Synthesis of fatty ketoesters by tandem epoxidation-rearrangement with heterogeneous catalysis

Article abstract of DOI:10.1039/c9cy01899a

Unsaturated fatty esters can be easily transformed into ketoesters through a two-step process. The highly efficient epoxidation is carried out with tert-butyl hydroperoxide (TBHP) in α,α,α-trifluorotoluene (TFT) using a Ti-silica heterogeneous catalyst. The formed epoxide is easily rearranged by a heterogeneous Br?nsted acid, with Nafion-silica SAC13 as the most efficient one. Both reactions can be combined in a tandem process, with separation of the Ti-silica catalyst by filtration from the reaction medium and addition of the second acid catalyst to perform the second reaction. Each catalyst is separated individually and can be reused, with or without re-activation, under the same conditions to maximize the productivity.

Full text of DOI:10.1039/c9cy01899a

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DOI: 10.1039/C9CY01899A
ARTICLE
Synthesis of fatty ketoesters by tandem epoxidation-
rearrangement with heterogeneous catalysis
Received 00th January 20xx,
Accepted 00th January 20xx
Vicente Dorado, Lena Gil, José A. Mayoral, Clara I. Herrerías* and José M. Fraile*
DOI: 10.1039/x0xx00000x
Unsaturated fatty esters can be easily transformed into ketoesters through a two-step process. The highly efficient
epoxidation is carried out with tert-butyl hydroperoxide (TBHP) in -trifluorotoluene (TFT) using a Ti-silica
heterogeneous catalyst. The formed epoxide is easily rearranged by a heterogeneous Brønsted acid, with nafion-silica
SAC 13 as the most efficient one. Both reactions can be combined in a tandem process, with separation of the Ti-silica
catalyst by filtration from the reaction medium and addition of the second acid catalyst to perform the second reaction.
Each catalyst is separated individually and can be reused, with or without re-activation, under the same conditions to
maximize
the
productivity.
Introduction
The use of renewable raw materials, which must reduce the
dependence on the fossil resources with a lower CO2 footprint
of the chemical industry, is one of the most important
principles of Green Chemistry.1 Catalysis, another important
principle of Green Chemistry, should play a very important role
in the transformation of the renewable materials into useful
compounds.2 Moreover, the combination of several reactions
in tandem processes (Scheme 1) should help to improve the
efficiency of those transformations.3
obtained by the Pd-catalyzed Wacker oxidation of oleate with
oxygen. In the first examples, high amount of PdCl2 (10 mol%)
and CuCl (50 mol%) as co-catalysts were employed with 1 bar
O2.21 Molecular oxygen and CuCl co-catalyst can be substituted
by benzoquinone and HBF4, together with the use of lower
amount of palladium catalyst (5 mol% of Pd(OAc)2).22 The use
of high pressure (10 bar) of oxygen also avoids the need for a
co-catalyst when the reaction is carried out in
dimethylacetamide, which also allows the recovery and reuse
of PdCl2 catalyst.23
Fatty esters are among the most useful raw materials,4,5 which
can be converted into compounds acting as biolubricants,6–9
biosurfactants,10,11 biopolymers12,13 and many other
applications. Although reactions on the ester group are the
most common in oleochemical industry, the reactivity of the
unsaturated hydrocarbon chains opens the way to a large
variety of compounds.14–16 Fatty ketoesters17 are however
rather unexplored as platform molecules, in spite of
possessing countless synthetic possibilities. 12-Ketostearate
has been obtained from ricinoleate by simultaneous
hydrogenation-dehydrogenation.18 On the other hand,
mixtures of 9- and 10-ketostearates can be obtained from the
oxidation of oleate. Apart from the use of bacteria for the
enzymatic oxidation,19 the direct oxidation of oleate to
ketostearates has been described in a catalyst-free method
using N2O as an oxidant at high temperature (220°C) and
pressure (4 MPa).20 Alternatively, the ketostearate has been
The other possibility is the two-step process of oleate
epoxidation and Meinwald rearrangement of the
epoxystearate to obtain the mixture of ketostearates.
