Chromium–aluminium orthophosphates. Part 1.—Structure, texture, surface acidity and catalytic activity in cyclohexene skeletal isomerization and cumene conversion of CrPO4–AlPO4catalysts

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Abstract
A series of CrPO4–AlPO4(5–10wt.% AlPO4, CrAlP) catalysts have been prepared by precipitation in aqueous ammonia or in aqueous ammonia–propylene oxide. The catalysts were calcined at 773–1273 K. All catalysts were amorphous for calcination temperatures below 1073 K. At 1273 K the β-CrPO4 structure was formed. CrAlP catalysts exhibited low surface areas which decreased as the calcination temperature increased. Moreover, surface acidity (acid amount and acid strength), measured by adsorption of pyridine and 2,6-dimethylpyridine at various temperatures (573–673 K) by using a pulse method, was better than for the individual components. This increased surface acidity is responsible for their better performance for cyclohexene skeletal isomerization (CSI) and cumene conversion (CC), reactions that were selected as dynamic methods for surface-acid characterization.