Simulation Study On The Reaction Of Si₂Cl₆ With HCl Catalyzed By Nitrogen-Containing Organic Compounds

Since photovoltaic energy has many advantages such as cheap,green and clean,so the development of photovoltaic energy is the general trend.And in recent years,the cost of photovoltaic power generation is also declining significantly,the rate of decline is much higher than other energy sources,which is also the reason for its vigorous development.China’s and the world’s polysilicon production and the cumulative installed capacity of photovoltaic are increasing year by year,and in 2022 the cumulative installed capacity of global photovoltaic reached 1TW.photovoltaic energy industry is full of vitality,playing an increasingly important role in the energy sector,and polysilicon is the basic raw material for the photovoltaic industry.Among them,the modified Siemens method stands out among several methods for producing polysilicon,accounting for about 80%of the market share.In response to the disadvantages of the modified Siemens method,which generates a large amount of residual liquid,our research is mainly based on solving the problem of high boiling matter in the kettle residual liquid.Si₂Cl₆ is a typical representative of perovskites,which are industrially cleaved to Si HCl₃ and Si Cl₄ by polysilicon companies mainly using HCl.There have been many studies on the effective cleavage of perovskites to monosilanes under the action of amine catalysts,but the relevant catalytic mechanism is not clear and difficult to be verified by purely experimental methods,and molecular simulation methods provide an important means to confirm the reaction mechanism.molecular simulation provides an important tool to confirm the reaction mechanism.The reaction of Si₂Cl₆ with HCl to produce Si HCl₃ and Si Cl₄ under the action of three types of amine catalysts（aliphatic amine,aromatic amine and cyclic imine）was studied by using density functional theory.All stagnation points involved in the reaction path are calculated at WB97XD/6-311++G（2d,p）level.In the absence of catalyst,Si₂Cl₆ reacts directly with HCl to generate Si HCl₃ and Si Cl₄.The activation energy of the reaction is 250.45 k J/mol.Two possible reaction pathways were investigated in the presence of the aliphatic amine catalyst.In the first pathway the aliphatic amine catalyst first physisorbed with Si₂Cl₆ and then continued to adsorb with HCl to form the intermediate complex IMB.After the transition state TSA,the intermediate complex IMC is formed,and IMC further obtains the intermediate complex IMD and Si Cl₄（P2）.IMD finally completes the desorption of catalyst NH₃ to generate Si HCl₃（P1）.The second path changes the reaction from one transition state to two transition states.The step to determine the reaction rate is to attack Si₂Cl₆with amine to generate Si HCl₃ and intermediate complex IM5.The next step is the reaction of the intermediate IM5 with HCl to form Si Cl₄ and amine catalyst.The two paths were compared and the first path was found to have a large energy barrier,which is thermodynamically unfavorable,and this path was reasonably eliminated.The energy barrier in the second path is significantly reduced,so that a better second path is selected.The order of catalytic activity can be deduced from the size of the rate-determining step energy barrier as:dimethylamine>propylamine>ethylamine>methylamine>ammonia.For aliphatic amine catalysts,the higher the number of alkyl substituents and the longer the alkyl chain,the better the catalytic activity.For aromatic amine catalysts,the order of catalytic activity is obtained based on the optimal pathway:N-methylaniline>p-methylaniline>o-methylaniline>m-methylaniline>aniline>diphenylamine.For aromatic amine catalysts,the catalytic activity is influenced by the side chain groups,and the connection of electron-absorbing group-C₆H₅ decreases the catalytic activity of the catalyst,while the attachment of the electron-donating group-CH₃ enhances the catalytic activity.In cyclic imine catalysts,the order of catalytic activity is:C₄H₈NH>C₅H₁₀NH>C₃H₆NH>C₆H₁₂NH>C₂H₄NH.As the carbon chain in the ring grows the catalytic ability increases and then decreases,with an optimum carbon chain length in the middle of the cyclic imine.In order to better understand the effect of temperature on the reaction,the Gibbs free energy distribution in the temperature range of 300～800 K was studied.We believe that the results of our calculations and discussions are vital to better understand how the reactions occur and to guide the development of catalysts.