Chitosan with Sulfonic Groups: A Catalyst for the Esterification of Caprylic Acid with Methanol

Esterification of caprylic acid with methanol was performed over chitosan with sulfonic acid groups, as a catalyst, at 60 °C. The sulfonic acid groups were introduced into chitosan (CH) by using chlorosulfonic acid. Catalysts were characterized by scanning electron microscopy (SEM), elemental analysis, thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and acid–base titration. Catalytic activity increased with the amount of sulfonic acid groups present on chitosan. The 4–CH–SO3H catalyst (chitosan with sulfonic acid groups—sample 4 prepared) showed the highest activity of all materials. The esterification of caprylic acid with methanol was optimized using a 4–CH–SO3H catalyst. Under optimized reaction conditions, it was found that, at 60 °C, with 0.2 g of catalyst loading and with a molar ratio methanol to caprylic acid equal 1:95, a caprylic acid conversion of about 83%, after 4 h could be obtained. Catalytic stability of the 4–CH–SO3H material was evaluated through consecutive batch runs. After the second batch, the catalytic activity stabilized.


Introduction
Biodiesel is a mixture of fatty acid methyl ester. It can be obtained by esterification of fatty acids with an alcohol (methanol, ethanol) and by transesterification of triglycerides with an alcohol (methanol, ethanol, propanol, or butanol) [1][2][3][4]. The transesterification of triglycerides has been performed using homogenous catalysts, such as NaOH or KOH. However, when the amount of free fatty acids present in vegetable oil or animal fats is high, the transesterification reaction cannot occur due to the formation of soap. It is very important to reduce the amount of the free fatty acids present in raw material so as to not influence the transesterification reaction. The fatty acids present in vegetable oil or animal fats can be reduced by an earlier reaction (an esterification reaction) between the fatty acids and an alcohol to produce biodiesel. Traditionally, this reaction is carried out in the presence of homogenous catalysts, such as H 2 SO 4 . However, homogenous catalysts have some disadvantages such as being difficult to recover the catalyst from the reaction mixture and its reutilization [1][2][3][4]. To overcome the problems associated with the homogenous catalyst, esterification reactions have been carried out using solid acid materials, which can be easily removed from the mixture and reused [5][6][7][8][9][10][11]. The esterification of palmitic acid with methanol was performed using poly (vinyl alcohol) (PVA) and polystyrene with sulfonic acid groups. The PVA catalysts showed catalytic activity higher than the polystyrene ones [5]. The esterification of oleic acid with ethanol was carried out over zeolite Y prepared from kaolin [6]. An oleic acid conversion of 85% was obtained, under optimal conditions (T = 70 • C, 5 wt.% catalyst loading and 6:1 ethanol to oleic acid molar ratio) [6]. The esterification of oleic acid with methanol was performed using tungstophosphoric acid supported on MCM-48 [7]. After 8 h of reaction, under optimized conditions (molar ratio of acid to alcohol 1:20; m = 0.1 g; T = 60 • C), the oleic acid conversion was 95% [7]. The esterification of palmitic, oleic, and stearic acid with 0.008, 0.015, 0.023 and 0.030 mol for the catalyst 1-CH-SO 3 H, 2-CH-SO 3 H, 3-CH-SO 3 H, 4-CH-SO 3 H, and 5-CH-SO 3 H, respectively) was added dropwise under stirring for 30 min in an ice-bath. The catalyst was then removed from the mixture by filtration. After this operation, the materials were washed with dichloroethane, and dried at room temperature.

Catalyst Characterization
The swelling degree of materials was performed according to Caetano et al. [17]. Acid capacities of material were determined by acid-base titration, according to Caetano et al. [17].
The FTIR spectra were recorded in a Perkin Elmer Spectrum 100 FTIR spectrometer (PerkinElmer, Shelton, WA, USA).
CHNS Elemental Analyzer 1112 series (Thermo Finnigan, San Jose, CA, USA) was used to determine the Sulphur.
NETZSCH STA 449F3 thermogravimetric analyzer (NETZSCH, Selb, Germany) was used to study the thermal stability CH and 4-CH-SO 3 H.
Zeiss Auriga equipment (Zeiss, Oberkochen, Germany) was used to obtain SEM images.

