Post-Modification Of ZSM-12Zeolites And Their Applications In The Hydroisomerization Of N-Hexane

The hydroisomerization of light n-alkanes provides a good choice for the production of high quality gasoline via improving the front-end octane rating of the gasoline. Due to the suitable pore structure and acidity, ZSM-12zeolite-supported platinum catalysts (Pt/ZSM-12) show excellent performances in the hydroisomerization of light n-alkanes. However, the presence of the mass transfer limitation in the zeolite reduces its catalytic activity. One of effective methods to improve the mass transport limitations in the zeolite is to introduce mesopores into the zeolite by alkaline treatments.In this thesis, two methods, including organic template-assisted and seeded growth methods, were employed to prepare ZSM-12zeolites. Consequently, the structures and morphologies of the synthesized samples were compared. Thereafter, the ZSM-12zeolites with different Si/Al ratios, prepared by the organic template-assisted method, were calcined, followed by an alkaline treatment to introduce mesopores into the zeolites. The effects of the alkaline treatment on the crystallinity and textural properties of the resultant ZSM-12zeolites and on the catalytic properties of the treated ZSM-12zeolites as catalyst supports in the hydroisomerization of n-hexane were discussed in detail. Additionally, the as-synthesized ZSM-12zeolites containing the template were alkaline treated as well, and the results show that the microporosity and acid sites of the resultant zeolites are preserved while the intercrystal mesoporosity is created. A comparison of the two different alkaline treatments was made in terms of the crystallinity, textural properties, morphologies, and acidity of the treated zeolites, and of the catalytic properties of the treated zeolites as the catalyst supports in the hydroisomerization of n-hexane. This thesis mainly consists of three parts as follows:(1) ZSM-12zeolites were synthesized using tetraethylammonium hydroxide (TEAOH) as the template, the so-called organic template-assisted method and the synthesized zeolites were used as the seeds to synthesize ZSM-12zeolites without using any templates, the so-called seeded growth method. Based on the characterization results, there are several significant differences in crystallinity, morphology, and textural properties between the resultant zeolites synthesized by the two different methods:the crystallinity of the zeolite synthesized by the seeded growth method is higher than that by the organic template-assisted method; the particles of the zeolite synthesized by the organic template-assisted method are agglomerated of nano-size crystals while the micro-sized crystals of the one synthesized by the seeded growth method is rodlike; the micropore volume of the zeolite synthesized by the organic template-assisted method is0.10cm3/g with some mesoporosity that decreases with decreasing Si/Al ratio in the zeolite while the the zeolite synthesized by the seeded growth method has a micropore volume of only0.01cm3/g, and the post acid treatment cannot enhance its microporosity.(2) The template-free ZSM-12zeolites with different Si/Al ratios (50,70, and100) were alkaline treated under the same conditions (0.2M NaOH at65℃for30min). Although plenty of mesoporosity can be introduced into the zeolite crystals due to the selective removal of Si, the significant loss of its microporosity leads to the reduction of the acid sites in the zeolite and the resultant zeolite as the support cannot significantly improve the catalytic activity in the hydroisomerization. Additionally, of the introduced mesoporosity cannot be increased with increasing the Si/Al ratio whilst the zeolite with a Si/Al of50possesses the highest mesoporosity after the alkaline treatment, which could be ascribed to the facts that with increasing Si/Al ratio the degree of agglomeration of the nano-sized crystals increases, leading to a pronounced difficulty for OH" to penetrate the micropores to leach Si. Therefore, the morphology of ZSM-12also plays an important role in the process of the alkaline treatment. (3) The as-synthesized (templated) and template-free ZSM-12zeolites were alkaline treated under the same conditions (0.2M NaOH at65℃for30min). Different techniques, such as N2adsorption-desorption, XRD, SEM, TEM, and NH3-TPD, were used to characterize the samples. The results show that after the alkaline treatment, mesoporosity is introduced into the templated zeolite, meanwhile its microporosity and acid sites are preserved, in comparison with those for the template-free zeolite. Finally, the templated zeolite post-treated with the alkaline solution was used as support to prepare Pt/ZSM-12catalyst, which was applied into the hydroisomerization of n-hexane. Under the optimized conditions, the catalytic activity of the catalyst prepared from the alkaline-treated templated zeolite as support is twice higher than that from the untreated zeolite.