Zeolitic Imadazolate frameworks Capable of Kinetic Separatio

Exceptional Framework Flexibility and Sorption Behavior of aMultifunctional Porous Cuprous Triazolate FrameworkJie-Peng Zhang*and Xiao-Ming Chen*MOE Laboratory of Bioinorganic and Synthetic Chemistry,School of Chemistry and ChemicalEngineering,Sun Yat-Sen Uni V ersity,Guangzhou510275,ChinaReceived January23,2008;E-mail:zhangjp7@(J.-P.Zhang);cxm@(X.-M.Chen)Abstract:The porous metal azolate framework[Cu(etz)]∞(MAF-2,Hetz)3,5-diethyl-1,2,4-triazole)processes an NbO type cuprous triazolate scaffold and a CsCl type hydrophobic channel system,in whichthe large cavities are interconnected by small apertures with pendant ethyl groups.Since the ethyl-blockedapertures behave as thermoactivated IRIS stops for the guest molecules,the gas sorption behavior ofMAF-2can be controlled by temperature,in which N2adsorption was observed at195K rather than77K.Single-crystal X-ray structural analysis revealed that the[Cu(etz)]∞host framework is not altered upon N2inclusion,confirming the occurrence of the so-called“kinetically controlledflexibility”.By virtue of thekinetically controlledflexibility and hydrophobic pore surface,MAF-2can adsorb large amounts of smallorganic molecules but excludes H2O.As demonstrated by single-crystal X-ray structural analyses,MAF-2shrinks,expands,or distorts its framework to accommodate the hydrogen-bonded hexamers of MeOH,EtOH,or MeCN,respectively.Moreover,MAF-2can also separate benzene and cyclohexane efficiently,as itsflexible scaffold can distort to a certain degree so that benzene can diffuse through the distortedapertures but cyclohexane cannot.Moreover,the adsorption/desorption of these organic vapors inducesreversible,multimode structural transformations.IntroductionThe construction and characterization of porous coordinationpolymers(PCPs)with useful functionalities have been the focusof intense research activity.To achieve permanent porosity andhigh gas storage capacity,highly robust metal-organic frame-works have been the major research goal during the pastdecade.1Recently,the advantages of dynamic frameworks forhigh performance molecular recognition,separation,and sensinghave attracted more and more attention.2–6Nevertheless,a givenflexible framework usually adopts a single deformation modesuch as expansion,contraction,or distortion upon guest inclu-sion.Obviously,aflexible PCP with a multimode structuraltransformation ability is more attractive as a multipurpose guest sensor and actuator.Thermodynamic(structural)multistability is regarded as a sign of frameworkflexibility,and most attention has been focused on the guest-induced structural transformations at equilibrium states.2–6However,someflexible frameworks may temporarily open their small apertures to allow the diffusion of oversize guest species and keep the host structure unchanged after guest inclusion.7–10The combination of structuralflexibility and thermodynamic monostability arises from the transient structural transformation to the thermodynamically and kineti-cally unstable state.In the energy diagram(Scheme1),a rigid framework is associated with a single energy minimum,but a flexible framework has two(or more)local energy minima. Usually,the local energy minima are well separated by energy barriers much higher than the thermal vibration energy(E a>>(1)Yaghi,O.M.;O’Keeffe,M.;Ockwig,N.W.;Chae,H.K.;Eddaoudi,M.;Kim,J.Nature2003,423,705–714.(2)(a)Kitagawa,S.;Kitaura,R.;Noro,S.I.Angew.Chem.,Int.Ed.2004,43,2334–2375.(b)Kitagawa,S.;Uemura,K.Chem.Soc.Re V.2005, 34,109–119.(3)Bradshaw,D.;Claridge,J.B.;Cussen,E.J.;Prior,T.J.;Rosseinsky,M.J.Acc.Chem.Res.2005,38,273–282.(4)Kepert,mun.2006,695–700.(5)Férey,G.Chem.Soc.Re V.2008,37,191–214.(7)(a)Barbour,mun.2006,1163–1168.(b)Dalgarno,S.J.;Thallapally,P.K.;Barbour,L.J.;Atwood,J.L.Chem.Soc.Re V.2007,36,236–245.(8)Cussen,E.J.;Claridge,J.B.;Rosseinsky,M.J.;Kepert,C.J.J.Am.Chem.Soc.2002,124,9574–9581.(9)(a)Zhao,X.;Xiao,B.;Fletcher,A.J.;Thomas,K.M.;Bradshaw,D.;Rosseinsky,M.J.Science2004,306,1012–1015.(b)Liu,Y.;Eubank, J.F.;Cairns,A.J.;Eckert,J.;Kravtsov,V.C.;Luebke,R.;Eddaoudi, M.Angew.Chem.,Int.Ed.2007,46,3278–3283.(c)Pan,L.;Olson, Scheme1.Energy Diagrams of the Rigid and FlexibleFrameworks Published on Web04/03/2008kT),so the structuralflexibility is dominated by thermodynamic multistability,and structural interconversion is controlled bythermodynamic factors such as adsorption/desorption of guestmolecules.When energy barriers are comparable to,or smallerthan,the thermal vibration energy(E a≈kT),the higher local energy minima become unstable and quickly relax to the singlethermodynamically and kinetically stable state.Consequently,no thermodynamic multistability is present,and structuralflexibility/interconversion is now dominated by kinetic factorssuch as the diffusion of guest molecules.Therefore,this uniquestructuralflexibility can be denoted as kinetically controlledflexibility(KCF),in contrast with the thermodynamically controlledflexibility(TCF)arisen from the thermodynamic multistability.The low energy barrier nature of this system implies that the frameworkflexibility can be easily tuned by external variables,11pointing to a novel family of porous materials whose porous properties can respond to physical stimuli such as an electricfield and heat.12The KCF of a dynamic framework is suggested by the smallersize of the channel apertures compared with the moleculardimensions of the guests and corroborated by the negligibleframework alteration after guest inclusion.Unfortunately,unambiguous KCF is difficult to be confirmed due to the lackof sufficient structural information of the guest-free and guest-included materials7–10,12or the coexistence of KCF and TCF.13,14KCF can arise from several dynamic mechanisms such asframework distortion,internetwork(intermolecular)displace-ment,and other molecular motions that dispense with theframework distortion or internetwork displacement.Frameworkdistortion and internetwork displacement can produce largestructural and energetic differences,which is a key feature fordynamic behaviors such as diffusion of oversize guests andabnormal sorption isotherms.However,these large differencesimply that the host framework can easily undergo permanentstructural transformation upon sorption,leading to TCF ratherthan KCF.Either permanent or momentary,these mechanismsinduce lattice distortions and stresses that tend to fracture thelarge single crystals,making direct/precise structural charac-terization difficult.6,15,16On the other hand,some molecularmotions,such as the rotation/swing of noncoordinated functionalgroups in PCPs,may dispense with framework distortion orinternetwork displacement,17–19avoiding the above-mentioneddisadvantages.Nevertheless,such dynamic motions usuallygenerate relatively small aperture variations7,12and/or disordered structures.17–19Therefore,rational design and a conclusive demonstration of a KCF dominated dynamic porous framework remain an ongoing challenge.Metal azolate frameworks(MAFs)have attracted much attention in the aspects of crystal engineering in recent years.14,19–21We have designed and synthesized a NbO-type22 cuprous triazolate framework{[Cu(etz)]·guest}∞(Hetz)3,5-diethyl-1,2,4-triazole)with cubic symmetry(MAF-2c·g)by the aid of a solvothermal in situ ligand/metal reaction of cupric salt, propionitrile,and ammonia.19MAF-2c possesses a3D intersect-ing pore system whose large cavities are interconnected via short,small channels.Interestingly,the apertures of these channels are blocked by pendant,disordered ethyl groups with rotation/swingflexibility and may be continuously tuned from closed to open by cooperative motions of the ethyl groups without disrupting the[Cu(tz)]∞(tz)triazolato ring)scaffold, being similar to the IRIS stop used in optical cameras(Figure 1).Nevertheless,besides the symmetry-imposed disorder,10,18 MAF-2c·g loses guest molecules and single crystallinity to form a new material with an unknown structure,19precluding further verification of