The effect of promoters on the catalytic activity of γ-Al_2O_3 for deamination of aniline to diphenylamine (DPA)

摘要

Catalytic deamination of aniline to diphenylamine has been studied on γ-Al_2O_3 + H_3BO_3, γ-Al_2O_3 + H_3BO_3 + HCl and γ-Al_2O_3 + H_3BO_3 + AlF_3 catalysts in a tubular reactor at fixed set of reaction conditions. It is observed that the catalyst containing AlF_3 as promoter exhibits the highest activity for the reaction and the activity decreases in the order, γ-Al_2O_3 + H_3BO_3 + AlF_3 > γ-Al_2O_3 + H_3BO_3 + HCl > γ-Al_2O_3 + H_3BO_3. The wt.% DPA in product versus time on stream plots are found to contain three well defined deactivation steps. Times taken by the catalysts for the deactivation down to 20 wt.% DPA in product level are also found to depend on the initial activities of the catalysts and decrease in the same order as the activities of the catalysts; the three deactivation steps fully appearing in these periods. The three deactivation steps are attributed to structure sensitivity of adsorption and deamination of aniline molecules and to coke deposition on the three Miller index planes of γ-Al_2O_3, (440), (400) and (311). The action of promoters in enhancing the deamination activity of γ-Al_2O_3 is explained by considering the electronegativities of the promoter elements, B, Cl and F.