Yongli Shen

Dimethyl carbonate synthesis from carbon dioxide and methanol over CeO2 versus over ZrO2: comparison of mechanisms

A comparison of the mechanisms of dimethyl carbonate (DMC) formation directly from carbon dioxide and methanol over CeO2 versus over ZrO2 is made through in situ Fourier transform infrared spectroscopy (FTIR). During the reaction involving methanol and CO2 adsorption over CeO2, a new band appears at around 1295 cm−1. Combining this result with in situ FTIR results of methyl formate adsorption, this band is assigned to carbomethoxide, which is taken as the intermediate in DMC formation over the ceria surface. Carbomethoxide originates from the reaction of methanol and adsorbed carbon dioxide; its formation is followed by reaction with a methoxy group to form DMC. This mechanism differs from that occurring on zirconium oxide, in which DMC is formed by the reaction between monodentate methyl carbonate and methanol.

Reaction mechanism of dimethyl carbonate synthesis on Cu/β zeolites: DFT and AIM investigations

The mechanism of dimethyl carbonate (DMC) synthesis on Cu-exchanged zeolite β has been investigated employing density functional theory (DFT) calculations and a double numerical plus polarization (DNP) basis set. The adsorption energy (ΔE) and decomposition activation energy (Ea) for O2 are −1.84 and 1.72 eV, respectively, suggesting that the decomposition of O2 occurs readily under reaction conditions on the Cu site. The formed O atom further reacts with methanol to form surface-bound (CH3O)(OH)–Cu(I)/β, in which CH3O and OH were coadsorbed on the Cu+ of the catalyst; this process proceeds without an activation barrier and with an energy release of 1.23 eV. The (CH3O)(OH)–Cu(I)/β species then reacts with another methanol molecule and carbon monoxide to produce DMC through two different reaction pathways. In path I, insertion of carbon monoxide into the (CH3O)(OH)–Cu(I)/β leads to the formation of monomethyl carbonate species (CH3OCOOH), which then reacts with methanol to produce DMC and H2O. The activation energies for both steps are 0.97 and 0.65 eV, respectively. In path II, (CH3O)(OH)–Cu(I)/β reacts with methanol first to produce a dimethoxide species ((CH3O)(CH3O)–Cu(I)/β), and the formation of DMC is via the insertion of carbon monoxide into the (CH3O)(CH3O)–Cu(I)/β. The activation energies for these elementary reactions are 0.65 and 0.70 eV, respectively. The topological properties of electron density distributions for all the related stationary points involved in this reaction have also been examined using the atoms in molecule (AIM) theory for the illustration of the bond paths and weak interactions of all the stationary points in the reaction path.

Mechanistic Insights into Selective Oxidation of Ethanol on Au(111): A DFT Study

Abstract The reaction mechanism of selective oxidation of ethanol on Au(111) covered with atomic oxygen was described employing density functional theory calculations. The first step, dissociation of the O–H bond in ethanol, has lower barrier for transfer of the H to adsorbed oxygen or surface hydroxyl than to gold with a barrier of 0.20 or 0.17 eV, respectively. The two lower activation energies for the β-H elimination of ethoxyl, 0.29 and 0.27 eV, come from transfer of H atom to surface O atom or another ethoxyl. Ethyl acetate is formed through the β-H elimination of ethoxy hemiacetal. The formation of acetic acid is not facile through the reaction between the surface hydroxyl and acetaldehyde or between the surface oxygen and acetaldehyde at low temperature due to a high activation barrier. Except for surface oxygen and bidentate acetate, all other surface species have low diffusion barriers, suggesting that rearrangement and movement of these species from the preferred adsorption sites to the ideal configurations for reactions are facile.

Synergy between Palladium and Potassium Species for Efficient Activation of Carbon Monoxide in the Synthesis of Dimethyl Carbonate

A potassium ion containing Pd/NaY catalyst is introduced that effectively activates the CO molecule for the carbonation of methyl nitrite to synthesize dimethyl carbonate. The potassium ions play a dual role during the activation of CO. Doping the catalyst with potassium enhances the electron density of Pd active species, which strengthens the PdC atoms interaction and thus more strongly activates the CO molecule. Further, the charge‐balancing potassium cations in the zeolite interact with the O atom of linear adsorbed carbonyls to form a PdCO⋅⋅⋅K+ structure, which further activates the CO bond. Both experimental analyses and density functional theory calculations indicated that the combination of potassium and Pd species facilitates the activation of CO in the carbonylation reaction.

DFT and DRIFTS studies of the oxidative carbonylation of methanol over γ-Cu2Cl(OH)3: the influence of Cl

This paper describes a detailed fundamental study regarding the influence of Cl species in γ-Cu(OH)3Cl catalyzed oxidative carbonylation of methanol employing density functional theory (DFT) calculations as well as in situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) experiments. The methanol was found positioned with the H atom towards the Cl atom—which in the first layer of the γ-Cu2Cl(OH)3(021) surface—in a close a-top position with a hydrogen bond between them at all adsorption sites; while the Cl atom plays a key role in the pre-reaction for H–O bond activation. The methoxide, which was formed through a substitution reaction, adsorbed on the surface through two O–Cu bonds; while the formed HCl weakly adsorbed on the surface and can easily escape from the surface. One new intermediate product was found during the calculation of minimum energy path (MEP), which connects the adsorbed methanol and coadsorbed methoxide and HCl, and its existence was confirmed through in situ DRIFTS experiments. During the reaction, the Cl atom escaped from the surface and bonded with the methanol first (forming CH3OH⋯Cl) and then reacted with the methanol to form adsorbed methoxide and HCl. The existence of Cl seriously decreased the energy cost for methanol oxidation to methoxide.

In situ DRIFTS study on the oxidative carbonylation of methanol to dimethyl carbonate over Cuβ catalyst

Abstract The mechanism of oxidative carbonylation of methanol to dimethyl carbonate (DMC) over Cuβ catalyst was investigated by using in situ DRIFTS; the adsorption of single methanol, carbon monoxide and DMC as well as their mixtures on the Cuβ catalyst were considered. The results indicated that methoxide species are formed when methanol is adsorbed on the catalyst due to presence of CuOx. Only one type of active sites that are located in the six-membered ring of β zeolite is found, over which adsorbed methanol can be oxidized to methoxide and water. DMC is adsorbed on the catalyst through the contact of the oxygen atom in carbonyl group with the active sites. There were two pathways for the oxidative carbonylation: by the mono-methoxide pathway, carbon monoxide can react with mono-methoxide species to form monomethyl carbonate (MMC) and MMC then reacts with methoxide to form DMC, or by the di-methoxide pathway, DMC is formed through inserting of carbon monoxide in the di-methoxide species; latter one is more favorable over the Cuβ catalyst.