Heterogenization of Shvo-type Ruthenium Catalysts and Their Application in Hydrogen Transfer Reactions


Student thesis: Doctoral Thesis

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  • Dongmei HE


Awarding Institution
Award date10 Jun 2016


Shvo’s catalyst is a very efficient homogeneous ruthenium catalyst used in various hydrogen transfer reactions including reactions for biomass conversion under mild conditions. Facile recycling of the catalyst is an important component of the design for sustainable processes. Heterogenization of homogeneous catalysts is an effective way for developing easily recyclable catalysts. We developed heterogenized homologues of Shvo’s catalyst for biomass conversion by modifying the ligand to facilitate the covalent attachment to the surface or the incorporation into the framework of solid supports.
We have synthesized and characterized the hydroxyl-functionalized cyclopentadienones 3,4-(p-HOPh)2-2,5-Ph2(C4CO) (7a) and 3-(p-HOPh)-4- (p-MeOPh)-2,5-Ph2(C4CO) (7b), and the triethoxysilylpropoxy-functionalized cyclopentadienones 3,4-[p-(EtO)3Si(CH2)3OPh]2-2,5-Ph2(C4CO) (8a) and 3-[p-(EtO)3Si(CH2)3OPh]-4-(p-MeOPh)-2,5-Ph2(C4CO) (8b). The reaction of Ru3(CO)12 with 8a and 8b resulted in the formation of the monoruthenium complexes {3,4-[p-(EtO)3Si(CH2)3OPh]2-2,5-Ph2(η4-C4CO)}Ru(CO)3 (9a) and {3-[p-(EtO)3Si(CH2)3OPh]-4-(p-MeOPh)-2,5-Ph2(η4-C4CO)}Ru(CO)3 (9b), respectively. The complexes 9a and 9b were converted to the diruthenium complexes {{3,4-[p-(EtO)3Si(CH2)3OPh]2-2,5-Ph2(η5-C4CO)}2H}Ru2(CO)4(μ-H) (10a) and {{3-[p-(EtO)3Si(CH2)3OPh]-4-(p-MeOPh)-2,5-Ph2(η5-C4CO)}2H}Ru2- (CO)4(μ-H) (10b), respectively, by refluxing them in isopropanol.
All of the four triethoxysilylpropoxy-functionalized ruthenium complexes (9a, 9b, 10a, and 10b) were tested as homogeneous catalysts for the transfer hydrogenation of levulinic acid with formic acid to give 4-hydroxyvaleric acid (4-HVA), which was in situ dehydrated to form gamma-valerolactone (GVL). The catalytic activities of these were much lower than that of the precursors of Shvo’s catalyst: {[3,4-(p-MeOPh)2-2,5-Ph2(η5-C4CO)]2H}Ru2(CO)4(μ-H) (1a) and [3,4-(p-MeOPh)2-2,5-Ph2(η4-C4CO)]Ru(CO)3 (4a).
The Shvo-type complexes 9a, 9b, 10a, and 10b were attached to the surface of silica using the covalent grafting method; these complexes were incorporated into the framework of silica by using the sol–gel method to form eight heterogenized Shvo-type catalysts (11a, 11b, 12a, 12b, 13a, 13b, 14a, and 14b). The catalysts were characterized by Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) surface area analysis and Barrett-Joyner-Halenda (BJH) pore size and volume analysis. The catalytic performances of the prepared catalysts were tested in the transfer hydrogenation reaction of levulinic acid with formic acid to form GVL. The sol–gel-based catalysts showed higher activity than covalently grafted catalysts. Over 90% of GVL was formed within 50 hours in the presence of sol–gel-based catalysts, while it took more than 80 hours to reach that high yield for covalently grafted catalysts. Hot filtration test for the catalysts 11a and 13a confirmed that the catalytic activity came from the supported catalyst and not from some active species leached from the solid support to the solution on the reaction conditions. The recyclability of 13a was investigated under batch conditions and significant loss of catalytic activity was observed after three cycles.
We successfully obtained the covalent attachment of complex 9a on the surface of magnetic nanoparticles, that is, magnetite (Fe3O4), magnetite covered by one layer of silica (Fe3O4@SiO2), magnetite covered by two layers of silica (Fe3O4@SiO2@SiO2), and magnetite covered by three layers of silica (Fe3O4@SiO2@SiO2@SiO2). The magnetic nanocatalysts obtained were Fe3O4@Ru (15a0), Fe3O4@SiO2@Ru (15a1), Fe3O4@SiO2@SiO2@Ru (15a2), and Fe3O4@SiO2@SiO2@SiO2@Ru (15a3). The catalysts were characterized by FT-IR spectroscopy, transmission electron microscopy (TEM), SEM, energy dispersive X-ray (EDX) spectroscopy, powder X-ray diffraction (XRD), BET surface area analysis, and BJH pore size and volume analysis. The catalytic performances of the catalysts were also tested in the conversion of levulinic acid to GVL with formic acid as the hydrogen source. Catalysts can be simply isolated from the reaction mixture by using an external magnet that allows fast and efficient separation of the product and catalyst compared to traditional methods. Catalysts supported on the silica-coated magnetic nanoparticles showed higher activity than that supported on Fe3O4. There were no obvious differences in TONs, TOFs, and the yield of GVL among 15a1, 15a2, and 15a3. Leaching tests for the four catalysts showed that the coating of silica layer on the magnetite prevented the leaching of Fe from the Fe3O4 core to some extent. When the number of silica layers coated on the surface was high, there was less leaching of Fe. The recyclability of 15a2 was investigated; it was found that 15a2 can be reused several times without any significant loss of catalytic activity. Hot filtration tests for 15a2 and 15a3 also guaranteed that catalytic activity came from the supported catalyst and not from some active species leached from the solid support to the solution on reaction conditions. The catalyst 15a2 also showed good activity in real biomass conversion to GVL and could be facilely recovered from black humins via magnetic concentration.