Modification of the Pore Structure

Thick and hydrophobic walls are present in these materials and, in the case of the CH=CH units, a bromination can be achieved without modification of the pore structure. Such a result reveals the accessibility of the organic group. However, its distribution either at the surface of the solid or in the framework of the walls is not yet clearly demonstrated. The reactivity is of limited interest for this demonstration, but microscopic pictures are more convincing. 

Gas/vapor phase modifications. Many inorganic membrane materials display functional groups that have chemical affinity to selected chemical agents. A well known example is a gamma-alumina membrane which has hydroxyl groups on the surfaces of the alumina crystallites. These hydroxyl groups present on the pore walls and the macroscopic surface of the membrane can act as the reactive sites for modifications of the pore structure with a chemical agent such as the diversified family of silane compounds (chloro- or alkoxy>silanes). 

K. K. Kunze and D. Segal, Modification of the pore structure of sol-gel derived ceramic oxide powders by water-soluble additives. Colloids Surfaces, (1991) 328-337. 

The "chemicai activation" is done at a iower temperature with specific chemicais to achieve specific ends - inciuding modification of the pore structure and minimization of non-porous tar structures. Phosphoric acid, potassium or sodium hydroxide, and zinc, caicium, or aiuminum chioride popuiate the formuiary of the chemists who manage this step. 

In a similar experiment, 2.5 g of zinc oxide prepared by precipitation from zinc nitrate solution by sodium carbonate, calcination, and attempted reduction under similar conditions as previously employed, gave a catalyst of surface area of 40 m2/g, which yielded less than 10 9 kg of methanol per square meter of the catalyst per hour under the standard conditions used for the testing of the copper catalyst. The zinc oxide was in its wurtzitic crystal modification as in most laboratory as well as industrial catalysts, was free of surface impurities, and had a morphology shown in Fig. 4. Details of the pore structure of this catalyst are given in reference (38). 

While oxidation is mainly used for purification of carbon nanostructures, it may also be an efficient tool for size control and surface modification. By selectively oxidizing, for example, smaller carbon nanotubes or diamond crystals, it can provide a simple technique for narrowing size distributions in carbon nanomaterials. Finally, modification of the porosity (activation) of carbon is another example of controlled oxidation and may allow optimization of the pore structure and surface area of nanoporous carbide-derived carbon (CDC) for various applications. 

It is quite challenging to rmderstand in what way the zeolite influences the metal compared to other supports. The electronic changes that could be induced by the pore system are quite subtle and metal particle size effects may overrule these changes [200]. hi comparison to metal-support interactions on macroporous oxides, the interaction between metal particles and the supporting zeolite matrix seems more pronounced. This may be because the metal particles interact with the zeolite lattice over a substantial fraction of their surfece. It has also been suggested that in addition to the intrinsic electronic effects due to the small size of the metal particles in the zeolite cage, a modification of the electronic structure of the metal by the acidic zeolite framework has to be considered [201,202]. 

FIGURE 14.4 Modification of interlayer pore structure by the postintercalation route. 

Before going further, however, two remarks should be made. First the importance of certain structural modifications of the surface of the irradiated catalysts should be stressed. Among these modifications we cite the sometimes considerable transformations of the physical texture of the solid (modifications of the pore spectrum, etc.) ( 4, 7 S) due to sintering, as well as the modifications of the chemical nature of the surface (decomposition of oxides, of silanol groups,. . . ) (74). These structural modifications may have a particularly important influence in the case of large surface solids such as the silica and alumina used in our experiments. 

Effects of Mercury Intrusion. The facts that there is very little residual mercury in the solids after extraction and that the extraction method used does not cause any detectable structural modification have been established now the earlier suggestion (I) that mercury intrusion causes compression of the pore structures can be examined. 

Hydrothermal treatment can however be used constructively to modify the properties of an adsorbent. Perhaps the best example is the formation of ultrastable Y by hydrothermal treatment of sodium ammonium Y zeolite. The change is accompanied by a contraction in the unit cell parameter and an increase in the Si/Al ratio due to elimination of aluminum from the lattice. The resulting product shows greater thermal stability than Y zeolites of similar composition which have not been subjected to the hydrothermal treatment. However, with X zeolite the usual result is a loss of crystallinity with attendant deterioration of the adsorptive properties while with A zeolite a more subtle effect referred to as pore closure occurs. Hydrothermally treated 4A zeolite behaves as if the window aperture is somewhat smaller than in normal 4A sieve. This effect can be useful since a pore-closed 4A does not admit chlorinated hydrocarbons and is therefore useful for drying freon refrigerants. If a wider pore sieve is used for this purpose premature breakdown and loss of capacity may occur due to formation of HF and/or HCl by hydrolysis. The precise mechanism of pore closure has not yet been established and it remains uncertain whether it involves a true modification of the ciystal structure or merely a rearrangement of the surface layers. 

Macropore control of silica and siloxane gels is now well-established method to fabricate gel-based devices in various forms. With an extension of the material shape and size, the effect of spatial confinement on the structure development becomes significant. Deeper understanding is still needed to completely control the morphology even in the miniaturized spaces. Further development of chemical modification of the pore surfaces as well as impregnating functional molecules will enhance the application of the well-defined porous material. 

Future work is likely to see a greater emphasis on control of where the modifications occur. There are already examples of different PSM processes occurring on crystal surfaces and the interior, as well as a few instances in which reaction is limited to specific sites within a MOF. A growing emphasis on this area will eventually lead to greater control of the pore structure and provide the capability to tailor pores with a range of complementary functionalities. This structural control will in turn lead to greater control over properties such as catalysis and separations. 

The connectivity (topology) of the zeolite framework is characteristic for a given zeolite type, whereas the composition of the framework and the type of extra-framework species can vary. Each zeolite structure type is denoted by a three-letter code [4], As an example, Faujasite-type zeolites have the structure type FAU. The pores and cages of the different zeolites are thus formed by modifications of the TO4 connectivity of the zeolite framework. 

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