Production of Valuable Esters from Oleic Acid with a Porous Polymeric Acid Catalyst without Water Removal

Article abstract of DOI:10.1055/s-0035-1560584

We demonstrate the use of porous phenolsulfonic acid-formaldehyde (PSF) resins as solid-acid catalysts, which showed excellent performance in the esterification of fatty acids without using any solvent or introducing a water-removal process. The catalyst was reused up to 30 times without significant loss of activity.

Full text of DOI:10.1055/s-0035-1560584

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© Georg Thieme Verlag Stuttgart · New York
2016, 27, 29–32
letter
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Y.-H. Kim et al.
Letter
Synlett
Production of Valuable Esters from Oleic Acid with a Porous Poly-
meric Acid Catalyst without Water Removal
Yo-Han Kima
Jusung Hana
Byeong Yeon Junga
Heeyoel Baekb
Yoichi M. A. Yamadab
Yasuhiro Uozumib
Yoon-Sik Lee*a
a School of Chemical and Biological Engineering, Seoul National
University, Gwanak-Gu, Seoul 151-744, Republic of Korea
b RIKEN Center for Sustainable Resource Science, 2-1 Hirosawa,
Wako, Saitama 351-0198, Japan
yslee@snu.ac.kr
Dedicated with respect to Professor Steven Ley on the occasion
of his 70th birthday
Received: 16.06.2015
Accepted after revision: 04.10.2015
Published online: 03.11.2015
lite (H-ZSM-5),15 and sulfonated multiwall carbon nano-
tubes (MWCNT).16 Although most of the catalysts have
exhibited considerable catalytic activity, their reliability in
industrial applications is still limited due to the instability
of their active sites and their sensitivity to water.
DOI: 10.1055/s-0035-1560584; Art ID: st-2015-d0448-l
Abstract We demonstrate the use of porous phenolsulfonic acid–
formaldehyde (PSF) resins as solid-acid catalysts, which showed excel-
lent performance in the esterification of fatty acids without using any
solvent or introducing a water-removal process. The catalyst was re-
used up to 30 times without significant loss of activity.
Polymer-based solid acids are promising for esterifica-
tion reactions, since they show cost efficiency, durability,
and high catalytic activity.8,17 As Frechet et al. reported,18
these advantages can be fully achieved by mimicking an en-
zymatic reaction mechanism. The catalytic performance of
the polymer-based solid acids strongly depends on their
thermal stability as well as their surface properties that
govern the accessibility of reactant to acid active sites. Vari-
ous attempts have been made to enhance the catalytic per-
formance of a polymer-based solid acid such as improving
its compatibility to oil/alcohol substrates, enhancing the
loading level of active sites, increasing the porosity, and
modifying the degree of cross-linking.17 Considering all
these factors, phenolsulfonic acid–formaldehyde (PSF) res-
ins could be a suitable acid catalyst, as they possess desir-
able surface properties and a porous structure.19 As illus-
trated in Figure 1 (a), the slightly hydrophilic meso-/macro-
porous structure of the resins can absorb fairly hydrophilic
substrates and can transform them into more hydrophobic
esters, which in turn could readily be excluded from the
catalyst surface. These intrinsic physical properties of the
resins can enhance the yield by shifting the equilibrium to-
ward esterification. In this study, porous PSF resin catalysts
were synthesized, and their esterification of oleic acid with
various alcohols was evaluated. Since these resin catalysts
possess a micro-/macroporous structure with strong acidic
sites, the applicability of the catalysts for industrial pro-
cesses was also systematically studied. The PSF resin cata-
lysts showed excellent esterification activity with 1.4 mol%
Key words heterogeneous catalyst, polymer-based solid acid, esterifi-
cation, fatty acid alkyl ester
Fatty acid esters are important compounds for a variety
of applications.1 They are used as emulsifiers,2 fuels,3 lubri-
cants,4 and plasticizers5 and are used in the industrial pro-
duction of alkyl resins.6 As such, the esterification of fatty
acids with alcohols is one of the most important chemical
transformations in both organic synthesis and industrial
chemistry. However, traditional esterification methods re-
quire high energy input with an additional water-removal
process. Furthermore, when the acid catalysts are neutral-
ized, this can generate a large amount of waste.7 The re-
maining or unseparated catalysts can result in degradation
of the valuable ester products, which causes depreciation in
their economic value. Thus, many researchers have shown
increasing interest in developing heterogeneous solid-acid
catalysts that feature easy separation of the catalyst, reus-
ability, low corrosive effect, and minimal energy require-
ments.8,9 This type of esterification has also been the focus
in chemical engineering with an attempt to develop green
production processes.10 Various solid-acid catalysts have
been developed for esterification reactions, including cat-
ion-exchange resins,11 sulfated silica, WO3/ZrO2,12 sulfonat-
ed-ZrO2 (s-ZrO2),13 supported heteropoly acids (HPA),14 zeo-
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Y.-H. Kim et al.
