Mechanistically Significant Details of the H/D Exchange Reactions of Propene over Acidic Zeolite Catalysts

Abstract

Although H/D exchange is indispensable to the study of reaction mechanisms in solution, they have been underexploited in studies of heterogeneous catalysis. Spectroscopic studies of H/D exchange at low temperatures and long reaction times on solid catalysts have been reported, [1,2] and more realistic temperatures have been used for studies of equivalent exchange sites as in methane.[3] We were drawn to the possibilities of studying mechanisms by observing exchange into structurally distinct sites within the constraints of high temperatures and several-second reaction times used for these reactions in actual practice. [4] We have found that by careful experimental design of H/ D exchange reactions, including appropriate reagent compositions or concentrations, and quantitative fitting of the ion mass distributions of product species, [5] we can sometimes reveal a surprising level of complexity in reactions that at first glance would appear to be simple or even elementary. We have carried out experiments pulsing controlled amounts of protonated reactants onto deuterated catalysts (as shown here) as well as equimolar mixtures of fully protonated and fully deuterated reagents onto half-deuterated catalysts in either pulse or continuous flow mode. As the first contribution from these studies, we report on H/D exchange for propene on three microcrystalline solid acids: the aluminosilicate zeolite DZSM-5, its boron analogue D[B]ZSM-5, and the silico-aluminophosphate DSAPO-34. In every case herein the inventory of exchangeable deuterium atoms in the catalyst beds vastly exceeded that of the hydrogen atoms in the propene reagent pulses by a factor of at least 300 in all cases but one, and in that instance there was still more than a 13-fold excess. Whereas an oversimplified view of olefin H/ D exchange would predict that we would observe [D 6 ]propene as the exclusive equilibrium product of this reaction, our observations of incomplete deuteration and

Insights into the Mechanism of Methanol‐to‐Olefin Conversion at Zeolites with Systematically Selected Framework Structures

“…These findings, especially from the toluene H/D exchange study on ZSM‐22 at all temperatures at which no olefin was produced, might provide clues to the detailed mechanism of MTO conversion. 1) As surface methoxy groups or the methyl group on methanol cannot directly undergo H/D exchange with acid‐site protons,25 toluene H/D exchange did not result from direct H/D exchange between protons on the benzene ring and deuterated acid sites. 2) As no olefin was produced on ZSM‐22, partially deuterated toluene could not be derived from other highly methylated benzenes that have undergone H/D exchange 11.…”

The fast Z-scan method for studying working catalytic reactors with high energy X-ray diffraction: ZSM-5 in the methanol to gasoline process

“…The process and its zeolite catalyst have been extensively studied, particularly with respect to the mechanism of the MTG process. [37][38][39][40][41][42] Computational methods, 43,44 microscopy, [45][46][47][48] isotopic labelling/ switching, [49][50][51] catalyst dissolution 52 and spectroscopies [53][54][55][56][57][58][59] have all been used. Of particular interest here are the numerous powder and single crystal diffraction structural studies carried out on adsorption of hydrocarbons into ZSM-5.…”