ANION TREATMENT (F- OR SO42-) OF ALPO4-AL2O3 (25 WT-PERCENT AL2O3) CATALYSTS .3. ANION EFFECT ON SURFACE BASIC PROPERTIES

The effect of both anion type and loading on the amount and strength o f basic sites of a series of AlPO4 -Al2O3 (APAl-P, 25 wt.% Al2O3) modi fied with F- or SO42- ions (1-3 wt.%) was studied. Surface basicity wa s measured by studying the adsorption of acidic molecules [benzoic aci d (BA, pK(a) = 4.2) and phenol (PH, pK(a) = 9.9)], from cyclohexane so lutions. 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(a) of the acid probe used. The amount of basic sites of anion-treated APAl-P cat alysts is lower than that of the unmodified one, and on increasing the anion loading, the basicity of the catalysts gradually decreases. Fur thermore, the decrease in surface basicity by anion loading is more ac centuated in the case of fluoride ion and, thus, the number of stronge st basic sites (measured vs. PH) sharply decreases.