Lixia Ling

Pyrolysis Mechanisms of Quinoline and Isoquinoline with Density Functional Theory

Abstract The pyrolysis mechanisms of quinoline and isoquinoline were investigated using the density functional theory of quantum chemistry, including eight reaction paths and a common tautomeric intermediate 1-indene imine. It is concluded that the conformational tautomerism of the intermediate decides the pyrolysis products (C 6 H 6 HC≡C—C≡N, C 6 H 5 C≡N and HC≡CH) to be the same, and also decides the total disappearance rates of the reactants to be the same, for both original reactants quinoline and isoquinoline during the pyrolysis reaction. The results indicate that the intramolecular hydrogen migration is an important reaction step, which often appears in the paths of the pyrolysis mechanism. The activation energies of the rate determining steps are obtained. The calculated results are in good agreement with the experimental results.

A theoretical study on the hydrolysis mechanism of carbon disulfide.

AbstractThe hydrolysis mechanism of CS2 was studied using density functional theory. By analyzing the structures of the reactant, transition states, intermediates, and products, it can be concluded that the hydrolysis of CS2 occurs via two mechanisms, one of which is a two-step mechanism (CS2 first reacts with an H2O, leading to the formation of the intermediate COS, then COS reacts with another H2O, resulting in the formation of H2S and CO2). The other is a one-step mechanism, where CS2 reacts with two H2O molecules continuously, leading to the formation of H2S and CO2. By analyzing the thermodynamics and the change in the kinetic function, it can be concluded that the rate-determining step involves H and OH in H2O attacking S and C in CS2, respectively, causing the C=S double bond to change into a single bond. The two mechanisms are competitive. When performing the hydrolysis of CS2 with a catalyst, the optimal temperature is below 252°C. FigureThe hydrolysis mechanism of CS2

Density functional theory analysis of carbonyl sulfide hydrolysis: effect of solvation and nucleophile variation

AbstractThe detailed mechanisms of the hydrolysis of carbonyl sulfide (OCS) by nucleophilic water and hydroxide ion in both the gas phase and bulk water solvent have been investigated using density functional theory. Various reaction channels on the potential surface have been identified. The thermodynamic results demonstrate that the hydrolysis of OCS by nucleophilic water and hydroxide ion should proceed more favorably at low temperature. The hydrolysis of OCS by the hydroxide ion is the main reaction channel from thermodynamic and kinetic perspectives, and the bulk solvent can influence the rate-determining step in this channel. However, the solvent barely modifies the activation energy of the rate-determining step. For the hydrolysis of OCS by nucleophilic water, the solvent does not modify the rate-determining step, and the corresponding activation energy of the rate-determining step barely changes. This bulk solvent effect suggests that most of the contribution of the solvent is accounted for by considering one water molecule and a hydroxide ion. FigureReaction energy profiles for the H2O and OH- attack on OCS

DFT study on the effects of defect and metal-doping on the decomposition of H2S on the α-Fe2O3(0001) surface

The adsorption and decomposition mechanisms of H2S on different α-Fe2O3(0001) surfaces, including Fe-vacancy, O-vacancy, sulfurized and Cu-, Zn- and Co-doped surfaces, have been studied systematically using periodic density functional calculations. The results show that the Fe-vacancy surface exhibits an excellent catalytic activity towards the decomposition of H2S, which is favorable for the desulfurization. Both O-vacancy and sulfurized surfaces have negative effects on the desulfurization. The doping of Cu, Zn and Co on the α-Fe2O3(0001) surface is beneficial to enhance the desulfurization performance of the hematite sorbent, of which Zn addition is a comparatively good candidate taking desulfurization efficiency and economic factors into account.

Theoretical studies on reaction mechanism of H2 with COS

The reaction mechanisms of H2 with OCS have been investigated theoretically by using density function theory method. Three possible pathways leading to major products CO and H2S, as well as two possible pathways leading to by-product CH4 have been proposed and discussed. For these reaction pathways, the structure parameters, vibrational frequencies and energies for each stationary point have been calculated, and the corresponding reaction mechanism has been given by the potential energy surface, which is drawn according to the relative energies. The calculated results show that the corresponding major products CO and H2S as well as by-product CH4 are in agreement with experimental findings, which provided a new illustration and guidance for the reaction of H2 with OCS.

Insight into the preferred formation mechanism of long-chain hydrocarbons in Fischer–Tropsch synthesis on Hcp Co(10−11) surfaces from DFT and microkinetic modeling

