Ceria nanostructured catalysts for conversion of methanol and carbon dioxide to dimethyl carbonate

Platelet Ceria Catalysts from Solution Combustion and Effect of Iron Doping for Synthesis of Dimethyl Carbonate from CO2

Solution combustion (SC) remains among the most promising synthetic strategies for the production of crystalline nanopowders from an aqueous medium, due to its easiness, time and cost-effectiveness, scalability and eco-friendliness. In this work, this method was selected to obtain anisometric ceria-based nanoparticles applied as catalysts for the direct synthesis of dimethyl carbonate. The catalytic performances were studied for the ceria and Fe-doped ceria from SC (CeO2-SC, Ce0.9Fe0.1O2-SC) in comparison with the ceria nanorods (CeO2-HT, Ce0.9Fe0.1O2-HT) obtained by hydrothermal (HT) method, one of the most studied systems in the literature. Indeed, the ceria nanoparticles obtained by SC were found to be highly crystalline, platelet-shaped, arranged in a mosaic-like assembly and with smaller crystallite size (≈6 nm vs. ≈17 nm) and higher surface area (80 m2 g-1 vs. 26 m2 g-1) for the undoped sample with respect to the Fe-doped counterpart. Although all samples exhibit an anisometric morphology that should favor the exposition of specific crystalline planes, HT-samples showed better performances due to higher oxygen vacancies concentration and lower amount of strong basic and acid sites.

Roles of carbon dioxide in the conversion of biomass or waste plastics

Under the current trend of pursuing sustainable development and environmental protection, the important application of carbon dioxide (CO2) in the conversion process of biomass or waste plastics has become a research direction of concern. The goal of this conversion process is to achieve the efficient use of carbon dioxide, providing a process for the efficient use of biomass, and solving the environmental problems caused by plastics. Remarkable progress has been made in the study of the reaction of CO2 with other substances to produce methane, low-carbon hydrocarbons, methanol, formic acid, and its derivatives, as well as ethers, aldehydes, gasoline, low-carbon alcohols, and other chemicals. In this paper, the important role of CO2 in the conversion of alcohol, sugar, cellulose, and waste plastics was reviewed, with emphasis on the important applications of CO2 as a carbon source, reactant, reaction medium, enhancing agent, solvent, and carrier gas in the conversion of biomass or waste plastics and the basic insights of the reaction mechanism. The emerging CO2 new roles not only put forward the green application of CO2 but also have guiding significance for the utilization of biomass resources and the treatment of waste plastics.

Dimethyl Carbonate Synthesis from CO2 over CeO2 with Electron-Enriched Lattice Oxygen Species

Direct synthesis of dimethyl carbonate (DMC) from CO2 plays an important role in carbon neutrality, but its efficiency is still far from the practical application, due to the limited understanding of the reaction mechanism and rational design of efficient catalyst. Herein, abundant electron-enriched lattice oxygen species were introduced into CeO2 catalyst by constructing the point defects and crystal-terminated phases in the crystal reconstruction process. Benefitting from the acid-base properties modulated by the electron-enriched lattice oxygen, the optimized CeO2 catalyst exhibited a much higher DMC yield of 22.2 mmol g-1 than the reported metal-oxide-based catalysts at the similar conditions. Mechanistic investigations illustrated that the electron-enriched lattice oxygen can provide abundant sites for CO2 adsorption and activation, and was advantageous of the formation of the weakly adsorbed active methoxy species. These were facilitating to the coupling of methoxy and CO2 for the key *CH3OCOO intermediate formation. More importantly, the weakened adsorption of *CH3OCOO on the electron-enriched lattice oxygen can switch the rate-determining-step (RDS) of DMC synthesis from *CH3OCOO formation to *CH3OCOO dissociation, and lower the corresponding activation barriers, thus giving rise to a high performance. This work provides insights into the underlying reaction mechanism for DMC synthesis from CO2 and methanol and the design of highly efficient catalysts.

Reactive Capture and Conversion of CO2 into Hydrogen over Bifunctional Structured Ce1-xCoxNiO3/Ca Perovskite-Type Oxide Monoliths

