MICROCALORIMETRIC AND CATALYTIC STUDIES OF THE ACIDIC CHARACTER OF MODIFIED METAL-OXIDE SURFACES .1. DOPING IONS ON ALUMINA, MAGNESIA, AND SILICA

Microcalorimetry of ammonia and sulfur dioxide adsorption and catalytic reaction of 2-propanol conversion have been used to study the effects on the acid/base properties of adding small amounts of ions (Ca2+, Li+, Nd3+, Ni2+, SO42-, Zr4+) on gamma-alumina, silica, or magnesia surfaces. It was found that the modification of gamma-Al2O3 surface properties with small amounts of the above ions changed moderately its amphoteric properties. More substantial changes were observed on magnesia which consisted in formation of centers of moderate and weak basic strength. The number of acid/base centers on doped silica was strongly affected by the presence of introduced ions. The catalytic test of 2-propanol conversion led to results in agreement with the acidity determination performed by adsorption microcalorimetry. The intermediate acidity strength played a central role in 2-propanol dehydration. We proposed a classification of the modified catalysts according to an acidity/basicity scale defined by the specific effect of ions. The acidity of the catalysts correlated with the charge/radius ratio and with the generalized electronegativity of the doping ions. The basicity correlated with the partial oxygen charge of the corresponding oxides.