<p>A kinetic investigation of the vapor phase Hofmann elimination of
tert-butylamine over H-ZSM-5 reveals a carbocation mediated E1-like mechanism,
where isobutene and ammonia are exclusively produced over Brønsted acid sites.
Hofmann elimination kinetics are found to be insensitive to Al content or
siting, varying only with alkylamine carbocation stability (r<sub>tertiary</sub>
> r<sub>secondary</sub> > r<sub>primary</sub>). Under conditions of
complete tert-butylamine surface coverage, experimentally measurable apparent
kinetics are directly equivalent to the intrinsic kinetics of the rate
determining unimolecular surface elimination. The direct measurement of
elementary step kinetics served as a water-free reactive probe, providing a
direct measurement of the impact of water on solid Brønsted acid catalyzed
chemistries at a microscopic level. Over a range of temperatures (453‒513 K)
and tert-butylamine partial pressures (6.8×10<sup>-2</sup>‒6.8 kPa), water
reversibly inhibits the rate of Hofmann elimination. Despite expected changes
in aluminosilicate hydrophobicity, the water-induced inhibition is found to be
insensitive to Al content, demonstrated to be due to one water molecule per Brønsted
acid site. Regardless of the significant reduction in the rate of Hofmann
elimination, kinetic interrogations and operando spectroscopic measurements
reveal that the coverage of TBA adsorbed on H-ZSM-5 is unaltered in the
presence of water. Cooperative adsorption between the tert-butylammonium
surface reactant and water adsorbed on a neighboring framework oxygen is proposed
to be responsible for the observed rate inhibition, the surface dynamics of
which is quantitatively captured through kinetic modeling of experimental rate
measurements.</p>
Solvating Alkylamine Hofmann Elimination in Zeolites Through Cooperative Adsorption
