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

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 Angstrom. 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 here-RHO and LTA-these maximum void volumes are the alpha-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.