Methyl modified MOF-5 a water stable hydrogen storage materialw

Cite this:mun .,2011,47,5244–5246Methyl modified MOF-5:a water stable hydrogen storage material wJie Yang,a Anna Grzech,b Fokko M.Mulder b and Theo J.Dingemans*aReceived 22nd February 2011,Accepted 18th March 2011DOI:10.1039/c1cc11054cWater stable methyl modified MOF-5s have been synthesized via a solvothermal route.Methyl-and 2,5-dimethyl-modified MOF-5s show the same topology and hydrogen uptake capability as that of MOF-5.The H 2uptake capacity of MOF-5,however,drops rapidly when exposed to the ambient air,whereas the H 2uptake capacities of the methyl modified MOF-5s remain stable for 4days.Hydrogen,which has an exceptional energy density and little or no harmful emissions,is currently under consideration as a future energy carrier.1However,the emergence of a hydrogen economy requires the development of new materials capable of safely storing hydrogen in a compact and light weight fashion.2Metal–organic frameworks (MOFs)are crystalline materials consisting of metal ions,or metal ion clusters and organic ligands,which are linked together to form extended porous frameworks.Over the last decade,MOFs have attracted considerable interest because of their large surface area,adjustable pore size and controllable properties,as well as their unusual thermal stability.These properties make them interesting candidates for a wide range of applications,including gas sorption,catalysis,drug delivery and sensing.3–6MOFs con-taining Zn,Mn,Cr,and Cu show outstanding hydrogen adsorp-tion behavior at 77K and are among the most promising hydrogen storage materials.7Maybe the best known MOF to date is MOF-5.This three-dimensional network has a crystal structure where metal clusters [Zn 4O]6+are joined to an octa-hedral array of benzene-1,4-dicarboxylate (BDC)groups to form a porous cubic Zn 4O(BDC)3framework.8MOF-5displays an excess H 2uptake capacity of 7.1wt%at 77K and 40bar.At 100bar,the total hydrogen uptake of MOF-5can reach up to 10wt%,corresponding to a record volumetric storage density of 66g L À1.9However,most MOFs,in particular zinc-based MOFs,are moisture-sensitive because the relative weak metal–oxygen coordination allows for attack by water molecules,resulting in the phase transformation and decomposition of the framework.10Structural decomposition,caused by the presence of water,always leads to poor reproducibility and a decrease in gas sorption capacities in MOFs.Discrepancies in hydrogen uptake capacities have been found for MOF-5and are thought to be due to the presence of water.9High quality MOF-5can only be obtained when the exposure to water and ambient air was minimized.9Much care must be taken to avoid the structural collapse of MOFs,since it will limit their potential commercial application.Therefore it is desirable to enhance the structural stability of MOFs towards moisture,but at the same time,the excellent H 2uptake capability of MOFs should remain unaffected.Very recently,researchers have demonstrated that it is possible to build water-stable MOF structures by introducing hydrophobic functional groups into the frame-work structures.11,12Yet,no results have been published on the hydrogen storage capability of such functionalized MOFs.In this communication,the effect of moisture on the hydrogen storage capability of methyl (CH 3)and dimethyl (DiCH 3)modified MOF-5has been investigated.The obtained results suggest that by simply introducing methyl groups the hydrogen uptake capacity of MOF-5remains uncompromised and less sensitive to the presence of moisture,even after 4days exposure to air with a relative humidity of 32–37%.The methyl and 2,5-dimethyl MOF-5analogs were synthesized using a well-known solvothermal route.The samples were activated by immersing them in dried CHCl 3followed by heating at 1601C in vacuum for 24h.The samples thus obtained were denoted as CH 3MOF-5and DiCH 3MOF-5.MOF-5was synthesized for reference purposes according to a literature procedure.13The