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 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 an E1-like unimolecular decomposition as the rate determining step (<b>Figure 1A</b>). In the presence water, the rate of Hofmann elimination was significantly reduced, the extent of which was unaffected by Al content (<b>Figure 1B</b>). The observed reduction in catalytic activity was found to be reversible; the dry rate of Hofmann elimination was readily re-established upon the removal of water from the vapor phase. A combination of kinetic measurements and in-situ spectroscopy reveal that water reduces the rate of Hofmann elimination, without competing for adsorption on the catalytically active Brønsted acid sites. Reaction order studies in water implicate only one water molecule per active site for the observed loss in activity, despite the ability of water clusters to form under reaction conditions investigated. We propose two hypotheses based on the results: water preferentially de-stabilizes the kinetically relevant transition state through solvation; alternatively, water occupies a neighboring framework site crucial to the Brønsted acid catalytic cycle.

Kinetic investigation into the effect of water on alkylamine Hofmann elimination over H-ZSM-5

ACS Spring Meeting 2021

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