Available online at Recent development of in silico molecular modeling for gas and liquid separations in metal–organic frameworksJianwen JiangAs a new family of nanoporous materials,metal–organic frameworks(MOFs)are considered versatile materials for widespread applications.Majority of current studies in MOFs have been experimentally based,thus little fundamental guidance exists for the judicious screening and design of task-specific MOFs.With synergistic advances in mathematical methods,computational hardware and software,in silico molecular modeling has become an indispensable tool to unravel microscopic properties in MOFs that are otherwise experimentally inaccessible or difficult to obtain.In this article,the recent development of molecular modeling is critically highlighted for gas and liquid separations in MOFs.Bottom-up strategies have been proposed for gas separation in MOFs,particularly CO2capture.Meanwhile, interest for liquid separation in MOFs is growing and modeling is expected to provide in-depth mechanistic understanding. Despite considerable achievements,substantial challenges and new opportunities are foreseeable in more practical modeling endeavors for economically viable separationsin MOFs.AddressDepartment of Chemical and Biomolecular Engineering,National University of Singapore,117576,SingaporeCorresponding author:Jiang,Jianwen(chejj@.sg)Current Opinion in Chemical Engineering2012,1:138–144This review comes from a themed issue onNanotechnologyEdited by Hua Chun ZengAvailable online23rd December20112211-3398/$–see front matter#2011Elsevier Ltd.All rights reserved.DOI10.1016/j.coche.2011.11.002IntroductionDuring the past decade,metal–organic frameworks (MOFs)have emerged as a new family of nanoporous materials[1,2].In remarkable contrast to traditional inor-ganic zeolites,MOFs can be synthesized from various inorganic clusters and organic linkers,thus possess a wide range of surface area and pore size.More fascinatingly, the judicious selection of building blocks allows the pore volume and functionality to be tailored in a rational manner.With such salient features,MOFs are considered versatile materials for widespread potential applications [3,4]as illustrated in Figure1.Indeed,MOFs have been identified as a topical area in materials science and technology because of their implications for global and national economies[5].To date,thousands of MOFs have been synthesized in this vibrantfield and several(Cu-BTC,ZIF-8,MIL-53,etc.) are commercially available under the trade name Basoli-te TM[6].However,massive research efforts on MOFs have been primarily based on experiments.It is impractical to search for task-specific MOFs by trial-and-error from infinitely large number of possible candidates.Therefore, quantitative guidelines are desired for the high-throughput screening of enormous MOFs and the rational design of new MOFs towards practical applications.In this context, clear and deep microscopic understanding from a molecu-lar level is indispensable.With synergistic advances in mathematical methods,computational hardware and soft-ware,in silico molecular modeling has played an increas-ingly important role in unraveling microscopic properties in MOFs[7 ,8 ,9 ].Sophisticated modeling and simu-lation provide molecular insights that are experimentally intractable,if not impossible,thus elucidate underlying physics from bottom-up.Among many potential appli-cations of MOFs,separations are of central importance in chemical industry and have been actively investigated [10].In this article,the recent development of molecular modeling is critically highlighted for both gas and liquid separations in MOFs,and the foreseeable challenges and opportunities are discussed.Gas separationThe overwhelming majority of studies for gas separation in MOFs have been focused on CO2capture.This is because the combustion of fossil fuels produces a huge quantity of CO2emissions into the atmosphere.Carbon capture and sequestration is crucial to environmental protection and sustainable economy.As an essential pre-requisite,CO2has to be captured fromflue gas/ shifted syngas in post-/pre-combustion processes. Another important gas separation involving CO2is puri-fication of natural gas,in which impurities such as CO2 need to be separated to enhance calorie content.MOF adsorbentsMost synthesized MOFs are crystallites and tested as adsorbents for gas separation.Several reviews have sum-marized numerous experimental studies for CO2capture in MOF adsorbents[11–13].Nevertheless,nearly all these experiments examined the adsorption of pure gases (e.g.CO2,N2,CH4,and H2)due to the formidable difficulty associated with mixtures.By contrast,simu-lation can be readily used for single or multi-componentsystems.Thus,quantitative understanding of mixture adsorption in MOFs has been obtained,to a large extent,from simulation studies.Several bottom-up strategies as illustrated in Figure 2have been proposed to tune CO 2capture performance,for example,using specific MOFs with small pores,catenation,functionalization,ionic fra-meworks,exposed metals or metal doping.Yang and Zhong [14]simulated the adsorption of CO 2/CH 4/H 2mixture in two MOFs (IRMOF-1and Cu-BTC)and found pore size strongly affects separation efficiency.However,IRMOF-1and Cu-BTC do not possess iden-tical topology,leading to ambiguous interplay with the effect of pore size.In this regard,Babarao et al.