(631d) Pt Catalysts for Efficient Aerobic Oxidation of Glucose to Glucaric Acid in Water

Jechan Lee, Basudeb Saha, and Dionisios G. Vlachos

Catalysis Center for Energy Innovation & Department of Chemical & Biomolecular Engineering, University of Delaware

Non-food biomass is an appealing and sustainable starting material for renewable liquid fuels and chemicals that can minimize our dependence on petroleum and mitigate concerns on greenhouse gas emissions. One such chemical is glucaric acid which has use in detergents and polymeric products. Glucaric acid can be upgraded to adipic acid (AA) for the production of nylon 6,6, polyurethanes and adipic esters. The current manufacturing process of AA (>2.3 million tons per year) from petroleum-based KA oil^1-3 employs corrosive nitric acid and generates greenhouse gas, N₂O.^4,5 Therefore, development of sustainable manufacturing processes for glucaric acid for use in detergents and for upgrading to AA is a viable strategy.⁶ However, efficient aerobic oxidation of glucose to glucaric acid has proven to be challenge because of slow and difficult oxidation of gluconic acid, an intermediate oxidation product of glucose.⁷

We studied aerobic oxidation of glucose to glucaric acid using activated carbon, SiO₂ and Al₂O₃ supported Pt catalysts in water. The Pt/C catalyst exhibits the highest activity of these in glucose conversion and glucaric acid yield. Experiments are conducted systemically in a wide range of pH, O₂ pressure, temperatures and glucose/Pt to optimize the reaction conditions. The reactions in base-free and mild basic conditions give best yield to glucaric acid as compared to acidic and highly basic (pH 13.2) media. A maximum 74% glucaric acid yield at initial pH of 7.2, 80 °C, 13.2 bar O₂ and glucose/Pt molar ratio of 54 was achieved, which is the highest reported to date. Câ??C bond scission of glucaric acid in basic pH via retro-aldol results in low carbon chain carboxylic acids. Higher temperatures and higher Pt loadings cause degradation of glucaric acid, resulting in lower yields. Importantly, the Pt/C catalyst is stable after five consecutive catalytic cycles. Recyclability and characterization studies reveal that the catalyst is stable after five cycles with no sign of Pt leaching into the solution.

Mechanistic studies of the oxidation process entails rapid conversion of the aldehyde group of glucose and much slower oxidation of the â??CH₂OH group of gluconic acid, limiting the overall process efficiency. In order to rationalize the observed phenomenon, we have estimated the free energy of adsorption of various compounds. Kinetically, the transformation of an alcohol group â??CH₂OH to an acid requires first dehydrogenation to â??CHO followed by oxidation to â??COOH.^8-10 Upon dehydrogenation, the chemistry becomes facile. These fundamental findings are consistent with the general knowledge that aldehydes are much more reactive than alcohols. We propose that the oxidation of gluconic acid is slow due to the (oxidative) dehydrogenation of â??CH₂OH to â??CHO being slow; this dehydrogenation step, which is important for activating the molecule, is unnecessary in oxidation of glucose to gluconic acid. We believe high pressure of O₂ facilitates oxidation of gluconic acid to glucaric acid in our study. This explanation is in line with previous work where the formation of glucaric acid is reported at high pressure of O₂.¹¹

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