C-Arylcalix[4]pyrogallolarene sulfonic acid: A novel and efficient organocatalyst material for biodiesel production

Full text of DOI:10.1246/BCSJ.20190275

Advance Publication Cover Page
The C-Arylcalix[4]pyrogallolarene Sulfonic Acid: A Novel and Efficient
Organocatalyst Material for Biodiesel Production
Jumina,* Hamid Rohma Setiawan, Sugeng Triono, Yehezkiel Steven Kurniawan,
Yoga Priastomo, Dwi Siswanta, Abdul Karim Zulkarnain, and Naresh Kumar
Advance Publication on the web December 6, 2019
doi:10.1246/bcsj.20190275
© 2019 The Chemical Society of Japan
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The C-Arylcalix[4]pyrogallolarene Sulfonic Acid: A Novel and Efficient
Organocatalyst Material for Biodiesel Production
Jumina1,*, Hamid Rohma Setiawan1, Sugeng Triono1, Yehezkiel Steven Kurniawan1,2, Yoga Priastomo1, Dwi
Siswanta1, Abdul Karim Zulkarnain3 and Naresh Kumar4
1 Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Gadjah Mada, Yogyakarta 55281, Indonesia
2 Ma Chung Research Center for Photosynthetic Pigments, Universitas Ma Chung, Villa Puncak Tidar N-01, Malang 65151, Indonesia
3 Pharmaceutical Laboratory, Faculty of Pharmacy, Universitas Gadjah Mada, Yogyakarta 55281, Indonesia
4 School of Chemistry, The University of New South Wales, Sydney NSW 2033, Australia,
E-mail: jumina@ugm.ac.id
Jumina
Jumina received Ph.D. degree in Organic Chemistry from University of New South Wales, Sydney-Australia in
1997. He has been working as a lecturer in the Department of Chemistry, Faculty of Mathematics and Natural
Science, Universitas Gadjah Mada, Yogyakarta-Indonesia since 1989 and was appointed as a full professor at the
same department since 2008. His research field is organic synthesis with particular interest in drug development
and biofuel.
Abstract
The synthesis of C-arylcalix[4]pyrogallolarene sulfonic
biodegradable, non-toxic, and environmentally friendly.2
Biodiesel is usually produced by the esterification reaction of
fatty acids with alcohol.3,4
acid derivatives and their application as the organocatalyst
material for biodiesel production were investigated. The C-
arylcalix[4]pyrogallolarene derivatives were prepared in high
yields through a two-step reaction: condensation between
pyrogallol and aromatic aldehydes (i.e. benzaldehyde, 4-
hydroxy-3-methoxybenzaldehyde and 4-ethoxy-3-methoxy
benzaldehyde) and then followed by a sulfonation reaction with
sulfuric acid. They were evaluated as the organocatalyst for the
esterification reaction of palmitic acid with methanol as the
representation of biodiesel production. Using 4 mol% of C-4-
hydroxy-3-methoxyphenylcalix[4]pyrogallolarene sulfonic acid,
methyl palmitate was generated in up to 91.9% yield after 4 h at
65°C, making it similarly efficient to sulfuric acid that is used as
the catalyst in the conventional reaction under similar
conditions. However, different from sulfuric acid, the C-4-
hydroxy-3-methoxyphenylcalix[4]pyrogallolarene sulfonic acid
could be recovered by using a simple filtration technique making
it is better than sulfuric acid from the industrial and
environmental point of view. These results demonstrating that
the C-arylcalix[4]pyrogallolarene sulfonic acid derivatives are a
novel and potential organocatalyst for methyl palmitate biodiesel
production.
Many researches have been carried out to develop and
optimize biodiesel production. Arbain and Salimon (2011) used
sulfuric acid and p-toluenesulfonic acid as homogenous catalysts
for fatty acid esterification, however, these acids are corrosive
and damaging to the environment.5 Other catalysts, such as
enzymes, inorganic polymers and mesoporous materials have
been investigated, however, their catalytic performance is still
unsatisfied for industrial use.6-9 For example, Sherkhanov et al.
