Highly efficient sulfonated-polystyrene–Cu(II)@Cu3(BTC)2 core–shell microsphere catalysts for base-free aerobic oxidation of alcohols

Abstract

A novel catalyst consisting of a functional sulfonated-polystyrene (SPS) core, a porous Cu₃(BTC)₂ shell and an active Cu(II) interface between the core and shell was developed via a facile step-by-step assembly method. The polystyrene core was sulfonated first to achieve functional –SO₃H groups on its surface. The main function of the –SO₃H groups was to graft Cu(II) ions to generate an active Cu(II) interface, and the excess –SO₃H could provide acid conditions for the catalytic reaction. The Cu(II) interface along with the acid conditions and the co-catalyst 2,2,6,6-tetramethyl-piperidyl-1-oxy (TEMPO) enhanced the catalytic activity for the aerobic oxidation of alcohols to aldehydes by molecular oxygen under base-free conditions. A portion of Cu(II) ions on the SPS surface was then coordinated with H₃BTC (1,3,5-benzenetricarboxylic acid) to form a porous Cu₃(BTC)₂ shell, which could protect the active metal from leaching as well as provide porous channels for mass transfer, resulting in high stability and recyclability in the catalysis procedure. The SPS–Cu(II)@Cu₃(BTC)₂ catalyst could be recycled ten times without a significant loss in its activity and selectivity. Furthermore, the SPS–Cu(II)@CuBDC (BDC = 1,4-benzenedicarboxylate) composite was also synthesized and showed high efficiency for catalyzing the aerobic oxidation of alcohols and aerobic homocoupling of arylboronic acids, suggesting that the unique nanostructure of SPS–Cu(II)@MOFs can be easily extended to design complex catalysts with high efficiency and good stability for different catalytic reactions.