Hierarchical Porosity

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...

Brandhuber, D., Torma, V., Raab, C., Peterlik, H., Kulak, A. and Husing, N. (2005) Glycol-modified silanes in the synthesis of mesoscopically organized silica monoliths with hierarchical porosity. Chemistry of Materials, 17, 4262 1271. 

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. 

Friedel-Crafts acylation is widely used for the production of aromatic ketones applied as intermediates in both fine chemicals and pharmaceutical industries. The reaction is carried out by using conventional homogenous catalysts, which represents significant technical and environmental problems. The present work reports the results obtained in the Friedel-Crafts acylation of aromatic substrates (anisole and 2-methoxynaphthalene) catalyzed by Beta zeolite obtained by crystallization of silanized seeds. This material exhibits hierarchical porosity and enhanced textural properties. For the anisole acylation, the catalytic activity over the conventional Beta zeolite is slightly higher than with the modified Beta material, probably due to the relatively small size of this substrate and the weaker acidity of the last sample. However, the opposite occurred in the acylation of a bulky substrate (2-methoxynaphthalene), with the modified Beta showing a higher conversion. This result is interpreted due to the presence of a hierarchical porosity in this material, which favors the accessibility to the active sites. 

The catalytic activity of hierarchical and conventional Beta zeolites for acylation of 2-MN is displayed in Figure 2(a) The Beta (PHAPTMS) sample shows a superior catalytic activity than the conventional one, due to its enhanced textural properties. In this case, the bulky nature of both substrate and products may cause the existence of diffusional problems inside the zeolitic channels, which are attenuated in the modified Beta sample due to the presence of the hierarchical porosity. Regarding the product distribution (Figure 2(b)), two main products are observed and a third isomer, 8-A,2-MN isomer is produced just in minor amounts. Interestingly, the selectivity towards the desired isomer increases in the material obtained from silanized seeds, reaching values around 75%. Probably, the active sites located on the surface of the secondary porosity are able to catalyze also the formation of 6-A,2-MN by transacylation. However, this reaction is expected to be strongly hindered in the conventional Beta zeolite since it requires the participation of two bulky molecules as reactants. 

The Beta material prepared by seed silanization show interesting catalytic properties in aromatic acylation reaction, especially when using a bulky substrate, such as 2-methoxynaphthalene. The superior activity and selectivity exhibited by this sample has been related to the presence of a hierarchical porosity, which decreases the steric and diffusional hindrances, favoring the accessibility to the active sites and allowing the occurrence of the transacylation reaction. 

Taguchi et al. [97] and Liang et al. [98,99] reported on the preparation of monolithic carbon columns, which exhibit a hierarchical, fully interconnected porosity. Silica particles (10 pm) have been suspended in an aqueous solution, containing ethanol, FeClj, resorcinol, and formaldehyde. After polymerization, the solid rod was dried, cured, and carbonized by raising temperature to 800°C and finally up to 1250°C. Finally, concentrated FIF was used to remove silica and iron chloride. Even if carbon have been shown to possess a high specihc surface area (up to lllSmVg), their chromatographic efficiency is moderate (FIETP of 72 pm). 

Douglas, J. and Arbogast, T. (1990) Dual porosity models for flow in naturally fractured reservoirs, in Dynamics of Fluid in Hierarchical Porous Media, editor J.H. Cushman, Academic Press, New York, pp. 177-222... 

Figure 10.1 Examples of siliceous exoskeletons of diatoms, showing hierarchical porosity. Photograph courtesy of Prof). Livage, College de France.

Groen, J. C., Peffer, L. A. A., Moulijn, J. A. and Perez-Ramirez, J. Mechanism of hierarchical porosity development in MFI zeolites by desilication the role of aluminium as a pore-directing agent, Chem. Eur. J., 2005, 11, 4983 1994. 

Figure 3.5.13 (A) Equilibrium times for diffusion on macroscopic (1 mm) and nanoscopic (10 nm) length scales. (B) Illustration of ionic and electronic wiring, with hierarchical porosity as Li+ distribution network and a carbon second-phase e- distribution network. Reprinted from [58] with permission, copyright 2007 John Wiley Sons.

Figure 3.5.14 Carbon monolith with hierarchical porosity. Reprinted from [69] with permission, copyright 2007 Wiley-VCH.

Beyond nanosizing, the optimization of morphology is possible (e.g. by directed introduction of porosity into materials) [58, 69, 70], In combination with a liquid electrolyte, which can penetrate the pores, distribution networks for Li+ ions can be formed. With hierarchical pores, such distribution networks resemble the network of blood vessels in the human body, which by itself is a highly efficient distribution network (see Figure 3.5.14). 

Mesostructures prepared through S°I° or N°F pathways have either wormhole6,7 or lamellar framework structures8,9. The wormhole structures possess a three-dimensional channel structure. Moreover, the framework domain size can be made very small (e.g., 20-200 nm), which introduces an intraparticle textural porosity that is complementary to the framework porosity. The combination of wormhole framework pores and textural pores can greatly facilitate access to catalytic centers in the framework walls. Lamellar frameworks, on the other hand, can be folded into vesicular particles with very thin mesostructured shells and hollow cores. These hierarchical structures also can facilitate access to reactive catalytic centers in the framework walls by minimizing the diffusion path length. 

Hierarchical porous materials are sohds that are ordered at different length scales. Materials with multiple porosities are of high interest for apphcations in catalysis and separation, because these apphcations can take advantages of different pore structures. For example, microporous mesoporous composites have shown superior catalytic activities by the combination of strong acidity from zeohtes 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. 

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. ... 

With conventional sol-gel routes, the pore size distribution is usually broad and the tortuosity of the pore network is important with the presence of constrictions. Thus ordered interconnected pore networks with constant pore size are strongly attractive. Hierarchical porosity and adaptive porosity are also fascinating approaches to increase or manage the permeability of ceramic membranes. 

The resulting mesoporous layers don t usually exhibit extraporosity at a larger scale, but as smaller pores in the oxide walls (Figure 25.24). ° Due to limitations associated with intrinsic mesostructure characteristics, anisotropy resulting from preferential orientations, or boundaries between ordered domains, the tem-plated mesoporosity is usually not directly interconnected. " In such situations, it does directly define the selectivity of the membrane, which depends on the pore size of the oxide walls. However, the resulting hierarchical porosity (templated mesopores and smaller pores of the oxide walls) favors a decrease in layer permeability. The templated mesoporosity can also be used to functionalize the membrane. 

Recent interest is being devoted to the development of hierarchically ordered porous structures (ordered on multiple lengthscales with controlled multiscale porosity), because... 

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