Introduction

As the global population continues to grow, the imperative to achieve sustainability across economic, environmental, and social dimensions becomes increasingly obvious [1]. Biofuels play a pivotal role in establishing a sustainable society and are poised to have a profound impact on the foreseeable future, potentially surpassing traditional fossil fuels. The European Union (EU) has taken a significant step toward sustainability through the implementation of the Renewable Energy Directive EC/2009/28, which aims to achieve a 10% adoption of renewable energy in the transportation sector by 2020 [2]. This commitment is projected to increase to 14% by 2030 [3]. The global biofuels market is expected to reach a market size of 153.8 billion U.S. dollars by 2024 [4].

Glycerol, because of its affordability and possession of multiple reactive (-OH) groups, presents an attractive option for the production of valuable chemicals such as propanediols, syngas, acrolein, and glycerol carbonate. Glycerol carbonate, in particular, holds significance in industries such as food, pharmaceuticals, cosmetics, and chemicals because of its beneficial properties such as high boiling and flash points and low volatility. Its potential as a low-VOC solvent positions it as a promising bio-based substitute for traditional organic solvents, showcasing glycerol’s potential as a sustainable feedstock for eco-friendly chemical production [5,6,7].

Various methods for the conversion of glycerol to glycerol carbonate (GC) currently exist, including photo gasification with phosgene [8], direct carbonylation with CO2 [9], pyrolysis with urea [10], and transesterification with propylene carbonate (PC) or dimethyl carbonate (DMC) [11]. Comparing the pros and cons of each of these four methods such as the toxicity of co-reactant, thermodynamic equilibrium limitation of the reaction, difficulties in by-product separation, and the reaction conditions GL conversion with DMC appears to be the most promising route to form GLC, as DMC has been considered as a green chemical and this conversion route can be conducted at relatively mild operational conditions.

Several catalysts, such as zeolites [17], metal salts [18], zinc catalysts [19], ionic liquids immobilized on resins [20, 21], metal oxides [22], and hydrotalcites [23, 24], have been explored for glycerol carbonate synthesis. Compared to acid catalysts, the presence of base catalysts can lead to relatively high yield and selectivity of glycerol carbonate, and also fast reaction rate in the transesterification of GL and DMC [12], which has been reported that the basic site of a catalyst is responsible for activating GL by cleaving its O–H bonds [13, 14]. Liu et al. [15] established a good correlation between catalytic activity and surface basicity. The transesterification of glycerol with DMC over alkaline catalysts is considered a more effective method for the synthesis of GC under mild conditions [13, 14, 16].

These studies underline the crucial link between catalytic activity and the balance of basic sites, leading to increased glycerol carbonate yields. Bimetallic mixed oxide catalytic systems, particularly those involving magnesium derived from hydrotalcites, have exhibited strong basic properties, serving as effective alternatives to homogeneous catalysts. These mixed oxides feature diverse basic sites, including hydroxyl groups, O2−, Mn+ acid–base pairs, and O2− anions, making them suitable for various organic reactions and glycerol transesterification. Importantly, their materials exhibit a dense and robust distribution of basic sites, highlighting their promising catalytic potential [25,26,27,28,29,30,31,32,33,34,35,36].

Other bimetallic oxide catalytic systems have been studied for their basic properties. Modified magnesia–strontia catalysts demonstrate effectiveness in the transesterification of palm olein for biodiesel production, with an optimal strontia content of 5.0 mmol/g and a calcination temperature of 600 °C [37]. Magnesia-supported SrO catalysts have been found to be suitable for biodiesel production from soybean oil, achieving a 93% yield within 30 min [38, 39]. Additionally, the Ca1−xMg1+xO2 mixed oxide and various magnesium– lanthanum mixed oxide catalysts have shown promise in glycerol carbonate synthesis through transesterification [40, 41]. However, a comprehensive study exploring the synergies between different combinations of Mg and M (metal) mixed oxides to enhance MgO catalyst performance is currently unavailable.

