Journal Pre-proof Catalytic activity of heterogeneous acid catalysts derived from corncob in the esterification of oleic acid with methanol Suppasate Dechakhumwat, Plaifa Hongmanorom, Chachchaya Thunyaratchatanon, Siwaporn Meejoo Smith, Supakorn Boonyuen, Apanee Luengnaruemitchai PII:
To appear in:
Received Date: 25 September 2018 Revised Date:
16 August 2019
Accepted Date: 31 October 2019
Please cite this article as: Dechakhumwat S, Hongmanorom P, Thunyaratchatanon C, Smith SM, Boonyuen S, Luengnaruemitchai A, Catalytic activity of heterogeneous acid catalysts derived from corncob in the esterification of oleic acid with methanol, Renewable Energy (2019), doi: https:// doi.org/10.1016/j.renene.2019.10.174. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Catalytic activity of heterogeneous acid catalysts derived from
corncob in the esterification of oleic acid with methanol
Suppasate Dechakhumwata, Plaifa Hongmanoroma, Chachchaya Thunyaratchatanona,
Siwaporn Meejoo Smithc, Supakorn Boonyuend, Apanee Luengnaruemitchaia,b,*
10330, Thailand b
Center of Excellence on Catalysis for Bioenergy and Renewable Chemicals (CBRC), Chulalongkorn University, Phayathai Rd., Pathumwan, Bangkok 10330, Thailand
The Petroleum and Petrochemical College, Chulalongkorn University, Soi Chula 12, Phyathai Road, Bangkok
Center of Alternative Energy, Faculty of Science and Department of Chemistry, Faculty of Science, Mahidol University, 999 Phuttamonthon Sai 4 Rd., Salaya, Nakhon Pathom 73170, Thailand
Department of Chemistry, Faculty of Science and Technology, Thammasat University, Rangsit Centre, Khlong Luang, Pathum Thani 12120, Thailand
Sulfuric acid (H2SO4) pretreated corncob-derived residue was used as a starting material to prepare
solid acid catalysts using different sulfonation chemicals (H2SO4, p-toluenesulfonic acid (TsOH)
and H2SO4/TsOH mixtures) for biodiesel production from the esterification of oleic acid with
methanol. Effects of the different sulfonation agents on the properties of the derived carbon-based
materials were investigated using various characterization techniques. Lignin residues were
obtained after the H2SO4 pretreatment step, and high lignin-containing carbon-based catalysts of ca.
69% (w/w) lignin were derived after sulfonation with H2SO4. Employing TsOH or H2SO4/TsOH
mixtures for sulfonation gave materials with a higher carbon/hydrogen (C/H) ratio, indicating a
relatively effective carbonization compared to that with H2SO4 sulfonation. The catalytic activity of
the sulfonated corncob in the esterification of oleic acid with methanol was influenced by the acid
density, acid strength and porous structure of the sulfonated materials. High methyl oleate yields (>
80% after 8 h at 60 °C) were achieved using the acid catalyst obtained from either H2SO4 or TsOH
sulfonation, whereas those from H2SO4/TsOH sulfonation gave slightly lower yields. Thus, the use
of the non-volatile TsOH solid as a ‘greener’ sulfonating agent for the production of carbon-based
solid acid catalysts with a high catalytic activity in the esterification reaction is supported.
Keywords: FAME; carbon based acid catalyst; sulfonation; p-Toluenesulfonic acid; esterification;
Tel.: 662-218-4148; Fax: 662-611-7220; e-mail address: [email protected]
Address author correspondence:
Green economy, an initiative that aims to lead to a low-carbon industrial revolution, should
primarily focus on “improved human well-being and social equity, while significantly reducing
environmental risks and ecological scarcities”, according to the UNEP . Being aligned well with
this sustainable initiative, bioeconomy has attracted increasing interest as it involves the use of
renewable natural resources as raw materials in several manufacturing industries, such as the
production of food, feed, bio-based products and biofuels . Although biodiesel had a small
production level from 2001–2005 (< 100 million gallons), a significant (more than 15-fold) increase
in the annual biodiesel production has occurred from 2013 onwards, with a recorded 1,556 million
gallons being produced in 2016 . Based on current projections (2010–2025) of the European
Union and the US EIA [4, 5], demand for biodiesel will continue to escalate each year.
