Cracking of C9 paraffins over xylene isomerization catalysts

The mechanism for cracking C₉ paraffins was studied in a series of experiments using feeds containing approximately 1.0% of one paraffin in a mixture of the three xylene isomers. Over a catalyst containing the medium pore molecular sieve AMS-1B, linear paraffins cracked more rapidly than branched paraffins. This difference was moderated somewhat by the addition of a hydrogenation metal to the catalyst. A similar effect is noted for catalysts containing the large pore zeolite Y where cracking of branched paraffins was favored in the absence of a hydrogenation metal. These shifts in cracking rates are shown to be consistent with a mechanism which involves an increase in the cracking rate due to the dehydrogenation activity of the metal and a decrease in cracking due to the hydrogenation activity of the metal. The distribution of cracked products indicates that the C₉ paraffins isomerize to an equilibrium distribution prior to cracking. The absence of isomerized C₉ paraffins in the product stream indicates that hydride abstraction is slow compared to either isomerization or cracking.