The present study focuses on the bimetallic oxide catalysts used in the synthesis of GC from glycerol and DMC. We prepared and evaluated Mg/Al, Mg/Zr, Mg/La, and Mg/Sr mixed oxide catalysts, focusing on the most effective Mg/Sr catalyst. We investigated various parameters such as the calcination temperature of the catalyst, Mg/Sr molar ratio, reaction time, reaction temperature, and amount of catalyst. The results, along with the correlations between the catalyst structural features obtained from the XRD, FT-IR, and CO2-TPD characterization techniques, are presented.

Experimental

Catalyst preparation

Bimetallic mixed oxide catalysts were prepared using the co-precipitation method. Catalysts with different Mg/M (M = Al, Zr, La, Sr) molar ratios (1:1, 1:2, and 1:3) were prepared using KOH as the precipitating agent. The same procedure was followed for the catalysts starting from their nitrate salts (Mg(NO3)2·6H2O, Al(NO3)3·9H2O, Zr(NO3)4, La(NO3)3·6H2O and Sr(NO3)2 all metal nitrates purchased Sigma-Aldrich purity 98 to 99.9%). Typically, for the preparation of the Mg–Al catalyst, calculated amounts of Mg(NO3)2·6H2O and Al(NO3)3·9H2O were first dissolved in distilled water, and the precipitate was obtained by adding KOH solution dropwise. After aging the mixture for 18 h at 80 °C, the solid was filtered and washed with excess water to remove alkali ions. Drying and calcination were performed at 120 °C overnight and 450 °C for 4 h in air. In addition, in the case of Mg/Sr, the 1:3 sample was calcined at different temperatures from 450 to 750 °C.

Characterization of catalysts

Phase composition of catalysts was determined by X-ray diffraction patterns of the catalysts recorded on a Rigaku Miniflex diffractometer using Cu Kα radiation (1.5406 Aº) at 40 kV and 30 mA. The measurements were obtained in steps of 0.045º with account time of 0.5 s and in the 2θ range of 10–80°.

TPD of CO2 carried out basicity measurements. The sample was pre-treated in helium at 300 °C for 1 h and then cooled at 50 °C prior to the adsorption of CO2 at this temperature. After the adsorption of CO2 for 30 min, the sample was flushed with He for 1 h at 100 °C to remove the physisorbed CO2 from the surface. The desorbtion pattern was recorded at a heating rate of 10 °C min−1 from 100 to 800 °C using a recorder connected to a GC equipped with a TCD detector.

FT-IR spectra were recorded on Biorad Excalibur series in the range 400–4000 cm−1 employing the KBr disk method.

Glycerol carbonate was synthesized in the liquid phase at a reaction temperature of 75 °C and atmospheric pressure. In a typical experiment, 2 g of glycerol and 9.72 g of dimethyl carbonate were added to 100 ml of RB, and 0.2 g of catalyst was added. To monitor the reaction, samples were withdrawn after 90 min.

The products were analyzed using a gas chromatograph (Shimadzu 2010) equipped with a flame ionization detector and a DB-1 wax capillary column (diameter: 0.25 mm, length 30 m). The products were also identified by GC–MS (Shimadzu, GCMS–QP2010S) analysis.

Results and discussion

X-ray diffraction studies

The XRD patterns of the Mg/Al (1:1), Mg/Zr (1:1), Mg/La (1:1), and Mg/Sr (1:1) oxides are shown in Fig. 1. Characteristic peaks related to the cubic form of MgO are present, as identified by their 2θ values at 36.8°, 43.8° 62.4° [JCPDS data card # 45-946, 37, 42, 43]. Diffraction peaks at 23.5°, 34.6°, 44.3° and 56.5° corresponding to Al2O3 [44, 45], peaks at 30.3°, 50.4° and 60.2° corresponding to the tetragonal phase of ZrO2 [46, 47], peaks at 15.6°, 27.9°, 39.4°, 48.6° and 22.1°, 31.3°, 38.7° corresponding to La(OH)3 (JCPDS 36-1481) and La2O3 [JCPDS card # 49-0981, 41, 48,49,50], and peaks at 2θ of 27.2°, 28.7°, 36.6°, 38.6°, 43.6°, 48.6° and 30.1°, 34.8°, 50.2°, 59.4°, 62.4° corresponding to Sr(OH)2 and SrO [37] are present in the XRD patterns of Mg/Al, Mg/Zr, Mg/La, and Mg/Sr oxides.

