# Direct Synthesis of Dimethyl Carbonate from Methanol and CO2 over ZrO2 Catalysts Combined with a Dehydrating Agent and a Cocatalyst

D. C. Shi, Svetlana Heyte Dyshlovenko, Mickael Capron, Sébastien Paul

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D. C. Shi, Svetlana Heyte Dyshlovenko, Mickael Capron, Sébastien Paul. Direct Synthesis of Dimethyl Carbonate from Methanol and CO2 over ZrO2 Catalysts Combined with a Dehydrating Agent and a Cocatalyst. Catalysts, 2024, Catalysts, 14, 10.3390/catal14100657. [https://lilloa.hal.science/hal-05129842v1](https://lilloa.hal.science/hal-05129842v1)

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Article

# Direct Synthesis of Dimethyl Carbonate from Methanol and CO₂ over ZrO₂ Catalysts Combined with a Dehydrating Agent and a Cocatalyst

Dichao Shi<fnref n="1" />,*, Svetlana Heyte<fnref n="2" />, Mickaël Capron<fnref n="2" /> and Sébastien Paul<fnref n="2" />,†

**Abstract:** Zirconia nanocrystals as catalysts for the direct synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide have received significant interest recently. In this paper, three zirconia-based catalysts presenting different monoclinic and tetragonal phase contents are prepared and characterized by X-ray diffraction (XRD), N₂ adsorption–desorption, transmission electron microscopy (TEM), and temperature-programmed desorption of NH₃ and CO₂ (NH₃-TPD and CO₂-TPD). The catalytic performances of these solids are evaluated in terms of DMC production. This production is low when using the bare zirconias, but it is significantly increased in the presence of 1,1,1-trimethoxymethane (TMM) playing the role of a dehydrating agent, which shifts the thermodynamic equilibrium. Moreover, the production of DMC is further improved by adding a second solid catalyst (cocatalyst), the molecular sieve 13X, to accelerate the hydration of TMM. Hence, the molecular sieve 13X plays a dual role by trapping water molecules formed by the reaction of DMC synthesis and providing strong acidic sites catalyzing TMM hydrolysis. To the best of our knowledge, the combination of two solid catalysts in the reaction medium to accelerate the water elimination to obtain higher DMC production from CO₂ and methanol has never been reported.

**Keywords:** zirconia nanocrystals; dimethyl carbonate; TMM; second solid catalyst

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**Citation:** Shi, D.; Heyte, S.; Capron, M.; Paul, S. Direct Synthesis of Dimethyl Carbonate from Methanol and CO₂ over ZrO₂ Catalysts Combined with a Dehydrating Agent and a Cocatalyst. *Catalysts* **2024**, *14*, 657. https://doi.org/10.3390/catal14100657

Academic Editor: Jaime Soler

Received: 21 August 2024

Revised: 20 September 2024

Accepted: 23 September 2024

Published: 24 September 2024

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**Copyright:** © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
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4.0/).

## 1. Introduction

A significant increase in CO₂ emissions caused by the intensive consumption of coal, oil, and gas has led to a dramatic environmental issue, the so-called global warming [1–4]. Therefore, efficient measures must be taken to significantly reduce CO₂ emissions by trapping it and valorizing it into useful materials. Dimethyl carbonate (DMC) has been given much attention because it can be used as a gasoline and diesel additive [5,6] or as an ideal solvent for lithium-ion batteries [7]. In addition, the usage of DMC as a green reagent for the replacement of toxic and hazardous chemicals, such as dimethyl sulfate, phosgene (COCl₂), and acetate esters, is also a potential application because of its low toxicity, low viscosity, and good dissolving ability [8,9]. Hence, the development of an environmentally benign and human-friendly route for DMC synthesis using CO₂ as feedstock is essential. Direct synthesis of DMC from CO₂ and methanol drew much attention recently because of its compliance with the concept of Green Chemistry in terms of the abundance of the feedstock CO₂, production of valuable chemical DMC, and absence of toxicity.

However, there are two main drawbacks to the direct synthesis of DMC from CO₂ and methanol. CO₂ is a very stable compound (ΔG°₂₅°C = -396 kJ/mol) that requires a high amount of energy input to be activated [10]. Various heterogeneous catalysts have been reported to decrease the energy barrier that must be overpassed to convert CO₂ to DMC, such as H₃PMo₁₂O₄₀ [11], CuCl₂/AC [12], Cu-Ni/Graphite [13], CeO₂ [14–17], and

<footnote n="1">¹ School of Chemical and Environmental Engineering, Liaoning University of Technology, Jinzhou 121001, China; shidichao@lnut.edu.cn</footnote>

