Catalytic Enhancement of Aldol Condensation by Oxygen Vacancy on CeO₂ Catalysts

Abstract

Aldol condensation is a very useful reaction for biomass upgrading by coupling small molecule platform compounds into high value-added products. Here we choose acetaldehyde (AcH) condensation, the rate-determining step in bioethanol transformations, as a targeted reaction, and prepared four CeO 2 catalysts with different concentrations of surface oxygen vacancies to investigate the role of oxygen vacancies in this reaction. To the best of our knowledge, it is the first time to demonstrate that there is a linear correlation between the activity of aldol condensation and the concentration of oxygen vacancies. Based on in-situ FTIR studies, we conclude that oxygen vacancies are the Lewis acid sites for activation and stabilization of AcH, while lattice oxygen act as base sites for the formation of enolate species which are the key intermediates for AcH coupling reactions. Moreover, the proposed reaction mechanism indicates that Ce cations which are weak Lewis acid sites are not involved in the AcH condensation.

Defect Engineering on CeO2‐Based Catalysts for Heterogeneous Catalytic Applications

“…Recently, defective‐CeO 2 ‐based nanomaterials have attracted extensive attention in aldol condensation, organic oxidation, hydrogenation reduction, and other reactions of organic compounds [12b,124] . Liang et al controlled the oxygen vacancy concentration on the surface of CeO 2 by controlling the morphology and the atmosphere treatment conditions and then found for the first time that the oxygen vacancy concentration was linearly positively correlated with the aldehyde hydroxyl condensation reaction activity of acetaldehyde.…”

“…Liang et al controlled the oxygen vacancy concentration on the surface of CeO 2 by controlling the morphology and the atmosphere treatment conditions and then found for the first time that the oxygen vacancy concentration was linearly positively correlated with the aldehyde hydroxyl condensation reaction activity of acetaldehyde. According to the results of in situ infrared study, they suggest that the oxygen vacancies of CeO 2 act as Lewis acid sites, playing a crucial role in activating and stabilizing acetaldehyde molecules [124a] . Recently, plenty of other works have been done on toluene oxidation [12a,125] .…”

“…Through the combination of experiments and theoretical simulation, they attributed the high performance of CuO x –CeO 2 ‐catalyzed hydrogenation of p ‐nitrophenol to the fact that it promotes hydrogen dissociation to form H* species, which contributes to the efficient reduction of p ‐nitrophenol (Figure 14d). [124c] In general, although the oxygen defect of CeO 2 is of signality in catalyzing organic synthesis reactions, its reaction mechanism is very complex and needs to be further studied.…”

A Brief Summary of Research Progress on the Application of Rare Earth Materials in Heterogeneous Catalysis

“…同时, 还可以将稳定性优良 的稀土元素引入反应体系充当结构助剂 [77] , 对载体的 结 构 和 形 貌 进 行 调 控 , 起 到 增 加 体 系 稳 定 性 的 作 用 [22][23][24] . 例如, Cunha 等 [76] [74] Figure 5 The linear correlation between the yields of ethanol condensation and the concentration of oxygen vacancies based on various CeO 2 catalysts (■) [74] 另外, 从表 图 6 La2O3 修饰的不同 Fe/Al 合金催化剂(50Fe(☆), 50FeLa(5)(○), 50FeLa(10)(□), 50FeLa(20)(△), 50FeLa(50)(◇))催化甲烷分解反应活 性随时间变化曲线 [76] Figure 6 Methane conversion comparison on La2O3 modified Fe/Al alloy catalysts (50Fe(☆), 50FeLa(5)(○), 50FeLa(10)(□), 50FeLa(20)(△) and 50FeLa(50)(◇)) [76] Lercher 团队 [25] The conversion (red •) and product selectivities (H 2 (olive ■), CO 2 (black ◆), CO(wine ◇), (CH 3 ) 2 CO (olive □), CH 4 (dark cyan •)) as a function of time (T＝650 ℃, S/C＝3, gas hourly space velocity (GHSV)＝28000 h -1 ) [25] 有下降, 这是因为 [26][27][28][29] (CH 4 ＋ CO 2 →2CO＋2H 2 ), 但由于镍表面容易积碳, 导致其很 容易失活, 因此需要找到合适的助催化剂来抑制表面积 碳的产生 [30] . 大多数稀土金属和氧化物都可以用作镍 基催化剂的助剂, 钆、铈和镧等稀土元素被广泛用于修 饰镍基催化剂 [31][32][33] , 其中借助镧氧化物来修饰镍基催化 剂最为常见 [32][33][34] .…”

datasheet1.docx

“…It is known that Lewis bases or Lewis acid-base pairs are active for acetaldehyde condensation. For a Lewis base-catalyzed reaction, Lewis bases abstract α-H from an acetaldehyde molecule, generating an enolate species that attacks another acetaldehyde molecule for the condensation to occur (Liang et al, 2019). However, catalysts with strong Lewis bases are prone to deactivate due to the acetaldehyde polycondensation (Ordomsky et al, 2010).…”