Crystal Facet-Dependent Reactivity of Cu₂O in Direct Synthesis of Tetraethoxysilane from Silicon and Ethanol

Abstract

Ethanol can react with active metals to form alkoxides and hydrogen gas, but metallic silicon exhibits relatively stable chemical properties and does not react with ethanol significantly under catalyst-free and low-temperature conditions. Previous research on organosilicon synthesis has primarily focused on the Rochow reaction, in which metallic silicon reacts with methyl chloride. The extension of copper-based catalysts from the Rochow reaction to enable the direct reaction of metallic silicon with ethanol, in which silicon replaces the hydroxyl hydrogen of ethanol to form silanes, has attracted widespread interest. In this study, we synthesized Cu₂O with well-defined exposed crystal facets via facet engineering, modulating the surface facet ratios and systematically investigating the effects of these Cu₂O with different morphologies on the direct reaction. Combining advanced spectroscopic techniques, density functional theory calculations, and molecular dynamics simulations, we revealed that Cu₂O (111) exhibits the highest surface oxygen vacancy concentration and hydroxyl density. The oxygen vacancies on the Cu₂O (111) surface facilitated the adsorption of ethanol, while the surface hydroxyl groups effectively promoted ethanol dehydrogenation. These two effects work in concert to render the Cu₂O (111) facet the highest facet-dependent chemical reactivity in the direct reaction of metallic silicon and ethanol relative to those from other facets. Our study provides crucial insights into the cooperative roles of exposed active sites and surface crystal planes that are potentially relevant to heterogeneous catalysis. Furthermore, this work proposes an understanding of product selectivity to guide the development of other copper-based catalysts for the direct synthesis strategy.

Keywords

Metallic silicon / Rochow reaction / Cu2O / Direct reaction / Oxygen vacancy

References

[1] Lin YF, Kuo JW. Mesoporous bis(trimethoxysilyl)hexane (BTMSH)/tetraethyl orthosilicate (TEOS)-based hybrid silica aerogel membranes for CO₂ capture. Chem. Eng. J. 2016, 300, 29-35. DOI:10.1016/j.cej.2016.04.119

[2] Longhi M, Kunsta SR, Beltrami LVR, Kerstner EK, Filho CIS, Sarmento VHV, et al. Effect of tetraethoxy-silane (TEOS) amounts on the corrosion prevention properties of siloxane-PMMA hybrid coatings on galvanized steel substrates. Mater. Res. 2015, 18, 1140-1155. DOI:10.1590/1516-1439.321614

[3] Smith SA, Williams BP, Joo YL. Effect of polymer and ceramic morphology on the material and electrochemical properties of electrospun PAN/polymer derived ceramic composite nanofiber membranes for lithium ion battery separators. J. Membr. Sci. 2017, 526, 315-322. DOI:10.1016/j.memsci.2016.12.052

[4] Temnikov MN, Zhiltsov AS, Kotov VM, Krylova IV, Egorov MP, Muzafarov AM. Comparison of effectiveness of various approaches to direct synthesis of alkoxysilanes. Silicon 2015, 7, 69-78. DOI:10.1007/s12633-014-9236-9

[5] Adonin NY, Prikhod’ko SA, Shabalin AY, Prosvirin IP, Zaikovskii VI, Kochubey DI, et al. The “direct” synthesis of trialkoxysilanes: New data for understanding the processes of the copper-containing active sites formation during the activation of the initial silicon based contact mass. J. Catal. 2016, 338, 143-153. DOI:10.1016/j.jcat.2016.03.012

[6] Laine RM, Furgal JC, Doan P, Pan D, Popova V, Zhang X. Avoiding carbothermal reduction: distillation of alkoxysilanes from biogenic, green, and sustainable sources, Angew. Chem. Int. Ed. 2016, 55, 1065-1069. DOI:10.1002/anie.201506838

[7] Chibiryaev AM, Kozhevnikov IV, Martyanov ON. High-temperature reaction of SiO₂ with methanol: Nucleophilic assistance of some N-unsubstituted benzazoles. Appl. Catal. A Gen. 2013, 456, 159-167. DOI:10.1016/j.apcata.2013.03.001

