Effect of Low Aluminum Silicon on the Direct Process - Industrial

Ind. Eng. Chem. Res. 2003, 42, 3555-3565

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Effect of Low Aluminum Silicon on the Direct Process J. M. Bablin,† A. C. Crawford,‡ D. C. DeMoulpied,† and L. N. Lewis*,† GE Global Research, 1 Research Circle, Niskayuna, New York 12309, and GE Silicones, 260 Hudson River Road, Waterford, New York 12188

GE Silicones ran a commercial direct process production unit with low aluminum content silicon, and very poor reaction yields were obtained. The direct process is used to react silicon with methyl chloride to produce methylchlorosilanes, which are used to produce silicone materials. Laboratory experiments were conducted that allowed GE to generate a plausible hypothesis to explain the commercial failure. The commercial and laboratory results have been used by GE to diagnose additional direct process reaction problems. Aluminum is an important impurity found in chemical-grade silicon. Aluminum acts as one of many promoters in the direct process. Laboratory experiments indicated that the presence of aluminum is required to increase the vapor pressure of the zinc promoter at direct process reaction conditions. Increased vapor pressure allows movement of zinc within the solid reaction mass present in the fluid-bed reactor. Insufficient zinc vapor pressure causes severe problems with raw material yields and product quality that translates to uneconomic operation of the commercial direct process reaction unit. Introduction

Table 1. Major Products of the Direct Process

The economic production of organohalosilanes, particularly dimethyldichlorosilane (Di), is at the heart of the multibillion dollar global silicone industry. In 1939 Rochow1 discovered what is now known as the direct process: the synthesis of methylchlorosilanes (MCSs) directly from silicon. On the basis of Rochow’s discovery, the silicone industry was launched after World War II, and Rochow’s direct process remains the only economical route to MCSs.2 Seyferth3 recently published a history of the direct process. The direct process reaction (the MCS reaction) is shown in eq 1 with general composition of the crude MCSs. The mixture of MCSs is called MCS crude or simply crude. In general terms the goal of the direct process is to maximize the amount of Di that is contained in the crude. The byproducts have uses, and at times it is desired to produce higher than normal amounts of some such as Mono to meet cyclic business demands. The term “crude” quality is used to define the usefulness of the crude stream to the silicone producer. For the purposes of this paper, high crude quality is crude that contains a maximize amount of Di with minimal amounts of all byproducts. A listing of major products of the direct process is provided in Table 1. Cu (0.5-5%) Zn (100-5000 ppm)

Si + MeCl 9 8 Sn (5-100 ppm) Al (500-4000 ppm) 280-350 °C

Me2SiCl2 (Di; 70-90%) MeSiCl3 (Tri; 4-12%) Me3SiCl (Mono; 1-5%) (1) MeHSiCl2 (MH; 0.5-5%) Me2HSiCl (M2H; 0.1-1.0%) other low boilers (0.1-0.5%) residue (high boilers/polysilanes, 0.5-10%) † ‡

GE Global Research. GE Silicones.

chemical name

formula

abbrevn

tetramethylsilane trichlorosilane dimethylchlorosilane methyldichlorosilane silicon tetrachloride trimethylchlorosilane methyltrichlorosilane dimethyldichlorosilane hexamethyldisilane pentamethylchlorodisilane sym-tetramethyldichlorodisilane trimethyltrichlorodisilane sym-dimethyltetrachlorodisilane

(CH3)4Si HSiCl3 (CH3)2HSiCl (CH3)HSiCl2 SiCl4 (CH3)3SiCl CH3SiCl3 (CH3)2SiCl2 (CH3)6Si2 (CH3)5Si2Cl (CH3)4Si2Cl2 (CH3)3Si2Cl3 (CH3)2Si2Cl4

S TCS M2H MH Q Mono Tri Di

normal bp (°C) 26.6 31.5 36.0 40.7 57.6 57.9 66.4 70.3 ∼113 ∼130 ∼150 ∼156 ∼158

The direct process is carried out commercially in fluidbed reactors using powdered silicon at approximately 300 °C. Rochow’s great discovery was copper catalysis. The reactants are essentially inert without copper. A huge amount of research has been done to find promoters to optimize the direct process to produce higher amounts of Di.4 Di is the monomer used to form the basic silicone polymer used in the silicone industry. Many different promoters have been described. Zinc was the first effective promoter. The benefit of zinc was discovered by Gilliam in 1949.5 Ward et al. found that tin was critical to the process and that there was an optimum composition of copper, zinc, and tin that maximized both the yield of Di and the reaction rate.6 At the end of the 1980s, many believed that the direct process had been fully optimized with regard to added promoters such as copper, zinc, and tin. However, in 1990, Dow Corning reported that phosphorus was an important promoter for Di.7,8 The benefit of aluminum as a promoter for the direct process was first reported by Hurd9 in 1947. Aluminum is a promoter for the MCS reaction, but unlike copper, zinc, tin, and others which are added to the reaction mass, it is usually present in the silicon. However, aluminum may at times be present in insufficient quantities to be effective. Chemical-grade silicon is a heterogeneous material. Pure silicon crystals are surrounded by intermetallic phases. Intermetallic phases can be thought of as solid solutions and contain the

