Hierarchical zeolites

4 Hierarchical Zeolites One of the most successful strategies for improving accessibility is the case of hierarchical zeolites [52, 53]. These zeolites are characterized by the presence of a bimodal pore size distribution formed by both micropores and mesopores. The microporous structure is the one inherent to the classical zeolite topology, while the secondary mesopore structure can be generated by a variety of specific synthetic procedures. The presence of secondary porosity in 

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]. 

By demetallation, one constituent is preferentially extracted from a preformed zeolite material to form mesoporous zeolite crystals. Existing and emerging demetallation strategies basically comprise dealumination, detitanation, and desilication. 

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Keywords Friedel-Crafts acylation, Beta zeolite, hierarchical zeolites. 

L. Huang, Z. Wang, H. Wang, J. Sun, Q. Li, D. Zhao, and Y. Yan, Hierarchical Zeolite Structures with Designed Shape by Gel-casting of Colloidal Nanocrystal Suspensions. Chem. Commun., 2001, 1364—1365. 

Wang, H. Huang, L. Wang, Z. Mitra, A. Yan, Y. Hierarchical zeolite structures with designed shapes by gelcasting of colloidal nanocrystal suspensions. Chem. Comm. 2001, 1364-1365. 

Thus, collidine does not affect the bridging hydroxyls in conventional HZSM-5 materials. However, a large fraction of these hydroxyls are accessible to coUidine in a nanosheet ZSM-5 material (J94). It was concluded that in this material most of the acidic hydroxyls, while located inside the zeoHte channels, are in close proximity to the channel mouths. Similar high accessibility was found for a two-dimensional zeo-hte (188). It was reported that the ACI linearly correlated with the external surface area of a hierarchical zeolite (584). 

Figure 8.4 Tailored design of zeolites for the enhancement of the accessibility to active sites, (a) Ultralarge-pore size zeolites, (b) nanocrystal zeolites, (c) delaminated zeolites, and (d) hierarchical zeolites.

The presence of a bimodal inicro-/mesoporous structure provides improved accessibility to the active sites of hierarchical zeolites, which, in many cases, impacts positively on their catalytic activity compared to conventional zeoHtes with micrometer crystal sizes. Hierarchical zeolites possess a collection of singular properties ... 

Increase in Total Available Surface Area For conventional zeolites, having crystal sizes in the micrometer range, the proportion of external surface area is usually negligible, that is, the Brunauer-Emmett-TeUer (BET) surface area corresponds almost completely with the surface area associated to the micropores. However, in the case of hierarchical zeolites, the presence of mesoporosity implies that a great part of the surface area is related to the latter, while a reduction is usually observed in the micropore surface area compared with the standard zeolites. 

Improvement of the BET surface area, as weU as the presence of a high proportion of nonmicroporous surface area, opens the possibility of functionalizing hierarchical zeolites with different agents. Organic-functionalized hierarchical zeolites have the added advantages of their high hydrothermal stability and reusability. 

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. 

Improved Dispersion of Active Phases The presence of secondary meso-porosity is expected to provide better dispersion of the active phase. For instance, the presence of mesoporosity in hierarchical zeolites offers great opportunities for... 

Hierarchical zeolites can be prepared by several different procedures based on distinct features. Although all of them lead to materials with bimodal pore size distributions, the features and contribution of the generated secondary mesoporosity strongly depend on the chosen route. 

Hard Templating by Carbon Materials The work on carbon templating by Jacobsen and coworkers [156] has stimulated dramatic growth in the synthesis of mesoporous/hierarchical zeolites. This approach uses an excess of zeolite gel inside an inert, mesoporous carbon matrix, wherein the zeohte grows over and... 

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... 

Carbon templating offers many possibilities for the synthesis of hierarchical zeolites because of the large availabihty of carbon materials that can be employed. 

S.3.2.2.4 Hard Templating by Polymers Another option for the synthesis of hierarchical zeohtes is the apphcation of polymers as hard templates, a strategy closely related to the use of carbons as hard templates. Several polymers have been used for this purpose. For example, polystyrene beads have been employed for obtaining hierarchical zeolites with a microporous/macroporous structure [161]. 

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 ... 

Zhang et al. [169] employed bacteria (Bacillus subtilis) as templates in the synthesis of microporous/macroporous MFI hierarchical zeolites. The zeolite nanounits... 

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). 

Catalytic Applications of Hierarchical Zeolites for Cleaner Technologies... 

The remarkable and singular properties shown by hierarchical zeolites have brought about potential catalytic applications for these materials in numerous reactions, especially those wherein steric or diffusion limitations are encountered. Table 8.3 summarizes the literature dealing with the application of hierarchical zeolites in a variety of reactions (oil refining and petrochemical reactions, and fine chemicals reactions). 

In addition to the above-mentioned applications, environmental catalysis is another field where hierarchical zeolites may have great potential. Among the investigated reactions, the following can be highlighted ... 

Aromatization and isomerization of 1-hexene Hierarchical zeolite prepared by desilication with 0.5 M NaOH Selectivity toward aromatics of 19.1% while the selectivity values obtained with the conventional ZSM-5 drop to 5.1% [178]... 

Decomposition of NO over hierarchical Cu-ZSM-11 and Cu-ZSM-5 [186]. The improved accessibiHty of the hierarchical zeolites causes an enhanced activity because of the formation of dimeric and oHgomeric Cu species within the mesopores instead of the preferential formation of monomeric Cu species over conventional Cu-ZSM-11 and Cu-ZSM-5. In addition, hierarchical Cu-ZSM-11 was twice as active as mesoporous Cu-ZSM-5 because of the occurrence of solely straight microporous charuiels, wherein the active sites are preferentially located. 

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. 

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