Effect of mesopore spatial distribution of HZSM-5 catalyst on zinc state and product distribution in 1-hexene aromatization

Abstract

1-hexene aromatization is a promising technology to convert excess olefin in fluid catalytic cracking (FCC) gasoline to high-value benzene (B), toluene (T), and xylene. Besides, the increasing market demand of xylene has put forward higher requirements for new generation of catalyst. For increasing xylene yield in 1-hexene aromatization, the effect of mesopore structure and spatial distribution on product distribution and Zn loading was studied. Catalysts with different mesopore spatial distribution were prepared by post-treatment of parent HZSM-5 zeolite, including NaOH treatment, tetra-propylammonium hydroxide (TPAOH) treatment, and recrystallization. It was found the evenly distributed mesopore mainly prolongs the catalyst lifetime by enhancing diffusion properties but reduces the aromatics selectivity, as a result of damage of micropores close to the catalyst surface. While the selectivity of high-value xylene can be highly promoted when the mesopore is mainly distributed interior the catalyst. Besides, the state of loaded Zn was also affected by mesopores spatial distribution. On the optimized catalyst, the xylene selectivity was enhanced by 12.4% compared with conventional Zn-loaded parent HZSM-5 catalyst at conversion over 99%. It was attributed to the synergy effect of mesopores spatial distribution and optimized acid properties. This work reveals the role of mesopores in different spatial positions of 1-hexene aromatization catalysts in the reaction process and the influence on metal distribution, as well as their synergistic effect two on the improvement of xylene selectivity, which can improve our understanding of catalyst pore structure and be helpful for the rational design of high-efficient catalyst.

1. Introduction

The global wave of environmental protection and carbon dioxide emission reduction has put forward higher requirements for traditional vehicle fuels [1e3]. For example, on the one hand, various economies have stricter restrictions on olefins and aromatics content in gasoline; on the other hand, the popularity of new energy vehicles has made the demand growth for gasoline weak. To meet these challenges, traditional refineries producing fluid catalytic cracking (FCC) gasoline need to find new ways to convert excess olefins. Benzene (B), toluene (T), and xylene are important chemical raw materials, which can be used to produce fabric, rubber, medicine, etc. [4e6]. Among them, the demand for xylene has increased greatly in recent years due to the rapid growth in demand for its downstream products [7e9]. Therefore, the aromatization of olefins has become a necessary choice for refineries to reduce olefin content and improve economic benefits.

HZSM-5 is one of the most commonly used 1-hexene aromatization catalysts, due to its special pore structure has high selectivity to aromatics [10,11]. However, highly selective ten-membered ring channels also have diffusion limitations on aromatics that are detrimental to the catalyst's lifetime [12e14]. To prolong the lifetime of HZSM-5 in aromatization, an efficient solution is to introduce intracrystalline mesopores inside the zeolite. After mesopores are introduced, the diffusion of reaction intermediates and products can be enhanced and the coke formation can be suppressed. Groen et al. [15] synthesized HZSM-5 zeolite with intracrystalline mesopores by post-treatment with 0.2 mol·L−1 NaOH solution. It was found the characteristic diffusion path length of neopentane in the mesoporous zeolite was dramatically shortened by approximately 2 orders of magnitude. Li et al. [16] introduced mesopores into HZSM-5 by alkali treatment and found the aromatization stability, which is defined as the ratio of aromatics selectivity at 8 h time on stream (TOS) to that at 2 h TOS, was enhanced by more than 3 times. However, due to the difference in the protocols used in creating mesopores, the introduced mesopores may vary in spatial distribution and volume. Besides, the introduced mesoporous structure will inevitably affect the yield and distribution of products. Therefore, against the backdrop of increasing emphasis on the transformation of excess fuel oil to high-value-added products, it is more urgent to study the influence of the introduction of mesopores on product distribution in 1-hexene aromatization.