Table 2 gives the commonly seen Zn- and Cu-based oxide catalysts for the glycerol carbonylation with CO₂. Mota et al. prepared ZnO, SnO₂, Fe₂O₃, CeO₂ and La₂O₃ via the sol-gel method using polyvinyl alcohol as a template^[25]; they found that ZnO exhibited relatively high catalytic activity and the used ZnO could maintain favorable catalytic performance even after calcination at 450 °C.

The introduction of other metals to form composite metal oxides can modulate the surface alkalinity of ZnO, which facilitates the adsorption and activation of CO₂ and enhances the reaction activity. Li et al.^[26] prepared a series of Zn-La composite oxide catalysts with different Zn/La ratios via the co-precipitation method and investigated the effect of calcination temperature on the structural properties of the catalysts. The results indicated that when calcined at 500  °C, La mainly existed in the form of La₂O₂CO₃. The formation of La₂O₂CO₃ species helps to improve the surface alkalinity of the catalyst, thereby promoting the adsorption and activation of CO₂ and facilitating the reaction. Furthermore, Park et al.^[27] prepared the ZnO-La₂O₂CO₃ composite oxide catalysts with different morphologies via co-precipitation and glycol combustion methods, and investigated the effects of oxide ratio and crystal structure on the catalytic performance. The results showed that the catalyst synthesized by the glycol combustion method exhibited superior product selectivity, which was mainly attributed to the monoclinic crystal structure of La₂O₂CO₃. After the reaction, the catalyst prepared by co-precipitation showed significant accumulation of zinc glycerolate intermediates on its surface, while the material prepared by glycol combustion only had a small amount of zinc glycerolate. This difference is primarily due to the fact that the monoclinic La₂O₂CO₃ facilitates the reaction between zinc glycerolate and CO₂ to form glycerol carbonate, thereby enhancing the selectivity toward glycerol carbonate.

Liu et al.^[28] prepared a series of ZnWO₄-ZnO catalysts with different tungsten loadings using the impregnation method and evaluated their activity using N, N-dimethylformamide as the reaction solvent. Under optimal reaction conditions, the glycerol carbonate yield reached 6.5% with 100% selectivity on the 10%W-Zn catalyst. The catalytic performance of the Zn-W composite oxide catalysts was attributed to the formation of ZnWO₄, the interaction between ZnWO₄ and ZnO, the moderate acid-base sites, and the specific surface area of the catalysts. DRIFTS results indicated that on the ZnO surface, CO₂ primarily existed in the form of chelating bidentate carbonates, whereas after the introduction of W, bridged bidentate carbonates became the dominant adsorption form of CO₂. Infrared spectroscopic characterization of the catalyst after the reaction revealed the formation of zinc glycerolate species, based on which a possible reaction mechanism was proposed, as shown in Figure 4.

To further increase the specific surface area of the catalysts and expose more active sites, Li et al.^[29−30] prepared a series of Zn-Al-La catalysts derived from hydrotalcite precursors, as well as cation-modified (Li, Mg, Zr) and anion-modified (F, Cl, Br) Zn-Al-La catalysts. Under the condition of using acetonitrile as the coupling agent, favorable results were achieved. In situ infrared spectroscopy results indicated that ZnO on the catalyst surface activated glycerol to form zinc glycerolate. The activated CO₂ on basic sites was then inserted into zinc glycerolate, generating a seven-membered ring intermediate species, which subsequently underwent intramolecular rearrangement to form the target product, glycerol carbonate (GC). The detailed mechanism is illustrated in Figure 5.

Furthermore, Cao et al.^[31] synthesized the ZnAlCe composite metal oxide catalysts via the co-precipitation method. Compared with pure ZnO, the introduction of Al increased the specific surface area of the catalyst, thereby exposing more active sites. The XPS results revealed that Zn in the Zn₄AlCe catalyst promoted the conversion of Ce⁴⁺ to Ce³⁺ in CeO₂. The incorporation of CeO₂ significantly enhanced the lattice oxygen content of the catalyst and the lattice oxygen, acting as a Lewis base site, facilitates the activation of CO₂. The solid Zn₄AlCe_x catalyst possesses abundant acid-base sites, which are spatially constrained and unable to freely combine, forming frustrated Lewis pairs (FLPs). The activity test results indicated that the ZnAlCe composite oxide catalyst exhibited favorable performance, primarily due to the introduction of Ce, which helped to increase the lattice oxygen on the catalyst surface and form a higher number of FLPs, thereby promoting the reaction. As the Ce content increased, the number of FLPs decreased, leading to a decrease in the catalytic activity. The FT-IR results showed that CO₂ was mainly adsorbed and activated on the material surface in the form of monodentate and bidentate carbonates and the introduction of Ce could enhance the adsorption intensity. Additionally, the formation of the intermediate species zinc glycerolate was observed.

Compared with the thermal catalysis, Li et al.^[32] conducted a study on the photothermal catalytic synthesis of glycerol carbonate using ZnO-La₂O₂CO₃ under light irradiation. The results showed that the introduction of La enhanced the number of basic sites on the catalyst surface, which contributed to the thermal catalytic reaction. Simultaneously, the incorporation of La also improved the light absorption capacity of the catalyst, promoting the generation of electron-hole pairs. The electrons facilitated the activation of CO₂, while the holes promoted the activation of glycerol, thereby enhancing the overall reaction performance. The photothermal synergistic effect significantly improved the catalytic performance, nearly doubling the yield of glycerol carbonate. Furthermore, they prepared a ZnO-Co₃O₄ catalyst. Under photothermal synergistic conditions, the introduction of Co enhanced the surface basicity (both strength and quantity) of the catalyst. The synergistic interaction between ZnO and Co₃O₄ reduced the bandgap of Co₃O₄-ZnO and improved its visible-light absorption capacity, enabling the catalyst to generate electron-hole pairs through visible light excitation^[33]. The photothermal synergy contributed to increased reaction activity, while the strong visible-light harvesting ability and efficient electron-hole separation achieved by incorporating Co enhanced the reactivity under visible light irradiation.

