# Efficient Production of Methyl Oleate Using a Biomass-Based Solid Polymeric Catalyst with High Acid Density

Authors: Anping Wang; Heng Zhang; Hu Li; Song Yang
Journal: Advances in Polymer Technology
 (2019)

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## Abstract

Biomass-based polymers are eco-friendly, nontoxic and biodegradable materials. In this work, in order to prepare green, low-cost and high-efficient catalysts under mild conditions, we chose biomass-based chitosan as raw material and prepared a new solid acidic catalyst by an acid functionalization method. FT-IR, XRD, SEM, TGA, BET, neutralization titration and other analytical methods were used to characterize the catalyst. The results showed that CS-SO3H morphology exhibited a sphere of about 10 μm diameter, and the acid density was as high as 3.81 mmol/g. The catalyst exhibits good catalytic activity in the esterification of oleic acid and methanol, which is a model reaction of the pre-esterification process in the preparation of biodiesel from feedstocks with high acid values. Under the optimum reaction conditions (15/1 methanol/oleic acid mole ratio and 3 wt% catalyst dosage at 75°C for 3 h), the yield of methyl oleate can reach 95.7%. Even if the mass of oleic acid in the reactant increased to 20 g, solid acid showed good catalytic performance, and the yield of methyl oleate was 94.4%. After four times of reuse, the yield of the catalyst can still reach 85.7%, which indicates that the catalyst has good catalytic activity and stability, and has potential application prospects.


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## Body

## 1. Introduction

Biodiesel is a green, biodegradable and renewable fuel. As a competitive alternative to fossil fuels, it attracts more and more attention [1–3]. The main component of biodiesel is free fatty acid methyl ester or ethyl ester, which is a mixture of various esters and determined by the composition diversity of raw materials [4]. Biodiesel was originally made from edible oils, such as soybean oil, rapeseed oil, and sunflower oil. However, the use of edible oil to produce biodiesel may affect food security in populous countries [5]. Therefore, researchers gradually explore the use of nonedible oil to synthesize biodiesel, such as Jatropha oil, Euphorbia lathyris oil [6], Koelreuteria integrifoliola oil [7] and so on. Meanwhile, waste cooking oils have gradually begun to recycle, which not only greatly reduces the cost of attractive raw materials, but also can solve the problem of environmental pollution [8, 9]. Unfortunately, nonedible oils and waste cooking oils generally have high acid value, which is disadvantageous in the preparation of biodiesel.In order to convert raw materials with high acid values into biodiesel efficiently and quickly, researchers have being explored more effective catalysts to deal with this problem. Homogeneous acids and bases, such as H2SO4, NaOH, and KOH, were initially used in the preparation of biodiesel [10]. However, homogeneous catalysts are difficult to separate from the reaction products, which has an impact on the quality of biodiesel. Moreover, the catalyst cannot be recycled. Simultaneously, homogeneous catalysts can also cause corrosion to the reaction equipment. Therefore, heterogeneous catalysts have attracted the attention of researchers [11]. In addition, for the high acid value raw materials, alkali catalysis will lead to serious saponification, so pre-esterification is necessary before transesterification. The esterification and transesterification can be carried out simultaneously by acid catalysis in one pot [8]. Therefore, using acid materials to catalyze the preparation of biodiesel from raw materials with high acid values is a good choice. Researchers reported a series of applications in the preparation of acidic catalysts for biodiesel, including SO3H-ZnO [12], SO4
2−/ZrO2[13], supported acidic ionic liquids [8], HPW-PGMA-MNPs [14], C18-SiO2-SO3H [15], carbon-based solid acids [16], MOF-supported heteropoly acids [17], and other heterogeneous acid catalysts. However, solid acid catalysts generally need higher reaction temperature and longer reaction time to obtain high biodiesel yield by high acid value raw materials. Thereby, in order to reduce the production cost of biodiesel, it is necessary to explore novel and efficient solid acid catalysts, which can pre-esterification oils with high acid value under mild conditions or simultaneously perform esterification and transesterification by one-pot method.With the development of the times, it is recognized that the development of renewable and biodegradable biomass-based catalysts are more in line with the concept of green chemistry [2]. Biomass-based catalysts are mainly obtained by carbonization and post-modification of biomass. However, high-temperature carbonization is not economical in practical application. Therefore, it is necessary to prepare functionalized biomass-based catalysts directly in a simple approach to meet the needs of production [18]. Chitosan is a kind of macromolecule polymer biomass material made from shrimp shell and crab shell, which has rich natural reserves. Chitosan has many advantages, such as biological function, compatibility, safety, and microbial degradation. It is widely used in medicine, food, chemical industry, cosmetics, water treatment and other fields [19, 20]. Chitosan has strong modifiability due to its active hydroxyl and amino groups. At the same time, chitosan is a cheap polymer, which makes it a natural advantage in industrial production as a catalyst.A solid acid catalyst for biomass polymer was prepared from chitosan and chlorosulfonic acid, to obtain high acid density, stability, and high-efficiency catalyst because sulfonic acid catalyst was a kind of relatively stable catalytic material [11, 21, 22]. The preparation method is simple, mild and eco-friendly. Because oleic acid is an important free fatty acid, it can be used to simulate high acid value raw materials to produce fatty acid methyl ester by esterification, which can effectively detect the catalytic activity of the catalyst in the preparation of biodiesel by pre-esterification and one-pot reaction. Thus, methyl oleate was synthesized by the reaction of oleic acid (OA) with methanol (MeOH), and the catalytic performance of CS-SO3H was evaluated.


## 2. Materials and Methods

### 2.1. Materials

Potassium hydroxide (KOH, AR, ≥85.0%), sodium chloride (NaCl, ≥99.5%), chlorosulfonic acid (AR, ≥97%), and oleic acid (AR, ≥99%, AV = 184.52 KOH mg/g) were obtained from Shanghai Macklin Biochemical Co. Ltd. Chitosan (>99%, The degree of deacetylation ≥95% ) was purchased from Shanghai Aladdin Biochemical Technology Co. Ltd. Methanol (AR, ≥99.5%), ethanol (AR, ≥99.7%), petroleum ether (AR, 60–90°C), and dichloromethane (AR, ≥99.5%) were purchased from Chuandong Chemical Reagent Co. Ltd.


### 2.2. Preparation of Catalyst

The preparation method of chitosan sulfonic acid catalyst (CS-SO3H) with high acid density was modified with reference to the literature [23]. Specifically, 5.0 g chitosan powder was added into 50 mL dichloromethane and stirred to form a suspension. Then, 5 mL chlorosulfonic acid was dispersed in 25 mL dichloromethane and slowly dripped into suspended chitosan (about 30 min) under vigorous stirring at room temperature. Subsequently, the mixture was stirred for 6 h at 25°C, filtered and washed to be neutral alternately with dichloromethane and absolute ethanol. Finally, the catalyst CS-SO3H was obtained after drying overnight at 80°C.


