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 (ACrPA) and a series of chromia-pillared clay materials (XSCr) for the deep oxidation of methylene chloride has been studied. Both types of catalysts showed a high activity (conversion > 80%) at T > 350 degrees C, The ACrPA sample showed a virtually constant catalytic activity (conversion > 99%) in the 300-400 degrees C temperature range while the activity of the XSCr samples markedly increased with the temperature reaching an almost total conversion at 400 degrees C. XPS (X-ray photoelectron spectroscopy) and temperature programmed reduction (TPR) studies demonstrated that ACrPA is a Cr(VI)/Cr(III) mixed valence catalyst, whereas XSCr samples contain only chromium(III) ion. The high activity shown by ACrPA at low temperatures was attributed to the presence of highly dispersed Cr(VI) species, which exhibited an unusually high stability. The metal oxidation state of the two chromium based systems seemed to be important in determining the catalytic activity. The increase of the activity with the Cr content in XSCr samples suggested that the acid sites in these catalysts were located on the chromia pillars.