reactions, many MOF-derivedporous materials have also been applied for gas-phase oxidationreactions, such as CO oxidation. A number of porous Cu-basednanocomposites, such as CuO/CeO 2 , 326,327 CeO 2 :Cu

2 + , 328

CuO/TiO 2 , 329 CuO/Cu 2 O

330 and Cu/CuO X /C

331 derivedfrom Cu-MOF or Cu-MOF-based composites, have beenreported for catalytic CO oxidation. Chen and co-workers havesuccessf ully sy nthesized Cu/CuO x nanojunctions stabilized andsupported by porous carbon through direct annealing of Cu-BTCin N 2 . 331 The unique structure prevented the fast oxidation ofcopper and allowed the co-existence of Cu, Cu 2 O and CuO onthe surface. The Cu/CuO x /C nanocomposites showed a remark-able catalytic activity towards CO oxidation with a complete COconversion temperature ( T 100 ) of 155 °C under both 1 vol% and5 vol% CO, and it maintained long-term durability even after 40 hunder 1 vol% CO. Very recently, some Co-based nanocomposites,such as CeO 2 –Co 3 O 4 mixed metal oxides, 332 Co/C-600, 333 ex-hibited excellent catalytic performance for preferential oxidationof CO. Wang et al. have demonstrated a novel approach for theeffective synthesis of CeO 2 –Co 3 O 4 using uniform short CeO 2 nanowires self-inserted into ZIF-67 nanocrystals as precursorsfollowed by a thermal annealing treatment. 332 The obtainedCeO 2 –Co 3 O 4 hybrids displayed excellent catalytic performancefor NO reduction by CO, which was much better than the currentperformance reported for Cu–Ce mixed metal oxides.

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Table 4. Summary of the MOF derivatives for catalysis.

Catalyst MOF precursor Reaction Ref.

Co-CoO@NC ZIF-67 Alcohol esterification 319

Ru 3 /CN Ru 3 (CO) 12 @ZIF-8 Alcohol oxidation 324

Cu/CuO x /C Cu-BTC CO oxidation 331

Pd@NC ZIF-8 Hydrodeoxygenation 334

Co@Pd/NC ZIF-67 Hydrogenation 339

ZnO@NPC-O x ZIF-8 CO 2 cycloaddition 350

Fe@C Fe-BTC Fischer–Tropsch 351

Cu/Zn@C Zn–Cu-BTC CO 2 hydrogenation 355

Fe 2 O 3 @TiO 2 Fe-MIL-101 Photocatalytic H 2 evolution 374

T-CoO x -C ZIF-67 Photocatalytic water oxidation 382

POM@Co 3 O 4 POM@ZIF-67 Photocatalytic water oxidation 383

Fe@C Fe-MIL-101 Photocatalytic CO 2 Reduction 384

In 2 S 3 -CdIn 2 S 4 MIL-68 Photocatalytic CO 2 Reduction 385

Co-N 4 Cobalt imidazolate framework Electrocatalytic ORR

386

Fe/N-GPC FeD@MIL-101-NH 2 Electrocatalytic ORR

398

Fe SA –N–C PCN-222 Electrocatalytic ORR

399

Co@BCN ZIF-67 Electrocatalytic HER

400

MoO 2 @PC-RGO Cu-based POMOFs/GO Electrocatalytic HER

403

Co 3 O 4 /NiCo 2 O 4 ZIF-67/Ni-Co Electrocatalytic HER

404

CoP/rGO ZIF-67/GO Electrocatalytic HER/OER

407

Ni SAs/N–C Ni 2 + @ZIF-8 Electrocatalytic CO 2 Reduction 408

C-AFC@ZIF-8 AFC@ZIF-8 Electrocatalytic CO 2 Reduction 409

CoP HNC ZIF-67 Electrocatalytic N 2 Reduction 411

Ru SAs/N–C ZIF-8 Electrocatalytic N 2 Reduction 413

Fig. 17. (a) Schematic illustration of the synthesis of Co-CoO@N-doped porous carbon nanocomposites via the pyrolysis of ZIF-67. Adapted with permission. 319 Copyright 2015, The Royal Society of Chemistry. (b) Schematic illustration of the Ru 3 /CN preparation process. Reproduced with permission. 324 Copyright 2017, American Chemical Society.

