Ca2+-exchanged ferrierite: Quasi one-dimensional zeolite for highly selective and stable formation of light alkenes in catalytic cracking of n-octane

The catalytic cracking of n-octane was carried out as a model reaction of naphtha cracking to produce light alkenes on proton-form zeolites of various pore dimensions such as ZSM-5, ferrierite, ZSM-12, ZSM-22, SAPO-11 and SAPO-31. Among them, ferrierite is of good activity. However, the selectivity to light alkenes and the stability of conversion were not satisfactory. After ion exchange with Ca2+ ions, a great improvement has been obtained both in the selectivity to light alkenes and the stability of conversion. TG analysis showed that the amount of coke formed decreased with the increase in the Ca2+-exchange level. Structure analysis using powder XRD data revealed that Ca2+ ions are located at the center of 8-MR channels near the intersection of 8-MR and 10-MR channels (Ca1) and in the wall between 5-MR pores and 8-MR channels (Ca2). Ca2+ ions in Ca1 sites reduce the dimension of ferrierite pores partly to form quasi one-dimensional pore structure. This change in pore dimension would reduce the hydride transfer to consume initially formed alkenes and to form coke precursors. NH3-TPD showed that the ion exchange with Ca2+ reduced the acid density and the amount of strong acid sites; these changes in acidity can also reduce the hydride transfer.

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► In Ca2+-exchanged ferrierite, exchanged cations block 8-MR channels and make ferrierite quasi one-dimensional.
► Ca2+-exchange into ferrierite reduces the bimolecular hydride transfer to increase the selectivity to light alkenes greatly.
► Ca2+-exchange into ferrierite suppresses the coke formation to improve its stability greatly.
► Ca2+-exchange into ferrierite decreases the acid density and acid strength.