Carbon-Based Solid Acid Catalyzed Esterification of Soybean Saponin-Acidified Oil with Methanol Vapor for Biodiesel Synthesis

Abstract

In this study, carbon-based solid acids were used to catalyze the esterification of soybean saponin-acidified oil (SSAO) with methanol vapor for the synthesis of biodiesel. The esterification conversion under different conditions was determined, and the catalyst components were determined using acid-base titration, elemental analysis, and inductively coupled plasma spectroscopy. The results showed that the conversion of SSAO under the optimal esterification conditions (i.e., catalyst loading of 6 wt%, methanol/oil molar ratio of 50:1, reaction temperature of 76 °C, and reaction time of 4 h) was 98.9%. The conversion was still higher than 80% after the catalyst was reused for four batches. The methanol vapor esterification (MVE) effectively mitigated the leaching of sulfonic acid groups and the production of sulfonate esters, while the activated white clay adsorption can significantly reduce the metal ion content in SSAO, which weakens its ion exchange with sulfonic acid groups. MVE for biodiesel synthesis is less costly compared to liquid methanol esterification because of the high recovery of methanol and the improved catalyst stability. Therefore, the addition of methanol in the form of vapor in the carbon-based solid acid-catalyzed esterification system is an effective way to maintain the catalyst activity and reduce the production cost of biodiesel.

1. Introduction

As one of the non-petroleum based fuels, because it is considered to be an excellent alternative to fossil fuels, biodiesel has attracted wide attention [1]. Compared to traditional fossil diesel, it has the advantages of better lubricity, a high flash point, a high cetane number, and being renewable [2]. Biodiesel is generally synthesized from animal and vegetable fats and oils through esterification or transesterification reactions [3]. However, in industrial production, the cost of production is a major constraint to the development of biodiesel. Among them, the cost of raw materials accounts for about 60% to 80% of the total cost [4]. Therefore, various cheap waste oils and fats have become an important part of the feedstock for biodiesel [5].

As the world’s largest soybean oil producer, China could produce 17.938 million tons of soybean oil in 2021 [6]. Soybean crude oil in the refining process will produce a large number of by-products, such as soybean saponins, accounting for about 5% to 10% of the total mass of crude oil, whose annual production is more than one million tons [7]. However, it is prone to deterioration and smell and requires a simple acidification pretreatment to make soybean saponin acidified oil (SSAO) before being treated by other methods. Considering that soybean saponin generally consists of 8% to 27% neutral oil, 40% to 60% fatty acid salts, and a small amount of water, SSAO also has the potential to be a feedstock for biodiesel [8]. However, SSAO contains large amounts of free fatty acids (FFA), which must be converted to fatty acid esters (i.e., acid value < 2 mg KOH/g) by an acid-catalyzed esterification reaction, followed by a transesterification reaction (usually alkali-catalyzed) to achieve the utilization of neutral oils and fats [9]. Homogeneous acid catalysts (e.g., concentrated sulfuric acid) with low cost and high efficiency are often used in the esterification reaction process [10]. However, this also brings about inevitable problems such as difficult catalyst separation and recovery, serious equipment corrosion, and waste liquid environmental pollution [11]. In contrast, the use of heterogeneous acid catalysts can effectively avoid these disadvantages [12].

Common heterogeneous acid catalysts include multiphase heteropoly acids [13], metal oxides and complexes [14], and strong acid cation exchange resins [15]. However, the application prospects are also limited to some extent due to the higher cost and complicated synthesis steps [16]. In contrast, sulfonic acid-functionalized carbon-based solid acid catalysts (CSACs) are widely available, have more acidic sites, and can be considered an excellent alternative to concentrated sulfuric acid [17]. It is generally synthesized from hydrocarbons such as glucose, starch, or low-cost waste biomass by in situ sulfonation (i.e., simultaneous carbonization and sulfonation) or stepwise carbonization sulfonation under the action of sulfonating agents such as concentrated sulfuric acid or benzene sulfonic acid [18]. CSACs have a hydrophobic charcoal structural backbone and hydrophilic acidic groups, which allow them to exhibit good catalytic activity in biodiesel production [19].

