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Article

CeO2-ZrO2 Solid Solution Catalyzed and Moderate Acidic–Basic Sites Dominated Cycloaddition of CO2 with Epoxides: Halogen-Free Synthesis of Cyclic Carbonates

by
Jie Gao
1,
Chengguang Yue
1,
Hao Wang
1,
Jiaxin Li
1,
He Yao
1,
Mei-Yan Wang
1,2,3,* and
Xinbin Ma
1,2,3
1
Key Laboratory for Green Chemical Technology of Ministry of Education, Collaborative Innovation Center of Chemical Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
2
Zhejiang Institute, Tianjin University, Ningbo 315201, China
3
Joint School of National University of Singapore and Tianjin University, International Campus of Tianjin University, Binhai New City, Fuzhou 350207, China
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(6), 632; https://doi.org/10.3390/catal12060632
Submission received: 19 May 2022 / Revised: 2 June 2022 / Accepted: 2 June 2022 / Published: 9 June 2022
(This article belongs to the Special Issue Catalytic Transformations of CO2 into High Valuable Products)
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Abstract

:
For the production of cyclic carbonates from the cycloaddition of CO2 with epoxides, halogen pollution and product purity are two of the most common problems due to the usage of homogeneous halogen-containing catalysts such as ammonium salt and alkali metal halide. Hence, the development of a novel, halogen-free and efficient catalyst for the synthesis of high-purity cyclic carbonates is significant. Here, a series of acid–base bifunctional Ce1-xZrxO2 nanorods were successfully prepared. The Ce1-xZrxO2 nanorods could catalyze the cycloaddition of CO2 with epoxides efficiently without any halogen addition. Especially for the Ce0.7Zr0.3O2 catalyst, a conversion of 96% with 100% 1,2-butylene carbonate selectivity was achieved. The excellent catalytic performance of Ce1-xZrxO2 nanorods is attributed to the formation of the CeO2-ZrO2 solid solution, which contributes to abundant moderate acidic–basic active sites on the catalyst surface. It is the synergistic effect of moderate acidic–basic sites that dominates the conversion of CO2 with epoxides, which will supply important references for the synthesis of efficient metal oxide catalyst for the cycloaddition of CO2 with epoxides.

