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Article

On the Alkalinity of Solid Catalysts for Transesterification of Dimethyl Carbonate and Ethanol

1
State Key Laboratory of Chemical Engineering, School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China
2
ZJU-Hangzhou Global Scientific and Technological Innovation Center, Zhejiang University, Hangzhou 310058, China
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2025, 15(13), 7225; https://doi.org/10.3390/app15137225
Submission received: 16 April 2025 / Revised: 13 May 2025 / Accepted: 24 June 2025 / Published: 26 June 2025

Abstract

In this study, Mg-Al-Zn, MgO, Al2O3, and ZnO were synthesized via the co-precipitation method and evaluated as catalysts for the transesterification reaction of dimethyl carbonate (DMC) and ethanol. The crystal structure, morphological characteristics, pore structure properties, and alkaline properties of the catalysts were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area analysis, temperature-programmed desorption of CO2 (CO2-TPD), and Fourier transform infrared spectroscopy (FTIR). The surface alkali strength and alkalinity of the solids were determined using the Hammett indicator method and non-aqueous titration. When Al2O3 and ZnO are used as catalysts for this transesterification, the conversion rate of dimethyl carbonate is relatively low. When MgO and Mg-Al-Zn are used as catalysts, the conversion rate of dimethyl carbonate is higher. This indicates that the alkali strength of the catalyst for the transesterification reaction needs to be greater than 9.3. Additionally, the activity of the catalysts is also related to the amount of the alkaline sites on the solid surface. The alkali strength of MgO is greater than 11; its excessively high alkali strength will react with CO2 and H2O during use, resulting in a reduction in the number of alkaline sites and thus showing unsatisfactory reactivity. The alkaline strength of the Mg-Al-Zn catalyst ranges from 9.3 to 11.0. When used for the first time, the number of alkaline sites decreases, and then the alkalinity remains at a certain value. Therefore, the alkaline strength of the solid catalyst for the transesterification reaction between DMC and ethanol needs to be between 9.3 and 11.0 so that the number of alkaline sites on the catalyst surface remains unchanged and the catalytic activity remains stable.

1. Introduction

Ethyl methyl carbonate (EMC), recognized as an environmentally friendly solvent and an important electrolyte solvent for lithium-ion batteries [1,2,3], has garnered extensive attention [4,5] due to its biodegradability, low toxicity, electrochemical stability, and safety [6,7]. Currently, the synthesis of EMC primarily involves the phosgene method, oxidative carbonylation, and transesterification. The phosgene method is incongruent with the principles of green chemistry due to the high toxicity of the raw material phosgene and the intermediate methyl chloroformate [8]. The oxidative carbonylation reaction is mainly used for the synthesis of symmetric esters. It is a safer and cleaner production method. Nonetheless, this approach usually requires the use of precious metal catalysts and operates under strict conditions, thus posing safety concerns [9]. The transesterification of dimethyl carbonate (DMC) with ethanol to produce EMC is carried out under mild and easily controllable reaction conditions. The raw materials have low toxicity, reducing the safety risks during operation. Although there is a kinetic limitation of the reaction equilibrium for this transesterification reaction [10], the kinetic equilibrium can be disrupted by removing the product to achieve higher yields. Moreover, the byproduct diethyl carbonate (DEC), an excellent solvent, exhibits broad application potential, thereby conferring high atom economy to this method [11,12].
Numerous alkali catalysts have been reported for the transesterification of DMC and ethanol. Homogeneous catalysts, such as ionic liquid systems synthesized from alkaline ionic precursors [13,14], demonstrate high reactivity but suffer from separation challenges. In contrast, heterogeneous catalysts have the characteristic of easy separation but often suffer from the issue of catalyst deactivation, such as metal oxide CaO [15], supported metal oxides [16], and immobilized ionic liquid catalysts [17]. Therefore, the synthesis of high-stability solid catalysts is critical for commercialization [18,19].
Shen et al. [20] reported that MgO exhibited superior catalytic performance among MgO, ZnO, La2O3, and CeO2 catalysts. However, single metal oxide catalysts generally suffer from poor stability [21,22]. Therefore, to enhance the stability of catalysts, researchers have optimized the composition and structure of catalysts [23,24], constructed bimetallic oxide catalysts [25,26], and even trimetallic oxide catalysts [27], achieving better catalytic effects and stability [11]. To further improve the stability of catalysts, researchers have also adopted methods such as doping non-metallic elements into the catalysts [28].
In addition to the preparation of catalysts, the deactivation mechanisms of alkali catalysts are also crucial. Many reasons, such as the reaction of H2O [29], the collapse of the catalyst framework [30], the poisoning of active sites [31], the leaching of active components, and the decrease in catalyst surface area [32], are possible for the deactivation of solid catalysts. However, most researchers believe that the primary reason for catalyst deactivation is the reaction of alkali sites with H2O and CO2 to form carbonates [21,29,33]. For instance, in the transesterification of soybean oil to produce biodiesel using CaO as a catalyst, Hu et al. [21] investigated the causes of catalyst deactivation and found that CaO is highly sensitive to H2O and CO2, easily forming hydroxides and carbonates. Yu et al. [29] found in the transesterification reaction of DMC and ethanol that the alkali strength of the catalyst is directly related to its catalytic activity, with H2O being the key factor causing the deactivation of CH3ONa. Almerindo Ge I et al. [33] found in the study of the deactivation of magnesium oxide catalysts in the esterification reaction of soybean oil that solid alkali catalysts are prone to be poisoned by certain components in the air, such as CO2 and H2O, because they interact with alkali sites and reduce the activity of the catalyst. There is limited research on the alkaline deactivation of DMC and ethanol transesterification. From the previous research, it is clear that the alkali sites of the solid catalyst are responsible for this reaction, and some alkali sites deactivate during the reaction due to the formation of carbonates. However, it is still unclear how strong the alkali sites of the solid catalyst are in catalyzing this reaction. It is also unclear which kinds of alkali sites are stable and active during this reaction. Additionally, the effect of alkali-site amount is also unclear for this reaction.
In this study, we used four catalysts of ZnO, MgO, Al2O3, and mixed Mg-Al-Zn oxide for this reaction. The catalytic activities and cycling stability of the four catalysts were systematically evaluated, and the underlying mechanisms enabling their stability retention were investigated.