In typical CSAC-catalyzed esterification reactions of waste fats and alcohols (mostly methanol), excess liquid methanol is often added to speed up the reaction [20]. This not only reduces the utilization of methanol but also complicates the separation step of biodiesel. In contrast, numerous previous studies have shown that prolonged contact of large amounts of liquid methanol with the catalyst accelerates the leaching of acidic groups from CSACs [21]. Meanwhile, for CSACs with more sulfonic acid groups per unit area (such as catalysts synthesized by concentrated sulfuric acid sulfonation or hydrothermal in situ sulfonation), some of the sulfonic acid groups on their surfaces will directly synthesize sulfonate esters with methanol, which also causes a reduction of catalytic activity [22]. In our previous study, CSAC was synthesized to catalyze the esterification of methanol vapor with oleic acid for the synthesis of biodiesel [23]. The mechanism by which methanol vapor enhances the stability of CSAC in the reaction was also investigated while optimizing the reaction conditions. However, neither our previous work nor recent studies have used CSACs to catalyze the synthesis of biodiesel from waste oils with complex compositions using methanol vapor esterification (MVE). The effect of MVE on the stability of CSACs and the deactivation mechanism of CSACs in this process need to be further explored.

In this study, a carbon-based solid acid catalyst was prepared from waste bamboo by in situ sulfonation and used to catalyze the esterification reaction of SSAO and methanol vapor. The reaction conditions for the MVE were optimized using the conversion yield of FFAs in SSAO as an index. Meanwhile, the conversion yields of FFAs and the physicochemical properties of catalysts in different batches of reactions were measured for the reuse process of catalysts, and the main deactivation mechanisms of catalysts were investigated. In addition, biodiesel was also synthesized from pretreated SSAO by MVE, and the effect of the pretreatment method on the catalyst stability was investigated. The results of this study can provide technical support for the process feasibility of CSAC-catalyzed esterification of high acid-value waste oils with methanol vapor for biodiesel synthesis.

2. Materials and Methods

2.1. Materials

The bamboo used in this experiment was agricultural waste from Zhejiang Province, China. The SSAO used was obtained from the Fangchenggang Zhongneng Bioenergy Investment Co., Ltd. in Fangchenggang, Guangxi, China, and its physicochemical properties are shown in Table S1. The methanol, anhydrous ethanol, concentrated sulfuric acid, potassium hydroxide, n-hexane, and other reagents are all pure analytical grades, and they were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd, Beijing, China.

2.2. Catalyst Preparation

The CSAC was synthesized by in situ sulfonation, as mentioned in our previous study [24]. Briefly, the bamboo powder was mixed with concentrated sulfuric acid at 1:10 (w/v) and stirred, followed by sulfonation at 150 °C for 4 h. After the reaction, the mixture in the container was poured into a beaker containing approximately 500 mL of deionized water and stirred with a glass rod. Subsequently, the mixture was vacuum filtered, and the solids were continuously washed with deionized water at 60 °C until the pH of the filtrate was nearly neutral. The solid obtained was placed in an oven and dried to a constant weight at 105 °C to remove residual moisture. The catalyst was named C150-4, and its physicochemical properties have also been fully investigated in previous studies by various characterization methods [24].

2.3. Pretreatment of Soybean Saponin-Acidified Oil

The pretreatment methods of SSAO include the water washing method and the activated white clay adsorption method. The water washing method was performed as follows: deionized water and SSAO were mixed in a beaker at 2:1 (w/w), stirred for 0.5 h, and then the water was separated from the oil by centrifugation; the washed SSAO was subsequently dried in an oven at 105 °C to a constant weight. For the activated white clay adsorption method, a certain mass of SSAO and 10 wt% activated white clay were placed in a single flask and stirred magnetically in a water bath at 80 °C for 0.5 h. Finally, the pretreated SSAO was separated by centrifugation for the esterification reaction.

2.4. Esterification Experiments

The methanol vapor esterification reaction unit consists of three parts: methanol heating, esterification, and methanol collection (Figure S1). First, a single-neck flask containing liquid methanol was heated at a certain temperature to produce methanol vapor; the vapor passed into the acidified oil contained in the three-neck flask through a tube consisting of a combination of a glass tube, a silicone tube, and a slender stainless steel tube. The three-neck flask was used as the esterification reaction vessel, and the esterification reaction temperature was controlled by heating the mixture in a water bath after adding the acidified oil and catalyst. The excess methanol vapor involved in the reaction overflowed from the other port of the flask and was collected as liquid methanol in another single-port flask using a condensing device. Based on our previous studies on the esterification of oleic acid with methanol vapor, a range of esterification reaction conditions was determined for this study [23]. The conditions for the esterification reaction were the following: a catalyst loading of 2 to 8 wt% (based on the SSAO mass), a methanol-oil molar ratio of 30:1 to 70:1, a reaction temperature of 72 to 80 °C, and a reaction time of 2 to 5 h. The products were titrated with a KOH-ethanol solution, and the acid value was calculated using Equation (1) [25]:

$$\mathrm{Acid}\mathrm{value}=\mathrm{V}\cdot \mathrm{c}\cdot 56.11/\mathrm{m},$$

(1)

where V is the volume of the KOH solution, c is the concentration of the KOH solution, 56.11 g/mol is the molar mass of KOH, and m is the mass of the sample.

