Esterification, transesterification and hydrogenation reactions of polyunsaturated compounds catalyzed by a recyclable polymer supported palladium catalyst

Article abstract of DOI:10.1016/j.jorganchem.2016.06.001

Aliphatic and aromatic carboxylic acids were converted into their corresponding esters using a polymer supported palladium(II) β-ketoesterate complex under hydrogen atmosphere in the presence of catalytic bromobenzene in alcohols. This method was also applicable to the transesterification of esters. Good to excellent yields were obtained for different aliphatic or aromatic starting materials. The esterification (or transesterification) was promoted by the in situ generation of HBr from bromobenzene, which provided a mild acidic reaction environment. Pd(II) centers were converted into polymer stabilized metal nanoparticles (the true active species) under reaction conditions. The palladium catalyst exhibited a remarkable activity and was reusable for eight consecutive cycles. The present system was also tested for the preparation of partially hydrogenated fatty acid methyl esters, starting from a mixture composed by highly polyunsaturated esters and free carboxylic acids, taken as a model acidic feedstock for biodiesel upgrading.

Full text of DOI:10.1016/j.jorganchem.2016.06.001

Accepted Manuscript
Esterification, transesterification and hydrogenation reactions of polyunsaturated
compounds catalyzed by a recyclable polymer supported palladium catalyst
Maria Michela Dell'Anna, Vito Filippo Capodiferro, Matilda Mali, Piero Mastrorilli
PII:
S0022-328X(16)30251-0
10.1016/j.jorganchem.2016.06.001
JOM 19518
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 12 April 2016
Revised Date: 31 May 2016
Accepted Date: 1 June 2016
Please cite this article as: M.M. Dell'Anna, V.F. Capodiferro, M. Mali, P. Mastrorilli, Esterification,
transesterification and hydrogenation reactions of polyunsaturated compounds catalyzed by a recyclable
polymer supported palladium catalyst, Journal of Organometallic Chemistry (2016), doi: 10.1016/
j.jorganchem.2016.06.001.
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ACCEPTED MANUSCRIPT
one-pot
esterification/transesterification/hydrogenation
Pd-pol
Highly Polyunsaturated
Fatty Acid Ethyl Esters and Free
H2 (2 bar)
PhBr (additive)
CH3OH
Partially and Fully
Hydrogenated
Fatty Acid Methyl Esters
carboxylic acids
40 °C
100
Conv. (%)
80
60
40
20
0
1
2
3
4
5
6
7
8
Cycle

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transesterification and hydrogenation
Esterification,
reactions
of
polyunsaturated compounds catalyzed by a recyclable polymer supported
palladium catalyst.
Maria Michela Dell’Anna,*a Vito Filippo Capodiferro,c Matilda Mali,a Piero Mastrorillia,b
a DICATECh, Politecnico di Bari, via Orabona 4, 70125 Bari, Italy.
bConsiglio Nazionale delle Ricerche, Istituto di Chimica dei Composti Organometallici (CNR-
ICCOM), via Orabona 4, 70125 Bari, Italy.
c Department of Pharmacy, University of Bari “Aldo Moro”, Via Orabona, 4, 70126 Bari, Italy
*Corresponding author: e-mail address: mariamichela.dellanna@poliba.it, tel: +39 080 5963695,
fax: +39 080 5963611
Abstract
Aliphatic and aromatic carboxylic acids were converted into their corresponding esters using a
polymer supported palladium(II) β-ketoesterate complex under hydrogen atmosphere in the
presence of catalytic bromobenzene in alcohols. This method was also applicable to the
transesterification of esters. Good to excellent yields were obtained for different aliphatic or
aromatic starting materials. The esterification (or transesterification) was promoted by the in situ
generation of HBr from bromobenzene, which provided a mild acidic reaction environment. Pd(II)
centers were converted into polymer stabilized metal nanoparticles (the true active species) under
reaction conditions. The palladium catalyst exhibited a remarkable activity and was reusable for
eight consecutive cycles. The present system was also tested for the preparation of partially
hydrogenated fatty acid methyl esters, starting from a mixture composed by highly polyunsaturated
esters and free carboxylic acids, taken as a model acidic feedstock for biodiesel upgrading.
