Highly Stable Crystalline Catalysts Based on a Microporous Metal−Organic Framework and Polyoxometalates

A series of remarkable crystalline compounds [Cu2(BTC)4/3(H2O)2]6[HnXM12O40]·(C4H12N)2 (X = Si, Ge, P, As; M = W, Mo) were obtained from the simple one-step hydrothermal reaction of copper nitrate, benzentricaboxylate (BTC), and different Keggin polyoxometalates (POMs). In these compounds, the catalytically active Keggin polyanions were alternately arrayed as noncoordinating guests in the cuboctahedral cages of a Cu-BTC-based metal−organic framework (MOF) host matrix. X-ray crystallographic analyses, TG, FT-IR, UV−vis, N2 adsorption studies, and acid−base titration demonstrated their high stability and toleration for thermal and acid−base conditions. No POM leaching or framework decomposition was observed in our study. The representative acid catalytic performance of a compound containing PW12 species was assessed through the hydrolysis of esters in excess water, which showed high catalytic activity and can be used repeatedly without activity loss. Moreover, catalytic selectivity, which is dependent on the molecular size of substrates, and substrate accessibility for the pore surface were observed. It is the first time that the well-defined, crystalline, MOF-supported POM compound has behaved as a true heterogeneous acid catalyst. The unique attributes of MOF and well-dispersed level of POMs prohibited the conglomeration and deactivation of POMs, which allowed for the enhancement of their catalytic properties.

通过铜硝酸盐、苯甲酸乙酯（BTC）及多种Keggin型多酸氧簇（POMs）的简单一步水热反应，成功获得了一系列令人瞩目的晶体化合物[Cu2(BTC)4/3(H2O)2]6[HnXM12O40]·(C4H12N)2（X = Si, Ge, P, As；M = W, Mo）。在这些化合物中，催化活性显著的Keggin型多阴离子交替排列于以Cu-BTC为基的金属有机框架（MOF）主基质的三八面体笼中，充当非配位客体。X射线晶体学分析、热重分析（TG）、傅里叶变换红外光谱（FT-IR）、紫外-可见光谱（UV−vis）、氮气吸附研究以及酸碱滴定实验均证实了其优异的稳定性和对热及酸碱条件的耐受性。本研究中未观察到POM的溶出或框架分解现象。含PW12物种的化合物代表性酸催化性能通过过量水的酯水解进行了评估，结果表明其具有高催化活性，且在重复使用过程中活性无显著下降。此外，观察到催化选择性，该选择性依赖于底物的分子大小，以及底物对孔表面的可及性。这是首次将定义明确的、晶态的、MOF负载的POM化合物作为真正的非均相酸催化剂进行应用。MOF的独特属性和POMs的均匀分散性有效地阻止了POMs的聚集和失活，从而提升了其催化性能。