1. Introduction

It is of great interest to utilize CO2 as a cheap, abundant, and non-toxic C1 resource to produce high-value-added chemicals such as organic carbonates [1,2,3], urea derivatives [4,5], methanol [6,7], carboxylic acid [8,9], and heterocycles [10,11]. Among these examples, the cycloaddition of CO2 with epoxides is one of the more economical and green pathways [12,13], and the resulting cyclic carbonates are widely applied as vital additives for electrolytes in lithium batteries, excellent aprotic polar solvents for coatings and adhesives, and raw materials for the synthesis of fine chemicals and polymers. [14,15]. In recent years, with the boom of new energy vehicles and battery energy storage, the consumption of cyclic carbonates has been increasing rapidly because of the growth in demand for lithium batteries.
Over the past decades, many homogeneous catalysts, such as quaternary ammonium salt [16,17], alkali metal halide [18,19], ionic liquid [20,21,22], and metal salen complex [23,24], have been developed for the cycloaddition of CO2 with epoxides to cyclic carbonates. These catalysts exhibit high catalytic activity; even ammonium salt and alkali metal halide have already been used for industrial production. However, these catalysts are mostly halogen-containing, causing halogen pollution and making the product purity unsatisfactory [25,26]. As we all know, halogens at ppm level in an electrolyte will seriously affect the performance of lithium batteries [27,28]. Although the high purity of cyclic carbonate can be accessed by a series of complicated separation processes, this involves a high separation cost. Heterogeneous catalysts such as supported ionic liquids [29,30], ion-exchange resins [31,32], functionalized polymers [33,34], metal–organic framework (MOF) materials [35,36], and zeolitic imidazolate frameworks (ZIFs) [37,38] were also developed. However, most of these catalysts have the problems of either low activity, owing to the deficiency of active sites, or low stability caused by the leaching of active components. Most notably, the active catalytic component of these catalysts is still the halogen anion.
The application of metal oxide catalysts supplies a halogen-free strategy for the synthesis of cyclic carbonates. MgO and Al2O3 have been reported to catalyze the cycloaddition of CO2 with epoxides but showed poor catalytic activity due to their single strong basic or acidic active sites [39,40,41]. Although ZnO [42,43], La2O3 [44], Nb2O5 [45], and Cs-P-Si [46] all have both acidic sites and basic sites, their intensity of acidity and basicity were so weak that the addition of halogens as co-catalysts or harsh reaction conditions of supercritical carbon dioxide was essential to attaining a good cyclic carbonate yield. Compared to monometallic oxides, multimetal oxides, e.g., Li-Mg [47], Mg-Al [48], Zn-Mg-Al [49], Mn-Ba [50], La-Zr [51], and Ce-La-Zr [52,53] possess more flexible acidic and basic sites, thus exhibiting good catalytic activity for the cycloaddition of CO2 with epoxides without additives of halogen, and over 70% yield of cyclic carbonate product could be delivered. Despite the above few examples of metal-oxide-catalyzed cycloaddition of CO2 with epoxides, the intrinsic correlation between activities and acidic–basic sites of the catalyst is still unclear, and the catalytic activity of metal oxides needs to be enhanced. Hence, developing a novel, halogen-free, and efficient metal oxide catalyst with feasible and bifunctional acidic–basic active sites is meaningful. It is also of great significance to reveal the interconnectedness between acidic–basic properties and the activities of the catalyst for the design and synthesis of metal oxide catalysts with specific active sites.
On the other hand, as a readily available metal oxide, CeO2 shows excellent Lewis acidic–basic properties at the same time, and its acidic–basic sites can be easily tuned by morphological adjustment or incorporation with other metals [54,55]. Among dispersible CeO2 nanocrystals of various morphologies, including cube, rod, polyhedra, and octahedra [56], the rod-shaped CeO2 was reported to possess the largest surface area and the largest number of acidic–basic sites on the surface. [57,58] Therefore, CeO2 nanorods may show good catalytic activity in the cycloaddition of CO2 with epoxides. Meanwhile, Zr4+ with a smaller ionic radius can easily enter the lattice of CeO2 and form a CeO2-ZrO2 solid solution, which will promote the surface acidic–basic properties of CeO2 [59]. A commercially available ceria- and lanthana-doped zirconia (Ce-La-Zr-O) catalyst has been applied for the cycloaddition of propylene oxide with CO2 by Saha et al. [52]. The conversion of propylene oxide can be achieved to 92% under conditions of 170 °C and 7 MPa CO2, while only 72% selectivity of propylene carbonate was presented. Here, by tuning the doping content of Zr, we prepared a series of rod-like CeO2-ZrO2 solid solution catalysts with various acidic–basic properties (Ce1-xZrxO2, x is the mole ratio of Zr to Ce+Zr in the catalysts, x ranges from 0.1 to 0.5). The Ce1-xZrxO2 nanorods, particularly Ce0.7Zr0.3O2, exhibited high activity for the synthesis of the cyclic carbonate from CO2 and epoxides, even under halogen-free conditions. It was found that the Ce1-xZrxO2 catalysts were rich in moderate acidic–basic sites, and the amount of active sites varies with Zr content, which is crucial to determining the reactivity of cycloaddition of CO2 with epoxides.

2. Results

A series of Zr-doped CeO2 nanorods, as well as monometallic CeO2 and ZrO2, were synthesized via the hydrothermal method [60]. The morphology and physical–chemical properties of different catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), Brunner–Emmet–Teller (BET), inductively coupled plasma-atomic emission spectrometry (ICP-AES), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption of ammonia and carbon dioxide (NH3-TPD and CO2-TPD).

