The metal-organic framework ZIF-8, which undergoes hydrolysis under hydrothermal conditions, is endowed with high water-resistance after a shell-ligand-exchange-reaction. The stabilized ZIF-8 retains its structural characteristics with improved application performances in adsorption and membrane separation. © 2013 The Royal Society of Chemistry

Ban, Yujie

Bux, Helge

Caro, Juergen

Jin, Hua

Li, Yanshuo

Liu, Xinlei

Peng, Yuan

Xu, Longya

Yang, Weishen

李砚硕

杨维慎

Caro, Jürgen

Institutional Repository of Leibniz Universität Hannover

9140 Chem. Commun., 2013, 49, 9140--9142 This journal is c The Royal Society of Chemistry 2013Cite this: Chem. Commun.,2013,49, 9140Improvement of hydrothermal stability of zeoliticimidazolate frameworks†Xinlei Liu,ab Yanshuo Li,*a Yujie Ban,ab Yuan Peng,ab Hua Jin,ab Helge Bux,aLongya Xu,a Jürgen Caroc and Weishen Yang*aThe metal–organic framework ZIF-8, which undergoes hydrolysis underhydrothermal conditions, is endowed with high water-resistanceafter a shell-ligand-exchange-reaction. The stabilized ZIF-8 retains itsstructural characteristics with improved application performances inadsorption and membrane separation.Metal–organic frameworks (MOFs) or porous coordination polymers(PCPs) are an emerging class of nanoporous materials showing awide range of potential applications, e.g. gas storage, molecularseparation, chromatography, heterogeneous catalysis and smartsensoring.1 Yet for most of the MOFs, one of the major drawbacksis their poor hydrothermal stability, which is clearly a limitation totheir practical applications.2 Development of novel MOF structureswith high stability is an important research topic.3,4 To date, only afew MOFs are reported to possess satisfactory hydrothermal stability,e.g. the ZIF (zeolitic imidazolate framework) family,4 MIL (MatérialInstitut Lavoisier) analogues3b and some zirconium3c–e and pyrazo-late3a based MOFs. On the other hand, from a practical point ofview, post-enhancement of hydrothermal stability of the alreadyexisting MOF materials is a more attractive option and one of themost active research domains nowadays.5 However along withimproved stability, the application properties of the parent MOFswere usually impaired, e.g. considerable decrease of the specificsurface area (SSA) after different post-treatments.5b–dIn this study, the hydrothermal stability of ZIF-8, one proto-typical member of the ZIF family, was intensively studied. ZIF-8crystallizes with a sodalite (SOD)-related structure with a formula ofZn (2-methylimidazolate)2.4a,b ZIF-8 has drawn extensive researchinterest from both academic4 and industrial societies2a due to itsexceptional thermal and chemical stability. In this work, however,we observed experimentally that ZIF-8 underwent hydrolysis underhydrothermal conditions, regardless of its size (nano- or micro-crystals, Fig. S1 and S2, ESI†) and origin (e.g. even the ZIF-8synthesized in water showed instability after exhaustive washing,Fig. S3 and S4, ESI†). This is also found to be a general pheno-menon for other types of ZIFs (Fig. S5 and S6, ESI†). The observeddifference in ZIF’s stability can be attributed to the careful elimina-tion of the protective effect of released ligands in our stability tests(ESI†), which has never been noticed before.Herein we report our strategy for improving the hydrothermalstability of ZIF-8 via a shell-ligand-exchange-reaction (SLER). Anumber of excellent advancements in ligand exchange of MOFshave been made in the very recent years.6 Different from the abovecases in most of which complete ligand exchange is desired andattempted, in the SLER employed here, the ligand exchange reactionmainly occurs in the outermost shell of ZIF-8 particles as shown inScheme 1. After SLER, the ZIF-8 (termed ZIF-8–DMBIM) retainedits original crystal structure and exhibited remarkably enhancedhydrothermal stability without sacrificing, or even