Evaluating Simulation Accuracy for the Prediction of MFI-Zeolite Phase Transitions

We studied the applicability of three popular classical models for zeolites, which include the effects of flexibility in their ability to reproduce the temperature-dependent orthorhombic-monoclinic phase transition in silicious MFI zeolites using molecular dynamics simulations. Work and research on structural changes under these conditions are limited; therefore, we investigated this under isothermal–isobaric ensemble conditions with adjustable cell lengths using computational X-ray diffraction (XRD) patterns and infrared spectra. None of the tested force fields were able to correctly reproduce the phase changes. We found that the structure collapsed using the force field by Demontis et al., while the force fields by Nicholas et al. and Hill and Sauer proved too flexible, creating new stable states dependent on the initial phase of MFI. With these equilibria as new baselines, further simulations were performed to find if any further phase changes were possible. Using XRD and a geometric criterion we introduced to quantify divergences from reference phases, we did find transitions, but again with dependencies on the initial phase’s symmetry. Systems initially in more symmetric orthorhombic states were found to not transition away from those configurations, while those in initially monoclinic states did, but did so to completely new phases.