Silanol Networks Control Methanol Reactivity in Nano- and Micron-sized Silicalite-1.

The catalytic activity of silanol groups in zeolites has long been overlooked because of their much lower acidity compared to Brønsted acid sites. Here, we demonstrate their non-negligible catalytic role in methanol conversion using pure silica MFI zeolite (Silicalite-1). Two silicalite-1 samples with different crystal sizes, micron-sized (Sil1_micro) and nano-sized (Sil1_nano), were synthesized and investigated by in situ and operando FTIR spectroscopy using probe molecules (carbon monoxide, pyridine, and methanol). multivariate-curve regression by alternating least squares was applied to elucidate the influence of the silanol hydrogen-bonding network on catalytic reactivity. Distinct acid-base behaviors were identified: isolated and weakly hydrogen-bonded silanols are active at low temperatures, whereas strongly hydrogen-bonded silanols become catalytically relevant at higher temperatures. Methanol adsorption studies reveal a unique bidentate coordination mode in Sil1_nano, associated with weakly hydrogen-bonded geminal silanols, which promotes the formation of reactive intermediates and dual coke species. In contrast, Sil1_micro, characterized by a higher fraction of strongly hydrogen-bonded silanols, exhibits a different methanol reactivity pattern. These results demonstrate that silanol groups are active functional entities rather than passive defects, capable of driving methanol reforming-like transformations and steering coke formation pathways.