31 Gas Adsorption and Storage in Metal-Organic Framework MOF-177

Gas Adsorption and Storage in Metal-Organic Framework MOF-177Yingwei Li and Ralph T.Yang*Department of Chemical Engineering,Uni V ersity of Michigan,Ann Arbor,Michigan48109Recei V ed August10,2007.In Final Form:October1,2007Gas adsorption experiments have been carried out on a zinc benzenetribenzoate metal-organic framework material, MOF-177.Hydrogen adsorption on MOF-177at298K and10MPa gives an adsorption capacity of∼0.62wt%, which is among the highest hydrogen storage capacities reported in porous materials at ambient temperatures.The heats of adsorption for H2on MOF-177were-11.3to-5.8kJ/mol.By adding a H2dissociating catalyst and using our bridge building technique to build carbon bridges for hydrogen spillover,the hydrogen adsorption capacity in MOF-177was enhanced by a factor of∼2.5,to1.5wt%at298K and10MPa,and the adsorption was reversible. N2and O2adsorption measurements showed that O2was adsorbed more favorably than N2on MOF-177with a selectivity of∼1.8at1atm and298K,which makes MOF-177a promising candidate for air separation.The isotherm was linear for O2while being concave for N2.Water vapor adsorption studies indicated that MOF-177adsorbed up to∼10wt%H2O at298K.The framework structure of MOF-177was not stable upon H2O adsorption,which decomposed after exposure to ambient air in3days.All the results suggested that MOF-177could be a potentially promising material for gas separation and storage applications at ambient temperature(under dry conditions or with predrying).1.IntroductionThe development of a safe and efficient hydrogen storage system is urgently needed for the realization of hydrogen as a clean energy carrier for automobiles.1,2Recently,a new class of porous materials assembled with metal ions and organic linkers, known as metal-organic frameworks(MOFs),was developed as potential candidates for hydrogen storage.3-34Because of their low densities and unusually high surface areas,these new materials exhibited exceptional H2storage capacities by mass at 77K.For example,a microporous metal-organic framework composed of metal Mn and1,3,5-benzenetristetrazolate(BTT3-) was reported to adsorb up to6.9wt%H2at77K and90bar.6 Ferey et al.reported a high H2uptake of6.1wt%at77K and 8MPa on MIL-101that was built up from benzene-1,4-dicarboxylate and trimetric chromium(III)octahedral cluster.26 To date,the highest H2adsorption capacity of7.5wt%was achieved at77K and70bar on MOF-177,a framework consisting of tetrahedral[Zn4O]6+clusters linked by the tritopic linker BTB (1,3,5-benzenetribenzoate).22However,the hydrogen adsorption in MOFs is mostly by weak van der Waals interactions with a low heat of adsorption(normally4-7kJ/mol).26,29,31Therefore, cryogenic temperatures(typically77K)are needed to obtain high H2storage capacities on MOFs.It has been shown that no significant amounts of hydrogen were adsorbed on the MOFs at room temperature.26,30The structure stability of MOFs upon water adsorption is an important issue for potential applications of MOFs for gas adsorption and storage materials because H2O is very difficult to fully remove from industrial gas resources.However,this aspect has received little attention in the literature.Recently,the Panella Hirscher and Huang et al.groups both observed a different*Corresponding author.Fax:(743)764-7453.E-mail address: yang@.(1)Schlapbach,L.;Zu¨ttel,A.Nature2001,414,353-358.(2)Dillon,A.C.;Heben,M.J.Appl.Phys.A2001,72,133-142.(3)Yaghi,O.M.;O’Keefee,M.;Ockwig,N.W.;Chae,H.K.;Eddaoui,M.; Kim,J.Science2003,423,705-714.(4)Rosi,N.L.;Eckert,J.;Eddaoudi,M.;Vodak,D.T.;Kim,J.;O’Keefee, M.;Yaghi,O.M.Science2003,300,1127-1129.(5)Snurr,R.Q.;Hupp,J.T.;Nguyen,S.T.AIChE J.2004,50,1090-1095.(6)Dinca,M.;Dailly,A.;Liu,Y.;Brown,C.M.;Neumann,D.A.;Long,J. R.J.Am.Chem.Soc.2006,128,16876-16883.