Study on hydrogen transfer reaction in C₅ hydrocarbons catalytic pyrolysis

The cracking reaction mechanism of C₅ hydrocarbons(n-pentane, 1-pentene) was analyzed. It is found that according to the ideal carbanion reaction mechanism and free radicals reaction mechanism, the molar selectivity of the cracking of n-pentane and 1-pentene to lower olefins (C₂H₄+C₃H₆+C₄H₈) is 50% and 100%, respectively. However, using MFI-30 zeolite, the molar selectivity of catalytic cracking of n-pentane and 1-pentene to light olefins at 650 ℃ is 23.41% and 56.79%, respectively, suggesting that 26.59% and 43.21% of light olefins have undergone hydrogen transfer reactions. The effects of different zeolites and key reaction temperature on the hydrogen transfer reaction during the catalytic pyrolysis of C₅ hydrocarbons were further investigated. The results show that the zeolite with small pore structure and low acid density and higher reaction temperature can inhibit the hydrogen transfer reaction to varying degrees, thereby increasing the selectivity of light olefins. At 650 ℃, as the zeolite changes from the FAU with a large pore structure and high acid content to the MFI-120 with a small pore structure and low acid content, the hydrogen transfer coefficient HTC of the catalytic pyrolysis of n-pentane and 1-pentene is reduced by 96.86% and 50.58%, respectively, and the coke selectivity is reduced from 11.91% and 20.77% to 0.75% and 0.89%, respectively. However, the selectivity of the lower olefins increases from 14.25% and 25.14% to 46.28% and 62.58%, respectively.