Sustainable Catalytic Synthesis of Diethyl Carbonate

Observed kinetics for the production of diethyl carbonate from CO2 and ethanol catalyzed by CuNi nanoparticles supported on activated carbon

Monometallic and bimetallic Cu:Ni catalysts with different Cu:Ni molar ratios (3:1, 2:1, 1:1, 1:2, 1:3) were synthesized by wetness impregnation on activated carbon and characterized by TPR (temperature programmed reduction), XRD (X-ray diffraction) and XPS (X-ray photoelectron spectroscopy). The synthesized catalysts were evaluated in the gas phase production of diethyl carbonate from ethanol and carbon dioxide. The largest catalytic activity was obtained over the bimetallic catalyst with a Cu:Ni molar ratio of 3:1. Its improved activity was attributed to the formation of a Cu-Ni alloy on the surface of the catalyst, evidenced by XPS and in agreement with a previous assignment based on Vegard law and TPR analysis. During the reaction rate experiments, it observed the presence of a maximum of the reaction rate as a function of temperature, a tendency also reported for other carbon dioxide-alcohol reactions. It showed that the reaction rate-temperature data can be adjusted with a reversible rate equation. The initial rate as a function of reactant partial pressure data was satisfactorily adjusted using the forward power law rate equation and it was found that the reaction rate is first order in CO2 and second order in ethanol.

Product Yield Increasing More Than 20 Times Achieved by Reducing Water Poisoning for Direct Diethyl Carbonate Synthesis

Diethyl carbonate (DEC) synthesis from CO2 and ethanol is a typical green chemical route for sustainable development and cleaner production. However, the bottleneck problem of a low DEC yield is still not solved at present. The DEC yield still maintains 0.5-5.0% unless expensive third additives are added. Compared to the cheap price of DEC, the third additives are generally more expensive. It is not advisible to use expensive chemicals to prepare cheap chemicals. In this work, the (ZrO2)1.0/(Fe2O3)1.0 catalyst was prepared by a novel template-precipitation method and the DEC yield of the DEC direct synthetic reaction only from CO2 and ethanol without third additives obtained 1.95%. After a series of supported catalysts were prepared, the DEC yield reached up to 46.1% for the (ZrO2)1.0/(Fe2O3)1.0@3A-CaO-CaC2 catalyst. This carrier can also be applied to other catalyst systems. CO2-TPD, NH3-TPD, and XRD were measured to analyze the microstructure and catalytic mechanism. Active center site theory was proposed to explain the intrinsic mechanism of catalyst activity. This new discovery of the mechanism and the DEC yield removed obstacles for the application of this reaction.

Direct Conversion of Low-Concentration CO2 into N-Aryl and N-Alkyl Carbamic Acid Esters Using Tetramethyl Orthosilicate with Amidines as a CO2 Capture Agent and a Catalyst

Herein, we report the direct conversion of low-concentration CO₂ (15 vol %), equivalent to the CO₂ concentration in the exhaust gas from a thermal power station, into carbamic acid esters (CAEs), which are precursors for pharmaceuticals, agrochemicals, and isocyanates. The reaction was performed using Si(OMe)₄ as a nonmetallic regenerable reagent and 1,8-diazabicyclo[5.4.0]undec-7-ene as a CO₂ capture agent and catalyst. This reaction system does not require the addition of metal complex catalysts or metal salt additives and is therefore simpler than our previously reported reaction system involving Ti(OR)₄ and a Zn(II) catalyst. A variety of N-aryl, N-alkyl, and bis CAEs (precursors of polyurethane raw materials) were obtained in moderate to high yields (45-77% for 6 examples, 84-89% for 7 examples). In addition, bis CAEs were successfully synthesized from simulated exhaust gas containing impurities such as SO₂, NO₂, and CO or on a gram scale. We believe that this method can eliminate the use of toxic phosgene as the raw material for isocyanate production and mitigate CO₂ emissions.

Synthesis of organic carbamates as polyurethane raw materials from CO2: the quest for metal alkoxides as regenerable reagents

It is well known that the utilization of carbon dioxide (CO₂) for chemical materials is attracting research attention from the viewpoint of the carbon cycle. To contribute to the reduction of CO₂ emission through such CO₂ utilization reactions and counteract global climate change, the target compounds should be core chemical products that are distributed in large quantities and used for a long time. One such synthetic target is isocyanates that are used as raw materials for the production of polyurethanes, which are versatile polymeric materials with a service life of approximately 10 years. However, since direct synthesis of isocyanate from CO₂ is quite difficult due to equilibrium constraints, a method via the use of its alcohol adduct, organic carbamate, as a precursor has been proposed. In this Perspective, we present regenerative metal alkoxide reactants, such as tin alkoxide, titanium alkoxide, and alkoxysilane, as environmentally benign reactants for the synthesis of organic carbamates from CO₂. We also present a practical and environmentally friendly method for the highly efficient synthesis of various organic carbamates, including industrially important diisocyanate precursors, from 1 atm CO₂ using alkoxysilanes. Furthermore, prospects for the practical application of these carbamate synthesis reactions are also discussed.

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.

From SiO2 to Alkoxysilanes for the Synthesis of Useful Chemicals

The transformation of silica (SiO₂) to useful chemicals is difficult to explore because of the strength of the Si-O bond and thermodynamic stability of the SiO₂ structure. The direct formation of alkoxysilanes from SiO₂ has been explored as an alternative to the carbothermal reduction (1900 °C) of SiO₂ to metallic silicon (Si_met) followed by treatment with alcohols. The base-catalyzed depolymerization of SiO₂ with diols and monoalcohols afforded cyclic silicon alkoxides and tetraalkoxysilanes, respectively. SiO₂ can also be converted to alkoxysilanes in the presence of organic carbonates, such as dimethyl carbonate. Alkoxysilanes can be further converted to useful chemicals, such as carbamates, organic carbonates, and chlorosilanes. An interesting and highly efficient pathway to the direct conversion of SiO₂ to alkoxysilanes has been discussed in detail along with the corresponding economic and environmental implications. The thermodynamic and kinetic aspects of SiO₂ transformations in the presence of alcohols are also discussed.

N-Aryl and N-Alkyl Carbamates from 1 Atmosphere of CO2

We have successfully isolated and characterized the zinc carbamate complex (phen)Zn(OAc)(OC(=O)NHPh) (1; phen=1,10-phenanthroline), formed as an intermediate during the Zn(OAc)₂ /phen-catalyzed synthesis of organic carbamates from CO₂ , amines, and the reusable reactant Si(OMe)₄ . Density functional theory calculations revealed that the direct reaction of 1 with Si(OMe)₄ proceeds via a five-coordinate silicon intermediate, forming organic carbamates. Based on these results, the catalytic system was improved by using Si(OMe)₄ as the reaction solvent and additives like KOMe and KF, which promote the formation of the five-coordinated silicon species. This sustainable and effective method can be used to synthesize various N-aryl and N-alkyl carbamates, including industrially important polyurethane raw materials, starting from CO₂ under atmospheric pressure.