Carbon capture, utilization, and storage (CCUS) technologies are pivotal for transitioning to a net-zero economy by 2050. In particular, conversion of captured CO2 to marketable chemicals and fuels appears to be a sustainable approach to not only curb greenhouse emissions but also transform wastes like CO2 into useful products through storage of renewable energy in chemical bonds. Bifunctional materials (BFMs) composed of adsorbents and catalysts have shown promise in reactive capture and conversion of CO2 at high temperatures. In this study, we extend the application of 3D printing technology to formulate a novel set of BFMs composed of CaO and Ce1-xCoxNiO3 perovskite-type oxide catalysts for the dual-purpose use of capturing CO2 and reforming CH4 for H2 production. Three honeycomb monoliths composed of equal amounts of adsorbent and catalyst constituents with varied Ce1-xCox ratios were 3D printed to assess the role of cobalt on catalytic properties and overall performance. The samples were vigorously characterized using X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), N2 physisorption, X-ray photoelectron spectroscopy (XPS), H2-TPR, in situ CO2 adsorption/desorption XRD, and NH3-TPD. Results showed that the Ce1-xCox ratios-x = 0.25, 0.50, and 0.75-did not affect crystallinity, texture, or metal dispersion. However, a higher cobalt content reduced reducibility, CO2 adsorption/desorption reversibility, and oxygen species availability. Assessing the structured BFM monoliths via combined CO2 capture and CH4 reforming in the temperature range 500-700 °C revealed that such differences in physiochemical properties lowered H2 and CO yields at higher cobalt loading, leading to best catalytic performance in Ce0.75Co0.25NiO3/Ca sample that achieved 77% CO2 conversion, 94% CH4 conversion, 61% H2 yield, and 2.30 H2/CO ratio at 700 °C. The stability of this BFM was assessed across five adsorption/reaction cycles, showing only marginal losses in the H2/CO yield. Thus, these findings successfully expand the use of 3D printing to unexplored perovskite-based BFMs and demonstrate an important proof-of-concept for their use in combined CO2 capture and utilization in H2 production processes.

The effects of Fe, Mg, and Pt-doping on the improvement of Ni stabilized on Al2O3-CeO3 catalysts for methane dry reforming

Herein, the promotional effects of Mg, Fe, and Pt on Ni-based catalysts supported on Al2O3-CeO2 (Ni/Al2O3-CeO2) were investigated in the dry reforming of methane (DRM) reaction. The interaction of a suitable amount of MgO and FeO with Ce2O3 stabilized in the catalysts was demonstrated by the temperature-programmed desorption of CO2 (CO2-TPD). Ce2O3 has a high basicity for adsorbing CO2, generating a monoclinic Ce2O2CO3 species in the DRM reaction. Surface oxygen ions were also produced by adding MgO and FeO, as demonstrated by the temperature-programmed reduction of H2 (H2-TPR). Monoclinic Ce2O2CO3 and surface oxygen may both be used to oxidize and remove the carbon that was deposited, maintaining the high activity and stability of the metal Ni and Pt catalysts. The high dispersion and synergistic interactions between the platinum and oxide phases, which are associated with the decrease in reduction temperature and the rise in the number of basic sites, are responsible for the increased activity of Pt with M-Ni/Al2O3-CeO2 catalysts. The co-doped Ni/Al2O3-CeO2 catalysts with Mg and Fe significantly enhanced the activity (more than 80% methane and 84% CO2 conversion), the selectivity toward syngas (∼90%), and maintained the H2/CO ratio at about 0.97 at 700 °C.

Atom-Economical Synthesis of Dimethyl Carbonate from CO2 : Engineering Reactive Frustrated Lewis Pairs on Ceria with Vacancy Clusters

The chemical conversion of CO₂ to long-chain chemicals is considered as a highly attractive method to produce value-added organics, while the underlying reaction mechanism remains unclear. By constructing surface vacancy-cluster-mediated solid frustrated Lewis pairs (FLPs), the 100 % atom-economical, efficient chemical conversion of CO₂ to dimethyl carbonate (DMC) was realized. By taking CeO₂ as a model system, we illustrate that FLP sites can efficiently accelerate the coupling and conversion of key intermediates. As demonstrated, CeO₂ with rich FLP sites shows improved reaction activity and achieves a high yield of DMC up to 15.3 mmol g^-1 . In addition, by means of synchrotron radiation in situ diffuse reflectance infrared Fourier-transform spectroscopy, combined with density functional theory calculations, the reaction mechanism on the FLP site was investigated systematically and in-depth, providing pioneering insights into the underlying pathway for CO₂ chemical conversion to long-chain chemicals.

Ultrasonically assisted surface modified CeO2 nanospindle catalysts for conversion of CO2 and methanol to DMC

This study developed a facile and effective approach to engineer the surface properties of cerium oxide (CeO2) nanospindle catalysts for the direct synthesis of dimethyl carbonate (DMC) from CO2 and methanol. CeO2 nanospindles were first prepared by a simple precipitation method followed by wet chemical redox etching with sodium borohydride (NaBH4) under high intensity ultrasonication (ultrasonic horn, 20 kHz, 150 W/cm2). The ultrasonically assisted surface modification of the CeO2 nanospindles in NaBH4 led to particle collisions and surface reduction that resulted in an increase in the number of surface-active sites of exposed Ce3+ and oxygen vacancies. The surface modified CeO2 nanospindles showed an improvement of catalytic activity for DMC formation, yielding 17.90 mmol·gcat-1 with 100 % DMC selectivity. This study offers a simple and effective method to modify a CeO2 surface, and it can further be applied for other chemical activities.

