Hydrogenolysis of polyols on supported transition metal catalysts.

摘要

Biorenewable resources such as carbohydrates are alternative feedstocks for the production of oxygenated chemicals. In some processing schemes, conversion of carbohydrates can involve the initial hydrogenation of a simple sugar, such as glucose, to the sugar alcohol sorbitol. The subsequent catalytic hydrogenolysis of sorbitol can yield lower molecular weight polyols, such as glycerol and glycols, along with acids such as lactic acid. Glycerol, which is also a byproduct from biodiesel production, can undergo further hydrogenolysis to yield glycols and lactic acid. These reactions are typically conducted in the aqueous phase over supported transition metal catalysts; however, the presence of base has also been shown to enhance the rate of hydrogenolysis. This suggests that the mechanism of hydrogenolysis is complex, involving both metal- and base-catalyzed reactions, along with reactions that may occur in solution. Gaining a deeper understanding of this mechanism could enable the design of more active and/or selective catalysts. Therefore, the goal of this dissertation is to better understand the mechanism of polyol hydrogenolysis in order to aid in the design of more active and/or selective catalysts for the process.; Activated carbon-supported Ru, Pt, and Re were evaluated as catalysts for the batchwise hydrogenolysis of sorbitol in aqueous solution at 473-498 K and 40 bar H2 under neutral conditions and in the presence of CaO. Both Ru and Pt were identified as promising catalysts for additional studies involving the hydrogenolysis of polyols, while Re was rather inactive and aqueous-phase processing promoted leaching of the metal. In the presence of base, Pt was more active than Ru by almost an order of magnitude. Under these conditions, lactate was the primary liquid-phase product formed over both Ru/C and Pt/C, followed by propylene glycol, ethylene glycol, and glycerol. As expected, Ru promoted significant methane formation. In the absence of base, Ru was more active than Pt, however, CS and C4 polyols were the primary products of hydrogenolysis.; The same carbon-supported Ru and Pt catalysts were also evaluated in the batchwise hydrogenolysis of glycerol in aqueous solution at 473 K and 40 bar H2, with and without added base. At neutral pH, Ru was more active than Pt at converting glycerol to glycols. However, Ru favored the production of ethylene glycol over propylene glycol and also catalyzed methane formation. Although less active, Pt catalyzed propylene glycol formation with high selectivity. Addition of base enhanced the reactivity of Pt to a greater extent than Ru, but lactate formation was significant at high pH in the presence of either Pt or Ru. The cleavage of C-C bonds leading to the formation of ethylene glycol from glycerol is proposed to occur primarily through a metal-catalyzed reaction on Ru, whereas this cleavage is thought to occur through a base-catalyzed reaction in the presence of Pt. An overall reaction network for glycerol hydrogenolysis is presented.; Bimetallic PtRu and AuRu catalysts were prepared by a surface redox method in which Pt or Au was deposited onto the surface of carbon-supported Ru nanoparticles having an average diameter between 2 and 3 nm. Characterization by HZ chemisorption, analytical TEM, and X-ray absorption spectroscopy at the Ru K-edge, Pt L m-edge, and Au Lm-edge, confirmed that Pt and Au were successfully deposited onto Ru without disrupting the Ru particles. Depression of the ethane hydrogenolysis rate over Ru after addition of Au was further evidence of successful deposition. The bimetallic particles were subsequently evaluated in the aqueous-phase hydrogenolysis of glycerol at 473 K and 40 bar H2 at neutral and elevated pH. Although monometallic Pt and Ru exhibited different activity and selectivity to products, the bimetallic PtRu catalyst functioned more like Ru. A similar result was obtained for the AuRu bimetallic catalyst. The PtRu catalyst was stable under the aqu