Research Note pubs.acs.org/IECR
Synthesis of SiCl4 from Gaseous HCl and Si(OMe)4. Reaction Development and Kinetic Studies John M. Roberts,* Donald V. Eldred, and Dimitris E. Katsoulis Dow Corning Corporation, 2200 W. Salzburg Road, Auburn, Michigan 48611, United States S Supporting Information *
ABSTRACT: This report describes a method to synthesize SiCl4 from alkyl orthosilicates and gaseous HCl. Reacting tetramethyl orthosilicate with HCl gas at 0 °C in the presence of a catalytic amount of hexamethyl phosphoramide (10 mol %) and four equivalents of acetonitrile afforded an 86% yield of SiCl4 after 6 h. Exchange between HCl and alkoxy groups on silicon during the reaction generates methanol. The methanol then reacts with acetonitrile (in the presence of HCl) to form an imidate, thus removing it from the reaction mixture. Other Lewis acid and base catalysts were also observed to accelerate the reaction. A kinetics experiment using 29Si NMR to monitor reaction intermediates in situ was conducted to determine the rates of chloride/ alkoxide exchange on silicon.
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INTRODUCTION Developing new, energy-efficient routes to chlorosilanes remains an elusive goal for the polysilicon and silicone industries. Traditionally, the first step in chlorosilane production has been the reaction of zerovalent silicon with either methyl chloride (the Rochow−Müller “Direct Process” reaction to produce methylchlorosilanes)1 or HCl (to produce HSiCl3 and SiCl4).2 The zerovalent silicon (“metallurgical grade” elemental silicon is mainly used industrially) is in turn produced via an energy-intensive carbothermic reduction of a mineral silicon source (typically SiO2). Research has shown that silica (and other mineral silicon sources) can be converted to tetraalkyl orthosilicates (which refer to compounds of the composition Si(OR)4 in this paper) and related compounds under comparatively mild conditions despite these minerals generally being unreactive.3−14 Orthosilicates such as tetramethyl orthosilicate (TMOS) and tetraethyl orthosilicate (TEOS) are indispensable for sol−gel chemistry15 but find limited application in the greater silicone/polysilicon industry. Unfortunately, silica cannot be directly converted to the corresponding chlorosilane, SiCl4, which finds usage as both a raw material in the synthesis of high-purity polysilicon16−19 and high-purity silica (used as a filler in the silicone industry).20 SiCl4 is currently produced as a byproduct of the polysilicon industry2 or from the chlorination of ferrosilicon with chlorine.21 While robust, these methods to produce SiCl4 rely on the aforementioned carbothermic reduction. Thus, a lowenergy route to SiCl4 would be desirable to the polysilicon/ silicone industry from both an economic and environmental perspective. As low-energy routes to orthosilicates from silica already exist, if it was possible to chlorinate an orthosilicate to give SiCl4, a two-step, redox neutral process to access SiCl4 from silica could be realized (Scheme 1). The chlorination of various alkyl-substituted orthosilicates (RxSi(OR′)4‑x) has been known for over 50 years,22−31 and two reports describe the chlorination of tetraalkyl orthosilicates22,32a more difficult transformation than chlorinating related alkyl-substituted © 2016 American Chemical Society
Scheme 1. Two-Step Synthesis of SiCl4 from SiO2
orthosilicates. However, the reagents which can currently chlorinate orthosilicates via known methods (e.g., SOCl2, acetyl chloride, oxalyl chloride) are prohibitively costly on a large scale. Thus, a cheaper chlorinating agent is needed. Hydrogen chloride gas would be the most desirable chloride source for orthosilicate chlorination from a cost perspective and because polysilicon/silicone firms produce HCl on large scales (HCl is generated from chlorosilane hydrolysis−condensation to form siloxane polymers in the silicone industry and from HSiCl3 decomposition to polysilicon in the polysilicon industry).2,33 Few reports detail the reaction of orthosilicates with HCl, and challenges surrounding its usage include removing alcohol byproducts to prevent reversion of SiCl4 to the orthosilicate starting materials.22,32,34 This communication details an innovative synthesis of SiCl4 by reacting tetraalkyl orthosilicates with gaseous HCl and removing byproduct methanol (MeOH) from the reaction mixture.
