Reaction Intermediates Modulate Framework Flexibility in Zeolite ZSM-5 during the Methanol-to-Hydrocarbons Reaction

Understanding how zeolite frameworks evolve during catalytic reactions is critical for improving their performance. Framework flexibility (the ability of zeolite T-O-T linkages to undergo angular and bond length variations that transiently adapt pore geometries) is increasingly recognized as key factor in adsorption and diffusion, yet its role remains unclear. Here, we investigate the dynamic behavior of zeolite ZSM-5 during the methanol-to-hydrocarbons (MTH) reaction, focusing on how reaction intermediates affect framework flexibility and pore accessibility. Using polarization-dependent attenuated total reflection infrared (ATR-PDIR) spectroscopy on b-oriented ZSM-5 thin films, where the straight channels align perpendicular to the substrate, we directly separated the responses of straight and sinusoidal pores. We correlate framework distortions and molecular orientation with catalytic function. The formation and orientation of key intermediates (methoxy, carbonyl, allylic carbocations, and aromatic precursors) coincide with zeolite stiffening within minutes on stream, while external pore blockage continues even after internal stiffening stabilizes. These effects are absent in silicalite controls. These findings indicate that early-formed intermediates modulate zeolite flexibility and initiate the formation of coke intermediates and hence the deactivation process. Flexibility thus emerges as an underexplored parameter for designing more resilient, adaptive zeolite catalysts.