(7)Kaye,S.;Long,J.R.J.Am.Chem.Soc.2005,127,6506-6507.(8)Fe´rey,G.;Mellot-Draznieks,C.;Serre,C.;Millange,F.;Dutour,J.;Surble´, S.;Margiolaki,I.Science2005,309,2040-2042.(9)Suh,M.P.;Ko,J.W.;Choi,H.J.J.Am.Chem.Soc.2002,124,10976-10977.(10)Frost,H.;Duren,T.;Snurr,R.Q.J.Phys.Chem.B2006,110,9565-9570.(11)Pan,L.;Sander,M.B.;Huang,X.Y.;Li,J.;Smith,M.;Bittner,E.;Bockrath,B.;Johnson,J.K.J.Am.Chem.Soc.2004,126,1308-1309.(12)Dinca,M.;Long,J.R.J.Am.Chem.Soc.2005,127,9376-9377.(13)Zhao,X.B.;Xiao,B.;Fletcher,A.J.;Thomas,K.M.;Bradshaw,D.; Rosseinsky,M.J.Science2004,306,1012-1015.(14)Rowsell,J.L.C.;Millward,A.R.;Park,K.S.;Yaghi,O.M.J.Am.Chem. Soc.2004,126,5666-5667.(15)Dybtsev,D.N.;Chun,H.;Yoon,S.H.;Kim,D.;Kim,K.J.Am.Chem. Soc.2004,126,32-33.(16)Kesanli,B.;Cui,Y.;Smith,M.R.;Bittner,E.W.;Bockrath,B.C.;Lin, W.Angew.Chem.,Int.Ed.2005,44,72-75.(17)Kubota,Y.;Takata,M.;Matsuda,R.;Kitaura,R.;Kitagawa,S.;Kato, K.;Sakata,M.;Kobayashi,T.C.Angew.Chem.,Int.Ed.2005,44,920-923.(18)Lin,X.;Jia,J.H.;Zhao,X.B.;Thomas,K.M.;Blake,A.J.;Walker,G. S.;Champness,N.R.;Hubberstey,P.;Schroder,M.Angew.Chem.,Int.Ed.2006, 45,1-7.(19)Chen,B.;Ockwig,N.W.;Millward,A.R.;Contreras,D.S.;Yaghi,O. M.Angew.Chem.,Int.Ed.2005,44,4745-4749.(20)Lee,J.Y.;Li,J.;Jagiello,J.J.Solid State Chem.2005,178,2527-2532.(21)Lee,J.Y.;Pan,L.;Kelly,S.R.;Jagiello,J.;Emge,T.J.;Li,J.Ad V.Mater. 2005,17,2703-2706.(22)Wong-Foy,A.G.;Matzger,A.J.;Yaghi,O.M.J.Am.Chem.Soc.2006, 128,3494-3495.(23)Dinca,M.;Yu,A.F.;Long,J.R.J.Am.Chem.Soc.2006,128,8904-8913.(24)Sun,D.;Ma,S.;Ke,Y.;Collins,D.J.;Zhou,H.C.J.Am.Chem.Soc. 2006,128,3896-3897.(25)Rowsell,J.L.C.;Yaghi,O.M.J.Am.Chem.Soc.2006,128,1304-1315.(26)Latroche,M.;Surble,S.;Serre,C.;Mellot-Draznieks,C.;Llewellyn,P. L.;Lee,J.H.;Chang,J.S.;Jhung,S.H.;Ferey,G.Angew.Chem.,Int.Ed.2006, 45,8227-8231.(27)Chen,B.L.;Ma,S.Q.;Zapata,F.;Lobkovsky,E.B.;Yang,J.Inorg. Chem.2006,45,5718-5720.(28)Dailly,A.;Vajo,J.J.;Ahn,C.C.J.Phys.Chem.B2006,110,1099-1101.(29)Panella,B.;Hirscher,M.;Putter,H.;Muller,U.Ad V.Funct.Mater.2006, 16,520-524.(30)Panella,B.;Hirscher,M.Ad V.Mater.2005,17,538-541.(31)Surble,S.;Serre,C.;Millange,F.;Duren,T.;Latroche,M.;Ferey,G.J. Am.Chem.Soc.2006,128,14889-14896.(32)Krungleviciute,V.;Lask,K.;Heroux,L.;Migone,A.D.;Lee,J.Y.;Li, J.;Skoulidas,ngmuir2007,23,3106-3109.(33)Jiang,J.W.;Sandler,ngmuir2006,22,5702-5707.(34)Seayad,A.M.;Antonelli,D.M.Ad V.Mater.2004,16,765-777.12937Langmuir2007,23,12937-1294410.1021/la702466d CCC:$37.00©2007American Chemical SocietyPublished on Web11/22/2007X-ray diffraction pattern after exposing MOF-5(i.e.,IRMOF-1) in air for several weeks.30,35The results indicated that the moisture in the air was adsorbed on MOF-5that caused decomposition of the structure of MOF-5.The decomposition of the structure could also be observed on other MOFs because of their structures similar to MOF-5.Herein we report experimental data on the gas adsorption and storage in the metal-organic framework MOF-177.MOF-177 has been shown as an excellent sorbent for CO2and H2storage.22,36 At77K,MOF-177exhibited the highest hydrogen storage capacity among the known MOF materials.However,there is still no report on the hydrogen adsorption nature on MOF-177 at room temperature.In this work,we synthesized the MOF-177 sample and examined its hydrogen adsorption isotherms at298 K up to100atm.Hydrogen storage by spillover at room temperature on this sample was also examined,as recent reports have shown that the hydrogen storage capacities at room temperature in nanostructured materials including MOFs,carbon nanostructures,and zeolites could be enhanced significantly by hydrogen spillover.37-43In addition,the adsorption isotherms of other gases,such as H2O vapor,N2,and O2,were investigated on the MOF-177sample.The structure stability of MOF-177 upon exposure to indoor ambient air has been studied in