Heterogeneous addition of H2 to double and triple bonds over supported Pd catalysts: a parahydrogen-induced polarization technique study
Heterogeneous addition of H2 to double and triple bonds over supported Pd catalysts: a parahydrogen-induced polarization technique study
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DOI：
10.1039/c2cp40690j
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发表时间：
2012-01-01
影响因子：
3.3
通讯作者：
Koptyug, Igor V.
中科院分区：
文献类型：
--
作者：
Kovtunov, Kirill V.;Beck, Irene E.;Koptyug, Igor V.
在这项工作中，以负载型 Pd 催化剂上丙烯和丙炔的氢化为例，在 Pd 粒径和催化剂载体性能的系统变化下研究了成对 H-2 加成对整个反应机理的贡献。对于负载在氧化铝、二氧化硅和氧化锆上的钯，只有丙炔与对-H-2氢化形成的丙烯表现出超极化。相反，丙炔或丙烯氢化中形成的丙烷没有超极化。这证明了负载型 Pd 催化剂上双键和三键上 H-2 加成的不同途径的存在。 Pd/TiO2 催化剂不仅能够以成对的方式将 H-2 添加到三键上，而且还能添加到双键上，从而具有独特的能力。这一发现表明，Pd-载体相互作用不仅在决定反应产物 NMR 谱线超极化的大小方面至关重要，而且在决定成对 H-2 加成的参与以及因此决定多相氢化机制方面也至关重要。对成对H-2加成选择性的比较分析表明，存在负责所有三种反应路线的不同表面活性位点：丙炔直接全加氢成丙烷、选择性加氢成丙烯和丙烯加氢成丙烷。提出了一种反应方案，该反应方案解释了在负载型 Pd 催化剂上使用对 H-2 进行丙炔和丙烯氢化中观察到的超极化和非极化反应产物的形成。 PHIP 技术的应用首次证明，在负载型 Pd 催化剂上丙炔存在下，丙烯的氢化不会发生。
In this work, the contribution of the pairwise H-2 addition to the overall reaction mechanism was studied under the systematic variation of both the Pd particle size and the properties of the catalyst support using the hydrogenation of propene and propyne over supported Pd catalysts as representative examples. For Pd supported on alumina, silica and zirconia, only propene formed upon hydrogenation of propyne with para-H-2 exhibits hyperpolarization. In contrast, propane formed in hydrogenation of propyne or propene is not hyperpolarized. This demonstrates the existence of different routes of H-2 addition to double and triple bonds on supported Pd catalysts. The unique ability of Pd/TiO2 catalysts to add H-2 in a pairwise manner not only to the triple but also to the double bond is demonstrated. This finding indicates that the Pd-support interaction is of primary importance in determining not only the magnitude of the hyperpolarization of the NMR lines of the reaction products but even the involvement of the pairwise H-2 addition and hence the mechanism of heterogeneous hydrogenation. The comparative analysis of the selectivities toward pairwise H-2 addition suggested the existence of different surface active sites responsible for all three reaction routes: the direct total hydrogenation of propyne into propane, its selective hydrogenation into propene, and hydrogenation of propene into propane. A reaction scheme which accounts for the formation of the observed hyperpolarized and non-polarized reaction products in propyne and propene hydrogenation with para-H-2 over supported Pd catalysts is suggested. For the first time, application of the PHIP technique allowed us to demonstrate that hydrogenation of propene does not take place in the presence of propyne on supported Pd catalysts.