Zinc-based bimetallic composite metal oxides such as ZnO-La₂O₂CO₃, Co₃O₄-ZnO, ZnWO₄-ZnO, as well as trimetallic composite oxides like ZnAlLa and ZnAlCe, have all shown certain effectiveness in this reaction. The increase in catalyst specific surface area, oxygen vacancies and the number of Lewis acid-base sites contributes to the enhanced reaction performance. Using N,N-dimethylformamide as a solvent promotes the adsorption and activation of CO₂, facilitating the reaction. Employing acetonitrile as a coupling agent significantly improves the conversion of glycerol, albeit with lower selectivity toward glycerol carbonate. The formation of the intermediate species zinc glycerolate has been observed in all Zn-based catalytic systems. CO₂ predominantly exists in a bidentate carbonate adsorption form on the catalyst surface, which aligns with the reaction mechanisms proposed by other researchers (Figures 4 and 5). Photothermal synergy markedly enhances the performance of the catalytic reaction. Zn-based catalysts represent one of the earlier studied metal oxide catalysts. Through approaches such as metal modification, morphology control to increase the specific surface area, and photothermal synergy, the reaction activity has been significantly improved. However, the highest yield of glycerol carbonate achieved so far remains only 28%, indicating that further optimization of Zn-based catalysts is still required to enhance their activity. In future catalyst design, consideration could be given to preparing composite metal oxides with specific structures such as perovskite or spinel, aiming to further increase the specific surface area and modulate the surface acidity and alkalinity. Alternatively, exploring other coupling agents or combinations of reaction solvents and coupling agents may help improve the reaction activity.

Ke et al.^[34] prepared metal oxides such as CuO, NiO, Co₃O₄, ZrO₂ and Al₂O₃ via the hydrothermal method. Among them, CuO exhibited a porous flake-like stacked structure with relatively small specific surface area, pore size and pore volume. Lattice oxygen contributes to the activation of glycerol, while oxygen vacancies facilitate the adsorption and activation of CO₂. The CuO material showed a higher number of lattice oxygen and oxygen vacancies, with their proportions close to 1∶1. Furthermore, CuO and Co₃O₄ possessed few acidic sites on their surfaces, as strong acidic sites tend to cause aggregation of active components on the catalyst surface, thereby reducing catalytic activity. The CO₂-TPD results indicated that CuO had a higher proportion of strong basic sites compared to other metal oxides, which aids in the activation of glycerol. Consequently, CuO demonstrated superior catalytic activity relative to other oxide catalysts and could be reused for multiple cycles. Under optimal reaction conditions, the glycerol conversion reached 89%, with a glycerol carbonate yield of 61.7%.

To further enhance the dispersion and catalytic activity of Cu, a series of supported catalysts were prepared using the impregnation method. Zhang et al.^[35] synthesized Cu/La₂O₃ catalysts with different Cu loadings and reduced the active Cu component under a hydrogen atmosphere, investigating the effects of reaction conditions on the catalytic performance. The results demonstrated that Cu and La₂O₃ synergistically promoted the activation of CO₂ and glycerol and the 2.3% Cu/La₂O₃ catalyst exhibited the best performance. There was a certain correlation between the size effect of Cu nanoparticles and their activity: as the Cu particle size increased from 1.7 to 11.9 nm, the turnover frequency decreased from 0.0096 to 0.0001 s⁻¹. Additionally, for the catalysts such as Cu/CaO, Cu/MgO, Cu/La₂O₃, Cu/Al₂O₃, Cu/ZrO₂, Cu/Mg-Al-Zr, Cu/CNTs, Cu/MCM-41 and Cu/HZSM-5 prepared via impregnation^[39], the distribution and particle size of Cu on the carriers varied due to differences in the interaction between Cu and the supports, as did the number of acid-base sites on the support surfaces. The results indicated that both Cu particle size and the acid-base properties of the catalyst influenced the conversion of glycerol to glycerol carbonate and glyceryl monoacetate. As the Cu particle size increased, the reaction rate of glycerol decreased. Moreover, the catalysts containing Lewis basic sites favored the formation of glycerol carbonate, whereas those containing Lewis acidic sites promoted the formation of glyceryl monoacetate.

Al-Kurdhani et al.^[36] prepared the Cu catalysts supported on different carriers (graphene oxide, graphene, activated carbon, SiO₂, Al₂O₃) using the impregnation method, among which the Al₂O₃-supported catalyst exhibited the best activity. The studies on the effects of Cu loading and calcination temperature showed that the optimal catalytic performance was achieved with a Cu loading of 30% and a calcination temperature of 700 °C. This is primarily attributed to the fact that the catalyst calcined at 700 °C demonstrated the largest specific surface area and total pore volume, which likely led to greater exposure of active sites and consequently higher activity. Additionally, increasing the calcination temperature facilitated the formation of CuAl₂O₄, resulting in an increased number of moderate to strong basic sites on the catalyst surface, which promoted the activation of glycerol and CO₂, thereby enhancing the activity. They also proposed a possible reaction mechanism (Figure 6): First, the O–H bond of glycerol is adsorbed and activated by the strong basic sites of the CuO/Al₂O₃ catalyst. Subsequently, the second O–H bond of the glycerol molecule is also activated, forming Cu(C₃O₃H₆), while the CO₂ molecule interacts with 2-cyanopyridine to form a five-membered ring intermediate (I). Next, the activated glycerol molecule undergoes nucleophilic attack on the carbonyl carbon atom of the intermediate, forming intermediate (II). Finally, the oxygen atom from the second O–H bond of glycerol performs an intramolecular nucleophilic attack on the carbonyl carbon, yielding glycerol carbonate.

