Kinetic Investigation on Solvating Alkylamine Hofmann Elimination over Brønsted Acidic Zeolites

The presence of solvents can significantly alter the reaction kinetics of catalytic conversions, yet the mechanisms of the solvent effect remain widely debated and could greatly vary between different chemistries. We use vapor phase Hofmann elimination of tert-butylamine (TBA) to systematically investigate how solvents participate catalyzed reaction over Brønsted acid sites of solid acid catalysts and affect a catalytic cycle. Twenty commonly used organic and inorganic solvents were tested to investigate the relationship between solvent identity and their influence on reaction kinetics. Kinetic measurements showed a significant and reversible inhibition in rates of Hofmann elimination in the presence of every tested solvent, with the degrees of reduction vary from less than 10% for non-polar solvents (carbon dioxide, benzene, hexane) to more than 90 % (methanol, ethanol, 1-propanol, acetonitrile), which was found to exhibit a roughly linear against the molecular dipole moment of solvents. Additionally, the degree of reduction is not affected by the size of molecule as the carbon chain length increases, but is negatively related to the degree of substitution by alkyl group on the α-carbon as well as halogen atom on the β-carbon. Despite the significant kinetic inhibition and a 30 kJ mol-1 increase in the measured apparent activation energies, the temperature programmed surface reaction profiles performed with pre-adsorbed TBA with feeding of acetonitrile and methanol showed no change in the peak temperature from the catalyst without solvent feed. Cooperative adsorption complex formed between the surface tert-butylammonium and solvent molecule adsorbed on a neighboring framework oxygen, which is far less reactive towards Hofmann elimination, is proposed to be responsible for the observed rate inhibition. Also, the stability of adsorption complex formation is found to be affected by the proton affinity of solvent, as revealed by the kinetic modeling of experimental rate measurements.