Solvating Alkylamine Hofmann Elimination in Zeolites Through Cooperative Adsorption

Abstract

<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>

On the Acid Sites of Phosphorous Modified Zeosils

“…To quantitatively probe why these changes in desorption rates are only consequential for P-SPP and not Al-MFI, we compared the rate of reactive desorption via alkylamine Hofmann elimination relative to that of molecular desorption. Considering the Hofmann elimination of tertbutylamine as a representative case, the Arrhenius plots of isobutene formation rates are shown for Al-MFI (same data as reported by Chen et al 32 ) and P-SPP in Figure 1B; the apparent elimination activation energy remains within experimental error across the two materials. The observation of similar activation energies is consistent with previous studies suggesting that the kinetics of Hofmann elimination is independent of acid site strength (Psites and aluminosilicates in this case).…”

“…To understand this seemingly contradictory result, it is necessary to consider the 'apparent' nature of the reported activation energies; macroscopically measurable apparent activation energies frequently include thermodynamic contributions like reactant adsorption energies (Ea,app = f(∆Hads)). 32,51,52 Computationally calculated adsorption energies of relevant surface species (IPA and DIPE) are significantly different on aluminosilicates and phosphorous containing materials (∆∆Eads,IPA = 13 kJ mol -1 and ∆∆Eads,DIPE = 59 kJ mol -1 , Table 2), making the direct comparison of measured activation energies across the two classes of materials non-trivial. Under identical reaction conditions, barriers over aluminosilicates are likely measured from an adsorbed state due to high surface monomer and dimer coverages (in black, Figure 4A), and from the gas-phase as reference on P-zeosils due to low coverages (in red, Figure 4A).…”