Pore size and crystal size effects on the selective hydroisomerisation of C-8 paraffins over Pt-Pd/SAPO-11, Pt-Pd/SAPO-41 bifunctional catalysts

The hydroconversion of n-octane over bifunctional Pt-Pd/SAPO-11 and Pt-Pd/S APO-41 catalysts was studied. High selectivity, 90% towards isomerisation, was observed even at high conversion. SAPO-41 was found slightly more selec tive than SAPO-11. The grain sizes have a profound effect on conversion, sm all grains were more active than large grains, but had almost no significan t effect on the selectivity and product distributions. It was concluded tha t the reaction of II-octane occurs in the SAPO's channels but the molecules did not penetrate too far in the channels. Restricted transition shape sel ectivity at the SAPO pore mouth is responsible for the product distribution s observed. These hypotheses were further supported by the study of monoeth yl heptane isomers and of dimethyl hexane isomers. Pore size effects and pa rticle size effects indicate that it is highly probable that the reactions occurred in the channels not too deep from the surface. Restricted transiti on-state selectivity was the main factor governing the extent of methyl deb ranching in branched C-8 isomers. A relatively small difference in the size of the channels resulted in a significant difference in the constraint exe rted by the pore dimension, smaller the pore dimension higher the tendency for the catalyst to form monobranched isomer with the methyl at terminal po sition. Also high selectivity for debranching the methyl branched C-8 isomers is ob tained with these medium monodimensional pore SAPO materials. Small-grain m olecular sieves were more effective than large-grain samples particularly i f amount of acid sites on the outer surface is small. (C) 1999 Elsevier Sci ence B.V. All rights reserved.