Sorbitol and glycerol are listed as two of the Department of Energy's top 12 chemicals that will serve as key feedstocks in future biorefineries. This work applies heterogeneous catalysis to the aqueous phase hydrogenolysis of sorbitol and the aqueous phase oxidation of glycerol for the production of value-added chemicals. Heterogeneous catalysis operation in the aqueous phase can be complicated by metal particle instability, resulting in particle growth or leaching. Moreover, catalysis may occur on the solid catalyst surface or in the surrounding solution.
To evaluate the stability of Ru particles operating under aqueous conditions at 473 K a variety of catalysts (Ru/SiO2, Ru/TiO2, Ru/Al2O3, and Ru/C) were characterized in-situ with X-ray absorption spectroscopy (XAS). Treatment of the samples at 473 K with 40 bar H2-saturated water resulted in significant particle growth for the Ru/SiO2 and Ru/Al2O3 samples, while both the Ru/TiO2 and Ru/C samples remained highly dispersed. Addition of a 0.4 M NaOH solution to the previous conditions resulted in continued Ru particle growth for the alumina sample while the carbon and titania supported samples remained stable. We then prepared a number of Ru-Pt/C bimetallic catalysts and characterized them with XAS. Results for the as-prepared samples revealed that for a Pt rich sample (11.1 wt% Pt, 3.2 wt% Ru) the Ru and Pt are well alloyed, while a 5.3 wt% Pt and 5.3 wt% Ru bimetallic catalyst had Ru segregated to the particle surface.
The hydrogenolysis of sorbitol (10 wt% sorbitol, 473 K, 40 bar H2, CaO added to maintain pH = 11.9) has been performed over monometallic Ru (5 wt%) and Pt (3 wt%) catalysts. Following 5 h of reaction, the conversion of sorbitol over both catalysts was similar: 71% over Ru/C and 82% over Pt/C. The product selectivities, defined as the molescarbon in product / molescarbon reacted in feed, varied slightly between the two catalysts. The major products observed during the reaction were lactic acid, propylene glycol, ethylene glycol, methanol, glycerol, and methane. Interestingly, the hydrogenolysis of sorbitol over the Pt/C was observed at temperatures as low as 423 K whereas a minimum temperature of 473 K was necessary over the Ru/C catalyst. In the absence of base, the Ru/C catalyzed approximately 60% conversion of sorbitol to lower molecular weight C4 and C5 polyols along with carbon dioxide and methane, while over the Pt/C a low conversion of sorbitol was observed (<15%) with no gas phase products detected.
The aqueous phase oxidation of glycerol over carbon-supported Au catalysts (333 K, 1-10 atm O2, 0.3 M glycerol, 0.6 M NaOH) has also been investigated. Initial rates are in good agreement with the published literature, however the observed selectivities to glyceric acid are poor for a wide range of Au particle sizes. To investigate the effect of a second metal on activity and selectivity, a Au on Pd bimetallic catalyst was prepared via a surface redox process. The XAS results showed that Au was coordinated to both Pd and Au. Reactivity studies suggest the bimetallic system is more stable than monometallic Pd. Additional kinetic studies are currently underway.