Interestingly, the US biodiesel production in 2016 surpassed the forecasted value by 1.7-fold,
indicating the outstanding growth of the US biodiesel sector. This significant growth in biodiesel
production in the US could be attributed to the acceptable operational performance and
environmentally friendly perception of bio-based fuels, as well as an important tax incentive
Fatty acid methyl esters (FAMEs), or biodiesel, can be produced via either transesterification of
triglycerides or esterification of free fatty acids (FFAs) with a short-chain alcohol in the presence of
a catalyst. Esterification of FFAs and alcohol generally requires an acid catalyst to achieve a high
FAME yield. On the other hand, basic catalysts are not ideal as they often react with FFAs to form
soap and so give a low FAME yield and purity. Previous works have reported the utilization of
homogeneous acid catalysts in the production of FAMEs from poor quality feedstocks, particularly
for triglycerides with low FFA contents [7-9]. However, using a homogeneous catalyst requires large
volume reactors, a large amount of catalyst. In addition, catalyst recyclability is a formidable
problem since homogeneous catalysts are miscible with the reaction mixture, causing a large
amount of waste water from the production of FAMEs. Thus, the use of heterogeneous catalysts has
gained considerable attention, owing to the reduced occurrence of corrosion, an improved
environmentally friendly processes and the possibility to reuse or recycle the solid catalyst.
Generally, homogeneous Bronsted acids, such as sulfuric acid (H2SO4) and hydrogen fluoride,
have been applied onto a solid support to increase the acid sites, stability and reusability of
heterogeneous catalysts in many acid-catalyzed reactions. However, most heterogeneous catalysts
developed so far are expensive and require complex synthetic procedures. Recently, sulfonated
carbon-based materials derived from H2SO4 treatment have shown great promise as effective
catalysts in the esterification of FFAs [10,11]. Nevertheless, to produce sulfonated materials from
sulfonating agents like H2SO4 requires special safety guidelines, as well as a high pressure-
withstanding reactor that resists damaging corrosion. Alternative sulfonating agents with a low
vapor pressure would be preferable to establish a ‘greener’ sulfonation process, which is more
environmentally benign in numerous industrial processes. In this vein, the sulfonation of sugar and
extracted lignin was performed using the non-volatile p-toluenesulfonic acid (TsOH), which
resulted in a material with a high acid site content and strong acidity [12-14]. Furthermore, a
carbon-silica composite bearing -SO3H functional groups was obtained through TsOH treatment
followed by H2SO4 sulfonation , where the TsOH treatment reduced the amount of corrosive
H2SO4 reagent required in the sulfonation process.
Corn (or maize) is one of the most common biomass used to produce ethanol in the US (ranked
1st in the world for ethanol production), as well as in China and Canada. Note that, in 2016, the US
retained its position as the top ethanol producer at around 60% of the world’s ethanol production
. In the ethanol production industry, machines are used to separate corn kernels from corncobs
during harvesting, and the kernels are then stored for further liquefaction, saccharification,
fermentation, distillation and dehydration . The post-harvested corn leaves, stems, husks and
cobs are typically left in the fields, for boosting up the soil’s nutrients, or in some countries like
Thailand, are often burnt. Corn plantation is a major source of grains supplied in the animal feed
sector in Thailand . Thus, an appropriate management of the large amount of agricultural waste
is necessary to prevent air pollution caused by the burning of the waste prior to planting a new crop.
In this work, corncob residue was used as a starting material for the preparation of solid acid
catalysts. The simple treatment of corncobs with H2SO4 can produce a series of porous solid
Brønsted acids. Several attempts have been made towards the synthesis of solid acid catalysts using
organosulfonic acid-functionalized catalysts on corncob residues. The sulfonating agent TsOH is an
aromatic ring linked to a long alkyl chain that endows the acid catalyst with an amphiphilic nature.
The main objective of this present work was to compare the catalytic performance in the oleic acid-
methanol esterification reaction of the corncob materials functionalized with sulfonic acid by
H2SO4, TsOH or H2SO4/TsOH mixtures. The optimum conditions of the catalyst preparation, in
terms of the type and amount of reagent and ratio of acid, were determined and benchmarked with
those in the previous reported works. As a comparison, commercial lignin was also treated in a
similar way with H2SO4 or TsOH. The catalytic activity of the carbon based materials in the
esterification of oleic acid with methanol was evaluated in terms of the methyl oleate (FAME)
yield. Note that oleic acid was selected as an unsaturated FFA model as it is the major FFA in
commonly available vegetable oils, such as 37% by weight (wt.%) in palm oil, the world’s most
widely used oil. The characteristics of the obtained sulfonated materials, in terms of their
morphology, functional group, surface area, pore size, internal structure, bulk composition, types of
acid site, acid site concentration and sulfur content, were examined. The reusability of the
sulfonated corncob-derived materials was also investigated.
The physical and chemical properties of the sulfonated corncob-derived materials were
characterized using thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron
microscopy (SEM), Brunauer-Emmett-Teller (BET) specific surface area, elemental analysis,
Fourier transform-infrared spectrophotometry (FT-IR) and ammonia-temperature-programmed
2. Materials and methods
Corncobs were supplied as complimentary samples by the Betagro Company, Thailand. The
average particle size (homogenized in a single lot) of the corncob was 1.6 mm. They were dried at
105 °C in an oven overnight and stored at room temperature for further use. Previously, when
corncobs were used as a raw material to produce biobutanol, they were first pretreated by dilute
acid treatment followed by enzymatic hydrolysis and acetone-butanol-ethanol fermentation .
