Process for separation of methyltrichlorosilane from dimethyldichlorosilane

A process for separating methyltrichlorosilane from dimethyldichlorosilane in a mixture. The process comprises contacting a mixture comprising methyltrichlorosilane and dimethyldichlorosilane with activated carbon, where the methyltrichlorosilane is selectively adsorbed by the activated carbon. The process allows for the recovery of a dimethyldichlorosilane fraction reduced in methyltrichlorosilane concentration. The methyltrichlorosilane can be recovered by desorption from the activated carbon. The present process is especially useful for removing low levels of methyltrichlorosilane present as a contaminate in dimethyldichlorosilane.

BACKGROUND OF INVENTION 
The present invention is a process for separating methyltrichlorosilane 
from dimethyldichlorosilane in a mixture. The process comprises contacting 
a mixture comprising methyltrichlorosilane and dimethyldichlorosilane with 
activated carbon, where the methyltrichlorosilane is selectively adsorbed 
by the activated carbon. The process allows for the recovery of a 
dimethyldichlorosilane fraction reduced in methyltrichlorosilane 
concentration. The methyltrichlorosilane can be recovered by desorption 
from the activated carbon. The present process is especially useful for 
removing low levels of methyltrichlorosilane from dimethyldichlorosilane. 
The commercial production of methylchlorosilanes involves the contact of 
methyl chloride with silicon metalloid in the presence of a catalyst 
comprising copper at temperatures within a range of about 300.degree. C. 
to 350.degree. C. Typically this process is optimized for the production 
of dimethyldichlorosilane, with lessor amounts of methylsilanes, 
methylchlorosilanes, methylhydrosilanes, C.sub.2 to C.sub.7 hydrocarbons, 
polysilanes, polysiloxanes, silylmethylenes, and other species being 
formed. This product mixture usually undergoes a series of process steps 
such as distillation, condensation, and the like to effect separation and 
recovery of commercially important individual components of the product 
mixture. However, standard separation techniques based on the difference 
in the boiling point between compounds become difficult and expensive when 
the compounds have similar boiling points. This situation exists with the 
separation of methyltrichlorosilane (b.p. 66.1.degree. C.) and 
dimethyldichlorosilane (b.p. 70.1.degree. C.). The present inventors have 
found that activated carbon selectively adsorbs methyltrichlorosilane when 
in mixture with dimethyldichlorosilane and therefore provides an 
alternative method for separation of these two methylchlorosilanes. The 
present process has been found particularly effective for removing trace 
amounts of methyltrichlorosilane from dimethyldichlorosilane, thereby 
providing dimethyldichlorosilane essentially free of methyltrichlorosilane 
contamination. 
Wilkman et al., U.S. Pat. No. 5,290,342, describe a process for separating 
ethylsilane from silane by selective adsorption of the ethylsilane onto 
activated carbon. 
Bothe et al., U.S. Pat. No. 5,445,742, describe a process for purification 
of halosilanes. The process consists of contacting a mixture comprising a 
halosilane and a hydrocarbon with an adsorbent selective for the 
hydrocarbon. Examples of useful adsorbents taught by Bothe et al. include 
activated carbon, carbon molecular sieves, and high silica zeolite. 
The cited art does not recognize that activated carbon can be used as a 
selective adsorbent to separate methyltrichlorosilane from 
dimethyldichlorosilane. 
SUMMARY OF INVENTION 
The present invention is a process for separating methyltrichlorosilane 
from dimethyldichlorosilane in a mixture. The process comprises contacting 
a mixture comprising methyltrichlorosilane and dimethyldichlorosilane with 
activated carbon, where the methyltrichlorosilane is selectively adsorbed 
by the activated carbon. The process allows for the recovery of a 
dimethyldichlorosilane fraction reduced in methyltrichlorosilane 
concentration. The methyltrichlorosilane can be recovered by desorption 
from the activated carbon. The present process is especially useful for 
removing low levels of methyltrichlorosilane present as a contaminate in 
dimethyldichlorosilane. 
