Roles of ethanol in coke formation and HZSM-5 deactivation during n-heptane catalytic cracking
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Abstract
In this work, the effects of ethanol on n-heptane catalytic cracking over HZSM-5 zeolites were explored at 550 °C for 6 h on stream. Particular attention was paid to the measurement of product distribution, especially alkene and aromatic products. Due to the unique structure, ethanol easily interacts with the acid sites over HZSM-5 zeolites. It favored the dehydration to ethene formation and prevented alkene secondary reaction to aromatic formation. Thus, ethanol addition promoted alkene selectivity while reducing aromatic selectivity in n-heptane catalytic cracking. However, ethanol addition accelerated the deactivation of HZSM-5 zeolites. XRD, 27Al MAS NMR, TG-DSC, N2 adsorption–desorption, and NH3-TPD were employed to characterize the fresh and spent HZSM-5 zeolites and elucidate the roles of ethanol in coke formation and HZSM-5 deactivation during n-heptane catalytic cracking. It was found that alkene promotion caused by ethanol addition enhanced coke formation and its migration from the micropores to the external surface of HZSM-5 zeolites. The formation and accumulation of external coke sharply blocked the pore openings and reduced the accessibility of the acidic site, accelerating HZSM-5 deactivation. Moreover, ethanol was more harmful to the framework of HZSM-5 zeolites compared to n-heptane. It accelerated the collapse of HZSM-5 crystallite, which was negative to the catalytic performance.
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