Solventless esterification of fatty acids with trimethylolpropane using sulfonated amorphous carbons derived from wood powder

Article abstract of DOI:10.1016/j.catcom.2017.03.015

Polyolesters synthesized by an esterification between polyols and fatty acids are value-added oleochemicals widely used as lubricants, cosmetics, and food additives among other applications. However, homogeneous acid catalysts are still preferred in their industrial production, despite their associated energy cost and the environmental issues that they present. In this paper, we describe lignocellulose-derived amorphous carbons with a high loading level of SO₃H and their application to the synthesis of polyolesters as a biobased heterogeneous acid catalyst. These sulfonated amorphous carbons could be readily prepared via (i) heat treatment at 400?°C for 1?h and (ii) sulfonation with chlorosulfuric acid. XRD and BET analyses demonstrated that these carbonaceous materials were not crystalline but were amorphous structures with a low surface area. The attachment of SO₃H groups was confirmed by FT-IR and XPS, and the loading level of SO₃H was determined by CHNS elemental analysis. Chlorosulfuric acid gave a higher loading level of SO₃H than other sulfating agents, such as conc. sulfuric acid and fuming sulfuric acid. The higher SO₃H-loaded amorphous carbons exhibited a greater catalytic activity for esterification between trimethylolpropane and fatty acids under solventless conditions. Esterification of oleic acid derived from vegetable oil with trimethylolpropane using this catalyst afforded the desired biolubricant at over 93% yield in 3?h. The sulfonated amorphous carbons could be reused three times without any significant loss of catalytic activity.

Full text of DOI:10.1016/j.catcom.2017.03.015

Catalysis Communications 96 (2017) 32–36
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Solventless esterification of fatty acids with trimethylolpropane using
sulfonated amorphous carbons derived from wood powder
Dasom Mun a,b, Anh Thi Hoang Vo c, Bora Kim a, Yong-Gun Shul b, Jin Ku Cho a,c,
⁎
a
Green Material and Process R&D Group, Korea Institute of Industrial Technology (KITECH), Cheonan, Chungnam 31056, Republic of Korea
b
c
Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul 03722, Republic of Korea
Green Process and System Engineering Department, Korea University of Science and Technology (UST), Cheonan, Chungnam 31056, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Polyolesters synthesized by an esterification between polyols and fatty acids are value-added oleochemicals
widely used as lubricants, cosmetics, and food additives among other applications. However, homogeneous
acid catalysts are still preferred in their industrial production, despite their associated energy cost and the envi-
ronmental issues that they present. In this paper, we describe lignocellulose-derived amorphous carbons with a
high loading level of SO3H and their application to the synthesis of polyolesters as a biobased heterogeneous acid
catalyst. These sulfonated amorphous carbons could be readily prepared via (i) heat treatment at 400°C for 1h
and (ii) sulfonation with chlorosulfuric acid. XRD and BET analyses demonstrated that these carbonaceous mate-
rials were not crystalline but were amorphous structures with a low surface area. The attachment of SO3H groups
was confirmed by FT-IR and XPS, and the loading level of SO3H was determined by CHNS elemental analysis.
Chlorosulfuric acid gave a higher loading level of SO3H than other sulfating agents, such as conc. sulfuric acid
and fuming sulfuric acid. The higher SO3H-loaded amorphous carbons exhibited a greater catalytic activity for es-
terification between trimethylolpropane and fatty acids under solventless conditions. Esterification of oleic acid
derived from vegetable oil with trimethylolpropane using this catalyst afforded the desired biolubricant at over
93% yield in 3h. The sulfonated amorphous carbons could be reused three times without any significant loss of
catalytic activity.