The pretreated corncobs were then filtered from the supernatant and were further treated by
enzymatic hydrolysis, which had a synergetic effect on the overall yields of reducing sugars. Solid
acid catalysts in the present work were, accordingly, prepared by the same H2SO4 acid pretreatment of
the corncobs but then used for the subsequent sulfonation. All supplied chemicals were AR grade
except as described otherwise.
2.1. Solid acid catalyst preparation
Following the reported optimum conditions , 10 g of dried ground corncob was suspended
in 100 mL of 2 wt.% H2SO4 in an 80-mL Teflon-lined stainless-steel autoclave with stirring for 15
min. The suspension was heated in a temperature-controlled oven at 120 ºC for 5 min, cooled and
then washed with de-ionized water (DW) to eliminate excess ions. The solid sample was separated,
dried in an oven at 110 ºC for 12 h, and kept in a desiccator and refereed to hereafter as pretreated
corncob (or C).
The pre-treated corncob was acidified by sulfonation using H2SO4 (98 wt.%, Sigma-Aldrich) or
TsOH (98 wt.%, Sigma-Aldrich) at varying concentrations, or H2SO4/TsOH mixtures at varying
weight ratios as follows. The H2SO4 sulfonation was performed in a closed-system reactor at 110 °C
for 5 h, using 5 g of pretreated corncob and 50 mL of H2SO4, with the obtained product denoted as
C-H2SO4. The TsOH sulfonation was performed by mixing 5 g of pretreated corncob and TsOH
powder (5, 10, 15 or 20 g) in an acid digestion bomb reactor at 180 °C. The obtained sulfonated
samples were denoted as C-TsOH-x, where x represents the weight of TsOH. Finally, the
H2SO4/TsOH sulfonation of 5 g of pretreated corncob was performed by 15 min mixing on a
magnetic stirrer, followed by heating in an acid digestion bomb reactor at 180 °C for 24 h. The
sulfonated samples derived from the H2SO4/TsOH treatments at H2SO4: TsOH (w/w) ratios of 1:3
and 3:2 were denoted as C-M-3-10 and C-M-15-10, respectively.
In addition, the sulfonation of commercial lignin (kraft and low sulfonate alkali, Sigma Aldrich)
was performed in a similar procedure to that used to prepare C-H2SO4 and C-TsOH-10, with the
sulfonated lignin samples denoted as L-H2SO4 and L-TsOH-10, respectively. At the end of the
respective sulfonation process, each sample was washed by warm DW until the filtrate had a neutral
pH, and the conductivity of supernatant was about 0–10 mS. Then, the separated solid sample was
dried in an oven at 110 °C for 12 h, and subsequently stored in a silica gel desiccator until use.
2.2. FAME production
Esterification of oleic acid with methanol catalyzed by the sulfonated materials was performed
using a 1-L Parr reactor under 0.3 MPa of nitrogen (N2) gas with stirring at 300 rpm. For each
reaction, a certain amount of oleic acid was added in a suspension of solid catalyst (of specific
weight) in methanol. The effects of the three operating parameters of the reaction time (up to 8 h),
reaction temperature (60, 80 and 100 ºC) and catalyst loading level, on the FAME yield were
investigated by univariate analysis. Unless stated otherwise stated, the reaction was performed at 60
ºC with a methanol: oil molar ratio of 15:1 and a catalyst loading of 5 wt.% compared to vegetable
After the reaction, the samples drawn from the reaction mixture were centrifuged and excess
methanol was removed at 90 °C in a water bath. The solid catalyst was filtered from the reaction
solution. The methyl oleate (FAME product) content was kept in a vial prior to analysis by gas
chromatography using a Hewlett Packard GC model 5890 equipped with a flame ionization
detector, as previously reported . The methyl oleate yields were determined using Eq. (1); C=
(∑ A) − A
C EI × VEI × 100 m
∑ A is the overall area of methyl ester from C
where C is the methyl ester content,
the peak area of that which is aligned with the methyl heptadecanoate solution, C EI and VEI are the
concentration (mg/mL) and volume, respectively, of the methyl heptadecanoate solution (mL) and
m is the weight (mg) of the sample.
to C 24 , AEI is
All experiments were repeated at least twice. To demonstrate the reusability of the prepared
catalyst, it was separated by filtration after completion of the esterification reaction and reused in a
2nd and 3rd successive run under the same conditions without regeneration.
2.3. Catalyst characterization
All samples were characterized as follows. The crystallinity of the acid catalysts was
characterized by XRD analysis using a Rigaku Smartlab powder X-ray diffractometer. Each
powdered sample was placed on a glass sample holder to record the XRD profile range from 10–
90° at a scanning speed of 2°/min, step of 0.01° and acceleration voltage of 40 kV.
8 181 182
The morphological structure of the synthesized catalysts was characterized by SEM using a Hitachi TM 3000 scanning electron microscope operated at an acceleration voltage of 30 kV.
