Zeolite adsorption, simulations methane

Yashonath etal. (46) used a Metropolis Monte Carlo method to simulate the infinite-dilution adsorption of methane in NaY zeolite. The lattice had a Si/Al ratio of 3.0 and was treated as rigid, whereas methane was modeled... 

Adsorption isotherms of methane in silicalite have also been predicted in a number of calculation studies (62, 155, 156). Goodbody et al. (62) predicted a heat of adsorption of 18 kJ/mol and simulated the adsorption isotherm up to 650 bar. From the adsorption isotherm, they found that the sinusoidal pore volume contains more methane molecules at all pressures. Snurr et al. (155) performed GC-MC and MD simulations over a wide range of occupancies at several temperatures. The intermolecular zeolite-methane potential parameters were taken from previous MD studies (11, 87) and the methane-methane parameters from MD simulations were adjusted to fit experimental results for liquid methane (157). Electrostatic contributions were neglected on account of the all-silica framework, and methane was represented by a rigid, five-center model. 

Shah M, Tsapatsis M, Siepmann JI, Ilja J (2015) Monte carlo simulations probing the adsorptive separation of hydrogen sulfide/methane mixtures using all-silica zeolites. Langmuir. Ahead of Print... 

Let us illustrate the above with a specific example. Although this example is strongly application driven, the conclusions are equally valid for more fundamental problems in simulations of complex fluids. There is considerable interest in studying the adsorption of alkanes in the pores of a zeolite. Zeolites are microporous materials which are used as catalysts in petrochemical applications (see Zeolites Applications of Computational Methods). A prerequisite for an understanding of the catalytic activity of these zeolites is a knowledge of the behavior of the molecules adsorbed in their narrow pores. Since this type of information is extremely difficult to obtain experimentally, simulations appear to be an attractive alternative. Indeed, over the last decade many simulation studies on the behavior of molecules in zeolites have been published (for a review see Ref. 3). A more careful look at these studies reveals that most simulations concern the adsorption of noble gases or methane, only a few studies of ethane or propane have been published. In petrochemical applications of zeolites, however, we are interested in the behavior of much longer alkanes such as octane and decane. 

Again, utilization of this type of unnatural Monte Carlo move turns out to be limited to small molecules. For example, Goodbody et al. have used this Monte Carlo trick to determine the adsorption isotherms of methane in a zeolite. In such a simulation one can observe that out of the 1000 attempts to move a methane molecule to a random position in the zeolite-999 attempts will be rejected because the methane molecule overlaps with a zeolite atom. If we were to perform a similar move with an ethane molecule, we would need 1000 x 1000 attempts to have one that was successful. Clearly, this random insertion scheme will break down for any but the smallest alkanes. 

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