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. 2020 Sep 11;5(37):23542–23548. doi: 10.1021/acsomega.0c01660

Synergizing Sulfonated Hydrothermal Carbon and Microwave Irradiation for Intensified Esterification Reaction

Laddawan Tumkot †, Armando T Quitain ‡,§,*, Panatpong Boonnoun ∥, Navadol Laosiripojana ⊥, Tetsuya Kida ‡, Artiwan Shotipruk †
PMCID: PMC7512435  PMID: 32984673

Abstract

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The synergy of sulfonated hydrothermal carbon and microwave (MW) irradiation was applied for the esterification of oleic acid with methanol (MeOH) to produce biodiesel. The effects of temperature, reaction time, ratio of oleic acid to methanol, and catalyst loading were investigated at a fixed MW power of 400 W. The addition of hexane, serving as a co-solvent and separator, was also investigated. The optimum conditions for the proposed process were oleic acid-to-methanol molar ratio of 1:5 and hexane-to-methanol ratio of 0.5 (v/v) in the presence of a 5 wt % catalyst, at 100 °C for 60 min, obtaining a 97% yield of oleic acid methyl ester. The addition of slight amounts of hexane resulted into an eightfold reduction in the amount of MeOH needed to obtain a yield above 90%, which normally required a MeOH-to-oil ratio of 40:1. This proposed novel approach could provide a more cost-effective method for the esterification of oil to produce biodiesel, that is, reactive separation utilizing carbon-based catalysts under MW irradiation.

1. Introduction

Biodiesel fuel (BDF) is a promising alternative fuel which is environmentally friendly because of its biodegradability, low carbon monoxide emission, low unburned hydrocarbons, free sulfur, and nontoxic characteristics.1,2 BDF, also known as fatty acid (FA) methyl ester (FAME), can be produced by either the transesterification of triglycerides or esterification of FAs with alcohol, usually methanol, because of its high reactivity. In the esterification of free FA present in oil, it has been widely reported that acid catalysts could effectively catalyze the reaction. Unfortunately, the use of homogeneous acids (e.g., H2SO4, HCl, etc.) has several drawbacks because it would require tedious washing procedures in purifying the products. Following the washing steps is the treatment of the resulting wastewater, which would add up to the total production cost.3 In addition, homogeneous catalysts cannot be regenerated and can cause equipment to corrode.4 As an alternative, the use of heterogeneous catalysts is being considered to avoid the abovementioned problems associated with the use of homogeneous catalysts. The comparison of the activities of various solid catalysts has been reported by Sani et al.5 Furthermore, acid functionalization of the surface of the catalysts could further enhance the activity.6

Heterogeneous carbon-based catalysts have been gaining attention recently to replace the conventional ones,7−11 with the advantages of low-cost carbon sources, simple preparation, high thermal stability, and high acid density. The conventional preparation process for synthesizing carbon-based catalysts involves dehydration and carbonization, which can be achieved by incomplete carbonization/pyrolysis. However, this conventional approach would require relatively higher temperatures (400–800 °C). As an alternative, a greener and milder hydrothermal carbonization process has been observed during sugar dehydration in hot compressed water (150–250 °C). Titirici et al. then synthesized carbonaceous materials using water at lower operating temperatures (150–250 °C) under a self-generated pressure.12 In this method, water originally present in the sample can be utilized, thus avoiding the costly drying step of the raw materials.12,13 Moreover, the hydrothermal carbon (HTC) can be functionalized with specific functional groups suitable for the reaction. One example is by attaching acid functional groups to the HTC using concentrated sulfuric acid, resulting in what is called nowadays as sulfonated HTC. The use of this type of catalyst in synergy with microwave (MW) is expected to give a higher yield of the target products.