2.1. Characterization of Ce1-xZrxO2 Nanorods

XRD analysis was performed initially to investigate the crystal structure of synthesized Ce1-xZrxO2 catalysts. As shown in Figure 1, all samples exhibited the characteristic cubic fluorite structure of CeO2 crystal (JCPDS no. 43–1002). The Ce1-xZrxO2 showed broader peaks which shifted to a higher angle compared to pure CeO2 (Table 1), suggesting that Zr was successfully doped into the ceria lattice structure. The calculated lattice parameter values for pure CeO2, Ce0.9Zr0.1O2, Ce0.8Zr0.2O2, Ce0.7Zr0.3O2, Ce0.6Zr0.4O2, and Ce0.5Zr0.5O2 are 5.39 Å, 5.38 Å, 5.37 Å, 5.37 Å, 5.36 Å, and 5.34 Å, respectively (Table 1). The lattice parameter decreased because Zr4+ with a smaller ionic radius (0.84 Å) replaced Ce4+ with a larger ionic radius (0.97 Å), which further confirmed the formation of CeO2-ZrO2 solid solution, accompanied by the shrinking interplanar spacing, which decreased from 3.11 to 3.06 Å [61]. By adjusting the Zr content, a phase-pure CeO2-ZrO2 solid solution formed in the catalysts of Ce0.9Zr0.1O2, Ce0.8Zr0.2O2, and Ce0.7Zr0.3O2. Excessive Zr will lead to the occurrence of non-negligible ZrO2 particles, since a weak peak assigned to the (220) crystal plane of ZrO2 appeared at 50.5° in Ce0.6Zr0.4O2 and Ce0.5Zr0.5O2.
A thorough inspection was further performed using TEM and HRTEM analysis to study the detailed microstructure of Ce1-xZrxO2. The TEM gave a panoramic overview, which suggested that Ce0.9Zr0.1O2, Ce0.8Zr0.2O2, and Ce0.7Zr0.3O2 maintained good rod-like morphology, as shown in Figure 2a–d. According to the HRTEM images, the rod Ce1-xZrxO2 exhibited (111), (200), and (220) lattice planes of CeO2 (Figure 2e,f and Figure S1c,d), which accorded well with the XRD results. Agglomerates appeared on Ce0.6Zr0.4O2 and Ce0.5Zr0.5O2, leading to morphological irregularity (Figure S1a,b). EDS characterization of the agglomerated particle of Ce0.5Zr0.5O2 (Figure S2) indicated the agglomerate was ZrO2, which was consistent with the results in XRD. The Ce0.7Zr0.3O2 was then singled out as the representative for EDS-Mapping characterization (Figure S3). The results demonstrated that all the constituent elements, i.e., Ce, Zr, and O, presented a homogeneous distribution (except for the agglomerates in Ce0.6Zr0.4O2 and Ce0.5Zr0.5O2).
The bulk phase and surface compositions of prepared Ce1-xZrxO2 were also investigated by ICP-AES and XPS, respectively (Table 1). For Ce0.9Zr0.1O2, Ce0.8Zr0.2O2, and Ce0.7Zr0.3O2 nanorods, it could be found that the content of Zr matched well with the added proportion of raw materials, both on the surface and in the bulk phase. However, the surface Zr/(Ce+Zr) ratios were slightly higher than the bulk ratio for Ce0.6Zr0.4O2 and Ce0.5Zr0.5O2, further indicating the aggregation of Zr on the surface of the catalysts.
BET surface area analysis indicated that the samples were typical mesoporous materials with one-dimensional pore structures (Figure S4). The textures of the catalysts are listed in Table 1. The specific surface area and the pore volume of CeO2 nanorods did not change much upon increasing the Zr doping ratio to 0.3, indicating that a suitable content of Zr has almost no effect on the surface area. When further increasing the doping amount of Zr (Ce0.6Zr0.4O2 and Ce0.5Zr0.5O2), the area decreased rapidly, caused by the aggregation of Zr on the surface of the catalysts.
Considering that the surface acidic–basic characteristics are crucial to the cycloaddition of CO2 with epoxides, NH3-TPD and CO2-TPD for Ce1-xZrxO2 nanorods were studied (Figure 3 and Figure 4). We can see that the synthesized Ce1-xZrxO2 catalysts possessed both acidic and basic sites. As Figure 3 shows, for all nanorods, three types of acidic sites were contained. Desorption peaks in the range of 100–200 °C, 200–400 °C, and 400–600 °C were assigned to weak, moderate, and strong adsorption, respectively [62]. The amount of NH3 desorbed from various acidic sites was calculated by the Gauss curve fitting method and listed in Table 2. As expected, the acidic properties of the Ce1-xZrxO2 catalysts were strongly affected by the Zr doping into CeO2. An obvious improvement in the amount of weak and moderate acidic sites in Ce1-xZrxO2 compared to CeO2 was observed (entries 2–6 vs. entry 1), which can be ascribed to a synergic effect between ZrO2 and CeO2 induced by the unbalance of local electronic conditions as well as the distortion of the crystal structure with the introduction of Zr. The amounts of NH3 adsorption for all catalysts showed a volcano-shaped curve with respect to Zr content. It has to be emphasized that the Ce0.6Zr0.4O2 and Ce0.7Zr0.3O2 catalysts possessed larger amounts of weak and moderate acidic sites compared to other samples (entries 4, 5 vs. entry 1, 2, 3, 6). Especially for Ce0.7Zr0.3O2, the highest density of moderate acidic sites was displayed, and the moderate adsorption quantity of NH3 was 0.339 mmol/g (entry 4).
Similar to NH3-TPD results, the desorption peaks at 100–200 °C, 200–400 °C, and 400–600 °C in CO2-TPD were attributed to weak, moderate, and strong adsorption (Figure 4) [63]. The surface basicity of Ce1-xZrxO2 nanorods was slightly higher than pure CeO2, especially for the moderate basic sites. At the same time, for all Ce1-xZrxO2 catalysts, more basic sites were formed compared to CeO2 (Table 3, entries 2–6 vs. entry 1), attributed to the facile oxygen mobility in ceria–zirconia solid solution. The doping of ZrO2 caused shrinkage of the fluorite-type lattice of CeO2, which generated extra surface oxygen anions as basic sites in Ce1-xZrxO2 catalysts [64]. The amount of weak and moderate basic sites increased first and then decreased with higher Zr content. The Ce0.7Zr0.3O2 nanorod also possessed the largest amount of moderate basic sites (entry 4). Aside from this, negligible strong acidic and strong basic sites were found in the Ce1-xZrxO2 solid solution, which may avoid the occurrence of the side reactions, e.g., polymerization of epoxides.

