Influence of coke formation on the conversion of hydrocarbons I. Alkanes on a USY-zeolite

The conversion of,l-hexane and 2,2,4 triMe-pentane was studied at 723 K on a USY-zeolite (Si/Al: frame=30; bulk=2.7) in an electrobalance reactor with external recirculation. The influence of the coke content on the reaction paths involved in the conversion of paraffinic model components was evaluat ed. Coke formation from propylene and i-butene, the main olefinic products formed during the conversion of the two paraffinic model components, was in vestigated. Under the conditions used, an increase in the molar WC ratio of the product s as a function of coke yield can be observed due to hydride transfer react ions with coke. Coke molecules cannot be considered as being inert with res pect to the cracking reactions and their formation leads to the occurrence of reactions that can strongly influence the catalyst performance. In parti cular, the potential of coke molecules to form highly delocalized carbenium ions and their ability to act as hydride donor towards surface carbenium i ons provides reaction paths to paraffinic reaction products. The effect of coke is not identical for all the elementary reactions involv ed in the conversion of the paraffinic model components. (C) 2000 Elsevier Science B.V. All rights reserved.