Research On Direct Synthesis Of Triethoxysilane And Reaction Deactivation Mechanism

Trialkoxysilane is an important chemical intermediates in organosilicon industry. Among the trialkoxysilane, the trimethoxysilane and triethoxysilane are the most important, both of them can be prepared to many kinds of silane coupling agents by hydrosilylation reaction, or can be prepared to high pure monosilane gas by disproportionation, and they can also be prepared to organic-inorganic hybrid materials and so on. The direct process of synthesis alkoxyssilane gets widely concerned for its simple route, environmental protection and other advantages.Orthogonal test in slurry reactor is carried out to study the condition of direct synthesis of triethoxysilane. The reaction temperature, catalyst content, promoter content, ethanol adding rate and solvent content are investigated. The best technology condition in slurry reactor is got. When the reaction temperature is 230 oC, the catalyst content is 14%, the promoter content is 2.8%, the adding rate of ethanol is 20 ml/h, the solvent content is 7%, the highest yield of triethoxysilane is 85.38% and the selectivity of triethoxysilane is 93.22%. As the slurry reactor has many disadvantages, the investigation of technology conditions in fixed bed is carried out. The particles size of silicon, pretreatment temperature for contact mass, reaction temperature, catalyst content, material of reactor are investigated. The experiments show that although under the best conditions, the effect of synthesis is not ideal. When the particles size of silicon is from 100 mesh and 200 mesh, the contact mass is not pretreated, catalyst content is 10%, promoter content is 2.8%, the yield of triethoxysilane is 15.2% and the selectivity of triethoxysilane is 95%.The physical characterization and fluidization experiment were carried out in the PMMA fluidized bed. In the experiment the bulk density, experimental formula of elutriation velocity and expansion rate of the bed were investigated. The critical fluidization velocity was confirmed through the methods of standard warp of pressure fluctuation and bed pressure drop. The critical fluidization velocity is 0.0137m/s when the silicon powder from 100 mesh to 200 mesh. Base on the reaction conditions from direct synthesis of triethoxysilane and the fluidization parameters from experiments of PMMA fluidized bed, the stirred fluidized bed system is designed. In the stirred fluidized bed reactor, the water content in ethanol, the aluminum content in silicon and the iron content in silicon are investigated. The experiment shows that the content of water in ethanol must be lower than 0.2%, otherwise the yield and selectivity of triethoxysilane will decrease, because of the problems of the silicon passivation and the product hydrolysis. The content of aluminum in silicon should be below 0.1% , or it is unfavorable to the reaction no matter the content of aluminum is higher or lower. And the content of iron must be below 0.5% in silicon, or it is the serious inhibition to the reaction.The reaction effect of different reactors under the best technology condition is compared. The experiment shows that the reaction effect on stirred fluidized bed is better than fixed bed reactor. But the yield of triethoxysilane in the stirred bed reactor is too low to be used on industrial scale. Only by changing the catalyst synthesis method, reaction temperature, adding rate of ethanol and other technology conditions can not reach good yield. So this paper carries out the experiments of the influence of promoters. The promoters of HF, C2H5Cl, ZnCl2, FeCl3, CuCN and KHF2 are investigated to get the best yield and selectivity of triethoxysilane. It has been found that adding double promoters of HF and C2H5Cl, and the yield of triethoxysilane reaches 42.35%, and the selectivity of triethoxysilane reaches 81.45%. The CuCN and KHF2 are also good promoters, the yield of triethoxysilane can reach 44.38% and 48.70%, and the selectivity of triethoxysilane can reach 85.79% and 80.59% respectively. Through the experiment, we find that Lewis acid has the function of inhibition for the direct synthesis of triethoxysilane and Lewis alkali has the function of promotion for triethoxysilane and tetraethoxysilane. So adding special and fit content Lewis alkali can promote the yield and selectivity of triethoxysilane.Through scanning electron microscopy combined with EDS, X-ray diffraction and X-ray photoelectron spectroscopy, catalyst and contact mass are investigated. There are three factors that influence the stability of the reaction, the damage of anion balance, the active center covered by carbon deposition and the passivation of silicon surface by silica. Because the anion departs from the system as low boiling point species, the balance of anion is damaged. And there is no anion supplement from external system, and then the inactive copper can not return to reaction system, so it accumulates. The inactive copper can catalyze the ethanol to form acetaldehyde, and the acetaldehyde reacts with ethanol to form 1, 1-diethoxyethane easier. This reaction prevents the reaction between silicon and ethanol.The ethanol is carbonized by the inactive copper catalysis at high temperature. When the contact mass is covered by the inactive carbon species, the deactivation occurred. The silica is formed when silicon reacts with the water which is from reactants and generates from side reaction. So the problems of the three aspects above must be solved to guarantee the reaction stable, both the yield and selectivity of triethoxysilane can be improved.