The esterification conversion yield was calculated from the acid value of the product after the reaction between oleic acid and methanol, as shown in Equation (2):

$$\mathrm{Conversion}\mathrm{yield}(\%)=\frac{\mathrm{A}{\mathrm{V}}_{\mathrm{i}}-\mathrm{A}{\mathrm{V}}_{\mathrm{f}}}{\mathrm{A}{\mathrm{V}}_{\mathrm{i}}}\times 100\%,$$

(2)

where AV_i and AV_f are the acid values of the initial and final reactants, respectively. The calculations were carried out in triplicate, and average values were obtained.

2.5. Reuse of Catalysts

After each esterification reaction, the catalysts were separated from the liquid products. Subsequently, they were washed with n-hexane and ethanol, filtered, and dried at 105 °C for 12 h to remove any residual moisture. The washing method helps to remove SSAO and biodiesel remaining on the surface of the catalyst, which eliminates the effect of the residue on the reusability of the catalysts [26]. The dried catalysts were reused in esterification, and the steps after the reaction are described above.

2.6. Analytical Methods

The sulfur content of the catalyst was determined using an organic elemental analyzer (Elementar Vario EL). The metal elements contained in the SSAO and catalysts were analyzed using inductively coupled plasma-optical emission spectroscopy. The sulfonic acid density and total acid density of the catalysts were measured using the acid-base titration method [27]. Approximately 0.5 g of the catalyst was mixed with 50 mL of a NaCl solution (2 M), after which the mixture was stirred for 6 h at room temperature. After filtration, 20 mL of filtrate aliquots with a phenolphthalein solution were titrated with NaOH solution (0.02 M) to a neutral point.

The production cost per unit mass of biodiesel synthesized from SSAO before and after pretreatment was calculated [23]. The cost of SSAO was set at USD 0.766 kg, C150-4 at USD 1.20 kg, methanol at USD 0.38 kg, and the unit energy cost at USD 0.091 kw·h. The energy consumption during the heating and temperature maintenance of methanol and SSAO was calculated using Equations (3)–(6):

$${\mathrm{Q}}_{\mathrm{h}}=\mathrm{M}{\mathrm{C}}_{\mathrm{p}}(\mathrm{t}-{\mathrm{t}}_0),$$

(3)

$$\mathrm{A}=\mathrm{D}\left(\mathrm{R}+\mathrm{H}\right),$$

(4)

$${\mathrm{Q}}_{\mathrm{m}}=\frac{\mathsf{\lambda }\mathrm{A}\mathrm{T}\left(\mathrm{t}-{\mathrm{t}}_0\right)}{\mathsf{\delta }},$$

(5)

$${\mathrm{Q}}_{\mathrm{t}}={\mathrm{Q}}_{\mathrm{h}}+{\mathrm{Q}}_{\mathrm{m}},$$

(6)

where Q_h (kJ) is the energy required to raise the feedstock (i.e., methanol or SSAO) to the target temperature and M (kg) is the mass of the feedstock. In this study, it was assumed that the mass of SSAO was 1 t, and the mass of methanol was calculated using the optimal methanol/oil molar ratio. C_p (kJ/(kg·°C)) is the specific heat capacity of the feedstock, which is 2.02 and 2.51 kJ/(kg·°C) for SSAO and methanol, respectively; t (°C) is the reactor heating temperature; and t₀ (°C) is the average environmental temperature, which was assumed to be 20 °C. The reactors were assumed to be cylindrical, and A (m²), D (m), R (m), and H (m) were the total heated area, the circumference of the outer wall, the inner diameter, and the height of the reactor, respectively. Taking into account the different reaction volumes required for the two esterification methods, the design parameters of the reactors in this study are shown in Table S2. Q_m (kJ) is the energy required to maintain the reactor temperature, and T (s) is the reaction time. λ (kW/(m·°C)) is the heat conductivity, and δ (m) is the thickness of the reactor wall. It was assumed that the reactor material in this study is 304 stainless steel with a λ of 16.3 kW/(m·°C), while δ is assumed to be 0.1 m. Q_t (kJ) is the total energy required for esterification.

3. Results and Discussions

3.1. Optimization of Esterification Conditions of SSAO with Methanol Vapor

In our previous study, the factors influencing the methanol vapor rate were explored, and the esterification reaction of oleic acid with the methanol vapor was catalyzed using C150-4 [23]. On this basis, biodiesel was synthesized by methanol vapor esterification (MVE) using soybean saponin acidified oil (SSAO), which has more complex components, as a feedstock.

