(603b) Acidity of Niobium Oxides from Catalytic Model Reactions Under Co-Presence of Water

Niobic acid and niobium oxide involving varying amounts of water (Nb₂O₅×xH₂O) are well documented in literature as water-tolerant solid acids for catalysis in water or under presence of water/steam/moisture, owing to their excellent water-resistant surface acidity. Three model reactions, namely Brønsted-acidity-specific 3,3-dimethylbut-1-ene (33DMB) skeletal isomerization for evaluating medium to strong Brønsted acidity and methylenecyclohexane (MECH) double-bond isomerization for relatively weak Brønsted acidity, and 2-propanol (2PO) dehydration for the overall acidity (Lewis + Brønsted) at the solid catalyst surface, were conducted herein under co-presence of gaseous water (steam or moisture) for understanding response to water co-presence of the surface acidic property of niobic acid and niobium oxide catalysts. Four Nb₂O₅-T samples (T = 300, 350, 450 and 500) were prepared by varying the calcination temperature T in Celsius (°C) of niobic acid to mimic hydrated Nb₂O₅×xH₂O catalysts of varying hydration degrees. The co-presence of water significantly enhanced the isomerization reaction rates of 33DMB and MECH over the Nb₂O₅-T catalysts, and the enhancements appeared to increase with increasing the reaction temperature and water partial pressure as well. Furthermore, the enhancement was always significantly higher for MECH isomerization than for 33DMB. The activation energies measured for these two isomerization reactions also increased with water partial pressure in the reactions. These observations clearly uncover that water co-presence resulted in generation of Brønsted acidic sites (BAS) over Nb₂O₅, and most of these newly generated BAS did not have the acidic strength for catalyzing the isomerization of 33DMB. The generation of BAS at the catalyst surface also depended sensitively on T of the Nb₂O₅-T sample. It is therefore that the interaction between water molecules and relatively stronger surface LAS would result in generation of weak to medium strong BAS being active for catalyzing the isomerization of MECH and/or 33DMB.