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eduction reactions ot limited to oxidation reactions, MOF-derived materials are

ble to stable active species for reduction reactions, such asnsaturated bonds hydrogenation, hydrogenation of diverseitro compounds and so on. 334–344 Jiang and co-workerseported ZIF-8 pyrolysis product, N-doped carbon, can bemployed to stabilize ultrafine Pd NPs for hydrodeoxygenationf vani l lin. 334 Given the high cost of noble metals, transitionetal catalysts, including Co

336,337,345-347 and Ni 338,348 basedaterials, have been developed. Recently, Liu et al. synthesizedo@NC catalyst by using N-containing Co-MOF for highly

elective hydrogenation of α, β-unsaturated aldehydes under

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ild conditions. 346 Kang et al. proposed a novel strategy forynthesizing non-noble metal catalysts that were supported onorous SiO 2 using MOFs as metal precursors, and Ni/SiO 2 nd Co/SiO 2 catalysts were synthesized with an averageetal particle size of 0.9 nm by this method ( Fig. 18 ). 338 The

atalysts exhibited excellent activity and stability for benzeneydrogenation to cyclohexane in liquid phase below 100 °C.part from the utilization of monometallic NPs, Li and co-orkers have synthesized Co@Pd core-shell NPs embedded in

he N-doped carbon matrix (Co@Pd/NC) with an average size ofa. 9.4 nm and a ultrathin Pd shell by using ZIF-67 and Pd(NO 3 ) 2 s the precursor and Pd source, respectively. 339 The Co@Pd/NC

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Fig. 18. Scheme of the preparation of Ni/SiO 2 catalysts. Adapted with permission. 338 Copyright 2016, Wiley-VCH.

F

exhibited much higher activity than MOFs supporting noble NPsfor hydrogenation of nitrobenzene under identical conditions.

Additionally, the incorporation of metal oxide onto thecarbon matrix was also attempted for catalytic reductionreactions. Recently, Jiang and co-workers developed γ -

e 2 O 3 @porous carbon nanocatalyst for the hydrogenationof nitro compounds via pyrolysis of Fe-MIL-88A. 341 Theγ -Fe 2 O 3 -based nanocatalyst exhibited excellent catalyticactivity, chemoselectivity and magnetic recyclability forthe hydrogenation of diverse nitro compounds under mildconditions. Later, they synthesized Co-CoO@NC catalyst withZIF-67 for effective tandem dehydrogenation of NH 3 BH 3 andhydrogenation of nitro compounds at room temperature. 342

Aside from metal/metal oxide NPs embedded in the carbonmatrix, some MOF-derived metal-free porous carbon-basedmaterials are active for catalytic hydrogenation as well. 343,344

Jiang and coworkers obtained metal-free porous carbon-basedmaterials with various forms and contents of nitrogen dopant,degrees of graphitization, porosities, and surface areas uponpyrolysis of different MOF precursors including MOF-5, ZIF-8,and PCN-224 for the catalytic reduction of 4-nitrophenol to4-aminophenol, and found that the pyrrolic nitrogen speciesplayed an important role for the catalytic efficiency. 343

Other reactions Except for the organic catalytic oxidation and reduction reactions,many other types of catalytic reactions, including CO 2 fixation re-action, 349,350 Fischer–Tropsch (FT) synthesis 351–354 CO 2 hydro-genation reaction, 345 etc. have been intensively investigated withthe utilization of MOF-derived composites. Jiang and co-workershave developed ZnO NPs encapsulated N-doped porous carbon(ZnO@NPC-O x ) by the pyrolysis of ZIF-8 and subsequentoxidation treatment. 350 The resulting catalyst exhibited excellentactivity, selectivit y, and recyclabilit y in the CO 2 cycloadditionreactions with epoxides under mild conditions. Fischer–Tropsch(F–T) synthesis has been recognized as one of the most feasiblemethods for turning syngas into liquid fuels. Santos et al.synthesized highly dispersed iron carbides embedded in a matrixof porous carbon using MOFs as catalyst precursors. 351 Very highiron loadings ( > 40 wt%) were achieved while maintaining anoptimal dispersion of the active iron carbide phase. The unique

