DOI: 10.1002/cphc.200800065 Regioselective H/D Exchange at the Side-Chain of Ethylbenzene on Dealuminated Zeolite H-Y Studied by In Situ MAS NMR–UV/Vis Spectroscopy Jun Huang, Yijiao Jiang, V. R. Reddy Marthala, Yean Sang Ooi, and Michael Hunger* [a] A number of heterogeneously catalyzed reactions (e.g. crack- ing, isomerization, dehydrogenation and alkylation) of hydro- carbons are initiated or promoted on solid acid catalysts by ac- tivation of the C ÀH bonds of the reactants. [1] Studies of H/D ex- change between the reactants and the Brønsted 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 stud- ies. [2, 3] The main routes include pentavalent carbonium ions formed via protonation of alkanes on Brønsted acid sites and trivalent carbenium ions due to hydride abstraction by Lewis acid sites. [2,3] Sommer et al. concluded that the H/D exchange via the first route requires much higher temperatures com- pared to the second one, and that it proceeds via direct proton transfer between the solid surface and the alkane. [3a] The second route via a carbenium ion may result in regioselec- tive H/D exchange following Markovnikov’s rule. [3a, c, e] On sulfat- ed zirconia, only the methyl group of propane exchanges hy- drogen atoms at 323 K, but both methyl and methylene groups are involved in H/D exchange at higher tempera- ACHTUNGTRENNUNGtures. [3a] Stepanov and Freude et al. reported regioselective H/D exchange between methyl and methylene groups of pro- pane on the zeolite H-ZSM-5, and showed that the H/D ex- change rate of the methyl groups is much higher than that of the methylene groups. [3b] Bucko et al. suggested that an en- tropic effect is responsible for the regioselective H/D exchange of propane and isobutane on zeolite clusters. [2c] The probability of adsorption of propane via the methylene group is seven- teen 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. [2c] Herein, H/D exchange at the side-chain of ethylbenzene adsorbed on deal- uminated deH-Y zeolites is studied and preferred regioselective exchange at the methyl group (b-carbon) is found at low tem- peratures. Based on the recently introduced in situ MAS NMR–UV/Vis spectroscopy, [4] 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 Brønsted acid sites in the H/D ex- change reaction is suggested. The combination of complemen- tary spectroscopic techniques helps gaining deeper under- standing of catalyzed reactions. As an important advantage, in situ pulsed-flow (PF) 1 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. Simul- taneously, the formation of cyclohexadienyl and arylcarbenium ions (Scheme 1) is studied via their UV/Vis bands. The applica- tion of the pulsed-flow technique allows the study of H/D ex- change kinetics at elevated temperatures with a well-defined starting point. Dealuminated zeolite deH-Y (n Si /n Al = 5.4) is obtained by steaming zeolite H-Y (n Si /n Al = 2.7), leading to a material with 22 extra-framework aluminum species per unit cell (u.c.) acting as Lewis acid sites. [5] The residual number of Brønsted acid sites is 10.9 SiOHAl/u.c. The details of catalyst preparation and characterization and the setup of the in situ PF MAS NMR– UV/Vis experiments are described in the Supporting Informa- tion. In Figure 1, 1 H MAS NMR and UV/Vis spectra of dehydrated (723 K) zeolite deH-Y recorded 10 to 15 min after injection of ethyl-d 5 -benzene (A, the deuterated ethyl group in Scheme 1) are shown. The 1 H MAS NMR signals (left) occurring at 1.2, 2.7, and 7.3 ppm are due to methyl groups, methylene groups, and non-deuterated aromatic rings, respectively, of A. The signals of non-deuterated bridging OH groups (SiOHAl) acting as the Brønsted acid sites of the zeolite catalyst are too weak and Scheme 1. LAS: Lewis acid site, BAS: Brønsted acid site. [a] J. Huang, Dr. Y. Jiang, V. R. R. Marthala, Dr. Y. S. Ooi, Prof. Dr. M. Hunger Institute of Chemical Technology University of Stuttgart, Stuttgart, 70569 (Germany) Fax: (+ 49)711-685-64081 E-mail: michael.hunger@itc.uni-stuttgart.de Supporting information for this article is available on the WWW under http://www.chemphyschem.org or from the author. ChemPhysChem 2008, 9, 1107 – 1109 # 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1107