N-alkylation of aniline with methanol over AlPO4-Al2O3 catalysts

Amorphous AlPO4-Al2O3 (5-25 wt% Al2O3, APAl-A) systems were selective catalysts for the N-alkylation of aniline with methanol. The methylation reaction (that was carried out in a continuous down-flow reactor operating at atmospheric pressure, 523 K and aniline feed rates over 3 x 10(-6) mol s(-1)) is a pseudo-first order process with respect to aniline concentration, taking place through consecutive formation of N-methyl and NN-dimethylanilines. Also, there is always more N-methylaniline (NMA) than N,N-dimethylaniline (NNDMA). Increasing reaction temperature enhances the selectivity of NNDMA. AlPO4 and Al2O3 are slightly active, but AlPO4-Al2O3 catalysts exhibit significantly higher activity (an order of magnitude more active) and, besides, there is a slight increase in activity on increasing Al2O3 loading up to 15 wt%, thereby showing that APAl-A catalysts are suitable for selective obtaining N-alkylation products. The effect of surface acidity on aniline alkylation activity is discussed. Strong acid sites are not required for the N-methylation of aniline, whereas, weak to moderate acid sites seem to be responsible for this reaction. Furthermore, there is no catalyst deactivation by coke deposition, whereas, the addition of pyridine or 2,6-dimethylpyridine deactivate the catalyst. (C) 1998 Elsevier Science B.V.