Mechanochemistry: an efficient and versatile toolbox for synthesis…bib.irb.hr/datoteka/1059738.Stolar_Uarevi.pdf · 2020. 4. 28. · Synthesis at Ruđer Bošković Institute in

CrystEngComm

HIGHLIGHT

Cite this: DOI: 10.1039/d0ce00091d

Received 20th January 2020,Accepted 25th March 2020

DOI: 10.1039/d0ce00091d

rsc.li/crystengcomm

Mechanochemistry: an efficient and versatiletoolbox for synthesis, transformation, andfunctionalization of porous metal–organicframeworks

Tomislav Stolar and Krunoslav Užarević *

Mechanochemistry is nowadays recognized as a green approach to chemical synthesis, but the full scope

of the accompanying solid-state reactivity is just beginning to be unravelled. This is especially true for the

preparation of porous metal–organic frameworks, whose synthesis by the use of mechanical force lagged

behind that of other relevant classes of materials. Nevertheless, mechanochemical procedures have rapidly

evolved from a mere synthetic curiosity towards efficient methods for obtaining high-quality MOFs on

different scales. This Highlight is dedicated to the functional approach of using mechanochemistry for the

preparation of catalytically active MOF composites and mixed-metal and mixed-ligand MOFs with

synergistic properties as well as fine-tuning of the MOF performance via defect engineering and

amorphization.

1 Introduction

Chemical reactivity induced by mechanical action, such asmilling, shearing, grinding, scratching, etc., on bulk solidshas become a well-recognized alternative to traditional

solution synthesis methods.1–3 Mechanochemistry has beenused for chemical extraction already in prehistoric times,4

and is widespread in the metallurgy and cement industry.However, despite being sporadically used in laboratories inthe 20th century, the first systematic mechanochemicalstudies were carried out in the early eighties onsupramolecular materials.5–7 Reports on mechanochemical

CrystEngCommThis journal is © The Royal Society of Chemistry 2020

Tomislav Stolar

Tomislav Stolar is a PhDcandidate at Ruđer BoškovićInstitute in Zagreb, Croatiaunder the supervision of Dr.Krunoslav Užarević. He startedhis scientific career as anundergraduate volunteer at thesame institution in 2013. Hisresearch interest spans solid-state chemistry, crystalengineering, prebiotic chemistry,mechanochemistry and MOFs.

Krunoslav Užarević

Krunoslav Užarević graduated in2009 at the University of Zagrebin the field of structural andinorganic chemistry. After aMarie-Curie NewfelproFellowship with Prof. TomislavFriščić at McGill University inMontreal, he started working asa Head of Laboratory for GreenSynthesis at Ruđer BoškovićInstitute in Zagreb in 2016. Hismain scientific focus is ondeveloping mechanochemicaland solvent-free procedures for

the synthesis and transformation of various classes of functionalmaterials, from supramolecular receptors and organic compoundsto porous metal–organic frameworks (MOFs). A particular part ofhis research involves designing new milling reactors anddeveloping new methodologies for in situ monitoring of millingand aging reactions.

Ruđer Bošković Institute, Bijenička c. 54, 10000 Zagreb, Croatia.

E-mail: [email protected]

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organic synthesis followed soon after,8,9 and the fielddeveloped at such a pace that in their comprehensive reviewof organic solvent-free synthesis in 2000, Toda and Tanakaconcluded: “Anyway, the typical organic synthetic procedurewhich has long been carried out in solvent throughout the19th and 20th centuries, should be modified to moremodern, elegant, and safe versions in the 21st century”.10

At that time, wider implementation of solvent-freemechanochemical techniques also happened due todevelopment of green chemistry principles wheremechanochemistry complied well with requirements for moresustainable and safer synthetic procedures.11 Thepharmaceutical industry quickly recognized grinding as a fastand efficient technique for the screening and preparation ofsolid pharmaceutical forms.12 Subsequently,mechanochemical methods rapidly emerged as a powerfultool for synthesis and transformation of various classes ofmaterials.13–19

One class of materials that lagged behind in terms ofpreparative mechanochemistry is metal–organic frameworks(MOFs). MOFs are porous coordination polymers built frommetal nodes and organic linkers.20–22 Due to variousarchitectures, permanent porosity and modular structures,they are one of the most studied classes of materials incontemporary chemical sciences and offer a diverse range ofpotential applications.23–30 Whereas mechanochemistry andMOFs were in 2019 proclaimed by the IUPAC as two of thetop ten emerging technologies in chemistry,31 it was not until2006 that the first official report on using mechanochemistryfor the preparation of moderately porous copperisonicotinate MOFs was published by the James group.32 Thisdelay is not surprising when one considers that MOFs arehighly porous compounds with ordered structures, and theuse of ball-milling as a method for the synthesis of thesecompounds is somewhat counter-intuitive.

The need for reduction of solvents used in chemicalsynthesis goes hand-in-hand with the social drive towardssustainability and innovation. These principles have beenimplemented in legislation of many developed countries ledby the United Nations “Transforming our World: the 2030Agenda for Sustainable Development”.33 As a result, therehave been specific actions taken such as the EuropeanCooperation in Science and Technology (COST) funded:CA18112 – Mechanochemistry for Sustainable Industry(MechSustInd).34

Using mechanochemical milling as an efficient, green andrapid synthetic technique for obtaining MOFs is now wellrepresented in the literature.35 However, the field has evolvedin many ways, thus creating the need for writing thishighlight, which focuses on the functional approach of usingmechanochemical techniques in the MOF context. It canserve as a reference for anyone working with MOFs who isinterested to see how the mechanochemical toolbox can addmore or create new values to their research. The first part ofthis Highlight is dedicated to the historical overview ofmechanochemical synthesis of MOFs, together with possibly

the first synthesis of MOF material in general, dating back to1968. After that, the timeline for the development of in situmonitoring methodologies for the mechanochemicalsynthesis of MOFs is presented. Furthermore, we show howball-milling is utilized to control topology transformations,prepare MOF composites, create mixed-ligand and mixed-metal MOFs, introduce defects, and collapse structures viaamorphization. Finally, the industrial potential of the large-scale mechanochemical preparation of MOFs by twin-screwextrusion (TSE) is highlighted at the end.

