Energy Fuels 2010, 24, 2170–2171 Published on Web 02/12/2010
: DOI:10.1021/ef9013373
Co-processing of Carbohydrates and Lipids in Oil Crops To Produce a Hybrid Biodiesel Mark Mascal* and Edward B. Nikitin Department of Chemistry and Bioenergy Research Group, University of California, Davis, 1 Shields Avenue, Davis, California 95616 Received November 11, 2009. Revised Manuscript Received February 5, 2010 Scheme 1
While it is a fact that a range of biomass-derived motor fuels are currently being developed and may eventually be brought to market, biodiesel is at present the only drop-in fuel registered with the United States Environmental Protection Agency (EPA) for use in unmodified diesel engines at any blend level up to 100% (B100). As a result, there has been a scramble in the renewable energy community to identify the most economical means of producing biodiesel from feedstocks that do not compete with the human food chain, and biodiesel derived either from algae or oil crops that grow well on marginal land hold considerable promise in this regard.1 Be that as it may, the economics of biodiesel production are burdened by the fact that the yield of oil per unit area of land is generally much less than that of cellulose from “energy crops”, such as switchgrass (Panicum virgatum). A potentially ideal situation would arise if both the triglyceride and carbohydrate content of plant biomass could be co-exploited in a single process to produce a hybrid lipidic/cellulosic biodiesel. Herein, we report the application of the biphasic acid/solvent biomass digester that we have described in previous work2 to the processing of oil seed feedstocks, resulting in a substantial increase in fuel production from this type of biomass. At present, biodiesel is produced by the transesterification of mainly plant-derived oils. Oil seeds of course also contain significant carbohydrate profiles, in the form of starch, cellulose (“fiber”), hemicellulose, and free sugars. We have previously shown that hexoses in any form, whether mono-, di-, or polysaccharides, can be converted into a mixture of 5-(chloromethyl)furfural (CMF) 1 and levulinic acid 2 in combined yields up to 95% (Scheme 1).2 The major product of the reaction is CMF 1, which accounts for between 70 and 90% of the organic material isolated, depending upon the reactor loading, while compound 2 comprises less than 10% of the product mixture. To our knowledge, this level of conversion of carbohydrate feedstocks into simple organic molecules is unrivaled in the literature. The question arose as to how this method would perform on high-oil-content biomass. We envisaged that concurrent extraction of the oil and conversion of the saccharides into CMF 1 and levulinic acid 2 was a plausible outcome, although exposure of unsaturated lipids to strongly acidic conditions left open the possibility of hydrolysis or other side reactions. The experiment consisted of introducing the feedstocks into a biphasic reactor containing aqueous hydrochloric acid and
Table 1. Yields of Lipid and CMF 1 (g) per Dry Kilogram Biomass Feedstocks lipid CMF 1 total organic extract soybean (Glycine max) sunflower (Helianthus annuus)a jatropha (Jatropha curcas) camelina (Camelina sativa) safflower (Carthamus tinctorius) canola (Brassica napus) a
67.9 110.2 78.6 102.5 113.0 120.0
324.9 468.9 470.7 477.0 484.3 568.3
Includes hull.
1,2-dichloroethane and heating the mixture at 80 °C for 3 h with periodic extraction of the aqueous phase with fresh solvent. Full experimental details of the procedure can be found in ref 2. Where necessary, the feedstock was first reduced to a coarse powder (ca. 10 mesh) by grinding in a mortar and pestle. The results for a number of typical oil crops are presented in Table 1. Because the cellular matrix is dissolved, we observe complete extraction of the lipids into the organic phase. When the experiment is conducted at 80 °C, the triglyceride structure remains fully intact. Raising the temperature to g100 °C led to the appearance of a minor side product resulting from the cleavage of a single fatty acid chain and substitution of the free glyceride hydroxyl group with chlorine. Because working at higher temperatures afforded no real advantages in terms of carbohydrate to CMF 1 conversion, reactions were generally run below 100 °C to avoid this hydrolysis reaction. As can be seen in the table, useful quantities of carbohydratederived CMF 1 were produced from each feedstock investigated. Bearing in mind that only the seeds of the crops in Table 1 were used in this study, the fraction of compound 1 could in principle be substantially increased if the entire plant, including stalks and leaves, were likewise processed. Although this idea is not relevant to jatropha, which is a perennial, one could otherwise envisage simply harvesting whole crops at maturity and grinding the plant matter and seeds together for conversion into CMF 1 and lipids. In a utilitarian sense, compound 1 can be thought to be much like plant oils, which are likewise not appropriate for use directly in automobiles, but can be converted into a suitable fuel by treatment with alcohols. The two are complementary in that they are ultimately sourced from different biological pathways in plants. The conversion of compound 1 and triglycerides 3 into a hybrid carbohydrate/lipid-based fuel cocktail is described
*To whom correspondence should be addressed. E-mail: mascal@ chem.ucdavis.edu. (1) Recent reviews: (a) Pinzi, S.; Garcia, I. L.; Lopez-Gimenez, F. J.; Luque de Castro, M. D.; Dorado, G.; Dorado, M. P. Energy Fuels 2009, 23, 2325–2341. (b) Demirbas, A. Energy Convers. Manage. 2009, 50, 14– 34. (2) Mascal, M.; Nikitin, E. B. ChemSusChem 2009, 2, 859–861. r 2010 American Chemical Society
257.0 358.7 392.1 374.5 371.3 448.3
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pubs.acs.org/EF
Energy Fuels 2010, 24, 2170–2171
: DOI:10.1021/ef9013373 Scheme 2a
a
R = fatty acid alkyl chain.
