Templates removal

The high temperature used to calcine these materials invariably leads to a contraction of the observed pore spacings due to the further condensation of the silicate species in the walls during this treatment. The amount of contraction varies with synthesis method, and can be up to 25 but even materials with good structures and well condensed walls undergo contractions of 2-3 Materials which were not initially 

Materials templated with polymeric surfactants can also be successfully solvent extracted using ethanolJ It has been shown that the templating surfactant can be recovered after solvent washing, and used to synthesise further mesoporous materials. This recycling of surfactant reduces the costs of making these materials, and would also be environmentally beneficial in industrial production processes. The main drawback of solvent 

Even milder conditions for template removal can be achieved using supercritical fluids, such as carbon dioxide. Treatment with supercritical CO2 alone is effective only in removing triblock copolymer templates from templated silicates, however inclusion of an alcohol improves the extraction efficiency for ionic surfactant templates.Washing with methanol modified supercritical CO2 was shown to remove 76-95 % of the surfactant template from MCM-41 and MCM-48 films, powders and SBA-1, SBA-3 powders at 85 Other groups prefer supercritical 

CO2 modified with a methanol/dichloromethane mixture and showed that the template extracted in this manner could be re-used for further syntheses of mesoporous silica. Treatment with supercritical CO2 alone leads to pore swelling with retention of mesostructural ordering in 2D hexagonal phase silica triblock copolymer surfactant composites. Pore swelling with supercritical CO2 has also been observed for materials templated with fluorinated surfactants.  

The use of UV/ozone treatment to break down the surfactant at room temperature in air has also been shown to completely remove surfactant from the pores both for mesoporous powders, and for thin films. Silica condensation is promoted by the UV light, simultaneously with surfactant removal. UV light in conjunction with dilute hydrogen 

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Controlled removal of the template is especially important when zeolite based membranes are involved consisting of a continuous MFI layer on a ceramic or sintered metal support (ref. 14). In these novel composite ceramic membranes the formation of cracks during template removal would be detrimental. The unique properties (ref. 14) of metal-supported MFl-layer membranes prove that indeed crack formation can be essentially prevented. 

Thus zeolite ZSM-5 can be grown (ref. 15) onto a stainless steel metal gauze as shown in Figure 6. Presumably the zeolite crystals are chemically bonded to the (chromium-) oxide surface layer of the gauze. After template removal by calcination and ion exchange with Cu(II) a structured catalyst is obtained with excellent performance (ref. 15) in DeNOx reactions using ammonia as the reductant. 

In situ thiol oxidation can also be done just after the tetraethoxysilane hydrolysis. Both MPTMS and H2O2 are added simultaneously before aging. This was demonstrated by Stucky et al. [69] on SBA-15 containing ffS03 and additional alkyl groups, followed by Mbaraka et al. [63], and Yang et al. [70], the latter including template removal. In situ oxidation avoids residual unreacted thiols, typically observed by postsynthetic routes. 

Third Concept in Catalyst Design. Fenton Detemplation. Mild Organic Template Removal in Micro- and Mesoporous Molecular Sieves... 

MicrocrystalUne zeolites such as beta zeolite suffer from calcination. The crystallinity is decreased and the framework can be notably dealuminated by the steam generated [175]. Potential Br0nsted catalytic sites are lost and heteroatoms migrate to extra-framework positions, leading to a decrease in catalytic performance. Nanocrystals and ultrafine zeolite particles display aggregation issues, difficulties in regeneration, and low thermal and hydrothermal stabilities. Therefore, calcination is sometimes not the optimal protocol to activate such systems. Application of zeolites for coatings, patterned thin-films, and membranes usually is associated with defects and cracks upon template removal. 

Hence, for many promising materials milder template removal strategies are needed. 

The application of zeolite membranes in microreactors is still in an early stage of development, and suffers sometimes from unexpected problems arising from template removal [70]. However, several application examples of zeolite membranes in microstructured devices have been demonstrated yielding similar advantages as were to be expected from experiences on the macroscale. Because of the high surface to volume ratio of microreactors, the application of zeolite membranes in these systems has great potential. 

The added value, variety of use, and methods to apply zeohte coatings or films in sensor apphcations has been convincingly demonstrated. Although current trends focus on miniaturization of sensors and creating smaUer zeohte crystals and thinner films, to decrease the response time of the sensor [79], often thick-film technology is sufficient to apply zeohte films for this type of application. Some sensor materials cannot withstand the high temperatures necessary for template removal by air calcination. Recent work demonstrated that ozonication yields... 

FIG. 11 TEM images of (a) a [(Si02/PDADMAC)2]-coated PS particle and hollow silica capsules produced from PS latices coated with (b) one, (c) two, or (d) three Si02 layers. The hollow silica capsules maintain the shape of the original PS particle template. Removal of the core by calcination is confirmed by the reduced electron density in the interior of the capsules (compare b-d with a). The images of the hollow silica capsules show the nanoscale control that can be exerted over the wall thickness and their outer diameter. (From Ref. 106.)... 