2.2. Catalytic Activity of Ce1-xZrxO2 Nanorods

The catalytic activities of the as-prepared CeO2, ZrO2, and Ce1-xZrxO2 nanorods for cycloaddition of CO2 with epoxides were then evaluated by using 1a as a model substrate. As shown in Table 4, all the catalysts performed excellent selectivity to target product 2a (entries 1–7). However, an obvious difference in the conversion of 1a among various metal oxides was observed. As we predicted, the Ce1-xZrxO2 nanorods could smoothly catalyze the cycloaddition of CO2 with 1a to afford high conversions. Among the catalysts examined, the Ce0.7Zr0.3O2 nanorod catalyst showed the highest conversion of 90% with 100% selectivity of 2a, much higher than that of the neat CeO2 and ZrO2 nanorods, which showed a conversion of 71% and 62%, respectively (entries 3 vs. 6, 7). N,N-dimethylformamide (DMF) itself was reported to catalyze the cycloaddition of CO2 with styrene oxide to obtain a yield of 67.7% [45], a large amount of DMF was required (10 mL), and the yield was greatly reduced to 8% if the styrene oxide was replaced with propylene oxide. To explore the effect of solvent DMF, an experiment of cycloaddition of CO2 with 1a under DMF without any catalyst was carried out (entry 8). It is shown that the solvent of DMF does not play a decisive role. When CeO2 and ZrO2 were physically mixed in the mol ratio of 7:3, only 67% of 1a could be turned into the carbonate, indicating that the formation of CeO2-ZrO2 solid solution could effectively improve the catalytic performance (entries 3 vs. 9). In addition, the catalytic activity of Ce1-xZrxO2 nanorods was affected by the CO2 pressure and reaction temperature. Elevated CO2 pressure and temperature are in favor of the transformation of CO2 with 1a (entries 3, 10–14). An excellent conversion of 96% with 100% selectivity was attained at conditions of 6 MPa CO2 and 150 °C (entry 11). As the temperature rose to 160 °C, a decrease in selectivity was observed. The solvent-free experiment was then carried out, delivering 2a with a good conversion of 84% and exclusive selectivity (entry 15).
It is worth noting that the total amount of acidic–basic sites of the Ce0.6Zr0.4O2 nanorod was higher than that of the other catalysts (Table 2, entry 5 and Table 3, entry 5), while a slightly lower conversion of 1a was obtained using Ce0.6Zr0.4O2 as a catalyst compared to Ce0.7Zr0.3O2 (Table 4, entries 4 vs. 3). To further clarify the correlation between the acidic–basic performance of Ce1-xZrxO2 nanorods and their catalytic activities, we evaluated the activity of all the catalysts for the cycloaddition of CO2 with 1a within 6 h, while the active sites are all efficient (Figure S5). Strong linear relationships between conversion and the number of moderate acidic–basic sites were presented where R2 was 0.93 and 0.97, separately (Figure 5a,b; for details, see Supporting Information, Table S1). The positive correlation indicated that it was the moderate acidic and basic sites that acted as the major active sites in catalyzing the cycloaddition of CO2 with epoxides. Aside from this, although the amount of both total acidic–basic sites and weak acidic–basic sites for Ce0.7Zr0.3O2 were less than Ce0.6Zr0.4O2 (Table 2, entries 4 vs. 5 and Table 3, entries 4 vs. 5), the best conversion was achieved for Ce0.7Zr0.3O2 (Table 4, entry 3), further confirming that the catalytic activity of Ce1-xZrxO2 nanorods was attributed to the synergistic interaction between their moderate acidic and moderate basic sites primarily. That is, the epoxide was activated by the moderate acidic site, and the CO2 molecule interacted with the moderate basic site. For weak acidic and basic sites, their acidity and basicity may not be strong enough to activate epoxides and CO2 effectively, whereas strong acidity and basicity induce side reactions. Combining TPD, XRD, and N2 adsorption results, the structure of a CeO2-ZrO2 solid solution favors the formation of moderate acidic and basic sites; thus, Ce1-xZrxO2 nanorods had better catalytic activity compared to pure CeO2 and ZrO2. However, with the increase in Zr content, a decrease in the specific surface area occurred in Ce0.6Zr0.4O2 and Ce0.5Zr0.5O2 nanorods, reducing the amount of surface moderate acidic–basic sites and leading to lower catalytic activity.
To explore the general applicability of a Ce0.7Zr0.3O2 catalyst, the cycloaddition of CO2 with various epoxides was investigated under the optimized reaction conditions, as shown in Table 5. All the epoxides studied could be smoothly converted to the corresponding cyclic carbonates in good to excellent yields (72–96%). Epoxides with low steric hindrance, e.g., propylene oxide and 3-hydroxy-1,2-propenoxide, showed high reactivity, with product yields over 90% (entries 1–4). Styrene oxide and 2-((propenyloxy)methyl)oxirane could also be efficiently transformed into the corresponding products (entries 5 and 6). Even using 1,2-epoxycyclohexane with bulky steric hindrance as the substrate, satisfactory catalytic activity was exhibited with a competitive conversion (72%) of hexahydro-1,3-dioxaindan-2-one compared to the reported literature [47,50,65].

