Preparation of contact mass for alkylhalosilane production and process for producing alkylhalosilanes

Abstract: Alkylhalosilanes are produced by first fluidizing a metallic silicon powder with an inert gas, preheating the silicon powder at a temperature between 200°C and a steady reaction temperature while keeping the silicon powder fluidized, adding a copper catalyst to the preheated silicon powder to form a contact mass, and feeding an alkyl halide into the contact mass whereby the alkylhalosilanes are formed by direct synthesis. This process prevents the copper catalyst from being sintered by thermal hysteresis and activates a high catalysis on the contact mass at the start of reaction. The desired dialkyldihalosilane can be produced at a high selectivity.

Description: BACKGROUND This invention relates to a method for preparing a contact mass for use in the production of alkylhalosilanes by the direct process and a process for producing alkylhalosilanes using the contact mass. More particularly, it relates to a process for continuously producing alkylhalosilanes by effecting gas-solid contact reaction between metallic silicon powder and an alkyl halide in the presence of a copper catalyst. With respect to the synthesis of alkylhalosilanes, Rochow first disclosed in US-A-2,380,995 direct synthesis reaction between metallic silicon and alkyl halides in the presence of copper catalysts. Since then, there have been reported a number of research works relating to various cocatalysts used together with copper catalysts, various copper catalysts and treatment thereof, reactors, additives used during reaction, and the like. The direct synthesis process involves activating a contact mass comprising metallic silicon and a copper catalyst and introducing an alkyl halide into the activated contact mass for accomplishing direct gas-solid contact between metallic silicon and alkyl halide, thereby producing alkylhalosilanes. The period required to activate to the contact mass is defined by R. J. H. Voorhoeve, Organohalosilanes, 1967, pp. 250-251, as a time taken until a phase of Cu 3 Si active in direct synthetic reaction (η phase) is formed in the contact mass. This is generally known as an "induction period." Various methods have been proposed for reducing the induction period. JP-B-63 043400, for example, discloses an alkylhalosilane producing method involving charging a reactor with silicon, catalyst and co-catalyst, passing nitrogen through the reactor, heating the reactor up to 200°C, admitting methyl chloride, heating the reactor up to 345°C, maintaining the temperature for 1 hour, and thereafter, cooling the reactor to 330°C. This method allows a larger amount of methyl chloride to be admitted. It is described that the reaction mass should be treated above 340°C for about 20 to 30 minutes when reaction is done at a temperature below 340°C, and that such treatment is unnecessary when reaction is done at a temperature above 340°C or when copper chloride is used. JP-A-54 078390 discloses the steps of commencing reaction at 360°C in order to reduce the induction period, lowering the temperature by 10°C in each of initial 2-hour periods and thereafter, lowering the temperature at a rate of 15°C/hr. until 280°C is reached. According to these methods, the contact mass is once heated to a temperature above the steady reaction temperature, and the temperature is then lowered for reaction to continue. Once heated to the elevated temperature, however, the copper catalyst is spent and likely to lower its catalytic activity at lower temperatures. It is also known that copper and similar solid catalysts become sintered owing to thermal hysteresis at elevated temperatures. With the progress of sintering, the catalyst life becomes shorter and the yield of the desired dialkyldihalosilane becomes lower. The procedure of once heating to a higher temperature and then cooling to the steady reaction temperature is undesirable from the energy standpoint partly because the reaction of alkyl halide with metallic silicon is exothermic. This is disadvantageous especially in the case of commercial scale reactors. Additionally, the cooling step takes a long time. If a long time passes before the steady reaction temperature is reached, there is a likelihood of failure to activate to the catalyst. JP-A-61 187933 discloses another alkylhalosilane producing method involving passing nitrogen through a reactor charged with metallic silicon powder and a copper catalyst for fluidization, heating the reactor at 325°C, passing HCl gas for 75 minutes to produce trichlorosilane, thereafter charging the reactor with an accelerator, and feeding methyl chloride for reaction to continue. This method, however, uses HCl as one reactant, on account of which tetrachlorosilane and trichlorosilane form. These chlorosilanes are generally difficult to separate from other useful silanes. Furthermore, JP-B-1 040035 discloses pretreatment by heating a mixture of metallic silicon powder, catalyst, and