The BET specific surface area, pore volume and Barrett-Joyner-Halenda (BJH) pore diameter
were evaluated from the results of N2-desorption analysis using a Thermo Finnigan Sorptomatic
1990 Series analyzer. Before analysis, the volatile species adsorbed on the catalyst surface were
eliminated by heating 1 g of the catalyst under a vacuum atmosphere at 300 °C for 18 h. Helium gas
was used as an adsorbate for the blank analysis, and N2 gas was used as the adsorbate.
The atomic carbon, hydrogen, nitrogen and sulfur composition of each sample was evaluated by
CHNS and X-ray fluorescence (XRF; Panalytical Axios PW 4400) analyses, while FT-IR analysis,
using a Nicolet Nexus 670 spectrometer, was employed to identify the chemical functional groups
in the samples using the KBr pellet method and recording the FT-IR spectra over the range of
The weight change of a material as a function of temperature and time in a controlled
atmosphere was measured by TGA using a Perkin: Pyris Diamond instrument. It is ideally used to
assess the volatile content, thermal stability, degradation characteristics, aging/lifetime breakdown,
sintering behavior and reaction kinetics of sample. The respective sample (10 mg) was heated from
50 °C up to 700 °C at a 20 °C/min. The TGA was performed at atmospheric pressure in a N2 flow
of 20 mL/min.
The acidity of the solid acid catalysts was evaluated by NH3-TPD / reduction / oxidation analysis
(NH3-TPD/R/O) using a Thermo Finnigan TPDRO 1100 instrument. The acidity was calculated
from the derived temperature profile by integration of the peak area compared with standard
In addition, an acid-base titration method was utilized to quantify the total acid density of each
spent catalysts. In brief, 0.05 g sample was suspended in 15 mL of 2 M NaCl solution and kept in
an ultrasonic bath for 30 min. The ultrasonic treatment should allow an exchange between H+ ions
existing in sulfonated (–SO3H) catalyst and Na+ ions. Next, the ultrasonically treated sample was
titrated with 0.02 M NaOH, using phenolphthalein as an indicator. The total acidity of each
sulfonated sample was quantified from the concentration of the NaOH solution multiplied by the
volume of the NaOH solution consumed and divided by the weight of catalyst used .
3. Results and Discussion
TGA analysis. Three decomposition curves were observed in the TGA plots (Fig. 1) and
utilized to determine the amount of cellulose, hemicelluloses, lignin and residue components in
each sample. The compositions of dried biomass based samples were estimated (wt.%) by
optimization of the reported decomposition model . Fresh corncob had cellulose,
hemicelluloses, lignin and residue compositions of 32.15, 27.31, 39.41 and 1.21 wt.%, respectively.
An initial weight loss was observed up to 100 °C due to water evaporation. Hemicellulose
depolymerization occurs at 200–300 °C and cellulose degradation occurs at 300–400 °C . The
weight loss observed for fresh corncob was observed at 320 °C whereas at this temperature no
weight loss was observed for C-TsOH-10, probably due to its low content of cellulose. Lignin
degradation occurs at 200–500 °C, which covers a wide range of the cellulose and hemicellulose
degradation temperature [23-26]. Sulfonation of the corncob led to an increased residual lignin
(except for C-TsOH-10) and residue materials. The samples with a high lignin content (> 60 wt.%),
C-H2SO4 and L-H2SO4, were comparable to that in the commercial lignin. The sulfonation
treatments generally led to complete decomposition of hemicellulose as well as cellulose removal.
Only the C-H2SO4 material had some residual cellulose, at around 37% of the initial cellulose
content in the fresh corncob. Therefore, it can be concluded that the sulfonation using TsOH
effectively removed both cellulose and hemicellulose in the biomass material.
XRD analysis. Figure 2 shows representative XRD profiles of the powdered fresh and
sulfonated corncob samples. The three diffraction peaks at a 2θ of around 16.0°, 22.6° and
35.0°attributed to the crystalline regions of cellulose type I were observed in the fresh sample .
Sharper XRD peaks corresponding to cellulose were observed from the pretreated corncob, possibly
due to the surface damage of the biomass that allowed effective X-ray exposure . This indicates
that acid pretreatment disrupted the native cellulose crystalline structure and increased the porosity,
surface area and crystallinity level compared to the untreated samples . Poorly crystalline
materials were obtained after sulfonation by either H2SO4 or TsOH. After sulfonation, the
amorphous carbon structure (at 2θ = 15−35°) was observed, assigned to the (002) plane of the
carbon , and the weak diffraction band located at 2θ = 40−50° becomes more visible. Note that
the characteristics of the amorphous carbon in the sulfonated corncob are in good agreement with
previous work that reported on sulfonated carbon base materials derived from glucose . In
addition, the XRD results agree well with those obtained from the TGA analyses.
