Pore-size adjustment

A pillared clay (c. Fig. 1) has lately attracted considerable attention as a pore-size adjustable molecular sieve. The intercalate of oligomeric metal hydroxide cations is dehydrated by calcination to give a porous material in... 

This method is to insert guest molecules or clusters into zeolite channels so as to make the channels narrower and to achieve the goal of pore-size adjustment. This method is also called internal surface modification to stress the variation of the inside of channels, but in fact both the internal and external surfaces are modified during the treatment process. J... 

In early 1980s, Vansant proposed the silylation method to modify zeolite channels.[46 52] The principle is to use silicanes to react with the surface hydroxygroups in H-form zeolites, and after hydrolysis the formed oxides narrow the channels so as to achieve the goal of pore-size adjustment. Figure 6.18 shows the Xe-adsorption-dynamic curves for the H-form mordenite samples silylated to various degrees. From the Figure... 

The CVD technique can be effective for zeolite pore-size adjustment, and the zeolites after modification through this technique exhibit distinctly enhanced shape-selective adsorption separation and catalytic performances. However, this technique requires vacuum apparatus and the monetary investment for this technique is large. In addition,... 

After CLD modification, the zeolites exhibit excellent shape selectivity for separation and purification of various isomers. For instance, the pore-size-adjusted NaY zeolite after Si(OCH3)4 modification is very effective for the separation of methylnaphthalene and trimethylbenzene isomers. Because the pore size of NaY itself is large, before modification the zeolite shows no shape selectivity for the two methylnaphthalene isomers, and the adsorption capacities for 1-methylnaphthalene and 2-methylnaphthalene are similar. However, as the pore size decreases, the zeolite exhibits increasing adsorption capacity... 

The MCM-41 material has attracted a lot of attention, and it is the most widely studied mesoporous material. It possesses a hexagonal array of unidirectional tubular pores, very high surface area and porosity, a narrow pore size distribution, and pore size adjustable from 2 to 10 nm. In this connection, a study of thermal and mechanical stability of MCM-41 materials containing different titanium contents was performed [9] and it was verified that at aU temperatures the structural changes are less pronounced for Ti-MCM-41, and the complete collapse of the ordered... 

In 1992, Kresge and coworkers at Mobil invented the so-called MCM-41 mesoporous materials that have a hexagonal arrangement of monosized mesopores with pore size adjustable from 1.5 nm to 10 nm (Kresge et al., 1992). 

It should be noted that, due to their great similarity to zeolites, MOFs can have the same area of applications. In particular, owing to their fascinating structures and unusual properties, such as permanent nanoscale porosity (up to 90 % free volume), high surface area, tunable pore size, adjustable... 

Chemical deposition of the vapors of organic substances within the pores of carbonized materials for pore size adjustment. In this process, CMS are generally prepared by carbonization of the raw precursors, activation for the enlargement of pore structure, and finally chemical deposition for tailoring pore apertimes. Suitable carbonaceous substances that can be... 

The relation between the dusty gas model and the physical structure of a real porous medium is rather obscure. Since the dusty gas model does not even contain any explicit representation of the void fraction, it certainly cannot be adjusted to reflect features of the pore size distributions of different porous media. For example, porous catalysts often show a strongly bimodal pore size distribution, and their flux relations might be expected to reflect this, but the dusty gas model can respond only to changes in the... 

The porosity of polymer beads is controlled by the ratio of diluents (poro-gen) to monomers in the organic phase. The increase in the ratio of diluents to monomer in the monomer mixture increases the porosity of polymer beads. The pore size can be manipulated by adjusting the ratio of nonsolvating and solvating diluents in the monomer mixture. The increase in the ratio of nonsolvating diluent (precipitant) in the monomer mixture increases the pore sizes and vice versa. 

Silica is the support of choice for catalysts used in processes operated at relatively low temperatures (below about 300 °C), such as hydrogenations, polymerizations or some oxidations. Its properties, such as pore size, particle size and surface area are easy to adjust to meet the specific requirements of particular applications. Compared with alumina, silica possesses lower thermal stability, and its propensity to form volatile hydroxides in steam at elevated temperatures also limits its applicability as a support. Most silica supports are made by one of two different preparation routes sol-gel precipitation to produce silica xerogels and flame hydrolysis to give so-called fumed silica. 

The most commonly employed crystalline materials for liquid adsorptive separations are zeolite-based structured materials. Depending on the specific components and their structural framework, crystalline materials can be zeoUtes (silica, alumina), silicalite (silica) or AlPO-based molecular sieves (alumina, phosphoms oxide). Faujasites (X, Y) and other zeolites (A, ZSM-5, beta, mordenite, etc.) are the most popular materials. This is due to their narrow pore size distribution and the ability to tune or adjust their physicochemical properties, particularly their acidic-basic properties, by the ion exchange of cations, changing the Si02/Al203 ratio and varying the water content. These techniques are described and discussed in Chapter 2. By adjusting the properties almost an infinite number of zeolite materials and desorbent combinations can be studied. 

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