10.1021/ie020334s CCC: $25.00 © 2003 American Chemical Society Published on Web 06/28/2003

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Ind. Eng. Chem. Res., Vol. 42, No. 15, 2003

Figure 1. GE Silicones MCS reaction unit.

various impurities present in chemical-grade silicon. The main impurities to consider are Fe, Al, and Ca. The predominant intermetallic phases found in chemicalgrade silicon are10,11 Si2Ca, FeSi2.4, Si2Al2Ca, FeSi2Al3, Fe4Si6Al6 (Fe5Si7Al8), and Si8Al6Fe4Ca. Rong and Sorheim12 have also discussed the fact that aluminum must be in the proper form to be effective. Silicon used for the MCS reaction may have a sufficient quantity of aluminum but an insufficient quantity of the correct (promoting) aluminum. The use of aluminum as a promoter has been discussed by others as well both as it is present in silicon13 and when it is added to a MCS reactor bed.14 Commercial MCS reactor trials with low aluminum content silicon metal were run at GE Silicones. The commercial trials were dramatic failures. The commercial failures were confirmed with a series of reactions carried out in a laboratory-scale MCS reactor. A hypothesis was developed and confirmed by experimentation to explain the poor results of the direct process. Sufficient amounts of aluminum are required to increase the vapor pressure of promoters. Unlike many problems experienced in the commercial operation of the direct process, the low aluminum silicon trials were unique because GE Silicones learned why the commercial trials failed. Details of the commercial and laboratory experiments are described. Commercial Results Introduction. Most parts of commercial MCS process operations are patent protected. Reduction of technology to commercial practice is closely guarded by silicone producers. Published results of contemporary commercial MCS reactor operations are very limited. To those skilled in the art, this paper provides interesting insight into the GE Silicones commercial MCS process. This paper also provides a view of how the understanding of MCS technology is advanced by a combination of commercial and laboratory experimentation. Commercial Trial Plan. Commercial MCS reactor trials at GE Silicones require significant planning with a broad range of disciplines before the trial is run. GE Silicones follows a strict management of change (MOC) program that is mandated by GE Silicones and the United States Occupational Health and Safety Administration. Representatives from manufacturing (production engineers, hourly control operators, and shift supervisors), technology (engineers and chemists), environmental, health & safety, and sourcing provide input as the trial is being designed and the MOC is

completed. Multiple-level sign-offs are required before the trial is run. Commercial silicon trials start several days after a reactor run has been started. Experience shows that a known period of time exists before steady state is achieved in reactor operation with the trial silicon. Trials are designed to exceed this known period of time to provide useful data. Commercial reactor operating conditions are determined by the joint manufacturing/ technology trial team. In most cases the goal is to run the reactor at standard operating conditions. It is preferred that reactor operation continues after the trial silicon has been consumed. This allows the reactor to transition back to standard silicon feed, and operation during this transition gives important information about the reactor response during the trial. MCS Reactor Operation. GE Silicones MCS reactors are operated for a set period of time that is determined by crude chlorosilane inventory. Mechanical or reactor performance problems can necessitate an early, unplanned shutdown. MCS reaction units consist of three major sections: reaction, solids separation, and methyl chloride recovery. Figure 1 shows a block diagram of a typical MCS unit. The units are controlled remotely via a distributed control system. Ground silicon, methyl chloride gas, copper catalyst, and promoters are fed to the fluid-bed reactor. A heat removal system is used to control the exothermic reaction to a desired temperature. Crude chlorosilanes, unreacted methyl chloride, and entrained solids exit the reactor. Solids are removed from the hot gas stream in a multipart separation process. Unreacted methyl chloride is separated from crude chlorosilane product by distillation. Crude chlorosilanes are transferred to storage tanks while unreacted methyl chloride is combined with fresh makeup methyl chloride for recycle to the reactor. Crude chlorosilanes are purified in a series of distillation columns located downstream from the MCS units. Ground silicon is prepared in an on-site silicon grinder. Methyl chloride is produced on-site and purchased from external suppliers. Copper oxide catalyst, zinc, and tin are purchased from external suppliers. MCS Reactor Performance Parameters. A commercial MCS reaction process has a significant number of variables that are used to determine the reactor performance and economics of individual runs. These variables include crude composition (GC analysis to determine M2H, MH, Mono, Tri, Di, and residue contents), reaction rate, silicon feed, methyl chloride feed, catalyst consumption, process vent rate (measure of hydrocarbons formed in the MCS reactor), reaction mass composition, and run length. Reaction mass composition

