Direct Synthesis of Dimethyl Carbonate from Methanol and Carbon Dioxide Catalyzed by Cerium-Based High-Entropy Oxides

Abstract

Fluorite high entropy oxides (HEOs) were synthesized by anchoring method and Ce_0.5(LaPrSmY)_0.5O_2-y used as catalysts for synthesis of dimethyl carbonate (DMC) from methanol (CH₃OH) and carbon dioxide (CO₂), the yield of DMC was up to 7.6 mmol/g at 140 ℃ and 8.0 MPa. Under the same reaction conditions, the time to reach reaction equilibrium was reduced by half compared HEOs with CeO₂. The structure of HEOs were characterized by X-ray diffraction (XRD), specific surface area and pore size analyzer (BET), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The results showed that there are five metal elements of Ce, La, Pr, Sm and Y in the lattice of layered fluorite-type HEOs Ce_x(LaPrSmY)_1-xO_2-y. The synergistic effect among the elements led to the lattice distortion and the generation of a large number of oxygen vacancies, which improved the reaction rate.

Graphical Abstract

The rare earth metal ions were anchored on the graphene oxide by electrostatic interaction and then calcinated to obtain fluorite high entropy oxides (HEOs) Ce_x(LaPrSmY)_1-xO_2-y. Ce_0.5(LaPrSmY)_0.5O_2-y were used as catalysts for synthesis of dimethyl carbonate (DMC) from methanol (CH₃OH) and carbon dioxide (CO₂), the yield of DMC was up to 7.6 mmol/g at 140 °C and 8.0 MPa. Under the same reaction conditions, the time to reach reaction equilibrium was reduced by half compared with CeO₂ attributed to the abundant oxygen vacancies on the surface and the excellent electron transfer ability.

1 Introduction

HEOs are single phase solid solutions consisting of five or more elements [1, 2]. The first appearance of HEOs in the literature was in 2015 [3], it has received increasing attention due to its high stability, low thermal conductivity, high corrosion resistance, oxidation resistance, and high catalytic activity [4, 5]. HEOs prepared by non-traditional synthesis methods (arc melting and spark plasma sintering) possessed an entropy stable system with a specific composition and structure [6, 7]_, but the catalytic performance of HEOs were limited, by only part of active sites in HEOs being exposed to the surface of the material due to high temperature. The cations in HEOs are composed of same or similar proportions incorporated into the cationic sub-lattice, and the average radius of cations is one of the most important factors affecting the crystal of HEOs[7,8,9].

Since 2000, rare earth oxide materials have received widespread attention in various research areas such as photocatalysis, energy storage and conversion, and electrochemistry, and a series of far-reaching research results have been achieved [10,11,12]. Rare-earth elements exhibit highly similar ionic radii and physicochemical properties[13]. Most of the high entropy compositions that were successfully prepared as single phases are within the 15% limit of atomic radii differences known as a Hume-Rothery [14] rule to metallurgists and as a Goldschmidt [15] limit for isomorphic mixtures to mineralogists. The combination of several (five or more) rare earth elements with highly similar ionic radii and physicochemical properties into a specific structure allows better formation of high entropy oxides with a single crystalline phase[16].

DMC as a ‘green solvent’ in the chemical industry, is an important chemical intermediate with a wide range of applications in many fields [17,18,19]. The demand for DMC is increasing year by year due to its wide range of application scenarios. Traditional synthetic routes suffered from various drawbacks [19,20,21,22,23,24]. The direct synthesis of DMC from CH₃OH and CO₂ has the advantages of abundant raw materials, environmental friendliness and high atomic economy. However, the direct synthesis method was limited by kinetic and thermodynamic factors, and the conversion of CH₃OH as well as the reaction rate was low [25,26,27]. The Ce catalyst possessed excellent catalytic performance in the direct synthesis of DMC from CO₂ and CH₃OH [28,29,30].

In this paper, rare earth element fluorite HEO was synthesized by anchoring method, and its catalytic performance in the direct synthesis of DMC from CO₂ and CH₃OH was investigated. During the formation process of HEOs, rare earth metal cations were adsorbed by the negative functional groups on the surface of graphene oxide, which could prevent effectively the agglomeration of precursor nanoparticles during heating [31]. Then, a porous single-phase fluorite-type HEOs are formed by high-temperature calcination.