(2016) employed the juvenile hormone acid O-methyltransferase
from the fruitfly Drosophila melanogaster to produce fatty acid
methyl esters, however, the amount of the produced biodiesel
was too low.6 Pappalardo et al. (2017) reported the esterification
of palmitic acid with lipase enzyme, however, only 20.7% yield
was obtained even after 24 h of reaction at 60°C.7 Another
drawback of enzymes is their high cost as well as low stability
to pH and temperature changes. Other catalytic materials have
been also been evaluated for biodiesel production. Grigoreva et
al. (2014) employed H-beta zeolite as a heterogeneous catalyst
for the esterification reaction of palmitic acid with methanol,
however, the reaction yield was only 54% due to the unsuitable
microporous size of the catalyst material for catalytic process.8
Dang and Chen (2013) utilized AmberliteTM IR-120 (H) for the
esterification reaction of palmitic acid with methanol.10 Even
though 98.3% yield of methyl palmitate was attained, the
reaction required heating at 61°C at for 10.5 h which is time-
consuming. Therefore, the development and optimization of new
catalyst materials for biodiesel production are highly required.
Calix[4]arene derivatives have attracted interest for over a
century due to their unique characteristics, particularly in the
field of host-guest chemistry. The upper and lower rims of
calix[4]arene derivatives can be modified to achieve specific
purposes and applications.11 Calix[4]arene derivatives have been
exploited for molecular discrimination, as well as their catalytic
activity for a wide variety of reactions such as indole and
xanthone synthesis, nucleophilic substitution reactions of
Keywords: Calix[4]pyrogallolarene sulfonic acid,
organocatalyst, biodiesel.
1. Introduction
Indonesia annually produced 33 million metric tons of
palm oil in 2015, which accounted for 54% of global demand. In
general, palm oil contains 44% of palmitic acid, 39% of oleic
acid, 8% of linoleic acid and 6% of stearic acid.1 Because of that,
palm oil becomes as one of the most important feedstocks for
biodiesel production, and it has been widely applied in the food,
pharmaceutical and energy industries. Biodiesel is a form of
renewable energy that attracts global attention because it is

alcohol, and heterocyclic coupling reactions.12-20 Fernandes et al.
(2012) reported that calix[4]arene sulfonic acid can be used to
produce ethyl esters of various carboxylic acids in medium to
high yield after 4 h.21,22 According to Almeida et al. (2015),
calix[4]arene sulfonic acids can be used as the organocatalysts
due to the sulfonic acid group behaving as a Bronsted acid.23
Calix[4]pyrogallolarenes, as one of the calix[4]arene subfamilies,
have been reported for their application as metal ion adsorbents,
gas adsorbents, chemosensors, as well as for their drug
encapsulation and ion transport properties.24-29 Therefore, the
sulfonation of calix[4]pyrogallolarene was thought to potentially
provide a novel organocatalyst material for the esterification of
fatty acids for biodiesel production.
In this work, a series of calix[4]pyrogallolarene sulfonic
acids were synthesized from pyrogallol and benzaldehyde
derivatives, i.e. unsubstituted benzaldehyde, 4-hydroxy-3-
methoxybenzaldehyde and 4-ethoxy-3-methoxy benzaldehyde.
The reaction scheme is shown in Figure 1. The chemical
structures of the synthesized products were characterized by
FTIR, MS, 1H-NMR and 13C-NMR. The calix[4]pyrogallolarene
sulfonic acid derivatives were investigated as organocatalysts for
the esterification of palmitic acid, and the effect of the
substituent on the benzaldehyde moieties on the catalytic activity
was explored. The reaction time, catalyst loading, and
temperature were also optimized to identify the best conditions
for methyl palmitate production.
NMR (δ/ppm): 5.62 (s, 4H, C-H methine), 6.19 (s, 4H, aromatic
proton of pyrogallol), 6.66 and 6.86 (d and m, 20H, aromatic
protons of benzaldehyde), 7.34 and 7.61 (s and s, 12H, OH). 13C-
NMR (δ/ppm): 35.20 (C-H methine), 119.04, 121.68 and 133.49
(aromatic carbons of benzaldehyde), 125.52, 128.22, 142.04 and
142.66 (aromatic carbons of pyrogallol). MS: found as 855.57
(M+) for C52H40O12.
Synthesis of C-4-hydroxy-3-methoxyphenylcalix[4]
pyrogallolarene (Calix-B). Vanillin (2.3 g, 15 mmol) and
pyrogallol (1.9 g, 15 mmol, 1 equivalent) were dissolved in
ethanol (30 mL) and the mixture was stirred at room temperature.
