Zeolites hierarchical porous

Different synthetic methodologies can be pursued to prepare hierarchical porous zeolites, which can be discriminated as bottom-up and top-down approaches. Whereas bottom-up approaches frequently make use of additional templates, top-down routes employ preformed zeolites that are modified by preferential extraction of one constituent via a postsynthesis treatment For the sake of conciseness, we restrict ourselves here to the discussion of the latter route. Regarding bottom-up approaches, recently published reviews provide state-of-the-art information on these methodologies [8, 9,17-19]. 

The search for better catalysts has been facilitated in recent years by molecular modeling. We are seeing here a step change. This is the subject of Chapter 1 (Molecular Catalytic Kinetics Concepts). New types of catalysts appeared to be more selective and active than conventional ones. Tuned mesoporous catalysts, gold catalysts, and metal organic frameworks (MOFs) that are discussed in Chapter 2 (Hierarchical Porous Zeolites by Demetallation, 3 (Preparation of Nanosized Gold Catalysts and Oxidation at Room Temperature), and 4 (The Fascinating Structure... 

A hierarchically macro-/meso-/microporous structured catalyst, Hp-ZSM, has been recently reported by. Li et al. [173]. The catalyst was synthesized using cetyltrimethylammonium bromide (CTAB) and TBAOH as the meso- and micropore templates. Ethanol was used to generate macropores, probably via an ethanol-in-water microemulsion mechanism. The hierarchical porous zeolite shows higher hydrothermal stability and sustained higher catalytic activity than either the amorphous aluminosilicate ZSM-5 or meso-ZSM-5 catalysts used in the reactions involving large molecules. 

Table 8.3 Applications of hierarchical porous zeolites in cleaner technologies for oil refining, petrochemical, and fine chemistry processes. 

Colloidal zeolites have been used as building blocks to fabricate hierarchical porous materials. Infiltrating ethanol sol of zeolite nanoparticles into an ordered array of polystyrene spheres resulted in macroporous zeolites, which involves a self-assembly process. After ethanol evaporation, zeolite nanoparticles were aggregated by capillary forces. High concentration of external silanol groups favored the formation of hydrogen bonds between particles and eventually Si-O-Si bonds after calcination. The method has been further developed to produce transparent and self-standing zeolite membranes with controlled mesoporosity. Concurrently, the preformed zeolite-coated polystyrene spheres have been... 

L Huang, Z. Wang, H. Wang, J. Sun, Q. Li, D. Zhao, and Y. Yan, Hierarchical Porous Structures by Using Zeolite Nanocrystals as Building Blocks. Microporous Mesoporous Mater, 2001, 48, 73-78. 

Zeolite nanocrystals have been demonstrated to be versatile building blocks for constructing hierarchical porous structures. " The use of nanoparticles as building blocks allows mild processing conditions... 

Hierarchical porous materials are solids that are ordered at different length scales. Materials with multiple porosities are of high interest for applications in catalysis and separation, because these applications can take advantages of different pore structures. For example, microporous-mesoporous composites have shown superior catalytic activities by the combination of strong acidity from zeolites with high reactant or product mobility due to large uniform mesopores. Several approaches have been reported on the design and synthesis of hierarchical porous materials, as discussed below. 

Fujita S, Nakano H, Ishii M, Nakamura H, Inagaki S (2006) Preparation of hierarchical porous silica and its optical property. Microporous Mesoporous Mater 96 205 Holland BT, Abrams L, Stein A (1999) Dual templating of macroporous silicates with zeolitic microporous frameworks. J Am Chem Soc 121 4308 Yamauchi Y, Kuroda K (2006) Fabrication of a Pt film with a well-defined hierarchical pore system via solvent-evaporation-mediated direct physical casting. Elec-trochem Commun 8 1677... 

Huang L, Wang Z, Wang H, Sun J, Li Q, Zhao D, Yan Y. Hierarchical porous structures by using zeolite nanocrystals as building blocks. Micropor Mesopor Mater 2001 48(1-3) 73-78. 