<footnote n="2">² Univ. Lille, CNRS, Centrale Lille, Univ. Artois, UMR 8181—UCCS—Unité de Catalyse et Chimie du Solide, F-59000 Lille, France; svetlana.heyte@univ-lille.fr (S.H.); mickael.capron@univ-lille.fr (M.C.)</footnote>

<footnote>* Correspondence: sebastien.paul@centralelille.fr</footnote>
ZrO₂ [18–20]. Among them, ZrO₂ nanocrystals showed a promising potential due to their acid–base properties [21]. ZrO₂ exhibits three polymorphs: monoclinic (m-ZrO₂), tetragonal (t-ZrO₂), and cubic (c-ZrO₂). The amount of acid–base sites for different polymorphs of ZrO₂ decreases in the order of *t*-ZrO₂ > *m*-ZrO₂ > *c*-ZrO₂ [18]. Tomishige et al. have demonstrated in their early study that, under 50 bar of CO₂ at 170 °C, a ZrO₂-based catalyst containing a major part of the monoclinic phase and a minor part of the tetragonal phase on its surface exhibited some DMC production (0.6 mmol/gcat.), whereas a pure monoclinic phase was inactive [19,20]. In this work, ZrO₂ nanocrystals were prepared, varying the ratios between the monoclinic and the tetragonal phases to examine the effect on DMC production from CO₂ and methanol.

A strong limitation encountered for the direct synthesis of DMC from CO₂ and methanol is the thermodynamic equilibrium. Whatever the catalyst used, the production of DMC is far from satisfactory because the reaction is always limited. One way to obtain a higher production of DMC is to add an efficient dehydration agent in the reaction medium to shift the equilibrium by eliminating in situ the water molecules formed by the reaction. Several dehydration agents have been employed to promote the formation of DMC, such as molecular sieve 3A/4A/5A/13X [22,23], ionic liquids (ILs) [24], butylene oxide (BO) [25,26], 2,2-dimethoxypropane (DMP) [27,28], 1,1,1-trimethoxymethane (TMM) [29,30], and 2-cyanopyridine (2-CP) [31–33]. Among them, 2-CP allowed to reach excellent catalytic performances, namely 92% of methanol conversion with higher than 99% selectivity to DMC, when using CeO₂ as a catalyst under 30 bar of CO₂ at 120 °C [34]. Nevertheless, the main drawback of using 2-CP as a dehydration agent is the hazardous character of this molecule and, hence, the difficulty of operation, which limits its application in industry. BO is not a better choice as it may present complex secondary and/or tertiary reactions [25]. DMP and TMM are potential green alternatives that have been commonly used due to their relatively high reactivity with water and less production of byproducts [29,35,36]. Tomishige and co-workers showed that the DMC production was 13.8 mmol/gcat. over CeO₂-ZrO₂ catalyst used in combination with DMP under 60 bar of CO₂ at 110 °C, which was almost 10 times higher than that observed without a dehydration agent (1.5 mmol/gcat.) [37]. Xuan et al. presented a much higher DMC production (6.6 mmol/gcat.) over MOF-808-4 with TMM as a dehydration agent under 120 bar of CO₂ at 140 °C compared with that observed without adding TMM as dehydrate (0.24 mmol/gcat.) [30]. It should be noted that, to the best of our knowledge, no publication reported the DMC synthesis from CO₂ and methanol over a bare ZrO₂ catalyst combined with DMP or TMM as dehydration agents.

Interestingly, Choi et al. found that by adding a second homogeneous catalyst, namely [Ph₂NH₂]OTf, the DMC production reached up to 40 mmol/gcat. over the Bu₂SnO catalyst in the presence of DMP, which was much higher than that observed without adding a second catalyst (namely 17 mmol/gcat.) [38]. The principle of adding a second acid catalyst (called cocatalyst in the following) is to accelerate the reaction of hydration of DMP. It should be noted that traditional Brönsted acids such as HCl, H₂SO₄, and H₃PO₄ have no remarkable promotion to DMC production. Moreover, they cannot be easily separated from the medium after the reaction. Therefore, it would be interesting to identify solid cocatalysts with acidic properties able to accelerate efficiently the DMP production or the TMM hydration reaction.