Pawar et al.^[37] found that single metal oxides such as ZnO, CuO, CeO₂ and ZrO₂ exhibited certain catalytic activity for this reaction. Subsequently, bimetallic oxide catalysts CuO/ZnO and CeO₂/ZnO were prepared, and their activity was significantly enhanced compared to the single metal oxides. Following this, a trimetallic oxide catalyst CuO/CeO₂/ZnO was synthesized, and the loadings of Ce and Cu were optimized. The effects of reaction temperature, time, pressure, and catalyst dosage on the activity were investigated and the results showed that under 160 °C, 6 MPa CO₂, and a catalyst dosage of 10% for 6 h, the yield of glycerol carbonate reached 57.2% with nearly 100% selectivity to carbonate. Moreover, the catalyst could be reused for multiple cycles.

Sarkar et al.^[38] prepared In₂O₃-ZnO composite metal oxides via the precipitation method, followed by the synthesis of CuO/In₂O₃-ZnO catalysts with different Cu loadings using the impregnation method. As the Cu content increased, the catalyst pore size gradually expanded and the number of acid-base sites on the surface also increased. The Cu sites played a crucial role in the activation of CO₂ molecules, and the synergistic interaction between Cu and Lewis bases (In₂O₃/ZnO) activated CO₂, enabling its insertion into the bond between Cu and the Lewis base to form a Cu-O-CO-(In₂O₃/ZnO) complex. The generated bridged bidentate carbonate facilitated the progression of the reaction. Among various catalysts, 2% Cu/In₂O₃/ZnO exhibited the best catalytic activity. Under optimal conditions, the glycerol conversion reached 38%, and the glycerol carbonate yield was 24.46%.

The aforementioned Cu-based catalysts, primarily including CuO, bimetallic oxides (Cu/La₂O₃, CuO/Al₂O₃), and trimetallic composite oxides (CuO/CeO₂/ZnO, CuO/In₂O₃-ZnO), have been applied to this reaction. The effects of coupling agents such as acetonitrile and 2-cyanopyridine on the reaction were also investigated. The results indicate that many factors such as the number of strong basic sites, specific surface area, and pore size in CuO materials significantly influence the reaction activity. Furthermore, the use of suitable reaction solvents helps to enhance the catalytic performance.

Additionally, for the Cu-supported catalysts, a correlation exists between Cu particle size and catalytic activity. The reaction mechanism over the Cu-based catalysts is similar to that over the Zn-based catalysts, involving the intermediate species of copper glycerolate. Although the impregnation method improves the dispersion of Cu and the introduction of other metals modifies the surface acidity and alkalinity of the material, the catalytic activity has not been significantly enhanced. In the future, in-depth research into the mechanism of Cu-based catalysts is needed to enable targeted adjustments of the catalyst’s structural properties, thereby improving its catalytic activity.

Table 3 gives the commonly seen rare earth-based oxide catalysts for the glycerol carbonylation with CO₂. Su et al.^[22] investigated the activity of different oxides (ZnO, TiO₂, ZrO₂, Al₂O₃, CeO₂) in this reaction using 2-cyanopyridine as the coupling agent and observed that CeO₂ exhibited the best activity. Subsequently, they evaluated the catalytic performance of CeO₂ with different morphologies (mesoporous, nanotubes, nanorods, nanowires, nanopolyhedra). Under the conditions of using 2-cyanopyridine as the coupling agent, 170 °C, 10 MPa CO₂, and a reaction time of 12 h, the yield of glycerol carbonate reached 14.2% with CeO₂ nanopolyhedra. Additionally, Liu et al.^[40] prepared the CeO₂ catalysts with various morphologies via precipitation, hydrothermal, and citrate gel methods, and investigated the effect of calcination temperature on the catalyst morphology. The results showed that CeO₂ nanorods synthesized via the hydrothermal method and calcined at 400 °C exhibited a relatively high specific surface area, pore volume, and pore size, along with smaller crystallite size and a greater number of surface oxygen vacancies, leading to superior catalytic activity. The redox properties and acid-base characteristics of CeO₂ significantly influence its activity. Under optimal reaction conditions, using 2-cyanopyridine as the dehydrating agent and dimethylformamide as the solvent, the glycerol conversion reached 38%, with a glycerol carbonate yield of 33.3%. Gao et al.^[41] prepared the CeO₂ nanocrystals with different crystal structures, among which the CeO₂ nano-octahedra dominated by the [111] crystal plane demonstrated better catalytic performance than the CeO₂ nanorods [110] and the nanocubes [100]. Simultaneously, computational simulations confirmed that the cyclization step is the rate-determining step, with the CeO₂[111] surface exhibiting the lowest overall activation energy, which aligns with the superior catalytic performance of CeO₂ nano-octahedra. The catalyst’s activity shows a certain linear correlation with the surface base density. This study provides guidance for the future development of CeO₂-based catalysts. Additionally, mesoporous CeO₂ single-crystal particles prepared via ionic liquid regulation also demonstrated favorable catalytic performance for the glycerol carbonate synthesis. Under relatively low CO₂ pressure (0.5 MPa), a glycerol carbonate yield of 59.1% was achieved. The catalytic activity was positively correlated with the concentration of weak acidic and weak basic sites on the catalyst surface, indicating that weak basic sites (surface hydroxyl groups) and surface defects can act as active centers to promote the activation of polyols and CO₂, synergistically facilitating the formation of cyclic carbonates^[42].

