Homogeneous catalysis and heterogeneous recovery: Thermo-responsive copolymer visible light catalyst with phthalocyanine as side groups

Metal phthalocyanines (MPcs) have attracted more and more attention in recent years for the catalytic degradation of pollutants in water driven by visible light. However, they are easy to aggregate in the aqueous phase and difficult to recycle, which seriously limits their photocatalytic activity and applicability. Introducing phthalocyanine into polymers may be an effective way to solve the above problems. Herein, three kinds of copolymers PNIPAMx-co-PDUPcZny with different molecular weights and monomer ratios were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization, with S-1-dodecyl-S'-(alpha,alpha'-dimethyl alpha"-acetic acid)trithiocarbonate (DDMAT) as chain transfer agent, 3-undecenamide-9(10),16(17),23(24)-decyloxy-zinc phthalocyanine (DUPcZn) and N-isopropyl acrylamide (NIPAM) as monomers, 2,2' -azobis(2-methylpropionitrile) (AIBN) as initiator. The polymerization process shows the characteristics of living polymerization, and the polymer dispersity index (PDI) values are <1.1. The three copolymers exhibit obvious thermo-responsive characteristics and the lower critical solution temperatures (LCSTs) are between 32.7 C and 33.1 C-degrees. The photocatalytic activity of the copolymers was evaluated by using 1,5-dihydroxynaphthalene (DHN) and Cr(VI) as model pollutants. It was found that the three copolymers had photocatalytic oxidation performance for DHN and photocatalytic reduction performance for Cr(VI), and their photocatalytic property was affected by the pH of the model pollutant solution and the catalytic temperature. Among them, PNIPAM57-co-PDUPcZn7 exhibit a photocatalytic oxidation rate of 93.0% for DHN and the photocatalytic reduction rate of 70.0% for Cr(VI), respectively, under the visible light irradiation for 120 min at pH = 5 and 32(degrees) C. More importantly, the homogeneous catalysis and heterogeneous recovery are achieved in the catalytic process through simple temperature regulation. In addition, the copolymers exhibited excellent chemical stability and reusability after three cycles. Finally, O21/e- was determined to be the active species that plays a crucial role in the photocatalytic oxidation/reduction process, upon which a possible photocatalytic mechanism was proposed. This research provides a promising strategy for the synthesis of environmentally friendly visible light catalysts for the sewage (waste) water treatment.