Fig. 1
figure 1

XRD patterns of Mg-M oxide catalysts a Mg/Al b Mg/Zr c Mg/La d Mg/Sr

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Along with these peaks, there are many other peaks corresponding to the mixed metal oxides. The Mg–Al oxide catalyst has peaks at 18.9°, 31.1° and 47.5° corresponding to the cubic form of MgAl2O4 mixed metal oxide. The pattern of the Mg/Zr oxide catalyst shows peaks at 50.8° and 63.3°, corresponding to Mg0.2Zr0.8O1.8 mixed metal oxide [51]. The Mg/La oxide catalyst has prominent peaks at 15.3°, 27.2°, and 48.8°, corresponding to La2MgOx [41]. The XRD pattern of the Mg/Sr oxide catalyst displays intense peaks at 25.1°, 48.8°, and 47.2° corresponding to Mg1−xSrxO mixed metal oxide [37, 39]. These patterns suggest that all bimetallic oxide catalysts contain individual and mixed metal oxides.

Temperature programmed desorption studies

CO2-TPD measurements were performed for the Mg/M (M = Al, Zr, La and Sr) oxide catalysts to determine the total basicity and basic strength distribution of the catalysts. The CO2 desorption profiles of the catalysts are presented in Fig. 2, and the calculated densities of the basic sites are presented in Table 1. CO2-TPD pattern of Mg/Al shows a small peak between 350 and 450 °C, indicating sites of medium strength. A small desorption peak appearing at 400–500 °C is related to the desorption of CO2 from the medium basic sites present in the Mg/Zr metal oxide catalyst. CO2-TPD profile of the Mg/La catalyst also possesses a very small peak between 650 and 700 °C, which indicates the existence of very few strong basic sites in catalyst. CO2-TPD profile of the Mg/Sr catalyst shows a very intense peak around 600 °C, which indicates that this catalyst has strong basic sites. The observed TPD pattern of the Mg/Sr oxide catalyst suggests that it is the strongest base compared with Mg/Al, Mg/Zr, and Mg/La oxide catalysts.

Fig. 2
figure 2

CO2-TPD profiles of Mg-M catalysts a Mg/Al b Mg/Zr c Mg/La d Mg/Sr

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Table 1 Densities of basic sites of Mg/M (M = Al, Zr, La and Sr) oxide catalysts
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FT-IR spectroscopic studies

The FT-IR spectra of the Mg/M (M = Al, Zr, La and Sr) oxide catalysts are shown in Fig. 3. All four spectra have a broad absorption band around 3500 cm−1 which is attributed to the O–H vibration mode of the hydroxyl group and water molecules in interlayer region [19]. Mg/Al, Mg/Zr and Mg/Sr oxide catalysts show a 2500 cm−1 band which is related to carbonate ion in the interlayer region. All four catalysts show a band around 1800 cm−1. This is attributed to C=O bond stretching in the carbonate ion in interlayer region. The Intensity of this band is greater for the Mg/Sr oxide catalyst and it is very low for Mg/La catalyst. All four catalysts show a small band around 1200 cm−1 related to the carbonate ion.

Fig. 3
figure 3

FT-IR spectra of different Mg/M oxide catalysts a Mg/Al b Mg/Zr c Mg/La d Mg/Sr

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Catalyst evaluation for glycerol carbonates synthesis

Effect of combination of metals on glycerol carbonate yield

The different Mg/M (M = Al, Zr, La and Sr) oxide catalysts synthesized were screened for the formation of GC by the transesterification of glycerol with DMC and the yields of GC are shown in Table 2. Among the four bimetallic oxide catalysts, Mg/Sr oxide catalyst show the highest activity (expressed in terms of GC yield) of 48.2% when compared with the other three catalysts. Mg/Al, Mg/Zr and Mg/La mixed oxide catalysts have shown the GC yield of less than 20% and the observed order of catalytic activity is Mg/Sr >  > Mg/Zr > Mg/Al > Mg/La oxides. Among the four catalysts Mg/La oxide has the least activity. The transesterification reaction in the absence of catalyst yielded negligible amount of glycerol carbonate. These results suggest that the activity of mixed oxide catalysts towards transesterification reaction vary with the changing combination of metals.