2 Historical overview

The first reported synthesis of an extended coordinationpolymer by manual grinding likely dates to 1968, more than20 years before Robson's seminal work,20 and it probablyrepresents also the first synthesis of a porous MOFmaterial.36 Authors Musgrave and Mattson did not recognizeat the time the potential of the prepared materials, whichtwo decades later expanded to a field of porous MOFs. Theydid recognize, however, the coordination abilities of one ofthe most popular ligands for the preparation of MOFsnowadays, 4,4′-bipyridine (bipy), stating it to be “particularlyinteresting because its structure should preclude chelationand favor coordination of the nitrogen atoms to two differentmetal ions; i.e., it should coordinate so as to form polymericcomplexes”.36 Mixing ethanol solutions of metal nitrates andbipy resulted in the rapid formation of 1D-coordinationpolymers with the general formula MĲbipy)ĲNO3)2. However,grinding several metal nitrates (Ni, Co, and Cu) in a mortarwith an excess of bipy (50–100 times) and a small amount ofmethanol afforded a new type of coordination polymerswhich differed in color from the MĲbipy)ĲNO3)2 products(Fig. 1a). The authors concluded from their analytical data

Fig. 1 First MOFs prepared by grinding or milling procedures: (a)dissolving CuĲNO3)2 and bipy in EtOH results in the formation of a 1D-coordination compound whereas grinding them in the presence ofMeOH results in a squarate coordination framework (1968); (b)HKUST-1 (CCDC FIQCEN) from dry milling of copper acetate and BTC(2008), and (c) [CuĲina)2]ĲH2O)2 (CCDC BAHGUN) from dry millingcopper acetate and ina (2006).

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that two bipy molecules coordinated to a single metal cationand that the new products are octahedral complexes of thegeneral formula MĲbipy)2ĲNO3)2. The molar ratio of bipy tometal cation and the presence of “entrapped” solventmolecules point to the formation of square networkscharacteristic of MOFs containing copper-, cobalt-, or nickelnitrates and bipy bridges (Fig. 1a). The following systematicstudy of the advantages of mechanochemical procedures overthe conventional solution synthesis of squarates took morethan 50 years, but the authors expanded their scope also tocontinuous mechanochemical procedures such as TSE.37

In 2006, the James group reported the first intentionalmechanochemical synthesis of a porous MOF.32 This ground-breaking contribution was focused on the dry millingreaction between copperĲII) acetate monohydrate andisonicotinic acid (ina). Milling for 10 minutes at a 25 Hzoscillation rate resulted in the quantitative formation of ahighly crystalline [CuĲina)2] MOF, with water and acetic acidby-products trapped in the pores (Fig. 1c). As a comparison,previously reported solvothermal synthesis of the sameproduct included 48 hours of heating the reaction mixture at180 °C. The PXRD pattern of the product obtained by millingwas in good accordance with the calculated pattern for[CuĲina)2]ĲH2O)2 (Cambridge Crystallographic Data Centre,CCDC code BAHGUN). The stability of themechanochemically prepared MOF was shown to be similarto the one from solution, and it was possible to remove theincluded guest molecules from the pores by heating at 200°C, resulting in the formation of the empty [CuĲina)2] (CCDCUFUMUD).38 The James group was also involved in otherearly studies of the mechanochemical preparation of anarchetypal MOF, HKUST-1, by dry milling of copperĲII) acetateand 1,3,5-benzenetricarboxylic acid (BTC) (Fig. 1b).39,40 Asignificant follow-up from the Emmerling group41

emphasized the importance of liquid additives and liquid-assisted grinding (LAG)42,43 methods for the preparation ofhigh-quality HKUST-1 material, with the Brunauer–Emmett–Teller (BET) surface corresponding to HKUST-1 preparedsolvothermally.

The real change in the synthetic paradigm came in 2009when Friščić and coworkers showed how metal oxides,precursors which are usually avoided in solution synthesisdue to their low solubility, can be exploited as startingreagents for the rapid and clean synthesis of six differentpillared Zn-MOFs. The developed LAG procedures are acombination of acid–base reactions and coordination-drivenself-assembly and, importantly, produce water as the sole by-product of the reaction.44

The following studies also showed that the type of theMOF product could be governed by the choice of grindingliquid and ionic additives. During mechanochemicalsynthesis of pillared MOFs from ZnO, fumaric acid and 1,4-diazabicycloĳ2.2.2]octane (dabco), it was discovered that theaddition of an ionic salt (5 mol percent) in the ion-and-liquidassisted grinding (ILAG) would inadvertently govern thereaction towards a specific topology, namely the tetragonal or

hexagonal form of the [ZnĲfum)Ĳdabco)] MOF (Fig. 2a).45

These templating effects were further confirmed by 15N solid-state NMR and Fourier-transform infrared spectroscopy(FTIR) studies, which revealed that the ions were included inthe pores of the product. A similar effect was also shown forZnĲ2-ethylimidazolate)2 [ZnĲEt–Im)2], a MOF belonging to thefamily of zeolitic imidazolate frameworks (ZIFs),46 where theionic additives directed the mechanochemical formationtowards three different topologies (Fig. 2b).47 The presentedresults provided the first indication of how templating andself-organization could affect the synthesis of extendedcoordination frameworks in the solid state. These earlystudies established a firm ground for the mechanochemicalsynthesis of MOFs to become a viable alternative toconventional solution synthesis and have, without doubt,inspired the rapid development of the field in the followingyears.

3 Advances in mechanochemicalsynthesis of MOFs and in situmonitoring of ball-milling reactions

After the initial reports on mechanochemical synthesis ofMOFs, it was evident that ball-milling is contributing toreducing the amount of used solvents, energy requirements,and the duration of the synthetic processes while at the sametime offering high-quality MOFs. However, the mechanisms

Fig. 2 Catalytic addition of different salts results in different MOFtopologies in (a) pillared MOFs and (b) ZIFs.

