Pore Size Dependence of Adsorption and Separation of Thiophene/Benzene Mixtures in Zeolites

Grand canonical ensemble Monte Carlo simulations were applied to study the adsorption and separation of thiophene from binary and ternary mixtures of benzene and n-alkanes in nine types of zeolites. Adsorption of thiophene is important for removing sulfur from gasoline. The computed pure component adsorption isotherms are in good agreement with experimental results for FAU zeolites. The calculated Q_st for thiophene in FAU were found to be 33-46 kJ/mol depending on uptake, in good agreement with the experimental data. The adsorption heats as a function of zeolite pore diameter suggest high thiophene affinity when the pore diameter is between 4.6 and 5 Å. Binary mixture simulations at two different compositions show that FER is very selective toward thiophene over benzene at 373 K. The thiophene selectivity shows a significant dependence on pore size in binary mixtures, and the smaller thiophene molecule can replace benzene at elevated pressures due to size entropy effects for some zeolites. For ternary mixtures of thiophene, benzene, and alkanes, the results show that the selectivity is low in AET, MWW, BOG, and FAU zeolites, while the presence of n-hexane has insignificant effects on the thiophene selectivity in FER and AEL zeolites.