INTRODUCTION
Vegetable oils are part of a larger family of chemical compounds known as fats
or lipids. They are made up predominantly of triesters of glycerol with fatty
acids and commonly are called triglycerides. Lipids are widely distributed in
nature; they are derived from vegetable, animal and marine sources and often
are by-products in the production of vegetable proteins or fibers and animal
and marine proteins. Fats of all types have been used throughout the ages as
foods, fuels, lubricants and starting materials for other chemicals. This wide
utility results from the unique chemical structures and physical properties
of fats. The chemical structures of lipids are very complex owing to the combination
and permutations of fatty acids that can be esterified at the three (enzymatically
non-equivalent) hydroxyl groups of glycerol. A generalized triglyceride has
the structure shown in Fig. 1 (Wallace,
1978).
Many naturally occurring fats are made up of fatty acids with chain length
greater than 12 carbon atoms; the vast majority of vegetable and animal fats
are made up of fatty acid molecules of more than 16 carbon atoms. Marine fats
are characterized by their content of larger-chain (up to C24) fatty
acids. Thus, because the fatty acid portions of the triglycerides make up the
larger proportion (ca 90% fatty acids to 10% glycerol) of the fat molecules,
most of the chemical and physical properties result from the effects of the
various fatty acids esterified with glycerol (Wallace, 1978).
|
| Fig. 1: |
General structure of a triglyceride |
Non-edible oils uses depend mainly on the properties of the various fatty acids
and fats although, some triglycerides are directly used as specialized lubricants.
The many derivatives of fatty acids manufactures are used in surface coatings,
plastics, detergents, lubricants, etc, where the long hydrocarbon chain confer
needed plasticity, surface activity, or lubricity. In some instances, eg, drying
oils, the reactive unsaturation of certain fatty acids is exploited and by means
of catalyzed, oxidative polymerization, tough, flexible surface coatings are
developed (Wallace, 1978).
The industrial chemistry of oils and fats is a mature technology, with decades
of experience and refinement behind current practices. Environmental pressures
demand cleaner processes and there is a market for new products. Current developments
are in three areas: green chemistry, using cleaner processes, less energy and
renewable resources; enzyme catalyzed reactions, used both as environmentally
friendly processes and to produce tailor-made products and novel chemistry to
functionalize the carbon chain, leading to new compounds. Changing perceptions
of what is nutritionally desirable in fat-based products also drives changing
technology; interesterification is more widely used and may replace partial
hydrogenation in the formulation of some modified fats.
The coverage in this study is necessarily selective, focusing on aspects of fatty acid and lipid chemistry relevant to the epoxidation and industrial exploitation of oils and fatty acids. The emphasis is on fatty acids and acylglycerols found in commodity oils and the reactions used in the food and oleochemical industries. Current areas of research, either to improve existing processes or to develop new ones, are also covered, a common theme being the use of chemical and enzyme catalysts.
Compounds of second-row transition metals rhodium and ruthenium and the oxides
of rhenium and tungsten have attracted particular interest as catalysts for
diverse reactions at double bonds. Recent interest in developing novel compounds
by functionalizing the fatty acid chain is also mentioned. To date, few of these
developments have found industrial use, but they suggest where future developments
are likely. A number of recent studies and books cover and expand on topics
discussed here (Akoh and Min, 2002; Gunstone
and Hamilton, 2001; Gunstone and Padley, 1997; Gunstone,
1996).
FATTY ACIDS
The number of triglycerides in a given natural fat is a function of the number
of fatty acids present and the specificity of the enzyme system involved in
the particular fat synthesis reaction. Litchfield (1972)
points out that many plant seed fats have the potential to provide 125-1,000
different triglycerides, animal fats contain potentially 1000-64000 triglycerides
species and butter fat could generate 2863288 triglycerides from 142 different
fatty acids.
Most of the fatty acids in fats are esterified with glycerol to form glycerides.
However, in some fats, particularly where abuse of the raw materials has occurred
leading to enzymatic activity, considerable (>5%) free fatty acid is found.
Hydrolysis occurs in the presence of moisture. This reaction is catalyzed by
some enzymes, acids, bases and heat. Most producers of fats attempt to prevent
the formation of free acids because certain penalties are assessed if they are
present in the trading of crude and refined fats. As in the case of triglycerides,
the number of known fatty acids is very large. Ca 900 vegetable and 500 animal
fats have been analyzed Table 1 lists fatty acid prevalent
in fats with their principal natural source and technical designation (Wallace,
1978).