Concentrated hydrochloric acid (1.5 mL) was added dropwise
into the mixture and the mixture was refluxed for 24 h. After the
reaction, the mixture was cooled down to room temperature and
stored for 24 h. The precipitate formed was filtered and washed
with ethanol:distilled water 1:1 to obtain compound Calix-B as
a pink solid (3.6 g, 93.3%). Melting point: >280°C. FTIR (ν/cm-
1): 3395 (O-H), 2924 (C-H sp3), 1605 and 1512 (C=C aromatic),
and 1273 (C-O ether). 1H-NMR (δ/ppm): 3.17 (s, 12 H, OCH3),
5.54 (s, 4H, C-H methine), 6.04 (s, 4H, aromatic proton of
pyrogallol), 6.08-6.30 (m, 12H, aromatic protons of
benzaldehyde), 7.76 and 7.95 (s and s, 12H, OH). 13C-NMR
(δ/ppm): 39.85 (C-H methine), 55.23 (OCH3), 113.22, 114.01,
121.13, 121.57, 122.65 and 131.56 (aromatic carbons of
benzaldehyde), 134.80, 141.30, 143.56 and 146.37 (aromatic
carbons of pyrogallol). MS: found as 1054.09 [M+14H]+ for
C56H62O20.
2. Experimental
Synthesis of C-4-ethoxy-3-methoxyphenylcalix[4]
pyrogallolarene (Calix-C). 4-Ethoxy-3-methoxybenzaldehyde
(2.7 g, 15 mmol) and pyrogallol (1.9 g, 15 mmol, 1 equivalent)
were dissolved in ethanol (30 mL) and the mixture was stirred at
room temperature. Concentrated hydrochloric acid (1.5 mL) was
added dropwise into the mixture and the mixture was refluxed
for 24 h. After the reaction, the mixture was cooled down to
room temperature and stored for 24 h. The formed precipitation
was filtered and washed with ethanol:distilled water 1:1 to obtain
compound C as a pink solid (4.3 g, 98.8%). Melting point:
>280°C. FTIR (ν/cm-1): 3456 (O-H), 2924 (C-H sp3), 1628 and
1474 (C=C aromatic), and 1288 (C-O ether). 1H-NMR (δ/ppm):
1.27 (t, 12H, OCH2CH3), 3.16 (s, 12 H, OCH3), 3.88 (q, 8H,
OCH2CH3), 5.74 (s, 4H, C-H methine), 6.02 (s, 4H, aromatic
proton of pyrogallol), 6.19 and 6.28 and 6.41 (d and s and d, 12H,
aromatic protons of benzaldehyde), 7.62 and 7.77 (s and s, 12H,
OH). 13C-NMR (δ/ppm): 15.05 (OCH2CH3), 39.85 (C-H
methine), 55.02 (OCH3), 63.52 (OCH2CH3), 111.64, 112.98,
131.65, 145.52 and 148.01 (aromatic carbons of benzaldehyde),
120.45, 121.62, 138.23 and 142.09 (aromatic carbons of
pyrogallol).
Synthesis of C-phenylcalix[4]pyrogallolarene sulfonic
acid (Calix-A-SA). Compound Calix-A (1.7 g, 2.0 mmol) was
directly reacted with concentrated sulfuric acid (4.9 g, 50 mmol,
25 equivalent) at 80°C for 4 h. After the reaction, the mixture
was cooled down to room temperature and the desired product
was triturated from the mixture by using ethyl acetate:methanol
3:2 (50 mL). The precipitate obtained was dried to obtain Calix-
A-SA as a black solid (1.9 g, 82.5%). Melting point: >280°C.
FTIR (ν/cm-1): 3372 (O-H), 2924 (C-H sp3), 1597 and 1466
(C=C aromatic), 1157 (C-O ether), 1034 (S=O), 840 (S-O), and
579 (C-S). 1H-NMR (δ/ppm): 4.11 (s, 4H, C-H methine), 6.73 (s,
4H, aromatic proton of pyrogallol), 7.26 and 7.69 (d and d, 16H,
aromatic protons of benzaldehyde), 7.44 (q, 12H, C-OH), 8.30
(s, 4H, SO3H).
Materials: Used materials in this work were purchased
from Merck at analytical grade and used without any further
purification.
Instrumentation: FTIR spectra were recorded using a
Shimadzu Prestige 21. The purity and mass spectra of the
products were determined from either GC-MS using a
Shimadzu-QP 2010S with Agilent GC type 6890-MS type 5973
or LC-MS using Acquity HPLC-SQD. NMR spectra were
recorded on H- (500 MHz) and 13C- (125 MHz) NMR JEOL
1
JNM ECZ500R/S1. Morphological images and elemental
analysis were recorded using Scanning Electron Miscrocpoe
(SEM) with EDX (Phenom ProX). Particle size analysis was
taken by Horiba Laser Scattering Particle Size Distribution
Analyzer LA-960.
