Study On The Effect Of Surface Acidity Of ZSM-5 Zeolite On Its Performance In N-hexane Catalytic Cracking

Catalytic cracking technology is expected to be a key technology for future low carbon olefin production due to its low energy consumption and adjustable product selectivity.Zeolite catalysts are widely studied as the core of catalytic cracking technology,where the influence of zeolite acidity,especially surface acidity,is particularly important.To investigate the effect of zeolite acidity on the reaction path of alkane catalytic cracking,the reaction path was identified by comparing the product distribution of n-hexane catalytic cracking on three different HZSM-5 zeolites with different Si/Al ratios and different Si/Al ratios.In order to exclude the effect of thermal cracking,the reaction temperature in this study was chosen to be 300°C.It was found that all reaction systems mainly underwent acid-catalyzed cracking,isomerization and hydrogen transfer reactions based on the carbon positive ion mechanism,while side reactions such as polymerization reactions accounted for very little,and no thermal cracking reactions occurred,which indicated that the total amount of catalyst acid had a significant effect on the reaction path of n-hexane catalytic cracking;and the selectivity of C2 and C4 products was almost unaffected by acidity,which indicated that n-hexane cracking to generate carbon tetra The C₂H₅⁺carbon ions generated during the cracking of n-hexane to produce carbon tetra are difficult to undergo hydrogen transfer reactions to form ethylene and ethane molecules,but are easily converted to other product molecules by forming new carbonium ions with other n-hexane molecules.In order to investigate the effect of zeolite surface barriers on the catalytic cracking behavior of alkanes on zeolites,HZSM-5 zeolites were treated with TEOS and Si Cl₄to increase and decrease the zeolite surface barriers,respectively.It was found that the increase of zeolite surface barriers barrier hindered the molecular out-diffusion and increased the secondary reaction in the catalytic cracking process.The effect of the outer surface barriers of the zeolites on the mass transfer of the guest molecules was investigated by frequency response technique using propane as the probe molecule.All three groups of zeolites showed a triple-peak effect,where the high frequency was attributed to the propane external diffusion process,and the signal value there was proportional to the size of the surface barriers;this indicates that the size of the surface barriers affects the mass transfer performance of the guest molecule in the zeolite,which in turn affects the product selectivity.Taken together,these results show that the zeolite surface barriers dominates the overall molecular sieve mass transfer rate,rather than the previously perceived internal zeolite diffusion process.