Influence of activation conditions of ce–X zeolite on its oxidizing properties as shown by infrared and electron spin resonance spectroscopies

Abstract

The oxidizing properties of cerium-exchanged X zeolites towards hydrogen and pyridine were investigated by e.s.r. and i.r. spectroscopies. After oxygen treatment at 500°C, cerium (III) ions at cationic sites were oxidized to the IV oxidation state. Hydrogen reduction above 200°C produced OH groups, which were attributed to OH groups bounded to the framework (3640 and 3590 cm^–1) and to OH groups attached to Ce^III ions (3518 cm^–1). The former OH groups were accessible to pyridine and acidic, whereas the latter were not accessible to pyridine and may not be acidic. Adsorption of pyridine at room temperature onto oxidized and dehydroxylated samples led to the formation of hydroxyl groups linked to the cation and to the appearance of Brönsted acidity. Ce^IV cations were thus reduced by pyridine, the organic base being partially transformed into dipyridyl. Particular care in the activation conditions of cerium-exchanged zeolites must be taken, especially the presence of reducing agents, such as grease from the vacuum line, since these can induce a reduction process resulting in the formation of Ce^III ions and hydroxyl groups bonded both to the zeolite framework and to the Ce cation.