Herein, ZrO₂ nanocrystals with different monoclinic and tetragonal phase contents were prepared, and their catalytic activity for the synthesis of DMC from CO₂ and methanol was investigated in the presence of a dehydration agent (TMM). Adsorption of NH₃ and CO₂ on the surface of the tested catalysts varies with monoclinic and tetragonal phase contents, showing the variety of acid–base properties, which are shown to be crucial to determining the level of production of DMC. Additionally, a solid cocatalyst (13X) was introduced in the reaction medium to promote the formation of DMC further by accelerating the hydration of TMM. Finally, 61 mmol/gcat. of DMC production was achieved on the ZrO₂ catalyst in the presence of TMM with adding molecular sieve 13X as a second catalyst.
## 2. Results and Discussion

### 2.1. Characterization of ZrO₂ Nanocrystals

The crystal structure of the three zirconia catalysts presenting a mixture of tetragonal and monoclinic phases in various ratios was characterized by XRD, as shown in Figure 1. Intensive diffraction peaks at 2θ = 34.6°, 35.2°, 50.2°, 59.3°, and 60.0° in Figure 1a are assigned to the tetragonal phase (PDF 01-081-1544), but they overlap with the diffraction peaks at 2θ = 34.1°, 34.4°, 35.3°, 49.4°, 50.2°, 50.5°, and 59.8° in Figure 1b,c that are assigned to the monoclinic phase (PDF 00-065-0728), thereby making clear identification of the two phases rather difficult. The quantification of both phases was conducted using the diffraction peak at 2θ = 30.2°, which can be assigned to the tetragonal phase, whereas the monoclinic phase has three distinct peaks at 2θ = 24.0°, 28.2°, and 31.5°. An intensive diffraction peak at 2θ = 30.2° appears with a very small shoulder at 2θ = 28.2° for the ZrO₂-1 catalyst, indicating the formation of a well-crystalized ZrO₂ that contains a phase mixture presenting a majority of tetragonal phase and a minority of monoclinic phase. The ZrO₂-2 catalyst presents two diffraction peaks at 2θ = 28.2° and 31.5° with a tiny peak at 2θ = 28.2° in between, meaning that the content of the monoclinic phase is much higher than that of the tetragonal phase. The intensity of the diffraction peaks at 2θ = 28.2° and 31.5° is found attenuated for the ZrO₂-3 catalyst, while the peak at 2θ = 28.2° becomes more intense in comparison with the ZrO₂-2 catalyst. It demonstrates that the content of the monoclinic phase decreases, whereas the content of the tetragonal phase increases for ZrO₂-3 compared to the ZrO₂-2 catalyst.

<figure><img src="image_4.png" /><figcaption>**Figure 1.** XRD patterns of ZrO₂-1 (a), ZrO₂-2 (b), and ZrO₂-3 (c).</figcaption></figure>

Rietveld refinement was carried out for ZrO₂-1, ZrO₂-2, and ZrO₂-3 to determine their phase contents and lattice parameters, as shown in Table 1. According to the refinement, the ratio between the monoclinic and the tetragonal phases is 3%/97% for ZrO₂-1, 69%/31% for ZrO₂-2, and 64%/36% for ZrO₂-3.

The porous structures of the three synthesized ZrO₂ were determined from N₂ adsorption-desorption isotherms and pore size distributions, as presented in Figure 2. The adsorption-desorption isotherms are identified as type IV for the three catalysts, which is characteristic of mesoporous materials [39,40]. The hysteresis loops for samples ZrO₂-1
and ZrO₂-2 are of H₂-type, typically indicating a mesoporous pore network. The absence of a plateau at high P/P₀ for ZrO₂-3 can be described by an H₄-type hysteresis, which is associated with narrow slit-like pores [41,42]. The pore size distribution obtained from the isotherms gives a single peak at 4 nm for ZrO₂-1, at 5.6 nm for ZrO₂-2, and at 4.5 nm for ZrO₂-3. The highest BET surface area is obtained for ZrO₂-3 (223 m²/g), whereas it is 115 m²/g for ZrO₂-1 and 101 m²/g for ZrO₂-2.