samples exposed to ambient air for 4days were labelled CH 3MOF-5-4d and DiCH 3MOF-5-4d.The hydrogen uptake capacities of MOF-5,CH 3MOF-5and DiCH 3MOF-5were measured on a homemade Sieverts setup.The elemental analysis of all samples and their hydrogen uptake capacities are summarized in Table 1.The methyl modified MOF-5s show the same topology as that of MOF-5,which is supported by the high degree of correspondence between the PXRD patterns of methyl modified MOF-5and MOF-5(Fig.S1,ESI w ).MOF-5samples were exposed to ambient air and the structural stability under ambient conditions was examined by PXRD.With respect to MOF-5,our findings are in agreement with results reported in the literature.When exposed to ambient air,with a relative humidity of 32–37%,an extra peak at 2y =8.81appears after only 45min.In addition,the intensity of the original peaks decreases dramatically with exposure time and after 1day,aFaculty of Aerospace Engineering,Delft University of Technology,Kluyverweg 1,Delft,The Netherlands.E-mail:t.j.dingemans@tudelft.nl;Fax:+31(0)152784472;Tel:+31(0)152784520bDepartment of Radiation,Radionuclides and Reactors,Faculty of Applied Sciences,Delft University of Technology,Mekelweg 15,2629JB Delft,The Netherlands.E-mail:f.m.mulder@tudelft.nl;Fax:+31(0)152783803;Tel:+31(0)152784870w Electronic supplementary information (ESI)available:Full synthetic procedure and characterization data including PXRD patterns and hydrogen isotherms.See DOI:10.1039/c1cc11054cChemCommDynamic Article Links/chemcommCOMMUNICATIOND o w n l o a d e d o n 09 J a n u a r y 2012P u b l i s h e d o n 30 M a r c h 2011 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 1C C 11054CView Online / Journal Homepage / Table of Contents for this issuethe structure of MOF-5completely transformed to ZnBDC ÁX H 2O (Fig.S2,ESI w ).9,14Interestingly,the PXRD pattern of CH 3MOF-5remains unchanged even after 4days exposure to air (CH 3MOF-5-4d).Similar results were obtained for DiCH 3MOF-5,as shown in Fig.1.Clearly,the structures of CH 3MOF-5and DiCH 3MOF-5remain stable up to 4days after exposure to ambient air.11,12These results strongly imply that introducing hydrophobic methyl groups enhances the structural stability of MOF-5type framework.TGA curves of CH 3MOF-5and DiCH 3MOF-5both exhibit weight loss events starting at B 4001C,which is due to the decomposition of the organic linkers (Fig.2).Compared to the decomposition temperature of unsubstituted MOF-5,the decomposition temperatures for CH 3MOF-5and DiCH 3MOF-5are decreased by B 501C,suggesting that the thermal stability of MOF-5is reduced somewhat by introducing methyl (CH 3)groups on the terephthalic acid linker.A marginal difference in decomposition behavior was observed between CH 3MOF-5and DiCH 3MOF-5,implying that the thermal stability of the MOF-5framework is insensitive to doubling the concentration of methyl groups present in the framework.Compared with fresh samples,a weight loss of B 3%was observed for CH 3MOF-5-4d at about 1001C (inset Fig.2),which is due to the removal of water.DiCH 3MOF-5-4d,on the other hand,shows a loss of water of B 0.25%,which is the result of the more hydrophobic character of this MOF.Both MOFs show some outgassing above 1001C,which appears to be diethylformamide (DEF).Both outgassing of water and DEF was confirmed by mass spectroscopy.According to molecular dynamics simulation results on the interaction of water and MOF-5,MOF-5is only stable at a very low water content but unstable when exposed to more than 4wt%water.10e These results are in line with our findings:when the concentration of CH 3functionalities increases (DiCH 3MOF-5-4d vs.CH 3MOF-5-4d)the water uptake decreases and stays well below 4wt%.Therefore,introducing hydrophobic CH 3groups is an effective approach towards the design of water-stable metal–organic frameworks.It was demonstrated by us that methyl modified MOF-5,very much like MOF-5,can load hydrogen within several minutes.CH 3MOF-5shows almost the same hydrogen uptake