| Organosilicon materials have been widely used in all aspects of life and industry due to their unique structure that gives them many excellent properties.Organosilicon materials are mainly prepared by the several monomers.Among them,dimethyldichlorosilane??CH3?2SiCl2,referred to as dimethyl?is the most important and the most widely used of silicone monomers.The disproportionation for producing dichlorodimethylsilane could be prepared with methyltrichlorosilane?CH3SiCl3,referred to as methyl?and trimethylchlorosilane??CH3?3SiCl,referred to as trimethyl?.This method not only produces a large number of dimethyl,but also can solve the problem of waste accumulation and improve the economic efficiency.Previous studies have shown that the selection of the catalyst is very important in the disproportionation of dimethyldichlorosilane.The microporous HZSM-5,mesoporous?-Al2O3 and AlCl3 are all good solid acid catalysts.However,the molecular sieve activity of different cluster models is different due to the confinement effect of molecular sieve pore confinement effect.The surface activity of 5T cluster HZSM-5 is better,while the activity are poor of 24T and 7T clusters HZSM-5,basically no activity.After the study found that the activity of the catalyst mainly in the surface location.Therefore,in order to further determine the surface reaction properties of HZSM-5,it is important to investigate the active sites on the surface of HZSM-5 small clusters?for example 4T and 5T clusters?.There are few studies on the catalytic mechanism of the new core-shell catalysts are formed by small clusters molecular sieves and?-Al2O3?combining HZSM-5 with?-Al2O3?.In particular,there are few theoretical studies on the disproportionation for producing dichlorodimethylsilane by use these catalysts.Therefore,in this paper,the theoretical calculations of the reaction mechanism for producing dimethyldichlorosilane catalyzed by 4T,5T clusters HZSM-5@?-Al2O3 are carried out.This paper selects the catalyst models such as 4T cluster HZSM-5@?-Al2O3?2,3,4,5 different active sites?,5T cluster HZSM-5@?-Al2O3?1,3,4 different active sites?,4T cluster AlCl3/ZSM-5@?-Al2O3?1,3 positions and 2,4 positions?,and 5T cluster AlCl3/ZSM-5@?-Al2O3?1,3 positions and 2,4 positions?.Researching the reaction mechanism of disproportionation for producing dichlorodimethylsilane by using Density Functional Theory?DFT?at the B3LYP/6-311++G?3df,2pd?level.Finally,the energy of each structure is calculated by using second-order Moller-Plesset perturbation theory?at MP2/6-311++G?3df,2pd?level?and the Zero Point Energy?ZPE?correction is considered.Conclusions are as follows:?1?The reaction mechanism of disproportionation for producing dichlorodimethylsilance by 4T cluster HZSM-5@?-Al2O3?2,3,4,5 different active sites?catalyst is studied.The activation energies of the rate-determining step of main reaction are:165.88 kJ·mol-1,129.99 kJ·mol-1,118.66 kJ·mol-1,and 145.55 kJ·mol-1,respectively.The activation energy of the rate-determining step of 4 active site is lower,and the reaction is easy to happen.?2?The reaction mechanism of disproportionation of dimethyldichlorosilance preared with methyltrichlorosilane and trimethylchlorosilane catalyzed by 5T cluster HZSM-5@?-Al2O3?1,3,4 different active sites?catalyst is studied.The activation energies of the rate-determining step of main reaction are:123.85 kJ·mol-1,112.39kJ·mol-1,and 105.42 kJ·mol-1,respectively.The activation energy of the rate-determining step of 4 active site is minimal,which is favorable for the formation of dimethyldichlorosilane.Comparing with the catalytic activity of the 4T cluster structure,the 5T cluster catalyst is better.?3?The reaction mechanism of disproportionation for producing dichlorodimethylsilance by 4T cluster AlCl3/ZSM-5@?-Al2O3?1,3 positions and 2,4positions?catalyst is studied.The activation energies of the rate-determining step of main reaction are:110.39 kJ·mol-1,and 105.12 kJ·mol-1,respectively.The results show that the activation energy of AlCl3 loaded on the No.2,4 positions is smaller than that on the No.1,3 positions,and the activity is better.Meanwhile,the catalytic effect is better than that the unloaded condition.?4?The reaction mechanism of disproportionation for producing dichlorodimethylsilance by 5T cluster AlCl3/ZSM-5@?-Al2O3?1,3 positions and 2,4positions?catalyst is studied.The activation energies of the rate-determining step of main reaction are:103.89 kJ·mol-1,and 98.54 kJ·mol-1,respectively.The energy analysis,bond order analysis,ELF analysis,and LOL analysis indicate that the order of catalytic activity is:5T cluster AlCl3/ZSM-5@?-Al2O3?2,4 positions?>5T cluster AlCl3/ZSM-5@?-Al2O3?1,3 positions?>4T cluster AlCl3/ZSM-5@?-Al2O3?2,4positions?>4T cluster AlCl3/ZSM-5@?-Al2O3?1,3 positions?.In summary,whether or not the catalyst is modified,the catalytic effect of the 5T cluster catalyst is better than the 4T cluster structure and it is more conducive to the reaction.In summary,whether HZSM-5 itself or HZSM-5@?-Al2O3 core-shell catalyst,the active center of 5T cluster has better catalytic effect than 4T cluster structure,which is more conducive to the disproportionation reaction. |
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