Synthesis of 4-ethoxy-3-methoxybenzaldehyde. Vanillin
(3.8 g, 25 mmol) and sodium hydroxide (3.7 g, 93 mmol, 3.7
equivalents) were dissolved in distilled water (50 mL) and
heated to 60°C. Diethyl sulfate (11.6 g, 75 mmol, 3 equivalents)
was added dropwise and the mixture was refluxed for 2 h. After
the reaction, the mixture was cooled to -15°C and left to stand
for 24 h at -15°C. The precipitate formed was filtered to obtain
4-ethoxy-3-methoxybenzaldehyde as a yellow solid (3.3 g, 73%
yield). Melting point: 58-60°C. FTIR (ν/cm-1): 3078 (C-H sp2),
2986 (C-H sp3), 2855 and 2770 (C-H aldehyde), 1682 (C=O),
1589 and 1512 (C=C aromatic), and 1265 (C-O ether). GC:
single peak at retention time (tR) = 27.09 min. MS: 180 (M+),
151 (base peak), 137, 123, 109, 95, 81, 65, 51, 29.
Synthesis of C-phenylcalix[4]pyrogallolarene (Calix-A).
Benzaldehyde (1.6 g, 15 mmol) and pyrogallol (1.9 g, 15 mmol,
1 equivalent) were dissolved in ethanol (30 mL) and the mixture
was stirred at room temperature. Concentrated hydrochloric acid
(1.5 mL) was added dropwise into the mixture and the mixture
was refluxed for 24 h. After the reaction, the mixture was cooled
down to room temperature and stored for 24 h. The precipitate
formed was filtered and washed with ethanol:distilled water 1:1
to obtain compound Calix-A as a pink solid (3.2 g, 99.7%).
Melting point: >280°C. FTIR (ν/cm-1): 3456 (O-H), 2924 (C-H
sp3), 1612 and 1504 (C=C aromatic), and 1285 (C-O ether). 1H-
Synthesis of C-4-hydroxy-3-methoxyphenylcalix[4]
pyrogallolarene sulfonic acid (Calix-B-SA). Compound Calix-
B (2.1 g, 2.0 mmol) was directly reacted with concentrated
sulfuric acid (4.9 g, 50 mmol, 25 equivalent) at 80°C for 4 h.

A
B
X=Y=SO3H
Figure 1. (A) Synthesis of 4-ethoxy-3-methoxybenzaldehyde and (B) Synthesis of C-arylcalix[4]pyrogallolarene sulfonic acid (Calix-
A-SA, Calix-B-SA, Calix-C-SA) derivatives.
After the reaction, the mixture was cooled down to room
temperature and the desired product was triturated from the
mixture by using ethyl acetate:methanol 3:2 (50 mL). The
precipitate obtained was dried to obtain B-SA compound as a
black solid (2.4 g, 88.9%). Melting point: >280°C. FTIR (ν/cm-
1): 3387 (O-H), 2924 (C-H sp3), 1620 and 1466 (C=C aromatic),
temperature and the desired product was triturated from the
mixture by using ethyl acetate:methanol 3:2 (50 mL). The
obtained precipitation was dried to obtain C-SA compound as a
black solid (2.5 g, 85.3%). Melting point: >280°C. FTIR (ν/cm-
1): 3387 (O-H), 2924 (C-H sp3), 1612 and 1466 (C=C aromatic),
1
1150 (C-O ether), 1034 (S=O), 856 (S-O), and 584 (C-S). H-
1
1173 (C-O ether), 1051 (S=O), 880 (S-O), and 579 (C-S). H-
NMR (δ/ppm): 2.06 (t, 12H, OCH2CH3), 3.74 (s, 12 H, OCH3),
3.77-4.02 (broad singlet, 24H, OCH2CH3, C-H methane and C-
OH), 7.07 (s, 4H, aromatic proton of pyrogallol), 6.99-7.39 (m,
8H, aromatic proton of benzaldehyde), 8.10 and 10.2 (s and s,
4H, SO3H). 13C-NMR (δ/ppm): 15.05 (OCH2CH3), 39.85 (C-H
methine), 55.02 (OCH3), 63.52 (OCH2CH3), 111.64, 112.98,
131.65, 145.52 and 148.01 (aromatic carbons of benzaldehyde),
120.45, 121.62, 138.23 and 142.09 (aromatic carbons of
pyrogallol).
NMR (δ/ppm): 3.74 (s, 12 H, OCH3), 3.80-4.03 (broad singlet,
12H, C-H methane and C-OH), 7.01 (s, 4H, aromatic proton of
pyrogallol), 7.09-7.41 (s and s and s, 8H, aromatic protons of
benzaldehyde), 8.22 and 10.5 (s and s, 4H, SO3H).