improving, theapplication performances in adsorption and membrane separation.The optimized recipe for SLER is: DMBIM (5,6-dimethylbenz-imidazole), methanol, triethylamine (TEA) and fresh ZIF-8 nano-crystals were dispersed in methanol in a glass bottle (weightcomposition: ZIF-8–DMBIM–TEA–MeOH = 1 : 1 : 0.7 : 160) and there-upon mixed. After being heated at 60 1C for 15 h, the product waswashed with methanol for later use.Scheme 1 Schematic representation of the shell-ligand-exchange-reaction(SLER) process of ZIF-8.a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, CAS,Zhongshan Road 457, 116023, Dalian, China. E-mail: leeys@dicp.ac.cn,yangws@dicp.ac.cnb University of Chinese Academy of Sciences, Beijing 100049, Chinac Institute of Physical Chemistry and Electrochemistry, Leibniz University Hannover,Callinstrasse 3A, 30167, Hannover, Germany† Electronic supplementary information (ESI) available: Experimental and char-acterization details. See DOI: 10.1039/c3cc45308aReceived 14th July 2013,Accepted 6th August 2013DOI: 10.1039/c3cc45308awww.rsc.org/chemcommChemCommCOMMUNICATIONPublished on 07 August 2013. Downloaded by Technische Informationsbibliothek (TIB) on 26/10/2017 07:48:10. View Article OnlineView Journal  | View IssueThis journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 9140--9142 9141The success of SLER was verified by FTIR-ATR and UV-vis Ramanspectroscopy. As shown in the FTIR spectra of ZIF-8–DMBIM (Fig. 1a),two new peaks appeared at 813 cm1 and 854 cm1, corresponding tothe C–H out-of-plane deformation vibrations in the phenyl ringsof DMBIM. A considerable red shift was observed compared withpure DMBIM, implying deprotonation of DMBIM and subsequentcoordination of imidazole nitrogen to zinc ions. The disappearance ofthe broad band at 3100 cm1 (N–H stretching vibrations) providesfurther evidence for the deprotonation–coordination process (Fig. S7and S8, ESI†). UV-vis Raman spectroscopy, employing exciting laserwith different wavelengths, is a powerful tool for studying the differentcompositions of the surface and bulk of a solid sample.7 In the Ramanspectra (Fig. 1b), the intensity of the in-plane deformation vibrations(720 cm1) and stretching vibrations (1293 cm1) originating from thefused rings of DMBIM decreases remarkably upon increasing thewavelength of the exciting laser (from 325 to 514 nm) comparedwith the ring-stretching vibrations originating from MIM (2-methyl-imidazolate, 1450 cm1). For this given material, the penetration depthis directly proportional to the exciting wavelength because of decreasedabsorbance (Fig. S9, ESI†). This signifies that the ligand exchange takesplace mainly in the outermost layer of ZIF-8 nanoparticles (Fig. S10,ESI†). This might be because the bulky DMBIM is too big to passthrough the narrow windows of ZIF-8 due to the molecular sievingeffect. The ligand exchange molar ratio (MDMBIM/(MDMBIM + MMIM)) is9.1%, as identified by 1H NMR (Fig. S11, ESI†).The structural integrity of ZIF-8–DMBIM after SLER treat-ment was evidenced by the unaltered powder X-ray diffraction(PXRD) patterns (Fig. 2a and b). The ZIF-8–DMBIM nano-particles maintained a rhombic dodecahedron shape with anarrow size distribution of around 40 nm (Fig. 2a and b insets).The hydrolysis-resistance abilities of ZIF-8 and ZIF-8–DMBIMwere compared via hydrothermal tests in distilled water at 80 1C for24 hours. A very small amount of crystals (18 mg of the sampleimmersed in 30 ml of water, 0.060 wt%) was used in order to excludethe protective effect of released ligands (Fig. S1–S4, ESI†). In contrastto the original ZIF-8 nanoparticles, which completely transformedinto ZnO (Fig. 2c), the crystal structure and morphology of the ZIF-8–DMBIM nanoparticles remained essentially unchanged after the test(Fig. 2d). Water contact angle measurements were carried out toassess the changes in hydrophobicity after SLER treatment (Fig. 2aand b