DESCRIPTION OF INVENTION 
The present invention is a process for separating methyltrichlorosilane 
from dimethyldichlorosilane in a mixture. The process comprises (A) 
contacting a mixture comprising methyltrichlorosilane and 
dimethyldichlorosilane with activated carbon, where the 
methyltrichlorosilane is selectively adsorbed by the activated carbon and 
(B) recovering dimethyldichlorosilane reduced in methyltrichlorosilane 
concentration. The process can further comprise (C) recovering the 
adsorbed methyltrichlorosilane by desorption from the activated carbon. 
In a preferred process the dimethyldichlorosilane is present as a major 
component of the mixture while the methyltrichlorosilane is present as a 
minor component of the mixture. More preferred is when the 
methyltrichlorosilane comprises less than about 10 weight percent of the 
mixture. The mixture may comprise as minor components other compounds 
including methylchlorosilanes and similar boiling hydrocarbons. 
The mixture comprising the methyltrichlorosilane and dimethyldichlorosilane 
can be contacted with the activated carbon by standard methods. The 
process can be run as a batch process, semi-continuous, or continuous 
process. The mixture comprising the methyltrichlorosilane and 
dimethyldichlorosilane can be contacted in the liquid or gas phase with 
the activated carbon. Preferred is when the mixture is in the liquid phase 
when contacted with the activated carbon. 
The temperature at which the mixture comprising the methyltrichlorosilane 
and dimethyldichlorosilane is contacted with the activated carbon is not 
critical and can generally be within a range of about 0.degree. C. to less 
than 180.degree. C. A preferred temperature is within a range of about 
10.degree. C. to 50.degree. C. The pressure at which the mixture 
comprising methyltrichlorosilane and dimethyldichlorosilane is contacted 
with the activated carbon is not critical and can generally be within a 
range of about 0.1 atm. to 10 atm. Preferred is when the process is 
conducted at a pressure of about 1 atm. to 5 atm. 
In a preferred process the mixture is contacted with one or more 
packed-beds of the activated carbon operated in a temperature swing 
adsorption (TSA) mode. For example, the mixture can be passed through a 
first packed-bed of activated carbon until the activated carbon is nearly 
saturated with methyltrichlorosilane, the mixture can then be diverted to 
second packed-bed of activated carbon. The methyltrichlorosilane can then 
be recovered from the nearly saturated bed of activated carbon by heating 
the bed to a temperature causing desorption of the methyltrichlorosilane 
from the activated carbon. A heated inert sweep gas such as nitrogen may 
be passed through the saturated bed to facilitate desorption and recovery 
of the methyltrichlorosilane. Typically, desorption of the 
methyltrichlorosilane can be effected at a temperature above about 
60.degree. C. Preferred is when the desorption temperature is within a 
range of about 100.degree. C. to 400.degree. C. The mixture can then be 
switched between the activated carbon beds allowing for the activated 
carbon beds to be alternated between an adsorption and desorption mode, 
thereby providing for a continuous process. 
The physical form of the activated carbon is not critical to the present 
invention and can be, for example, flakes, chips, pellets, and powder. By 
"activated carbon" it is meant a microcrystalline, nongraphite form of 
carbon, having an internal porosity, the carbon having been activated by 
standard methods known in the art for producing activated carbon. For 
Example, the activated carbon can be formed by chemical or gas activation 
processes as described in Kirk-Othmer, Concise Encyclopedia of Chemical 
Technology, John Wiley & Sons, New York, p. 204-205, 1985. The activated 
carbon can be, for example, bituminous coal-based coconut shell-based, 
wood-based, or peat-based. 
Since chlorosilanes rapidly hydrolyze upon contact with water to form gels, 
it may be desirable to dry the activated carbon prior to use in the 
present process. The activated carbon can be dried by standard techniques 
such as heating or heating along with reduced pressure to reduce residual 
water. An example of useful conditions for drying the activated carbon is 
described in the Examples herein. 
The activated carbon selectively adsorbs methyltrichlorosilane relative to 
dimethyldichlorosilane in a mixture. This selectivity allows for recovery 
of dimethyldichlorosilane reduced in methyltrichlorosilane concentration. 
Recovery of the dimethyldichlorosilane reduced in methyltrichlorosilane 
can be effected by standard methods for separating gases or liquids from 
solids. In the preferred process using a packed-bed of activated carbon, 
recovery of the dimethyldichlorosilane can consist of collecting the 
effluent from the column in a suitable container for chlorosilanes. 