The epoxidation of oleates by different procedures has been
described in the literature.24 Some of them are performed in
aqueous or aqueous/organic biphasic media, such as those
using H2O2 or oxone as an oxidant. These methods do not
seem to be adequate for a tandem process with an acid-
catalyzed rearrangement due to the probable epoxide ring
opening to diol. In some cases molecular oxygen has been
used as an oxidant in the presence of a sacrificial reagent,
either an aldehyde25 or cumene.26 Under such conditions, at
least part of the reaction takes place through a radical
mechanism, leading to mixtures of trans and cis epoxides. The
most efficient method for stereospecific epoxidation in
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anhydrous medium with heterogeneous catalysis seems to be compounds (TFT, tert-butanol, decane) wereVieewvAartpicolerOantleinde
DOI: 10.1039/C9CY01899A
the use of alkyl hydroperoxides, such as tert-butyl under reduced pressure to obtain the methyl cis-9,10-
hydroperoxide (TBHP), in combination with Ti catalysts. The epoxystearate with 97% isolated yield, which was used
grafting of Ti sites on silica supports leads to active catalysts without further purification.
for epoxidation of fatty acid methyl esters, with higher activity The epoxidations of methyl elaidate (24 h of reaction), erucate
in the case of the structured mesoporous Ti-MCM-41.27 and ricinoleate were carried out in the same way. In the case
However, amorphous Ti-SiO2 catalysts have also shown of methyl linoleate, the amount of TBHP was increased to 0.56
interesting features, such as higher activity per exposed Ti mL (3 mmol).
site.28 More recently a sponge-like hierarchical titanium
Rearrangement reactions
silicalite has been also described as catalyst for the
epoxidation of methyl oleate.29
SAC-13 (58.8 mg, 10
mol -SO3H sites) was added to a solution
With respect to rearrangement of epoxystearate, not many of methyl cis-9,10-epoxystearate (312.5 mg, 1 mmol) and
examples have been described in the literature. Conventional benzonitrile (42.5 mg, internal standard) in 
-
Lewis acids, such as BF3,17 and less common ones, such as trifluorotoluene (TFT, 5 mL), and the mixture was heated
30
Bi(OTf)3 and YCl3 31, have been used for this purpose with under reflux and Ar atmosphere for 1 h. The reaction was
high conversion and selectivity. On the other hand, monitored by GC. The catalyst was filtered off and washed
heterogeneous sulfonic solids have been shown to be efficient with dichloromethane. The methyl oxostearates were
Brønsted catalysts for this reaction.32
obtained by evaporation of the solvent under reduced
In this paper, we present our results on the epoxidation of pressure, without further purification, to get 92% isolated
unsaturated fatty esters with a heterogeneous Ti-silica catalyst yield.
and the Meinwald rearrangement of the epoxides, as well as The same method was used for the other fatty epoxides. In the
the combination of both reactions in a tandem process case of methyl cis,cis-9,10:12,13-diepoxystearates, 117.6 mg of
(Scheme 2).
SAC-13 (20 mol -SO3H sites) were used.
Tandem processes
Experimental
Preparation of the Ti-silica catalyst
The required amount of Ti-silica (15 mol Ti) was added to a
solution of methyl oleate (296.5 mg, 1 mmol) in 
-
trifluorotoluene (TFT, 5 mL) and the mixture was heated under
reflux. TBHP (0.28 mL of 5.5 M solution in decane, 1.5 mmol)
was added and the full conversion was checked by GC after 2
h. The catalyst was filtered off. Then, SAC-13 (58.8 mg, 10
10 g of pre-dried Merck 60 silica (0.063-0.200 mm particle size,
surface area 512 m2/g, pore volume 0.816 cm3/g)33 were
treated with HNO3 1 M (300 mL) at room temperature for 3 h.
The silica was thoroughly washed with water until neutral pH,
and then dried successively under vacuum at 150°C overnight,
at 500°C for 2 h under air and for 2 h under vacuum. In a
glovebox, titanocene dichloride (Cp2TiCl2, 940 mg, 3.8 mmol)
was added to the cold calcined silica and 170 mL of anhydrous
CH2Cl2 were added via cannula. The mixture was stirred under
Ar at room temperature for 2 h, then 8.6 mL of pyridine were
added and stirring was maintained for 20 h. The resulting
mol -SO3H sites) and benzonitrile (42.5 mg, internal standard)
were added to the filtrate and the mixture was heated under
reflux and Ar atmosphere for 1 h.
