Acid–base reactions on alumina-supported niobia

Catalysts containing from 8 to 28% wt. of niobia supported on γ-alumina by impregnation with niobium pentaethoxide solutions were characterized by infrared spectroscopy of adsorbed pyridine as a probe for acidic sites and CO2 adsorption as a probe for basic sites. The catalysts had their activity measured in isopropanol dehydration at 453 K, 1-butene isomerization at 348 K and in cumene dealkylation at 728 K. The density and strength of the Lewis acid sites and of the basic sites decreased with niobium content. On the other hand, the density of Brønsted acid sites increased in the same direction. Different reactions responded differently to niobium addition. In isopropanol dehydration, the main factor responsible for the observed decrease in catalytic activity was the decrease in concentration of basic sites. Activity for 1-butene isomerization also decreased with niobium addition, but the main factor seemed to be the decrease in concentration of alumina-associated Lewis acidic sites. Evidence from cis-/trans-2-butene ratio indicated that niobium addition modifies the properties of neighboring aluminum sites. Brønsted acidic sites created by niobium addition are responsible for the development of activity in the cumene dealkylation reaction, but the sites associated with tridimensional niobia species seem to be more effective in this reaction.