Study On Oxidation Mechanisms Of Sulfur Compounds In The Oil By Quantum Chemistry

The problem of high selectivity and activity of Catalysts in oxidative desulfurization has not been solved perfectly, which attributed to some relevant reaction mechanisms being not studied deeply. The paper studied the oxidation mechanisms of sulfur compounds in the oil by quantum chemistry, in order to understand such mechanisms thoroughly,as well as to promote the manufacture of clean fuel oils.Quantum chemistry method was employed to study the reaction mechanisms of sulfur compounds (CH3SH, CH3SCH3 and C4H4S) and H2O2. Geometries of the stationary points were completely optimized. Activation energies were calculated. The transition states were validated by the vibrational analysis and the internal reaction coordinate (IRC) calculations. Feasible reaction pathway of this reaction has been studied. Topological analysis of electronic density was used to analyse some pathways. The results showed that the three kinds of reactions were similar to each other, which might involve the attack of O to S, O—O bond cleavage and hydrogen transfer. Moreover, it might involve isomerization in the reaction of CH3SH and H2O2 as well. For the first step of three reactions, the reaction of C4H4S and H2O2 had the highest activation energy (157.3 kJ/mol), which indicated the oxidative desulfurization of C4H4S by H2O2 can occur most difficultly. As to mechanism of DMS and hydrogen peroxide, the solvent effect was also investigated in water and toluene, respectively. The results indicated that the solvent effect lowed the activation energies, also we found the blue-shifts of transition states comparing to the gas phase.Quantum chemistry method was employed to study the reaction mechanisms of sulfur compounds (CH3SH, CH3SCH3 and C4H4S) and CH3CO3H. Geometries of the stationary points were completely optimized. Activation energies were calculated. The transition states were validated by the vibrational analysis and the internal reaction coordinate (IRC) calculations. Feasible reaction pathway of this reaction has been studied. Topological analysis of electronic density was used to analyse some pathways. The results showed that the three kinds of reactions were similar to each other, which might involve the attack of O to S, O—O bond cleavage and hydrogen transfer. But there still were some differences among them. The reaction of C4H4S and CH3CO3H had the highest activation energy (87.9 kJ/mol), which indicated the oxidative desulfurization of C4H4S by CH3CO3H can occur most difficultly. When the oxidant was changed from H2O2 to CH3CO3H, the activation energies of first step of three reactions were decreased greatly, which showed that the desulfurization effect of the latter was better. Also, it showed the catalytic action of the CH3CO2H. Moreover, the change of oxidant didn't change the reaction course greatly, which indicated that they belong to the same system called hydrogen peroxide system.In order to study the properties of important radical intermediate produced in the course of oxidative desulfurization, density function theory (DFT) B3LYP method was employed to study the mechanism of the reaction of CH3S·and HO2·with the 6-311++G(d, p) basis sets. Geometries of the stationary points were completely optimized. The transition states were validated by the vibrational analysis and the internal reaction coordinate (IRC) calculations. The vibration analysis and the IRC analysis testified the authenticity of intermediates and transition states. Five feasible reaction pathways of this reaction have been studied. The results indicated that the main reaction pathway is single spin pathway CH3S·+HO2·→CH3SOOH (1P), which has no activation energy and is beneficial from the aspects of reaction kinetics and thermodynamics. For triple spin reaction pathways, CH3S·+HO2·→COM11→TS1→COM12→CH3SH+O2 (3P) is the main pathway, and the corresponding activation energy is 53.5 kJ/mol. This exothermic pathway has the lowest activation energy, which is beneficial from the aspects of reaction kinetics and thermodynamics.Metal ions play a role in catalytic oxidation desulfurization as catalysts. The study of the interaction of metal ions and sulfur compounds can also contribute to understanding the nature of the mechanism for sulfur compounds'oxidation. The demethanation reactions of dimethyl sulfide by Fe+ on both quartet and sextet potential energy surfaces have been investigated using density functional theory (B3LYP) in conjunction with the DGDZVP and 6-311+G(d, p) basis sets. The calculation results indicated that, along the energetically preferable sextet pathway, the demethanation reaction can occur through four elementary steps, that is, encounter complexation, C-S activation, ?-H shift, and nonreactive dissociation. The first step C-S activation process constitutes the rate-determining step, and the corresponding activation energy was 144.4 kJ/mol. Whereas along the quartet PES, both the C-S and methyl C-H activation could result in the demethanation reaction, of which, the reaction involving C-S activation is calculated to be the main pathway with relative lower energy barriers and intermediates.