CATALYTIC BEHAVIOR OF CHROMIA AND CHROMIUM-DOPED ALUMINA PILLARED CLAY MATERIALS FOR THE VAPOR-PHASE DEEP OXIDATION OF CHLORINATED HYDROCARBONS

The catalytic behavior of a chromium-doped alumina pillared clay (ACrP A) and a series of chromia-pillared clay materials (XSCr) for the deep oxidation of methylene chloride has been studied. Both types of catal ysts showed a high activity (conversion > 80%) at T > 350 degrees C, T he ACrPA sample showed a virtually constant catalytic activity (conver sion > 99%) in the 300-400 degrees C temperature range while the activ ity of the XSCr samples markedly increased with the temperature reachi ng an almost total conversion at 400 degrees C. XPS (X-ray photoelectr on spectroscopy) and temperature programmed reduction (TPR) studies de monstrated that ACrPA is a Cr(VI)/Cr(III) mixed valence catalyst, wher eas XSCr samples contain only chromium(III) ion. The high activity sho wn by ACrPA at low temperatures was attributed to the presence of high ly dispersed Cr(VI) species, which exhibited an unusually high stabili ty. The metal oxidation state of the two chromium based systems seemed to be important in determining the catalytic activity. The increase o f the activity with the Cr content in XSCr samples suggested that the acid sites in these catalysts were located on the chromia pillars.