[15]examined the separation of CO 2/CH 4mixture in isostruc-tural MOFs (Cu-BTC and PCN-60)and observed that the selectivity in Cu-BTC with small pores is nearly twice of that in PCN-60.This strategy of small pores is also reflected in framework catenation that can induce con-stricted pores and greater potential overlaps.For example,catenated IRMOF-13and PCN-6exhibit a larger selectivity for CO 2/CH 4mixture than non-cate-nated counterparts [15].An appealing strategy is to use ionic MOFs as demonstrated by Jiang and co-workers [16,17 ,18]for the separation of CO 2-containing mixtures.Simulation reveals that CO 2molecules are strongly adsorbed onto the ionic frameworks and nonframeworkions,and the predicted selectivity is significantly higher than in neutral MOFs and many other nanoporous materials.On the other hand,Yazaydin et al.[19]screened a diverse set of 14MOFs for low-pressure CO 2capture from flue gas combining simulation and experiment.The results show that M/DOBDC (M =Zn,Mg,Co or Ni)with high density of exposed metals strongly interact with CO 2.By physical and chemical doping,Xu et al.[20 ]estimated the separation of CO 2/CH 4mixtures in Li-modified MOF-5.Owing to the enhancement of electro-static potentials,adsorption selectivity was predicted to be much higher than in MOF-5.In a separate study,Lan et al.[21 ]simulated CO 2capture in covalent-organic frameworks doped by alkali,alkaline-earth and transition metals,and concluded that Li is the best surface modifier for CO 2capture.The strategies outlined in Figure 2have been compre-hensively discussed [24 ,25 ].Two of them (ionic fra-meworks and metal doping)appear to be more efficient to enhance CO 2capture.It should be noted that these strategies also can tune the separation of other mixtures,for example,the selectivity of alkane isomers was found to be enhanced by framework catenation [26].In a recent perspective,Krishna and van Baten [27 ]highlighted the potency of simulation in screening of best MOFs for CO 2capture and hydrocarbon separation,and they furtherRecent development of in silico molecular modeling Jiang 139Figure 1Purification Toxics RemovalDrug DeliveryFuel Cell SystemsStorageStorage and SeparationCarbon SequestrationSensingMOPWidespread potential applications of MOFs (/ees6/clathrates/index.shtml ).compared MOFs against traditional zeolites with regard to separation characteristics.As an alternative to simulation,analytical theories have been developed for gas separation in MOFs.Liu et al.[28,29]proposed a density functional theory (DFT)in 3D-nanoconfined space.The theory was applied to adsorption and separation in 3D-MOFs with complex pore networks,whereas most DFT studies are limited in simple confined geometries (e.g.slit and cylindrical pores).Good agree-ment was obtained between theoretical predictions,simu-lation and experimental data.Coudert et al.[30]developed the osmotic framework adsorbed solution theory (OFAST)in terms of a competition between host’s free energy and adsorption energy.This theory is based exclusively on pure-component adsorption and has the superior capability to describe flexible MOFs.For illustration,the authors used the OFAST to examine the effect of breathing on separation of CO 2/CH 4mixtures in MIL-53.The modeling studies discussed above for gas separation in MOF adsorbents are primarily focused on adsorption selectivity.However,several other factors (e.g.working capacity,regenerability,etc.)should be included in prac-tice as discussed by Bae and Snurr [31 ].Another crucial issue is how moisture in gas mixtures would affect sep-aration performance?From systematical simulation stu-dies in various neutral and ionic MOFs,Jiang and coworkers observed four different intriguing effects ofH 2O on CO 2capture [25 ].It is also instructive to examine structural change in flexible MOFs that might occur upon adsorption [32].The incorporation of flexi-bility to simulate structural change would need a robust force field.However,a general force field is currently unavailable for MOFs and first-principles modeling is expected to play a pivotal role [33 ].In addition,the chemical and thermal stability of MOFs are important for separation [34].A large number of MOFs are unstable in atmosphere or under moisture,which impedes their util-ization.Therefore,it is indispensable to develop molecu-lar guidelines for the design of stable MOFs.Nevertheless,unraveling what govern the stability of MOFs at a microscopic level is a challenge.MOF membranesCompared with adsorptive separation,membrane-based separation is considered to be energetically more effi-cient,lower capital cost and larger separation capability.However,the fabrication of MOF membranes is a for-midable task [35].Only in recent years,have there been active experimental endeavors to explore MOF mem-branes for gas separation [36 ].Since both equilibrium and dynamic properties are required,simulation for gas separation in MOF mem-branes is more time-consuming than in MOF adsorbents.Nevertheless,a handful of simulation studies have been reported.Keskin and Sholl [37 ]examined the separation140NanotechnologyFigure 2functionalizationmetal dopingionic frameworksexposed metalssmall porescatenationBottom-up strategies to tune CO 2capture performance.The representative MOFs are from [15,17 ,20 ,22,23].performance of diverse MOFs for CO2/CH4and CO2/H2 mixtures.They found that all the MOFs examined exhi-bit unfavorably low CO2selectivities and mixture effects play a crucial role in determining the performance.By combining simulation and IR microscopy,Bux et al.