Development and application of a ReaxFF reactive force field for hydrocarbon cracking on an aluminiosilicate zeolite catalyst

A ReaxFF reactive force field method was used to investigate catalytic cracking of hydrocarbons inside HZSM-5 zeolite. The force field parameters for Al/Si/O interactions were trained against a DFT data set consisting of equations of states and heats of formation of bulk zeolite crystal phases. The accuracy of trained force field parameters was tested by comparing ReaxFF activation energies with QM calculations for the most important chemical reactions that occur during catalytic cracking of alkanes and alkenes. Before simulating hydrocarbon cracking reactions at elevated temperatures, MD calculations were performed to investigate the effect of temperature and dopants on the stability and acidity of HZSM-5 zeolite. The simulations indicate that iron and aluminum doped HZSM-5 zeolite is stable below 3500K on MD time-scale. Beyond 3500K, the framework starts melting; this melting process is characterized by sudden inward collapse of the framework. The acidity of the framework is temperature dependent and heating HZSM-5 zeolite above 1000K can significantly alter the hydrocarbon cracking chemistry. Adsorption energies of linear alkanes inside the pores of HZSM-5 zeolite were evaluated using MD-base energy minimizer. The current set of force field parameters accurately predict that adsorption energy increases with the increase in the alkane chain length. MD simulations were performed to evaluate diffusion coefficients of butane, hexane, octane and decane molecules inside MFI and TON zeolite. For fast moving shorter alkanes, ReaxFF's predictions of diffusion coefficients are in good agreement with experimental results. However, for bigger chain alkanes, it was observed that MD simulations longer than 5ns are essential for prediction of accurate diffusion behavior. beta-scission cracking of C8H17 carbenium ions was simulated using MD-NVT ensemble at elevated temperatures (1300K and 1400K). The beta-scission mechanisms observed in the MD simulations support the classical mechanism [1] over cyclic carbocation mechanism [2] and SN2-alkoxide mechanism [3].