Submitted to: Crop Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/16/2007
Publication Date: 7/11/2008
Citation: Weston, T.R., Derner, J.D., Murrieta, C.M., Rule, D.C., Hess, B.W. 2008. Comparison of catalysts for direct transesterification of fatty acids in freeze-dried forage samples. Crop Science 48:1636-1641. Interpretive Summary: Boron-trifluoride (BF3) has proven to be an effective catalyst for single-step direct transesterification of fatty acids in forages, but it is quite volatile, can be toxic if inhaled, and requires storage in cool and dark environment to maintain reactivity. In contrast, methanolic HCl is less volatile, maintains a long shelf-life without special preparations, and is cheaper than boron-trifluoride. Comparisons between boron-trifluoride and methanolic HCl as catalysts for single-step direct transesterification of fatty acids in forages, however, have not been conducted. We harvested biomass from seven primary species of the northern mixed-grass prairie, including warm-season grasses, cool-season grasses and forbs, and freeze-dried this material for testing the two catalysts. Both catalysts completely converted total lipid extracts to fatty acid methyl esters, and fatty acid concentrations did not differ between catalysts. Methanolic HCl costs $0.19 less per sample than boron-trifluoride, thereby making it a more cost-effective catalyst.
Technical Abstract: Preparation of fatty acid methyl esters from forages comparing BF3 in CH3OH to HCl in CH3OH as a catalyst in single-step direct transesterification has not been reported. Our objective was to compare 1.09 M methanolic HCl to 7% BF3 in CH3OH as catalysts for direct transesterification of fatty acids in freeze-dried forage samples. Samples included blue grama (Bouteloua gracilis), fringed sage (Artemisia frigida), western wheatgrass (Pascopyrum smithii), needle-and-thread (Hesperostipa comata), Dalmatian toadflax (Linaria dalmatica), needleleaf sedge (Carex eleocharis), and scarlet globemallow (Sphaelercea coccinea). Thin layer chromatographic evaluation revealed complete conversion of total lipid extracts to fatty acid methyl esters using both catalysts. Additionally, gas-liquid chromatography analysis confirmed similar (P=0.96) total fatty acid concentrations for both catalysts. Concentrations of most identified fatty acids were similar (P=0.17 to 0.99) for both catalysts. Total weight percentages of identified fatty acids and unidentified fatty acids were not affected (P=0.37) by catalyst (91.2 and 8.8% vs. 90.6 and 9.4% for HCl and BF3, respectively). Boron-trifluoride costs $0.19 per sample more than HCl. We conclude that 1.09 M methanolic HCl is both a cost-effective and appropriate substitute for 7% BF3 in CH3OH for preparation of fatty acid methyl esters from freeze-dried forage samples.