2. Materials and Methods

2.1. Chemicals

The chemicals used in this experiment are as follows: Mg(NO3)2·6H2O(≥99.0%), Al(NO3)3·9H2O (≥99.0%), Zn(NO3)2·6H2O (≥99.0%), NaOH (≥98.0%), and Na2CO3 (≥99.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Anhydrous ethanol (≥99.7%), DMC (≥99.0%), 4-methyl-2-pentanone (≥99.5%), bromothymol blue, phenolphthalein, alizarin yellow R, cyclohexane, 2,4-dinitroaniline (≥98.0%), and benzoic acid (≥98.0%) were all purchased from Shanghai Titan Scientific Co., Ltd. (Shanghai, China).

2.2. Catalyst Preparation

The catalyst with a molar ratio of Mg/Al/Zn = 1:2:1 was prepared using the co-precipitation method. During the preparation process, 0.01 mol of Mg(NO3)2·6H2O, 0.02 mol of Al(NO3)3·9H2O, and 0.01 mol of Zn(NO3)2·6H2O were dissolved in 100 mL of deionized water to form mixed solution A. Separately, NaOH and Na2CO3 were thoroughly dissolved in 100 mL of deionized water to prepare solution B. Solutions A and B were then mixed at the same drip rate under continuous stirring at room temperature while maintaining the pH of the mixture at 10. Subsequently, the mixture was further reacted in a water bath at 70 °C for 12 h. The precipitate was filtered and washed with deionized water until neutral, yielding the catalyst precursor. The solid was then transferred to an oven at 100 °C and dried for 12 h. The catalyst of Mg-Al-Zn was obtained by calcination at 550 °C (3 °C/min) for 12 h. MgO, Al2O3, and ZnO were synthesized using the same precipitation method as the Mg-Al-Zn catalyst with different precursors.

2.3. Reaction Test

The reaction was carried out in a 250 mL three-neck flask as the reactor. The central neck of the flask was equipped with a 500 mm spherical condenser for reflux condensation. The experiment utilized magnetic stirring with DMC/C2H5OH = 1 (molar ratio), catalyst loading: 3.0 wt.% (mass ratio of catalyst to reaction solution), reaction temperature: 78 °C, and reaction time: 3 h. The specific procedure was as follows: DMC and the catalyst were added to the flask and heated for 30 min. After reaching the target temperature, ethanol was added to initiate the reaction. After 3 h, samples were taken from the reaction mixture for quantitative analysis.
The quantitative analysis of reactants and products was performed using a gas chromatograph (GC-2060) from Shanghai Ruimin Instrument Co., Ltd. (Shanghai, China). The capillary column used was SE-54 (50 m × 0.32 mm × 0.5 μm) from Agilent (Beijing, China), and the detector was an FID hydrogen flame ionization detector. The conversion rate and selectivity were quantified using the internal standard method, with 4-methyl-2-pentanone selected as the internal standard solvent.
The specific calculation formulas are as follows:
x D M C = ( 1 m D M C m 0 D M C ) × 100 %
S E M C = M D M C × m E M C M E M C × m 0 D M C × x D M C × 100 %
S D E C = M D M C × m D E C M D E C × m 0 D M C × x D M C × 100 %
XDMC represents the conversion rate of DMC, MDMC denotes the mass of the remaining DMC, M0DMC indicates the initial mass of DMC, SEMC represents the selectivity of the product EMC, SDEC represents the selectivity of the product DEC, MEMC denotes the molar mass of EMC, MDMC denotes the molar mass of DMC, MDEC denotes the molar mass of DEC, MEMC represents the mass of the produced EMC, and MDEC represents the mass of the produced DEC.

2.4. Characterizations

The XRD patterns of the catalysts were obtained using a Bruker D8 Advance diffractometer (Woltzbach, Bruker Company, Karlsruhe, Germany) with Cu Kα radiation, scanned over a 2θ range of 5–85° and at a scanning rate of 10°/min. The SEM of the catalysts was examined using a field emission scanning electron microscope (SEM, Nova NanoSEM 450, FEI, Hillsboro, OR, USA) with a magnification of 50,000 times. The BET specific surface area was determined by N2 adsorption-desorption isotherm measurements performed on a Micromeritics ASAP 2460 instrument (Micromeritics Company, Norcross, GA, USA). The sample was first vacuum-dried at 60 °C for over 12 h, then approximately 100 mg was weighed and placed in a U-type tube. It was degassed under vacuum at 120 °C for 6 h as pretreatment. Subsequently, N2 adsorption and desorption were carried out at 77 K. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) equation, while the pore size distribution was determined from the desorption data via the Barrett-Joyner-Halenda (BJH) method. The CO2-TPD of the catalyst was performed using the VDSorb-91i chemisorption analyzer (Ward Instrument Co., LTD, Quzhou, Zhejiang, China). Approximately 60 mg of the sample was pretreated at 500 °C for 1 h, cooled to 40 °C, and then exposed to CO2 (30 mL/min). Helium purging was performed until the baseline stabilized, followed by heating to 750 °C for testing (10 °C/min). The Diffuse Reflectance Infrared Fourier Transform Spectra (DRIFTS) were conducted using the Thermo Scientific Nicolet iS20 instrument (Thermo Fisher Scientific, Waltham, MA, USA). In these experiments, an in situ diffuse reflectance cell was used and detected by an MCT/A detector (Thermo Fisher Scientific, Waltham, MA, USA). Spectral analysis was carried out within the wavenumber range of 4000 cm−1 to 650 cm−1, with high precision of 4 cm−1 and 32 scans. Approximately 20 to 30 milligrams of catalyst were loaded into the in-situ battery. The sample was first introduced with pure argon gas, heated up to 400 °C at a rate of 10 °C/min for 1 h, and then slowly cooled to 20 °C. Meanwhile, the background spectrum at the corresponding temperature was recorded. After continuing to purge with argon gas for 10 min, CO2 gas was introduced. After adsorbing CO2 for 20 min, Ar is blown into the battery. After blowing for 5 min, it is heated to 400 °C at a rate of 10 °C/min. The real-time spectrum of the temperature rise process after adsorbing CO2 was recorded.
The alkali strengths of the solid catalysts were determined using the Hammett indicator method, and the surface alkalinity was investigated using non-aqueous titration (specific operational steps can be found in the Supporting Information). Following the representation method of the Hammett function, the surface alkali strength of solid catalysts is denoted by H0. Indicators employed for measuring surface alkalinity included bromothymol blue (Brb, pKa = 7.2), phenolphthalein (Phe, pKa = 9.3), alizarin yellow R (AyR, pKa = 11.0), and 2,4-dinitroaniline (Din, pKa = 15.0). When titrating the alkalinity, cyclohexane was used as the solvent and phenolphthalein as the indicator. The solid surface was titrated with a benzoic acid/ethanol solution until it turned colorless, indicating the endpoint. The alkali content was then calculated based on this titration [22,34].