<table><caption>**Table 1.** Rietveld refinement results for ZrO₂-1, ZrO₂-2 and ZrO₂-3.</caption><thead><tr><th rowspan="2">Catalysts</th><th rowspan="2">Phase</th><th rowspan="2">Lattice Parameters</th><th rowspan="2">Interfacial Angle (°)</th><th colspan="4">Ionic Position and Coordinate</th></tr><tr><th>Atoms</th><th>x</th><th>y</th><th>z</th></tr></thead><tbody><tr><td rowspan="6">ZrO₂-1</td><td rowspan="3">Tetragonal</td><td>a = 3.62440</td><td rowspan="3">α = β = γ = 90</td><td>Zr</td><td>0.75000</td><td>0.25000</td><td>0.75000</td></tr><tr><td>b = 3.62440</td><td>O</td><td>0.25000</td><td>0.25000</td><td>0.04462</td></tr><tr><td>c = 5.15600</td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="3">Monoclinic</td><td>a = 5.14500</td><td>α = 90</td><td>Zr</td><td>0.27831</td><td>0.00025</td><td>0.20448</td></tr><tr><td>b = 5.20750</td><td>β = 99.23</td><td>O1</td><td>0.19088</td><td>0.49701</td><td>0.13474</td></tr><tr><td>c = 5.31070</td><td>γ = 90</td><td>O2</td><td>0.42585</td><td>0.88364</td><td>0.36247</td></tr><tr><td rowspan="6">ZrO₂-2</td><td rowspan="3">Tetragonal</td><td>a = 3.62440</td><td rowspan="3">α = β = γ = 90</td><td>Zr</td><td>0.75000</td><td>0.25000</td><td>0.75000</td></tr><tr><td>b = 3.62440</td><td>O</td><td>0.25000</td><td>0.25000</td><td>0.00345</td></tr><tr><td>c = 5.15600</td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="3">Monoclinic</td><td>a = 5.14500</td><td>α = 90</td><td>Zr</td><td>0.28329</td><td>0.04115</td><td>0.21327</td></tr><tr><td>b = 5.20750</td><td>β = 99.23</td><td>O1</td><td>0.06090</td><td>0.33632</td><td>0.34648</td></tr><tr><td>c = 5.31070</td><td>γ = 90</td><td>O2</td><td>0.39521</td><td>0.73584</td><td>0.47151</td></tr><tr><td rowspan="6">ZrO₂-3</td><td rowspan="3">Tetragonal</td><td>a = 3.62440</td><td rowspan="3">α = β = γ = 90</td><td>Zr</td><td>0.75000</td><td>0.25000</td><td>0.75000</td></tr><tr><td>b = 3.62440</td><td>O</td><td>0.25000</td><td>0.25000</td><td>0.03912</td></tr><tr><td>c = 5.15600</td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="3">Monoclinic</td><td>a = 5.14500</td><td>α = 90</td><td>Zr</td><td>0.27732</td><td>0.03893</td><td>0.21013</td></tr><tr><td>b = 5.20750</td><td>β = 99.23</td><td>O1</td><td>0.07258</td><td>0.34447</td><td>0.34290</td></tr><tr><td>c = 5.31070</td><td>γ = 90</td><td>O2</td><td>0.41583</td><td>0.74603</td><td>0.48318</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Nitrogen adsorption–desorption isotherms of ZrO₂-1 (a), ZrO₂-2 (b), and ZrO₂-3 (c). The insert shows the pore size distributions.</figcaption></figure>
Crystal sizes of all ZrO₂ catalysts were investigated by TEM and HRTEM, as shown in Figure 3. TEM images of ZrO₂-1 (Figure 3a) and ZrO₂-2 (Figure 3c) show that the particle sizes are in the range of 10–15 nm in all cases and the particles present a strong aggregation. The interplanar spacing of the mainly exposed planes is 0.3 nm, corresponding to (101) planes of the tetragonal phase for ZrO₂-1 (Figure 3b), and 0.32 nm corresponding to (−111) planes of the monoclinic phase for ZrO₂-2 (Figure 3d), implying that ZrO₂-1 is a tetragonal phase-rich crystal, whereas ZrO₂-2 is a monoclinic phase-rich crystal. This result is in good agreement with the conclusion made according to XRD patterns. However, the ZrO₂-3 sample shows less aggregation and presents a smaller particle size (8–10 nm, Figure 3e) in comparison with the other two samples. This is also consistent with the higher surface area of ZrO₂-3 as compared with those of ZrO₂-1 and ZrO₂-2. In addition, the presence of interplanar spacing of 0.3 nm and 0.32 nm, which are attributed to the (101) and (−111) planes of the tetragonal phase and the monoclinic phase, respectively (Figure 3f), implies that ZrO₂-3 contains more tetragonal phase compared to ZrO₂-2.

<figure><img src="image_6.png" /><figcaption>**Figure 3.** TEM and HRTEM images of ZrO₂-1 (a,b), ZrO₂-2 (c,d), and ZrO₂-3 (e,f).</figcaption></figure>

It is known that the amounts of acidic and basic sites are crucial to the formation of DMC from CO₂ and methanol [14]. Therefore, NH₃-TPD and CO₂-TPD measurements were used to investigate the acid-base properties of zirconia nanocrystals. The desorption peaks with peak maximum located at <200 °C, 200–400 °C, and >400 °C are attributed to weak, moderate, and strong acidic/basic sites, respectively, as summarized in Table 2. The total amounts of NH₃/CO₂ desorbed from acid/base sites increased in the order: ZrO₂-3 > ZrO₂-2 > ZrO₂-1. It should be noted that the total amounts of desorbed NH₃/CO₂ are mainly derived from moderate and strong acidic/basic sites. The ZrO₂-1 sample, which mainly contains a tetragonal phase, presents stronger acid sites, while ZrO₂-2, containing mainly
a monoclinic phase, has more moderate acid sites. On the contrary, the ZrO₂-1 sample contains more moderate basic sites, whereas ZrO₂-2 has stronger basic sites. However, the ZrO₂-3 sample, which contains certain contents of the tetragonal phase and monoclinic phase, presents the highest total amounts of moderate and strong acidic/basic sites as compared to the other two samples. The high total amount of acidic/basic sites for ZrO₂-3 may be due to its contents of the tetragonal phase and monoclinic phase, as well as its high surface area. This is consistent with the results reported previously in the literature [19].

