451830 Copper Aluminate: A Potential Catalyst to Make Dimethyldichlorosilane in a 2-Step Process
Dimethyldichlorosilane (Me2SiCl2) is typically produced by Rochow direct process , which comprises the reaction of methyl chloride with elemental silicon in the presence of promoted copper catalyst. However, the feedstock for direct process to produce Me2SiCl2, metallurgical grade silicon, is produced by carbothermic reduction of silica. This is a very energy intensive process and requires 12 kWh of electrical energy and 12 kWh of carbon feedstock based energy, thus leading to relatively high production cost, per kg of Si produced.
An alternate process to produce silicones was investigated by Dow Corning [2-5] and the process is energy conservative in polydimethylsiloxanes (PDMS) production. The process involves the production of chlorosilanes from silica followed by reaction with methyl chloride to generate the PDMS. This new process capability showed a range of potential impacts from flexibility of monomers produced, improved selectivity of Me2SiCl2and more efficient raw material utilization.
The following reactions are carried out on a metal or metal supported catalyst (CAT).
H2 + SiCl4 + CAT --> Si-CAT + HCl + HSiCl3 (>600oC) ------ step 1
CH3Cl + Si-CAT --> (CH3)xSiCl4-x + CAT (300oC), x=1-4 ---- step 2
The catalyst selection for the 2-step process was challenging as the process involves severe reaction conditions which include reacting with H2 and SiCl4 feed gases at above 650oC in a fluid bed reactor. Copper based catalysts were evaluated in the 2-step process due to potential similarities of step-2 reaction with the direct process. High surface area, large pore volume and homogeneous dispersion are all important characteristics of the catalyst for high activity and cycle stability.
g-Al2O3 is the most widely used as a support material for several hydrotreating commercial processes. Notable features of alumina supports are their ability to disperse high loading of active metal phase, good mechanical properties, and high thermal stability and can interact with active phase resulting in less sintering of active phase in hydrogen atmosphere [6-9]. However a considerable drawback with g-Al2O3 is that it is not resistant to SiCl4 during the course of reaction and will be transformed to aluminum chloride. Thermodynamic calculations show the reaction between Al2O3 and SiCl4 can take place around 600oC to make SiO2 and AlCl3. It has been reported that silicon tetrachloride can de-aluminate aluminum from the Al2O3 support causing AlCl3formation . However, one would not expect this reaction to take place in the presence of hydrogen.
Alumina has several advantages as a support material and if all these advantages associated with this support are incorporated in a catalyst system, it will be an ideal catalyst for the 2-step process. If g-alumina support is transformed in to alpha-alumina (or a more stable phase) and associate with copper, the resulting catalyst may inherit the favorable physical properties and retaining hydrogenation activity of SiCl4in step-1.
In this study, various compositions of copper aluminate type spinel materials were synthesized by wet impregnation and physical mixing methods and investigated in the 2-step process. The catalytic performance of these copper aluminates was superior to those of conventional copper catalysts. More uniform distribution of CuAl2O4 and complete formation of CuAl2O4 phase in the catalyst/ supports due to high calcination temperatures thus higher Cu mobility/diffusion are believed to help strengthen the particle integrity and could minimize AlCl3formation in the process.
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