Keywords
Esterification; palladium nanoparticles; transesterification; recyclable catalyst; hydrogenation;
biodiesel upgrade.
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1. Introduction
Esterification of carboxylic acids and transesterification of esters are important reactions having
wide industrial applications for the synthesis of fragrances, flavors, polymers, pharmaceutical and
agricultural compounds [1,2,3]. In this regard, many useful and reliable catalytic systems for the
synthesis of esters [4,5,6,7] and for transesterification reactions [8,9,10,11,12,13] have been
reported in the literature. In addition, esterification and transesterification reactions are of great
interest for the preparation of biodiesel [14], which is commonly produced from either
transesterification of natural triglycerides (animal fats or vegetable oils) or from esterification of
free fatty acids (FFAs) with methanol to give fatty acid methyl esters (FAMEs) [15]. Another
important aspect in biodiesel synthesis is the amount of polyunsaturated compounds in the
feedstock. Products that comprise large amounts of highly unsaturated FAMEs are more susceptible
to oxidation (rancidity) but, on the other hand, have better cold flow properties than highly saturated
products [16], so a useful biodiesel should have a large amount of monounsaturated FAMEs and
low quantities of both saturated and polyunsaturated FAMEs. Many efforts have been recently
made to design catalytic systems aiming at biodiesel upgrading by partial hydrogenation of FAMEs
[17,18], also with palladium based catalysts [19,20,21,22].
On the basis of the above considerations, we could argue that a catalytic system able to
simultaneously promote esterification, transesterification and partially hydrogenation reactions
would be interesting both for fine chemicals synthesis and for biodiesel production.
Recently, it has been reported that the esterification of carboxylic acids and the transesterification of
esters could be smoothly achieved using Pd/C under hydrogenation conditions (1-4 bars H2) in the
presence of small amount of bromobenzene in alcohols [23]. However, the reported Pd/C catalytic
system could be improved (Pd/C is even pyrophoric) mainly from a recyclability point of view,
being the catalyst re-usable only up to three times [23].
Aiming at developing a more active system, we decided to evaluate the catalytic activity of a
polymer supported palladium catalyst (in the following Pd-pol) for the esterification of carboxylic
acids and transesterification of esters. In order to obtain a material with a uniform distribution of the
active sites, the pre-catalyst was synthesized in an unusual procedure [24], i.e., by co-
polymerization of the β-ketoesterate metal-containing monomer Pd(AAEMA)2 [AAEMA−=
deprotonated form of 2-(acetoacetoxy)ethyl methacrylate] with suitable co-monomer (ethyl
methacrylate, EMA) and cross-linker (ethylene glycol dimethacrylate, EGDMA) [25] (Scheme 1).
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O
O
CO
CO
CO CO CO CO
OEt OEt OEt OEt
O
OEt
OEt
O
O
O
O
Pd
O
O
OEt
CO
OEt OEt OEt OEt
CO CO CO CO
O
O
C
O
O
O
OEt
O
O
CO CO
OEt OEt
CO CO CO
OEt OEt OEt
Scheme 1: synthesis of Pd-pol.
Pd-pol pre-catalyst was successfully employed in several palladium promoted reactions
[26,27,28,29,30,31,32,33,34,35]. In all cases, the pristine Pd(II) supported complex was reduced in
situ under reaction conditions to Pd(0), forming palladium nanoparticles (NPs) (the real active
species) immobilized onto the insoluble support (Pd-pol, Scheme 1) and stabilized by the macro-
porous and reticulated polymer matrix [36]. These NPs showed a size distribution centered around 4
nm (when formed under 1 atm H2 in methanol as the solvent), retaining their morphology and
roughly their dimension even after several catalytic runs in hydrogenation reactions [30].
