The first solid state porphyrin-weak acid molecular complex: A novel metal free, nanosized and porous photocatalyst for large scale aerobic oxidations in water

Abstract

The first solid state porphyrin dication of a weak carboxylic acid was synthesized through the reaction of meso-tetrakis(N-methylpyridinium-4-yl)porphyrin (H2TMPyP) supported on the sodium salt of Amberlyst 15 nanoparticles (nanoAmbSO3Na) with formic acid. The polymer-porphyrin hybrid compound, nanoAmbSO3@H2TMPyP(HCOOH)2, was characterized by FT-IR and diffuse reflectance (DR) UV–vis spectroscopy, EDX, BET, DLS, FESEM and TGA methods. Interestingly, the diacid was quite stable towards decomposition to the corresponding porphyrin and carboxylic acid in neat water as well as in acetonitrile. The protonation reaction was accompanied with the shift of the Soret band from 435 to 454 nm in DR UV–vis spectrum and a rapid color change from red brown to green. A particle size of 190 and <50 nm was estimated for the nanoparticles by DLS and FESEM, respectively. Also, the macroporous structure of the catalyst was revealed by BET experiments. nanoAmbSO3@H2TMPyP(HCOOH)2 was used as photosensitizer for the highly chemoselective and large scale aerobic oxidation of sulfides to the corresponding sulfoxide in water within 3 h at room temperature. A conversion of ca. 95% was observed for the oxidation of sulfides to their corresponding sulfoxide in water with a selectivity of ca. 100%. The use of acetonitrile as solvent led to a significant decrease in the conversion values while retaining selectivity. The higher efficiency of the catalyst in water was in accord with greater singlet oxygen quantum yield (ΦΔ) value of the photosensitizer in this solvent (ΦΔ = 0.41) relative to that in acetonitrile (ΦΔ = 0.08). Apparently, cooperative acid catalysis caused by the protic solvent and porphyrin diacid is involved in the preference of water over acetonitrile as the solvent for efficient oxidation of sulfides to sulfoxides. Furthermore, the photosensitizer was used for the efficient oxidation of 1,5-dihydroxynaphthalene (DHN) in water within 15 min. The photocatalyst was remarkably stable towards oxidative degradation so that it could be recovered and reused for at least 5 times without loss of activity. It is noteworthy that large scale photooxidation of sulfides (TON ≈ 5 × 3000) and DHN (TON ≈ 6 × 1000) were achieved in water, using nanoAmbSO3@H2TMPyP(HCOOH)2 as the photosensitizer.