23

iron spatial confinement and the absence of large iron particlesin the obtained solids minimized catalyst deactivation, resultingin high active and stable operation for FT reaction. In orderto improve Cu-based catalysts for CO 2 hydrogenation reaction,Zhang et al. prepared hierarchical porous Cu@C and Cu/Zn@Cstructures with Zn doped Cu-BTC. 355 Benefiting from thesynergistic effects between Cu and Zn, catalyst Cu/Zn@Cexhibited superior activ ity w ith 5.0% CO 2 conversion and 100%CO selectivity at 500 °C under a total pressure of 1 bar.

Photocatalysis In recent years, except for the photodegradation of dyes, 356–373

MOF-derived porous composites have also been developed forphotocatalysis, including photocatalytic water splitting and CO 2 reduction. 374–385 The MOF-derived photocatalysts possess theirunique merits, such as, facilitating the diffusion of substrates andproducts and suppressing the volume recombination of electronsand holes.

Photocatalytic water splitting

Titanium dioxide, as a well-known photocatalysts, has beenintensively investigated for photocatalytic water splitting.However, TiO 2 can harvest UV-light only. Lin and co-workershave developed a novel MOF-templated approach to synthesizecrystalline octahedral nanoshells composed of hematite Fe 2 O 3 NPs embedded in anatase TiO 2 with some Fe dopant. 374 Thismaterial enabled photocatalytic hydrogen production fromwater using visible light, which neither component alone wasable to do so. In addition to metal oxides, metal sulfides withphotocatalytic activity can also be synthesized from MOFs. Jiangand co-workers have rationally fabricated hierarchically porousmetal oxides and sulfides based on thermally stable MOFs as hardtemplates ( Fig. 19 (a)). 379 The obtained hierarchically porousCdS possessed remarkably higher photocatalytic hydrogenproduction activity than the corresponding bulk or nanosizedcounterparts because of its nanosize effect and porosity.

MOF-derived nanocomposites have also been employed forwater oxidation. Lu and co-workers developed a porous cobaltoxide–carbon hybrid, named T-CoO x –C, which was preparedby pyrolysis of ZIF-67. 382 The optimal 700-CoO x –C exhibitedexcellent photocatalytic activity for water oxidation with highTOF of 0.039 ± 0.03 s −1 by using Ru(bpy) 3 (PF 6 ) 3 as anoxidant. Later, Lan et al. successfully prepared a porous hollowmolecule@oxide catalyst using POM@ZIF-67 composites asthe precursors. 383 The resultant POM@Co 3 O 4 exhibited asignificantly enhanced activity compared to pure porous Co 3 O 4 materials in the photocatalytic water oxidation reaction.

Photocatalytic CO 2 reduction

Recently, Ye’s group realized the solar-driven CO 2 conversion intoCO using a MOF-derived Fe@C catalyst, which was composedof an iron core < 10 nm and ultrathin carbon layers. 384 Due tothe photothermal effect, the local surface-plasmon resonancesof Fe, the confinement of the carbon layer, the carbon layer-coated catalysts exhibited improved catalytic performance inthe solar-driven CO 2 conversion by H 2 compared with Fe/SiO 2 and Fe/CNT catalysts. Wang et al. fabricated MOF-derivedhierarchical In 2 S 3 –CdIn 2 S 4 heterostructured nanotubes througha self-templated strategy for photocatalytic reduction of CO 2 w ith v isible light ( Fig. 19 (b)). 385 The nanotubes exhibitedsuperior performance for deoxygenative CO 2 reduction withhigh CO generation rate and outstanding stability, which wasassociated to their unique structural and compositional features.

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Fig. 19. (a) Schematic illustration for the synthesis of hierarchically porous metal oxides/sulfides templated by MOFs by a nanocasting process. Adapted with permission. 379 Copyright 2017, Wiley-VCH. (b) Schematic illustration of the synthetic process of the hierarchical In 2 S 3 -CdIn 2 S 4 heterostructured nanotube. Reproduced with permission. 385 Copyright 2017, American Chemical Society.

