(741e) Kinetic Investigation into the Effect of Water on Alkylamine Hofmann Elimination over H-ZSM-5

Catalytic transformations over solid acid catalysts frequently involve water as a solvent, particularly when considering sustainable chemical conversions related to biomass upgrading. While it is well recognized that water can significantly affect catalytic kinetics over solid acids, the means by which this occurs remains debated. The challenge usually arises from the highly non-ideal thermodynamics of the aqueous phase, further complicated by reactions that already involve water as a reactant or product. We present a kinetic investigation of the vapor phase Hofmann elimination of tert-butylamine (TBA) over H-ZSM-5 as a model reaction for deciphering the effect of water on solid acid catalysis. The Hofmann elimination offers a purely Brønsted acid catalyzed and water-free chemistry, allowing us to systematically study the effect of a solvent like water in the more thermodynamically ideal vapor phase. Kinetic measurements in the absence of water reveal a tert-butylammonium adsorbate as the most abundant reaction intermediate, with its E1-like unimolecular decomposition as the rate determining step (Figure 1A). Water reduces the rate of Hofmann elimination (Figure 1B) and increases the apparent activation energy. Rate measurements with increasing total pressure but at a fixed ratio of P_water/ P_TBA reveal a mechanism by which water hinders the rate of reaction, without competing for adsorption on the catalytically active Brønsted acid sites. We propose two hypotheses based on the results: water preferentially de-stabilizes the kinetically relevant transition state; alternatively, water occupies a neighboring framework site crucial to the Brønsted acid catalytic cycle.