Monte Carlo simulations on the relation between the structure and properties of zeolites: the adsorption of small hydrocarbons

Abstract

In order to understand the high selectivities observed in adsorption processes and catalytic conversions of hydrocarbons using zeolites, it is necessary to study the behavior of molecules inside the pores of a zeolite. This kind of information is difficult to obtain from direct experiments. We report Monte Carlo computer simulations on the adsorption of methane in the zeolites faujasite, mordenite, and ZSM-5. For all compounds our simulations yield good agreement with the available experimental data on the heat of adsorption. In addition, the present calculations predict that the heat of adsorption of methane in mordenite should decrease steeply with increasing Al/Si ratio. Analysis of the distribution of CHI in mordenite suggests a simple explanation for this peculiar effect, which may appear to be of great importance for the catalytic activity of mordenite.Living organisms, employing enzymes, can synthesize molecules with essentially 100% selectivity. Compared to this elegance, chemical reactions in the process industries are rather crude ways of producing useful chemicals. However, recent improvements in the design of zeolite catalysts suggest that much higher selectivities may be achieved in the near future.'Zeolites are microporous crystalline materials usually consisting of silicon, aluminum, oxygen, and sodium. Certain positions on the inner walls of the micropores appear to behave as active sites, where catalytic conversions can take place. Selectivity can be obtained by adjusting the size of the micropores or cavities and by modifying the location of the active sites in such a way that, ideally, only one type of molecular species can reach these active sites. Of course, the performance of currently available zeolites is still far removed from this "enzyme-like" selectivity, but it is clear that a better understanding of the microscopic basis for the catalytic action of zeolites may help us approach this ultimate goal of zeolite chemistry. One of the first factors that must be understood is the relation between the molecular structure of a zeolite and the adsorption of specific molecules. One particularly promising tool to obtain such information is computer simulation .2-J Recently, Yashonath et aL2 have reported a simulation study on the adsorption of methane in faujasite, with emphasis on the influence of the temperature on the adsorption. In this work we have used the methods pioneered by Yashonath et aL2 to gain a better understanding of the effect of the chemical composition of mordenite, which is indicated by the Al/Si ratio, on the adsorption process. The latter parameter is known to have a pronounced effect on the catalytic activity of zeolitese6We have carried out simulations at low coverage of methane (zero filling). This allows us to ignore methane-methane interactions. In our simulations (cf. we assume that 3 , can be written as the sum of a Lennard-Jones potential and a Coulombic interaction where qi is the charge of atom i and rij the distance between the atoms i and j . The values of A...

Adsorption of Isobutene on NanZSM-5 Type Zeolite with Various Si/Al Ratios: Molecular Simulation Study

“…The optimized structure parameters of Na n ZSM-5 type zeolite with various Si/Al ratios used in this work are listed in Table 2. Along with those in literature for comparison, the bond length values were consistent with those predicted by molecular dynamic calculation 19 and ab initio cluster calculations, 23 and the atomic charges on Na, O, Si, Al were in agreement with those used by Smit, 24 Demontis, 25 and Klemm.…”

“…The structure parameters of these Na n ZSM-5 type zeolites were consistent with those predicted by molecular dynamic calculation 19 and ab initio cluster calculations. [23][24][25] Our simulation results indicate that the charges of Si atoms were shifted to higher values when the degree of Na exchange was increased in Na n ZSM-5. Then, the adsorption of isobutene on Na n ZSM-5 zeolite with various Si/Al ratios has been investigated by grand canonical Monte Carlo simulation and Cvff-300-1.01 forcefield.…”

Zeolite microporosity studied by molecular simulation

“…Tetrahedral atoms, e.g. Si, Al, are not considered as for most zeolites they are well screened by adjacent oxygen atoms [37]. (3) The diameter of the test particle is recorded for all grid points that are inside the test particle.…”

Simulating the Behavior of Organic Molecules in Zeolites

“…In practice the necessary configurational integrals are performed numerically using the Metropolis (27) importance sampling scheme. Such procedures have been extensively employed in the simulation of zeolite-sorbate interactions (28)(29)(30)(31)(32)(33). Monte Carlo procedures within the Grand Canonical ensemble also provide a means of estimating the loading favored by zeolites interacting with potential sorbates at a particular temperature and pressure.…”