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of acidic sites without requiring water removal. Moreover,
due to their simple separation by filtration, the PSF resin
catalysts successfully prevented the degradation of ester
products.
ter the esterification reaction showed that the morphologi-
cal homogeneity of the polymer catalyst did not change sig-
nificantly (Figure 1, c). However, elemental analysis on the
PSF resin showed 30% degradation of the sulfonic acid
groups via thermal decomposition during esterification.
Hence, the PSF resins catalyst was treated with concentrat-
ed sulfuric acid after every catalyst recovery in order to re-
generate the active sites of sulfonic acid.
Acid-catalyzed esterification is an equilibrium-limited
reaction due to the formation of water. Reducing the energy
consumption in the water-removal process is essential to
produce cost-effective, high-yielding esterification process-
es. The catalytic performance of the PSF resin catalyst as
well as its optimal reaction conditions in the esterification
of oleic acid and methanol was investigated by a model re-
action. Figure S1 (Supporting Information) shows the sum-
marized results, where the esterification proceeded with
0.7–2.8 mol% PSF resin catalyst. The conversion of oleic acid
to ester increased with an increasing amount of PSF resin,
and the conversion reached almost 100% when 1.4 mol% of
catalyst was used. Moreover, the 1.4 mol% PSF resin catalyst
showed higher activity when it was subjected to different
reaction temperatures in the range of 30–90 °C. As shown
in Figure S2 (Supporting Information), the conversion of
oleic acid dramatically increased when the temperature
was above 50 °C. The ratio of oleic acid to alcohol is one of
the critical factors for industrial applications because a
higher yield of ester can be easily obtained by removing the
minimal amount of starting materials. Curiously enough,
the optimal ratios of oleic acid to alcohols were found to be
different depending on the kind of alcohols used. As shown
in Figure S3 (Supporting Information), 2-ethylhexanol was
well transformed into the corresponding ester at a ratio of
1:1.2 (alcohol to oleic acid), with minimal remaining alco-
hol. This tendency was followed by other primary alcohols,
including benzyl alcohol, hexanol, and 2-ethylhexanol.
However, 1,1,1-tris(hydroxymethyl)ethane showed good
oleic acid diester conversion at a ratio of 1:2 (alcohol to ole-
ic acid).
We compared the catalytic ability of the PSF resin cata-
lyst with a variety of commercially available heterogeneous
solid-acid catalysts through the esterification of oleic acid
and methanol, without water removal at 90 °C.20 As shown
in Table 1,PSF resin catalyst exhibited the highest catalytic
activity in terms of conversion and yield, when compared
to sulfonated ZrO2 (75%), H-ZSM-5 (12%), Montmorillonite
K-10 (11%), Amberlyst IRA 400 (14%), and Amberlyst 15
(20%) under the same reaction conditions. Moreover, the
PSF resin catalyst showed an excellent esterification perfor-
mance when compared to that of PTSA, which is routinely
used for industrial purposes (97% vs. 90% yields). This is
mainly related to the surface properties of the PSF resin
containing strong acidic active sites.
Figure 1 (a) Scheme of porous phenol sulfonic acid–formaldehyde
(PSF) resin structure and esterification reaction on the PSF resin; (b)
SEM image of freshly prepared PSF resin; (c) SEM image of the used PSF
resin.
