Tri- and tetradentate copper complexes: a comparative study on homogeneous and heterogeneous catalysis over oxidation reactions

Abstract

Catalytic liquid phase oxidation of benzene, phenol, styrene and cyclohexene was carried out using Schiff-base copper(II) complexes of ligands 4-(2-amino-ethylimino)-pent-2-en-2-ol (HL¹) and (4-[2-(3-hydroxy-1-methyl-but-2-enylideneamino)-ethylimino]-pent-2-en-2-ol (H₂L²) in homogenous and heterogeneous media. Treatment of Cu-Y-zeolite with ligand solutions led to insertion of the flexible ligands into zeolite super-cages and in situ formation of encapsulated copper(II) complexes inside the zeolite cavities. The encapsulated complexes [Cu(L²)]-Y and [CuCl(L¹)]-Y were characterized using FT-IR, UV-vis, Raman spectrometry, BET, XRD and thermal analysis techniques, substantiating the formation of intrazeolitic copper complexes without loss in the crystallinity for the zeolite framework structure. Catalytic activities of the encapsulated complexes and their “neat” analogues were investigated for the oxidation of benzene, phenol, styrene and cyclohexene using H₂O₂ as an oxidant. Conversion efficiencies were optimized by varying reaction parameters, i.e. temperature, oxidant concentration, catalyst amounts and solvent nature and volume. In all oxidation reactions, the neat complexes exhibited higher conversion efficiency than the encapsulated complexes at shorter reaction times under optimum reaction conditions. Although the zeolite-encapsulated complexes required longer reaction times to reach maximum conversion, they were beneficial towards enhancing the selectivity and TOF values as compared to the neat analogues. Furthermore, the heterogeneous catalytic system can be recovered and reused without activity loss. The possible reaction pathway for phenol oxidation catalysed by these complexes was investigated by UV-visible spectroscopy and electrochemical methods.