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of these efficient processes were still not clear. This allchanged with the development of in situ techniques forgaining real-time insight into the course of mechanochemicalreactions without the need to interrupt the millingprocess.48,49 The technique relies on high-energy synchrotronX-rays which pass through the walls of the reaction vessel,interact with the milled sample and can be detected on ahigh-resolution 2D detector (Fig. 3a). Highly crystalline ZIFswere ideal model compounds for the development of in situdiffraction methods (Fig. 3b and c), showing dynamicchanges of crystalline phases with time-resolution in secondsand consequently enabling the optimization of millingreactions. The technique was also extended to other relevantMOF materials, and some results will be presentedthroughout this Highlight.

In the first contribution describing the in situ PXRDmonitoring,48 it was revealed that the ionic additive in themilling of 2-methylimidazole and ZnO indeed had atremendous accelerating effect on the product formationrate. Monitoring the change in the intensity of thecharacteristic (211) reflection of ZIF-8 showed a sigmoidalpattern characteristic of nucleation and growth of a crystalphase in both the LAG and ILAG procedures (Fig. 3d–f). Themonitoring thus proved that the ILAG, besides being an atomand energy economical process, also results in a significantshortening of the reaction, from 29 hours by solvothermalsynthesis46 to 4 minutes by milling.48

The influence of liquid additives on themechanochemical synthesis of HKUST-1 was also studiedby in situ methods.50 It was shown that LAG additivesdirect the mechanosynthesis of HKUST-1 via completelydifferent reaction mechanisms. On the one hand, LAGusing MeOH or similar short-chained additives resulted inthe direct formation of HKUST-1 from the reactantmixtures (Fig. 4a). On the other hand, two different

discrete intermediates (INT1 and INT2) containing amononuclear copper core, and with the general formula[CuĲH2BTC)2 (solvent)2], were isolated when the millingprocess was interrupted (INT1), or when the additivescontained longer aliphatic chains (INT2). The study alsoshowed how mechanochemistry allows for the use of theweakly soluble precursor CuĲOH)2, yielding water as theonly by-product of the reaction.

Julien et al. have shown how Zn-MOF-74 can besynthesized from ZnO and 2,5-dihydroxyterephthalic acid (H4-DHTA) by milling under one hour under room temperatureconditions, using water as the liquid additive.51 The structureof MOF-74, also known as CPO-27, is characterized byhoneycomb channels, and belongs to a particularlyinteresting family of MOFs, widely investigated for adsorptionand catalysis applications due to their open metal-sites andlarge surface areas.52–55 In situ monitoring of themechanosynthesis revealed that the reaction proceeds in twosteps, first yielding a non-porous 2-D coordination polymer[ZnĲH2DHTA)ĲH2O)2] which then reacts with anotherequivalent of ZnO to form a porous [Zn2ĲDHTA)ĲH2O)2]-Ĳsolvent)n MOF-74 framework in 45–90 minutes of milling(Fig. 4b).51 Interestingly, water or methanol additives provedto be much more suitable for the mechanochemicalpreparation of Zn-MOF-74 than the conventionaldimethylformamide (DMF), for which the milling was longerand involved two different unknown types of 1 : 1 (Zn : DHTA)intermediates. This step-wise synthesis was later used for thecontrollable preparation of a series of bimetallic MOF-74materials56 (section 6).

Looking from a synthetic perspective, more complexgroups of MOFs are those based on discrete metal clusternodes since the assembly of a cluster is a necessaryprecondition for the formation of MOFs. In 2015, theLewiński group reported the successful ball-milling synthesisof one of the most famous MOFs, MOF-5 (IRMOF-1),59 byusing the “SMART” strategy (SBU-based MechanochemicalApproach for Precursor Transformation).57 MOF-5, which isbased on [Zn4O]

6+ cluster nodes, was not accessible bymilling of the simple zinc precursors and terephthaliclinker. The authors switched to discrete zinc precursorsbearing a similar [Zn4O]

6+ core, such as the benzoate[Zn4OĲO2CPh)6] derivative or more basic benzamidate[Zn4OĲNHOCPh)6] derivative (Fig. 4c). The SMART strategyproved to be a smart choice as it resulted in crystalline andhigh-quality MOF-5 after a mere 60 minutes of LAG at roomtemperature, strongly contrasting the harsh solutionapproaches needed for the preparation of this archetypalMOF. The basicity of the precursor was shown to beparticularly important for the milling reaction, and thebenzamidate precursor yielded crystalline MOF-5 even after30 minutes of dry milling. Solid amide, which is the onlyby-product of synthesis, could be recycled and used forgeneration of new cluster precursors.57 The SMARTapproach was later used for the preparation of otherIRMOFs, such as IRMOF-3, IRMOF-10, and MOF-177.60

Fig. 3 Real-time and in situ monitoring of the mechanochemicalmilling formation of ZIF-8 from zinc oxide revealed the acceleratingeffect of ionic additives.49 (a) The experimental setup; (b) schematicrepresentation of the formation of ZIF-8 from ZnO and2-methylimidazole; (c) structure of ZIF-8; time-resolved in situ PXRDdata for the (d) LAG (EtOH) and (e) ILAG (NH4NO3 and EtOH)procedures for the mechanochemical preparation of ZIF-8; and (f)time-resolved variation of the intensity of the (211) reflection of ZIF-8in LAG (red) and ILAG (blue) synthesis. Reproduced from ref. 49 withpermission from the American Chemical Society, Copyright 2015.