Ind. Eng. Chem. Res., Vol. 42, No. 15, 2003 3557

includes the amount of carbon present. The carbon level in the reaction mass provides an indication of the amount of methyl chloride cracking that occurs as the reactor is operated. Most data are collected at GE Silicones by a data acquisition system that is interfaced with the distributed control system. These data are stored for historical trend analysis and other data manipulation. Other data are generated by an on-site quality control testing laboratory. Results and Discussion of Commercial Trial Results Trial Summary. GE Silicones recently ran two low aluminum silicon trials on a commercial MCS reaction unit. Both trials were run during a period of time when GE Silicones was evaluating the commercial use of phosphorus as a promoter for the direct process. The aluminum content of silicon was hypothesized as an important variable for successful use of phosphorus. It was believed that aluminum levels in silicon used at GE Silicones were too high; successful use of phosphorus required lower aluminum content silicon. Trials were designed to test the hypothesis. The two trials were run on the same commercial MCS reactor with silicon produced by a single supplier. The time between the two trials was approximately 1 month. One trial was run in the presence of phosphorus. Phosphorus was not used in the second trial, which served as a control. Reactor Performance Evaluation. Rigorous economic evaluations of commercial GE Silicones MCS reactor operations are made on a routine basis. The economic analysis of short silicon trials such as the low aluminum trials described in this paper is difficult as a result of the lack of sufficient (minimum 1 week) data. Short trials are evaluated based on reactor operation before, during, and after the trial. Focus is placed on significant events (good or bad) that occur during the trial and on how the reactor recovers after the trial. Parameters used to judge a short commercial trial include crude composition, reaction rate, catalyst consumption, and process vent rate. A successful short trial is one that yields statistically unchanged or improved reactor performance from periods before and after the trial. Positive results obtained during a short trial are used to justify future longer trials. Poor results are generally grounds for halting further commercial trials until the cause of the poor results can be explained. Commercial Data Presentation. Crude composition and reaction mass carbon levels are presented in this paper as normalized values to protect the proprietary nature of this information. Normalized crude composition data are presented for M2H, MH, Mono, Tri, and Di. The crude quality is measured on a routine frequency at GE Silicones by GC. Each actual crude analysis was normalized to the average crude composition for the year of the trial. Normalized values greater than 1 represent actual values greater than the yearly average, while normalized values less than 1 represent actual values less than the yearly average. Reaction mass carbon levels are measured on a routine frequency. The actual values were normalized to a carbon level that is the desired maximum. Normalized carbon values greater than 1 represent carbon levels that are higher than desired while normalized

values less than 1 represent low, and thus acceptable, carbon levels. The normalized data are plotted as a function of individual sequential data points. Pretrial data were adjusted to allow the graphs to indicate identical start times for both trials. This allows a comparison of reactor operation with and without phosphorus addition. Trial Silicon Material. GE Silicones has enjoyed long-term technical relationships with many silicon suppliers located around the world. These relationships allow procurement of commercial trial quantities of silicon with relative ease. A qualified silicon producer was contacted, and plans were made to produce the trial low aluminum silicon. The silicon supplier produced silicon with each furnace tap less than 0.10% aluminum. Levels of iron, calcium, titanium, and other minor trace elements were unchanged from normal levels supplied to GE Silicones by the selected supplier. Conventional casting techniques were used to produce ingots of about 100-150 mm thickness. The lump silicon was crushed and supplied in standard 90 metric tonne covered hopper railroad cars. The average aluminum content was 0.06-0.08%, which is well below the average aluminum in standard chemical-grade silicon consumed by GE Silicones. The oxygen content and intermetallic structure of the trial silicon were not characterized. Low Aluminum Silicon Trials. The first low aluminum silicon trial was run with copper oxide catalyst, zinc, tin, and phosphorus promoters. Solid copper phosphide, Cu3P, was the source of phosphorus. The second trial was run without phosphorus addition. Results of the two commercial trials were not anticipated and very poor based on significant crude-quality degradation. Qualitatively, degradation of crude quality was delayed when phosphorus was present. The behavior of zinc and reactor parameters associated with zinc were an area of interest because the results were analyzed after the commercial trials. Feed of low aluminum silicon to the MCS reactor began several days after the reactor runs were started for both trials. Reactor operation in both trials was in normal ranges prior to starting the trial. Phosphorus addition in the form of solid Cu3P began 12 h after feed of low aluminum silicon began in the first trial. The target level of phosphorus was 150 ppm in the reaction mass. Copper oxide, zinc, and tin promoters were adjusted to desired levels. The reaction rate decreased shortly after low aluminum silicon feed began for both trials. Some adjustments were made in reactor operation to compensate for the lower rate. Aluminum is known as a rate promoter,4 and the initial trial results confirmed this reported observation. It quickly became evident to the trial team that the major responses of reactor operation for both trials would be crude quality and composition of the reaction mass. Graphs showing normalized values of M2H, MH, Mono, Tri, and Di are presented in Figures 2-6, respectively. Data are provided for both commercial trials: with and without phosphorus. The data are arranged to show identical start times for both trials. This allows a comparison of the reactor response with and without phosphorus addition. The data clearly show that Mono was suppressed to extremely low levels for both trials. The presence of phosphorus does not appear to influence Mono suppression. Hurd9 discovered in 1947 that the lack of alumi-