detail in this paper.It was found that the MOF-177crystals were unstable in indoor ambient air and decomposed gradually to form an amorphous material.N2and O2adsorption measurements showed that O2was adsorbed more favorably than N2on MOF-177at 298K.By bridged spillover,the hydrogen adsorption capacity in MOF-177was enhanced by a factor of∼2.5,to1.5wt%at 298K and10MPa.In addition,the adsorption was reversible at room temperature.2.Experimental Methods2.1.Preparation of1,3,5-Tris(4-carboxyphenyl)benzene.1,3,5-Tris(4-carboxyphenyl)benzene(H3BTB)is the organic linker for constructing the MOF-177framework.At present,it is not commercially available.In this study,H3BTB was synthesized in our laboratory from1,3,5-tris(4-bromophenyl)benzene.44First,2g of1,3,5-tris(4-bromophenyl)benzene(Aldrich,97%)was added in 70mL of dry tetrahydrofuran(THF;Aldrich,g99.9%)under an atmosphere of helium.The stirred solution was cooled in a bath containing dry ice in acetone.An8mL aliquot of Bu n Li in n-hexane (Aldrich,1.6M)was added dropwise under vigorous stirring and an atmosphere of He.Then,gaseous CO2was passed into the solution to yield a white precipitate.The mixture was acidified with glacial acetic acid(Aldrich,g99.99%)at room temperature,and copious water was added to give a white precipitate.The precipitate was collected by repeated filtering,thorough washing with water,and recrystallized from glacial acetic acid.The solid was dried in an oven at60°C for at least24h.2.2.Synthesis of MOF-177.MOF-177was synthesized according to the reported procedures.45The difference between the reported synthesis and ours was that we used a hydrothermal bomb(300mL),while a Pyrex tube was used by the former.And because the volume of the bomb we used(300mL)was much larger than that in the reported synthesis;40×more starting materials were used in this study.In a typical synthesis,the prepared H3BTB(0.2g,0.46mmol) and Zn(NO3)2‚6H2O(0.8g,2.68mmol;Aldrich,99.999%)were dissolved in80mL of N,N-diethylformamide(DEF;Acros,99%). The mixture was introduced in a300mL hydrothermal bomb that was heated to100°C at a heating rate of2°C/min.The temperature was held at100°C for23h and then cooled to20°C at a rate of 0.2°C/min.The solution was aged in the bomb for almost30days, yielding block-shaped large crystals.The product was isolated by filtration and washed with DEF and chlorobenzene and then immersed in chlorobenzene(35mL)for2h.Then the solvent was decanted and replenished.The crystals were immersed in the solvent for another 24h,filtered,and washed again with chlorobenzene.The sample was dried in a desiccator overnight and then degassed at room temperature.The solvent was fully removed by degassing in vacuum (∼10-4Torr)at150°C for8h,yielding the porous material. 2.3.Preparation of Bridged Samples.An effective bridge building technique has been explored and described in detail in our previous publications.38,41,42A catalyst containing5wt%platinum supported on active carbon(Pt/AC,Strem Chemicals)was used as the source for hydrogen dissociation.Active carbon can be considered as the primary receptor for hydrogen atoms.Here MOF-177was the secondary spillover receptor.Carbon bridges between the source and receptor were formed by carbonization of sucrose that was previously introduced into a physical mixture of the two components. The receptor/precursor/source ratio was fixed at8:1:1on the basis of the complete carbonization of the precursor(into carbon).The ternary mixture was ground together for1h and then subjected to the heating treatment procedures as described in the previous paper for preparing the bridged IRMOF-8sample.38The formation of carbon bridges by using the bridge building technique has been confirmed by high-resolution transmission electron microscopy (HRTEM).422.4.Characterization.Powder X-ray