Abstract
A novel soft-template (ST) is fabricated and successfully employed as mesoporogen to synthesis hierarchical ZSM-5 zeolites with outstanding mesoporosity and high hierarchy factors. The as-produced soft-template can connect steadily with the MFI frameworks by covalent bonds of –Si–O–Si– during the high-temperature hydrothermal crystallization process. This type of connection mode can effectively avoid the formation of amorphous materials, and the specific structure of this soft-template can efficiently introduce plentiful of mesopores with few micropores being consumed. The particles of as-synthesized hierarchical ZSM-5 zeolites are in size of about 1 μm, which are made up of nanocrystals of 60–150 nm. The structure parameters of these samples are characterized with the techniques of X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, nitrogen sorption, scanning electron microscope (SEM), transmission electron microscope (TEM), NH3 temperature-programmed desorption (NH3-TPD) and thermogravimetric (TG). Due to the nature of zeolites and great microporosity, these hierarchical samples present great tolerance of hydrothermal treatment. And because of the intracrystalline mesopores, large external surface areas, and abundant accessible acid sites, whether in conversion rate of reactants or selectivity of products, the hierarchical samples exhibit excellent catalytic performance in the reactions of alkylation between benzene and benzyl alcohol, cracking of 1,3,5-tri-isopropylbenzene, and thermal cracking of low-density polyethylene (LDPE), respectively.
Highlights
Zeolites are widely used as adsorbents, ion exchangers, and heterogeneous catalysts in a variety of applications, due to the abundant surface acidity, large surface area, excellent hydrothermal stability, and particular molecule sieving ability [1]
In consideration of the importance of zeolite frameworks, the nanosized zeolites with short intracrystalline path length have been produced to solve the diffusion problem, whereas it is hard to recycle these nanocatalysts after heterogeneous catalysis [7] and the hydrothermal stability of nanosized zeolites is worthy of discussion [8]
The X-ray diffraction (XRD) spectrum of Mesoporous zeolite (MZ) samples (Fig. 1a) presents the same diffraction peaks with the sample Traditional zeolite (TZ) [33], further indicating that the as-synthesized MZ samples are of typical MFI structure and have high crystallinity in the presence of the mesoporogen ST
Summary
Zeolites are widely used as adsorbents, ion exchangers, and heterogeneous catalysts in a variety of applications, due to the abundant surface acidity, large surface area, excellent hydrothermal stability, and particular molecule sieving ability [1]. The diffusion limitations of bulky reactant molecules in crystals are a very severe problem during catalysis reactions, because of the small and sole even partial occlusive micropores in conventional zeolites [2]. In consideration of the importance of zeolite frameworks, the nanosized zeolites with short intracrystalline path length have been produced to solve the diffusion problem, whereas it is hard to recycle these nanocatalysts after heterogeneous catalysis [7] and the hydrothermal stability of nanosized zeolites is worthy of discussion [8]. Introducing a secondary pore system besides the micropores in zeolite crystals is becoming a major research hotspots [9, 10] and that is the synthesis of hierarchical zeolites.
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