DOI: 10.1002/cphc.201000126 New Microporous Materials for Acetylene Storage and C 2 H 2 /CO 2 Separation: Insights from Molecular Simulations Michael Fischer, Frank Hoffmann, and Michael Frçba* [a] 1. Introduction As a novel class of inorganic–organic hybrid materials, metal– organic frameworks (MOFs) have been proposed for numerous applications which are related to their permanent microporosi- ty, such as gas storage [1, 2] and separation, [3] catalysis, [4–6] and chemical sensing. [7] Most studies in the field of gas storage and separation in MOFs have focused on gases related to energy applications, such as storage of hydrogen [2, 8–10] or methane [11–13] and purification of these gases by adsorption separation, [14–18] or to environmental problems, for example, carbon dioxide storage. [19–22] Recently, a few studies have assessed the storage capacity of MOFs for acetylene, C 2 H 2 , [23–30] which is technologi- cally important due to the low compression limit of acetylene at room temperature (pure C 2 H 2 will explode at pressures above 2 bar). Owing to the similar fluid properties of acetylene and CO 2 , an efficient separation of C 2 H 2 /CO 2 mixtures is anoth- er technologically interesting issue, and there have been some attempts to assess the potential of MOFs in this field. [23, 24, 28] In addition to MOFs, the acetylene storage capacities of several other types of porous materials have been investigated, among them zeolite type A, [31] mesoporous silica SBA-15, [32] carbon molecular sieves, [33] and the porous organic molecular crystal cucurbit[6]uril. [34] Molecular simulations have been widely employed to study the adsorption of gases such as H 2 , CH 4 , or CO 2 , and separation of binary mixtures of CH 4 /CO 2 or alkanes in MOFs. Several re- views covering these topics have appeared recently. [35–39] While molecular dynamic methods have been used to elucidate the diffusion mechanism of C 2 H 2 in Na Y zeolite, [40, 41] no computa- tional investigations on acetylene sorption or C 2 H 2 /CO 2 separa- tion in novel materials, such as MOFs or organic porous materi- als, have been reported so far. Herein, the acetylene adsorption and C 2 H 2 /CO 2 separation properties of three different materials are studied by using force-field-based molecular simulations. The compounds under consideration are the metal–organic frameworks magnesium formate and Cu 3 (btc) 2 (btc = 1,3,5-benzenetricarboxylate) and the organic porous crystal cucurbit[6]uril (CB[6]). These materi- als were chosen for the following reasons: First, the materials can be synthesized from rather inexpensive chemicals, which is of importance for large-scale applications. Secondly, X-ray dif- fraction experiments on acetylene-loaded samples show that the C 2 H 2 adsorption does not induce significant distortions of the framework, whereas pronounced structural changes have been reported for other potentially interesting com- pounds. [25, 27, 28] The inclusion of these effects in the simulation would require a more elaborate approach. Finally, experimental C 2 H 2 adsorption data are available for all compounds, allowing for a direct validation of the simulation approach. Grand-canonical Monte Carlo (GCMC) simulations were em- ployed to obtain single-component C 2 H 2 and CO 2 adsorption isotherms and isosteric heats of adsorption, which are critically [a] M. Fischer, Dr. F. Hoffmann, Prof. Dr. M. Frçba Institute of Inorganic and Applied Chemistry University of Hamburg Martin-Luther-King-Platz 6, 20146 Hamburg (Germany) Fax: (+ 49) 40-42838-6348 E-mail : froeba@chemie.uni-hamburg.de Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201000126. Force-field based grand-canonical Monte Carlo simulations are used to investigate the acetylene and carbon dioxide uptake capacity, as well as the C 2 H 2 /CO 2 adsorption selectivity of three novel microporous materials: Magnesium formate, Cu 3 (btc) 2 , and cucurbit[6]uril. Because no comparable computational studies of acetylene adsorption have been reported so far, the study focuses on systems for which experimental data are available to permit a thorough validation of the simulation re- sults. The results for magnesium formate are in excellent agreement with experiment. The simulation predicts a high se- lectivity for acetylene over CO 2 , which can be understood from a detailed analysis of the structural features that determine the affinity of Mg-formate towards C 2 H 2 . For Cu 3 (btc) 2 , preliminary calculations reveal the necessity to include the interaction of the sorbate molecules with the unsaturated metal sites, which is done by means of a parameter adjustment based on ab- initio calculations. In spite of the high C 2 H 2 storage capacity, the C 2 H 2 /CO 2 selectivity of this material is very modest. The simulation results for the porous organic crystal cucurbit[6]uril show that the adsorption characteristics that have been ob- served experimentally, particularly the very high isosteric heat of adsorption, cannot be understood when an ideal structure is assumed. It is postulated that structural imperfections play a key role in determining the C 2 H 2 adsorption behavior of this material, and this proposition is supported by additional calcu- lations. 2220  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemPhysChem 2010, 11, 2220 – 2229