A Quantum Chemical Study On Desulfurization Mechanism Of Thiophene Comppounds And Molecular Structure Of Coal

In this paper,quantum chemistry methods(M06-2X,CBS-QB3)were used to calculate the molecular properties(BDE,ACID,ESP,Becke charge,HOMO and LUMO)of 2-methylthiophene.Besides,energy barrier(G?(35)),reaction energy(GP(35)),reaction thermodynamic properties(?H?,?G?)of 2-methylthiophene,thiophene and benzothiophene were also calculated.The energy diagrams of 2-methylthiophene,thiophene and benzothiophene about their desulfurization mechanism were also calculated under inert and oxidative atmosphere.The energy diagrams of the desulfurization mechanism of 2-methylthiophene,thiophene and benzothiophene have been characterized using M06-2X methods.The transition state theory is employed to determine the rate constant,k(T)and base element reactions.The desulfurization mechanism of thiophene compounds under inert and oxidative atmospheres is deeply investigated from the aspects of kinetics and thermodynamics.This study can provide the theoretical guidance at the molecular level for the desulfurization mechanism of other thiophene compounds in coal.The main conclusions of this paper are drawn as following:(1)Under inert atmosphere,the desulfurization reactions of 2-methylthiophene pyrolysis are endothermic,and the initial temperatures of their spontaneous reactions are much higher than 600 K.Under oxidative atmosphere,the desulfurization reactions of 2-methylthiophene pyrolysis are exothermic,and the initial temperatures of their pontaneous reactions are below 600 K.(2)Under inert atmosphere,H· is beneficial for H2 S formation during 2-methylthiophene pyrolysis.In addition,·S· is beneficial for CS2 formation.For COS formation,it is found kinetically more favorable for the reaction initiated by alkyl side chain than that by thiophene ring under oxidative atmosphere.For SO2 formation,the reaction intrigued by Diels-Alder cycloaddition between bimolecular 2-methylthiophene sulfones is more kinetically and thermodynamically favorable than by single 2-methylthiophene sulfone decomposition under oxidative atmosphere.(3)For 2-methylthiophene sulfone and thiophene sulfone,SO2 formation by their decomposition reaction is endothermic reaction with high energy barrier.However,SO2 formation by Diels-Alder reaction is exothermic reaction with low energy barrier.Thus,the main reaction of SO2 formation for 2-methylthiophene sulfone and thiophene sulfone is Diels-Alder reaction.(4)In the six-membered fused ring compounds,the most suitable site for the carboxyl group distribution is ? position of the fused ring.With the coal rank increasing,the amount of carboxyl substituents will gradually decrease.When five-membered aromatic ring combines with six-membered aromatic ring in a fused ring molecule,the ?-position of the five-membered ring is also easily substituted by a carboxyl group.The reason may be that the steric hindrance of the five-membered ring ?-substituted by the carboxyl group is smaller than that six-membered ring ?-substituted by the carboxyl group.The distribution of hydroxyl group is less affected by coal ranks and also exists in bituminous coal.The carboxyl group prefers to distribute at the ortho position in the same benzene ring substituted by the hydroxyl group,because a hydrogen bond can be formed between the hydroxyl group and the carboxyl group at ortho position,providing additional energy to stabilize the molecule.The hydroxyl group is more stably distributed in the coal-like compounds compared with the carboxyl group.The sulfur atom in the coal-like compound does not affect the preferential sites of the carboxyl group and the hydroxyl group.