insets). The ZIF-8–DMBIM showed a significant increase inthe contact angle of water (1211) compared with the original ZIF-8(601), which can be well explained by the effective introduction ofhydrophobicity within the outermost layer of the ZIF-8 nanoparticlesby DMBIM substitution. SLER with other kinds of imidazole deri-vatives was also attempted to enhance the hydrolysis-resistance ofZIF-8. Different degrees of enhancement were achieved dependingon the physicochemical properties of the ligands (Fig. S12, ESI†).The textural characteristics of ZIF-8 and ZIF-8–DMBIM werequantified by measuring the nitrogen sorption isotherms at 77 k.The isotherm curves (both adsorption and desorption branches) ofZIF-8–DMBIM are consistent with that of ZIF-8. Only a negligibledecrease in BET surface area and pore volume was observed afterSLER (Fig. S13 and S14, ESI†).ZIF-8 has been proven to be very promising for the efficientrecovery of bio-alcohols from aqueous solutions.8 In this work, theisobutanol sorption isotherms of ZIF-8–DMBIM were collected on anIntelligent Gravimetric Analyzer (IGA). After SLER treatment, ZIF-8–DMBIM showed only a slight decrease in the equilibrium adsorptioncapacity at 3.5 kPa (Fig. 3a), which coincides with the results fromBET. An interesting observation is the absence of the ‘‘gate opening’’effect (hysteretic adsorption behaviour)9 in the adsorption ofisobutanol on ZIF-8–DMBIM. This is in contrast to ZIF-8, for whicha threshold pressure (‘‘gate-opening’’ pressure) of 0.5 kPa wasobserved for the uptake of isobutanol (Fig. 3a). As presented inFig. 3b, although the DMBIM ligand is more bulky than MIM, theisobutanol transport diffusivity in ZIF-8 was obviously enhancedafter SLER. These phenomena observed on ZIF-8–DMBIM might bea result of the specific framework–guest interaction.9c No doubt, thesteep adsorption uptake of isobutanol at low pressure and theenhanced transport diffusivity are clearly of benefit for the applica-tions in adsorption and membrane based separation.The application performance of ZIF-8–DMBIM in liquid-phaseadsorption was evaluated employing the solution-depletion method(ESI†) with the emphasis on its hydrothermal stability. In order toFig. 1 (a) FTIR-ATR spectra of ZIF-8 (black), DMBIM (blue) and ZIF-8–DMBIM(red). (b) Raman spectra of ZIF-8–DMBIM using 325 nm (wine) and 514 nm (olive)exciting laser wavelength.Fig. 2 SEM images and XRD patterns of (a) ZIF-8, (b) ZIF-8–DMBIM, (c) ZIF-8 after thehydrothermal test and (d) ZIF-8–DMBIM after the hydrothermal test. The insets are thecorresponding TEM images and the contact angle images of water droplets.Communication ChemCommPublished on 07 August 2013. Downloaded by Technische Informationsbibliothek (TIB) on 26/10/2017 07:48:10. View Article Online9142 Chem. Commun., 2013, 49, 9140--9142 This journal is c The Royal Society of Chemistry 2013accelerate the evaluation process, the adsorbents were periodicallytreated under harsh conditions (0.060 wt% adsorbents in 3.0 wt%isobutanol solution at 80 1C for 24 h) after each adsorption–desorption cycle (ESI†). As shown in Fig. 3c, the uptake capacity ofZIF-8 decreased to around zero after 4 cycles due to the hydrolysisinduced amorphization of the SOD structure (Fig. S15, ESI†). Incontrast, no discernible degradation of the isobutanol uptakeoccurred on ZIF-8–DMBIM, thanks to its excellent hydrothermalstability (Fig. S15, ESI†).In our previous studies, by dispersing ZIF-8 nanoparticles intosilicone rubber (e.g. polymethylphenylsiloxane, PMPS), we got the so-called mixed-matrix-membranes (MMMs).8a,10 In the current work, aZIF-8–DMBIM–PMPS membrane was prepared and tested for perva-poration recovery of isobutanol from water (ESI†). When exposing theZIF-8–DMBIM–PMPS membrane to a feed containing 3.0 wt% iso-butanol, a permeate containing 58 2wt% isobutanol was obtained.This value is ca. 1.40  0.05 times higher than that obtained viaevaporation