The relationship between the oxidation activity and the acid-base properties of Fe 2O 3-based mixed oxides : II. The Fe 2O 3P 2O 5, Fe 2O 3K 2O, and Fe 2O 3Bi 2O 3P 2O 5 systems

Abstract

Both the acidity and basicity of Fe 2O 3P 2O 5 and Fe 2O 3K 2O, with different compositions, were directly measured by means of the adsorption of NH 3, pyridine, CO 2, and SO 2 from the gas phase. With the addition of P 2O 5 or K 2O, the acid-base properties of the Fe 2O 3-based catalysts vary as can be expected from the natures of P 2O 5 and K 2O. The acidic character of Fe 2O 3P 2O 5 is fairly low compared with that of Fe 2O 3V 2O 5 and Fe 2O 3MoO 3. The acidity data were confirmed by the catalytic activities for acid-catalyzed reactions such as isomerization of 1-butene and dehydration of isopropyl alcohol. Then, the acid-base properties of a series of ternary oxides, Fe 2O 3Bi 2O 3P 2O 5, with different Bi Fe compositions and a constant P 2O 5 content, P (Fe + Bi) = 1 10 , were also determined. The Bi 2O 3P 2O 5 ( P Bi = 1 10 ) is far more basic and less acidic than the Fe 2O 3P 2O 5 ( P Fe = 1 10 ). With the substitution of Bi 2O 3 for Fe 2O 3, the acidity of the mixed oxides decreases and the basicity increases steadily. The vapor-phase oxidation of n-hexane, 1-butene, butadiene, methanol, and formic acid was carried out in an ordinary continuous-flow reactor. Then, the relationship between the catalytic behavior and the acid-base properties of the catalysts was investigated. The results support an earlier proposal that the oxidation activity and selectivity can be interpreted in terms of the acid-base properties of the metal oxide catalysts and the reactants.