Zeolite-Catalyzed Isobutene Amination: Mechanism and Kinetics

Abstract

Amination of isobutene with NH 3 was investigated over Brønsted acidic zeolites at 1 atm and 453−483 K. To compare catalytic activities over different zeolites, the measured reaction rates are normalized by the number of active sites determined by tert-butylamine temperature-programmed desorption (TPD). Small-and medium-pore zeolites with one-dimensional channels exhibit low activity because of pore blockage by adsorbed tertbutylammonium ions. However, turnover frequencies and activation energies are not sensitive to framework identity, as long as the active site is accessible to isobutene and tertbutylamine. Kinetic measurements and FTIR spectroscopy reveal that the Brønsted acid sites in MFI are covered predominantly with tert-butylammonium ions under reaction conditions. The desorption of tert-butylamine is assisted by the concurrent adsorption of isobutene. DFT simulations show that at very low tert-butylamine partial pressures, for example, at the inlet to the reactor, tert-butylamine desorption is rate-limiting. However, at sufficiently high tert-butylamine partial pressures (>0.03 kPa), protonation of isobutene to the corresponding carbenium ion limits the rate of amination.

Solvating Alkylamine Hofmann Elimination in Zeolites Through Cooperative Adsorption

“…A TBA covered surface under our reaction conditions is also consistent with isobutene amination studies where TBA induces product poisoning, blocking the access of isobutene and ammonia to the acid sites. 57 Similar to the partial pressure dependence, the apparent activation energy (Ea,app) is insensitive to Al content, with an average value of 173 kJ mol -1 (Fig. 2B).…”

“…conditions. 54,57 In the presence of a 24 kPa water co-feed, no significant change in the integrated peak area was observed relative to the absence of water (Fig. 6B).…”

“…Bell et al reported the first-principle calculated energetics for the reverse reaction, isobutene amination, over aluminosilicate zeolites. 57 Given that microkinetic reversibility must be maintained, we estimate a barrier of 198 kJ mol -1 based on their reported calculations. We therefore take the overall rate of Hofmann elimination to be limited by the surface reaction (Step 2),…”

“…The vapor phase chemistry does not involve water as a reactant and/or product, nor require its presence as a solvent, allowing for precise control of the activity of water in the catalytic system. Alkylamines like TBA form stoichiometric adsorption complexes on a Brønsted acid site, 54 providing a well-defined and readily characterized adsorbate [55][56][57] to assess the impact of water on. A single carboncontaining product is selectively formed (isobutene) over a wide range of conditions, allowing us to more readily isolate the impact of water on Brønsted acid catalytic kinetics.…”

Solvating Alkylamine Hofmann Elimination in Zeolites Through Cooperative Adsorption

“…54,70 The presence of tert-butylammonium on the catalyst surface is consistent with infrared spectroscopy measurements that observe its formation upon TBA adsorption on H-ZSM-5 (~ 1500 cm -1 ). 54,57 A surface elimination reaction involving tert-butylammonium, proceeding through an E1-like mechanism, 74 leads to the formation of a tert-butyl carbocation (Scheme 2, path ii) and ammonia. The C4 carbocation then donates a β-hydrogen back to the aluminosilicate framework, desorbing as isobutene, and regenerating the BAS (Scheme 2, path iii).…”