Morphology and surface properties. The morphology of fresh corncob, pretreated corncob and
the C-H2SO4 catalyst are shown in Fig. 3. The fresh corncob had a less porous structure and
smoother surface compared with the others. Generated pores can be observed in the pretreated
corncob, and were more evident after the sulfonation with H2SO4. Small pores appeared on the
surface of the pretreated corncob, while larger pores were visible on the H2SO4 sulfonated sample.
Nevertheless, TsOH sulfonation of the corncob using varies amount of TsOH (5, 10, 15 or 20 g)
resulted in highly porous materials, as shown in Fig. 3(d-g). Similar results were obtained from the
sulfonation when using H2SO4/TsOH mixtures, as seen in Fig. 3(h-i). The surface characteristics of
the obtained C-H2SO4 material corresponded well to the porous sulfonated coffee residue reported
previously . Notably, sulfonation with H2SO4/TsOH mixtures changed the structure of the
catalysts less than with pure TsOH. The SEM images of commercial lignin after sulfonation with
H2SO4/TsOH mixtures also showed a porous structure (Fig. 3(j-l)).
The BET specific surface area, specific pore volume and pore diameter of the fresh corncob and
sulfonated carbonaceous materials derived from corncob are summarized in Table 1. The specific
surface areas of the sulfonated samples were much higher than that of the fresh corncob, in
excellent agreement with the porous structure observed in the SEM results. The H2SO4 sulfonation
was less effective at production of a porous material compared with the TsOH processes, and
increasing the TsOH content increased the surface area and pore volume of the obtained C-TsOH-X
materials in a dose-dependent manner. Utilizing the H2SO4/TsOH mixtures for sulfonation further
developed pores in the sulfonated materials, as suggested by the significantly higher values of the
surface area (> 360 m2/g) and pore volume (ca. 0.29 cm3/g) observed in the sulfonated materials
prepared with the H2SO4/TsOH mixtures. Overall, the pore diameters of the obtained sulfonated
materials were ranked with respect to the acid system used as H2SO4 >>TsOH > H2SO4/TsOH.
Notably, a low surface area and small pore volume were observed in the sulfonated carbonaceous
material derived from lignin, indicating the advantages of using corncob biomass as the starting
material for sulfonation.
Composition. The composition of light chemical elements (C, H, N and including S) in the
samples is summarized in Table 1. The elemental composition of fresh corncob and commercial
lignin were around 50% C and 5–6% H (C/H ratio of 8–9), but corncob had more N and less S than
lignin. The increased C/H values (15.6–17.6) found in the sulfonated samples, with C levels
increased to 56-72% and H levels decreased to 3.44–4.59%, are evidence that the carbonization
process occurred during sulfonation. The higher (14.3- to 23-fold) sulfur content found in the
sulfonated samples may represent the increased number of –SO3H active sites on the sulfonated
samples [12,31]. The S contents in the sulfonated samples were in the range of 3.22–5.98 wt.%,
where a 23-fold higher sulfur content was found in C-TsOH-10 (5.98 wt.%) than that in the fresh
corncob (0.26 wt.%). Note that commercial lignin contained quite a high sulfur content (4.81 wt.%)
and subsequent H2SO4 or TsOH sulfonation slightly enhanced its sulfur content to 6.44 and 7.90
Furthermore, the XRF analysis data (not shown) also confirmed the higher sulfur content in the
sulfonated samples than that in the fresh corncob (5.85 wt.% of sulfur oxides), supporting the
reported CHNS analysis results. Other chemical elements in the fresh corncob included Al, Si, P,
Cl, K and Ca at 6.97, 18.16, 1.69, 5.85, 19.01 and 44.09 wt.%, respectively. The very high sulfur
content detected in the C-H2SO4 and C-TsOH-10 samples (78.56 and 95.81 wt.%, respectively)
suggested the effective sulfonation processes used in this work.
From the FT-IR analysis (Fig. 4), the absorption peaks at 1420 cm-1 (CH2 bending vibration
mode, representing crystallinity band ) and at 1040 cm-1 (C-O stretching vibration mode) were
detected in the fresh and pretreated corncob samples. However, a greatly reduced intensity of these
peaks was observed in the sulfonated materials, C-H2SO4 and C-TsOH-10, implying the
modification and/or degradation of lignin units. After sulfonation, the appearance of absorption
bands at 1030 and 1155 cm-1 were attributed to the O=S=O symmetric stretching and SO3 stretching
, indicating that -SO3H groups were successfully incorporated into the framework in the form of
C-SO3H. The stronger absorption peaks around 1030 and 1155 cm-1 in C-TsOH-10 compared to C-
H2SO4 suggested a higher amount of sulfonic acid functional groups was obtained in C-TsOH-10
than in C-H2SO4.
Acidity properties. The acidity property of a solid catalyst is comprised of the number, strength
and type (Brønsted or Lewis) of acid sites. However, no single method can provide these properties.
The total acidity of the catalysts was obtained using acid-base titration , while the number and
strength of acid sites of the carbonaceous samples was examined based on the peak position and
peak area obtained from the NH3-TPD profiles. The disadvantages of this method are that NH3 can
be adsorbed on the non-acidic part of a surface, and the peak temperature does not show the acid
strength directly .
