A Comparative Study on the Catalytic Activity of Mn Containing MCM-41 Molecular Sieves for Oxidation of p-Cymene

Abstract

Mesoporous silica molecular sieves SiMCM-41, AlMCM-41 with Si/Al ratio equal to 158 and MnMCM-41 with Si/Mn ratio equal to 29 were synthesized by hydrothermal method. Manganese oxide impregnated on SiMCM-41 and on AlMCM-41 catalysts were also prepared by wet impregnation method. These catalysts referred as Mn/SiMCM-41 and Mn/AlMCM-41 (158). Both the manganese incorporated and impregnated catalysts structure was elucidated by XRD. Nitrogen adsorption isotherm was used to determine specific surface area, pore volume, and pore size distribution. The thermal stability of the as-synthesized catalyst was studied using TG-DTA. The presence of Mn²⁺ was evident through DR UV–VIS and ESR spectroscopy. Through ESR studies, MnMCM-41 (29) was assigned to have framework Mn(II) sites. Extra-framework manganese species with well-resolved sextet, centered at g = 2.00 had octahedral symmetry was observed in Mn/SiMCM-41 and Mn/AlMCM-41 (158). Mn retained its +2 oxidation state (ESR active) even upon calcination at 550 °C in presence of air. The catalytic activity of the above-mentioned manganese containing MCM-41 catalysts were tested by vapor phase oxidation of p-cymene with CO₂-free air in the temperature range from 200 to 400 °C in steps of 50 °C. The major products were 4-methylacetophenone (4-MAP), 4-isopropyl benzaldehyde (4-IPB), 1,2-epoxyisopropyl benzaldehyde or 1,2-epoxycumenaldehyde (1,2-ECA) and 4-methyl styrene (4-MS). Among the product selectivity, (4-MAP) was found to be higher than that of others. The order of the activity of catalysts followed Mn/SiMCM-41 > Mn/AlMCM-41 (158) > MnMCM-41 (29) > Mn/SiO₂.