Effect of external surface barriers over hierarchical ZSM-5 zeolite on n-heptane catalytic cracking

Abstract

To better understand external surface barriers on hierarchical ZSM-5 zeolite, SiO2 layers were constructed over zeolite nanocrystal by using chemical liquid depositions (CLD). Diffusion measurement showed that the apparent diffusion rate over the zeolite was increased by up to 142 % after SiO2 deposition, indicating that the external surface barriers had been significantly reduced. The performance of hierarchical ZSM-5 zeolite for n-heptane catalytic cracking indicated that the catalytic activity (TOF) was increased from 0.55 to 0.91 s−1 after 3 nm layer of SiO2 deposition, leading to a total yield increase of ethylene and propylene by up to 23 %. The strong positive correlation of catalytic activity with apparent diffusion rate suggests that reducing external surface barriers can improve the utilization efficiency of active sites in micropores and enhance the catalytic activity of the zeolite. These results demonstrate that CLD of SiO2 can reduce external surface barriers on hierarchical zeolite, thereby improving its cracking performance.