Application of two morphologies of Mn2O3 for efficient catalytic ortho-methylation of 4-chlorophenol

Abstract

Vapor phase ortho-methylation of 4-chlorophenol with methanol was studied over Mn₂O₃ catalyst with two kinds of morphologies. Here, Mn₂O₃ was prepared by a precipitation and hydrothermal method, and showed the morphology of nanoparticles and nanowires, respectively. XRD characterization and BET results showed that, with the increase of calcination temperature, Mn₂O₃ had a higher crystallinity and a smaller specific surface area. N₂ adsorption/desorption and TPD measurements indicated that Mn₂O₃ nanowires possessed larger external surface areas and more abundant acid and base sites. Simultaneously, in the fixed bed reactor, methanol was used as the methylation reagent for the ortho-methylation reaction of 4-chlorophenol. XRD, XPS, TG-MS and other characterizations made it clear that methanol reduced 4-chlorophenol and its methide, which were the main side-reactions. And Mn³⁺ was reduced to Mn²⁺ under the reaction conditions. Changing the carrier gas N₂ to a H₂/Ar mixture further verified that the hydrogen generated by the decomposition of methanol was not the reason for dechlorination of 4-chlorophenol compounds. Here we summarized the progress of 4-chlorophenol methylation based on the methylation of phenol. Also, we proposed a mechanism of the 4-chlorophenol dechlorination effect which was similar to the Meerwein–Ponndorf–Verley-type (MPV) reaction. The crystal phase and carbon deposition were investigated in different reaction periods by XRD and TG-DTA. The reaction conditions for the two kinds of morphologies of the Mn₂O₃ catalyst such as calcination temperature, reaction temperature, phenol–methanol ratio and reaction space velocity were optimized.