To explore the optimal catalytic performance of C150-4 in the MVE, the esterification reaction conditions were optimized. Catalyst loading is an important factor affecting the cost of biodiesel. The effect of catalyst loading on conversion was investigated at an esterification reaction temperature of 76 °C, a methanol/oil molar ratio of 50:1, and a reaction time of 4 h, as shown in Figure 1a. The results showed that the conversion of SSAO could reach 96.4% with a catalyst loading of 2 wt%, indicating the high catalytic activity of C150-4. The conversion was also further increased to 98.9% as the catalyst dosage was increased to 6 wt%. It is due to the addition of more active groups to accelerate the reaction process. However, when the addition was increased to 8 wt%, the conversion of SSAO was slightly reduced. It implies that the active groups provided by the catalyst at 6 wt% can result in essentially complete esterification of FFAs under these conditions. In addition, the excessive addition of C150-4 as a heterogeneous catalyst may reduce the mass transfer efficiency of the esterification reaction system [28].

The reaction temperature is also an important parameter that affects the esterification reaction process [29]. The effect of temperature on SSAO conversion was investigated at a catalyst loading of 6 wt% and a methanol/oil molar ratio of 50:1 for 4 h. Figure 1b shows that the conversion increased significantly when the esterification temperature was increased from 72 °C to 76 °C, from 95.7% to 98.9%. However, as the temperature continued to increase, the conversion showed a decreasing trend. This is because at lower temperatures, on the one hand, it tends to cause the conversion of methanol vapor to the liquid state, which reduces the inflow rate of gas; on the other hand, it reduces the rate of molecular motion and leads to a lower conversion [30]. Additionally, because the esterification reaction is an exothermic reaction, excessively high temperatures are also not conducive to the positive progression of the reaction [31].

In conventional liquid methanol esterification (LME) reactions, excess methanol is often added to speed up the reaction rate [32]. In contrast, more methanol is required for MVE, which mainly stems from the faster rate of flow provided by the pressure when methanol is heated to convert to vapor. However, according to our previous study, the recovery of methanol also increases with the rate of methanol vapor flow, so the actual methanol dosage is much lower than the vapor inflow. In this regard, the effect of the methanol/oil molar ratio on conversion was investigated by controlling the methanol vapor flow rate of 0.5 g/min (recovery of approximately 87%), the catalyst loading of 6 wt%, the reaction temperature of 76 °C, and the reaction time of 4 h, as shown in Figure 1c. The results showed that the conversion increased from 95.6% to 98.0% when the methanol/oil molar ratio changed from 30:1 to 50:1. However, when the methanol/oil molar ratio increased further to 70:1, the conversion also decreased slowly. This may be because when the methanol vapor was excessive relative to SSAO, the volume concentration of SSAO and the catalyst in the esterification reaction system was diluted, which reduced the collision frequency between the reactants and the active groups of the catalyst [33].

In comparison to the liquid esterification reaction, MVE has a greater mass transfer resistance as a three-phase reaction system. Therefore, MVE may require a longer time to ensure sufficient contact between methanol vapor, oil, and catalyst for the reaction to proceed completely. The effect of reaction time on conversion was investigated at a catalyst loading of 6 wt%, a reaction temperature of 76 °C, and a methanol/oil molar ratio of 50:1. As shown in Figure 1d, the conversion was 95.3% after 2 h of the reaction. When the time was further extended, the conversion showed a trend of increasing and then decreasing and reached the highest (98.0%) at 4 h. These results show that MVE can achieve efficient conversion of FFAs in a shorter period of time. However, considering that in industrial production, the fatty acid content in SSAO after the esterification reaction must meet the demand for alkali-catalyzed transesterification, 4 h was chosen as the optimal reaction time.

3.2. Catalyst Stability and Deactivation Mechanisms in Methanol Vapor Esterification

The reusability of heterogeneous catalysts is one of their main advantages compared to that of homogeneous catalysts [34]. It is also an important factor that affects the production cost of biodiesel in industrial production. In response, the reusability performance of C150-4 was investigated under optimal conditions for the MVE. As shown in Figure 2, the conversion of SSAO decreased slightly from 98.2% to 95.3% in the reactions of batches 1 to 3. However, the activity of C150-4 decreased significantly in the fourth reaction cycle until it was almost completely inactivated in the fifth reaction batch. The esterification of liquid methanol with SSAO was catalyzed using C150-4 at a methanol/oil molar ratio of 10:1, and the other reaction conditions were the same, also lasting for five cycles (Figure 2). The results showed that the activity of C150-4 was high in the first two cycles, with conversions exceeding 90%. However, the conversion dropped to 60.5% in the third batch of repeated esterification and finally decreased to 36.3% in the fifth cycle. This indicates that C150-4 can only be reused three times in the liquid phase esterification system, showing the same trend as previous studies [24]. In contrast, MVE is more beneficial for extending the catalyst’s lifetime. This is mainly attributed to the fact that MVE enables the catalyst to contact a large amount of methanol in a short time, rather than being in an overall system of excess methanol [35].