Herein, we report on the catalytic activity and recyclability of Pd-pol system in the esterification
(and transesterification) reaction by a simple condensation of a carboxylic acid with an alcohol
under mild hydrogenation conditions in the presence of catalytic amount of bromobenzene. Since
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the present system was incidentally able to catalyze also the C=C double bond hydrogenation, we
tested the developed protocol in the simultaneous esterification, transesterification and partially
hydrogenation of a mixture of free acids and polyunsaturated fatty esters, taken as a model acidic
feedstock for biodiesel upgrading.
2. Experimental
2.1. Materials
All chemicals were purchased from commercial sources and used as received. Pd-pol was
synthesized according to the procedure described in ref. [30]. Commercial edible oil was composed
by all cis-5,8,11,14,17-eicosapentaenoic acid ethyl ester (40%w), all cis-4,7,10,13,16,19-
docosahexaenoic acid ethyl ester (20%w), α−linolenic, linoleic and oleic acid ethyl esters (30%w),
excipients (10%w). Palladium content in Pd-pol was assessed after sample mineralization by atomic
absorption spectrometry using a Perkin–Elmer 3110 instrument. The experimental error on the
palladium percentage was ±0.3. Palladium amount in the crude product (after removal of the
catalyst) was assessed after sample mineralization by inductively coupled plasma-optical emission
spectroscopy (ICP-OES, Thermo ICAP 6000) analysis. For the corresponding calibration curve the
limit of detection was 50 ppb. Mineralization prior to Pd analyses was carried by microwave
irradiation with an ETHOS E-TOUCH Milestone applicator, after addition of 12 mL HCl/HNO3
(3:1, v/v) solution to each weighted sample. GC–MS data (EI, 70 eV) were acquired on a HP 6890
instrument using a HP-5MS crosslinked 5% PH ME siloxane(30.0 m × 0.25 mm × 0.25 mm)
capillary column coupled with a mass spectrometer HP 5973. Reactions were monitored by GLC or
by GC–MS analyzes. GLC analysis of the products was performed using a HP 6890 instrument
equipped with a FID detector and a HP-1 (Crosslinked Methyl Siloxane) capillary column (60.0 m
× 0.25 mm × 1.0 µm). Yields were calculated by GLC analysis as moles of ester product per mole
of starting acid by using biphenyl as internal standard.
2.2. General procedure for catalytic esterification of carboxylic acid and transesterification
reaction
In a typical run, a 50 mL stainless steel autoclave equipped with a transducer for online pressure
monitoring was charged under air of Pd-pol (10.6 mg, Pd: 0.5 mol%), the substrate (carboxylic acid
or ester, 1.0 mmol), bromobenzene (0.1 mmol, 15.7 mg) and alcohol (5.0 mL). The autoclave was
then purged three times with hydrogen, then pressurized with 2 bars H2, set on a magnetic stirrer
and heated to 40 °C. After the minimum time needed to reach reaction completion, the autoclave
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was let to reach room temperature, the hydrogen was vented and the autoclave opened. When
necessary, 3.0 mL diethyl ether was added to the mixture at the end of reaction in order to dissolve
organic reactants and products. The catalyst was recovered by centrifugation. The yields were
assessed by GLC analysis of the organic solution by using biphenyl (50.0 mg) as internal standard.
2.3. Recycling experiments
The catalyst recovered by centrifugation was washed with methanol and diethyl ether and dried
under vacuum. The recovered catalyst was thus weighed and reused for a new cycle employing
appropriate amount of carboxylic acid, assuming that the palladium content remained unchanged
with the recycles. Iteration of this procedure was continued for eight reuses of the catalyst.
2.4. Isolated Yield
3-phenylpropionic acid (751.0 mg, 5.0 mmol), bromobenzene (78.5 mg, 0.5 mmol), Pd-pol (53.0
mg, Pd: 0.5 mol%), and methanol (10 mL) were placed in autoclave vessel. Autoclave was
pressurized with 2 bars of hydrogen gas. The reaction mixture was then warmed to 40 °C
temperature and stirred for 3 h. After reaction, the catalyst was filtered through celite bed. Filtrate
was added of water (30 mL). The reaction mixture was extracted with ethyl acetate (3 x 15 mL).
