MICROCALORIMETRIC AND INFRARED SPECTROSCOPIC STUDIES OF GAMMA-AL2O3 MODIFIED BY BASIC METAL-OXIDES

Microcalorimetric and infrared spectroscopic studies of ammonia and carbon dioxide adsorption were used to study the surface acid/base properties of gamma-Al2O3 following addition of K2O, MgO, and La2O3. Microcalorimetric measurements of NH3 adsorption at 423 K showed that addition of basic metal oxide cations at loadings less than 200 mu mol/g converted the stronger acid sites (heats from 140-160 kJ/mol) to sites of intermediate strength (100-140 kJ/mol). Increasing the loadings of the basic oxides further eliminated the stronger acid sites and eliminated sites of intermediate strength. Infrared spectra of adsorbed NH3 showed that gamma-Al2O3 contained both Bronsted and Lewis acid sites after calcination at 723 K, and addition of potassium removed Bronsted acid sites. The addition of basic metal oxides at loadings less than 500 mu mol/g increased the number of weak basic sites (heats from 60-100 kJ/mol) and sites of intermediate strength (100-140 kJ/mol). Strong basic sites (140-160 kJ/mol) were formed at higher loadings of basic oxides on alumina. The effectiveness of basic metal oxides to neutralize acid sites and to generate basic sites on alumina can be related to the electronegativities of these oxides.