283366 Synthesis of Biodiesel Via Etherification of Biomass-Sourced Furanyl Alcohols
Synthesis of Biodiesel via Etherification of Biomass-Sourced Furanyl Alcohols
Eric Sacia, Balakrishnan Madhesan, and Alexis T. Bell*
Energy Biosciences Institute and Department of Chemical Engineering, University of California, Berkeley, CA 94720
The combustion of petroleum-sourced transportation fuels is one of the leading sources of anthropogenic CO2 emissions, and, therefore, is cited as a significant source of rising atmospheric CO2 concentrations.1 For this reason, significant emphasis has been placed in both the European Union2 and the United States3 to replace approximately 10-15% of the fuel supply with renewable sources in the next ten years. In order to meet these mandated targets, innovative schemes of fuel generation must be determined using the chemical functionality present in biomass derived molecules to replace not only gasoline blendstocks, but also diesel range molecules since diesel consumption far outpaces that of gasoline on a global level.4
In order to replace the C11-C22 alkanes and aromatics in current generation diesel fuels, molecules possessing similar properties with a similar cetane number must be synthesized. While transesterification of palm, rapeseed, or soybean oil to generate fatty acid methyl esters can produce fuels that meet cetane number requirements, they exhibit notable problems with pour point, oxidation stability, and raw material cost in addition to using a food crop as a fuel raw material. Diesel fuels can also be generated from 5-(hydroxymethyl)furfural (HMF) and furfural, the products of dehydrating glucose and xylose, sugars that can be sourced from lignocellulosic feedstocks.5 While furan condensation6 and aldol condensation7 derivatives provide intriguing diesel additives due to their formation of branched and linear alkanes, the high hydrogen input necessary to make these fuels is problematic. For this reason, we have investigated the generation of furanyl ethers as a source of biorenewable diesel fuel that has shown promise in diesel blends up to and exceeding 17%.8
Our work has focused on three primary pathways for forming furanyl ethers. First, we have evaluated the direct etherification of HMF to 5-(alkoxymethyl)furfurals using a variety of linear alkyl alcohols from methanol to 1-butanol as solvents. We discovered that acetal formation preceded ether formation for HMF etherification due to the mesomeric effect causing the aldehyde of HMF to be significantly electron withdrawing from the ring. High temperatures and longer reaction times led to decomposition of 5-(ethoxymethyl)furfural into ethyl levulinate.
In the second pathway, we began with furanyl alcohols that are formed from input of 1-2 moles of hydrogen into HMF and furfural. Hydrogenation of HMF generates bis-(hydroxymethyl) furan (BHMF) and methylfurfuryl alcohol (MFA). These molecules have already been shown to be byproducts in the formation of dimethylfuran (DMF), a candidate gasoline additive.9 The aldehyde linkage of furfural can readily be reduced to furfuryl alcohol, providing a pathway from hemi-cellulose to furanyl ethers as well. By performing etherification reactions on the entire spectrum of furanyl alcohols, we were able to gain valuable insight into trends in reactivity and key factors in selectivity of these reactions. By optimizing conditions, yields of furanyl ethers in excess of 98% were observed.
The third pathway we investigated was the direct conversion of fructose into furanyl ethers in ethanol and butanol as solvents. At temperatures of 110°C, fructose readily dehydrated to 5-(ethoxymethyl)furfural (EMF) (71%), the diethylacetal of EMF (10%) and ethyl levulinate (16%) for a combined yield of biofuel candidate molecules of 97% using Amberlyst-15, demonstrating an important pathway of forming attractive fuel candidates from biomass derived sugars.
Using the mechanism and rate law that were determined using heterogeneous sulfonic acid resin catalysts, specifically Amberlyst-15, we elucidated that furanyl alcohol etherification occurs via an SN1 reaction where the loss of water is the rate limiting step. This mechanism causes the kinetics to be first order in the furanyl alcohol and the acid catalyst and zero order in water and the alkyl alcohol. This mechanism further explains trends observed whereby decreasing alcohol solvent polarity caused an increase in reaction rate. The ring substituents of the furanyl alcohol were found to have a profound effect on the reactivity towards etherification. Electron donation to the ring greatly stabilizes the transition state in which the oxonium ion intermediate is formed. This causes MFA to have a higher reactivity than BHMF, which in turn is more active than HMF. It was also shown using 13C NMR spectroscopy that furanyl alcohol partition into the surface phase of the sulfonic acid resin plays an important role in the selectivity toward desired product formation. The results of the present study illustrate how compounds suitable as diesel can be produced in high yield from HMF and related furanyl alcohols.
1. Inventory of U.S. Greenhouse Gas Emissions and Sinks. EPA, Ed. Washington DC, 2011.
2. Renewable Energy Road Map, Renewable energies in the 21st century: building a more sustainable future. Brussels, 2007.
3. Regalbuto, J. R., Cellulosic Biofuels—Got Gasoline? Science 2009, 325 (5942), 822-824.
4. Namat Abu Al-Soof, F. S., Brahim Aklil, Mohammad Taeb, Mohammad Khesali, Mohammad Mazraati, Benny Lubiantara, Taher Najah, Amal Alawami, Claude Clemenz, Nadir Guerer, Garry Brennand, Jan Ban, Joerg Spitzy, Douglas Linton, James Griffin, Martin Tallett, Petr Steiner, Ula Szalkowska World Oil Outlook 2010; Organization of the Petroleum Exporting Countries: Vienna, Austria, 2010.
5. Dee, S.; Bell, A. T., Effects of reaction conditions on the acid-catalyzed hydrolysis of miscanthus dissolved in an ionic liquid. Green Chemistry 2011, 13 (6), 1467-1475.
6. Corma, A.; de la Torre, O.; Renz, M.; Villandier, N., Production of High-Quality Diesel from Biomass Waste Products. Angewandte Chemie International Edition 2011, 50 (10), 2375-2378.
7. Barrett, C. J.; Chheda, J. N.; Huber, G. W.; Dumesic, J. A., Single-reactor process for sequential aldol-condensation and hydrogenation of biomass-derived compounds in water. Appl Catal B-Environ 2006, 66 (1-2), 111-118.
8. Imhof, P.; Dias, A. S.; de Jong, E.; Gruter, G.-J., OA02 - Furanics: Versatile Molecules for Biofuels and Bulk Chemicals Applications. NAM Abstract 2009.
9. Chidambaram, M.; Bell, A. T., A two-step approach for the catalytic conversion of glucose to 2,5-dimethylfuran in ionic liquids. Green Chemistry 2010, 12 (7), 1253-1262.
See more of this Group/Topical: Catalysis and Reaction Engineering Division