Synthesis of small crystal ZSM-5 zeolite by solvent-free method and catalytic cracking performance of butene

Catalytic cracking is one of the important paths to achieve high value-added applications of C₄ hydrocarbon resources, and ZSM-5 zeolite is the most concerned catalytic material in the industrial application and research and development fields for C₄ olefin catalytic cracking technology. In this work, four types of small crystal ZSM-5 zeolites with different silicon aluminum ratios have been synthesized using a solvent-free method. The crystal structure and texture properties were characterized using XRD, SEM, and N₂ adsorption isotherms. The acid properties of these zeolites were investigated using NH₃-TPD and Py-FTIR, and key catalytic performance indicators such as butene conversion rate, product distribution, and hydrogen transfer index were systematically examined. The correlation between the acid properties of the zeolites and the butene conversion reaction pathway was systematically analyzed, with a focus on exploring the effect of acid density on product selectivity by regulating hydrogen transfer reaction. The research results indicate that the four H-ZSM-5 zeolites synthesized by solvent-free method have nearly spherical crystal with uniform particle size distribution (about 160 nm). Compared with the reported hydrothermal synthesis of small grain ZSM-5 zeolites in literature, the small grain ZSM-5 zeolites synthesized by solvent-free method showed significantly higher ethylene selectivity when the C₄ olefin conversion rate is slightly higher. Moreover, as the silicon aluminum ratio increased, the catalytic cracking activity of butene decreased, but the selectivity for ethylene and propylene increased significantly. Based on the analysis of product distribution and reaction pathways, it is inferred that the reaction pathways for catalytic conversion of butene on ZSM-5 samples with different silicon aluminum ratios are different. Among them, the acid density on ZSM-5 zeolites with low silicon aluminum ratios increases, and the synergistic effect between adjacent B acid centers in the pores facilitates the occurrence of hydrogen transfer reactions based on bimolecular mechanisms. This leads to the formation of alkanes through hydrogen transfer reactions of the light olefin product molecules produced by cracking reaction, thereby reducing the selectivity of light olefins in the target product. The single acid center catalytic conversion efficiency of ZSM-5 zeolite with high silicon aluminum ratio is significantly higher than that of low silicon aluminum ratio zeolite. It is proposed that the catalytic cracking pathway of butene on ZSM-5 zeolite is that butene or it’s isomers first oligomerizes on the B acid center to form C₈ carbocation intermediate, and then undergoes β-cracking to produce propylene/ethylene and C₅ carbocation intermediate. The catalytic cracking of ZSM-5 zeolite with high silicon aluminum ratio is more conducive to the direct cracking of C₅ carbocation intermediate into ethylene and propylene products. In addition, the samples with high silicon aluminum ratios are prone to undergo single-molecule acid catalyzed isomerization reactions of the butene skeleton or double bonds, resulting in more butene isomers. Furthermore, the deactivation rate of small crystal ZSM-5 zeolite is relatively low. This is because the reduction in crystal size helps the carbon deposition precursor to rapidly diffuse out of the zeolite crystal, slowing down the catalyst's carbon deposition deactivation rate and preventing rapid deactivation. The research results and conclusions of this article not only provide important data support for the development of efficient catalytic cracking ZSM-5 zeolite materials, but also have important guiding significance for understanding the regulatory mechanism of product distribution in the cracking process of C₄ olefins.