co-catalyst to 300°C and then introducing an equimolar mixture of dimethyldichlorosilane and methyl chloride. This method, however, is less effective to activate to the catalyst. Even if the reaction is activated, the dialkyldihalosilane used in the pretreatment goes to waste. In summary, existing alkylhalosilane producing methods suffer variously from problems associated with the activation of the contact mass, namely (1) the copper catalyst can be sintered during long-term thermal hysteresis at elevated temperatures, resulting in the catalyst losing lifetime and selectivity. (2) Excessive heating is disadvantageous from the energy standpoint. (3) A longer induction period fails to activate. (4) Extra HCl is used with formation of by-products which are difficult to separate, leading to a lowering of selectivity. A general aim herein is to provide new and useful methods of preparing a catalytically active contact mass comprising metallic silicon powder and copper catalyst, useful for preparation of alkylhalosilane, as well as corresponding methods going on to prepare alkylhalosilane by direct synthesis in said contact mass. Preferred aims include reduction or elimination of one or more of the prior art disadvantages referred to above, maintaining good catalytic lifetime and selectivity. Making investigations on the process for preparing alkylhalosilanes by commercially advantageous direct synthesis, especially with a view to activating catalysis by the contact mass while enabling adequate life and selectivity during reaction and reducing or eliminating problems arising from the sintering of the catalyst, we have found that useful results can be obtained by charging a heater or reactor with a metallic silicon powder, fluidizing the silicon powder with an inert gas, preheating the fluidised silicon powder at a temperature of at least 200°C and preferably below an ultimate reaction temperature (e.g. below 350°C or 300°C), then adding a copper catalyst to the preheated, fluidized silicon powder to form the contact mass. Feeding an alkyl halide into this contact mass in a reactor, thereby effecting continuous gas-solid contact reaction between the alkyl halide and the contact mass, produces alkylhalosilane. More particularly, by previously fluidizing only the metallic silicon powder, the heat amount given within the heater or reactor can be uniformly distributed throughout the silicon powder for fully drying the silicon powder. This eliminates moisture which is one reaction inhibiting factor. The late addition of the copper catalyst reduces sintering of the catalyst by thermal hysteresis although thorough mixing of the metallic silicon powder and the copper catalyst powder is still achieved. In a first aspect, the invention provides a method for preparing a contact mass for use in the production of alkylhalosilanes. A metallic silicon powder is fluidized with an inert gas. The silicon powder is preheated at temperature between 200°C and a later reaction temperature. while the silicon powder is kept fluidized. A copper catalyst is added to the preheated silicon powder, forming the contact mass. In a second aspect, the invention provides a method for producing alkylhalosilanes of the general formula: R n SiX 4-n wherein R is a lower alkyl group having 1 to 4 carbon atoms, X is a halogen atom, and letter n is an integer of 0 to 3, the method comprising the steps of fluidizing a metallic silicon powder with an inert gas; preheating the silicon powder at a temperature between 200°C and a steady reaction temperature while keeping the silicon powder fluidized; adding a copper catalyst to the preheated silicon powder to form a contact mass; and feeding a reactant gas containing an alkyl halide into the contact mass whereby the silanes are formed by direct synthesis. BRIEF DESCRIPTION OF THE DRAWINGS The only figure, FIG. 1 schematically illustrates one exemplary system for producing alkylhalosilanes. FURTHER EXPLANATIONS; PREFERRED AND OPTIONAL FEATURES The invention pertains to a process for producing alkylhalosilanes. Broadly stated, alkylhalosilanes of the general formula: R n SiX 4-n wherein R is an alkyl group having 1 to 4 carbon atoms, X is a halogen atom, and letter n is an integer of 0 to 3, are produced by introducing a reactant gas containing an alkyl halide into a contact mass comprising a metallic silicon powder and a copper catalyst whereby the alkylhalosilanes are formed by the direct synthesis process. According to the invention, while the metallic silicon powder is kept fluidized with an inert gas, the silicon powder is previously heated at a temperature of from 200°C to a steady reaction temperature. Thereafter, the copper catalyst is added to the preheated, fluidized silicon powder to form the contact mass. The alkyl halide-containing reactant gas is introduced into this contact mass whereby the alkylhalosilanes are produced. The metallic silicon used herein should preferably have a purity of at least 97% by weight, especially at