Evaluation of commercial FCC catalysts for hydrocarbon conversion: II. Time-on-stream behavior of n-hexane conversion and comparison of n-hexane conversion to MAT

Abstract

n-Hexane conversion over commercial FCC catalysts was investigated in a plug-flow reactor system allowing analysis of products with a high time resolution. The different decays of protolytic cracking, dehydrogenation and hydride transfer with time-on-stream suggests different and/or additional sites for the different reaction pathways. In particular, dehydrogenation proceeds via two pathways, of which one is only important at short times-on-stream (TOS). This reaction pathway is speculated to be affiliated with the extra-lattice aluminum in the zeolite. The variation of methane with TOS is correlated to the dehydrogenation, and a part of methane is produced by active sites also dehydrogenating n-hexane. Addition of water decreased the rates of all reaction pathways due to competitive adsorption. Under the reaction conditions used, the carbonaceous species responsible for deactivation seem to be rather small in molecular weight (e.g. cyclic alkenes or single-ring aromatics). The rate of n-hexane conversion is directly proportional to the activity evaluated by the microactivity test. The octane numbers and the amount of coke formed derived from this latter test can be well correlated to the isoparaffin-to- n-paraffin ratio and the rate of hydride transfer in n-hexane cracking, respectively.