From α-Al₂O₃ Phase Purity to Surface Reactivity: A Causal Design Framework for Selective Ethylene Epoxidation over Ag/α-Al₂O₃ Catalysts
DOI:
https://doi.org/10.66960/jof.3093-8899.00043Keywords:
ethylene oxide, silver catalyst, α-alumina, phase purity, surface hydroxyls, support reactivity, selectivity, catalyst designAbstract
Silver supported on α-alumina remains the industrially dominant catalytic platform for direct ethylene epoxidation, yet the support is often treated as a passive mechanical carrier even though recent studies show that phase purity, hydroxyl chemistry, exposed support area and Ag/support geometry can directly alter ethylene oxide (EO) selectivity and secondary EO loss. This critical review integrates evidence from phase-controlled α-Al₂O₃ synthesis, Ag particle and surface-ratio studies, EO degradation experiments, promoter studies, operando oxygen-species characterization and kinetic modeling. Four original synthesis contributions are proposed: (i) a Phase–Surface Selectivity Envelope that separates bulk phase purity from surface EO-loss propensity; (ii) a Support-Exposure Penalty concept that treats uncovered α-Al₂O₃ as an independent selectivity variable; (iii) a Causal Control Matrix for disentangling phase, surface modifier and Ag morphology effects; and (iv) a Minimum Evidence Set for claiming support-controlled selectivity. These contributions are not presented as experimentally validated laws; they are falsifiable design frameworks intended to guide the PhD program on phase-pure and surface-controlled Ag/α-Al₂O₃ catalysts. The review concludes that the most defensible route to higher selectivity is not simply to maximize α-phase fraction, but to combine quantified phase purity with low secondary EO reactivity, controlled Ag coverage and promoter history under matched conversion conditions.
References
Lohr TL, Lockemeyer JR, Bishopp SD, et al. Ethylene Oxide: A Catalyst and Process Development Success Story. Industrial & Engineering Chemistry Research. 2024;63(43):18221-18240. doi:10.1021/acs.iecr.4c02241. DOI: https://doi.org/10.1021/acs.iecr.4c02241
Jain D, Stangland E. A Perspective on the Role and Development of Supports for Maximizing the Efficiency of Catalytic Gas-Phase Ethylene Epoxidation. Energy & Fuels. 2025;39(26):12309-12322. doi:10.1021/acs.energyfuels.5c01329. DOI: https://doi.org/10.1021/acs.energyfuels.5c01329
Keijzer CJ, Dalebout R, de Rijk JW, Lockemeyer JR, Lohr TL, van den Brink P, de Jongh PE. Silver Catalysts Supported on High Surface Area α-Alumina: Effect of Carbohydrate Template Size and Heat Treatment on Phase Purity. ChemCatChem. 2023;15:e202300513. doi:10.1002/cctc.202300513. DOI: https://doi.org/10.1002/cctc.202300513
Keijzer CJ, Smulders LCJ, Wezendonk D, de Rijk JW, de Jongh PE. Influence of Ag particle size and Ag:Al2O3 surface ratio in catalysts for the chloride-promoted ethylene epoxidation. Catalysis Today. 2024;428:114447. doi:10.1016/j.cattod.2023.114447. DOI: https://doi.org/10.1016/j.cattod.2023.114447
Keijzer CJ, van de Ven NMT, Dalebout R, Lohr TL, Lockemeyer JR, van den Brink P, de Jongh PE. Influence of alkali and chloride promoters on silver catalysts in ethylene epoxidation. Journal of Catalysis. 2025;450:116206. doi:10.1016/j.jcat.2025.116206. DOI: https://doi.org/10.1016/j.jcat.2025.116206
Keijzer PH, van den Reijen JE, Keijzer CJ, de Jong KP, de Jongh PE. Influence of atmosphere, interparticle distance and support on the stability of silver on α-alumina for ethylene epoxidation. Journal of Catalysis. 2022;405:534-544. doi:10.1016/j.jcat.2021.11.016. DOI: https://doi.org/10.1016/j.jcat.2021.11.016
