Oxidative conversion of renewable feedstock: carbohydrate oxidation

Carbohydrates are the most abundant resource for the conversion of renewable feedstocks to useful chemicals and energy. Approximately 180 billion tons of biomass is produced from photosynthesis each year, including about 180 million tons of edible sugars and more than 1 billion tons of starch from grains. Carbohydrates can be transformed to fuels such as ethanol and ethyl tert-butylether, but triglycerides represent a better energy source owing to their high enthalpy and thus challenge petrol in diesel engines. The low enthalpy of carbohydrates does not suggest particular advantages in their direct utilization as a fuel. In recent applications, however, glucose has been successfully employed as a renewable feed for fuel cells. On the other hand, the six-carbon unit of glucose is one of the most prominent components of biomasses, is easy to produce, and represents one of the most attractive building blocks for the preparation of a variety of chemical intermediates. Thus, glucose will play a central role in feeding the so-called biorefinery, a future plant where chemical and biochemical processes are advantageously employed for the synthesis of intermediates and fine chemicals as an alternative to fossil-derived chemicals. Crystalline glucose and, preferably, its concentrated solution (syrup) are the starting materials for the production of several chemical intermediates, which represent the platform for other important chemicals derived from renewable biological sources according to standard chemical and biochemical processes. A variety of natural and genetically modified microorganisms allow the aerobic fermentation of glucose toward different products, whereas only a limited number of synthetic catalysts have been proven to be active in glucose-selective oxidation. As mentioned earlier, glucose is a strategic material for technological applications including chemical processes and alternative energy resources. Therefore, industrial and academic research is deeply involved in optimizing current technologies as well as developing novel processes. Particularly studied is the oxidation of glucose to gluconic acid and gluconates, nowadays performed by the enzymatic process based on Aspergillus niger mold. Besides positive aspects of this latter process, for example, mild and environmentally acceptable “quasi-green” conditions, some drawbacks arise mainly from the limited glucose concentration tolerated by the enzyme, thus necessitating large plants and the deep purification from the enzyme, due to the toxic effects of A. niger. This chapter focuses on the catalytic conversion of carbohydrates, in particular aerobic oxidations, with the aim of highlighting the potential of new emerging technologies.