Journal ofMaterials Chemistry A

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Department of Chemistry, TKL of Metal- a

Nankai University, Tianjin 300071, P. R. C

+86-22-23502458

† Electronic supplementary informationand crystallographic data in CIF or10.1039/c3ta01322g

Cite this: J. Mater. Chem. A, 2013, 1,4186

Received 1st December 2012Accepted 12th February 2013

DOI: 10.1039/c3ta01322g

www.rsc.org/MaterialsA

4186 | J. Mater. Chem. A, 2013, 1, 41

Bottom-up assembly of a porous MOF based onnanosized nonanuclear zinc precursors for highlyselective gas adsorption†

Yun-Wu Li, Kun-Huan He and Xian-He Bu*

A step-by-step assembly of a highly porous MOF from a predesigned

giant nanosized nonanuclear zinc precursor and a length-extendable

organic linker with functional triazole groups bearing exposed

uncoordinated nitrogen sites has been achieved. The open N-donor

sites lead to a highly selective CO2 over CH4 adsorption capacity.

Moreover, non-coordinating groups in the channel surfaces behave

as a gate function.

Great effort is being devoted to the design and synthesis ofporous metal–organic frameworks (PMOFs) for potentialapplications in molecular separation,1 catalysis,2 gas storageand capture,3 drug delivery,4 and sensor devices.5 Until now,current research focuses on two effective strategies to constructPMOFs with large pores and high porosity. The most commonstrategy is ligand extension which directly introduces larger orlonger organic linkers into their porous frameworks.6 The use oflarger or longer organic linkers would consequently increase thespace between metal vertices which benets us to expand largepore diameter or/and high porosity in target PMOFs. Theappropriate ligands can be designed, synthesized and modiedby the mature technique of organic synthesis. So this strategyhas found wide applications and has been applied successfullyin many examples or even several mesoporous MOFs.7 Thealternative strategy to achieve a large pore size is to constructthe PMOFs from large metal clusters. As references demon-strated, utilizing large polynuclear clusters as vertices canrelease amore sufficient capacity and can easily satisfy the stericdemands of organic linkers.8 The metal clusters can be prede-signed to possess the required geometry, accessible coordina-tion mode, and desired directionality, which provides thepossibility of rational synthesis. Very recently, this approach has

nd Molecule-Based Material Chemistry,

hina. E-mail: buxh@nankai.edu.cn; Fax:

(ESI) available: CCDC 887425. For ESIother electronic format see DOI:

86–4189

also been successfully implemented to construct highly porousMOFs exhibiting both the microporous andmesoporous range.9

The two synthetic routes can both produce independentframework units containing large pore metrics. However, untilrecently few of highly porous MOFs have been reported basedon the combination of the two strategies.10 Here, we deliberatelyconsidered the above two strategies and selected an elongatedligand and an enlarged metal cluster to construct a highlyporous MOF stepwise, {[Zn9Cl2(bcpt)2(Me2bta)12]$0.5DMF}n(H2bcpt ¼ 3,5-bis(3-carboxyphenyl)-1,2,4-triazole, Me2btaH ¼5,6-dimethyl-1H-benzotriazole) (1). This work not only intro-duces one nearly nanosized (ca. 1.0 nm) nonanuclear zinccluster (Fig. 1a) as a precursor in the highly porous MOFstructure but also designs a nanosized (ca. 1.43 nm) bridgingligand by incorporating three aromatic rings groups (Fig. 1c).The presynthesized nonanuclear zinc cluster exhibits elongateddistorted triangular prism geometry (Fig. 1b) with multiplesubstitutable apical sites. The selected ligand H2bcpt containsan uncoordinated triazole heterocycle with exposed N atoms,which can act as functional sites for enhancing adsorption ofsome specic gases.11 Our strategy integrates both the bottom-up concept and stepwise synthetic method instead of the one-pot synthesis which provides access to the logical design andsynthesis of robust PMOFs with predetermined structures andproperties.

