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