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EXPERIMENTAL SECTION General Information. All reactions were carried out under a nitrogen atmosphere with magnetic stirring unless otherwise stated. All reagents were purchased from Aldrich and used as received unless otherwise stated. Proton-decoupled 13C NMR spectra were recorded on a Varian Inova 400 spectrometer with a 13C operational frequency of 100 MHz and are reported in ppm using solvent as an internal standard (CDCl3 at 77.16 ppm and C6D6 at 128.6 ppm). 29Si NMR spectra were recorded on Varian Mercury 400, Varian Inova 400, and Agilent DD2 500 NMR spectrometers with 29Si operational frequencies of 80 and Received: Revised: Accepted: Published: 1813
December 10, 2015 January 26, 2016 January 28, 2016 January 28, 2016 DOI: 10.1021/acs.iecr.5b04720 Ind. Eng. Chem. Res. 2016, 55, 1813−1818
Research Note
Industrial & Engineering Chemistry Research
Table 1. Reaction of Si(OMe)4 with Gaseous HCl (50 sccm) for 3 h at 110° C (Yield in mol % from 29Si NMR)
a
entry
Cat.
SiCl4
SiCl3(OMe)
SiCl2(OMe)2
SiCl(OMe)3
Si(OMe)4
1 2 3 4a
FeCl3 ZnCl2 PPh3
0 0 0 0
0 0 0 0
0 0 0 0
5 0 0 28
95 3 46 72
Reaction ran 2.25 M in toluene under Dean−Stark conditions, 10 mol % cat.
through a bubbler. To a stirred solution of Si(OMe)4 (8 mL, 54.7 mmol) and acetonitrile (56.4 mL, 875.2 mmol) was added HMPA (0.96 mL, 5.47 mmol). HCl gas (50 to 40 sccm) was introduced from the sparge tube. A 2 mL sample of the reaction mixture was removed every hour, added to 3 mL of CDCl3, and 0.1 mL of SiEt4 was added. 29Si NMR was immediately taken. After every sample was taken the flow of HCl was decreased 2 sccm to keep the flow to volume ratio constant. The reaction stirred in this manner for 7 h.
100 MHz and are reported in ppm using SiEt4 (7.35 ppm) as an internal standard. GC−MS work was done on an Agilent 7890A GC with a 5975C MSD attached. An Agilent 190915433 30 m 250 μm × 0.25 μm LTM column was used. The conditions for the method used were as follows: oven held constant at 50 °C, the LTM ran for 1.9 min at 50 °C, then ramped at 140 °C/min to 200 °C, then held for 4 min at 200 °C for a total run time of 7 min. TMOS Chlorination without MeCN. The reaction was carried out in a 200 mL 3-neck 24/40 round-bottom flask. One inlet contained a glass gas sparge tube that was connected to a rotameter, which was in turn connected to an HCl cylinder. Another inlet was connected to a Dean-Strak trap and then to a reflux condenser. The condenser was connected to an empty Airfree bubbler which was cooled to −78 °C to trap any volatiles that remained in the reaction mixture. The trap was connected to a manifold under nitrogen, which vented to atmosphere through another bubbler. To a stirred solution of Si(OMe)4 (25 mL, 169 mmol) and toluene (75 mL) was added PPh3 (4.43g, 16.9 mmol). The solution was warmed to 130 °C, and then a stream of HCl (50 sccm) was added to the reaction via the sparge tube. The reaction mixture was stirred for 3 h. The contents of the trap were analyzed by 29Si NMR but not isolated. TMOS and SiCl(OMe)3 were observed in a molar ratio of 72:28 (see Supporting Information Figure S12). TMOS Chlorination with MeCN. The reaction was carried out in a 100 mL 2-neck 24/40 round-bottom flask. One inlet contained a glass gas sparge tube that was connected to a rotameter, which was in turn connected to an HCl cylinder. The other inlet was connected to a reflux condenser. The condenser was connected to an empty Airfree bubbler which was cooled to −78 °C to trap any volatiles that left the reaction mixture. The trap was connected to a manifold under nitrogen, which vented to atmosphere through another bubbler. To a stirred solution of Si(OMe)4 (4 mL, 27.35 mmol) and acetonitrile (23.4 mL, 437.6 mmol) was added hexamethyl phosphoramide (HMPA) (0.48 