Phospholopids (Fig. 2) occur in most natural fats with different amounts and compositions depending on the source of the fat. Owing to their complexity, these fat-soluble, biologically important compounds also have presented some intriguing analytical problems to chemists and biochemists.
| Table 1: |
Representative fatty acids |
 |
| aNo. or carbon atoms:number of unsaturation |
|
| Fig. 2: |
Structure of phospholipids |
From a technical standpoint, phospholipids, eg, from soybean, are composed
mainly of lecithin, cephalin (phosphatidyethanolamine) or phosphatidylinositol.
These complex mixtures (2-3% in soybean oil) are hydrated during the degumming
step, removed and dried. These products are sold as commercial lecithin used
in margarines, confections and shortenings where a fat-soluble emulsifier is
required.
Sterols are of minor importance in the technology of fats. Normally they are
removed in the refining and deodorization steps. The gross mixtures recovered
from these processes have been utilized as source of certain phytosterols that
are used as raw materials in the pharmaceutical industry (Seher
et al., 1976).
In addition to the materials of waxes, hydrocarbons, ketones, aldehydes and mono- and diglycerides are found in fats at varing but low levels. The waxes in some seed oils are troublesome and are removed to prevent haze formation in the finish product. The ketones and aldehydes probably arise from oxidative damage and can cause flavor and odors in fats. The mono- and diglycerides result from hydrolytic reactions either in the raw materials or during processing but do not pose particular problems in the end products. The hydrocarbons are mainly analytical curiosities.
EPOXIDATION
Epoxides are produced by reaction of double bonds with peracids (Fig.
3a, b). This proceeds by a concerted mechanism, giving
cis stereospecific addition. Thus, a cis olefin leads to a cis epoxide and a
trans olefin to a trans epoxide. The order of reactivity of some peracids is
m-chloroperbenzoic > performic > perbenzoic > peracetic; electron withdrawing
groups promote the reaction. The carboxylic acid produced is a stronger acid
than the strongly hydrogen bonded peracid and may lead to subsequent ring opening
reactions especially in the case of formic acid. Small scale reactions are carried
out with m-chloroperbenzoic acid in a halocarbon or aromatic solvent, in the
presence of bicarbonate to neutralize the carboxylic acid as it is formed (Karlson
et al., 1994).
|
| Fig. 3: |
Stereochemistry of epoxidation reactions with (a) KMnO4,
(b) m-chloroperbenzoic acid catalyzed hydrolysis |
Many studies have indicated the importance of using catalysts for epoxidation
purposes.
Acid catalysts: The epoxidation of mahua oil (Madhumica indica)
by hydrogen peroxide has been studied by Goud et al.
(2006). Mahua oil (Madhumica indica) was epoxidised in situ
with hydrogen peroxide as oxygen donor and glacial acetic acid as active oxygen
carrier in presence of catalytic amount of an inorganic acid. Catalytic loading
of two different acids, i.e., H2SO4 and HNO3
were studied and H2SO4 was found to be more effective
in terms of conversion to oxirane. The effects of these parameters on the conversion
to the epoxidised oil were studied and the optimum conditions were established.
Relative conversion data showed that it was possible to develop epoxides from
locally available natural renewable resources such as mahua oil. The epoxidation
of MO using in situ generated peracetic acid could be carried out at
moderate temperature range of 55-65°C. Higher temperatures and higher sulphuric
acid concentrations reduced reaction time and resulted in higher oxirane content
with lesser cleavage to glycol. H2SO4 was found to be
more effective in terms of oxirane conversion. The epoxidation reaction of mahua
oil fell into kinetically controlled regime at stirring speeds >1500 rev/min.
From the relative conversion data obtained for various reaction parameters,
it could be concluded that it was possible to develop value added products such
as epoxides from MO.
