An insight into the mechanism of selective mono-N-methylation of aniline on Cu_1-xZn_xFe₂O₄: a DRIFTS study

Abstract

Mechanism of selective mono-N-methylation of aniline with methanol on Cu_1-xZn_xFe₂O₄ catalysts was investigated in detail. The interaction of reactants (aniline, methanol and methanol:aniline) and possible products (N-methylaniline (NMA), N,N-dimethylaniline (DMA) and o-toluidine (OT)) on catalysts surface was studied by temperature-dependent in situ FTIR spectroscopy. Methanol adsorbs dissociatively over catalysts surface at 373 K as methoxy species and is oxidized to formate species at high temperature through dioxymethylene and/or formaldehyde as a surface intermediate species. On the other hand, adsorption of aniline:methanol mixtures shows that methanol oxidation was completely hindered in the presence of aniline. Aniline adsorbs on the Lewis acid sites at ≤373 K with phenyl ring oriented in a perpendicular manner to the catalyst surface; however, N—H bond scission occurs above 373 K. A comparison of adsorbed NMA and methanol:aniline (3:1) mixture on Cu_0.5Zn_0.5Fe₂O₄ shows NMA forms from the reaction mixture at 473 K. However, maximum activity at 573 K in catalytic reaction studies suggests that desorption limits the methylation kinetics. FTIR study displays stable aniline and methyl species on ZnFe₂O₄ even at 573 K; however, no methyl species is detected on Cu_0.95Zn_0.05Fe₂O₄ at 473 K due to methanol reforming reaction and that limits the overall reaction and hence low catalytic activity. It is proposed that methanol is protonated on catalysts surface by the labile H⁺ due to N—H bond scission. Co-adsorption of acidity probes with aniline and methanol indicates that aniline methylation takes place at single acid-base site.