The challenges associated with synthesizing porous materials1 mean that new classes of zeolites (zeotypes)—such as aluminosilicate zeolites2,3 and zeolite analogues4—together with new methods of preparing known zeotypes5, continue to be of great importance. Normally these materials are prepared hydrothermally with water as the solvent in a sealed autoclave under autogenous pressure6. The reaction mixture usually includes an organic template or ‘structure-directing agent’ that guides the synthesis pathway towards particular structures. Here we report the preparation of aluminophosphate zeolite analogues by using ionic liquids7 and eutectic mixtures8. An imidazolium-based ionic liquid acts as both solvent and template, leading to four zeotype frameworks under different experimental conditions. The structural characteristics of the materials can be traced back to the solvent chemistry used. Because of the vanishingly low vapour pressure of ionic liquids, synthesis takes place at ambient pressure, eliminating safety concerns associated with high hydrothermal pressures. The ionic liquid can also be recycled for further use. A choline chloride/urea eutectic mixture8 is also used in the preparation of a new zeotype framework.

Ionic liquids and eutectic mixtures as solvent and template in synthesis of zeolite analogues

This review focuses on the synthesis, crystallization mechanism, and application of colloidal zeolites. The synthesis formulations and features of different zeolite-type structures prepared in nanosized form are summarized. Special attention is paid to zeolites prepared as stable colloidal suspensions. Next, new insights into zeolite crystallization mechanism gained by using colloidal zeolites as model systems are discussed. Further, the methods for deposition of zeolite nanocrystals from suspensions onto supports of different shapes and compositions used for the fabrication of zeolite films and membranes are reviewed. The use of colloidal zeolites for the fabrication of hierarchical macrostructures is also described. Other uses of nanozeolites for the preparation of functionalized materials, for the synthesis of mesoporous silicas of improved hydrothermal stability, and as seeds for zeolite syntheses are illustrated. The emerging applications of nanozeolites in sensing, optoelectronics, and medicine cons...

Nanozeolites: Synthesis, Crystallization Mechanism, and Applications

The Kirkendall effect is a consequence of the different diffusivities of atoms in a diffusion couple causing a supersaturation of lattice vacancies. This supersaturation may lead to a condensation of extra vacancies in the form of so-called “Kirkendall voids” close to the interface. On the macroscopic and micrometer scale these Kirkendall voids are generally considered as a nuisance because they deteriorate the properties of the interface. In contrast, in the nanoworld the Kirkendall effect has been positively used as a new fabrication route to designed hollow nano-objects. In this Review we summarize and discuss the demonstrated examples of hollow nanoparticles and nanotubes induced by the Kirkendall effect. Merits of this route are compared with other general methods for nanotube fabrication. Theories of the kinetics and thermodynamics are also reviewed and evaluated in terms of their relevance to experiments. Moreover, nanotube fabrication by solid-state reactions and non-Kirkendall type diffusion processes are covered.

/pdf/formation-of-nanotubes-and-hollow-nanoparticles-based-on-8yxfpqf0n3.pdf

Formation of nanotubes and hollow nanoparticles based on Kirkendall and diffusion processes: a review.

There is a deep interest in methods to fabricate hollow nanocrystals for potential application as high-efficiency catalysts or drug-delivery agents. Tubular one-dimensional nanocrystals have been prepared for a wide variety of materials, including semiconductors1,2, metals3,4, ferroelectrics5,6 and magnetite7. They can be produced by rolling up layered materials or via an axial growth in a rolled-up form8,9,10, coating pores in templates11 or by eliminating the core of a core-shell nanowire1,7. The Kirkendall effect, a classical phenomenon in metallurgy12, was recently applied to explain the formation of hollow spherical nanocrystals13,14,15,16,17. Although the experimental demonstration and theoretical treatment mainly concern binary compounds and planar interfaces or nanoscale spherical interfaces, the fabrication route provided by the Kirkendall effect should be generic, and should also work for high-aspect-ratio hollow cylinders (that is, nanotubes) or even more complex superstructures. In this letter, we report, for the first time, on ultra-long single-crystal ZnAl2O4 spinel nanotubes (total diameter: ∼40 nm, wall thickness: ∼10 nm) fabricated through a spinel-forming interfacial solid-state reaction of core-shell ZnO–Al2O3 nanowires involving the Kirkendall effect. Our results simultaneously represent an extension of applying the Kirkendall effect in fabricating hollow nano-objects from zero-dimensional to multidimensional, and from binary to ternary systems.

Monocrystalline spinel nanotube fabrication based on the Kirkendall effect

In the past decade or so, small-pore zeolites have received greater attention than large- and medium-pore molecular sieves that have historically dominated the literature. This is primarily due to the commercialization of two major catalytic processes, NOx exhaust removal and methanol conversion to light olefins, that take advantage of the properties of these materials with smaller apertures. Small-pore zeolites possess pores that are constructed of eight tetrahedral atoms (Si4+ and Al3+), each time linked by a shared oxygen These eight-member ring pores (8MR) provide small molecules access to the intracrystalline void space, e.g., to NOx during car exhaust cleaning (NOx removal) or to methanol en route to its conversion into light olefins, while restricting larger molecule entrance and departure that is critical to overall catalyst performance. In total, there are forty-four structurally different small-pore zeolites. Forty-one of these zeolites can be synthesized, and the first synthetic zeolite (KFI, 1...

Small-Pore Zeolites: Synthesis and Catalysis

High-quality zeolite LTA and FAU single nanocrystals have been prepared in the reaction system SiO2−Al2O3−(TMA)2O−NaCl−H2O at 100 °C using tetraethyl orthosilicate (TEOS), and Al powder as the silicon and aluminum sources, respectively. The effect of various factors, such as the (TMA)2O/Al2O3, NaCl/Al2O3, and SiO2/Al2O3 ratios, on the crystal size is discussed. Powder X-ray diffraction (XRD), dynamic light scattering (DLS), and transmission electron microscopy (TEM) are used to characterize the crystallinities, crystal size distributions, and morphologies of the products.

Synthesis and Characterization of High-Quality Zeolite LTA and FAU Single Nanocrystals

The first titanium phosphate nanotubes with alternating interlayer spacings have been successfully prepared and characterized. The synthesis is accomplished in a reverse microemulsion formed in an amine extraction system. TEM data from samples made after different times of reaction suggest a scrolling-formation mechanism.

Microemulsion-based synthesis of titanium phosphate nanotubes via amine extraction system.

A new microporous aluminophosphate, designated AlPO-HDA, is prepared under solvothermal conditions using 1,6-hexanediamine as the template. AlPO-HDA is the first microporous aluminophosphate with A...

Al16P20O80H4.4C6H18N2: A new microporous aluminophosphate containing intersecting 12- and 8-membered ring channels

HREM Study of Fine Structure of Zeolites

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Synthesis and Characterization of High-Quality Zeolite LTA and FAU Single Nanocrystals

Microemulsion-based synthesis of titanium phosphate nanotubes via amine extraction system.

Al16P20O80H4.4C6H18N2: A new microporous aluminophosphate containing intersecting 12- and 8-membered ring channels

HREM Study of Fine Structure of Zeolites