1.1 Materials

Ce(NO₃)₃∙6H₂O, La(NO₃)₃∙nH₂O, Sm(NO₃)₃∙6H₂O, Pr(NO₃)₃∙6H₂O, Y(NO₃)₃∙6H₂O, CH₃OH, n-Butanol (CH₃(CH₂)₃OH), Ethylene glycol ((CH₂OH)₂), Dimethyl carbonate (C₃H₆O₃), were from, Ron company (Shang hai, China). All reagents were of analytical grade and were used as received. CO₂ (99.99%), N₂ (> 99.99%).

1.2 Preparation of Catalyst

The formation diagram of porous layered HEOs were shown in Fig. 1. The typical synthesis process was shown in Fig. 2: 100 mg of graphene oxide was dissolved in 50 mL of ethylene glycol, a certain molar ratio of Sm, La, Ce, Pr and Y nitrate were dissolved in 30 mL of ethylene glycol, and each of them was sonicated for 30 min. Then the two solutions were mixed and continued to be ultrasonically dispersed for 60 min to make the solution mixed uniformly. The the solution was poured into flasks and refluxed for 4 h in 170 ℃ oil bath. The product was separated by centrifugation, washed and precipitated three times with anhydrous ethanol, and then dried in a vacuum oven at 60 ℃ for 6 h. The obtained precursors were calcined at 500 ℃, 750 ℃ and 1000 ℃ for 2 h in air to obtain porous layered HEOs Ce_x(LaPrSmY)_1-xO_2-y (‘x’, the molar ratio of Ce ions in the metal cation of HEOs; ‘1-x’, the molar ratio of metal cation M (M = La, Pr, Sm and Y) in the metal cation of HEOs; ‘2-y’, the molar amount of oxygen atoms of HEOs to maintain electrical neutrality.). Prepared Ce_0.2(LaPrSmY)_0.8O_2-y were named HEO-500, HEO-750 and HEO-1000.

Fig. 1

The formation concept of HEOs

Fig. 2

Schematic illustration of the formation process for holey HEOs

1.3 Catalyst Evaluation

The catalyst performance test was carried out in an autoclave. A certain amount of catalyst and 15 mL CH₃OH were put into a high-pressure reactor with PTFE lining. The reactor was installed according to the operation and the gas tightness of the device was good during the reaction, and then CO₂ was introduced three times to replace the air in the autoclave. Finally, the reactor was filled with 4 MPa CO₂ (initial pressure). The temperature was set to rise from room temperature to 140 ℃ in 30 min and kept at 140 ℃ for a certain time. After reaction, the product was analyzed by gas chromatography (Agilent 7890B). The yield of DMC was quantified using the internal standard method, with n-butanol as the internal standard, and the calculation procedure is as follows [32].

$${W}_{DMC}={f}_{DMC}\frac{{A}_{DMC}}{\frac{{A}_{s}}{{W}_{s}}}$$

(1)

$${n}_{DMC}=\frac{{W}_{DMC}}{{M}_{DMC}}$$

(2)

$$Y_{DMC} =\left(\frac{{n}_{DMC}}{m_{catalyst}}\right)$$

(3)

${f}_{DMC}$: Relative mass correction factor of DMC. ${A}_{DMC}$: Peak area of DMC. ${A}_{s}$: Peak area of n-butanol. ${W}_{s}$: Mass of internal standard n-butanol in the sample. ${W}_{DMC}$: Mass of DMC in the sample. ${M}_{DMC}$: Molar mass of DMC. $n_{DMC}$: Amount of substance of DMC. $m_{catalyst}$: Quality of catalyst. $Y_{DMC}$: The yield of DMC

1.4 Characterization

An X-ray diffractometer manufactured by German Brook AXS Company was used to test and analyze the crystal structures of samples (wide-angle scanning range: 20°–90°; scanning speed: 6°/min). Nitrogen adsorption–desorption measurements were performed at 90 ℃ using an ASAP 2020 analyzer utilizing the BET model for the calculation of specific surface areas. Micromeritics Auto Chem 2920 automatic programmed temperature chemical adsorption instrument, test conditions: 90 ℃, vacuum degassing 8 h, adsorbate N₂, carrier gas He. The catalyst particles were observed using Talos F200S field emission high-resolution transmission electron microscope with the pressurization voltage set to 200 kV. The analysis of surface elements was performed on X-ray photoelectron spectroscopy (XPS), Thermo Fisher Scientific Escalab 250Xi spectrometer with Al Kalpha radiation as the excitation source.