3. Results

3.1. Catalysts Performance

As shown in Table 1, in the transesterification reaction of DMC and ethanol, the selectivity of EMC and DEC is greater than 99.80%, with essentially no other side reactions occurring. This is consistent with the relevant calculations of the kinetics of the transesterification reaction between DMC and ethanol [10], and the products basically contain no other substances. The experimental results indicated that the reaction of DMC and ethanol was negligible in the absence of a catalyst, with a DMC conversion rate of 0.57%. While Al2O3 and ZnO exhibited catalytic activity compared to the blank control, their performance remained suboptimal. MgO, Mg-Al, and Mg-Al-Zn as catalysts were all able to achieve more than 20% conversion of DMC, with the Mg-Al catalyst having the best catalytic effect. Therefore, only the cycling stability of MgO, Mg-Al, and the Mg-Al-Zn catalyst was investigated. For MgO and Mg-Al, the DMC conversion rate declined to below 6.00% after 6 cycles, prompting their exclusion from further cycling experiments. As illustrated in Figure 1, the Mg-Al-Zn catalyst exhibited an initial DMC conversion rate of 41.84%, which decreased to approximately 34.00% in the second cycle. Subsequently, the catalyst was stabilized in recycling, indicating good reusability of the catalyst. In addition, increasing the catalyst loading to 7.0 wt.% enhanced the DMC conversion rate to 56.53% (detailed information can be found in the Supporting Information).

3.2. The Morphology and Structure of Different Catalysts

The crystal structure of the catalysts was characterized by XRD, as shown in Figure 2. The peak positions of MgO, Al2O3 and ZnO correspond to their respective standard cards. From the analysis, it can be concluded that MgO has a cubic crystal system, γ-Al2O3 has a cubic spinel structure, and ZnO has a fibrillar zincite structure, all of which were successfully prepared. From the XRD pattern of the Mg-Al-Zn catalyst, peaks of MgO appear at around 36.8° and 44.0°, while the characteristic peak of ZnO appears at around 62.9°. Additionally, each peak position shows characteristic peaks of MgAl2O4. Based on the XRD pattern analysis of the Mg-Al-Zn catalyst, it can be concluded that MgAl2O4 has a cubic spinel structure (311 crystal plane), with an estimated grain size is approximately 25 nm. MgO has a cubic structure (200 crystal plane), and its estimated grain size is approximately 15 nm. ZnO has a hexagonal wurtzite structure (101 crystal plane), with an estimated grain size of approximately 30 nm. Phase analysis reveals that the positions and intensities of the XRD diffraction peaks for these four catalysts show no significant changes before and after use, indicating that the catalysts maintain excellent structural stability during the reaction. However, it can be seen from the results in Figure 1 that the catalytic performance of MgO has continuously decreased during use, and the catalytic activity of the Mg-Al-Zn catalyst has also changed significantly from the first to the second use. This indicates that the change in the activity of the catalyst during its use has no direct relationship with the crystal structure of the catalyst.
From the SEM images in Figure 3, it can be observed that the Mg-Al-Zn catalyst exhibits loosely distributed crystal particles with relatively uniform sizes. This is similar to the SEM characterization of Mg-Al-Zn composite oxides described in other published articles [35]. After the Mg-Al-Zn catalyst was used 10 times, the SEM characterization of the catalyst showed that there was no obvious change in its microstructure, and the particle size remained basically the same. This is consistent with the BET characterization in Table 2 and the analysis results of the XRD pattern in Figure 2, indicating that the change in activity of the Mg-Al-Zn catalyst from the first to the second use is independent of the microstructure of the solid.
The N2 adsorption- desorption isotherms of MgO, Al2O3, Mg-Al-Zn catalysts, and the Mg-Al-Zn catalyst after use are shown in Figure 4, and their structural properties are listed in Table 2. Based on the data in the table, it can be concluded that the pore size distribution of MgO is concentrated around 9–13 nm, while that of Al2O3 is around 4–8 nm. The pore size distribution of the Mg-Al-Zn catalyst is entirely concentrated around 2–5 nm. Compared with the BET characterization of Mg-Al-Zn composite oxides in other literature [35], the specific surface area of the Mg-Al-Zn catalyst in this paper is relatively slightly larger, but the average pore size of this catalyst is smaller. Moreover, the specific surface area and pore size of the catalyst after 10 cycles of use show little difference compared to the unused catalyst, indicating that the pores of the catalyst are not blocked. This indicates that the decrease in the activity of Mg-Al-Zn during the first two uses has no direct relationship with the pore structure and specific surface area of the catalyst.

