Gas phase hydroxylation of benzene with air–ammonia mixture over copper-based phosphate catalysts

• Sponge-like Ca–Cu phosphate were prepared.
• They catalyze selective oxidation of benzene to phenol.
• The air–ammonia mixture is the oxidant.
• >95% selectivity and 4.2 mol% yield was achieved.
• During 37 h on stream the catalyst was not notably deactivated.

The direct gas phase hydroxylation of benzene to phenol over copper-based phosphate catalysts has been studied. Nitrous oxide and air–ammonia mixtures were used as oxidizing agents. It has been found that nitrous oxide can be replaced by air–ammonia mixture without significant deterioration of the phenol yield. Nanostructured Ca–Cu phosphate catalysts with a dominant mixed pyrophosphate structure, prepared in the presence of suitable surfactants were found highly active and selective. Using surfactant-modified phosphate catalysts with sponge-like structure above 95% selectivity and about 6.5% and 4.2% phenol yields were achieved at the benzene WHSV 1 h−1 and under not optimized reaction conditions using N2O and air–ammonia mixture as oxidizing agents, respectively. These catalysts, compared with unmodified ones, were found to be much more resistant toward deactivation caused by the chemical reduction of catalytically active copper phase during the catalytic test. The activity of the spongy Ca–Cu phosphate catalyst after 37 h on stream decreased only by about 25%. In contrast, over similar unmodified Ca–Cu phosphate catalyst the phenol yield already after 8 h on stream drops to about one third of the initial value.

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