Brillis, Aristidis; (2002) Catalytic cracking of C8 aliphatic hydrocarbons over ultrastable y zeolite and its deactivation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Catalytic cracking of hydrocarbons is an important industrial reaction, whose fundamental understanding and quantification of its elementary processes are far from complete. Next to the main reactions, a major side effect is the formation of high molar mass, low volatile hydrocarbons, the so-called coke, that blocks the active sites and deactivates the catalyst. The aim of this work is to gain a fundamental understanding of the deactivation of zeolite catalysts as a result of coke deposition. Also, of special importance are the product selectivities as well as the understanding of the individual reaction mechanisms of catalytic cracking. The effect of changing the residence time or the reactant composition is also studied. The experiments performed include the use of ultrastable H-Y zeolite (USHY) as catalyst in a fixed bed reactor, with n-octane, isooctane and 1-octene as reactants, at a range of 523-623 K, reactant feed partial pressures of 0.05, 0.1 and 0.2 bar and different reactant flow rates. The relative yields, as well as the tendency towards coking for the three reactants in increasing order are n-octane isooctane 1-octene. Moreover, coking during cracking of isooctane and 1-octene at 5% and 10% compositions decreases with temperature. However, this unexpected trend is reversed at the higher reactant compositions. The dominant product is isobutane with a yield of almost 0.9 when using isooctane and much less when n-octane or 1-octene are used as reactants. Other major products are propane, n-butane, isopentane and isobutene. The cracking of n-octane can proceed via β - scission and/or hydride transfer reactions, while isooctane cracking proceeds via the protonation of the quaternary carbon atom over the Broensted acid sites. However, 1-octene cracking shows a high extent of isomerisation reactions. Generally olefins show higher ability to undergo cracking reactions compared with paraffins.

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