Deactivation of solid acid catalysts for butene skeletal isomerisation: on the beneficial and harmful effects of carbonaceous deposits Original Research Article

Skeletal isomerisation of n-butene to isobutene is mainly controlled by catalyst pore topology, acid strength, acid site density and location of the acid sites. It is established that the pore structure of the catalyst is the most important feature with regard to isobutene selectivity and stability. The most favourable activity versus selectivity and stability characteristics are displayed by the zeolite ferrierite, for which the presence of carbonaceous deposits coincides with the selective performance in butene skeletal isomerisation. By-product formation, mainly propene, pentenes and octenes, as well as isobutene production initially takes place via oligomerisation and cracking throughout the ferrierite crystals. After some time-on-stream the pore system of ferrierite is largely filled by aliphatic carbonaceous deposits, with catalysis primarily occurring at the pore mouths of the channels. At this stage cracking of these aliphatic deposits is the origin of small amounts of by-products. Slowly the deposits are converted into aromatic coke, thus, further reducing reactivity and concomitant formation of non-selective products. It is emphasised that the observed increase of isobutene selectivity with time-on-stream is a consequence of the decrease of cracking reactions. Final deactivation of the catalyst is due to blockage of the pore mouth inlets by poly-aromatic compounds formed after a prolonged time-on-stream.