The combined organic layers were washed with 5% aqueous sodium bicarbonate solution (3 x 15
mL), dried over anhydrous Na2SO4 and filtered. The filtrate was evaporated under vacuum to give
methyl 3-phenylpropionate as a pale yellow oil (763.5 mg, yield = 93 %). Comparison of its MS
and NMR features with those reported in literature [37] confirmed the purity of the obtained
product.
2.5. One pot hydrogenation, transesterification and esterification
A 50 mL stainless steel autoclave equipped with a transducer for online pressure monitoring was
charged under air with Pd-pol (10.6 mg, Pd %w = 5.0), the commercial oil (323.5 mg), octanoic acid
(0.50 mmol, 72.1 mg, when present), bromobenzene (0.10 mmol, 15.7 mg) and methanol (5.0 mL).
The autoclave was then purged three times with hydrogen, then pressurized with 2 bars H2, set on a
magnetic stirrer and heated to 40 °C. The reaction course was monitored by submitting to GLC
analysis 0.2 mL samples (diluted with 3.0 mL diethyl ether) drawn at proper time from the reaction
mixture.
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3.Results and Discussion
3.1. Esterification
In preliminary experiments we selected the esterification of 3-phenylpropionic acid to the
corresponding methyl ester as the model reaction to study the catalytic activity and selectivity of
Pd-pol, being 3-phenylpropionic acid methyl ester an intermediate for drug synthesis [38]. The
relevant results are reported in Table 1.
Table 1. Esterification of 3-phenylpropionic acid to methyl 3-phenylpropionate under different
reaction conditions a
O
O
Pd-pol
(0.5% mol)
OH
OCH3
CH3OH
H2, additive
Entry
P (bar)b
T (°C)
t (h)
Conv. (%)c
Selectivity
(%)c
0
1d
2e
3
5
5
40
40
40
40
40
RT
40
40
3
3
0
1
>99
f
-
3
0
0
4
5
3
5
2
2
2
>99
>99
83
90
>99
>99
5
2.5
2.5
2.5
3
>99
6
>99
7
8g
>99
>99 (93)h
a3-phenylpropionic acid (1.0 mmol), CH3OH (5.0 mL), Pd-pol (Pd: 0.5 mol% of Pd, 10.6 mg),
bromobenzene (0.1 mmol, 15.7 mg) under H2. b H2 pressure. c Calculated by GLC analysis. d In the
e
f
g
absence of Pd-pol. In the absence of bromobenzene. Under air. Bold typing refers to best
reaction conditions. h Isolated yield of the reaction carried out on a 5.0 mmol scale.
The best conditions were found to be those employed in entry 8, that is: 3-phenylpropionic acid (1.0
mmol), Pd-pol (0.5 mol% of Pd), hydrogen (2 bar) in methanol (5 mL) at 40 °C for 3 h. The
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preliminary experiments revealed that in methanol at 40 °C after 3 h the conversion into the ester
product was negligible in the absence of Pd-pol (entry 1), or in the absence of a catalytic amount of
bromobenzene (entry 2) or when the reaction was carried out under air (entry 3), thus confirming
that Pd-pol, bromobenzene and hydrogen gas were all essential for the desired transformation. On
the contrary, by increasing the pressure of H2 up to 5 bars in the presence of both Pd-pol and
bromobenzene, the acid conversion raised up to 99% at 40 °C after 2 h (entry 4) and up to 83% at
room temperature after 2.5 h (entry 6). By decreasing the dihydrogen pressure to 3 bars the
minimum time to reach quantitative conversion was 2.5 h at 40 °C (entry 5). Since in terms of
“safe” chemistry the use of a low hydrogen pressure is more convenient, we decided to carry out the
catalytic esterification of 3-phenylpropionic acid under 2 bars hydrogen at the temperature of 40 °C,
achieving quantitative substrate conversion after 3 hours (entries 7 and 8). Using the optimized
reaction conditions, the activity and the scope of the catalytic system was explored in the
esterification reaction of some aliphatic and aromatic acids in different alcohols as solvents (Table
2).