<table><caption>**Table 2.** Acidic and basic amounts of ZrO₂ catalysts were determined from NH₃-TPD and CO₂-TPD measurements.</caption><thead><tr><th rowspan="2">Samples</th><th colspan="4">NH₃ Absorption (µmol/g) <sup>a</sup></th><th colspan="4">CO₂ Absorption (µmol/g) <sup>b</sup></th></tr><tr><th>Weak (&lt;200 °C)</th><th>Moderate (200-400 °C)</th><th>Strong (&gt;400 °C)</th><th>Total</th><th>Weak (&lt;200 °C)</th><th>Moderate (200-400 °C)</th><th>Strong (&gt;400 °C)</th><th>Total</th></tr></thead><tbody><tr><td>ZrO₂-1</td><td>5</td><td>68</td><td>87</td><td>160</td><td>0</td><td>70</td><td>50</td><td>120</td></tr><tr><td>ZrO₂-2</td><td>21</td><td>168</td><td>81</td><td>270</td><td>24</td><td>63</td><td>103</td><td>190</td></tr><tr><td>ZrO₂-3</td><td>2</td><td>162</td><td>186</td><td>350</td><td>39</td><td>149</td><td>182</td><td>370</td></tr></tbody></table>

## 2.2. Catalytic Performance of ZrO₂ Nanocrystals

The catalytic performances of zirconia nanocrystals were examined for the direct synthesis of DMC from methanol and CO₂. In order to identify the optimal reaction temperature for a high DMC production, different reaction temperatures have been used to evaluate the catalytic activity of the ZrO₂-3 catalyst, as shown in Figure 4. DMC production increased dramatically with the reaction temperature to reach 0.2 mmol/gₐt. at 130 °C. It decreased slightly when further increasing the reaction temperature. This may be explained by the preferential formation of dimethyl ether (DME) at high reaction temperatures [43]. Therefore, a reaction temperature of 130 °C turned out to be optimal and was kept for the rest of the study.

<figure><img src="image_7.png" /><figcaption>**Figure 4.** DMC production as a function of the reaction temperature on the ZrO₂-3 catalyst. Reaction conditions: 2 mL of methanol, 30 mg of catalyst, 38 bar CO₂ pressure, 6 h reaction time. The error bars correspond to the standard deviation from three independent measurements.</figcaption></figure>

<footnote><sup>a</sup> As determined from NH₃-TPD measurement. <sup>b</sup> As determined from CO₂-TPD measurement.</footnote>
Figure 5 depicts that the three zirconia catalysts exhibited different catalytic performances (blue bars) in the order of ZrO₂-3 > ZrO₂-2 > ZrO₂-1, which is consistent with the amounts of acidic and basic sites measured for each catalyst. The ZrO₂-3 catalyst shows the best DMC production as compared to the other two catalysts. However, the performances are still very low. Finding an efficient dehydrating agent that could react with water formed by the reaction and hence shift the thermodynamic equilibrium and promote DMC production is therefore necessary.

<figure><img src="image_8.png" /></figure>

**Figure 5.** DMC production on three zirconia-based catalysts (blue bars), in the presence or absence of 13X (red bars) and TMM as dehydration agent (violet bars). Reaction conditions: 2 mL of methanol for the reactors with zirconia as catalyst and for the reactors with zirconia as catalysts in combination with 13X, 1 mL of methanol for the reactors with zirconia as catalyst in combination with TMM, 1.3 mL of TMM (molar ratio methanol/TMM is 2:1) if present, 30 mg of catalyst, 150 mg of 13X if present, under 38 bar CO₂ pressure, 130 °C, and 6 h reaction time.

Molecular sieves have been widely reported as inorganic dehydration agents for the formation of DMC from CO₂ and methanol due to the advantages of easy operation, no byproduct formation, and recyclability [27,44]. In our study, molecular sieve 13X was tested alone and in combination with TMM, but no DMC was produced. When molecular sieve 13X was combined with zirconia as a catalyst (as shown in Figure 5 red bars), it did not present any promotion of DMC production. However, it did not degrade DMC either. TMM was then tested as a dehydrating agent. It is of particular interest to note that when TMM reacts with water, it forms methanol, which is one of the main reactants. It turned out that TMM is a very efficient dehydrating agent to promote DMC production when combined with all three zirconia catalysts (violet bars), suggesting that TMM has a relatively high reactivity with water. Specifically, for the ZrO₂-3 catalyst, the DMC production reaches up to 1.4 mmol/gcat., showing that TMM is a suitable dehydration agent for the synthesis of DMC.