The kinetics of epoxidation of cottonseed oil by peroxyacetic acid generated
in situ from hydrogen peroxide and glacial acetic acid in the presence
of liquid inorganic acid catalysts were studied by Dinda
et al. (2008). It was possible to obtain up to 78% relative conversion
to oxirane with very less oxirane cleavage by in situ technique. The
order of effectiveness of catalysts was found to be sulphuric acid > phosphoric
acid > nitric acid > hydrochloric acid. Acetic acid was found to be superior
to formic acid for the in situ cottonseed oil epoxidation. The epoxidation
of cottonseed oil using in situ generated peroxyacid could be carried
out at moderate temperature of about 60°C. CH3COOH was found
to be more effective oxygen carrier than HCOOH, in the present work. Out of
all the liquid inorganic acid catalysts studied, H2SO4
was found to be the most efficient and effective. Higher temperature and higher
acid concentrations reduced the reaction time needed to reach the maximum conversion
to oxirane value; however, it simultaneously increased the extent of oxirane
ring cleavage to glycols. The reaction was kinetically controlled beyond a stirring
speed of about 1800 rev/min. Maximum yield of oxirane with negligible amount
of oxirane cleavage could be obtained if the epoxidation of cottonseed oil,
using in situ generated peroxyacetic acid, is carried out at optimum
conditions. These optimum conditions include a temperature range of 50-60°C,
H2O2-to-unsaturation mole ratio range 1.5-2.0, CH3COOH-to-
unsaturation mole ratio of about 0.5 and H2SO4 loading
of about 2% (by weight) of the aqueous phase.
Improved low temperature properties of 2-ethylhexyl 9(10)-Hydroxy-10(9)-Acyloxystearate
Derivatives has been done by Salimon and Salih (2009a-d)
The epoxidation of oleic acid and other unsaturated hydrocarbon chains using
H2SO4 as catalyst constitutes one of the most useful reactions
in organic synthesis. As the epoxide group is an active intermediate, it can
be readily transformed to the required functionality. Here, we report the synthesis
of seven useful branched 2-ethylhexyl α-hydroxy stearate esters from commercially
available oleic acid and common organic acids. The common organic acids used
here in were octanoic, nananoic, lauric, myristic, palmitic, stearic and behenic
acids. One of the products, the behenic ester of 2-ethylhexyl hydroxy stearate
showed to have pour point, flash point and viscosity indices of -53, 161°C
and 215 cp, respectively, which is favourable properties in the synthesis of
a bio-based lubrication base fluid. Overall, the epoxidation of oleic acid using
H2SO4 as catalyst was successfully proceed and the data
indicated that most of these synthesized derivative compounds have significant
potential as lubricant base oil.
The impact of the relevant process variables on the reaction of soybean oil
fatty acid methyl esters with performic acid (PFA) generated in situ using
concentrated hydrogen peroxide (up to 60 wt.%), to produce an epoxidized product
in high yield, is studied in detail (Campanella et al.,
2008). The degree of mixing, temperature, concentration and molar ratios
of reactants and/or use of a diluents were considered. Temperature increases
are significantly detrimental for achieving high oxirane numbers, as the selectivity
to ring-opening reactions increases. Higher concentrations of either formic
acid or H2O2 are also harmful (particularly, the carboxylic
acid) but much less than temperature. A proposed alternative process, employing
moderate temperature (up to 40°C) and concentrated H2O2,
compares favorably with the conventional one; higher conversion combined with
high epoxide productivity and selectivity are attainable. Using economically
sound reactants molar ratios, under well-mixed regimes, in which the immiscible
polar and organic phases are well dispersed, the epoxidation process can be
adequately described using an (equilibrated) two-phase reaction model. The model
accounts for both the reversible per acid formation (in the aqueous phase) and
the epoxidation reaction proper, together with the attacks on the epoxide ring
by formic acid and performic acid (in the organic phase). In the epoxidation
of soybean fatty acid methyl esters with PFA generated in situ from FA
using highly concentrated hydrogen peroxide (up to 60 wt.%) and moderate temperature
(preferably 40°C), the impact of temperature increases is significantly
detrimental for achieving high yields and/or high oxirane-number. Higher concentrations
of either formic acid or hydrogen peroxide are also harmful (particularly, the
carboxylic acid) but much less than temperature. Nevertheless, this process
alternative compares favorably to the conventional one, which is conducted at
60°C using 30 wt.% H2O2; higher double bond conversion
combined with high epoxide productivity and excellent selectivity are attainable.
Using economically sound FA-to-double bonds and H2O2-todouble
bonds molar ratios, under well-mixed regimes, in which the immiscible polar
and organic phases are always well dispersed, the epoxidation process can be
adequately described using an (equilibrated) two-phase reaction model accounting
for just the reversible per acid formation (in the aqueous phase) and the epoxidation
reaction proper, together with the attacks on the epoxide ring by FA and PFA
(in the organic phase).
