Transition metal complexes enslaved in the supercages of zeolite-Y: DFT investigation and catalytic significance

Abstract

A series of zeolite-Y encapsulated hybrid catalysts, [M(STCH)·xH2O]-Y have been prepared by encapsulating Schiff base complexes [where M = Mn(II), Fe(II), Co(II), Ni(II); (x = 3) and Cu(II); (x = 1); H2STCH = salicylaldehyde thiophene-2-carboxylic hydrazone] in zeolite-Y matrix by flexible ligand method. These hybrid materials have been characterized by various physico-chemical techniques such as ICP-OES, elemental analyses, (FT-IR and electronic) spectral studies, BET, scanning electron micrographs, thermal analysis and X-ray powder diffraction patterns. X-ray powder diffraction analysis reveals that the structural integrity of the mother zeolite in the hybrid material remained intact upon immobilization of the complex. Density functional theory is employed to calculate the relaxed structure, bond angle, bond distance, dihedral angle, difference of highest occupied molecular orbital and lowest unoccupied molecular orbital energies gap and electronic density of states of ligand and their neat transition metal complexes. The hybrid materials are active catalysts for the hydroxylation of phenol using hydrogen peroxide (30% H2O2) as an oxidant in order to selectively synthesize catechol or hydroquinone, amongst them [Cu(STCH)·H2O]-Y shown the highest % of selectivity towards catechol (81.3%).