[8] Fukaya N, Choi SJ, Horikoshi T, Kumai H, Hasegaw M, Yasuda H, et al. Synthesis of tetramethoxysilane from silica and methanol using carbon dioxide and an organic dehydrating reagent. Chem. Lett. 2016, 45, 828-830. DOI:10.1246/cl.160329

[9] Nguyen TTH, Fukaya N, Sato K, Choi J-C, Kataoka S. Technoeconomic and environmental assessment for design and optimization of tetraethyl orthosilicate synthesis process. Ind. Eng. Chem. Res. 2018, 57, 2192-2199. DOI:10.1021/acs.iecr.7b03895

[10] Putro WS, Lee VY, Sato K, Choi J-C, Fukaya N. From SiO₂ to alkoxysilanes for the synthesis of useful chemicals. ACS Omega 2021, 6, 35186-35195. DOI:10.1021/acsomega.1c05138

[11] Fukaya N, Choi SJ, Horikoshi T, Kataoka S, Endo A, Kumai H, et al. Direct synthesis of tetraalkoxysilanes from silica and alcohols. New J. Chem. 2017, 41, 2224-2226. DOI:10.1039/c6nj03897b

[12] Nguyen TTH, Fukaya N, Choi SJ, Sato K, Choi J-C. Impact of the water removal method on tetraethyl orthosilicate direct synthesis: Experiment and process assessment. Ind. Eng. Chem. Res. 2019, 58, 19997-20002. DOI:10.1021/acs.iecr.9b02887

[13] Feng J, Li Z, Liu X, Chi R. Preparation of silica gel from quartz for direct synthesis of alkoxysilanes. Silicon 2025, 17, 1767-1775. DOI:10.1007/s12633-025-03309-4

[14] Seyferth D. Dimethyldichlorosilane and the direct synthesis of methylchlorosilanes. The key to the silicones industry. Organometallics 2001, 20, 4978-4992. DOI:10.1021/om0109051

[15] Hurd DT, Rochow EG. On the mechanism of the reaction between methyl chloride and silicon-copper. J. Am. Chem. Soc. 1945, 67, 469-471. DOI:10.1021/ja01223a007

[16] Newton WE, Rochow EG. Direct synthesis of organic derivatives of silicon using nonhalogenated organic compounds. Inorg. Chem. 1970, 9, 1071-1075. DOI:10.1021/ic50087a014

[17] Han J, Gheyas SI, Wang Y, Strongin DR, Graham AP, Hinch BJ, et al. Thermal chemistry of CH₃ on Si/Cu(100) . J. Phys. Chem. B 2000, 104, 3078-3084. DOI:10.1021/jp9928863

[18] Acker J, Bohmhammel K. Thermodynamic assessment of the copper catalyzed direct synthesis of methylchlorosilanes. J. Organomet. Chem. 2008, 693, 2483-2493. DOI:10.1016/j.jorganchem.2008.04.026

[19] Adonin NY, Prikhod’ko SA, Shabalin AY, Prosvirin IP, Zaikovskii VI, Kochubey DI, et al. Synthesis and structural features of nanostructured cuprous chloride with high catalytic activity. Silicon 2015, 7, 79-87. DOI:10.1007/s12633-014-9272-5

[20] Zhang Z, Che H, Wang Y, She X, Sun J, Gunawan P, et al. Facile solvothermal synthesis of porous cubic Cu microparticles as copper catalysts for Rochow reaction. ACS Appl. Mater. Interfaces 2012, 4, 1295-1302. DOI:10.1021/am3002605

[21] Zhang Z, Che H, Wang Y, Gao J, She X, Sun J, et al. Flower-like CuO microspheres with enhanced catalytic performance for dimethyldichlorosilane synthesis. RSC Adv. 2012, 2, 2254-2256. DOI:10.1039/C2RA00923D

