Predicting diffusion barriers and diffusivities of C6–C12 methylbenzenes in MFI zeolites

Abstract

Mass transport plays an important role in zeolite catalyzed reactions and catalyst deactivation, yet experimental measurement of mass transport, particularly ultra-slow diffusion processes (e.g., of bulky aromatics), is limited because of time scale restraints. Here, we use density functional theory to overcome these limitations and calculate diffusion barriers of benzene and all twelve C 7 –C 12 methylbenzenes through the straight and sinusoidal channels of silicalite-1 (MFI framework). Straight and sinusoidal diffusion barriers are well-predicted by a critical diameter describing the minimum width of the molecule, where benzene, toluene, and para -xylene (all 6.6 Å) diffuse through both channels with barriers 200 kJ mol −1 lower than those of pentamethylbenzene (8.2 Å). The MFI framework distorts to accommodate species with larger critical diameters and this distortion correlates to activation barriers where smaller molecules, such as benzene, distort the framework to smaller extents compared to larger species, such as pentamethylbenzene. Diffusing through the straight channel of MFI is always more facile than via the sinusoidal channel, by an average of 39 kJ mol −1 because the tortuosity of the sinusoidal channels forces larger framework distortions than straight channel diffusion. We show that DFT-calculated straight channel diffusion activation barriers agree well with those reported by frequency response experiments and can be used to calculate self-diffusivities of molecules, with appropriate entropy corrections. Examining all aromatics provides insights to the role of molecule size, channel tortuosity, and entropy during intracrystalline diffusion to provide a reference point for the species that can reasonably diffuse through both channels (e.g., benzene, toluene, xylenes, durene), through straight channels only (e.g., 1,2,3-trimethylbenzene), or simply are ‘stuck’ within intersections (e.g., pentamethylbenzene) in MFI. • Diffusion barriers trend with critical diameter where aromatics with smaller critical diameters diffuse with lower barriers. • MFI distorts to accommodate aromatic diffusion and the extent of this distortion is predicted by critical diameter. • Diffusing through the straight channel occurs with barriers, on average, 39 kJ mol −1 lower than sinusoidal diffusion.