Nature of vanadium species on vanadium silicalite-1 zeolite and their stability in hydroxylation reaction of benzene to phenol

Abstract

The vanadium silicalite-1 (VS-1) zeolite catalysts with different vanadium contents were prepared by a modified hydrothermal synthesis method, and the one with the highest vanadium content (1.9 wt%) was treated by ammonium acetate (NH₄OAc), H₂O₂ and/or acetic acid (HOAc). The samples were characterized by ICP, XRD, XPS, TPR, and DR UV-Vis to reveal the effect of the state of the vanadium species on the catalysts and to examine their leaching behavior under different conditions. The presence of H₂O₂ removed the vanadium oligomer loaded on the extra-framework of the VS-1. Highly dispersed distorted octahedral V^V species located in the pore of zeolite were unstable in HOAc. The mononuclear octahedral V^V, tetrahedral V^V and V^IV species in the framework showed good stability under the hydroxylation conditions. The VS-1 catalyst with 1.0 wt% of vanadium incorporation in the framework showed a stable hydroxylation activity yielding about 11% phenol (TOF = 63.0) with above 90% selectivity.