[22] Zhang Z, Che H, Gao J, Wang Y, She X, Sun J, et al. Shape-controlled synthesis of Cu₂O microparticles and their catalytic performances in the Rochow reaction. Catal. Sci. Technol. 2012, 2, 1207-1212. DOI:10.1039/C2CY20070H

[23] Zhang Z, Che H, Wang Y, Gao J, Zhao L, She X, et al. Facile synthesis of mesoporous Cu₂O microspheres with improved catalytic property for dimethyldichlorosilane synthesis. Ind. Eng. Chem. Res. 2012, 51, 1264-1274. DOI:10.1021/ie2020747

[24] Li J, Ni Z, Ji Y, Zhu Y, Liu H, Zhang Y, et al. ZnO supported on Cu₂O{100} enhances charge transfer in dimethyldichlorosilane synthesis. J. Catal. 2019, 374, 284-296. DOI:10.1016/j.jcat.2019.02.029

[25] Krylova IV, Egorov MP, Nefedov OM. Reaction of silicon with alcohols in autoclave. Russ. Chem. Bull. 2017, 66, 260-266. DOI:10.1007/s11172-017-1726-7

[26] Su K, Ji Y, Zhou X, Jiang X, Li H, Zhu Y, et al. Approach to generating the right active phase in the “Direct” synthesis of trimethoxysilanes using the CuCl-Cu₂O catalyst. Appl. Surf. Sci. 2021, 544, 148915. DOI:10.1016/j.apsusc.2020.148915

[27] Su G, Wang A, Zhang M, Yin H, Wang G. Selective Synthesis of Triethoxysilane and Tetraethoxysilane through Direct Reaction between Ethanol and Silicon Catalyzed by CuCl and Metallic Cu0 Nanoparticles in Fixed-bed Reactor. Silicon 2020, 14, 573-580. DOI:10.1007/s12633-020-00830-6

[28] Gao Y, Zhang L, van Hoof AJF, Hensen EJM. On the surface-dependent oxidation of Cu₂O during CO oxidation: Cu²⁺ is more active than Cu⁺ . Appl. Catal. A Gen. 2020, 602, 117712. DOI:10.1016/j.apcata.2020.117712

[29] Huang W, Liu Q, Zhou Z, Li Y, Ling Y, Wang Y, et al. Tuning the activities of cuprous oxide nanostructures via the oxide-metalinteraction. Nat. Commun. 2020, 11, 2312. DOI:10.1038/s41467-020-15965-8

[30] Li J, He M, Yan J, Liu J, Zhang J, Ma J. Room temperature engineering crystal facet of Cu₂O for photocatalytic degradation of methyl orange. Nanomaterials 2022, 12, 1697. DOI:10.3390/nano12101697

[31] Sun X-L, Yuan H, Yang D, Hu Z. Synergism between {111} and {100} crystal facets on Cu₂O for boosted photothermal catalysis. Ind. Eng. Chem. Res. 2025, 64, 1046-1057. DOI:10.1021/acs.iecr.4c03649

[32] Teng Z, Yi X, Zhang C, He C, Yang Y, Hao Q, et al. CO self-sustained catalytic combustion over morphological inverse model CeO₂/Cu₂O catalysts exposing (100), (111) and (110) planes. Appl. Catal. B Environ. 2023, 339, 123119. DOI:10.1016/j.apcatb.2023.123119

[33] Wan L, Zhou Q, Wang X, Wood TE, Wang L, Duchesne PN, et al. Cu₂O nanocubes with mixed oxidation-state facets for (photo)catalytic hydrogenation of carbon dioxide. Nat. Catal. 2019, 2, 889-898. DOI:10.1038/s41929-019-0338-z

[34] Zhong W, Gong Z, He Z, Zhang N, Kang X, Mao X, et al. Modulating surface oxygen species via facet engineering for efficient conversion of nitrate to ammonia. J. Energy Chem. 2023, 78, 211-221. DOI:10.1016/j.jechem.2022.11.024

