We evaluated bismuth doped cerium oxide catalysts for the continuous synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide in the absence of a dehydrating agent. Bi_xCe_1−xO_δ nanocomposites of various compositions (x = 0.06–0.24) were coated on a ceramic honeycomb and their structural and catalytic properties were examined. The incorporation of Bi species into the CeO₂ lattice facilitated controlling of the surface population of oxygen vacancies, which is shown to play a crucial role in the mechanism of this reaction and is an important parameter for the design of ceria-based catalysts. The DMC production rate of the Bi_xCe_1−xO_δ catalysts was found to be strongly enhanced with increasing O_v concentration. The concentration of oxygen vacancies exhibited a maximum for Bi_0.12Ce_0.88O_δ, which afforded the highest DMC production rate. Long-term tests showed stable activity and selectivity of this catalyst over 45 h on-stream at 140 °C and a gas-hourly space velocity of 2,880 mL·g_cat⁻¹·h⁻¹. In-situ modulation excitation diffuse reflection Fourier transform infrared spectroscopy and first-principle calculations indicate that the DMC synthesis occurs through reaction of a bidentate carbonate intermediate with the activated methoxy (−OCH₃) species. The activation of CO₂ to form the bidentate carbonate intermediate on the oxygen vacancy sites is identified as highest energy barrier in the reaction pathway and thus is likely the rate-determining step.

Dimethyl carbonate (DMC) is widely employed as a versatile reagent and solvent for green organic synthesis and fuel additives. Owing to the environmental issues of the traditional methods for DMC production, the direct synthesis of DMC from carbon dioxide and methanol has received significant interest worldwide. Here, we demonstrate the catalytic effectiveness of monolithic titanium-doped cerium oxides, Ti_xCe_1−xO₂ nanocomposites, for the direct synthesis of DMC. The Ti_xCe_1−xO₂ nanocomposites are characterized by BET measurements, Raman spectroscopy, powder X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and H₂-temperature-programmed reduction analysis. To increase the reaction efficiency, the Ti_xCe_1−xO₂ composites are uniformly coated on honeycomb ceramics in a continuous tubular fixed-bed reactor. This leads to an extended surface area of the catalyst, and therefore the reaction conversion rate toward DMC production is significantly enhanced. Results indicate that the catalyst exhibits 24.3% methanol conversion with 79% selectivity for DMC synthesis at 140 °C in the absence of any dehydrating agents. The catalytic effectiveness of the Ti_xCe_1−xO₂ nanocomposites is found to be considerably higher than those of conventional catalysts. Analysis shows that the concentration of active sites can be systemically enriched by incorporating an optimal amount of Ti dopants into CeO₂ to improve the catalytic activity of the reaction. The results of this study demonstrate the promise of the Ti_xCe_1−xO₂ nanocomposites for the green production of DMC, and may offer guidelines for designing efficient catalysts for the selective conversion of methanol to other building blocks of chemical synthesis.

Heteroatom dopants can greatly modify the electronic and physical properties and catalytic performance of gold nanoclusters. In this study, we investigate the catalytic activity of [Au_25−x(PET)_18−xM]NH₃ (PET = 2-phenylethanethiolate, and M = Cu, Co, Ni, and Zn) nanoclusters in aerobic alcohol oxidation. The [Au_25−x(PET)_18−xM]NH₃ nanoclusters are thoroughly characterized by matrix assisted laser desorption ionization (MALDI) mass spectrometry, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), and inductively coupled plasma–mass spectrometry (ICP-MS). The XPS analyses suggest that the transition metals strongly interact with the gold atoms of the nanoclusters. The CeO₂-supported nanoclusters show catalytic activity, based on the conversion of benzyl alcohol, in the order, [Au_25−x(PET)_18−xNi] > [Au_25−x(PET)_18−xCu] > [Au_25−x(PET)_18−xZn] > [Au_25−x(PET)_18−xCo]. Regarding product selectivity, the [Au_25−x(PET)_18−xZn] and [Au_25−x(PET)_18−xCo] catalysts preferably yield benzaldehyde, [Au_25−x(PET)_18−xCu] yields benzaldehyde and benzyl acid, and [Au_25−x(PET)_18−xNi] yields benzyl acid. The exposed metal atoms are considered as the catalytic active sites. Also, the catalytic performance (including activity and selectivity) of the [Au_25−x(PET)_18−xM] catalysts is greatly turned and mediated by the transition metal type.

In this paper, a series of Zn_xCe_1−xO₂ (x = 0.00, 0.05, 0.10, 0.15, and 0.20) nanoparticles were synthesized via an aqueous-phase coprecipitation method. The Zn_xCe_1−xO₂ nanoparticles were coated on honeycomb ceramics to prepare monolithic catalysts, and the catalytic activity of the monolithic catalysts was investigated for the direct synthesis of dimethyl carbonate (DMC) from CO₂ and methanol in a continuous tubular fixed-bed reactor. The as-prepared catalysts were fully characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), N₂ adsorption/desorption isotherms, H₂-temperature-programmed reduction (H₂-TPR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and CO₂-temperature-programmed desorption (CO₂-TPD). The characterization results showed that doping Zn ions into the ceria lattice formed a fluorite-like solid solution and modified the structure and surface properties of the catalysts. Among all the monolithic Zn_xCe_1−xO₂ catalysts, the monolithic Zn_0.10Ce_0.90O₂ catalyst showed the highest catalytic activity with a methanol conversion of 20.5% with DMC selectivity of 82.1% at 160 °C and 2.4 MPa in the absence of any dehydrating agents. This superior catalytic activity was mainly attributed to a series of better properties, such as larger specific surface area (i.e. 179.9 m² g⁻¹), higher oxygen vacancy concentration, and the unique structure of the monolithic catalyst (e.g., removing by-product of water in time). This study provides a valuable design strategy for highly efficient direct synthesis of DMC using monolithic catalysts in the fixed-bed reactor.