Abstract
Acid strength and acidity of Mo-X (alkaline earth metal)-As oxide catalyst have been studied and discussed in connection with catalytic activity in vapor phase oxidation of 1-butene. The activity test was carried out at about 470°C, with 17% olefin in air. Acid strength and acidity were determined by the amine titration method with the set of Hammett indicators, in the range of -5.6<pKa<7.1
The acidity of Mo-Ba oxide is decreased remarkably by addition of As componen to the binary oxide, although As oxide has high acidity, especially in the range of strong acid strength, and the resultant Mo-Ba-As oxide has acidity of weak acid strength. A comparison of activity with acidic property of Mo-X-As (atomic ratio=1:1:1) type oxide catalyst containing different kinds of X component and Mo-Sr-As (As/Mo=1) oxide catalysts of different amounts of Sr showed that butadiene formation (partial oxidation) and 2-butene formation (isomerization) increased with increasing acidity in weak acid strength (1.5<pKa<7.1, while CO
2+CO formation increased with increasing acidity in strong acid strength (pKa<1.5).
It is considered that molybdate of alkaline earth metal exists in both Mo-X and Mo-X-As oxides, and plays important roles in the catalytic oxidation reaction. However, the coexistence of As component suppresses the crystallization of this salt, resulting in the decrease of isomerization and increase of selectivity for partial oxidation. By increasing atomic number of X component in Mo-X-As oxide catalyst, acidity and the activity for diene formation decreased but the selectivity for the formation increased. Propylene oxidation was also studied with regard to the acidic property of the catalyst.