The total acidity obtained from titration ranged from 0.32–1.93 mmol/g, where C-H2SO4 gave
the highest acidity (1.93 ± 0.01 mmol/g) and L-TsOH-10 gave the lowest acidity (0.315 ± 0.01
mmol/g). Whereas the amount of acid sites for C-H2SO4 was estimated from desorption peak area of
NH3 released through TPD at 1.17 mmol/g. As illustrated in Fig. 5, the desorption peaks occurring
between 100–300 °C (-OH, -COOH), 300–550 °C (-SO3H) and higher than 550 °C were assigned to
the weak, medium and strong acid sites on the surface, respectively . The peak at the higher
desorption temperature (620–700 °C) was clearly observed for C-H2SO4 and L-H2SO4 (Fig. 5 (a)
and (h)) corresponding to the stronger acid sites, which agrees well with the two largest acid
quantity obtained from titration method (Table 1). For the C-TsOH samples (Fig. 5 (b)-(e)), the total
acidity determined by NH3-TPD revealed that increasing the TsOH amount decreased the amount of
strong acid sites and the Tmax of the strong acid sites slightly shifted toward a higher temperature.
The material derived from corncob sulfonation with the H2SO4/TsOH, especially C-M-15-10,
showed the highest amount of acid sites. This result is consistent with previous work, which
reported that carbon based materials with a higher acid density were obtained when using an acid
mixture (ClSO3H/H2SO4) as the sulfonating agent rather than when using H2SO4 alone .
Surprisingly, the TPD profile of L-TsOH-10 showed the smallest amount of acid sites and the Tmax
of the strong acid sites was shifted slightly to the lowest temperature compared to the other
3.2. Effect of the sulfonation agent
The solid carbonaceous materials of this study were then employed as acid catalysts in the
esterification of oleic acid with methanol to produce methyl oleate (FAME), with the corresponding
yields reported in Table 1. The total acidity of the catalysts obtained from the titration ranged from
0.32–1.93 mmol/g, depending on the synthesis conditions, and gave a FAME yield in the range of
39.5 ± 2.0% to 86.5 ± 0.6%. The FAME yield was related to the total acid site obtained from
titration, where C-H2SO4 gave the highest FAME yield (86.5%) and L-TsOH gave the lowest
FAME yield (39.5%), suggesting that more acid sites and stronger acid strength (from NH3-TPD)
could activate the protonation of the carbonyl oxygen. However, the obtained FAME yields
suggested that the catalytic activity of the sulfonated carbon-based materials depended on not only
their acid property but also on their porous structure. It should be noted that the acidity present in
the catalysts in this work is low compared to the total acid density of the commercial Amberlyst-15
(4.2 mmol/g) , where a FAME yield of 85% was obtained with 7 wt.% Amberlyst-15 at a
7:1 molar ratio of methanol: oleic acid and 60 °C. Whereas H2SO4-Zr2O (total acidity by NH3-TPD
of 1.66 mmol/g) gave a 98% yield at 100 °C at a 10 h reaction time and HCl-SO3-ZrO2
(6.29 mmol/g) under the same condition gave a nearly 100% yield .
Catalysts prepared from TsOH sulfonation resulted in high FAME yields (≥ 73%) with C-
TsOH-10 achieving the highest FAME yield (80.4%) of the C-TsOH-X catalysts, but these FAME
yields were still lower than that with C-H2SO4. Note that the higher catalytic activity of the acid
catalysts derived from TsOH sulfonation were obtained from the catalyst having a higher amount of
strong acid sites, but, as already stated, C-H2SO4 gave a higher FAME yield despite having a lower
level of strong acid sites. Using the H2SO4/TsOH acid mixture as the sulfonating agent led to an
acidic carbonaceous catalyst with a markdly larger amount of strong acid sites, in agreement with
previous work , but with FAME yields of only around 75%. Thus, additional explanation is
required for the highest FAME yield being obtained with the C-H2SO4 catalyst. Catalysts with a
better performance have previously been reported to have larger pore diameters that allow the
reactants (methanol, fatty acid) to efficiently diffuse into the catalytically active sites . On this
basis, it was reasonable that the catalysts with the smaller pore diameters gave lower FAME yields
despite having a high number of strong acid sites. This could be the reason why the highest FAME
yield was obtained with C-H2SO4, as it had the largest pore diameter. The surface acidity obtained
in this work showed the same trend as that in other studies. The sulfonated solid acid catalyst
obtained from algae increased from 0.6 mmol/g to 1.46 mmol/g  and the density of –SO3H sites
in the rice husk char catalyst of 0.70 mmol/g  gave the best catalytic performance for
esterification at 90 °C, although this reaction temperature was higher than that used in the present
The presence of S in the C-H2SO4 and C-TsOH-10 samples was also confirmed by X-ray
photoelectron spectroscopy (XPS) analysis, as shown in Fig. 6. The main C 1s bands at 285 eV
(Fig. 6 (a) and (b)) were attributed to C–C bonding and the bands at 286.3, 287.7 and 289.1 eV are
attributed to C–O bonds, carbonyl and carboxylic acid groups, respectively, . The relative
amount of C–O bonding in the C-H2SO4 sample was higher than that in C-TsOH-10. The band at
169 eV attributed to the S in –SO3H groups was clearly seen in both catalysts (Fig. 6 (c) and (d)).