Table 2. Esterification of carboxylic acids with alcoholsa
entry
substrate
product
t(h)
Yield (%)b
1
2
98
6
COOH
2c
3
2
3
95
98
COOCH3
4
14
91
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5
14
58
6
7
14
4
87
95
8
9
4
5
85
10
10
4
3
70
95
11d
12e
3
94
acarboxylic acid (1.0 mmol), R’OH (5.0 mL), Pd-pol (Pd: 0.5 mol% of Pd), bromobenzene (0.10
mmol, 15.7 mg) under H2 (2 bars), T = 40 °C. b Calculated by GLC analysis. c diethyl ether (3.0 mL)
was added to the reaction mixture prior GLC analysis, in order to dissolve residual substrate and
methyl stearate. d reaction carried out in the presence of HBr (0.1 mmol) under air at T = 40 °C, in
e
the absence of both Pd-pol and PhBr. 3-phenylpropionic acid (5.0 mmol), CH3OH (5.0 mL), Pd-
pol (Pd: 0.10 mol% of Pd, 10.6 mg), bromobenzene (0.10 mmol, 15.7 mg) under H2 (2 bars), T = 40
°C.
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The esterification with methanol of octanoic acid, a substrate frequently taken as a model reactant
for the synthesis of “green diesel” [39], proceeded smoothly, producing the relevant methyl ester in
2 h (entry 1). On the other hand, the methylation of oleic acid (entry 2) occurred together with the
C=C bond hydrogenation, yielding a 95% of methyl stearate. This result is significant, because
there is an increasing interest in the fully hydrogenation process of FAMEs [40,41].
The methylation of 2-phenylpropionic acid was quantitative in 3 h (entry 3, Table 2), and followed
the same course observed for its isomer 3-phenylpropionic acid (entry 8, Table 1). As expected on
the basis of the proposed mechanism (vide infra), the methylation of aromatic acids was more
sluggish, requiring 14 h to achieve acceptable conversions (entries 4-6, Table 2). The esterification
of 1-naphtoic acid (entry 5, Table 2) was incomplete due to the detrimental steric effect of the
hydrogen atom in position 8 [42].
To explore the reaction scope, 3-phenylpropionic acid (the model acid) was reacted with different
alcohols (ethanol, n-propanol, i-propanol, benzyl alcohol) in the presence of bromobenzene. The
obtained results in the esterification of the model acid under the optimized reaction conditions were
excellent when linear alkyl alcohols were used (entries 7 and 8, Table 2). However, when a more
hindered hydroxyl group (entry 9, Table 2) was employed, poor results were observed. Finally, the
use of benzyl alcohol gave a good yield (entry 10, Table 2).
3.2. Recyclability
The reusability of Pd-pol catalyst in the esterification of 3-phenylpropionic acid to methyl 3-
phenylpropionate was investigated under the reaction conditions described for entry 8 of Table 1.
After the first use, the supported catalyst was recovered by simple centrifugation and reused in the
next run after a washing workup. The recovered catalyst was successfully employed in subsequent
eight cycles with a high catalytic activity, giving the product in excellent yields (94-99%, Figure 1).
The samples of Pd-pol recovered either during reaction (filtered off when the substrate conversion
was 30 % ca.) and after the first and the eighth runs were mineralized and analyzed by atomic
absorption spectrometry showing the same palladium content (in the range of the experimental
error) of the pristine catalyst (Table 3). In addition, the crude product (after removal of the
supported catalyst) was mineralized and submitted to ICP-OES analyses revealing the absence of
palladium in the organic phase (Pd amount was below the detection limit of 50 ppb). All these
experiments proved that the metal leaching into solution was negligible.
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100
80
60
40
20
0
Conv. (%)
1
2
3
4
5
6
7
8
Cycle
Figure 1: Recyclability of Pd-pol (0.5 mol% of Pd) in the esterification of 3-phenylpropionic acid
to methyl 3-phenylpropionate in methanol under H2 (2 bars) at 40 °C (t = 3 h) using bromobenzene
as additive.
