Proton-Exchange Reaction in Acidic Zeolites: Mechanism and Free Energetics

Conversion of alkanes to liquid fuels and other chemicals has a great demand in many industrial applications. The key challenge in the alkane conversion is the activation of the C-H bond. Protonated zeolites are known to activate the C-H bond in alkanes. Modeling of such catalytic reactions and estimation of reaction free energies especially when entropy has a non-negligible contribution in the reactant state due to the translational motion in the pores are computationally challenging. The large size of the zeolite systems also poses difficulty in modeling such reactions. We address these problems through extensive fully relaxed hybrid quantum mechanics-and molecular mechanics-based molecular dynamics simulations and free energy calculations using the temperature-accelerated sliced sampling approach. We model a proton-exchange reaction between methane and the Bronsted acid site of zeolite at 300 K. We investigate the differential reactivity of H-ZSM-5 and H-MCM-22 zeolites toward proton exchange and probe the role of acid strength, internal structure, and entropy.