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lectrocatalysis n reference to pristine MOFs, MOF derivatives featuring muchnhanced chemical stability and conductivity have been widelysed for electrocatalysis, mainly composing of oxygen reductioneaction (ORR), electrochemical water splitting (OER/HER),nd electrochemical CO 2 /N 2 reduction over the last severalears.

RR

t-based catalysts always possess excellent activity for ORR. Toeduce the high cost of noble metals, extensive studies havehown the MOF-derived metal/metal oxide-carbon nanohybridsMMCs) are able to provide a good alternative to the Pt-ased e lectrocatalysts for ORR. Among various transition metal-ased MMCs systems, Co–N–C and Fe–N–C are importantepresentatives. Ma et al. reported a seminal work in MOF-erived materials for ORR. 386 They selected a cobalt imidazolate

ramework as the precursor for the synthesis of electrocatalystsith high-density and regularly distributed Co-N 4 centers. The

atalyst showed an onset potential of 0.83 V vs a reversible hydro-en electrode (RHE). Since the first report on converting ZIFs toorous carbons with large surface areas by Xu and coworkers, 387

great many of ZIF-derived electrocatalysts were explored foriverse applications, including ORR. 388–394 Jiang and coworkersynthesized a series of bimetallic ZIFs (BMZIFs) based on thesostructural ZIF-8 and ZIF-67 with varied Zn/Co ratios. 391 Thenprecedented BMZIF-derived porous carbons inherited respec-ive advantages of carbons from ZIF-8 (large surface area) andIF-67 (high degree of graphitization and uniform distributionf N and CoN x active species), exhibiting excellent ORR activitypproaching the level of commercial Pt/C in alkaline media.

Given that the Fe-containing catalysts sometimes tend toe more active than Co-containing catalysts, the Fe-containingOFs as significant alternatives were widely investigated to

roduce active ORR catalysts. 395–398 Zhu et al. obtained hierar-hically graphitic porous carbon architectures with atomicallyispersed Fe and N doping through the introduction of guestpecies (FeCl 3 and dicyandiamide) into the pores of MIL-101-H 2 and subsequent pyrolysis. 398 The unique morphology and

tomically dispersed active sites with a strong synergetic effectnabled the obtained material excellent catalytic performance

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or ORR, outperforming the benchmark of Pt catalyst and manytate-of-the-art noble-metal-free catalysts in alkaline media,articularly in terms of the onset and half-wave potentials andurability. Recently, Jiang and coworkers used a mixed-ligandtrategy to incorporate iron into the porphyrinic MOF (PCN-22) to fabricate high-content (1.76 wt%) single-atom (SA) iron-

mplanted N-doped porous carbon (Fe SA –N–C) via pyrolysis Fig. 20 (a)). 399 The resulting Fe SA –N–C demonstrated excellentRR performance in both alkaline and acidic media, surpassingost of reported non-noble-metal catalysts and even the Pt/C.

ater splitting

ecently, MOF-derived nanomaterials were widely used ashe promising non-noble-metal electrocatalysts for the HER and

ER. Transition metal NPs and transition metal phosphides haveeen demonstrated as excellent electrocatalysts for HER. Zhangt al. fabricated a hybrid of cobalt encapsulated by N, B-codopedltrathin carbon cages (Co@BCN) via pyrolysis of H 3 BO 3 -

containing ZIF-67. 400 The catalysts exhibited remarkablelectrocatalytic performance for HER in both acidic and alkalineedium. Another porous CoP electrocatalyst with a concave

olyhedron (CPH) structure was facilely prepared using Co-OF (ZIF-67) polyhedrons as the precursor. 401 Comparedith the contrastive CoP NPs, the obtained porous CoP CPH

lectrocatalyst exhibited a remarkably enhanced electrocatalyticerformance for the HER in acidic media. Due to inexpensive,xcellent electrochemical performance and corrosion resistance,olybdenum-based compounds became one of the promising

lectrocatalysts for HER. 402,403 Lan, Yu and co-workersynthesized a porous Mo-based composite (MoO 2 @PC-RGO)y pyrolyzing a polyoxometalate-based MOF and graphene oxidePOMOFs/GO). 403 Owing to the synergistic effects amongighly dispersive MoO 2 particles, the obtained MoO 2 @PC-GO showed excellent HER activ ity w ith a very positive onsetlose to that of 20% Pt/C, low Tafel slope (41 mV/dec) andemarkable long-term cycle stability in 0.5 M H 2 SO 4 .

