Application of raman spectroscopy to monitor and quantify ethyl esters in soybean oil transesterification

Abstract

Biodiesel (FA esters) has become very attractive as an alternative diesel fuel owing to its environmental benefits. Transesterification is the most usual and important method to make biodiesel from vegetable oils. This article investigates the potential for using Raman spectroscopy to monitor and quantify the transesterification of soybean oil to yield ethyl esters. The differences observed in the Raman spectra of soybean oil after transesterification were a peak at 2932 cm −1 (ν CH 2 ), the displacement of the ν C=O band from 1748 to 1739 cm −1 , and the bands at 861 (ν R-C=O and ν C-C ) and 372 cm −1 (δ CO-O-C ). Uni-and multivariate analysis methods were used to build several analytical curves and then applied in known samples, treated as unknowns, to test their ability to predict concentrations. The best results were achieved by Raman/PLS calibration models (where PLS = partial least squares regression) using an internal normalization standard (ν =C-H band). The correlation coefficient (R 2 ) values so obtained were 0.9985 for calibration and 0.9977 for validation. Univariate regression analysis between biodiesel concentration and the increasing intensity of ν CH 2 band or ν C=O displacement showed R 2 values of 0.9983 and 0.9742, respectively. Although spectroscopic methods are less sensitive than chromatographic ones, the data obtained by spectroscopy can be correlated with other techniques, allowing biodiesel yield and quality to be quickly assessed.

Production, characterization and development of standards for biodiesel — A review

“…Hence, these chromatographic techniques are not suitable for online monitoring of a fast and reversible reaction like the transesterification reaction in the biodiesel production. The spectroscopic techniques, especially 1 H NMR [11], IR [11], Raman [12] and some physical measurements [13] have been employed for this purpose. Two most useful techniques are briefly described below.…”

A Raman spectroscopic approach to investigate the production of biodiesel from soybean oil using 1-alkyl-3-methylimidazolium ionic liquids with intermediate chain length

“…2) was analyzed, since it is the only relevant signature that does not interfere with those of the other compounds. The reliability of this vibrational signature for the evaluation of the transesterification progress has previously been validated by Ghesti et al, who have demonstrated a perfect match between reaction yields calculated from Raman spectroscopy and obtained from 1 H NMR or chromatographic experiments [30,31]. More precisely, we estimate the reaction yield (E) from the intensity of two contributions centered at 1732 cm À1 (for methyl esters or biodiesel) and at 1747 cm À1 (for soybean oil), where E is expressed as I 1732 =ðI 1732 þ I 1747 Þ Á 100, where I i are integrated Raman intensities and E is unit less and given in %.…”

“…Indeed, a shift from $1747 cm À1 to $1732 cm À1 , as well as an intensity increase of the m CH2 mode at about 2965 cm À1 (Fig. SI-2), is indicative of ester formation [30]. Even though not widely used yet, vibrational spectroscopy, in particular Raman spectroscopy, is a straightforward method to estimate the conversion from a vegetable oil into biodiesel [31].…”

Optimized production and advanced assessment of biodiesel: A review

“…The authors have pointed out that the spectroscopic methods are considered less sensitive than chromatography techniques, but the spectral data can be correlated with other techniques for the more rapid assessment of the qualitative and quantitative monitoring of the biodiesel. 70 Identification of the significant Raman spectral features associated with the samples under study is considered very important regarding the Raman spectral data analysis. Different properties of biodiesel, including density, viscosity, methanol content, and water concentration, are predicted by developing the prediction models on the basis of the different feature selection techniques.…”