Received 7 December 2016
Received in revised form 15 March 2017
Accepted 16 March 2017
Available online 18 March 2017
Keywords:
Solventless esterification
Fatty acids
Polyols
Sulfonated amorphous carbon
Biobased
Heterogeneous acid catalysts
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
bulky substrates (fatty acids and polyols) [4]. Esterification of fatty
acids using heteropolyacid (HPA) supported on clay K-10 [5] or meso-
Polyolesters (POEs) are oleochemicals widely used as lubricants,
cosmetics, and food additives among other applications, and they are
made by esterification of fatty acids with polyols [1]. Esterification is
performed in the presence of Brønsted acids, such as sulfuric acid, phos-
phoric acid, or p-toluenesulfonic acid. Although these homogeneous
acid catalysts are preferred in industrial production due to their low
cost and strong acidity, (i) they are difficult to separate from reaction
mixtures and reuse; (ii) a large amount of acidic waste water is
discharged, which causes environmental problems; and (iii) they
often cause serious corrosion in process units [2]. To overcome the
drawbacks of homogeneous acid catalysts, a variety of heterogeneous
acid catalysts were explored. Cation exchange resins with high loading
of sufficient sulfonic acid functionalities exhibit good performance in
certain cases, but they are expensive and have low thermal stability
[3]. Zeolitic solid acids including H-beta and H-ZSM-5 were attempted
but the small pore sizes were unsuitable for an effective access of
porous silica [6], an Fe-Zn double-metal cyanide complex [7], and
ZrOCl2·8H2O [8] as catalysts have been reported, but the studies were
confined to simple alcohol compounds used primarily to produce the
fatty acid methyl esters (FAME) of biodiesel. Additionally, immobilized
lipase has been applied to the esterification of fatty acids with long
chain alcohols, but there are still economic issues [9]. Currently, amor-
phous carbons (AC) containing SO3H groups attract considerable atten-
tion as biobased heterogeneous acid catalysts for esterification due to
their physical/chemical stability and high catalytic activity. These car-
bon materials are typically made from natural carbon sources, such as
glucose [10] or cellulose [11], via incomplete carbonization followed
by sulfonation. Incomplete carbonization results in AC composed of
small polycyclic aromatic carbon sheets in a three-dimensional sp3-
bonded structure, and these AC can afford highly acidic catalysts by sul-
fonation because there are numerous reactive sites where SO3H groups
may attach covalently, compared with crystalized carbon (CC) [12].
In this study, amorphous carbons containing SO3H (AC-SO3H) were
prepared from lignocellulose, the most abundant biomass, and they
were applied to the synthesis of polyolesters as biobased heterogeneous
acid catalysts. AC-SO3H could be readily obtained by the thermal
⁎
Corresponding author at: Green Material and Process R&D Group, Korea Institute of
Industrial Technology (KITECH), Cheonan, Chungnam 31056, Republic of Korea.
E-mail address: jkcho@kitech.re.kr (J.K. Cho).
http://dx.doi.org/10.1016/j.catcom.2017.03.015
1566-7367/© 2017 Elsevier B.V. All rights reserved.

D. Mun et al. / Catalysis Communications 96 (2017) 32–36
33
treatment of wood powder and then sulfonation with sulfonating
agents such as conc. sulfuric acid (H2SO4), fuming sulfuric acid (SO3-
H2SO4), and chlorosulfonic acid (ClSO3H). AC-SO3H was characterized
by XRD, FT-IR, XPS, BET, and elemental analysis. Esterification between
trimethylolpropane (TMP), a polyol containing three symmetric alcohol
moieties, and fatty acids, including 2-ethylhexanoic acid (2-EHA),
isononanoic acid (INA), and oleic acid, was performed without solvent
in the presence of as-synthesized AC-SO3H. The effects of the loading
level of SO3H, the temperature, and the structure of the fatty acids on
the synthesis of polyolesters were investigated. The reusability of AC-
SO3H was also tested.
gently stirred at 80°C in an oil bath for 3h and additionally at room tem-
perature (25°C) for 3h. The resulting sulfonated amorphous carbona-
ceous materials were washed with hot distilled water (70°C) until pH
paper was no longer changed to an acid-indicating color and then
washed with 1,4-dioxane. Afterwards, to remove the loosely bonded
materials, sulfonated amorphous carbonaceous materials were washed
by Soxhlet extraction with 1,4-dioxane for 24h and dried in vacuum
oven for one day. The AC-SO3H species obtained by treatment with
conc. sulfuric acid, fuming sulfuric acid, and chlorosulfonic acid were de-
noted as AC-co-SO3H, AC-f-SO3H, AC-ch-SO3H, respectively.
2.3.2. Esterification between fatty acids and TMP
2. Experimental procedures
Into a 250mL two-neck round-bottom flask were placed TMP
(3.7mmol, 1equiv) and an excess of fatty acid (12.3mmol, 3.3equiv),
and AC-SO3H (0.123mmol, 1mol% of fatty acid) was added as a catalyst.