mL, 2.73 mmol). The reaction mixture was cooled to 0 °C and then HCl gas (25 sccm) was introduced from the sparge tube. The reaction stirred in this manner for 6 h. After 6 h the contents of the trap were reintroduced to the flask and an internal standard (SiEt4, 1 mL, 5.29 mmol) was added. An aliquot was taken, diluted with CDCl3, and 29Si and 13C NMR were immediately taken. An 85% yield of SiCl4 was observed, along with a trace amount of SiCl3(OMe) (see Supporting Information, Figure S20). SiCl4 was not isolated from the reaction mixture. Kinetics Experiment. The reaction was carried out in a 100 mL 2-neck 24/40 round-bottom flask. One inlet contained a glass gas sparge tube that was connected to a rotameter, which was in turn connected to an HCl cylinder. The other inlet was connected to a reflux condenser. The condenser was connected to a manifold under nitrogen, which vented to atmosphere
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RESULTS AND DISCUSSION The first reactions performed toward this goal were carried out using TMOS as the orthosilicate, as it is generally the most Scheme 2. Imidate Formation from MeOH, HCl, and MeCN
reactive orthosilicate and can be produced using a major feedstock of the silicone industry, MeOH.5,8,13 Our initial hypothesis was that if we could remove MeOH from the reaction mixture we could drive the reaction to completion. We envisioned accomplishing this by running the reaction at reflux or by using a Dean−Stark trap. To facilitate exchange between the methoxide and chloride on silicon, we also planned to evaluate different Lewis acid and base catalysts. During these scouting studies no internal standards were used, the results are ratios of products observed by 29Si NMR and can be seen in Table 1. As can be seen in Table 1, very little chloride−methoxide exchange is observed, both in the neat reactions (entries 1−3) and the reaction that ran in toluene under Dean−Stark conditions (entry 4). 29Si NMR and GC−MS for Table 1 can be seen in the Supporting Information, Figures S8−S12. When the reaction was run neat with no solvent or catalyst (entry 1), trace SiCl(OMe)3 was produced. This indicates there was some base level of chloride/methoxide exchange (some condensation product was also observed). Employing a catalyst (FeCl3) in a neat reaction (entry 2) was not successful and resulted in oligomeric orthosilicates ((MeO)3SiOSi(OMe)3, (MeO)3SiOSi(OMe)2OSi(OMe)3, etc.nonhydrolytic condensation products detected via 29Si NMR). This phenomenon is known when orthosilicates are combined with Lewis acids, resulting in siloxane bond formation and methyl chloride as a byproduct.35 methyl chloride was observed in the reaction mixture by GC−MS when performing these reactions. When a weaker Lewis acid was used (ZnCl2, entry 3) roughly the same result was obtained as was observed in entry 2. Using a Lewis base catalyst, triphenylphosphine, under the Dean−Stark conditions saw the amount of SiCl(OMe)3 produced increase 1814
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Table 2. Reaction of Si(OEt)4 with Gaseous HCl (25 sccm) for 6 h. Reactions Either Run in a Solvent (0.25 M) or Neat, Yield (mol %) Determined by 29Si NMR
entry
MeCN (equiv)
solvent
temp. (° C)
SiCl4
SiCl3(OEt)
SiCl2(OEt)2
SiCl(OEt)3
1 2 3 4
8 8 16 16
toluene 1,4 dioxane
65 65 65 0
0 0 0 5
12 9 21 75
58 47 68 18
5 6 0 2
Table 3. Reaction of Si(OMe)4 with Gaseous HCl for 6 h under a Nitrogen Environment. Yield (mol %) Determined by 29Si NMR
a
entry
MeCN (equiv)
Cat.
temp (° C)
HCl (sccm)
SiCl4
SiCl3(OMe)
SiCl2(OMe)2
SiCl(OMe)3
1 2 3 4 5 6 7 8
16 16 16 16 8 8 8 8
LiCl ZnCl2 FeCl3a HMPA HMPA HMPA HMPA HMPA
0 0 0 0 0 −17 0 0
25 25 25 25 25 25 50 10
99 31 50 86 28 37 50 9
0 58 50 14 70 61 26 69
0 0 0 0 2 0 16 13
0 0 0 0 0 0 8 8
Reaction ran with 1 mol % catalyst.
