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

Conversion of Methanol to Hydrocarbons: How Zeolite Cavity and Pore Size Controls Product Selectivity

“…H/D exchange reactions have occasionally also been studied, but to a lesser extent. [28,29] As mentioned above, owing to the porous nature of the zeotype catalysts and the bulky nature of many suggested reaction intermediates, careful studies of these species are required and have been achieved by a procedure introduced by Guisnet and co-workers. [30] By employing thermal quenching of the reaction followed by dissolution of the encapsulating catalyst framework in hydrofluoric acid, these species are liberated and available to analysis (often by GC-MS) after extraction in an organic solvent.…”

Umwandlung von Methanol in Kohlenwasserstoffe: Wie Zeolith‐Hohlräume und Porengröße die Produktselektivität bestimmen

“…[28,29] Wie schon erwähnt, werden viele der angenommenen Zwischenprodukte aufgrund ihrer sperrigen Natur und der mikroporçsen Beschaffenheit der zeolithischen Katalysatoren in den Poren eingeschlossen. Die Methode erlaubt es, Informationen über primäre und sekundäre Produkte zu erhalten, ohne Stçrung der Reaktion durch die Einbeziehung anderer Reaktanten als Methanol.…”

“…Vereinzelt wurden auch H/D-Austauschreaktionen angewendet. [28,29] Wie schon erwähnt, werden viele der angenommenen Zwischenprodukte aufgrund ihrer sperrigen Natur und der mikroporçsen Beschaffenheit der zeolithischen Katalysatoren in den Poren eingeschlossen. Guisnet et al haben ein Verfahren eingeführt, um diese Strukturen detailliert untersuchen zu kçnnen.…”

Insights into the reaction mechanism of propene H/D exchange over acidic zeolite catalysts from theoretical calculations

“…Followed by the cracking reaction, the deuterated C 6 species will be converted into propene-D 4 , propene-D 5 and propene-D 6 , with the distribution ratio being 1:2:1, in good agreement with the experimental results. 20 The calculated activation energy barriers for the rate-determining step of two hydride transfer processes are 7.0 and 12.3 kcal/mol, respectively, which indicates that the intramolecular hydride transfer can readily occur.…”

“…20 In order to explore the influence of acid strength, a 72T boron-substituted ZSM-5 cluster model propene H/D exchange would be considerably improved as the decrement of the activation energy barrier is 12.3 kcal/mol. Therefore, the confinement of zeolite framework has a significant influence on the production of propene-D 5 .…”