The reaction mixture was heated to the desired temperature and stirred
at 300rpm under N2 flow for 24h. Volatile substances formed during the
reaction were removed by N2. At intervals of 1h, a small amount of sam-
ple was taken out from the reaction mixture using a syringe and then it
was diluted with dichloromethane (DCM) and submitted for GC–MS
(GCMS-QP2010 Ultra, Shimadzu, Japan) with an Rxi-5ms column
(50m in length). Helium was used as a carrier gas at 1mLmin−1of flow
rate. Injector temperature was 150°C. Column temperature was pro-
grammed from 150°C to 330°C at a rate of 5°Cmin−1 and held isother-
mal for 14min. The split ratio, ion source temperature, mass scan
range were 1:20, 300°C, 35–1000massunits, respectively.
2.1. Materials
A wood powder (pine tree, 50–100μm in diameter) used as a raw
material of the lignocellulose was supplied by G·biotech (Korea). The
fatty acids 2-ethylhexanoic acid (2-EHA), isononanoic acid, and oleic
acid, as well as trimethylolpropane (TMP), a polyol, were provided by
Oh Sung Chemical Industry Co., Ltd. (Korea). All other chemicals, includ-
ing conc. sulfuric acid (H2SO4, 95.0–98.0%), fuming sulfuric acid (SO3-
H2SO4, 28.0–32.0% free SO3), chlorosulfonic acid (ClSO3H, N98%), p-
toluenesulfonic acid monohydrate (PTSA, 99%), and 1,4-dioxane
(99.8%), were purchased from Sigma-Aldrich (USA) and directly used
without further purification.
The yield of polyolester was calculated by the following equation.
2.2. Analysis
Yield of polyolesterð%Þ ¼ Conversion of TMP
ꢀ Selectivity of polyolesters ðT−nFÞ
AC-SO3H was analyzed by XRD (Powder X-ray diffraction, D8 AD-
VANCE with DAVINCI, BRUKER, Germany), FT-IR (Fourier transform in-
frared spectroscopy, Nicolet 6700, Termo Scientific system, USA) and
XPS (X-ray photoelectron spectroscopy K-Alpha™+, Thermo Scientific,
USA). The loading amounts of sulfur were determined by elemental
analysis (Automatic Elemental Analyzer, FLASH 2000 Series, Thermo
Scientific, USA), and the surface areas were measured by BET analysis
(Brunauer-Emmett-Teller, ASAP2010, Micromeritics, USA), respective-
ly. XRD was conducted at 2θ from 2.5° to 30°, at a scanning step size
of 0.02° and a scan speed per step of 0.5s using Cu Kα radiation. FT-IR
samples were prepared in a pellet form by mixing the catalyst sample
with KBr, and spectra were recorded in ATR mode; 126 spectra were ac-
cumulated and averaged to improve the signal-to-noise ratio. XPS mea-
surements were performed on a Thermo Scientific K-alpha instrument
using monochromatized Al Kα radiation (hν=1486.6eV) and processed
using Thermo Avantage software. The calculated spectra represented
the transmittance. The specific surface area was determined on a BET
surface analyzer using N2 as the adsorbent at liquid nitrogen tempera-
ture (77K) in the relative pressure (P/P0) range of 0–0.25. The powder
samples were degassed in air over 12h at 100°C prior to analysis.
where T is an initial letter of trimethylolpropane; F is an initial letter of
fatty acid (E, I, O in the cases of 2-EHA, isononanoic acid, oleic acid, re-
spectively); and n is the number of fatty acids coupled with a TMP.
3. Results and discussion
3.1. Characterization of AC-SO3H
According to the previous report by Okamura et al., heat treatment
above 450°C afforded large carbon sheets in a well-crystallized form,
which indicated a lack of reactive sites for the attachment of SO3H
groups on the carbon sheets [13]. Therefore, the carbonization process
was conducted at a moderate temperature (under 450°C) in this
study. Wood powders were changed to black carbonaceous materials
after heat treatment under N2 at 400°C for 1h. In the XRD patterns of
the carbonaceous materials, two broad peaks were observed at 10–30°
and at 35–50°, corresponding to randomly oriented aromatic carbon
sheets [14] (Fig. S1a). The results indicated that the carbonaceous mate-
rials formed an amorphous structure due to incomplete carbonization.