To investigate the mechanism of catalyst deactivation in the MVE, the sulfonic acid groups and the sulfur content of 3 g of C150-4 in several esterification cycles were measured, as shown in

Table 1. Changes in the sulfonic acid and sulfur contents in different esterification reaction methods.

The Batch of Esterification Reaction		Sulfonic Acid Density (mmol/g)	Sulfur Density (mmol/g)	Catalyst Mass (g)	Total Sulfonic Acid (mmol)	Total Sulfur (mmol)
Fresh C150-4 catalyst	-	1.476	1.146	3.00	4.428	3.438
Liquid methanol esterification	3rd batch	0.167	0.706	3.11	0.519	2.198
	4th batch	0.143	0.681	3.14	0.449	2.142
Methanol vapor esterification	3rd batch	0.481	0.699	3.03	1.458	2.119
	4th batch	0.324	0.671	3.06	0.990	2.053
	5th batch	0.214	0.630	3.08	0.660	1.941
Methanol vapor esterification (SSAO with adsorption treatment)	6th batch	0.377	0.704	3.02	1.140	2.127

. The sulfonic acid density and sulfur density on the surface of CSAC decreased continuously with increasing catalyst reuse. The sulfonic acid density decreased from 1.476 mmol/g to 0.481 mmol/g, and the total amount of sulfonic acid groups decreased by 2.970 mmol after reuse three times. After four cycles, the sulfonic acid density decreased to 0.324 mmol/g and the total sulfonic acid continued to decrease by 0.468 mmol, while after five cycles (47.1% esterification conversion), the catalyst was considered deactivated, at which point the sulfonic acid density was 0.214 mmol/g and the total sulfonic acid decreased by 0.330 mmol compared to the previous batch reaction. After five repeated esterifications, the sulfonic acid group on the surface of C150-4 was reduced by a total of 3.768 mmol, accounting for 85.09% of the initial amount. In contrast, in the LME, the total amount of sulfonic acid was reduced to 0.449 mmol after four catalytic cycles, a reduction of 89.86% of the total amount. It can be demonstrated that the inflow of methanol into the reaction system in the form of vapor can alleviate the loss of sulfonic acid groups from the CSACs, thus delaying the deactivation of the catalyst.

Our previous study demonstrated that the deactivation of C150-4 during catalyzing the esterification reaction of SSAO with liquid methanol originated from the loss of sulfonic acid groups on the catalyst surface, manifested by the leaching and chemical derivatization of sulfonic acid [24]. Poorly grafted sulfonic acid groups on the catalyst will leach out into excess methanol as aromatic fragments during the esterification process [36]. The remaining sulfonic acid groups will also be esterified with methanol to form sulfonate esters or deactivated by ion exchange with the metal ions in SSAO [37]. To investigate the contribution of different causes of loss of sulfonic acid groups to catalyst deactivation, the variation of sulfonic acid amounts in the solid and liquid phases in different batches of C150-4-catalyzed MVE was further quantified in this study, as shown in Figure 3. Residual sulfuric acid leaching, sulfonic acid groups leaching, and the amount of chemically derived sulfonic acid were calculated from Equations (7)–(10) [38].

$$\mathrm{S}{\mathrm{O}}_3{\mathrm{H}}_{\mathrm{Dec}}=\mathrm{S}{\mathrm{O}}_3{\mathrm{H}}_{\mathrm{Tot}}-\mathrm{S}{\mathrm{O}}_3{\mathrm{H}}_{\mathrm{Cat}},$$

(7)

$${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4=2\left(\mathrm{S}{\mathrm{O}}_3{\mathrm{H}}_{\mathrm{Tot}}-{\mathrm{S}}_{\mathrm{Tot}}\right),$$

(8)

$$\mathrm{S}{\mathrm{O}}_3{\mathrm{H}}_{\mathrm{Lea}\mathrm{n}}=\mathrm{S}{\mathrm{O}}_3{\mathrm{H}}_{\mathrm{L}\mathrm{e}\mathrm{a}(\mathrm{n}-1)}+{\mathrm{S}}_{\mathrm{B}\mathrm{e}}-{\mathrm{S}}_{\mathrm{A}\mathrm{f}},$$