[38] simulated ethene/ethane separation in ZIF-8membrane. They found that ethane adsorbs more strongly than ethene,but ethene diffuses faster;and the interplay results in a membrane permeation selectivity for ethene. Krishna and van Baten[27 ]underlined the advantages of using simulation tools in the screening of MOF mem-branes for CO2capture.Along with considerable interest in MOF membranes, MOF-based composite membranes have received increasing attention for gas separation.In this emerging area,a handful of experiments have been conducted[39], but modeling studies are ing atomistic simulation and continuum model,Keskin and Sholl[40]attempted to select MOF/polymer membranes for high-perform-ance gas separation.A highly selective MOF was ident-ified and predicted to enhance the performance of Matrimid and other polymers for CO2/CH4separation. Chen et al.[41]proposed a composite with ionic liquid (IL)supported on IRMOF-1.The simulation reveals that ions in the composite act as favorable sites for CO2adsorption,and the selectivity for CO2/N2mixture is higher than in neat IL,IRMOF-1and many other supported IL membranes.It is worthwhile to note that defects and inter-crystalline interstices usually exist in synthesized MOF membranes. Nevertheless,most simulation studies use perfect and rigid models for MOF membranes.How to incorporate defects and interstices into practical modeling is challenging.On the other hand,theflexibility of MOF structures may have a larger influence in membrane separation than adsorbent separation[36 ],and should be implemented as well into modeling.Another essential issue is the mech-anical properties of MOFs[42].The high pressure exerted for membrane separation may distort MOF structures and deteriorate performance.It is thus crucial to quantitatively understand how pressure affects pore geometries and framework dimensionalities.For MOF-based composite membranes,microscopic insights into the interactions between MOF and other species(e.g.polymer or ionic liquid)are strikingly important and fundamental studies at a molecular level are desired.Liquid separationWhile gas separation in MOFs has been extensively investigated,endeavors for liquid separation are lagged behind[43 ].A recent trend has been to explore the use of MOF adsorbents and membranes for liquid separation. By combining chromatographic and breakthrough exper-iments,Alaerts et al.determined the adsorption and separation of ortho-substituted alkylaromatics(xylenes, ethylbenzene,ethyltoluenes and cymenes)in a column packed with MIL-53crystallites[44].Jin and coworkers tested the separation of water/organics mixtures in MIL-53membrane and observed a high selectivity for water removal from ethyl acetate solution[45].Simulation for liquid separation in MOFs is scarce owing to the significant amount of computational time required to sample liquid phase.Consequently,the microscopic understanding of liquid separation in MOFs is far from complete.To the best of our knowledge,only two simu-lation studies have been reported in this area,one for water desalination and the other for biofuel purification. Recent development of in silico molecular modeling Jiang141Figure3Selectivities of biofuel in Na-rho-ZMOF and Zn4O(bdc)(bpz)2by pervaporation[47 ].Specifically,Hu et al.[46]performed simulation on the desalination of NaCl aqueous solution through a ZIF-8 membrane by reverse osmosis.Because of the sieving effect of small apertures in ZIF-8,Na+and ClÀions could not transport through ZIF-8membrane and water desa-lination was observed.Theflux of water permeating the membrane was found to scale linearly with external pressure.In a separate study,Nalaparaju et al.[47 ] examined hydrophilic Na-rho-ZMOF and hydrophobic Zn4O(bdc)(bpz)2for biofuel purification.The selectiv-ities between water and ethanol in the two MOFs are largely determined by adsorption behavior.As indicated in Figure3,Na-rho-ZMOF is preferable to remove water, whereas Zn4O(bdc)(bpz)2is promising to enrich ethanol. The simulation provides molecular guidelines for the selection of appropriate MOFs towards efficient biofuel purification.Currently,modeling for liquid separation in MOFs is very limited.With increasing demands for clean water,liquid fuels and other liquid-based applications,more efforts are expected in order to provide deep molecular insights.A pre-requisite for liquid separation is that the MOFs used should be stable in water or other liquids[48],it is crucial to understand what factors govern the stability of MOFs, which would allow to produce stable MOFs for liquid separation.ConclusionAs a burgeoningfield,research activities in MOFs are rather hectic.In addition to enormous experimental stu-dies,we have witnessed the recent development of in silico molecular modeling for MOFs.Microscopic under-standing has been achieved for gas separation particularly CO2capture in MOFs,and bottom-up strategies have been proposed to enhance separation efficiency.How-ever,liquid separation in MOFs remains largely unex-plored at a molecular level and more endeavors are desired towards this end.It is obvious that current molecular modeling for separ-ations using MOFs is still in an infant stage.As discussed above,substantial challenges are foreseen for more prac-tical modeling and precise description.A number of issues should be considered in future modeling,such as the stability and mechanical properties of MOFs, structuralflexibility,material regenerability,and effect of moisture(in gas separation).These challenges provide new opportunities for modeling studies to unravel in-depth microscopic insights and thus provide quantitative guidelines on the rational screening and design of novel MOFs.Furthermore,for energy-efficient and cost-effec-tive separations,process requirements are essential to be integrated with material properties at a system level.In this context,molecular modeling,process optimization, as well as material synthesis,should be synergized holi-stically towards 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