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Study of the oxidation states of chromium in substituted and supported: Cr-silicoaluminophosphate molecular sieves

Abstract:

A chromium-substituted silicoaluminophosphate molecular sieve (CrAPSO-11) and a Cr-supported SAPO-11 molecular sieve (Cr/SAPO-11) were studied by XPS and reduction-oxidation (redox) cycles. The distribution of oxidation states as well as the average oxidation state for chromium were 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 (O2, 773 K) revealed a significantly higher percentage of Cr(III) species in the Cr/SAPO-11 catalyst. The difference between the two samples remained following reduction (H2, 773 K). For the CrAPSO-11 sample, Cr(VI) showed a higher stability towards reduction, compared to chromium supported on conventional supports. This result suggests the incorporation of Cr(VI) into the silicoaluminophosphate framework, as mentioned in the literature. For the Cr/SAPO-11 solid, Cr(III) was extremely stable towards oxidation at 773 K. Due to the redox characteristics and the XPS results, the formation of chromia species on this catalyst is suggested.