### 2.3. Catalyst Characterization

Fourier-transform infrared (FT-IR) spectrum is mainly used for functional characterization of catalytic materials. Through the appearance of new functional groups, we can judge whether the catalyst has been successfully prepared. The used infrared spectrometer is Nicolet 360 FT-IR apparatus. Crystallinity and structure of catalytic materials were characterized by means of the Cu Kα radiation X-ray diffractometer (XRD, Tongda TD-3500). Scanning electron microscopy (SEM) can provide morphological information of catalysts, including morphology, surface roughness, and voids. The scanning electron microscope we used was JSM-6700F (5 kV). Physical properties of polymer materials were determined at 77 K using Bruner-Emmett-Teller (BET) method and nitrogen adsorption-desorption device (ASAP 2460, Mcmerric Instruments Co. Ltd.). The instrument used for measuring the contact angle of catalyst water is from Dataphysics Corporation (OCA15EC, Germany). The thermal stability of the samples was determined by PerkinElmer TGA47 thermogravimetric analyzer in a nitrogen atmosphere.The acid density is an important parameter of catalyst. It is generally evaluated by acid-base neutralization titration [11, 20, 24]. In this work, the acid density of CS-SO3H is determined by the same method. In a general procedure, 40 mg catalyst was added into 20 mL 0.1 M NaCl solution. The suspension was stirred vigorously for 24 h, then centrifuged and separated. The solution after the catalyst removal was titrated to be neutral with 0.01 M NaOH solution. Phenolphthalein is an indicator of neutralization titration. The acid density of the catalyst was obtained by calculating the average value of the three parallel measurements.


### 2.4. Esterification of Oleic Acid

Methyl oleate was synthesized from OA and MeOH, and the catalytic activity of CS-SO3H was tested. Usually, a 15 mL pressure bottle is used as a reaction vessel (Beijing Synthware Glass Instrument Co. Ltd.). Methanol (1.70 g, 53 mmol), oleic acid (1.00 g, 3.5 mmol), and 0.03 g CS-SO3H were added to the reaction vessel. The reactor was placed into an oil bath at 75°C and lasted for 3 h under magnetic stirring (600 rpm). Subsequently, the mixture was cooled to room temperature and the catalyst was removed by centrifugation. Water and unreacted MeOH were removed by vacuum distillation. Nuclear magnetic resonance (NMR, 500 MHZ) was used as an instrument for measuring the yield of FAME, and CDCl3 is used as a solvent.


### 2.5. Expanded Reaction Scale for the Production of Methyl Oleate

In order to ensure that the catalyst has good catalytic performance in large-scale preparation of methyl oleate, the amount of oleic acid was increased to 20 g, with the temperature of 75°C, the MeOH to OA molar ratio of 15 : 1, and the time of 3 h. The yield was determined by NMR after post-treatment.


### 2.6. Hot Filtration and Water Tolerance Tests

In order to study heterogeneous catalytic behavior of catalysts, thermal filtration experiments were used to verify the catalytic performance. Under the optimal conditions, two groups of parallel experiments were carried out. One group continued to react after filtrating and removing the catalyst after 0.5 h, then sampled and tested the yield at 0.5 h, 1 h, 2 h, and 3 h, respectively. The control group did not do any treatment and with the yield being measured at the same time point.Water tolerance tests were determined by adding a certain amount of water (0–10% of oleic acid mass) to the reaction system, and then the change of methyl oleate yield was detected.


### 2.7. Reusability

The reusability test was still carried out under optimum conditions, and then the catalyst was washed repeatedly with MeOH and petroleum ether to remove adhering impurities. Subsequently, the solvent on the catalyst surface was removed at 80°C and applied in the next cycle.


## 2.1. Materials

Potassium hydroxide (KOH, AR, ≥85.0%), sodium chloride (NaCl, ≥99.5%), chlorosulfonic acid (AR, ≥97%), and oleic acid (AR, ≥99%, AV = 184.52 KOH mg/g) were obtained from Shanghai Macklin Biochemical Co. Ltd. Chitosan (>99%, The degree of deacetylation ≥95% ) was purchased from Shanghai Aladdin Biochemical Technology Co. Ltd. Methanol (AR, ≥99.5%), ethanol (AR, ≥99.7%), petroleum ether (AR, 60–90°C), and dichloromethane (AR, ≥99.5%) were purchased from Chuandong Chemical Reagent Co. Ltd.


## 2.2. Preparation of Catalyst

The preparation method of chitosan sulfonic acid catalyst (CS-SO3H) with high acid density was modified with reference to the literature [23]. Specifically, 5.0 g chitosan powder was added into 50 mL dichloromethane and stirred to form a suspension. Then, 5 mL chlorosulfonic acid was dispersed in 25 mL dichloromethane and slowly dripped into suspended chitosan (about 30 min) under vigorous stirring at room temperature. Subsequently, the mixture was stirred for 6 h at 25°C, filtered and washed to be neutral alternately with dichloromethane and absolute ethanol. Finally, the catalyst CS-SO3H was obtained after drying overnight at 80°C.


## 2.3. Catalyst Characterization

Fourier-transform infrared (FT-IR) spectrum is mainly used for functional characterization of catalytic materials. Through the appearance of new functional groups, we can judge whether the catalyst has been successfully prepared. The used infrared spectrometer is Nicolet 360 FT-IR apparatus. Crystallinity and structure of catalytic materials were characterized by means of the Cu Kα radiation X-ray diffractometer (XRD, Tongda TD-3500). Scanning electron microscopy (SEM) can provide morphological information of catalysts, including morphology, surface roughness, and voids. The scanning electron microscope we used was JSM-6700F (5 kV). Physical properties of polymer materials were determined at 77 K using Bruner-Emmett-Teller (BET) method and nitrogen adsorption-desorption device (ASAP 2460, Mcmerric Instruments Co. Ltd.). The instrument used for measuring the contact angle of catalyst water is from Dataphysics Corporation (OCA15EC, Germany). The thermal stability of the samples was determined by PerkinElmer TGA47 thermogravimetric analyzer in a nitrogen atmosphere.The acid density is an important parameter of catalyst. It is generally evaluated by acid-base neutralization titration [11, 20, 24]. In this work, the acid density of CS-SO3H is determined by the same method. In a general procedure, 40 mg catalyst was added into 20 mL 0.1 M NaCl solution. The suspension was stirred vigorously for 24 h, then centrifuged and separated. The solution after the catalyst removal was titrated to be neutral with 0.01 M NaOH solution. Phenolphthalein is an indicator of neutralization titration. The acid density of the catalyst was obtained by calculating the average value of the three parallel measurements.