On the other hand, the band at 164 eV attributed to S in the SH groups  was only observed in
the C-TsOH-10 sample.
More specifically, in the case of C-TsOH-X catalysts, it could be noticed that the catalytic
performance might directly correlated with acidity properties. For C-TsOH-10, it gave the quite
high FAME yield, albeit the acid quantity was merely 0.58 mmol/g, which was lower than that of
the other C-TsOH-X catalysts (Table 1). Nevertheless, it provided the highest level of S contents
which could refer to both SO3H and SH groups, and as a result, having more acidic sites to proceed
the esterification reaction. As presented in Figure 5, the NH3-TPD profile of C-TsOH-10 catalyst
also reveals the more distinct desorption peak centered around 150 °C than other catalysts,
attributed to the more existence of –OH and/or –COOH. These weak acid groups could enhance the
catalytic performance by acting as active sites, facilitating the adsorption of reactants on the catalyst
surface, and consequently, increasing the reaction possibility .
3.3. Effect of the reaction time and temperature
To determine the optimum condition for FAME production, the C-H2SO4 and C-TsOH-10
catalysts were chosen as they exhibited the highest catalytic activity. The esterification of methanol
and oleic acid was performed for 2, 4 and 8 h at 60 °C to evaluate the effect of the reaction time
(Fig. 7). Interestingly, after 2 h of reaction, C-TsOH-10 gave a higher (about 2.75-fold) FAME
yield than C-H2SO4, which was probably due to the higher amount of strong acid sites (Fig. 5 (b)).
At a longer reaction time of 4 h, a greater FAME yield was obtained with both catalysts than at 2 h,
but they both now gave a broadly similar FAME yield, while after 8 h C-H2SO4 gave a higher
FAME yield than C-TsOH-10. The large diameter pores in the C-H2SO4 catalyst, which will both
allow larger substrates to diffuse in and also become blocked with smaller by-product molecules
less easily, may facilitate the relatively high FAME yields at longer reaction times.
Considering the faster initial reaction kinetics of C-TsOH-10 than C-H2SO4, then C-TsOH-10
was selected to study the effect of the reaction temperature. The temperature-dependent FAME
yields as a function of reaction time in the esterification of oleic acid with methanol catalyzed by C-
TsOH-10 is shown in Fig. 8. At 60 °C, a 4 h esterification gave a FAME yield of ∼72%, being
comparable to the highest FAME yield obtained using sulfonated coffee residue as catalyst in the
esterification of caprylic acid, a shorter chain fatty acid, at a similar temperature and reaction time
. In addition, the C-TsOH-10 catalyst gave higher FAME yields and required a lower reaction
temperature, compared with that reported previously for various acid catalysts, such as Amberlyst
15, Nafion NR50 and sulfonated polydivinylbenzene .
For comparison, the sulfonated carbon synthesized by the hydrothermal carbonization of a
mixture of furfural/sodium dodecylbenzene sulfonate (SDBS)/H2SO4 at 180 °C, with a surface area
of 40 m2/g and total acid level of 2.3 mmol/g, gave a 87.3% FAME yield for the esterification of
oleic acid and methanol at 65 °C within 4 h , whereas the chitosan sulfonate bead with 0.34
mmol/g gave 70% methyl oleate at 50°C within 4 h . As expected, higher temperatures
enhanced the oleic acid conversion rate . Here, the C-TsOH-10 catalyst was relatively active at
80 °C and 100 °C giving FAME yields of ca. 70% and 83% after 1 and 4 h, respectively.
Temperature was the important variable with the greatest effect on FAMEs yield, which was
increased by about 1.6-fold when increasing the reaction temperature from 60 °C to 80 °C after 1 h.
Since it is economically desirable to conduct high-yield reactions at the lowest temperature, the
catalyst reusability was further examined at 80 °C.
3.4. Reusability of the C-TsOH-10 catalyst
The reusability of the C-TsOH-10 catalyst was investigated in the esterification of oleic acid and
methanol. The reaction was performed under the same reaction conditions for four cycles. After
each cycle, the catalyst was separated from the mixture of reactant and products and retested in a
fresh reaction without regeneration.
Unlike synthetic catalysts, sulfonated mesoporous polymer  showed a great promise as an
effective and reusable catalyst in esterification of oleic acid (> 90%) for up to four cycles, though
with the requirement of a high temperature (100 °C) and high methanol: oil molar ratio (30:1).
