One-pot oxidation of cellobiose to gluconic acid. Unprecedented high selectivity on bifunctional gold catalysts over mesoporous carbon by integrated texture and surface chemistry optimization

Abstract

Bifunctional catalysts coupling acid sites for activation of glycosidic bond via hydrolysis with metallic sites for further oxidation of glucose intermediate offer an advanced technological solution toward direct conversion of cellulose to platform chemicals. Gold (metallic functionality) was supported on pristine and functionalized mesoporous carbons including; carbon xerogels (CXs) with distinct morphologies and ordered mesoporous carbons (OMCs). Phenolic groups (acidic sites) were introduced on the surface of these carbons by air treatment, which was confirmed by XPS, TPD and IR results. The bifunctional Au catalysts were applied in the direct tandem oxidative conversion of cellobiose to gluconic acid. A remarkably high selectivity of nearly 80% to gluconic acid was obtained in a short reaction time of only 75min using Au catalyst supported on functionalized CX presenting larger average mesopores size. The adsorption of the substrate (cellobiose), intermediate (glucose) and product (gluconic acid), influenced by the polarity of the carbon support, was found to have a significant effect on the selectivity to gluconic acid. The combined effect of the adequate texture and surface chemistry of the support played a vital role in the performance of the bifunctional catalyst in this process. It was proposed that a conformational change in cellobiose exposing glycosidic bond for hydrolysis, was induced upon adsorption in the larger mesopores of CX, leading to higher selectivity to glucose. On the other hand, the phenolic groups on the carbon surface played a double role as binding sites for cellobiose and as catalytic sites for the selective hydrolysis of cellobiose to glucose. The tandem reaction and its consecutive steps (cellobiose hydrolysis and glucose oxidation) were modeled, and the rate constants were derived and compared. The oxidation of glucose by Au nanoparticles was found to take place directly on the surface of the bifunctional catalysts without desorbing to the reaction medium. The present findings contribute to the advancement in understanding of the role of carbon acidic groups in the catalytic performance of bifunctional carbon supported gold catalysts in one-pot tandem reactions of biomass valorization.