(9)

$$\mathrm{S}{\mathrm{O}}_3{\mathrm{H}}_{\mathrm{D}\mathrm{e}\mathrm{r}}=\mathrm{S}{\mathrm{O}}_3{\mathrm{H}}_{\mathrm{Dec}}-\mathrm{S}{\mathrm{O}}_3{\mathrm{H}}_{\mathrm{Lea}}-{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$$

(10)

where SO₃H_Tot is the total sulfonic acid in the catalyst, SO₃H_Cat is the remaining sulfonic acid in the catalyst, SO₃H_Dec is the reduction of sulfonic acid, H₂SO₄ is the amount of sulfuric acid remaining in the catalyst (assuming that all sulfuric acid is leached after the first esterification reaction), S_Tot is the total sulfur content of the catalyst, SO₃H_{Lea n} is the cumulative amount of sulfonic acid leached in the esterification reactions, S_Be and S_Af are the sulfur content of the catalyst before and after the current esterification reaction, respectively, and SO₃H_Der is the cumulative amount of chemically derived sulfonic acid during esterification.

The results showed that the leaching of residual sulfuric acid during catalyst reuse was 1.980 mmol, representing 44.7% of the total amount. The same phenomenon was observed in previous studies, which may be caused by the difficulty in achieving complete removal of partial sulfuric acid from the pores of C150-4 during the extraction and washing process [39]. After the third batch reaction, the leaching of sulfonic acid from the catalyst was 0.329 mmol and the loss of sulfonic acid due to chemical derivatization was 0.661 mmol, which represented 14.2% and 28.6% of the total loss (2.309 mmol), respectively. In contrast, the total loss of sulfonic acid groups reached 3.438 mmol after four catalyst reuses, of which the leached amount was 0.395 mmol, while the chemically derived amount increased significantly to 1.063 mmol. At the end of the fifth batch reaction, the amount of leached and chemically derived sulfonic acid increased further to 0.507 and 1.281 mmol, respectively, representing 13.5% and 34.0% of the total loss. This means that the chemical derivatization of the sulfonic acid group is the main cause of its loss in the reactions of batches 3 to 5, and this leads to a significant decrease in the conversion of SSAO in batches 4 and 5 (Figure 2).

To investigate the main pathways by which sulfonic acid groups are chemically derived, the metal ion content in SSAO and C150-4 used in the MVE was determined by ICP and converted to H⁺ equivalents (i.e., the amount of H in the sulfonic acid groups that can be substituted by metal cations), as shown in

Table 2. Changes in metal ion mass equivalents in raw oil (30 g) and reused catalysts (3 g) after esterification.

Metal Ions	SSAO					SSAO with Absorption Treatment
	Raw Oil	Fresh Catalyst	Catalyst Used 3 Batches	Catalyst Used 4 Batches	Catalyst Used 5 Batches	Raw Oil	Catalyst Used 6 Batches
Na+	19.86	1.53	1.71	1.83	1.79	11.39	1.81
Ca2+	6.97	0.58	0.65	0.68	0.73	5.36	0.82
Al3+	7.41	0.34	0.43	0.50	0.50	1.96	0.55
Mg2+	5.88	0.46	0.60	0.66	0.73	2.55	0.46
K+	0.39	0.025	0.030	0.031	0.032	0.155	0.030
Zn2+	0.26	0.017	0.020	0.022	0.025	0.079	0.039
Mn2+	0.0037	0.0020	0.0023	0.0021	0.0033	0.0022	0.0026
Ba2+	0.10	0.0063	0.0072	0.0087	0.0092	0.023	0.0010
Sum	40.87	2.96	3.45	3.74	3.82	21.52	3.72

. The results showed that the metal ion content of the catalyst increased by 0.49 mmol H⁺ equivalent after three cycles compared to fresh C150-4. Assuming that all added metal cations underwent ion exchange reactions with the H⁺ of the sulfonic acid group, the resulting loss of sulfonic acid accounted for 74.13% of total chemical derivatization (0.661 mmol). Therefore, the amount of sulfonic acid groups converted to sulfonate esters by reaction with methanol was approximately 0.171 mmol, which is significantly less than the number of sulfonate esters formed during the LME in our previous study [24]. This may be attributed to the inflow of vapor, which reduces the residual methanol in the esterification system. In contrast, the metal ion content of the catalyst increased by 0.29 and 0.08 mmol H⁺ equivalent after four and five cycles, respectively, accounting for 84.79% and 96.38% of the chemical derivatives of the current batch. It further indicates that the loss of the sulfonic acid group with increasing reuse batches is mainly due to its chemical derivatization with the metal ions in SSAO.

