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).