which is determined by the vapor–liquid equilibrium(VLE). Compared with the ZIF-8–PMPS membrane, the ZIF-8–DMBIM–PMPS membrane exhibited improved selectivity towardsisobutanol while keeping the isobutanol flux (productivity) constant(Fig. 3d, Fig. S16–S18, ESI†). This performance also ranks amongthe highest values for organophilic pervaporation membranes.8a Theimproved selectivity results from the increased hydrophobicity,the decreased threshold pressure for isobutanol adsorption and theenhanced transport diffusivity after SLER treatment of ZIF-8.In summary, ZIF-8 is currently one of the most stable metal–organic frameworks.2a,4a,c Nevertheless, our experimental results pro-vided compelling evidence that ZIF-8 will undergo hydrolysis underhydrothermal conditions, where transformation to ZnO will be thermo-dynamically preferred.2a The hydrothermal stability of ZIF-8 can beremarkably improved via shell-ligand-exchange-reaction (SLER), takingadvantage of the hydrophobicity (water-repellent) effect and the sterichindrance effect of DMBIM. After SLER treatment, the ZIF-8–DMBIMretains the structural characteristics of ZIF-8 with improved applicationperformances in adsorption and membrane separation. Besides ZIF-8,the SLER methodology was also successfully applied to stabilize othertypes of ZIFs, e.g. ZIF-7 (SOD topology, Zn (benzimidazolate)2)4a,b andZIF-93 (RHO topology, Zn(4-methylimidazolate-5-carbaldehyde)2)11(Fig. S5 and S6, ESI†). The stabilized ZIF materials are expected to besuitable for various applications under aqueous conditions, such asadsorbents, membranes, and heterogeneous catalysts.This work was supported by the National Science Fund(21176231, 21006101, 21276249 and 21361130018) and the DICPIndependent Research Project (No. R201006). The authors thankProf. Dr Baokun Huang and Dr Min Wang for their kind support inRaman spectra characterization and contact angle measurements,respectively.Notes and references1 H. C. Zhou, J. R. Long and O. M. Yaghi, Chem. Rev., 2012, 112, 673.2 (a) J. J. Low, A. I. Benin, P. Jakubczak, J. F. Abrahamian, S. A. Faheemand R. R. Willis, J. Am. Chem. Soc., 2009, 131, 15834;(b) K. A. Cychosz and A. J. 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Hayashi,Y. Houndonougbo, M. Asta, B. B. Laird and O. M. Yaghi, J. Am.Chem. Soc., 2010, 132, 11006.Fig. 3 (a) isobutanol (40 1C) sorption isotherms and (b) transport diffusivity onZIF-8 (black circles) and ZIF-8–DMBIM (blue squares). Filled and open symbolsrepresent the adsorption and desorption branches, respectively. (c) Uptake ofisobutanol (3.0 wt%, 25 1C) on ZIF-8 (black) and ZIF-8–DMBIM (blue) over fivecycles. (d) isobutanol concentration in the permeate (downstream side) atdifferent feed concentrations (upstream side, at 80 1C) for ZIF-8 (black) andZIF-8–DMBIM (blue) derived mixed matrix membranes (MMMs). Vapor–liquidequilibrium (VLE) data (gray) are also shown for comparison.ChemComm CommunicationPublished on 07 August 2013. Downloaded by Technische Informationsbibliothek (TIB) on 26/10/2017 07:48:10. View Article Online

English

Improvement of hydrothermal stability of zeolitic imidazolate frameworks

Xinlei Liu

Yanshuo Li

Yujie Ban

Yuan Peng

Hua Jin

Helge Bux

Longya Xu

Jürgen Caro

Weishen Yang

Crossref

Chemical Communications

The metal-organic framework ZIF-8, which undergoes hydrolysis under hydrothermal conditions, is endowed with high water-resistance after a shell-ligand-exchange-reaction. The stabilized ZIF-8 retains its structural characteristics with improved application performances in adsorption and membrane separation

Institutional Repository of Dalian Institute of Chemical Physics, CAS

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Improvement of hydrothermal stability of zeolitic imidazolate frameworks

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Institutional Repository of Dalian Institute of Chemical Physics, CAS