Bimodal mesoporous silica

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. 

These two kinds of surfactants are mutually insoluble in aqueous systems, and they can form separated hydrogenated surfactant micelles that coexist with fluorinated surfactant micelles. Consequently, mesoporous silica with a bimodal pore-size distribution can be obtained, because of the coexistence of these two different types of aggregates that template two different pore sizes. An example of bimodal mesoporous silica is shown in Figure 11.13. 

Xing, R. Lehmler, H.-J. Knutson, B. L. Rankin, S. E., Synthesis and Tuning of Bimodal Mesoporous Silica by Combined Hydrocarbon/Fluorocarbon Surfactant Templates. Langmuir 2009,25, 6486-6492. 

Gao, L. Sun, J. Li, Y. Zhang, L., Bimodal Mesoporous Silicas Functionalized with Different Level and Species of Amino Groups for Adsorption and Controlled Release of Aspirin. J. Nanosci. Nanotech. 2011,11,6692-6697. 

May, A. Stebe, M. J. Gutierrez, J. M. Blin, J. L., Coexistence of Two Kinds of Fluorinated Hydrogenated Micelles as Building Blocks for the Design of Bimodal Mesoporous Silica with Two Ordered Mesopore Networks. Langmuir 2011,27,1400-1404. 

Mixed solutions of cationic surfactants and nonionic poly(ethylene glycol) or block copolymers were employed for the synthesis of monolithic trimodal porous silica.[176] Lyotropic mixtures of block copolymers of different lengths with hydrophilic linear PEO chains were also applied to their nanocasting into bimodal micro-mesoporous silica to formulate the dependence of the mesopore sizes and the microporosity on the lengths or sizes of the hydrophobic and the hydrophilic blocks, but the mesostructures were worm-type in morphology and several hundred nanometers or more in size. 

M. Antonietti, B. Berton, C. Goltner, and H.P. Hentze, Synthesis of Mesoporous Silica with Large Pores and Bimodal Pore Size Distribution hy Templating of Polymer Latices. Adv. Mater., 1998, 10, 154-159. 

Mesoporous carbon was obtained by sucrose carbonization in the pores of MCM-4 silica spheres with subsequently dissolution of the silica. The carbon was impregnated with the ZSM-5 synthesis gel and the crystallization was carried out under hydrothermal conditions. After burning off the carbon, ZSM-5 with a bimodal mesopore system showing mean diameters around 2 and 30 nm was obtained. Nevertheless, the hexagonal pore array of the MCM-41 was not reproduced in the ZSM-5. 

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. 

Antonietti M, Berton B, Goltner C, Hentze HP (1998) Synthesis of mesoporous silica with large pores and bimodal pore size distribution by templating of polymer latices. Adv Mater 10 154... 

Suzuki and Sinha have prepared novel bimodal mesoporous crystalline ceria nanoparticles and evaluated their performance in VOC removal (106). The mesoporous ceria showed 92% acetaldehyde removal with 33% CO2 conversion at ambient temperature after 24 h. This acetaldehyde removal performance is nearly twice as high as that for conventional VOC removal using materials such as activated carbon or mesoporous silica. 

The first impregnation of mesoporous silica gel by furfuryl alcohol resulted in bimodal mesoporous carbon with the maximum pore size of 2.9 and 16 nm and further impregnation yielded unimodal carbons with the pore size of 2.8 nm. 5 was 1540-1810m /g and more than half the overall pore volume was attributed to... 

Suzuki, N. Kiba, S. Yamauchi, Y., Bimodal Filler System Consisting of Mesoporous Silica Particles and Silica Nanoparticles Toward Efficient Suppression of Thermal Expansion in Silica/Epoxy Composites. J. Mater. Chem. 2011,21, 14941-14947. 

Fuertes and Nevskaia [139] developed a vapor deposition polymerization (VDP) method to prepare OMCs. Carbon precursor FA was infiltrated into the pores via vapor-phase adsorption at room temperature. When ordered SBA-15 silica was used as a template, the resultant carbon possessed a unimodal pore structure similar to that of CMK-3. However, when a disordered mesoporous silica was used as a template, mesoporous carbon with a well-defined bimodal pore system (mesopores centered at 3 and 12 nm) was obtained, as can be seen from Figure 2.19. A mechanism responsible for the formation of such carbons was subsequently proposed [140] based on the degree of carbon infiltration, which can be controlled with the VDP method. Kruk et al. [141] described a polymerization method for carbon infiltration, which was believed to ensure uniform filling [142] and avoid the formation of nontemplated carbon. 

The difficulty in direct synthesis of mesoporous transition metal oxides by soft templating (surfactant micelles) arises from their air- and moisture-sensitive sol-gel chemistry [4,10,11]. On the other hand, mesoporous silica materials can be synthesized in nimierous different solvent systems (i.e., water or water-alcohol mixtures), various synthetic conditions (Le., acidic or basic, various concentration and temperature ranges), and in the presence of organic (Le., TMB) and inorganic additives (e.g., CT, SO, and NOs ) [12-15]. The flexibility in synthesis conditions allows one to synthesize mesoporous silica materials with tunable pore sizes (2-50 nm), mesostructures (Le., 2D Hexagonal, FCC, and BCC), bimodal porosity, and morphologies (Le., spheres, rods, ropes, and cubes) [12,14,16-19]. Such a control on the physicochemical parameters of mesoporous TM oxides is desired for enhanced catalytic, electronic, magnetic, and optical properties. Therefore, use... 

Based on the control of sol-gel deposition, hierarchically porous silica-based materials with a bimodal pore system (mesopores/large meso/macropores) and a diversity of dopant elements (Al, Ti, V, and Zr) could be prepared by using a one-pot surfactant-assisted procedure [78]. Another example includes the preparation of nonionically templated [Si]-MSU-X mesoporous silicas with bimodal pore systems by adding dilute electrolytes. 

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). 

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