Dehydrogenation of propane over Znsingle bondMOR. Static and dynamic reaction energy diagram

The dehydrogenation of propane over Zn2+-exchanged mordenite has been studied theoretically using ab initio density-functional calculations at different levels of theory. We compare (i) total-energy calculations based on semilocal exchange-correlation functionals with those adding semi-empirical corrections for dispersion forces, (ii) calculations based on a large periodic model of the zeolite with calculations based on small and large finite cluster models, and (iii) calculations of the free energies of activation and of the reaction rates based on harmonic transition state theory (hTST) with those based on thermodynamic integration over free-energy gradients determined by constrained ab initio molecular dynamics. Dehydrogenation proceeds in four steps: (i) adsorption of propane on the Zn2+ cation, (ii) dissociation of a hydrogen atom leading to the formation of a Zn-propyl complex and a Brønsted acid site, (iii) reaction of the acid proton and a β–H atom of propyl, resulting in the elimination of a hydrogen molecule, and (iv) desorption of propene from the Zn2+ cation.