New Strategies on Green Synthesis of Dimethyl Carbonate from Carbon Dioxide and Methanol over Oxide Composites

Dimethyl carbonate is a generally used chemical substance which is environmentally sustainable in nature and used in a range of industrial applications as intermediate. Although various methods, including methanol phosgenation, transesterification and oxidative carbonylation of methanol, have been developed for large-scale industrial production of DMC, they are expensive, unsafe and use noxious raw materials. Green production of DMC from CO2 and methanol is the most appropriate and eco-friendly method. Numerous catalysts were studied and tested in this regard. The issues of low yield and difficulty in tests have not been resolved fundamentally, which is caused by the inherent problems of the synthetic pathway and limitations imposed by thermodynamics. Electron-assisted activation of CO2 and membrane reactors which can separate products in real-time giving a maximum yield of DMC are also being used in the quest to find more effective production method. In this review paper, we deeply addressed green production methods of DMC using Zr/Ce/Cu-based nanocomposites as catalysts. Moreover, the relationship between the structure and activity of catalysts, catalytic mechanisms, molecular activation and active sites identification of catalysts are also discussed.

Dimethyl Carbonate: Review of Synthesis Routes and Catalysts Used

This review summarizes the performance of potential catalysts for the synthesis of dimethyl carbonate (DMC) using CO2 as a feedstock by two major processes—the direct route of carbonylation of alcohols and the indirect route of alcoholysis of urea. The reaction mechanisms and corresponding catalysts that were previously investigated are discussed. The major challenges associated with the conversion of CO2 to DMC are the low yields, low DMC selectivity, and thermodynamic limitations (alcohol carbonylation) of reversible reactions with low equilibrium constants. This occurs mainly due to the highly stable carbon dioxide molecules. The development of novel catalysts with high yields and high selectivity needs to be studied to overcome these technical challenges. In addition, to enhance the DMC yield, the use of dehydrating agents for water removal from the reaction mixture is recommended and discussed in detail in this article. This review critically examines the different catalysts used by investigators, along with their respective operating conditions and suitability in applications for the commercial synthesis of CO2-based DMC.

Sustainable Synthesis of Dimethyl- and Diethyl Carbonate from CO2 in Batch and Continuous Flow-Lessons from Thermodynamics and the Importance of Catalyst Stability

Equilibrium conversions for the direct condensation of MeOH and EtOH with CO₂ to give dimethyl- and diethyl carbonate, respectively, have been calculated over a range of experimentally relevant conditions. The validity of these calculations has been verified in both batch and continuous flow experiments over a heterogeneous CeO₂ catalyst. Operating under optimized conditions of 140 °C and 200 bar CO₂, record productivities of 235 mmol/L·h DMC and 241 mmol/L·h DEC have been achieved using neat alcohol dissolved in a continuous flow of supercritical CO₂. Using our thermodynamic model, we show that to achieve maximum product yield, both dialkyl carbonates and water should be continuously removed from the reactor instead of the conventionally used strategy of removing water alone, which is much less efficient. Catalyst stability rather than activity emerges as the prime limiting factor and should thus become the focus of future catalyst development.

A CFD study on the performance of CO2 methanation in a water-permeable membrane reactor system

A water-permeable membrane reactor is proposed to promote CO2 methanation with in situ removal of H2O. A two-dimensional CFD simulation model is built up and the effects of GHSV, H2O permeance and CO2/H2 permeation selectivity are investigated.

Bimetallic MOF-Derived Synthesis of Cobalt-Cerium Oxide Supported Phosphotungstic Acid Composites for the Oleic Acid Esterification

The impregnation of phosphotungstic acid (HPW) with porous cobalt-cerium oxide (HPW@CoCeO) has been prepared by pyrolysis of CoCe-MOF and used for the production of methyl oleate from oleic acid and methanol. FTIR, XRD, SEM, TEM, N2 adsorption/desorption, and NH3-TPD were characterized for the prepared composites. Simultaneously, the effects of reaction time, substrate molar ratio, temperature, and catalyst loading on catalytic activity were highlighted, and the conversion of 67.2% was reached after 4 h at 60°C. Importantly, HPW@CoCeO was reusabe and reused more than eight times, and the oleic acid conversion could be maintained at 61.8% without significant activity loss. Thus, the HPW@CoCeO composite could be used as acid catalysts for sustainable energy production.

Enhancing the Dispersion of Cu-Ni Metals on the Graphene Aerogel Support for Use as a Catalyst in the Direct Synthesis of Dimethyl Carbonate from Carbon Dioxide and Methanol

Graphene has attracted attention because of its interesting properties in catalyst applications including as a catalyst support; however, it is known that the graphene can be restacked, forming a graphite-like structure that leads to poor specific surface area. Hence, the high-porosity graphene aerogel was used as a Cu-Ni catalyst support to produce dimethyl carbonate (DMC) from carbon dioxide and methanol. In this work, we have introduced a new synthesis route, which can improve the dispersion of metal particles on the graphene aerogel support. Cu-Ni/graphene aerogel catalysts were synthesized by a two-step procedure: forming Cu-Ni/graphene aerogel catalysts via hydrothermal reduction and then Cu-Ni loading by incipient wetness impregnation. It is found that the catalyst prepared by the two-step procedure exhibits higher DMC yield (25%) and MeOH conversion (18.5%) than those of Cu-Ni loading only by an incipient wetness impregnation method. The results prove that this new synthesis route can improve the performance of Cu-Ni/graphene aerogel catalysts for DMC production.