Vapour-phase oxidation of ethylbenzene with air over Mn-containing MCM-41 mesoporous molecular sieves

Abstract

The vapour-phase oxidation of ethylbenzene has been carried out over unwashed (U) Mn-MCM-41, Mn-AlMCM-41 (100), Mn-AlMCM-41 (150) and washed (W) Mn-MCM-41, Mn-AlMCM-41 (100) and Mn-AlMCM-41 (150) using CO 2-free air as oxidant at 250–400 °C. The mesoporous structure of these catalysts was confirmed by XRD technique. Surface area, pore size and wall thickness were calculated from BET equation and BJH method using nitrogen sorption technique. FT-IR studies showed that Si and Al ions were incorporated into the hexagonal mesoporous structures of MCM-41 and AlMCM-41. The thermal stability of the as-synthesised materials was studied using thermogravimetric–diffenential thermal analysis (TG–DTA); diffuse reflectance (DR) UV-Vis spectroscopy confirmed that manganese ion could be isolated in framework positions of the MCM-41 mesostructure by this approach. Mn(II) species with a well-resolved sextet centred at g=2.0 has distorted octahedral symmetry and is observed in hydrated impregnated Mn-MCM-41 and Mn-AlMCM-41. Reaction conditions were optimised for oxidation by varying temperature, weight hourly space velocity and time on stream. The major products were acetophenone, benzaldehyde and styrene. The yield of acetophenone, which is the expected product in this investigation, increases with increase in the metal content of the catalyst. Among the six catalysts studied, ethylbenzene conversion and yield of acetophenone are higher over Mn-MCM-41 (U) catalyst with more metal content.