Restructuring of MFI Framework Zeolite Models and Their Associated Artifacts in Density Functional Theory Calculations

This study compares and evaluates multiple orthorhombic silicalite MFI framework structures using periodic density functional theory (DFT) calculations implemented with a wide range of exchange–correlation functionals and dispersion-correction schemes. Optimization of the structure available from the International Zeolite Association (IZA) yields only metastable forms, which restructure to arrangements 18–156 kJ mol–1 lower in energy (55 kJ mol–1 on average) through annealing and adsorption/desorption processes without altering their connectivity. These restructuring events can occur unintentionally during DFT studies of adsorptive and catalytic properties, leading to very large artifacts in DFT-predicted adsorption, reaction, and activation energies. Pre-annealing the IZA structure prevents restructuring and these artifacts but forms MFI structures which do not conform to the Pnma spacegroup symmetry and have significantly perturbed sinusoidal and straight channel geometries. These issues persist across a wide range of exchange–correlation functionals, including common choices such as the Perdew–Burke–Ernzerhof and Bayesian error estimation functionals, and dispersion-correction schemes such as the D3 method. Direct optimization of structures generated from the work of van Koningsveld et al. and Olson et al., in contrast, yields structures that are extremely similar across all functionals, restructure less often during annealing, and have smaller energy shifts when they do restructure (5 kJ mol–1, on average). Optimizing the unit cell parameters of these structures without constraining atoms or the unit cell shape also yields more stable structures, though often with unit cell parameters that do not closely match structures found experimentally. Annealing of other commonly studied zeolites (BEA, CHA, and LTA) does not yield structures with energy decreases or structural changes as significant as those for MFI. This study thus illuminates a potential source of significant error for DFT studies of MFI and provides evidence-based solutions for a variety of DFT methods.