Effect of coke on surface barriers over H-ZSM-5 zeolites during methanol-to-olefins
The surface barriers of guest molecules in zeolites have a significant impact on catalytic reactions,and their change can affect the diffusion rate of guest molecules,thereby altering the catalytic performance of zeolites.In the methanol-to-olefins(MTO)process,coke inside the zeolite crystals is a key factor influencing product selectivity.It affects the methanol conversion reaction pathway on the one hand and the mass transfer performance of the zeolites on the other.Although the influence of coke on the reaction mechanism has been widely studied,research on the effect of coke on the surface barriers of zeolites is scarce.In this article,we investigate the mutual influence between surface barriers and coke on zeolites during the MTO reaction process:firstly,by modifying the external surface of H-ZSM-5 zeolites using a chemical deposition method with SiO2 to adjust the surface barriers;then,by combining diffusion measurement with zero length column(ZLC)method and reaction evaluation results,we analyze the mutual influence between the surface barriers of propane and coke at different stages of the MTO reaction.The results show that a small amount of coke on the original sample helps reduce the surface barriers of propane,but when the amount of coke exceeds a certain value,the surface barriers of propane show an increasing trend.Samples with a small amount of SiO2 deposition also reduce the surface barriers of propane,while samples with a larger amount of SiO2 deposition increase the surface barriers of propane.Samples with SiO2 deposition deactivate faster,and the higher the deposition amount,the faster the deactivation.Detailed analysis reveals that the surface barriers of guest molecules on zeolites do not have a simple monotonically increasing relationship with coke amount but are related to the initial state of the crystal surface.It provides important insights into the changes in surface barriers of zeolites during the MTO reaction process.
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