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

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.