Synthesis of C-4-ethoxy-3-methoxyphenylcalix[4]
pyrogallolarene sulfonic acid (Calix-C-SA). Compound Calix-
C (2.3 g, 2.0 mmol) was directly reacted with concentrated
sulfuric acid (4.9 g, 50 mmol, 25 equivalent) at 80°C for 4 h.
After the reaction, the mixture was cooled down to room

Biodiesel
production
using
C-arylcalix[4]
methoxybenzaldehyde at 2770 and 2855 cm-1, while a new peak
corresponding to the methine C-H appeared at 1466 cm-1.
Moreover, the methine group generated signals at 5.74 ppm in
pyrogallolarene sulfonic acid organocatalysts. Palmitic acid
(0.6 g, 2.5 mmol) was dissolved in methanol (8.0 g, 250 mmol,
100 equivalent). Organocatalyst (Calix-A-SA, Calix-B-SA, or
Calix-C-SA) (4 mol%) or concentrated sulfuric acid was added,
and the mixture was heated at 65°C for 4 h. After the reaction,
the mixture was cooled down to the room temperature and the
organocatalyst was recovered by using a simple filtration method.
The filtrate was neutralized with 10% sodium hydroxide in
distilled water, extracted with ethyl acetate (3×10 mL) and the
organic phase was washed with distilled water (10 mL) three
times. The organic phase was dried over anhydrous sodium
sulfate and the solvent was removed in vacuo to obtain methyl
palmitate as a yellow liquid.. FTIR (ν/cm-1): 2916 (C-H sp3),
1743 (C=O), and 1332 (C-O ester). GC: single peak at retention
time (tR) = 39.64 min. MS: 270 (M+), 239, 227, 199, 185, 171,
157, 143, 129, 115, 101, 87, 74 (base peak). The recovered calix-
B-SA organocatalyst was re-used for the esterification of
palmitic acid with methanol under the similar condition to afford
methyl palmitate.
1
the H-NMR spectrum and at 39.85 ppm on the 13C-NMR
spectrum. The FTIR, 1H-NMR and 13C-NMR analyses
confirmed the chemical structure of either Calix-A or Calix-B or
Calix-C in tetrameric macrocyclic structure.
Synthesis of Calix-A-SA, Calix-B-SA and Calix-C-SA.
Sulfonation reaction of Calix-A with sulfuric acid gave Calix-A-
SA in 82.5% yield. The existence of the sulfonic acid group was
indicated by the presence of S=O absorption peaks at 1157 and
1034 cm-1, an S-O absorption peak at 840 cm-1 and a C-S
absorption peak at 579 cm-1 in the FTIR spectrum. Furthermore,
1
a new singlet appeared at 8.30 ppm in the H-NMR spectrum
with an integration ratio of 4, suggesting that each benzaldehyde
ring contained one sulfonic acid group. Moreover, as the other
aromatic protons of the benzaldehyde group appeared as a set of
two doublets at 7.26 and 7.44 ppm (each 2H), it was deduced
that the sulfonic acid group was located at the para position.
Calix-B-SA was synthesized from the sulfonation reaction
of Calix-B with sulfuric acid. The FTIR spectrum of the product
showed S=O peaks at 1173 and 1051 cm-1, an S-O peak at 880
cm-1, and a C-S peak at 579 cm-1. The protons of the -SO3H
group appeared as two distinct signals at 8.22 and 10.45 ppm in
3. Results and Discussion
Synthesis of 4-ethoxy-3-methoxybenzaldehyde. The 4-
Ethoxy-3-methoxybenzaldehyde has been successfully
synthesized by the alkylation of vanillin with diethyl sulfate
under alkaline condition in 73% yield (Figure 1). The
consumption of the starting material was indicated by the
absence of the hydroxyl stretching peak at 3600-3300 cm-1 in the
FTIR spectrum. Furthermore, the FTIR spectrum of 4-ethoxy-3-
methoxybenzaldehyde matched the reference spectrum of the
standard compound. The purity of the product was confirmed by
the appearance of a single peak on GC chromatogram at a
retention time of 27.09 min. In the mass spectrum, the molecular
ion peak was found at m/z = 180, and it fragmented by loss of an
ethyl radical to give the base peak at m/z = 151.
1
the H-NMR spectrum, suggesting that the sulfonic group was
not located in the same position for each aromatic ring.
