Encapsulation of Cu(II)[2-(2′-hydroxyphenyl)benzimidazole]2 within zeolite nano-cavity: Structural properties and its catalytic activity towards phenol and styrene oxidation. Issue 4 (December 2016) - Shilpa, E.R.
Graphical abstract: Oxidation of phenol and styrene using Cu(opbmzl)2 complex encapsulated within α-cage of zeolite-Y. Highlights: The successful encapsulation of the complex was attested by XRD studies and cyclic voltammetry. Encapsulated Cu(opbmzl)2 within the cavity of zeolite-Y showed better catalytic activity than Cu(opbmzl)2, Cu-Y and NaY towards phenol and styrene oxidation. The reactions proceed via [(HOO)-Cu(opbmz)2 ] intermediate. The catalyst was stable and retained its activity up to few consecutive runs. Abstract: The Cu(opbmzl)2 [ohpbmzl = 2-(2′-hydroxyphenyl)benzimidazole] was encapsulated within the super-cage of zeolite-Na-Y through a facile ligand approach and characterized by various analytical techniques such as elemental analysis, X-ray diffraction, inductively coupled plasma-atomic emission, FT-IR, UV–vis-DRS and EPR spectroscopy, thermogravimetric analysis (TGA), BET surface area measurements, pore volume by Horvath-Kawazoe (HK) method and cyclic voltammetry (CV). The shifting of the absorption bands and changes in redox properties of intra-zeolite complex compared to non-encapsulated complex implicated that the zeolite matrix remarkably influenced the structure-electronic properties. The peak potentials of Cu(opbmzl)2 -Y in cyclic voltammogram were independent of scan rates, further corroborating the intrazeolite mechanism for electron-transfer-pathway in the zeolite-Y. The catalytic activity of Cu-Y, non-encapsulated and encapsulated complex was evaluated for phenol and styrene oxidation. As very low selectivity and formation of tarry products (for phenol oxidation) prevailed with t-butyl hydroperoxide (TBHP), H2 O2 was reported to be the reasonable oxidant for the probe reactions. The influence of reaction parameters such as catalyst dosage, substrate concentration, substrate: H2 O2 mole-ratio, temperature, time and solvent effects were investigated in detail. All the reaction parameters investigated were very crucial in tuning the product selectivity and conversion efficiency. The encapsulated complex exhibited better catalytic activity and selectivity than the non-encapsulated complex for the probe reactions, associated with exceptional structural stability during the recycling process. From the results, it was derived that the reactions proceeded without the involvement of free radicals and the probable mechanistic pathways based on the intermediate complexes associated with Cu-O species were proposed tentatively.