Chromia-chrominum phosphates as solid acid catalysts

A series of chromia-chrominum phosphate catalysts with various P/Cr ratios were prepared with the coprecipitation method. The surface acidic properties of these catalysts were examined by temperature-programmed desorption (TPD) of ammonia and the dehydration reaction of isopropanol. The dehydration reaction was carried out in a continuous-flow microreactor at 220 °C under atmospheric pressure. The results of TPD of ammonia indicated that the surface acidity of chromia-chrominum phosphate is medium strong. The acidic strengths are approximately the same for all the samples. However, the acid concentration is increased with increasing the P/Cr ratio of the catalyst. The dehydration activity of the catalyst is also increased with increasing the P/Cr ratio of the sample. The results indicated that the addition of phosphorus on chromia, even in a small amount, could significantly enhance the acidities of the catalysts. In addition, the acid amount of mixed oxide is decreased in the order: iron oxide-iron phosphate > chromia-chrominum phosphate > alumina-aluminum phosphate.