Abstract
Epoxidized methyl esters (EMO) with their high oxirane ring reactivity, acts as a raw material in the synthesis of various industrial chemicals including polymers, stabilizers, plasticizers, glycols, polyols, carbonyl compounds, biolubricants etc. EMO has been generally quantified by the gas chromatography (GC) and high performance liquid chromatography (HPLC) techniques. Taking into the account of the limitations of these techniques, two qHNMR based equations have been proposed for the quantification of EMO in the mixture of EMO and methylesters (MO). The validity of the proposed method was determined using standard mixtures of MO and EMO having different molar concentrations. The developed equations have been applied on the samples of EMO prepared from oleic acid in two step process viz ., esterification followed by epoxidation. The qHNMR based EMO quantification showed acceptable agreement with the results obtained from HPLC analysis.
Key words: Oleic acid, Methyl oleate, Epoxidized methyl oleate,1H NMR Quantification, Epoxidation, Fatty acid methyl esters.
Introduction
1H NMR spectroscopy is one of the most commonly used spectroscopy technique for the structure elucidation of the synthetic and natural organic compounds (Elyashberg 2015). 1H NMR has conjointly been applied as a quantitative analytical tool for the quantification owing to the fact that the signal intensity in1H NMR corresponds directly to the number of proton nuclei responsible for that signal (Rizzo & Pinciroli 2005). Quantitative 1H NMR (qHNMR) has emerged as an effective analytical tool for quality analysis in various fields such as natural drugs (Yan et al. 2016), food and beverages (Cao et al. 2014), PLGA based microspheres (Zhang et al. 2017), medicinal components (Göğer et al. 1999; Hollis 1963) and dietary supplements (Phansalkar et al. 2017). Apart from qHNMR, HPLC and GC are widely used chromatographic techniques for the quantification of the molecules (Gelbard et al. 1995). GC with its destructive nature required complex operational procedures including handling of explosive H2 gas, volatile substances and mass spectra for product conformation (Monteiro et al. 2008). On the other hand in case of HPLC, requirement of reference standards, HPLC grade solvents, specific detector for the compound of interest and equilibration of the columns has made the technique costly and time consuming (Sun et al. 2017).The limitations of both the chromatographic techniques can be overcome by qHNMR as it is rapid technique with no specific requirement of reference standard and offers recovery of the analyte after analysis (Cerceau et al. 2016). Moreover the amount of solvent (deuterated CDCl3, DMSO, D2O) required for each qHNMR analysis is minimum (~0.5 ml) as compared to that for HPLC and GC methods.
In the past decade, fatty acid methyl esters (FAMEs) popularly known as biodiesel (BD) has gained the attention in the automobile industry worldwide as a sustainable, non-toxic and biodegradable substitute for the diesel fuel (Su & Guo 2014). However, a high degree of unsaturation in the fatty acid chain of BD has lead to a decrease in its oxidative stability thus limiting its applicability as bio-lubricants (Kumar & Ali 2012). Epoxidation of the double bond of the unsaturated fatty acid chain to form epoxidized fatty acid methyl esters (EFAMEs) or epoxidized methyl oleate (EMO) is one of the alternatives to improve the oxidative stability of biodiesel. This approach has opened avenues for exploring the use of formed oleochemicals as lubricants (Suarez et al.2009). Beside acting as lubricants, EFAMEs such as epoxidized methyl oleate (EMO) have been utilized as a building blocks for the synthesis of various products such as stabilizers in resins, substitutes to phthalates as plasticizer (Di Serio et al. 2012), surfactants (Doll & Erhan 2006), asphalt additives and transformer fluids (Milchertet al. 2015) as well as antifoaming compounds (Tiozzo et al. 2013), in cosmetics and pharmaceutical industry (Kumar & Ali 2012).
In the literature, the prepared epoxides have generally been quantified by the evaluation of oxirane number of the modified product (Di Serioet al. 2012) or by GC technique (Gorla et al. 2013). HPLC has also been used for the quantification (Orellana-Coca et al.2005) but requirement of sample derivatization has made this technique a tedious one (Dupard-Julien et al. 2007). As an alternative,1H NMR has also been utilized for the quantification of the epoxides. Although the NMR technique exhibit expensive instrumentation and requirement of expertise for carrying out the analysis but in comparison to HPLC it is less time consuming and require little of the solvent for carrying out one single analysis. Considering the above advantages, 1H NMR based derivation for the quantification of mono and di-epoxides from sunflower oil was repored by Aerts and Jacobs (2004). However, the authors proposed an extended formula utilizing peaks at 2.01 ppm and 2.90 ppm along with a separate peak at 0.88 ppm as internal standard. Moreover, the results obtained are not consistent with the recent studies on epoxides (Xia et al. 2016). Williamson and Hatzakis (2019) has also utilized the NMR characterization described by Xie et al. for the analysis of the epoxidised product formed from the extracted coffee oil which are potential candidates as bioplastic precursors. Considering the importance of the epoxides in the industrial field it is important to find out a simple reliable method for easy quantification of the epoxides in the reaction mixture.
To the best of our knowledge, qHNMR technique has not been further explored for the EMO quantification. To make the quantification process simple, quick and easy, we herein propose a qHNMR based derivation for the determination of the epoxidized methyl oleate (EMO) derived from oleic acid (OA).