Postsynthesis treatments

Different synthetic methodologies can be pursued to prepare hierarchical porous zeolites, which can be discriminated as bottom-up and top-down approaches. Whereas bottom-up approaches frequently make use of additional templates, top-down routes employ preformed zeolites that are modified by preferential extraction of one constituent via a postsynthesis treatment For the sake of conciseness, we restrict ourselves here to the discussion of the latter route. Regarding bottom-up approaches, recently published reviews provide state-of-the-art information on these methodologies [8, 9,17-19]. 

PolycrystalHne membrane growth proceeds by initial formation of a gel layer on the surface of the support crystallization takes place at the interface between the bulk Hquid phase and the gel layer, resulting in deposition of zeolite nuclei and crystals formed [8]. Concurrently, the crystals deposited onto the support surface continue to grow, eventually resulting in a continuous membrane layer. Postsynthesis treatment is necessary when a template is used in synthesis to activate the zeolite and open the pores. Usually this is accomplished through calcination or burn out of the organic molecule. 

For hydrocarbons of more than three carbons, mulhple isomers are possible. Among those isomers, the natural or equilibrium distributions rarely match the commercial demand. Isomerization technology provides the means to convert the less valuable isomers into more valued ones. Specific isomerization reaction mechanisms involve species of relatively similar size, so zeolites, with their precise morphologies, can be made into exceptional catalysts with high selectivity. The ability to adjust zeolite chemistry through innovative synthesis or postsynthesis treatments further enhances their versatihty in isomerization applicahons. 

Finally, it must be mentioned that catalytic interest has promoted the preparation of many high-silica zeolitic-like phases modified to include other T atoms such as B, Ti, Fe, or Ga. This modification can be at synthesis, or by postsynthesis treatment to replace Si and/or A1 from existing high-silica frameworks. Not all these are recorded here because these modified materials again may not be frilly characterized however. Table 21 lists well-characterized examples, for example, analogs of the MFI (ZSM-5) framework. 

In general, methods for the large-scale production of advanced materials by combustion synthesis consist of three main steps (1) preparation of the green mixture, (2) high-temperature synthesis, and (3) postsynthesis treatment. A schematic diagram of these steps is presented in Fig. 4. The first step is similar to... 

The third main step of combustion synthesis technologies is postsynthesis treatment. This step is optional, since not all products require additional processing after synthesis. Powder milling and sieving are used to yield powders with a desired particle size distribution. Annealing at elevated temperatures (800-1200°C) removes residual thermal stress in brittle products. The synthesized materials and articles may also be machined into specified shapes and surface finishes. 

The bulk chemistries, surface chemistries, and morphologies of silicon carbide whiskers vary widely depending on the type of process used, the stage of the process development, and the postsynthesis treatments practiced by the producer. The synthesis of the whiskers as described earlier is... 

Films are typically allowed to dry immediately after deposition to drive the silica condensation reaction toward completion. However, the film may not be fully condensed and stable after drying, and postsynthesis treatments are often employed to improve film stability, reduce shrinkage upon template removal, and drive the condensation reaction toward completion. 

Thermal postsynthesis treatments have also been studied over a range of temperatures. These treatments are normally carried out well below the temperature required to remove the template and strive to strengthen the silica network while the template is still in place. In one report, the samples were treated at increasing temperatures, and it was shown that between the temperatures of 150°C and 175°C the order of the mesostructure increased. " ... 

Gravimetric measurements were executed by Pradhan et al. on SWNTs with different postsynthesis treatments. They reported hydrogen storage capacities at approximately 0.2 MPa ranging from approximately 1 to 6wt%, depending on the processing of the material. The sample was first oxidized in dry air to remove the amorphous carbon, then refluxed in HCl or HNO3 and finally annealed at different pressure. 

Another method to tailor pore size of mesoporous solids is to perform restructuring upon hydrothermal treatment. Applying aging treatments at different temperatures and for prolonged periods (from 24 hours up to several days) can efficiently modulate the nature of the mesophase. Such a treatment can either be performed directly in the mother liquor or at a different pH in fresh solutions (typically water or alcohol). For example, Kushalani et al. (185) proved that sUiceous MCM-41-type mesophases could be restmctured at elevated temperatures in its mother liquor resulting in pore size expansion from 3.7 to 5.9 nm. Also, postsynthesis treatments often improve the thermal stabUity of samples normally obtained at room temperature (with less shrinkage... 