3.3. Alkalinity of Different Catalysts

According to the CO2-TPD results shown in Figure 5, it can be inferred that MgO has a weak desorption peak at 250–450 °C. Its alkali strength is stronger than that of MgO obtained under other preparation conditions [20], but the number of alkali sites is relatively small. However, the alkaline strength of ZnO, Al2O3, and Mg-Al-Zn catalysts is relatively weak. Under the test conditions of CO2-TPD, no desorption peak occurred, indicating that there was no adsorption or the amount of adsorbed CO2 was extremely small. This shows that the catalyst was less affected by CO2 [21]. Therefore, in order to determine the CO2 adsorption and desorption on the solid surface of the catalyst more accurately, the DRIFTS method was employed to characterize the adsorption and desorption process.
Figure 6 presents the infrared spectra recorded during the CO2 adsorption and desorption processes for the pre-treated Mg-Al-Zn catalyst. Gas-phase CO2 peaks are observed at 2360 cm−1 and 2295 cm−1 [36], attributed to the introduction of CO2 gas. Concurrently, characteristic peaks for carbonate and bicarbonate species appear at 1516 cm−1 and 1681 cm−1 [37], respectively, indicating that CO2 was adsorbed onto the catalyst surface, forming carbonate and bicarbonate. At 25 min, the CO2 gas flow was terminated and replaced with Ar gas, resulting in a sharp decline in the CO2 gas peaks. Subsequently, the temperature began to rise. The CO2 gas peaks were still detectable at this stage. Observing the carbonate and bicarbonate peak intensities revealed that during the heating process, these species decomposed to CO2, leading to CO2 desorption. The DRIFTS results demonstrate that the Mg-Al-Zn catalyst adsorbs CO2. To determine the alkali strength and alkalinity on the solid catalyst surface, measurements were conducted using the Hammett indicator method.
The MgO solid catalyst caused bromothymol blue to change from yellow to blue, phenolphthalein from colorless to red, and alizarin yellow R from yellow to red, but it did not induce a color change in 2,4-dinitroaniline. Therefore, it can be concluded that the highest alkali strength on the surface of the MgO solid catalyst lies within the range of 11.0 to 15.0. On the other hand, the Mg-Al-Zn solid catalyst caused bromothymol blue to change from yellow to blue and phenolphthalein from colorless to red, but it did not alter the color of alizarin yellow R. Thus, the highest alkali strength on the surface of the Mg-Al-Zn solid catalyst is estimated to be between 9.3 and 11.0. This indicates that the alkali strength of the Mg-Al-Zn catalyst is slightly weaker than that of MgO. In addition, the highest alkali strength on the surfaces of Al2O3 and ZnO solid catalysts is both between 7.2 and 9.3. Combining the experimental results in Table 1 and Table 3, it can be seen that the alkali strength (H0) of the solid catalyst required for the transesterification reaction system between DMC and ethanol must be greater than 9.3. Otherwise, the catalytic performance will be poor, as seen with ZnO and Al2O3. The conclusion drawn from this part of the experiment is consistent with the relationship between the activity of the catalyst and the alkalinity when CaO catalyzes the transesterification reaction [22].
From Figure 7, it can be observed that the surface alkalinity of the Mg-Al-Zn solid is 3.267 mmol/g, while the MgO solid is 0.53 mmol/g. This difference in alkalinity may explain why the catalytic activity of Mg-Al-Zn is superior to MgO. With the increase in the number of uses, the conversion rate of MgO gradually decreases, and the number of alkali sites on the solid surface of MgO also progressively diminishes. This indicates that the decrease in the activity of the MgO catalyst is directly related to the reduction of surface alkaline sites. For the Mg-Al-Zn catalyst, the number of alkali sites decreased from 3.27 mmol/g to approximately 2.70 mmol/g after the first reaction and then tended to stabilize. The catalytic activity of the Mg-Al-Zn catalyst also slightly decreased after the first use and then remained basically unchanged. This indicates that the stability of the Mg-Al-Zn catalyst is directly related to the alkalinity of the solid surface.

4. Conclusions

Due to the hydrolysis of DMC with reagents or water in the atmosphere to produce CO2 [38], then CO2 and H2O react with solid alkalis to form carbonates [21,33]; both the alkali strength and the number of alkali sites of the catalyst will decrease, resulting in a decline in the catalytic activity of the catalyst. In this study, through the determination of the alkali strength and the analysis of catalytic activity of MgO, Al2O3, ZnO, and Mg-Al-Zn catalysts, it was proved that when the alkali strength is greater than 9.3, the transesterification reaction between DMC and ethanol can achieve a certain catalytic effect. When the surface alkali strength of MgO is greater than 11.0, it reacts with CO2 [33] to form carbonates, resulting in a decrease in catalytic activity. The alkaline strength of the solid surface of the Mg-Al-Zn catalyst is less than 11.0 and is less affected by CO2. Furthermore, according to the conclusion drawn in Section 3.3, the surface alkali strength of the Mg-Al-Zn catalyst is greater than 9.3; therefore, it has good catalytic activity. The alkaline sites of the Mg-Al-Zn catalyst are less affected by CO2, so after the first use, the alkalinity of the catalyst slightly decreases. During the second use, the number of alkali sites on the solid surface remains basically unchanged. Therefore, the Mg-Al-Zn catalytic activity can remain stable, and the catalyst can be reused multiple times. We can conclude that the catalytic system of the transesterification reaction between DMC and ethanol requires a solid alkali catalyst with an alkali strength greater than 9.3. If the alkali strength on the surface of the catalyst is greater than 11.0, it will gradually deactivate with an increase in the number of uses. The surface alkali content of solid catalysts is directly related to the stability of their catalytic performance. For the transesterification reaction between DMC and ethanol, when the alkali strength of the solid catalyst is between 9.3 and 11.0, a better catalytic effects and stability can be achieved. Similar situations may also exist in other transesterification reactions; as long as the appropriate range of the alkali strength of the catalyst is explored, a catalyst with stable catalytic activity can be prepared [22].
Through a series of characterizations in this study, it was confirmed that when the surface alkali strength of the solid catalyst is between 9.3 and 11.0, it can stably catalyze the transesterification reaction system of DMC and ethanol. The Mg-Al-Zn ternary composite oxide catalyst meets the above conditions (9.3 < H0 < 11.0) and demonstrates notable cycling stability as a solid catalyst. Under the conditions of a reaction temperature of 78 °C, a reaction time of 3 h, DMC/C2H5OH = 1, and a catalyst amount of 3.0 wt.%, the conversion rate of DMC of the Mg-Al-Zn catalyst remained stable at around 34.00% during 10 cycles of use. When the catalyst load increased to 7.0 wt.%, the conversion rate of DMC reached 56.53%. Compared with the existing industrial catalysts (such as CaO [22] and ILs [13,14]), the catalytic activities of Mg-Al-Zn catalysts may be lower than those of CaO and ILs. However, the stability of many existing industrial catalysts is difficult to maintain, and there will be labor costs and catalyst costs when replacing the catalysts. In addition, both the ionic liquid and the reaction system are in the liquid phase, which involves costs of separation and treatment of waste liquid. The Mg-Al-Zn catalyst has good stability and is easy to separate, which can reduce costs to a certain extent and achieve long-term use.
This study systematically explored the relationship among the surface alkali strength, alkalinity, and catalytic performance of solid catalysts, providing a theoretical foundation for the industrial design of heterogeneous transesterification catalysts.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/app15137225/s1, Figure S1: Effects of catalyst loading (A), reaction time (B), and reaction temperature (C) on the conversion rate of dimethyl carbonate (DMC); Table S1: The color changes of Hammett indicators; Table S2: The catalytic performance of Mg-Al-Zn catalysts under CO2 and H2O conditions.

