Comparison of Cu3Si and Cu5Si reactivities in direct synthesis of dimethyldichlorosilane

The reactivity of Cu3Si and Cu5Si in the direct synthesis of dimethyldichlorosilane (Me2) was studied on a fixed bed under atmospheric conditions. The reactive mass was characterized after reaction by XRD and scanning electron microscopy. On Cu3Si at 315 °C, the transformation of silicon is high: it reaches 80% after 5 h reaction. But, because of the coking, the Me2 selectivity after 2 h reaction is negligible, the main product being the methyltrichlorosilane. At 200 °C, the Si transformation rate is lower, but the Me2 selectivity remains as high as 60% after 5 h and finally the Me2 yield is larger. On Cu5Si at 200 and 250 °C, the Me2 selectivity, and the yield as well, are higher than on Cu3Si, but the reaction stops after a half an hour. Clearly, the Si surface atoms in Cu5Si are initially more reactive than in Cu3Si to produce Me2. During the direct synthesis there is a progressive Cu enrichment and XRD patterns show the following succession of phases: Cu3Si → Cu15Si4→ Cu5Si → Cu0.