HoUow TS-1 MFI TBOT TEOS TPAOH With postsynthesis treatment in a solution containing TPAOH 170 C, 3 days Wang, 2007 (21)... 

There is stiU a dispute as to whether the catalytic activity of iron-containing zeotype materials, for example, Fe-ZSM-5, should be attributed to isomorphously substituted framework iron or to extra-framework iron oxide or iron hydroxide species that are highly dispersed in the material. These extra-framework iron species are present for two reasons, either because they were not incorporated into the framework during the synthesis or because they were ejected from the framework during postsynthesis treatments (such as calcination or other heat treatments). The unresolved issue of the origin of catalytic activity continues to be the subject of research, whereby state-of-the-art characterization techniques are being applied. 

With the exception of the mineral melanophlogite, all porosils are synthetic products. All porosils were directly synthesized with structure-directing agents (SDA) as templates, in general, organic molecules. There are a few more cases where postsynthesis treatment of alu-minosilicate zeolites leads to the complete dealumina-tion of silicate framework structures, yielding all-silica... 

Fine tuning of chemical properties through isomorphous substitution or various postsynthesis treatments. 

For catalyst preparation all techniques are available that are used in standard lab preparation of inorganic materials. Most of the preparation steps can be automated and can be done in parallel using synthesis robots. In the next sections, the most frequently used preparation routes are discussed. Postsynthesis treatment steps common to many different preparation routes are discussed separately at the end of the section. 

Alumina prepared by conventional hydrolysis of aluminum compound precursors is covered by surface hydroxyl groups. High-temperature calcination is needed in order to expose the surface coordination unsaturation of A1 ions. A new method, which involves stoichiometric hydrolysis of an amine-Al alkoxide monomeric complex, can generate alumina with a surface that is covered with far fewer hydroxyls without high-temperature postsynthesis treatment. In this method, the coordination unsaturation site of A1 is protected with an amine throughout the preparation process. The bound amine on the alumina surface can be exchanged with other bases, and the final solid is a Lewis-acid catalyst and catalyzes reactions such as aminolysis of epoxide. The chemistry in the preparation of such an alumina is described. 

Figure 16.1 is a schematic summary of the different preparation approaches it should be taken into account, however, that a postsynthesis treatment, most commonly a thermal treatment under controlled atmosphere, is required to induce phase separation, to promote the formation of the desired oxide or metallic nanophase, and to obtain the desired crystallinity, as will be discussed later. 

It thus becomes obvious that when one new material is discovered, there arc two possible approaches to develop possible catalytic uses. One approach consists in starting from an interesting and relevant reaction, and if the material is not completely adequate, then one may try to modify it by further work on synthesis and postsynthesis treatments in order to improve its adequacy. There is, however, another approach which can lead to shorter term benefits, and this is to investigate first the characteristic of the material and then to look for the reaction and processes for which those characteristics are most appropriate. There is no doubt that there is still much scope for this second approach as far as the applications of MCM-41-type materials in acid catalysis are concerned. 

This chapter has discussed the diversity and potential of CDCs. The available precursor carbides, chlorine treatment temperatures, postsynthesis treatment methods, presynthesis template methods, synthesis configurations, and many other parameters give rise to the diversity of chemistries and morphologies found in CDCs. These parameters are studied so that scientists and engineers can have a better understanding of the mechanisms involved in the synthesis of CDC and to provide them with model materials to study other phenomena such as electroadsorption. 

Depending on the carbon material and parameters such as synthesis conditions and possible postsynthesis treatment, the geometry of pores inside carbon particles may vary greatly. Abstracted, the pore shapes may be approximated as spherical, cylindrical, or slit shaped as the simplest geometries. Indeed, many templated carbons show cylindrical pores and for most activated carbons we assume slit-shaped pores. More complex shapes are also possible the space between dense nanoparticles shows pore walls with a positive curvature. ... 

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