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Energetics of n-alkanes in zeolites : a configurational-bias Monte Carlo investigation into pore size dependence

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Energetics of n-Alkanes in Zeolites: A Configurational-Bias Monte Carlo Investigation into Pore Size Dependence Simon P. Bates,*,† Willy J. M. van Well,† Rutger A. van Santen,† and Berend Smit‡ Contribution from the Schuit Institute of Catalysis, Laboratory for Inorganic Chemistry & Catalysis, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands, and Shell International Oil Products, B.V., Shell Research and Technology Centre, Amsterdam, P.O. Box 38000, 1030 BN Amsterdam, The Netherlands ReceiVed NoVember 17, 1995. ReVised Manuscript ReceiVed May 8, 1996X Abstract: A recently-developed Monte Carlo method is used to simulate the energetics of n-alkanes from butane to decane in a variety of different all-silica zeolite structures (MFI, MOR, FAU, RHO, LTA, and FER). Where possible, the predicted values of the heat of adsorption are compared to experimental data and are generally found to be in good agreement. On the basis of the energetic data, the graphs of heat of adsorption as a function of mean pore diameter appear to show a maximum between 4 and 5 Å. However, close inspection of the location and conformation of the alkanes in small pore zeolites reveals that the molecules adopt highly coiled conformations localized exclusively in regions of maximum void volume. In the case of the small pore zeolites studied heresRHO and LTAsthese maximum void volumes are the R-cages and the alkanes “feel” a larger pore diameter than that generally used to characterize the zeolite (that of the channels). It is necessary to obtain information on the location and conformations of sorbed molecules to fully understand the trends in the heat of adsorption as a function of pore diameter. 1. Introduction Zeolites are highly crystalline inorganic materials composed of corner-sharing AlO4 and SiO4 tetrahedra. The linking of these tetrahedra to form extended three-dimensional structures naturally gives rise to a series of windows, channels, or cages within the zeolite. This porosity me

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