## 2.4. Esterification of Oleic Acid

Methyl oleate was synthesized from OA and MeOH, and the catalytic activity of CS-SO3H was tested. Usually, a 15 mL pressure bottle is used as a reaction vessel (Beijing Synthware Glass Instrument Co. Ltd.). Methanol (1.70 g, 53 mmol), oleic acid (1.00 g, 3.5 mmol), and 0.03 g CS-SO3H were added to the reaction vessel. The reactor was placed into an oil bath at 75°C and lasted for 3 h under magnetic stirring (600 rpm). Subsequently, the mixture was cooled to room temperature and the catalyst was removed by centrifugation. Water and unreacted MeOH were removed by vacuum distillation. Nuclear magnetic resonance (NMR, 500 MHZ) was used as an instrument for measuring the yield of FAME, and CDCl3 is used as a solvent.


## 2.5. Expanded Reaction Scale for the Production of Methyl Oleate

In order to ensure that the catalyst has good catalytic performance in large-scale preparation of methyl oleate, the amount of oleic acid was increased to 20 g, with the temperature of 75°C, the MeOH to OA molar ratio of 15 : 1, and the time of 3 h. The yield was determined by NMR after post-treatment.


## 2.6. Hot Filtration and Water Tolerance Tests

In order to study heterogeneous catalytic behavior of catalysts, thermal filtration experiments were used to verify the catalytic performance. Under the optimal conditions, two groups of parallel experiments were carried out. One group continued to react after filtrating and removing the catalyst after 0.5 h, then sampled and tested the yield at 0.5 h, 1 h, 2 h, and 3 h, respectively. The control group did not do any treatment and with the yield being measured at the same time point.Water tolerance tests were determined by adding a certain amount of water (0–10% of oleic acid mass) to the reaction system, and then the change of methyl oleate yield was detected.


## 2.7. Reusability

The reusability test was still carried out under optimum conditions, and then the catalyst was washed repeatedly with MeOH and petroleum ether to remove adhering impurities. Subsequently, the solvent on the catalyst surface was removed at 80°C and applied in the next cycle.


## 3. Results and Discussion

### 3.1. Catalyst Characterization

As in Figure1(a), the FT-IR spectra show that the absorption bands at 1591 and 1377 cm−1 correspond to the peaks of N-H and C-O functional groups, respectively. Those belong to the characteristic peaks of chitosan. The peaks at 1160 and 1033 cm−1 are characteristic absorption bands of -SO3H group [25]. This is consistent with Dawodu et al. [21] report that the absorption bands at 1178 and 1026 cm−1are attributed to O=S=O stretching vibrations. Compared with the FT-IR spectra of CS and CS-SO3H, a new absorption band appeared at 808 cm−1 of CS-SO3H, which belongs to the characteristic peak of C-O-S [26–28]. This indicates that -SO3H functional group has been grafted onto –OH of chitosan, and the preparation of the catalyst is successful.Figure 1
The CS and CS-SO3H FT-IR spectra (a), XRD (b), SEM (c) and (d), water contact angle (e) and (f).
(a)
(b)
(c)
(d)
(e) CS 90.3°
(f) CS-SO3H 32°XRD spectra (Figure1(b)) show that chitosan was amorphous powder, and the characteristic peak of chitosan appeared at 19.9° [23]. The position of the diffraction peak of CS-SO3H is at 20.6°, which indicates that the structure of sulfonated SO3H has changed.As can be seen from Figure1(c), CS has irregular morphology and rough surface with a size of about 10–20 μm, which conforms to the structural characteristics of polymers. Figure 1(d) shows that after sulfonation, CS-SO3H becomes a more regular spherical structure with a relatively smooth surface and a size of about 10 μm. This indicates that the sulfonic acid functional groups are mainly concentrated on the surface of the catalyst, resulting in changes in the surface morphology.In the production of methyl oleate, hydrophobic materials can catalyze raw materials containing water. If hydrophobic materials are strong, the yield of methyl oleate will be higher. The water contact angle of CS is 90.3° and CS-SO3H is 32.7° (Figures 1(e) and 1(f)). Because chitosan itself is insoluble in water, and the sulfonic acid group belongs to the hydrophilic group, which results in a smaller contact angle. When the contact angle is greater than 90°, it is generally considered that catalyst has good hydrophobic properties. While the contact angle is between 30° and 90°, it is thought that the material has certain wettability, but the wettability is not good [8, 29]. However, in the process of preparing methyl oleate, a certain contact angle can meet the needs of production. Therefore, the water tolerance of catalyst was explored in the follow-up study.Table1, Figures 2(a) and 2(b) show that the specific surface area of CS and CS-SO3H decreases from 2.18 to 1.21 m2/g, the pore volume reduces from 0.006 to 0.003 cm3/g, while the mean pore size remains unchanged at 11 nm. This is mainly due to the fact that the surface area of biomass-based polymer CS is relatively small. However, the pore size distribution of chitosan from 2 to 50 nm, while that of sulfonated chitosan pore diameter distribution from 2 to 20 nm, indicating that the sulfonation process changes the pore structure of chitosan. As shown in Figure 2(c), thermogravimetric analysis shows that the mass loss of CS-SO3H is 14.7% when the temperature is lower than 223°C, which may be mainly due to the loss of water in the catalyst. Subsequently, in the range of 223–305°C, the mass of catalyst decreased sharply, indicating that polymeric network was decomposed and carbon materials were gradually formed [28]. However, the mass of CS-SO3H decreased less after 305°C, which proved that the carbon material tended to be stable, and the decomposition of residual organic components reduced slightly. Overall, the thermal stability of CS-SO3H is acceptable and has no effect on catalytic esterification.Table 1
The BET and acid density comparison different catalysts.
Sample
SBET (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
Total acid density (mmol H+·g-1)
Ref.
HS/C-SO3H
470
0.40
4.2
2.10
[11]
FCHC-SO3H
41.4
0.085
6.4
1.20
[18]
β-Cyclodextrin
-SO3H
0.18
-
4.74
3.00
[21]
Bamboo acid
0.25
-
-
1.74
[30]
CS
2.18
0.006
11.4
-
This work
CS-SO3H
1.21
0.003
11.1
3.81
This workFigure 2
(a) Nitrogen adsorption-desorption isotherms of CS and CS-SO3H, (b) BJH pore-size distributions of CS and CS-SO3H, and (c) thermogravimetric analysis of the CS-SO3H catalyst.
(a)
(b)
(c)Because the active amino and hydroxyl groups on chitosan can react with chlorosulfonic acid, the same unit can contain 2–4 sulfonic groups, so CS-SO3H catalyst has high acid density. At the same time, because of the small specific surface area, the -SO3H are mainly concentrated on the surface of the polymer acidic materials.