The rate constants of epoxidation were determined by reacting formic acid with
a number of oils, pure TAG and pure FAME (Scala and Wool,
2002). These results showed that FA composition had a significant effect
on the value of the rate constant. In TAG, the double bonds of oleic acid and
linoleic acid were equally reactive and the double bonds of linolenic acid were
approximately three times more reactive than oleic and linoleic acids. For FAME,
the rate constants of epoxidation increased as the level of unsaturation increased.
Furthermore, the rate constants of epoxidation for the FAME were higher than
their respective TAG. We conclude that steric and electronic effects caused
FA with different levels of unsaturation to have different reactivities. These
results were used to derive a model that predicts the epoxidation kinetics of
oils from their FA composition. The predictions of the model closely match the
experimentally determined rate constants.
The kinetics epoxidation of Rubber Seed Oil (RSO) by peroxyacetic acid generated
in situ were studied at various temperatures (Okieimen
et al., 2002). It was found that epoxidation with almost complete
conversion of unsaturated carbon and negligible oxirane cleavage can be attained
by the in situ technique. The rate constant for epoxidation of RSO was
found to be of the order of 10-6 L/mol/sec and activation energy
of epoxidation of 15.7 kcal mol-1 was determined. Some thermodynamic
parameters: enthalpy, entropy and free energy activation of 15.2, -31.94 and
25.44 kcal mol-1, respectively were obtained for the epoxidation
of RSO. The results from this study show that the epoxidation of RSO by peroxyacetic
acid generated in situ can be carried out at moderate temperatures with
minimum epoxide degradation. The kinetic and thermodynamic parameters of epoxidation
obtained from this study indicate that an increase in the process temperature
would increase the rate of epoxide formation.
Enzymes: Epoxy alkylstearates were synthesized by lipase catalysed esterification
and perhydrolysis followed by epoxidation of oleic acid in a one-pot process
(Orellana-Coca et al., 2007). Immobilized Candida
antarctica lipase (Novozym® 435) was used as the catalyst.
The esterification reaction occurred relatively quickly and was followed by
epoxidation of the alkyl ester and the remaining fatty acid. Higher degree of
esterification was achieved with n-octanol, n-hexanol and n-butanol as compared
to that with ethanol and iso-propanol. The rate and yield of epoxidation was
enhanced with iso-propanol but was lowered with the other alcohols. The lipase
suffered significant loss in activity during the reaction primarily due to hydrogen
peroxide. The presence of alcohols, in particular ethanol, further contributed
to the enzyme inactivation. The epoxidation reaction could be improved by step-wise
addition of the lipase. The enzymatic approach for the synthesis of alkyl epoxystearates
is a simpler and energy efficient alternative to the chemical process and the
solvent-free conditions and good product yields further result in savings in
product separation processes. The main limitation, however, is the low stability
of the lipase under the reaction conditions employed. Possibility to use anhydrous
reaction conditions and to minimize the exposure of the enzyme to the peroxide
would be beneficial for improving product yields and performance of the biocatalyst.
Unsaturated carboxylic acids are converted to percarboxylic acids catalyzed
by an immobilized lipase from Cundid~z Antarctica (Novozym 435R) (Warwel
and Klaas, 1995). These unsaturated percarboxylic acids are only intermediates
and epoxidized themselves in good yields and almost without consecutive reactions.
The mechanism of the oxygen-transfer is found to be predominantly intermolecular
and the formation of the percarboxylic acids proceeds via two different catalytic
reactions. The lipase is surprisingly stable under the reaction conditions;
it is recovered and reused fifteen times to produce epoxy-steak acid on a multi-gram
scale.
The effect of reaction parameters on lipase-mediated chemo-enzymatic epoxidation
of linoleic acid was investigated by Orellana-Coca et
al. (2005). Hydrogen peroxide was found to have the most significant
effect on the reaction rate and degree of epoxidation. Excess of hydrogen peroxide
with respect to the amount of double bonds was necessary in order to yield total
conversion within a short time period, as well as at temperatures above 50°C
to compensate for hydrogen peroxide decomposition. However, prolonged incubation
with high excess of hydrogen peroxide leads to the accumulation of peracids
in the final product. The reaction rate increased also with increasing hydrogen
peroxide concentration (between 10 and 50 wt. %); however, at the expense of
enzyme inactivation. Linoleic acid was completely epoxidized when used at a
concentration of 0.5-2 M in toluene at 30°C, while in a solvent-free medium,
the reaction was not complete due to the formation of a solid or a highly viscous
oily phase, creating mass transfer limitations. Increasing the temperature up
to 60°C also improved the rate of epoxide formation.
