Synthesis of SiCl4 via the Chloride Salt-Catalyzed Reaction of

Research Note pubs.acs.org/IECR

Synthesis of SiCl4 via the Chloride Salt-Catalyzed Reaction of Orthosilicates with SOCl2 John M. Roberts,* Jessica L. Placke, Donald V. Eldred, and Dimitris E. Katsoulis The Dow Chemical Company, 2200 W. Salzburg Road, Auburn, Michigan 48642, United States S Supporting Information *

ABSTRACT: This paper details a method to chlorinate tetraalkyl orthosilicates in the presence of a catalyst using SOCl2 as the chloride source/deoxygenating agent. Several inexpensive catalysts were screened, and it was found that soluble chloride salts performed better than Lewis base catalysts. The optimized reaction employed a widely used and commercially available soluble chloride salt catalyst (e.g., NBu4Cl, 0.4 equiv), 16 equiv of SOCl2, and afforded quantitative yield of SiCl4 after 3 h. As the bulk of the orthosilicate substrate increased, the yield of SiCl4 decreased. A reaction mechanism has been proposed.

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INTRODUCTION Developing energy-efficient routes to chlorosilanes is an area of ongoing research in the polysilicon and silicone industry. One such silane, SiCl4, can be used as a raw material in the synthesis of high-purity silicon1−4 and silica.5 SiCl4 is currently produced as a byproduct of the polysilicon industry6 or from the chlorination of ferrosilicon with chlorine.7 While robust, these methods to produce SiCl4 require zerovalent forms of silicon as their starting material. These reduced silicon forms (ferrosilicon and elemental silicon) are made via energy intensive carbothermic reductions. Thus, developing more efficient routes to chlorosilanes could result in large cost savings while increasing the sustainability of the silicone and polysilicon industries. We have proposed a two-step, redox neutral process as an alternative to accessing SiCl4 from elemental silicon.8 This route would first convert silica (SiO2) to a tetraalkyl orthosilicate (Si(OR)4) and then chlorinate the orthosilicate to yield SiCl4 (Scheme 1). Silica, despite being generally

orthosilicates using SOCl2 and catalytic pyridinium chloride to generate SiCl4 from tetramethyl orthosilicate (TMOS) but their procedure afforded a mixture of partially chlorinated products and required long reaction times.23 However, this report did highlight an added benefit of using SOCl2, the production of methyl chloride (MeCl) as a byproduct of TMOS chlorination. Methyl chloride is a key raw material for the silicone industry, where it is reacted with elemental silicon in the Rochow−Müller “Direct Process” reaction to form methylchlorosilanes.33 This communication details an improved synthesis of SiCl4 from tetraalkyl orthosilicates and SOCl2 using inexpensive catalysts.

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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 Sigma-Aldrich and used as received unless otherwise stated. Isopropoxy orthosilicate was obtained from TCI. 29Si NMR spectra were recorded on Varian Mercury 400 with a 29Si operational frequency of 80 MHz and are reported in ppm using either SiCl4 (−19.65 ppm), SiMe4 (0.0 ppm) or SiEt4 (7.35 ppm) as internal standards. Example of a Typical Reaction. Table 1, Entry 1. The experimental setup was as follows: A 24/40 100 mL 2-neck flask was prepared by stoppering one neck and connecting the other to a reflux condenser. The reflux condenser was attached to a bubbler that was cooled to −15 °C in a saline/ice bath, and this bubbler was connected to a manifold under a nitrogen atmosphere. To the flask was added SOCl2 (25 mL, 342 mmol, 16 equiv). Then dimethylformamide (0.66 mL, 8.55 mmol, 0.4 equiv) and Si(OMe)4 (3.17 mL, 21.37 mmol, 1 equiv) were

Scheme 1. Two-Step Synthesis of SiCl4 from SiO2

unreactive, can be converted to tetraalkyl orthosilicates and related compounds under relatively mild conditions using inexpensive reagents.9−22 Regarding the second step of the sequence, the chlorination of various orthosilicates has been known for over 50 years.23−32 Recently we showed that orthosilicates can be chlorinated with HCl gas in the presence of a stoichiometric methanol trapping reagent.8 SOCl2 has several advantages over HCl as a chlorinating reagent: it can be used in smaller excess, the absence of protons drastically reduces the potential for condensing orthosilicates, and it is much easier to handle than HCl. A previous report attempted to chlorinate © 2017 American Chemical Society

Received: Revised: Accepted: Published: 11652

August 9, 2017 September 21, 2017 September 25, 2017 September 25, 2017 DOI: 10.1021/acs.iecr.7b03301 Ind. Eng. Chem. Res. 2017, 56, 11652−11655

