Cationic mercaptopyridine-substituted phthalocyanine/MCM-41: an efficient photocatalyst for visible-light synthesis of juglone

Selective oxidation of naphthol derivatives under visible light presents an environmentally friendlier alternative to conventional oxidation methods. Here, we report the preparation of a heterogeneous photocatalyst, Pc-3/MCM-41, by immobilizing a cationic zinc phthalocyanine (Pc-3) onto mesoporous MCM-41 silica via electrostatic interactions. Spectroscopic, thermal, and microscopic analyses indicated that Pc-3 was successfully incorporated into the silica support, with reasonably uniform dispersion and minimal aggregation. Under red LED irradiation, the catalyst promoted the oxidation of 1,5-dihydroxynaphthalene (1,5-DHN) to juglone, achieving ca. 80-85% conversion under certain conditions. The catalyst can be recovered by filtration and reused, although its activity declines over several cycles. Kinetic investigations under these conditions revealed a maximum pseudo-first-order rate constant of 0.5484 min-1. Langmuir-Hinshelwood modeling yielded adsorption (Kad) and reaction rate (kr) constants of 140.48 mmol-1 and 0.0104 mmol min-1, respectively. Because substrate adsorption caused deactivation, regeneration via washing with N,N-dimethylformamide partially restored catalytic activity. These results highlighted Pc-3/MCM-41 as a promising visible-light photocatalyst for oxidation reactions, though improvements in stability and cycle performance remain necessary.