STUDY OF THE OXIDATION-STATES OF CHROMIUM IN SUBSTITUTED AND SUPPORTED CR-SILICOALUMINOPHOSPHATE MOLECULAR-SIEVES

A chromium-substituted silicoaluminophosphate molecular sieve (CrAPSO- 11) and a Cr-supported SAPO-11 molecular sieve (Cr/SAPO-11) were studi ed by XPS and reduction-oxidation (redox) cycles. The distribution of oxidation states as well as the average oxidation state for chromium w ere determined for both preparations. The XPS results showed that two chromium 2p doublets (Cr(VI) and Cr(III)) were necessary to curve fit the experimental data. Comparison of curve-fitting results for the two oxidic samples (O-2, 773 K) revealed a significantly higher percentag e of Cr(III) species in the Cr/SAPO-11 catalyst. The difference betwee n the two samples remained following reduction (H-2, 773 K). For the C rAPSO-11 sample, Cr(VI) showed a higher stability towards reduction, c ompared to chromium supported on conventional supports. This result su ggests tbe incorporation of Cr(VI) into the silicoaluminophosphate fra mework, as mentioned in the literature. For the Cr/SAPO-11 solid, Cr(I II) was extremely stable towards oxidation at 773 g. Doe to the redox characteristics and the XPS results, the formation of chromia species on this catalyst is suggested.