Design and synthesis of Ga-doped ZSM-22 zeolites as highly selective and stable catalysts for n-dodecane isomerization

Abstract

Ga-Doped ZSM-22 zeolites were synthesized successfully via a hydrothermal method by directly incorporating Ga species into the zeolite framework. It appeared that the as-prepared Ga-substituted zeolites possessed high crystallinity, uniform morphology and open pore structure. The XRD, FTIR, XPS as well as ⁷¹Ga and ²⁹Si MAS NMR results confirmed the presence of Ga species in the framework of zeolites and the formation of the Ga–OH–Si group, thus leading to a significant decrease of Brønsted acidity because of the lower acidity of the Ga–OH–Si bridge hydroxyl compared to that of the Si–OH–Al group. These characteristics ensured that the Pt-loading [Al, Ga]-ZSM-22 catalyst exhibited lower cracking activity and higher selectivity (about 78.2% at 90.5% of conversion) in the n-dodecane isomerization than the parent Pt/ZSM-22 catalyst, due to the more balanced bifunctionality between acid sites and metal sites. DFT calculations revealed that Al and Ga incorporation was mostly prevalent at the T2, T3 and T4 sites, while Fe heteroatom substitution occurred equivalently in four sites. Bader charge analysis suggested that the acidity decreased in the order of Al-ZSM-22 > Ga-ZSM-22 > Fe-ZSM-22, and the cracking activity exhibited the order of Al-ZSM-22 ≫ Ga-ZSM-22 > Fe-ZSM-22. Therefore, it could be concluded that the isomerization was facilitated by the combination of the heteroatom location and the acidity strength. Remarkably, no significant decline of crystallinity and special surface area of Ga-doped zeolites was observed after the high temperature calcination, revealing higher stability of Ga than Fe in the zeolite framework.