To further increase the DMC production, the effect of reaction time was investigated for the three zirconia catalysts (Figure 6). The DMC production increased remarkably on all three zirconia catalysts, with increasing the reaction time when combined with TMM as a dehydration agent. ZrO₂-3 catalyst exhibited a 24 mmol/gcat. DMC production after 48 h of
reaction, which is 17 times higher than after 6 h of reaction. The DMC production after 48 h on the ZrO₂-3 catalyst is significantly higher than that on the ZrO₂-1 and ZrO₂-2 catalysts.

<figure><img src="image_9.png" /><figcaption>**Figure 6.** Effect of reaction time on the production of DMC on different types of zirconia catalysts with TMM as dehydration agent. Reaction conditions: 1 mL of methanol, 1.3 mL of TMM (molar ratio methanol/TMM is 2:1), 30 mg of zirconia, under a 38 bar CO₂ pressure, at 130 °C reaction temperature, with 6 h, 12 h, and 48 h reaction times. The error bars correspond to the standard deviation calculated from three independent measurements.</figcaption></figure>

However, even if the ZrO₂-3 catalyst presents a high DMC production after 48 h of reaction with TMM as a dehydration agent, a lot of TMM remains in the medium (conversion of TMM is only 31% after 48 h). Therefore, adding a second catalyst in the medium (cocatalyst) to promote the hydration of TMM is proposed to accelerate what is believed to be the limiting step of the whole process. The difficulty of selecting a cocatalyst is to avoid the degradation of DMC while accelerating the hydration of TMM. It has been reported that TMM can easily be hydrolyzed in the presence of strong acidic sites [45–47]. Hence, solid catalysts with strong acid sites should be first tested. Molecular sieve 13X, which was investigated as a dehydration agent in our work, came to our mind because of its water adsorption capacity and strong acidic properties [48–50]. Moreover, 13X is available on a commercially large scale [51].

Figure 7 shows that the DMC production improved to a certain extent with increasing the mass of the second catalyst, 13X, on the ZrO₂-3 catalyst with TMM as a dehydration agent. A total of 30 mg of the cocatalyst presents the highest catalytic performances after 6 h of reaction. DMC production decreases when more cocatalyst is added, even if the conversion of TMM continues to increase linearly. It is believed that DMC is decomposed by a too large number of acidic sites introduced by supplementary 13X [52]. In our reaction, 30 mg of the second catalyst has been chosen for the rest of the study. It has to be noted that 13X has a dual role, as it can also trap water during the reaction. To the best of our knowledge, the combination of two solid catalysts in the reaction medium to accelerate in situ water elimination and obtain high DMC production has never been reported.
<figure><img src="image_10.png" /><figcaption>**Figure 7.** Production of DMC (blue squares) and TMM (red dots) conversion as a function of the mass of 13X added to the ZrO₂-3 catalyst using TMM as a dehydrating agent. Reaction conditions: 1 mL of methanol, 1.3 mL of TMM (molar ratio methanol/TMM is 2:1), 30 mg of catalyst, 38 bar CO₂ pressure, 130 °C, 6 h reaction time. The error bars correspond to the standard deviation calculated from three independent measurements.</figcaption></figure>

The effect of the methanol/TMM molar ratio on the DMC production on ZrO₂-3 with the addition of 13X as cocatalyst has also been investigated by varying the volume of TMM added in a fixed volume of methanol solution (1 mL). It should be noted that even if a stoichiometric molar ratio of methanol and TMM of 2:1 is used, the TMM is not completely consumed at the end of the reaction, even when 13X is added as a second catalyst to promote hydration. DMC production decreased dramatically when reducing the TMM amount in the reaction medium, and finally, it reached a plateau when the molar ratio of methanol and TMM was 6:1, as shown in Figure 8. Therefore, the stoichiometric ratio of methanol and TMM is optimal and used in the following of the study, even if it has to be noted here that the presence of unconverted TMM in the reactive medium after the reaction is not desired as it must be separated from DMC by distillation, which is an energy-intensive separation method.

The influence of the reaction time on DMC production was studied in the presence of a ZrO₂-3 catalyst using 13X as a cocatalyst and TMM as a dehydration agent, as shown in Figure 9. DMC production increased remarkably with prolonged reaction time, and it arrived at a high value of 61 mmol/gₐt. after 48 h of reaction, which is more than 2.5 times more than without adding 13X as a cocatalyst (see Figure 6). DMC production is therefore significantly improved on the ZrO₂-3 catalyst by adding 13X as a cocatalyst to promote the hydrolysis of TMM used as a dehydration agent.