Aer careful consideration, we chose the nanosized metalcluster [Zn9(Me2bta)12(NO3)6]$ 3DMF (denoted as {Zn9}) as aninitial reaction precursor because similar pentanuclear clustershave proven to achieve the substituted process both in theexperimental and theoritical study by Tao and Su.12 The {Zn9}cluster has six Zn2+ ions at each vertex of the triangular prismwhich bears a chelating nitrate group (Fig. 1a). In principle,these labile, peripheral nitrate groups could be fully substitutedby stronger bonding ligands which have been demonstrated inthe literature.12 The {Zn9} precursor was prepared according tothe literature.13 Block-like colorless crystals of 1 were obtainedvia a solvothermal reaction of the {Zn9} precursor andH2bcpt$HCl (1 : 5.4 in molar ratio) in DMF solution (For details

This journal is ª The Royal Society of Chemistry 2013

Fig. 1 (a) The structure and size of the nanosized {Zn9} cluster SBU; (b) each {Zn9}cluster can be treated as a distorted triangular prism; (c) the elongated nanosizedligand H2bcpt; (d) the six substituted sites by four bcpt ligands and two Cl� (allhydrogen atoms are omitted for clarity).

Fig. 2 (a) The 2Dwavelike framework of 1withmesoporous rectangle windows;(b) the 3D packing structure of 1 showing 1D open channels along the c axis (allsolvent and hydrogen atoms are omitted for clarity).

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see the Experimental section, ESI†). Phase purity of the bulkycrystals was conrmed by the similarity between the experi-mental and simulated powder X-ray diffraction (PXRD) patterns(Fig. S7†).

The single-crystal X-ray diffraction study revealed that 1crystallizes in themonoclinic space group C2/c (number 15) (Fordetails see the X-ray crystallography section, ESI†). As expected,all the nitrate groups bonding to the apical sites of {Zn9} arereplaced by four bcpt linkers and two terminal Cl� ions. Thefour substituted sites by bcpt ligands are in the diagonal posi-tions of the distorted triangular prism (Fig. 1d). Based on theabove coordination modes, all the nonanuclear zinc clusters arelinked by the bcpt ligands into a 2D wavelike framework withmesoporous rectangle windows ca. 17.75 A � 33.45 A (Fig. 2a).Such an undulated sheet with very large rectangle pores offersan ideal condition for the interpenetration. Actually, two iden-tical sets of 2D layers in 1 are interlocked with each other toform a two-fold interpenetration framework (see Fig. S1 andS2†). The two-fold interpenetration 2D layers can be coded as Aand B according to the different orientations of nonanuclearzinc clusters. Aer interpenetration, the nonanuclear zincclusters in one layer occupy some space of another layer whichreduces the pore size (Fig. S1 and S2†). These two kinds of layersare stacked together in an ABAB/mode along the c axis and arenotably well-overlapped (see Fig. 2b and S3†). Hence, in the 3Dpacking arrangement, compound 1 possesses two sorts ofpotential 1D channels along the b axis (Fig. S3†) and the c axis(Fig. 2b) that are partly occupied by several highly disordered

This journal is ª The Royal Society of Chemistry 2013

solvent DMF molecules (see the Experimental section, ESI†).PLATON analysis showed that the effective free volume of 1reached 41.0% of the crystal volume (8190.6 A3 out of the19996.0 A3 unit cell volumes).14

In order to conrm this bottom-up strategy, a similarsubstituted compound should be isolated. So another dicar-boxylic acid ligand H2BDA (1,4-benzenedicarboxylic acid) waschosen to substitute the corresponding vertex of the {Zn9}precursor and compound 2 (Fig. S4, ESI†) was obtained. Thiscompound exhibits a 1D puckered chain. It is not a highdimensional structure mainly because the giant aggregated{Zn9} units occupy certain spaces which cannot accommodateenough room and hinder the short ligands H2BDA to connectmore sites (Fig. S5, ESI†). But this result further proved thepossibility of substituting the apical sites by carboxylic acidgroups which is consistent with our proposition.

The high porosity of the PMOF 1 provides opportunities forprobing gas adsorption properties. Thermogravimetric analysisindicates that compound 1 is thermally stable until around325 �C (Fig. S6†). And the PXRD pattern indicates that theactivated 1a has crystalline form (Fig. S7†). So, N2, H2, CO2 andCH4 sorption isotherms were carried out to examine its gasesloading capacity. N2 sorption measurement for activated 1a wasperformed at 77 K and 87 K at 1 atm, respectively as shown inFig. 3a. The uptake amount of N2 increases abruptly at the start

J. Mater. Chem. A, 2013, 1, 4186–4189 | 4187

Fig. 3 (a) N2 and H2 sorption isotherms for activated 1a at 77 K and 87 K; (b)selective uptake of CO2 over CH4 for activated 1a at 195 K.

