Communication pubs.acs.org/JACS
Silicone Electrosynthesis from Silica Raw Materials at Room Temperature Jeffrey E. Dick§ and Daesung Chong* Department of Chemistry, Ball State University, Muncie, Indiana 47305, United States S Supporting Information *
Scheme 1. Summary of the Current Industrial Production of Silicones through the Carbothermic Reduction of Silica,1,6,7 Followed by Rochow’s Direct Synthesis for the Production of Organosilicon Compounds5
ABSTRACT: Here, we describe a method of synthesizing silicones at room temperature in a one step, one pot electrochemical reaction. The current energy intensive method to produce silicones involves carbothermic reduction at temperatures exceeding 2000 °C over coal, which consequentially produces environmentally harmful gases, CO and CO2. Even after this, the metallic Si is subjected to follow up chemical reactions described by Eugene Rochow in 1945 to produce the final silicones. The electrochemical synthesis is achieved by bulk cathodic electrolysis of methanol in the presence of raw SiO2 materials (sand or quartz) at Eappl = −2.7 − −3.0 V vs Fc/Fc+ couple in two organic solvent/electrolyte systems. Electrodes are a nickel wire cloth, a nickel−chromium wire, a graphite plate, or a Pt basket. The major products in order of relative yields were octamethylcyclotetrasiloxane (D4), hexamethylcyclotrisiloxane (D3), and decamethylcyclopentasiloxane (D5). Ranges for relative yields, or the ratio of the D3, D4, or D5 to the total amount of silicones produced, are 17−28%, 64−80%, and 5−8%, respectively. We also provide evidence for a possible mechanistic pathway involving the initiation of a methyl radical followed by radical propagation to the final products through the intermediate dimethyldimethoxysilane. To the best of our knowledge, this is the first report of the reduction of silicon-containing raw materials at room temperature to yield silicones.
One of the greatest challenges, therefore, in silicon chemistry is to discover low cost and sustainable methods to synthesize silicones from earth-abundant, silicon-containing raw materials. Here, we present an electrochemical method for the direct production of silicones from silica (SiO2: sand or quartz) at room temperature. We found that silicones can be directly manufactured from raw materials in a reusable organic solvent and electrolyte system with an electrochemically reducible alkylating agent, such as methanol, for the production of cyclic polydimethylsiloxanes (PDMS) and methylmethoxysilanes (Scheme 2). Most importantly, this methodology completely bypasses the production of metallurgical grade silicon from excessive burning of coal and, therefore, drastically cuts the energy cost and CO/CO2 emissions. This process (Scheme 2) also eliminates the use of chloride/hydrochloric acid cycle used in the direct synthesis. Our results demonstrate that elevated temperatures are not required to produce silicones and that electrochemistry provides a pivotal foundation upon which the conversion of other raw materials to useful products can be potentially built. Recently there has been growing interest in the use of tetramethoxysilane (Si(OMe)4, TMOS)8,9 and other silanes, like dimethyldimethoxysilane, (SiMe2(OMe)2, DMDMS)10,11 as valuable precursors to silicones, arising from the efforts to bypass the carbothermic reduction process;7,9,12 however, elevated temperatures, reflux, and reaction workup are still required. We demonstrated that TMOS (Epc = −2.9 V vs ferrocene, (C5H5)2Fe0/+, Figure 1) is reducible at room temperature under similar conditions to what is presented in this report,13 producing hexamethoxydisiloxane as the major product and DMDMS as the minor product (see GC-MS data in Supporting Information (SI)). Chibiryaev et al. reported the
T