Table 2 Activity of Mg-M oxide catalysts
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Detailed study on highly active Mg/Sr oxide catalyst

Catalyst characterization

The XRD patterns of the catalysts with different Mg/Sr molar ratios are shown in Fig. 4. The patterns show diffraction peaks at 2θ of 27.2°, 28.7°, 36.6°, 38.6°, 43.6°, 48.6° and 30.1°, 34.8°, 50.2°, 59.4°, 62.4° corresponding to Sr(OH)2 and SrO [37]. Along with these peaks, prominent peaks related to the cubic form of MgO are also present at 2θ = 36.8°, 43.8° and 62.3º [37, 42, 43]. The remaining peaks may be attributed to the Mg/Sr mixed oxides, Mg1-xSrxO [37, 39]. The decrease in peak intensities suggests a decrease in the crystallite sizes of the Sr and Mg oxides with an increase in Sr in the Mg/Sr oxide catalysts.

Fig. 4
figure 4

X-ray diffraction patterns of the Mg/Sr catalysts with different molar ratio a 1:1 b 1:2 c 1:3

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CO2-TPD measurements were carried out for different Mg/Sr oxide catalysts to evaluate the total basicity and basic strength distribution of the catalysts. The desorption profiles of the catalysts are presented in Fig. 5. The desorption peak centered at 550 ºC can be attributed to CO2 desorbed from sites with strong basic sites. Mg/Sr (1:1), Mg/Sr (1:2) and Mg/Sr (1:3) oxide catalysts have this peak around 550 °C. So, all the three catalysts have strong basic sites. But Mg/Sr (1:3) oxide catalyst shows a more intense broad peak indicating that it has greater number of strong basic sites than the remaining two catalysts. The basicity of the catalyst is seen to vary with the Sr content in Mg/Sr oxide catalysts.

Fig. 5
figure 5

CO2-TPD patterns of the Mg/Sr oxide catalysts with different Mg/Sr molar ratio a 1:1 b 1:2 c 1:3

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Effect of molar ratio of Mg/Sr

The Mg/Sr oxide catalysts were evaluated further for the transesterification of glycerol and the results are shown in Table 3. Mg/Sr (1:3) oxide catalyst shows good activity towards transesterification of glycerol with DMC compared to its other analogues with different molar ratio [Mg/Sr (1:2), Mg/Sr (1:1)]. Formation of GC from glycerol transesterification without using any catalyst is negligible (0.71%). The formation of GC increased with increase the amount Sr in Mg/Sr oxide catalyst. Among these catalysts, the catalyst Mg/Sr (1:3) shows the highest transesterification activity. These results suggest that the activity of the base catalyzed reaction can be enhanced by using mixed oxides.

Table 3 Effect of Mg/Sr molar ratio on transesterification of glycerol
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The transesterification activity observed for the Mg/Sr (1:3) catalyst can be interpreted from the observed catalyst characteristics. The densities of basic sites of the three Mg/Sr catalysts are shown in Table 4. From this data it is apparent that Mg/Sr (1:3) catalyst contains the highest number of strong basic sites.

Table 4 Densities of basic sites of Mg/Sr catalysts
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The presence of strong basicity is responsible for high transesterification activity as this reaction is facilitated by the base sites. Other catalysts show lower activity due to the presence of fewer amounts of basic sites than Mg/Sr (1:3) catalysts. These results support the fact that the basicity of the catalyst is the main factor in obtaining high GC yield.

Effect of calcination temperature of catalyst

The variation in catalytic properties with thermal treatment and its effect on glycerol conversion were studied using a Mg/Sr (1:3) catalyst by calcination at different temperatures. Because the Mg/Sr (1:3) oxide catalyst showed the highest GC yield, this catalyst was further studied to understand its surface-structural properties and their relation to transesterification activity. In the case of mixed metal oxides, the acidic and basic properties vary with the change in the pretreatment temperature, apart from the composition. The Mg/Sr (1:3) oxide catalyst was subjected to different calcination temperatures in the range of 450 to 750 ºC. Mg/Sr (1:3) oxide catalysts calcined at different temperatures were studied for the transesterification of glycerol, and the results are given in Table 5.

