MCS reaction

Scheme 1. Summary of reactions between NbCls, NbBrj, and TaBrj and Nadtc, B = benzene, MC = dichloromethane all NbX,/Nadtc B reactions also produce NbdtC4 (not shown), all MXj/Nadtc/MC reactions also produce dtcjCH (not shown).

B. Recycle of Solid Waste Products from the MCS Reaction... 1593... 

From the 1940s through the 1970s, while the MCS reaction was practiced industrially on a large scale, it was frequently the source of frustrating irreproducibility. The major treatises on the subject of Direct Process failed to mention the importance of promoter elements in the catalyst7-10. In addition, the use of substrates other than MeCl was explored only occasionally. Finally, there were few successful attempts to understand the Direct Process on a fundamental level before 1980. 

II. THE EFFECT OF PROMOTERS ON THE DIRECT PROCESS A. Promoters in the MCS Reaction... 

Other main group elements have been used as promoters in MCS beds containing copper, tin and zinc. Phosphorus and indium have been used in the MCS reaction. The reaction of silicon (40 g) with MeCl (2 bar) at 300 °C in the presence of Cu (3.2 g), ZnO (0.05 g), In (0.004 g) and P (0.056 g) gave 1.7% of Me3SiCl, 0.017% mixture of MeSiCl3/Me2SiCl2 and 3.7% of polysilanes34. Antimony, just below phosphorus in... 

III. THE EFFECT OF SILICON, CATALYST AND PROMOTER MORPHOLOGY ON THE MCS REACTION... 

Without zinc, silicon diffusion to the surface is slow under MCS conditions. Furthermore, when only tin is used as a promoter, no Cu is observed at the surface. Table 3 shows the elemental concentration under various conditions. Under MCS reaction conditions, when zinc was present silicon was not depleted from the subsurface, and when zinc was absent the subsurface was depleted in silicon. Zinc causes the rate of silicon diffusion and copper dispersion to increase. Zinc accumulates at grain boundaries and lowers the free energy of CU3SL Tin and zinc appear to work synergistically but tin does not enhance silicon diffusion on its own. Tin does appear to lower the surface energy of silicon/copper. 

Further surface studies have addressed the effect of different forms of copper as a catalyst in the direct process65 -69. Analyzing Si(100) surfaces it was shown that the form of copper is critical for selectivity in the MCS reaction. The most effective combination was 82% Cu and 18% Q12O (neither worked very well alone). There was no correlation between the amount of CusSi on the surface and rate or selectivity for the direct process. 

Lewis and coworkers have also made significant contributions to the understanding of the MCS reaction via the use of surface studies72. XPS and AES analysis of catalytically active surfaces showed that zinc causes a restructuring of the Cu3Si surface. Additionally, zinc enrichment is enhanced by the addition of SnCLt. Lead is a well known poison for the direct reaction and the Lewis group found that lead suppressed enrichment of the Cu3Si surface in zinc and silicon. 

More recent support has appeared for the importance of the copper-silicon rich phase on the silicon surface in the MCS reaction. Lieske and coworkers74 showed that redispersion of the eta phase can be an element of the induction period of the MCS reaction and seems to be brought about by the reaction itself. The Cu-Si surface species, perhaps Cu-Si surface compounds or extremely small Cu-Si particles, seem to be of similar importance as X-ray detectable Cu-Si phases. 

VI. RECOVERY AND USE OF BY-PRODUCTS FROM THE MCS REACTION A. High Boiling Residues from the Direct Process... 

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