diffraction(XRD)patterns were recorded on a Rigaku Miniflex diffractometer operating at30 kV,15mA for Cu K R(λ)0.1543nm)radiation,with a scan speed of2°/min and a step size of0.02°in2θ.Infrared spectra(IR)were recorded on a Nicolet Impact400FT-IR spectrometer with a TGS detector.Scanning electron microscopy(SEM)images were obtained on a Philips XL30FEG SEM instrument equipped with UTW Si-Li solid-state X-ray detector(XEDS)using a15kV accelerating voltage.Gravimetric analysis was performed on a TGA-50ther-mogravimetric analyzer(Shimadzu).Brunauer-Emmett-Teller (BET)surface areas and pore volumes were measured on a Micromeritics ASAP2020sorptometer using nitrogen adsorption at77K.2.5.Hydrogen Isotherm Measurements.Low-pressure H2,N2, or O2adsorption isotherms at77or298K were measured with a standard static volumetric technique(Micromeritics ASAP2020). Approximately200mg of sample was used for each measurement. Samples were degassed in vacuum(∼10-4Torr)at150°C for at least12h prior to measurements to remove any residual guest molecules in order to obtain the highest gas adsorption capacity. Hydrogen adsorption at298K and pressures greater than0.1 MPa and up to10MPa was measured using a static volumetric technique with a specially designed Sievert’s apparatus.In the apparatus,valves with high-pressure bellows seals were employed. Pressures at various points were monitored and automatically recorded with pressure transducers,which were later used for calculations on hydrogen uptake by our computer code.More details were given earlier.41The apparatus was previously tested to prove to be leak-free and proven for accuracy through calibration by using LaNi5, AX-21,zeolites,and IRMOFs at298K.All isotherms matched the known values.Typically,approximately200mg of sample was used for each high-pressure isotherm measurement.Prior to measurements,the samples were degassed in vacuum(∼10-2Torr) at150°C for at least12h.(35)Huang,L.M.;Wang,H.T.;Chen,J.X.;Wang,Z.B.;Sun,J.Y.;Zhao,D.Y.;Yan,Y.S.Microporous Mesoporous Mater.2003,58,105-114.(36)Millward,A.R.;Yaghi,O.M.J.Am.Chem.Soc.2005,127,17998-17999.(37)Li,Y.W.;Yang,R.T.J.Am.Chem.Soc.2006,128,726-727.(38)Li,Y.W.;Yang,R.T.J.Am.Chem.Soc.2006,128,8136-8137.(39)Li,Y.W.;Yang,F.H.;Yang,R.T.J.Phys.Chem.C2007,111,3405-3411.(40)Lueking,A.;Yang,R.T.J.Catal.2002,206,165-168.(41)Lachawiec,A.J.;Qi,G.S.;Yang,ngmuir2005,21,11418-11424.(42)Li,Y.W.;Yang,R.T.J.Phys.Chem.B2006,110,17175-17181.(43)Li,Y.W.;Yang,R.T.J.Phys.Chem.C2007,111,11086-11094.(44)Weber,E.;Hecker,M.;Keopp,E.;Orlia,W.J.Chem.Soc.,Perkin Trans.1988,1251-1257.(45)Chae,H.K.;Siberio-Perez,D.Y.;Kim,J.;Go,Y.;Eddaoudi,M.;Matzger,A.J.;O’keeffe,M.;Yaghi,O.M.Science2004,427,523-527.12938Langmuir,Vol.23,No.26,2007Li and Yang3.Results and Discussion3.1.Characterization Results.Powder X-ray diffraction pattern for the synthesized MOF-177sample (washed with DEF and then dried in a desiccator)is shown in Figure 1.The extremely high intensities of the diffraction peaks indicated the good crystallinity of the MOF-177crystals.The strongest peak at 2θ)5.2and the peak at 2θ)10.8matched well with the already published XRD pattern on the wet as-synthesized MOF-177material (sample I).45The small new peak at 2θaround 7.0in Figure 1in comparison with the reported XRD pattern on sample I can be attributed to the extent of the removal of guest molecules in the frameworks of the synthesized sample,because it has been reported by Chae et al.45that some new peaks (including the new peak at 2θ)7.0in Figure 1)could be observed upon sample drying.The sample prepared by Chae et al.was dried in air for only 1min,while our sample was dried in a desiccator overnight.The difference in the drying time for the wet samples could result in various contents of guest molecules in the frameworks.Figure 2shows the IR spectrum of the