Table 3 shows a comparison of the catalytic performances of our best catalysts (ZrO₂-3) with other Zr-based catalysts used in the literature for the direct synthesis of DMC from CO₂ and methanol. The production of DMC obtained in this work is significantly higher than the previous reports, indicating the efficiency of the solid cocatalyst approach.
<figure><img src="image_11.png" /><figcaption>**Figure 8.** Effect of the methanol/TMM molar ratio on the DMC production using ZrO₂-3 catalyst and 13X as second catalyst. Reaction conditions: 1 mL of methanol, 30 mg of zirconia, 30 mg of 13X, 38 bar CO₂ pressure, 130 °C, and 6 h reaction time. The error bars correspond to the standard deviation calculated from three independent measurements.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 9.** Effect of reaction time on the production of DMC using ZrO₂-3 catalyst with 13X as second catalyst and TMM as dehydration agent. Reaction conditions: 1 mL of methanol, 1.3 mL of TMM (molar ratio methanol/TMM is 2:1), 30 mg of zirconia ZrO₂-3, 30 mg of 13X, 38 bar CO₂ pressure, 130 °C, and 6 h, 24 h, and 48 h of reaction times. The error bars correspond to the standard deviation calculated from three independent measurements.</figcaption></figure>
<table><caption>**Table 3.** Comparison of the catalytic performances of ceria-based catalysts using organic dehydration agents.</caption><thead><tr><th>Entry</th><th>Catalyst</th><th>Dehydration Agent</th><th>Temp. (°C)</th><th>Time (h)</th><th>DMC Production (mmol/g<sub>cat.</sub>)</th><th>Ref.</th></tr></thead><tbody><tr><td>1</td><td>ZrO<sub>2</sub></td><td>-</td><td>160</td><td>16</td><td>10.5</td><td>[20]</td></tr><tr><td>2</td><td>Ce<sub>0.1</sub>Zr<sub>0.9</sub>O<sub>2</sub></td><td>MS 3A</td><td>170</td><td>70</td><td>6.2</td><td>[22]</td></tr><tr><td>3</td><td>CeO<sub>2</sub>-ZrO<sub>2</sub></td><td>DMP</td><td>110</td><td>140</td><td>13.8</td><td>[37]</td></tr><tr><td>4</td><td>Ce<sub>0.5</sub>Zr<sub>0.5</sub>O<sub>2</sub></td><td>DMP</td><td>140</td><td>3</td><td>23.6</td><td>[39]</td></tr><tr><td>5</td><td>ZrO<sub>2</sub></td><td>TMM</td><td>130</td><td>48</td><td>61</td><td>This work</td></tr></tbody></table>

## 3. Materials and Methods

### 3.1. *Materials*

Zirconium carbonate, nitric acid (70%), ammonia solution (28%), methanol (≥99.8%), 2,2-dimethoxypropane (98%), and trimethyl orthoformate (also called 1,1,1-trimethoxymethane for synthesis) were all purchased from Sigma Aldrich (St. Louis, MO, USA) and used as received. CO₂ (99.9%) used as a reagent for the reaction was supplied by Air Liquide (Paris, France). A molecular sieve 13X (100–120 mesh, abbreviated as 13X in this paper) was supplied by Chromoptic (Villejust, France).

### 3.2. *Synthesis of ZrO₂ Nanocrystals*

A 4.6 M solution of zirconium nitrate was formulated by dissolving 1 kg of zirconium carbonate into nitric acid (HNO₃). Zirconium nitrate solution was subsequently added to 1.5 L of ammonia solution (NH₃·H₂O) at a rate of 10 mL/min with constant stirring to precipitate. The resulting precipitate was allowed to age within the mother liquor for durations of 48 h, 24 h, or no aging (0 h) at a constant temperature. Following the designated aging period, the precipitate underwent multiple washes before being dried overnight at 120 °C in an oven. All synthesized samples were subjected to calcination at 400 °C for 2 h in a static air atmosphere with a heating rate of 5 °C/min, followed by cooling to room temperature inside the oven. In this study, the sample aged for 48 h was denoted as ZrO₂-1, the sample aged for 24 h as ZrO₂-2, and the sample with no aging (0 h) as ZrO₂-3.