its KCF.We report herein the direct synthesis,structure,and property of this new material.Guest-free[Cu(etz)]∞was obtained as a trigonal phase(MAF-2)without any symmetry-imposed disor-der.This material demonstrated exceptional frameworkflex-ibility and sorption properties,e.g.,temperature-controllable sorption behavior,superior organic solvent/water and benzene/ cyclohexane selectivity,and multimode structural transformations. Experimental SectionMaterials and Physical mercially available reagents were used as received without further purification.Hetz was synthesized by a reported method.23Elemental analyses(C, H,N)were performed on a Perkin-Elmer240elemental analyzer. Thermal gravimetric analysis was performed under N2using a NETZSCH TG209system.Powder X-ray diffraction patterns were measured at a Rigaku D/M-2200T automated diffractometer.Gas and solvent vapor sorption isotherms were measured on a volumetric adsorption apparatus(Bel-max).Synthesis of[Cu(etz)]∞(MAF-2).A solution of Hetz(4.0mmol) in methanol(100mL)was dropwise added into a solution of [Cu(NH3)2]OH(4.0mmol)in aqueous ammonia/methanol(1/1,100 mL)with stirring at room temperature under N2atmosphere.The mixture was aged at50°C for3h and then evacuated under vacuum until half of the solvent was recovered.The white crystalline powder wasfiltered,washed with methanol,and dried in vacuum to give MAF-2(yield80–100%depending on the solvent recovery ratio). Anal.Calcd(%)for C6H10CuN3:C,38.39;H,5.37;N,22.39. Found:C,38.35;H,5.43;N,rge single crystals of MAF-2(11)(a)Wu,C.-D.;Hu,A.;Zhang,L.;Lin,W.-B.J.Am.Chem.Soc.2005,127,8940–8941.(b)Llewellyn,P.L.;Bourrelly,S.;Serre, C.;Filinchuk,Y.;Férey,G.Angew.Chem.,Int.Ed.2006,45,7751–7754.(c)Maji,T.K.;Matsuda,R.;Kitagawa,S.Nat.Mat.2007,6,142–148.(d)Dincaˇ,M.;Long,J.R.J.Am.Chem.Soc.2007,129,11172–11176.(e)Mulfort,K.L.;Hupp,J.T.J.Am.Chem.Soc.2007,129, 9604–9605.(f)Kawano,M.;Kawamichi,T.;Haneda,T.;Kojima,T.;Fujita,M.J.Am.Chem.Soc.2007,129,15418–15419.(12)(a)Chen,B.-L.;Ma,S.-Q.;Zapata,F.;Fronczek,F.R.;Lobkovsky,E.B.;Zhou,H.-C.Inorg.Chem.2007,46,1233–1236.(b)Ma,S.-Q.;Sun,D.-F.;Wang,X.-S.;Zhou,H.-C.Angew.Chem.,Int.Ed.2007, 46,2458–2462.(13)Atwood,J.L.;Barbour,L.J.;Jerga,A.;Schottel,B.L.Science2002,298,1000–1002.(14)Zhang,J.-P.;Kitagawa,S.J.Am.Chem.Soc.2008,130,907–917.(15)Rather,B.;Zaworotko,mun.2003,830–831.(16)Vittal,J.J.Coord.Chem.Re V.2007,251,1781–1795.(17)Horike,S.;Matsuda,R.;Tanaka,D.;Matsubara,S.;Mizuno,M.;Endo,K.;Kitagawa,S.Angew.Chem.,Int.Ed.2006,45,7226–7230. (18)Eddaoudi,M.;Kim,J.;Rosi,N.;Vodak,D.;Wachter,J.;O’Keeffe,(20)(a)Masciocchi,N.;Galli,S.;Sironi,ments Inorg.Chem.2005,26,1–37.(b)Zhang,J.-P.;Chen,mun.2006,1689–1699.(21)(a)Huang,X.-C.;Lin,Y.-Y.;Zhang,J.-P.;Chen,X.-M.Angew.Chem.,Int.Ed.2006,45,1557–1559.(b)Park,K.S.;Ni,Z.;Côté,A.P.;Choi,J.Y.;Huang,R.;Uribe-Romo,F.J.;Chae,H.K.;O’Keeffe, M.;Yaghi,O.M.Proc.Natl.Acad.Sci.U.S.A.2006,103,10186–10191.(c)Tian,Y.-Q.;Zhao,Y.-M.;Chen,Z.-X.;Zhang,G.-N.;Weng,L.-H.;Zhao,D.-Y.Chem.s Eur.J.2007,13,4146–4154.(d) Hayashi,H.;Côté,A.P.;Furukawa,H.;O’Keeffe,M.;Yaghi,O.M.Nat.Mater.2007,6,501–506.(22)(a)Chen,B.-L.;Ockwig,N.W.;Millward,A.R.;Contreras,D.S.;Yaghi,O.M.Angew.Chem.,Int.Ed.2005,44,4745–4749.(b)Lin, X.;Jia,J.-H.;Zhao,X.-B.;Thomas,K.M.;Blake,A.J.;Walker,G.S.;Champness,N.R.;George,M.W.;Hubberstey,P.;Schröder,M.Multifunctional Porous Cuprous Triazolate Framework A R T I C L E Swere obtained by heating the mixture in a Teflon-lined autoclave at120°C for2days,followed byfiltration and drying in vacuum. X-Ray Crystallography.The solvent-loaded single crystals were prepared by exposing MAF-2in the corresponding solvent vapor for a few days.Diffraction intensities were collected on a Bruker Apex CCD area-detector diffractometer(Mo K R).The structures were solved with direct methods and refined with a full-matrix least-squares technique with the SHELXTL program package.24Dini-trogen molecules were refined as a2-fold disorder with a0.25+ 0.25occupancy and a N-N distance of1.10(1)Å.Anisotropic thermal parameters were applied to all non-hydrogen atoms except the disordered EtOH molecules.The organic hydrogen atoms were generated geometrically.Solvent accessible volumes(void/%)for the[Cu(etz)]∞frameworks were calculated by the SOLV routine of PLATON2003.25Crystal data and refinement parameters for the complexes are summarized in Table1.Results and DiscussionMaterial Synthesis.The cubic phase MAF-2c·g was origi-nally synthesized in low yield(10–15%)by a solvothermal in situ metal/ligand reaction of cupric salt,propionitrile,and ammonia.19Due to the symmetry-imposed disorder of the ethyl groups,it is difficult to make a conclusive structural interpreta-tion of the ethyl conformations around each aperture.Moreover, MAF-2c·g easily loses guest molecules and single-crystallinity,Figure1.(a)NbO type coordination framework(2-fold disordered ethyl groups are shown as dashed bonds in red)and bcu type pore network(large yellow sphere:large pore inside the NbO cage;green column:short channel with double gate)structures of[Cu(etz)]∞.Each gate can tune its aperture size independently by rotation/swing motions of the three pendant ethyl groups without disrupting the[Cu(tz)]∞scaffold.(b-e)Perspective views of four representative channel states derived from the crystal structure of MAF-2c,i.e.,double closed,double open,closed-open,and open-closed,respectively(ethyl groups are colored in red and green to highlight the two different gates).Note that all channels can simultaneously adopt the closed-open/open-closed state,giving rise to a nonpermeable structure in the static point of view.Table1.Crystallographic Data and Refinement Parameters aMAF-2c·g19MAF-2MAF-2·N2MAF-2·MeOH MAF-2·EtOH MAF-2t·MeCN formula C6H12CuN3O C6H10CuN3C6H10CuN4C7H14CuN3O C8H16CuN3O C24H39Cu3N12 FW205.73187.71201.72219.75233.78686.29 space group Im3j R3j R3j R3j R3j P1jT/K123(2)123(2)93(2)123(2)123(2)123(2) a/Å14.9159(7)19.6891(8)19.6052(5)19.429(2)19.810(3)11.949(2) b/Å14.9159(7)19.6891(8)19.6052(5)19.429(2)19.810(3)12.263(4) c/Å14.9159(7)14.1971(7)14.2420(4)14.321(2)14.180(2)13.213(4) R/deg9090909090108.677(2)/deg9090909090108.348(1)γ/deg90120120120120104.458(3) V/Å33318.6(3)4766.3(4)4740.7(2)4681.7(8)4819.2(12)1603.2(7) Z12181818182D c/g cm-3 1.236 1.177 1.272 1.403 1.450 1.422µ/mm-1 1.934 2.007 2.025 2.061 2.007 2.005R1(I>2σ)0.05310.04020.03270.02660.03630.0436 wR2(all data)0.15720.07700.07610.05870.09930.0887 GOF 1.082 1.014 1.015 1.037 1.031 1.010void/%39.733.432.732.235.437.3a R1)ΣF o|-|F c/Σ|F o|,wR2)[Σw(F o2-F c2)2/Σw(F o2)2]1/2.A R T I C L E S Zhang and Chentransforming into a guest-free crystalline phase(MAF-2)with an unknown structure.The preparative difficulty is commonly encountered in the production of coordination polymers.Actually,the simple chemical compositions of binary metal-azolate frameworks imply that these materials can be facilely synthesized by simple acid–base reactions.Simply mixing[Cu(NH3)2]OH and Hetz in aqueous ammonia/methanol produces white crystalline pow-ders.After removal of the solvent,the binary cuprous triazolate can be isolated in quantitative yield.According to the elemental analysis and PXRD patterns,the product displays a composition and diffraction pattern identical to those of the guest-free rge,high-quality single crystals of MAF-2can be obtained by a solvothermal treatment of the reaction mixture. Framework Structure.The cubic MAF-2c possesses a regular (Im3j)NbO-type[Cu(tz)]∞scaffold,which is constructed by interconnection of square-planar nodes({Cu2(tz)2}fragments) with a90°interplanar angle between adjacent{Cu2(tz)2} fragments.The structural information of the3D NbO network can be simply visualized from its cagelike fragments(NbO cages).The large cavities(d)9Å)inside the NbO cages interconnect via eight hexagonal{Cu6(etz)6}rings(chair conformation)to form a3D intersecting pore system(void) 39.7%)with a regular bcu topology(dual net of NbO).26The chairlike,hexagonal{Cu6(etz)6}ring defines a short channel with a double gate.From the static point of view,all channels of MAF-2c