He C, Lu X, Liu Y, Yan Z, et al. Stability enhancement for silver catalyst in ethylene epoxidation by support treatment. Catalysis Communications. 2023;178:106669. doi:10.1016/j.catcom.2023.106669. DOI: https://doi.org/10.1016/j.catcom.2023.106669
Chu Y, Liang H, Lin Q, Huang H, Che P, Han Y. Effect of water vapor on the structure and performance of silver catalysts for ethylene oxidation. Chemical Engineering Journal. 2025;518:164442. doi:10.1016/j.cej.2025.164442. DOI: https://doi.org/10.1016/j.cej.2025.164442
Miller JH, Joshi A, Li X, Bhan A. Catalytic degradation of ethylene oxide over Ag/α-Al2O3. Journal of Catalysis. 2020;389:714-720. doi:10.1016/j.jcat.2020.07.008. DOI: https://doi.org/10.1016/j.jcat.2020.07.008
Egelske B, Xiong W, Zhou H, Monnier JR. Effects of the method of active site characterization for determining structure-sensitivity in Ag-catalyzed ethylene epoxidation. Journal of Catalysis. 2022;410:221-235. doi:10.1016/j.jcat.2022.03.021. DOI: https://doi.org/10.1016/j.jcat.2022.03.021
Sun L, Huang H, Che P, Lin Q, et al. In situ discovery on the formation of supported silver catalysts for ethylene epoxidation. Advanced Powder Technology. 2022;33(9):103732. doi:10.1016/j.apt.2022.103732. DOI: https://doi.org/10.1016/j.apt.2022.103732
Pu T, Setiawan A, Mosevitzky Lis B, Zhu M, Ford ME, Rangarajan S, Wachs IE. Nature and Reactivity of Oxygen Species on/in Silver Catalysts during Ethylene Oxidation. ACS Catalysis. 2022;12(8):4375-4381. doi:10.1021/acscatal.1c05939. DOI: https://doi.org/10.1021/acscatal.1c05939
Pu T, Setiawan A, Foucher AC, Guo M, Jehng JM, Zhu M, Ford ME, Stach EA, Rangarajan S, Wachs IE. Revealing the Nature of Active Oxygen Species and Reaction Mechanism of Ethylene Epoxidation by Supported Ag/α-Al2O3 Catalysts. ACS Catalysis. 2024;14(1):406-417. doi:10.1021/acscatal.3c04361. DOI: https://doi.org/10.1021/acscatal.3c04361
Guo M, Dongfang N, Iannuzzi M, van Bokhoven JA, Artiglia L. Structure and Reactivity of Active Oxygen Species on Silver Surfaces for Ethylene Epoxidation. ACS Catalysis. 2024;14(13):10234-10244. doi:10.1021/acscatal.4c01566. DOI: https://doi.org/10.1021/acscatal.4c01566
Brandão L, Reece C. Non-steady state validation of kinetic models for ethylene epoxidation over silver catalysts. Catalysis Science & Technology. 2024;14(13):3596-3608. doi:10.1039/D4CY00052H. DOI: https://doi.org/10.1039/D4CY00052H
Yu Z, Zhu L, Xu H, Cheng D. Selective Enhancement of Ethylene Epoxidation via Directing Reaction Pathways over Ag Single-Atom Catalyst. Industrial & Engineering Chemistry Research. 2024;63(7):3044-3056. doi:10.1021/acs.iecr.3c04600. DOI: https://doi.org/10.1021/acs.iecr.3c04600
Salaev MA, Salaeva AA, Vodyankina OV. Towards the understanding of promoting effects of Re, Cs and Cl promoters for silver catalysts of ethylene epoxidation: A computational study. Catalysis Today. 2021;375:585-590. doi:10.1016/j.cattod.2020.04.057. DOI: https://doi.org/10.1016/j.cattod.2020.04.057
Wu S, Tatarchuk BJ, Adamczyk AJ. Ethylene oxidation on unpromoted silver catalysts: Reaction pathway and selectivity analysis using DFT calculations. Surface Science. 2021;708:121834. doi:10.1016/j.susc.2021.121834. DOI: https://doi.org/10.1016/j.susc.2021.121834
van Hoof AJF, Hermans EAR, van Bavel AP, Friedrich H, Hensen EJM. Structure Sensitivity of Silver-Catalyzed Ethylene Epoxidation. ACS Catalysis. 2019;9(11):9829-9839. doi:10.1021/acscatal.9b02720. DOI: https://doi.org/10.1021/acscatal.9b02720