least 98% by weight. Preferred is metallic silicon powder obtained by grinding metallic silicon to an appropriate particle size. Where the reactor used is a fluidized bed reactor or agitation reactor, metallic silicon powder having a particle size of 5 to 150 µm is preferred in order that the metallic silicon powder be appropriately fluidizing in the reactor. Note that the term "particle size" used herein is a particle size corresponding to 50% of a mass base cumulative oversize distribution curve by sieve analysis. For the copper catalyst, any form of copper may be used, for example, elemental copper such as granular copper and stamped copper, copper alloys such as Cu-Si, and copper compounds such as cuprous oxide, cupric oxide, and copper chloride. The loading of the copper catalyst is preferably about 0.1 to 10 parts, especially about 2 to 8 parts by weight of copper per 100 parts by weight of the metallic silicon powder. Together with the copper catalyst, promoters such as metallic zinc, antimony and tin powders may be used according to a well-known technique. The promoter may be used separately or as an alloy with copper, for example, Cu-Zn, Cu-Sn or Cu-Zn-Sn. Alkyl halides are reacted with metallic silicon to form alkylhalosilanes. The alkyl halides used herein are those having 1 to 4 carbon atoms, for example, methyl chloride, ethyl chloride, propyl chloride, methyl bromide, and ethyl bromide. Among these, methyl chloride is commercially most useful. Dimethyldichlorosilane prepared using methyl chloride finds numerous applications as a raw material for a variety of silicone resins. Desirably the alkyl halide reactant is previously heated and gasified before it is fed into the reactor. The alkyl halide gas may be used alone or in admixture with an inert gas. The feed amount of alkyl halide gas is calculated as an amount (combined with the inert gas) necessary to fluidize the contact mass and hence, properly determined from the diameter of a reactor used and a superficial velocity in a column. According to the invention, the alkylhalosilanes are produced as follows. First, a metallic silicon powder is heated at 200°C or higher while keeping the silicon powder fluidized with an inert gas. This heating may be carried out by means of a heater or by passing a hot heat carrier through a jacket around a reactor or an internal heat exchanger in a reactor. It is preferred to heat the metallic silicon powder at a higher temperature. If the temperature exceeds the reaction temperature, that is, the temperature for reaction of the metallic silicon powder with the alkyl halide to take place, there is a greater tendency for the copper catalyst to be sintered or reduced in lifetime when it is charged. For this reason, the preheating temperature preferably does not exceed the appropriate steady reaction temperature, at least when the catalyst is added. The term "steady reaction temperature" designates the temperature of the contact mass in a continuous reactor during the period when reaction of the metallic silicon powder with the alkyl halide proceeds in a steady state, that is, the reaction period excluding the operation starting and ending stages. It is believed that in conventional direct synthesis processes, the steady reaction temperature is in the range of 250°C to 350°C. 350°C is a permissible limit, but a more preferable steady reaction temperature is in the range from 280°C to 300°C. Therefore, in the contact mass preparation method it is most favourable that the temperature at which the metallic silicon powder is fluidized and heated is in the range of from 200°C to 300°C, especially from 240°C to 280°C. The metallic silicon powder is kept fluidized in the preheating stage for about 1 to 15 hours, especially about 2 to 10 hours. The next step of adding the copper catalyst to the preheated silicon powder is preferably carried out in the above temperature range. In the step of activating a catalysis on the contact mass, the contact mass is advantageously kept at a temperature ± 10°C within the above temperature range for a certain time, preferably about 1 to 8 hours, more preferably about 2 to 5 hours because a catalytic activity ensuring a longer lifetime and a higher selectivity can be activated. In the step of heating the metallic silicon powder or activating a catalysis, an inert gas is used for fluidizing the silicon powder or contact mass in the reactor. The inert gases usable include e.g. argon and nitrogen, with nitrogen being preferred for economy. The flow velocity of inert gas may be at least the velocity at which the metallic silicon powder starts fluidizing, especially about 5 times the fluidization starting velocity. If the flow velocity of inert gas is below this range, uniform fluidization of the metallic silicon powder would be difficult. If the flow velocity of inert gas is beyond this range, more metallic silicon powder would scatter and the losses of inert gas and heat would increase. It is preferred to flow the inert gas