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Silicon for the Chemical and Solar Industry XIII
Kristiansand, Norway, June 13 – 16, 2016
Mechanistic Aspects of the Rochow Direct Process
Genqiang Xue, Van V. Green
Dow Corning Corporation, 4770 Highway 42 East, Carrollton, Kentucky 41008, USA
Abstract
The Rochow Direct Process involves reaction of silicon with methyl chloride to
synthesize methyl chlorosilanes (MCS), which are used in various silicone products.
Understanding fundamental mechanisms of this reaction is critical for improving
product selectivity and material efficiency. The first part of this manuscript surveys
literature about side-reactions and by-products related to methyl group decomposition.
The formation of coke deposition, hydrocarbons, hydrogen gas, and MCS containing
a Si-H bond or a Si-CH
2
-Si moiety is discussed. The involvement of C-H bond
cleavage as a rate-determining-step for methyl group decomposition is also assessed.
The second part reports new methods for studying the reaction of silicon with cuprous
chloride (CuCl). Silicon tetrachloride (SiCl
4
) yield from this reaction was measured
by a GC method with short cycle-time. The onset temperature was also studied by
coupling a chemisorption analyser with a mass spectrometer.
Introduction
The reaction of methyl chloride (CH
3
Cl) with silicon to synthesize methyl
chlorosilanes (MCS), also known as the Muller-Rochow Synthesis or the Direct
Process, is a key pillar for the silicone industry. Dimethyldichlorsilane ((CH
3
)
2
SiCl
2
or simply Me2) is the major product and used to manufacture polydimethylsiloxane
fluid or elastomer. Some MCS by-products are valuable for different applications. In a
commercial process, a balanced Me2-to-by-product ratio might be more important
than a maximized Me2 selectivity for assessing process performance.
1
Mechanistic study of the direct process is an active research area. Key
topics/approaches include: (a) surface chemistry study (see ref. 2 for examples); (b)
thermodynamic/kinetic modeling for understanding product selectivity (see ref. 3 for
examples); (c) roles played by different promoters (see ref. 4 for examples).
The main reaction of forming MCS, particularly Me2, has been the focus for
mechanistic studies. On the other hand, knowledge about methyl group decomposition
and associated side-reactions within the direct process is still limited. Reaction of
CH
3
Cl on pristine surfaces such as Cu, single crystal Si and copper silicide has been
extensively studied (see references 5-7 for examples) These studies provided a wealth
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