Author Contributions

T.Z. wrote the manuscript and contributed partially to the manuscript revision. S.W. helped with literature searching and data analysis. W.S. and Y.F.: writing—review and editing, funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (Project No. 21576083) and supported by the Shaanxi Coal Industry Joint Fund Project (No. 2019JLM-20).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Mterial. Further inquiries can be directed to the corresponding authors.

Acknowledgments

I would like to express my gratitude to all the members who participated in the Green Chemistry Project Group of the East China University of Science and Technology.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Pan, R.; Cui, Z.; Yi, M.; Xie, Q.; Manthiram, A. Ethylene Carbonate-Free Electrolytes for Stable, Safer High-Nickel Lithium-Ion Batteries. Adv. Energy Mater. 2022, 12, 202103806. [Google Scholar] [CrossRef]
  2. Grégoire, C.M.; Cooper, S.P.; Khan-Ghauri, M.; Alturaifi, S.A.; Petersen, E.L.; Mathieu, O. Pyrolysis study of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate using shock-tube spectroscopic CO measurements and chemical kinetics investigation. Combust. Flame 2023, 249, 112594. [Google Scholar] [CrossRef]
  3. Li, C.; Chen, Y.; Chen, Y.; Lao, B.; Qi, L.; Wang, H. Stability analysis for 5V high energy density pouch batteries of Si anode and SL/EMC electrolytes. J. Alloys Compd. 2019, 773, 105–111. [Google Scholar] [CrossRef]
  4. Fiorani, G.; Perosa, A.; Selva, M. Dimethyl carbonate: A versatile reagent for a sustainable valorization of renewables. Green Chem. 2018, 20, 288–322. [Google Scholar] [CrossRef]
  5. Huang, S.; Yan, B.; Wang, S.; Ma, X. Recent advances in dialkyl carbonates synthesis and applications. Chem. Soc. Rev. 2015, 44, 3079–3116. [Google Scholar] [CrossRef]
  6. Shang, Y.; Zheng, M.; Zhang, H.; Zhou, X. The Guanidine-Promoted Direct Synthesis of Open-Chained Carbonates. Aust. J. Chem. 2019, 72, 933–938. [Google Scholar] [CrossRef]
  7. Song, Y.; He, X.; Yu, B.; Li, H.-R.; He, L.-N. Protic ionic liquid-promoted synthesis of dimethyl carbonate from ethylene carbonate and methanol. Chin. Chem. Lett. 2020, 31, 667–672. [Google Scholar] [CrossRef]
  8. Shi, J.; Wang, H.; Lin, Y.; Cui, Y.; Xue, X.; Hao, X.; Zhang, Z.; Liu, C. Mg and Al dual-metal functionalized mesoporous carbon as highly efficient heterogeneous catalysts for the synthesis of ethyl methyl carbonate. New J. Chem. 2021, 45, 21199–21205. [Google Scholar] [CrossRef]
  9. Zhou, X.; Zhang, C. Effect of Preparation Method on the Catalytic Property of Calcined Ca–Al Hydrotalcite for the Synthesis of Ethyl Methyl Carbonate. ACS Omega 2021, 6, 5056–5060. [Google Scholar] [CrossRef]
  10. Keller, T.; Holtbruegge, J.; Niesbach, A.; Górak, A. Transesterification of Dimethyl Carbonate with Ethanol to Form Ethyl Methyl Carbonate and Diethyl Carbonate: A Comprehensive Study on Chemical Equilibrium and Reaction Kinetics. Ind. Eng. Chem. Res. 2011, 50, 11073–11086. [Google Scholar] [CrossRef]
  11. Wang, H.; Liu, W.; Wang, Y.; Tao, N.; Cai, H.; Liu, J.; Lv, J. Mg–Al Mixed Oxide Derived from Hydrotalcites Prepared Using the Solvent-Free Method: A Stable Acid–Base Bifunctional Catalyst for Continuous-Flow Transesterification of Dimethyl Carbonate and Ethanol. Ind. Eng. Chem. Res. 2020, 59, 5591–5600. [Google Scholar] [CrossRef]
  12. Schäffner, B.; Schäffner, F.; Verevkin, S.P.; Börner, A. Organic Carbonates as Solvents in Synthesis and Catalysis. Chem. Rev. 2010, 110, 4554–4581. [Google Scholar] [CrossRef] [PubMed]
  13. Qi, Z.; Li, S.; Cai, Y.; Cui, R.; Chen, J.; Ye, C.; Qiu, T. DBU-assisted expeditious synthesis of highly stable IL@ZIFs intergrowth composite: A nanocatalyst for EMC production. Fuel 2023, 334, 126659. [Google Scholar] [CrossRef]
  14. Chen, J.; Huang, H.; Gong, W.; Chen, Y.; Dong, R.; Ren, L.; Qiu, T. Fine-Tuning Electron–Donor Capability in the Basic Anion of Poly(ionic liquid) Frameworks for Revolutionizing Catalytic Synthesis of Ethyl Methyl Carbonate with Both Ultrahigh Catalytic Activity and Selectivity. Langmuir 2024, 40, 9233–9243. [Google Scholar] [CrossRef] [PubMed]
  15. Song, Z.; Jin, X.; Hu, Y.; Subramaniam, B.; Chaudhari, R.V. Intriguing Catalyst (CaO) Pretreatment Effects and Mechanistic Insights during Propylene Carbonate Transesterification with Methanol. ACS Sustain. Chem. Eng. 2017, 5, 4718–4729. [Google Scholar] [CrossRef]
  16. Sun, H.; Li, H.; Chang, X.; Miao, S.; Yuan, X.; Zhang, W.; Jia, M. Nitrogen-doped carbon supported ZnO as highly stable heterogeneous catalysts for transesterification synthesis of ethyl methyl carbonate. J. Colloid Interface Sci. 2021, 581, 126–134. [Google Scholar] [CrossRef]