The parameters affecting the lipase activity and operational lifetime during
chemo-enzymatic epoxidation of fatty acids were investigated (Tornvall
et al., 2007). Immobilized Candida antarctica lipase B (Novozym®
435) was incubated in the presence of various reaction components (i.e., toluene,
water, H2O2, oleic acid, perpalmitic acid and epoxystearic
acid, respectively) at temperatures between 20 and 60°C followed by measurement
of residual enzyme activity. Epoxystearic acid was shown to slightly inactivate
the enzyme at 50°C, while oleic acid and perpalmitic acid did not. No deactivation
of the enzyme was observed in presence of toluene/water mixture within 48 h
at 20-60°C. In the presence of 6-12 M hydrogen peroxide, the enzyme was
rather stable at 20°C, while at 60°C the enzyme lost activity rapidly,
with the rate of deactivation increasing with increasing hydrogen peroxide concentration.
In the work presented here, the parameters found to be most crucial for the
activity and hence operational stability of Novozym®435 in chemo-enzymatic
epoxidation of fatty acids, are hydrogen peroxide at high concentrations, together
with elevated temperatures. For epoxidation processes run at elevated temperatures,
controlled addition of H2O2 is hence important for enzyme
stability, especially in the beginning before the formation of water is sufficient
to dilute the added H2O2. Since, the reaction is exothermic,
a large-scale process would probably be most efficient if a temperature program
is used. Concurrently with improving the process design, development of a more
stable biocatalyst preparation would be an alternative strategy.
Enzymes were found to have significant effect on the reaction rate and degree of epoxidation with good yields.
Ti(IV)-grafted silica catalysts: The liquid-phase epoxidation of mixtures
of Fatty Acid Methyl Esters (FAMEs) over titanium-containing silica materials,
using tertbutylhydroperoxide (TBHP) as oxidant, is here reported (Guidotti
et al., 2006). The mixtures were obtained from vegetable renewable
source, i.e., from high-oleic sunflower oil, coriander oil, castor oil and soya-bean
oil. The influence of the nature and the position of functional groups on the
C-18 chain of the FAMEs was studied. Very high activity and selectivity were
obtained in the epoxidation of castor and soya-bean oil methyl esters in a reaction
medium free from organic acids. Ti-MCM-41 (an ordered mesoporous titanium-grafted
silica) displayed in this case, for the first time, superior performances, from
a synthetic point of view, with respect to non-ordered mesoporous titanosilicates.
Titanium-grafted mesoporous silica materials showed to be suitable catalysts
for the epoxidation of unsaturated FAME mixtures in a reaction medium that is
completely free from organic acids. They are also versatile, as they have been
used over a series of four FAME mixtures obtained from different vegetable sources
exhibiting interesting performances. In particular, very high conversion and
selectivity values were obtained with Ti-MCM-41 in the epoxidation of castor
oil or soya-bean oil FAME mixtures. In such cases, the superior catalytic performances
displayed by this ordered mesoporous titanosilicate with respect to the other
non-ordered materials can be explained by the concurrence of various favourable
factors, such as the presence of large amounts of highly accessible and well
defined Ti(IV) tetrahedral active sites and the peculiar environment around
the Ti(IV) sites (i.e., the high density of silanols surrounding the Ti(IV)
sites), which accounts for the enhanced formation of epoxidised species when
highly polar moieties (for instance, hydroxy-group in methyl ricinoleate or
an epoxy-ring in methyl monoepoxyoctadecenoate) are already present on the substrate
molecules. However, interesting performances were also recorded over grafted
non-ordered silicas and, over these materials, the titanium sites (considered
singularly) showed remarkable turnover frequency values, even higher than those
obtained over Ti–MCM-41.