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Soon to follow, in 2016, another large and importantfamily of MOFs with discrete oxocluster nodes have beensuccessfully synthesized. MOFs based on bridging carboxylateligands and zirconium oxocluster nodes attracted attentionimmediately upon their discovery in 2008,61 since theyoffered unprecedented stability under atmosphericconditions and in a wide range of pH. This, together with ahigh density of metal cations in [Zr6O4ĲOH)4]

12+ nodes,motivated a large number of studies that establishednumerous new topologies62 and potential applications63 ofthis exciting family of MOFs. The solution syntheses involvedthe extensive use of formamide solvents and acidic orcorrosive reactants. The first mechanochemical synthesistackled these issues (Fig. 4d) by developing a LAG procedurewhere ZrĲi-propoxide)4 as a zirconium source and methanolas an additive were used, yielding poorly crystalline UiO-66MOF in 45 minutes of milling.58 Switching to preorganized

benzoate [Zr6O4ĲOH)4ĲC6H5COO)12] or methacrylate [Zr6O4-ĲOH)4ĲC2H3COO)12] clusters yielded highly crystalline, active,and porous products by LAG (MeOH) or accelerated agingprocesses on a gram-scale. The curiosity in these syntheseswas that methanol and ethanol yielded much morecrystalline UiO-products than formamide additives.58

The UiO-66-F4 derivative, with a fluorinated terephthalicacid linker, was accessible from the methacrylate Zr6 clusterby LAG with a water additive and in several minutes, thusavoiding the need for organic solvents in the preparation andpurification of the product. Unfortunately, the samemethodology was not applicable for other, more conventionalUiO-66 derivatives.64 The mechanochemical synthesis of theUiO-67 MOF, based on 4,4′-biphenyldicarboxylic acid, andNU-901, based on the tetratopic pyrene linker, failed whenusing methacrylate- or benzoate-Zr6 precursors.65 To try toamend this, authors switched to the new dodecanuclear

Fig. 4 (a) In situ monitoring of HKUST-1 formation revealed rich mechanochemical reactivity and diverse reaction pathways depending majorly onthe additives used. (b) Mechanochemical formation of MOF-74 is a two-step process involving the formation of a 1 : 1 Zn :H4DHTA intermediate.51

The process can be followed visually by monitoring the change of color and the rheology of the milled reaction mixture. (c) The “SMART” strategyfor the mechanochemical synthesis of MOF-5 uses preorganized Zn4O-clusters as precursors.57 (d) The first mechanochemical formation of UiO-66 and UiO-66-NH2 from preorganized Zr6-oxoclusters allows for the use of “greener” liquid additives.58 (d) is reproduced from ref. 58 withpermission from The Royal Society of Chemistry.


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acetate cluster [Zr12O8ĲOH)8ĲCH3COO)24] with more basic andmobile acetate ancillary ligands. This strategy was successful;pure, highly porous, and crystalline UiO-67 and NU-901 weresynthesized. The measured porosity of the UiO-67 materialwas 2250 m2 g−1, establishing a new benchmark for theporosity of the material prepared by ball-milling.65

In situ monitoring has also shown how solid-statereactivity can be profoundly different compared to that insolution, gaining solid phases not accessible by any othersynthetic procedures (Fig. 5a).66 Milling of ZnO and2-methylimidazole with a small amount of diluted acetic acidafforded ZIF-8 almost instantaneously, which was followed byits amorphization after 20 minutes of milling. Unexpectedly,the continuation of milling resulted in the recrystallization ofthe amorphous matrix into a new topological polymorph ofZIF-8 named katsenite (kat). The kat polymorph was reportedfor the first time despite numerous studies focused on thecrystallization of ZIF-8, and this phase is still reported solelyby milling. It proved to be unstable and converted into a

non-porous and thermodynamically more stable diamondoid(dia) phase upon further milling.66 In some experimentshowever, the recrystallization of amorphous ZIF-8 proceededdirectly towards the dia phase, revealing the stochastic natureof nucleation and crystallization from the amorphous matrix.Previously, quantitative PXRD monitoring methods revealedthat during the mechanochemical formation of zeoliticframeworks, almost 40% of the reaction mixture isamorphous.67

4 Topology transformations by ball-milling

The above-described synthesis of the kat polymorph of ZIF-8,66 which proceeds in the order ZIF-8–amorph–kat–dia, isalso a nice example of MOF topology transformation by ball-milling. Akimbekov et al. provided the first combinedexperimental and theoretical evaluation of this reaction,showing how the differences in ligand structure andframework topology affect the relative stability ofisocompositional ZIFs.68 The results show that the observedmechanochemical synthesis and transformations ofpolymorphs of ZIF-8 (ref. 66) are consistent with Ostwald'srule of stages69 and proceed toward increasinglythermodynamically stable and denser phases (Fig. 5b).

The ZnĲEt–Im)2 phases displayed similar behavior. Besidesthe ammonium salts as ionic additives, which directed theformation of the RHO, ANA, or qtz topology in the ZnĲEt–Im)2ZIF,47 the volume of added liquid additives was shown to playan essential role in their mutual transformations.48,49 In situPXRD data revealed that during the ball-milling, differenttopologies of ZnĲEt–Im)2 occur in the reaction vessel, and theoccurrence and life-span of the respective form is directlylinked to the amount of the liquid additive in the reactionmixture.48 By prolonged milling, the polymorphs of ZnĲEt–Im)2 transform in the sequence RHO–ANA–qtz, from the mostporous one (and thermodynamically least stable) to the leastporous one (and thermodynamically most stable phase),70

thus following Ostwald's rule of stages (Fig. 5c). Very recently,the Friščić group reported the phenomenon of“disappearing” polymorphism in mercuryĲII)-ZIFs.71 In theirattempt to prepare a known dia-phase of the ZIF, the authorssynthesized a new layered phase with the sql-topology. Thediamondoid phase became elusive; it was detected as a shortintermediate in the milling process, whereas the solutionprocedures showed no evidence of it. This ZIF system thusshows behavior that is opposite to that of the known Zn-ZIFsdescribed above, achieving a less stable phase by millingprocessing. The theoretical studies rationalized this behavioras to stem from the weak interactions stabilizing the layeredsql-phase, indicating that the non-covalent interactions canalso play a role in the mechanochemical control over theMOF polymorphs.