in a circulating manner. After the metallic silicon powder is preheated as above, the copper catalyst is admitted into the heater or reactor where it is mixed with the metallic silicon powder to form a contact mass. Where a promoter is used, it is added at the same time as the copper catalyst whereupon they are concurrently heated and mixed. The foregoing steps may be carried out in a separate heater or directly in a reactor for alkylhalosilane production. After the contact mass is given catalytic activity as mentioned above, the alkyl halide is introduced into the reactor whereby gas-solid contact reaction takes place between the alkyl halide and metallic silicon to form alkylhalosilanes. This reaction can be conventionally carried out. The heater or reactor used in the alkylhalosilane producing process according to the invention may be any of well-known reactors including fluidized bed reactors and agitating fluidized bed reactors. For industrial manufacture, a fluidized bed reactor is used with the advantages of a high yield, a high selectivity, and effective utilization of energy. More desirably, the reactor is externally equipped with means (including a condenser, piping and a compressor) for circulating the unreacted alkyl halide gas and a cyclone or filter means for collecting scattered contact mass. Referring to FIG. 1, there is illustrated an exemplary industrial scale system having a fluidized bed reactor incorporated for the production of alkylhalosilanes. When the alkylhalosilane is produced by this system, a metallic silicon powder is first fed from a silicon reservoir tank 1 to a fluidized bed reactor 3 through a source feed line 2, and an inert gas is introduced into the reactor 3 at its bottom through a gas feed line 4 for starting fluidization of the metallic silicon powder. A mixture of a small amount of metallic silicon powder and a cocatalyst may be fed. Then, a heat carrier is passed through a jacket 6 around the reactor 3 from an inlet 5 to an outlet 7 for heating the reactor 3 for thereby heating the metallic silicon powder at the preselected temperature. Thereafter, a catalyst mixture is fed from its source (not shown) into the reactor 3 through the source feed line 2 where the catalyst is mixed with the silicon powder to form a contact mass. After the contact mass is activated, an alkyl halide is fed from a reactant feed line 8 to a heater 9 where it is heated and then into the reactor 3 through the line 4. In the reactor, gas-solid contact reaction is effected between the alkyl halide and the contact mass to produce the alkylhalosilane. The system shown in FIG. 1 includes an output/circulating line from the reactor 3 including a cyclone 10 at the top of the reactor 3, another cyclone 11 for separating particles, a reservoir 12 for receiving the separated particles, condensers 13 and 14 for condensing silane gases, a reservoir 15 for receiving the condensed silanes, a compressor 16 for compressing the inert gas for circulation, and a return pipe 17 for feeding back the unreacted alkyl halide-carrying gas. EXAMPLE Examples of the invention are given below by way of illustration and not by way of limitation. All parts are by weight. Example 1 A fluidized bed reactor of carbon steel having a diameter of 8 cm and a height of 1 m as shown in FIG. 1 was charged with 100 parts of a metallic silicon powder. While nitrogen gas was passed through the reactor at a flow velocity of 2 cm/sec for fluidizing the silicon powder, the reactor was heated to raise the interior temperature to 260°C over 3-1/2 hours. At the end of preheating, 4 parts of a catalyst mixture composed mainly of metallic copper and brass powders was admitted into the reactor. Thereafter, the reactor interior temperature was gradually increased to 290°C while methyl chloride was introduced into the reactor at a flow velocity of 7 cm/sec. The time taken until the temperature of 290°C was reached was 2 hours. Reaction was continued at the interior temperature of 290°C. The reaction was terminated when the conversion of the metallic silicon powder exceeded 20% of the metallic silicon powder charge. Table 1 shows the running time from the start of methyl chloride introduction to the end of reaction. Table 1 also shows STY as well as the proportions of monosilanes and high-boiling products relative to the entire methylchlorosilane product. It is noted that the methylchlorosilane product contains dimethyldichlorosilane (D), methyltrichlorosilane (T), other methylchlorosilanes, and high-boiling products (R) having a boiling point above 80°C under atmospheric pressure (e.g., disilanes). The amounts of these components are expressed by percents by weight based on the total weight of the methylchlorosilane product. The formation rate of methylchlorosilane is represented by a space time yield (STY) which is equal to the weight of methylchlorosilanes produced per unit time relative to the weight of metallic silicon held in the reactor, that is, {weight (g) of methylchlorosilanes}/{weight (kg) of metallic silicon) x (time (hr.)