2 Results and Discussion

2.1 Catalyst Characterization

2.1.1 XRD

XRD patterns of CeO₂ and a series of HEOs prepared at different calcination temperature were shown in Fig. 3. There was no obvious characteristic peak observed when HEOs being calcined at 500 ℃, indicating that the sample was not strongly crystalline.When the calcination temperature was increased to 750 ℃, a series of multi-phase mixture peaks appeared in the XRD pattern. At this time, the graphene oxide had not been completely removed from the surface of the HEOs crystals. Moreover, the characteristic absorption peak intensity of HEOs in the figure was weakened and the half-peak width was widen, indicating that the sample was weakly crystalline and the nanoparticles were not completely dispersed. When the calcination temperature was increased to 1000 ℃, the HEOs exhibited the same characteristic peaks as CeO₂, which could indicate HEOs were transformed from a multiphase mixture to a face-centered cubic structure with a single crystalline phase, and the graphene oxide was completely removed. In addition, the XRD pattern of HEO-1000 illustrates that all the rare earth metal elements entered the fluorite crystalline phase with small half-peak widths of characteristic peaks and good crystallinity [33, 34]. This is in agreement with the previous report of Hao Chen et al., that the key to the formation of high entropy oxides is being treated at high temperature [4]. From the XRD pattern and the calculation of Scherrer's formula in Jade, it could be concluded that the sample lattice parameter is 5.35 Å, which was smaller than the lattice parameter of CeO₂ (5.411 Å), probably due to the fact that the radii of all other ions are slightly smaller than Ce⁴⁺.

Fig. 3

XRD patterns of CeO₂ and holey HEOs prepared at different calcination temperature: (a) CeO₂, (b) HEO-1000, (c) HEO-750, (d )HEO-500

2.1.2 BET

The N₂ adsorption–desorption test of HEOs catalysts prepared at different calcination temperature was carried out, and the results were shown in Fig. 4. It could be seen from the adsorption isotherms of HEOs calcined at 500 ℃ that there was no saturated adsorption platform and hysteresis loop. It showed that the pore structure of the catalyst was very irregular, only a few of the catalyst particles were accumulated into slit pores, which was still in the multiphase amorphous state, and the specific surface area of the solid solution was only 37 m²·g⁻¹. The adsorption isotherms of HEOs calcined at 750℃ and 1000℃ showed IV type characteristics at P/P₀ = 0.6, indicating that the catalysts formed a mesoporous structure and an H2 type hysteresis loop[35]. The hysteresis loop of HEOs calcined at 1000 ℃ appears later than that of HEOs calcined at 750 ℃. When the pressure was enough to cause capillary condensation, the larger the pore size on the surface of the solid solution, the higher the pressure at which capillary condensation occurred, indicating that the pore size of the solid solution calcined at 1000 ℃ is bigger [36, 37]. However, the layered structure of HEOs calcined at 1000 ℃ was more regular, and the specific surface area of solid solution was 100 m²·g⁻¹, while the specific surface area of HEOs calcined at 750 ℃ was 64 m²·g⁻¹.

Fig. 4

N₂ adsorption–desorption isotherms of holey HEOs prepared at different calcination temperature: (a) HEO-500, (b) HEO-750, (c) HEO-1000

2.1.3 TEM and EDS

The TEM images of porous layered HEOs calcined at 1000 ℃, 750 ℃ and 500 ℃ were shown in Fig. 5(a)–(c), respectively. When the calcination temperature of 500 ℃ and 750 ℃, it could be clearly observed that the HEOs particles were not completely dispersed. The black shadows in Fig. 5(b) and 5 (c) showed that some atoms of HEOs were still in the state of aggregation at this time, and there were some collapses and graphite residues between layers. The uniformly dispersed nanoparticles were formed after HEOs were calcinated at 1000 ℃, and the particles were interconnected to form the pore walls of porous structure, which was consistent with the results of XRD analysis. Attributed to ethylene glycol as a surfactant in the preparation of the precursor, not only the size of the nanoparticles could be well controlled, but also the spontaneous agglomeration of the synthesized nanoparticles could be reduced, ensuring the high dispersion of metal ions [38, 39].The average particle size of HEO-1000 particles was about 15–25 nm.

