Pore size and crystal size effects on the selective hydroisomerisation of C8 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/SAPO-41 catalysts was studied. High selectivity, 90% towards isomerisation, was observed even at high conversion. SAPO-41 was found slightly more selective than SAPO-11. The grain sizes have a profound effect on conversion, small grains were more active than large grains, but had almost no significant effect on the selectivity and product distributions. It was concluded that 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 selectivity at the SAPO pore mouth is responsible for the product distributions observed. These hypotheses were further supported by the study of monoethyl heptane isomers and of dimethyl hexane isomers. Pore size effects and particle size effects indicate that it is highly probable that the reactions occurred in the channels not too deep from the surface. Restricted transition-state selectivity was the main factor governing the extent of methyl debranching in branched C-8 isomers. A relatively small difference in the size of the channels resulted in a significant difference in the constraint exerted by the pore dimension, smaller the pore dimension higher the tendency for the catalyst to form monobranched isomer with the methyl at terminal position. Also high selectivity for debranching the methyl branched C-8 isomers is obtained with these medium monodimensional pore SAPO materials. Small-grain molecular sieves were more effective than large-grain samples particularly if amount of acid sites on the outer surface is small. (C) 1999 Elsevier Science B.V. All rights reserved.