Catalytic Cracking Behavior of n-Heptane over HZSM-5 Catalyst

Abstract

The catalytic cracking of n-heptane, which was selected as a model compound for the paraffin of light straight-run naphtha, was studied over HZSM-5 catalyst in a fixed-bed microreactor. Its catalytic behavior was compared with that of 1-heptene and the effects of hydrothermal treatment and catalyst carrier were evaluated. The results showed that compared with the cracking of 1-heptene, the concentration of hydrogen, methane, and ethane was much higher in the cracked gas obtained from the cracking of n-heptane. We concluded that this mainly originated from the monomolecular cracking pathway while the content of propylene and butylene in liquefied petroleum gas (LPG) was much lower. Upon hydrothermal treatment, the total amount of acid decreased sharply, especially the strong Br[o]nsted acid (Bacid) sites, leading to a steep decline in catalytic activity. This was accompanied by the improved propylene/propane and butylene/butane molar ratios in the products. Meanwhile, the ratio between the C3 and C4 products decreased, suggesting a decrease in the occurrence of monomolecular cracking. The carrier also significantly influenced the cracking behavior of n-heptane. We found that the presence of Lewis acid (Lacid) sites in the carrier improved the n-heptane conversion and promoted the bimolecular cracking pathway. Generally, compared with the olefin reactant, paraffin usually shows much lower reactivity and light olefin selectivity and, therefore, it is not a desirable feed for the catalytic cracking reaction over the zeolite catalyst for the purpose of light olefin production.