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A study of the feasibility of incorporation of chromium into the molecular sieve framework: The transformation of 1-butene over Cr-Silicoaluminophosphate molecular sieves

Abstract:

The catalytic transformations of 1-butene were performed over a chromium-substituted silicoaluminophosphate molecular sieve (CrAPSO-11), over a Cr-supported SAPO-11 molecular sieve (Cr/SAPO-11), and over two SAPO-11 samples. For times-on-stream under 2.5 h, the CrAPSO-11 catalyst showed a higher skeletal isomerization efficiency (SIE) than the two prepared SAPO-11 samples and the supported chromium system. After 2.5 h a drastic decrease in the SIE occurred (for the CrAPSO-11 sample) with a concomitant increase in the formation of butadiene and 2-butenes. The formation of butadiene was considerably hindered over the supported system compared to that of the CrAPSO-11 sample. The cracking reactions as well as the formation of C5+ hydrocarbons were also suppressed over the supported system. The catalysts were recently characterized by XPS and redox cycles (12) and presently studied by XRD, DRS, and NO chemisorption followed by IR spectroscopy and acidity measurements performed with pyridine chemisorption. A larger amount of Cr(VI) was found by DRS and XPS (∼70%) for the oxidic CrAPSO-11 (O2, 773 K), compared to the supported system. For the latter, Cr(III) was the main species present. The NO chemisorption experiments showed the presence of high chromium oxidation states on the oxidic CrAPSO-11. After reduction (H2, 773 K), the distribution of oxidation states of chromium (XPS and NO chemisorption experiments) for the reduced CrAPSO-11 solid, was different ∼40% Cr(VI) and ∼60% Cr(III) compared to the oxidic sample. Cr(VI) showed a higher stability towards reduction in the CrAPSO-11 catalyst when compared to chromium supported on conventional supports. The distribution of oxidation states of chromium was very similar for the oxidic and the reduced Cr/SAPO-11 catalyst. The oxidic CrAPSO-11 catalyst showed an increase in the number of medium + strong acid sites compared to the other catalysts. The reduction process, however, decreased the Brønsted strong acid sites by a factor of two. A concomitant decrease was observed for the medium + strong Lewis acid sites, upon reduction. These results suggest that a partially unsaturated Cr(VI) in the vecinity of P-OH groups may act as strong Lewis sites, generating Brønsted acidity by Brønsted-Lewis interaction, as suggested in the literature. The supported sample showed a decrease of medium + strong Brønsted acid sites compared to the CrAPSO-11 solid. In this case, however, the reduction process did not cause major changes in the acidity distribution. The characterization as well as the catalytic data support and reinforce the model presented by Chen and Sheldon (J. Catal. 153, 1 (1995)) for the related CrAPO-5 system. The results suggest the incorporation of chromium into the molecular sieve framework for the CrAPSO-11 catalyst. © 1997 Academic Press.