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Figure 2. Normalized M2H in commercial MCS crude with/without phosphorus.

Figure 3. Normalized MH in commercial MCS crude with/without phosphorus.

Figure 4. Normalized Mono in commercial MCS crude with/without phosphorus.

num suppresses Mono formation. The recent trials confirm this result. Trends for M2H, MH, Tri, and Di show the same pattern: the presence of phosphorus appears to have delayed eventual degradation of crude

quality. Crude quality eventually degraded to the same poor levels for both trials; the nonphosphorus run was stopped sooner based on the phosphorus trial experience. The significant changes in M2H, MH, Tri, and Di


Figure 5. Normalized Tri in commercial MCS crude with/without phosphorus.

Figure 6. Normalized Di in commercial MCS crude with/without phosphorus.

Figure 7. Normalized carbon in commercial MCS reaction mass with/without phosphorus.

content of the MCS crude was unexpected in both commercial trials. The residue content of the crude increased for both trials, but the trends are more variable. Processes downstream from the MCS reactor can and do influence the residue content measured in the crude product. The carbon level in the reaction mass increased to extremely high levels for both trials. The presence of phosphorus did not appear to have an impact on the amount of carbon generated. Figure 7 shows the normalized reaction mass carbon level as a function of

sequential process data on the same time scale as that used for crude quality. The zinc level in the reaction mass initially began to increase after low aluminum silicon feed began for both trials. The zinc increase was recognized as a potential problem, and changes were made in reactor operation that stopped the zinc increase. Without these operational changes, projected zinc levels would have increased at a similar rate as carbon. Small amounts of hydrocarbon are always formed in the direct process reaction. These hydrocarbons are

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Figure 8. Fixed-bed laboratory reactor.

removed from the MCS process as noncondensables in the crude-methyl chloride separation process. The vent rate during both low aluminum trials increased along the same time scale as reaction mass carbon levels increased. Final vent rates were 4 times higher than normal. The first trial with phosphorus added was ultimately stopped as a result of excess vent rate. At some point in both commercial trials, reactor operation reached a point of no return. It was not possible to return the reactor to pretrial operation. The presence of phosphorus in the first trial delayed the degradation, but the final outcome of both trials was the same. The large amounts of Si-H compounds (M2H and MH) are indications of excess methyl chloride cracking. High levels of carbon in the reaction mass and high process vent rates also indicate excess methyl chloride cracking. The cause of the methyl chloride cracking was not discovered until a series of laboratory experiments were run. Laboratory experiments would indicate that the interaction between zinc and aluminum was at the center of the poor commercial results. Subsequent data analysis disclosed that intentional changes to the zinc addition rate to the commercial reactor obscured the zinc-aluminum interaction. The clear result of the two commercial low aluminum silicon trials was that 0.060.08% Al silicon does not contain enough aluminum to sustain commercial direct process reaction at GE Silicones. Laboratory Results Introduction. The poor commercial MCS low aluminum silicon trial results with and without phosphorus were not expected. Prior to the commercial trials, several laboratory runs were made in a stirred-bed reactor with the low aluminum silicon used in the plant trials. Results of these runs were satisfactory and provided no indication of the poor trial results. A pilotscale MCS reactor does not exist at GE Silicones. Following the plant experiments, a series of laboratory experiments were run in a fixed-bed reactor. The purpose of these experiments was to duplicate the commercial results and look for a failure hypothesis. Historically, there is poor translation of the laboratory results to commercial MCS operations as a result of