appear to be simultaneously closed(d)1.5Å, smaller than any real molecule)by the pendant ethyl groups. Considering the rotation/swingflexibility of ethyl groups,the apertures may be continuously tuned to the open state,which permits the passage of a sphere with d<3.6Åor an irregular particle with the smallest cross-sectional dimensions<4.2Å(such as methane and nonbranched hydrocarbons).The suitable aperture-size variation implies that MAF-2c may potentially show KCF for a variety of guest molecules(Figure1).Nevertheless,symmetry-imposed disorder of the ethyl groups precludes any conclusive interpretation based on the crystal structure of MAF-2c.Single-crystal X-ray diffraction analysis of MAF-2revealed one Cu(I)cation and one etz anion in the crystallographically asymmetric unit and a distorted NbO type[Cu(etz)]∞network, without any symmetry-imposed disorder(R3j).As the local coordination geometries of MAF-2c and MAF-2are very similar,the network distortion should mainly arise from the rotation between adjacent{Cu2(tz)2}moieties,which reduces the interplanar angles from90°to79°.Straightforwardly,the unit-cell parameters of MAF-2(a,19.6891Å;c,14.1971Å; and V,4766.3Å3)are significantly different from the trigonal setting ones of MAF-2c(a,21.0943Å;c,12.9175Å;and V, 4977.8Å3).The transformation from MAF-2c to MAF-2 includes a large expansion in the c-axis(9.9%)and a large contraction in the ab plane(6.7%in axis length,12.9%in area), resulting in an anisotropic contraction(4.2%in volume).The NbO cages and cavities also distort in the same manner.The eight equivalent{Cu6(etz)6}rings of MAF-2c disproportion into two types in MAF-2.Thefirst type consists of two{Cu6(etz)6} rings(running along the c-axis),which are identical to the double-closed channels of MAF-2c.The other six{Cu6(etz)6} rings form the second type of channels,which are distorted in the same manner as the lattice,but are completely blocked by only two ethyl groups(Figure2).Consequently,the unoccupied space inside MAF-2can be safely described as isolated cavities, from the static point of view.On the other hand,the momentary rotation/swing motion of the ethyl groups can allow the passage of small guest molecules(as large as nonbranched hydrocar-bons),without disrupting the[Cu(tz)]∞scaffold.Therefore, MAF-2represents as a suitable candidate for the study of KCF. Framework Stability.According to thermal gravimetric(TG) analysis and X-ray diffraction studies,the guest-free MAF-2 does not change its structure(except small thermal expansion that has negligible impact on the static aperture size)from cryogenic to high temperature until decomposition above∼280°C.Although MAF-2is constructed by three-coordinated(or coordinatively unsaturated)cuprous centers,its chemical stability is remarkable.14The microcrystalline powder sample of the guest-free MAF-2can be handled in air,and its color changes from white to light green after several weeks depending on the humidity,implying the oxidation of Cu(I)in air is promoted by moisture.However,after prolonged exposure,the PXRD pattern of the light-green product still resembles that of MAF-2 (except some peak weakening),implying that the oxidation occurred on the particle surface rather than the pore surface. The hydrophobic,isolated pore system of MAF-2is responsible for these observations,as the water molecules may hardly pass through the hydrophobic,closed channels.Figure2.Perspective views of the distorted NbO type[Cu(tz)]scaffold (a)and the ethyl-blocked channels(b:running along the c-axis,c:running slanted to the c-axis)ofMAF-2.Figure3.N2sorption isotherms of MAF-2.Inset:Characteristic adsorption isobars for rigid porous materials(a)andflexible porous materials with temperature-dependent aperture size(b:for small guest,c:for large guest).Multifunctional Porous Cuprous Triazolate Framework A R T I C L E Sat 77K.On the other hand,MAF-2can readily adsorb supercritical N 2at 195K.The adsorption amount at 1atm (49mL/g)corresponds to 0.41N 2per formula unit or 2.5N 2per NbO cage,indicating the N 2molecules are adsorbed into the cavities of MAF-2rather than the particle surface.Because the static aperture size of MAF-2is too small for any real molecule,the apertures must open during the sorption process.The abnormal temperature-dependent sorption behavior of MAF-2is in contrast with those of rigid porous frameworks.When the temperature increases,the adsorption capacity of a rigid porous material at a given guest pressure decreases monotonously because the saturation pressure of the guest increases and the host–guest interaction decreases.However,flexible frameworks may start to adsorb a given gas until the temperature is high enough to open their apertures to allow the diffusion of the guest molecules.In contrast to the monotonously decreasing isobar of a rigid porous material,there is a characteristic onset adsorption (gate opening)temperature in the isobar of such a flexible material.In the case of MAF-2,the temperature-dependent gate-opening behavior should arise from the ethyl-blocked apertures,which behave as thermoactivated IRIS stops.As the effective aperture sizes of these flexible materials vary with temperature,size-different guest molecules could have different onset adsorption temperatures.Obviously,these flexible materials are of great importance in multipurpose gas separa-tion.12It should be noted that,the temperature-dependent gate-opening behavior could be derived from not only KCF,but also temperature-driven structure variation of the guest-free host framework.Nevertheless,the temperature-driven structure varia-tion of the guest-free MAF-2has been ruled out by single-crystal and powder X-ray diffraction studies.We also determined the structure of the gas-loaded single crystal to confirm that thermodynamic bistability is not involved in the gas sorption process of MAF-2.Direct structural information of the absorbed gas molecules is fundamental to the understanding of adsorption mechanism and rational design of porous frameworks.Due to the extremely weak host–guest interaction,crystal structure determination of the gas-loaded phases remains a great chal-lenge.Although some successes have been achieved for the gas-loaded PCPs with very small pores (strong confinement for the gas),27–29crystal structures concerning gas molecules absorbed in large pores are still very scarce.30In our experiment,a fully degassed single crystal of MAF-2was filled with N 2and flame-sealed in a glass capillary and then the temperature was decreased very slowly to 93K in a cold dinitrogen flow,ensuring as much N 2as possible is adsorbed at high temperature but the diffraction data were collected at low temperature (Table 1).Except some negligible thermal contraction,the host framework of the N 2-loaded MAF-2is identical to that of the guest-free one,precluding the occurrence of thermodynamic bistability during the adsorption process.Therefore,the gas sorption mechanism of MAF-2can be safely assigned to the KCF originated from the momentary rotation/swing motions of the ethyl groups.Interestingly,although the large cavity ofMAF-2is not expected to strongly confine the partially loaded N 2molecules at 93K,the N 2molecules were nicely refined in a 2-fold disordered model.The N 2molecules form close contacts to the triazolato rings,and each N 2is either parallel (N ···N 3.33–3.41Å)or staggered (N ···N 3.52–3.54Å)in relation to the 1,2-nitrogen atoms of etz (Figure 4).The intermolecular contacts between the N 2molecules (shortest N ···N 4.17Å,center ···center 4.79Å)are substantially longer than other known values,30indicating relatively strong adsorbent–adsorbate interactions in MAF-2·N 2.Vapor Sorption.Recovery,removal,and detection of small amounts of organic solvents from water,which rely on the selective binding of organic solvents over water,are of great technical,industrial,and environmental importance.Hydrophilic/hydrophobic affinity and pore size considerations are generally used to engineer porous materials with different