16060 | Phys. Chem. Chem. Phys., 2014, 16, 16060--16066 This journal is © the Owner Societies 2014 Cite this: Phys. Chem. Chem. Phys., 2014, 16, 16060 Molecular simulation of gas adsorption and diffusion in a breathing MOF using a rigid force field† E. Garcı ´ a-Pe ´ rez,* a P. Serra-Crespo, a S. Hamad, b F. Kapteijn a and J. Gascon a Simulation of gas adsorption in flexible porous materials is still limited by the slow progress in the development of flexible force fields. Moreover, the high computational cost of such flexible force fields may be a drawback even when they are fully developed. In this work, molecular simulations of gas adsorption and diffusion of carbon dioxide and methane in NH 2 -MIL-53(Al) are carried out using a linear combination of two crystallographic structures with rigid force fields. Once the interactions of carbon dioxide molecules and the bridging hydroxyls groups of the framework are optimized, an excellent match is found for simulations and experimental data for the adsorption of methane and carbon dioxide, including the stepwise uptake due to the breathing effect. In addition, diffusivities of pure components are calculated. The pore expansion by the breathing effect influences the self-diffusion mechanism and much higher diffusivities are observed at relatively high adsorbate loadings. This work demonstrates that using a rigid force field combined with a minimum number of experiments, reproduces adsorption and simulates diffusion of carbon dioxide and methane in the flexible metal–organic framework NH 2 -MIL-53(Al). A Introduction Metal–organic frameworks (MOFs) consist of metal or metal clusters (‘‘connectors’’) linked by organic ligands (‘‘linkers’’) 1–6 forming nanoporous crystalline solid materials. The ligands act as spacers, creating an open porous structure with very high pore volume and surface area. Due to their unusual variety in terms of chemical composition, accessibility and pore dimen- sions as well as to their low densities (0.2–1 g cm À3 ) and high surface areas (500–4500 m 2 g À1 ), MOFs are considered promis- ing candidates in applications involving adsorption and separa- tion of strategic gases such as carbon dioxide, methane, and hydrogen. 3,4,6–10 The first report on gas adsorption using MOFs was pub- lished in 1997 11 and since then, MOFs have become an active field of research, resulting in numerous publications. 12–16 In case of carbon dioxide adsorption, several approaches have been followed to improve selectivity: adsorbate–surface inter- actions, flexibility/gate opening mechanisms, and cooperative effects 14 (i.e. flexibility and specific interactions) have been reported as very effective ways of improving selectivity. In view of the high selectivities, relatively low enthalpies of adsorption, and fast kinetics, MOFs are among the most promising solids for carbon dioxide capture and separation. Several authors have applied molecular simulations to study the adsorption and separation of carbon dioxide in metal– organic frameworks, i.e. Wang et al., 17 Babarao et al., 18 Keskin and Sholl, 19 Walton et al., 20 Yang and Zhong, 21–23 Calero et al., 24 Chen and Jiang, 25 Liu et al., 26 and Wells and Chaffee. 27 Also combined experiment and modeling techniques are used in work by Pera-Titus et al., 28 Lescouet et al., 29 Serra-Crespo et al., 30 Chen et al., 25 Stavitski et al., 31 Boutin et al., 32 and Montoro et al. 33 on functionalized MOFs. Although the origin of the improved separation performance in functionalized MOFs is not yet fully understood, it is clear from the above mentioned works that a delicate interplay of weak dispersion forces with framework polarity plays an important role. A special class of MOFs is formed by those whose pore dimensions may change without breaking chemical bonds within the framework. This results in special properties like the breathing effect 8,34 and the gate phenomenon 35,36 where pores contract or open during molecule adsorption. An example of a breathing type material is the MIL-53 series. MIL-53 is built from MO 6 octahedra (where M can be three-valent ions Fe 3+ , Cr 3+ , Al 3+ , Ga 3+ , In 3+ or Sc 3+ ) formed from trans bridging OH ions and the oxygens of coordinate, bridging 1,4-benzene- dicarboxylate linkers. 34 In this way a crystalline material with 1-D diamond shaped pores is formed. Amino-MIL-53(Al) 37 is a material with the topology of MIL-53. During the synthesis of a Catalysis Engineering-Chemical Engineering Department, Delft University of Technology, Julianalaan, 136, 2628 BL Delft, The Netherlands. E-mail: e.garciaperez81@gmail.com; Fax: +31 152785006; Tel: +31 152786976 b Department of Chemical, Physical, and Natural System, University Pablo de Olavide, Ctra. Utrera km. 1, 41013, Seville, Spain † Electronic supplementary information (ESI) available: The definitions of the crystallographically different atoms for the (a) np form and (b) lp form of NH 2 -MIL-53(Al). Complete set of parameters and charges for adsorbates and adsorbents. See DOI: 10.1039/c3cp55416c Received 23rd December 2013, Accepted 11th June 2014 DOI: 10.1039/c3cp55416c www.rsc.org/pccp PCCP PAPER Open Access Article. Published on 11 June 2014. Downloaded on 18/02/2016 15:24:11. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online View Journal | View Issue