Hollow Zeolite Single Crystals: Synthesis Routes and Functionalization Methods

Abstract

when reaction intermediates or products have not enough space to form. [6][7][8] However, the ultra-microporous dimension of zeolite pores makes that catalytic activity is often limited by diffusion and that active sites deeply buried under the surface of the crystals are hardly accessible to molecules. As a consequence, only a fraction of the crystals are effectively used in catalysis, which represents a major drawback in many reactions catalyzed by zeolites, as they do not operate at their full volume. [9] Many strategies have been developed to minimize the effect of internal diffusion on reaction rates and the subject is still of great interest for both academic and industrial researchers. Transport in zeolite micropores can be significantly improved using nanosized zeolites, typically with individual crystals smaller than a few hundreds of nanometers. [10][11][12] Nanosized crystals are generally obtained by modifying standard hydrothermal crystallization conditions for example by decreasing temperature, shortening the crystallization period, increasing supersaturation, or by adding growth inhibitors to the synthesis gel. One of the most famous examples of growth inhibition concerns the preparation of zeolite nanosheets with specific structure directing agents containing a diquaternary ammonium head group and a long hydrophobic surfactant chain. [13][14][15] These molecules force the zeolite framework to grow perpendicularly to the alkyl chains, leading to a multilamellar stacking of sheets with a thickness corresponding to a few unit cells. However, the synthesis of zeolite crystals with a size below 500 nm is not always obvious. In addition, it is neither evident to reduce the size of the crystal while keeping all other structural parameters the same. Indeed decreasing the size often results in changes in the zeolite composition and thus properties. [16][17][18] Transport can also be greatly facilitated by using "mesoporous" zeolites, i.e., zeolites that possess a secondary array of pores in addition to structural micropores. Such zeolites are usually obtained by dissolving some parts of the framework under alkaline (desilication), acidic (dealumination), or hydrothermal conditions. [9,[19][20][21][22][23][24][25][26] Since diffusion in mesopores is generally several orders of magnitude faster than in micropores, those microporous-mesoporous hierarchical zeolites showed improved catalytic properties for many reactions involving large molecules. [27][28][29][30][31][32][33] However, the mesopore size distribution is difficult to control and the benefits on catalytic reaction rates can be partially annihilated by significant changes in framework Hollow zeolite single crystals represent a new class of materials that have received considerable attention over the last decade. Besides facilitating the transport of molecules in zeolite crystals, the cavity present in hollow crystals offers many possibilities for encapsulating active species and constitutes a closed space that can be used as a nanoreactor. However,...

Revival of Zeolite‐Templated Nanocarbon Materials: Recent Advances in Energy Storage and Conversion

“…recorded by IZA to date. [103][104][105] Herein, a brief introduction is provided from the following aspects of: i) structures, ii) Lewis acidity, and iii) ion-exchange capacity for ease of understanding the merits of zeolites as templates for synthesis of nanocarbons.…”

“…Zeolite molecular sieves, a family of aluminosilicates and their analogues, are crystalline, highly porous materials, which were originally observed in 1756 and 248 framework types have been recorded by IZA to date. [ 103–105 ] Herein, a brief introduction is provided from the following aspects of: i) structures, ii) Lewis acidity, and iii) ion‐exchange capacity for ease of understanding the merits of zeolites as templates for synthesis of nanocarbons.…”

Variation of Aluminium Distribution in Small‐Sized ZSM‐5 Crystals during Desilication

“…If hollow crystals possess zeolite shells, encapsulated nanoparticles can be protected from external poisoning due to the molecular sieving effect . Recently, typical strategies of making hollow zeolite crystals have been reviewed . Hollow crystals produced by template‐based methods are usually large, in the micrometer range with polycrystalline walls.…”

“…[5,6] Recently,t ypical strategies of making hollow zeolite crystalsh ave been reviewed. [7,8] Hollow crystalsp roduced by template-based methodsa re usually large, in the micrometer range with polycrystalline walls.M oreover,t oe lectrostatically adsorb zeolitic species, the property of the template surface sometimes needst ob em odified by multistepa lternatively coating process which indeed is complicated and time consuming. [9,10] In contrast, by meanso fc ontrolled dissolution, hollow zeolite crystals can be easily produced.…”

Synthesis of all-silica hollow zeolites by selective demetallation

“…If the shell part possesses a regular pore structure, such as in the case of hollow zeolites, features of size-selectivity and poison-resistance for catalysts can be achieved. 3,4 With respect to the synthesis of hollow zeolite crystals, different synthetic routes have been explored, such as by using sacrificial templates 5,6 or exploiting zeolite crystallization patterns. 7,8 A recent trend to synthesize hollow zeolites is synthesizing zeolite crystals first and then introducing hollow structures by selectively removing the core part.…”