Zeolites with Hierarchical Porous Structure Combining Microporous with Meso-/Macro porous... 

An improved polymerization-induced colloid aggregation (im-PlCA) method was developed to prepare zeolite microspheres with hierarchical porous stractures and a uniform size, which could easily be carried out by adding urea and formaldehyde to an acidic pH precontrolled colloidal solution, as obtained from a hydrothermal crystallization process. After removing the polymeric component, solid and hollow zeolite microspheres can be obtained under different preparation conditions [172]. 

Besides CNTs, another ID carbon nanostructure is carbon nanoflbers. For example, Shen et al. [156] prepared a series of hierarchical porous carbon libers with a BET surface area of 2,231 m g and a pore volume of 1.16 cm g. In this synthesis method, the polyacrylonitrile (PAN) nanofibers (prepared by dry-wet spinning) were selected as precnrsors, and pre-oxidation and chemical activation were involved to get the developed porosities. This type of material contained a large amount of nitrogen-containing groups (N content >8.1 wt%) and consequently basic sites, resulting in a faster adsorption rate and a higher adsorption capacity for CO2 than the commercial zeolite 13X that is conventionally used to capture CO2, in the presence of H2O (Fig. 2.27). 

Hierarchical porous materials are predominately based on zeolitic systems where mainly mesopores have been introduced in the microporous framework of a zeolite crystal in a similar way as motorways would intersect a narrow road system of a downtown area [110-113]. The hierarchy in such materials will result in an optimized performance in transport-limited applications. Thus, hierarchical zeolite-containing materials combine characteristics of pore size regimes of at least two different length scales [113, 114]. It has already been proved that such micro/mesoporous bimodal systems reduce the diffusion limitations for molecules within zeolite catalysts [115-118]. Several methods for the implementation of additional transport pores have been developed during the past few years, however. 

It seems that the zeolites allow a better adsorption of alkanes by using a confinement effect. A similar effect was observed on hierarchical porous silica membranes with mesopores of 4nm. These mesopores allowed a better stabilization of Pt particles and preferential adsorption of the reactants. 

Figure 32.5 Proposed microemulsion mechanism for the formation of hierarchical trimodal porous zeolites [63].

Figure 2.2 Classification of different types of porous materials, (a) A purely microporous zeolite is considered as a non-hierarchical system according to the single level of porosity, (b) Fragmentation of the zeolite into nanocrystals engenders a network of mesopores constituting the intercrystalline space, leading to an interconnected hierarchical system. Intraconnected...

In view of catalytic potential applications, there is a need for a convenient means of characterization of the porosity of new catalyst materials in order to quickly target the potential industrial catalytic applications of the studied catalysts. The use of model test reactions is a characterization tool of first choice, since this method has been very successful with zeolites where it precisely reflects shape-selectivity effects imposed by the porous structure of tested materials. Adsorption of probe molecules is another attractive approach. Both types of approaches will be presented in this work. The methodology developed in this work on zeolites Beta, USY and silica-alumina may be appropriate for determination of accessible mesoporosity in other types of dealuminated zeolites as well as in hierarchical materials presenting combinations of various types of pores. 

Finally, zeolite nanoparticles have been used as building blocks to construct hierarchical self-standing porous stmctures. For example, multilayers of colloidal zeolite crystals have been coated on polystyrene beads with a size of less than 10 p,m [271,272]. Also, silicalite-1 membranes with a thickness ranging from 20 to several millimeters and controlled mesoporosity [273] have been synthesized by the self-assembly of zeolite nanocrystals followed by high-pressure compression and controlled secondary crystal growth via microwave heating. These structures could be useful for separation and catalysis applications. 

The first successful preparation of micro/mesoporous or micro/macroporous molecular sieves as well as mesoporous zeolite single crystals started an intensive search of optimization procedures for their synthesis, to increase their thermal stability and to tailor their acid, base and redox properties for possible applications in heterogeneous catalysis. There is no doubt that mastering of synthesis of these hierarchic materials is an important challenge in the area of porous materials. 

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