The BET surface area, under 100m2g−1 of the carbonaceous materials,
confirmed that they are not crystalline materials. Next, SO3H groups
were attached to the aromatic rings of the amorphous carbon (AC) to
impose acidic character. Three sulfonating agents, specifically sulfuric
acid, fuming sulfuric acid, and chlorosulfonic acid, were examined for
efficient sulfonation. Sulfonation of AC was analyzed by FT-IR and XPS.
FT-IR spectra of the sulfonated amorphous carbon (AC-SO3H) contained
bands at 1377 and 1040cm−1, which were assigned as O_S_O and
SO−3 stretching bands, respectively (Fig. S1b) [14]. A peak appearing at
168.8eV in the XPS spectrum corresponded to the S 2p binding energy
of the SO3H groups [13] (Fig. S1c). The XRD spectra of the AC-SO3H
were similar to that of AC. It was understood that no structural change
occurred during sulfonation. The BET surface areas were also sustained
after sulfonation, except for AC-ch-SO3H sulfonated with chlorosulfuric
acid (286m2g−1). It could be explained that further carbonization oc-
curred during treatment with chlorosulfuric acid, and the crystallinity
2.3. Procedure
2.3.1. Preparation of AC-SO3H
Amorphous carbonaceous materials were prepared directly from
wood powder. A wood powder was carbonized by heating at 400°C
under N2 for 1h. Fifty grams of wood powder (WP) were placed into a
rectangular shaped ceramic alumina crucible (10×10×5cm3), and the
WP-containing crucible was placed in a chamber-type electric furnace.
The furnace was heated to 400°C over 80min under flowing N2, and
the temperature was maintained for 1h to afford approximately 15g of
amorphous carbonaceous materials (weight yield: ~30%). For the at-
tachment of SO3H groups onto the aromatic rings of the amorphous car-
bonaceous materials, sulfonation proceeded as follows: In the 500mL
round bottom flask were placed 10g of amorphous carbonaceous mate-
rials, and 100mL of a sulfonating agent such as conc. sulfuric acid, fuming
sulfuric acid, or chlorosulfuric acid were added. The mixtures were

34
D. Mun et al. / Catalysis Communications 96 (2017) 32–36
Fig. 1. Comparison of AC-co-SO3H and AC-ch-SO3H in the esterification between polyol and fatty acid.
increased. However, this surface area was still considerably lower than
that of activated carbon. The loading level of \\SO3H groups on AC-
SO3H after sulfonation was determined by elemental analysis and the
amount of sulfur per AC-SO3H was calculated to mmolg−1. The loading
levels of \\SO3H on AC-co-SO3H, AC-f-SO3H, and AC-ch-SO3H were
0.67mmolg−1, 1.29mmolg−1, and 1.57mmolg−1, respectively. The
total Brønsted acidity of AC-ch-SO3H was measured by an acid-base titra-
was carried out without a solvent at 180°C for 24h. Interestingly, AC-
ch-SO3H with a high loading level of SO3H showed a much faster reac-
tion rate, even though the total amount of SO3H is the same as that in
AC-co-SO3H. The half-life (t1/2) of TMP in the reaction using AC-co-
SO3H was 31.8min, while the t1/2 of TMP was only 11.4min in the case
of AC-ch-SO3H (Fig. 2). The yields of esterification products were also
enhanced when AC-ch-SO3H was used. A tri-ester (T-3E) was obtained
as the major product in 58.1% yield, and N95% of the mono-ester (T-
1E) was consumed after 7h (Fig. S2b). However, the reaction in the
presence of AC-co-SO3H gave di-ester (T-2E) as the major product in
52.9% yield and a significant amount of the mono-ester (T-1E), over
10%, remained even after 24h (Fig. S2a). All results were summarized
in Table 1. Moreover, a trace amount of unknown products (about 1–
2%) was found in GC–MS when using AC-ch-SO3H. It is due to the high
activity of AC-ch-SO3H and the reason about formation of unknown
products will be discussed in Section 0.
tion using a 10mM NaOH aqueous solution to give 2.02mmolofH+g−1
.
This finding indicated that approximately 30% of the total Brønsted
acidity would be from other acidic moieties, such as\\CO2H, rather
than\\SO3H.
