A new mixed-linker Fe-MOF as a multifunctional bio-photocatalyst for tandem photo-oxidation condensation reaction

Abstract

A novel mixed-linker of iron-based metal–organic frameworks named Fe(INA)(H2N-BDC) has been directly synthesized by mixing 2-amino-1,4-benzenedicarboxylic acid (NH2-BDC) and isonicotinic acid (INA) using hydrothermal method. Its characterization was fully performed by a number of methods such as FT-IR, PXRD, XPS, FESEM, TEM, TGA, ICP and BET and used as a multifunctional bio-photocatalyst for the preparation of 2,4,6-triarylpyridines (TAPs) via tandem photo-oxidation condensation reactions starting from alcohols under sunlight irradiation and air atmosphere in ethanol. Mixed-linker Fe-MOF showed enhanced catalytic activity compared with its analogues with one single ligand, which indicates the synergistic effect of two linkers and Fe-O clusters in Fe(INA)(H2N-BDC) as a photocatalyst with lower band gap. This observation confirmed by an important reduction in the band gap energy and a rise in the characteristic emission of mixed-linker Fe-MOF compared with Fe-MIL-101-NH2 according to the DRS analysis and fluorescence spectrophotometry, respectively. EPR studies clearly indicated that Fe(INA)(H2N-BDC) produces 1O2 and O2•- as active oxygen units under sunlight illumination. Moreover, amine groups in NH2-BDC provide basic functional groups, which in cooperation with acidic sites (Fe in the metal nodes) promote the condensation of the in-situ formed aldehydes with acetophenone and ammonium acetate. Using sunlight irradiation as an energy-saving source, air as a safe oxidant, good yields, reusability of the photocatalyst and any requirement of additives make the present method as an environmentally-friendly method for the preparation of these important scaffolds. This is the first report of the synthesis of TAPs starting from alcohols catalyzed by MOFs.