Research Note

Industrial & Engineering Chemistry Research Table 1. Product Distribution (mol % via 29Si NMR) after 3 h

entry

X equiv SOCl2

SiCl4

SiCl3OMe

SiCl2(OMe)2

1 2 3 4 5a 6b 7c 8d 9e 10f

16 8 6 4.4 4.4 16 16 16 16 16

95 94 80 73 9 29 0 34 15 99

3 4 5 24 57 36 0 45 45 0

0 2 0 3 33 24 0 21 40 0

Scheme 2. Chlorination of TMOS with SOCl2 and DMF. R = OMe or Cl

Reaction 1.0 M in toluene. bReaction ran at 23 °C. cReaction ran with no DMF. dReaction run with 0.1 equiv DMF. eReaction run with 0.04 equiv DMF. fReaction run using 0.4 equiv of the Vilsmeier Reagent instead of DMF. a

added to the flask. The solution was warmed to reflux (90 °C) and stirred at 750 rpm for 3 h. After 3 h the solution was cooled to room temperature, the contents of the trap were added back to the reaction mixture, and an internal standard (SiEt4, 0.2 mL, 1.07 mmol) was added. After this addition, ∼3 mL of the reaction mixture was added to a 15 mm Teflon NMR tube. An amount of ∼3 mL of CDCl3 was also added to the tube. The 29 Si NMR spectrum was immediately recorded. Comparison between integration of the SiEt4 reference peak and SiCl4 (−19.65 ppm) showed a yield of 95% SiCl4.

chlorinated orthosilicates was detected (29% yield, entry 6). The balance of the silicon in all of these reactions was either in the form of SiCl(OMe)3, TMOS, or hexavalent silicate salts. When no DMF was present, the reaction was unsuccessful, affording only trace amounts of SiCl(OMe)3 (entry 7). It was also observed that decreasing DMF loading decreased the yield of SiCl4 (34% and 25%, entries 8 and 9). Finally, to help shed light on the proposed reaction mechanism the Vilsmeier reagent was used as a catalyst (entry 10). Comparable reactivity to DMF was observed (quantitative yield), indicating that the Vilsmeier reagent is likely being formed in situ when DMF is used. This result also suggests that the group transfer on silicon is not facilitated by DMF acting as a Lewis base, and the absence of hexavalent salts suggests the mechanism may be different. The effect of orthosilicate size upon SiCl4 yield was also investigated. Orthosilicates of varying sizes were subjected to the standard reaction conditions (16 equiv SOCl2 and 0.4 equiv of DMF at 90 °C for 3 h) (Table 2). The size of the alkyl

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RESULTS AND DISCUSSION We began our investigation by testing dimethylformamide (DMF) as a catalyst to validate proof of concept work that was recently reported.34 It was hypothesized that by using DMF (vs pyridinium chloride) better yields could be obtained as acidic protons (which can catalyze condensation reactions between orthosilicates) would no longer be present in the reaction mixture. A proposed catalytic cycle is shown in Scheme 2. In the proposed cycle, DMF is first converted to (chloromethylene)dimethyliminium chloride (the Vilsmeier reagent) by SOCl2 (I). The chloride from the Vilsmeier reagent would then add to an orthosilicate, generating methoxide from σ bond metathesis with chloride (II). The methoxide would then add to the Vilsmeier reagent forming an imidate and chloride (III). Finally, the resulting imidate would undergo demethylation by chloride attack on the methyl group to furnish MeCl and regenerate DMF (IV). The results of the initial reaction screen are listed in Table 1. All reactions were run for 3 h at 90 °C. The first reaction was carried out neat with 0.4 equiv of DMF (10 mol % based on methoxide) and used 16 equiv of SOCl2 (4 equiv based on methoxide). A 95% yield of SiCl4 was observed by 29Si NMR (using SiEt4 as an internal standard). Decreasing the excess of SOCl2 resulted in lower yields (94% and 80%, entries 2 and 3). When the reaction became too concentrated, a phase separation in the reaction mixture occurred (73% yield, entry 4). Using a solvent (toluene) for the reaction proved unsuccessful, as diminished chlorination was observed (9% yield, entry 5). Running the reaction at room temperature significantly decreased the yield and a mixture of partially

