Catalytic nitroxidation of 1-methylnaphtalene. I. Preparation, characterisation and NO-surface interactions of chromia/alumina-based catalysts

In view of their use in the nitroxidation of 1-methylnaphthalene into 1-naphthonitrile, chromia/alumina and K-containing chromia/alumina catalysts were prepared. They were characterised by X-ray diffraction, scanning electron microscopy, nitrogen physical adsorption, UV–vis diffuse reflectance spectroscopy, adsorption microcalorimetry of ammonia and carbon dioxide, temperature-programmed reduction. The interaction of NO with the catalyst surface was investigated by temperature-programmed methods. Both Cr(III) and Cr(VI) species were found on the catalysts, the content of the latter increasing along with the K loading. A bichromate phase was detected in the K-containing catalysts. Cr(VI) species underwent (incomplete) reduction under hydrogen atmosphere. The reduction extent was higher at the higher K loadings, though higher energy barriers needed to be overcome for its occurrence. All the catalysts were able to dissociate NO into atomic nitrogen and oxygen and also to convert NO into N2O and O2 in the temperature range 623–657 K. NO disproportionation occurred on the K-containing chromia/alumina catalysts even at relatively low temperatures (372–407 K), but not on chromia/alumina. Chromia/alumina showed an acidic as well as a basic character, the acid features being however predominant. At the lowest K content acidity appeared remarkably lowered and basicity significantly increased. Further addition of K originated a catalyst somewhat balanced in its (weak) acid–base character. At the highest K loadings the original acidic and basic features of chromia/alumina were completely lost. Such different redox and acid–base features, as well as the different interaction modes with NO, are expected to influence deeply the catalytic behaviour in 1-methylnaphthalene nitroxidation.