Table 3. Palladium amount on the supported catalyst recovered after different cycles of the model
reaction.
Pd-pol
Pd wt%
5.0 ± 0.3
5.1 ± 0.3
4.9 ± 0.3
4.8 ± 0.3
Pristine
Recovered during reaction (30% conv.)
Recovered after the first run
Recovered after the eight run
3.3. Mechanistic considerations
To gain insights into the reaction mechanism, the following test was performed. The solid catalyst
was hot filtered off after the esterification of 3-phenylpropionic acid in methanol under dihydrogen
had run for 15 min and the conversion had reached 30% ca. The clear filtrate was then stirred under
air at 40 °C. After 3 h, the GLC analysis of the catalyst-free mixture showed quantitative
conversion into methyl 3-phenylpropionate, and the atomic adsorption analysis carried out on the
digested filtered catalyst revealed a palladium content equal to that found in Pd-pol before use.
However, a color change of the palladium based polymer from yellow (before use) to dark gray was
observed at the very beginning of the reaction, suggesting that dihydrogen transformed the pre-
catalyst into the active species, i.e. palladium nanoparticles. In fact, the polymeric support of Pd-pol
was proven to stabilize palladium nanoparticles (with diameters ranging from 2 to 6 nm) formed
under dihydrogen in methanol, as revealed by TEM analysis [30]. It is thus clear that the
catalytically active species were not the original monomeric Pd(II) species, i.e. [Pd(AAEMA)2], but
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the in situ generated palladium nanoparticles. The reaction pathway could be similar to the one
proposed by Aavula and co-workers [23] starting with an oxidative addition of bromobenzene
additive to the coordinately unsaturated palladium(0) at the nanoparticle edge, affording Pd(II)-
phenyl bromide intermediate, which in contact with hydrogen gas leads to generation of HBr and
Pd(II) phenyl hydride species that, in turn, restores the initial Pd(0) by elimination of benzene
(Scheme 2). Proton transfer from the generated HBr to the carbonyl oxygen of the carboxylic acid
increases the electrophilicity of the carbonyl carbon, promoting the well-known Fischer
esterification with alcohol [43] (Scheme 2).
Scheme 2: Possible reaction mechanism for the esterification with alcohol promoted by Pd-pol.
This mechanism was confirmed by the high catalytic activity showed by the hot filtered mother
liquor let under stirring at 40 °C under air in the absence of metal catalyst. The following
observations also substantiated the proposed reaction pathway: the esterification of 3-
phenylpropionic acid with methanol proceeded smoothly i) when the methanol solution containing
the substrate was stirred under air at 40 °C without the Pd catalyst in the presence of catalytic
amount of HBr (entry 11, Table 2), and ii) when in the model reaction system (3-phenylpropionic
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acid, methanol, PhBr additive, dihydrogen atmosphere) the amount of Pd-pol was decreased up to
0.10 mol% of Pd (entry 12, Table 2).
In addition, the proposed mechanism explains why aromatic acids took longer time to react (entries
4-6, Table 2). In fact, the electro-donating capability of the aromatic group decreases the
electrophilicity of the carboxylic carbon, thus slowing the reaction rate.
3.4. Transesterification
Since it is known that the acid promoted esterification runs the same pathway followed by the
transesterification reaction in the presence of excess alcohol, the present catalytic system was
applied to the transesterification of ethyl octanoate with methanol. The obtained results were
excellent (yield = 95 % after 3 hour reaction, Scheme 3).
Scheme 3: Transesterification with methanol promoted by Pd-pol.
The transesterification carried out on ethyl benzoate gave poor results (yield = 15 % after 20 hours).