### 3.2. Yield Analysis of Methyl Oleate

NMR1H spectroscopy is an effective and simple method for quick detection of methyl oleate yields [29–32]. Generally, TMS is used as the internal standard marker for 0 point, and CDCl3 is used as the solvent. As can be seen from Figures 3(a) and 3(b), oleic acid and methyl oleate 1H NMR spectra are basically the same. The only difference is that methyl oleate has a significant new single peak at 3.66 ppm, which is -OCH3 peak after esterification. Meanwhile, the triple peaks at 2.30 ppm did not change before and after the reaction, which is a typical α-CH2 peak. Therefore, the yields of methyl oleate can be determined by unchanged α-CH2 and emerging -OCH3 peak areas. The following formula is used to evaluate methyl oleate yield.(1)
C
=
100
×
2
A
M
e
3
A
C
H
2
,Figure 3
The1H spectra of oleic acid (a), and methyl oleate (b).
(a)
(b)whereA
M
e represents the integral area of -OCH3 and A
C
H
2 is the area of α-CH2.


### 3.3. Esterification of Oleic Acid to Methyl Oleate

The cost of preparing methyl oleate is a primary consideration in industrial production. Therefore, the parameters of the OA and MeOH esterification reaction were optimized. The reaction temperature (55–95°C), reaction time (1–5 h), MeOH/OA molar ratio (5/1–25/1), and catalyst dosage (1–5 wt% oleic acid) were studied by single factor experiment. Under the optimized conditions, the yield was examined and compared with the results of chitosan catalysis and blank experiments (Table2).Table 2
The methyl oleate yield comparison different catalysts.
Catalyst
Total acid density (mmol H+·g−1)
Methyl oleate yield (%)
Blank
-
0.4
CS
-
0.9
CS-SO3H
3.81
95.7


#### 3.3.1. Reaction Temperature

High temperature is conducive to accelerating the reaction rate and reaching equilibrium quickly [33]. However, excessive temperature increases the production cost. Therefore, it is the first thing to determine the appropriate reaction temperature through experiments. The yield of methyl oleate increased from 75.2% to 95.9% due to the change in temperature (55–75°C) (Figure 4(a)). With the increase of temperature to 95°C, the yield reached 97.9%, and the change of yield was not obvious. The reaction temperature can be determined at 75°C.Figure 4
Performance optimization of oleic acid (AV = 184.52 mg KOH/g) with the CS-SO3H catalyst: (a) reaction temperature, (b) reaction time, (c) methanol/oleic acid molar ratio, and (d) catalyst amount.
(a)
(b)
(c)
(d)


#### 3.3.2. Reaction Time

Esterification reaction needs enough time to ensure mass transfer, but once the equilibrium state is reached, increasing reaction time has little effect on the yield [34]. As shown in Figure 4(b), the yield of methyl oleate varies greatly with time. When the time is within 1–3 h, the yield ranges from 81.3% to 95.9%. However, when time is 4 h and 5 h, the yield of methyl oleate did not increase significantly (96.3%–97.6%). Therefore, it is more reasonable to select a reaction time of 3 h for the following optimization.


#### 3.3.3. Methanol/Oleic Acid Molar Ratio

As can be seen from Figure4(c), with the increase in a molar ratio of MeOH/OA, the yield of methyl oleate gradually increases. The molar ratio of MeOH/OA has little effect on the yield of methyl oleate. The yield of 73.3% can be obtained at the molar ratio of 5 : 1. The yield of 95.8% is obtained when the molar ratio is 15 : 1. When the highest molar ratio is 25 : 1, the yield reaches 96.6%, almost unchanged. While esterification itself is a type of reversible reaction, more methanol can accelerate the reaction rate before reaching equilibrium. However, a 15 : 1 molar ratio is desirable in view of cost.


#### 3.3.4. Catalyst Dosage

In the esterification reaction, a relatively large number of active sites can promote the positive direction of the reaction [18]. As shown in Figure 4(d), When the amount of catalyst was increased from 1% to 3% the weight of OA, methyl oleate yield increased from 84.8 to 95.2%. However, when the amount of catalyst was increased to 4% and 5%, the yield hardly changed. Therefore, a suitable amount of catalyst will accelerate the reaction rate, and the yield will reach a certain level, and increasing the amount of the catalyst has no practical significance for the reaction. Moreover, from the viewpoint of actually controlling the production cost, 3 wt% of the catalyst is the optimal condition.In summary, considering the cost of production, the optimized reaction conditions are as follows: temperature 75°C, time 3 h, methanol/oleic acid molar ratio 15/1, catalyst dosage 3 wt%, and the yield of methyl oleate is 95.7%. As a contrast, the yield of the blank experiment without a catalyst is only 0.4%, while that of the control group with chitosan as a catalyst is 0.9% (Table2). Therefore, it can be concluded that CS-SO3H catalyst has an excellent catalytic effect and is a simple, economical and green catalyst for methyl oleate production.


### 3.4. Expanded Reaction Scale for the Production of Methyl Oleate

High yield could be obtained with fewer substrates, but when the amount of substrates multiplied, the catalytic activity may be affected by mass transfer resistance [35–37]. In order to ensure that the catalyst still has excellent catalytic performance in the presence of a large number of substrates, the large-scale reaction was carried out. Under the optimized conditions of the oleic acid amount of 20 g, the temperature of 75°C, the methanol to oleic acid molar ratio of 15 : 1, and the reaction time of 3 h, a high methyl oleate yield of 94.4% could be obtained. It shows that chitosan sulfonic acid catalyst still has a good catalytic effect in large scale reactions. This provides a basis for the development of new polymer-based high-efficiency acid catalysts.


