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.

一系列引人注目的晶态化合物[Cu2(BTC)4/3(H2O)2]6[HnXM12O40]·(C4H12N)2（X = Si, Ge, P, As；M = W, Mo）通过铜硝酸盐、苯甲酸甲酯（BTC）及不同凯哥林多氧金属酸（POMs）的简单一步水热反应获得。在这些化合物中，催化活性凯哥林多聚阴离子交替排列于Cu-BTC基金属有机框架（MOF）主基质的三棱八面体笼中作为非配位客体。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、氮气吸附研究以及酸碱滴定实验均证实了它们的高稳定性和对热及酸碱条件的耐受性。本研究中未观察到POM的浸出或框架分解。含有PW12物种的化合物的代表性酸性催化性能通过过量水中酯的水解进行评估，显示出高催化活性和可重复使用而不失活的特点。此外，还观察到了催化选择性与底物分子大小以及底物对孔表面的可及性。这是首次明确界定、结晶状、MOF负载的POM化合物表现出真正的非均相酸催化剂的特性。MOF的独特属性以及POMs的良好分散度阻止了POMs的聚集和失活，从而增强了它们的催化性能。