Selective N-alkylation of aniline with methanol over gamma-alumina

The alkylation of aniline with methanol over gamma-alumina was studied in a fixed-bed, integral-flow reactor at atmospheric pressure and 300-425 degrees C. N-methylation is the predominant reaction with a selectivity higher than 98 mol-% over this temperature range. Increasing reaction temperature and methanol/aniline molar ratio enhances the selectivity of N,N-dimethylaniline. Consecutive methylation of aniline produces N-methylaniline and N,N-dimethylaniline; the corresponding apparent activation energies are 62.7 +/- 2.1 and 48.3 +/- 2.9 kJ mol(-1). N-methylaniline reacts faster than aniline due to the electron donating effect of its methyl group on the nitrogen atom, that facilitates the alkylation with methanol, Based on the FT-IR results and those reported in the literature, it is proposed that both aniline and methanol are adsorbed undissociatively on the Lewis acid-base dual sites of gamma-alumina. The electrophilic attack of the methyl group of methanol on the nitrogen atom of aniline or N-methylaniline yields N-methylaniline and N,N-dimethylaniline, respectively.