binding abilities between water and organic solvents.Because hydrophilic building blocks are commonly used,many inorganic and metal -organic porous materials are known to adsorb water over organic molecules.Moreover,because the molecular size or kinetic diameter of H 2O is smaller than those of organic solvent molecules,size selective porous materials can only adsorb small water molecules over large organic molecules.Nevertheless,for PCPs,hydrophobic pore surfaces can be rationally introduced to adsorb organic molecules over H 2O.Li et al.have reported a hydrophobic PCP,zinc 5-tert -butyl isophthalate,which exhi-bits essentially zero water adsorption (∼1mg/g)up to the humidity P /P 0)0.65but significant MeOH adsorption (110mg/g at P /P 0)0.73).31Nevertheless,due to the large pore size required for the accommodation of organic molecules,capillary condensation of H 2O usually occurs for these hydrophobic porous materials at relatively high humidity and significantly reduces the organic/water selectivity.For example,Kitagawa et al.have demonstrated that a PCP with a hydrophobic pore surface does not show water uptake until P /P 0)0.59,which then abruptly rise to a high value (128mg/g)at higher humidity.32Inspired by the hydrophobic and closed channel structure,we anticipated that MAF-2could selectively separate organic solvents from water.Water,methanol,ethanol,and acetonitrile(27)(a)Matsuda,R.;Kitaura,R.;Kitagawa,S.;Kubota,Y.;Belosludov,R.V.;Kobayashi,T. C.;Sakamoto,H.;Chiba,T.;Takata,M.;Kawazoe,Y.;Mita,Y.Nature 2005,436,238–241.(28)Kubota,Y.;Takata,M.;Kobayashi,T.C.;Kitagawa,S.Coord.Chem.Re V .2007,251,2510–2521.(29)Takamizawa,S.;Nakata,E.-I.;Akatsuka,T.Angew.Chem.,Int.Ed.(31)Pan,L.;Parker,B.;Huang,X.-Y.;Olson,D.L.;Lee,J.Y.;Li,J.Figure 4.Perspective views of the absorbed N 2molecules (2-folddisordered sites are represented in red and green)in space-filling (left,with an NbO cage)and ball-and-stick modes (right).A R T I C L E S Zhang and Chenvapor sorption isotherms for MAF-2were measured at298K. As shown in Figure5,MAF-2exhibits a remarkable water resistance in the whole measured humidity range.The extremely low water uptake of MAF-2(3mg/g at P/P0)0.97)can be attributed to particle surface adsorption on uncoordinated defects,indicating essentially no water molecules can diffuse through the ethyl-blocked channels into the NbO cages.On the other hand,MAF-2can adsorb large amounts of MeOH,EtOH, and MeCN even at low relative pressures.The existence of onset adsorption pressures of these organic solvents near P/P0)0.02 indicates that the adsorption processes are restricted by diffusion barriers and/or weak host–guest interactions below the critical points.Obviously,the diffusion barriers are related to the ethyl-blocked apertures.Nevertheless,these organic solvents can open the ethyl-blocked channels to access the large internal cavities, but water cannot.Therefore,organic solvent/water separation can be performed at very high humidity(i.e.,small amounts of organic solvents vs large amount of water),which is close to the practical requirements.The weaker permeability of water compared to small organic solvents should be ascribed to the hydrophobicity of the ethyl groups,which interact very differ-ently to water and organic molecules.This unique instance demonstrated a viable strategy for designing porous materials with high organic molecule/water selectivity,in whichflexible, hydrophobic apertures are utilized to block the large internal cavities.To elucidate the adsorption mechanism of MeOH,EtOH,and MeCN,we also measured the crystal structures of the solvent-loaded MAF-2.Interestingly,the single crystals of the guest-free MAF-2have a high tendency to crack into smaller pieces in the MeCN vapor,while MeOH and EtOH vapors do not induce a significant change on the single crystal morphology. These phenomena are correlated to the degree of structural transformation upon solvent inclusion.The crystal structures of the MeOH-and EtOH-loaded materials are isostructural to the guest-free MAF-2,and the ethyl-blocked channels remain closed,confirming that KCF is dominant in the sorption processes of MeOH and EtOH.Nevertheless,small framework distortions are evident by comparing their unit-cell parameters. The inclusion of MeOH induces an anisotropic contraction (1.8%in volume),including a1.3%contraction in the a-axis and a0.9%expansion in the c-axis.In contrast,EtOH induces an anisotropic expansion(1.1%in volume),including a0.6%in which the largest distortions(ab plane contraction,c-axis expansion,and volume contraction)are observed for MAF-2·MeOH,while MAF-2·EtOH adopts the smallest distortion. Inside the large NbO cage of MAF-2·MeOH,six MeOH molecules form a chairlike hydrogen-bonded(O···O2.69Å) hexagon with S6symmetry,and the hydrophobic methyl groups are exposed on the outer sphere with weak interactions to the triazolato rings(CH3···N3.66Å)across the ab plane(Figure 6).33The framework contraction in the ab plane indicates that the MeOH hexamer is smaller than the NbO cage.More interestingly,the hydrogen-bonded MeOH hexamer behaves as a single molecule,which is relatively stiff to contract theflexible host framework rather than just expanding itself.The EtOH molecules also form similar hydrogen-bonded hexamers(dis-ordered)in MAF-2·EtOH,but host framework must expand to accommodate the enlarged guest hexamers.The adsorption of MeCN produces a triclinic structure, namely MAF-2t·MeCN,with highly unsymmetrical framework distortions,which can be evaluated by comparing its unit-cell parameters with the primitive unit-cell parameters of the cubic and trigonal phases(see Supporting Information).These distor-tions arise again from the rotations between adjacent{Cu2(tz)2} fragments,as MAF-2t·MeCN still retains the original local coordination geometries and overall coordination topology. Moreover,some of the ethyl groups adopt new conformations, reflecting their rotation/swingflexibilities.MAF-2t possesses four(crystallographic)types of channels,which are all blocked by the ethyl groups.These structural features reveal that the adsorption mechanism involves not only TCF but also KCF. Interestingly,six MeCN molecules located inside the NbO cage of MAF-2t construct a trigonal prism cluster by weak C-H···N interactions(Figure7).The shortest host–guest contacts are C-H···π(C···N3.68Å)and C-H···Cu(C···Cu3.79Å). To our knowledge,this is thefirst structurally characterized, matrix-isolated MeCN hexamer,which would be useful for the study of the structure and properties of acetonitrile.The frameworkflexibility of MAF-2,as revealed from the diverse structural transformations,implies the hexagonal {Cu6(tz)6}windows may undergo drastic distortions and cooperate with the KCF of ethyl groups to allow the passage of molecules which are larger than the above-studied guests, but other molecules exceeding the upper limit of theseflex-ibilities will be excluded.Consequently,this hypothesis naturallyFigure5.H2O,MeOH,EtOH,and MeCN sorption isotherms measured at 298K.P0is the saturation pressure(H2O3.17kPa,MeOH16.94kPa,EtOH 7.87kPa,MeCN11.80kPa).Figure6.Perspective views of the MeOH hexamer in space-filling(left, with a NbO cage)and ball-and-stick modes(right).Multifunctional Porous Cuprous Triazolate Framework A R T I C L E S。

CHEMICAL INDUSTRY AND ENGINEERING PROGRESS 2016年第35卷第6期·1794·化 工 进 展柔性金属有机骨架材料（MOFs ）用于气体吸附分离李立博，王勇，王小青，陈杨，杨江峰，李晋平（太原理工大学精细化工研究所，山西 太原 030024）摘要：柔性金属有机骨架材料（MOFs ）具有高度有序的网络结构与可变形的骨架，其骨架结构会对外界的温度、压力及客体分子的刺激产生独特的结构响应。

近几年来，柔性MOFs 在气体吸附、气体分离、传感等领域显示出巨大的应用潜力。

截至目前，研究者们对柔性MOFs 的研究仅局限于对其结构形变的机理解释，而缺乏对柔性MOFs 应用于相关化工过程的性能研究。

本文着重对近年来柔性MOFs 在气体吸附分离领域的研究进展进行了综述，并详细地分析了柔性MOFs 结构与其气体吸附分离性能之间的构效关系。

通过分子模拟结合实验，讨论了柔性MOFs 结构对气体分子的平衡吸附与动力学扩散的影响。

分析表明，设计合成具有良好吸附选择性与扩散性能的柔性MOFs 是其应用于绿色、高效气体分离过程的重要发展方向。

