On the "active spacer and stabilizer" role of Zn in Cu_1-xZn_xFe₂O₄ in the selective mono-N-methylation of aniline: XPS and catalysis study

Abstract

A systematic catalytic methylation study on ferrospinel materials led to the selective production of N-methylaniline (NMA) with Cu_1-xZn_xFe₂O₄. Aniline methylation was carried out on Cu_1-xZn_xFe₂O₄ with a feed composition of CH₃OH:PhNH₂:H₂O = 3:1:1 at 513-633 K. NMA was formed selectively on all of the catalyst compositions, with trace amounts of secondary products under most of the conditions. Cu_0.5Zn_0.5Fe₂O₄ composition showed high catalytic activity and stability up to 100 h. Although the Cu²⁺ was responsible for methylation activity, Zn²⁺ enhanced the overall stability of the catalyst system. XPS investigations revealed that the degree of Cu²⁺ reduction decreased dramatically from x=0.05/0.25 to 0.5/0.75 on spent catalysts. TPR studies indicated that the reducibility of Cu²⁺ decreased from fully reducible at 523 K with Cu-rich compositions to partially reducible at 573 K on x=0.5. Stable activity observed on Cu_0.5Zn_0.5Fe₂O₄ can be attributed to the highly heterogeneous distribution of metal ions. This heterogeneous distribution indicates an important role of zinc, likely as an "active spacer cum stabilizer" that hinders the reduction of active Cu²⁺ and contributes to prolonged activity.