Pu T, Tian H, Ford ME, Rangarajan S, Wachs IE. Overview of Selective Oxidation of Ethylene to Ethylene Oxide by Ag Catalysts. ACS Catalysis. 2019;9(12):10727-10750. doi:10.1021/acscatal.9b03443. DOI: https://doi.org/10.1021/acscatal.9b03443
Yang H, Li G, Liu Q, Cheng H, Wang X, Cheng J, Jiang G, Zhang F, Zhang Z, Hao Z. Tailoring the Electronic Metal-Support Interactions in Supported Silver Catalysts through Al modification for Efficient Ethylene Epoxidation. Angewandte Chemie International Edition. 2024;63(16):e202400627. doi:10.1002/anie.202400627. DOI: https://doi.org/10.1002/anie.202400627
van den Reijen JE, Kanungo S, Welling TAJ, Versluijs-Helder M, Nijhuis TA, de Jong KP, de Jongh PE. Preparation and particle size effects of Ag/α-Al2O3 catalysts for ethylene epoxidation. Journal of Catalysis. 2017;356:65-74. doi:10.1016/j.jcat.2017.10.001. DOI: https://doi.org/10.1016/j.jcat.2017.10.001
van den Reijen JE, Versluis WC, Kanungo S, d'Angelo MF, de Jong KP, de Jongh PE. From qualitative to quantitative understanding of support effects on the selectivity in silver catalyzed ethylene epoxidation. Catalysis Today. 2019;338:31-39. doi:10.1016/j.cattod.2019.04.049. DOI: https://doi.org/10.1016/j.cattod.2019.04.049
van den Reijen JE, Keijzer PH, de Jongh PE. Pore structure stabilization during the preparation of single phase ordered macroporous α-alumina. Materialia. 2018;4:423-430. doi:10.1016/j.mtla.2018.10.016. DOI: https://doi.org/10.1016/j.mtla.2018.10.016
Ayame A, Uchida Y, Ono H, Miyamoto M, Sato T, Hayasaka H. Epoxidation of ethylene over silver catalysts supported on α-alumina crystal carriers. Applied Catalysis A: General. 2003;244:59-70. doi:10.1016/S0926-860X(02)00595-1. DOI: https://doi.org/10.1016/S0926-860X(02)00595-1
Christopher P, Linic S. Shape- and Size-Specific Chemistry of Ag Nanostructures in Catalytic Ethylene Epoxidation. ChemCatChem. 2010;2:78-83. doi:10.1002/cctc.200900231. DOI: https://doi.org/10.1002/cctc.200900231
Iyer KR, Bhan A. Particle size dependence of ethylene epoxidation rates on Ag/α-Al2O3 catalysts: Why particle size distributions matter. Journal of Catalysis. 2023;420:99-109. doi:10.1016/j.jcat.2023.02.008. DOI: https://doi.org/10.1016/j.jcat.2023.02.008
Bukhtiyarov VI, Prosvirin IP, Kvon RI, Goncharova SN, Bal'zhinimaev BS. XPS study of the size effect in ethene epoxidation on supported silver catalysts. Journal of the Chemical Society, Faraday Transactions. 1997;93:2323-2329. doi:10.1039/A608414A. DOI: https://doi.org/10.1039/a608414a
Mao CF, Vannice MA. High surface area α-aluminas III. Oxidation of ethylene, ethylene oxide, and acetaldehyde over silver dispersed on high surface area α-alumina. Applied Catalysis A: General. 1995;122:61-76. doi:10.1016/0926-860X(94)00214-2. DOI: https://doi.org/10.1016/0926-860X(94)00214-2
Kim YC, Park NC, Shin JS, Lee SR, Lee YJ, Moon DJ. Partial oxidation of ethylene to ethylene oxide over nanosized Ag/α-Al2O3 catalysts. Catalysis Today. 2003;87:153-162. doi:10.1016/j.cattod.2003.09.012. DOI: https://doi.org/10.1016/j.cattod.2003.09.012
Lee JK, Verykios XE, Pitchai R. Support and crystallite size effects in ethylene oxidation catalysis. Applied Catalysis. 1989;50:171-188. doi:10.1016/S0166-9834(00)80834-9. DOI: https://doi.org/10.1016/S0166-9834(00)80834-9
Goncharova SN, Paukshtis EA, Bal'zhinimaev BS. Size effects in ethylene oxidation on silver catalysts. Influence of support and Cs promoter. Applied Catalysis A: General. 1995;126:67-84. doi:10.1016/0926-860X(95)00036-4. DOI: https://doi.org/10.1016/0926-860X(95)00036-4