Tungsten-based catalyst: A solvent-free, rather complete and selective
cis epoxidation of Methyl Oleate (MO) using a tungsten containing catalyst called
Tetrakis (Poliet al., 2009). High epoxide yields
have been obtained by adjusting the reaction parameters (reaction time, temperature,
gas phase, oxidant molar ratio and concentration). The highest selectivities
are the result of a synergetic effect of hydrogen peroxide and air (or oxygen)
used as oxidizing agents that decreases the MO dimerization and then favour
the complete conversion of MO into its epoxide. The use of the Tetrakis phosphotungstate
catalyst leads to a rather complete and selective transformation of cis MO into
cis epoxide under greener conditions than what was obtained in previous studies
because it is performed with a stoichiometric amount of hydrogen peroxide, without
solvent and at a lower temperature leading to a maximal H2O2
efficiency. Such a high yield is the result of a synergetic effect of hydrogen
peroxide and air (or oxygen) used as oxidizing agents at a rather low temperature
(313 K). This really high yield is also probably due to the low MO dimerization
under these conditions because of the catalyst saturation by oxygen and also
because of the modification of the H2O2 decomposition
equilibrium. Finally, this epoxidation method can be extended to other unsaturated
fatty compounds and crude vegetable oils.
Transition metal complexes: The epoxidation of methyl linoleate was
examined using transition metal complexes as catalysts (Du
et al., 2004). With a catalytic amount of methyltrioxorhenium (4
mol%) and pyridine, methyl linoleate was completely epoxidized by aqueous H2O2
within 4 h. Longer reaction times (6 h) were needed with 1 mol% catalyst loading.
Manganese tetraphenylporphyrin chloride was found to catalyze the partial epoxidation
of methyl linoleate. A monoepoxidized species was obtained as the major product
(63%) after 20 h.
Ion exchange resin: Canola oil with an iodine value of 112/100 g and
containing 60% oleic acid and 20% linoleic acid, was epoxidised using a peroxyacid
generated in situ from hydrogen peroxide and a carboxylic acid (acetic
or formic acid) in the presence of an Acidic Ion Exchange Resin (AIER), Amberlite
IR 120H (Mungroo et al., 2008). Acetic acid was
found to be a better oxygen carrier than formic acid, as it produced about 10%
more conversion of ethylenic unsaturation to oxirane than that produced by formic
acid under otherwise identical conditions. A detailed process developmental
study was then performed with the acetic acid/AIER combination. The parameters
optimised were temperature (65°C), acetic acid to ethylenic unsaturation
molar ratio (0.5), hydrogen peroxide to ethylenic unsaturation molar ratio (1.5)
and AIER loading (22%). An iodine conversion of 88.4% and a relative conversion
to oxirane of 90% were obtained at the optimum reaction conditions. The heterogeneous
catalyst, AIER, was found to be reusable and exhibited a negligible loss in
activity. The epoxidation of canola oil using peroxycarboxylic acid generated
in situ was carried out most effectively using the acetic acid-acidic
ion exchange resin combination. The epoxidation process with minimum oxirane
cleavage was, therefore, optimized using acetic acid and the Amberlite IR- 120H
(AIER) catalyst system. It was found that the epoxidation reaction occurred
optimally at a temperature of 65°C, an acetic acid to ethylenic unsaturation
molar ratio of 0.5:1, a hydrogen peroxide to ethylenic unsaturation molar ratio
of 1.5:1 and a catalyst (AIER) loading of 22 wt.% of total canola oil used.
Under these optimum conditions, 90% conversion of ethylenic unsaturation to
oxirane was obtained, with a similar conversion of iodine. The AIER catalyst
was found to be reusable. The formation of the epoxide adduct of canola oil
was confirmed by FTIR and 1H NMR spectral analysis. From the relative
conversion data obtained, it can be concluded that it is possible to develop
value-added products, such as epoxide, from canola oil.
The kinetics of the epoxidation of soybean oil in bulk by peracetic acid formed
in situ, in the presence of an ion exchange resin as the catalyst, was
studied by Sinadinovic-Fiser et al. (2001). The
proposed kinetic model takes into consideration two side reactions of the epoxy
ring opening involving the formation of hydroxy acetate and hydroxyl groups
as well as the reactions of the formation of the peracid and epoxy groups. The
catalytic reaction of the peracetic acid formation was characterized by adsorption
of only acetic acid and peracetic acid on the active catalyst sites and irreversible
surface reaction was the overall rate-determining step. Kinetic parameters were
estimated by fitting experimental data using the Marquardt method. Good agreement
between the calculated and experimental data indicated that the proposed kinetic
model was correct. The effect of different reaction variables on epoxidation
was also discussed. The conditions for obtaining optimal epoxide yield (91%
conversion, 5.99% epoxide content in product) were found to be: 0.5 mole of
glacial acetic acid and 1.1 mole of hydrogen peroxide (30% aqueous solution)
per mole of ethylenic unsaturation, in the presence of 5 wt.% of the ion exchange
resin at 75°C, over the reaction period of 8 h.