Sulfonated HTC, being carbon-based and having a sulfonic group (−SO3H) attached on the surface, has an excellent MW absorptivity. If a material has high MW absorptivity, “hot spots” could form on the surface upon MW irradiation because of localized superheating.14 The “hot spot” temperature on the surface of the catalyst may be higher than the bulk temperature of the reaction medium. On the contrary, the conventional conductive heating method limits the heat transfer onto the surface of the solid catalyst. The esterification reaction is considered an equilibrium reaction and is normally a slow process; thus, intensification methods are necessary to accelerate the reaction. The use of MW as one of the possible approaches has been proposed by many researchers to overcome these limitations and to enhance reaction rates.15−18 Moreover, the use of MW could also entail less energy requirement for the reaction because it can selectively and directly heat materials by the realignment of polar molecules and ions with the electromagnetic waves upon irradiation.14 The realignment of molecules with MW would also give the right molecular orientation for the reaction to proceed. This has been demonstrated in our recent studies on the synergy between MW and graphene oxide catalysis.19−23

Furthermore, the addition of MW-transparent solvents like hexane will further drive higher yields as the product could be immediately separated from the reaction mixtures upon its formation. The nonpolar characteristic of hexane is capable of separating FAME, also a nonpolar compound. Besides, being nonpolar, it is MW-transparent and would not add up to the energy requirement for the reaction. If FAME could be separated from the reaction zone the moment it is produced, the yield is expected to be higher because of the forward shift in equilibrium. Other than the role of separating the product, hexane can also act as a co-solvent that could enhance the mutual solubility of oil and alcohol during the reaction.24,25

This research focuses on the application of the synergy of sulfonated HTC and MW irradiation to intensify the catalytic esterification of FA, taking oleic acid as a representative starting material. The operating conditions of temperature, reaction time, molar ratio of oil to methanol, and catalyst loading were investigated. In addition, the role of hexane as a co-solvent and extractor of the products from the reaction mixture was also investigated. Lastly, the stability and reusability of the spent catalyst were evaluated. To the best of our knowledge, the application of the synergy of sulfonated HTC and MW irradiation in the presence of hexane as a co-solvent and separator has not been investigated for the esterification of oleic acid to methyl oleate. This will lead to a new concept of integrated MW-assisted carbon-based catalyzed reactive separation approach for BDF production.

2. Materials and Methods

2.1. Materials and Chemicals

The carbon precursor, d-glucose, and sulfuric acid (laboratory grade, 98%) were purchased from Fluka, Singapore. Methanol (laboratory grade, 99% purity) was purchased from Fisher Scientific, UK. Methyl oleate, n-hexane, and 2,6-dimethylnaphthalene (DMN) were purchased from Sigma-Aldrich (USA). n-Hexane is of high-performance liquid chromatography-grade with 99+% purity. All reagents used in this study were properly stored to avoid the absorption of moisture from the atmosphere.

2.2. Preparation and Characterization of Sulfonated HTC-Based Catalysts

2.2.1. HTC Preparation

The hydrothermal carbonization process was performed in a 600 mL high-pressure reactor (Parr, USA) filled with 30 g of glucose and 300 mL of deionized water, following the procedure previously reported by Wataniyakul et al..26 The operating temperature was 220 °C for 6 h under nitrogen atmosphere. The resulting HTC was washed with distilled water until no change in pH was observed in the filtrate and then oven-dried at 110 °C overnight.

2.2.2. Acid-Functionalized HTC Preparation

Sulfonic acid groups were attached onto the surface of HTC by heating 10 g of carbon in 100 mL of concentrated sulfuric acid in a three-neck round-bottom flask at 150 °C for 15 h, under nitrogen flow. A 1 L flask containing activated carbon was connected to the reaction flask to adsorb the acid vapor. The nitrogen inlet flow was switched off when the reaction reached the desired reaction time. After cooling, 1 L of distilled water was added to the mixture and filtered. The black solid was then repeatedly washed with hot distilled water until no sulfate ions were detected. The catalysts were then dried in an oven at 110 °C overnight.

