Zeolite deactivation during hydrocarbon reactions: characterisation of coke precursors and acidity, product distribution

摘要

The catalytic conversion of hydrocarbons over zeolites has been applied in large scale petroleum-refining processes. However, there is always formation and retention of heavy by-products, called coke, which causes catalyst deactivation. This deactivation is due to the poisoning of the acid sites and/or pore blockage. The formation of coke on hydrocarbon processing catalysts is of considerable technological and economic importance and a great deal of work has been carried out to this study. The main aim of this work is to understand the deactivation of zeolite catalysts as a result of coke deposition. The deactivation by coke of USHY zeolite was investigated during catalytic conversion of hydrocarbons – 1-pentene, n-heptane and ethylbenzene – as representatives of olefins, paraffins and aromatics respectively, at different reaction temperatures, time-on-streams and composition. Three novel techniques, coke classification, thermogravimetric method for characterising coke precursors and indirect temperature programmed desorption (TPD) for catalyst acid sites characterisation were developed to further study catalyst deactivation mechanism. Product distribution, coke formation, characterisation of coke precursors, as well as the role of strong acid sites on hydrocarbon reactions are presented and discussed. During catalytic reactions of 1-pentene over USHY zeolite, cracking and hydride transfer were the predominant reactions in initial stage which deactivated rapidly allowing isomerisation to become the main reaction afterwards. Deactivation studies showed that coke formation was very strong initially which is in good correlation with the initial rapid deactivation. The hydrogen freed during this initial time from the formation of high C/H ratio coke components contributed to the formation of hydride transfer products. The amount of coke precursors decrease with increasing reaction temperature due to the higher desorption of coke precursors into gas phase while hard coke amount increased with temperature as expected from an activated process. The coke amount formed was not proportional to the reactant feed composition, because of a strong pseudo-zeroth- order initial coking on strong acidic sites. The thermogravimetric method provides insight into the chemical character of coke precursor components in terms of the mode of their removal and allows further classification of coke precursors into small and large coke precursors. The concentration and strength of acid sites of coked catalysts were studied by the TPD methodology. Besides, characterisation of coke precursors was also revealed. The initial deactivation preferentially on strong acid sites is very fast. The concentration of free acid sites is inversely correlated well with the total concentration of coke rather than individual coke groups. Coke precursors tend to be more stable at higher reaction temperatures. Furthermore, by selectively poisoning strong acid sites of USHY zeolite, it shows conclusively that strong acid sites are responsible for cracking and hydride transfer reactions as well as strong coke formation while weak acid sites can only catalyse double bond isomerisation.