Zeolite encapsulated Ni(ii) and Cu(ii) complexes with tetradentate N2O2 Schiff base ligand: catalytic activity towards oxidation of benzhydrol and degradation of rhodamine-B

Abstract

Nickel(II) and copper(II) complexes of the Schiff base ligand O,O′-trimethyl bis (salicylidene isonicotinylhydrazone) (H₂L) were synthesized and characterized. The encapsulation of these complexes in the cavities of the zeolite was achieved by a fixed ligand method (FLM). The free complexes (FC) and the retention of the zeolite encapsulated metal complexes [ZEMC; i.e., Ni(II)L–Y, Cu(II)L–Y] were investigated by several spectroscopic, thermal analysis, sorption and microscopic techniques [i.e., FTIR, NMR, ESI-mass, conductivity, DRS/UV-Vis, EPR, XPS, AAS, TGA, XRD, nitrogen isotherm, magnetic and (SEM/TEM)EDX]. This study reveals the formation and stability of the metal complexes in the cavities of the zeolite. The catalytic activities of free complexes and encapsulated complexes were studied in the liquid phase oxidation of benzhydrol/H₂O₂ and the photodegradation of rhodamine-B (RhB) under UV/visible (H₂O₂) irradiation. The catalytic activities of benzhydrol (BH) oxidation and RhB degradation were higher with free complexes and encapsulated complexes, respectively. In most cases, the copper(II) [Cu(II)L·2ClO₄, Cu(II)L–Y] complexes showed better activity than the nickel(II) complexes [Ni(II)L·2ClO₄, Ni(II)L–Y] in both BH and RhB degradation reactions. The enhancement of Cu(II)L–Y activity was mainly due to the metal present in the cavities and on the surface. Hence, in the case of FC, rate of the reaction (% of conversion) for the copper(II) complexes is more than that for nickel(II) in BH oxidation, but, in the case of ZEMC, the photocatalytic activity and rate of reaction (% of conversion) were more for Cu(II)L–Y than Ni(II)L–Y. These results show that both the oxidation reactions follow pseudo first order kinetics. The catalytic activities of the recovered and purified ZEMC were compared with the fresh catalyst.