3.3. Effect of SSAO Pretreatment on Catalyst Activity

To reduce the effect of metal ions on the catalyst’s reusability performance, SSAO was pretreated with water washing, activated white clay adsorption, and a combination of water washing and activated white clay adsorption, respectively. The acid and saponification values of the pretreated SSAO are shown in Table S3. The reduction in acid value of pretreated SSAO was within 5 mg KOH/g, indicating that the three pretreatment methods caused little loss of FFAs. The esterification reaction of pretreated SSAO with methanol vapor was catalyzed using C150-4 under optimal conditions, and several cycles were performed. Figure 4 shows that C150-4 has the best catalytic stability in catalyzing SSAO treated by adsorption of activated white clay. A conversion of 98.2% was achieved in the first reaction and remained at 95.6% in the fourth batch. However, the catalyst activity decreased significantly with a further increase in the number of reactions. The conversion was reduced to 83.6% in batch five, compared to 73.6% in batch 6.

In contrast, the conversion of pretreated SSAO with water washing was 98.1% in the initial esterification reaction. However, it decreased to 92.3% and 76.8% in the third and fourth batch reuse, respectively, both less than its conversion when catalyzing the unpretreated SSAO. Similarly, for the combined washing-adsorption pretreatment of SSAO, the reusability performance of C150-4 in catalyzing its esterification reaction was worse than that of the adsorption-only SSAO. The above results indicate that activated white clay adsorption treatment is beneficial in improving the catalytic activity of C150-4 in esterification, while the water washing treatment of SSAO, on the contrary, negatively affects the stability of the CSAC. This may be because a certain amount of water may remain in the washed SSAO after centrifugation and drying. The moisture would also combine with the acidic groups on the catalyst in the esterification reaction and accelerate their leaching [40].

To investigate the effect of metal ions on the catalyst reuse performance, the metal content in the pretreated SSAO as well as in C150-4 after six batch cycles was determined by ICP (Table 2). The results showed that the total content of metal cations in SSAO decreased from 40.87 to 21.52 mmol H⁺ equivalent after the adsorption of activated white clay, with a reduction of nearly 50%, among which the content of Na⁺, Ca²⁺, Al³⁺, and Mg²⁺ decreased significantly. Meanwhile, the total sulfonic acid and sulfur contents of C150-4 were measured to be 1.140 mmol and 2.127 mmol, respectively, after six batches of reuse (Table 1). As a result, the sulfonic acid was reduced by 3.288 mmol and the sulfur content was reduced by 1.311 mmol relative to the fresh catalyst. Table 2 shows that the metal ion content of C150-4 after the sixth batch reaction is 3.72 mmol, which is 0.76 mmol more than that of fresh C150-4 and accounts for 23.1% of the total loss of sulfonic acid. Moreover, this is lower than its metal ion content after four replicate batches when catalyzing the esterification of unpretreated SSAO. This indicates that the adsorption pretreatment could alleviate the deactivation of the catalyst by reducing the amount of metal ions in SSAO.

3.4. Comparison of Methanol Vapor Esterification with Other Liquid Methanol Esterification Studies

Table 3. Carbon-based solid acid synthesis by in situ sulfonation of concentrated sulfuric acid for esterification reaction.

Catalyst Raw Materials	Feedstock Oil	Catalyst Loading (wt%)	Methanol/Oil Molar Ratio	Reaction Temperature (°C)	Reaction Time (h)	Conversion (%)	Number of Reuses	Ref.
Bagasse	Oleic acid	10	20	45	24	85.1	4	[41]
Cacao shell	Oleic acid	7	7	65	24	77	4	[16]
Cellulose	Rapeseed oil fatty acids	20	6	65	6	87.5	4	[42]
Peanut shell	Oleic acid	4	10	85	3	98	6	[43]
Coconut coir husk	Waste palm oil	10	12	130	3	89.8	4	[44]
Bamboo	Soybean Saponin acidified oil	6	50	76	4	98.2	4	This study

summarizes the catalytic performance of carbon-based solid-acid catalysts also synthesized by in situ sulfonation using concentrated sulfuric acid and compared with the present study. It can be seen that the catalytic efficiency of the catalysts is related to the properties of the feedstock and the esterification conditions. Compared with other esterification conditions, the catalyst loading, reaction temperature, and reaction time required for methanol vapor esterification in this study were close to or below average. Although the methanol/oil molar ratio reached 50:1, the total consumption was also significantly lower than in other studies, considering that the recovery of methanol was close to 87%. Under this condition, the conversion reached 98.2%, showing the highest catalytic activity. As for the catalyst stability, C150-4 can be reused for four cycles, which is the same as most of the studies but still has much potential for improvement.