2.3. Reusability of Catalyst

The recyclability and reusability of the Ce0.7Zr0.3O2 catalyst were also tested. As shown in Figure 6, the Ce0.7Zr0.3O2 could be recycled at least six times in the cycloaddition of CO2 with 1a without a significant decrease in reaction conversion and selectivity. The reused catalyst was characterized by XRD (Figure S6) after each run. There was no difference in crystal structure between the fresh and reused catalyst, illustrating the good stability of the Ce0.7Zr0.3O2 catalyst.

2.4. Plausible Mechanism

According to the above experimental results and previous reports [47,49,51], a plausible reaction mechanism for the Ce1-xZrxO2 nanorods catalyzed cycloaddition of CO2 with epoxides was proposed (Figure 7). The prepared Ce1-xZrxO2 nanorods possessed abundant moderate acidic and basic sites. The epoxide was adsorbed by a moderate acidic site, while the CO2 was activated by a moderate basic site to form a carbonate anion species, which attacks the sterically less-hindered carbon of activated epoxide via electrophilic interaction, furnishing the intermediate of oxygen–anion and carbon–cation pairs. The subsequent intermolecular cyclization of oxygen–anion and carbon–cation pairs delivered the cyclic carbonate product, and the catalyst was regenerated to involve in the next catalytic cycle.

3. Experimental Procedures

3.1. Preparation of Zr-Doped CeO2 Nanorods

The nanorods were synthesized by the hydrothermal process. Different mole ratios of Ce(NO3)3·6H2O and ZrO(NO3)2 were dissolved in 10 mL of deionized water under vigorous stirring until the solids were dissolved completely. At the same time, 16.8 g of NaOH was dissolved in 70 mL of deionized water. The two solutions were mixed in a Teflon bottle, and the mixture was kept stirring for 30 min. Then, the Teflon bottle was transferred into a stainless steel autoclave, which was kept in an oven at 100 °C for 24 h. After this finished, the catalysts were separated by centrifugation (10,000 r/min for 7 min), washed with deionized water and ethanol three times separately, and dried at 60 °C overnight. The Ce1-xZrxO2 nanorods were finally obtained by calcination at 600 °C for 5 h in a muffle furnace. The neat CeO2 and ZrO2 were synthesized by the same process.