Scheme 3. Reaction of TMOS with Ethyl Isocyanate
critical to the success of the reaction and that another method to remove MeOH was needed. Next, we hypothesized that MeOH could be removed from the reaction mixture by reacting it with a chemical “trap”. The product from trapping the MeOH would ideally not boil under the reaction conditions, be easily hydrolyzed to regenerate methanol, and be inexpensive to regenerate. In reviewing the literature we found that MeOH could be added to acetonitrile (MeCN) in the presence of HCl to form an imidate salt (Scheme 2),36 thus, we decided to test our hypothesis by using MeCN as a trap and/or solvent. Reaction optimization using TEOS as the orthosilicate can be seen in Table 2 (TEOS was used vs TMOS because the mixed chloro-ethoxysilanes are less volatile than their chloro-methoxysilane counterparts). The spectra for Table 2 can be seen in the Supporting Information in Figures S13−S16. When attempting to optimize reaction conditions several different solvents (and equivalents of MeCN per reaction) were screened. When a 2-fold excess of MeCN was used, toluene and dioxane showed little difference in terms of product selectivity (entries 1 and 2). Notably, the degree of chlorination was drastically increased when MeCN was added to the reaction mixture (vs results in Table 1) and the presence of the imidate was confirmed by 13C NMR (Figure S26). When the solvent was removed and the number equivalents of MeCN increased to a 4-fold excess (16 equiv) the amount of SiCl3(OEt) doubled and the total chlorination also increased (entry 3). When the temperature was lowered to 0 °C, 5 mol % of SiCl4 was formed. Gasses are more soluble at lower temperatures so increased HCl solubility at 0 °C could
Figure 1. Relative concentration of silicon species in the chlorination reaction as a function of time. Relative concentrations determined via 29 Si NMR using an internal standard (SiEt4).
Table 4. Rate Constants for the Formation of Intermediates and SiCl4 (Rate Constants Shown in h−1) reactant
SiCl(OMe)3
SiCl2(OMe)2
SiCl3(OMe)
SiCl4
rate
2.5
0.99
0.46
0.12
to ∼30:70 (vs TMOS). It is unclear whether the increase in chlorination was due to the Lewis base catalyst or the Dean− Stark conditions. At this point, though, it became apparent that effectively removing MeOH from the reaction mixture was 1815
DOI: 10.1021/acs.iecr.5b04720 Ind. Eng. Chem. Res. 2016, 55, 1813−1818
Research Note
Industrial & Engineering Chemistry Research Scheme 4. Proposed Reaction Mechanism
Scheme 5. Recycling of Reaction Byproducts
play a role in the increased reactivity observed (vs 65 °C). Having optimized some reaction conditions, we next sought to evaluate different catalysts, HCl flow rates, and reaction temperatures in the hope of further increasing the amount of SiCl4 produced over a 6 h time span. Shown in Table 3 are catalyst comparison studies; the spectra associated with these results can be seen in Figures S17−24 in the Supporting Information. As seen in Table 2, this reaction can proceed without any catalyst. However, we hypothesized that by adding a catalyst which increases the rate of exchange between chloride and methoxide on silicon, we could accelerate the rate of reaction. We also switched back to TMOS as the orthosilicate as the reaction was now being run at lower temperatures. Entries 1−3 show Lewis acid catalysts, all which give greater amounts of SiCl4 than the uncatalyzed reaction, including quantitative conversion in the case of LiCl (entry 1). FeCl3 was an effective catalyst at loadings as low as 1 mol % (vs 10 mol % for entries 1 and 2) and still afforded high levels of SiCl4. However, analyzing the reaction mixture using 29Si NMR was problematic in the presence of paramagnetic FeCl3 and the possibility of nonhydrolytic condensation with Lewis acid catalysts remained. Thus, we switched to a Lewis base catalyst. Entry 4 shows this result, which afforded an 86% yield of SiCl4 after 6 h of being reacted with HCl (25 sccm) and MeCN (16 equiva 4-fold excess) at 0 °C when using HMPA as a catalyst. With conditions in hand that gave a good yield of SiCl4, we attempted to further optimize this reaction by first decreasing the equivalents of MeCN in the reaction mixture from 16 to 8 equiv (entry 5). We chose to use HMPA as the catalyst despite LiCl’s superior performance because it negated the possibility of nonhydrolytic condensation. The amount of SiCl4 generated after 6 h sharply decreased from 86 to 28 mol % yield. While the catalyst loading and MeCN equivalents was held constant, the temperature was further decreased to −17 °C (entry 6) which resulted in a modest increase in the amount of SiCl4
