Академия наук республики узбекистан

 
(1.4)
 
Moreover, increasing the number of carbon atoms in R reduces the reaction 
ability of alcohols. Therefore, all the technical solutions useful for reactions with 
ethyl alcohol a priori are also operable for methyl alcohol. 
For a long time the development and application of the alkoxysilane method 
was restricted by the presence of an induction period of the direct reaction of 
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metallurgical silicon and low alcohols. As a result
, the reactions were carried out, 
at best, in a continuous
-
cyclic mode for low reaction rates, and the process 
consumes a lot of energy per unit of product. 
As known, the induction period of the direct reaction of 
metallurgical silicon 
with alcohol can be 
from several hours to tens of hours. The main cause of the 
induction period is an oxide film on the silicon surface due to rapid oxidation of 
silicon with atmospheric oxygen. In order to reduce the induction period duration 
an additional step of reaction medium activation was proposed to be introduced 
into the alkoxysilane synthesis process [Pat.US 5783720] where the activation is 
made at temperatures up to 400° C and hydrogen and nitrogen are proposed to be 
used as an activating agent. The authors state th
at the activation process in practice 
takes much time from 5 to 10 hours, which affects the technology efficiency. To 
shorten the induction period, silicon in form of powder was recommended to be 
preliminary treated with hydrofluoric acid to remove an oxid
e film from the 
surface of particles [Pat. JP511692, Pat. US5177234]. These solutions lead to 
additional complications because after treatment with hydrofluoric acid the silicon 
powder must be cleaned and dried in an inert atmosphere, which dramatically 
complicates the synthesis process. 
Some papers
proposed to make the activation of 
the reaction mass by holding it at higher temperature in the atmosphere of nitrogen, 
argon and others [US5177234, US4727173] and to preliminarily mix silicon with a 
catalyst in 
the inert atmosphere for 8 hours [US4487949]. To activate silicon 
before synthesis, haloids: 
chlorine
alkily, hydrogen chlorides, ammonium chlorides 
[US5177234] or NH4HF
2
[ER517398] were proposed to be introduced.
It should be noted that introduction of substances such as haloids or 
alkylhaloids into the reactor before synthesis, there is an additional operation of 
cleaning of the target product from these impurities, for example by distillation, 
which reduces productivity and complicates the technology of 
alkoxysilane 
production. Thus, the analysis shows that there is no clear understanding of the 
causes and nature of the induction period during the direct synthesis of 
alkoxysilanes, especially when there are no effective solutions of this problem. The 
prop
osal of additional reagents to be used in synthesis naturally requires their 
removal from the final product, but i
t leads to additional operations in the 
manufacturing process, which complicates the technology of alkoxysilane 
production and inevitably increases the cost of the final product.
We have proposed to exclude the causes of this problem 
– 
no oxide film 
formation on the surface of just ground particles of silicon during the milling 
process.
There is also a problem of moisture. The point is that the
silicon powder 
is very hygroscopic and intensively absorbs moisture from the environment. In the 
reaction this moisture behaves, on the one hand, as a factor blocking the start of the 
target reaction and, on the other one, causes undesirable side reactions. Thus, the 
preparation of silicon in a protective environment, in our case in a solvent, 
eliminates not only from oxide film formation, but also from excess of moisture in 
the reaction medium. 
It is also known that besides the main reactions of alkoxysil
ane synthesis in 
the reactor there are side reactions leading to formation of oligoalkoxysiloxane, 
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water and other by-products that are gradually accumulated in the reaction medium 
and reduces the process rate [US5783720, JP511692, US6090965, US4931578]. 
Many of these side reactions are catalyzed by metals, most of which is present in 
initial silicon as impurities. The used silicon mass comprised by initial silicon, 
impurities and alkoxyisiloxane is accumulated in the reactor, which reduces the 
reaction rat
e. These processes make it necessary to regenerate the solvent for 
further use in the synthesis of alkoxysilanes.
Thus, a key challenge in the practical implementation of the alkoxysilane 
process is to develop a method allowing the synthesis reaction with high 
conversion in silicon and alcohol, regulation of a composition of the reaction 
products and multiple recovery of solvent and its reuse. This problem can be 
solved by eliminating or minimizing the induction period of the synthesis reaction, 
activating 
the reaction medium, simplifying the technology and running the 
reaction of alkoxysilane synthesis in a continuous mode.
As a result, a new technology of alkoxysilane synthesis by direct reaction of 
metallurgical 
silicon and low alcohols (methanol or ethanol) has been developed. 
The physical basis of the proposed technical solutions of how to activate the
reagents is that the milling of raw silicon is performed in the environment of a high 
boiling solvent, not in the air, which makes it possible to avoid 
a layer of natural 
oxide SiOx inevitably formed on the surface of just ground particles of 
metallurgical 
silicon during their contact with atmospheric oxygen. This reaction 
runs at any temperature, including room one, and is independent of chemical purity 
of silicon. A scheme of the alkoxysilane synthesis process is presented in Fig. 1.1.
and 
General view of the experimental setup for the implementation of the process 
of synthesis of monosilane 
is presented in Fig. 1.
2.