Study On The Synthesis And Application Of Hierarchical ZSM-11 With Intergrowth Morphology

High-silica shape-selective zeolite ZSM-5 has been widely applied in the production of propylene from the catalytic pyrolysis or methanol conversion process owing to its special pore structure, pore size and high thermal stability. However, the commercial ZSM-5 zeolites often possess a large crystal size of about 3-5 μm and the resulting long diffusion path limits the fast diffusion of olefins from the micropore channels, which leads to their transformation into paraffins. Besides, the structure of ZSM-5 is composed of the intersecting straight and sinusoidal(Zig-Zag) channels. Compared with the straight channels, the sinusoidal channels have larger diffusion resistance, which also does a great contribution to the occurrence of secondary reaction. If the Zig-Zag channels could be replaced by straight channels, the diffusion resistance could be greatly reduced and then the olefin products could be well retained. ZSM-11 is an end member of the pentasil family and shows similar framework density and pore size with another member ZSM-5. Unlike that ZSM-5 zeolite has intersecting straight and sinusoidal channels, ZSM-11 zeolite has only straight channels. Therefore, the diffusion resistance in ZSM-11 channels should be much less than that in ZSM-5 channels. Using zeolite nanocrystals may be viewed as a useful strategy to increase accessibility to the internal surface of zeolites. Nevertheless, the reduction of crystal size to nano-scale also causes some other problems such as the difficulty of filtration, decline of hydrothermal stability, drop of shape selectivity and considerable loss of crystallinity during the chemical treatment. Therefore, it is highly desired to synthesize nanocrystalline zeolite aggregates to achieve the aim of both shortening diffusion path length and curtailing filtration difficulties. In this paper, we prepared hierarchical ZSM-11 zeolite with nanosized rod-like crystals inserting morphology by a facile method. The small sized nanorods have an obviously shortened diffusion path, while the intergrowth morphology not only curtail the filtration difficulties but also improve the hydrothermal stability of materials. This paper mainly focuses on the synthesis and application of hierarchical ZSM-11 with intergrowth morphology, which will be clarified in details as following.The synthesis of hierarchical ZSM-11 with intergrowth morphology in the system containing cheap materials as well as a spot of template(TBABr) as a single template was studied first. XRD, FTIR and NMR analysis confirms the purity and the high crystallinity of ZSM-11, while the SEM/TEM and N2 adsorption analysis reveals that each individual particle is composed of nanosized rod crystals inserting each other and the intercrystalline voids existing among rods give a significant mesopore size distribution. Steam treatment experiment demonstrates the excellent hydrothermal stability of this hierarchical ZSM-11. Moreover, when applied as an active component for the catalytic pyrolysis of heavy oil, the hierarchical ZSM-11 catalyst exhibits a high selectivity to desired products(LPG+gasoline+diesel), as well as a much lower dry gas and coke yield, plus a high selectivity and yield of light olefins(C3=-C4=) and very poor selectivity to benzene. In addition, after being used as a catalyst for methanol conversion, this material shows excellent crystallinity and mesoporous properties after operation for a month. More importantly, this material exhibits a much higher selectivity to propylene and the C3=/C2= ratio reaches 4.36, which is larger than that of commercial ZSM-5 catalyst(1.36).In order to have a deep understanding on the formation of the intergrowth morphology of ZSM-11, the types of materials and crystallization conditions were investigated in detail. It was found that the existence of TBABr in the system played a dual function in directing the formation of both micropore and mesopore structures, while the appearance of Al species played animportant role in promoting the dual function of template. Based on these researches above, a mechanism was proposed for the formation of hierarchical ZSM-11.Hierarchical submicron ZSM-11 particles with intergrowth morphology and intercrystalline mesopores were prepared by adding micron ZSM-11 microspheres(6.5-7.5 μm) to the TBA-gel precursors via an aging – crystallizing process. It was found that the existence of seed particles did a great contribution to the fast crystallization rate of systems. The particle size of products could be feasibly tuned from 100 nm to 1000 nm either by adjusting the seeds content or changing the synthesis conditions.Based on the above research about the seed-assisted synthesis of submicron ZSM-11 particles, the effect of seeds on the crystallization process was comprehensively investigated in the system without templates. It was found that well crystallized ZSM-5 with uniform crystal size(1-2 μm) could be prepared in a short period by seeding hierarchical ZSM-11 seeds in an organic template-free initial gel. More importantly, this ZSM-5 sample performed a higher selectivity to C2=-C4= and a lower deactivation rate in the methanol conversion reaction. In spite of adding ZSM-11 as seeds, ZSM-5 and HY were also introduced into the synthesis system for comparison. It was elucidated that the actual role of seed crystals was mainly to provide surface for secondary nucleation and thus inhibited the formation of impure phases. Both the framework similarity between seed and target zeolites and the existence of terminal TOH in seed zeolites were proved as key factors for the nucleation process. The exposed surface of the seed crystals showed the ability of recognition to the building units which constituted their frameworks, while the existence of terminal TOH on the seed surface had an important impact on attaching the composite building units in the liquid phase. In addition, the textural properties and morphologies of seed crystals had no obvious effect on the morphology of the product zeolites, but had a significant influence on the crystal size by providing different amount of specific sites for nucleation on the external surface of seeds.