Abstract
The current chemical industry relies largely on limited fossil resources. In the last two centuries, abundant and cheap coal and oil were driving the economic development. In future, more sustainable solutions are needed. The growing population of the planet with its growing demand for energy will lead to shortage of fossil resources, and thereby, make other feedstock economically more attractive.
The only alternative carbon sources are CO2 and biomass. CO2 can be reduced to versatile molecules, but these processes require large amounts of hydrogen and/or energy, which is in the near future not available in excess and therefore of limited economical prospect. However, should energy or H2 be abundant, CO2 utilization should then become a key technology, providing carbon feedstock for the chemical industry.
In biomass, the CO2 is already partially reduced via photosynthesis according to Eq. 1:
n CO2 + n H2O -> (CH2O)n + n O2
In the near future, biomass has a huge potential as carbon feedstock, due to its abundance. The amount of yearly produced biomass is 1.7-2.0·1011 tons, and only a small fraction is currently used. This number excludes marine biomass.[1]
The concept of biorefineries, facilities that convert biomass into fuel, power, and chemicals, in contrast to today’s petroleum refineries, has evolved to aim at conversion of lignocellulosic biomass. For that, two pathways are discussed, one via the syngas platform, and the other via the sugar platform. The advantage of the syngas platform is that syngas is already used today widely as feedstock for well-established processes. The advantage of the sugar platform is the wide scope that modern biotechnology offers.
The conversion of sugar, starch, or oil crops, is chemically relatively easy, but requires expensive feedstock that is produced at low yield per agricultural area, causing N2O emissions, and posing competition to food production. The conversion of lignocellulosic biomass is chemically more challenging, but offers a number of advantages over the conversion of sugar, starch or oil crops. The utilization of cellulose, hemicellulose, and lignin results in higher yields per area, since the whole plant and not only the fruit is converted. In turn, lignocellulosic feedstock is more abundant and cheaper, especially considering the utilization of waste and the cultivation of unsuitable land. Therefore, the overall costs of the feedstock are lower, and fewer greenhouse gases are emitted. Importantly, no competition to the food supply arises.
In this work, the utilization of lignocellulosic feedstock was investigated. The main hurdles for efficient cellulose conversion, the insolubility and recalcitrance, were overcome through mechanocatalytic depolymerization, yielding water-soluble oligosaccharides (WSO). The WSO can be converted into sugars and sugar alcohols under mild conditions. Thus, the hydrolysis of WSO and the hydrolytic hydrogenation of WSO were investigated.
By hydrolysis of WSO sugars are formed, which can be further converted into platform chemicals by fermentation or catalytic reactions. Focus of this work was on hydrogenation of glucose and the hydrolytic hydrogenation of WSO using Ru/C as a catalyst forming sugar alcohols. The main product of the reaction is sorbitol. Figure 1.1 illustrates an overview of the reactions.
