The effect of immobilization/mobilization processes on the temperature onset of a catalyst bed production studied with ethylene oligomerization on HZSM-5 zeolites

Immobilization of ethylene in the channel system of HZSM-5 was evaluated from the breakthrough curves of ethylene and desorption curves of ethylene transformation products measured on a zeolite bed between 308 K and 523 K. The kinetics of the overall immobilization/mobilization reaction was measured on a series of HZSM-5 samples in an integral flow reactor with a fixed bed of zeolite at 623 K. The aluminum content of the samples ranged from 3.2 to 6.4 Al atoms per unit cell. The zeolites were characterized by XRD, SEM, chemical analysis, Al-27 MAS NMR, water sorption, TPD of NH3 and FTIR spectroscopy. The results of kinetic measurements are represented by a family of S-shaped curves plotted as ethylene conversion versus contact time. The form of the curves suggests a contribution of an autocatalytic rate step to the kinetics. The length of the induction period increases with increasing crystal size and decreasing aluminum content in the catalyst. The kinetic curves obtained for crystals of the same aluminum content are strongly influenced by the crystal size. The ethylene breakthrough curves exhibit at the beginning a sharp breakthrough peak which decreases with accelerating oligomerization. The temperature threshold of zeolite bed production was estimated to lie at 418 K. At this temperature C-2-C-5 hydrocarbons were found in the gas phase in the gas leaving the reactor. The curve plotted as the amount of immobilized species against the temperature of the zeolite bed exhibits a steep increase to a flat maximum occurring between 353 K and 433 K, followed by a steep decrease. Adsorption of water on samples loaded with the immobilized species provides information about the space accessible from the gas phase and indicates differences in the nature of immobilized species formed at different temperatures. (c) 2006 Elsevier B.V. All rights reserved.