3.5. Simultaneous hydrogenation, transesterification and esterification
This catalytic system is able also to reduce C=C double bonds, as it is clear from entry 2 of Table 2,
thus promoting simultaneously esterification and hydrogenation reactions, which is really
interesting for production of high quality biodiesel. In fact, for example fatty acid methyl esters
derived from algae or fish oils have good cold flow properties, but suffer from very poor oxidation
stability, which makes these products unsuitable for the use as biodiesel. All these features arise
from the presence of highly polyunsaturated esters. Many efforts have been made to date to partially
hydrogenate polyunsaturated methyl esters (biodiesel upgrading) in order to increase the oxidation
stability, while maintaining satisfying cold flow properties [44]. Another important problem in
biofuel production is the undesirable presence of large amount of FFAs in certain biodiesel (acidic
oils) feedstocks (mostly those coming from waste cooking oils), which lowers the fuel quality [13].
For this reason, we decided to test the catalytic activity of Pd-pol system in the simultaneous partial
hydrogenation, transesterification and esterification with methanol of a mixture constituted by an
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edible commercial oil containing highly polyunsaturated fatty acid ethyl esters (FAEEs) added of
octanoic acid, in order to simulate an acidic biodiesel feedstock. The majority of the esters in the
chosen edible oil included acids with many unsaturations, such as eicosapentaenoic acid (EPA,
C20:5, 40%w) and docosahexaenoic acid (DHA, C22:6, 20%w) (scheme 4). The remaining part of
the oil comprised ethyl esters of α−linolenic acid (ALA, C18:3, scheme 4), linoleic acid (C18:2)
and oleic acid (C18:1) and minor amounts of ethyl esters of C17 and C16 fatty acids.
O
9
15
12
1
OC2H5
ALA (C18:3)
O
11
8
17
19
14
5
1
OC2H5
EPA (C20:5)
16
4
13
10
7
OC2H5
1
DHA (C22:6)
O
Scheme 4: ALA, EPA and DHA ethyl esters.
The model mixture was suspended in methanol and subjected to Pd-pol catalytic
hydrogenation/esterification/transesterification using PhBr as the additive.
The octanoic acid was quantitatively converted into its corresponding methyl ester after 2 hours.
Time course of the reaction for C18, C20 and C22 species is reported in Figures 2a, 2b and 2c,
respectively.
100%
(a)
FAME C18:0
80%
FAME C18:1
60%
FAME C18:2
FAEE C18:1
FAEE C18:2
FAEE C18:3
40%
20%
0%
0 h
2 h
22 h
25 h
46 h
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100%
80%
60%
40%
20%
0%
(b)
FAME C20:0
FAME C20:1
FAME C20:2
unresolved mixture
FAEE C20:4
FAEE C20:5
0 h
2 h
22 h
25 h
46 h
100%
80%
60%
40%
20%
0%
(c)
FAME C22:0
FAME C22:1
FAME C22:2
FAME C22:6
unresolved mixture
FAEE C22:6
0 h
2 h
22 h
25 h
46 h
Figure 2: Percentage composition during reaction of (a) C18 ; (b) C20; (c) C22 species.
Before the reaction started (t = 0 h, Figure 2a), the C18 fraction of the mixture was constituted by
α−linolenic acid ethyl ester (34%, FAEE C18:3), linoleic acid ethyl ester (13%, FAEE C18:2) and
oleic acid ethyl ester (53%, FAEE C18:1). For α-linolenic acid ethyl ester (FAEE C18:3) the C=C
double bond hydrogenation reaction was faster than the transesterification, and after 2 h the
conversion of the initial C18:3 FAEE into C18:2 and C18:1 methyl and ethyl esters was quantitative
(Figure 2a). The transesterification of ethyl esters into methyl esters reached completion after 22
hours, when C18:1 FAME isomers and methyl stearate (FAME C18:0) were the sole C18 species,
being in 4:1 ratio. Their relative ratio remained almost the same even after 46 h reaction.