Results and discussion
Epoxidation reactions
yellow solid was filtered, washed with CH2Cl2 until uncolored One of the main issues about heterogeneous catalysis is the
solution and dried under vacuum. Finally, the catalyst was reproducibility in the preparation of the catalysts and hence
calcined at 550°C under flow (80 mL/min) of Ar for 1 h and their catalytic performance. In this case, Ti-silica catalyst was
then of air for 20 h, to get a white solid.
prepared following a
procedure previously described,34
although with some minor modifications. The silica support
was treated with HNO3 in order to eliminate all the traces of Fe
Epoxidation reactions
The required amount of Ti-silica (15
mol Ti) was added to a and to prevent in this way the possible formation of radicals
solution of methyl oleate (296.5 mg, 1 mmol) in 
-
with the hydroperoxide. TiCp2Cl2 was used as the Ti precursor,
trifluorotoluene (TFT, 5 mL) and the mixture was heated under and the reaction with the silanol groups on the silica surface
reflux. TBHP (0.28 mL of 5.5 M solution in decane, 1.5 mmol) was carried out at room temperature using pyridine as a base
was added and the reaction was monitored by GC. Once the to favor the reaction. The solid was calcined under Ar and then
reaction had finished (1 to 2 h) the catalyst was filtered off and air to form the final grafted Ti species and eliminate all the
washed with dichloromethane. The solvent and volatile traces of organic matter. Along this work, 8 batches of catalyst
were prepared using the same methodology, with a certain
variability in the obtained Ti content, from 0.18 to 0.30
mmol/g (average 0.225 mmol/g, standard deviation 0.048).
This can be due to uncontrolled slight changes in temperature
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and gas flow. Hence, Ti analysis is compulsory before using
ARTICLE
irreversible way. Owing to the low amount of catalyst used, it was
View Article Online
DOI: 10.1039/C9CY01899A
each batch of catalyst.
Optimization of epoxidation conditions with methyl oleate. 
-
Trifluorotoluene (TFT) was chosen as solvent for the
epoxidation of methyl oleate (Scheme 3), given its efficiency
with other substrates.35,36 Reactions were carried out under
reflux, as temperatures lower than 100°C were unable to reach
quantitative conversions in short reaction times (up to 2 h).
Given that Ti-silica catalysts had shown activity in the
Meinwald rearrangement of aromatic epoxides,35 the reaction
was tested for a long time period in order to see the
performance of this single catalyst in the two-steps one-pot
process. As expected, the epoxidation reaction was fast, with
total conversion after 2 h, and selective to the epoxide,
whereas the catalyst showed poor activity for Meinwald
rearrangement, with only 34% selectivity to ketostearates
after 1 week. This result evidences the need for a second
catalyst for the rearrangement.
not possible to analyze the solid after each run. However, the
analysis of the catalyst after the 8th run showed a content of
titanium of only 0.003 mmol/g, which implies an important loss of
titanium, between 80 and 90%, responsible for the second
deactivation mechanism. Taking these values, the productivity of
the Ti sites after 8 runs can be estimated, considering only 90%
conversion, to be around 850 mol of epoxide per mol of Ti.
Epoxidation of other unsaturated fatty esters. The same
methodology was applied to the epoxidation of four other
unsaturated fatty esters (Scheme 4): one with a longer chain with
cis alkene (erucate, C22:1 cis-13), one with a trans alkene (elaidate,
C18:1 trans-9), one with an additional functionality (ricinoleate,
(R)-12-hydroxy-9-cis-octadecenoate) and one with two cis non-
conjugated double bonds (linoleate, C18:2 cis,cis-9,13). Under the
Thus, those conditions (reflux for 2 h) were kept to test the
effect of the amounts of tert-butyl hydroperoxide (TBHP) and
catalyst. The nearly stoichiometric amount of TBHP (1.1 eq) led
to conversions in the range of 95-100%, irrespective from the
amount of catalyst (1.2 to 2.5 mol%). The result did not show
any relationship with the Ti loading of the catalyst, and hence,
the amount of TBHP was increased to ensure the total
conversion of methyl oleate. On the contrary, the use of 1.5 eq
of TBHP was enough to get total conversion in 2 h using 1.5
mol% of Ti-silica catalyst, in reaction scales from 1 to 10 mmol
of methyl oleate. Thus, these conditions of 1.5 mol% catalyst,
1.5 eq TBHP, and TFT under reflux were taken as the standard
ones for the subsequent reactions.
