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

TLDR

A one‑step hydrothermal synthesis of crystalline compounds [Cu₂(BTC)₄/₃(H₂O)₂]₆[HₙXM₁₂O₄₀]·(C₄H₁₂N)₂ (X = Si, Ge, P, As; M = W, Mo) was achieved by reacting copper nitrate, benzenetricarboxylate, and Keggin polyoxometalates, with the POMs incorporated as noncoordinating guests in cuboctahedral cages of a Cu‑BTC MOF host. X‑ray crystallography, TG, FT‑IR, UV‑vis, N₂ adsorption, and acid–base titration revealed exceptional thermal and acid–base stability with no POM leaching or framework decomposition, while the PW₁₂‑containing compound displayed high, reusable acid catalytic activity in ester hydrolysis, with selectivity governed by substrate size and pore accessibility, constituting the first true heterogeneous acid catalyst based on crystalline MOF‑supported POMs.

Abstract

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.