Topology transformation by milling is not limited to ZIFs;very recently, mechanochemical procedures were used forrapid synthesis of phase-pure products and topology

Fig. 5 (a) Phase transition in the ZnĲMe–Im)2 system when water ordilute acetic acid is used as a liquid additive in the LAG process.Comparison of the experimentally observed mechanochemical phasetransitions to the measured relative enthalpy of the respectivepolymorphs of the (b) ZnĲMe–Im)2 and (c) ZnĲEt–Im)2 ZIFs.68 Theenthalpies were calculated relative to the densest phase for each MOF.Fig. 5(b) and (c) are reproduced from ref. 68 with permission from theAmerican Chemical Society, Copyright 2017.


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transformation in the 12-coordinated porphyrinic zirconiumMOF polymorphic pair, MOF-525 and PCN-223.72 MOFsbased on the tetrakisĲ4-carboxyphenyl)porphyrin (TCPP)ligand and zirconium oxoclusters represent a particularlyinteresting subclass of zirconium carboxylate MOFs due totheir occurrence in several different topologies, while the vastmajority of known zirconium MOFs have been isolated andreported as one sole crystalline phase.62 Mechanochemicalprocedures were shown to be very suitable for the green andrapid production of MOF-525 and PCN-223 polymorphs,72

gaining pure phases in 25–60 minutes of ball-milling, andmending the problematic solution procedures which oftenyielded mixed phases.73 The resolution between thepolymorphic phases in LAG processes was directed by liquidadditives and the type of preorganized zirconium clusters.Moreover, when using zirconium dodecanuclear acetatecluster [Zr12O8ĲOH)8ĲCH3COO)24]

65 and a MeOH additive, insitu PXRD monitoring revealed that MOF-525 formed rapidlyand then transformed into a pure PCN-223 form in a mere 30minutes of milling (Fig. 6). This transformation that occursduring the in situ formation of MOF-525 is in strong contrastwith long solution or slurry procedures, pointing towards theimportance of plastic deformations, defects, and flexibility ofpartially coordinated zirconium clusters that form during the

mechanochemical nucleation and growth of a MOF phase. Asimilar phenomenon was also observed for themechanochemical synthesis of UiO-66 from preorganizedzirconium clusters, where the unit cell parameters changeduring the mechanochemical nucleation and growth of MOFparticles.74

5 MOF composites

Utilizing tunable and flexible MOFs as solid substrates forfunctional heterogeneous catalysis is an important directionof research, although it is still in its early stage.75 Some ofthe benefits that MOF catalysts offer are a high density ofcatalytic sites, post-synthetic modifications, and poreconfinement. On the other hand, benchmark industrialcatalysts have been developed for decades and offer highmechanical, chemical, and thermal stability. Therefore, highproduct selectivity for the synthesis of fine chemicals is aprerequisite for the industrial application of functional MOFcatalysts.76 MOF composites, from controllable integration ofMOFs and other materials, mitigate disadvantages that MOFsmight have and can lead to synergistic performances.77

Wei et al. have shown how mechanochemical milling canbe used to prepare MOF composites consisting of enzymes asbiocatalysts and MOFs.78 Several enzymes, includingβ-glucosidase, invertase, β-galactosidase, and catalase havebeen encapsulated into ZIF-8, UiO-66-NH2, or Zn-MOF-74during solid-state synthesis (Fig. 7), using the advantages ofrapid mechanochemical reactions to minimize damage toenzyme catalytic activity. MOFs not only served as hosts butboosted the resistance to proteases and stability under acidicconditions (Fig. 7). This work has shown how solvent-freeball milling can be utilized as a tool for making MOF bio-composites that can be of great interest in differentindustrial processes.

Loading of catalytically active metals into a MOF supportproved to be highly effective for the preparation of potentMOF catalysts.79–87 Jiang et al. directly deposited Aunanoparticles (NPs) onto ZIF-8 by solid grinding with avolatile organogold complex and without using organicsolvents (Fig. 8a).86 Au@ZIF-8 samples have shown highcatalytic activity in gas phase CO oxidation after thermal

Fig. 6 Mechanochemical polymorph resolution in porphyrinic Zr-MOFs. Transformation of MOF-525 into PCN-223 occurs under mildmilling conditions despite requiring a significant rearrangement innode- and linker geometry.72 The effective separation of paramagneticcenters in porphyrin rings leads to interesting and rare electron-spinspectra of the two MOF polymorphs. Reproduced from ref. 72 withpermission from the American Chemical Society, Copyright 2019.

Fig. 7 Mechanochemical procedure for the rapid preparation of bio-composites based on ZIF-8, Zn-MOF-74, or UiO-66-NH2 via theencapsulation of enzymes into the MOF cavities. The prepared MOFbio-composites displayed high biological activity, while also providingsuperior stability of the enzymes.78


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activation at around 300 °C, enabled by the high thermalstability of the ZIF-8 support. It is expected that thisapproach can be extended to loading of other noble metalNPs into MOF supports.86,87

Furthermore, Bhattacharyya et al. used ball-milling toprepare perovskite quantum dot (PQD) MOF-composites.88

Solvent-free synthesis of a PQD@MOF composite wasachieved using the metallated anionic AMOF-1 [(NH2Me2)2]-ĳZn3ĲL)2]·9H2O (L = 5,5′-(1,4-phenylenebisĲmethylene))bisĲoxy)-diisophthalate) as the template. The procedure includes anexchange of the organic cations with PbĲII), followed bygrinding of PbII@AMOF-1 with CsX (X = Cl, Br, and I) for 15–20 min (Fig. 8b). The in situ formation of PQDs wasevidenced by characteristic perovskite emission using a UVlamp. CsPbX3 PQD nanocrystals are formed on the surface ofAMOF-1 with size dependence on AMOF-1, thus showing aquantum confinement effect. The authors have shown thatthe CsPbX3@AMOF-1 samples have good stability in severalsolvents and can be easily processed as a colour-tuned ink.