The aforementioned studies indicate that the catalytic activity of CeO₂ is related to its morphologies and crystal structures, which can modulate the specific surface area, crystallite size, pore size, and other properties. Additionally, introducing a second component to prepare materials with special structures can also enhance the catalytic performance. Liu et al. prepared CeO₂-ZrO₂ solid solution catalysts with varying ratios via the hydrothermal method. Compared with pure CeO₂, an initial increase followed by a decrease in the specific surface area, pore volume, and surface acid-base quantity was observed with the increase of the Zr content. When the Zr content was 0.02, the catalytic activity reached its peak, with glycerol conversion at 40.9% and glycerol carbonate yield at 36.3%. Fourier transform infrared spectroscopy (FT-IR) results indicated that CO₂ was mainly adsorbed and activated on the catalyst surface in the form of chelating bidentate carbonate. Post-reaction characterization revealed the presence of cerium glycerolate intermediate species on the catalyst surface, based on which a reaction mechanism was proposed (Figure 7). CO₂ and glycerol are adsorbed and activated on the CeO₂ surface, and the activated CO₂ inserts into the activated glycerol to form a seven-membered ring intermediate species. This intermediate then undergoes intramolecular rearrangement to produce glycerol carbonate, while 2-cyanopyridine reacts with water under the influence of the catalyst to form 2-picolinamide^[43].

Kulal et al.^[44] synthesized CeO₂ nanorods modified with other metals (Ni, Cu, Zn) via the hydrothermal method. With the introduction of Ni, Cu and Zn, the crystallite size of CeO₂ decreased, indicating the formation of more defects on the CeO₂ surface. Raman spectroscopy revealed that the incorporation of Zn generated a higher number of oxygen vacancies in CeO₂, which improved the adsorption and activation of CO₂. Furthermore, Zn promoted the transformation of Ce⁴⁺ to Ce³⁺ in CeO₂, partially compensating for lattice contraction through the insertion of Zn with a smaller ionic radius. The chemical adsorption of CO₂ on the catalyst increased with the concentration of surface oxygen vacancies associated with Ce³⁺. The catalytic activity followed the order: Zn/CeO₂ > Ni/CeO₂ > Cu/CeO₂ > CeO₂ nanorods, which aligned with the trend in the CO₂ adsorption capacity. The 10.5% Zn/CeO₂ catalyst exhibited the best performance, achieving a glycerol conversion of 90.4% and a glycerol carbonate yield of 89.5% under optimal reaction conditions.

CeO₂ exhibits significant advantages in the direct catalytic synthesis of glycerol carbonate from glycerol and CO₂ due to its tunable acid-base properties and abundant surface oxygen vacancies. Studies indicate that the catalytically active centers are positively correlated with the concentration of surface oxygen vacancies (Ce³⁺), weak basic sites (such as surface hydroxyl groups), and weak acidic sites.

These sites can synergistically activate CO₂ (forming chelating bidentate carbonates) and glycerol (forming cerium glycerolate intermediates), thereby reducing the activation energy of the rate-limiting cyclization step. The introduction of metal ions such as Zn and Ni can effectively reduce the crystallite size of CeO₂, induce more oxygen vacancies and Ce³⁺ species, and enhance the chemical adsorption and activation capacity of CO₂. Among these, Zn doping has shown particularly outstanding performance, significantly increasing the yield of glycerol carbonate to nearly 90%. Future research could focus on fine-tuning the concentration of surface oxygen vacancies and the Ce³⁺/Ce⁴⁺ ratio through post-synthesis treatments such as calcination in different atmospheres, plasma treatment, or reduction treatments, aiming to maximize active sites. Building on the notable success of Zn doping, further exploration of other low-cost doping metals (e.g., Co, Mn, Fe) that can effectively modulate the electronic structure of CeO₂ and promote oxygen vacancy generation would be valuable.

La₂O₃ has also been applied in the carboxylation of glycerol with CO₂ to synthesize glycerol carbonate. Using hexanedinitrile as the coupling agent, the effects of crude glycerol (containing impurities such as water, methanol, sodium methoxide, and fatty acid methyl esters) and pure glycerol as feedstocks on the catalytic performance were investigated. For the crude glycerol, the presence of methanol helped to enhance the selectivity toward the target product, whereas impurities such as water, sodium methoxide, and fatty acid methyl esters led to a rapid increase in the by-products, negatively impacting the glycerol carboxylation reaction^[45].

Razali et al.^[46] investigated the influence of different coupling agents (acetic anhydride, benzonitrile, acetonitrile, and hexanedinitrile) on the reaction activity using La₂O₃ as the catalyst. Compared with the most commonly used dehydrating agent, acetonitrile, hexanedinitrile increased the yield of glycerol carbonate by fivefold. This effect can be partially attributed to the dinitrile structure of hexanedinitrile. Additionally, hexanedinitrile plays a key role in suppressing the by-product formation and enhancing the mass transfer effect resulting from improved CO₂ solubility. Furthermore, researchers prepared La₂O₂CO₃ via co-precipitation, hydrothermal, and sol-gel methods and found that the catalyst synthesized by co-precipitation exhibited the best activity, achieving a glycerol conversion of 57% and a glycerol carbonate selectivity of 18% when using hexanedinitrile as the coupling agent.

Hu et al.^[47] prepared La nanocluster-modified ZIF-8; the large specific surface area of ZIF-8 facilitates uniform distribution of La on the zeolite surface, while the La³⁺–O²⁻ pairs enhance the acidity and basicity of the catalyst, improving its ability to activate glycerol and CO₂. Using acetonitrile as the coupling agent and 5La/ZIF-8 as the catalyst, the glycerol conversion and glycerol carbonate yield reached 46.5% and 35.3%, respectively. Additionally, the investigation of the effects of coupling agents such as MgCO₃, H₂SO₄ and CaCO₃ on the reaction indicated that MgCO₃ as a coupling agent significantly enhanced product selectivity. When CH₃CN and MgCO₃ were used as the dehydrating agents, the CO₂/glycerol conversion followed a second-order reaction, with activation energies of 7.83 and 5.42 kJ/mol, respectively. Acetonitrile as a coupling agent requires a higher activation energy and reaction temperature, leading to increased by-product formation and reduced product selectivity. In contrast, MgCO₃ as a coupling agent could improve the product selectivity.

