Abstract
One of the most important challenges in the 21st century will be an intelligent change from a fossil-carbon-based economy to a sustainable economy based on renewable sources. Alternative energy sources are urgently needed, both to decrease the emission of greenhouse gases and to cover the world’s energy requirements. Renewable energy sources, such as solar and wind energy, geothermics, water power and biomass, can give important contributions to the energy supply of the future. In particular, biomass is interesting not only as energy source but also as a renewable raw material for the chemical industry.
Cellulose is the most abundant source of renewable carbon and does not directly compete with food supply. Therefore, this biopolymer is considered a promising feedstock for fuels, chemicals and materials. While cellulose-to-sugar is a “dream reaction”, the polymer is resistant against chemical, biological and mechanical processing.
For more than 100 years, researchers have been working on the depolymerization of cellulose to transform the polysaccharide into sugars. Producing cost-competitive sugars from cellulose, however, requires still efficient routes for its depolymerization. Most of the processes for hydrolyzing cellulose require harsh reaction conditions, under which undesired byproducts are formed. In addition, the generation of large amounts of industrial wastewater adds to the already high costs of current saccharification processes.
Recently, our group found that solid acids are efficient catalysts for the depolymerization of cellulose in ionic liquids (ILs).[1] The preliminary results arouse interest to expand research on depolymerization of cellulose, as an entry point process for lignocellulose-based biorefinery. In this PhD work, different routes for the catalytic depolymerization of cellulose were exploited, as depicted in Figure 1.1.
Chapter 2 provides an overview of the state of the art for the hydrolysis of cellulose. In addition, the specific task of ILs in the depolymerization of cellulose is discussed. Furthermore, a brief introduction to the broader field of mechanochemistry with focus on mechanical treatment of cellulose is given.
In the chapter 3, depolymerization of cellulose in ILs is presented. This process is different from that taking place in aqueous phase because cellulose is fully dissolved in the IL and, therefore, exhibits high reactivity. Once cellulose is in solution, the development of solid catalysts for this system emerges as an interesting target. Therefore, the performance and role of solid catalysts for this reaction were studied. The effect of common impurities in the ILs, e.g. imidazoles, exerted on the reaction performance was also investigated. Because cellulose is a cheap raw material, particular attention was paid to the ILs in this process, so the recycling of the IL was evaluated.
In chapter 4, the thermal stability of imidazolium-based ILs is addressed. Thermal gravimetric analysis is often used to determine the decomposition temperature of ILs, but this method revealed to be a suboptimal tool to assess the long-term stability of ILs. For this reason, different analytical methods were explored in order to verify their ability to determine, precisely and quickly, even low levels of degradation of ILs.
In chapter 5, considering the drawbacks of using ILs in cellulose processing, solvent-free mechanically assisted routes for the depolymerization of cellulose were explored. Different inorganic and organic acids were tested in the solid-state depolymerization of cellulose. To get a better understanding of the reaction, further experiments starting from cellobiose and glucose were performed. The depolymerization products were thoroughly analyzed by several techniques. Moreover, the products obtained by the mechanical processing were subjected, in a subsequent step, to enzymatic and dilute acid hydrolysis. Finally, the mechanically assisted depolymerization route was applied to practical lignocellulosic feedstocks, such as wood, sugarcane bagasse and switchgrass.
In chapter 6, the effect of salts on the hydrolysis of cellulose was studied. Experiments were carried out with cellobiose and maltose dissolved in aqueous solutions of metal halides. In addition, hydrogels of cellulose were used as substrates for the salt-assisted, acid-catalyzed hydrolysis. Furthermore, the depolymerization of cellulose dissolved in organic electrolytes comprising different mole fractions of IL was carried out to assess the effect of the IL on the reaction performance. As a final point, the catalytic activity of salts was studied also in the solid state reaction, i.e. in the mechanically assisted depolymerization of cellulose.