  17. Zhao, Z.; Liu, M.; Wang, Y.; Yan, Z.; Xu, G.; Guo, J.; Shi, L. One-step embedding method for immobilized bifunctional and alkaline ionic liquids as effective catalysts applied in transesterification. React. Chem. Eng. 2023, 8, 1654–1664. [Google Scholar] [CrossRef]
  18. Ralphs, K.; Hardacre, C.; James, S.L. Application of heterogeneous catalysts prepared by mechanochemical synthesis. Chem. Soc. Rev. 2013, 42, 7701–7718. [Google Scholar] [CrossRef]
  19. Šepelák, V.; Düvel, A.; Wilkening, M.; Becker, K.-D.; Heitjans, P. Mechanochemical reactions and syntheses of oxides. Chem. Soc. Rev. 2013, 42, 7507–7520. [Google Scholar] [CrossRef]
  20. Shen, Z.L.; Jiang, X.Z.; Zhao, W.J. A New Catalytic Transesterification for the Synthesis of Ethyl Methyl Carbonate. Catal. Lett. 2003, 91, 63–67. [Google Scholar] [CrossRef]
  21. Hu, M.; Pu, J.; Qian, E.W.; Wang, H. Biodiesel Production Using MgO–CaO Catalysts via Transesterification of Soybean Oil: Effect of MgO Addition and Insights of Catalyst Deactivation. BioEnergy Res. 2023, 16, 2398–2410. [Google Scholar] [CrossRef]
  22. Praikaew, W.; Kiatkittipong, W.; Aiouache, F.; Najdanovic-Visak, V.; Termtanun, M.; Lim, J.W.; Lam, S.S.; Kiatkittipong, K.; Laosiripojana, N.; Boonyasuwat, S.; et al. Mechanism of CaO catalyst deactivation with unconventional monitoring method for glycerol carbonate production via transesterification of glycerol with dimethyl carbonate. Int. J. Energy Res. 2021, 46, 1646–1658. [Google Scholar] [CrossRef]
  23. Lv, J.; Cai, H.; Guo, Y.; Liu, W.; Tao, N.; Wang, H.; Liu, J. Selective Synthesis of Ethyl Methyl Carbonate via Catalytic Reactive Distillation over Heterogeneous MgO/HZSM-5. ChemistrySelect 2019, 4, 7366–7370. [Google Scholar] [CrossRef]
  24. Miao, Y.-N.; Wang, Y.; Pan, D.-H.; Song, X.-H.; Xu, S.-Q.; Gao, L.-J.; Xiao, G.-M. Zn-Co@N-Doped Carbon Derived from ZIFs for High-Efficiency Synthesis of Ethyl Methyl Carbonate: The Formation of ZnO and the Interaction between Co and Zn. Catalysts 2019, 9, 94. [Google Scholar] [CrossRef]
  25. Liu, P.; Li, H.; Zhou, X. Clean synthesis of ethyl methyl carbonate using Mg-Al mixed oxide as catalyst. IOP Conf. Ser. Earth Environ. Sci. 2021, 687, 012063. [Google Scholar] [CrossRef]
  26. Bing, W.; Zheng, L.; He, S.; Rao, D.; Xu, M.; Zheng, L.; Wang, B.; Wang, Y.; Wei, M. Insights on Active Sites of CaAl-Hydrotalcite as a High-Performance Solid Base Catalyst toward Aldol Condensation. ACS Catal. 2017, 8, 656–664. [Google Scholar] [CrossRef]
  27. Liao, Y.; Li, F.; Dai, X.; Zhao, N.; Xiao, F. Solid base catalysts derived from Ca-M-Al (M = Mg, La, Ce, Y) layered double hydroxides for dimethyl carbonate synthesis by transesterification of methanol with propylene carbonate. Chin. J. Catal. 2017, 38, 1860–1869. [Google Scholar] [CrossRef]
  28. Li, F.; Liao, Y.-h.; Zhao, N.; Xiao, F.-k. The effect of NaF amount on solid base catalysts derived from F-Ca-Mg-Al layered double hydroxides and dimethyl carbonate synthesis. J. Fuel Chem. Technol. 2022, 50, 80–88. [Google Scholar] [CrossRef]
  29. Yu, Y.; Shi, L.; Guo, J.; Li, X.; Yang, W.; Zhang, Z.; Xu, G. In-depth understanding of soluble base deactivation during the carbonate transesterification process. Fuel 2021, 285, 119201. [Google Scholar] [CrossRef]
  30. Hernández-Giménez, A.M.; Hernando, H.; Danisi, R.M.; Vogt, E.T.C.; Houben, K.; Baldus, M.; Serrano, D.P.; Bruijnincx, P.C.A.; Weckhuysen, B.M. Deactivation and regeneration of solid acid and base catalyst bodies used in cascade for bio-oil synthesis and upgrading. J. Catal. 2022, 405, 641–651. [Google Scholar] [CrossRef]
  31. Xu, M.-X.; Hu, Z.; Zhou, J.-L.; Cai, Q.; Zhang, X.-Y.; Li, M.-X.; Wu, Y.-W.; Wang, T.-P.; Lu, Q. Synergistic poisoning of KCl and PbCl2 on commercial V2O5-MoO3/TiO2 catalysts for MSW incineration flue gas denitrification. Catal. Commun. 2022, 172, 106545. [Google Scholar] [CrossRef]
  32. Shan, R.; Shi, J.; Yan, B.; Chen, G.; Yao, J.; Liu, C. Transesterification of palm oil to fatty acids methyl ester using K2CO3 /palygorskite catalyst. Energy Convers. Manag. 2016, 116, 142–149. [Google Scholar] [CrossRef]
  33. Almerindo, G.I.; Probst, L.F.D.; Campos, C.E.M.; de Almeida, R.M.; Meneghetti, S.M.P.; Meneghetti, M.R.; Clacens, J.-M.; Fajardo, H.V. Magnesium oxide prepared via metal–chitosan complexation method: Application as catalyst for transesterification of soybean oil and catalyst deactivation studies. J. Power Sources 2011, 196, 8057–8063. [Google Scholar] [CrossRef]
