A novel route to produce phthalic anhydride by oxidation of o-xylene with air over mesoporous V-Mo-MCM-41 molecular sieves

Mesoporous V-Mo-MCM-41 with Si/(V + Mo) ratio equal to 46, 92, 184 and 230 were synthesized using cetyltrimethylammonium bromide surfactant as template under hydrothermal conditions while the mesoporous materials were characterized using several techniques e.g., ICP-AES, XRD, N₂-adsorption, FTIR, TG/DTA, ⁵¹V-MAS-NMR, ⁹⁵Mo-MCM-41, ²⁹Si-MAS-NMR, SEM and TEM. The d-spacing value and unitcell parameter in the V-Mo-MCM-41 increases by increasing the metal ions content. FTIR studies showed that V- and Mo-ions were incorporated into the hexagonal mesoporous MCM-41 materials, however, the wavenumber of the anti-symmetric Si-O-Si vibration bands (1097 cm^-1) in V-Mo-MCM-41 are higher than those in Si-MCM-41 (1081 cm ^-1). These bands should be due to the increase of the mean Si-O distance in the wall caused by the substitution of the small ionic radii of silicon by the larger ionic radii of vanadium and molybdenum. The surface area, pore diameter, pore volume, hydrothermal stability and acid sites in the V-Mo-MCM-41 decreases by increasing the metal ions content. The incorporated metal ions as V⁵⁺ and Mo⁶⁺ in V-Mo-MCM-41 are coordinated to Si(IV) by tetrahedral environments. The synthesized V-Mo-MCM-41 materials were found to be effective as catalysts in the oxidation of alkyl aromatics, e.g., in the oxidation of o-xylene using O₂ gas as the oxidizing agent for the production of phthalic anhydride. The effect of reaction temperature, feed flow rate ratio (o-xylene:O₂ gas), contact time and recyclability on the selectivity of phthalic anhydride were studied. Thus, the selectivity of phthalic anhydride in the presence of V-Mo-MCM-41(46) catalyst was higher than that of other V-Mo-MCM-41 catalysts due to increasing the catalytic activity on the inner side pores of silica surface.