Also for EPA ethyl ester (FAEE C20:5), the C=C double bond hydrogenation was fast, and the
conversion into less unsaturated species was 83% after 2 h reaction (Figure 2b). Actually, a plethora
of species formed during reaction due to partial hydrogenation, isomerization and esterification. It
was not possible to distinguish these species by GLC techniques, because they gave rise to
overlapping broad peaks. They were classified in figure 2b as “unresolved mixture”. Their quantity
decreased down to 10% of the total amount of C20 species after 22 hour reaction and disappeared
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after 46 hours, when the C20 fraction was made up mainly of di-unsaturated FAMEs (48%) and
monounsaturated FAMEs (32%), being minor (20%) the amount of fully hydrogenated FAME.
DHA ethyl ester (FAEE C22:6) disappeared after 2 h stirring under reaction conditions (Figure 2c).
Also in this case, as it happened for EPA derivatives, an “unresolved mixture” formed, whose
amount decreased down to 13% of the total amount of C22 species only after 22 h. After 46 hour
reaction, the C22 portion contained di-unsaturated FAMEs (62%), monounsaturated methyl ester
isomers (23%) and a minor quantity of fully hydrogenated product (15%).
For all polyunsaturated FAEEs, this catalyst provided satisfactory transesterification capability and
good partial hydrogenation activity with consecutive C=C double bond reductions, which might
result in an improvement of the oxidative stability of the obtained FAME mixture. The lower
catalytic activity showed by Pd-pol system in hydrogenating the present mixture with respect to the
activity observed in hydrogenating (and esterifying) oleic acid (entry 2, Table 2) might be found in
the low purity of the considered mixture. Palladium based catalysts are known to be sensitive to
impurities. In fact, it has been reported that, in the presence of a Pd sol-gel heterogeneous catalyst,
squalene (a terpene with six C=C double bonds) was fully hydrogenated in 4 h if its purity was > 98
%w, while complete hydrogenation did never occur under the same reaction conditions, if the
substrate was the 82 %w squalene olive oil (after 24 hour reaction only four double bonds were
hydrogenated, and no further reaction took place even at longer reaction times) [45].
In order to ascertain whether the presence of free octanoic acid could affect the course of the
hydrogenation/transesterification reaction of the model mixture, the same experiment was carried
out in the absence of octanoic acid. The product distribution did not change, revealing that the
initial presence of a free carboxylic acid had no influence on the overall reaction, being the acid
quickly esterified.
4.Conclusion
Polymer supported palladium nanoparticles promoted the esterification of carboxylic acids with
alcohol under dihydrogen gas in the presence of catalytic amount of bromobenzene. This procedure
could also be applicable to the transesterification of esters. The role of bromobenzene consisted in
generating in situ catalytic amount of HBr which provided a mild acidic environment and started
the Fischer esterification (or transesterification) reaction. The palladium catalyst exhibited a
remarkable activity and was reusable for eight consecutive cycles. The present method resulted also
able to simultaneously catalyze the hydrogenation and the esterification of oleic acid to methyl
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stearate, a valuable industrial product. The strength of this system stands in its simplicity and
environmentally benignity (avoid of strong acidic conditions and use of a recyclable catalyst). The
Pd-pol method was also tested in the one-pot partial hydrogenation, transesterification and
esterification with methanol of a low purity mixture of ethyl esters of highly polyunsaturated fatty
acids, added of a free carboxylic acid (octanoic acid), in order to simulate a FFA containing
biodiesel feedstock to be upgraded. Esterification of the carboxylic acid and transesterifications
were quantitative, while the hydrogenation led to mono- and di-unsaturated products, which are
more suitable for biofuel application than fully hydrogenated or highly polyunsaturated FAMEs.
Therefore, this study suggests that the addition of catalytic amount of bromobenzene to acidic oils
eliminates the undesirable free fatty acids, when upgrading biodiesel under palladium catalyzed
hydrogenation conditions.
Acknowledgments
The Politecnico di Bari is gratefully acknowledged for financial support (FRA funds).
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Highlights
•
•
•
•
•
Esterification and transesterification were promoted by a Pd catalyst (Pd-pol).
Reactions occurred in the presence of PhBr (additive) under H2.
The catalyst could be reused at least eight times without loss of activity.
The system was able to catalyze also hydrogenation reactions.
The system killed free carboxylic acids when used for upgrading acidic biodiesel.