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 E1-like elimination mechanism with tert-butylammonium adsorbate as reaction intermediate. Under controlled water partial pressures, the rate of Hofmann elimination was significantly reduced, the extent of which was unaffected by Al content, and such loss in catalytic activity was found to be reversible upon removal of water. A combination of kinetic measurements, in-situ spectroscopy, and kinetic modeling reveal the formation of a water-TBA complex that inhibits Hofmann elimination. The stability of the inhibiting complex is found to vary with the choice of cooperative adsorbate (solvent identity), revealing a non-linear trend with the relative proton affinities of the alkylamine and cooperative adsorbate.

Cooperative adsorption and the solvation of Brønsted acid sites in zeolites

ACS Spring 2022

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