Graphical Bifunctional catalysts exhibiting significantly enhanced isomerization yields were synthesized by combining Pt-impregnated γ-Al 2 O 3 with ZSM-23 zeolite. The structures of the di-branched products were meticulously characterized. Furthermore, the influences of thermodynamic stability, zeolite pore structure, and substrate carbon number on product distribution were analyzed concerning the spacing of branched chains. Hydroisomerization of long-chain alkanes is a crucial process in producing high-performance fuel and lubricant base oils. The challenge in this process is enhancing catalytic performance by optimizing the intimacy between metal and Brønsted acid sites in bifunctional catalysts. In this study, a series of bifunctional catalysts with varying metal–acid site intimacy was prepared by depositing platinum (Pt) on the ZSM-23 zeolite or γ-Al 2 O 3 surface. The Pt/A + Z catalyst, exhibiting moderate intimacy, demonstrated superior catalytic performance in the hydroisomerization of n -hexadecane, achieving up to 64 ± 0.8% i-C 16 yield at 89 ± 0.5% conversion. The optimized metal–acid site intimacy facilitated the desorption of reaction intermediates, minimizing cracking at the acid sites and thus improving selectivity. Further analysis revealed that product distributions were closely related to the pore structure of zeolites. Specifically, the formation of di-branched isomers during the isomerization of n -hexadecane, n -dodecane, and n -octane was influenced by the type of zeolite support (ZSM-23, ZSM-48). The present study proposes a direct method to enhance the hydroisomerization performance while reducing the cost of the catalyst. By determining di-branched products more precisely, it introduces a novel approach for evaluating products based on branch chain spacing. Additionally, the combined effects of catalyst structure and thermodynamic stability on product distribution were investigated, along with the influence of reactant stay time.
Highly crystallized ZSM-23 zeolite samples were synthesized by employing the dual structure directing agents pyrrolidine (PY) and isopropylamine (IPA). The existence of two types of structure directing agents widened the synthesis composition ratio range and speeded up the crystallization. The samples obtained showed larger micropore surface areas and volumes compared with the products synthesized with single SDA. It was found that the use of the dual-structure directing agent had no effect on the total acidity but altered the acid site distribution where more weak and medium acidic sites were exposed and was found to be more suitable for n-hexadecane hydroisomerization. The bifunctional catalysts prepared from the supports using the dual-structure directing agent showed excellent catalytic performance under certain reaction conditions. It was noteworthy that Pt/PI-200 (Pt supported on the dual-structure directing agent with a SiO 2 /Al 2 O 3 ratio of 200) showed a constant selectivity to iso-hexadecane ca. 69% with contact time of 0.75 min and longer.
The seed-assisted synthesis of ZSM-23 zeolite in the absence of alkali metal ions (denoted as SNNa) was investigated. Highly crystallized SNNa with needle-shape morphology and size of 500–1100 nm in length and 50–200 nm in diameter can be synthesized. Much attention was paid to the roles of alkali metal ions during the crystallization process . The absence of Na + can effectively avoid the impurity-quartz formation and promote SNNa crystallization in a broad range of SiO 2 /Al 2 O 3 ratio between 80 and 200, whereas the crystallization rate was slowed down at the same time. On the contrary, the presence of Na + had a significant effect both on the nucleation and crystal growth processes. The bifunctional catalyst Pt/H-SNNa prepared, showed good catalytic performance in the hydroisomerization of n-hexadecane due to its small crystal size, appropriate acidity and acid site distribution.
Siyan Yan,Jiale Zhu,Jianguo Zhuang,Xinhui Liu,Yitao Zhao,Yuan Qian,Xuedong Zhu,Fan Yang
ZSM-23 zeolite with a reduced a -axis is synthesized using trimethoxy[3-(phenylamino)propyl]silane (TPPS) and applied in n -hexane cracking. Once nuclei are formed, TPPS primarily attaches to the ends of rod-like ZSM-23 crystals, effectively hindering elongation growth and significantly promoting the diffusivity of ZSM-23 zeolite. The carbonium pathway is favored over the carbenium pathway in n -hexane cracking when reduced a -axis ZSM-23 is applied, leading to higher ethene yield instead of propene. The fast diffusion also shifts deactivation from an inside-out process to an outside-in one due to the different precursors for carbon deposition. The obtained short a -axis ZSM-23 exhibits excellent catalytic performances in n -hexane cracking, displaying better activity (98.4% vs. 86.4%) and greater stability (2400 min vs. 120 min) than traditional ZSM-23. These findings deepen the understanding of organosilane’s role in unidimensional zeolite synthesis and unveil the structure-performance relationship and deactivation mechanism in n -hexane cracking.
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