significant differences in the two processes. Commercial trials that are run outside the standard operating parameters provide a unique chance to conduct applied research on a full-scale unit. Significant advances in the understanding of the MCS process are achieved when commercial trial results are fully analyzed. GE Silicones experience shows that understanding unsuccessful commercial trials provides the greatest insight into the MCS process. Experimental Procedures. A fixed-bed reactor was used for the laboratory MCS experiments.15 A drawing of the reactor is shown in Figure 8. The reactor was a vertical glass tube of 20 cm length and 1.3 cm outside diameter with a glass frit located 6 cm from the bottom to support the bed. The diameter of the bed was limited by heat-transfer considerations because the MCS reaction is exothermic and the reaction mass was not stirred. The thermal conductivity of the silicon was assumed to be the same as that of sand, namely, 6 × 10-4 cal/(cm s C). The highest anticipated reaction rate was 1 g of crude/(h g of silicon). For this rate, a temperature rise of ∼10 °C along the center line of the reactor was calculated. Over this temperature range, no changes in crude composition were observed. The other reactor design consideration was pressure drop. The reaction mass was limited to 6 g to provide a safe maximum pressure drop of approximately 6 psi. The reactor was centered vertically in a 5 cm glass tube wrapped with Nichrome heating ribbons. Two separate Nichrome heating zones were provided with temperature control. Two pairs of electrodes were fitted to the Nichrome to create two heated zones. The top zone was used to preheat the MeCl feed, while the reactor was contained in the bottom heating zone. The 5 cm heated glass tube was centered in a 6.4 cm glass tube used for insulation of the reactor and safety. Silicon, copper catalyst, and promoters were well mixed and loaded into the fixed-bed reactor. The powder was lightly tapped so that the bed height was 4.7-4.9 cm. The reactor was installed in the system, and an argon flow was started. The argon flow was used to ensure that there were no leaks in the system and to purge air and moisture from the equipment. The bed was purged with argon for about 30 min at an argon flow of 40 cm3/min. The heating system was then turned


Figure 9. Low Al silicon: effect of the rate (Al ) 1000 ppm Cu-Al; P ) 500 ppm P). Table 2. Experimental Silicon Used for Laboratory Runs silicon

% Fe

% Al

% Ca

standard low aluminum zero aluminum

0.47 0.36

0.160 0.059

0.02 0.11

on, and typically within 30 min the reaction mass temperature had reached the desired 300 °C operating temperature. The gas flow was then switched from argon to 35 cm3/min of MeCl. MCS vapor exiting the reactor was condensed with a condenser operating at -20 °C. Liquid crude was collected in a receiver, and samples were periodically manually removed from the system. The liquid crude samples were weighed and analyzed by a gas chromatograph (GC) to determine the amounts of S, M2H, MH, Mono, Tri, Di, and Residue. GC measurements were made using a Hewlett-Packard model 6890 instrument with a thermal conductivity detector and a 60 m SPB210 Supelco methylsilicone capillary column. The fixed-bed laboratory reactor is a batch process operated for a specific amount of time. Crude composition data and the reaction rate are plotted as a function of percent silicon utilization (conversion). Silicon utilization is equivalent to the reaction time. The contents of Di and MH are the focus of fixed-bed reactor data analysis. Di is the desired commercial reaction product, and MH provides a measure of methyl chloride cracking reactions. Materials. Fixed-bed laboratory runs were all made at 5% Cu, 2000 ppm Zn, and 50 ppm Sn relative to silicon at the start of each run. Copper flake was the source of copper catalyst for all laboratory runs. Tindoped brass flake was the source of zinc and tin promoters. The composition of the brass flake was 80% Cu, 19.5% Zn, and 0.5% Sn. All runs were made with a weight ratio of 4 parts Cu flake and 1 part tin-doped brass flake. Triethylphosphine [(CH3CH2)3P] was the source of phosphorus. Aluminum powder and a Cu-Al alloy (60% Cu/40% Al) were the external sources of aluminum. Three different silicon materials were used and are summarized in Table 2. Silicon was ground in a Retsch laboratory grinder to a d50 of approximately 50 µm. The low aluminum silicon used for laboratory runs contains an elevated level of calcium. Higher calcium may amplify the low aluminum effect.

