Faculty of Chemistry, University of Bucharest, 4-12 Regina Elisabeta Avenue, 030018 Bucharest, Romania.
Research Center for Catalysts & Catalytic Processes, Faculty of Chemistry, University of Bucharest, 4-12 Regina Elisabeta Avenue, S3, 030018 Bucharest, Romania.
Molecules. 2022 Dec 1;27(23):8391. doi: 10.3390/molecules27238391.
A rapid, cheap and feasible new approach was used to synthesize the Mg0.375Fe0.375Al0.25-LDH in the presence of tetramethylammonium hydroxide (TMAH), as a nontraditional hydrolysis agent, applying both mechano-chemical (MC) and co-precipitation methods (CP). For comparison, these catalysts were also synthesized using traditional inorganic alkalis. The mechano-chemical method brings several advantages since the number of steps and the energy involved are smaller than in the co-precipitation method, while the use of organic alkalis eliminates the possibility of contaminating the final solid with alkaline cations. The memory effect was also investigated. XRD studies showed Fe3O4 as stable phase in all solids. Regardless of the alkalis and synthesis methods used, the basicity of catalysts followed the trend: mixed oxides > parent LDH > hydrated LDH. The catalytic activity of the catalysts in the Claisen−Schmidt condensation between benzaldehyde and cyclohexanone showed a linear dependence to the basicity values. After 2 h, the calcined sample cLDH-CO32−/OH−-CP provided a conversion value of 93% with a total selectivity toward 2,6-dibenzylidenecyclohexanone. The presence of these catalysts in the reaction media inhibited the oxidation of benzaldehyde to benzoic acid. Meanwhile, for the self-condensation of cyclohexanone, the conversions to mono- and di-condensed compounds did not exceed 3.8%.
采用机械化学（MC）和共沉淀法（CP），在四甲基氢氧化铵（TMAH）作为非传统水解剂的存在下，快速、廉价且可行的新方法合成了 Mg0.375Fe0.375Al0.25-LDH。为了进行比较，这些催化剂也使用传统无机碱合成。机械化学方法具有几个优点，因为与共沉淀法相比，步骤数和涉及的能量更小，而使用有机碱消除了最终固体被碱性阳离子污染的可能性。还研究了记忆效应。XRD 研究表明，所有固体中均存在 Fe3O4 作为稳定相。无论使用的碱和合成方法如何，催化剂的碱性遵循以下趋势：混合氧化物>母体 LDH>水合 LDH。催化剂在苯甲醛和环己酮的 Claisen-Schmidt 缩合反应中的催化活性与碱性值呈线性关系。2 小时后，煅烧样品 cLDH-CO32−/OH−-CP 提供了 93%的转化率和 2,6-二苄基环己酮的总选择性。这些催化剂在反应介质中的存在抑制了苯甲醛氧化为苯甲酸。同时，对于环己酮的自缩合，单缩合和双缩合化合物的转化率不超过 3.8%。
