Adsorption and Separation of Noble Gases by IRMOF-1: Grand Canonical Monte Carlo Simulations Jeffery A. Greathouse,* ,† Tiffany L. Kinnibrugh, †,‡ and Mark D. Allendorf § Geochemistry Department, Sandia National Laboratories, Albuquerque, New Mexico 87185-0754, and Microﬂuidics Department, Sandia National Laboratories, LiVermore, California 94551 The gas storage capacity of metal-organic frameworks (MOFs) is well-known and has been investigated using both experimental and computational methods. Previous Monte Carlo computer simulations of gas adsorption by MOFs have made several questionable approximations regarding framework-framework and framework-adsorbate interactions: potential parameters from general force ﬁelds have been used, and framework atoms were ﬁxed at their crystallographic coordinates (rigid framework). We assess the validity of these approximations with grand canonical Monte Carlo simulations for a well-known Zn-based MOF (IRMOF-1), using potential parameters speciﬁcally derived for IRMOF-1. Our approach is validated by comparison with experimental results for hydrogen and xenon adsorption at room temperature. The effects of framework ﬂexibility on the adsorption of noble gases and hydrogen are described, as well as the selectivity of IRMOF-1 for xenon versus other noble gases. At both low temperature (78 K) and room temperature, little difference in gas adsorption is seen between the rigid and ﬂexible force ﬁelds. Experimental trends of noble gas inﬂation curves are also matched by the simulation results. Additionally, we show that IRMOF-1 selectively adsorbs Xe atoms in Xe/Kr and Xe/Ar mixtures, and this preference correlates with the trend in van der Waals parameters for the adsorbate atoms. Introduction Metal-organic frameworks (MOFs) are now well-established nanoporous materials and are attracting considerable attention with respect to storage of gases such as H 2 , CH 4 , and CO 2 . 1-6 The science of MOF-based chemical separations is less well developed, but their tailorable pore sizes and pore chemistry make them attractive for both membrane- and molecular-sieve separations. 4,7-10 Several examples of MOFs demonstrating molecular separation potential have been reported, including separation or preferential adsorption of linear vs branched hydrocarbons, 11,12 xylenes, 13 benzene, 14 alcohols, 15,16 CO 2 , 17-19 and H 2 . 18-21 The puriﬁcation of rare gases is also a problem of industrial interest, since there are both medical and lighting applications of xenon. Mueller et al. measured the volume- speciﬁc uptake of rare gases by IRMOF-1, showing that this MOF exhibits preferential adsorption of xenon over the lighter rare gases. 22 In addition, they found that Cu-BTC (also known as HKUST-1) can be used to separate xenon from krypton, with a calculated capacity of more than 60 wt %, exceeding high- surface-area carbons. Although chemical intuition built upon expanded knowledge of the geometric aspects of MOF coordination chemistry developed over the past several years has led to an explosion of new MOFs, it is still largely unclear how MOF structure and pore chemistry inﬂuence the resulting gas adsorption properties. Theory is now beginning to play a constructive role in guiding and accelerating synthetic efforts. Several recent investigations focused on identifying design criteria for gas storage, 1,23 and separations 24,25 using MOFs have been pub- lished. In particular, Keskin and Sholl (KS) showed that mixture effects play a very important role in determining the separation efﬁciency of MOF-5 but nevertheless were able to combine single-component adsorption isotherms with a thermodynamic theory to predict membrane separation ability. 24 This is not always true, however. Karra and Walton (KW) modeled gas sorption in the MOF Cu-BTC 25 and predicted from grand canonical Monte Carlo (GCMC) calculations that mixture selectivities are consistent with pure-component gas isotherms. KS obtained good agreement with experimental single- component isotherms using a rigid force ﬁeld. Computational methods employing force ﬁelds are powerful tools for assessing the importance of such effects. In particular, GCMC simulations have been used extensively to predict the gas adsorption properties of MOFs, including those containing aluminum, 26,27 zinc, 1,23,28-37 copper, 38-43 cobalt, 44 and vanadium. 27 Standard potentials, such as the DREIDING, 45 OPLS, 46 and UFF 47 were used in these atomistic simulations thus far, and in all cases, the positions of the MOF atoms were ﬁxed. Numerous investigations make clear, however, that MOFs are structurally ﬂexible and can exhibit substantial changes in unit cell parameters upon adsorption or desorption of guest molecules. Many examples of structural changes in MOFs upon adsorption of guest species are known, 48,49 and ﬂexible structures have been designed and synthesized to enhance selectivity for a particular guest. 14,21,50 Recently, Allendorf et al. 51 showed that detectable stresses are induced at an interface between a Cu-BTC ﬁlm and a microcantilever as a result of the adsorption of CO 2 and alcohols. This suggests that gas adsorption can induce small structural changes in MOFs, supporting the notion that a ﬂexible force ﬁeld approach could be useful. Fixed-atom force ﬁelds obviously cannot capture these effects. A recent review article points to the need for additional simulations on adsorbate mobility to assess the effects of framework ﬂexibility, which has implications for the ability of MOFs to separate one gas from another. 52 To assess the importance of structural ﬂexibility in the modeling of MOF adsorption properties, as well as the potential * To whom correspondence should be addressed. E-mail: jagreat@sandia.gov. † Geochemistry Department, Sandia National Laboratories. ‡ Present Address: Department of Chemistry, Texas A&M University, P. O. Box 30012, College Station, Texas 77842. § Microﬂuidics Department, Sandia National Laboratories. Ind. Eng. Chem. Res. 2009, 48, 3425–3431 3425 10.1021/ie801294n CCC: $40.75  2009 American Chemical Society Published on Web 03/09/2009