3.5. Costing of Biodiesel Synthesis from Soybean Saponin-Acidified Oil

In industrial production, the cost of biodiesel production is one of the most important factors to consider. Compared to C150-4-catalyzed esterification of liquid methanol with SSAO, MVE has a higher biodiesel yield and extends the catalyst lifetime; however, it also has a higher energy cost due to the need for an additional methanol heating reactor. The esterification using adsorption-pretreated SSAO can also further improve the reusability of the catalyst, but the cost incurred by the adsorbent cannot be ignored. Therefore, to objectively evaluate the cost of C150-4-catalyzed esterification for biodiesel synthesis in the three modalities, it was assumed that the reaction was repeated for six batches (i.e., the maximum number of repetitions of C150-4-catalyzed pretreatment of SSAO in MVE) with 1000 kg of SSAO as feedstock under the optimal conditions of each of the three. The average mass of the synthesized biodiesel in each batch reaction was calculated from the conversion of esterification. Furthermore, a life cycle evaluation of the entire energy production chain is beyond the scope of this study.

Based on the results of this study, the production cost of biodiesel synthesized in the pilot plant by simulating the three methods is shown in

Table 4. The production cost of C150-4-catalyzed SSAO for the synthesis of biodiesel by esterification.

Esterification Method	LME	MVE	MVE (SSAO with Adsorption Treatment)
SSAO mass (kg)	1000	1000	1000
SSAO costs (USD)	766	766	766
Adsorbent costs (USD)	-	-	23
Catalyst costs (USD)	40	30	20
Methanol inflow mass (kg)	1.25 × 103	5.71 × 103	5.71 × 103
Methanol recovery mass (kg)	-	5.18 × 103	5.18 × 103
Methanol costs (USD)	475	201.4	201.4
Methanol heating energy consumption (kJ)	-	8.03 × 105	8.03 × 105
Esterification heating energy consumption (kJ)	8.06 × 105	6.59 × 105	6.59 × 105
Adsorption treatment energy (kJ)	-	-	1.77 × 105
Energy costs (USD)	20.4	37.0	40.4
The average mass of biodiesel produced per batch (kg)	703.8	684.1	672.3
The average production cost of biodiesel (USD/kg)	1.85	1.51	1.56

. When catalyzing unprepared SSAO for LME and MVE, the catalytic efficiency of C150-4 decreased significantly after three and four cycles, respectively, and can be considered a failure. Therefore, their catalyst costs in each batch were 2 and 1.5 times higher than those of pretreated SSAO for MVE, respectively. Meanwhile, although the methanol/oil molar ratio of MVE is much higher than that of LME, it requires less methanol because the recovery of methanol vapor is close to 90%. However, the generation of methanol vapor requires additional energy consumption, which also significantly increases the energy cost of MVE. In this regard, the energy cost per batch reaction for LME and MVE with SSAO as feedstock and MVE with pretreated SSAO as feedstock were USD 20.4, 37.0, and 40.4, respectively. The final cost of LME for biodiesel production was calculated to be USD 1.85 kg, while that of MVE was USD 1.51 kg, which was 18.4% lower than the former. Additionally, the cost of biodiesel synthesis by MVE of adsorbed pretreated SSAO is USD 1.56 kg, which is slightly higher than that of the MVE process of ordinary SSAO.

4. Conclusions

In this study, a carbon-based solid acid catalyst, C150-4, was synthesized by in situ sulfonation of waste bamboo for the catalytic esterification of SSAO with methanol vapor for the synthesis of biodiesel. The optimal esterification conditions obtained were a catalyst loading of 6 wt% of SSAO mass, a reaction temperature of 76 °C, a methanol/oil molar ratio of 50:1, and a reaction time of 4 h. Under these conditions, the conversion of SSAO catalyzed by C150-4 could reach 98.9% and remained higher than 80% at reuse up to the fourth batch reaction. Compared to LME under the same reaction conditions, MVE can extend the lifetime of the catalysts by mitigating the loss of sulfonic acid groups from C150-4. The chemical derivatization of sulfonic acid groups, including the formation of sulfonate esters and ion exchange with metal ions, is the main way sulfonic acid is lost during the reuse of catalysts. Adsorption treatment of SSAO with activated white clay can effectively reduce the content of metal ions within it. This method weakens the degree of reaction between the sulfonic acid group on the surface of C150-4 and metal ions, which can further enhance the stability of the catalyst. Furthermore, the esterification of SSAO with methanol vapor significantly reduced the consumption of methanol, and the costs of biodiesel synthesis were USD 1.51 kg, which was 18.4% lower than that of LME. Therefore, methanol vapor esterification provides a greater possibility for the synthesis of biodiesel from high acid-value waste oils.