On the “Active spacer and stabilizer” role of Zn in Cu1−xZnxFe2O4 in the selective mono-N-methylation of aniline: XPS and catalysis study

A systematic catalytic methylation study on ferrospinel materials led to the selective production of N-methylaniline (NMA) with Cu1−xZnxFe2O4. Aniline methylation was carried out on Cu1−xZnxFe2O4 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. Cu0.5Zn0.5Fe2O4 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=0.05/0.25x=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=0.5x=0.5. Stable activity observed on Cu0.5Zn0.5Fe2O4 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.