FT-Raman Spectroscopy quantification of biodiesel in a progressive soybean oil transesterification reaction and its correlation with 1H NMR Spectroscopy methods

Biodiesel fuel (fatty acid esters) has become more and more attractive due to its environmental benefits. Transesterification is the most common and important method for making biodiesel from vegetable oils or animal fats. Several studies have focused on the development and improvement of analytical methods for monitoring biodiesel production and determining the fuel quality. Analytical procedures reported in the literature include chromatographic methods (e.g., gas chromatography, high-performance liquid chromatography, gel permeation chromatography, etc.) and spectroscopic methods [e.g., H-1 and C-13 NMR, near infrared, Fourier transform infrared spectroscopy, and recently, Fourier transform (FT)-Raman]. The study presented in this paper expands our previous research, in which FT-Raman spectroscopy combined with partial least squares (PLS) multivariate analysis was successfully applied to the quantification of soybean oil/ethyl ester mixtures. The FT-Raman/PLS methods developed by our group were used to monitor and quantify a transesterification reaction process involving soybean oil and ethanol to produce fatty acid ethyl esters (biodiesel) over 22 h catalyzed by a heterogeneous Lewis acid catalyst. The results were successfully correlated with two 1H NMR spectroscopic methods reported in the literature and a new I H NMR method proposed in this work that can be easily extended to other vegetable oils. The correlation coefficients (R-2) obtained from the linear fit between FT-Raman measurements and the above H-1 NMR methods were 0.9974, 0.9847, and 0.9972, respectively.