Photochemical fixation and reduction of sulfur dioxide to sulfide by tetraphenylporphyrin magnesium: Spectroscopic and kinetic studies

The photochemical reaction of sulfur dioxide (SO2) with tetraphenylporphyrin magnesium (MgTPP) has been investigated in dichloromethane (CH2Cl2) solution at room temperature with illumination by visible light. Conventional fluorescence, UV-vis, and MS spectral analyses showed that under these conditions, SO2 was initially photochemically fixed by MgTPP to form a 1:1 molecular adduct. On continued irradiation and maintaining the flow of SO2, MS and XRD results showed that MgTPP is remarkably effective in the photochemical reduction of SO2 to sulfide (S2-). The kinetics of the photochemical reaction of MgTPP with SO2 was studied in a SO2-saturated solution. Under irradiation, the reaction follows pseudo first order kinetics for MgTPP, having a half-life decreasing from 106 to 57 min as the illumination intensity is increased from 350 to 600 Lm. This investigation of the photochemical fixation and reduction of SO2 by MgTPP is of key interest in elucidating fundamental photochemical reaction mechanisms associated with porphyrins in the presence of SO2; furthermore, the analysis of the photochemical reaction may offer new opportunities for the fixation and reduction of SO2 to less harmful species.