in Scheme 2. Although in previous reports we focused on the ethanolysis of CMF 1 into 5-(ethoxymethyl)fufural as a potential new-generation biofuel,3 we have recently determined that compound 1 can similarly be converted into ethyl levulinate 4 in high yield.4 Thus, heating mixtures of compounds 1 and 3 in ethanol at 200 °C for 6 h results in the conversion of compound 1 into ethyl levulinate 4, while at the same time, compound 3 is transformed into biodiesel ethyl ester 5. The latter reaction is the result of the in situ production of catalytic acid during the ethanolysis of compound 1. Putting this into practice, a 10 g sample of safflower seeds (dry weight) was extracted with hexane and the resulting oil submitted to standard base-catalyzed transesterification in ethanol to give 3.82 g of biodiesel ethyl ester. Alternatively, digestion of an equivalent sample in the acid/solvent reactor followed by heating of the organic extract in ethanol as described above gave a mixture of 3.77 g of biodiesel ethyl ester plus 0.92 g of ethyl levulinate, amounting to 24% more fuel for the same mass of feedstock. Ethyl levulinate is a nontoxic, γ-keto pentanoate ester with a boiling point of 206 °C and flash point of 91 °C. It is has good lubricity and has been tested in blends with petroleum diesel up to 10% with no change in the cetane number.5 Its shorter chain length suggests the potential to favorably impact the properties of biodiesel in terms of its cold performance issues (cloud point, pour point, and viscosity).6 A potential drawback of the process in Scheme 2 is that the glycerol byproduct is degraded into an unidentified mixture of compounds during the ethanolysis reaction. An alternative is to heat the mixture of compounds 1 and 3 in water, which leads to the analogous conversion of the CMF 1 into levulinic acid 2 but leaves the oil 3 completely intact. Levulinic acid 2 is considered an important renewable platform chemical and appears on the list of the top 12 value-added chemicals from biomass published by the Department of Energy’s National Renewable Energy Laboratory (NREL).7 Of course, the
resulting mixture of compounds 2 and 3 could also be converted into fuel esters, such as compounds 4 and 5, in a separate step using an alcohol and any of the acid catalysts currently being developed for lipid transesterification.8 Finally, evaporation of the aqueous phase of the reaction mixture during recovery of the hydrochloric acid gives a mixture of amino acid salts and small peptides, alongside trace amounts of levulinic acid 2. Standard amino acid analysis of the hydrolysate on a cation-exchange column with spectrophotometric identification of the ninhydrin derivatives indicated substantial fragmentation of the proteins. This was confirmed by mass spectrometry of the mixture, which shows individual amino acid ions as well as less abundant, higher mass peaks. Protein hydrolysates have a value-added market in nutraceutical, livestock feed, veterinary, aquaculture, cosmetic, and detergent-foaming agent applications. In summary, we report the application of our biphasic acid/solvent biomass reactor to the processing of soybean, sunflower, jatropha, camelina, safflower, and canola seeds, resulting in the conversion of the carbohydrates present in these feedstocks into the biofuel precursor CMF 1. Mixtures of the seed oil and CMF 1 can be submitted to ethanolysis to give ethyl levulinate 4 and biodiesel ethyl ester 5. Alternatively, heating seed oil-CMF 1 mixtures in water results in the hydrolysis of 1 to levulinic acid 2 while leaving the oil intact. Levulinate esters are short-chain oxygenates, which can be blended with diesel fuel and may improve its cold-performance properties. The methodology described here has the potential to revolutionize biodiesel production by merging lipid- and cellulose-based biomass conversion technologies and thereby substantially increasing the overall yield of fuel produced from oil crops. Acknowledgment. This work was supported by the U.S. Department of Energy (Award Number DE-FG36-08GO88161) and the Nevada Institute for Renewable Energy Commercialization (Award Number 2008/11/002). We also thank Stephen Kaffka for supplying samples of camelina, canola, and safflower and Sham Goyal for supplying a sample of jatropha.
(3) (a) Mascal, M.; Nikitin, E. B. Angew. Chem., Int. Ed. 2008, 47, 7924–7926. (b) Mascal, M.; Nikitin, E. B. ChemSusChem 2009, 2, 423–426. (4) Mascal, M.; Nikitin, E. B. Green Chem. 2010, in press. (5) McCormick, B. Renewable diesel fuels: Status of technology and R&D needs. Presentation at the 8th Diesel Engine Emissions Reduction Conference, Coronado, CA, Aug 25-29, 2003 (http://www1.eere.energy. gov/vehiclesandfuels/pdfs/deer_2002/session4/2002_deer_mccormick.pdf). (6) Chevron Corporation. Diesel fuels technical review, 2007 (http:// www.chevron.com/products/ourfuels/prodserv/fuels/documents/Diesel_Fuel_ Tech_Review.pdf). (7) Pacific Northwest National Laboratory (PNNL) and National Renewable Energy Laboratory (NREL). Top value added chemicals from biomass. Volume I-Results of screening for potential candidates from sugars and synthesis gas. Technical report identifier PNNL-14804, Aug 2004 (http://www1.eere.energy.gov/biomass/pdfs/35523.pdf).
Supporting Information Available: General experimental procedures for the processing of oil seeds into triglycerideCMF 1 mixtures, the ethanolysis of triglyceride-CMF 1 mixtures into ethyl levulinate 4-biodiesel ethyl ester 5 mixtures, and the hydrolysis of triglyceride-CMF 1 mixtures into triglyceridelevulinic acid 2 mixtures. This material is available free of charge via the Internet at http://pubs.acs.org. (8) Melero, J. A.; Iglesias, J.; Morales, G. Green Chem. 2009, 11, 1285– 1308.
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