The role of spatial constraints and entropy in the adsorption and transformation of hydrocarbons catalyzed by zeolites

Abstract

The competing influences of enthalpy and entropy on the adsorption and transformation of hydrocarbon molecules in zeolites have been investigated using dispersion-corrected density-functional theory in combination with advanced statistical-mechanical techniques. At the example of propane in protonated mordenite, it is demonstrated that while enthalpy favors adsorption in the narrower side pockets (SP) due to the stronger interaction with the framework, the loss of entropy is smaller for molecules in the wider main channel (MC). At ambient and elevated temperatures, the free energy favors adsorption in the MC (in agreement with experiment) and diffusion to the SP is an activated process. On the other hand, the free energy of activation for monomolecular cracking is lower for Brønsted acid (BA) sites in the SP, if the reactant is already located there. Cracking at a BA in the MC is a simple one-step reaction but as the SP is accessible to molecules only via the MC, cracking at a BA site in the SP is possible only after overcoming the barrier for diffusion from the MC to the SP. Thus, the difference in the free energies of adsorption in the MC and the SP increases the effective free-energy of activation for the reaction in SP and the reaction in the MC is favored. This result contradicts the interpretation of recent experiments on hydrocarbon transformations catalyzed by mordenite-containing BA sites and Na+ counterions in varying proportions. This experimental interpretation is based on the assumption that Na+ counterions preferentially replace BA sites located in the SP. This assumption has been critically examined using ab-initio calculations and found to be inconsistent with our theoretical predictions. It is demonstrated that the experimentally observed decrease of the reaction rate with increasing Na+/H+ ratio arises from the strongly attractive nature of the Na+ counterions, which makes the approach of the reactant to the BA site more difficult and reduces the reaction rate. We suggest that our results on the competing influence of enthalpy and entropy arising from the confinement of the reactant in cavities of different diameters have general validity for the adsorption and acid-catalyzed reactions of hydrocarbon molecules in zeolites with a complex framework structure.