2.3. BDF Production and Characterization of FAME

2.3.1. Esterification Reaction under MW Irradiation

To investigate the applicability of the proposed approach, the esterification of oleic acid and methanol was used as a model reaction. The reaction was carried out using MW irradiation (MARS 6, CEM) under the following conditions: reaction temperature of 50–100 °C, reaction time of 30 and 60 min, 1:1 to 1:40 molar ratio of oil to methanol, and catalyst loading of 1–10 wt %. The temperature in the control vessel was monitored by a fiber-optic thermocouple and that of the other vessels by an infrared (IR) sensor. First, oleic acid, methanol, hexane, and the catalyst were mixed and then irradiated at specified experimental conditions. After the reaction, the system was cooled down gradually to reach a temperature below 50 °C, before removing the samples from the vessel. The samples were then allowed to settle naturally to form two phases. After reaching equilibrium, the volume of the two phases were recorded, and then the samples were collected for further analysis. The top phase consists of hexane and oleic acid methyl esters (OAMEs) as products. The bottom phase consists of unreacted methanol and water as by-products.

2.3.2. Characterization of FAME

The OAME product was drawn for gas chromatography–flame ionization detection (GC–FID) component analysis (GC-14B: Shimadzu). A HP5-MS capillary column with a length of 30 m, thickness of 0.25 μm, and internal diameter of 0.25 mm was used. The temperatures of the injector and detector were kept at 270 and 310 °C, respectively. Helium was used as a carrier gas. The sample was prepared by adding 20 μL of OAME to 180 μL of n-hexane, and DMN was used as an internal standard. The yield was calculated based on the total amount of OAME obtained in the two phases, using the following equation

2.3.2.

where OAME represents the amount of OAME determined by the GC–FID analysis.

All experiments were carried out in duplicates or triplicates, and the values represent the average of these independent experimental runs.

3. Results and Discussion

3.1. Catalytic Activity of Sulfonated HTC

3.1.1. Preliminary Experiment

This research employed a previously developed methodology for the HTC synthesis, which was reported extensively by Wataniyakul et al..26 Then, the resulting carbon-based catalyst was functionalized by concentrated sulfuric acid. The catalytic activities of sulfonated HTC were first evaluated for the esterification of oleic acid with methanol using hexane as the co-solvent and separator under MW irradiation, as shown in Scheme 1. In a typical experimental run, a magnetic stirring bar was placed inside the MW reactor to homogenize the mixture of reactants at a constant rate. Oleic acid reacts with methanol utilizing the surface functionalities of the catalyst to produce OAMEs. Hexane, serving as a separator (nonpolar phase), helps in dissolving the synthesized OAMEs, whereas water and unreacted methanol remain in the polar phase. In this system, reaction and separation take place at the same time. This process not only simplifies the separation scheme but also drives higher yields as the target product is immediately removed from the reaction zone, which causes the forward shift in the reaction.

Scheme 1. Schematic Diagram Showing the Concept of Reactive Separation for Esterification Using Sulfonated HTC under MW.

Scheme 1

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To verify the abovementioned hypothesis, preliminary experiments on the effect of parameters (sulfonated HTC, hexane, and MW) affecting the reaction were carried out using oleic acid and methanol at a molar ratio of 1:10 at 100 °C for 60 min as control experiments (Figure 1). In the absence of sulfonated HTC, only methanol would absorb MW, as oleic acid being nonpolar has lesser interaction with MW. Thus, only an OAME yield of 28% was obtained. Apparently, MW alone is not capable of catalyzing the reaction under this condition to give a high yield. However, in the presence of 5 wt % sulfonated HTC, and also under MW, the yield increased threefold to 80.5%. This shows the inherent property of the carbon-based catalyst to be a very good MW-absorbing material and a good catalyst for esterification. This provides a high surface temperature to catalyze the reaction, as also reported by Menéndez et al..27 Moreover, the presence of sulfonic acid functionalities, being a Brønsted acid and polar, would add up to the MW-absorbing properties of the sulfonated HTC. Thus, in this regard, the synergism of the sulfonated HTC and MW can significantly accelerate the esterification reaction.

Figure 1.

Figure 1

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Results of preliminary tests on the parameters affecting the esterification reaction. [Conditions: T = 100 °C, 60 min, oleic acid to methanol 1:10, hexane/methanol (H/M) = 0.5 (v/v), 5 wt % catalyst loading, and MW power set = 400 W].