Fig. 5

TEM images of holey HEOs at different calcination temperature: (a) 1000 ℃, (b) 750 ℃, (c) 500 ℃, (d) HRTEM images

The microstructure of HEO-1000 at the atomic scale was visualized by HRTEM, and the image was shown in Fig. 5(d). HEO-1000 showed only one type of lattice stripe, indicating that HEO-1000 is a single-crystal structure with a lattice spacing of 0.241 nm, corresponding to the (111) lattice with the highest intensity of diffraction peaks in the high-entropy oxide [40]. Combined with the XRD patterns, it was concluded that high-temperature calcination was necessary for the formation of a single fluorite type HEOs.

Elemental dispersion played an important role for the formation of high entropy structures in HEOs, the element mapping diagram of HEO-1000 were shown in Fig. 6. The distribution of Ce, La, Pr, Sm and Y in the material could be seen in the Fig. 6, and no local aggregation of one or several elements was found on the microscopic scale. It shown that the elements in the sample were uniformly distributed and formed a single-phase high entropy structure. Meanwhile, it could be speculated that the uniform distribution of Ce, La, Pr, Sm and Y in the fluorite unit cell is consistent to the highest configuration entropy model theory. That is, the stability of the whole system is the strongest when the configuration entropy of HEOs is the biggest [41, 42].

Fig. 6

Elemental mapping images of HEO-1000

2.1.4 XPS

The surface valence of the elements in the HEO-1000 catalyst were determined by XPS, and the results were shown in Fig. 7. The O 1 s peaks can be well fitted with three peaks due to the lattice oxygen located at 528.5 eV, oxygen vacancies located at 531.0 eV and surface oxygen located at 533 eV [43]. The intensity of the oxygen vacancy peak represents the content of oxygen vacancies on the catalyst surface, indicating the strength of the redox ability of the catalyst [44, 45]. By XPS quantitative analysis of these three types of oxygen, it was found that the oxygen vacancy content in porous layered HEOs were as high as 29%, while higher oxygen vacancy content increased the ability of the catalyst to adsorb CO₂ and activate CH₃OH. The formula is as follows:

Fig. 7

O1s XPS spectra of HEO-1000 sample: (a) O_C, (b) O_V, (c) O_L

$$C[{O}_{V}\%]=\left(\frac{A {O}_{V}}{A {O}_{L}+A {O}_{C}+A {O}_{V}}\right)\times 100\%$$

(4)

Figure 8 was the XPS spectra of HEO-1000, including Ce, La, Pr, Sm and Y, indicating the formation of a good five-element solid solution structure, which was consistent with the element mapping results in Fig. 6. In the XPS spectra of the metal elements, the Ce 3d spectra could be fitted to eight peaks, four V peaks and four U peaks, namely, V (~ 882.4 eV), V′ (~ 883.2 eV), Vʺ (~ 888.7 eV), V‴ (~ 898.1 eV), U (~ 900.9 eV), U′ (~ 902.8 eV), Uʺ (~ 907.4 eV) and U‴ (~ 916.4 eV). where V, Vʺ, V‴ and U, Uʺ, U‴ denoted the electronic states of Ce⁴⁺ and V′ and U′ denoted the electronic states of Ce³⁺ [30]. La 3d_3/2 and La 3d_5/2 at 853.1 and 835.7 eV characteristic peaks attributed to La³⁺ [46]. The characteristic peaks located at 933.8 eV and 954.3 eV should be attributed to Pr 3d_2/5 and Pr 3d_2/3 of Pr³⁺ in HEO-1000. The other two surrounding satellite peaks might be due to the presence of unpaired electrons in the Pr hybrid orbitals and spin states[47]. The characteristic peaks located at about 1083.3 eV and 110.4 eV 1were attributed to Sm 3d_5/2 and Sm 3d_3/2 of Sm³⁺ [47]. the characteristic peaks located at 156.5 eV and 158.8 eV were attributed to Y 3d_5/2 and Y 3d_3/2 of Y³⁺ [46]. In addition, XPS spectra showed that the binding energy of each element in HEO-1000 was strong compared to that of oxides composed of a single rare earth element[48,49,50,51,52]. The shift of elemental bond energy to higher positions indicated not only the formation of alloy-like species, but also a higher positive charge of all metal elements, which made it easier for the reactants to react on HEO-1000 and improved the catalytic performance.