MOFs and their composites are considered to be suitablerecursors for preparing highly efficient OER catalysts to replaceoble-metal-based OER electrocatalysts (RuO 2 and IrO 2 ). Lound co-workers synthesized Co 3 O 4 /NiCo 2 O 4 double-shelledanocages (DSNCs) by annealing ZIF-67/Ni–Co layered double

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Fig. 20. (a) Illustration of the rational fabrication of single Fe atoms-involved Fe SA –N–C catalyst via a mixed-ligand strategy. Adapted with permission. 399

Copyright 2018, Wiley-VCH. (b) Fabrication of hybrid Co 3 O 4 -carbon porous nanowire arrays. Reproduced with permission. 405 Copyright 2014, American Chemical Society. (c) Schematic showing the formation of Ni SAs/N–C. Reproduced with permission. 408 Copyright 2017, American Chemical Society.

hydroxide nanocomposites in air. 404 The Co 3 O 4 /NiCo 2 O 4 DSNCs exhibited much better electrocatalytic activity for OERthan single-shelled Co 3 O 4 nanocages. Not limited to the MOF-derived powders, MOF nanoarrays assembled on substrateshave also been fabricated to give MOF-derived porous arraysattached on the substrates via pyrolysis. Qiao and coworkersprepared hybrid porous nanowire arrays composed of Co 3 O 4 and carbon by a facile carbonization of Co-MOF hybrid arraysgrown on Cu foil ( Fig. 20 (b)). 405 The resultant hybrid materialexhibited higher OER activity, more favorable kinetics, andstronger durability in comparison to those of IrO 2 /C. In alater publication, Jiang, Yu and coworkers developed a versatilestrategy for the controllable synthesis of three dimensional MOFhybrid arrays by utilizing semiconducting nanostructures asself-sacrificing templates. 406 Particularly, the MOF hybrid-array-derived carbon-based composites could be directly applied toboth anodes and cathodes for water splitting and exhibited muchhigher activity, more favorable kinetics, and better durability thanthe corresponding counterparts.

Considering the demand of practical application, theintegration of OER and HER functionalities into a single elec-trocatalysts to realize the overall water splitting is highly desired.Recently, a number of transition metal phosphides/sulfides/selenides, have been widely investigated for electrocatalyticwater splitting. Jiang’s group successfully prepared a series oflayered CoP/rGO composites via pyrolysis of sandwich-typeZIF-67/GO, followed by subsequent phosphating process. 407

The optimized catalyst is able to be utilized as a bifunctionalcatalyst on both the anode and cathode for overall water splittingin basic media, even displaying superior performance to theintegrated Pt/C and IrO 2 catalyst couple.

25

CO 2 reduction

In addition to electrocatalytic ORR, HER and OER describedabove, CO 2 reduction has also been conducted over MOF-derived catalysts with limited examples. Zhao et al. adoptedZIF-8 to assist the preparation of single nickel atoms dispersedin N-doped porous carbon (denoted as Ni SA s/N–C) v iapyrolysis of Ni 2 + @ZIF-8 composites ( Fig. 20 (c)). 408 Comparedto Ni NPs and Ni foam, the resultant Ni SAs/N–C was capableof selectively reducing CO 2 with excellent current densityand Faradaic efficiency. Later, Ye et al. selectively confinedammonium ferric citrate on the surface of ZIF-8 NPs. Afterpyrolysis, isolated iron-nitrogen sites were generated and locatedon the carbon matrix surface (denoted as C-AFC@ZIF-8). 409

Due to the highly exposed Fe-N active sites on the surface,C-AFC@ZIF-8 showed superior selectivity and mass activitytowards CO 2 electroreduction compared to most reportednoble metal catalysts. Recently, Pan et al. developed an effectivesynthesis approach to prepare atomically dispersed Fe and Co-based M–N–C model catalysts via pyrolysis of the correspondingFe- or Co-doped ZIF-8. 410 They found that Fe was intrinsicallymore active than Co in M-N 4 for the electrochemical reduction ofCO 2 to CO with a larger current density and higher CO Faradaicefficiency (FE) (93% vs. 45%). The experimental observationwas well explained by theoretical calculations.