3.2. General Procedure of Ce1-xZrxO2 Nanorods Catalyzed Cycloaddition of CO2 with Epoxides

Taking the Ce0.7Zr0.3O2 catalysis in cycloaddition of CO2 with 1,2-butene oxide (1a), for instance, 1 g Ce0.7Zr0.3O2, 3 mL DMF, and 1 mL 1,2-butylene oxide 1a (11.42 mmol) were successively added into a 25 mL stainless-steel autoclave with a magnetic stirrer, and then carbon dioxide was charged into a feasible pressure at room temperature. The autoclave was heated up to the desired temperature, and the reaction was carried out for 24 h. After the reaction finished, the autoclave was cooled in an ice bath, and extra CO2 was vented slowly. The conversion and selectivity were analyzed by gas chromatography (GC) using diphenyl as an internal standard. The crude product was filtrated and then purified by extraction (H2O/EtOAc = 3:1, V:V) to remove DMF. The collected organic phase was concentrated in a vacuum to provide the pure product 4-ethyl-1,3-dioxolan-2-one (2a) with an isolated yield of 90%.

4. Conclusions

In this study, a series of acid–base bifunctional Ce1-xZrxO2 nanorods, which could catalyze the cycloaddition of CO2 with epoxides efficiently under halogen-free conditions, were successfully prepared by doping ZrO2 in CeO2. Among the Ce1-xZrxO2 catalysts, Ce0.7Zr0.3O2 exhibited the best catalytic activity, giving various cyclic carbonates with good yield and exclusive selectivity. The high catalytic performance of Ce1-xZrxO2 nanorods is attributed to the abundant moderate acidic–basic active sites on the catalyst surface due to the formation of a CeO2-ZrO2 solid solution. Investigations into the relationship between catalytic activity and the surface acidity/basicity of Ce1-xZrxO2 catalysts reveal that it is the synergistic interaction between the moderate acidic sites and moderate basic sites that dominates the conversion of CO2 with epoxides. Additionally, the catalyst can be recycled at least six times without a significant loss of activity, illustrating the good stability of the CeO2-ZrO2 solid-solution catalysts. Most importantly, this study supplies an efficient and environmentally friendly method for the halogen-free synthesis of cyclic carbonates with high purity via metal-oxide-catalyzed cycloaddition of CO2 with epoxides.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12060632/s1, Figure S1: TEM and HRTEM images of Ce0.6Zr0.4O2 and Ce0.5Zr0.5O2; Figure S2: EDS spectrum of agglomerating particles in Ce0.5Zr0.5O2; Figure S3. EDS-Mapping images of Ce0.7Zr0.3O2; Figure S4: N2 adsorption–desorption isotherm and BJH pore size distribution of CeO2 and Ce1-xZrxO2 nanorods; Figure S5: Effect of time on the activity of Ce0.7Zr0.3O2; Figure S6: The XRD patterns of reused Ce0.7Zr0.3O2; Table S1: Effect of the solvents and catalyst amount on the cycloaddition of CO2 with 1a; Tables S2: Effect of different catalysts on the cycloaddition of CO2 with 1,2-butylene oxide at 6 h.

