Oxidation of benzene to phenol with N2O over a hierarchical Fe/ZSM-5 catalyst

Abstract

Catalytic oxidation of benzene with N₂O to phenol over the hierarchical and microporous Fe/ZSM-5-based catalysts in a continuous fixed-bed reactor was investigated. The spent catalyst was in-situ regenerated by an oxidative treatment using N₂O and in total 10 reaction-regeneration cycles were performed. A 100% N₂O conversion, 93.3% phenol selectivity, and high initial phenol formation rate of 16.49 ± 0.06 mmol_phenol g_catalyst⁻¹ h⁻¹ at time on stream (TOS) of 5 min, and a good phenol productivity of 147.06 mmol_phenol g_catalyst⁻¹ during catalyst life-time of 1800 min were obtained on a fresh hierarchical Fe/ZSM-5-Hi2.8 catalyst. With the reaction-regeneration cycle, N₂O conversion is fully recovered within TOS of 3 h, moreover, the phenol productivity was decreased ca. 2.2 ± 0.8% after each cycle, leading to a total phenol productivity of ca. 0.44 ton_phenol kg_catalyst⁻¹ estimated for 300 cycles. Catalyst characterizations imply that the coke is rapidly deposited on catalyst surface in the initial TOS of 3 h (0.28 mg_c g_catalyst⁻¹ min⁻¹) and gradually becomes graphitic during the TOS of 30 h with a slow formation rate of 0.06 mg_c g_catalyst⁻¹ min⁻¹. Among others (e.g., the decrease of textural property and acidity), the nearly complete coverage of the active Fe-O-Al sites by coke accounts for the main catalyst deactivation. Besides these reversible deactivation characteristics related to coking, the irreversible catalyst deactivation is also observed with the reaction-regeneration cycle. The latter is reflected by a further decreased amount of the active Fe-O-Al sites, which agglomerate on catalyst surface with the cycle, likely associated with the hard coke residue that is not completely removed by the regeneration.