The SMART approach was also used for loading ofibuprofen into a HKUST-1 network, where the authorsmanaged to efficiently encapsulate ibuprofen into the MOF

by a one-pot mechanochemical procedure involving a pre-assembled [Cu2Ĳibuprofen)4] cluster.89 The in situ loadingduring the mechanosynthesis enabled room-temperaturesynthesis of drug-loaded MOF composites for slow release ofibuprofen in buffered solutions.

6 Mixed-metal and mixed-ligandMOFs

Panda et al. reported a mechanical alloying method for thepreparation of mixed-metal MOFs,90 which was firstdemonstrated on two isostructural MOFs with a dicarboxylicacid linker, [AlĲOH)Ĳ1,4-naphthalenedicarboxylate)] (Al-ndc)and [GaĲOH)Ĳ1,4-naphthalenedicarboxylate)] (Ga-ndc). The

Fig. 8 a) Au NP loading into ZIF-8 by grinding and chemical reduction.TEM images of Au@ZIF-8 before (left) and after (right) catalyticreaction.86 Adapted from ref. 86 with permission from the AmericanChemical Society, Copyright 2009. b) Efficient formation ofPQDs@AMOF-1 nanocrystals.88 Reproduced from ref. 87 withpermission from The Royal Society of Chemistry.

Fig. 9 (a) Mechanochemical alloying of MĲIII) carboxylate MOFsthrough amorphization and recrystallization of two isostructuralMOFs.90 Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproducedwith permission from ref. 88. (b) “High-entropy”-ZIF with a randomdistribution of metal centers was made by the LAG method using amixture of “green” metal precursors and 2-Me–Im.92 Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission from ref.90.


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methodology involves dry milling of both MOFs under anargon atmosphere, which was shown to induceamorphization of the reaction mixture. The mechanicalalloying methodology allowed for different ratios of twoMOFs to be mixed, each resulting in crystalline and porousMOFs after the accelerated aging91 in a humid atmosphere (3days in a 95% RH atmosphere at 25 °C). X-ray and STEM-EDX studies revealed that mechanical milling promotes highdiffusivity of each metal ion in an amorphous solid matrix,allowing for the preparation of mixed-metal MOFs with ahomogeneous distribution of metal centers, at least at theresolution offered by STEM-EDX (Fig. 9a). The methodologywas also applied to ZIF-8 and MOF-74 to show the generalityof the principle, and the results indicated that the success ofthe method depends on various parameters, such as ionicradii, valence states, coordination preferences, theelectronegativity of metal ions, and the MOF topology.90

Lee et al. showed that mechanical alloying of isolated[FeĲ1,2,3-triazolate)2] with isostructural [CuĲ1,2,3-triazolate)2]results in a bimetallic MOF with a homogeneous distributionof the two metal centers. This hybrid material, whenpyrolyzed, retains the bimetallic homogeneity and can beused as a oxygen reduction reaction (ORR) catalyst.93 The twometals, included in the MOF structure, work in synergy, andshow exceptional ORR performance, outperforming thebenchmark Pt/C. This behavior was attributed to thehomogeneous distribution and control of the Fe2+ and Cu2+

composition within the MOF crystals, which cannot beobtained by solution-based synthesis. The findings of thisstudy showed that MOFs can be alloyed together by theapplication of mechanochemical techniques and that thesenew materials may show synergistic properties thatoutperform the individual components.

Ball-milling was successfully applied for the synthesis of aZIF-8 material composed of five divalent metal cations, ZnĲII),NiĲII), CdĲII), CuĲII), and CoĲII), dispersed randomly throughoutthe 3D-network in the sod topology of the MOF (Fig. 9b).92

The authors named the new multi-metal ZIF-8 “high-entropyzeolitic imidazolate framework, HE-ZIF”, inspired by high-entropy alloys, where five or more metal species areincorporated deliberately into a single lattice with randomoccupancy. The distribution of specific metals wasestablished by EDS microscopy, and inductively coupledplasma atomic emission spectrometry (ICP-AES) showed thatHE-ZIF contains all five metals in much higher content thanthose assembled from solution, in which zinc was theprincipal constituent. HE-ZIF was highly porous and showedenhanced catalytic activity compared with ZIF-8, which theauthors presumed to be a consequence of the synergisticeffect of the five metal-constituents.92 The results of thisstudy showed how mechanochemistry can be used as apowerful tool for the preparation of multimetallic MOFs, butcontrol of the spatial distribution of heterometallic centersremained a challenge.

Ayoub et al. demonstrated how the nature of the ligandcould be exploited for targeted, rapid, and controllable

mechanochemical synthesis of bimetallic MOF-74 materials(formula [M1M2ĲDHTA)·nĲsolvent)]) with a homogeneousdistribution of metal centers and controlled 1 : 1stoichiometric composition of heterometallic nodes.56 Theauthors used the ditopic nature of the DHTA ligand tocontrol the metal positioning in the MOF; the first metal(M1) reacts with the carboxylic functionalities of H4DHTA,forming nonporous 1 : 1 M1ĲH2DHTA) solid coordinationcomplexes of ZnĲII), MgĲII), NiĲII), and CoĲII) in the firstsynthetic step (Fig. 10a). These coordination polymers thenreact in the second step with an equimolar amount of thesecond metal (M2), and the prolonged LAG (alcohols)resulted in open and crystalline MOF-74. This approachresults in the assembly of various mixed-metal MOF-74materials composed of pairs of d-block or main group metalsin a predetermined 1 : 1 stoichiometric ratio, includingZnMg-, ZnCo-, ZnCu-, MgZn-, MgCo-, NiZn-, NiMg-, NiCo-,CoZn-, CoMg-, CoCu-, and MgCa-MOF-74. Similar attempts insolution failed, and control of the metal distribution was notachievable by conventional methods.

