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.

En Route to CO2-Based (a)Cyclic Carbonates and Polycarbonates from Alcohols Substrates by Direct and Indirect Approaches

This review is dedicated to the state-of-the art routes used for the synthesis of CO2-based (a)cyclic carbonates and polycarbonates from alcohol substrates, with an emphasis on their respective main advantages and limitations. The first section reviews the synthesis of organic carbonates such as dialkyl carbonates or cyclic carbonates from the carbonation of alcohols. Many different synthetic strategies have been reported (dehydrative condensation, the alkylation route, the “leaving group” strategy, the carbodiimide route, the protected alcohols route, etc.) with various substrates (mono-alcohols, diols, allyl alcohols, halohydrins, propargylic alcohols, etc.). The second section reviews the formation of polycarbonates via the direct copolymerization of CO2 with diols, as well as the ring-opening polymerization route. Finally, polycondensation processes involving CO2-based dimethyl and diphenyl carbonates with aliphatic and aromatic diols are described.