Under the same preliminary experimental conditions, the OAME yield further increased to 93.3%, with the addition of hexane. Hexane acts as a co-solvent during the reaction and as a separator of the product right after the reaction. As a co-solvent, it enhances the mutual solubility of oleic acid and methanol during the reaction. On the other hand, being nonpolar, it is capable of separating OAME from the reaction mixture. It was demonstrated that the addition of hexane provided a higher yield of OAME as it is simultaneously removed from the reaction zone, thereby shifting the reaction equilibrium forward. This result confirms our hypothesis on the advantages of the synergistic effect of MW, sulfonated HTC, and the addition of hexane as the co-solvent.

3.1.2. Detailed Investigation on the Effect of Reaction Conditions

To study in detail how reaction conditions would affect esterification, the effects of temperature, time, oleic acid-to-methanol ratio, catalyst loading, and amount of hexane on the OAME yield were investigated. Experiments were conducted using oleic acid and methanol with the molar ratio of 1:5 to 1:40, reaction temperatures from 50 to 100 °C, and reaction times of 30–60 min, with hexane, as shown in Figure 2.

Figure 2.

Figure 2

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Results of the esterification reaction under various conditions of (a) catalyst loading, (b) reaction temperature, (c) reaction time, and (d) molar ratio of oleic acid to methanol [conditions: hexane/ethanol = 1.8 (v/v) and MW power set = 400 W].

At first, the amount of catalysts was varied from 1 to 10% at T = 100 °C and reaction time of 60 min. Based on the results in Figure 2a, the yield increased significantly to 79.1 from 13.6% with the addition of 1% sulfonated HTC. However, further increasing the amount of catalysts to 2.5 up to 5 wt % did not show any significant effect on the yield. Thus, we focused on using 1–5 wt % sulfonated HTC as the amount of catalyst in the succeeding runs, under the above standard operating conditions, unless otherwise specified.

As for the effect of temperature, apparently, the OAME yield increased with increasing reaction temperature, as shown in Figure 2b. This is due to the resulting higher molecular collision and mass transfer rates16,17 as the temperature increases. Next, the effect of reaction time was evaluated from 30 to 60 min at 100 °C in the presence of 1 wt % catalyst loading, with the molar ratio of oleic acid to methanol at 1:10 and the H/M ratio of 1.8 (v/v), as shown in Figure 2c. The effect of reaction time is the same as that of the reaction temperature, where the yield of OAME increases along with increasing time.

Moreover, we studied the effect of the molar ratio of oleic acid to methanol, which is one important variable affecting the esterification, as shown in Figure 2d. The yield increased from 37 to 91% with an increase in the amount of methanol from 1:5 to 1:40 at T = 100 °C for 60 min in the presence of 1 wt % catalyst loading and H/M ratio of 1.8 (v/v). As expected, the large amount of methanol could drive the equilibrium reaction forward.

Furthermore, we investigated was the effect of the amount of hexane on the reaction, and the results are summarized in Figure 3. It should be noted that for the study on the effects of hexane, the amount of catalysts was increased from 1 to 5 wt %, in order to compensate for the possible decrease in the molecular collision rates with the catalysts, with the addition of hexane. To verify this, additional experiments on the effects of catalysts from 0 to 10 wt %, in the presence of high amounts of hexane, were carried out under similar conditions. The results in Figure S1a show no significant effect of the amount of catalysts on the yield; thus, succeeding experiments were carried out using 5 wt % catalyst loading.

Figure 3.

Figure 3

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Effect of different H/M ratios (v/v) on esterification. [Conditions: T = 100 °C, 60 min, oleic acid to methanol 1:10, and 5 wt % catalyst loading].

The amount of hexane to methanol (v/v) was varied; for example, at H/M = 0.5 (meaning the volume of hexane is half the volume of methanol), the addition of hexane increases the yield by 10% as compared to its absence. OAME was present in both phases of the products (unreacted methanol phase and hexane phase), with only a small amount of OAME present in the unreacted methanol phase.