Perovskite-structured LaMO₃ (M = Fe, Co, Ni) catalysts were prepared via the sol-gel method. The transition metals in these catalysts exhibit multiple valence states, leading to unique surface properties such as oxygen defects. Among them, LaNiO₃ showed the highest concentration of oxygen vacancies, while La₂O₃ had the fewest. Additionally, the CO₂-TPD results indicated that the number of weak and medium basic sites for different catalysts followed the order of LaCoO₃ > LaNiO₃ > LaFeO₃ > La₂O₃. LaCoO₃ demonstrated the best catalytic performance, primarily due to its higher number of weak and medium basic sites. However, strong basic sites tend to promote side reactions, reducing the selectivity of glycerol carbonate^[48].

The La-based catalysts demonstrate potential in the synthesis of glycerol carbonate from glycerol and CO₂, with their performance influenced by both the catalyst structure (basicity, oxygen vacancies) and the type of coupling agent (hexanedinitrile, acetonitrile, MgCO₃, 2-cyanopyridine). By adjusting the catalyst structure (e.g., perovskite-type LaMO₃), enhancing La dispersion (e.g., La/ZIF-8), and improving surface acidity-basicity, the catalytic activity can be significantly enhanced. Moreover, employing different coupling agents can optimize the product selectivity. For instance, using MgCO₃ as a coupling agent achieves a glycerol carbonate selectivity up to 93%, markedly outperforming acetonitrile, hexanedinitrile, and 2-cyanopyridine. Future research should focus on the precise regulation of the surface properties of La-based catalysts, such as through the fabrication of composite metal oxides with tailored morphologies, to further improve the catalytic activity.

Table 4 gives the commonly seen Mg-based catalysts for the glycerol carbonate synthesis from glycerol and CO₂. Among MgO, CaO and Al₂O₃ prepared using the precipitation method, MgO exhibited a relatively large specific surface area and pore volume, facilitating the exposure of more active sites. Additionally, MgO displayed the highest total basicity, primarily composed of strong and weak basic sites. CaO showed comparable total basicity to MgO, but its basic sites were mainly of medium strength. γ-Al₂O₃ had the lowest total basicity, and similar to MgO, its basic sites were predominantly strong and weak.

The catalytic activity of these three oxides followed the order of MgO > CaO > Al₂O₃, which correlated with the number of basic sites on the catalyst surface. It was also observed that both the quantity of basic sites and the BET surface area of the catalyst significantly impact the catalytic performance for this reaction^[49].

To further modulate the surface acidity-basicity and specific surface area of the catalysts, bimetallic composite oxide catalysts including MgCa, MgAl and CaAl were prepared via the co-precipitation method. The MgAl oxide mainly existed in the cubic phase of MgAl₂O₄^[50]. In the CaAl oxide, in addition to CaO, phases such as Ca₁₂Al₁₄O₃₃ and CaAl₂O₄ were also observed. The MgCa oxide primarily consisted of MgO and CaO phases. The MgCa catalyst exhibited a relatively large specific surface area, allowing for the exposure of more active sites. The MgAl and CaAl catalysts had a higher number of surface basic sites, predominantly strong basic sites. In contrast, although the total number of basic sites on the MgCa surface was relatively lower, they consisted mainly of weak and medium-strength basic sites, with little strong basic sites. Under identical compositional conditions, the MgCa catalyst demonstrated the best performance. Furthermore, the optimization results of MgCa composition indicated that as the Mg content increased, the specific surface area gradually increased, with Mg_0.75Ca_0.25O exhibiting a relatively large specific surface area, pore size and pore volume. The basicity of the catalyst increased with higher Mg content, and the strength of the basic sites was also enhanced. The number of strong basic sites in Mg_0.75Ca_0.25O rose rapidly, reaching 1904.5 µmol/g. The higher the Mg content in the MgCa catalyst, the higher the reaction activity. When the Mg/Ca ratio increased from 1∶3 to 1, the glycerol conversion increased from 14.8% to 18.2%. The catalytic activity was positively correlated with the specific surface area, surface basicity, and quantity of basic sites. By optimizing the reaction conditions at 180 °C and a CO₂ pressure of 7 MPa, the glycerol conversion reached 25.7%, and the glycerol carbonate yield was 16%^[50].

To increase the number of oxygen vacancies in the material, researchers introduced other valence-variable metals to prepare trimetallic composite oxide catalysts. Chen et al.^[51] synthesized highly dispersed MgInCe composite metal oxides using hydrotalcite as the precursor. After calcination at 500 °C, well-crystallized MgIn₂O₄ was formed, while lower calcination temperatures resulted in a poorer crystal structure of MgIn₂O₄. The incorporation of In led to the formation of more In–O vacancies on the material surface, facilitating the adsorption and activation of CO₂. The introduction of Ce created Mg-O-Ce acid sites, enhancing the activation capability for glycerol. As a result, under the synergistic effect of acid-base sites, the catalyst exhibited excellent activity, with glycerol carbonate selectivity reaching up to 99%. The activation process of CO₂ and glycerol on the catalyst was investigated using FT-IR, and a possible reaction mechanism was proposed (Figure 8): adjacent Mg-O-Ce sites and In-O vacancy sites provide synergistic effects. (1) Mg-O-Ce sites activate glycerol in a dual-activation mode, (2) In-O vacancy sites simultaneously adsorb CO₂ in a bidentate activation mode, and (3) high dispersion at the atomic level promotes the insertion of activated CO₂ into the five-membered ring glycerol intermediate.

Furthermore, Pattanaik et al.^[52] prepared the Mg-Zn-Ce composite metal oxide catalysts using the co-precipitation method. The introduction of an appropriate amount of Mg into Zn/Ce oxides promoted the formation of a solid-solution structure, which facilitated the generation of more oxygen vacancies on the surface and also improved the specific surface area. The surface of the Mg-Zn-Ce catalyst exhibited both Brønsted and Lewis acid structures. Compared to other materials, the 10Mg/10Zn/CeO₂ catalyst possessed the highest number of acid and base sites. The ternary oxides, 10Mg/10Zn/CeO₂, in particular, demonstrated higher glycerol conversion and glycerol carbonate yield. A high concentration of surface oxygen vacancies, sufficient acid-base strength, and a larger surface area are effective means to enhance catalytic performance. Under optimal reaction conditions, the glycerol conversion reached 90%, with a glycerol carbonate yield of 58%. The catalyst was easily separable and reusable, showing no significant loss in activity after five consecutive cycles.