Table 5 Effect of calcination temperature on activity of Mg/Sr (1:3) catalyst
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The uncalcined Mg/Sr (3:1) catalyst showed a very low GC yield of about 2.71%. The activity of the catalyst has increased with increase in calcination temperature up to 650 ºC. Further increase in temperature resulted in a decrease in activity. The catalyst calcined at 650 ºC shows the highest activity among all the catalysts. In order to understand the variation in activity with change in calcination temperature these catalysts were further characterized in terms of XRD and CO2-TPD studies and the details are given below.

The XRD patterns of the Mg/Sr (1:3) oxide catalyst calcined at different temperatures are shown in Fig. 6. The patterns suggest well MgO and SrO crystalline phases in the catalyst calcined at 750 °C. The patterns of catalysts calcined at other temperatures indicate low crystallinity. The catalyst calcined at 750 °C has more peaks than the remaining three catalysts. This indicates that at higher temperatures, more mixed oxides are formed, leading to a decrease in the activity of the Mg/Sr (1:3) catalyst.

Fig. 6
figure 6

XRD profiles of the Mg/Sr (1:3) catalyst calcined at a 450 °C b 550 °C c 650 °C d 750 °C

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CO2–TPD patterns of the catalysts calcined at different temperatures are shown in Fig. 7. The patterns suggest that the catalyst calcined at 450 °C has a peak at around 500 °C. A very small peak around 600 °C is also seen indicating that this catalyst has medium and strong basic sites. The catalyst calcined at 550 °C shows only one peak at around 500 °C with considerable intensity; thus, it has medium basic sites only. Catalyst calcined at 650 ºC shows a large peak around 700 °C and 750 °C apart from a small peak around 350 °C and a peak around 600 °C. The densities of basic sites are shown in Table 6. From the data it can be seen that the amount of basicity is more for the catalyst calcined at 650 °C. The density of strong basic sites present in this catalyst is very high when compared with that of catalysts calcined at 450 °C and 550 °C. The catalyst calcined at 750 °C shows a different TPD pattern exhibiting a small desorption peak at 350 °C which can be attributed to the sites of moderate basic strength present in the catalyst. This catalyst also has a peak at 650 °C which indicates the presence of strong basic sites. But the number of strong basic sites and total number of basic sites in this catalyst are less than that of the catalyst calcined at 650 °C. Mg/Sr (1:3) catalyst calcined at 650 °C is therefore more active than the catalyst calcined at 750 °C. Hence, among the four catalysts studied the one calcined at 650 °C is the best catalyst.

Fig. 7
figure 7

CO2-TPD patterns of the Mg/Sr (1:3) catalyst calcined at a 450 °C b 550 °C c 650 °C d 750 °C

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Table 6 Densities of basic sites of Mg/Sr (1:3) oxide catalyst calcined at different temperatures
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Effect of various reaction parameters on transesterification of glycerol with dimethyl carbonate

The mixed oxide catalyst with Mg/Sr (1:3) and calcined at 650 °C showed the highest activity among all the other Mg/Sr catalysts. It was selected as a system catalyst for the evaluation of reaction parameters to optimize the reaction conditions for the high yield of glycerol carbonate. The effects of various parameters, namely reaction temperature, reaction time, glycerol to DMC molar ratio, and the amount of catalyst, were studied.

Influence of reaction temperature

Effect of reaction temperature on transesterification of glycerol was studied in the temperature range of 50 to 110 °C and the results are shown in Fig. 8. As can be seen from the results, GC yield is found to increase with increase in temperature up to 90 °C and thereafter there is no considerable change in the yield The GC yield has increased by about 15% per every 20 °C rise in reaction temperature up to 90 °C, beyond which the increase in yield is only about 0.6% per 20 °C rise in reaction temperature. The catalyst shows maximum activity at a reaction temperature of 90 °C.

