A STUDY OF THE FEASIBILITY OF INCORPORATION OF CHROMIUM INTO THE MOLECULAR-SIEVE FRAMEWORK - THE TRANSFORMATION OF 1-BUTENE OVER CR-SILICOALUMINOPHOSPHATE MOLECULAR-SIEVES

The catalytic transformations of 1-butene were performed over a chromi um-substituted silicoaluminophosphate molecular sieve (CrAPSO-11), ove r a Cr-supported SAPO-11 molecular sieve (Cr/SAPO-11), and over two SA PO-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) wi th a concomitant increase in the formation of butadiene and 2-butenes. The formation of butadiene was considerably hindered over the support ed system compared to that of the CrAPSO-11 sample. The cracking react ions 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 pe rformed with pyridine chemisorption. A larger amount of Cr(VI) was fou nd by DRS and XPS (similar to 70%) for the oxidic CrAPSO-11 (O-2, 773 K), compared to the supported system. For the latter, Cr(III) was the main species present. The NO chemisorption experiments showed the pres ence of high chromium oxidation states on the oxidic CrAPSO-11. After reduction (H-2, 773 K), the distribution of oxidation states of chromi um (XPS and NO chemisorption experiments) for the reduced CrAPSO-11 so lid. was different similar to 40% Cr(VI) and similar to 60% Cr(III) co mpared to the oxidic sample. Cr(VI) showed a higher stability towards reduction in the CrAPSO-11 catalyst when compared to chromium supporte d on conventional supports. The distribution of oxidation states of ch romium was very similar for the oxidic and the reduced Cr/SAPO-11 cata lyst. The oxidic CrAPSO-11 catalyst showed an increase in the number o f medium + strong acid sites compared to the other catalysts. The redu ction process, however, decreased the Bronsted strong acid sites by a factor of two. A concomitant decrease was observed for the medium + st rong Lewis acid sites. upon reduction. These results suggest that a pa rtially unsaturated Cr(VI) in the vecinity of P-OH groups may act as s trong Lewis sites, generating Bronsted acidity by Bronsted-Lewis inter action, as suggested in the literature. The supported sample showed a decrease of medium + strong Bronsted 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 wel l as the catalytic data support and reinforce the model presented by C hen and Sheldon (J. Catal. 153, 1 (1995)) for the related CrAPO-5 syst em. The results suggest the incorporation of chromium into the molecul ar sieve framework for the CrAPSO-11 catalyst. (C) 1997 Academic Press .