DFT calculations, together with microkinetic modeling, have been employed to probe into the preferred mechanism of hydrocarbon C–C chain growth on Co(10−11) surfaces during Fischer–Tropsch synthesis. The results show that both CH and CH2 are favored CHx (x = 1–3) monomers, and are much easier to be formed than CH4 and CH3OH. CH and CH2 self-coupling via a carbide mechanism realizes the initial C–C chain formation, rather than via a CO/CHO insertion mechanism. Meanwhile, CH3CH2 is the favored C2 monomer, and is predominantly formed via a carbide mechanism rather than via a CO/CHO insertion mechanism, leading to C2H5OH formation. Starting from CH3CH2 intermediates, CH3CH2 coupling with CH2 to form CH3CH2CH2 realizes further C–C chain growth from C2 to C3 species, instead of a CO/CHO insertion mechanism leading to C3H7OH formation. Thus, the proposed mechanism of C–C chain growth is that RCH2CH2 coupling with CH2 to R′CH2CH2 (R′ = RCH2) realizes C–C chain growth. Meanwhile, CHO insertion into RCH2CH leads to RCH2CHCHO, followed by its hydrogenation to an alcohol. However, microkinetic modeling shows that the effect of CH4 formation on the production of C2+ hydrocarbons should be considered, whereas alcohols have a negligible effect on the selectivity of C2+ hydrocarbons. Our results confirm that Co(10−11) surfaces exhibit a better catalytic activity and selectivity toward C2+ hydrocarbon formation.

Source and major species of CHx (x = 1–3) in acetic acid synthesis from methane–syngas on Rh catalyst: a theoretical study

Density functional calculations have been carried out to investigate the source and major species of CHx (x = 1–3) involved in acetic acid synthesis from methane–syngas on the Rh(111) surface. All possible formation pathways of CHx (x = 1–3) from methane and syngas have been systematically investigated. For CHx formation from methane, our results show that CH is the most abundant species; for CHx formation from syngas, all CHx (x = 1–3) species form from CHO by CO hydrogenation, and the optimal formation routes of CHx show that CH and CH3 are the most abundant species rather than CH2 and CH3OH. On the other hand, CH formed by methane is more favourable than CH and CH3 formed by syngas; meanwhile, CO insertion into CHx species to form C2 oxygenates as acetic acid precursors is more favourable than CO hydrogenation to CH and CH3. As a result, in acetic acid synthesis from methane–syngas, CHx (x = 1–3) species come from methane rather than syngas, and the corresponding primary species is CH. In addition, the CO in syngas is predominantly responsible for insertion reactions that produce CHCO, which is a C2 oxygenate precursor leading to the formation of acetic acid. Furthermore, microkinetic modelling analysis shows that the major product of acetic acid synthesis from methane–syngas on the Rh(111) surface is CH3COOH, and that the production of CH3OH cannot compete with that of CH3COOH.

Formation mechanism of methane during coal evolution: A density functional theory study

The formation mechanism of methane (CH4) during coal evolution has been investigated by density functional theory (DFT) of quantum chemistry. Thermogenic gas, which is generated during the thermal evolution of medium rank coal, is the main source of coalbed methane (CBM). Ethylbenzene (A) and 6,7-dimethyl-5,6,7,8-tetrahydro-1-hydroxynaphthalene (B) have been used as model compounds to study the pyrolysis mechanism of highly volatile bituminous coal (R), according to the similarity of bond orders and bond lengths. All possible paths are designed for each model. It can be concluded that the activation energies for H-assisted paths are lower than others in the process of methane formation; an H radical attacking on β-C to yield CH4 is the dominant path for the formation of CH4 from highly volatile bituminous coal. In addition, the calculated results also reveal that the positions on which H radical attacks and to which intramolecular H migrates have effects on methyl cleavage.

Quantum Chemistry Studies on the Free-radical Growth Mechanism of Polycyclic Arenes from Benzene Precursors

The free-radical growth mechanisms for the formation of polycyclic arenes (PCAs) were constructed based on the block unit of benzene, and were calculated by the quantum chemistry PM3 method. Two kinds of reaction paths are proposed and discussed. The calculation results show that the formation of PCAs is only controlled by the elimination of H atom from benzene, and the corresponding activation energy is 307.60 kJ.mol(-1). H(2) is only the effluent gas in our proposed reaction mechanism, and the calculation results are in accord with the experimental facts.

Cost-Effective Palladium-Doped Cu Bimetallic Materials to Tune Selectivity and Activity by using Doped Atom Ensembles as Active Sites for Efficient Removal of Acetylene from Ethylene

The catalytic activity and selectivity of cost‐effective noble‐metal‐doped common metal materials strongly depend on the doped atom ensemble in specific arrangements to provide active sites. In this study, aiming at insight into the doped atom ensembles as active sites for tuning the selectivity and activity towards the target reaction, different doped noble metal Pd atom ensembles for cost‐effective Pd‐doped Cu catalysts act as active sites to investigate the activity and selectivity towards the efficient removal of acetylene from ethylene by using density functional theory calculations. The results show that an ensemble composed of one surface and its joint sublayer Pd atoms in the Cu catalyst as active sites enhance both the selectivity and activity of C2H4 formation caused by adjusting the catalyst surface electronic structure. Moreover, the surface d‐band center of the Pd‐doped Cu catalyst can act as an effective “descriptor” for the rapid screening of catalytic activity in the design of improved catalysts with the noble‐metal‐doped common metal. Further, the ensemble composed of one surface and its joint sublayer doped Pd atoms as active sites in the cost‐effective Pd‐doped Cu bimetallic catalysts is an efficient approach to finely tune the activity and selectivity towards the efficient removal of acetylene from ethylene.