Recovery and reuse. One important feature of the heterogeneous
catalysts is the possibility of recovery and reuse of them to increase
the productivity. The results with Ti-silica catalyst are represented
in Figure 1. The recovered catalyst was less active than the freshly
prepared one in runs 2 and 3. It required around 5 h to reach 90%
conversion, and 8 h to reach 95%. However, the drop in activity was
more important in the fourth run, with only 80% conversion after
8h, whereas nearly 24 h were necessary to reach 95% conversion.
Then, the catalyst was reactivated by calcination after the fourth
run, recovering in this way the activity shown in runs 2 and 3.
Similar results were observed in run 6, with a drop in activity, and
run 7, with a partial recovery of the activity after reactivation by
calcination. These results indicate that there exists a deactivation
mechanism due to adsorption of organic matter, which is reversible
by calcination. However, another part of the activity was lost in an
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same conditions with oleate, the result with methyl erucate was
analogous, total conversion in 2 h. However, the lower reactivity of
the trans alkene in elaidate slows down the reaction and it takes 24
h for complete conversion. The presence of the hydroxyl group in
methyl ricinoleate does not produce any modification of the
reactivity, leading to total conversion in 2 h. Although only one peak
in GC was detected, the formation of the two possible
diastereomers in nearly equimolecular amounts was inferred by the
duplicity in the signals of the 13C-NMR spectrum (see Supporting
Information). In the case of linoleate, it was not possible to obtain
the monoepoxides selectively, even using a low amount of TBHP.
The selectivity to monoepoxides was high only at very low
conversions, as the diepoxides were formed even at low
concentration of the monoepoxides. Double amount of oxidant was
used to get full conversion to the diepoxides in only 2 h. Two peaks
were detected in GC, in agreement with the two possible
diastereomers that can be formed.
strongest perfluoroalkylsulfonic resins were VmiewucAhrticlemOnolirnee
efficient than the other weaker sulfonicDsOoIl:id10s.,10a3s9i/tC9hCaYd01b8e9e9nA
already described by Ríos et al in the comparison of SAC-13
and Amberlyst 15.32
The cross-linking degree was important in the case of
poly(styrene-divinylbenzene) resins, and the gel type Dowex
50W×2, with only 2% cross-linking agent, was more efficient
than the macroreticular Amberlyst 15, with 20% cross-linking
divinylbenzene. This effect has been already observed in other
reactions catalyzed by this kind of solids,37,38 and it has been
ascribed to accessibility problems to some of the sulfonic sites
in the inner part of the resin, which is probably the reason for
the poor results previously described with this catalyst.32
Regarding the perfluoroalkylsulfonic resins, both Nafion and
Table 1. Results of the Meinwald rearrangement of methyl cis-9,10-
epoxystearate with different catalysts.a
Conv.
(%)b
Catalyst
mol%
Solvent
T (°C)
t (h)
Meinwald rearrangement of fatty epoxides
Homogeneous Lewis acids
Optimization with methyl cis-9,10-epoxystearate. Given that the
Ti-SiO2 epoxidation catalyst is not acidic enough to efficiently
promote the Meinwald rearrangement of the cis-9,10-
epoxystearate to a mixture of 9-oxo- and 10-oxostearates (Scheme
5), a variety of homogeneous Lewis as alternatives to the already
described,30,31 and heterogeneous Brønsted acid catalysts, including
SAC-1332 and other sulfonic solids, was tested with this purpose
(Table 1). Regarding reaction conditions, TFT was used as solvent in
most of the reactions in order to check the possibility of carrying
out the two-steps process in one-pot or tandem. It is worth noting
that no other products but methyl ketostearate were detected in
the reaction mixtures, by either GC or NMR, indicating that the
reaction has 100% selectivity to the rearrangement.
Cu(OTf)2
Zn(OTf)2
Yb(OTf)3
5
5
5
TFT
TFT
TFT
120
120
120
80
2
2
>99
>99
>99
>99
>99
>99
>99
91
2
2
50
3
30
6
Co(ClO4)2
Zn(ClO4)2
5
5
TFT
TFT
120
120
24
24
Heterogeneous Brønsted acids
K10 montm.
