Oxidovanadium(IV) Complexes of Tetradentate Ligands Encapsulated in Zeolite‐Y as Catalysts for the Oxidation of Styrene, Cyclohexene and Methyl Phenyl Sulfide

Abstract

AbstractThe reaction of oxidovanadium(IV)‐exchanged zeolite‐Y with N,N′‐ethylenebis(pyridoxyliminato) (H2pydx‐en, I), N,N′‐propylenebis(pyridoxyliminato) (H2pydx‐1,3‐pn, II) and H2pydx‐1,2‐pn (III) in methanol heated at reflux leads to the formation of the corresponding complexes, abbreviated herein as [VIVO(pydx‐en)]‐Y (4), [VIVO(pydx‐1,3‐pn)]‐Y (5) and [VIVO(pydx‐1,2‐pn)]‐Y (6) in the supercages of zeolite‐Y. The neat complexes [VIVO(pydx‐en)] (1), [VIVO(pydx‐1,3‐pn)] (2) and [VIVO(pydx‐1,2‐pn)] (3) were also prepared. Spectroscopic studies (IR, UV/Vis and EPR), elemental analyses, thermal studies, field‐emission scanning electron micrographs (FESEM) and X‐ray diffraction patterns were used to characterize these complexes. Oxidations of styrene, cyclohexene and methyl phenyl sulfide were investigated using these complexes as catalyst precursors in the presence of H2O2 as oxidant. Under the optimized reaction conditions, a maximum of 85.5 % conversion of styrene was obtained with 4, 84.6 % conversion with 5 and 82.9 % conversion with 6 in 6 h of reaction time. The selectivity of the various products was similar for the catalyst precursors 4–6 and followed the order: benzaldehyde > 1‐phenylethane‐1,2‐diol > benzoic acid > phenyl acetaldehyde. With cyclohexene, a maximum conversion of 95.9 % was achieved with 4, 94.5 % with 5 and 94.2 % conversion with 6, also in 6 h of reaction time. The selectivity of the various products was similar for the three catalysts: 2‐cyclohexen‐1‐one > 2‐cyclohexen‐1‐ol > cyclohexane‐1,2‐diol. The oxidation of methyl phenyl sulfide was achieved with 4, 5 and 6 in 2.5 h of reaction time with 85.5, 82.1 and 80 % conversion, with higher selectivity towards sulfoxide. Overall, the encapsulated catalysts were significantly more active than their neat counterparts and have the further advantage of being recyclable. No relevant difference in activity was found due to a change in the diamine in the Schiff base ligands I–III. UV/Vis and 51V NMR spectroscopic experiments with 1 confirmed the plausible formation of VVO(O2)L as intermediates in the catalytic oxidations.