Preparation of organohalosilanes

Details Preparation of organohalosilanes Preparation of organohalosilanes

2002-08-29

2004-04-27

O'Sullivan, Peter (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Silicon containing

Reexamination Certificate

active

06727376

ABSTRACT:

BACKGROUND OF THE INVENTION
With respect to the synthesis of alkylhalosilanes, Rochow first disclosed in U.S. Pat. No. 2,380,995 direct synthesis reaction between metallic silicon and alkyl halide in the presence of a copper catalyst. Since then, there have been reported a number of research works relating to various co-catalysts used together with copper catalysts, reactors, additives used during reaction, and the like. In the industrial synthesis of organohalosilanes, the selectivity of diorganodihalosilane which is most widely used in silicone resins, the formation rate of silanes, and the percent conversion of metallic silicon into useful silane are crucial. The selectivity of diorganodihalosilane is evaluated in terms of a weight or molar ratio of dialkyldihalosilane to the silanes produced and a T/D ratio.
Organohalosilane products contain diorganodihalosilane (D), triorganohalosilane (M), organotrihalosilane (T), etc. as well as other by-products such as organohydrodihalosilane (H) and organohalodisilane. In particular, disilanes are known as a high-boiling fraction among silicone manufacturers using direct method organohalosilanes because few processes are available for the effective utilization of disilanes, and most disilanes are discarded. The T/D ratio is a compositional ratio of organotrihalosilane to diorganodihalosilane in the entire organohalosilanes produced, with a lower T/D ratio being preferred. The formation rate of organohalosilane is represented by a space time yield (STY) which is the weight of crude organohalosilane produced per unit time relative to the weight of metallic silicon held in the reactor. In order to improve the content of diorganohalosilane produced, reduce the T/D ratio or increase the STY, various research works have been made with a focus on the catalyst and co-catalyst.
USSR Application Specification No. 617,569 (Certificate of inventorship No. 122,749) dated Jan. 24, 1959 discloses reaction in the presence of metallic silicon-copper alloy with 20 to 40 ppm of antimony added. Allegedly, the dimethyldichlorosilane content is improved from 40% to 60%. U.S. Pat. No. 4,500,724 discloses use of a copper/zinc/tin catalyst containing 200 to 3,000 ppm of tin, thereby achieving an improvement of T/D to 0.037. Japanese Patent Publication (JP-B) No. 6-92421 discloses reaction using copper arsenide having an arsenic concentration of at least 50 ppm. It is described in these patent references that reactivity, more specifically the rate of reaction of metallic silicon is improved by adding these tin, antimony and arsenic co-catalysts to a reaction contact mass comprising metallic silicon and copper.
USSR Application Specification No. 903,369 (Certificate of inventorship No. 178,817) dated Jun. 2, 1964 discloses that a co-catalyst selected from the group consisting of zinc, bismuth, phosphorus (200 ppm), arsenic, tin, and iron improves the dimethyldichlorosilane content to 72.1% from the value of 40-60% achieved by the above-referred Application Specification No. 617,569 (Certificate of inventorship No. 122,749). Also USSR Application Specification No. 1,152,943 (Certificate of inventorship No. 237,892) dated Nov. 20, 1969 discloses to add a phosphorus-copper-silicon alloy to a contact mass so as to give 2,500 to 30,000 ppm of phosphorus, thereby improving the dimethyldichlorosilane content to 82.3%. Moreover, U.S. Pat. No. 4,602,101 corresponding to JP-B 5-51596 discloses that 25 to 2,500 ppm of a phosphorus compound capable of generating elemental phosphorus in the reactor is added to a contact mass. Although the results of reaction according to this US patent are improved over the last-mentioned USSR patent, there still remain many problems including hazard imposed by spontaneously igniting elemental phosphorus and increased cost of raw materials. Then this US patent is also unsuitable to apply to commercial scale reactors. Also, F. Komitsky et al., Silicon For the Chemical Industry IV, Geiranger, Norway (1998), page 217, proposes the addition of phosphorus in the form of copper phosphide, leaving problems including a low percent conversion, ineffective utilization of phosphorus, and difficult control of a phosphorus concentration. U.S. Pat. No. 6,025,513 intends to add boron to a contact mass wherein the boron concentration is controlled so as to improve productivity. U.S. Pat. No. 5,059,706 discloses to introduce a phosphorus compound in a vapor phase into a reactor for increasing selectivity. U.S. Pat. No. 6,005,130 discloses to introduce organomonophosphine for increasing selectivity.
However, the phosphorus base additives used in the prior art have an outstanding trade-off between activity and composition selectivity. In particular, it is pointed out that oxide originating from phosphorus can exacerbate flow on the particle surface. Therefore, the conventional phosphorus base additives offer few merits on the continuous operation of commercial scale reactors. Other additives are known from L. Rosch, W. Kalchauer et al., Silicon for the Chemical Industry IV, Sandefjord, Norway (1996) wherein monomethyldichlorosilane is introduced for improving activity. This additive is effective only at the initial period, but not regarded as exerting a lasting effect during the continuous operation of commercial scale reactors.
Also, since the gas phase low-molecular weight compounds used in the prior art have a low evaporation temperature and lack thermal stability, it is difficult to precisely control the reaction at elevated temperatures. Under such circumstances, Ueno et al. proposed from a different point of view which had never been taken in the prior art, an industrial process using organophosphino compounds as the activating agent (see U.S. Pat. Nos. 6,215,012 and 6,242,629).
SUMMARY OF THE INVENTION
An object of the invention is to provide a novel and improved process for preparing organohalosilanes at a drastically increased formation rate without lowering the selectivity of useful silane, thereby increasing the utilization of silicon.
The present invention provides a process for preparing organohalosilanes of the following general formula (I):
R
1
n
H
m
SiX
4-n-m
  (I)
wherein R
1
is a monovalent hydrocarbon group, X is a halogen atom, n is an integer of 1 to 3, m is an integer of 0 to 2, and the sum of n and m is an integer of 1 to 3, by reacting metallic silicon particles with an organohalide in the presence of a copper catalyst, wherein a contact mass composed of the metallic silicon and the catalyst further contains an effective amount of a phosphonium compound having on the molecule at least one group of the following general formula (II):
[R
2
R
3
R
4
P—]
+
Y
−
  (II)
wherein R
2
, R
3
and R
4
are each independently a monovalent hydrocarbon group and Y is a halogen atom or acid group.
In summary, in the synthesis of organohalosilanes by reaction of metallic silicon with organohalide, the present invention incorporates an effective amount of a specific phosphonium compound in the contact mass for the purpose of increasing the formation rate of useful silane. More particularly, conventional additives which are known effective to improve the useful silane content are phosphorus compounds including metallic phosphorus, phosphorus oxide, copper phosphide, tin phosphide, zinc phosphide, aluminum phosphide, antimony phosphide, phosphorus trichloride, trimethylphosphine, and triphenylphosphine. We addressed the actual drawback of the direct method or Rochow method using such phosphorus compounds as a co-catalyst, that is, the problem that the phosphorus compounds serve to increase the diorganodihalosilane content, but reduce the reaction rate and hence, the productivity of useful silane. We also intended to realize in a commercial plant an increase of production rate which has never been accomplished when the direct method is carried out using as an activator conventional additives known to improve activity or such compounds as methyldichlorosilane. In such efforts, we have found that the above objects a

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