Biomass Conversion in Ionic Liquids

Biofuels obtained from conversion of straw and other agricultural and forestry waste products is known as second generation biofuels. For economic and ethical reasons, second generation biofuels have preference over first generation biofuels which are derived from agricultural products that could also serve as food. Conversion of cellulose is a key challenge in this respect. The thesis investigates catalytic conversion of cellulose into sugars and the important platform chemical 5-hydroxymethylfurfural (HMF).

Several concepts for cellulose conversion catalysis are available. The project focusses on ionic liquid (IL) catalysts. ILs are salts that melt at relatively low temperatures, typically comprising a bulky organic cation and a smaller inorganic anion. They are very tunable solvents and a wide range of properties can be achieved by combining different cat- and anions. Some ILs have been shown to dissolve cellulose in rather high amounts, as the IL anions are able to break up the crystalline structure of the cellulose crystals. When cellulose is dissolved, the glucoside bonds become exposed and can easily be hydrolyzed by acidic catalysts. The cellulose can also be precipitated from the IL by addition of small amounts of water, yielding amorphous cellulose, which is significantly easier to hydrolyze enzymatically. Such pre-treatment could become commercially attractive in future second generation biofuel plants.

In the project, the kinetics of hydrolysis and the simultaneous HMF formation was investigated using sulfuric acid, solid acids and Lewis acidic chromium(III)chloride as catalysts. A new in-situ Fourier Transform Infrared (FTIR) spectroscopic method successfully determined activation energies for hydrolysis to be 92-96 kJ/mol regardless of the catalyst used. The activation energies of HMF formation could be determined to 84 and 102 kJ/mol for Bronsted and Lewis acidic catalysis, respectively. The low activation energies suggest that the IL acts co-catalytic by stabilizing an oxocarbenium transition state.

Further, conversion of glucose with chromium(III)chloride or chromium(II)chloride as catalysts was investigated. The Cr(III) catalyst exhibited high initial conversion rates but suffered from pronounced product inhibition. Activation energies were found to be 100-102 kJ/mol. For Cr(II) the initial rates were around 8 times lower but the activation energy was identical. Notably, the Cr(II) showed no sign of production inhibition and followed an apparent first order kinetics, which resulted in high conversion at longer reaction times compared to Cr(III). In a proposed mechanism, this was suggested to be due to a Cr(II)/Cr(III) synergy involving electron transport between the Cr centers.

A kinetic model based on active monomeric [CrCl3]3- species was proposed showing that the product inhibition resulted in second order like kinetic behavior. The thesis identifies product inhibition as a major challenge for the utilization of chromium catalysts in biomass conversion.