Pellet Size-Induced Increase in Catalyst Stability and Yield in Zeolite-Catalyzed 2-Butene/Isobutane Alkylation

Zeolite-catalyzed alkylation of isobutane is a promising alternative to conventional H₂SO₄ or HF-catalyzed processes to produce high octane fuels. Fast catalyst deactivation by deposits of heavy hydrocarbons, blocking pores, and acid sites limits industrial applications. Increasing the catalyst pellet size is reported here to be able to reduce catalyst deactivation and to increase the total yield during its lifetime. On nearly fully La-exchanged X zeolite (LaX), the catalyst lifetime was increased more than threefold, while the product yield increased six times when the catalyst pellet diameter was increased from 0.15–0.25 to 1.6 mm. This positive effect is caused by limiting mass transport of butene in large pellets, decreasing its concentration at the acid sites in the inner side of the particle. The lower butene concentration suppressed multiple alkylation and so also the formation of high molecular weight hydrocarbons blocking active sites. Characterization of used LaX pellets by microscopy of cross sections and IR spectroscopy showed a core–shell structure of carbon deposits. This demonstrates unequivocally that only the outer zone of the particle is deactivated by the heavy hydrocarbons, while the core remained nearly unaffected.