he pursuit of environmentally benign and low-cost methods for the production of useful materials from earth-abundant raw materials is of vital importance in our growing society. One such useful material is silicone, and other organosilicon compounds which are used widely in many common household materials, including paints,1 personal care products,1 electronics,1 textiles,1 glass,2 ceramics,2,3 and potentially, lithium-ion batteries.4 Bulk production of silicones follows the carbothermic reduction process, where raw SiO2 (∼60% of Earth’s crust) is reduced to elemental silicon, which is then reoxidized to methylchlorosilanes at 200−400 °C.5 Condensation of these products ultimately produces silicones.5 Carbothermic reduction relies on the burning of coal at temperatures exceeding 2000 °C, releasing environmentally and physiologically harmful CO and CO2 gases;1,6 moreover, the follow-up reactions require copper and zinc promoters and temperatures between 200 and 400 °C5 as shown in Scheme 1. © XXXX American Chemical Society
Received: February 6, 2014
A
dx.doi.org/10.1021/ja5011724 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Journal of the American Chemical Society
Communication
on different electrode materials in both THF and MeCN (Table 1 in SI). Throughout electrolysis, peaks in GC-MS began to emerge, the most prominent one being [M−CH3+] = 281, which corresponded to octamethylcyclotetrasiloxane, D4, and then [M−CH3+] = 208, which corresponded to hexamethylcyclotrisiloxane, D3, followed by an [M−(Me2SiO−H)+] of 295, which is attributed to decamethylcyclopentasiloxane, D5, formed by decomposition during ionization. We directly compared the data of D3, D4, and D5 and DMDMS products in GC-MS with authentic samples of D3, D4, and D5 (Gelest, 99.99%) to ensure the mass fragmentation and retention times were identical. It is of interest to note that the product distribution of D3, D4, and D5 cyclic silicones in GC-MS consistently fell in the range of 17−28%, 64−80%, and 5−8%, respectively, despite changing the electrode material and applied potential. The isolated product was dissolved in deuterochloroform (CDCl3, Cambridge Isotope Laboratories, TMS free, 99.96%). 1 H NMR chemical shifts consistently were around the following values: δ = 0.0738, 0.0619, and 0.0471 ppm, which agree well with Si−CH3 moieties in siloxane and silane chemical shift data. There were up to three prominent peaks in 29Si NMR at δ = −21.0, −19.8, and −8.66, which agree well with data for D5, D4, and D3, respectively.17 FT-IR data showed a strong ν(Si−C) stretch at 1261 cm−1 and a strong ν(Si−O) stretch at 1080 cm−1. All silicone data reported in this work corresponded well with accepted values, as cited, authentic samples run in-house, or NIST library values. Although methanol (MeOH) has been known as a methylation agent18 and fuel source via its decomposition,19 the cathodic reduction process in organic solvent systems has not yet been well studied. Cyclic voltammetry (CV) at Ni disk electrode shows a chemically irreversible reduction event at Epc = −2.54 V vs ferrocene (scan rate, v = 0.5 V s−1, SI) at room temperature. Bulk cathodic electrolysis of 2 mM MeOH at Eappl = −2.9 V in THF/0.1 M [NBu4][OTf] at room temperature was carried out, and 2 F/equiv was observed. We carried out the cathodic reduction of MeOH under these conditions and attempted to apply this cathodic reduction system in the presence of various silica sources (SiO2 sand and spectrophotometric quartz cuvettes). The electrolysis of MeOH with silica provided polydimethylsiloxanes as major products and DMDMS as the minor product in accordance with the characterizations of the reaction products by GC-MS, FT-IR, and NMR data. Other methylmethoxysilane products (Scheme 2) were also detected in GC-MS; however, we deem these products as unquantifiable because of their low intensities in GC-MS. No observable peaks for these trace species were observed in 29Si NMR. We now turn to mechanistic considerations of the cathodic reduction of methanol in the presence of some SiO2 source. Surface-esterification reaction of silica surfaces is welldefined. For instance, the silica surface functionalization from silanol groups on the silica surface, Si−OH, with a variety of alcohols to surface-alkoxylated silica, Si−OR, under autoclave conditions has been reported some time ago.20 A base (KOH)-catalyzed formation of TMOS from silica with dimethylcarbonate at 320 °C has been reported.15 The direct synthesis of methylmethoxysilane from metallic silicon and MeOH (∼83 kPa) with Cu(I) catalyst at 380−450 °C is also well-known.21 However, a chemical reaction of methanol with