关键词：柔性金属有机骨架；吸附；分离；天然气；碳氢化合物中图分类号：TQ 028.1 文献标志码：A 文章编号：1000–6613（2016）06–1794–10 DOI ：10.16085/j.issn.1000-6613.2016.06.020Selective gas adsorption and separation in flexible metal-organicframeworksLI Libo ，WANG Yong ，WANG Xiaoqing ，CHEN Yang ，YANG Jiangfeng ，LI Jinping（Research Institute of Special Chemicals ，Taiyuan University of Technology ，Taiyuan 030024，Shanxi ，China ）Abstract ：Flexible metal-organic frameworks （MOFs ）have both highly ordered coordination network and cooperative structural transformability. Their strutures can respond to temperature ，pressure ，guest adsorption/desorption ，and other external stimuli. In recent years ，flexible MOFs has showed great potential in gas adsorption ，gas separation and sensing. However ，most reports on flexible MOFs are limited to the mechanism study on structural transformation ，while their applications on chemical industry has been insufficiently investigated. In this review ，emphasis is given on the recent progress in the gas adsorption and separation on flexible MOFs. And the relationships between adsorption/separation properties and framework features are detailed analyzed. In addition to the experimental aspect ，theoretical investigations of adsorption equilibrium and di ffusion dynamics via molecular simulations are also briefly reviewed. Therefore ，more efforts should be made to design and synthesis new flexible MOFs with highly adsorption selectivity and diffusion properties for green and e fficient gas separation process.Key words ：flexible metal-organic frameworks ；adsorption ；separation ；natural gas ；hydrocarbons金属有机骨架材料（MOFs ）是由金属离子与有机配体通过特定的配位方式形成的一类具有高孔隙率、高比表面积的多孔材料。

Enhancement of CO2Adsorption and CO2/N2Selectivity on ZIF-8via Postsynthetic ModificationZhijuan ZhangSchool of Chemistry and Chemical Engineering,South China University of Technology,Guangzhou,510640,P.R.ChinaDept.of Chemistry and Chemical Biology,Rutgers University,Piscataway,New Jersey,08854Shikai Xian,Qibin Xia,Haihui Wang,and Zhong LiSchool of Chemistry and Chemical Engineering,South China University of Technology,Guangzhou,510640,P.R.ChinaJing LiDept.of Chemistry and Chemical Biology,Rutgers University,Piscataway,New Jersey,08854DOI10.1002/aic.13970Published online January11,2013in Wiley Online Library()Imidazolate framework ZIF-8is modified via postsynthetic method using etheylenediamine to improve its adsorption per-formance toward CO2.Results show that the BET surface area of the modified ZIF-8(ED-ZIF-8)increases by39%,and its adsorption capacity of CO2per surface area is almost two times of that on ZIF-8at298K and25bar.H2O uptake on the ED-ZIF-8become obviously lower compared to the ZIF-8.The ED-ZIF-8selectivity for CO2/N2adsorption gets significantly improved,and is up to23and13.9separately at0.1and0.5bar,being almost twice of those of the ZIF-8. The isosteric heat of CO2adsorption(Q st)on the ED-ZIF-8becomes higher,while Q st of N2gets slightly lower com-pared to those on the ZIF-8Furthermore,it suggests that the postsynthetic modification of the ZIF-8not only improves its adsorption capacity of CO2greatly,but also enhances its adsorption selectivity for CO2/N2/H2O significantly. V C2013American Institute of Chemical Engineers AIChE J,59:2195–2206,2013Keywords:ZIF-8,modification,adsorption/gas,isosteric heat of adsorption,selectivityIntroductionCO2has often been cited as the primary anthropogenicgreenhouse gas(GHG)as well as the leading culprit inglobal climate change.The development of a viable carboncapture and sequestration technology(CCS),is therefore,ascientific challenge of the highest order.1–4Currently,a vari-ety of methods,such as membrane separation,chemicalabsorption with solvents,and adsorption with solid adsorb-ents,have been proposed to sequester CO2from thefluegases of power plant.Thereinto,the adsorption is consideredto be one of the most promising technologies for capturingCO2fromflue gases because of their easy control,low oper-ating and capital costs,and superior energy efficiency.5–7Many adsorbents have been investigated for CO2adsorptionincluding activated carbons,zeolites,hydrotalcites and metaloxides.8–14However,although some zeolite materials havebeen claimed to be most adequate for CO2separation fromflue streams,it is difficult to regenerate them without signifi-cant heating which leads to low productivity and greatexpense.15,16Recently,metal-organic frameworks(MOFs)haveattracted great attention and present a promising platform forthe development of next-generation capture materialsbecause of their high capacity for gas adsorption and tunablepore surfaces that can facilitate highly selective binding ofCO2.17–26To optimize a MOF for a particular application,itis important to be able to tailor its pore metrics and function-ality in a straightforward fashion.However,tailoring MOFsmaterials by modifying their textural properties(e.g.,surfacearea and pore volume)and surface chemistry(acid–baseproperties,functional groups)for adsorption application isstill a difficult task.27Many researchers have given insightinto modification of the MOF materials so as to develop newand better adsorbents.Strategies reported include ligandfunctionalization,24,28–39framework interpenetration,22,23introduction of alkali-metal cations,40–42control of poresize32,43–47and incorporation of open metal sites(OMSs).39,48–51However,because of the instability underconditions for the synthesis of MOFs or the competitivereaction with some framework components,it may be diffi-cult for certain functional groups to incorporate into MOFsusing aforementioned strategies.Another strategy for gener-ating desired functionalities in MOFs is the postsynthesis Additional Supporting Information may be found in the online version of thisarticle.Correspondence concerning this article should be addressed to Z.Li atcezhli@.V C2013American Institute of Chemical EngineersAIChE Journal2195June2013Vol.59,No.6modification of preconstructed,robust precursor MOFs.52–55 For example,An et al.33demonstrated that postsynthetic exchange of extra-framework cations within anionic bio-MOF-156can be used as a means to systematically modify its pore dimensions and metrics.Farha et al.57synthesized a series of cavity modified MOFs by replacing coordinated sol-vents with several different pyridine ligands.They found that a p-(CF3)NC5H4-modified MOF showed considerable improvements in the CO2/N2selectivities compared to the parent framework.46Long and his coworkers58previously reported the grafting of ethylenediamine(en)within a water-stable MOF H3[(Cu4Cl)3(BTTri)8](CuBTTri),and found that the en modified sample had more greater attraction of CO2 at low pressures and the CO2/N2selectivity also increased over the entire pressure range measured.More recently, Long and coworkers59incorporated the N,N0-dimethylethyle-nediamine(mmen)into the CuBTTri MOF,and showed that the CO2uptake was drastically enhanced.Zhang et al.60 reported that after ZIF-8was modified by ammonia impreg-nation,the surface basicity was greatly increased and there-fore the CO2uptake was enhanced.Park et al.61reported a postsynthetic reversible incorporation of organic linkers3,6-di(4-pyridyl)-1,2,4,5-tetrazine(bpta)into SNU-30 [Zn2(TCPBDA)(H2O)2]Á30DMFÁ6H2O through single-crystal-to-single-crystal transformations,and found that the desol-vated SNU-310exhibited enhanced selective adsorption of CO2over N2.Xiang et al.62incorporated the CNTs into HKUST-1,and then modified it with Li1.The results showed that the hybrid Li@CNT@[Cu3(btc)2],which is formed by the combination of Li doping and CNT incorpora-tion,having an enhancement of CO2uptakes by about 305%.However,to this date,no work has been reported out on the postsynthetic modification of ZIF-8to enhance its functionality.In this work,the postsynthetic modification of the ZIF-8is proposed to prepare a novel adsorbent with higher CO2 adsorption capacity and CO2/N2selectivity.The postsyn-thetic modification of the ZIF-8crystals would be carried out by using ethylenediamine treatment.Then the surface groups of the modified ZIF-8samples(ED-ZIF-8)would be characterized.Single-component isotherms of CO2and N2 on the modified ZIF-8samples would be measured sepa-rately.Furthermore,the CO2/N2selectivity is estimated by using IAST on