Charif et al.^[53] prepared phosphorus (P), sulfur (S), magnesium (Mg), and lithium (Li) doped graphitic carbon nitride (g-C₃N₄). Doping with these elements reduced the crystallinity of C₃N₄, decreased crystallite size, widened pore channels, and increased pore volume. Compared to pure C₃N₄, the incorporation of phosphorus and sulfur altered the electronic properties and enhanced Lewis basicity. Relative to LiPSCN, the MgPSCN catalyst exhibited better thermal stability and demonstrated superior catalytic performance, achieving a maximum glycerol conversion of 70.05% and a maximum glycerol carbonate (GC) yield of 45.85%. The gradual decline in activity after catalyst reuse was primarily attributed to the detachment of active particles from the carbon nitride surface.

Compared to MgO, composite metal oxides exhibited enhanced catalytic activity. Among binary composite oxides such as Mg-Ca and Mg-Al, the Mg-Ca system, dominated by weak to medium-strength basic sites, demonstrated superior performance to Mg-Al and Ca-Al systems containing strong basic sites. This indicates that the distribution of base strength is more critical than the total base quantity alone. A larger specific surface area, appropriate pore size, and pore volume are essential for exposing sufficient active sites and promoting reactant diffusion. Multicomponent composite oxides (e.g., Mg-Zn-Ce) possess characteristics such as a high concentration of oxygen vacancies, bifunctional acid-base sites, and a large specific surface area, achieving high activity (glycerol conversion up to 90%) and good stability. Heteroatom-doped g-C₃N₄ (e.g., with P, S, Mg, Li) also displayed favorable catalytic activity by modulating the electronic structure and basicity, though its stability (e.g., active component detachment) remains a key challenge to address.

Future research could focus on designing acid-base sites with atomic-scale proximity and spatially matched structures, such as metal-oxygen pairs (acidic sites) coupled with basic sites/oxygen vacancies, by utilizing well-defined precursors like hydrotalcite, spinel, or perovskite to fabricate highly dispersed and structurally stable bifunctional or multifunctional active centers. Building on the proven effectiveness of systems like Mg-Ca and Mg-Zn-Ce, further exploration of introducing other metal elements (e.g., Fe, Co, Ni, Mn) to modulate the surface oxygen vacancies and acid-base properties could lead to further improvements in the catalytic activity.

Table 5 gives the commonly seen noble metal catalysts for the glycerol carbonate synthesis from glycerol and CO₂. Ozorio et al. prepared AgY, ZnY and SnY catalysts by impregnating metal nitrates on the surface of NaY zeolite. Among them, AgY exhibited the highest glycerol conversion, primarily attributed to the high dispersion of Ag on the zeolite surface^[54].

Lukato et al.^[55] loaded Au onto a cerium-based metal-organic framework (MOF) using different methods. When MgCO₃, acetonitrile, and hexanedinitrile were used as the dehydrating agents, MgCO₃ showed the best performance. After reduction with hydrogen, the 4% Au@MOF catalyst achieved a glycerol conversion of 52% and a glycerol carbonate yield of 44% under a CO₂ pressure of 1.5 MPa. The high selectivity of this catalyst is likely related to the small and uniformly distributed Au nanoparticles within the MOF cavities.

Au nanoparticles exhibit strong surface plasmon resonance (SPR) in the visible region, enabling efficient capture of visible light to generate high-energy hot electrons and holes, thereby enhancing catalytic activity through photothermal synergy. Liu et al.^[56] prepared a series of Au/ZnWO₄-ZnO catalysts via the impregnation method. The results showed that a certain interaction was formed between Au and Zn. As the Au content increased, the ZnO particle size gradually increased, while the specific surface area and total surface basicity of the catalyst decreased. Furthermore, the introduction of Au enhanced the visible-light absorption intensity of the catalyst. Excitation by visible light promoted the generation of photogenerated electron-hole pairs, which facilitated the adsorption and activation of CO₂ and glycerol. This process suppressed the desorption of CO₂ under high temperature and improved the selectivity toward the target product. Building on this, they further prepared a series of Au/Co₃O₄-ZnO catalysts and found that the heterojunction structure of Au/Co₃O₄-ZnO facilitated the visible-light absorption and promoted the separation of electron-hole pairs. Additionally, the loading of Au altered the electron density of Co and Zn, enhancing the reducibility of Co and increasing the number of oxygen vacancies on the Co₃O₄-ZnO surface. The localized surface plasmon resonance (LSPR) effect of Au nanoparticles strengthened the visible-light absorption capacity of Au/ Co₃O₄-ZnO and improved the separation of photogenerated electron-hole pairs, thereby enhancing the photothermal catalytic performance^[57]. This approach, leveraging photothermal synergy, offers a new direction for the development of highly efficient catalysts.

Currently, research on the noble metal catalysts mainly focuses on Au-based systems. By supporting Au on metal oxides or MOF materials, the dispersion of Au can be enhanced. Additionally, the electronic interaction between Au and the carrier modulates the reducibility of the support and the concentration of surface oxygen vacancies. The localized surface plasmon resonance (LSPR) effect of Au enhances visible light absorption and promotes the separation of photogenerated carriers, while photothermal synergy improves the reaction efficiency. The introduction of coupling agents significantly increases glycerol conversion and glycerol carbonate yield. However, due to the high cost of noble metals, future efforts should prioritize rational design strategies to improve the dispersion of noble metals, thereby increasing atomic utilization efficiency and reducing loading levels.

