Transition metal complexes of 4-hydroxy-3-methoxybenzaldehyde embedded in fly ash zeolite as catalysts for phenol hydroxylation

Abstract

The focus of this work is to use fly ash, a waste generated by thermal power plants, to synthesize an economical and efficient heterogeneous catalyst for the fine chemicals industry. To achieve this goal, fly ash zeolite (FAZ) was prepared from fly ash and Cu(II), Ni(II) and Co(II) complexes of 4-hydroxy-3-methoxybenzaldehyde [M(VAN)-FAZ] were loaded in the channels of zeolite matrix by flexible ligand strategy. The prepared FAZ was characterized by XRD, SEM, FT-IR, TGA, XRF and BET analyses. Encapsulation of metal complexes in zeolite gains the advantage of heterogeneity thereby facilitating easy separation of products and selectivity. The incorporation of metal complexes in the framework of FAZ was further assured by FT-IR, XRD, TGA, AAS and UV–Visible analyses of M(VAN)-FAZ. Thermographs indicated a loading of 6.70–16.09% of the metal complexes in FAZ. The –OH stretching reported at 3184 cm−1 for the free ligand was absent in the FT-IR spectra of the encapsulated metal complexes, indicating the binding of this group with the metal ion. The loading of metal complexes in the pores of FAZ have been further confirmed by AAS reports. The FAZ encapsulated transition metal complexes of vanillin have been established as catalysts for phenol hydroxylation. The extent of phenol conversion increased with time, after 240 min, 90% phenol conversion was observed and the preferential catalytic activity of [M(VAN)-FAZ] was observed as: [Cu(II)(VAN)-FAZ] > [Co(II)(VAN)-FAZ] > [Ni(II)(VAN)-FAZ]. The products were characterized by GC-MS analysis and the recyclability of the prepared catalyst was assessed up to three cycles.