Results and Discussion of Laboratory Results The fixed-bed laboratory reactor was used to investigate the poor commercial trial results obtained when a low aluminum content silicon was run. Three groups of laboratory reactions were run. The first group was run with low aluminum content silicon and different amounts of added aluminum and phosphorus. The second group was run with zero aluminum pure silicon (99.9995% Si) and different amounts of added aluminum and phosphorus. A third series of experiments were run to investigate different forms of added aluminum. A primary goal of the laboratory experiments was to duplicate the commercial results. Phosphorus was used in the experimental plan given the qualitative commercial result that use of phosphorus with low aluminum silicon delayed degradation of crude quality. The fixed-bed reactor used has been found by GE Silicones to provide very repeatable data for crude composition as a function of silicon utilization. The reactor was not designed to produce a repeatable reaction rate. Order of magnitude differences in rate may be considered significant when different experiments are compared. The rate is the least translatable parameter between a batch fixed-bed laboratory reactor and a continuous commercial fluid-bed reactor. Rate data are provided to show major differences between neat low aluminum or zero aluminum silicon and experiments run with varying amounts of added aluminum and/or phosphorus. Low Aluminum Silicon Laboratory Runs. Several laboratory runs were made with the low aluminum silicon described in Table 2. The low aluminum silicon gave very poor crude quality (low % Di and high % MH) and low rate compared to the standard silicon when Cu-Zn-Sn was used. Addition of 500 ppm P (relative to starting silicon) to the low aluminum silicon improved the reaction rate and crude quality (higher % Di and lower % MH). Addition of aluminum in the form of a Cu-Al alloy (1000 ppm Cu-Al alloy relative to starting silicon) also improved the reaction rate and crude quality when added to the low aluminum silicon. The total aluminum content relative to silicon increased from 0.059% to 0.099% when a 1000 ppm Cu-Al alloy was added.

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Figure 10. Low Al silicon: effect on % Di (Al ) 1000 ppm Cu-Al; P ) 500 ppm P).

Figure 11. Low Al silicon: effect on % MH (Al ) 1000 ppm Cu-Al; P ) 500 ppm P).

Figure 12. Zero Al silicon: effect on the rate (standard ) 0.16% Al; P ) 500 ppm P).


Figure 13. Zero Al silicon: effect on % Di (standard ) 0.16% Al; P ) 500 ppm P).

Figure 14. Zero Al silicon: effect on % MH (standard ) 0.16% Al; P ) 500 ppm P).

Addition of both 500 ppm phosphorus and 1000 ppm Cu-Al alloy to the low aluminum silicon resulted in the highest reaction rate and the best crude quality (highest % Di and lowest % MH) compared to the results obtained with either phosphorus or aluminum alone. Reaction rates as a function of percent silicon utilization are provided in Figure 9 for the various runs with low aluminum silicon: one run with neat low aluminum silicon, one run with 500 ppm phosphorus added, two replicate runs at 1000 ppm added Cu-Al alloy, and two replicate runs with 1000 ppm Cu-Al alloy and 500 ppm phosphorus added. The lowest rates were obtained when low aluminum silicon without addition of phosphorus or aluminum were made. Rates are higher when runs were made with the addition of Cu-Al alloy or a combination of Cu-Al alloy and phosphorus. A run made with only the addition of phosphorus yielded a rate higher than that of neat low aluminum silicon but lower than runs where Cu-Al was added. Figure 10 shows % Di composition of crude produced as a function of silicon utilization for the same runs as those presented in Figure 9. The lowest % Di crude was produced with neat low aluminum silicon. The highest % Di crude was produced with a combination of 1000 ppm Cu-Al alloy and 500 ppm phosphorus. Figure 11 shows % MH composition of crude as a function of silicon

utilization for the same runs as those presented in Figures 9 and 10. High MH is a problem for GE Silicones. Neat low aluminum silicon produced the highest amount of MH. Runs with 500 ppm phosphorus and a combination of 1000 ppm Cu-Al alloy/500 ppm phosphorus produced the lowest amount of MH. Zero Aluminum Silicon Laboratory Runs. Several experiments were performed using silicon with zero aluminum (99.9995% Si). These experiments showed that phosphorus alone can restore crude quality that is provided by standard silicon (see Table 2) that contains 0.16% Al. Phosphorus cannot increase the reaction rate to levels expected when standard aluminum content silicon is run. Figure 12 shows reaction rates for various runs with zero aluminum silicon. A single control run of standard silicon with 0.16% aluminum is provided for reference. Two replicate runs were made with zero aluminum silicon and the standard Cu-Zn-Sn catalyst system. The rate is very low compared to 0.16% aluminum standard silicon. Addition of 500 ppm phosphorus to the zero aluminum silicon yielded a slight rate increase in two replicate runs. Phosphorus is not a substitute for aluminum as the rate promoter. Figure 13 shows % Di composition of crude as a function of silicon utilization for the same runs as those