as-synthesized MOF-177sample.For comparison,the IR spectra of the commercial raw material 1,3,5-tris-(4-bromophenyl)benzene and the prepared H 3BTB linker are also presented in Figure 2.The observation of the strong vibrational bands around 1710and 1300cm -1and the strong broad bands between 2500and 3300cm -1confirmed the presence of carboxyl groups in the synthesized H 3BTB from1,3,5-tris-(4-bromophenyl)benzene.The presence of the bands characteristic of the framework -(O -C -O)-groups around 1550and 1416cm -1indicated the presence of the dicarboxylate within the MOF-177sample.8The strong peak at 1660cm -1was the characteristic peak for DEF (C d O)solvent that was not totally removed after drying in a desiccator.45The N 2adsorption isotherm at 77K was measured on the MOF-177sample in which the pores were fully evacuated.As seen in Figure 3,the N 2adsorption on the sample showed a reversible type I isotherm characteristic of a microporous material.No hysteresis was observed upon desorption of gas from the pores,indicating the stability of the pores.The BET surface area was about 3100m 2/g.By assuming a monolayer coverage of N 2and applying the Langmuir model,the Langmuir surface area could be calculated.The reported Langmuir surface areas on MOF-177by Yaghi et al.varied quite substantially.They reported a value of ∼4500m 2/g in their two earlier papers with a much higher one of 5640m 2/g in a later paper.22,36,45But the three MOF-177samples were prepared by the same method under the same conditions.In this work,the Langmuir surface area of the synthesized MOF-177sample was ∼4300m 2/g,which was very close to the value reported in Yaghi’s earlier papers.36,45We synthesized the MOF-177samples several times using the same procedures as Yaghi,yet each time we obtained nearly the same result on the Langmuir surface area.None of the samples we measured showed the high Langmuir surface area of 5,640m 2/g reported by Yaghi et al.22The pore volume and the pore diameter of the MOF-177sample was 1.58cm 3/g,and 10.6Å,respectively.3.2.H 2O Adsorption Studies.The studies on the H 2O adsorption/desorption on the materials for future hydrogen storage applications are very important because the adsorption of H 2O on the material will decrease the hydrogen capacity when refilling.Moreover,the structures of some materials such as MOFs could be destroyed upon adsorption of water.Unfortunately,the reported promising porous materials for hydrogen adsorption all adsorb significant amounts of H 2O.For example,zeolites can adsorb ∼25wt %of H 2O,while a superactivated carbon (AX-21)can adsorb up to 65wt %of moistures at room temperature.In addition,the adsorbed H 2O cannot be easily desorbed from the materials at room temperature.Thus,the hydrogen uptakes of the materials will decrease gradually with refilling times.The materials can be regenerated by heating to release the adsorbed H 2O.However,if the material is not stable upon the adsorption of H 2O,the original highest hydrogen uptakes cannot be regenerated.In this case,additional special procedures to remove any trace of H 2O existing in the hydrogen sources would be needed to meet the U.S.Department of Energy (DOE)durability criteria for hydrogen storage applications for transportation.46H 2O vapor adsorption isotherm at 298K on the MOF-177sample after the removal of guest molecules is presented in Figure 4.It was observed that MOF-177adsorbed up to ∼10wt %H 2O at 298K at P/P 0)0.1(P 0is the saturation pressure of H 2O at 298K).The H 2O uptake in the MOF-177sample was relatively low with such a high surface area and pore volume.Previous thermogravimetric analysis (TGA)revealed that a metal -organic framework MIL-101with similar surface area and pore volume as MOF-177could adsorb up to 40wt %H 2O.26The much lower H 2O adsorption capacity in