Hierarchical zeolites method

In addition, experiments of isobutane diffusion reveal that the effective diffusivity is three times higher over a carbon-templated hierarchical zeolite [143]. Similar conclusions were drawn by Groen et al. [144] with hierarchical zeolites synthesized using the desilication method, where a two-order-magnitude improvement was observed in the diffusion of neopentane inside desiUcated ZSM-5 because of the shorter diffusion path length and presence of an accessible network of mesopores. 

Ordered mesoporous carbons, prepared by nanocasting, have also been used successfully as templates for the synthesis of hierarchical zeolites [159, 160). This is the case of CMK-3, an ordered mesoporous carbon attained by nanoreplication of pure silica SBA-15. The hierarchical zeolites obtained by employing CMK-3 as a template mainly present supermicropores or small mesopores with a size around 2 nm. The textural properties of the hierarchical zeohtes can be tuned by changing the type of CMK-3 carbon used. A modification of this method consists of directly impregnating the composite SBA-15-carbon or MCM-41-carbon with TPAOH and leaving the mixture crystallizing hydrothermally under steam. After calcination, a mesoporous ZSM-5 is formed with mesopore sizes around 3.5 and... 

Other Methods Several other methods have been reported for the preparation of hierarchical zeolites, not encompassed by any of the preceeding strategies. Herein, we highlight some interesting examples and they are listed as follows ... 

The use of leaves from the plant Equisetum arvense as a template is another effective method to prepare hierarchical zeolites [170]. The presence of siHca in the plant promotes zeolite crystallization, leading to a microporous/mesoporous zeolite (roughly 0.79 cm g of intracrystalline mesoporosity). 

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. 

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. 

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... 

The strategy of this method is to utilize the inherent porosity of bulky substrates in the construction of hierarchical stractures by incorporating additional pore systems. Diatoms are unicellular algae whose walls are composed of silica with an internal pore diameter at submicron to micron scales. Zeolitization of diatoms, in which zeolite nanoparticles are dispersed on the surface of diatoms followed by a hydrothermal conversation of a portion of the diatom silicas into zeolites, resulted in the formation of a micro/mesoporous composite material. Similarly, wood has also been used as a substrate to prepare meso/macroporous composites and meso/macroporous zeolites. After the synthesis, wood is removed by calcination. ... 

These experimental results indicate that hierarchical pore zeolite materials can be prepared through a secondary growth method by using wood tissue as the template. Because the woods used are easily obtained, and the product overcomes the limitation of the single micropore of zeolites on mass transfer, there are potential applications in the adsorption and catalysis fields. It also provides a precedence for the further exploration of this kind of material. 

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). 

Various groups have reported promising EC electrode materials using hierarchical nanostructured carbons with well-organised pore structures, large specific surface areas and, particularly, an interconnected pore network. Portet et al. has reported 146 F g for zeolite-templated hollow-core mesoporous carbon s)mthesised by chemical vapor deposition (CVD) method [43]. Murali et al. has also reported hollow carbon capsules with specific capacitance of... 

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