the basis of single-component isotherms of CO2and N2.The influence of the textural structures and sur-face chemistry of the original and modified ZIF-8samples on their adsorption capacities for CO2and selectivity of CO2/N2would be discussed and reported here.This informa-tion will be valuable for selecting appropriate adsorbents for CO2capture process.Methods and MaterialsMaterials and instrumentsZinc nitrate hexahydrate(Zn(NO3)2Á6H2O,98%,extra pu-rity)and2-methylimidazole(H A MeIM)(99%purity)were purchased from J&K Chemicals.N,N A Dimethylacetamide (DMF)was purchased from Qiangshen Chemicals Co.,Ltd. of Jiangshu(Jiangshu,China),and it was further purified by 4A molecular sieve to eliminate the water.Maganetic suspension balance RUBOTHERM was sup-plied by Germany.Its precision was0.000001g.ASAP 2010sorptometer was supplied by Micromeritics Co., Norcross,GA,USA.AdsorbentsSynthesis of ZIF-8was performed following the reported procedures63with a few modifications.First,a solid mixture of zinc nitrate hexahydrate Zn(NO3)2Á6H2O(0.956g, 3.2 mmol)and2-methylimidazole(H A MeIM)(0.24g, 3.4 mmol)was dissolved in70mL of DMF solvent.The mixture was quickly transferred to a100mL autoclave and sealed. Second,the autoclave was heated at a rate of5K/min to 413K in a programmable oven and held at this temperature for24h under autogenous pressure by solvothermal synthe-sis,followed by cooling at a rate of0.3K/min to room tem-perature.Third,after removal of mother liquor from the mix-ture,chloroform(40mL)was added to the autoclave.The as-synthesized ZIF-8crystals were then isolated byfiltration. Colorless polyhedral crystals were collected from the upper layer,washed with DMF(10mL33),and dried at383K overnight.To further remove the guest species from the framework and prepare the evacuated form of ZIF-8crystals for modifi-cation and gas-sorption analysis,the as-synthesized ZIF sam-ples were immersed in methanol at ambient temperature for 48h,and evacuated at ambient temperature for5h,and sub-sequently at an elevated temperature673K for2h. Postsynthetic modification of adsorbentsThe as-synthesized ZIF-8crystals(labeled as ZIF-8)were dried at383K for24h for postsynthetic modification.The subsequent treatment applied to the modification of ZIF-8crystals consists of the following steps:The modified ZIF-8sample(labeled as ED-ZIF-8)was synthesized using ethylenediamine as a linker.In a typical procedure,the ZIF-8sample was added to30%ethylenediamine solution and then the mixture was placed in a stainless high-pressure autoclave.The autoclave was heated in an oven at416K for 1h and then381K for6h.The light yellow product wasfil-tered and washed with deionized water.Finally,the sample was dried at383K overnight.Characterization of adsorbentsThe specific surface area and pore volume of original ZIF-8and modified ZIF-8crystals were measured on a Micrometrics gas adsorption analyzer ASAP2010instrument equipped with commercial software for calculation and analysis.Powder X-ray diffraction data were collected using a D8 advance h-2h diffractometer(Bruker)in reflectance Bragg-Brentano geometry employing Cu K a line focused radiation with40kV voltages and40mA current.The X-ray scanning speed was set at2 /min and a step size of0.02 in2h.A Jade5XRD pattern processing software(MDI,Inc.,Liver-more,CA)was used to analyze the XRD data collected on the ZIF-8samples.The surface organic molecules were analyzed by taking FTIR spectra on a Bruker550FTIR instrument equipped with a diffuse reflectance accessory that included a reaction cell.Data acquisition was performed automatically using an interfaced computer and a standard software package.The samples were dried in vacuo at423K prior to mixing with KBr powder.The samples were run in ratio mode allowing for subtraction of a pure KBr baseline.The sample chamber2196DOI10.1002/aic Published on behalf of the AIChE June2013Vol.59,No.6AIChEJournalwas kept purged with nitrogen during the entire experiment.The spectrometer collected 64spectra in the range of 400–4,000cm 21,with a resolution of 4cm 21.CO 2and N 2adsorption measurementsThe CO 2and N 2adsorption-desorption isotherms at 298K,308K,318K,and 328K were obtained on a RUBO-THERM magnetic suspension balance.The initial activa-tion of the modified sample was carried out at 423K for 12h in a vacuum environment.He (ultra-high purity,U-sung)was used as a purge gas in this study.The adsorp-tion processes were carried out using high purity CO 2and N 2(99.999%)gas.A feed flow rate of 60mL/min of CO 2,40mL/min of N 2and 30mL/min of He,respec-tively,were controlled with the mass flow controllers (MFC)to the sample chamber.Both adsorption and de-sorption experiments were conducted at the same tempera-ture.The temperature of the sorption chamber can be adjusted and maintained constant by an internal tempera-ture sensor.However,the pressure can be changed step-wise through the gas flow rate.Typically,there are four steps for finishing determination of an isotherm of CO 2or N 2by using Rubotherm magnetic suspension balance.These detail steps are shown by the operation manual of Rubotherm maganetic suspension balance.H 2O adsorption measurementsThe water adsorption measurements were conducted on a computer-controlled DuPont Model 990TGA.The partial pressure of water was varied by changing the blending ratios of water-saturated nitrogen and pure nitrogen gas streams.Before measurement,the modified ZIF-8samples were acti-vated at 423K for 6h.Results and DiscussionStructure and pore characterizationFigure 1exhibits the adsorption-desorption isotherms of N 2at 77K on the two samples ZIF-8and ED-ZIF-8.It can be seen that both samples show type-I behavior,indicating they are microporous in nature.Table 1lists structure param-eters of the two samples.These data indicate that the BET surface area and micropore volume of the ED-ZIF-8sample are significantly higher than those of the original ZIF-8sam-ple,with an increase of $39%and 35.6%,respectively.Yaghi and his coworkers reported a pore volume of 0.66cm 3/g for ZIF-8from the single crystal structure.For the ZIF-8sample,the total pore volume is calculated to be 0.54cm 3/g,because part of the pores might be blocked.However,after the postsynthetic modification,the blocked pores were reopened,and at meanwhile,some new pores were formed.64,65Thus,the total pore volume of the ED-ZIF-8sample is greatly improved.Figure 2shows the powder X-ray diffraction (PXRD)pat-tern of the modified ZIF-8sample.It can be seen that the main peaks of the modified ZIF-8sample are very clear,and similar to those of the original ZIF-8sample,indicating that the integrity of the modified ZIF-8sample maintains well af-ter the postsynthetic modification.However,for a deep look-ing,it can be found that the major peaks of ED-ZIF-8all shifted to the left side (low-angle area)a little bit,which means after modification,the lattice distance increased.In order to obtain information concerning changes in the surface groups,FTIR experiments were carried out to char-acterize the samples.Figure 3a shows the FTIR spectra of the original ZIF-8and the ED-ZIF-8sample.It is noticed that the spectra for the two samples show high similarities,and the main peaks of both ZIF-8samples match well with the published FTIR spectra for the ZIF-8.However,someTable 1.Porous Structure Parameters of the Modified ZIF-8CrystalsSample BET surface area (m 2.g 21)Langmuir surface area (m 2.g 21)Micropore volume(cm 3.g 21)Total pore volume (cm 3.g 21)Micropore diameter (nm)Mesopore diameter (nm)ZIF-8102513520.450.540.352 4.43ED-ZIF-8142818970.610.750.5444.53Figure 1.N 2adsorption-desorption isotherms of ZIF-8and ED-ZIF-8samples.[Color figure can be viewed in the online issue,which is available at .]Figure2.PXRD patterns of ZIF-8and ED-ZIF-8samples.