Catalytic Pathways towards the Synthesis of Fuels and Specialty Chemicals from Biomass-Derived Platform Molecules

Production of alternative fuels and chemicals from biomass sources remains one of the greatest contemporary challenges for the field of catalysis. Due to its many advantages for growth and production, lignocellulosic biomass has been identified as a beneficial source of chemical energy for biofuels and biomass-derived chemicals as accessing petroleum reservoirs becomes more difficult. However, a major obstacle is to generate fuels that are compatible with current generation engines and distribution networks from the oxygen-rich sugars present in biomass./nTo satisfy various fuel requirements, we have explored multiple routes towards the generation of gasoline, diesel, and jet fuel replacements from biomass-derived platform molecules. In order to achieve this goal, we focused on utilization of furans and furan-derivatives as platform molecules since they are available by dehydration of biomass-derived sugars. Furanyl ethers were identified as attractive additives for diesel fuels due to the low H2 inputs required to produce them, the high cetane number of ethers, and the high energy density the resultant furanyl ethers. We were able to develop a two-step, one-pot reaction scheme to produce targeted furanyl ethers directly from fructose, as reported in Chapter 2. Elucidation of the mechanism of furanyl alcohol etherification was completed through detailed kinetic studies, and key factors governing activity and selectivity are presented in Chapter 3. As part of this effort, the beneficial influence of surface partitioning on maintaining high selectivity in the reaction over a polymeric resin catalyst, Amberlyst-15, was demonstrated using NMR./nIn addition to etherification of furanyl alcohols, various C-C bond formation reactions were also studied due to their utility in fuel upgrading reactions. This work is reported in Chapters 4-8. An emerging class of fuel precursors that can be formed through acid-catalyzed C-C bond-forming condensations of furanyl aldehydes with 2-methylfuran are described in Chapter 4. We were able to design effective solid acids that could carry out this reaction rapidly and selectively under solvent-free conditions. Similar to furanyl alcohol etherification, the catalyst surface environment strongly influenced the reaction. Using furan condensation, we synthesized a variety of pathways to C11-C21 compounds that are also good diesel precursors but have too low of a cetane number. We hypothesized that selective hydrogenation of these aromatic intermediates to the cyclic ether analogs would generate fuels with high cetane numbers and significantly better viscosity and chemical stability. This idea was later confirmed by detailed fuel testing and is reported in Chapter 5. To form these products, we identified catalysts that could selectively hydrogenate the furan ring without cleaving the C-O bond to produce alcohols via hydrogenolysis. Tuning the properties of the surface greatly affected the selectivity of the catalyst. By forming this previously unexplored class of molecules, our scheme avoids hydrogen-intensive hydrodeoxygenation to alkanes, a process which would require 55-60% more hydrogen./nBase-catalyzed condensation reactions have also been found to be highly useful in producing replacements for petroleum-derived fuels. These products are typically stable throughout the process of hydrodeoxygenation and can be used to make hydrocarbons that require little to no further regulatory approval. Starting from 2,5-hexanedione (HD), a platform molecule available from 5-hydroxymethylfurfural, we identified a heterogeneous base catalyst for the self-condensation of HD to 3-methylcyclopent-2-enone. This work is discussed in Chapter 6. Hydrogenation of this product produced methylcyclopentane, a gasoline additive with a high blending octane number (103) that is fully compatible with current gasoline./nn-alkyl methyl ketones, derived from either furanic compounds or compounds obtained via the fermentation of sugars, undergo base-catalyzed condensation under conditions similar to those used for the self-condensation of HD to yield cyclic trimer condensates in exceptionally high yields (95-98%, see Chapter 7 and 8). After hydrodeoxygenation, the resulting C12-C18 alkanes exhibit exceptional cold-flow properties and high energy density, making them ideal as jet fuel replacements. The scope of n-alkyl methyl ketones available from biomass is very broad, also allowing for production of higher-margin products, such as automobile lubricants. Jet fuel and lubricant products produced in this manner are demonstrated to result in a reduction of greenhouse gases of up to 87% compared to production of these products from petroleum. The results of this work are presented in Chapter 7. By understanding the mechanism and the reaction kinetics, we were able to create an alkane mixture that reproduces the entire boiling range of jet-fuel. These results are presented in Chapter 8. Using the suite of technologies developed in this work, varieties of novel, drop-in products could be made for gasoline, diesel, jet fuel, and even lubricants, thereby exemplifying the versatility of biomass-derived platform molecules.

Advisor: Bell, Alexis T.