3.2. Effect of the SO3H loading level on the esterification between TMP and
2-EHA using AC-SO3H
In the previous section, we found that the sulfonation of AC relied on
sulfonating agents, and chlorosulfuric acid resulted in a higher loading
level of SO3H than any of the other sulfonating agents. The higher load-
ing level of SO3H means that the acid sites exist more densely on the
support. To examine the effect of the SO3H loading level on the esterifi-
cation between a polyol and a fatty acid, two catalysts (AC-co-SO3H and
AC-ch-SO3H) with different loading level of SO3H (0.67mmolg−1 vs
1.57mmolg−1) were compared under identical conditions (Fig. 1). The
polyol and fatty acid sources used were 3.7mmol of TMP (500mg) and
2-EHA (12.3mmol, 3.3equiv of TMP), respectively. TMP not containing
a β-H was employed owing to its thermal and hydrolytic stability [15].
The amount of catalyst was determined as 1mol% of the fatty acid
(0.123mmol of SO3H). In other words, 184mg of AC-co-SO3H and
78mg of AC-ch-SO3H were added to the reaction. The esterification
3.3. Effect of the temperature on the esterification between TMP and 2-EHA
using AC-co-SO3H
The effect of the reaction temperature on the esterification of TMP
and 2-EHA in the presence of AC-co-SO3H was investigated. Prior to
test, the activity of AC-co-SO3H was compared with that of PTSA as a ref-
erence. AC-co-SO3H showed a little bit higher activity than PTSA in both
reaction rate (t1/2=30.6min for AC-co-SO3H vs 36.6min for PTSA) and
yield of T-3E (36.5% for AC-co-SO3H and 35.4% for PTSA) (see Figs. S3
and S4). From the results, we can understand that AC-co-SO3H has a
comparative activity to homogeneous acid catalysts. As expected, the
esterification progressed more rapidly as the reaction temperature in-
creased (180°C–220°C), and the t1/2 of TMP was shortened from
31.8min to 13.2min (Fig. S5). The yields of the esterification products
were increased at 220°C. T-3E was obtained in 51.4% yield and T-1E al-
most disappeared after 24h (Fig. S6). However, a couple of unknown
compounds began to form after 3h at 220°C (Table S2). From GC–MS,
the molecular weights of unknown compounds were same as T-2E
and T-3E, respectively. It indicates that unknown compounds are diaste-
reomers of T-2E and T-3E because 2-EHA has a chiral center. Probably,
these diastereomers seem to be more steric hindered than major prod-
ucts, T-2E and T-3E because the steric hindered diastereomers of T-2E
Table 1
Characteristic data and the esterification results of AC-co-SO3H and AC-ch-SO3H.
Catalyst
Surface
area
Loading
Amount t1/2 of TMP Product yields
level of SO3H used
(min)
(%)
(m2g−1
)
(mmolg−1
)
(mg)
T-1E T-2E T-3E
10.4 52.9 36.5
AC-co-SO3Ha 63
AC-ch-SO3Hb 286
0.67
1.57
184
78
31.8
11.4
0.7
39.3 58.1
a
Reaction conditions: TMP (3.7mmol), 2-EHA (12.2mmol, 3.3equiv of TMP), AC-co-SO3H
(0.12mmol, 1mol% of 2-EHA), N2 flow, 180°C, 24h.
Reaction conditions: TMP (3.7mmol), 2-EHA (12.2mmol, 3.3equiv of TMP), AC-ch-SO3H
(0.12mmol, 1mol% of 2-EHA), N2 flow, 180°C, 24h.
Fig. 2. Comparison of the TMP (reactant) profiles according to the time of the esterification
between TMP (3.7mmol) and 2-EHA (3.3equiv of TMP) using AC-co-SO3H and AC-ch-
SO3H (1mol% of 2-EHA each) at 180°C.
b

D. Mun et al. / Catalysis Communications 96 (2017) 32–36
35
Fig. 3. Reaction scheme for the esterification of TMP with fatty acids and the structures of the fatty acids.
and T-3E requiring higher activation energy can be produced only at
high temperature.
though its chain length is the longest. The tri-ester of oleic acid (T-3O)
can be obtained in a 93% yield after 3h (Fig. 4). Following reaction, the
catalyst (AC-ch-SO3H) was filtered out and then the filtrate was passed
through Celite layer to afford the products.