Table 6 gives the commonly seen modified zeolite catalysts for the glycerol carbonate synthesis from glycerol and CO₂. The Co-ZIF-67 material prepared by Hu et al.^[58] exhibits high crystallinity, a large specific surface area (1250 m²/g), good thermal stability, facile ligand-to-metal electron transfer, and abundant Lewis basic sites, which contribute to its high catalytic activity in the glycerol carbonylation with CO₂. Using acetonitrile as a coupling agent under 0.3 MPa CO₂, the selectivity to glycerol carbonate reached 92%, with a carbonate yield of 29%. The reaction followed a third-order kinetics and proceeded rather slowly; a longer reaction time favored the progress of this reaction.

Gao et al.^[59] prepared a series of Co, Zn, Cu, Ni, Zr, Ce and Fe-modified ETS-10 catalysts. The introduction of metals led to a slight decrease in the specific surface area of the catalysts, while the number of basic sites on the catalyst surface increased, and the alkali strength decreased. Among them, the Co-modified zeolite displayed little structural damage to the zeolite support. Moreover, the moderate Lewis basic sites of ETS-10 and the synergistic effect of Co played a positive role in activating CO₂ and glycerol. In addition, Co/ETS-10 exhibited good structural integrity and suitable Lewis basic and acidic strengths. Under 170 °C, an initial pressure of 4.0 MPa, 5 mL of CH₃CN and a reaction time of 6 h, the Co/ETS-10 catalyst showed the highest activity, with a glycerol conversion of 35.0% and a carbonate yield of 12.7%.

Hu et al.^[60] prepared Ce-, Cu-, and Mg-modified ZIF-8 catalysts. The introduction of Ce and Cu resulted in isomorphic substitution of Zn in ZIF-8, maintaining the structural integrity of ZIF-8. In contrast, the introduction of Mg disrupted the ZIF-8 structure, leading to the formation of rod-like structures accompanied by irregular and small particles, which exhibited characteristics of a ZIF-8 and metal oxide composite material. The surface alkalinity of all metal-doped catalysts increased, with Mg-modified ZIF-8 showing the most significant increase. The catalytic activity of all metal-modified ZIF-8 catalysts was improved, with Mg-ZIF-8 demonstrating the best reaction performance. The effects of coupling agents such as acetonitrile, CaCO₃, MgCO₃, Na₂SiO₃, CaF₂, CaCl₂ and NaHCO₃ were also investigated, which revealed that using acetonitrile as the coupling agent gave a relatively high glycerol conversion; however, the selectivity to glycerol carbonate was low due to the formation of by-products. In contrast, when NaHCO₃ was used as the coupling agent, the product selectivity reached 91%.

The introduction of other metal oxides can significantly modulate the zeolite surface acidity and alkalinity, offering considerable advantages for the design and optimization of the catalysts for glycerol carbonylation. Currently, the research on zeolite modification remains limited. Although Co-modified ZIF-67 and ETS-10 displayed comparable glycerol conversion, they were rather different in the selectivity to glycerol carbonate. Future studies could focus on these two catalysts to investigate the influence of the physicochemical properties of zeolites on the side reactions, aiming to enhance the selectivity for the target product. Experimental results indicate that the coupling agents have a significant impact on the product selectivity. Although acetonitrile facilitates the primary reaction, it may also promote the side reactions. In contrast, inorganic coupling agents such as NaHCO₃ can efficiently remove water molecules, shift the reaction equilibrium toward the product side, and achieve high selectivity (> 90%) to glycerol carbonate. Future work could explore modifications using different basic zeolites (e.g., SBA-15, ZSM-5, X- and Y-type zeolites) and the effects of various coupling agents (e.g., 2-cyanopyridine, adiponitrile) on the glycerol carbonylation reaction.

Besides abovementioned catalysts, some other catalysts are also used in the glycerol carbonate synthesis from glycerol and CO₂, as given in Table 7. The Ti-Al-Mg, Ti-Cr-Mg and Ti-Fe-Mg series catalysts were prepared via co-precipitation^[61]. The trivalent metal ions Al, Cr and Fe can modify the acid-base properties and electronic environment around Mg on the catalyst surface, thereby modulating the catalytic performance. Compared to Ti-Cr-Mg and Ti-Fe-Mg, a ternary oxide MgAl₂Ti₃O₁₀ was formed in Ti-Al-Mg, where Mg²⁺ ions had different neighboring cations (Al³⁺ and Ti⁴⁺), creating a more electron-rich environment and donating electrons to the Mg²⁺ ions through an inductive effect; this could weaken the basic characteristic of Mg²⁺. Ti-Al-Mg exhibited the highest low-temperature CO₂ adsorption capacity and the lowest CO₂ binding energy on its surface, thus facilitating CO₂ adsorption and activation. The activated CO₂ was also easily desorbed and could subsequently participate in the reaction. The Ti-Cr-Mg and Ti-Fe-Mg catalysts exhibited limited porosity and a dense structure, which restricted the diffusion of reactants; in contrast, the surface of the Ti-Al-Mg catalyst was porous, rough, and composed of irregularly shaped particles, featuring a layered yet dense structure with a relatively large specific surface area. The extended pore network and interparticle voids facilitated effective molecular diffusion, while the roughness and porosity provided a substantial number of exposed active sites, enabling efficient adsorption and activation of reactants. Consequently, the Ti-Al-Mg catalyst demonstrated the highest catalytic activity, achieving a glycerol carbonate yield of 36.1%^[61].

Furthermore, mesoporous CeLaCo perovskite-type composite metal oxides have also been employed in the glycerol carbonylation reaction. After calcination at 700 °C, this material develops a higher number of strong basic and acidic sites on its surface. By using ZSM-5 zeolite membranes with different Si/Al ratios as the physical water-removing membrane, the catalytic reaction efficiency is significantly enhanced. As the Si/Al ratio of the ZSM-5 membrane decreases, the increased concentration of Na⁺-gated channels facilitates preferential water separation, leading to continuous improvement in the glycerol conversion and glycerol carbonate yield. Additionally, the ZSM-5 membrane demonstrates good reusability in the coupled reaction-separation process^[62]. This research provides novel insights for the development of physical dehydration processes.

