Regioselective H/D Exchange at the Side‐Chain of Ethylbenzene on Dealuminated Zeolite H‐Y Studied by In Situ MAS NMR–UV/Vis Spectroscopy

Abstract

A number of heterogeneously catalyzed reactions (e.g. cracking, isomerization, dehydrogenation and alkylation) of hydrocarbons are initiated or promoted on solid acid catalysts by activation of the C H bonds of the reactants. Studies of H/D exchange between the reactants and the Bronsted acid sites of solid catalysts at the early stages of acid-catalyzed reactions provide useful information concerning activation mechanisms and intermediates. These activation mechanisms have been the topic of a number of theoretical and experimental studies. 3] The main routes include pentavalent carbonium ions formed via protonation of alkanes on Bronsted acid sites and trivalent carbenium ions due to hydride abstraction by Lewis acid sites. Sommer et al. concluded that the H/D exchange via the first route requires much higher temperatures compared to the second one, and that it proceeds via direct proton transfer between the solid surface and the alkane. The second route via a carbenium ion may result in regioselective H/D exchange following Markovnikov’s rule. c, e] On sulfated zirconia, only the methyl group of propane exchanges hydrogen atoms at 323 K, but both methyl and methylene groups are involved in H/D exchange at higher temperaACHTUNGTRENNUNGtures. Stepanov and Freude et al. reported regioselective H/D exchange between methyl and methylene groups of propane on the zeolite H-ZSM-5, and showed that the H/D exchange rate of the methyl groups is much higher than that of the methylene groups. Bucko et al. suggested that an entropic effect is responsible for the regioselective H/D exchange of propane and isobutane on zeolite clusters. The probability of adsorption of propane via the methylene group is seventeen times lower than that of adsorption via a methyl group; this entropy contribution leads to a higher free-energy barrier for proton-exchange via the methylene group. Herein, H/D exchange at the side-chain of ethylbenzene adsorbed on dealuminated deH-Y zeolites is studied and preferred regioselective exchange at the methyl group (b-carbon) is found at low temperatures. Based on the recently introduced in situ MAS NMR–UV/Vis spectroscopy, which herein is combined with the injection of short pulses of partially deuterated reactant molecules onto the catalyst at the reaction temperature, a reaction mechanism involving both Lewis and Bronsted acid sites in the H/D exchange reaction is suggested. The combination of complementary spectroscopic techniques helps gaining deeper understanding of catalyzed reactions. As an important advantage, in situ pulsed-flow (PF) H MAS NMR–UV/Vis spectroscopy can probe routes of hydrogen transfer via the characteristic NMR signals of the reactants before and after the exchange. Simultaneously, the formation of cyclohexadienyl and arylcarbenium ions (Scheme 1) is studied via their UV/Vis bands. The application of the pulsed-flow technique allows the study of H/D exchange kinetics at elevated temperatures with a well-defined starting point.