Fig. 8
figure 8

Effect of reaction temperature GC yield on Mg/Sr (1:3) catalyst calcined at 650 °C. Reaction conditions Glycerol: 2.0 g; Dimethyl carbonate: 9.783 g; Catalyst: 0.300 g; and Reaction time: 90 min

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Effect of molar ratio of glycerol to dimethyl carbonate

The molar ratio of glycerol/dimethyl carbonate (GL/DMC) also affects the conversion of glycerol to GC. The effect of GL/DMC molar ratio from 1:1 to 1:10 was evaluated using Mg/Sr (1:3) catalyst, and results are shown in Fig. 9. The results obtained suggest that at lower molar ratios the yield of the product is poor. The yield increases with increasing molar ratio and reaches a maximum value at 1:5. Further increase in the ratio leads to decrease in GC yield.

Fig. 9
figure 9

Effect of glycerol /DMC mole ratio on synthesis of glycerol carbonate on Mg/Sr(1:3) catalyst calcined at 650 °C. Reaction conditions Catalyst: 0.300 g; Reaction time: 90 min and Reaction temperature: 90 °C

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Effect of catalyst amount

Synthesis of GC from glycerol and DMC was carried out with different amounts of the Mg/Sr (1:3) catalyst varying from 100 to 500 mg. The activity results are shown in Fig. 10. From the figure, it is evident that the GC yield increases gradually with increasing amount of catalyst up to 300 mg, after that there is no considerable increase. The difference in GC yield is only about 0.9% from 300 (83.3) to 500 mg (84.2%). Therefore, it may be concluded that 300 mg is the optimum amount of catalyst.

Fig. 10
figure 10

Effect of catalyst weight on synthesis of glycerol carbonate on Mg/Sr (1:3) catalyst calcined at 650 °C. Reaction conditions Glycerol: 2.0 g; Dimethyl carbonate: 9.783 g; Reaction time: 90 min and Reaction temperature: 90 °C

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Effect of reaction time

Synthesis of GC from glycerol and DMC by transesterification reaction over Mg/Sr (1:3) catalyst was carried out for different reaction times ranging 30 min to 120 min with 30 min intervals. The change in the GC yield with reaction time is presented in Fig. 11. These results suggest that the GC yield increases with increasing time initially and it reaches it maximum at 90 min. Beyond 90 min there is a small increase in GC yield. At 90 min 83.3% GC yield was obtained after reaching 150 min yield was 84.0%. So, there is no considerable change in yield beyond 90 min. Hence 90 min is the optimum reaction time for transesterification reaction over Mg/Sr (1:3) catalyst.

Fig. 11
figure 11

Effect of reaction time on synthesis of glycerol carbonate on Mg/Sr (1:3) catalyst calcined at 650 °C. Reaction conditions Glycerol: 2.0 g; Dimethyl carbonate: 9.783 g; Catalyst: 0.300 g and Reaction temperature: 90 °C

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Conclusion

On the basis of the findings above, it can be concluded that the Mg/Sr mixed oxide exhibits superior efficiency compared with Mg/Al, Mg/Zr, and Mg/La mixed binary oxide catalysts. The effectiveness of the Mg/Sr catalyst relies on the mole ratio of its constituents and the pre-treatment temperature. Notably, the Mg/Sr ratio of 1:3 demonstrates the highest activity among the various Mg/Sr catalysts. The investigation into the calcination temperature (ranging from 450 to 750 °C) reveals that the sample with a Mg/Sr ratio of 1:3, calcined at 650 °C, exhibits the highest activity due to the abundant presence of robust basic sites.

The yield of glycerol carbonate (GC) is also influenced by the reaction temperature, glycerol to dimethyl carbonate ratio, reaction time, and the quantity of catalyst. A comprehensive analysis of these parameters indicates that the optimal reaction conditions involve a 90-min reaction time, a reaction temperature of 90 °C, 300 mg of catalyst, and a GL/DMC molar ratio of 1:5. When compared with the results obtained using monometallic oxide catalysts, the use of the Mg/Sr bimetallic oxide catalyst leads to a significant increase in the yield, from 40.5 to 83.3%.