Etherification of Biomass Derived Molecules for Effective Diesel Fuel Production

Etherification of Biomass Derived Molecules for Effective Diesel Fuel Production

Eric Sacia and Alex T. Bell*

Energy Biosciences Institute and Department of Chemical Engineering, University of California, Berkeley, CA 94720

Email: alexbell@uclink.berkeley.edu

The Environmental Protection Agency has estimated that anthropogenic CO₂ emissions due to the transportation sector in the United States account for nearly 1/3 of net CO₂ emissions (1.7 billion metric tons in the year 2009)¹. Since the combustion of fossil fuels is considered to be a major contributor to the growth of atmospheric CO₂ levels, there is currently significant interest in finding ways to reduce the net carbon footprint from transportation fuels. One approach is to convert biomass to both gasoline and diesel range fuels. Toward this aim, the EU has set a target that 10% of transportation fuels should be biomass sourced by 2020 in addition to the DOE's estimate that 20% of U.S. fuels will be sourced from biomass by 2030.

To produce high quality diesel fuels, compounds with high cetane numbers must be formed that can replace the C11-C22 hydrocarbons derived from petroleum. While biodiesel can be produced by transesterification of palm, rapeseed, or soybean oil, the supply of these oils is limited. Further, there are challenges in scalability and in the development of effective heterogeneous catalysts for transesterifaction². An alternative method is to form diesel components from sugars generated through the hydrolysis of the cellulosic components of lignocellulosic biomass. The reaction pathways in this work have been  focused on producing furanic ethers since it has been shown that such products have high cetane numbers and can be blended into petroleum-derived diesel up to 17 wt% ³.

 Our work has focused on two paths of utilizing the hydroxymethyl furfural (HMF) that is generated from dehydration of glucose. First, we have carried out the direct etherification of HMF with each primary alcohol from methanol through 1-butanol with emphasis on ethanol and 1-butanol since these alcohols can already be readily formed from biomass. The second pathway involves etherification of the hydrogenation products of HMF as the aldehyde is reduced to an alcohol, forming bis-hydroxymethyl furan (BHMF), then to a methyl group, forming methylfurfuryl alcohol (MFA) ⁴. This effort has examined both liquid and solid acids as catalysts and the effects of solvent on the rates of etherification. By examining the effect of acid choice, it was determined that specific solid acids could be used to increase selectivity beyond the homogeneous result through catalyst shape selectivity and environment. Under optimum conditions ether yields in excess of 98% have been achieved.

 The kinetics of etherification have been investigated and found to be first order in the furfuryl alcohol and the acid catalyst but zero order in ethanol. Further, the rate of etherification is highly sensitive to the ring substituents. As the substituent on the 2 position of HMF is altered from an aldehyde to a hydroxymethyl group (BHMF), and finally to a methyl group (MFA), the substituent becomes more electron donating. Increasing electron donation to the ring decreases the activation energy of the etherification and increases the rate significantly. Decreasing solvent polarity has also been observed to increase reaction rate by less stabilizing charged reaction intermediates. Rate parameters for each step of etherification have been measured and the variations in these parameters can be interpreted in terms of the structure and composition of the reactants and the solvent. The results of this work will contribute to a better understanding of the means for converting biomass-derived sugars into products suitable for blending into diesel.

1.            Inventory of U.S. Greenhouse Gas Emissions and Sinks:. Agency, U. S. E. P., Ed. Washington DC, 2011.

2.            Huber, G. W.; Iborra, S.; Corma, A., Synthesis of transportation fuels from biomass: Chemistry, catalysts, and engineering. Chem Rev 2006, 106 (9), 4044-4098.

3.            Imhof, P.; Dias, A. S.; de Jong, E.; Gruter, G.-J., OA02 - Furanics:  Versatile Molecules for Biofuels and Bulk Chemicals Applications. NAM Abstract 2009.

4.            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.