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 CO 2 ) 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 Cu 2+ 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 squarepyramidal Cu 2+ species was suggested to be the active species for the phenol formation.

Aerobic one-step oxidation of benzene to phenol on copper exchanged HZSM5 zeolites: A mechanistic study

“…It is obvious that a high copper loading promotes the formation of the deep oxidation products. These results correspond with the result of oxygen activation on the copper centers and with the observation that was made previously in literature [3][4][5]. The more copper centers are available in the zeolite the more oxygen molecules can be activated.…”

“…The catalysts which provide the best yields of phenol can be identified from figure 4. Obviously, the obtained amounts of phenol were not as high as it has been reported in literature [3][4][5]. However the main aim of this study is to understand the reaction mechanism on a molecular scale, so the small yields of phenol did not diminish our results.…”

“…The isolated Cu ions and the peroxo di-copper complex were both mentioned previously in literature in context with oxygen activation [3,12,13]. Hamada et al suggested isolated, squarepyramidal Cu 2+ ions to be the active sites for the phenol formation on Cu/HZSM5 zeolites [3].…”

“…The isolated Cu ions and the peroxo di-copper complex were both mentioned previously in literature in context with oxygen activation [3,12,13]. Hamada et al suggested isolated, squarepyramidal Cu 2+ ions to be the active sites for the phenol formation on Cu/HZSM5 zeolites [3]. The mechanism of oxygen activation on di-copper complexes was studied for the enzyme tyrosinase [12,13], and the Cu-Cu distance in Cu/HZSM5 was found to be similar to that in tyrosinase [7], suggesting that oxidation on a zeolite may occur by the same mechanism as in the enzyme.…”

“…Hamada et al explored the gas-phase reaction on several Cu/HZM5-zeolites under different reaction conditions and reported a phenol yield up to 5% with a selectivity of nearly 30% [2][3][4]. Meanwhile phenol yields up to 10% have been reported [5], but the detailed reaction mechanism for the phenol formation is still unknown.…”