Confined hot-pressurized water in Brønsted-acidic Beta zeolite speeds up the O-demethylation of guaiacol

New lignocellulose biorefinery technologies that enable the conversion of lignin into platform chemicals are essential to reduce our future dependence on fossil resources. In this study, we investigate the Brønsted acid-catalyzed O-demethylation of guaiacol in hot-pressurized water (HPW) as a model reaction for transforming lignin-derived phenolic substrates featuring ortho methoxy groups. We compare the effects of Brønsted mineral acid (HCl) and microporous solid acid (H-BEA zeolite) in water to elucidate the hydrolysis mechanism and the impact of zeolite microporosity on reaction rates. Operando molecular modeling combined with experimental kinetic studies reveals that, regardless of the catalyst type, O-demethylation follows a concerted, one-step O-activated SN2 mechanism. This mechanism involves a strong hydrogen bond between guaiacol and a hydronium ion as an ionic contact pair. Protons confined within the zeolite form more active undercoordinated hydronium ions, which are associated with lower enthalpic requirements and thus accelerate the hydrolysis. The molecular organization of solvent and reactants around the confined catalytic active site plays a crucial role in modulating the association of the reacting species. These proof-of-concept results demonstrate the significant influence of solvent (water) coordination on acid-catalyzed bimolecular reactions, such as hydrolysis, within confined spaces.