silicon for the chemical and solar industry XIII

of information about interaction of CH

Cl (or methyl groups from other sources) with

these surfaces and subsequent reactions. However, it remains a challenge to connect

these results with the observations on actual direct process reaction mass, which is a

very complex solid mixture of silicon with Cu catalyst, several promoters, and more

than 10 metallic impurities.

The required information for making the connection

includes (a) which is the key active site causing the majority of methyl group

decomposition? And (b) how does that relate to the active site that catalyzes MCS

formation?

The first part of this manuscript surveys literature about side-reactions of methyl

groups and related by-products. The second part reports new methods for studying the

reaction of Si with CuCl, which is a key step for forming the active catalyst of the

direct process.

Methyl group decomposition in the direct process

During the direct process reaction, it is desirable to keep the per-pass conversion of

Cl well below 100%. Excess CH

Cl improves the reaction rate of silicon. The un-

converted CH

Cl is separated from products and recycled back to the process.

Of all

methyl groups from the converted portion of CH

Cl, however, only part of them are

incorporated into MCS. The other methyl groups go through different side-reactions

and generate different by-products. For convenience, they are collectively termed as

“unproductive” methyl groups in this manuscript. Unproductive methyl groups reduce

material efficiency of the direct process. Some unproductive methyl groups go

through C-H bond cleavage (decomposition reaction) to generate coke and hydrogen

ligands incorporated into hydrogen (H

), hydrocarbons and MCS products with a Si-H

bond. These side-reactions and by-products, as well as possible reaction mechanisms,

are summarized below.

By-products associated with methyl group decomposition

Some unproductive methyl groups lose hydrogen atoms and become carbonaceous

residue (coke) deposits on the reaction mass. Coke formed in the direct process is not

purely carbon based. Coke with hydrogen-to-carbon molar ratio in the range of 0.5-

1.8 has been observed.

It was proposed that coke can catalyze formation of more

coke or even deactivate the direct process reaction.

8,9

Olakangil systematically studied

coke formation in the direct process.

The impurities present in chemical grade

silicon were found insignificant for coke formation. On the other hand, phosphorus, a

promoter used in the direct process, was found to suppress coke formation at higher

temperature and pressure. CO

profiles obtained from temperature programmed

oxidation (TPO) of coke deposition showed two major peaks, which were attributed

to coke formed near two different sites. The lower oxidation temperature peak was

attributed to coke formed near a site rich in copper and/or chlorine, while the higher

temperature one was attributed to coke formed near a site lean in copper and/or

chlorine. The assignment was based on observation that copper and chlorine catalyzed

oxidation of carbon black and reduced oxidation temperature. However, the

possibility of generating two forms of coke was not ruled out.

Based on kinetic

study, Olakangil proposed coke formation mechanism consists of reactions 1-6 (S as

an unoccupied active site consisting of Cu, Si or promoters). The proposed

mechanism include key assumptions such as (a) reaction 2 was a rate-determining-

step; and (b) reaction intermediates were adsorbed on reaction mass surface.

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