Deloxan
5
5
TFT
TFT
120
120
80
24
24
24
3
41
>99
96
As can be seen, all the homogeneous Lewis acids (5 mol%) led
to conversions over 90%, with triflates as the most efficient
ones, irrespective from the metal used (copper, zinc,
ytterbium), whereas with perchlorates the reaction required
24 h to reach similar conversion values. With Yb(OTf)3 it was
shown that the reaction also is efficient at lower
temperatures, although below 50°C the reaction is slightly
slower.
Regarding the heterogeneous Brønsted acid catalysts, the
acidic K10 montmorillonite was not efficient, with only 41%
conversion after 24 h. Thus, sulfonic solids with sites of
different strength were tested: i) Deloxan (alkylsulfonic
polysiloxane); ii) Amberlyst 15 and Dowex 50W×2 (arylsulfonic
poly(styrene-divinylbenzene) resins); iii) Nafion, SAC-13
(nafion-silica composite) and Aquivion (perfluoroalkylsulfonic
resins). All of them were active at 120°C, although the
Toluene
PhCl
DMF
TFT
120
140
160
120
120
120
120
120
120
50
>99
>99
7
3
7
Amberlyst 15
Dowex 50W×2
Nafion NR-50
SAC-13
5
5
5
5
2
1
24
24
2
74
TFT
>99
>99
>99
>99
>99
>99
>99
>99
>99
5
TFT
TFT
2
SAC-13
TFT
2
SAC-13
TFT
1
TFT
3
TFT
30
24
1
Toluene
PhCl
DMF
TFT
120
140
160
120
120
120
1
7
5
1
5
1
>99
>99
99
Aquivion
PW79S
TFT
1
Aquivion P98
TFT
8
a
b
Scale: epoxide (1 mmol), solvent (5 mL).
chromatography.
Determined by gas
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Recovery of SAC-13 in the Meinwald rearranVgieewmAertnictle. OnTlhinee
DOI: 10.1039/C9CY01899A
possibility of recycling and reuse was checked in the case of SAC-13.
As can be seen in Figure 2, the catalyst kept its activity in four
consecutive runs, with a slight drop in activity in the fifth one (2 h
required for total conversion). The deactivation was more
important in the two subsequent runs, but it was possible to
regenerate the catalyst by treatment with HCl.
Meinwald rearrangement of different fatty epoxides. The same
methodology, SAC-13 1 mol% in TFT under reflux, was applied to
the other prepared epoxides. The simple epoxides (Scheme 6),
methyl trans-9,10-epoxystearate and cis-13,14-epoxydocosanoate
(or epoxybehenate) led to the expected ketones with total
conversion in 2 h, showing no effect of the stereochemistry of the
oxirane ring.
Aquivion PW79S performed similarly, with total conversion
after 2 h, as well as the nafion-silica composite SAC-13. Due to
the problems found in the filtration of Aquivion PW79S
(powder), another analogous catalyst in the form of pellets
was prepared from Aquivion P98 by hydrolysis39 (see
Supporting Information). However, the efficiency of this solid
was lower, probably by accessibility problems, as pointed for
Amberlyst 15. SAC-13 was considered as the most convenient
catalyst and, in fact, it was possible to decrease the amount of
catalyst from 5 mol% to only 1 mol% with the same results,
reaching total conversion in only 1 h.
On the contrary, the epoxides with additional functionalization
led to complex mixtures of products, which were separated by
column chromatography on silica and characterized by NMR.
The
rearrangement
of
methyl
cis,cis-9,10:12,13-
diepoxystearates (Scheme 7) produced 50% yield of furanic
fatty esters, with furan in positions (9,12) and (10,13). These
products had been already described as the only ones
obtained in early attempts of Meinwald rearrangements,40 and
more recently as by-products in the ring opening with alcohols
catalyzed by either Amberlyst 15 or H2SO4.41 Besides the
The effect of solvent and temperature was checked with
Deloxan (5 mol%) and SAC-13 (1 mol%). As observed in the
case of homogeneous Lewis acids, the heterogeneous
Brønsted acids
can also promote the rearrangement at lower temperature,
although it was slower, in agreement with results described in
the literature.32 Non-polar solvents, such as toluene or
chlorobenzene, were suitable for this reaction, but a polar
solvent, such as dimethylformamide, produced the
deactivation of the catalyst, probably because its weak
basicity. Anyway, TFT under reflux (120 °C) was the solvent and
temperature of choice to make compatible both epoxidation
and Meinwald rearrangement.
