A1PO4-Al2O3 catalysts with low alumina content. III. Surface basicity of catalysts obtained in aqueous ammonia

The amount of basic sites of A1PO₄-Al₂O₃ (APA1-A, 5–15 wt% Al₂O₃) catalysts at two basic strengths was measured by studying the liquid-phase adsorption of two acidic molecules (benzoic acid (BA, pK = 4.2) and phenol (PH, pKa = 9.9) from cyclohexane solutions, through the application of a spectrophotometric method. The data obtained follow the Langmuir adsorption isotherm and the monolayer coverage at equilibrium (at 298 K),X _m, is assumed as the amount of basic sites corresponding to the specific pK of the acid used as titrant. The amount of basic sites of any AlPO₄-Al₂O₃ catalyst is higher than that of AlPO₄, but lower than that of Al₂O₃. Besides, an increase in the Al₂O₃ content from 10 wt% gradually increases the basicity of the APA1-A catalyst. Moreover, calcination at increasing temperatures does not practically affect the surface basicity of APAl-A-5 and APAl-A-10 catalysts. However, for AlPO₄ content higher than 10 wt% we observe a decrease in surface basicity, this decrease depends on alumina content, i.e. it is higher as the amount of alumina increases. The basic sites of APAl-A systems catalyze the Knoevenagel condensation ofp-methoxybenzaldehyde and malononitrile at room temperature and in the absence of solvent.