Selective ortho-methylation of phenol with methanol over copper manganese mixed oxide spinel catalysts

Abstract

Methylation of phenol with methanol as an alkylating agent to produce 2,6-xylenol was investigated over copper manganese mixed oxide spinels, Cu_xMn_3-xO₄ (x=0, 0.25, 0.5, 0.75, and 1) prepared through co-precipitation. The catalytic activity strongly depends on the composition, acid-base properties, and structural stability. Various parameters, including catalyst composition, reaction temperature, feed composition, and durability of the catalyst during methylation, were investigated. Mainly o-cresol and 2,6-xylenol, along with small amounts of mesitol, were found in the product. A high ortho-selectivity of 100%, with 2,6-xylenol selectivity of 74%, was observed over Cu_0.25Mn_2.75O₄ at 673 K. These catalysts were investigated using various techniques, including BET surface area, XRD, DRS UV-vis, TPD of NH₃ and CO₂, TPR, and X-ray photoemission spectroscopy (XPS). Powder XRD of the catalysts revealed the formation of copper-manganese spinels with hausmannite (Mn₃O₄) tetragonal structure, for x=0-0.5, whereas an increase in copper content (x>0.5) led to the formation of cubic Cu_1.5Mn_1.5O₄ phase. DRS UV-vis, and FTIR further supported the changes in structural phases observed by XRD. Temperature-programmed desorption of CO₂ and NH₃ showed that the catalysts have strong basicity along with weak acidity when x=0 and 0.25. XPS and XAES analysis revealed the presence of only Cu²⁺ ions in fresh sample with x=0.25, whereas for x=1.0, both Cu¹⁺ and Cu²⁺ were observed. The deactivation of the catalysts is attributed to structural changes occurring during the reaction. Catalytic activity is correlated with structure, as well as with acid-base properties.