The PSF resin catalyst was prepared using a previously
reported method,19 namely the condensation polymeriza-
tion of p-phenolsulfonic acid and formaldehyde. The resin
became a reddish brown solid after polymerization and
was scarcely soluble in any solvent. Unlike commercially
available geltype solid-acid catalysts, the PSF resin catalyst
was compatible with hydrophilic solvents including metha-
nol, ethanol, and even water. The compatibility of these sol-
vents enabled hydrophilic substrates such as alcohols, to
diffuse into the resin matrix and to react with the sulfonic
acid active site. After esterification, the more hydrophobic
ester products could be spontaneously excluded from the
PSF resin matrix.19
As shown in Figure 1 (b), the PSF resin catalyst has a bot-
ryoidal macroporous structure with a particle size of 0.5–1
μm (Figure 1, b), which increases the surface area and im-
proves the accessibility of the reactants. The resin has a po-
rous structure (ca. 13 nm pore size) with a surface area of
2.1 m2 g–1, determined by N2 absorption BET analysis. The
results from energy dispersive spectrometry (EDS) con-
firmed that the PSF resins catalyst contained sulfur, which
was further identified via ICP-AES. The loading of sulfur on
the PSF resins was 1.4–2.0 mmol g–1. SEM images taken af-
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Table 1 Esterification of Oleic Acid with Methanol by Using Various
Table 2 Esterification of Oleic Acid with Methanol by Using Various
Acid Catalystsa
Acid Catalystsa
Entry
Substrate
Conversion (%)
Yield (%)
Entry Substrate
Conversion (%) Yield (%)
1
2
3
4
5
6
7
8
none
11
97
11
20
14
75
12
90
11
97
11
20
14
75
12
90
1
2
3
4
5
MeOHb
95
98
95
PSF resin
hexanol
93
Montmorillonite K-10
Amberlyst 15
Amberlite IRA 400
sulfonated ZrO2
H-ZSM-5
2-ethylhexanol
benzyl alcohol
1,1,1-tris(hydroxymethyl)ethane
>99
95
96
91
83
42/36/5c
a Reaction conditions: catalyst (1.4 mol%), oleic acid (10 mmol), alcohol sub-
strates (12 mmol), 90 °C, 6 h. Yields were determined via GC analysis with
an internal standard: methyl heptadecanoate.
b MeOH (100 mmol), 50 °C.
PTSA
c Mono-/di-/triester.
a Reaction conditions: catalyst (1.4 mol%), oleic acid (10 mmol), MeOH (50
mmol), 90 °C, 6 h. Yields were determined via GC analysis with internal
standard: methyl heptadecanoate.
cant loss in its activity. The results show that the PSF-resin
catalyst is capable of transforming methanol and 1,1,1-
tris(hydroxymethyl)ethane into the corresponding esters,
which are in great demand for industrial uses.
The loading level of the acid sites on PSF-resin catalyst
was not higher than those of other solid-acid catalysts. The
order of the loading level from highest to lowest is as fol-
lows: H-ZSM-5 > Montmorillonite K-10 >> Amberlyst 15 >>
PSF-resin catalyst > s-ZrO2. In heterogeneous solid-acid-cat-
alyzed esterification, the accessibility of substrates to active
sites is important. When excellent accessibility is provided,
the catalysts show optimum catalytic performance. Despite
the fact that the amount of active sites on the PSF-resin cat-
alyst was less than that of others; based on the catalytic
performance, we can conclude that the porous structure of
the PSF-resin catalyst provided an optimal environment for
enhanced catalytic activity.
The catalytic performance of PSF-resin catalyst was in-
vestigated in the esterification of oleic acid and other alco-
hols, including aliphatic alcohols, aromatic alcohols, and
polyol, without applying the water-removing procedure
(Table 2). The PSF-resin catalyst showed good performance
in esterification of short alkyl chain, as well as long chain,
and branched primary alcohols (Table 2, entries 1–3). The
aromatic primary alcohol was also transformed well into its
corresponding ester (Table 2, entry 4). Furthermore, this
catalytic system was suitable for triglyceride synthesis,
with an excellent conversion rate (85%, Table 2, entry 5) al-
though a monoester was predominantly obtained.
In order to examine the reusability of the catalyst, the
resin catalyst was separated via filtration after esterifica-
tion, washed with ethanol, and treated with concentrated
sulfuric acid. Then, the catalyst was rinsed with methanol
and water to remove the remaining sulfuric acid, and dried
under vacuum before recycling.21 The changes in activity
upon recycling during the synthesis of methyl oleate are
presented in Figure 2. It is noteworthy that the PSF-resin
catalyst was recyclable up to 30 times without any signifi-
Figure 2 Recycling results of PSF-resin catalyst after esterification of
2-ethylhexanol and oleic acid. Reagents and conditions: catalyst (1.4
mol%), oleic acid (10 mmol), 2-ethylhexanol (1.2 equiv, 1.9 mL), 90 °C,
6 h.
In conclusion, phenolsulfonic acid–formaldehyde resin
is an efficient heterogeneous solid-acid catalyst for esterifi-
cation. The porous structure of the resin enhances catalytic
performance, despite the loading of active sites not being
higher than those of other commercially available solid-
acid catalysts. These properties induced an excellent esteri-
fication reaction of oleic acid with various alcohol sub-
strates. Finally, the catalyst can easily be separated and re-
used without any loss of catalytic activity. We believe that
application of the phenolsulfonic acid–formaldehyde resin
catalyst will facilitate the development of green chemical
processes for the production of industrial ester compounds.
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Acknowledgment
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This work was supported by the WCU (World Class University) pro-
gram (R32-2010-000-10213-0) through the National Research Foun-
dation of Korea (NRF) funded by the Ministry of Education, Science
and Technology.
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Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0035-1560584.
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, 29–32