Coke Formation on Zeolite Catalysts in Naphtha Cracking

Coke formation was investigated in the catalytic cracking of alkanes into light alkenes over ferrierite and ZSM-5 zeolites. H-ferrierite had the highest alkene selectivity in heptane cracking because of its smaller pore size. Ca2+-exchange into H-ferrierite improved the selectivity and controlled coke formation. Ca2+ ions located at the center of the 8-membered ring converted ferrierite pores into one-dimensional channels of 10-membered rings, which suppressed the bimolecular hydride transfer to form alkanes and coke precursors. H-ZSM-5 zeolites having various extents of coke deposited in hexane cracking were characterized by adsorption measurements of various alkane molecules. Small amounts of coke did not affect the hexane conversion and micropore volume, whereas the adsorption rate of 2,3-dimethylbutane decreased significantly. These results suggest that coke is accumulated on the external surface of H-ZSM-5. Most coke would be formed via aromatic hydrocarbons. Selectivity for the transformation of aromatics into coke correlated with crystallite size of H-ZSM-5, indicating that aromatics formed in short channels can diffuse out of the crystallite immediately without coke precursor formation. H-ZSM-5 with smaller crystallite size is likely to be a stable catalyst for naphtha cracking into light alkenes with adequate catalyst life.