[35] Xiong W, Gu X-K, Zhang Z, Chai P, Zang Y, Yu Z, et al. Author correction: fine cubic Cu₂O nanocrystals as highly selective catalyst for propylene epoxidation with molecular oxygen. Nat. Commun. 2021, 12, 5921. DOI:10.1038/s41467-021-26257-0

[36] Zhao X, Susman MD, Rimer JD, Bollini P. Synthesis, structure, and catalytic properties of faceted oxide crystals. ChemCatChem 2020, 13, 6-27. DOI:10.1002/cctc.202001066

[37] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865-3868. DOI:10.1103/PhysRevLett.77.3865

[38] Li L, Zhang R, Vinson J, Shirley EL, Greeley JP, Guest JR, et al. Imaging catalytic activation of CO₂ on Cu₂O (110): A first-principles study. Chem. Mater. 2018, 30, 1912-1923. DOI:10.1021/acs.chemmater.7b04803

[39] Huang W-C, Lyu L-M, Yang Y-C, Huang M-H. Synthesis of Cu₂O nanocrystals from cubic to rhombic dodecahedral structures and their comparative photocatalytic activity. J. Am. Chem. Soc. 2012, 134, 1261-1267. DOI:10.1021/ja209662v

[40] Chen B-H, Kumar G, Wei Y-J, Ma H-H, Kao J-C, Chou P-J, et al. Experimental revelation of surface and bulk lattices in faceted Cu₂O crystals. Small 2023, 19, 230491. DOI:10.1002/smll.202303491

[41] Zhang D-F, Zhang H, Guo L, Zheng K, Han X-D, Zhang Z. Delicate control of crystallographic facet-oriented Cu₂O nanocrystals and the correlated adsorption ability. J. Mater. Chem. 2009, 19, 5220-5225. DOI:10.1039/B816349A

[42] Yin Y, Liu K, Gao M, Zhang L, Su H, Zeng S. Influence of the structure and morphology of CuO supports on the amount and properties of copper-cerium interfacial sites in inverse CeO₂/CuO catalysts. J. Mol. Catal. A Chem. 2015, 404-405, 193-203. DOI:10.1016/j.molcata.2015.05.001

[43] Biesinger MC, Lau LWM, Gerson AR, Smart RSC. Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn. Appl. Surf. Sci. 2010, 257, 887-898. DOI:10.1016/j.apsusc.2010.07.086

[44] Yuan Q, Chen L, Xiong M, He J, Luo S-L, Au C-T, et al. Cu₂O/BiVO₄ heterostructures: Synthesis and application in simultaneous photocatalytic oxidation of organic dyes and reduction of Cr(VI) under visible light. Chem. Eng. J. 2014, 255, 394-402. DOI:10.1016/j.cej.2014.06.031

[45] Li S, Wang H, Li W, Wu X, Tang W. Effect of Cu substitution on promoted benzene oxidation over porous CuCo-based catalysts derived from layered double hydroxide with resistance of water vapor. Appl. Catal. B Environ. 2015, 166-167, 260-269. DOI:10.1016/j.apcatb.2014.11.040

[46] Zhu Y, Zhang L, Zhao B, Chen H, Liu X, Zhao R, et al. Improving the activity for oxygen evolution reaction by tailoring oxygen defects in double perovskite oxides. Adv. Funct. Mater. 2019, 29, 1901783.1-1901783.12. DOI:10.1002/adfm.201901783

[47] Liu B, Yao X, Zhang Z, Li C, Zhang J, Wang P, et al. Synthesis of Cu₂O nanostructures with tunable crystal facets for electrochemical CO₂ reduction to alcohols. ACS Appl. Mater. Interfaces 2021, 13, 39165-39177. DOI:10.1021/acsami.1c03850

[48] Nandy S, Thapa R, Kumar M, Som T, Bundaleski N, Teodoro OMND, et al. Efficient field emission from vertically aligned Cu₂O₁-_δ(111) nanostructure influenced by oxygen vacancy. Adv. Funct. Mater. 2015, 25, 947-956. DOI:10.1002/adfm.201402910