The present process can further comprise recovery of the 
methyltrichlorosilane by desorption from the activated carbon. Desorption 
of the activated carbon can be effected by standard means such as using 
elevated temperatures, reduced pressure, or a combination of both as 
described above for the preferred continuous process.

The following examples are provided to illustrate the present invention. 
These examples are not intended to limit the scope of the claims herein. 
EXAMPLE 
Several activated carbons were evaluated for their ability to selective 
adsorb methyltrichlorosilane from a mixture consisting of 
dimethyldichlorosilane containing either about 1500 ppm or about 2100 ppm 
methyltrichlorosilane. Each activated carbon sample was placed in a 50 ml 
flask. The carbon sample size ranged between 10 to 20 grams. The flask was 
heated at about 350.degree. C. under vacuum at about 30 mm Hg for six to 
eight hours to dry the activated carbon. The flask was cooled and purged 
with dry nitrogen. The chlorosilane mixture was then injected into the 
cooled flask through a Viton rubber septum. An additional seal was 
provided by a teflon stop-cock. The flask was shaken at room temperature 
for about 16 hours. A liquid sample was taken from the flask and analyzed 
by gas chromatography using a flame ionization detector (GC-FID). The 
result of this analysis is reported in Table 1 under the heading "1st 
Pass". The activated carbon was then regenerated by heating at 350.degree. 
C. under vacuum at about 30 mm Hg for six to eight hours. The flask was 
cooled, purged with dry nitrogen, and a second aliquot of the chlorosilane 
mixture injected into the flask. After shaking the flask for about 16 
hours at room temperature a liquid sample was taken from the flask and 
analyzed by GC-FID. The results of this analysis are reported in Table 1 
under the heading "2nd Pass". The activated carbons tested were products 
of either Calgon, Pittsburgh, Pa.; or Norit America Atlanta, Ga. as 
indicated in Table 1. The manufacturer's designation for each of the 
activated carbons is also provided in Table 1 along with the identity of 
the carbonaceous base material from which the activated carbon was formed. 
Because of the variability in adsorbent to liquid ratios used in the 
testing of the activated carbons the results of the adsorption tests are 
reported in Table 1 as the effective Henry's Law coefficient. The 
effective Henry's Law coefficient is calculated as H=X.sub.c /Y.sub.c, 
where X.sub.c is the adsorbent's equilibrium loading of 
methyltrichlorosilane (i.e. mass of methyltrichlorosilane adsorbed per 
unit mass of adsorbent) and Y.sub.c is the methyltrichlorosilane liquid 
equilibrium concentration (mass/mass). All activated carbons described in 
Table 1 were effective in selectively adsorbing methyltrichlorosilane. The 
larger the Henry's Law coefficient the higher the adsorption capacity of 
the activated carbon for the methyltrichlorosilane. 
TABLE 1 
______________________________________ 
Activated Carbon Selectivity For 
Methyltrichlorosilane Adsorption 
Henry's Coefficient 
Manufacturer 
Designation 1st Pass 2nd Pass 
______________________________________ 
Calgon .sup.(a) PCB-G Pulv 
4.4 1.6 
Norit .sup.(b) Norit C 
3.7 1.9.sup.a 
Calgon .sup.(c) CPG 12 X 40 
2.7 0.7 
Calgon .sup.(b) 114A-AWD 
2.6 1.2 
Norit .sup.(c) GCW 12 X 40 
2.3 1.1.sup.a 
Calgon .sup.(c) XtruSorb 600 
2.0 0.7 
Calgon .sup.(c) APA 12 X 40 
2.0 1.0 
Calgon .sup.(c) XtruSorb 700 
1.9 0.7 
Norit .sup.(d) PK 1-3 
1.0.sup.a 
-- 
Calgon .sup.(c) CAL 12 X 40 
0.6 0.3 
______________________________________ 
Base material = .sup.(a) coconut shell, .sup.(b) wood, .sup.(c) bituminou 
coal, .sup.(d) peat. 
.sup.a Initial concentration of methyltrichlorosilane about 2,100 ppm