Factors affecting coke formation on H-ZSM-5 in naphtha cracking

• During catalytic cracking of naphtha, BTX is formed in the micropores of H-ZSM-5.
• A part of BTX is converted into coke precursor inside the micropores.
• Na+-exchange and phosphorus addition effectively reduce the BTX formation.
• Small crystallite size H-ZSM-5 effectively reduces the formation of coke precursor.

The coke formation on H-ZSM-5 zeolite during the catalytic cracking of alkanes constituting naphtha was investigated with a focus on the reaction route and the role of acid concentration and crystallite size of H-ZSM-5. To reveal the coke formation route, cracking of n-hexane, methylcyclopentane or methylcyclohexane was carried out on H-ZSM-5(Si/Al = 107). Cracking of n-hexane produced benzene, toluene and xylene (BTX) as secondary products from successive reaction routes through light alkenes. Only in the cracking of methylcyclopentane and methylcyclohexane, direct reaction routes partially contributed to the BTX formation. In any cases, most of the coke would be formed through BTX. The reaction of BTX into coke was analyzed from the catalytic results on Na+-exchanged and phosphorus embedded H-ZSM-5 with various crystallite sizes. The ratio of accumulated amounts of coke and BTX (coke/BTX ratio) was obtained as a measure of selectivity for coke formation. The coke/BTX ratio did not show a significant correlation with the acid concentration of catalysts, whereas the ratio gave a strong correlation with the crystallite size. H-ZSM-5 with smaller crystallite sizes would help BTX molecules escaping immediately out of micropores before being converted into coke precursor, which minimizes the coke formation.

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