Substrate Size-Selective Catalysis with Zeolite-Encapsulated Gold Nanoparticles (original) (raw)
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Advances in nanosized zeolites
Nanoscale, 2013
This review highlights recent developments in the synthesis of nanosized zeolites. The strategies available for their preparation (organic-template assisted, organic-template free, and alternative procedures) are discussed. Major breakthroughs achieved by the so-called zeolite crystal engineering and encompass items such as mastering and using the physicochemical properties of the precursor synthesis gel/ suspension, optimizing the use of silicon and aluminium precursor sources, the rational use of organic templates and structure-directing inorganic cations, and careful adjustment of synthesis conditions (temperature, pressure, time, heating processes from conventional to microwave and sonication) are addressed. An ongoing broad and deep fundamental understanding of the crystallization process, explaining the influence of all variables of this complex set of reactions, underpins an even more rational design of nanosized zeolites with exceptional properties. Finally, the advantages and limitations of these methods are addressed with particular attention to their industrial prospects and utilization in existing and advanced applications.
Nanosized zeolites: Quo Vadis?
Comptes Rendus Chimie, 2016
This feature article highlights recent developments in nanosized zeolites and addresses the question: what will be next? Until recently the limiting step to practical applications was determined by the challenging synthesis of nanosized zeolites in high yield and adequate form. The advances in the synthesis of nanozeolites that provide the basis for future developments and for diverse practical applications are reviewed. The main trends and challenges in processing, characterization and applications of nanosized zeolites are summarized. Particular attention is paid to the processing of nanosized zeolites in terms of their further use. The extending applications of nanosized zeolites in nanotechnology, medicine, pharmaceutical, and food industry, are discussed.
Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived catalysts
Nature, 2009
Zeolites-microporous crystalline aluminosilicates-are widely used in petrochemistry and fine-chemical synthesis 1-3 because strong acid sites within their uniform micropores enable size-and shape-selective catalysis. But the very presence of the micropores, with aperture diameters below 1 nm, often goes hand-in-hand with diffusion limitations 3-5 that adversely affect catalytic activity. The problem can be overcome by reducing the thickness of the zeolite crystals, which reduces diffusion path lengths and thus improves molecular diffusion 4,5 . This has been realized by synthesizing zeolite nanocrystals 6 , by exfoliating layered zeolites 7-9 , and by introducing mesopores in the microporous material through templating strategies 10-17 or demetallation processes . But except for the exfoliation, none of these strategies has produced 'ultrathin' zeolites with thicknesses below 5 nm. Here we show that appropriately designed bifunctional surfactants can direct the formation of zeolite structures on the mesoporous and microporous length scales simultaneously and thus yield MFI (ZSM-5, one of the most important catalysts in the petrochemical industry) zeolite nanosheets that are only 2 nm thick, which corresponds to the b-axis dimension of a single MFI unit cell. The large number of acid sites on the external surface of these zeolites renders them highly active for the catalytic conversion of large organic molecules, and the reduced crystal thickness facilitates diffusion and thereby dramatically suppresses catalyst deactivation through coke deposition during methanol-togasoline conversion. We expect that our synthesis approach could be applied to other zeolites to improve their performance in a range of important catalytic applications.
Catalysis Today, 2012
Zeolites and related materials (including a wide range of microporous and mesoporous materials with ordered pore structure) have been one of the areas in the field of materials and catalysis with the largest impact on science, technology and industrial processes. We discuss here some recent developments in this field, with particular references how to tailor and design zeolite and related material properties to control/enhance the catalytic performances. Four main topics have been addressed. (i) The recent progress and perspectives in the field of tailored syntheses, with selected examples showing the trend and prospects to develop new structures, control the location of active sites, and the crystal size and morphology, including nanoarchitecture of the final catalysts. (ii) The development and prospects of two-dimensional zeolites presenting an extended view/concept of zeolite structures integrating the classical 3D frameworks and the various lamellar forms. (iii) The progresses in the design and synthesis of hierarchical zeolites, with discussion on the still existing challenges related to the synthesis, characterization and catalytic application. (iv) Novel opportunities and needs in terms of zeolite multifunctional design for catalytic applications, with a discussion of the critical issues related to the use in the field of fine chemicals, organic industrial syntheses and biorefinery, and the prospects for the use in two novel challenging areas of the direct conversion of CO 2 to light olefins and methane to methanol.
Progress in zeolite synthesis promotes advanced applications
Microporous and Mesoporous Materials, 2014
This article outlines the importance of zeolite synthesis and their unique physicochemical characteristics promoting advanced applications. The main strategies for preparation of zeolites including organic-template assisted, organic-template free and alternative procedures are considered for synthesis of crystallites offering control and fine-tuning of their properties. Besides, rational design of zeolites with pre-determined structure, porosity, size, morphology, and composition are more viable by studying carefully the chemical and physical parameters controlling the zeolite synthesis and understanding the crystallization mechanism. Finally, a particular attention to the preparation of zeolites with nanosized dimensions and their utilization in innovative applications including photovoltaic, medicine and holographic sensors are presented.
Hollow Zeolite Single Crystals: Synthesis Routes and Functionalization Methods
Small Methods, 2018
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. [10-12] 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. [13-15] 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. [16-18] 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. [9,19-26] 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. [27-33] 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, despite the simplicity of the concept, the synthesis of hollow zeolites is difficult, as it necessitates to control the chemistry and the spatial distribution of elements throughout the crystals. Here, the different strategies that can be used to transform bulk zeolite crystals into hollow analogs are reported. In particular, selective dissolution as well as dissolution-recrystallization methods are discussed in detail in the case of zeolites with mobile five (MFI) framework type. More sophisticated fabrication strategies are also described for two other major zeolites, namely, zeolites Beta and Y. Additionally, methods to introduce metal nanoparticles with controlled size and composition in the cavity are also addressed. The review ends with concluding remarks, including major challenges and future outlooks in the field. Zeolite Single Crystals
Harvesting and Manipulating Zeolite Nanocrystals
Advances in Science and Technology, 2006
Recently, we have developed techniques to synthesize, isolate, harvest, and assemble silicalite nano-crystals into tightly bound aggregates having zeolitic domains interspersed by mesoporous areas. Crystals with well-defined zeolite structure, as small as 45 nm, have been synthesized then assembled into aggregate particles or planar films. This talk will touch briefly on two of these synthesis techniques, some characterizations of these aggregated nanocrystals, and some projections for future work in the area.