多级孔分子筛合成策略

2. Soft template method
2.2. Silanization templating methods
D. Serrano, J. Aguado, G. Morales, J. Rodriguez, A. Peral, M. Thommes, J. Epping, B. Chmelka,Molecular and meso-and macroscopic properties of hierarchical nanocrystalline ZSM-5 zeolite prepared by seed silanization, Chem. Mater. 21 (2009) 641–654.
➢ Advantage flexibility and diversity
➢ Soft templates Surfactants and polymers are the two most
common and important soft templates. Novel soft templates such as silanized
I. Schmidt, A. Krogh, K. Wienberg, A. Carlsson, M. Brorson, C.J. Jacobsen, Catalytic epoxidation of alkenes with hydrogen peroxide over first mesoporous titanium-containing zeolite, Chem. Commun. 2157–2158 (2000)
Report on synthesis of Hierarchical zeolites
Zhang
2020-04-30
Байду номын сангаас
Background
Zeolites are well known for their ordered microporous networks, good hydrothermal stability, large surface area, high acidity and selectivity.
1. Hard template method
1.2. Carbon nanotubes and nanofibres
I. Schmidt, A. Boisen, E. Gustavsson, K. Ståhl, S. Pehrson, S. Dahl, A. Carlsson, C. J. Jacobsen, Carbon nanotube templated growth of mesoporous zeolite single crystals, Chem. Mater. 13 (2001) 4416–4418.
3. Template-free methods
➢ Strategies
intercrystalline mesoporosity generated by nanocrystals aggregation
intracrystalline mesoporosity formed by controlling the crystallization of amorphous gels
In other words, hierarchical zeolites not only have inherent microporosity, but also have mesoporosity or even macroporosity. For most cases, however, the secondary porosity lies in the mesoporous range.
K. Na, C. Jo, J. Kim, K. Cho, J. Jung, Y. Seo, R.J. Messinger, B.F. Chmelka, R. Ryoo, Directing zeolite structures into hierarchically nanoporous architectures,Science 333 (2011) 328–332
Hierarchical zeolites can generally be obtained by (i) generating intracrystalline mesopores inside the microporous zeolite structure or (ii) introducing intercrystalline mesopores into spaces that are intergrown by nanosized zeolite crystals.
zeolitic seeds, organosilanes, silylane cationic polymers, and amphiphilic organosilanes
2. Soft template method
2.1. Templating with surfactants
(a) 18–N3–18 surfactant (white spheres, hydrogen; gray spheres, carbon; red spheres, nitrogen). (b) SEM, (c and d) TEM, [(c) and (d), insets] Fourier diffractogram, and (e) XRD pattern of hexagonally ordered crystalline MMS after surfactant removal.
➢ Hard templates Carbonaceous templates include carbon
nanoparticles, nanotubes, nanofibres, mesoporous carbons, aerogels, etc.
1. Hard template method
D. Xu, G.R. Swindlehurst, H. Wu, D.H. Olson, X. Zhang, M. Tsapatsis, On the synthesis and adsorption properties of singleunit-cell hierarchical zeolites made by rotational intergrowths, Adv. Funct. Mater. 24 (2014) 201–208.
1. Hard template method
1.3. Ordered mesoporous carbons
synthesized by the recrystallization of SBA-15 using in situ-formed CMK-5 as the carbon template
1.1. Space-confined method
first reported by Madsen and Jacobsen in 1999; little connection—instability and no durability
Growth of zeolite crystals around carbon particles.
However, the presence of only microporous channels also restricts the diffusion of reactants and products into and out of the microporous networks, especially limiting zeolite applications involving bulky molecules.
Background Some general advantages
➢ Increased intracrystalline diffusion ➢ Reduced steric limitation ➢ Changed product selectivity ➢ Decreased coke formation ➢ Acid properties and hydrothermal stability.
Y. Fang, H. Hu, An ordered mesoporous aluminosilicate with completely crystalline zeolite wall structure, J. Am. Chem. Soc. 128 (2006) 10636–10637
2. Soft template method
Outline
1 Hard templates 2 Soft templates 3 Template-free methods 4 Dealumination
5 Desilication
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1. Hard template method
➢ General procedures mixing - ageing – crystallization - removal
mesoporosity created between self-pillared zeolite nanosheets, which can be made from rotational intergrowths of zeolites
3. Template-free methods
X. Zhang, D. Liu, D. Xu, S. Asahina, K.A. Cychosz, K.V. Agrawal, Y. Al Wahedi, A. Bhan, S. Al Hashimi, O. Terasaki, Synthesis of self-pillared zeolite nanosheets by repetitive branching, Science 336 (2012) 1684–1687
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Background Three different types of shape selectivity in zeolites
Background Significance
These excellent properties make zeolites extremely useful for petrochemical processes and refining.
The importance of developing hierarchical zeolites has attracted great attention in recent years due to the prospect of increased accessibility for bulky molecules/enhanced the mass transport of molecules while maintaining the intrinsic size, shape and transition state selectivity of zeolite
• The presence of aluminium in the zeolite framework can generate negative charges, which can be balanced by the small cations of hydroxyl protons, providing weak Lewis or strong Brønsted acid sites, respectively, which makes zeolites excellent catalysts and ion exchangers
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Background Definition
Zeolites with at least two levels of pore diameters, i.e., <2 nm (micropores) and >2 nm (meso-/macropores) are considered as ‘‘hierarchical structured zeolites”.