Surface Species Formed during Aniline Methylation on Zeolite H–Y Investigated by in Situ MAS NMR Spectroscopy

Aniline alkylation with methanol on zeolite H–Y has been studied using in situ13C MAS NMR spectroscopy under batch conditions. To clarify the main reaction pathways, the conversion of methanol as well as the interaction of aniline with surface methoxy groups were investigated under similar conditions. Methanol-13C and methyl iodide-13C were used as labeled reactants. Co -adsorption of aniline and methanol-13C on zeolite H–Y led to strongly adsorbed aniline molecules, assigned to aniline H-bonded to zeolite Brønsted acid sites, and three types of methanol species of different mobility: mobile methanol molecules with a liquid-like characteristics and two types of rigid methanol species with solid-like characteristics attributed to a methanol adsorption complex with aniline and surface methoxy groups, respectively. Among all the methanol species observed, only surface methoxy groups were shown to be responsible for aniline alkylation which takes place at temperatures from 373 to 523 K. The formation of surface methoxy groups was found to be a limiting step of the overall reaction. The primary alkylation product is N-methylaniline. Toluidines and N-methyltoluidines are formed at temperatures from 523 to 623 K after complete conversion of methanol to N-methylaniline. Therefore, isomerization or disproportionation of N-methylaniline was proposed to account for their formation.