Cooperative Adsorption: Solvating the Hofmann Elimination of Alkylamines

A kinetic investigation of the vapor-phase
Hofmann elimination
of tert-butylamine (TBA) 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 is found
to be insensitive to Al content, varying only with alkylamine carbocation
stability (rtertiary > rsecondary > rprimary).
Under
conditions of complete TBA surface coverage, experimentally measurable
apparent kinetics is 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 TBA partial pressures
(6.8 × 10–2–6.8 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.