### 3.5. Hot Filtration and Water Tolerance Tests

Thermal filtration is a simple and easy way to distinguish the types of catalysts. By comparing the yields after thermal filtration, the homogeneous/heterogeneous solid catalysts can be quickly judged [38]. Generally speaking, the heterogeneous behavior of catalysts is measured under the same conditions of comparative experiments. One group filtered the catalyst after 0.5 h and kept the reaction for another 2.5 h, while the other group did not have any treatment under the same conditions (Figure 5(a)). The results show that after removal of the catalyst, the yield remains basically unchanged, that is, the active site of the catalyst is not lost. This proves that CS-SO3H is an excellent heterogeneous biomass-based polymer catalyst.Figure 5
(a) Hot filtration test of CS-SO3H in the esterification of oleic acid; (b) Water tolerance study of the CS-SO3H catalyst. Reaction conditions: temperature 75°C, the CS-SO3H catalyst of 3 wt%, time of 3 h, and methanol/oil molar ratio 15/1.
(a)
(b)Owing to the esterification reaction is reversible, water will be formed in the reaction process. However, water, as a by-product of esterification, will lead to inactivation of the active site of the acid catalyst with poor water resistance, thus reducing the reaction rate, resulting in a lower yield of ester in certain reaction time. This will lead to poor reusability of the catalyst. Thus, the water tolerance of the catalyst is good, which is beneficial to esterification [8]. We added a certain amount of water into the system to determine the water tolerance of the catalyst. At the same time, it can also achieve the same effect for feedstocks which contain a small amount of water.(2)
Oleic acid (A)
+
Methanol (B)
⇋
Methyl oleate (C)
+
Water (D)
.As can be seen from Figure5(b), when the water content is 4 wt% of oil quality, the yield of methyl oleate is very close to 90%. When the water content is 8 wt%, the yield of methyl oleate can still reach 80%. With water content reaching 10 wt%, the yield can be reduced to 72.4%. This indicates that the catalyst has excellent water tolerance.


### 3.6. Reusability of Catalyst

For heterogeneous catalysts, their recyclability can not only reduce the production cost but also maximize environmental protection [14]. Therefore, it is necessary to explore the reusability of catalysts. The reusability of catalysts was determined under the optimal conditions. After each reaction cycle, the catalyst was centrifuged and washed alternately with petroleum ether and ethanol three times to remove the organic matter adhering on the catalyst. After drying overnight at 80°C, the regenerated catalyst was used again for the next reaction.As can be seen from Figure6(a), the active site of sulfonic acid still exists, but the characteristic infrared absorption band moves from 1160 and 1033 cm−1 to 1210 and 1046 cm−1. This indicates that the catalyst is affected by impurities, and it is necessary to remove by repeated use of petroleum ether scrubbing. The absorption band at 808 cm−1 belongs to C-O-S, which indicates that -SO3H is still bonded to chitosan [26–28]. By comparing the XRD peaks of CS-SO3H before and after the reaction (Figure 6(b)), no significant change in the shape of the peaks was observed, indicating that the catalyst was stable. The methyl oleate yield can still reach 85.7% after the catalyst is reused for four times (Figure 6(c)). Generally speaking, the sulfonic acid catalyst prepared by grafting method has good catalytic activity and stability, and the leaching amount of the active site is relatively small [25, 28]. For example, after five cycles of use, the yield of HS/C-SO3H catalyst is still as high as 90% [11]. Moreover, the CS-SO3H catalyst prepared by us has a high acid density (3.81 mmol/g), which makes the catalyst maintain good reusability.Figure 6
The fresh and reused CS-SO3H (a) FT-IR; (b) XRD; (c) reusability of the CS-SO3H catalyst. Reaction conditions: reaction temperature 75°C, the CS-SO3H catalyst of 3 wt%, reaction time of 3 h, and methanol/oil molar ratio 15/1.
(a)
(b)
(c)As can be seen from Table3 , entry 1 and 2 are bio-matrix composite catalysts with good reusability. Relative to entry 3, 4, 5, the reuse efficiency of biomass-based catalysts is poor. This is mainly due to the sulfonation reagent being generally sulfuric acid. Although it has a high acid density, the active sites are easy to leach in the reaction, resulting in a low reuse rate. However, CS-SO3H is a stable biomass-based polymer material because it is directly bonded by chlorosulfonic acid and CS in the form of a covalent bond, rendering the catalyst of strong stability. After four cycles, the yield of CS-SO3H can still reach 85.7%. Compared with entry 1 and 2, the yield of CS-SO3H after reuse is roughly the same, but the synthesis method of CS-SO3H is simpler and milder. This is an advantage for the polymer catalyst. Meanwhile, the reaction temperature of CS-SO3H in esterification reaction is mild, the reaction time is short, and the amount of catalyst needed is less. This may be mainly due to the high acid density which provides more active sites and speeds up the reaction rate (Scheme 1).Scheme 1
Synthesis of the biomass-based acid catalyst CS-SO3H.Table 3
Comparison of the performance of different heterogeneous catalysts in esterification.
NO.
Catalyst
Feedstock
Reaction conditions
Methyl oleate yield (Conversion) (%)
Reuse cycle (Conversion after last run)
Ref.
1
HS/C-SO3H
Oleic acid
T=80°C, CA = 3.6 wt%, M/O = 10 : 1, t = 5 h
96.9
5 (90)
[11]
2
FCHC-SO3H
Oleic acid
T = 80°C, CA = 4 wt%, M/O = 15 : 1, t = 3 h
96.7
5 (84.3)
[18]
3
Bamboo Ph-SO3H
Oleic acid
T=85°C, CA = 12 wt%, E/O = 7:1, t  =  3ha
96.0
5 (28)
[39]
4
Sugarcane bagasse -SO3H
Oleic acid
T = 50°C, CA = 0.1 wt%, M/O = 20:1, t = 24 h
85.1
5 (66.6)
[40]
5
Sulfonated carbon
FFAb
T = 68°C, CA = 6 wt%, M/O = 15:1, t = 3 h
94.4
5 (60)
[41]
6
CS-SO3H
Oleic acid
T = 75°C, CA = 3 wt%, M/O = 15:1, t = 3 h
95.7
4 (85.7)
This work
aThe reaction substrates are ethanol and oleic acid. bFFA denotes free fatty acids.