One of the first MOF families prepared by milling, pillaredZn-MOFs,45 which are discussed in more detail above, arebuilt by the bridging carboxylate and the dabco linkers, thusalso presenting an example of mixed ligand MOFs. Roztockiet al. have shown how MOFs with more complex mixedlinkers can be obtained quantitatively in a mechanochemicalreaction.94 The authors were studying the Zn-MOFs builtfrom 5-substituted isophthalic acids (Xiso) and4-pyridinecarbaldehyde isonicotinoyl hydrazone (pih),containing N and O donors. These MOFs, with the general

Fig. 10 (a) Mechanochemical approach towards mixed-metal MOF-74materials by exploiting the ditopic nature of the H4DHTA linker. (b) Thelength of the linkers can be decisive for the dimensionality of preparedCd-MOFs.


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formula [Zn2ĲXiso)2Ĳpih)2Ĳguest)]n, are accessible fromsolution, but also from rapid LAG procedures fromZnĲCH3COO)2·2H2O using a small amount of DMF or DMA.94

In a continuation of their research, the same group ofauthors reported the mechanochemical synthesis of twononisoreticular cadmium–organic frameworks built from 4,4-oxybisĲbenzenedicarboxylic) acid and two ditopicacylhydrazone linear linkers: shorter4-pyridinecarboxaldehyde isonicotinoylhydrazone or longerterephthalaldehyde di-isonicotinoylhydrazone, respectively(Fig. 10b). The frameworks possess the same type ofsecondary building units including Cd2 clusters but differ indimensionality.95 The study showed the ecological benefitsthe mechanochemical approach provided, where the days-long solution syntheses with soluble metal salts at elevatedtemperatures could be accomplished in under one-hour ofmilling at room temperature.

7 Amorphous and defective MOFs

Although the vast majority of the discovered MOFs arecrystalline solids, amorphous MOFs are an emerging area ofMOF research.96 Amorphization is a process where long-range order is lost and replaced by the local ordering of thesolid. It is accompanied by the loss of porosity due to thecollapse of the structure. This methodology can be used toentrap dangerous chemicals like radioactive iodine,97,98

encapsulate drug molecules,99 or produce gas separationmembranes.100 On the other hand, with thousands of MOFcompounds being known, there is a growing interest inmodifying the chemical and physical properties of thealready synthesized MOFs.101 Indeed, the introduction ofdefects by either de novo synthesis or post-syntheticmodification offers the possibility of tailoring MOFproperties, for example in heterogeneous catalysis.102

Having already been known that mechanochemical ball-milling is a fast and an efficient technique for obtaininghighly crystalline ZIFs,47 Bennet et al. showed thatamorphous ZIFs (am-ZIFs) could also be obtained in bulk bymechanosynthesis.103 Starting from evacuated ZIF-4, am-ZIF-4is obtained after only 30 min of milling at room temperatureand without crystalline impurities, Fig. 11a. The amorphousphase obtained by ball-milling is indistinguishable from theone obtained by thermal treatment for 5 hours at 300 °C.

Orellana-Tavra et al. developed a drug delivery system(DDS) based on the amorphous UiO-66 framework.99

Hydrophilic and fluorescent model drug calcein (cal) wasloaded into UiO-66 by soaking it in a calcein solution.cal@UiO-66 was subsequently ball-milled, and the processresulted in the encapsulation of calcein due to the structuralcollapse of the framework. By doing this, the authors wereable to significantly increase drug release times from 2 to upto 30 days in a DDS with a particle size of 272 ± 157 nm(Fig. 11b). Furthermore, as evidenced by confocal microscopy,both cal@UiO-66 and cal@am-UiO-66 were successfullyincorporated into HeLa cells with the slower release of the

drug from the amorphous material. This showed that thekinetic characteristic of the delivery was preserved for boththe crystalline and amorphous MOF-based DDSs.

Ye et al. reported the de novo synthesis of UiO-66 withincreasing defects and without the addition of amonocarboxylic acid or solvent.105 The procedure includedmilling ZrOCl2·8H2O and 1,4-benzenedicarboxylic acid (BDC)using a mortar and pestle. Such a pre-milled reaction mixturewas then transferred to an autoclave and crystallized at acertain temperature and for a certain time. After optimizingthe reaction conditions, the maximum number of missinglinkers per Zr6O4ĲOH)4ĲBDC)6 reached 2.16, much higher thanthat in UiO-66 prepared by a solvothermal method. The roleof defects was verified by studying the oxidativedesulfurization (ODS) reactions of dibenzothiophene (DBT)and 4,6-dimethyldibenzothiophene (4,6-DMDBT) as well asthe oxidation of benzyl alcohol. It was shown how theintroduction of defects enhanced the catalytic performancewhen compared to defect-free UiO-66.

Ogiwara et al. have shown how amorphization can be usedto modify the molecular motion of 2-methylimidazolate (Me–

Fig. 11 a) Solid-state transformations of ZIF-4.103 Reprinted withpermission from the American Chemical Society, Copyright 2011. b)Comparison of calcein release times from crystalline UiO-66 (black)and amorphous UiO-66 (red) DDS.99 Reproduced from ref. 97 withpermission from The Royal Society of Chemistry.


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Im) linkers in ZIF-8.106 The authors have taken advantage ofsolid-state 2H NMR spectroscopy where 2H line shape, spin–lattice and spin–spin relaxation (T1 and T2) are defined byintramolecular quadrupole interaction, making themconsequently sensitive to the type and the rate of themolecular motion. Firstly, the authors deuterated Me–Imlinkers and used them to synthesize ZIF-8 crystals. Secondly,deuterated ZIF-8 was evacuated, and the amorphization wasperformed by ball-milling. Solid-state 2H NMR studies haverevealed that amorphization significantly affected themobility of the Me–Im linkers, including the rotational angleand speed. Surprisingly, even a new librational modeappeared in am-ZIF-8.