### 3.3. *Characterization*

X-ray diffraction patterns (XRD) were recorded in ambient conditions using the Cu Kα radiation (λ = 1.5418 Å; 40 kV, 30 mA) on a D8 Discovery diffractometer from Bruker AXS GmbH (Karlsruhe, Germany) equipped with Siemens X-ray tube. The 2θ of the wide angle ranged from 10° to 70°. The N₂ adsorption–desorption measurement was carried out using nitrogen adsorption at liquid-nitrogen temperature (77 K). The sample was degassed at 75 °C for 1 h and at 130 °C for 2 h. The specific surface area was evaluated with the Brunauer–Emmett–Teller (BET) model, and the pore size distribution was measured with the Barrett–Joyner–Halenda (BJH) model over the range P/P₀ = 0.05–0.30. The total pore volume was measured from the volume of N₂ adsorbed at P/P₀ = 0.95. Temperature-programmed desorption of NH₃ and CO₂ (NH₃-TPD and CO₂-TPD) was performed to quantify the amount and strength of the acid–base sites of the catalysts. A typical test was as follows: 50 mg catalysts were pre-treated in a He flow (30 mL/min) at 130 °C for 2 h in order to remove the physisorbed water. Then, NH₃ (CO₂) was absorbed at the surface by pulsed injections at 130 °C until saturation was stated from the MS signal. The TPD profiles were monitored by MS and thermal conductivity detector and recorded from 130 to 650 °C at a heating rate of 10 °C/min. Transmission electron microscopy (TEM) images were performed on a Tecnai G2 20 microscope (Waltham, MA, USA) equipped with LaB6 filament and operating at 200 kV. Samples for TEM analyses were prepared by dispersing the powder products as a slurry in ethanol and then deposited on a carbon film coated on a copper grid.
### 3.4. Catalytic Evaluation

The catalytic activity for the direct synthesis of DMC from CO₂ and methanol was evaluated on the REALCAT platform using a Screening Pressure Reactor (SPR) system from Unchained Labs (Pleasanton, CA, USA) equipped with 24 parallel stainless steel batch reactors of 6 mL each [53]. Typically, 30 mg of ZrO₂ nanocrystals or/and 30 mg of molecular sieve 13X were placed into the reactors with 1 mL of methanol and 1.3 mL of TMM (the molar ratio of methanol and TMM is 2:1). Before the reaction, the reactors were purged with CO₂ several times to remove air and then pressurized up to 20 bar with CO₂ at room temperature. The reaction temperature and pressure of the reactor were raised to 130 °C and 38 bar for carrying out the reaction for a given time with a continuous stirring of 700 rpm and then cooled down. Reaction products were analyzed using Shimadzu GC-2010 Plus and GC-FID-2010 Plus AF Ultra EI gas chromatographs, equipped with a FID detector (Shimadzu Corporation, Kyoto, Japan) and a ZB-WAX Plus column (30 m × 0.25 mm × 0.25 µm) (Phenomenex, Torrance, CA, USA). Solutions after the reaction were filtered to eliminate the solid and analyzed without further dilution.

### 4. Conclusions

In this work, three zirconia-based catalysts with different monoclinic and tetragonal phase contents were prepared and characterized. The ZrO₂-3 catalyst, presenting a mixture of monoclinic and tetragonal phases, presented the highest DMC production because of its large surface area and high total amount of acidic and basic sites. The DMC production was significantly increased by using TMM as a dehydration agent, which reacts with the water molecules formed by the reaction, hence shifting the thermodynamic equilibrium. Most importantly, this shift was further enhanced by the addition of a second solid catalyst molecular sieve, 13X, to accelerate the reaction of TMM hydrolysis. The catalytic effectiveness of the molecular sieve 13X is due to its strong acidic sites and water adsorption capacity.

At a temperature of 130 °C, a pressure of 38 bar of CO₂, a molar ratio of MeOH/TMM of 2:1, using 30 mg of ZrO₂-3 catalyst and 30 mg of 13X, 61 mmol DMC/gₐt. could be produced after 48 h of reaction. This is 2.5 times higher than without adding 13X as a cocatalyst.

**Author Contributions:** Conceptualization, D.S. and S.P.; methodology, D.S. and S.H.; validation, D.S., S.H., M.C. and S.P.; formal analysis, D.S.; investigation, S.P.; resources, S.P.; data curation, D.S.; writing—original draft preparation, D.S.; writing—review and editing, S.P.; supervision, S.P.; project administration, S.P.; funding acquisition, S.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was carried out in the frame of the VULCADIME project funded by the European Community through the Eurostars 113359 (call21) program. The REALCAT platform used in this work benefits from a state subsidy administered by the French National Research Agency (ANR) within the frame of the 'Future Investments' program (PIA) with the contractual reference ANR-11-EQPX-0037.

**Data Availability Statement:** Data are contained within the article.

**Acknowledgments:** The authors thank all the partners of the project for the very fruitful discussion and collaboration. The Carbon Recycling International company is acknowledged for supplying the zirconia catalysts used in this paper. The European Union, through the ERDF funding administered by the Hauts-de-France Region, Centrale Lille, the CNRS, and Lille University, as well as the Centrale Initiatives Foundation, is thanked for their financial contributions to the acquisition and implementation of the equipment of the REALCAT platform.

**Conflicts of Interest:** The authors declare no conflicts of interest.
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