## 3.1. Catalyst Characterization

As in Figure1(a), the FT-IR spectra show that the absorption bands at 1591 and 1377 cm−1 correspond to the peaks of N-H and C-O functional groups, respectively. Those belong to the characteristic peaks of chitosan. The peaks at 1160 and 1033 cm−1 are characteristic absorption bands of -SO3H group [25]. This is consistent with Dawodu et al. [21] report that the absorption bands at 1178 and 1026 cm−1are attributed to O=S=O stretching vibrations. Compared with the FT-IR spectra of CS and CS-SO3H, a new absorption band appeared at 808 cm−1 of CS-SO3H, which belongs to the characteristic peak of C-O-S [26–28]. This indicates that -SO3H functional group has been grafted onto –OH of chitosan, and the preparation of the catalyst is successful.Figure 1
The CS and CS-SO3H FT-IR spectra (a), XRD (b), SEM (c) and (d), water contact angle (e) and (f).
(a)
(b)
(c)
(d)
(e) CS 90.3°
(f) CS-SO3H 32°XRD spectra (Figure1(b)) show that chitosan was amorphous powder, and the characteristic peak of chitosan appeared at 19.9° [23]. The position of the diffraction peak of CS-SO3H is at 20.6°, which indicates that the structure of sulfonated SO3H has changed.As can be seen from Figure1(c), CS has irregular morphology and rough surface with a size of about 10–20 μm, which conforms to the structural characteristics of polymers. Figure 1(d) shows that after sulfonation, CS-SO3H becomes a more regular spherical structure with a relatively smooth surface and a size of about 10 μm. This indicates that the sulfonic acid functional groups are mainly concentrated on the surface of the catalyst, resulting in changes in the surface morphology.In the production of methyl oleate, hydrophobic materials can catalyze raw materials containing water. If hydrophobic materials are strong, the yield of methyl oleate will be higher. The water contact angle of CS is 90.3° and CS-SO3H is 32.7° (Figures 1(e) and 1(f)). Because chitosan itself is insoluble in water, and the sulfonic acid group belongs to the hydrophilic group, which results in a smaller contact angle. When the contact angle is greater than 90°, it is generally considered that catalyst has good hydrophobic properties. While the contact angle is between 30° and 90°, it is thought that the material has certain wettability, but the wettability is not good [8, 29]. However, in the process of preparing methyl oleate, a certain contact angle can meet the needs of production. Therefore, the water tolerance of catalyst was explored in the follow-up study.Table1, Figures 2(a) and 2(b) show that the specific surface area of CS and CS-SO3H decreases from 2.18 to 1.21 m2/g, the pore volume reduces from 0.006 to 0.003 cm3/g, while the mean pore size remains unchanged at 11 nm. This is mainly due to the fact that the surface area of biomass-based polymer CS is relatively small. However, the pore size distribution of chitosan from 2 to 50 nm, while that of sulfonated chitosan pore diameter distribution from 2 to 20 nm, indicating that the sulfonation process changes the pore structure of chitosan. As shown in Figure 2(c), thermogravimetric analysis shows that the mass loss of CS-SO3H is 14.7% when the temperature is lower than 223°C, which may be mainly due to the loss of water in the catalyst. Subsequently, in the range of 223–305°C, the mass of catalyst decreased sharply, indicating that polymeric network was decomposed and carbon materials were gradually formed [28]. However, the mass of CS-SO3H decreased less after 305°C, which proved that the carbon material tended to be stable, and the decomposition of residual organic components reduced slightly. Overall, the thermal stability of CS-SO3H is acceptable and has no effect on catalytic esterification.Table 1
The BET and acid density comparison different catalysts.
Sample
SBET (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
Total acid density (mmol H+·g-1)
Ref.
HS/C-SO3H
470
0.40
4.2
2.10
[11]
FCHC-SO3H
41.4
0.085
6.4
1.20
[18]
β-Cyclodextrin
-SO3H
0.18
-
4.74
3.00
[21]
Bamboo acid
0.25
-
-
1.74
[30]
CS
2.18
0.006
11.4
-
This work
CS-SO3H
1.21
0.003
11.1
3.81
This workFigure 2
(a) Nitrogen adsorption-desorption isotherms of CS and CS-SO3H, (b) BJH pore-size distributions of CS and CS-SO3H, and (c) thermogravimetric analysis of the CS-SO3H catalyst.
(a)
(b)
(c)Because the active amino and hydroxyl groups on chitosan can react with chlorosulfonic acid, the same unit can contain 2–4 sulfonic groups, so CS-SO3H catalyst has high acid density. At the same time, because of the small specific surface area, the -SO3H are mainly concentrated on the surface of the polymer acidic materials.


## 3.2. Yield Analysis of Methyl Oleate

NMR1H spectroscopy is an effective and simple method for quick detection of methyl oleate yields [29–32]. Generally, TMS is used as the internal standard marker for 0 point, and CDCl3 is used as the solvent. As can be seen from Figures 3(a) and 3(b), oleic acid and methyl oleate 1H NMR spectra are basically the same. The only difference is that methyl oleate has a significant new single peak at 3.66 ppm, which is -OCH3 peak after esterification. Meanwhile, the triple peaks at 2.30 ppm did not change before and after the reaction, which is a typical α-CH2 peak. Therefore, the yields of methyl oleate can be determined by unchanged α-CH2 and emerging -OCH3 peak areas. The following formula is used to evaluate methyl oleate yield.(1)
C
=
100
×
2
A
M
e
3
A
C
H
2
,Figure 3
The1H spectra of oleic acid (a), and methyl oleate (b).
(a)
(b)whereA
M
e represents the integral area of -OCH3 and A
C
H
2 is the area of α-CH2.


## 3.3. Esterification of Oleic Acid to Methyl Oleate

The cost of preparing methyl oleate is a primary consideration in industrial production. Therefore, the parameters of the OA and MeOH esterification reaction were optimized. The reaction temperature (55–95°C), reaction time (1–5 h), MeOH/OA molar ratio (5/1–25/1), and catalyst dosage (1–5 wt% oleic acid) were studied by single factor experiment. Under the optimized conditions, the yield was examined and compared with the results of chitosan catalysis and blank experiments (Table2).Table 2
The methyl oleate yield comparison different catalysts.
Catalyst
Total acid density (mmol H+·g−1)
Methyl oleate yield (%)
Blank
-
0.4
CS
-
0.9
CS-SO3H
3.81
95.7


### 3.3.1. Reaction Temperature

High temperature is conducive to accelerating the reaction rate and reaching equilibrium quickly [33]. However, excessive temperature increases the production cost. Therefore, it is the first thing to determine the appropriate reaction temperature through experiments. The yield of methyl oleate increased from 75.2% to 95.9% due to the change in temperature (55–75°C) (Figure 4(a)). With the increase of temperature to 95°C, the yield reached 97.9%, and the change of yield was not obvious. The reaction temperature can be determined at 75°C.Figure 4
Performance optimization of oleic acid (AV = 184.52 mg KOH/g) with the CS-SO3H catalyst: (a) reaction temperature, (b) reaction time, (c) methanol/oleic acid molar ratio, and (d) catalyst amount.
(a)
(b)
(c)
(d)


### 3.3.2. Reaction Time

Esterification reaction needs enough time to ensure mass transfer, but once the equilibrium state is reached, increasing reaction time has little effect on the yield [34]. As shown in Figure 4(b), the yield of methyl oleate varies greatly with time. When the time is within 1–3 h, the yield ranges from 81.3% to 95.9%. However, when time is 4 h and 5 h, the yield of methyl oleate did not increase significantly (96.3%–97.6%). Therefore, it is more reasonable to select a reaction time of 3 h for the following optimization.


