Abstract
The rapid decline of conventional energy sources coupled with a rapid increase in greenhouse gas production and thereby increasing ozone layer depletion and incessant rise in sea levels has ushered in a mad rush for renewable and CO2-neutral energy sources. There has thus been a growing interest for utilizing biomass as an energy source. Biomass derived from agricultural residues and energy crops such as corn are food-competing, and their use is therefore controversial and much debated about. As a result the focus of the industry has now shifted to utilize energy derived from non-food competing biomass feedstocks, such as forest residues and urban wastes. The biomass can be converted to bio-oils through a pyrolysis treatment in absence of oxygen. Biofuels derived from these sources are called second generation biofuels. Pyrolysis bio-oils have, in comparison to petroleum-based fuels, poor chemical properties, due to high water and oxygen content. Further upgrading to remove water and oxygen is needed to improve the bio-oil properties. A hydrotreating reaction to remove oxygen from bio-oils, hydrodeoxygenation (HDO), is carried out in this thesis. Previous research has shown that iron nanoparticles supported on mesoporous silica nanoparticles (MSN) denoted as (Fe-MSN) catalyzes the hydrotreatment of fatty acids with high selectivity for HDO over decarbonylation and hydrocracking. The catalysis is likely to involve a reverse Mars–Van Krevelen mechanism, in which the surface of iron is partially oxidized by the carboxylic groups of the substrate during the reaction. The strength of the metal–oxygen bonds that are formed affects the residence time of the reactants facilitating the successive conversion of carboxyl first into carbonyl and then into alcohol intermediates, thus dictating the selectivity of the process. The selectivity is also affected by the pretreatment of Fe-MSN, the more reduced the catalyst the higher the yield of HDO product. For this study the reaction system for the hydrotreatment of oleic acid using supported iron nanoparticles was adopted. Commercial SBA-15 was used as the support for impregnating iron nanoparticles for this study. SBA-15 has a hexagonal symmetry with pore diameters in the range of 5-8 nm and can be a perfect support for immobilization of metal nanoparticles. Also SBA-15 is thermally stable and can retain its structure at high temperatures used for the HDO reactions. Although HDO reactions are now being studied in detail all over the world, till now there has been no report of the effect of using CO2 alongside H2 while carrying out HDO reactions. The hydrotreatment of oleic acid in a green reaction media to produce the major HDO product octadecane was demonstrated in this thesis. The data show that hexane containing pressurized CO2 could dictate the selectivity of the hydrotreatment of oleic acid with high selectivity for HDO over decarboxylation/decarbonylation. Since the decarboxylation/decarbonylation pathway involves the removal of CO2 and CO, respectively, according to Le Chatelier’s principle, it was hypothesized that hexane containing CO2 could increase the yield of HDO product by preventing the forward reaction for decarboxylation/decarbonylation. It was also speculated that hexane containing pressurized CO2 could offer further beneficial effects on the yield of HDO products. One is that the presence of pressurized CO2 in hexane could reduce the viscosity of the reactant solution. As a result more uniform dispersion of the catalyst occurred in the solution and the diffusion resistances were reduced as well due to the increase in mass transfer. It is also believed that the solubility of H2, the reacting gas was also enhanced in hexane containing CO2 allowing for more H2 to bind to the surface of the iron nanoparticles. The effects of three individual operation variables namely, temperature, CO2 pressure and time and their interactions on the hydrotreatment of oleic acid were studied using a central composite design. The results showed that all these three variables were significant factors for increasing the yield of octadecane while CO2 pressure was the most significant variable in decreasing the yield of heptadecane, the major decarboxylation/decarbonylation product.