Similarly, Calix-C-SA was produced by the sulfonation of
Calix-C with sulfuric acid. The FTIR spectrum revealed the
appearance of new peaks at 1150 and 1034 (S=O), 856 (S-O) and
584 (C-S) cm-1. Similar to Calix-B-SA, the 1H-NMR spectrum
showed two new singlet peaks at 8.10 and 10.2 ppm belonging
to the -SO3H group. Therefore, the spectrometry analysis
confirmed the chemical structure of Calix-A-SA, Calix-B-SA
and Calix-C-SA, demonstrating that they have been successfully
synthesized.
Synthesis of Calix-A, Calix-B and Calix-C.
Biodiesel production by using Calix-SA as the novel
organocatalyst material. In this work, a preliminary study of
biodiesel production was carried out by using palmitic acid as a
representative fatty acid as it is found as a major component of
palm oil. Hence, the Calix-SA products were evaluated as
organocatalysts for the esterification of palmitic acid with
methanol. Concentrated sulfuric acid was used as a comparative
control. The effect of reaction time (1, 2, 4 or 8 h) on the yield
of the esterification reaction of palmitic acid was first
investigated. Under these conditions, all catalysts gave the
highest yield of methyl palmitate at 4 h reaction time as shown
in Figure 2(a). Moreover, the yield did not increase when the
reaction was extended to 8 h. At 4 h reaction time, Calix-A-SA,
Calix-B-SA, Calix-B-SA, and sulfuric acid gave 80.0, 88.9, 85.9
and 93.3% yield of methyl palmitate respectively, which is
remarkable.
Calix[4]pyrogallolarene
A
(Calix-A) was obtained in
quantitative yield (99.7%) by reacting benzaldehyde and
pyrogallol under acidic condition. As a homogenous acid
catalyst, HCl protonates the carbonyl group of benzaldehyde to
facilitate electrophilic substitution of the electron-rich pyrogallol.
In the FTIR spectrum of the product, the C-H aldehyde peaks of
benzaldehyde at 2738 and 2820 cm-1 disappeared. The existence
of the methine bridges ins Calix-Awas indicated by the presence
1
of a singlet at 5.62 ppm in the H-NMR spectrum and peak at
35.20 ppm in the 13C-NMR spectrum. Further confirmation of
the structure was provided by MS, which showed a peak at m/z
= 855.57 corresponding to the molecular ion of Calix-A.
Moreover, the purity of the product was confirmed using LC
which showed a single peak with a retention time of 0.12 min.
Similarly, condensation reaction between vanillin and
pyrogallol in the presence of ethanolic HCl produced calix-B as
the product. The FTIR spectrum revealed the absence of the
vanillin C-H aldehyde peaks at 2742 and 2848 cm-1, indicating
that the condensation reaction had occurred. The methine proton
100
80
60
40
20
0
100
80
60
40
20
0
(a)
(b)
PgPhS
Calix-A-SA
PgPh4OEt3OM
Calix-C-SA
1
appeared at 5.54 ppm on the H-NMR spectrum, while the
PgPh4OH3OMe
Calix-B-SA
methine carbon appeared at 39.85 ppm on the 13C-NMR
spectrum. The MS spectra found the [M+14H]+ ion fragment for
calix-B at m/z = 1054.09 at 14.32 minutes as the retention time.
Therefore, the chemical structure of calix-B was confirmed from
these FTIR, 1H-NMR, 13C-NMR and LC-MS elucidation
analyses.
Asam sulfat
Sulfuric acid
1
2
4
8
1
2
4
8
Reaction time (h)
Catalyst amount (mol%)
Figure 2. (a) Effect of the reaction time on the yield of methyl
palmitate. Catalyst amount: 2 mol%. Reaction temperature:
65°C. (b) Effect of the catalyst loading on the yield of methyl
palmitate. Reaction time: 4 h. Reaction temperature: 65°C.
Meanwhile, Calix-C was prepared through a condensation
reaction between 4-ethoxy-3-methoxy benzaldehyde and
pyrogallol in similar fashion to above. The FTIR spectrum
revealed the absence of C-H aldehyde peaks of 4-ethoxy-3-
The effect of the catalyst amount in methyl palmitate
production was next investigated. It was found that the optimum

amount of all the catalyst materials was 4 mol% at a temperature
of 65°C and a reaction time of 4 h as shown in Figure 2(b). Under
these conditions, Calix-A-SA, Calix-B-SA, Calix-B-SA, and
sulfuric acid gave 83.0, 91.9, 87.4 and 94.8% yields of methyl
palmitate, respectively. Furthermore, yields did not improve
when the catalyst amount was increased up to 8 mol%. This
remarkable catalytic activity reflected the advantages of using
calix[4]pyrogallolarene framework, such as preorganized
structure with multiple phenolic functional groups and the
existence of pKa values enhance the acidity properties of the
calix[4]pyrogallolarene.
