Bifunctional gold catalysts: Relationship between preparation method and catalytic performance in tandem cellobiose valorization

Abstract

Au was supported on carbon xerogel (CX) using various techniques and tested in tandem oxidation of cellobiose to gluconic acid (combining hydrolysis and oxidation steps in one-pot) in order to establish the relationship between the physicochemical properties of these materials and their performance as bifunctional catalysts. Notably higher selectivity to gluconic acid was obtained by catalysts with larger Au particle size showing moderate TOF. The performance of Au/CX catalysts was also affected by changes in surface chemistry of CX, introduced during deposition of Au. A direct link between the catalyst modifications and the reaction pathway was established by applying a simple reaction model to compare the rate constants of the intermediate processes. The experimental and modelled results revealed that reduction with citric acid was the most suitable method of preparation of the bifunctional catalyst. This catalyst was almost inactive in conversion of glucose and gluconic acid to side products, resulting in a 4 times higher yield of the desired product as compared to its counterpart prepared by sol-immobilization method (SI). On the other hand, the presence of PVA stabilizer on the surface of the SI catalyst resulted in the preferential oxidation of alcohol over aldehyde group in glucose, leading to poor selectivity of the cascade process. The reaction kinetics was examined and the apparent activation energy of the one-pot oxidation of cellobiose to gluconic acid was determined.