Bimodal mesoporous structures

Encapsulation via the layer-by-layer assembly of multilayered polyelectrolyte (PE) or PE/nanoparticle nanocomposite thin shells of catalase in bimodal mesoporous silica spheres is also described by Wang and Caruso [198]. The use of a bimodal mesoporous structure allows faster immobilization rates and greater enzyme immobilization capacity (20-40 wt%) in comparison with a monomodal structure. The activity of the encapsulated catalase was retained (70 % after 25 successive batch reactions) and its stability enhanced. 

The synthesis of mesoporous silicas was performed from an isotropic reaction mixture using cationic surfactant as a structure directing agent. The decrease in pH, which causes the formation of solid particles, was achieved by hydrolysis of methyl acetate. The procedure enabled to obtain not only siliceous MCM-41 but also a less well-ordered hexagonal silica with extraordinary large surface area and silica with bimodal mesoporous structure containing the MCM-41 mesopore system and a system of mesopores with a mean diameter of 14 nm. 

Tin incorporated mesoporous Sn-MFI catalysts with different Si/Sn ratio using microwave were synthesized with carbon as hard template. These tin MFI catalysts were characterized using various physicochemical techniques XRD reviled the formation of more crystalline MFI structures which was further supported by the SEM and TEM imaging which clearly showed well ordered zeolite single crystals with mesoporosity. The N2 sorption isothers reviled the formation of bimodal mesoporous zeolites and the presence of tin in tetrahedral site was confirmed by FTIR (970 cm 1) and XPS (3ds/2 and 3 dj 2 electronic states). The thus synthesized mesoporous Sn-MFI catalysts with different Si/Sn ratios were used in studying the catalytic Baeyer-Villiger Oxidation (BVO) of cyclic ketones... 

N2 adsorption-desorption isotherms and pore size distribution of sample II-IV are shown in Fig. 4. Its isotherm in Fig. 4a corresponds to a reversible type IV isotherm which is typical for mesoporous solids. Two definite steps occur at p/po = 0.18, and 0.3, which indicates the filling of the bimodal mesopores. Using the BJH procedure with the desorption isotherm, the pore diameter in Fig. 4a is approximately 1.74, and 2.5 nm. Furthermore, with the increasing of synthesis time, the isotherm in Fig. 4c presents the silicalite-1 material related to a reversible type I isotherm and mesoporous solids related to type IV isotherm, simultaneously. These isotherms reveals the gradual transition from type IV to type I. In addition, with the increase of microwave irradiation time, Fig. 4c shows a hysteresis loop indicating a partial disintegration of the mesopore structure. These results seem to show a gradual transformation... 

Bimodal pore size distribution in MCM-4I has been observed by several groups in the last few years [22-24], However, the relation between two types of mesopores were never fully understood. In a recent TEM study of an MCM-41-type silicate with a bimodal mesopore system, a paint-brush like morphology of the particles was observed (Figure 7) [25], It was then proposed that the two types of pores with the pore diameters of 2.5 nm and 3.5 nm respectively coexist and are parallel to each other in the particles. Due to different rates of crystal growth, the lengths of these two groups of mesopores are different, resulting in such a novel structure only on the (001) surface. 

Another approach is the use of monolithic columns consisting of silica based rods of bimodal pore structure. They contain macropores (-1-2 pm) and smaller mesopores ( 10-20nm) [38]. The macropores allow for low backpressure at high flow rates. The mesopores provide the needed surface area for interactions between the solute and stationary phase. The macropores result in higher total porosity as compared to porous silica particles. Flow rates of 5 mL/min can be tolerated on a 10-cm column without an appreciable loss in... 

Monolithic columns are another approach to provide lower pressure drops and higher rates of mass transfer. These are continuous solid columns of porous silica stationary phase instead of packed particles. Like perfusion packings, they have a bimodal pore structure (Figure 21.7). Macropores, which act as flowthrough pores, are about 2 fim in diameter. The silica skeleton contains mesopores with diameters of about 13 nm (130 A). It can be surface modified with stationary phases like Cig. The rod is shrink-wrapped in a polyetheretherketone (PEEK) plastic holder to prevent waU effects of solution flowing along the walls. The surface area of the mesopores is about 300 mVg, and the total porosity is 80%, compared with 65% for packed particles. The colunm exhibits a van Deemter curve approximating... 

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... 

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 ... 

FIGURE 12.17 Illustration of the preparation of a bimodal mesoporous-macroporous carbon by dual-phase separation. The macropores are formed from the spinodal decomposition of glycolic solvents (a). Bicontinuous structure, framework structure, and the large macropores left by the solvent after annealing and carbonization (b). The carbon walls display large amounts of mesopores (c) templated by the triblock copolymer. (From Liang, C. D. et al., Chemistry of Materials, 21, 2115, 2009. With permission.)... 

Monolilhic silica columns for high performance liquid chromatography (HPLC) are prepared by a unique sol-gel process utilizing polycondensation of silica and phase separation (Nakanishi, 1991). The monolithic silica columns have a bimodal pore structure, i.e. micropores and mesopores, which contribute to high separation efficiency of the liquid chromatography (Ishizuka, 2000 Lubda, 2002). 

The major design concept of polymer monoliths for separation media is the realization of the hierarchical porous structure of mesopores (2-50 nm in diameter) and macropores (larger than 50 nm in diameter). The mesopores provide retentive sites and macropores flow-through channels for effective mobile-phase transport and solute transfer between the mobile phase and the stationary phase. Preparation methods of such monolithic polymers with bimodal pore sizes were disclosed in a US patent (Frechet and Svec, 1994). The two modes of pore-size distribution were characterized with the smaller sized pores ranging less than 200 nm and the larger sized pores greater than 600 nm. In the case of silica monoliths, the concept of hierarchy of pore structures is more clearly realized in the preparation by sol-gel processes followed by mesopore formation (Minakuchi et al., 1996). 

Taking into account the bimodal structure of the catalyst of this study, in which microporous crystals of zeolite are agglomerated with a binder (bentonite) and with alumina (inert charge), both of high mesopore proportion, the limitation to internal diffusion of oxygen in both regions (in series) of the porous structure has been quantified. 

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