(532n) Investigating the Effect of Solvents over Brønsted Acid Sites of Zeolite Using Alkylamine Hofmann Elimination

Solvents are ubiquitous parts in catalytic transformations over solid acid catalysts, yet how solvents participate and affect the catalytic cycles is still under debate. Challenges in deciphering effects of solvent in catalyzed reactions usually arise from the highly non-ideal thermodynamics of the liquid phase, further complicated by reactions that already involve solvent as a reactant or product. By using the vapor phase Hofmann elimination of tert-butylamine (TBA) over H-ZSM-5 as model reaction, we first investigated the effect of water, with the simplest molecular structure and the most common industrial application, 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 in the more thermodynamically ideal vapor phase. Kinetic measurements revealed a significant decrease in the rate of Hofmann elimination with cofed water, which was reversible upon removal of water. The extent of kinetic inhibition increases 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 gained understanding, we extended our scope of study over multiple solvents across a wide range of molecular size, dipole moment and proton affinity. Under the same reaction condition, varying extents of inhibition was observed for different solvents, with none of the proposed parameters being the sole descriptor of their kinetic behaviors. A roughly linear relationship was observed between degree of inhibition and adsorbate dipole moment, as well as a volcano-shaped trend with proton affinity, albeit exceptions exist for both cases (Figure 1B).