MOF-177than MIL-101can be attributed to the difference in the number of hydrophobic benzene rings in the frameworks.There are four benzene rings in the linker for MOF-177,while only one for MIL-101.The adsorption(46)U.S.Department of Energy,Energy Efficiency and Renewable Energy (EERE),Hydrogen,Fuel cells &Infrastructure Technologies Program,Multi-Year RD&D Plan,2005./hydrogenandfuelcells/mypp/pdfs/storage.pdf (accessed June2005).Figure 1.X-ray diffraction pattern of the MOF-177(assynthesized).Figure 2.Infrared spectra of MOF-177(as synthesized),the synthesized linker H 3BTB (1,3,5-tris(4-carboxyphenyl)benzene),and the (purchased)raw material (1,3,5-tris(4-bromophenyl)benzene).Gas Adsorption and Storage in MOF-177Langmuir,Vol.23,No.26,200712939rate of H 2O at P/P 0)0.1and 298K was studied on a fresh MOF-177sample and was shown in Figure 5.It can be seen that ∼90%of the saturation point (∼9wt %)was reached in 4h.From the kinetics data in Figure 5,the diffusion time constant (D/R 2)can be approximated by using the following equation when M t /M ∞<0.3:where D is diffusivity,R is diffusion distance or radius,and M t and M ∞denote the total amounts of H 2O at time t and at equilibrium,respectively.47The value of D/R 2was calculated as 5.5×10-6s -1.Thus,a saturation adsorption of H 2O would take about 50h at P/P 0)0.1and 298K.The slow uptake rates canbe attributed to the large crystal sizes of the MOF-177sample.The crystal size reported by Yaghi et al.was about 0.3mm.45The crystal sizes of the synthesized MOF-177sample in this study were even larger.Figure 6A shows the SEM image of the MOF-177sample after the removal of guest molecules.It is seen that the crystal sizes of the dried sample were in the range of 0.5-1.0mm.The desorption rates were much faster than adsorption;however,∼6.5wt %H 2O did not desorb from the MOF-177sample at 298K and in an atmosphere of helium,as can be seen in Figure 5.The results indicated that the adsorption of H 2O on MOF-177was not reversible at 298K.To study the stability of MOF-177upon H 2O adsorption,the MOF-177sample after evacuating the guest molecules was exposed to indoor ambient air at room temperature,at a nominal relative humidity of 40%(air-conditioned).The XRD patterns of the samples after exposing to air for 1and 3days are compared in Figure 7with the as-synthesized MOF-177sample.The XRD pattern of the sample changed significantly after exposure to air for 1day.The intensity of the peak at 2θ)5.2°decreased(47)Crank,J.The Mathematics of Diffusion ;Clarendon:Oxford,U.K.,1979;pp 89-92.Figure 3.N 2isotherm of MOF-177at 77K:circles,fresh sample;triangles,after exposure to ambient air (RH ∼40%)at 298K for 3days;open symbols,adsorption;filled symbols,desorption.Figure 4.H 2O isotherm at 298K for MOF-177:O ,adsorption;2,desorption.P/P 0is the ratio of H 2O vapor pressure (P )to saturation pressure of H 2O at 298K (P 0).M t M ∞)4 π(Dt R2)1/2(1)Figure 5.H 2O vapor adsorption rates on the MOF-177sample at 298K and P/P 0)0.1.12940Langmuir,Vol.23,No.26,2007Li and Yangsubstantially with several new peaks appearing at 2θfrom 5to 15°.This revealed that the crystals of the MOF-177sample were decomposing,but some crystallinity still remained.It has been reported that some new peaks at 5-15°could also be observed when the MOF-177was heated to 325°C,which was very close to the decomposing temperature of MOF-177(350°C).45After 3days,the sample mainly existed as an amorphous phase,as shown in Figure 7.The results indicated that the framework structure of MOF-177was degraded by H 2O after exposure to indoor ambient air for 3days.The SEM images of the MOF-177sample in air for 3days are also shown in Figure 6.It can be seen that the particle sizes of the decomposed sample were much smaller than that of the fresh sample.This indicated the breakup of the