N 2 reduction

Electrochemical N 2 reduction (NNR) under ambient conditionsis a promising energy- and environmental-friendly method forammonia synthesis, the related reports on MOF derivativesstarts very recently. 411–414 Guo et al. reported the synthesis of

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noble-metal-free CoP hollow nanocage (CoP HNC) catalystrom a MOF precursor through a layered-doublehydroxidentermediate, and the use as the cathode for electrochemical

RR411. In another work, Geng et al. developed Ru singletoms on nitrogen-doped carbon (denoted as Ru SAs/N–C) forRR with a high-recorded activity, 413 which was prepared via

yrolyzing the Ru-containing derivative of ZIF-8. Despite the excellent catalytic performance of MOF-derived

orous materials, most of them suffer from multiphase and broadore distributions due to the hardly controlled in the structuralerivation process. Therefore, much more efforts should beevoted for rationally controlled fabrication of MOF-derivedaterials with desired pore character and structures.

CONCLUSION AND PERSPECTIVE

ver the past two decades, MOFs have been rapidly developeds an emerging class of heterogeneous catalysts. The rationallyesigned and fabricated MOF-based materials are equipped with variety of catalytic centers for broad catalytic applications. Inhis review, we have briefly summarized MOF-based materials foratalysis according to the structural features of MOFs and the cat-gory of active site origin. As for pristine MOFs, they contain co-rdinatively unsaturated metal sites on the metal nodes/clustersnd active groups on the organic linkers, serving as catalytic sites.he metal nodes/clusters and organic linkers are easily function-

lized by post-synthetic modification and/or pre-introductionith particular sites/molecules. Furthermore, encapsulating the

uest catalytic centers, for instance, metal-based NPs, functionalolecules/complexes, and enzymes, etc., into the pore/defect

paces of MOFs is a very effective and feasible strategy to synergis-ically enhanced catalysis, by the integration of strengths between

OFs and guest species. The MOF derivatives, such as dopedorous carbons, metal-based compounds and their related com-osites, obtained by the chemical/thermal conversion of MOFs,re also very promising candidates for multifarious catalyticpplications. We should say sorry for the missed references in thiseview article, as it is almost impossible to be exhaustive for allelated publications given the extremely wide range of reports onhis topic. Overall, compared with the traditional catalysts, MOF-ased materials have their particular advantages: i) high disper-ion and isolation of active sites which can maximize the atomtilization; ii) highly uniform and tunable pore structure and sizeenefiting mass transfer and size sieving; iii) well-defined andevisable crystalline structures that are critical to the understand-

ng of structure-function relationship and catalytic mechanismnvestigation; iv) the facile functionalization of metal nodes andrganic ligands making it easy to the rational design/introductionf multifunctional active sites; v) the readi ly tai lorable chemicalnvironment around active sites favoring the catalytic activ-t y/selectivit y; vi) ultrahigh surface area greatly facilitating theccess of catalytic sites and substrate concentration; vii) the porepace provides congenital conditions for the fabrication of MOFomposites by incorporating guest species; and so on.

Though tremendous advances have been achieved in recentears, MOF catalysts are sti l l in its infancy, great challenges alsoxist: i) The intrinsic growth mechanism of MOF remains uncleart this stage. Getting insightful understanding of the self-assemblyechanism of MOFs is of vital importance to the construction

f pre-designed MOFs with targeted structures, regular or hier-rchical pores and adjustable composition. ii) We are sti l l on theoad to go for low-cost and large-scale production of MOFs withigh yields, though some “star” MOFs, such as ZIF-8 (also called