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of the experiment and reaches a plateau at ca. 78 cm3 g�1 (STP)at 77 K and ca. 63 cm3 g�1 (STP) at 87 K. According to N2

adsorption data, the apparent Brunauer–Emmett–Teller (BET)and Langmuir surface areas are ca. 206 and 274 m2 g�1,respectively. As we can see, the N2 adsorption and desorption donot exhibit the classically reversible type-I isotherm but displayan obvious hysteresis instead. The H2, CO2 and CH4 sorptionisotherms also display small hysteresises. The hysteresis curvesshould arise from the hindered escape of adsorbed gases in thepores during the desorption process due to the benzene ringsand methyl groups of Me2bta ligands in the channel surfacesmay function as gates and render the famous gate-opening andgate-closing behaviors.15 The pore size distributions determinedby analyzing the N2 isotherm at 77 K are between ca. 0.51 nmand 1.78 nm, and the median pore size is about 0.53 nm for 1a(Fig. S9†). To further monitor the attractive energy carrier gasstorage capacity and behaviors of 1a, H2 adsorption isothermswere also recorded at 77 K and 87 K. As shown in Fig. 3a, the H2

uptake of 1a reach ca. 76 cm3 g�1 (STP) at 77 K and 1 atm, andca. 64 cm3 g�1 (STP) at 87 K and 1 atm, respectively. The valuesof H2 uptake are moderate.16 The H2 adsorption enthalpies wereestimated from the H2 isotherms at 77 and 87 K by using amodied version of the Langmuir–Freundlich equation.16 Theenthalpies of the adsorption are 12.6–5.8 kJ mol�1 for 1a(Fig. S10†), which are relatively higher than the values of manyreported MOFs.16

4188 | J. Mater. Chem. A, 2013, 1, 4186–4189

To evaluate the adsorption selectivity and capacity of 1a, theadsorption isotherms of CO2 and CH4 at 195 K were measured(Fig. 3b). It is interesting to nd that 1a shows highly selectivegas adsorption for CO2 over CH4 at 195 K. As shown in Fig. 3b, itadsorbs a comparatively high amount of CO2 at 195 K and 1 atm(ca. 113 cm3 g�1 (STP)), while adsorbs only a limited amount ofCH4 (ca. 11.8 cm3 g�1 (STP)) under the same conditions,respectively. The CO2 uptakes are nearly 9.6 times higher thanthat of CH4 for 1a at 195 K and 1 atm. This selectivity of CO2 overCH4 is greatly higher than that in many reported MOFs even thefamous MOF-5 and MFI zeolites, the values of which arereported to be 2 and 2.5 times, respectively.17 It is worth notingthat the selectivity of CO2 over CH4 of 1a is up to 6000 at 0.1 atmwhich is a typical partial pressure of CO2 in industrial ue gas.18

The CO2 adsorption amounts achieve ca. 60 cm3 g�1 (STP) andthere are almost no uptakes for CH4 (less than 0.01 cm3 g�1

(STP)). This selectivity is very high and the separation rate isquite fast.19 The signicant CO2 sorption selectivity in 1amay beattributed to the favorable interaction between adsorbed CO2

molecules and uncoordinated N atoms with lone electron pairsfrom N-rich aromatic rings which not only increases CO2

absorbance, but also is benecial to gas separation.11 Theexcellent CO2 adsorption selectivity and capacity of 1a canpromise its utilization in natural gas purication for energyproduction and greenhouse gas capturing for environmentalprotection.

In conclusion, a highly porous MOF has been constructedfrom a predesigned nanosized giant nonanuclear zinc precursorand a length-extendable organic linker with functional triazolegroups bearing exposed uncoordinated nitrogen sites by using astepwise synthesis strategy. The gases adsorption resultsdemonstrate a high capability in the selective separation of CO2

over CH4, which provides insight into the potential applicationsof this material in gas separation. This result highlights thehigher degree of design and prediction achieved on the result-ing structures of highly porous MOFs using the bottom-upapproach. More research work will be focused on the reactionbetween various enlarged cluster-based precursors and theother elongated ligands with open N-donor sites in order toisolate new PMOF species with interesting properties andfunctionalities.

This work was nancially supported by the 973 Program ofChina (2007CB815305), the NNSF of China (21031002, 20801029and 51172102), and the Natural Science Fund of Tianjin, China(10JCZDJC22100).

Notes and references

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