produced (vs entry 5). When the HCl feed to the reactor was increased from 25 sccm (1.1 mmol/min) to 50 sccm (2.2 mmol/min, entry 7), SiCl4 production increased by a factor of 1.5. Not surprisingly, when the HCl flow was decreased to 10 sccm the amount of SiCl4 produced in 6 h also decreased (entry 8). Next, we conducted three experiments to probe the scope of the chlorination reaction. We found that a 14% yield of SiCl4 (plus other chlorinated species) could be synthesized using the reaction conditions from “TMOS Chlorination with MeCN” from the Experimental section but substituting MeCN with ethyl isocyanide (Scheme 3) (29Si NMR yield, Figure S25). In addition to TMOS and TEOS being used as substrates for this reaction, Ge(OMe)4 and Sn(OMe)2 were also screened using the reaction conditions from “TMOS Chlorination with MeCN” from the Experimental section. Both GeCl4 and SnCl4 (indicating that an oxidation of tin took place under these reaction conditions) were observed in the reaction mixtures after 6 h by GC−MS (Figures S27 and 28) but these species were not isolated from the reaction mixture to furnish yield data. Future directions in the development of this reaction would focus on reaction engineering to turn the current batch process to a continuous one, scale up the reaction, and ultimately isolate SiCl4. In hope of better understanding this reaction a kinetics experiment was carried out (see Experimental). Samples were taken every hour, and with the aid of an internal standard (SiEt4), the progress of the reaction was tracked using 29Si NMR. A detailed procedure of how the data was worked up can be found in the Supporting Information. Shown in Figure 1 is the relative concentrations of the different reactants as a function of time. This information was then converted into rate constants (via line fitting and assuming a pseudo-first-order approximation) for each subsequent chlorination reaction on the way to forming SiCl4 (Table 4). Anecdotal evidence of the trends shown in Figures 1 and Table 4 can be seen in 1816
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orthosilicate chlorination prior art;22−32 for example, mono and dichlorination of orthosilicates is a relatively facile process vs adding three and finally four chloride groups to an orthosilicates. On the basis of the data obtained during these studies we now propose a reaction mechanism for the chlorination reaction (Scheme 4). In this scheme we propose that HCl adds to an orthosilicate forming first a pentacoordinate silicate intermediate and then the chlorinated product after methoxide leaves. The reaction is driven by the subsequent reaction of byproduct MeOH with MeCN (and another equivalent of HCl) to yield the imidate salt byproduct. This process is repeated with additional equivalents of HCl and MeCN until the silicon is fully chlorinated. Regarding the byproducts of this reaction, the imidate salt can be hydrolyzed with water to form acetamide, methanol, and HCl, of which the latter two can be directly recycled to either the chlorination reaction (in the case of HCl) or TMOS synthesis (in the case of MeOH) (Scheme 5).5,8,13 Acetamide can also be recycled back to MeCN by dehydrating it, although this chemistry is not industrially practiced 37,38 (MeCN is produced as a byproduct of acrylonitrile synthesis).39
CONCLUSION We have demonstrated that orthosilicates can be chlorinated with HCl to yield SiCl4 by adding MeCN to the reaction mixture in the presence of a Lewis acid or base catalyst. The MeCN in the reaction mixture reacts with MeOH (generated from the reaction of HCl and the orthosilicate) to form an imidate byproduct. Removal of MeOH from the system in this manner is essential in driving the reaction to completion. This reaction can take place without a catalyst but does not proceed as quickly. Without the addition of MeCN, however, the degree of chlorination is not of synthetic utility for our desired application. The reaction is best run at 0 °C, presumably due to the increased solubility of HCl at lower temperatures. Kinetic data obtained from 29Si NMR experiments confirm anecdotal results and show that each subsequent chloride addition to an orthosilicate is more difficult than the previous one. Finally, this work has shown that a two-step sequence to synthesize SiCl4 from SiO2 is indeed possible using cheap reagents and mild reaction conditions. ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b04720. Detailed experimental procedures, spectra, and diagrams of reaction setups (PDF)
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REFERENCES
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Research Note
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded internally by Dow Corning Corporation. The Authors would like to thank Dow Corning Analytical Sciences. 1817
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