Evaluation of the acid-base surface properties of several oxides and supported metal catalysts by means of model reactions

Acid-base properties of oxides (Al2O3, SiO2, ZrO2, CeO2, MgO, SiO2-Al2O3 and CeO2-Al2O3) were investigated by means of model reactions: 3,3-dimethylbut-1-ene isomerization (33DMB1), methylene cyclohexane isomerization (MECH), cyclohexanol conversion (CHOL) and CO2 chemisorption at room temperature. The effect of acid (Cl-, SO42-) and basic (K+) promoters on certain oxides (Al2O3, ZrO2) was also studied. Surface acidity was evaluated by means of 33DMB1 or MECH isomerization and of CHOL dehydration into cyclohexene. CO2 chemisorption as well as the cyclohexanone to cyclohexene ratio in CHOL conversion were used to measure the surface basicity of the solids. Except for very few cases, all the tests gave coherent results which led to well-defined scales of acidity and of basicity. Contrary to what could be observed with the conventional isopropanol test, the CHOL test proves to be little sensitive to the: redox sites of the oxides. The presence of metals can create significant perturbations in these acid-base tests, except in the case of 33DMB1 isomerization. The latter reaction is not catalyzed by Pt and Rh surfaces, which allows to measure (in the presence of these metals) the acid properties of the support and to investigate the changes in surface acidity, resulting from the metal impregnations (metal --> support electronic effect, presence of anions,...).