furanic
fatty
esters,
24%
yield
of
3,5-
dihydroxytetrahydropyrans was also obtained. This type of
products had been proposed, on the basis on mass spectra of
TMS derivatives, as products in the reaction of the threo
isomer of methyl cis,cis-9,10:12,13-diepoxystearate with acetic
or trichloroacetic acids,42 although they had not been isolated
and fully identified to the best of our knowledge. Moreover,
17% yield of methyl 9,13-dioxostearate (1,5-diketone) was also
obtained. The remaining 9% yield corresponds to a complex
mixture of tetrahydrofuranic derivatives, although they could
not be separated in pure form and unequivocally identified.
The treatment of methyl cis-9,10-epoxy-12-hydroxystearates
with a Brønsted acid (Scheme 8) promotes the intramolecular
attack of the protonated epoxide by the hydroxyl group,
leading to a 3-hydroxytetrahydrofuran structure. Cyclizations
leading to hydroxytetrahydrofurans had been reported in the
acid catalyzed ring opening of linoleate diepoxide,43,44 and in
its biocatalytic rearrangement.45 The formation of
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tetrahydrofurans without hydroxyl group had been also
described in the functionalization of ricinoleate by
oxymercuriation-demercuriation.46 However, the presence of
the aromatic furan was rather unexpected. It seems that the
strong Brønsted acid is able to dehydrate the
hydroxytetrahydrofuran to dihydrofuran, which is finally
aromatized in a process through an unknown mechanism.
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DOI: 10.1039/C9CY01899A
One-pot and tandem processes
The sequence of the two reactions was first tested with methyl
oleate in one-pot, by addition of all the components (oxidant,
Ti-silica catalyst and SAC-13 acid catalyst) to the reaction
mixture from the beginning. After 2 h, only 23% conversion
was observed, 19% to oxostearate, much lower than in the
sequence of the two single reactions. After 48 h the
conversion only increased to 65%, but due to the isomerization
of the cis double bond of oleate with the strong acid catalyst
to the most stable trans double bond of methyl elaidate. The
detected problem was the decomposition of TBHP in the
presence of a strong Brønsted acid, given that further
additions of TBHP made the reaction progress until total
conversion to oxostearate with a total amount of 4.5 eq of
TBHP added into three portions of 1.5 eq at reaction times of
0, 1 and 2 h. In fact, the same effect can be obtained by
addition of the acid catalyst after the epoxidation reaction has
finished. However, with this system, the advantage of the one-
pot method (just putting all the components from the
beginning) is lost and both catalysts are recovered as a mixture
at the end of the reaction, without possibility of reuse under
the same conditions.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was financially supported by the Spanish Ministerio
de Ciencia, Innovación y Universidades (project number
RTI2018-093431-B-I00), the Gobierno de Aragón (E37_17R
group) and co-financed with Feder 2014-2020 “Construyendo
Europa desde Aragón”. Authors would also like to
acknowledge the use of Servicio General de Apoyo a la
Investigación-SAI, Universidad de Zaragoza.
Notes and references
‡ Footnotes relating to the main text should appear here. These
might include comments relevant to but not central to the
matter under discussion, limited experimental and spectral data,
and crystallographic data.
Thus, it would be advantageous the use of the tandem
procedure,47,48 that is the filtration of the Ti-silica catalyst at
the end of the first reaction and then addition of SAC-13 to
perform the second one. In this way, total conversion to
oxostearate is obtained in a total reaction time of 3 h (2 + 1),
with 90% isolated yield, and each catalyst can be recovered
and reused with or without reactivation, in a similar way to the
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6
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of separation and purification; hence,
a sustainability
improvement of the overall process has been achieved. In
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Catalysis Science & Technology
Page 8 of 8
View Article Online
DOI: 10.1039/C9CY01899A
Synthesis of fatty ketoesters by tandem epoxidation-rearrangement
with heterogeneous catalysis
Vicente Dorado, Lena Gil, José A. Mayoral, Clara I. Herrerías* and José M. Fraile*
Table of contents
Fatty ketoesters are obtained from unsaturated fatty esters in a tandem two-step process with a
combination of two heterogeneous catalysts, without intermediate purification, and with maximum
productivity of the catalysts through recycling and reuse.