1 Centre for Catalysis and Sustainable Chemistry, Department of Chemistry, Technical University of Denmark2 Department of Chemistry, Technical University of Denmark
Due to rising oil prices and global warming caused by CO2 emissions, there is an increased demand for new types of fuels and chemicals derived from biomass. This thesis investigates catalytic conversion of cellulose into sugars in ionic liquids and the important platform chemical 5-hydroxymethylfurfural (HMF). The thesis focuses on kinetic and mechanistic investigations using new in-situ FTIR spectroscopic methods based on the ATR-principle. At first the kinetics of cellulose hydrolysis and the simultaneously HMF formation was investigated in the ionic liquid 1-butyl-2,3-dimethylimidazolium chloride using sulfuric acid, solid acids and Lewis acidic chromium(III)chloride as catalysts. Initially the important glycosidic group vibration was located at 1155 cm-1. The new in-situ spectroscopic method successfully determined activation energies for hydrolysis to be 92-96 kJ/mol regardless of the catalyst used. The often used cellulose model cellobiose was found to hydrolyze substantially easier with an activation energy of only 69 kJ/mol. The activation energies of HMF formation could simultaneously be determined to be 84 and 102 kJ/mol for Brønsted and Lewis acidic catalysis respectively. The low activation energies suggest that the ionic liquid acts co-catalytic by stabilizing the oxocarbenium transition state. The chromium catalyzed conversion of glucose to HMF in ionic liquid 1-butyl-3-methylimidazolium chloride with CrCl3⋅6H2O and CrCl2 as catalysts was investigated. The CrCl3⋅6H2O catalyst exhibited high initial conversion rates but suffered from pronounced product inhibition. The rates were 2-3 higher if water was removed simultaneously during reaction. Independent of whether water was presence or not activation energies energies were found to be 100-102 kJ/mol. For CrCl2 the initial rates were around 8 times lower but the activation energy was identical the the ones found for CrCl3⋅6H2O. Thus the activity was attributed to around 12 % of chromium(III) that was found to present in the sample. The CrCl2 showed no sign of product inhibition and followed first order kinetics, which resulted in high conversion at longer reaction times compared to CrCl3⋅6H2O. In a proposed mechanism this was suggested to be due to a CrII/CrIII synergy. A kinetic model based on active monomeric [CrCl6]3- species was proposed showing that the product inhibition resulted in second order like kinetic behavior. The fructose dehydration was investigated in both the presence and absence of CrCl3⋅6H2O. The partly dehydrated fructose intermediates were accumulated in the absence of chromium and water, leading to formation of humins. In the presence of CrCl3⋅6H2O the reaction was selective and the rates were 6-30 times higher with an activation energy of 74 kJ/mol. The thesis identifies the product inhibition as a major challenge for technical utilization of chromium catalysts in biomass conversion.