3.4. Effect of the chemical structure of the fatty acids on the esterification
with TMP using AC-ch-SO3H
The reason why the reactivity of oleic acid is much higher than the
other fatty acids (2-EHA and INA) can be understood due to the less ste-
ric hindered alkyl chain of oleic acid. Although the alkyl chain length of
oleic acid is nearly twice longer than that of 2-EHA and INA, the bulki-
ness of oleic acid is relatively low because the alkyl chain of oleic acid
is linear without any branch and cis-form double bond in the middle
of chain prevents free rotation of the chain. At a prolonged reaction
time, it was found that a small portion of T-3O (about 3%) was hydro-
lyzed and converted back to T-2O (see Table S3 and Fig. S7a for details).
2-EHA and INA showed a lower reactivity than oleic acid due to the
bulkiness caused by the branching and the flexibility of their alkyl
chains. The reactivity of 2-EHA was lower than that of INA to give T-
3E in only 40.6% yield after 24h. Meanwhile, T-3I was obtained in
59.3% yield. It is believed that branch of 2-EHA has negative effect on
the esterification compared with that of INA because ethyl group is big-
ger sized branch than methyl branch. In addition, ethyl group of 2-EHA
is much closer to carboxylic acid that is the reactive site of esterification
than methyl of INA (Fig. 4).
Polyolesters coupled by multiple numbers of fatty acids are highly
bulky compounds. Therefore, the chemical structures of fatty acids can
affect their reactivity during esterification. In this study, the reactivity
of three fatty acids, including 2-EHA, INA, and oleic acid, in an esterifica-
tion with TMP was evaluated in the presence of AC-ch-SO3H (Fig. 3). All
the reactions were carried out under solventless conditions. 2-EHA and
INA are synthetic fatty acids. 2-EHA has a C8 alkyl chain and the ethyl
branch is adjacent to the carboxylic acid moiety. INA has a C9 alkyl
chain and the isopropyl group is located at the end of the alkyl chain.
Meanwhile, oleic acid is a fatty acid derived from vegetable oil. Oleic
acid has a C18 linear alkyl chain and one double bond that is located
at the middle of the chain. Therefore, the alkyl chain of oleic acid pro-
duces a lesser steric factor than those of 2-EHA and INA. Particularly,
polyolesters made with oleic acid are known as biolubricants and they
have several significant advantages: non-toxicity, biodegradability, car-
bon-neutrality, high temperature stability, and no need for various addi-
tives such as anti-oxidants, pour point depressants, emulsion stabilizers,
and detergents [16–21]. Oleic acid showed the highest reactivity even
Fig. 4. Yields of the products from the esterification of TMP with 2-EHA, INA, and oleic acid.
Reaction conditions: TMP (3.7mmol), fatty acid (3.3equiv of TMP), AC-ch-SO3H (1mol% of
fatty acid), N2 flow, 24h for T-nE and T-nI and 3h for T-nO.
Fig. 5. Reusability test of AC-ch-SO3H in the esterification of TMP with 2-EHA. Reaction
conditions: TMP (3.7mmol), 2-EHA (3.3equiv of TMP), AC-ch-SO3H (1mol% of 2-EHA),
N2 flow, 24h.

36
D. Mun et al. / Catalysis Communications 96 (2017) 32–36
3.5. Reusability test of AC-ch-SO3H in the esterification between TMP and 2-
EHA
and Technology) (CAP-11-04-KIST) of the Republic of Korea and KITECH
(Korea Institute of Industrial Technology) and from the Internal Re-
search Program (PEO17250) of KITECH.
The reusability of AC-ch-SO3H was evaluated as a biobased heteroge-
neous acid catalyst in the esterification between a polyol and a fatty
acid. TMP and 2-EHA were used as substrates, and the reaction was per-
formed in the presence of 1mol% of the catalyst without solvent at 180°C
for 8h. After each run, AC-ch-SO3H was filtered and washed with 1,4-
dixoane. Any further regeneration of the catalyst was not conducted.
AC-ch-SO3H can be reused 3 times without any significant loss of activ-
ity (Fig. 5). This result was confirmed by FT-IR analysis of the fresh and
the reused AC-ch-SO3H. The peaks corresponding to\\SO3 and O_S_O
were retained after the reaction (Fig. S5).
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2017.03.015.
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We would like to acknowledge the financial support from the R&D
Convergence Program of NST (National Research Council of Science