Scheme 2. Summary of the New Electrosynthetic Route from Silica to Silicones and Methylated Silanes through Bulk Cathodic Reduction of Methanol at Room Temperature
formation of TMOS from the reaction of methanol with raw silicon-containing materials at 350 °C.8 Furthermore, Schattenmann et al. reported reduction of TMOS to form MeSi(OMe)3 at 300 °C over NaH.9 Resulting from this and coupled with the electrochemical reducibility of TMOS11 at room temperature, we believed that the reduction of raw materials, such as sand and quartz, should be possible through the reduction of methanol as the limiting reagent. There are other possible methyl sources, such as methyl iodide14 and dimethylcarbonate.15 In terms of cost and environmental friendliness, however, methanol is a better methylation agent compared to the aforementioned alternatives. Reproducibly, there were no silicones or silanes detected from the pre-electrolysis sample, silica (sand or quartz)/ MeOH/THF (or MeCN)/[NBu4][OTf] (e.g., see GC-MS backgrounds in SI). When the solution was bulk electrolyzed at Eappl = −2.7 − −3.0 V at a nickel wire cloth, a nickel− chromium wire, a graphite plate, or a Pt basket, the concentration of MeOH (monitored voltammetrically) fell to nearly zero after ∼2 h, with a total coulomb count of ∼avg. 0.3 F/equiv of MeOH (see Table 1 in SI). The low coulomb count suggests that, after initial cathodic reduction of MeOH, radical chain processes are involved in the net conversion of silica to methylated organosilicon products. It is well-known that accurate coulomb counts can only be generated with electrochemical reactions that have no follow up chemical reactions.16 In our system, this is not the case, which further leads to a reduction in the theoretical coulomb count. The mechanistic aspects are discussed later (vide inf ra). Each product was first characterized using GC-MS. Percent conversion data were calculated based on the −OSi(Me)2 moiety and the amount of methanol consumed in the reaction, which was 100%, and the concentrations of products were calculated based on a GC-MS calibration curve. The conversion in terms of silicon consumption is low, but we believe that our process is much more energy efficient and environmentally friendly than the state of the art. The synthesis of silicones using this electrochemical method provided yields of 56−74% B
dx.doi.org/10.1021/ja5011724 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Journal of the American Chemical Society
Communication
Si−OH + MeO− → Si−OMe + OH−
the copper-activated silicon at 220−270 °C mainly produced HSi(OMe)3 and TMOS.22 According to precedent, the dissolution of silica surface ( Si−OH) to Si−Me and Si−OMe with a methylation or methoxylation agent is one of the most essential components of the chemical conversion from silica/heat/MeOH/catalyst to DMDMS and ultimately to organosilicones; however, water must be involved in the overall mechanism for the hydrolytic polymerization reaction to occur. Based on data analysis and observations presented here, an electron transfer/chemical reaction (EC) mechanism is proposed. Initially, a methanol radical anion, [Me−O−H]•−, most likely will be generated at the reduction potential of MeOH at the electrode surface. One possible occurrence is a dissociation of this putative radical species into methyl radical (Me•) and hydroxide anion (OH−) (eq 1). Ethane gas production via a homocoupling of the two Me• species is possible but was not observed in our reaction system. On the other hand, we cannot rule out the possibility that the methanol radical anion, [Me-O-H]•−would decompose to methoxide species (MeO−) and hydrogen radical (H•) (eq 1).
