Arzumanov Sergei S, Reshetnikov Sergei I, Stepanov Alexander G, Parmon Valentin N, Freude Dieter
Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Prospekt Akademika Lavrentieva 5, Novosibirsk 630090, Russia.
J Phys Chem B. 2005 Oct 27;109(42):19748-57. doi: 10.1021/jp054037n.
The kinetics of hydrogen (H/D) exchange between Brønsted acid sites of zeolite H-ZSM-5 and variously deuterated propanes (propane-d(8), propane-1,1,1,3,3,3-d(6), propane-2,2-d(2)) have been monitored in situ by (1)H MAS NMR spectroscopy within the temperature range of 503-556 K. The contribution of intramolecular hydrogen transfer to the H/D exchange in the adsorbed propane was estimated by monitoring the kinetics of (13)C-labeled carbon scrambling in propane-2-(13)C in situ with (13)C MAS NMR at 543-573 K. Possible mechanisms of the exchange have been verified on the basis of the analysis of the variation of protium concentration in both the methyl and the methylene groups of propane in dependence of the reaction time. The main route of the exchange consists of a direct exchange of the acidic OH groups of the zeolite with either the methyl groups or the methylene group presumably with a pentacoordinated carbonium ion intermediate. The assumption that the intramolecular H scrambling between the methyl groups and the methylene group of propane via carbenium-ion-type intermediates is the fastest process among the other possible routes does not account for the experimental kinetics of H/D exchange for propanes with different initial contents and locations of deuterium in a propane molecule. The rate constant (k(3)) for intramolecular H/D exchange between the methyl and the methylene groups is 4-5 times lower compared to those of the direct exchange of both the methyl (k(1)) and the methylene (k(2)) groups with Brønsted acid sites of the zeolite, the k(1) being ca. 1.5 times higher than k(2). At lower temperature (473 K), the exchange is slower, and the expected difference between k(1) and k(2) is more essential, k(1) = 3k(2). This accounts for earlier observed regioselectivity of the exchange for propane on H-ZSM-5 at 473 K. Faster direct exchange with the methyl groups compared to that with the methylene groups was attributed to a possible, more spatial accessibility of the methyl groups for the exchange. Similar activation energies for H and C scramblings with a 2 times more rapid rate of H scrambling was rationalization by the proceeding of these two processes through an isopropyl cation intermediate, as in classical carbenium ion chemistry.
通过(^{1}H) MAS NMR光谱在503 - 556 K温度范围内原位监测了沸石H-ZSM-5的布朗斯台德酸位点与各种氘代丙烷（丙烷-d(8)、丙烷-1,1,1,3,3,3-d(6)、丙烷-2,2-d(2)）之间氢（H/D）交换的动力学。通过在543 - 573 K下用(^{13}C) MAS NMR原位监测丙烷-2-(^{13}C)中(^{13}C)标记的碳重排动力学，估算了吸附丙烷中分子内氢转移对H/D交换的贡献。基于对丙烷甲基和亚甲基中氢浓度随反应时间变化的分析，验证了可能的交换机制。交换的主要途径包括沸石的酸性OH基团与甲基或亚甲基直接交换，可能通过五配位碳正离子中间体。认为丙烷甲基和亚甲基之间通过碳正离子型中间体进行分子内H重排是其他可能途径中最快过程的假设，无法解释具有不同初始氘含量和位置的丙烷的H/D交换实验动力学。甲基和亚甲基之间分子内H/D交换的速率常数（(k(3))）比甲基（(k(1))）和亚甲基（(k(2))）与沸石布朗斯台德酸位点直接交换的速率常数低4 - 5倍，(k(1))比(k(2))高约1.5倍。在较低温度（473 K）下，交换较慢，(k(1))和(k(2))之间的预期差异更显著，(k(1)=3k(2))。这解释了早期在473 K下观察到的H-ZSM-5上丙烷交换的区域选择性。与亚甲基相比，与甲基的直接交换更快，这归因于甲基在交换中可能具有更大的空间可及性。H和C重排具有相似的活化能，H重排速率快2倍，这可以通过这两个过程像经典碳正离子化学中那样通过异丙基阳离子中间体进行来解释。