26

AF-4), HKUST-1, MIL-100, etc., are commercially available.ome organic linkers should be synthesized by several steps ofrganic synthesis and some metal precursors are of relatively highrice; the preparation of MOFs is usually an energy-consumingrocess. iii) The stability, mainly water/chemical, thermal andechanical stability of MOFs are sti l l far from satisfactory.hough great progress on MOF stability and the successful

eport of stable MOF structures in recent years, 415,416 and alsoost-treatment to improve MOF water/moisture stability, 56 morefforts should be devoted to chemical stability of MOFs. It isequired that the catalysts are able to retain the activity andelectivity during the time-on-stream process under real reactiononditions. The mechanical stability of MOFs has been seldomtudied. 417 This is of great importance in MOF catalysis, as the

echanical molding process of catalysts is generally necessary foreal catalysis in practical applications. iv) Regeneration issue. Dueo the low thermal stability of MOFs in reference to traditionalatalysts like zeolites, the high-temperature regeneration processo remove the adsorbed substrates/intermediates/products isossibly challenging for some MOF-based catalysts. v) MOFsre mostly microporous materials. Mass transfer resistance mightxist for various large substrates/products. To address this issue,he development of hierarchically porous MOFs or delaminatedD MOFs might be an effective solution. vi) The contradictionetween stability and function of MOFs. There are limitedristine MOFs featuring both high stability and abundant activeites, which are highly expected for catalytic study. This is alsohy the property (catalysis) studies have been focused on some

star” MOFs, such as MIL-101, PCN-222, etc., as they are able toatisfy the multiple requirements. vii) Precisely control over thessembly of MOFs and guests at the molecular level. Determininghe exact position of guests and the fine structure of guests in

OFs would be of great significance to get insightful informationbout the synergetic effect between guests and MOFs. viii) The

OF-based photocatalysts for some important redox reactions,uch as H 2 production, oxygen evolution, CO 2 reduction, etc.,lmost require sacrificial agents to promote the reaction. Morefforts should be paid to change the situation. ix) High conduc-ivity is preferred for rapid charge transfer in both photocatalysisnd electrocatalysis. Therefore, the poor conductivity of MOFatalysts should be improved towards these applications. x) Mostf current studies over MOF-based catalysts are based on modeleactions. More investigations on the industrially important andhallenging reactions over MOF-based materials are expected.i) The mechanism understanding and accurate control overhe MOF-derived materials. Despite the rapid progress in theabrication of MOF-derived nanomaterials, the intrinsic mech-nism of their transformation progress is sti l l unclear, severelyestricting their rational design and functional application. xii) In-

situ or in-operando experiments and advanced characterizationechniques should be introduced for better understanding theatalytic conversion process and related mechanisms.

Despite the above, as a class of heterogeneous porous catalystsith well-defined and readi ly tai lored structures, MOF-basedaterials afford great advantages in fundamentally catalytic

esearch. More than that, by virtue of the particular strengthsf MOF structures, we believe MOF-based materials shouldresent special advantages and bright future towards catalysis,ossibly in but not limited to the following aspects: 1) Chiralatalysis. Compared to other porous materials such as zeolites,he structural tai lorabi lity makes it very convenient to fabricatehiral MOFs, in either pre-assembly or post-synthesis route.he current reports have sufficiently demonstrated that the