  34. Boz, N.; Kara, M. Solid Base Catalyzed Transesterification of Canola Oil. Chem. Eng. Commun. 2008, 196, 80–92. [Google Scholar] [CrossRef]
  35. Hao, Q. Synthesis of Dimethyl Carbonate via Transesterification Catalyzed by Solid Base. Master’s Dissertation, Dalian University of Technology, Dalian, China, 2023. [Google Scholar]
  36. Almusaiteer, K.A.; Al-Mayman, S.I.; Mamedov, A.; Al-Zeghayer, Y.S. In Situ IR Studies on the Mechanism of Dimethyl Carbonate Synthesis from Methanol and Carbon Dioxide. Catalysts 2021, 11, 517. [Google Scholar] [CrossRef]
  37. Guo, Y.; Mei, S.; Yuan, K.; Wang, D.-J.; Liu, H.-C.; Yan, C.-H.; Zhang, Y.-W. Low-Temperature CO2 Methanation over CeO2-Supported Ru Single Atoms, Nanoclusters, and Nanoparticles Competitively Tuned by Strong Metal–Support Interactions and H-Spillover Effect. ACS Catal. 2018, 8, 6203–6215. [Google Scholar] [CrossRef]
  38. Li, F.; Wang, X.; Li, H.; Xue, W.; Wang, Y.; Zhao, X. Facile and Green Preparation of Zinc Oxyacetate. Chem. Lett. 2017, 46, 1151–1154. [Google Scholar] [CrossRef]
Figure 1. Catalytic performance and durability of the catalysts (reaction conditions: DMC/C2H5OH = 1, catalyst dosage: 3.0 wt.%, reaction temperature = 78 °C, reaction time: 3 h). Since the conversion rate of DMC dropped below 6.00% after the sixth use of MgO and Mg-Al as catalysts, there was no need to continue the repeated experiments. The experiment was repeated three times for each data point. The error bar is the error range of the data point obtained based on the calculated standard deviation, and the upper and lower horizontal lines represent the upper and lower limits of the 95% confidence interval of the mean.
Figure 1. Catalytic performance and durability of the catalysts (reaction conditions: DMC/C2H5OH = 1, catalyst dosage: 3.0 wt.%, reaction temperature = 78 °C, reaction time: 3 h). Since the conversion rate of DMC dropped below 6.00% after the sixth use of MgO and Mg-Al as catalysts, there was no need to continue the repeated experiments. The experiment was repeated three times for each data point. The error bar is the error range of the data point obtained based on the calculated standard deviation, and the upper and lower horizontal lines represent the upper and lower limits of the 95% confidence interval of the mean.
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Figure 2. XRD patterns of the catalysts before and after use for MgO (A), Al2O3 (B), ZnO (C), and Mg-Al-Zn (D). (a: after used, 10Ca: the catalyst after 10 cycles) (test conditions: a 2θ range of 5–85° and a scanning rate of 10°/min). The blue lines represent the standard cards of the crystal images corresponding to each oxide.
Figure 2. XRD patterns of the catalysts before and after use for MgO (A), Al2O3 (B), ZnO (C), and Mg-Al-Zn (D). (a: after used, 10Ca: the catalyst after 10 cycles) (test conditions: a 2θ range of 5–85° and a scanning rate of 10°/min). The blue lines represent the standard cards of the crystal images corresponding to each oxide.
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Figure 3. Scanning electron microscopy (SEM) images of the samples: Mg-Al-Zn (A), Mg-Al-Zn-10Ca (B). (×50,000).
Figure 3. Scanning electron microscopy (SEM) images of the samples: Mg-Al-Zn (A), Mg-Al-Zn-10Ca (B). (×50,000).
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Figure 4. N2 adsorption-desorption isotherms and pore size distribution diagrams of different catalysts. (The test sample was vacuum dried at 60 °C for more than 12 h, weighed 100 mg, and placed in a U-shaped tube at 120 °C for 6 h of vacuum degassing pretreatment, followed by N2 adsorption and desorption at 77 K.).
Figure 4. N2 adsorption-desorption isotherms and pore size distribution diagrams of different catalysts. (The test sample was vacuum dried at 60 °C for more than 12 h, weighed 100 mg, and placed in a U-shaped tube at 120 °C for 6 h of vacuum degassing pretreatment, followed by N2 adsorption and desorption at 77 K.).
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Figure 5. CO2-TPD profiles of different catalysts. (Approximately 60 mg of the sample was pretreated at 500 °C for 1 h, cooled to 40 °C, then exposed to CO2 at a flow rate of 30 mL/min. Helium purging was performed until the baseline stabilized, followed by heating to 750 °C for testing 10 °C/min).
Figure 5. CO2-TPD profiles of different catalysts. (Approximately 60 mg of the sample was pretreated at 500 °C for 1 h, cooled to 40 °C, then exposed to CO2 at a flow rate of 30 mL/min. Helium purging was performed until the baseline stabilized, followed by heating to 750 °C for testing 10 °C/min).