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presented in Figure 12. Two replicate runs made with neat zero aluminum silicon yielded crude with the lowest amount of Di. Addition of 500 ppm phosphorus to zero aluminum silicon increased the % Di composition of crude to similar levels as standard 0.16% aluminum content silicon yielded. Two replicate runs of zero aluminum silicon and 500 ppm phosphorus are shown in Figure 13. Figure 14 shows the % MH composition of crude as a function of silicon utilization. Zero aluminum silicon produced very high amounts of undesired MH as shown by two replicate runs. When 500 ppm phosphorus was added to the zero aluminum silicon, the amount of MH produced in two replicate runs was the same as that for standard 0.16% aluminum silicon. Phosphorus is an effective replacement for aluminum to improve crude quality (higher amounts of Di and lower amounts of MH). Forms of Aluminum. The addition of a 1000 ppm Cu-Al alloy to standard silicon with 0.16% aluminum content had no effect. Crude quality and rate remained unchanged. Aluminum powder was used in place of a Cu-Al alloy in one run of low aluminum silicon, but it was not as effective as a Cu-Al alloy. Presumably, rapid formation of AlCl3, followed by vaporization out of the reactor bed, would preclude aluminum powder from being an effective promoter. Cu-Al may decompose to AlCl3 under MCS conditions more slowly than metallic Al, thus explaining the improved promoter performance of Cu-Al vs Al. Investigation of different forms of phosphorus was beyond the scope of the laboratory experiments. Solid Cu3P is the only form of phosphorus that was used commercially. Triethylphosphine [(CH3CH2)3P] was used in the reported laboratory experiments. GE knows that many forms of phosphorus including Cu3P have the same positive effect on laboratory MCS reactions. Origin of the Low Aluminum Effect. One observation in both commercial low aluminum silicon trials was the increased accumulation of zinc in the reaction mass. This observation was partially masked by significant changes in commercial zinc addition rates in response to the increased accumulation. Another significant commercial reactor parameter indicated the lack of zinc movement during both trials. Our working hypothesis was that aluminum played a role in increasing the vapor pressure of the key MCS catalyst metals, namely, copper and zinc. A detailed development of this hypothesis was published by Oye et al.16 Movement of copper and zinc by the vapor phase within the MCS reactor is important. Five vaporpressure experiments were run to investigate the hypothesis: (i) ZnCl2 only; (ii) CuCl only; (iii) ZnCl2 + CuCl (50:50 mole ratio); (iv) AlCl3 + CuCl (50:50 mole ratio); (v) AlCl3 + ZnCl2. A small amount (ca. 50 mg) of ZnCl2 was placed in a sealed tube under vacuum. The end of the tube containing zinc chloride was heated to ca. 150 °C, and the other end of the tube was cooled with liquid nitrogen. The tube was then broken open and the cold end analyzed for the presence of metal. Four additional experiments were run with the combinations listed above. Zinc chloride and copper chloride were only found at the cold end of the tube when aluminum chloride was present. We speculate that the more volatile copper or zinc derives from formation of chlorobridged dimers with aluminum chloride as shown in Figure 15.

Figure 15. Possible forms of volatile forms of CuCl and ZnCl2 in the presence of AlCl3.

CuCl was included in the experiments because copper is the primary MCS catalyst. Phosphorus and tin (both are MCS promoters) were not included in the vaporpressure experiments. The composition of phosphorus (when used) and tin in commercial reaction masses is an order of magnitude less than that of copper, zinc, and aluminum. Future metal chloride vapor-pressure experimentation could include phosphorus, tin, and other trace elements present in silicon such as calcium, iron, and titanium. GE elected to not perform quantitative experiments to measure the vapor pressure of the CuCl-ZnCl2AlCl3 system. Results from the qualitative experiments were sufficient to confirm observations from the commercial trials to propose the hypothesis described in this section. Conclusions Two commercial trials were recently run at GE Silicones with low aluminum silicon (0.06-0.08% furnace tap average). Crude quality quickly degraded to extremely low levels of Di and very high levels of Si-H compounds (M2H and MH). Specific laboratory reactor runs were made in an attempt to explain the commercial trial results. The hypothesis investigated in the laboratory was that aluminum is a critical component to successful operation of the direct process by its interactions with zinc (and other metals). Aluminum increases the vapor pressure of zinc, which, in turn, allows zinc to be mobile. Insufficient amounts of aluminum results in low zinc vapor pressure and the inability to transfer zinc to desired reaction sites. Laboratory results indicated that some actions of aluminum can be achieved by the use of phosphorus. An experiment was run to measure the vapor pressure of zinc and copper with and without the presence of aluminum. The experiment confirmed the hypothesis that aluminum increases the vapor pressure of zinc. The experiments did explain the commercial trial results. Zinc is a good catalyst for cracking methyl chloride. Use of the 0.06-0.08% Al silicon did not provide sufficient aluminum to provide zinc mobility. As a result, zinc present in the reaction mass acted to crack methyl chloride on a higher degree than participated in the MCS reaction. Excess methyl chloride cracking resulted in the production of high amounts of Si-H compounds and Tri along with high levels of hydrocarbons. A byproduct of methyl chloride cracking is carbon which accumulated in the reaction mass at very high levels. High levels of carbon contributed to the “death spiral” of the reactor as copper and promoters were deposited on the surface of carbon instead of on the surface of fresh silicon. These metals deposited on the carbon surface contributed to the methyl chloride cracking reaction. GE has used the results of the low aluminum silicon trials described in this paper to diagnose subsequent


