Monomolecular cracking of propane over acidic chabazite: An ab initio molecular dynamics and transition path sampling study

The monomolecular Haag–Dessau mechanism for propane cracking over acidic chabazite has been studied using dispersion-corrected periodic DFT calculations in combination with ab initio molecular dynamics (AIMD) simulations, transition path sampling (TPS), and free-energy integrations. The AIMD simulations show that due to the weak specific interaction of the saturated molecule with Brønsted acid sites, the adsorption energy is considerably reduced at elevated temperature and that only a fraction of the molecules adsorbed within the zeolite is sufficiently close to the acid site to form a reactant complex for protonation. TPS shows that the preferred reaction mechanism is the protonation of a terminal methyl group. The direct proton attack on the C–C bond between the methyl and methylene groups is not excluded but occurs with lower probability. The intrinsic reaction parameters such as free energy and entropy of activation are determined using thermodynamic integration based on constrained molecular dynamics simulations.