Author Contributions

Conceptualization, methodology, validation, J.G., M.-Y.W. and X.M.; formal analysis, data curation, writing—original draft preparation and visualization, J.G.; investigation, J.G. and H.W.; resources, J.L. and H.Y.; writing—review and editing, J.G., M.-Y.W. and C.Y.; supervision, project administration, M.-Y.W. and X.M.; funding acquisition, M.-Y.W. and X.M. 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, grant number U21B2096, the Tianjin Key Science and Technology Project, grant number 19ZXNCGX00030, and the Natural Science Foundation of Tianjin City, grant number 21JCQNJC00620.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 00632 g001 550
Figure 1. XRD patterns of the Ce1-xZrxO2 and CeO2 nanorods.
Figure 1. XRD patterns of the Ce1-xZrxO2 and CeO2 nanorods.
Catalysts 12 00632 g001
Catalysts 12 00632 g002 550
Figure 2. TEM images of (a) CeO2, (b) Ce0.9Zr0.1O2, (c) Ce0.8Zr0.2O2 and (d) Ce0.7Zr0.3O2; HRTEM images of (e) CeO2, (f) Ce0.9Zr0.1O2, (g) Ce0.8Zr0.2O2, and (h) Ce0.7Zr0.3O2.
Figure 2. TEM images of (a) CeO2, (b) Ce0.9Zr0.1O2, (c) Ce0.8Zr0.2O2 and (d) Ce0.7Zr0.3O2; HRTEM images of (e) CeO2, (f) Ce0.9Zr0.1O2, (g) Ce0.8Zr0.2O2, and (h) Ce0.7Zr0.3O2.
Catalysts 12 00632 g002
Catalysts 12 00632 g003 550
Figure 3. NH3-TPD profiles of Ce1-xZrxO2 nanorods.
Figure 3. NH3-TPD profiles of Ce1-xZrxO2 nanorods.
Catalysts 12 00632 g003
Catalysts 12 00632 g004 550
Figure 4. CO2-TPD profiles of Ce1-xZrxO2 nanorods.
Figure 4. CO2-TPD profiles of Ce1-xZrxO2 nanorods.
Catalysts 12 00632 g004
Catalysts 12 00632 g005 550
Figure 5. Correlation between the conversion of 1a and number of active sites: (a) moderate acidic sites, (b) moderate basic sites.
Figure 5. Correlation between the conversion of 1a and number of active sites: (a) moderate acidic sites, (b) moderate basic sites.
Catalysts 12 00632 g005
Catalysts 12 00632 g006 550
Figure 6. Reusability of Ce0.7Zr0.3O2 catalyst.
Figure 6. Reusability of Ce0.7Zr0.3O2 catalyst.
Catalysts 12 00632 g006
Catalysts 12 00632 g007 550
Figure 7. Mechanism of cycloaddition of CO2 with butylene oxide.
Figure 7. Mechanism of cycloaddition of CO2 with butylene oxide.
Catalysts 12 00632 g007
Table
Table 1. Textural data of CeO2 and Ce1-xZrxO2 nanorods.
Table 1. Textural data of CeO2 and Ce1-xZrxO2 nanorods.
Catalyst2θ (o) aInterplanar
Spacing (Å) a		Lattice Parameter (Å) bMol Ratio of Zr/(Ce+Zr)SBET (m2/g)VPore (cm3/g)
Bulk Phase cSurface d
CeO228.583.115.39--770.59
Ce0.9Zr0.1O228.703.105.380.090.11850.60
Ce0.8Zr0.2O228.723.105.370.180.20810.59
Ce0.7Zr0.3O228.843.095.370.270.28760.58
Ce0.6Zr0.4O229.073.075.360.350.39660.53
Ce0.5Zr0.5O229.123.065.340.450.51530.38
a Calculated by the (111) plane from XRD. b Calculated by the cubic phase of Ce1-xZrxO2 catalysts. c Content of Zr/(Ce+Zr) in the bulk phase calculated according to the ICP-AES results. d Content of Zr/(Ce+Zr) on the surface calculated according to the XPS results.
Table
Table 2. Acidic amounts of the catalysts calculated from NH3-TPD measurement.
Table 2. Acidic amounts of the catalysts calculated from NH3-TPD measurement.
EntryCatalystNH3 Adsorption (mmol/g) a
Weak
(100–200 °C) 		Moderate
(200–400 °C)		Strong
(>400 °C)		Total
1CeO20.0540.1980.0260.278
2Ce0.9Zr0.1O20.0750.2400.0410.356
3Ce0.8Zr0.2O20.0820.3020.0120.398
4Ce0.7Zr0.3O20.0900.3390.0050.434
5Ce0.6Zr0.4O20.0910.3250.0320.449
6Ce0.5Zr0.5O20.0680.3210.0210.411
a Calculated by peak fitting of NH3-TPD.
Table
Table 3. Basic amounts of the catalysts calculated from CO2-TPD measurement.