silica surface
(3b)
Owing to the rationale in this study, the formation of TMOS is not favorable, despite the evolution of TMOS in previous studies.8,9 In order to support the hypothesis that TMOS is not an active reaction intermediate or silicone precursor in our system, bulk cathodic electrolysis of TMOS under similar conditions was carried out as outlined earlier. This cathodic electrolysis, Eapp = −3.1 V, passed 2 F/equiv (Figure 1). A
e−
− Me −O−H → [Me ··· O ··· H]•− ⇄ Me• + −OH • (or MeO + H)
(1)
In the first case, though, the generation of hydroxide anion would deprotonate methanol, producing a methoxide moiety. Thus, the reduction of one methanol will afford a methyl radical and a methoxide anion in a 1:1 stoichiometric ratio, which could interact on the silica surface producing DMDMS. DMDMS could then hydrolyze into the polydimethylsiloxanes reported using the water produced from the initial reduction. Because water is required for the follow-up chemical reaction to hydrolytically polymerize DMDMS, the generation of methyl radical species and hydroxide anion is likely the dominant path due to the formation of water in situ. Another adventitious pathway to form water molecules could be an esterification of a silica surface.23 The esterification reaction between silanols on the silica surface and MeOH through an initial hydrogen bonding could provide water as shown in eq 2, which would allow for hydrolysis.
Figure 1. Cyclic voltammograms of 2.1 mM Si(OMe)4 in 0.12 M [NBu4][OTf]/THF, where OTf is −OSO2CF3, at a 2 mm Pt electrode; v = 0.5 V s−1. Black solid line: before electrolysis, black dashed line: during the electrolysis, and red solid line: after exhaustive electrolysis of TMOS.
quantitative amount of hexamethoxydisiloxane ((MeO)3Si−O− Si(OMe)3) was observed as the major electrolysis product. This product was characterized via GC-MS data analysis, where preand postelectrolysis samples were taken to ensure no silane/ siloxane products were in the background. Thus, the direct reduction of TMOS does not afford similar products or product distributions as we observe with the reduction of MeOH in the presence of a SiO2 source. We may therefore conclude that TMOS does not play an active role in the proposed process. This implies that the generation of only methoxide in situ is unlikely and that the C−O bond is likely broken by one electron. Additionally, TMOS has not been detected through any of our analytical studies. We cannot rule out the possibility that impurities on the surface of SiO2 are the cause of the catalytic conversion to organosilicones. In the case of the spectrophotometric quartz cuvette, thorough washing in aqua regia and piranha solutions conducted before the experiment was carried out. Also, it should be noted that the quartz cuvette could be reused for a number of experiments, with similar results. In conclusion, our results indicate that under mild conditions, a silica (sand or quartz) surface may act as an electroauxiliary, upon cathodic reduction of MeOH, thereby providing a novel starting point for simple Me−Si coupling reactions. Future work will focus on broadening the scope of these electrosyntheses to include different alcohols as well as tuning the electrochemical process to produce different types of silicones and to differentiate between silicones and other organo-silicon products. The potential synthetic impact of the electrochemical method is substantial, considering that relatively high product yields may be obtained under time
H
Si−OH + MeOH ⇌ Si−O ··· H −OMe ⇌ Si−OMe + H 2O silica surface
(2)
The DMDMS generated will proceed with follow-up hydrolytic polymerization to yield polydimethylsiloxanes because of catalytic polymerization of DMDMS in the presence of the water generated in situ. To rule out the possibility of DMDMS undergoing a follow-up electrochemical reduction at the applied potential, we investigated the redox reactivity of DMDMS in both dried THF and MeCN. We observed no cathodic reduction or anodic oxidation, which implies that the polymerization is chemical, rather than electrochemical, in nature, supporting the claim that DMDMS is the active precursor to the silicone products. Therefore, both MeO− and Me• are seen to interact on the surface of silica (sand or quartz) to afford the modified (i.e., methylated and methoxylated) silica surface (Si−Me and Si−OMe) (eqs 3a and 3b). Because of these observations, we believe that DMDMS is the active precursor to silicones. Si−OH + Me• → Si−Me + OH• silica surface
(3a) C
dx.doi.org/10.1021/ja5011724 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Journal of the American Chemical Society
Communication
(21) Okamoto, M.; Abe, H.; Kusama, Y.; Suzuki, E.; Ono, Y. J. Organomet. Chem. 2000, 616, 74. (22) Lewis, K. M.; Mereigh, A. T.; O’Young, C.-L.; Cameron, R. A. U.S. Patent, US 7,652,164, 2010. (23) Ossenkamp, G. C.; Kemmitt, T.; Johnston, J. H. Chem. Mater. 2001, 13, 3975.
and intensive energy requirements that are minimal compared to those necessary for the thermally induced multistep processes of silica to organosilicones that is presently the state of the art.
