几何效应促进水滑石催化剂实现对羟醛缩合反应性能提升

Geometric effect promoted hydrotalcites catalysts towards aldol condensation reaction

近年来,绿色发展观念深入人心.与液体碱相比,固体碱催化剂由于其环境友好、腐蚀性小、易于回收等优点引起了科研工作者的广泛关注.但是,在固体碱催化领域,如何对碱性位点进行调控从而使羟醛缩合反应获得优异性能仍然是一个很大挑战.甲醛和异丁醛反应生成的产物羟基新戊醛是精细化工合成中一类非常重要的有机中间体,广泛应用于药物、润滑油、聚酯树脂等化工产品生产.目前,应用于该反应的固体碱催化剂催化活性较低,性能有待进一步提升.因此,设计一类结构可调、性能优异的固体碱催化剂材料迫在眉睫.本文利用水滑石材料结构特有的记忆效应制备了一系列掺杂镓、铟的钙铝水滑石催化剂,并将其应用于甲醛与异丁醛缩合生成羟基新戊醛反应,并探讨了不同离子半径的元素掺杂对钙铝水滑石结构产生的影响和作用.结果表明,复原后的re-Ca4Al0.90Ga0.10-LDH对羟基新戊醛的生成表现出优异的性能(HPA产率为72%),达到固体碱催化剂催化该反应的最高水平,甚至与液体碱催化剂水平相当.通过氘代氯仿原位红外光谱对催化剂的活性位点进行表征,结果证明掺入镓之后使弱碱性位点的相对浓度增加,而掺入铟后使弱碱性位点的相对浓度减小.进而将弱碱性位点的数量和中强碱性位点的数量与催化性能结果进行构效关联,证明了该反应中的活性中心为弱碱性位点(7配位Ca-OH).该弱碱性位点不仅促进了产物羟基新戊醛的脱附,还提高了反应的活性和选择性,使得羟基新戊醛的产率大大提升.通过EXAFS手段对催化剂的精细结构进行表征,证明了掺入镓和铟后,钙铝水滑石晶格发生了膨胀,Ca-O键长增加.其中,掺杂铟元素使得铟与水滑石层间羟基形成额外的In-OH化学键,从而使七配位的Ca-OH位点浓度降低.通过XPS和DFT计算证明了掺入镓和铟后催化剂的碱性强度没有发生变化,说明镓和铟的掺入只改变了碱性位点数量而对碱性位点强度没有明显影响.最后,对催化剂结构和性能进行了关联,证明了镓作为结构助剂掺入到水滑石层板中产生的几何效应使得晶格发生膨胀从而暴露了更多的活性位点,实现了催化反应活性的极大提升.该工作揭示了不同离子半径元素的掺杂所产生的几何效应对催化活性位点的有效调控作用,从而大大提升了羟醛缩合反应性能,为制备新型优异的固体碱催化剂提供了思路和依据.

In solid basic catalysis field,how to achieve optimized activity and desired stability through elaborate control over basic site properties remains a challenge.In this work,taking advantage of the structure memory effect of layered double hydroxides(LDHs),rehydrated Ca4 Al1-x Gax-LDHs and Ca4 Al1-x Inx-LDHs catalysts were prepared and applied in aldol condensation reaction that isobutyraldehyde(IBD)reacts with formaldehyde(FA)to obtain hydroxypivalaldehyde(HPA).Notably,the resulting re-Ca4 Al0.90Ga0.10-LDHs exhibits an extraordinarily-high catalytic activity(HPA yield:72%),which is to our best knowledge the highest level in this reaction.The weak Br?nsted basic site,7-coordinated Ca-OH group,which serves as an active site,catalyzes the condensation process and promotes the product desorption.Studies on structure-property correlations demonstrate that Ga as a structural promoter induces a moderate expansion of the laminate lattice,which results in a significant increase in the concentration of weak basic sites in re-Ca4Al0.90Ga0.10-LDHs,accounting for its high catalytic activity.This work illuminates that geometric structure of basic active sites can be tuned via introducing catalyst additive,which leads to a largely improved performance of hydrotalcite solid basic catalysts towards aldol condensation reaction.