(349l) Solvatation of the Hofmann Elimination of Alkylamines through Cooperative Adsorption

Water is frequently involved in solid acid catalyzed transformations either as solvent, and its influence towards catalytic kinetics has long been investigated, the means by which this occurs however is still under debate. Potential challenges within studies are resulted from the highly non-ideal thermodynamics of the aqueous phase, and the complication of water itself being a reactant or product in the reaction. We hereby present a kinetic investigation of the vapor phase Hofmann elimination of tert-butylamine (TBA) over H-ZSM-5 as a probe chemistry for studying water’s effect on solid acid catalysis. Advantages of the Hofmann elimination include a purely Brønsted acid catalyzed and water-free chemistry, which enable studying the effect of a solvent like water in the more thermodynamically ideal vapor phase. The elimination was found to undergo through a E1-like mechanism with tert-butylammonium adsorbate as reaction intermediate. In the presence of controlled water partial pressures, the reaction rate was significantly reduced, although the extent of which was unaffected by Al content (Figure 1A). Also, when water is removed from the vapor phase, the dry rate of Hofmann elimination was readily re-established, indicating such activity loss is reversible. Kinetic measurements, in-situ spectroscopy, and kinetic modeling together reveal the formation of a water-TBA complex over the acid site that inhibits Hofmann elimination (Figure 1B). Various solvents were tested as cooperative adsorbates and manifested a non-linear trend between the stability of the inhibiting complex and the relative proton affinities of alkylamine and cooperative adsorbate (PA_amine – PA_{co-operative adsorbate}).