The increase in the amount of hexane added to the mixture was expected to give a higher yield because of its role as a separator. On the contrary, as shown in Figure 3, the OAME yield decreased with the increasing amount of hexane. It was likely that hexane in excessive amounts could inhibit the reaction by shadowing the acid functionality of the catalyst, thereby hindering the reactants’ accessibility to the surface of the catalyst, obtaining a lower yield. Other than the shadowing effect of hexane on the catalysts, this also decreases the collision frequency among the catalysts and the reactants, thus decreasing the yield of the target products. In addition, limitations to the energy dissemination into the reaction vessel with excessive amounts of hexane may also result into a decrease in the yield of the products.

For this reaction in particular, only the reactant methanol could absorb MW energy because of its high polarity and dielectric loss constant (the capability of the substance to convert the absorbed MW energy into heat), whereas hexane does not absorb MW owing to its nonpolar nature. However, a separation efficiency of more than 94% was achieved. This result confirmed the role of hexane as a good product separator in the esterification reaction process. The separation efficiency was calculated by the following equation

3.1.2.

Moreover, the effect of addition of hexane was studied at different molar ratios of 1:10 and 1:25 (oleic acid to methanol), at various catalyst loadings, as shown in Figure S1. Based on the result, 100% yield of OAME was obtained at 100 °C for 60 min, 1:25 molar ratio of oleic acid to methanol, and 10% catalyst loading by weight of oleic acid. The excessive use of methanol at 1:25 molar ratio of oleic acid to methanol (Figure S1b) was able to provide a high yield, as expected. In particular, about 30% OAME was obtained in the methanol phase under 1:25 molar ratio of oleic acid to methanol. The obtained results are similar to the findings of Sajjadi et al.28 and Yuan et al.,29 which reported that the excess methanol leads to BDF and glycerol (polar molecules) being miscible, thereby reducing the yield. On the other hand, Figure S1a shows the presence of a small amount of OAME (<5%) in the unreacted methanol phase under the 1:10 molar ratio of oleic acid to methanol as in the same way of OAME yield. These results show the role of hexane as a separator, resulting in a higher selectivity of OAME.

To further investigate the effect of hexane in the context of being a co-solvent, the molar ratio of oil to methanol was varied under the optimum value of each parameter. Figure 4 shows the relatively higher yield with hexane at the oil-to-methanol molar ratios of 1:2.5 and 1:5. A 97% OAME yield was obtained at a 1:5 molar ratio of oil to methanol with hexane. These results confirmed the role of hexane as a co-solvent to increase the solubility of oil and methanol.25 At a higher oil-to-methanol molar ratio of 1:10, the excess methanol can drive the reaction forward to the right-hand side; however, this may require a higher energy because of the polar nature of methanol.

Figure 4.

Figure 4

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Effect of hexane at different molar ratios of oleic acid to methanol. [Conditions: T = 100 °C, 60 min, H/M = 0.5 (v/v), and 5 wt % catalyst loading].

In summary, the optimum conditions to catalyze esterification by sulfonated HTC, with hexane as a solvent and separator at the same time, under MW irradiation are 1:5 molar ratio of oleic acid to methanol and H/M ratio of 0.5 (v/v) in the presence of 5 wt % catalyst loading at 100 °C for 60 min to get 97% yield of OAME.

Another advantage of adding hexane is the decrease in the total energy requirement for the reaction. In short, lowering the amount of methanol (1:5 molar ratio of oil to methanol) showed almost the same yield of OAME when compared with the 1:10 molar ratio of oil to methanol (97 and 98%, respectively). This indicated that lesser energy is needed to absorb the MW power because hexane is a non-MW absorptive material. Therefore, the results show that the use of hexane makes the proposed process more energy-efficient when integrated in reactive separation.

Considering the overall process of BDF production, including the separation of the products from the mixture, the presence of water (also a by-product) in the methanol phase would require a more complicated procedure in the separation of OAME and will make it more energy-intensive. The addition of hexane, serving as a co-solvent and separator for the reaction, would also have an added benefit of easily separating OAME from hexane, thus resulting in a lesser energy consumption in the overall process.

