Synthesis Of Hierarchically Porous ZSM-5Molecular Sieve And Its Performance In Catalytic Cracking

Hierarchically porous zeolites, combing good catalytic performance of microporous zeolites and excellent mass transport of mesoporous materials, exhibit better catalytic effect in the reactions involving bulky molecules. Silane coupling agents with organic functional groups can be implanted into zeolite phase through chemically bonding with aluminosilica species. In such way, nanocrystals aggregates can be obtained by inhibiting crystal growth. In this thesis, hierarchically porous ZSM-5zeolite were prepared through hydrothermal method with tetraethyl orthosilicate as silicon source, aluminium isopropoxide, aluminum nitrate and sodium aluminate as aluminum source, tetrapropylammonium hydroxide (TPAOH) as template, silane coupling agent3-aminopropyltriethoxysilane (APTES) as soft template. The prepared samples have been evaluated in cracking reaction of n-heptane. The detailed results are summarized as follows:(1) Some factors affecting the synthesis were investigated, such as silica/alumina ratio, APTES content, pre-crystallization, alkalinity, aluminum source, etc. The structure and morphology of the samples were characterized by XRD, FT-IR, SEM, TEM and N2adsorption-desorption. The results indicate that the morphologies of the zeolites change from nanocrystals aggregates into single crystals when increasing silica/alumina ratio from200to∞at constant APTES content of0.05, the external specific surface area and mesopore decrease as well. When increasing amount of APTES at silica/alumina ratio of200, the external surface area and mesopore volume of the samples increase, while the degree of crystallinity decrease, indicating that the crystal growth is inhibited by APTES. When synthesizing Silicalite-Ⅰ, the morphology of the samples is coffin-type single crystal and do not change by APTES addition. The external specific surface area and mesopore volume also increase with increasing APTES content. Pre-crystallization has a certain impact on crystallinity, morphology, pore structure and yield of samples, especially under the condition of higher APTES addition. Improved alkalinity shortens the crystallization time, but the sample size increases and the external specific surface area and mesopore volume reduce. When using aluminum nitrate and sodium aluminate as aluminum source, samples are single crystals with a few hundred nanometers size and still have high specific surface area and mesopore volume. When synthesizing at low silica/alumina ratio (SiO2/Al2O3=60) with sodium aluminate as aluminum source, under the condition of shorter pre-crystallization, silylation and crystallization time, hierarchically porous ZSM-5zeolites are successfully obtained.(2) We name the hierarchically porous zeolite synthesized by different process parameters (such as aluminum source, APTES addition) as H-1, H-2and H-3. Their acidic properties were characterized by NH3-TPD. H-2and H-3have equivalent acid strength.H-1contains stronger acid site, in addition to acid sites equivalent with acid strength of H-2and H-3. The total amount of acid:H-3> H-2> H-1. Activity and stability of the above catalysts were evaluated in the n-heptane cracking reaction. The results indicate that all the catalysts exhibit very high initial activity, wherein stability of H-1catalyst is very good, but the stability of the other two is not.