Title : Selective oxidation of free mono- and oligosaccharides using photocatalysis
Abstract:
Thanks to their renewability and abundance, the use of carbohydrates as biomass feedstock and their eco-friendly conversion into useful chemicals are becoming increasingly attractive to prepare products capable of replacing petroleum-based chemicals. Sugars are polyfunctional and so, can lead to a wide variety of high-value products such as detergents, pharmaceuticals and cosmetics after chemical modifications. However, usually, glycochemistry does not satisfy the chemistry principles and shows difficulties to be transferred into industrial levels: multi-step protocols and hazardous chemicals are often required. Therefore, the development of novel oxidation methodologies using reusable heterogeneous catalysts and eco-friendly procedure is a very interesting alternative. Heterogeneous photocatalytic transformation constitutes a viable and environmentally friendly procedure since it can be ideally driven only by sunlight absorption. One of the best and most versatile photocatalyst developed so far is the anatase TiO2, which suffers from several drawbacks such as its large band-gap value of 3.20 eV restricting its absorption to UV light and its poor selectivity. The poor light harvesting can be overcome by the introduction of noble metal nanoparticles such as Au which affords to extend light absorption up to ca. 550 nm thanks to concomitant surface plasmon resonance effect (SPR). In addition, gold NPs in contact with an adequate semi-conductor prolongs the excited-state lifetime of gold as a result of the improved charge separation process. These photocatalytic properties open new perspectives for photo-assisted organic chemistry and for the selective photo-oxidation of organic compounds. We present herein a fast and efficient methodology for the selective photo-oxidation of mono- and oligosaccharides into corresponding sodium aldonates in only 10 minutes under standard incident sunlight irradiation. Three photocatalysts will be compared, namely Au/Al2O3, Au/TiO2 and Au/CeO2, the latter being the most efficient (TOF > 750 000 h-1) and perfectly selective. The versatility of this methodology will be discussed and the reusability of the photocatalysts demonstrated. We will also discuss the nature of the intermediate species governing the mechanism, thanks to complementary investigations.