[Color figure can be viewed in the online issue,which is available at .]AIChE Journal June 2013Vol.59,No.6Published on behalf of the AIChE DOI 10.1002/aic2197differences are also observed.For example,the spectrum of the ED-ZIF-8sample is different from that of the ZIF-8sam-ple in that (1)as shown in Figure 3b there is a new peak at 3381cm 21which is assigned to N A H group appeared on the spectrum of the ED-ZIF-8sample,suggesting some N A H groups have been introduced on the surfaces of the sample ED-ZIF-8,and (2)a peak at 3626cm 21assigned to O A H of the adsorbed H 2O is present in the spectrum of the ZIF-8sample,which is absent in the spectrum of the ED-ZIF-8sample,as shown in Figure 3b.CO 2and N 2adsorption isothermsFor comparison,Figure 4shows the isotherms of CO 2on the ZIF-8and ED-ZIF-8samples.It is visible that the amount adsorbed of CO 2increases as temperature decreases.This suggests that the adsorption of CO 2is mainly physical adsorption.More importantly,it is found that the ED-ZIF-8sample had higher CO 2adsorption capacities compared to the ZIF-8sample,indicating that the adsorption capacities of the modified ZIF-8toward CO 2are greatly improved,nearly being twice as much as the ZIF-8.One of the reasons is that the surface area (BET)of the ED-ZIF-8increases by 39%,as indicated in Table 1.The other reason is that adsorptioncapacity per unit surface area of the ED-ZIF-8for CO 2increases due to an introduction of N A H groups by postsyn-thetic modification.To further understand that,Figure 4a and 4b are separately transferred into Figure 5a and 5b in which the equilibrium uptakes of CO 2based on unit surface area (BET)of the two samples are plotted as a function of CO paring Figure 5b and Figure 5a shows that the CO 2uptake per surface area (BET)of the ED-ZIF-8is obvi-ously higher than that of the ZIF-8,which is mainly ascribed to the introduction of N A H groups,as shown in Figure 3.Figure 6a and 6b show the N 2adsorption isotherms on the two samples.It is visible that the N 2uptakes on the modified ZIF-8samples are slightly higher than that on the ZIF-8due to its larger surface area and pore volume after modification.However,after Figure 6a and 6b are converted into Figure 7a and 7b in which the equilibrium uptakes of N 2based on unit surface area of the two samples are plotted as a function of pressure,it is found from Figure 7that the equilibrium uptakes of N 2per surface area of the ED-ZIF-8are slightly lower than that of the ZIF-8,which means that ED-ZIF-8sample has less affinity toward N 2than ZIF-8sample.This will be helpful to enhance the adsorption selectivity for CO 2/N 2.Figure 3.a.FTIR spectra of the modified ZIF-8crystalsbetween 4,000–400cm 21;b.FTIR spectra of the modified ZIF-8crystals between 4000–2,400cm 21.[Color figure can be viewed in the online issue,which is available at .]Figure 4.a.Isotherms of CO 2on the ZIF-8sample withdifferent temperatures; b.isotherms of CO 2on the ED-ZIF-8sample with different temperatures.[Color figure can be viewed in the online issue,which is available at .]2198DOI 10.1002/aicPublished on behalf of the AIChE June 2013Vol.59,No.6AIChEJournalMultiple cycles of CO 2adsorption-desorption on the ED-ZIF-8To evaluate the regeneration performance of the modified sample or the reversibility of CO 2adsorption on the modi-fied sample,the experiments of multiple cycles of CO 2adsorption-desorption on the ED-ZIF-8were performed in the Rubotherm system at 298K.For adsorption process,the adsorption pressure were targeted for 25bar;while for de-sorption process the system pressure was targeted for 1mbar,and then the desorption system was quickly depressur-ized by using vacuum pumping.Figure 8shows the variation curve of the amounts adsorbed of CO 2on the ED-ZIF-8dur-ing four consecutive cycles of CO 2adsorption-desorption experiments at 298K.It was visible clearly that during the desorption,the amounts adsorbed of CO 2on the ED-ZIF-8sample decreased sharply with time,and then reached a very low content,about 2.21wt %of residual CO 2which was present on the sample after desorption at 1mbar.The effi-ciency of CO 2desorption was nearly up to 98%over the entire four circles.It indicated further that CO 2adsorption was reversible with very little accumulation of irreversible bound CO 2on the ED-ZIF-8framework.In addition,it wasalso observed from Figure 8that the curves representing the cycles of CO 2adsorption-desorption experiments were very similar,suggesting that adsorption and desorption properties of the sample ED-ZIF-8for CO 2were stable or repeatable.It also proved that the pressure swing was effective in strip-ping adsorbed CO 2from the ED-ZIF-8.H 2O adsorption isothermsFigure 9shows the water isotherms on the modified ZIF-8samples at 298K.The water uptake on the ED-ZIF-8sample is less than that on the ZIF-8sample,indicating that the sur-face of the modified sample became more hydrophobic com-pared to the ZIF-8sample.It also means that the interaction of the water molecule with the modified sample became weaker as compared to that with the ZIF-8.Ideal adsorbed solution theory (IAST)selectivity of CO 2/N 2The ideal adsorbed solution theory (IAST)developed by Myers and Praunitz 66provides an effective method to predict the adsorption selectivity and the adsorption equilibrium of gas mixtures from the isotherms of the pure components.Figure 5.a.Isotherms of CO 2on the ZIF-8samplebased on unit surface area;b.isotherms of CO 2on the ED-ZIF-8sample based on unit surface area.[Color figure can be viewed in the online issue,which is available at .]Figure 6.a.Isotherms of N 2on the ZIF-8sample atdifferent temperatures;b.isotherms of N 2on the ED-ZIF-8sample at different temperatures.[Color figure can be viewed in the online issue,which is available at .]AIChE JournalJune 2013Vol.59,No.6Published on behalf of the AIChE DOI 10.1002/aic2199Previous work reported that the IAST can accurately predict gas mixture adsorption in a number of zeolites and MOF materials.10,48,67–70The IAST assumes that the adsorbed mixture is an ideal solution at constant spreading pressure and temperature,where all the components in the mixture conform to the rule analogous to Raoult’s law,and the chemical potential of the adsorbed solution is considered equal to that of the gas phase at equilibrium.From the IAST,the spreading pressure p is given byp 0i ðp 0i Þ5RT A ðp 0iqd ln p (1)p Ã5p A 5ðp 0i 0q i dp (2)Where A is the specific surface area of the adsorbent,p andp *are the spreading pressure and the reduced spreading pres-sure,separately.p 0i is the gas pressure of component i corre-sponding to the spreading pressure p of the gas mixture.At a constant temperature,the spreading pressure of single component is the samep Ã15p Ã25…5p Ãn 5p(3)For binary adsorption of component 1and 2,the IASTrequiresy 1p t 5x 1p 1ð12y 1Þp t 5ð12x 1Þp 2(4)Where y 1and x 1denote the molar fractions of component 1in the gas phase and in the adsorbed phase,respectively.p t is the total gas pressure,p 1and p 2are the pressures of com-ponent 1and 2at the same spreading pressure as that of the mixture,respectively.Adsorption selectivity in a binary mixture of component 1and 2is defined asS 125x 1x 2 y 2y 1 (5)For the application of IAST to predict adsorption separa-tion selectivity,the following two conditions are necessary:good quality adsorption data of each single component;and excellent curve fitting model for such data.48,71,72In order to perform the integrations of Eqs.(1)and (2)required by IAST,the single-component isotherms should be fitted by a proper isotherm model.In practice,several meth-ods are available.In this work,it is found that the dual-site Langmuir-Freundlich (DSLF)equation can be successful to fit this set of adsorption data.The dual-site Langmuir-Freundlich model can be expressed as followsq 5q m ;13b 1p 1=n 111b 1p 11q m ;23b 2p 1=n 211b 2p 2(6)Where p is the pressure of the bulk gas at equilibrium with the adsorbed phase (kPa),q m,1,q m,2are the saturation capaci-ties of sites 1and 2(mmol/g),b 1and b 2are the affinity coefficients of sites 1and 2(1/kPa),and n 1and n 2are the deviations from an ideal homogeneous surface.Figure 10shows a comparison of the model fits and the isotherm data.It is visible that the DSLF model can be applied favorably for fitting experimental data of CO 2and N 2adsorption.Table 2presents the fitting parameters ofFigure 7.a.Isotherms of N 2on the ZIF-8sample basedon unit surface area;b.isotherms of N 2on the ED-ZIF-8sample based on unit surface area.