However, this polymeric catalyst required several chemical reagents and well-controlled synthesis
steps to ensure sufficient yields of the catalyst end product. In contrast, the potential benefits of
corncob as the starting material for the production of solid acid catalysts is the natural and
renewable abundance of this non-toxic waste biomass.
The FAME yields obtained with C-TsOH-10 dropped substantially from 77.51% to 44.91 % and
28.76% in the 2nd and 3rd cycle, respectively. This was probably due to some leaching of acid sites
from the catalyst surface, with a reduction in the acid quantity obtained from titration of ca. 24% in
the 3rd cycle (from 0.58 to 0.50 and 0.44 mmol/g in the 2nd and 3rd cycle, respectively). Catalyst
deactivation may also be caused by poisoned active sites covered by the water by-product [43,48].
The deactivation of catalysts after the first cycle was also observed in the case of other sulfonated
biomass catalysts [48,49], where a washing step could be added to regenerate the catalyst.
This work systematically investigated the relationship between the properties and the catalytic
activity of carbonaceous acid catalysts derived from the sulfonation of corncob using various
sulfonating agent systems. Both solid TsOH and liquid H2SO4/TsOH mixtures can be used as
effective sulfonating agents in the preparation of biomass-based solid acid catalysts with a high
amount of strong acid sites. However, in terms of catalytic activity in the esterification of oleic acid
with methanol, only the H2SO4 and TsOH sulfonated materials gave methyl oleate (FAME) yields
of 80% or above. A faster initial reaction rate was observed in the esterification catalyzed by C-
TsOH-10 than C-H2SO4. The catalytic activity of the studied sulfonated corncob materials
depended not only on the quantity of strong acid sites, but also on their specific surface area and
pore sizes. The C-TsOH-10 catalyst had a high surface area of 240 m2/g, and suitable pore size of
3.5 nm in diameter, and that allowed long chain oleic acid to easily diffuse into the catalyst pore and
react with methanol at the catalytically active sites. Utilizing TsOH as a sulfonating agent could
dismiss the requirement for a large-volume reactor and lower the risk of the sulfonation process
compared to the use of H2SO4.
The authors thank: the Government Research Budget (Grant No. GRB_APS_49_59_63_08);
Chulalongkorn University (CU-GES-60-04-63-03); the Thammasat University Research Fund
under the Research University Network (RUN) Initiative (No.8/2560) for research funding.
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Table 1 Physical and chemical properties of the materials and FAME yield.
Fig. 1 Representative TGA profiles of the fresh corncob, commercial lignin and sulfonated
588 589 590
Fig. 2 Representative XRD patterns of the (a) fresh corncob, (b) pretreated corncob, (c) C-H2SO4 catalyst and (d) C-TsOH-10 catalyst.
Fig. 3 Representative SEM images at 3,000 x magnification of the (a) fresh corncob, (b) pretreated
corncob, (c) C-H2SO4, (d) C-TsOH-5, (e) C-TsOH-10, (f) C-TsOH-15, (g) C-TsOH-20, (h)
C-M-3-10, (i) C-M-15-10, (j) commercial lignin, (k) L-H2SO4 and (l) L-TsOH-10.
594 595 596 597 598 599 600 601 602 603
Fig. 4 Representative FT-IR spectra of the (a) fresh corncob, (b) pretreated corncob, (c) C-H2SO4 and (d) C-TsOH-10
Fig. 5 Representative NH3-TPD profiles of the (a) C-H2SO4, (b) C-TsOH-5, (c) C-TsOH-10, (d) CTsOH-15, (e) C-TsOH-20, (f) C-M-3-10, (g) C-M-15-10, (h) L-H2SO4 and (i) L-TsOH-10.
Fig. 6 Representative XPS spectra of C 1s of the (A) C-H2SO4 and (B) C-TsOH-10 catalysts and S 2p of the (C) C-H2SO4 and (D) C-TsOH-10 catalysts.
Fig. 7 Effect of the reaction time on the FAME yield in the presence of either the C-TsOH-10 or the C-H2SO4 catalyst at 60 °C.
Fig. 8 Effect of the reaction temperature on the FAME yield in the presence of the C-TsOH-10 catalyst.
Table 1 Physical and chemical properties of the materials and FAME yield Material
86.5 ± 0.6
73.2 ± 1.1
80.4 ± 0.9
77.6 ± 1.1
76.4 ± 2.1
74.2 ± 1.2
75.4 ± 1.4
1.20 ± 0.03
78.2 ± 0.7
0.32 ± 0.01
39.5 ± 2.0
measured by CHNS analyzer,
determined by titration method, measured by GC
Corncob-derived residue can be as a starting material for the production for the esterification.
Both TsOH and H2SO4/TsOH mixtures can be used as effective sulfonating agents.
TSsOH sulfonated sample presented a considerable increase in surface area.
H2SO4 and TsOH sulfonated materials gave high methyl oleate (FAME) yields.