Catalytic Experiments
The catalytic tests were performed using 8 mmol of caprylic acid, 30 mL of methanol, and 0.2 g of catalyst, at 60 • C.
Stability tests of the 4-CH-SO 3 H material were performed running five consecutive experiments. Between reactions, the material was washed with methanol and dried during 24 h, at 80 • C.
During the reaction, some samples were picked up. Undecane was used as the internal standard. The samples were analyzed using a GC (Hewlett Packard instrument (Hewlett-Packard Company, Palo Alto, CA, USA)).

Determination of Kinetic Constants, k (min −1 )
To determination of the value of kinetic constants, k (min −1 ), Equation (1), was applied according to the Pessoa Junior et al. [23]: where X t is the conversion of caprylic acid at time t Figure 1 shows the scheme of preparation of chitosan with sulfonic acid groups. Table 1 shows the characteristics of materials. It was observed that the acidity of the chitosan with -SO 3 H groups enlarged with the increase in the number of sulfonic acid groups introduced on the chitosan matrix. Table 1 also shows the sulfur amount in the chitosan with sulfonic acid groups, which was determined via elemental analysis. Similar results for the acid capacities, obtained by acid-base titration and those obtained by elemental analysis, were apparent [20,24]. The swelling degree of materials, for methanol and water, reduced when the sulfonic acid groups were increased (Table 1). This behavior could be due to the decrease in free volume [20,24]     The swelling degree of materials, for methanol and water, reduced when the sulfonic acid groups were increased (Table 1). This behavior could be due to the decrease in free volume [20,24]. Figure 2A shows the FT-IR spectra of chitosan ( Figure 2AI) and chitosan with sulfonic acid groups ( Figure 2AII). The spectrum of the 4-CH-SO 3 H sample ( Figure 2AII [20].