frameworks in the MOF-177sample.The destruction of the frameworks can befurther verified by observing the samples at a higher resolution.As shown in Figure 6B,the fresh synthesized MOF-177sample showed a latticelike morphology.However,the sample after exposure to air for 3days exhibited a strawlike morphology.The framework structures observed on the freshly synthesized sample vanished.The destruction of the frameworks resulted in a significant decrease in the gas adsorption capacities.As shown in Figure 3,the N 2adsorption amounts at 77K were much lower than that on the freshly synthesized sample.The BET surface area and pore volume for the sample after exposure to air for 3days were 30m 2/g and 0.1cm 3/g,respectively.The results indicated that the MOF-177sample could be decomposed significantly upon exposure to air for a short period of time.Greathouse and Allendorf reported the simulation results on the interaction of water with MOF-5by molecular dynamics simulation.48It was suggested that the relatively weak bonds between Zn ions and O atoms in MOF-5were easily attached by water molecules.A possible decomposition mechanism might beAnother possible mechanism for decomposition was that proposed by Huang et al.,35where the MOF undergoes hydrolysis (i.e.,the reverse reaction of the synthesis)to form Zn ions and acid (H 3-BTB,in this case).They proposed that this reverse reaction would proceed under acid conditions.It can be calculated that 10.1wt %H 2O will fully decompose the structure of MOF-177.The humidity level in indoor air (with air conditioning)is normally 30-40%(i.e.,P/P 0)0.3-0.4).The H 2O uptake rates in air would be much faster than that at P/P 0)0.1shown in Figure 4.Therefore,during the synthesis and gas storage applications,(48)Greathouse,J.A.;Allendorf,M.D.J.Am.Chem.Soc.2006,128,10678-10679.Figure 6.SEM images of the fresh as-prepared MOF-177after the removal of guest molecules (A,B),and the MOF-177sample after exposure to ambient air (RH ∼40%)at 298K for 3days (C,D).Figure 7.XRD patterns the fresh MOF-177(as synthesized),MOF-177after removing guests and exposing to ambient air (RH ∼40%)at 298K for 1and 3days.Zn 4O(BTB)2+4H 2O f [(Zn 4O)(H 2O)4(BTB)]3++BTB 3-(2)Gas Adsorption and Storage in MOF-177Langmuir,Vol.23,No.26,200712941exposing the MOF-177sample to humid air should be avoided to obtain the highest storage capacities.For application to hydrogen storage,an additional drying would be required before hydrogen is refilled to the storage system containing MOFs with structures similar to that of MOF-177as the storage materials.3.3.N 2,and O 2Adsorption Studies.Air separation is of importance for producing commercial N 2or O 2with high mercial sorption-based air separation is usually done using nitrogen selective zeolites in pressure swing adsorption (PSA)systems.49-51But separation of air by adsorption of the less abundant oxygen is more desirable because less work would be done to accomplish the same separation.The low-pressure N 2,and O 2adsorption isotherms at 298K on the MOF-177sample are shown in Figure 8.It can be seen that O 2was adsorbed more favorably than N 2on the MOF-177sample with a pure-component selectivity of ∼1.8at 1atm.The equilibrium capacities for O 2and N 2were 0.18and 0.10mmol/g at 1atm,respectively.The equilibrium adsorption capacity for O 2at 1atm is comparable to that obtained on AgBr/SiO 2(0.243mmol/g)with a high selectivity of 2.87for O 2at 295K.49Although the selectivity for O 2over N 2was not very high at low pressure,it is interesting to note that the adsorption isotherm was linear for O 2while being strongly concave for N 2,as shown in Figure 8.This behavior is unique among all known sorbents for air separation.50,51It can be expected that higher selectivities could be obtained at higher pressures on the MOF-177sample.Thus