■
ASSOCIATED CONTENT
S Supporting Information *
Experimental examples; summary table; electrode pictures, and CV of MeOH; full data set for the characterization of products (GC-MS, FT-IR, 29Si- and 1H NMR). This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
[email protected] Present Address §
Department of Chemistry and Biochemistry, Center for Electrochemistry, University of Texas, Austin, TX, 78712 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS Dedicated to Professors William E. Geiger and Allen J. Bard on the occasion of their 70th and 80th birthdays, respectively. We would like to acknowledge the invaluable advice of Dr. Adam Heller (Univ. of Texas). We thank the Ball State University start-up fund and Sigma Xi Research Society, which supported this research. We thank Mr. Joshua P. May for his experimental assistance.
■
REFERENCES
(1) Pouget, E.; Tonnar, J.; Lucas, P.; Lacroix-Desmazes, P.; Ganachaud, F.; Boutevin, B. Chem. Rev. 2010, 110, 1233. (2) Laine, R. M.; Blohowiak, K. Y.; Robinson, T. R.; Hoppe, M. L.; Nardi, P.; Kampf, J.; Uhm, J. Nature 1991, 353, 642. (3) Boudin, A.; Cerveau, G.; Chuit, C.; Corriu, R. J. P.; Reye, C. Agnew. Chem. Int. Ed. 1986, 25, 474. (4) Hooper, R.; Lyons, L. J.; Mapes, M. Schumacher, D.; Moline, D.; West, R. 2001, 34, 931. (5) (a) Rochow, E. J. Am. Chem. Soc. 1945, 67, 963. (6) Seyferth, D. Organometallics 2001, 20, 4978. (7) Lewis, L. N.; et al. Inorg. Chem. 2002, 41, 2608. (8) Chibiryaev, A. M.; Kozhevnikov, I. V.; Martyanov, O. N. Appl. Catal., A 2013, 456, 159. (9) Schattenmann, F. J.; Ligon, W. V.; Donahue, P.; Grade, H.; Abatto, K. Organometallics 2004, 23, 6202. (10) Nishimura, Y.; Fukunaka, Y. Electrochem. Acta. 2007, 53, 111. (11) Gobet, J.; Tannenberger, H. J. Electrochem. Soc. 1988, 135, 109. (12) Collinson, M. Acc. Chem. Res. 2007, 40, 777. (13) Chong, D.; Dick, J. E. U.S. Patents pending, 2013. (14) Pause, L.; Robert, M.; Saveant, J.-M. J. Am. Chem. Soc. 2000, 122, 9829. (15) Lewis, L. N.; Schattenmann, F. J.; Jordan, T. M.; Carnahan, J. C.; et al. Inorg. Chem. 2002, 41, 2608. (16) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.; John Wiley & Sons: New York, 2001; pp 427. (17) Nachod, F. C.; Zuckerman, J. J.; Randall, E. W. Determination of Organic Structures by Physical Methods; Academic Press, Inc.: Waltham, MA, 1976. (18) Porchet, S.; Kiwi-Minsker, L.; Doepper, R.; Renken, A. Chem. Eng. Sci. 1996, 51, 2933. (19) Yudanov, I. V.; Matveev, A. V.; Neyman, K. M.; Rösch, N. J. Am. Chem. Soc. 2008, 130, 9342. (20) Mertens, G.; Fripiat, J. J. J. Colloid Interface Sci. 1973, 42, 169. D
dx.doi.org/10.1021/ja5011724 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