Table 2. Product Distribution (mol % via 29Si NMR) after 3 h for Various Orthosilicates

entry

R=

SiCl4

SiCl3OR

SiCl2(OR)2

1 2 3 4

Me Et n-Pr iPr

95 31 23 10

3 21 22 14

0 41 55 76

substituent on the orthosilicate was directly linked to the yield of SiCl4 obtained. The difference between methyl and ethyl orthosilicates was quite pronounced (entries 1 and 2, 95% yield for R = Me, 31% for R = Et). When even larger groups were used the yield of chlorinated products decreased even further (entries 3 and 4, 23% yield for R = n-Pr and 10% for R = iPr). These results suggest that bulkier substituents would disfavor the exchange between chloride and alkoxide (Scheme 2, Step 11653

DOI: 10.1021/acs.iecr.7b03301 Ind. Eng. Chem. Res. 2017, 56, 11652−11655

Research Note

Industrial & Engineering Chemistry Research II) and/or dealkylation by chloride to turn over DMF (Scheme 2, Step IV). Next, we screened several Lewis base catalysts with varying nucleophilicity. The results of the screen are shown in Table 3.35 Triphenylphosphine oxide and hexamethyl phosphoramide

Scheme 3. Proposed Catalytic Cycle for the TBAC-Catalyzed Chlorination of TMOS with SOCl2 (R = Cl or OMe)

Table 3. Product Distribution (mol % via 29Si NMR) after 3 h for Different Catalyst

entry

cat.

SiCl4

SiCl3OMe

SiCl2(OMe)2

SiCl(OMe)3

1 2 3 4 5 6 7 8 9 10

DMF HMPA P(O)Ph3 PhS(O)Me C6H5N·HCl NBu4Cl NMe4Cl NBu4Br NMe4F NBu4I

95 85 99 0 63 99 99 99 99 10

3 0 0 0 28 0 0 0 0 15

0 0 0 4 9 0 0 0 0 71

0 0 0 36 0 0 0 0 0 4

decreased SiCl4 yields, as did employing Lewis base and Lewis acid catalysts (vs chloride salts). Solvents, lower catalyst loadings, and a smaller excess of SOCl2 in the reaction mixture also led to reduced yields. Exchange between chloride and alkoxide groups on silicon and the subsequent trapping of the alkoxide with SOCl2 drives the reaction to completion. The species formed from the reaction of the alkoxide with SOCl2 is easily dealkylated by excess chloride to yield MeCl (when TMOS is the substrate), which can be easily recycled to produce other silicone products.

(HMPA) were likely deoxygenated by SOCl2 under the reaction conditions forming soluble chloride salts. Lewis base precatalysts that could be deoxygenated by SOCl2 (entries 1−3, 95%, 85%, and 99% yield, respectively) were more reactive than those that could not be (entry 4, no conversion to SiCl4). Lewis acids did show some reactivity but are inferior catalysts and may also promote condensation side reactions.36 Pyridinium chloride, the catalysts in the initial report, showed decreased reactivity in our system when compared to aprotic chloride sources (entry 5, 63% yield). Given the observed trend that chloride salts formed in situ were more effective catalysts (vs Lewis bases), we now hypothesized that chloride was the actual catalyst and that the presence of an extra electrophile (like the Vilsmeier reagent) was unnecessary. An additional benefit of moving to tetrabutylammonium chloride (TBAC) or similar catalysts is they are much less harmful than HMPA or other Lewis bases. To test this hypothesis, TBAC was used as a catalyst (entry 6). Quantitative conversion of TMOS to SiCl4 was observed. Tetramethylammonium chloride gave similar results, as did tetrabutylammonium bromide and tetramethylammonium fluoride (entries 7−9). The only halide salt that did not work in the reaction was tetrabutylammonium iodide (entry 10, 10% yield). Combining these new results with our initial hypothesis led us to propose the following catalytic cycle (Scheme 3). The catalytic cycle begins with a reversible addition of chloride from TBAC to an orthosilicate yielding a pentavalent intermediate (I) and forming a Si−Cl bond. Methoxide can also leave from the pentavalent intermediate, but it is trapped by SOCl2 (II and III). Dealkylation of the resulting intermediate (IV) by chloride furnishes SO2, MeCl, and regenerates TBAC.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b03301. Detailed experimental procedures and spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

John M. Roberts: 0000-0001-7733-6301 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded internally by Dow Corning Corporation, a wholly owned subsidiary of The Dow Chemical Company. The authors would like to thank Dow Corning Analytical Sciences.

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CONCLUSION We have shown that orthosilicates (primarily TMOS) can be chlorinated with SOCl2 at moderate temperatures. The key to making the chlorination reaction higher yielding was using a catalytic amount of halide, whether in the form of a quaternary ammonium halide salt or that generated by the reaction of a Lewis base precatalyst with SOCl2. Bulky orthosilicates led to

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DOI: 10.1021/acs.iecr.7b03301 Ind. Eng. Chem. Res. 2017, 56, 11652−11655