The produced methyl palmitate was characterized by FTIR
and GC-MS together with the standard methyl palmitate as a
comparison. The FTIR and GC-MS spectra of the synthesized
methyl palmitate and the standard methyl palmitate are shown in
Figure 3. For both the products and the standard, the C-H, C=O
and C-O stretching peaks were found at 2916 and 2847, 1705-
1745 and 1296-1332 cm-1, respectively, in the FTIR analysis,
with only slight differences between both spectra. Furthermore,
the purity of the synthesized methyl palmitate was confirmed in
100% purity by the appearance of a single peak in GC
chromatogram, which matched the retention time of the standard
(39.64 min)1. Furthermore, the MS spectra of both compounds
gave signals at m/z = 270 for the molecular ion and at m/z = 74
for the base peak. Fragmentation of the molecular ion produced
signals at m/z = 227 (loss of ethyl radical) and m/z = 239 (loss
of methyl radical). The molecular ion also underwent the
McLafferty rearrangement to produce the fragment C3H6O2+ at
m/z = 74, which was the base peak.
These results also suggested that the presence of hydroxyl
and alkoxy groups on the benzaldehyde scaffold influence the
catalytic activity of C-arylcalix[4]pyrogallolarene sulfonic acids.
Calix-B-SA, with a hydroxy group and a methoxy group on each
benzaldehyde ring, exhibited the best catalytic activity out of the
three novel organocatalysts synthesized. This could be due to the
ability of Calix-B-SA to undergo more hydrogen bonding with
either palmitic acid or methanol compared to Calix-A-SA or
Calix-C-SA. Moreover, the phenolic group on Calix-B-SA can
also function as a Bronsted acid to catalyze the reaction. All of
the organocatalysts showed slightly lower (less than 10%)
catalytic activity than sulfuric acid. However, it should be noted
that the C-arylcalix[4]pyrogallolarene sulfonic acids could be
easily recovered by filtration after the completion of the
esterification reaction, while the sulfuric acid could not and may
pollute the environment.
A
Since Calix-B-SA exhibited the best catalytic activity, the
effect of the reaction temperature on the esterification of palmitic
acid was further investigated. The yield of methyl palmitate at
50, 55, 60 and 65°C were 32.6, 63.7, 84.4 and 91.9%,
respectively. This is likely because esterification reaction is an
endothermic process as well as the reaction rate is higher at
higher temperature, therefore the yield was increased by
increasing reaction temperature. From the overall results of
this work, it was concluded that the optimum reaction conditions
for the esterification of palmitic acid with methanol are catalyst
loading of 4 mol% Calix-B-SA, a temperature of 65°C, and a
reaction time for 4 h.
B
Figure 4. Effect of the reaction temperature on the yield of the
esterification of palmitic acid with Calix-B-SA as the catalyst.
Catalyst amount: 4 mol%. Reaction time: 4 h.
C
Figure 3. (A) The FTIR spectra, (B) GC chromatograms and (C)
MS spectra of the obtained methyl palmitate in the present work
(d), and the standard methyl palmitate (e).
The comparison of catalytic performance for the
esterification of palmitic acid with methanol is shown in Table 1.
The Calix-B-SA organocatalyst exhibits higher catalytic activity

for methyl palmitate biodiesel production compared with most
of the previously reported works. It should be noted that while
some reported catalysts showed higher yields of methyl
palmitate (Entries 4, 6 and 10), they also needed longer reaction
times and/or higher reaction temperature. For example, PW
12/Al-SBA-15 and [GlyH]1.0H2.0PW12O40 catalysts gave higher
reaction yields of 98.0% and 93.3% respectively, but both
required high temperature reactions (≥ 90°C) which is energy-
consuming (Entries 6 and 10). Moreover, the AmberliteTM IR-
120 catalyst gave 98.3% yield after a long reaction time of 10.5
h, which is undesirable for industrial applications. Notably,
Calix-B-SA also gave slightly higher yields than the p-sulfonic
acid calix[n]arenes reported earlier (Entries 12 and 13).
A
Table 1. Comparison of catalytic methods for methyl palmitate
ester biodiesel production.
Temperature
Reaction
Time (h)
Yield
(%)
20.7
54.0
90.7
Catalyst
Ref.