problems with commercial reactor operation related to silicon quality. The results show that an insufficient aluminum content of silicon metal provides a unique “fingerprint”, which is useful when commercial reactor problems occur. Acknowledgment The plant trials could not have been performed without the skilled assistance of Ed Mash, Tom Kotkoskie. and Ed Raymond at GE Silicones. The vaporpressure measurements were carried out by Jim Carnahan at GE Global Research. Literature Cited (1) Rochow, E. G. The Direct Synthesis of Organosilicon Compounds. J. Am. Chem. Soc. 1945, 67, 963. (2) Lewis, L. N. Recent Advances in the Direct Process. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; John Wiley & Sons: 1998; Vol. 2, Chapter 26, p 1581. (3) Seyferth, D. Dimethyldichlorosilane and the Direct Synthesis of Methylchlorosilanes. The Key to the Silicone Industry. Organometallics 2001, 20, 4978-4992. (4) Kanner, B.; Lewis, K. M. Catalyzed Direct Reactions of Silicon; Lewis, K. M., Rethwisch, D. G., Eds.; Elsevier: Amsterdam, The Netherlands, 1993; p 1. (5) Gilliam, W. F. Preparation of Dialkyl-Substituted Dihalogensilanes. U.S. Patent 2,464,033, 1949. (6) Ward, W. J.; Ritzer, A.; Carroll, K. M.; Flock, J. W. Catalysis of the Rochow Direct Process. J. Catal. 1986, 100, 240.

(7) Halm, R. L.; Peirce, A. B.; Wilding, O. K. Method for Preparation of Alkylhalosilanes. U.S. Patent 4,762,940, 1990. (8) Halm, R. L.; Wilding, O. K. Method of Direct Process Performance Improvement via Control of Silicon Manufacture. U.S. Patent 4,898,960, 1990. (9) Hurd, D. T. Preparation of Alkylhalogenosilanes. U.S. Patent 2,427,605, 1947. (10) Margaria, T. Proceedings of the Silicon for Chemical Industry, Loen, Norway, June 1994; pp 69-80. (11) Rong, H. M. Ph.D. Thesis, Institute of Inorganic Chemistry, The Norwegian Institute of Technology, Trondheim, Norway, 1992. (12) Rong, H. M.; Sorheim, H. In Silicon for the Chemical Industry III; Oye, H. A., Rong, H. M., Ceccaroli, B., Nygaard, L., Tuset, J. Kr., Eds.; Trondheim, Norway, 1996; p 199. (13) Gasper-Galvin, L. D.; Rethwisch, D. G.; Sevenich, D. M.; Friedrich, H. B. In Catalyzed Direct Reactions of Silicon; Lewis, K. M., Rethwisch, D. G., Eds.; Elsevier: Amsterdam, The Netherlands, 1993; p 279. (14) Joklik, J.; Bazant, V. Collect. Czech. Chem. Commun. 1961, 26, 417. (15) Lewis, L. N.; Ward, W. J. Ind. Eng. Chem. Res. 2002, 41 (3), 397-402. (16) Hoel, J.-O.; Frank, R.; Rong, H. M.; Oye, H. A. In Silicon Chemical Industry IV; Oye, H. A., Rong, H. M., Nygaard, L., Schussler, G., Tuset, J. Kr., Eds.; Trondheim, Norway, 1998; pp 201-215.

Received for review May 6, 2002 Revised manuscript received March 4, 2003 Accepted March 6, 2003 IE020334S

Effect of Low Aluminum Silicon on the Direct Process - Industrial

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