Fig. 8

XPS spectra of HEO-1000 sample: (a) Broad spectrum, (b) Ce 3d, (c) La 3d, (d) Pr 3d, (e) Sm 3d, (f) Y 3d

2.1.5 H₂-TPR

H₂-TPR experiments were performed in order to evaluate the redox ability of HEO-1000 and the results were shown in Fig. 9. The reduction peak at 462.4 ℃ was attributed to the removal of lattice oxygen from the surface and subsurface. The reduction peak at 757.5 ℃ could be interpreted as the elimination of lattice oxygen in HEO-1000. The reduction peak at 287.9 ℃ was attributed to the removal of adsorbed oxygen from the surface, which could indicate that HEO-1000 possesses abundant oxygen vacancies and therefore could capture a large amount of adsorbed oxygen[45, 53]. The three peaks appearing in Fig. 9 were all moving to the direction of low temperature during the reduction process compared with the peak of pure CeO₂. The layered structure on the surface of the HEOs benefited to the activation of various oxygen species and provided high performance.

Fig. 9

H₂-TPR profiles of samples: (a) CeO₂, (b) HEO-1000. (I) 287.9 °C, (II) 462.4 °C, (III)757.5 °C

2.2 Catalytic Performance

The catalytic performance of porous layered HEOs in the direct synthesis of DMC from CH₃OH and CO₂ was investigated, and the results were shown in Fig. 10. The yield of DMC was 7.2 mmol/g when 0.05 g HEO-1000 catalyst was used at 140 ℃ and 8 MPa (reaction pressure). The yield of DMC was 7.3 mmol/g when CeO₂ as catalyst under the same reaction conditions [29]. When HEO-1000 was used for the synthesis of DMC from CH₃OH and CO₂, the conversion of DMC increased exponentially with time. It took only 2–3 h for HEO-1000 to reach equilibrium at 140 ℃ and 8 MPa (reaction pressure), while it took about 5 h for CeO₂ to reach equilibrium under the same reaction conditions. The large pores, higher specific surface area, and the gap between the layers of HEO-1000 exposed more active sites, which ensured that CO₂ was sufficiently adsorbed by the catalyst, thus the catalytic efficiency of the catalyst was improved greatly. The HEO-1000 lattice was composed of five metal elements, and the differences in atomic radii and bond lengths can lead to severe distortion of the internal lattice of the solid solution [54, 55], which was called lattice distortion, as shown in Fig. 11. That is, enhancing S_config (configuration entropy), the binding of different types of cations in the same lattice was realized and enriched the number of oxygen vacancies in the active catalytic center. In addition, lattice distortion could also reduce the energy of the whole HEO system and promote and electron transfer of active species[56, 57].

Fig. 10

Effect of reaction time on the yield of DMC over different catalysts: (a) HEO-1000, (b) CeO₂, Reaction conditions: 0.05 g catalyst, 15 mL CH₃OH, 140 °C, 8 MPa (reaction pressure)

Fig. 11

Schematic diagram of lattice distortion of HEOs

2.3 Influence of Element Composition on Structure and Catalytic Performance of HEO

CH₃OH was dissociated into CH₃O⁻ and H⁺ on the catalyst[58, 59], while CO₂ was activated on the surface of the catalyst to produce a bidentate adsorption state and combined with CH₃O⁻ to form the methyl carbonate active intermediate [60, 61], and the catalyst could be considered as an ‘electron transfer station’.

Ce was in + 4 valence states in the prepared HEOs, while La, Pr, Sm and Y were in + 3 valence states. When the low-valence cations La, Pr, Sm and Y entered the high-valence cation Ce lattice to produce a covalent trend with the Ce⁴⁺ [62], due to the lack of an electron at this time, in order to maintain the electrical neutrality of the solid solution as a whole, a positively charged oxygen vacancy was formed by the lattice oxygen flowing to enhance the electron transfer capability[63], and the positively charged oxygen vacancies could play a role in transferring electrons from CH₃OH to CO₂ [60, 64]. The higher the content of positively charged oxygen vacancies in the catalyst, the stronger the ability of the catalyst to transfer electrons. Therefore, the content of tetravalent cerium in HEO were increased to form more positively charged oxygen vacancies on the catalyst surface, the conductivity of the catalyst and the electron transfer efficiency of CH₃OH and the catalyst surface were improved. In the direct synthesis of DMC from CO₂ and CH₃OH, the rate of CH₃OH dissociation and adsorption was much slower than that of CO₂. In terms of kinetics, the process of CH₃OH giving electrons was the rate-controlling step of the reaction [65]. To improve the activation ability of CH₃OH was more favorable to improve the overall reaction activity.

