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

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.