Submitted to: Fuel
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/1/2009
Publication Date: 6/1/2009
Citation: Nagy, M., Kerr, B.J., Ziemer, C.J., Ragauskas, A.J. 2009. Phosphitylation and Quantitative 31P-NMR Analysis of Partially Substituted Biodiesel Glycerols. Fuel. 88:1793-1797. Interpretive Summary: The main step during biodiesel production is the catalytic transesterification of triglycerides. Glycerol and fatty acids are by-products of the biodiesel production and considered as contaminants in the final biodiesel product. By selectively measuring the amount of fatty acids and glycerol during the production of biodiesel, the triglyceride transesterification yield and subsequent biodiesel quality can be determined. This study examined the application of phosphitylation of glycerol hydroxyl groups with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP) followed by 31P-nuclear magnetic resonance (NMR) spectroscopy analysis. Data show that this methodology provided a rapid quantitative analytical technique for the determination of hydroxyl patterns on partially esterified glycerols as well as the detection of fatty acids in biodiesel. This novel method is important for biodiesel production engineers showing that the TMDP/31P–NMR technique provides a rapid and accurate method for biodiesel evaluation.
Technical Abstract: There is a constant need for new and improved analytical techniques to monitor the quality of incoming feedstocks and their respective conversion products in the biofuels industry. Current analytical methods in the biodiesel industry include high performance liquid chromatography, gas chromatography, various types of spectroscopy, and wet chemical techniques, which are often time consuming, and, due to initial sample preparation, extend the time for analysis and complicate data analysis. Recently, an alternative approach to characterizing mixtures of multi-substituted glycerols which involves an initial phosphitylation of hydroxyl groups of substituted glycerol with 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP) followed by 31P-nuclear magnetic resonance (NMR) spectroscopy was conducted with analytical pure compounds to establish a database of chemical shifts and demonstration of the applicability to a commercial biodiesel sample. This study examined the application of this methodology to a broad spectrum of glycerol samples from commercial biodiesel operations and evaluated the applicability of alternative phosphitylation agent 2-chloro-1,3,2-dioxaphospholane (DOP) instead of TMPD. The data show that TMPD was a better phosphitylating agent than DOP for the characterization of biodiesel and its processing components, providing a spectrum with a good separation and high quantitative accuracy even in case of a complex sample. The transesterification yield and the quality of the final biodiesel product could be determined by measuring the free hydroxyl groups in the reaction mixture. The novel solvent mixture used in this study was capable of handling samples with high hydroxyl content (e.g., pure glycerol) suggesting that this technique could be used for samples throughout the whole production line, from the parent oil up to the biodiesel and glycerol streams. This novel method is important for biodiesel production engineers showing that the TMDP/31P–NMR technique provides a rapid and accurate method for biodiesel evaluation.