3.1.3. Catalyst Reusability and Spent Catalyst Characterization

In order to study the stability of sulfonic groups and the possibility of catalyst reuse, the sulfonated HTC was repeatedly used for the esterification reaction under MW irradiation under the same conditions. The sulfonated HTC was filtered and washed with methanol and reused in the next experimental runs. The results are shown in Figure 5. The yield of OAME decreased from 97 to 47% after five consecutive cycles, decreasing the activity by almost 50%. Comparing the Fourier transform (FT)-IR spectra (Figure S2) of the catalyst before and after the reaction, we confirmed the presence of O=S=O and −SO3H symmetric stretchings of the active sulfonic acid functional groups at 1020 and 1167 cm–1, respectively, following the results by Valle-Vigón et al.30 and Pileidis et al.31 The likely reason for the decreasing yield after the catalyst reuse was the leaching of the sulfonic groups. Deactivation of the sulfonic groups by the reaction with methanol to form sulfonate esters is also possible according to Fraile et al.32

Figure 5.

Figure 5

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Reusability of sulfonated HTC in esterification. [Conditions: T = 100 °C, 60 min, oleic acid-to-methanol ratio of 1:5, H/M = 0.5 (v/v), and 5 wt % catalyst loading].

4. Conclusions

The synergism of sulfonated HTC and MW irradiation using hexane as a co-solvent and separator showed a positive effect in the esterification of oleic acid to produce BDF. The sulfonated HTC was confirmed to have good MW absorptivity because of two Brønsted acid sites, namely the −COOH and −SO3H groups. Moreover, it was demonstrated that the presence of hexane could result in a higher yield as the product is immediately removed, thus enhancing the forward reaction to take place. The BDF yield of 97% was achieved under the optimum conditions at 100 °C for 60 min, 5% catalyst loading by weight of oleic acid, 1:5 molar ratio of oil to methanol, and H/M ratio of 0.5 (v/v).

This novel approach of applying the synergy of sulfonated HTC and MW irradiation in the presence of hexane as a co-solvent and separator could lead to a new concept of integrated MW-assisted carbon-based catalyzed reactive separation for BDF production. The application of this approach to reactions other than the one investigated in this study is also possible and will be explored in our future extended works.

While the preliminary results here were obtained from a laboratory-scale reactor, future works should also include the feasibility of the approach for a large-scale production. Although most of the conventional approaches consider scaling-up of the process, in this novel approach, because of the limitations of MW with regard to its penetration depth, maintaining the reactor as small as possible and simply “numbering it up” would rather make it cost- and energy-effective. Besides, this would also avoid the tedious and laborious steps involved in scaling up a chemical process.

With regard to the preparation of the catalyst, several methods to its synthesis should be explored to avoid the leaching of the sulfonic groups and to prevent the loss of activity. The suitability of the following approaches—grafting, impregnation, surface modification, and metal doping––is worthy of future investigation. Moreover, the calculation of energy requirements for the process in comparison with the conventional approach should also be carried out to further show the merits of the proposed methodology for industrial applications.

Acknowledgments

This work was partially supported by the Japan Science and Technology Agency, via the e-ASIA joint research program in the field of Functional Materials and Bioenergy (grant numbers 14528300, 18066393) and JST SICORP (grant number JPMJSC18E2), and the Japan Student Services Organization (JASSO) Scholarship for the research attachment of L.T. at Kumamoto University. The support from IRN grant (no. IRN62W0001) from Thailand Science Research and Innovation (TSRI) and National Research Council of Thailand (NRCT) is also acknowledged.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c01660.

  • Effect of different oleic acid-to-methanol ratios, 1:10 and 1:25, on catalyst loading under conditions of T = 100 °C, 60 min, and H/M = 1.8 (v/v) and FT-IR spectra of the sulfonated HTC and spent sulfonated HTC after five cycles in esterification (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c01660_si_001.pdf (262.9KB, pdf)

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