(°C)
60
117
60
7
8
9
Novozym 435
H-Beta Zeolite
Zirconia
24
3
8
sulphate
10
30
31
32
AmberliteTM IR-
120
61
130
100
65
10.5
98.3
79.0
98.0
91.6
Mesoporous Al-
MCM-41
PW 12/Al-SBA-
15
Phosphotungstic
acid-based
poly(ionic
liquid)
2
8
8
B
Figure 5. SEM Image of (A) Calix-B-SA and (B) recycled
Calix-B-SA
This findings perhaps showed that some sulfonic acid groups of
the calix-B-SA have undergone desulfonation during the
esterfication process. Such a phenomenon is not surprising as
there have been some reports describing the occurrence of
desulfonation reaction of aromatic sulfonic acids under the
influence of heat or water.37 When the recycled calix-B-SA was
used for the esterification of palmitic acid with methanol under
the same condition as that of calix-B-SA, an 88.8% yield of
biodiesel was obtained which was only slightly lower than that
of calix-B-SA (92.8%). Again, this finding is perhaps attributed
by the lower sulfur content of the recycled calix-B-SA in
comparison to that of calix-B-SA. Furthermore, the formation of
bigger aggregates of the recycled calix-B-SA in compariosn to
those of calix-B-SA was another reason of the slightly lower
yield of methyl palmitate catalyzed by recycled calix-B-SA
compared to that of calix-B-SA.
33
34
35
O.O5SZ
60
60
90
6
6
3
40.0
30.0
93.3
WOx/ZrO2
[GlyH]1.0H2.0P
W12O40
36
22
WS2
60
50
6
6
70.0
91.0
p-Sulfonic acid
calix[4]arene
p-Sulfonic acid
calix[6]arene
p-
22
22
50
50
6
6
90.0
31.0
Hydroxybenzen
e sulfonic acid
p-
Toluenesulfonic
acid
22
50
6
20.0
Sulfuric acid
Calix-A-SA
Calix-B-SA
Calix-C-SA
65
65
65
65
4
4
4
4
94.8
83.0
91.9
87.4
Present
Present
Present
Present
4. Conclusion
Three C-arylcalix[4]pyrogallolarene derivatives and three
C-arylcalix[4]pyrogallolarene sulfonic acid derivatives were
successfully synthesized in high to quantitative yields (82.5-
99.7%). The structure and purity of the products were confirmed
The surface morphology of calix-B-SAand recycled calix-
B-SA was measured by SEM (Figure 5). The surface of calix-B-
SA and recycled calix-B-SA both exhibited irregular shapes. In
addition, the SEM images also showed that the recycled calix-
B-SA formed a bigger degree of aggregation in comparison to
that of calix-B-SA. In term of particle size, the results of
measurement using Static Light Scattering method indicated that
the particle size of recycled calix-B-SA (17.08 µm) was smaller
than that of calix-B-SA(42.51 µm). This fact is consistent to the
result of sulfur content measurement using SEM-EDX which
showed that the sulfur content of the recycled calix-B-SA was
smaller (2.91%) than that of calix-B-SA (10.25%).
1
by FTIR, GC-MS or LC-MS, H-NMR and 13C-NMR. The C-
arylcalix[4]pyrogallolarene sulfonic acid derivatives were tested
as the organocatalyst material for the esterification reaction of
palmitic acid and methanol into methyl palmitate. Among these,
Calix-B-SAemerged as the most efficient organocatalyst, giving
91.9% yield of methyl palmitate at 65°C for 4 h with a catalyst
loading of 4 mol%. Compared with sulfuric acid or other
reported catalytic methods in the literature, the prepared
calix[4]pyrogallolarenes showed a desirable combination of
efficient activity at relatively low temperatures and short
reaction times, as well as recoverability. Therefore, these

findings are useful for organocatalyst design from available
natural products, and may also provide new scaffolds for large
scale biodiesel production process because of their promising
catalyst activity.
24. S.N. Pod’yachev, A.R. Mustafina, A.H. Koppehele, M.
Gruner, W.D. Habicher, B.I. Buzykin, and A.I.
Konovalov, Russ. Chem. Bull. 2004, 53, 1181-1188.
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G.W. Gokel, Inorg. Chim. Acta 2014, 417, 177-185.
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Inoue, and J. Mattay, Tetrahedron Lett. 2009, 50, 1374-
1376.
Acknowledgement
The authors sincerely thank the Directorate of Research
and Community Services, KEMRISTEKDIKTI for their
financial support through PSNI Scheme budget year 2018 and
2019.
27. S. Chandrasekran, and I.V.M.V. Enoch, J. Mol. Struct.
2015, 1102, 247-252.
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