MOF-177could be a promising sorbent for high-pressure air separation,particularly by PSA.50,51A similar linear increase with pressure was also found on the MOF-177material for CO 2storage.36The O 2isotherm on MOF-177is comparable to that on activated carbon and zeolites.50,51The O 2/N 2selectivity is caused by the higher magnetic susceptibility of O 2over that of N 2.The low N 2isotherm indicates a lack of electric charges on the surfaces of MOF-177;otherwise there would be strong interactions between the quadrupole moment of N 2and the charges.The strongly concave shape of the N 2isotherm on MOF-177would indicate early saturation.However,some reports have shown that MOFs could exhibit unusual isotherm shapes.For example,Bourrelly et al.observed a step increase with pressure in the adsorptionisotherm of CO 2on MIL-53.52And several mechanisms,such as molecular gate,structural disorganization/organization,and breathing type mechanism,for the steps in adsorption isotherms on MOFs have been proposed.13,52,53Therefore,additional high-pressure N 2adsorption experiments on MOF-177would be needed to predict the saturation adsorption amount of N 2in MOF-177.3.4.H 2Adsorption Studies.The hydrogen adsorption isotherm of the MOF-177sample at 77K and at pressures up to 1atm is shown in Figure 9.The adsorption of H 2on MOF-177was reversible,and no significant hysteresis was observed upon desorption.The highest H 2uptake at 1atm and 77K was ∼1.5wt %on the MOF-177sample synthesized in this study.Yaghi et al.reported a hydrogen capacity of 1.25wt %at 77K and 1atm on a MOF-177sample with a Langmuir surface area of ∼4500m 2/g.14However,in a later report a lower hydrogen adsorption capacity of ∼1wt %at 0.9bar was reported on a MOF-177sample with a higher Langmuir surface area of ∼5600m 2/g.22The H 2uptakes reported on MOF-177were relatively low for a material with the highest surface area because similar amounts of hydrogen can be adsorbed on other MOFs with much lower surface areas.For example,a 1.5wt %H 2uptake can be obtained on IRMOF-8with a Langmuir surface area of ∼1500m 2/g.14It is interesting to note that the isotherm on MOF-177was nearly linear at pressures >100Torr,as can be seen from Figure 9.Therefore,a high H 2capacity can be obtained on MOF-177at high pressures.At 70bar and 77K,the H 2uptake on the MOF-177sample was measured as 7.5wt %,which is the highest hydrogen storage capacity reported at 77K up to date.22The interaction of H 2with the adsorption material is very important to achieve a high capacity for H 2.To date,there is no report on measuring the heats of adsorption of H 2on MOF-177to the best of our knowledge.In this study,the heats of adsorption on MOF-177were calculated using the Clausius -Clapeyron equation from the H 2adsorption isotherms at three different temperatures,as shown in Figure 10.The isosteric heats of adsorption were determined by evaluating the slopes of the plot of ln(P )vs 1/T at the same adsorption amounts.Such plots are given in Figure 11.The heats of adsorption were calculated as -11.3kJ/mol (at 0.32cm 3/g),-10.1kJ/mol (at 0.49cm 3/g),-7.5kJ/mol (at 0.90cm 3/g),and -5.8kJ/mol (at 1.50cm 3/g).(49)Jayaraman,A.;Yang,R.T.Chem.Eng.Sci.2005,60,625-634.(50)Yang,R.T.Gas Separation by Adsorption Processes ;Butterworth:London,1987;Chapter 7.(51)Yang,R.T.Adsorbents:Fundamentals and Applications ;Wiley:New York,2003;Chapter 10.(52)Bourrelly,S.;Llewellyn,P.L.;Serre,C.;Millange,F.;Loiseau,T.;Ferey,G.J.Am.Chem.Soc.2005,127,13519-13521.(53)Sudik,A.C.;Millward,A.R.;Ockwig,N.W.;Cote,A.P.;Kim,J.;Yaghi,O.M.J.Am.Chem.Soc.2005,127,7110-7111.Figure 8.N 2,O 2adsorption isotherms on MOF-177at 298K and pressures up to 1atm.Figure 9.Hydrogen adsorption isotherm for MOF-177at 77K.Filled circles indicate the desorption branch.12942Langmuir,Vol.23,No.26,2007Li and Yang。