Defected and amine-loaded zirconium-based metal–organic frameworks (MOF) UiO-66(Zr) have been characterized and chemically assessed in a condensation reaction of acetone and 4-nitrobenzaldehyde. The present work focuses primarily on the production of the aldol adduct as the main product while minimizing the formation of the enone adduct as a side product. Four analogues with varying numbers of missing linkers in the hexanuclear secondary building units, from one to four out of a total of 12, were synthesized. Thorough characterization showed that an increased number of defects results in an enhanced acidity, a higher internal surface area, and an increased accessibility of coordinatively unsaturated Zr-sites. The catalytic performance assessment revealed that defects, to a certain extent, positively affect the selectivity towards the aldol adduct, . the catalyst, which is missing 3 of the 12 linkers, exhibits the highest activity in a solvent-free environment with a turnover frequency (TOF) amounting to 8.03 10 s. This TOF value is approximately double of the catalyst with 1.4 defects, which exhibited a TOF of only 4.41 10 s. Excluding more than three linkers more likely leads to 'missing cluster defects' and causes a decrease in the reaction rate towards the aldol adduct. A reduction in overall catalytic activity was observed upon functionalization with amine groups (UiO-66-NH). Functionalization with L-proline (UiO-66) resulted in a shift from an acid to a basic reaction mechanism which was reflected by an improved selectivity towards the aldol adduct while maintaining a reasonable overall activity level. The best performances were obtained working solvent-free or with hexane as a solvent, while polar/semi-polar solvents hindered the catalyst performance. An 18-hour stability test demonstrated the robustness of defected and functionalized UiO-66.

中文翻译：

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UiO-66金属有机骨架作为羟醛缩合催化剂：缺陷、溶剂、功能对催化活性和选择性的影响

缺陷型负载胺的锆基金属有机骨架 (MOF) UiO-66(Zr) 已在丙酮和 4-硝基苯甲醛的缩合反应中进行了表征和化学评估。目前的工作主要集中于作为主要产物的羟醛加合物的生产，同时最大限度地减少作为副产物的烯酮加合物的形成。合成了六核二级结构单元中具有不同数量缺失连接子的四种类似物，总共 12 个中的 1 到 4 个。全面的表征表明，缺陷数量的增加导致酸度增强、内表面积增加以及配位不饱和Zr位点的可及性增加。催化性能评估表明，缺陷在一定程度上对羟醛加合物的选择性产生积极影响。该催化剂缺少 12 个连接体中的 3 个，在无溶剂环境中表现出最高的活性，周转频率 (TOF) 达到 8.03 10 s。该TOF值大约是具有1.4个缺陷的催化剂的两倍，其TOF仅为4.41 10 s。排除三个以上的连接体更有可能导致“缺失簇缺陷”并导致羟醛加合物的反应速率降低。胺基 (UiO-66-NH) 官能化后观察到总体催化活性降低。 L-脯氨酸 (UiO-66) 的官能化导致从酸性反应机制转变为碱性反应机制，这反映在对羟醛加合物的选择性提高，同时保持合理的总体活性水平。在无溶剂或以己烷作为溶剂的情况下获得了最佳性能，而极性/半极性溶剂则阻碍了催化剂性能。 18 小时稳定性测试证明了缺陷和功能化 UiO-66 的稳健性。