(44f) Kinetic Investigation on the Solvation of Alkylamine Hofmann Elimination over Brønsted Acidic Zeolites

Solvents are frequently involved in catalytic transformations over solid acid catalysts, yet, how solvents participate and affect a catalytic cycle remains widely debated. Challenges in deciphering the effect of water on catalyzed reactions arises from the highly non-ideal thermodynamics of the aqueous phase, exacerbated by the use of probe chemistries that already involve water as a reactant or product. Here, we use the vapor phase Hofmann elimination of tert-butylamine (TBA) over H-ZSM-5 as a water-free probe and purely Brønsted acid-catalyzed chemistry to systematically investigate the effect of water in the more thermodynamically ideal vapor phase, offering a well-defined molecular adsorbate structure, to understand the role of aqueous solvation on solid acid catalysis. Kinetic measurements revealed a significant and reversible catalytic inhibition of the Hofmann elimination by water. The extent of kinetic inhibition increased monotonically with higher partial pressure of water but was unaffected by the Al content of zeolite (Figure 1A). A combination of kinetic measurements, in-situ spectroscopy, and kinetic modeling reveal the formation of a water-TBA complex that inhibits Hofmann elimination. Based on the understanding achieved, we extended our study to multiple solvents varying in molecular size, dipole moment and proton affinity. Various extents of inhibition were observed for different solvents, and both dipole moment and proton affinity of adsorbate appear to impact the degree of catalytic inhibition, albeit exceptions exist for both. A roughly linear relationship was observed between degree of inhibition and dipole moment, while solvents disobeying the relationship exhibit a volcano-shaped trend with proton affinity (Figure 1B).