Effect of spacer groups on the performance of MCM-41-supported platinum cluster-derived hydrogenation catalysts

Abstract

MCM-41 was functionalized with (EtO)₃SiCH₂Cl, (MeO)₃SiCH₂CH₂CH₂Cl, and (CH₃)Cl₂SiCH₂Cl. The functionalized materials were characterized by solid-state NMR (CPMAS, ²⁹Si and ¹³C) and XPS. The NMR data indicate that three new silicon environments were created by (EtO)₃SiCH₂Cl and (MeO)₃SiCH₂CH₂CH₂Cl, whereas with (CH₃)Cl₂SiCH₂Cl, two new silicon environments were obtained. XPS results from Si 2p core level and the valence band from the material functionalized by (MeO)₃Si(CH₂)₃Cl was found to be the same as that of the corresponding fresh catalyst (1a), in contrast to that of the materials functionalized by the other two silane reagents. After further functionalization with triethylamine, these materials were used as inorganic anion exchangers to support the cluster anion [Pt₁₂(CO)₂₄]^2-. Solid-state NMR (²⁹Si, ¹³C, ¹⁵N) was used to establish the presence of the quaternary ammonium group in the cluster-supported species. Analogous materials were also created using fumed silica as the support, and all of the cluster-supported materials were tested as catalysts for the hydrogenation of methyl pyruvate, acetophenone, nitrobenzene, benzonitrile, ethylacetoacetate, 4-nitrotoluene, cyclohexanone, allyl alcohol, and styrene. The best activity was obtained for the catalyst that had MCM-41 as the support and chloropropyl as the spacer group. TEM showed that the supports and the spacer groups had observable effects on the platinum crystallite size of the catalysts.