Application of anionic porphyrin as chromogenic indicator and metalloporphyrin catalysts for oxidation of benzyl alcohol

Abstract

In recent years, extensive research has been carried out on the synthesis of anionic watersoluble porphyrin, meso-tetrakis(4-sulphonatophenyl)porphyrin (TSPP) due to the potential applications in photodynamic therapy, sensors, optical devices and catalysis. However, the information on anion recognition properties of free-base TSPP and performance of TSPP metalloporphyrin complexes in heterogeneous catalytic oxidation are relatively scarce. Hence, this research is divided into two main parts. Firstly, free-base TSPP was utilized as chromogenic reagent for recognition of acetate and citrate ions in aqueous solutions and the optical properties were characterized by ultravioletvisible (UV-Vis) and fluorescence spectroscopies. The interaction of TSPP with the acetate and citrate ions was quantitatively analyzed at pH 4 using different acids (H2SO4, HCl, HNO3, H3PO4 and CH3COOH) for the pH adjustments. The anion sensing mechanism was based on protonation of the imino nitrogens of TSPP porphyrin core in the presence of acids (pH~4), whereby the optical changes observed in the absorption and emission spectra of the pH 4 TSPP solution were the effects of pH changes induced by acetate or citrate anion during deprotonation of TSPP. The Stern-Volmer constant based on the fluorescence quenching spectra revealed that the sensitivity of pH 4 TSPP was almost unaffected by the different acids used for the pH adjustment. For the UV-Vis spectroscopy method, the lowest limit of detection (LOD) of acetate ion (5.61  106 M) was obtained using nitric acid, while for the fluorescence spectroscopy method, the LOD (4.66  106 M) of acetate ion was obtained when sulphuric acid was used. Detection of citrate ion was only possible when acetic acid was used for the pH adjustment, with LOD of 9.59  106 M and 9.79  106 M for UV-Vis and fluorescence spectroscopies, respectively. Matrix anions (chloride, nitrate, phosphate, carbonate and sulphate ions) effects on the acetate and citrate detection were also studied. The linear range of the fluorescence detection was wider (2–19 × 10–5 M) as compared to the UV-Vis detection (4–18 × 10–5 M). Besides, no significant matrix anion effect could be observed in the fluorescence spectra. The microspecies distributions of TSPP, acetate and citrate ions were simulated using the Marvin Bean ChemSketch® analysis. The results suggested that the pH of the TSPP solution was instantaneously increased upon the addition of acetate or citrate anion. Consequently, the –2 charge TSPP was converted to –4 charge TSPP, accompanied with significant optical changes. In the second part of this work, TSPP was coordinated to Mn(III), Fe(II), Co(II) and Cu(II) ions to yield the meso-tetrakis(4-sulphonatophenyl)porphyrinato metal complexes (TSPP-M) for catalytic oxidation of benzyl alcohol to benzaldehyde with tert-butyl hydroperoxide as oxidant. The neat TSPP-M showed superior catalytic performance (~60% conversion) as compared to that of free-base TSPP (4.8% conversion). Subsequently, TSPP-M was immobilized on the mesoporous silica support Santa-Barbara Amorphous-15 (SBA-15) via post-synthesis method to afford the heterogeneous catalysts TSPP-M/SBA-15 (M: Mn, Fe, Co and Cu). The TSPP-M/SBA-15 catalysts exhibited high BET (Brunauer-Emmett-Teller) surface areas > 150 m2 g−1 with average pore diameters > 5 nm. The effects of reactant to oxidant molar ratio, reaction temperature, solvent, and reaction time were further investigated. Under the optimal conditions, the catalytic trend was in the order: Mn(III) > Fe(II) > Co(II) > Cu(II). The TSPP-Mn/SBA-15 catalysts gave > 99% selectivity and turn over frequency of 665 h−1 after 3 hours of reaction. In addition, all the TSPP-M/SBA-15 catalysts could be reused up to three recycles without significant loss of catalytic activity. The catalytically active species leached out of TSPP-M/SBA-15 surface was extremely small and generally, can be considered negligible. Findings of present studies are significant because the anionic free-base TSPP was reported for the first time as chromogenic reagent for anion recognition. Furthermore, the TSPP-M/SBA-15 has great potential as an environmental friendly heterogeneous catalyst that offers the prospect of commercialization in the production of chlorine-free benzaldehyde.