capacity as that observed for MOF-5in the studied pressure range (Fig.3).The hydrogen uptake capacities of CH 3MOF-5and MOF-5are 1.42wt%and 1.44wt%at 1bar and 77K,respectively.DiCH 3MOF-5demonstrates a lower hydrogen uptake capacity of 1.29wt%at 1bar and 77K.Previous studies have shown that chemical modifications of the organic linker have only little impact on the hydrogen uptake capacity of MOF-5.15The same seems to be true for our CH 3MOF-5compound.The methyl group does not affect the hydrogenTable 1Elemental analysis results (calculated values in parentheses)and hydrogen uptake capacitiesSamplesC (%)H (%)SSA BET /m 2g À1V p total /cm 3g À1H 2uptake a (wt%)MOF-537.6(37.5) 1.65(1.56)2750 1.15 1.44CH 3MOF-540.2(40.0) 2.24(2.22)2537 1.03 1.42CH 3MOF-5-4d 38.0b2.47b——0.97DiCH 3MOF-541.0(42.2) 2.89(2.81)19270.82 1.29DiCH 3MOF-5-4d41.0b2.93b——1.25a77K and 1bar.bValues are for reactivated samples.Fig.1PXRD patterns of (Di)CH 3MOF-5before and after exposure to ambient air.Fig.2TGA curves of (Di)CH 3MOF-5before and after exposure to ambient air.MOF-5is included for reference purposes.Fig.3Hydrogen uptake capacities of (Di)CH 3MOF-5before and after 4days exposure to ambient air.D o w n l o a d e d o n 09 J a n u a r y 2012P u b l i s h e d o n 30 M a r c h 2011 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 1C C 11054Cuptake capacity of MOF-5,even though it was claimed that the presence of a methyl group could have a positive effect on the hydrogen uptake capability of other MOFs.16It is noteworthy,however,that the decrease in hydrogen uptake of DiCH 3MOF-5compared to MOF-5is relatively less than the lowering of surface area and pore volume.Of significant interest is the hydrogen storage uptake capability of the methyl and dimethyl modified MOF-5after exposure to ambient air.It is well documented that MOF-5completely loses its hydrogen storage capability after exposure to ambient air due to a total collapse of the framework structure within 1day (Fig.3).The hydrogen uptake capacities of the methyl modified MOF-5s,on the other hand,can be recovered after exposure to ambient air with a relative humidity of 32–37%.Before each measurement,CH 3MOF-5-4d and DiCH 3MOF-5-4d were reactivated by heating the samples at 1601C in vacuum for 24h.The hydrogen uptake capacity of CH 3MOF-5is reduced to 70%after 4days exposure to ambient air,whereas the hydrogen uptake capacity of DiCH 3MOF-5-4d remains virtually the same.These results,again,imply that methyl substituents can play an important role in designing water-stable MOFs without compromising the hydrogen uptake capacity.According to the C and H elemental analysis results in Table 1,the H content in reactivated CH 3MOF-5-4d is higher than that of a fresh sample,which is due to the absorbed water and corresponds to a water content of about 3.2wt%.This CH 3MOF-5,with strongly adsorbed water,shows a lower H 2uptake capacity as compared to a fresh sample.Although the structure of CH 3MOF-5remains stable at low water concen-trations,distortions in the framework structure might occur as reported in a simulated study for MOF-5.10e Even 0.6wt%of water in the structure may lead to the distortion of the ZnO 4tetrahedron and this will result in a reduced hydrogen uptake capability.Introducing a second methyl group,as is the case for DiCH 3MOF-5,the water uptake is reduced even further,which can be concluded from the constant C/H ratio found for DiCH 3MOF-5and DiCH 3MOF-5-4d.This result confirms that incorporating hydrophobic methyl groups can indeed improve the water stability of MOF-5without seriously compromising the hydrogen uptake capability.Even after a total of 8days exposure to ambient air,the original structures of CH 3MOF-5and DiCH 3MOF-5are still maintained according to the PXRD patterns (see Fig.S3,ESI w ).About 65%of the hydrogen uptake capacities of CH 3MOF-5and DiCH 3MOF-5could be retained (Fig.S4,ESI w ).This exciting result further demonstrates the 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