Collett et al.^[63] utilized rice husks (RH), oilseed rape straw (OSR), softwood pellets (SWP), and wheat straw pellets (WSP) to prepare biochar materials for the catalytic synthesis of glycerol carbonate. The SWP-derived biochar did not contain metallic elements, whereas the carbon materials prepared from other feedstocks contained elements such as Si, K, Ca and Mg. Furthermore, significant differences exist in the specific surface area and micropore volume of the materials. Activity test results show that SWP biochar and activated carbon had no activity for this reaction, while other biochar materials demonstrated certain catalytic activity. Moreover, biochar materials with higher ash content showed better catalytic performance. The K content played a crucial role in the catalytic activity, with RH-550 demonstrating the most favorable catalytic performance.

Fao et al.^[64] prepared P- and S-doped graphitic carbon nitride (PSCN) catalysts. The doping of P and S improved the Lewis acid-base sites on the catalyst surface, thereby promoting the glycerol carbonylation reaction. The co-doping of P and S enhanced CO₂ adsorption and facilitated the separation of e⁻-h⁺ pairs, resulting in a higher reaction rate over PSCN compared to that over unmodified CN.

Furthermore, density functional theory (DFT) calculations indicated that the effective charge transfer induced by P and S doping could reduce the activation barrier for the carboxylation, enhancing the catalytic activity. This provides a promising catalyst for the CO₂/glycerol conversion.

Among the aforementioned catalysts, biochar prepared from biomass exhibits certain catalytic activity for this reaction, primarily attributed to the alkali metal ions, especially potassium, present in the biochar. This indicates that such catalysts are essentially metal-based in nature. Meanwhile, P- and S-doped C₃N₄ demonstrates a relatively high glycerol conversion under lower reaction pressure, offering new insights for the development of novel catalysts. Additionally, the use of ZSM-5 zeolite membranes to enhance water separation and improve reaction performance provides a novel pathway for reaction-separation coupling. Future research could focus on exploring the photothermal synergistic catalytic properties of C₃N₄ non-metallic catalysts, while optimizing coupling agents and reaction solvents to further enhance the catalytic activity. The ZSM-5 zeolite membrane reaction process is green and efficient, making it potentially applicable to other catalytic systems and thereby advancing the development of this synthesis route.

Table 8 summarizes the best-performing catalysts among Zn-, Cu-, Ce-, La- and Au-modified zeolites and non-metallic materials. Among these, ZnO-CeO₂ exhibits the highest activity, achieving a selectivity of 99% to glycerol carbonate and a yield of 89.5% under the conditions using 2-cyanopyridine as the coupling agent and N,N-dimethylformamide as the solvent. This performance is comparable to that of the homogeneous Zn(OTf)₂ catalyst. Under nearly identical conditions, CuO and MgZnCe catalysts show similar glycerol conversions, but they differ significantly in the product selectivity, indicating a substantial influence of the catalyst on product distribution. Zeolites, with their large specific surface areas, can significantly enhance the dispersion of active components, thereby improving catalytic activity. La₂O₃/ZIF-8 and Mg-ZIF-8 also demonstrate good activity for this reaction; however, compared to the Zn-based catalysts, they display much lower glycerol conversion and product yield, which may be attributed to the differences in the intrinsic properties of their active sites. Although Au, as a noble metal catalyst, achieves high dispersion when supported on MOF materials, it does not show a distinct activity advantage. The results above indicate that Zn-based catalysts exhibit a clear activity advantage for this reaction and currently hold the greatest potential for industrial application.

Furthermore, when La₂O₃/ZIF-8 was used as the catalyst and acetonitrile as the coupling agent, the highest product selectivity observed was 76%. In contrast, other coupling agents achieved a selectivity above 90%, with 2-cyanopyridine notably reaching 99%. Thus, acetonitrile is not an ideal choice as the coupling agent, which aligns with theoretical calculation results^[20]. Additionally, the use of polar solvents such as N,N-dimethylformamide or N-methylpyrrolidone resulted in significantly higher glycerol conversions, indicating that polar solvents facilitate the activation of glycerol or CO₂ and promote the progress of the carbonylation reaction.

Although ZnO-CeO₂ exhibited high catalytic activity under the conditions using 2-cyanopyridine as the coupling agent and dimethylformamide as the solvent, the reaction system is complex, involving lengthy subsequent product separation processes, and the high cost of 2-cyanopyridine poses a challenge.

For the catalyst optimization, future efforts could focus on modulating the preparation parameters and synthesis methods of ZnO-CeO₂ to achieve unique morphologies such as layered hydrotalcite, perovskite, or fluorite structures. Alternatively, mesoporous/porous composite oxides could be synthesized via templating methods or by supporting the active components on zeolite surfaces to increase specific surface area and expose more active sites. Additionally, introducing other components (e.g., Fe, Mn) or employing post-treatment techniques (e.g., calcination under different atmospheres, plasma treatment, reduction) could fine-tune the surface oxygen vacancy concentration and Ce³⁺/Ce⁴⁺ ratio, maximizing active site availability and further enhancing catalytic performance. The research on other catalyst types, such as rare earth oxides, zeolites, noble metals, and non-metallic catalysts, remains relatively limited. Systematic studies on their physicochemical properties and reaction mechanisms are needed to build a robust theoretical foundation for the catalyst design. An integrated experimental and computational approach could be adopted to rationally screen coupling agents such as CaC₂, 2-cyanopyridine, MgCO₃, and NaHCO₃. Moreover, novel zeolite membrane reactors could be designed to develop physical separation methods for efficient removal of byproduct water, promoting a greener and more sustainable process.