Results and Discussion
Polymers 2021, 13, x FOR PEER REVIEW 5 of 10 Figure 2A shows the FT-IR spectra of chitosan ( Figure 2AI) and chitosan with sulfonic acid groups ( Figure 2AII). The spectrum of the 4-CH-SO3H sample ( Figure 2AII) shows a band at 1226 cm −1 corresponding to -S = O stretching bands of -SO3H in -O-SO3H and a band at 1064 cm −1 -S = O stretching bands of −SO3H in NH−SO3H groups, which suggests the introduction of the sulfonic acid group onto chitosan [20].  Figure 2B shows the X-ray diffraction patterns of chitosan and the 4-CH-SO3H catalyst. The pattern of chitosan has a characteristic crystalline peak at 2θ degrees near to 20° [18]. The 4-CH-SO3H catalyst also has a peak at 2θ degrees of 20°, which is similar to that of chitosan. However, the intensity of the 4-CH-SO3H catalyst peak is lower than that of chitosan. A decrease in the degree of crystallinity was detected. This result could be due to the introduction of sulfonic acid groups on the chitosan structure [20].
The surface SEM micrographs for chitosan (CH) and chitosan with sulfonic acid (4-CH-SO3H sample) are shown in Figure 3. It can be seen that the 4-CH-SO3H catalyst shows voids and cracks in its structure ( Figure 3B) while the chitosan matrix (without sulfonic acid groups) exhibits a void-free dense structure ( Figure 3A). The presence of some voids and cracks in the 4-CH-SO3H material could improve the access of methanol and/or caprylic acid to the active sites of the catalyst [16,17].   Figure 2B shows the X-ray diffraction patterns of chitosan and the 4-CH-SO 3 H catalyst. The pattern of chitosan has a characteristic crystalline peak at 2θ degrees near to 20 • [18]. The 4-CH-SO 3 H catalyst also has a peak at 2θ degrees of 20 • , which is similar to that of chitosan. However, the intensity of the 4-CH-SO 3 H catalyst peak is lower than that of chitosan. A decrease in the degree of crystallinity was detected. This result could be due to the introduction of sulfonic acid groups on the chitosan structure [20].
The surface SEM micrographs for chitosan (CH) and chitosan with sulfonic acid (4-CH-SO 3 H sample) are shown in Figure 3. It can be seen that the 4-CH-SO 3 H catalyst shows voids and cracks in its structure ( Figure 3B) while the chitosan matrix (without sulfonic acid groups) exhibits a void-free dense structure ( Figure 3A). The presence of some voids and cracks in the 4-CH-SO 3 H material could improve the access of methanol and/or caprylic acid to the active sites of the catalyst [16,17].
Polymers 2021, 13, x FOR PEER REVIEW 5 of 10 Figure 2A shows the FT-IR spectra of chitosan ( Figure 2AI) and chitosan with sulfonic acid groups ( Figure 2AII). The spectrum of the 4-CH-SO3H sample ( Figure 2AII) shows a band at 1226 cm −1 corresponding to -S = O stretching bands of -SO3H in -O-SO3H and a band at 1064 cm −1 -S = O stretching bands of −SO3H in NH−SO3H groups, which suggests the introduction of the sulfonic acid group onto chitosan [20].  Figure 2B shows the X-ray diffraction patterns of chitosan and the 4-CH-SO3H catalyst. The pattern of chitosan has a characteristic crystalline peak at 2θ degrees near to 20° [18]. The 4-CH-SO3H catalyst also has a peak at 2θ degrees of 20°, which is similar to that of chitosan. However, the intensity of the 4-CH-SO3H catalyst peak is lower than that of chitosan. A decrease in the degree of crystallinity was detected. This result could be due to the introduction of sulfonic acid groups on the chitosan structure [20].
The surface SEM micrographs for chitosan (CH) and chitosan with sulfonic acid (4-CH-SO3H sample) are shown in Figure 3. It can be seen that the 4-CH-SO3H catalyst shows voids and cracks in its structure ( Figure 3B) while the chitosan matrix (without sulfonic acid groups) exhibits a void-free dense structure ( Figure 3A). The presence of some voids and cracks in the 4-CH-SO3H material could improve the access of methanol and/or caprylic acid to the active sites of the catalyst [16,17].   Figure 4 shows the TG analysis of CH ( Figure 4A) and 4-CH-SO 3 H samples ( Figure 4B). Two major stages of degradation at 90-200 • C and 200-450 • C were observed. The first weight loss is attributed to the desorption of physically absorbed water molecules and acetic acid. The second weight loss is attributed to the thermal degradation of chitosan with sulfonic acid groups [25]. As the reaction studied is performed at 60 • C, it is possible conclude that the 4-CH-SO 3 H sample is thermally stable. with sulfonic acid groups [25]. As the reaction studied is performed at 60 °C, it is possible conclude that the 4-CH-SO3H sample is thermally stable.

Catalytic Experiments
The esterification of caprylic acid (octanoic acid) with methanol was performed using chitosan with sulfonic acid groups, as heterogeneous catalysts, at 60 °C, obtaining methyl octanoate plus water. Figure 5A shows the conversion (%) of caprylic acid versus time (h). After 90 min of reaction, the caprylic acid conversion was 22, 30, 61, 75 and 72% for the 1-CH-SO3H, 2-CH-SO3H, 3-CH-SO3H, 4-CH-SO3H, and 5-CH-SO3H materials, respectively. The initial catalytic activities of chitosan with sulfonic acid in the esterification of caprylic acid with methanol are compared in Figure 5B (dark bars). It was observed that the initial activity of chitosan with sulfonic acid groups increased with the sulfonic group

Catalytic Experiments
The esterification of caprylic acid (octanoic acid) with methanol was performed using chitosan with sulfonic acid groups, as heterogeneous catalysts, at 60 • C, obtaining methyl octanoate plus water. Figure 5A shows with sulfonic acid groups [25]. As the reaction studied is performed at 60 °C, it is possible conclude that the 4-CH-SO3H sample is thermally stable.

