Metal–organic frameworks and related materials for hydrogen purification: Interplay of pore size and pore wall polarity{ Michael Fischer, Frank Hoffmann and Michael Fro ¨ba* Received 5th December 2011, Accepted 29th February 2012 DOI: 10.1039/c2ra01239a The separation of hydrogen from other weakly adsorbing gases is a topic of high industrial relevance. Microporous materials, such as zeolites, metal–organic frameworks (MOFs), and nanoporous molecular crystals, hold much promise as adsorbent materials for adsorption-based hydrogen separation units. However, most experimental and theoretical studies that have been reported so far have focused on relatively few gas mixtures (mainly CO 2 /H 2 and CH 4 /H 2 ). In this work, the suitability of five materials (zeolite: silicalite; MOFs: Mg-formate, Zn(dtp), Cu 3 (btc) 2 ; porous molecular crystal: cucurbit[6]uril) for the adsorption-based separation of carbon monoxide/hydrogen and oxygen/ hydrogen mixtures is assessed using force-field based grand-canonical Monte Carlo simulations. The simulations are employed to predict single-component and mixture isotherms, as well as adsorption selectivities. Moreover, a detailed analysis of the solid-fluid interactions is carried out on an atomistic level. The choice of materials is motivated by their structural properties: four systems contain relatively narrow channels (diameters , 6.5 A ˚ ), but differ in pore wall composition and polarity. The fifth system possesses coordinatively unsaturated metal sites, which can act as preferential adsorption sites for some guest molecules. The role of electrostatic interactions is fundamentally different for the two mixtures considered: for CO/H 2 separation, the employment of polar adsorbents is beneficial due to the enhanced electrostatic interaction with carbon monoxide. On the contrary, an increased polarity of the pore wall tends to reduce the O 2 /H 2 selectivity, because electrostatic interactions favour hydrogen over oxygen due to its larger quadrupole moment. In general, materials with narrow channels perform best in the separation of hydrogen from weakly adsorbing species, because the dispersive interactions are maximized in the channels. Moreover, they provide little space for the co-adsorption of hydrogen. 1. Introduction Due to the potential of hydrogen as a ‘‘clean’’ energy carrier, much scientific attention has been directed towards the devel- opment and optimization of hydrogen production, storage, and utilization technologies. 1 Currently, hydrogen is mostly pro- duced by steam-reforming of methane or higher hydrocarbons. For these large-scale plants, efficient technologies have been developed to purify the produced hydrogen, making use of pressure-swing adsorption (PSA). 2 This technology relies on a preferred adsorption of the impurities over hydrogen in porous adsorbents, typically using a combination of activated carbons and zeolites in the adsorbent bed. In order to use hydrogen as an energy carrier to propel vehicles, it may be advantageous to replace the production in large-scale chemical plants by other technologies that can be implemented in smaller, decentralized units, in order to avoid the necessity to transport the produced hydrogen over large distances. 1,3 In this context, it may be necessary to develop new purification technologies that are able to deliver hydrogen of the required purity in these small-scale units. Both adsorption-based separation processes and mem- brane-based separation processes could be employed for the removal of undesired byproducts from the hydrogen feed. In the former case, the adsorption selectivity and the working capacity are key quantities that define the suitability of a material. 4,5 In the latter case, the (ideal) membrane selectivity corresponds to the product of the adsorption selectivity and the ratio of the self- diffusivities. 6,7 The following discussion will focus exclusively on the adsorption selectivity, concentrating on the usage of materials in adsorption-based processes. It should, however, be mentioned that significant advances in the preparation of membranes that could be employed in the separation of hydrogen from other gases have also been reported. 3,8–11 The development of new classes of ordered microporous materials, namely metal–organic frameworks (MOFs) 12 and nanoporous molecular crystals (NMCs), 13 has stimulated Institute of Inorganic and Applied Chemistry, Department of Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, D-20146, Hamburg, Germany. E-mail: froeba@chemie.uni-hamburg.de; Fax: (+49)-40-42838-6348; Tel: (+49)-40-42838-3100 { Electronic Supplementary Information (ESI) available. See DOI: 10.1039/c2ra01239a/ RSC Advances Dynamic Article Links Cite this: RSC Advances, 2012, 2, 4382–4396 www.rsc.org/advances PAPER 4382 | RSC Adv., 2012, 2, 4382–4396 This journal is ß The Royal Society of Chemistry 2012