It is not necessary for a solid-solution single phase of a HEOs to have an equimolar ratio distribution. Since the configurational entropy S_config is proportional to the logarithm of the number of element species of HEOs. The entropy was increased more slowly, when the number of elemental species is higher. That is, when the element species is more than 5, it is possible to form a solid solution single-phase structure even if the HEOs element composition is not equimolar [33, 66]. Therefore, the influence of high valence cation Ce⁴⁺ content on the catalytic performance was studied by no changing others rare earth elements, and it was expected that the ability of catalysts to transfer electrons could be improved by exploiting the conductivity of high entropy materials. Ce_0.5(LaPrSmY)_0.5O_2-y and Ce_0.8(LaPrSmY)_0.2O_2-y were obtained by increasing the content of Ce⁴⁺ in HEO-1000 to 50% and 80% while keeping the other four low-valent cations in the same molar ratio.

The XRD patterns of HEOs with different Ce⁴⁺ contents were shown in Fig. 12. It is shown that Ce_0.5(LaPrSmY)_0.5O_2-y still maintained the cubic fluorite structure. However, when the content of Ce element was increased to 80%, Ce_0.8(LaPrSmY)_0.2O_2-y appeared miscellaneous peaks at around 40° and 60–90°. The Jade crystal plane analysis showed that the second phase appeared in the solid solution, indicating that the Ce⁴⁺ content was excessive. HEOs lost a single fluorite structure because the lattice was destroyed to form a second phase.

Fig. 12

XRD patterns of holey HEOs at varying Ce contents: (a) Ce_0.8(LaPrSmY)_0.2O_2-y, (b) Ce_0.5(LaPrSmY)_0.5O_2-y, (c) Ce_0.2(LaPrSmY)_0.8O_2-y

The N₂ adsorption–desorption isotherms of pore-shaped HEOs at different Ce contents, and the results were shown in Fig. 13. BET analysis showed that the adsorption isotherms of Ce_0.5(LaPrSmY)_0.5O_2-y and Ce_0.8(LaPrSmY)_0.2O_2-y still exhibited the type IV adsorption isotherm. However, the hysteresis loop in the N₂ desorption curve of Ce_0.8(LaPrSmY)_0.2O_2-y was greatly attenuated when P/P₀ was 0.8. It is presumed that the mesoporous structure of the catalyst was covered by the collapse of the layered structure (Fig. 13). At this point, the specific surface area of the catalyst decreased from 100 m²/g to 64 m²/g, then the collision probability of CH₃OH and CO₂ on the catalyst surface decreased, and the activity of the catalyst decreased.

Fig. 13

N₂ adsorption–desorption isotherms of holey HEOs at different Ce contents: (a) Ce_0.8(LaPrSmY)_0.2O_2-y, (b) Ce_0.5(LaPrSmY)_0.5O_2-y, (c) Ce_0.2(LaPrSmY)_0.8O_2-y

HEOs with different Ce content was used for synthesis of DMC from CH₃OH and CO₂, and the results were shown in Fig. 14. The results of the catalytic activity test showed that when Ce⁴⁺ content was increased from 20 to 50%, the DMC yield increased from 7.1 mmol/g to 7.6 mmol/g when the reaction time is 200 min. When the Ce content was further increased to 80%, the DMC yield and conversion rate decreased significantly, and the DMC yield decreased to 6 mmol/g.

Fig. 14

Effect of reaction time on the yield of DMC: (a) Ce_0.2(LaPrSmY)_0.8O_2-y, (b) Ce_0.5(LaPrSmY)_0.5O_2-y, (c) Ce_0.8(LaPrSmY)_0.2O_2-y Reaction conditions: 0.05 g catalyst, 15 mL CH₃OH, 140 ℃, 8 MPa (reaction pressure)