### 3.3.3. Methanol/Oleic Acid Molar Ratio

As can be seen from Figure4(c), with the increase in a molar ratio of MeOH/OA, the yield of methyl oleate gradually increases. The molar ratio of MeOH/OA has little effect on the yield of methyl oleate. The yield of 73.3% can be obtained at the molar ratio of 5 : 1. The yield of 95.8% is obtained when the molar ratio is 15 : 1. When the highest molar ratio is 25 : 1, the yield reaches 96.6%, almost unchanged. While esterification itself is a type of reversible reaction, more methanol can accelerate the reaction rate before reaching equilibrium. However, a 15 : 1 molar ratio is desirable in view of cost.


### 3.3.4. Catalyst Dosage

In the esterification reaction, a relatively large number of active sites can promote the positive direction of the reaction [18]. As shown in Figure 4(d), When the amount of catalyst was increased from 1% to 3% the weight of OA, methyl oleate yield increased from 84.8 to 95.2%. However, when the amount of catalyst was increased to 4% and 5%, the yield hardly changed. Therefore, a suitable amount of catalyst will accelerate the reaction rate, and the yield will reach a certain level, and increasing the amount of the catalyst has no practical significance for the reaction. Moreover, from the viewpoint of actually controlling the production cost, 3 wt% of the catalyst is the optimal condition.In summary, considering the cost of production, the optimized reaction conditions are as follows: temperature 75°C, time 3 h, methanol/oleic acid molar ratio 15/1, catalyst dosage 3 wt%, and the yield of methyl oleate is 95.7%. As a contrast, the yield of the blank experiment without a catalyst is only 0.4%, while that of the control group with chitosan as a catalyst is 0.9% (Table2). Therefore, it can be concluded that CS-SO3H catalyst has an excellent catalytic effect and is a simple, economical and green catalyst for methyl oleate production.


## 3.3.1. Reaction Temperature

High temperature is conducive to accelerating the reaction rate and reaching equilibrium quickly [33]. However, excessive temperature increases the production cost. Therefore, it is the first thing to determine the appropriate reaction temperature through experiments. The yield of methyl oleate increased from 75.2% to 95.9% due to the change in temperature (55–75°C) (Figure 4(a)). With the increase of temperature to 95°C, the yield reached 97.9%, and the change of yield was not obvious. The reaction temperature can be determined at 75°C.Figure 4
Performance optimization of oleic acid (AV = 184.52 mg KOH/g) with the CS-SO3H catalyst: (a) reaction temperature, (b) reaction time, (c) methanol/oleic acid molar ratio, and (d) catalyst amount.
(a)
(b)
(c)
(d)


## 3.3.2. Reaction Time

Esterification reaction needs enough time to ensure mass transfer, but once the equilibrium state is reached, increasing reaction time has little effect on the yield [34]. As shown in Figure 4(b), the yield of methyl oleate varies greatly with time. When the time is within 1–3 h, the yield ranges from 81.3% to 95.9%. However, when time is 4 h and 5 h, the yield of methyl oleate did not increase significantly (96.3%–97.6%). Therefore, it is more reasonable to select a reaction time of 3 h for the following optimization.


## 3.3.3. Methanol/Oleic Acid Molar Ratio

As can be seen from Figure4(c), with the increase in a molar ratio of MeOH/OA, the yield of methyl oleate gradually increases. The molar ratio of MeOH/OA has little effect on the yield of methyl oleate. The yield of 73.3% can be obtained at the molar ratio of 5 : 1. The yield of 95.8% is obtained when the molar ratio is 15 : 1. When the highest molar ratio is 25 : 1, the yield reaches 96.6%, almost unchanged. While esterification itself is a type of reversible reaction, more methanol can accelerate the reaction rate before reaching equilibrium. However, a 15 : 1 molar ratio is desirable in view of cost.


## 3.3.4. Catalyst Dosage

In the esterification reaction, a relatively large number of active sites can promote the positive direction of the reaction [18]. As shown in Figure 4(d), When the amount of catalyst was increased from 1% to 3% the weight of OA, methyl oleate yield increased from 84.8 to 95.2%. However, when the amount of catalyst was increased to 4% and 5%, the yield hardly changed. Therefore, a suitable amount of catalyst will accelerate the reaction rate, and the yield will reach a certain level, and increasing the amount of the catalyst has no practical significance for the reaction. Moreover, from the viewpoint of actually controlling the production cost, 3 wt% of the catalyst is the optimal condition.In summary, considering the cost of production, the optimized reaction conditions are as follows: temperature 75°C, time 3 h, methanol/oleic acid molar ratio 15/1, catalyst dosage 3 wt%, and the yield of methyl oleate is 95.7%. As a contrast, the yield of the blank experiment without a catalyst is only 0.4%, while that of the control group with chitosan as a catalyst is 0.9% (Table2). Therefore, it can be concluded that CS-SO3H catalyst has an excellent catalytic effect and is a simple, economical and green catalyst for methyl oleate production.


## 3.4. Expanded Reaction Scale for the Production of Methyl Oleate

High yield could be obtained with fewer substrates, but when the amount of substrates multiplied, the catalytic activity may be affected by mass transfer resistance [35–37]. In order to ensure that the catalyst still has excellent catalytic performance in the presence of a large number of substrates, the large-scale reaction was carried out. Under the optimized conditions of the oleic acid amount of 20 g, the temperature of 75°C, the methanol to oleic acid molar ratio of 15 : 1, and the reaction time of 3 h, a high methyl oleate yield of 94.4% could be obtained. It shows that chitosan sulfonic acid catalyst still has a good catalytic effect in large scale reactions. This provides a basis for the development of new polymer-based high-efficiency acid catalysts.