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Figure 6. DRIFTS of Mg-Al-Zn catalyst of CO2 adsorption followed by Ar sweeping. (Approximately 20–30 mg of catalyst is loaded into the in situ cell). The samples are first pretreated in an Ar atmosphere at 400 °C for 1 h. Then, the samples are slowly cooled to 20 °C while sweeping with Ar gas, and the background spectra are recorded. After a 10 min purge with Ar gas, CO2 gas is introduced. After 20 min of CO2 adsorption, the Ar is purged from the cell, and after 5 min of blowing, the cell is heated to 400 °C at 10 °C/min.
Figure 6. DRIFTS of Mg-Al-Zn catalyst of CO2 adsorption followed by Ar sweeping. (Approximately 20–30 mg of catalyst is loaded into the in situ cell). The samples are first pretreated in an Ar atmosphere at 400 °C for 1 h. Then, the samples are slowly cooled to 20 °C while sweeping with Ar gas, and the background spectra are recorded. After a 10 min purge with Ar gas, CO2 gas is introduced. After 20 min of CO2 adsorption, the Ar is purged from the cell, and after 5 min of blowing, the cell is heated to 400 °C at 10 °C/min.
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Figure 7. Relationship between surface alkalinity and DMC conversion rate for Mg-Al-Zn and MgO. (Reaction conditions: DMC/C2H5OH = 1, catalyst dosage: 3.0 wt.%, reaction temperature = 78 °C, reaction time: 3 h. The alkalinity was determined by the Hammett indicator titration method. The experiment was repeated three times for each data point. The error bar is the error range of the data point obtained based on the calculated standard deviation, and the upper and lower horizontal lines represent the upper and lower limits of the 95% confidence interval of the mean).
Figure 7. Relationship between surface alkalinity and DMC conversion rate for Mg-Al-Zn and MgO. (Reaction conditions: DMC/C2H5OH = 1, catalyst dosage: 3.0 wt.%, reaction temperature = 78 °C, reaction time: 3 h. The alkalinity was determined by the Hammett indicator titration method. The experiment was repeated three times for each data point. The error bar is the error range of the data point obtained based on the calculated standard deviation, and the upper and lower horizontal lines represent the upper and lower limits of the 95% confidence interval of the mean).
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Table 1. Effect of catalyst on the synthesis of EMC.
Table 1. Effect of catalyst on the synthesis of EMC.
CatalystDMC Con. (%)EMC Sel. (%)DEC Sel. (%)
Blank0.57--
MgO23.4995.254.67
Al2O37.0896.423.45
ZnO5.2595.024.86
Mg-Al51.7991.238.47
Mg-Al-Zn41.8491.588.32
Reaction conditions: DMC/C2H5OH = 1, catalyst dosage: 3.0 wt.%, reaction temperature = 78 °C, reaction time: 3 h. DMC: dimethyl carbonate, EMC: ethyl methyl carbonate, DEC: diethyl carbonate. “-”: No catalyst was added during the reaction process.
Table 2. Textural and Structural Properties of Catalysts.
Table 2. Textural and Structural Properties of Catalysts.
CatalystSBET (m2/g)Pore Volume (cm3/g)Pore Size (nm)
MgO81.760.5114.92
Al2O3186.900.436.08
ZnO0.61--
Mg-Al-Zn160.970.267.51
Mg-Al-Zn-10Ca163.940.257.87
The catalyst sample was first degassed under vacuum at 120 °C for 6 h; subsequently, N2 adsorption-desorption isotherms were measured at 77 K using liquid nitrogen.
Table 3. Alkali strength of different catalysts.
Table 3. Alkali strength of different catalysts.
CatalystBrbPheAyRDinAlkali Strength (H0)
MgO×11.0–15.0
Al2O3×××7.2–9.3
ZnO×××7.2–9.3
Mg-Al-Zn××9.3–11.0
The indicators were prepared as 0.5% ethanol solutions.
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Zhang, T.; Wu, S.; Shen, W.; Fang, Y. On the Alkalinity of Solid Catalysts for Transesterification of Dimethyl Carbonate and Ethanol. Appl. Sci. 2025, 15, 7225. https://doi.org/10.3390/app15137225

AMA Style

Zhang T, Wu S, Shen W, Fang Y. On the Alkalinity of Solid Catalysts for Transesterification of Dimethyl Carbonate and Ethanol. Applied Sciences. 2025; 15(13):7225. https://doi.org/10.3390/app15137225

Chicago/Turabian Style

Zhang, Tianyu, Shun Wu, Weihua Shen, and Yunjin Fang. 2025. "On the Alkalinity of Solid Catalysts for Transesterification of Dimethyl Carbonate and Ethanol" Applied Sciences 15, no. 13: 7225. https://doi.org/10.3390/app15137225

APA Style

Zhang, T., Wu, S., Shen, W., & Fang, Y. (2025). On the Alkalinity of Solid Catalysts for Transesterification of Dimethyl Carbonate and Ethanol. Applied Sciences, 15(13), 7225. https://doi.org/10.3390/app15137225

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