Catalytic Experiments
The esterification of caprylic acid (octanoic acid) with methanol was performed using chitosan with sulfonic acid groups, as heterogeneous catalysts, at 60 °C, obtaining methyl octanoate plus water. Figure 5A shows the conversion (%) of caprylic acid versus time (h). After 90 min of reaction, the caprylic acid conversion was 22, 30, 61, 75 and 72% for the 1-CH-SO3H, 2-CH-SO3H, 3-CH-SO3H, 4-CH-SO3H, and 5-CH-SO3H materials, respectively. The initial catalytic activities of chitosan with sulfonic acid in the esterification of caprylic acid with methanol are compared in Figure 5B (dark bars). It was observed that the initial activity of chitosan with sulfonic acid groups increased with the sulfonic group with sulfonic acid groups [25]. As the reaction studied is performed at 60 °C, it is possible conclude that the 4-CH-SO3H sample is thermally stable.

Catalytic Experiments
The esterification of caprylic acid (octanoic acid) with methanol was performed using chitosan with sulfonic acid groups, as heterogeneous catalysts, at 60 °C, obtaining methyl octanoate plus water. Figure 5A shows the conversion (%) of caprylic acid versus time (h). After 90 min of reaction, the caprylic acid conversion was 22, 30, 61, 75 and 72% for the 1-CH-SO3H, 2-CH-SO3H, 3-CH-SO3H, 4-CH-SO3H, and 5-CH-SO3H materials, respectively. The initial catalytic activities of chitosan with sulfonic acid in the esterification of caprylic acid with methanol are compared in Figure 5B (dark bars). It was observed that the initial activity of chitosan with sulfonic acid groups increased with the sulfonic group The initial catalytic activities of chitosan with sulfonic acid in the esterification of caprylic acid with methanol are compared in Figure 5B (dark bars). It was observed that the initial activity of chitosan with sulfonic acid groups increased with the sulfonic group numbers. This behavior could be due to the increase in value of the acidity (Table 1). However, when the sulfonic acid amount raised from the 4-CH-SO 3 H catalyst to the 5-CH-SO 3 H catalyst, there was no significant increase in acidity (Table 1) and caprylic acid conversion. Previously, Singh et al. [6] studied the esterification of oleic acid with methanol using heteropolyacids immobilized on MCM-48 as catalysts. The authors ascertained that the catalytic activity increased with the number of active sites [6]. The kinetic constants (k), calculated by Equation (1) after 90 min of reaction, are compared in Figure 5B (light bars). It is observed that k increased with the amount of −SO 3 H present on chitosan, which can be explained by the increase in acidity (Table 1). However, a slight decrease in k is observed from the catalyst sample 4-CH-SO 3 H to the catalyst 5-CH-SO 3 H.
Consequently, the 4-CH-SO 3 H material was used for our full study.

Effect of Catalyst Loading
The effect of the 4-CH-SO 3 H catalyst loading on caprylic acid conversion was studied at 60 • C ( Figure 6A). The increase in caprylic acid conversion with the 4-CH-SO 3 H concentration could be due to an increase in the number and availability of active sites [8,9,23]. With a m = 0.3 g of catalyst, a caprylic acid conversion of 87% was obtained after 90 min of reaction.
numbers. This behavior could be due to the increase in value of the acidity (Table 1). However, when the sulfonic acid amount raised from the 4-CH-SO3H catalyst to the 5-CH-SO3H catalyst, there was no significant increase in acidity (Table 1) and caprylic acid conversion. Previously, Singh et al. [6] studied the esterification of oleic acid with methanol using heteropolyacids immobilized on MCM-48 as catalysts. The authors ascertained that the catalytic activity increased with the number of active sites [6].
The kinetic constants (k), calculated by Equation (1) after 90 min of reaction, are compared in Figure 5B (light bars). It is observed that k increased with the amount of −SO3H present on chitosan, which can be explained by the increase in acidity (Table 1). However, a slight decrease in k is observed from the catalyst sample 4-CH-SO3H to the catalyst 5-CH-SO3H.
Consequently, the 4-CH-SO3H material was used for our full study.