## 3.5. Hot Filtration and Water Tolerance Tests

Thermal filtration is a simple and easy way to distinguish the types of catalysts. By comparing the yields after thermal filtration, the homogeneous/heterogeneous solid catalysts can be quickly judged [38]. Generally speaking, the heterogeneous behavior of catalysts is measured under the same conditions of comparative experiments. One group filtered the catalyst after 0.5 h and kept the reaction for another 2.5 h, while the other group did not have any treatment under the same conditions (Figure 5(a)). The results show that after removal of the catalyst, the yield remains basically unchanged, that is, the active site of the catalyst is not lost. This proves that CS-SO3H is an excellent heterogeneous biomass-based polymer catalyst.Figure 5
(a) Hot filtration test of CS-SO3H in the esterification of oleic acid; (b) Water tolerance study of the CS-SO3H catalyst. Reaction conditions: temperature 75°C, the CS-SO3H catalyst of 3 wt%, time of 3 h, and methanol/oil molar ratio 15/1.
(a)
(b)Owing to the esterification reaction is reversible, water will be formed in the reaction process. However, water, as a by-product of esterification, will lead to inactivation of the active site of the acid catalyst with poor water resistance, thus reducing the reaction rate, resulting in a lower yield of ester in certain reaction time. This will lead to poor reusability of the catalyst. Thus, the water tolerance of the catalyst is good, which is beneficial to esterification [8]. We added a certain amount of water into the system to determine the water tolerance of the catalyst. At the same time, it can also achieve the same effect for feedstocks which contain a small amount of water.(2)
Oleic acid (A)
+
Methanol (B)
⇋
Methyl oleate (C)
+
Water (D)
.As can be seen from Figure5(b), when the water content is 4 wt% of oil quality, the yield of methyl oleate is very close to 90%. When the water content is 8 wt%, the yield of methyl oleate can still reach 80%. With water content reaching 10 wt%, the yield can be reduced to 72.4%. This indicates that the catalyst has excellent water tolerance.


## 3.6. Reusability of Catalyst

For heterogeneous catalysts, their recyclability can not only reduce the production cost but also maximize environmental protection [14]. Therefore, it is necessary to explore the reusability of catalysts. The reusability of catalysts was determined under the optimal conditions. After each reaction cycle, the catalyst was centrifuged and washed alternately with petroleum ether and ethanol three times to remove the organic matter adhering on the catalyst. After drying overnight at 80°C, the regenerated catalyst was used again for the next reaction.As can be seen from Figure6(a), the active site of sulfonic acid still exists, but the characteristic infrared absorption band moves from 1160 and 1033 cm−1 to 1210 and 1046 cm−1. This indicates that the catalyst is affected by impurities, and it is necessary to remove by repeated use of petroleum ether scrubbing. The absorption band at 808 cm−1 belongs to C-O-S, which indicates that -SO3H is still bonded to chitosan [26–28]. By comparing the XRD peaks of CS-SO3H before and after the reaction (Figure 6(b)), no significant change in the shape of the peaks was observed, indicating that the catalyst was stable. The methyl oleate yield can still reach 85.7% after the catalyst is reused for four times (Figure 6(c)). Generally speaking, the sulfonic acid catalyst prepared by grafting method has good catalytic activity and stability, and the leaching amount of the active site is relatively small [25, 28]. For example, after five cycles of use, the yield of HS/C-SO3H catalyst is still as high as 90% [11]. Moreover, the CS-SO3H catalyst prepared by us has a high acid density (3.81 mmol/g), which makes the catalyst maintain good reusability.Figure 6
The fresh and reused CS-SO3H (a) FT-IR; (b) XRD; (c) reusability of the CS-SO3H catalyst. Reaction conditions: reaction temperature 75°C, the CS-SO3H catalyst of 3 wt%, reaction time of 3 h, and methanol/oil molar ratio 15/1.
(a)
(b)
(c)As can be seen from Table3 , entry 1 and 2 are bio-matrix composite catalysts with good reusability. Relative to entry 3, 4, 5, the reuse efficiency of biomass-based catalysts is poor. This is mainly due to the sulfonation reagent being generally sulfuric acid. Although it has a high acid density, the active sites are easy to leach in the reaction, resulting in a low reuse rate. However, CS-SO3H is a stable biomass-based polymer material because it is directly bonded by chlorosulfonic acid and CS in the form of a covalent bond, rendering the catalyst of strong stability. After four cycles, the yield of CS-SO3H can still reach 85.7%. Compared with entry 1 and 2, the yield of CS-SO3H after reuse is roughly the same, but the synthesis method of CS-SO3H is simpler and milder. This is an advantage for the polymer catalyst. Meanwhile, the reaction temperature of CS-SO3H in esterification reaction is mild, the reaction time is short, and the amount of catalyst needed is less. This may be mainly due to the high acid density which provides more active sites and speeds up the reaction rate (Scheme 1).Scheme 1
Synthesis of the biomass-based acid catalyst CS-SO3H.Table 3
Comparison of the performance of different heterogeneous catalysts in esterification.
NO.
Catalyst
Feedstock
Reaction conditions
Methyl oleate yield (Conversion) (%)
Reuse cycle (Conversion after last run)
Ref.
1
HS/C-SO3H
Oleic acid
T=80°C, CA = 3.6 wt%, M/O = 10 : 1, t = 5 h
96.9
5 (90)
[11]
2
FCHC-SO3H
Oleic acid
T = 80°C, CA = 4 wt%, M/O = 15 : 1, t = 3 h
96.7
5 (84.3)
[18]
3
Bamboo Ph-SO3H
Oleic acid
T=85°C, CA = 12 wt%, E/O = 7:1, t  =  3ha
96.0
5 (28)
[39]
4
Sugarcane bagasse -SO3H
Oleic acid
T = 50°C, CA = 0.1 wt%, M/O = 20:1, t = 24 h
85.1
5 (66.6)
[40]
5
Sulfonated carbon
FFAb
T = 68°C, CA = 6 wt%, M/O = 15:1, t = 3 h
94.4
5 (60)
[41]
6
CS-SO3H
Oleic acid
T = 75°C, CA = 3 wt%, M/O = 15:1, t = 3 h
95.7
4 (85.7)
This work
aThe reaction substrates are ethanol and oleic acid. bFFA denotes free fatty acids.


## 4. Conclusion

In this paper, a simple method was used to prepare biomass-based polymer catalyst CS-SO3H under mild conditions. The results showed that the catalyst had a high acid density (3.81 mmol/g), good catalytic performance in the esterification of OA and MeOH, and could efficiently prepare methyl oleate. Under the optimum reaction conditions, the yield of methyl oleate was up to 95.7%. In addition, the catalyst has good stability and reusability. After four cycles, the yield of methyl oleate can still as high as 85.7%. This indicates that CS-SO3H has good catalytic performance in the pre-esterification and esterification of biodiesel. Moreover, the preparation of catalysts from biomass raw materials not only greatly reduces the cost, but also can be used as a solution for the treatment of biomass waste. Therefore, the preparation of biomass-based polymer materials with high activity is of great significance in the production of biodiesel and has good application prospects in the field of cleaner production.



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*Source: 4041631-2019-12-08.xml*