Confinement-Driven “Flexible” Acidity Properties of Porous Zeolite Catalysts with Varied Probe-Assisted Solid-State NMR Spectroscopy

Although the acidity properties and pore confinement effect inside porous zeolite catalysts have been extensively explored to correlate with their catalytic performances, a comprehensive understanding regarding the influence of confinement effect on the acidity properties is still lacking. Herein, with the employment of two commonly used nuclear magnetic resonance (NMR) probe molecules, namely pyridine-d5 and trimethylphosphine oxide (TMPO) with different shapes and size dimensions, the influence of the confinement effect on the apparent acid strength of 10-membered ring ZSM-5 zeolite (MFI) has been comprehensively elucidated by using a combined solid-state NMR experiment and density functional theory (DFT) calculation approach. The confinement-driven subtle distinctions of the acidic features that disturbed by framework structures have been definitely identified. It is demonstrated that when the probe molecule is fitted perfectly into the zeolite channels, the pore confinement could significantly affect the electronic structures of adsorbed species by van der Waals (vdW) and electrostatic interactions, thus capable of compensating the weaker intrinsic acid strength of zeolites and possibly rendering a stronger apparent acidity in the catalytic processes. These results offer new insights into the detailed acidity properties of porous zeolite catalysts with diverse pore architectures, which is essential to the optimization of their catalytic performances.