[Color figure can be viewed in the online issue,which is available at .]Figure 8.Recycle runs of CO 2adsorption-desorptionon the ED-ZIF-8at 298K and 25bar for adsorption and 1mbar for desorption.[Color figure can be viewed in the online issue,which is available at .]2200DOI 10.1002/aicPublished on behalf of the AIChE June 2013Vol.59,No.6AIChEJournalDSLF equation as well as the correlation coefficients (R 2).Examination of the data shows that this DSLF model is able to fit the adsorption data well since the correlation coeffi-cients R 2are up to 0.9997.In this work,the equilibrium adsorption data of single component CO 2as well as N 2are available,and the DSLF model can fit the experimental isotherms of CO 2and N 2adsorption very well.Therefore,the DSLF model can be combined with the ideal adsorbed solution theory (IAST)to predict the mixture adsorption isotherms and calculate the selectivities of the two samples for CO 2/N 2adsorption.Figure 11a and 11b present,respectively,the adsorption isotherms predicted by IAST for equimolar mixtures of CO 2/N 2in the samples ZIF-8and ED-ZIF-8as a function of total bulk pressure.It can be seen that CO 2is preferentially adsorbed over N 2on the two samples because of stronger interactions between CO 2and the ZIF-8sample,and the amount adsorbed of N 2is much lower in the mixtures than that in single-component adsorption because of competition adsorption from CO 2,which adsorbs more strongly.Figure 12shows the IAST-predicted selectivities of the two samples for equimolar CO 2and N 2mixtures at 298K as a function of total bulk pressure.It can be seen that the adsorption selectivity of the two samples for CO 2/N 2dropped with an increase in the pressure.More importantly,Figure 9.H 2O adsorption isotherms on the modifiedZIF-8samples at 298K.[Color figure can be viewed in the online issue,which is available at .]Table 2.The Fitting Parameters of the Dual-site Langmuir-Freundlich Equations for the Pure Isotherms of CO 2and N 2at 298KZIF-8ED-ZIF-8CO 2N 2CO 2N 2R 20.99970.99990.99970.9999q m,1(mmol/g)27.2527.8748.8828.32q m,2(mmol/g) 2.122 1.919 4.672 1.847b 1(atm 21)0.015330.0011700.012590.001388b 2(atm 21)0.0068950.026090.029480.02504n 1 1.6000.7875 1.4040.7704n 20.32440.96340.44300.8671Figure 10.DSLF fitting of the CO 2and N 2isotherms onZIF-8and ED-ZIF-8at 298K.[Color figure can be viewed in the online issue,which is available at .]Figure11.a.The IAST -predicted isotherm forequimolar CO 2/N 2mixtures of the ZIF-8sample at 298K as a function of total bulk pressure; b.the IAST -predicted isotherm for equimolar CO 2/N 2mixtures of the ED-ZIF-8sample at 298K as a function of total bulk pressure.[Color figure can be viewed in the online issue,which is available at .]AIChE Journal June 2013Vol.59,No.6Published on behalf of the AIChE DOI 10.1002/aic2201the adsorption selectivity of CO 2/N 2on the sample ED-ZIF-8is always higher than that on the sample ZIF-8,especially in the low-pressure region.For example,at 0.1and 0.5bar,the selectivity of the sample ED-ZIF-8for CO 2/N 2were up to 23and 13.9separately,which is almost twice of those of the sample ZIF-8.Figure 13a and 13b show,respectively,the IAST-pre-dicted selectivities of the samples ZIF-8and ED-ZIF-8for CO 2/N 2at different mixture compositions and different pres-sures.It is noticed that the selectivity increases rapidly as the gas-phase mole fraction of N 2approaches unity.For example,at yN 250.9,a typical feed composition of flue gas,high selectivities are obtained.Even at yN 250.5,the selectivity of the ED-ZIF-8for CO 2/N 2is in the range of 6–24,much higher than those on the ZIF-8sample and many other MOF samples such as ZIF-7030,ZIF-6830and MOF-508b.73This property is very important since some separa-tion processes could be operated at low pressures,such as vacuum swing adsorption (VSA),which could be extremely efficient by using the sample ED-ZIF-8because its selectiv-ity increases dramatically with decreasing pressure.Ideal adsorbed solution theory (IAST)selectivity of CO 2/N 2/H 2OThe major challenge of CO 2capture from power plant flue gas wastes is the separation of CO 2/N 2.In addition,competition adsorption of water molecule must be taken into account,because these flue gas wastes are usually saturated with certain amount of water (5–7%by volume)for the industrial postcombustion processes.Thus,for real industrial use of adsorbents,the effect of water on CO 2/N 2selectivity is another crucial factor that needs to be considered and evaluated.Here,the IAST was adopted to evaluate the ter-nary mixture CO 2/N 2/H 2O adsorption on the modified ZIF-8samples.First,the experimental isotherms of water on the modified ZIF-8samples at 298K were fitted using the DSLF model.Table 3presents the fitting parameters of DSLF equation as well as the correlation coefficients.It can be seen that theDSLF model fits the H 2O adsorption on both samples very well.Second,the DSLF model was combined with the ideal adsorbed solution theory (IAST)to predict the mixture adsorption isotherms,and then calculate the selectivities of the two samples for CO 2/N 2adsorption.Figure 14shows the predicted isotherms of ternary mix-ture CO 2/N 2/H 2O on the modified ZIF-8samples at 298K.It can be observed that in comparison with the ZIF-8,after modification,the CO 2adsorption capacity of the ED-ZIF-8in the ternary mixture obviously increased,and its N 2adsorption capacity somewhat increased,which made CO 2/N 2adsorption selectivity of the ED-ZIF-8increase.More importantly,its water adsorption capacity in the ternary mix-ture became lower compared to the ZIF-8,and it was also lower than the single component water uptake.It means the competition adsorption of H 2O in the ternary mixture was weakened on the surfaces of the ED-ZIF-8sample.Figure 12.The IAST -predicted selectivity for equimolarCO 2and N 2at 298K as a function of total bulk pressure.[Color figure can be viewed in the online issue,which is available at .]Figure 13.a.The IAST predicted selectivities atdifferent mixture compositions and different pressures for the ZIF-8sample at 298K;b.the IAST predicted selectivities at different mixture compositions and different pressures for the ED-ZIF-8sample at 298K.[Color figure can be viewed in the online issue,which is available at .]2202DOI 10.1002/aicPublished on behalf of the AIChE June 2013Vol.59,No.6AIChEJournal。

第51卷第7期 辽 宁 化 工 Vol.51，No. 7 2022年7月 Liaoning Chemical Industry July，2022

收稿日期： 2022-04-21 作者简介： 刘青（1983-），女，贵州省安顺市人，讲师，硕士研究生，2011年毕业于贵州大学生态学专业，研究方向：现代分析技术、环境保护等。

金属-有机框架Ce-BTC对中性红的吸附研究 刘 青 （安顺学院 化学化工学院，贵州 安顺 561000） 摘 要： 印染废水的处理一直是当前环境污染的主要问题。其中，材料吸附是解决印染废水污染问题较为可行的方法之一，金属有机框架（MOFs）作为一类新兴的多孔材料已广泛应用于有机染料的吸附去除。以Ce(NO3)3·6H2O和均苯三甲酸(H3BTC)为原料，采用水热法制备了Ce-BTC，并应用于模

拟废水中中性红的吸附降解能力，系统研究了初始中性红溶液pH值、吸附剂用量以及吸附的时间、温度对吸附率的影响。结果表明：在50℃下用0.2 g的Ce-BTC吸附40 min，中性红吸附率为80.3%。 关 键 词：Ce-BTC；金属-有机框架；吸附性能；中性红 中图分类号：TQ424.25 文献标识码： A 文章编号： 1004-0935（2022）07-0906-04

随着城市化进程的加快，清洁的地表水已成为短缺，淡水资源的有限性，日益严重的水污染危机引起了研究人员的广泛关注。其中染料是多个行业排放的水污染物之一。染料在水中的污染由于颜色的变化及其毒性而剥夺了水质[1]。某些类型的染料含有致癌物质（如苯或萘）。如果没有有效的处理，这些水会对人类健康和生物体造成严重危害。此外，由于染料分子不会自降解且对光非常稳定，因此废水的修复带来了困难[2]。 目前处理废水中的染料常用的有膜过滤、离子交换、化学沉淀、活性吸附等方法[3-4]。在这些方法中，吸附由于其简单性和有效性而在染料去除应用中起着至关重要的作用[5]。 几十年前，多孔材料已被证实是一种很好的吸附剂，按时间顺序排列，从活性炭、沸石和 Zeotype材料等等[6-9]。多孔碳因其坚固、便宜且环保是使用最广泛的碳之一。然而，当多孔碳在低浓度下用作带电染料的吸附剂时，吸附效率低[10-11]。因此，寻找新的吸附材料是当前的趋势。近年来，金属有机骨架（MOFs），一种新型的高结晶多孔材料，已经出现并在吸附和分离方面显示出巨大的潜力[12]。MOFs 由金属离子和有机配体连接在一起形成巨大的多样性网络。与传统多孔材料相比，MOFs材料显示出一些优越的特性，包括高表面积、可设计的孔结构和通过金属离子和有机配体的变化可调节的物理化学性质[13]。由于其优异的性能，MOFs是气/液吸附和净化的有希望的候选者。肖蜜[14]等人设计并合成了Cd-MOF材料，并研究了Cd-MOF吸附剂和染料分子之间的匹配效应、尺寸排阻和电荷的相关性。X射线单晶衍射表明，Cd-MOF呈现出独特的双层螺旋配位框架，四聚体水部分容纳在开放通道中。Cd-MOF的Zeta电位表明它是一个带负电的框架，倾向于与阳离子染料颗粒进行静电相互作用，表明 Cd-MOF 对亚甲基蓝 (MB)阳离子染料具有良好的吸附能力。王文强等[15]以PVP为表面活性剂，通过简单的沉淀法成功合成了具有大表面积的普鲁士蓝类似物Zn3[Co(CN)6]2·nH2O纳米球。