Study On The Reaction Mechanism Of Trichlormethylsilane Disproportionation Catalyzed By AlCl₃/ZSM-5

Dichlorodimethylsilane is the most widely used monomer in organosilicon material.Most of the plant use direct-processing method to produce monomers, which would producelots of by-products, such as trichlormethylsilane （10¹5%） and chlorotrimethylsilane. Thecontents of these by-products accumulate quickly with the expanding of the producing. Butthe use of them is limited, the storing of them would not only rust the container but also wasteresource, and let them off directly would pollute the environment. This paper is intended totransfer them to dichlorodimethylsilane using redistribution method, which can change wasteinto valuable, and also accord with the requirement of green chemistry.The reaction of disproportionation preparation for dichlorodimethylsilane can proceed inthe presence of catalysts. Our early experiment found that AlCl₃and ZSM-5have certaincatalytic activity in the reaction. At present, the theoretical study on the disproportionatedreaction is less. Domestic and overseas scholars’ research focuses on the preparation,screening and modification of catalysts, and the improvement and innovation of productionprocess of dichlorodimethylsilane.The mechanism of the disproportionated reaction between trichlormethylsilane andchlorotrimethylsilane was investigate using density functional theory （DFT） with a5T clustermodel simulating ZSM-5zeolite, in order to supplement theory data and provide morereference data in the preparation, screening of catalysts, and the industrial production ofdichlorodimethylsilane.The main results and conclusions are as follows:（1） When HZSM-5is used as a catalyst, activation energy of each step is: Ea1=76.93kJ·mol^-1, Ea2=147.29kJ·mol^-1, Ea₃=181.68kJ·mol^-1, Ea4=118.67kJ·mol^-1, Ea5=166.98kJ·mol^-1, Ea6=202.68kJ·mol^-1, respectively, which indicates that the dispropor-tionatereaction can proceed smoothly. The first, second and fourth step are exothermic reactions, therest three steps are endothermic reactions. Enthalpy changes are:△H₁=-57.76kJ·mol^-1,△H₂=-37.28kJ·mol^-1,△H₃=85.06kJ·mol^-1,△H₄=^-19.95kJ·mol^-1,△H5=87.43kJ·mol^-1,△H6=138.36kJ·mol^-1, respectively. It accords with the conclusion that adsorptionis an exothermic process, and desorption is an endothermic process. The disproportionatereaction mainly proceed according to the first reaction channel, generating principal productdichlorodimethylsilane.（2） When AlCl₃/ZSM-5is used as a catalyst, activation energy of each step is relatively small, which shows that the disproportionated reaction can proceed smoothly. The tworeactions that catalyst reacts separately with （CH₃）₃SiCl and CH₃SiCl₃to form intermediateare endothermic reactions, the other two reactions in which intermediate reacts separatelywith CH₃SiCl₃and （CH₃）₃SiCl to reduce the catalyst are exothermic reactions, enthalpychanges are:△H₁*=82.97kJ·mol^-1,△H₂*=-98.98kJ·mol^-1,△H₃*=120.77kJ·mol^-1,△H₄*=-44.11kJ·mol^-1, respectively. The first step of the disproportionate reaction is theforming process of the intermediate, and it is also the rate determing step of the main reaction,activation energy of the step is92.68kJ·mol^-1. Then, the intermediate can separately reactwith trichlormethylsilane and chlorotrimethylsilane,and the difference of activation energiesthe two reactions is not significant, so both of them are easy to occur, and they are competing.Compared with the case of AlCl₃, ZSM-5are separately used as catalyst, AlCl₃/ZSM-5showsbetter catalytic effect on the disproportionation reaction, so that the reaction is easier to occur,which proved our previous experimental results.（3） When AlCl₃/Mg-ZSM-5is used as a catalyst, activation energies of the four steps areEa&11=162.52kJ·mol-, Ea&2=80.08kJ·mol^-1, Ea&3=191.92kJ·mol^-1, Ea&4=77.45kJ·mol^-1,respectively, which are not high, so the disproportionation reaction can carries out smoothly.The two reactions that catalyst reacts separately with （CH₃）₃SiCl and CH₃SiCl₃to formintermediate are endothermic reactions, the other two reactions in which intermediate reactsseparately with CH₃SiCl₃and （CH₃）₃SiCl to reduce the catalyst are exothermic reactions,enthalpy changes are:△H&&1=84.28kJ·mol^-1,△H&2=-72.99kJ·mol^-1,△H₃=106.33kJ·mol^-1,△H&4=-50.41kJ·mol^-1, respectively. The first step of the disproportionatedreaction is the forming process of the intermediate, and it is also the rate determing step of themain reaction. Then the intermediate can separately react with trichlormethylsilane andchlorotrimethylsilane, the difference of activation energies the two reactions is not significant,so both of them are easy to occur, and the two are competing. The catalytic activity ofAlCl₃/Mg-ZSM-5is not better than that of AlCl₃/ZSM-5, the reason may be the acid strengthof ZSM-5molecular sieves is reduced, due to the introduction of Mg.