Mesoporous networks

Acidic micro- and mesoporous materials, and in particular USY type zeolites, are widely used in petroleum refinery and petrochemical industry. Dealumination treatment of Y type zeolites referred to as ultrastabilisation is carried out to tune acidity, porosity and stability of these materials [1]. Dealumination by high temperature treatment in presence of steam creates a secondary mesoporous network inside individual zeolite crystals. In view of catalytic applications, it is essential to characterize those mesopores and to distinguish mesopores connected to the external surface of the zeolite crystal from mesopores present as cavities accessible via micropores only [2]. Externally accessible mesopores increase catalytic effectiveness by lifting diffusion limitation and facilitating desorption of reaction products [3], The aim of this paper is to characterize those mesopores by means of catalytic test reaction and liquid phase breakthrough experiments. 

Figure 16.2 Transmission electron microscopy image of steam-stabilized Y zeolite showing mesopore network.

In addition to azobenzene, coumarin-modified mesoporous networks have been used to create light-induced controlled release.63,64 The action of coumarin differs from that of azobenzene in that illumination drives a reversible bimolecular coupling reaction, creating a cyclobutane dimer that physically blocks pore egress. 

Jansen, J. C., Shan, Z., Marchese, L., Zhou, W., van der Puil, N. and Maschmeyer, T. A new templating method for three-dimensional mesopore networks, Chem. Commun., 2001, 713-714. 

The isotherms and as-plots in Figure 10.24 exemplify the adsorptive behaviour of three types of pore structure (a) a wide range of open mesopores in gel E (b) a well-defined mesoporous network in gel Al and (c) a distribution of supermicropores and some mesopores in gels A3 and C. 

Apart from recapture of the injected electrons by the oxidized dye, there is an additional loss channel in the nanocrystalline dye-sensitized cell which involves reduction of triiodide ions in the electrolyte present within the mesoporous network ... 

The above ideas can be readily extended to a three-level nesting of monolith porosity and meso -I- microporosity, as shown in Fig. 32, where the pore network has been relaxed to a more realistic random-pattern (irregular) configuration. Nesting the (Porosity as networks in networks should provide improved access and possibly additional convection through the larger macro(Pore channels, thereby enhancing the reactivity of the mesoporous network and any associated micro[X)re structure. 

In Fig. 9 three orthogonal slices through the reconstruction of the XVUSY crystal are displayed. The x-z projection shows a cylindrical mesopore that connects the interior of the crystal with the outside world . For one and the same mesopore marked with a white arrow in all three projections it is clear that no connection to the external surface via the mesopore network exists. In other words, this mesopore is a cavity in the crystal and will hardly contribute to reduction of mass transfer resistance. From independent measurements based on physisorption and mercury intrusion [29] it has been found that 30% of the mesopore volume in this material is present in cavities that are connected to the external surface only via micropores. More recently elegant proof from thermoporometry experiments for the existence of these cavities has been published [31]. 

Fig. 15. Volume rendering of a Pt/TUD-1 Fig. 16. Projection of the reconstructed volume of particle (height 216 run) showing the 3D PdRu/MCM-41 showing the hexagonal order of the mesopore network in orange with Pt particles mesopores and the metal particles [16]. in red.

The advances made by the use of CNTs and CNFs as supports for fuel cell applications are generally attributed to (1) the possibility of reaching high metal dispersion and high electroactive surface area values for Vulcan XC-72R the catalyst particles can sink into the microporosity, thus reducing the number of three-phase boundary active sites (2) the peculiar three-dimensional mesoporous network formed by these materials, which provides improved mass transport and (3) their excellent conducting properties, which improve electron transfer. 

In fuel cell applications, however, the presence of small micropores can reduce the accessibility of the liquid electrolyte to the metal particles placed within them. This may be avoided by developing carbon gels with the appropriate mesoporous network. Hence, the pore texture of carbon gels can be adapted to the reaction under question, and this is possible because of the pore texture flexibility of carbon gels, which can be tailored by controlling all the steps in carbon gel synthesis. This is an advantage over activated carbons, which are generally microporous solids with low meso- and macroporosity, which induces diffusional limitations and diminishes catalyst performance. 

In the MCNS, the microporous nanospheres are interlinked to form a three-dimensional mesoporous network. Their exceptional performance could be also related with the presence of mesopores which provide a rapid mass transport of ions within the electrode, facilitating the charging and discharging of the double layer. This effect has been already consider when studying activated carbons, which surface capacitance was found to increase with the mesopore content The optimal proportion of mesopores for using activated carbons in EDLCs was found to be between 20 and 50% [57]. 

Fig. 15 shows the variation of capacitance determined by cyclic voltammetry as a function of the potential sweep rate. It is clear that the presence of an ordered mesoporous network facilitates the formation of the double layer, especially at high potential sweep rates, that results in a higher capacitance for the mesoporous carbon. Upon gasification of the two carbons with CO2 for obtaining an almost equivalent increment of specific siuface area (carbons MCI and ACl in Table 2), the mesoporous carbon shows a larger increase of capacitance (Fig. 15). 

In conclusion, these results support the argument that the presence of an ordered mesoporous network interconnecting the micropores lowers the resistance for electrolyte migration and imjMOves the ultimate accessibility of micropore surface to electrolytes. 

Other rod-type carbons with various structures have been synthesized with the SBA-1, SB A-IS and SBA-16 (IfU gc cage-type, cubic /m3m) mesoporous silica templates. These carbons (designated as CMK-2 [4], CMK-3 [S] and CMK-6 [IS, 16], respectively) exhibit very similar XRD patterns to those for their silica templates, as shown in Fig. 4. This result indicates that the structures are maintained in the same space groups during the synthesis of the carbons from silicas, unlike the case of the CMK-1 synthesis. It is therefore reasonable that these silica templates are composed of 3-dimensional (3-D) mesoporous networks of the same continuity. The carbon synthesis within the mesoporous networks of the same continuity gives the CMK-2, CMK-3 and CMK-6 mesoporous carbons corresponding to faithful replication of the template pore systems. 

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