As the Ce⁴⁺ content increased from 20 to 80%, the oxygen vacancy content of the HEOs surface and the yield of DMC in the activity test changed, the results were shown in Fig. 15. With the increased of Ce⁴⁺ content from 20 to 50%, the oxygen vacancy content on the catalyst surface increased from 25.6% to 31.9%. However, when Ce⁴⁺ content increased from 50 to 80%, the oxygen vacancy concentration decreased to 22%. It could be seen that the oxygen vacancy concentration was positively correlated with the catalytic performance of the HEOs. The adsorption and activation of CH₃OH by some additional positively charged oxygen vacancies increased the ability to receive negatively charged methoxy, that is, the increased content of positively charged oxygen vacancies effectively enhanced the ability of the catalyst surface to transfer electrons. In addition, it could be seen from Fig. 15 that the Ce content has a significant influence on the yield of DMC which is due to the fact that the carbonylation process of DMC has a high spatial site resistance and the reduction in the content of oxygen vacancies in the catalyst reduced the adsorption capacity of CO₂ and CH₃OH molecules and hindered the formation of C-O bonds [67]. The proposed mechanism of DMC formation from CO₂ and CH₃OH at the surface oxygen vacancies over catalysts as shown in Fig. 16 [59].

Fig. 15

Effect of surface oxygen vacancy content of HEOs with different Ce content on the yield of DMC, Reaction conditions: 0.05 g catalyst, 15 mL CH₃OH, 140 ℃, 8 MPa (reaction pressure)

Fig. 16

Proposed mechanism of DMC formation from CO₂ and CH₃OH at the surface oxygen vacancies over catalysts

The TEM images of Ce_0.5(LaPrSmY)_0.5O_2-y and Ce_0.8(LaPrSmY)_0.2O_2-y were shown in Fig. 17. The Ce_0.8(LaPrSmY)_0.2O_2-y cannot form a well-defined high-entropy structure because the Ce content was too high to maintain the high dispersion of each atom on the molecular level. The shaded part of the picture might be a large agglomeration of Ce atoms. Combined with the BET results in Fig. 13, it could be seen that the pore of Ce_0.8(LaPrSmY)_0.2O_2-y was blocked, the lamellar structure was collapsed, and the specific surface area was decreased. The collapse of the structure and atomic aggregation lead to the coverage of oxygen vacancies on the catalyst surface, resulting in the inability of CH₃OH and CO₂ to be fully adsorbed by the catalyst, which reduced the catalyst activity. Combined with the XRD patterns of Fig. 12, it could be seen that Ce_0.8(LaPrSmY)_0.2O_2-y appeared a second phase at this time, resulting in lattice rupture. The conductivity of the catalyst was proportional to the hole concentration and hole mobility. The disruption of the lattice will reduce the rate of electron migration, which will affect the rate of electron transfer on the catalyst surface.

Fig. 17

TEM images of holey HEOs at different Ce contents: (a) Ce 50%, (b) Ce 80%

In summary, the content of positively charged oxygen vacancies on the catalyst surface could be increased by changing the content of high-valent cations in HEOs, and the catalytic performance of HEOs were improved. And excessive content of a single element will prevent various atoms from dispersing on the molecular level, the porous layered structure in HEOs will be destroyed, and the S_config of HEOs are reduced, then HEOs cannot maintain a single cubic fluorite structure. The specific surface area and oxygen vacancy content of HEOs in the disrupted single crystalline phase were reduced, resulting in lower catalyst performance as well as electron transfer efficiency.

3 Conclusions

Ce_x(LaPrSmY)_1-xO_2-y porous fluorite HEOs catalysts were synthesized by anchoring method. DMC was synthesized from CH₃OH and CO₂ by using Ce_0.5(LaPrSmY)_0.5O_2-y as a catalyst, and the yield of DMC was as high as 7.6 mmol/g at 140 ℃ and 8.0 MPa (reaction pressure). Under the same conditions, Ce_0.5(LaPrSmY)_0.5O_2-y reached the reaction equilibrium was in half the time of CeO₂. The reaction rate for the direct synthesis of DMC from CH₃OH and CO₂ on Ce_0.5(LaPrSmY)_0.5O_2-y was enhanced, attributed to the abundant oxygen vacancies on the surface and the excellent electron transfer ability. When the content of valent cation Ce⁴⁺ in HEOs increased from 20 to 50%, the catalytic performance was improved due to lattice distortion and more oxygen vacancies. When the content of Ce⁴⁺ increased to 80%, the fluorite crystal in HEOs were destroyed, a second crystal phase was produced, and the catalytic performance decreased.