Untangling Framework Confinements: A Dynamical Study on Bulky Aromatic Molecules in MFI Zeolites

Abstract

In MFI zeolites, differentiating the molecular dynamics as a function of pore structures, that is, straight-channel versus zigzag-channel versus channel-intersection, has always been challenging, mainly due to small differences in pore size but is of great interest because these subtle structural differences can remarkably influence shape selectivity. Herein, 1,2,4-trimethyl benzene (1,2,4-TMB), a characteristic molecule larger than the 10-MR channel diameter, while smaller than the channel intersection, is chosen to probe the pore-confined dynamical behaviors in MFI via 2H NMR spectroscopy and density functional theory calculations. Our results show that in the absence of acid sites, that is, in the siliceous MFI silicalite-1, 1,2,4-TMBs can only diffuse along the straight channels; while at equilibrium, they incline to occupy the channel intersections with structure-defined orientations. Furthermore, a series of dynamic motions of 1,2,4-TMBs under different types of pore confinements are revealed and evaluated at a molecular level over a wide range of timescales, concluded, in short, as methyl C3-rotation > 112°-flip > 90°-flip > translational diffusion. With the presence of acid sites, that is, in H-ZSM-5; however, 1,2,4-TMBs are strongly adsorbed on Brønsted acid sites and the confined motions are further impeded. The findings in this work may provide insights to the catalytic roles of polymethyl-benzene intermediates, including 1,2,4-TMB, which usually serve as active centers or deactivation precursors in zeolite-based hydrocarbon conversion processes.