Hierarchical nanoporous solid base catalysts for biodiesel synthesis
Woodford, Julia Jane
The discovery of alternative solid base catalysts to replace homogeneous catalysts currently used in the industrial synthesis of biodiesel could remove the need for atom and energy inefficient routes to the desired biofuel product, and allow for the possibility of a continuous production process.\ud Hydrotalcites have shown promise as catalysts in the transesterification of triglycerides with methanol to form biodiesel; however their activity is hampered by slow diffusion of the bulky triglycerides through the microporous hydrotalcite structure and poor accessibility of the active sites. This thesis has examined the synthesis of hydrotalcites via novel routes in an attempt to improve base site accessibility to triglycerides feedstocks in order to enhance catalytic performance. Macropore introduction into MgAl hydrotalcites helps to overcome mass transport limitations and increase their activity 10-fold for the transesterification of olive oil. Hydrotalcites prepared on an alumina support through a novel grafting and hydrothermal protocol form well-ordered crystallites on the high surface area oxide support. The resulting hydrotalcite-coated aluminas exhibit activities comparable to macroporous hydrotalcites of similar Mg:Al stoichiometries. Hydrotalcites prepared on alumina-grafted SBA-15 and macroporous-mesoporous SBA-15 employing the same grafting and hydrothermal synthesis are also extremely active in triglyceride transesterification, with the hierarchical macroporous-mesoporous outperforming the purely mesoporous SBA-15 support.\ud Comparative studies on non-porous solid bases derived from nanocrystalline MgO reveal that Cs doping via co-precipitation confers superior activity for tributyrin transesterification. X-ray absorption spectroscopy has been applied to probe the local chemical environment of Cs atoms within such Cs-doped MgO, and the catalytically active phase identified as Cs2Mg(CO3)2(H2O)4. Cs-MgO is an order of magnitude more active for the transesterification of bulky triglycerides and olive oil than the undoped, parent MgO nanocrystals.