Effect of Catalyst Loading
The effect of the 4-CH-SO3H catalyst loading on caprylic acid conversion was studied at 60 °C ( Figure 6A). The increase in caprylic acid conversion with the 4-CH-SO3H concentration could be due to an increase in the number and availability of active sites [8,9,23]. With a m = 0.3 g of catalyst, a caprylic acid conversion of 87% was obtained after 90 min of reaction.

Effect of Molar Ratio Alcohol to Caprylic Acid
The esterification reactions were performed using an excess of alcohol to favor the forward reaction, since these reactions are reversible. To study the effect of molar ratio caprylic acid to methanol on reaction, different experiments were carried out using 1:15, 1:63, and 1:95. The 4-CH-SO3H catalyst amount (0.2 g) and the reaction temperature (60 °C) were kept constant. It was observed that when the molar ratio increases from 1:15-1:63, the conversion of caprylic acid increases, at a fixed time ( Figure 6B). However, when the molar ratio value increases from 1:63-1:95, an increase in conversion was not observed. Similar results were also noted by S. Singh et al. [6]. Additional experiments were carried out with m = 0.3 g of catalyst. When the amount of catalyst increased from m = 0.2 g to m = 0.3 g, the conversion of caprylic acid also increased.

Effect of Molar Ratio Alcohol to Caprylic Acid
The esterification reactions were performed using an excess of alcohol to favor the forward reaction, since these reactions are reversible. To study the effect of molar ratio caprylic acid to methanol on reaction, different experiments were carried out using 1:15, 1:63, and 1:95. The 4-CH-SO 3 H catalyst amount (0.2 g) and the reaction temperature (60 • C) were kept constant. It was observed that when the molar ratio increases from 1:15-1:63, the conversion of caprylic acid increases, at a fixed time ( Figure 6B). However, when the molar ratio value increases from 1:63-1:95, an increase in conversion was not observed. Similar results were also noted by S. Singh et al. [6]. Additional experiments were carried out with m = 0.3 g of catalyst. When the amount of catalyst increased from m = 0.2 g to m = 0.3 g, the conversion of caprylic acid also increased. Figure 7A shows the conversion of caprylic acid with methanol and ethanol at 60 • C. The loading of 4-CH-SO 3 H catalyst (m = 0.2 g) and the temperature (T = 60 • C) were kept constant. A decrease in the caprylic acid conversion is observed when ethanol is used compared with the experiment using methanol. This result can be explained by ethyl nucleophile being less reactive than methyl nucleophile [26,27]. Likewise, steric hindrance increases with molecular size [21,23]. When the reaction is performed using ethanol at 80 • C, an increase in the reaction rate is observed. The conversion of caprylic acid is increased when the amount of catalyst increases from m = 0.2 g to m = 0.3 g. Figure 7A shows the conversion of caprylic acid with methanol and ethanol at 60 °C. The loading of 4-CH-SO3H catalyst (m = 0.2 g) and the temperature (T = 60 °C) were kept constant. A decrease in the caprylic acid conversion is observed when ethanol is used compared with the experiment using methanol. This result can be explained by ethyl nucleophile being less reactive than methyl nucleophile [26,27]. Likewise, steric hindrance increases with molecular size [21,23]. When the reaction is performed using ethanol at 80 °C, an increase in the reaction rate is observed. The conversion of caprylic acid is increased when the amount of catalyst increases from m = 0.2 g to m = 0.3 g.

