One-step gas-phase catalytic oxidation of benzene to phenol with molecular oxygen over Cu-supported ZSM-5 zeolites

Abstract

The gas-phase catalytic oxidation of benzene was carried out using molecular oxygen as an oxidant over Cu-supported catalysts. Phenol was formed only when using Cu-supported high-silica-type zeolites as catalysts. The Cu-supported H-ZSM-5 zeolite (CuH-ZSM-5) was the most active among those tested. Deep oxidation products (CO and CO₂) were also produced together with phenol and they were suggested to form either via phenol or directly from benzene, based on the activity test experiments by varying the contact time. The yield and selectivity of phenol had a maximum with respect to the Cu contents in the CuH-ZSM-5, and were optimized with ca. a 0.77 wt.% Cu loading (phenol yield ∼4.9%). The calcination of the CuH-ZSM-5 at the higher temperature of around 1123 K was found to be effective for the phenol formation. The H-ZSM-5 with lower Si/Al atomic ratios was an effective support for the phenol formation. The EPR study revealed that the increase in the square-pyramidal Cu²⁺ species was brought about when the CuH-ZSM-5 was calcined at higher temperatures or when HZSM-5 with lower Si/Al atomic ratios was used as a support. The isolated square-pyramidal Cu²⁺ species was suggested to be the active species for the phenol formation.