THERMODYNAMICS OF ADSORBED MOLECULES FOR A NEW ACID-BASE TOPOCHEMISTRY OF ALUMINA

In the present study, two methods have been applied for characterizing the surface acidity and basicity of an amphoteric γ-alumina: a titration method using Hammett indicators and a gas phase adsorption method using basic (i.e. piperidine, diethylamine, n-butylamine, ammonia, pyridine, aniline), amphoteric (i.e. pyrrole, water, methylalcohol tert-butyl alcohol, acetonitrile), and acidic (i.e. m-cresol, carbon dioxide, acetic acid, sulfur dioxide) probe molecules by means of combined volumetry-calorimetry technique. The scales of acid-base strength obtained by the two methods and their reliability have been compared. Both the strength and number of acidic sites were related to the pKa of the probes. When using weaker probes (pKa ≤ 0) only the very strong acid sites are titrated. At pKa ≥ 5 a plateau is reached. Moreover, in order to evaluate more precisely the gas-solid interactions, the thermodynamic behaviour of the adsorbed probes has been studied, including the isotherm, the differential heat, the integral heat and the differential molar entropies of adsorption as a function of the coverage. Integral heat corresponding to the chemisorbed volume has been taken as a criterion to compare the acid strength of the sites. Besides, integral heat curves describe a parabola which curvature has been related to the strength of the adsorbate-adsorbent system. The values of the differential molar entropies of adsorbate confirm an immobile layer in the range 0-0.4 degree of coverage.