8 Industrial potential of large-scalemechanochemical synthesis

In the 21st century and after the development of greenchemistry concepts, sustainable and scalable synthesis ofMOFs is one of the requirements for their successfulindustrial application.107 Generally, the majority oflaboratory-scale synthesis of MOFs is done by a solvothermalapproach using high-boiling and hazardous organic solventsthat will have to be washed away during later stages of MOF

activation. When thinking on the chemical design level, anyvolume reduction of organic solvent usage, or the usage ofwater instead, will have both environmental and economicalbenefits in the long run. Mechanochemical synthesis ofMOFs also requires the input of solvents/liquids by either theLAG or ILAG methods; however, they are added only incatalytic amounts. As an example, for the synthesis of 500 mgof a MOF product, approximately 200 μL should be addedand often simple alcohols (MeOH and EtOH) or water can beused as the liquid additive.104

Very important economic analysis of the large-scaleindustrial synthesis of MOFs was performed by DeSantiset al.108 It was shown that for the solvothermal synthesismethod, a large percentage of the overall costs is attributed tothe precipitation step (manufacturing cost) and solvent used(material cost). As a consequence and in direct comparison, adetailed cost breakdown for the synthesis of Mg-MOF-74 hasyielded $48.52 per kg for the solvothermal synthesis method,$17.96 per kg for the aqueous synthesis method and $11.88 perkg for the LAG mechanochemical synthesis method. As aconclusion, the authors reported that the potential costs for theLAG method are projected at $8.23 per kg which is below the$10 per kgMOF set by the US Department of Energy (DOE) as atechnical target for the industrial application of MOFs.108

Fig. 12 (a) The first methodology for the continuous solid-state synthesis of highly porous MOFs was performed by TSE (left). The prepared MOFs(right) displayed high porosity and crystallinity. We gratefully acknowledge the authors for providing us this figure. (b) Mechanochemical synthesisof several fcu-Zr-MOFs with water or alcohol additives was accomplished by using a Zr12-acetate precursor.104 (c) SEM images of the UiO-66-NH2

catalyst prepared on a 10 g scale in a planetary mill (left) and by TSE (right). Both samples are active for catalytic hydrolysis of the DMNPsimulant.104 (b) and (c) are reproduced from ref. 104 with permission from the American Chemical Society, Copyright 2018.


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Often overlooked, mechanochemical milling is also ascalable synthesis method. Planetary ball-milling offers thepossibility of synthesis on a tens-of-grams scale, whereasextrusion, in particular TSE, shows potential for industrial-scale output. A TSE is a device with a barrel having twointermeshing rotating screws that transport the reactionmixture along the barrel. The mixture is transported to thekneading segment, a distinct part of the screws where thematerial is subjected to mechanochemical treatment byshearing and kneading. The kneaded mixture is thentransported further, either to the end of the barrel or to thenext kneading section, until it reaches the end of the barreland exits into a container. The separate segments in thebarrel can be heated, and additives can be added to thereaction mixture at different points of the process, thusproviding many possibilities to alter and influence the TSEprocess.109 While a TSE was mainly used in the reactiveextrusion of polymers and organic materials, a realbreakthrough in the synthesis of MOFs was made in 2015 bythe James group. The authors have shown how TSE can beused for the synthesis of HKUST-1, ZIF-8 andAlĲfumarate)ĲOH) with a kg h−1 product output rate,amounting to a space time yield (STY) of 144 000 kg m−3 d−1

for HKUST-1 and even higher STYs for the other tested MOFs(Fig. 12a).110 Recently, Karadeniz et al. managed to scale-upthe synthesis of several zirconium-based MOFs which arecharacterized by high chemical stability and catalytic activityfor the degradation of chemical warfare agents.104 MOF-801,MOF-804, UiO-66 and UiO-66-NH2 were synthesized on a 500mg scale using water as the liquid additive. In the case ofUiO-66-NH2, its synthesis was achieved by TSE using MeOHas the liquid additive with a 1.4 kg h−1 output rate (Fig. 12b).Importantly, the adsorption and catalytic properties of themechanochemically synthesized MOFs were validated and aresimilar to their solvothermally synthesized MOFcounterparts.104 This showed that continuous-flow extrusionreactors, which are being used in many industrialprocesses,109 can be readily applied for large-scale productionof important MOFs.

9 Summary and outlook

As can be seen from this short overview, mechanochemistryand MOFs have, despite the relatively late start, becomerapidly intertwined in many new and exciting areas ofmaterials science. Ball-milling is not only being used for agreener and high-yielding approach towards relevant MOFs,but also offers a platform for the preparation of non-conventional MOFs and solid-phases not available by anyother synthetic methods. Unlike those for other materialclasses, the mechanisms of the synthetic grinding processesfor MOFs have been relatively well characterized thanks to insitu monitoring methods. The knowledge about the factorsgoverning the reactions towards specific phases can beutilized in the scale-up of the syntheses. On the one hand,two important examples in the literature have shown how

TSE is a cost-effective method that minimizes therequirements for costly soluble metal precursors, organicsolvents, and energy in the process while affording high-quality MOFs in kg per hour output rates. We expect thatthese studies will greatly contribute to achieving continuousmanufacturing of MOFs on industrial scales in a sustainablemanner. On the other hand, we feel that the full potential ofmechanochemical techniques is still not achieved, and therising number of articles describing the use ofmechanochemistry for the preparation of MOF hybridmaterials, polymorph control, and transformation of MOFsinto new phases promises the field to expand further, formechanochemistry to become more present in syntheticlaboratories and chemical industries focused on MOFchemistry.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are thankful to Dr. Stipe Lukin for critically readingthe manuscript. The work has been supported in part by the“Research Cooperability” Program of the Croatian ScienceFoundation funded by the European Union from the EuropeanSocial Fund under the Operational Programme Efficient HumanResources 2014-2020, through grant PZS-2019-02-4129.

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Mechanochemistry: an efficient and versatile toolbox for synthesis…bib.irb.hr/datoteka/1059738.Stolar_Uarevi.pdf · 2020. 4. 28. · Synthesis at Ruđer Bošković Institute in