UC Berkeley

Zeolites containing Brønsted or Lewis acid sites are extensively used catalysts in industrial processes. In this study, force field parameters are derived for the quantum mechanics/molecular mechanics (QM/MM) simulations of reactions in zeolites by reducing the deviation between QM/MM calculations and experimental data over a range of adsorption energies. The accuracy of the thermal correction for adsorption enthalpies determined by the rigid rotor-harmonic oscillator approximation (RRHO) is examined and shown to be improved by treating low-lying vibrational modes as free translational and rotational modes via a quasi-RRHO model. With the quasi-RRHO scheme, the QM/MM simulations can accurately reproduce experimental adsorption energies for both nonpolar and polar molecules adsorbed in MFI, H-MFI, and H-BEA.

The anharmonic effects of intramolecular nuclear motions, namely torsions and vibrations, are also examined in this study. Comparing with the harmonic oscillator approximation, the accuracy of the calculation of molecular partition functions, heat capacities, entropies, and enthalpies can be improved with an uncoupled mode (UM) model, where the full-dimensional potential energy surface for internal motions is modeled as a sum of independent one-dimensional potentials for each mode. However, the extent of improvements is very limited if the one-dimensional potentials are determined by the energy as a function of displacement along each normal mode. Significant improvement can be achieved by constructing the potentials for internal rotations and vibrations separately using the energy surfaces along the torsional coordinates and the remaining vibrational normal modes.

Three reactions catalyzed by BEA zeolite are investigated in this study using the QM/MM model, including the formation of p-xylene from ethylene and 2,5-dimethylfuran (DMF) in H-BEA as well as the isomerization of glucose to fructose and the synthesis of 4-(hydroxymethyl)benzoic acid (HMBA) from ethylene and 5-(hydroxymethyl)furoic acid (HMFA) in Sn-BEA. The pathways and energy barriers of these reactions derived by QM/MM simulations agree well with the available experimental results, which validates the realism of the QM/MM model. The influence of solvents and the effect of active site structures and heteroatoms on reaction barriers are investigated to derive criteria for future catalyst design.