Pore-Size Control

Several methods can be relied on to adjust the pore sizes of mesoporous silica molecular sieves, as mentioned above, such as the selection of surfactants, hydrothermal treatment, and organic additives. Table 13.1 illustrates the pore 

Surfactant Pore sizes of mesoporous sihca materials mainly depend on the hydrophobic groups of surfactants. Cationic quaternary surfactants with longer alkyl chains can yield larger pore sizes. The chain is, however, limited to C22, because surfactants with long alkyl chains are insoluble in water and lead to the formation of disordered products. As for the conventional PEO-PPO-PEO triblock copolymers, the pore 

2-5 Surfactants with different chain lengths including long-chain quaternary cationic salts and neutral organoamines 

4-7 Long-chain quaternary cationic salts as surfactants High-temperature hydrothermal treatment 

5-8 Charged surfactants with the addition of organic swelling agents such as TMB and midchain amines 

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N,]S2-diaHyltartardiamide (DATD) [58477-85-3] (37). The cross-linking of polymerized monomer with the comonomer is what controls the pore size of the gel polymer mesh. This level of pore size control makes polyacrylamide gel electrophoresis an effective analytical tool. 

One of the most promising applications of enzyme-immobilized mesoporous materials is as microscopic reactors. Galameau et al. investigated the effect of mesoporous silica structures and their surface natures on the activity of immobilized lipases [199]. Too hydrophilic (pure silica) or too hydrophobic (butyl-grafted silica) supports are not appropriate for the development of high activity for lipases. An adequate hydrophobic/hydrophilic balance of the support, such as a supported-micelle, provides the best route to enhance lipase activity. They also encapsulated the lipases in sponge mesoporous silicates, a new procedure based on the addition of a mixture of lecithin and amines to a sol-gel synthesis to provide pore-size control. 

Khodakov, A.Y., Griboval-Constant, A., Bechara, R., and Villain, F. 2001. Pore-size control of cobalt dispersion and reducibility in mesoporous silicas. J. Phys. Chem. B 105 9805-11. 

J. Hayashi, H. Mizuta, M. Yamamoto, K. Kusakabe and S. Morooka, Pore Size Control of Carbonized BPDA-pp ODA Polyimide Membrane by Chemical Vapor Deposition of Carbon, J. Membr. Sci. 124, 243 (1997). 

A monolithic hydrophobic polymer formed by photoinitiated polymerization for on-chip solid-phase extraction is shown in Figure 5.6. The polymer mixture includes butyl methacrylate (BMA) and ethylene dimethacrylate (EDMA), with the pore size controlled by the composition of the hexane/methanol porogenic mixture. The degree of pre-concentration depends on the flow rate, as shown in the pre-concentration of GFP at three flow rates (see Figure 5.7). The factors of pre-concentration were 355, 756, and 1002 for the flow rates of 3, 1.03, and 0.53 rE/min, respectively [342]. 

Endo, M., Takeda, T., Kim, Y.J., Koshiba, K., and Ishii, K. High power electric double layer capacitor (EDLC s) From operating principle to pore size control in advanced activated carbons. Carbon Sci. 1, 2001 117-128. 

Ohkuma, K. Hirayama, C. Pore-size controlled and 14. poly(s-lysine)-immobilized cellulose spherical particles... 

A Y-type zeolite membrane was formed on a porous a-alumina support tube. The membranes produced separated CO2 from N2 at a permeance of the order of 10- mol m-2 s-i Pa-i and a selectivity of 20-100 at 30°C. This rapid and selective permeation was due to the pore-size controlled adsorption. 

Although studies on the subject of ordered mesoporous materials were started about 15 years ago, the unique structure and the properties of these materials attracted many scientists in different fields of research. Their efforts resulted in fruitful results that have been reported in thousands of publications. The flexibility and complexity of their synthesis and structure, and the extensive application potentials of mesoporous materials, create a huge opportunity for researchers and developmental scientists. This chapter will summarize the research results on mesoporous materials from syntheses, structures, formation mechanisms, compositions, morphologies, pore-size control, modifications, applications, challenges, and so on. 

Aromatization of C4-C6 hydrocarbons to benzene, toluene and para xylene over pore size controlled ZnO-HZSM-5 zeolite... 

Fig. 4. Influence of reaction temperature on variation on conversion, and para-xylene selectivity of pore size controlled ZnO-HZSM-5 catalyst. WHSV = 1.1 /h...

The aromatization of C4-C6 hydrocarbons over the chemical vapour deposited ZnO-ZSM-5 zeolite enhanced paraxylene content in xylenes to a value as high as 99%. The high paraxylene selectivity was not affected much by varying WHSV in the range 0.5 to 2.35 and temperature from 773 to 813 K. The pore size controlled ZnO incorporated ZSM-5 zeolite has got good potential to be employed for aromatizing commercial feedstock C4-C6 hydrocarbons to value added benzene, toluene and paraxylene. 

Combining Pore Size Control and Thin Starting Zones... 

Pore size control in porous glass-ceramics with skeleton of NASICON-type crystal CaTi4(P04)6. J. Non-Cryst. Solids, 139, 90-92. 

At the same time, the understanding that the rate constant k, contains a term for the concentration of active sites per unit of catalyst volume or weight has led to efforts to increase the specific surface area of catalysts. The tensions introduced by the desire to increase surface area by increases in porosity and the need for pore size control to avoid difiusion constraints has resulted in a great deal of activity, understanding, and materials development. 

The good chemical and physical stability of many MOPs, coupled with synthetic diversity and potential for pore size control, bodes well for applications in heterogeneous catalysis. For example, microporous phthalocyanine and porphyrin network polymers are used as heterogeneous catalysts for the oxidation of cyclohexene, the decomposition of hydrogen peroxide, and the oxidation of hydro-quinone [52]. Enhanced catalytic activity was observed with respect to low molar mass analogues. There is considerable scope for future development here -for example, the design of electrocatalytic materials using CMPs [11, 12, 19]. 

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