(198b) Selective Oxidation of Diols Over Supported Pt Catalysts in Base-Free Aqueous Solution

Interest in producing commodity chemicals from biomass instead of fossil fuel continues to grow as the biological and chemical transformations of carbohydrate feedstocks become more economical. The selective oxidation of sugar-derived multi-functional molecules, such as glycerol and hydroxymethylfurfural, is one route to add value to biorenewable chemical feedstocks. Recent studies have demonstrated high activity of gold catalysts for alcohol oxidation reactions in the presence of added base, but that process suffers from the need to neutralize the product stream with acid to release the organic acid product. The fundamental barriers limiting the rate and selectivity during the oxidation of alcohols to acids in the absence of added base are unclear but are critical to the development of industrially-relevant biorenewable chemical oxidation processes.

This work explores the overall conversion, product selectivity, oxidation rate, and stability of supported metal catalysts as a function of pH and alcohol structure.   In particular, supported Pt catalysts were evaluated in the acidic aqueous-phase oxidation of diols to mono- and di-acids at 343 K in a semibatch reactor. The influence of solution pH (acidic versus basic) on the identifiable intermediate reaction products was also determined. The effect of diol chain length on the initial turnover frequency and the final selectivity to diacids was investigated at 10 bar O₂ and 0.1 M diol. Results from the kinetic studies of diols were compared to those from mono-alcohols and polyols at similar conditions. The initial rate of reaction was significantly higher for diols with four to six carbons in the chain compared to diols with only two or three carbons. In addition, the longer chain diols were very selective to the diacid at high conversions, while shorter chain diols were unable to selectively convert to the diacid, even at high catalyst loadings.

Deactivation of the Pt catalyst was observed at low pH conditions. A typical reaction profile at moderate catalyst loading showed considerable deactivation compared to that observed with supported gold catalysts in the presence of added base. Analysis of a used Pt catalyst by electron microscopy indicated that sintering of the metal was negligible. Moreover, elemental analysis of the reaction medium revealed that only a small amount of Pt leached into solution after 24 h, suggesting dissolution of Pt is not responsible for deactivation.  Adsorption of reaction byproducts on the active Pt catalyst was the most likely cause of catalyst deactivation.