}. Comparative Example 1 The same reactor as in Example 1 was charged with 100 parts of the metallic silicon powder and 4 parts of the catalyst mixture, both used in Example 1. While nitrogen gas was passed through the reactor for fluidizing the contact mass, the reactor was heated to raise the interior temperature to 260°C over 3-1/2 hours. Thereafter, methyl chloride was introduced into the reactor to initiate reaction while the reactor interior temperature was gradually increased to 290°C. The time taken until the temperature of 290°C was reached was 2 hours. Reaction was continued at the interior temperature of 290°C. The reaction was terminated when the conversion of the metallic silicon powder exceeded 20% of the metallic silicon powder charge. The results are shown in Table 1. Comparative Example 2 The same reactor as in Example 1 was charged with 100 parts of the metallic silicon powder. While nitrogen gas was passed through the reactor for fluidizing the silicon powder, the reactor was heated to raise the interior temperature to 180°C over 1 hour. Then 4 parts of the catalyst mixture used in Example 1 was admitted into the reactor. After 2 hours, methyl chloride was introduced into the reactor at 260°C to initiate reaction. The reactor interior temperature was gradually raised until the reactor interior temperature reached 290°C. The time taken until the temperature of 290°C was reached was 2 hours. Reaction was continued at the interior temperature of 290°C. The reaction was terminated when the conversion of the metallic silicon powder exceeded 20% of the metallic silicon powder charge. The results are shown in Table 1. E 1 CE 1 CE 2 Running time (hr) 6 8 8 Si consumption (%) 24.6 22.6 23.2 STY (g/kg·hr) 188 129 133 MeHSiCl 2 (wt%) 1.8 4.7 4.3 Me 3 SiCl (wt%) 2.6 2 2.2 MeSiCl 3 (wt%) 7.1 8.3 8.1 Me 2 SiCl 2 (wt%) 86 81.9 81.3 R (wt%) 2.5 3.1 4.1 A comparison of Example 1 with Comparative Example 1 reveals that Example 1 wherein the metallic silicon powder is preheated at a temperature above 200°C before the catalyst is added brings the consumption of metallic silicon powder above 20% within a shorter time and yields a significantly favorable silane composition. As compared with Example 1, Comparative Example 2 wherein the catalyst is added after the metallic silicon powder is preheated, but to an insufficient extent, is low in STY and dimethyldichlorosilane proportion. The present invention can help to avoid the copper catalyst from being sintered by thermal hysteresis while activating catalysis by the contact mass so that a high catalytic activity is available with high selectivity at the start of reaction. The desired dialkyldihalosilane can be produced at a high selectivity. Although some preferred embodiments have been described, many modifications and variations may be made thereto in the light of the general teachings herein. It is therefore to be understood that the invention may be practised otherwise than as specifically described in the embodiments.

Claims: A method of preparing a catalytic contact mass useful in the production of alkylhalosilane, in which metallic silicon powder is mixed with copper catalyst, characterised in that the metallic silicon powder is subjected to a preheating stage, in which it is heated at 200°C or more while being fluidised with a flow of inert gas, before addition of copper catalyst thereto. A method according to claim 1 in which the preheating stage lasts for from 1 to 15 hours. A method according to claim 1 or claim 2 in which the copper catalyst is added to the preheated silicon powder at a temperature not more than 300°C. A method according to any one of claims 1 to 3 in which the activation temperature, following addition of the copper catalyst, is maintained for at least one hour with not more than 20°C variation. A method of making one or more alkylhalosilanes of the formula R n SiX 4-n where R represents C 1-4 alkyl, X represents halogen and n is from 0 to 3, comprising preparing a catalytic contact mass in accordance with any one of claims 1 to 4 and feeding reactant gas containing alkyl halide into the contact mass to form said one or more alkylhalosilanes by direct synthesis. A method according to claim 5 in which the same reactor vessel is used for formation of the catalytic contact mass and synthesis of the one or more alkylhalosilanes.

Inventors: INUKAI TETSUYA, NAKANISHI TETSUO, NAKAYAMA HIROSHI, SATOH YUKINORI, TSUKIOKA KAZUMASA, INUKAI, TETSUYA, NAKANISHI, TETSUO, NAKAYAMA, HIROSHI, SATOH, YUKINORI, TSUKIOKA, KAZUMASA, Inukai, Tetsuya, c/o Shin-Etsu Chem. Co., Ltd., Nakanishi, Tetsuo, c/o Shin-Etsu Chem. Co., Ltd., Nakayama, Hiroshi, c/o Shin-Etsu Chem. Co., Ltd., Satoh, Yukinori, c/o Shin-Etsu Chem. Co., Ltd., Tsukioka, Kazumasa, c/o Shin-Etsu Chem. Co., Ltd.
Applicants: SHINETSU CHEMICAL CO, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1998
Main classification: B01J23
IPC codes: B01J37, C07F7, C07B61, B01J23