Deactivation studies during catalytic cracking of C 8 aliphatic hydrocarbons over ultrastable Y-zeolite. Conversion and product yield profiles with time onstream Aristidis A. Brillis and George Manos  Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, UK Received 17 June 2003; accepted 23 September 2003 The deactivation of an ultrastable Y-zeolite during cracking of n-octane, isooctane and 1-octene was studied in a fixed-bed reactor. The relative reactivities of the three reactants in increasing order were n-octane < isooctane < 1-octene. At high residence times, complete conversion was achieved followed by a relatively slow decline that, in fact, presented a falsified deactivation picture. On the other hand, a surprising olefin yield increase with time onstream was observed, i.e., isobutene during isooctane cracking and octene isomers in the case of 1-octene. The isobutene yield increase was assigned to preferential deactivation of strong acid sites that allowed primary products to desorb into the gas phase, increasing their apparent yield rather than undergo secondary reactions. Similarly, 1-octene isomers showed a yield increase with time onstream due to stronger deactivation of cracking reactions than isomerization. KEY WORDS: deactivation; catalytic cracking; hydrocarbons; zeolites; acid catalysis. 1. Introduction The cracking of hydrocarbons on acid solid catalysts is a long-established and a very important industrial process. Their cracking mechanisms run via carbonium or carbenium ion intermediates, similar to reactions catalyzed by strong acids in homogeneous media [1–3]. Carbonium ions play a dominant role in high-tempera- ture paraffin cracking, since they are the initiators of the cracking chain proceeding via complex ionic mechan- isms [4,5]. Carbenium ions, on the other hand, are involved in the chain propagation of cracking, skeletal and double-bond isomerization, alkylation and oligo- merization reactions [3]. Overall, for the cracking of paraffins, three different pathways have been suggested [6–9]. Albeit tremendous progress has been made in the development of cracking catalysts, they still suffer from strong side reactions that form heavy, low boiling point, high molar mass by-products, the so-called coke, that deposit on the active surface and deactivate the catalysts. The aim of this work was to study the effect of process variables, like residence time and reactant feed composition, in order to get further insight into the deactivation behavior of the chosen system. The reaction system chosen was the catalytic cracking of aliphatic hydrocarbons over an acidic ultrastable Y- zeolite (USHY). Simple one-component feed streams have been chosen with the same carbon number but with different structure, namely n-octane, isooctane (2,2,4 trimethylpentane) and 1-octene. Differences in profiles with time onstream of the product distribution from the three reactants were studied and discussed. 2. Experimental section 2.1. Materials The USHY zeolite catalyst was provided in calcined powder form of average crystallite size of 1 m, a bulk Si/Al ratio of 2.5 and a framework Si/Al ratio of 5.7. Its micropore area was 532:4m 2 =g and its micropore volume 0:26 cm 3 =g. The measured BET surface area was 590  23:5m 2 =g. The catalyst was pressed into pellets, crushed and sieved, producing particles in the size range of 1.0–1.7 mm. Before each experiment, the catalyst was dried in an oven at 473 K for 2 h. All hydrocarbons, n-octane, isooctane (2,2,4 tri- methylpentane) and 1-octene, were supplied by Fluka Chemicals (99% purity). Nitrogen (99% purity) was used as carrier gas. 2.2. Experimental procedure The experiments were performed in the temperature range of 523–623 K and at atmospheric pressure, in a stainless steel tubular fixed-bed reactor, with a total length of 25.5 cm and an inner diameter of 15 mm. The 1-cm-long catalyst bed was placed between two metal  To whom correspondence should be addressed. E-mail: g.manos@ucl.ac.uk Catalysis Letters Vol. 91, Nos. 3–4, December 2003 ( # 2003) 185 1011-372X/03/1200–0185/0 # 2003 Plenum Publishing Corporation