Vapor phase nitration of benzene using mesoporous MoO3/SiO2 solid acid catalyst

Abstract

Nitration of benzene has been carried out in the vapor phase, using dilute nitric acid as the nitrating agent, over mesoporous MoO₃/SiO₂ solid acid catalyst with more than 90% benzene conversion and 99.9% selectivity for mononitrobenzene. A series of MoO₃/SiO₂ catalysts with varying MoO₃ loadings (1–30 mol%) were prepared using a sol–gel technique and characterized using X-ray diffraction analysis (XRD), BET specific surface area analysis, temperature-programmed desorption (TPD) of ammonia, and FTIR spectroscopic analysis of adsorbed pyridine. XRD analysis revealed the amorphous nature of the catalyst for 1 and 10 mol% MoO₃ loading, and the formation of a crystalline α-MoO₃ phase on the amorphous silica support at higher MoO₃ loading. The BET surface area for pure silica support was 606 m² g⁻¹ and the pore size distribution in the range 40–80 Å, showing the mesoporous nature of amorphous silica. The BET surface area decreased to 583 m² g⁻¹ with incorporation of 1% MoO₃ and further decreased to 180 m² g⁻¹ with increasing MoO₃ loading up to 20%. Above 10% MoO₃ loading, formation of crystalline MoO₃ clusters on amorphous silica was observed. Ammonia-TPD showed a drastic increase in acid strength after addition of 1% MoO₃ compared to pure silica support and the acid strength increased with increasing MoO₃ content. Among the series of catalysts prepared, MoO₃/SiO₂ containing 20 mol% MoO₃ was found to be the most active catalyst for benzene nitration, without showing any deactivation after more than 1000 h, showing a very high potential for commercialization.