Catalyst Stability and Reusability
The 4-CH-SO3H material was reused for consecutive reactions. A stabilization of the initial activity after the second use was observed ( Figure 7B). A small amount of -SO3H groups cannot be linked to the chitosan. The loading of acid sites of 4-CH-SO3H material was determined by an acid-base titration and elemental sulfur analysis. After five cycles, the amount of S decreased slightly (Table 1). Table 2 shows the comparation of caprylic acid conversion using chitosan with sulfonic acid groups, as catalysts, with other catalysts reported in the literature. The 4-CH-SO3H catalyst shows a high catalytic activity. Chitosan is a biopolymer that can be used as a heterogenous catalyst for the esterification of caprylic acid with methanol.

Conclusions
The esterification of caprylic acid was carried out in the presence of chitosan with sulfonic acid at 60 °C. The catalytic activity of polymeric of materials increased with the amount of the sulfonic acid catalyst until a maximum was reached, which was obtained The esterification of caprylic acid (octan chitosan with sulfonic acid groups, as heter octanoate plus water. Figure 5A shows the conversion (%) of reaction, the caprylic acid conversion was 2 CH-SO3H, 3-CH-SO3H, 4-CH-SO3H, and 5  Figure 7A shows the conversion of caprylic acid with methanol and ethanol at 60 °C. The loading of 4-CH-SO3H catalyst (m = 0.2 g) and the temperature (T = 60 °C) were kept constant. A decrease in the caprylic acid conversion is observed when ethanol is used compared with the experiment using methanol. This result can be explained by ethyl nucleophile being less reactive than methyl nucleophile [26,27]. Likewise, steric hindrance increases with molecular size [21,23]. When the reaction is performed using ethanol at 80 °C, an increase in the reaction rate is observed. The conversion of caprylic acid is increased when the amount of catalyst increases from m = 0.2 g to m = 0.3 g.

Catalyst Stability and Reusability
The 4-CH-SO3H material was reused for consecutive reactions. A stabilization of the initial activity after the second use was observed ( Figure 7B). A small amount of -SO3H groups cannot be linked to the chitosan. The loading of acid sites of 4-CH-SO3H material was determined by an acid-base titration and elemental sulfur analysis. After five cycles, the amount of S decreased slightly (Table 1). Table 2 shows the comparation of caprylic acid conversion using chitosan with sulfonic acid groups, as catalysts, with other catalysts reported in the literature. The 4-CH-SO3H catalyst shows a high catalytic activity. Chitosan is a biopolymer that can be used as a heterogenous catalyst for the esterification of caprylic acid with methanol.

Conclusions
The esterification of caprylic acid was carried out in the presence of chitosan with sulfonic acid at 60 °C. The catalytic activity of polymeric of materials increased with the amount of the sulfonic acid catalyst until a maximum was reached, which was obtained

Catalyst Stability and Reusability
The 4-CH-SO 3 H material was reused for consecutive reactions. A stabilization of the initial activity after the second use was observed ( Figure 7B). A small amount of -SO 3 H groups cannot be linked to the chitosan. The loading of acid sites of 4-CH-SO 3 H material was determined by an acid-base titration and elemental sulfur analysis. After five cycles, the amount of S decreased slightly (Table 1). Table 2 shows the comparation of caprylic acid conversion using chitosan with sulfonic acid groups, as catalysts, with other catalysts reported in the literature. The 4-CH-SO 3 H catalyst shows a high catalytic activity. Chitosan is a biopolymer that can be used as a heterogenous catalyst for the esterification of caprylic acid with methanol.

Conclusions
The esterification of caprylic acid was carried out in the presence of chitosan with sulfonic acid at 60 • C. The catalytic activity of polymeric of materials increased with the amount of the sulfonic acid catalyst until a maximum was reached, which was obtained with the 4-CH-SO 3 H sample. Under optimized reaction conditions (m = 0.2 g of 4-CH-SO 3 H catalysts, T = 60 • C, and molar ratio caprylic acid to methanol 1:95), a 75% of caprylic acid conversion was obtained. The esterification of caprylic acid with methanol led to higher conversion when compared with ethanol at the same reaction temperature.
The catalytic stability of the 4-CH-SO 3 H material was evaluated through different consecutive batch runs with the same catalyst. The activity stabilized after the second batch.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.