Methyltrioxorhenium (VII) on nobia: Soybean oils (oleic, linoleic and
linolenic acids and their methyl esters) are epoxidized readily with Urea-Hydrogen
Peroxide (UHP) when methyltrioxorhenium(VII) supported on nobia is used as the
catalyst in chloroform. Simple alkenes are epoxidized by the same method (Bouh
and Espenson, 2003). The epoxide and not a diol is produced.
And also has been used the methyltrioxorhenium (MTO)-CH2Cl2/H2O2
biphasic system for epoxidizing soybean oil (Gerbase et
al., 2002). The reactions were optimized (reactant ratio, time and temperature),
which resulted in a better performance (higher conversion and selectivity) than
those described in the literature. Total doublebond conversion and 95% selectivity
were obtained in 2 h at room temperature. Furthermore, it was possible to reach
desired epoxidation degrees by changing the oxidant and MTO amounts. The rhenium-epoxidized
soybean oil remained stable in the absence of stabilizers for up to 30 day when
stored at mild conditions.
Methyltrioxorhenium (MTO) catalyses direct epoxidation by hydrogen peroxide. The reaction is carried out in pyridine, avoiding acidic conditions detrimental to high epoxide yield and uses less concentrated hydrogen peroxide (30%) than other methods. This method epoxidized soybean and metathesized soybean oil in high yield. The epoxidized metathesized oil was more stable to polymerization than that produced using m-chloroperbenzoic acid, presumably because it was free of acidic impurities. These and other novel approaches to epoxidation have recently been reviewed. None has yet found industrial application.
Amorphous Ti.SiO2: A study of the epoxidation of soybean
oil and soybean methyl esters with hydrogen peroxide in dilute solution (6 wt.%)
using an amorphous heterogeneous Ti/SiO2 catalyst in the presence
of tert-butyl alcohol has been studied by Campanella et
al. (2004). The influence of some relevant process variables such as
temperature and the hydrogen peroxide-to-double bond molar ratio on performance
is examined. The highest yields of epoxidized olefins were obtained upon using
a H2O2: substrate molar ratio of 1.1: 1. Higher ratios
than this were not effective for speeding up the reaction. Under the experimental
conditions employed in this work, no degradation of the oxirane ring was observed.
Alumina: Two commercial aluminas and one produced by the sol-gel process
were compared for the epoxidation of unsaturated fatty esters using anhydrous
or aqueous hydrogen peroxide as oxidant and ethyl acetate as solvent (Sepulveda
et al., 2007). The aluminas show a good catalytic activity and excellent
selectivity towards the epoxides. The sol-gel alumina was more efficient and
when using aqueous hydrogen peroxide could be recycled several times. Alumina
synthesized by the sol-gel method is shown to be an inexpensive and efficient
catalyst in the epoxidation of methyl oleate and soybean oil methyl esters with
aqueous hydrogen peroxide as oxidant. A conversion of 95% and selectivity >97%
for the epoxide were obtained after 24 h without the use of any kind of homogeneous
acid. After 4 cycles, a conversion of 87% was obtained. These results show that
sol-gel alumina is an alternative catalyst for the epoxidation of vegetable
oils.
CONCLUSIONS
Recent studies have attempted to improve the efficiency of epoxidation under milder conditions that minimize the formation of byproducts. Chemo-enzymatic epoxidation uses the immobilized lipase from Candida antartica (Novozym 435) to catalyze conversion of fatty acids to peracids with 60% hydrogen peroxide. The fatty acid is then self-epoxidized in an intermolecular reaction. The lipase is remarkably stable under the reaction conditions and can be recovered and reused 15 times without loss of activity. Competitive lipolysis of triacylglycerols is inhibited by small amounts of fatty acid, allowing the reaction to be carried out on intact oils.
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