Wu JC, Harriott P. The effect of crystallite size on activity and selectivity of silver catalysts. Journal of Catalysis. 1975;39:395-402. doi:10.1016/0021-9517(75)90306-1. DOI: https://doi.org/10.1016/0021-9517(75)90306-1
Verykios XE, Stein FP, Coughlin RW. Influence of metal crystallite size and morphology on selectivity and activity of ethylene oxidation catalyzed by supported silver. Journal of Catalysis. 1980;66:368-382. doi:10.1016/0021-9517(80)90040-8. DOI: https://doi.org/10.1016/0021-9517(80)90040-8
Kanoh H, Nishimura T, Ayame A. Supported silver catalysts for oxidation of ethylene: effects of thermal treatment of alumina as catalyst support. Journal of Catalysis. 1979;57:372-379. doi:10.1016/0021-9517(79)90003-4. DOI: https://doi.org/10.1016/0021-9517(79)90003-4
Fotopoulos AP, Triantafyllidis KS. Ethylene epoxidation on Ag catalysts supported on non-porous, microporous and mesoporous silicates. Catalysis Today. 2007;127:148-156. doi:10.1016/j.cattod.2007.03.061. DOI: https://doi.org/10.1016/j.cattod.2007.03.061
Shariati K, Mehrani A, Corsello T, Kelley M, Lauterbach J. Ethylene epoxidation over shape-selective silver-based catalysts. Applied Catalysis A: General. 2025;697:120200. doi:10.1016/j.apcata.2025.120200. DOI: https://doi.org/10.1016/j.apcata.2025.120200
Al Hadhrami A, Amoudi-Al MA, Nadeem MA, Ziani AA, D'Souza L. Synergistic Effects of Ceria and pH-Controlled Strong Electrostatic Adsorption on Silver/Alumina Catalysts for Ethylene Epoxidation. ChemistrySelect. 2025;10:e04199. doi:10.1002/slct.202504199. DOI: https://doi.org/10.1002/slct.202504199
Busca G. The surface of transitional aluminas: A critical review. Catalysis Today. 2014;226:2-13. doi:10.1016/j.cattod.2013.08.003. DOI: https://doi.org/10.1016/j.cattod.2013.08.003
Cao K, Huang M, Wang G, Li J, Li C. Engineering surface Si-OH species for highly effective ethylene epoxidation on Ag/SiO2 catalyst. Chemical Engineering Science. 2026;321:122652. doi:10.1016/j.ces.2025.122652. DOI: https://doi.org/10.1016/j.ces.2025.122652
Tsybulya SV. Study of the Real Structure of Silver Supported Catalysts of Different Dispersity. Journal of Catalysis. 1995;154:194-200. doi:10.1006/jcat.1995.1160. DOI: https://doi.org/10.1006/jcat.1995.1160
Atkins M, Couves J, Hague M, Sakakini BH, Waugh KC. On the Role of Cs, Cl and Subsurface O in Promoting Selectivity in Ag/α-Al2O3 Catalysed Oxidation of Ethene to Ethene Epoxide. Journal of Catalysis. 2005;235:103-113. doi:10.1016/j.jcat.2005.07.019. DOI: https://doi.org/10.1016/j.jcat.2005.07.019
Hassani SS, Ghasemi MR, Rashidzadeh M, Sobat Z. Nano-sized silver crystals and their dispersion over α-alumina for ethylene epoxidation. Crystal Research and Technology. 2009;44(9):948-952. doi:10.1002/crat.200900126. DOI: https://doi.org/10.1002/crat.200900126
Dellamorte JC, Lauterbach J, Barteau MA. Promoter-Induced Morphological Changes of Ag Catalysts for Ethylene Epoxidation. Industrial & Engineering Chemistry Research. 2009;48(13):5943-5953. doi:10.1021/ie801627k. DOI: https://doi.org/10.1021/ie801627k
Christopher P, Linic S. Engineering Selectivity in Heterogeneous Catalysis: Ag Nanowires as Selective Ethylene Epoxidation Catalysts. Journal of the American Chemical Society. 2008;130(34):11264-11265. doi:10.1021/ja803818k. DOI: https://doi.org/10.1021/ja803818k
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Copyright © 2026 Khikmatullo Nasullaev, Saidxon Shukurkhonov, Shuhratqodir Gulomov, Ruslan Khabipov, Nargiza Igamkulova
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