Chlorination of trichlorosilane/chlorodimethylsilane using metal chlorides: experimental and mechanistic investigations

Abstract

Removal of carbonaceous impurities from trichlorosilane (SiHCl₃) reduces the carbon content of solar grade polysilicon produced with the improved Siemens method. The separation of chlorodimethylsilane (CH₃)₂SiHCl from SiHCl₃ by distillation remains challenging due to the small difference in their boiling points. Herein, the chlorination of (CH₃)₂SiHCl/SiHCl₃ with metal chlorides (WCl₆, MoCl₅) were studied. The aim was to convert (CH₃)₂SiHCl into (CH₃)₂SiCl₂, increase the relative volatility of (CH₃)₂SiHCl and SiHCl₃ and facilitate the distillation. The optimum reaction conditions were 60 °C, 60 min and n(WCl₆ or MoCl₅): n(SiHCl₃ or (CH₃)₂SiHCl) = 0.7 at 0.8 MPa. Under these conditions, and when WCl₆ and MoCl₅ were used as the chlorine sources, the extents of (CH₃)₂SiHCl conversion were 22.7 and 18.5 times higher than those of SiHCl₃, respectively. In addition, a mechanistic study showed that the difference between the reactions of SiHCl₃ and (CH₃)₂SiHCl resulted from the different energy barriers for the reactions of the and (CH₃)₂SiCl· radicals with WCl_x or MoCl_x, and the barrier for the reaction was higher than that for the (CH₃)₂SiCl· reaction.