[49] Wang J, Chen L, Yue L, Filot IAW, Hensen EJM, Liu P. Unveiling the Au-Mn-Cu synergy in Au/LaMnCuO₃ catalysts for selective ethanol oxidation. Chin. J. Catal. 2025, 75, 34-48. DOI:10.1016/S1872-2067(25)64686-9

[50] Zhang R, Teoh WY, Amal R, Chen B, Kaliaguine S. Catalytic reduction of NO by CO over Cu/Ce_xZr_1−xO₂ prepared by flame synthesis. J. Catal. 2010, 272, 210-219. DOI:10.1016/j.jcat.2010.04.000

[51] Zhu Y, Zhong X, Jin S, Chen H, He Z, Liu Q, et al. Oxygen defect engineering in double perovskite oxides for effective water oxidation. J. Mater. Chem. A 2020, 8, 10957-10965. DOI:10.1039/D0TA04362A

[52] Zhu Y, He Z, Choi Y, Chen H, Li X, Zhao B, et al. Tuning proton-coupled electron transfer by crystal orientation for efficient water oxidization on double perovskite oxides, Nat. Commun. 2020, 11, 4299. DOI:10.1038/s41467-020-17657-9

[53] Wu C, Sun Y, Wen X, Zhang J-Y, Qiao L, Cheng J, et al. Adjusting oxygen vacancies in perovskite LaCoO₃ by electrochemical activation to enhance the hydrogen evolution reaction activity in alkaline condition, J. Energy Chem. 2023, 76, 226-232. DOI:10.1016/j.jechem.2022.08.035

[54] Fernández HM, Amirabbasi M, Mempin C, Jr., Trapletti A, Wartner G, Tesch MF, et al. Plasma-engineered hydroxyl defects in NiO: A DFT-Supported-Spectroscopic analysis of oxygen-hole states and implications for water oxidation. J. Am. Chem. Soc. 2026, 148, 15076-15091. DOI:10.1021/jacs.6c00210

[55] Pan Y, Li R, Zhang L, Liu J-X, Wang W, Zhang G-J. Synergistic mechanochemistry and facet engineering boost conversion of dilute CO₂ from industrial flue gases. Appl. Catal. B Environ. 2026, 391, 126686. DOI:10.1016/j.apcatb.2026.126686

[56] Liao S, Xu S, Ye L, Y-CG, Li J, Hu C. Facet-engineered Cu₂O microcrystals for tailoring biomass-polyol conversion to glycolic acid. Appl. Catal. B Environ. 2026, 393, 126762. DOI:10.1016/j.apcatb.2026.126762

[57] Lee HR, Shibata T, Kanezashi M, Mizumo T, Ohshita J. Pore-size-controlled silica membranes with disiloxane alkoxides for gas separation. J. Membr. Sci. 2011, 383, 152-158. DOI:10.1016/j.memsci.2011.08.046

[58] Vuyyuru KR, Pant KK, Krishnan VV, Nigam KDP. Recovery of nickel from spent industrial catalysts using chelating agents. Ind. Eng. Chem. Res. 2010, 49, 2014-2024. DOI:10.1021/ie901406e

[59] Tissot H, Wang C, Stenlid JH, Panahi M, Kaya S, Soldemo M, et al. Interaction of atomic hydrogen with the Cu₂O(100) and (111) surfaces. J. Phys. Chem. C 2019, 123, 22172-22180. DOI:10.1021/acs.jpcc.9b03888

[60] Kim JY, Rodriguez JA, Hanson JC, Frenkel AI, Lee PL. Reduction of CuO and Cu₂O with H₂: H embedding and kinetic effects in the formation of suboxides. J. Am. Chem. Soc. 2003, 125, 10684-10692. DOI:10.1021/ja0301673

[61] Nguyen TTH, Kataoka S, Fukaya N, Sato K, Choi J-C, Endo A. Feasibility study of new synthesis route of tetraethoxysilane from rice hull ash. Comput. Aided Chem. Eng. 2017, 40, 703-708. DOI:10.1016/B978-0-444-63965-3.50119-7