Host (nanopores of zeolite Y)-guest (oxovanadium(IV) tetradentate schiff-base complexes) nanocomposite materials: synthesis, characterization and liquid phase hydroxylation of phenol with hydrogen peroxide

摘要

Oxovanadium(IV) tetradentate Schiff-base complexes; [VO(X_2-haacac)] (X = H, Cl, CH_3 and NO_2), X_2-haacac = substituted bis(2-hydroxyanil)acet-ylacetone; and encapsulated in the nanopores of zeolite NaY; [VO(X_2-haacac)]-NaY; have been synthesized and characterized. The host-guest nanocomposite materials; [VO(X_2-haacac)]-NaY; was characterized by chemical analysis and spectroscopic methods (FT-IR, UV/VIS, XRD, BET and DRS). The analytical data indicated a composition corresponding to the mono-nuclear complex of Schiff-base ligand. The characterization data showed the absence of extraneous complexes, retention of zeolite crystalline structure and encapsulation in the nanopores. Substitution of the aromatic hydrogen atoms of the Schiff-base ligand by electron withdrawing groups like -Cl, and -NO_2 has two major effects: (1) retention and concentration of the oxovanadium(IV) complex in the zeolite cavities is enhanced (due to the larger size of the substituents) and (2) the electronic and spectral properties of the encapsulated complex are modified. Liquid-phase selective hydroxylation of phenol with H_2O_2 to a mixture of catechol and hydroquinone in CH_3CN has been reported using oxovanadium(IV) Schiff-base complexes encapsulated in zeolite-Y as catalysts. Reaction conditions have been optimized by considering the concentration of substrate and oxidant,rnamount of catalyst, effect of time, volume of solvent and temperature. Under the optimized reaction conditions, [VO((NO_2)_2-haacac)]-NaY has shown the highest conversion of 42.3% after 6 h. All these catalysts are more selective toward catechol formation. Encapsulated oxovanadium(IV) complex is catalyti-cally very efficient as compared to other neat complexes for the hydroxylation of phenol and is stable to be recycled without much deterioration.