(617fp) Copper Aluminate: A Potential Catalyst to Make Dimethyldichlorosilane in a 2-Step Process

The present study relates to the development of a new generation of copper aluminate type spinel catalysts for the production of dimethyldichlorosilane (Me₂SiCl₂) from SiCl₄ in a 2-step process. Copper aluminates were found to exhibit superior activity over other conventional catalysts and hold better particle integrity in lab scale fixed bed and fluid bed reactors. In the present study, copper aluminates were synthesized by incipient wetness impregnation of Cu(NO₃)₂ over Al₂O₃ support followed by air calcination as well as by physical mixing of CuO and Al₂O₃ followed by high temperature calcination. Both γ-alumina and copper aluminate possess the same structure (cubic-close packed), which results automatically in a dispersed CuO on alumina single phase, which was confirmed by XRD analysis. In the reduction of spinel compound, CuAl₂O₄ and part of CuO closely associated with CuAl₂O₄ were reduced much slower than bulk CuO or supported CuO catalysts. However, once reduced, a phase change is forced and most of the alumina is converted to alpha phase. Calcination at as high as 1000^oC has prevented sintering of Cu and minimized AlCl₃ formation in step-1 reaction with H₂ and SiCl₄. High rate of methylchlorosilanes production on spinel catalysts in step-2 was attributed to the smaller particle sizes and “meta-stable” copper silicide (Cu₃Si) that is formed during the step-1 reaction. The catalysts were characterized by XRD, SEM/EDS, N₂O pulse chemisorptions and particle size distribution analysis and correlated with activity results in the 2-step process to make Me₂SiCl₂ from SiCl₄. In-situ XRD analysis confirmed the high reactivity of copper in spinel compared to other conventional catalysts. SEM analysis has shown the Cu association with alumina in fresh catalysts and the dissociation of Cu from support was less in the spent catalysts compared to conventional silica supported copper catalysts. Spinel catalyst was found to be active even at as low as 240^oC step-2 reaction temperature with MeCl, producing high selectivity towards Me₂SiCl₂.

INTRODUCTION

Dimethyldichlorosilane (Me₂SiCl₂) is typically produced by Rochow direct process [1], 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 (>600^oC) ------ step 1

CH₃Cl + Si-CAT --> (CH₃)_xSiCl_4-x + CAT (300^oC), 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 650^oC 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 600^oC to make SiO₂ and AlCl₃. It has been reported that silicon tetrachloride can de-aluminate aluminum from the Al₂O₃ support causing AlCl₃formation [10]. 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.

REFERENCES

1. E.G. Rochow, J Amer. Chem. Soc., 67, 1945, 963-965

2. D. Katsoulis, R Larsen, US patent 8772525

3. A. Coppernoll, C. Horner, K. Janmanchi, D. Katsoulis, R. Larsen, US patent application 20150158890

4. K. Janmanchi, C. Horner, A. Coppernoll, WIPO patent application WO/2015/073213

5. A. Coppernoll, C. Horner, K. Janmanchi, WIPO patent application WO/2014/099125

6. B. Dhandapani, S.T. Oyama, Catal Lett., 35 (1995) 353

7. P. Bodnariuk, B. Coq, G. Ferrat, F. Figueras, J. Catal., 116 (1989) 459 

8. B. Coq, J.M. Cognion, F. Figueras, D. Tournigant, J. Catal., 141 (1993) 21

9. B. Coq, F. Figueras, S. Hub, D. Tournigant, J. Phys. Chem., 99 (1995) 1115

10. D. Gencev, K.S. nee Mogyorosi, S. Riederauer, J. Szepovolgyi, US patent 4,416,862