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chiral catalysis over MOFs are very promising for possiblepharmaceutical or related intermediate synthesis. 2) Selectivecatalysis. Almost all locations, including the surroundings ofactive sites, the internal surface of pore space, the external surface,etc., in MOFs can be modified with some particular functionalgroups/species. This modification wi l l not only be beneficial toenhanced activity, but also wi l l greatly improve the all kinds ofselectivity, including chemoselectivity, wettability selectivity, chi-ral selectivity, etc. Meanwhile, the uniform pore sizes wi l l endowMOFs size selectivity and/or regioselectivity in catalysis. 3) One-pot multi-component or multi-step (cascade) coupling reactions.The one-pot cascade reaction is of high value in industry as thatwi l l avoid the traditional separation process. MOFs are a classof very important platform materials that are able to integratedifferent components and functions into a single composite,which wi l l behave as multifunctional catalyst with diversifiedactive sites, activating different substrates and serving for not onlycascade reactions but also multi-component coupling reactions(for example, A 3 coupling). 418,419 4) Photo-assisted catalysis.Given the relatively low stability, introducing light irradiationwould be an important solution to allow the reactions to proceedunder moderate conditions that originally take place under harshconditions. Particularly, the photothermal effect wi l l create theheat around active sites to accelerate the reactions, 303,304 insteadof traditional heating way. 5) Single-site/atom catalysis. Single-site/atom catalysts can achieve the maximum utilization ofmetal active sites and usually exhibit ultrahigh activity/selectivitytowards diverse reactions. MOF-based single-site/atom catalysts,which can integrate the strengths of both MOFs and single-site/atom catalysts, wi l l present particular strengths towardcatalytic mechanism investigation and enhanced catalysis. 6)Multivariate (MTV) MOFs for catalysis. MTV MOFs representa class of MOFs with multiple functional sites such as differentmetal ions and linkers. Due to the variety of the building units,MTV MOFs wi l l open a new perspective in synergetic catalysisand tandem reactions by introducing multiple components.7) Electrocatalytic organic reaction. It has been proved thatelectrocatalysis paves a green, safe, and atomically efficient way tothe organic synthesis. MOFs for the photocatalytic conversion oforganic molecules have been reported recently. However, MOF-based electrocatalysts for organic catalysis have not attractedmuch attention ti l l now. 8) 2D MOFs or hierarchically porousMOFs for catalysis. 263,420 2D structure can cause the active sitesexposure on the external surface rather than inside pores andthe mesoporous structure can allow for fast mass transfer duringthe catalytic process. The construction of 2D or hierarchicallyporous MOFs wi l l greatly improve the accessibility of activesites and thus enhance the catalytic performance. 9) Quasi-MOFs for catalysis. 421 Quasi-MOFs are usually obtained viathermal treatment to partially remove the organic ligands whilethe porosity of the framework can be largely retained. Thiscould help enhance interaction between MOFs and guests byexposing the inorganic nodes to guests, which presents a newstrategy for improving the catalytic performance of MOF-basedmaterials.

In summary, with accumulated efforts in this research field, thedevelopment of MOF-based materials for catalysis has entereda new era at this stage, where challenges and opportunitiescoexist, as discussed above. With sustained research endeavortowards those challenges, we are confident to see more andmore important contributions pushing MOF-based materials tocatalysis science and look forward to industrial production andpractical applications based on MOFs in the near future.

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CREDIT AUTHORSHIP CONTRIBUTION STATEMENT

Dandan Li: Writing - original draft. Hai-Qun Xu: Writing -original draft. Long Jiao: Writing - original draft. Hai-LongJiang: Conceptualization, Superv ision, Writing - rev iew &editing.

ACKNOWLEDGMENTS

This work was supported by the NSFC ( 21725101 , 21871244 ,21673213 , 21701160 and 21521001 ), and the Hefei NationalLaboratory for Physical Sciences at the Microscale (KF2019002).

CONFLICT OF INTEREST

The authors declare no conflict of interest.

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AUTHOR BIOGRAPHIES

Dandan Li received her Ph.D. degree in inorganic chem- istry from Anhui University in 2016, and then worked at University of Science and Technology of China (USTC) as a postdoc under the guidance of Prof. Hai-Long Jiang in 2016–18. She is currently an associate professor at Anhui University. Her research work is focused on the development of MOFs for photocatalysis.

Hai-Qun Xu earned her Ph.D. degree from USTC under the supervision of Prof. Hai-Long Jiang in 2018. She currently works at Zhejiang Gongshang University. Her research work is focused on MOF-based materials for photocatalysis.

Long Jiao received his BS degree in materials physics from Shandong University in 2014. He is currently a PhD

student under the supervision of Prof. Hai-Long Jiang at University of Science and Technology of China (USTC). His research work is focused on MOF-based materials for electrocatalysis.

Hai-Long Jiang earned his Ph.D. in 2008 from Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. He subsequently worked at the National Institute of Advanced Industrial Science and Technology (AIST, Japan), first as an AIST Fellow and later as a JSPS Fellow during 2008–11. After a post- doctoral stint at Texas A&M University (USA), he became a fullprofessor at USTC in 2013. He is a Fellow of the Royal Society ofChemistry (FRSC) and was recognized as a highly cited researcher(2017 and 2018) in chemistry by Clarivate Analytics. His main researchinterest is in the development of crystalline porous and nanostructuredmaterials, crossing coordination chemistry and nanoscience, for energy-/environment-related catalysis.

DOI: 10.1016/j.enchem.2019.100005 EnergyChem 1 , 100005 (2019)