Table 3. Basic amounts of the catalysts calculated from CO2-TPD measurement.
EntryCatalystCO2 Adsorption (mmol/g) a
Weak
(100–200 °C)		Moderate
(200–400 °C)		Strong
(>400 °C)		Total
1CeO20.0490.2000.0250.273
2Ce0.9Zr0.1O20.0660.2370.0270.330
3Ce0.8Zr0.2O20.0790.2760.0250.383
4Ce0.7Zr0.3O20.0820.3370.0120.422
5Ce0.6Zr0.4O20.0990.3130.0110.423
6Ce0.5Zr0.5O20.0840.3070.0220.413
a Calculated by peak fitting of CO2-TPD.
Table
Table 4. Catalytic activity of various catalysts for the cycloaddition of CO2 with 1a a.
Table 4. Catalytic activity of various catalysts for the cycloaddition of CO2 with 1a a.
Catalysts 12 00632 i001
EntryCatalystConv. b/%Sel. b/%Yield b/%
1Ce0.9Zr0.1O273 ± 399 ± 172 ± 1
2Ce0.8Zr0.2O280 ± 199 ± 179 ± 2
3Ce0.7Zr0.3O290 ± 1>9990 ± 2
4Ce0.6Zr0.4O287 ± 199 ± 186 ± 2
5Ce0.5Zr0.5O282 ± 299 ± 281 ± 1
6CeO271 ± 199 ± 170 ± 3
7ZrO262 ± 398 ± 161 ± 2
8-32 ± 578 ± 125 ± 4
9 cCeO2 + ZrO267 ± 199 ± 266 ± 3
10 dCe0.7Zr0.3O262 ± 399 ± 261 ± 2
11 eCe0.7Zr0.3O296 ± 1>9996 ± 1
12 fCe0.7Zr0.3O258 ± 499 ± 157 ± 4
13 gCe0.7Zr0.3O279 ± 2>9979 ± 3
14 hCe0.7Zr0.3O299 ± 281 ± 280 ± 3
15 iCe0.7Zr0.3O284 ± 197 ± 081 ± 1
a Reaction conditions: 1 mL (11.42 mmol) 1a, 1 g catalyst, 3 mL DMF, 5 MPa CO2 150 oC, 24 h. b Determined by GC using biphenyl as the internal standard, the disparity values were calculated from the results of three parallel experiments. c CeO2 and ZrO2 were physically mixed in the mol ratio of 7:3. d 3 MPa CO2. e 6 MPa CO2. f 120 oC. g 140 oC. h 160 oC. i Solvent-free and 3 mL (34.26 mmol) 1a were added.
Table
Table 5. Ce0.7Zr0.3O2 catalyzed synthesis of various cyclic carbonates a.
Table 5. Ce0.7Zr0.3O2 catalyzed synthesis of various cyclic carbonates a.
Catalysts 12 00632 i002
EntryEpoxideProductConv. b/%Sel. b/%Yield b/%
1 Catalysts 12 00632 i003 Catalysts 12 00632 i00493 ± 2100 ± 093 ± 2
2 Catalysts 12 00632 i005 Catalysts 12 00632 i00696 ± 1100 ± 096 ± 1
3 Catalysts 12 00632 i007 Catalysts 12 00632 i00891 ± 199 ± 190 ± 2
4 Catalysts 12 00632 i009 Catalysts 12 00632 i01091 ± 299 ± 190 ± 3
5 Catalysts 12 00632 i011 Catalysts 12 00632 i01290 ± 297 ± 187 ± 1
6 Catalysts 12 00632 i013 Catalysts 12 00632 i01486 ± 198 ± 084 ± 1
7 Catalysts 12 00632 i015 Catalysts 12 00632 i01672 ± 397 ± 170 ± 1
a Reaction conditions: 1 mL epoxides, 1 g Ce0.7Zr0.3O2 catalyst, 3 mL DMF, 6 MPa CO2 at 150 °C, 24 h. b Determined by 1H-NMR using 1,1,2,2-tetrachloroethane as internal standard, the disparity values were calculated from the results of three parallel experiments.
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Gao, J.; Yue, C.; Wang, H.; Li, J.; Yao, H.; Wang, M.-Y.; Ma, X. CeO2-ZrO2 Solid Solution Catalyzed and Moderate Acidic–Basic Sites Dominated Cycloaddition of CO2 with Epoxides: Halogen-Free Synthesis of Cyclic Carbonates. Catalysts 2022, 12, 632. https://doi.org/10.3390/catal12060632

AMA Style

Gao J, Yue C, Wang H, Li J, Yao H, Wang M-Y, Ma X. CeO2-ZrO2 Solid Solution Catalyzed and Moderate Acidic–Basic Sites Dominated Cycloaddition of CO2 with Epoxides: Halogen-Free Synthesis of Cyclic Carbonates. Catalysts. 2022; 12(6):632. https://doi.org/10.3390/catal12060632

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Gao, Jie, Chengguang Yue, Hao Wang, Jiaxin Li, He Yao, Mei-Yan Wang, and Xinbin Ma. 2022. "CeO2-ZrO2 Solid Solution Catalyzed and Moderate Acidic–Basic Sites Dominated Cycloaddition of CO2 with Epoxides: Halogen-Free Synthesis of Cyclic Carbonates" Catalysts 12, no. 6: 632. https://doi.org/10.3390/catal12060632

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