Figure 3. N2 isotherms before (curve 1) and after (curve 2) template removal...

On the intergrowth structure of zeolite crystals as revealed by wide field and confocal fluorescence microscopy of the template removal processes... 

Keywords zeolites fluorescence confocal fluorescence microscopy template removal intergrowth... 

Figure 5. a) Fluorescence microphotographs of ZSM-5 crystals taken during template removal and b) confocal fluorescence images taken at 700 K (561-nm laser, detection at 575-635 nm, intensity presented as a gray scale). 

Mesoporous zirconia has been prepared using anionic surfactants containing reactive oxygens that could bind Zr2+ (224-226). Mesoporous zirconia was obtained using alkyl phosphate amphiphiles but they were not stable to template removal. 

Isoelectronic mesoporous Ti02 has been prepared by the same method but it is also not stable to template removal (224, 227). Hexagonal and cubic manganese oxide mesostructures (MOMS) have been prepared (228). Layered Mn(OH)2 is combined with CTAB and stirred at 75°C for 12 h. Depending on the CTAB concentration, either hexagonal MOMS-1 or cubic MOMS-2 is formed. The MOMS phases are apparently stable to calcination and exhibit semiconducting properties. 

Ordered mesoporous materials of compositions other than silica or silica-alumina are also accessible. Employing the micelle templating route, several oxidic mesostructures have been made. Unfortunately, the pores of many such materials collapse upon template removal by calcination. The oxides in the pore walls are often not very well condensed or suffer from reciystallization of the oxides. In some cases, even changes of the oxidation state of the metals may play a role. Stabilization of the pore walls in post-synthesis results in a material that is rather stable toward calcination. By post-synthetic treatment with phosphoric acid, stable alumina, titania, and zirconia mesophases were obtained (see [27] and references therein). The phosphoric acid results in further condensation of the pore walls and the materials can be calcined with preservation of the pore system. Not only mesoporous oxidic materials but also phosphates, sulfides, and selenides can be obtained by surfactant templating. These materials have pore systems similar to OMS materials. 

Dong, J., lin, Y.S., Hu, M.Z.C., Peascoe, R.A., and Payzant, E.A. (2000) Template-removal-associated microstructural development of porous-ceramic-sup-ported MFI zeolite membranes. 

Wang, H., Holmberg, B.A., and Yan, Y. (2002) Homogeneous polymer-zeolite nanocomposite membranes by incorporating dispersible template-removed zeolite nanocrystals. /. Mater. Chem.,... 

The strategy used to design active and selective catalysts was based on the following five factors for regulation, (i) conformation of ligands coordinated to Rh atom (ii) orientation of a vacant site on Rh (iii) cavity with the template molecular shape for reaction space produced behind template removal (iv) architecture of the cavity wall and (v) micropore in inorganic polymer-matrix overlayers stabilizing the active species at the surface [46, 47, 71]. 

XRD and TEM analysis on template-removed MSU-Ge-2 evidenced the presence of a well-defined, long-range periodicity of the hexagonal pore structure (Fig. 3). The low-angle powder XRD pattern of as-prepared and template-removed mesoporous MSU-Ge-2 indicates a pore periodicity of 4.8 and 4.0 nm, respectively. The pore-to-pore distance (4.0 nm) determined from XRD... 

Schistosoma japonicum. The carbobenzoxy (CBz) protected template 160 was initially converted to the a, p-dehydrolactone 161 via the phosphate ester, before undergoing cycloaddition to ylide 162, generated in situ by acidic treatment of A(-benzyl-A(-(methoxymethyl)trimethylsilyl amine. The resultant cycloadduct (163) was isolated in 94% yield as a single diastereoisomer. Destructive template removal, by catalytic hydrogenation, released (5)-( )-cucurbitine, after ion-exchange chromatography, as the free amino acid in 90% yield (Scheme 3.46). 

The reaction scope was further developed with a range of aromatic and aliphatic aldehydes to furnish a range of p-hydroxy-a-amino acids, which are important constituents of many natural products (62). The resultant bicyclic adducts were obtained in good to excellent yield and represent differentially and orthagonally protected p-hydroxy-a-amino acids. The template removal was easily conducted by... 

The reaction protocol was further extended to the concise synthesis of poly-oxamic acid, the unique polyhydroxyamino acid side-chain moiety of the antifungal polyoxin antibiotics (63). Treatment of the template 205 under standard thermal cycloaddition conditions with (5)-glyceraldehyde acetonide led to the formation of a single diastereoisomer 208 in 53% yield. Subsequent template removal released polyoxamic acid 209 in essentially quantitative yield. This represents a matched system, with the mismatched system leading to more complex reaction mixtures (Scheme 3.70). 

Regiocontrol was also high with unsymmetrical dipolarophiles, as was the endo/exo selectivity. Simple destructive template removal followed by Boc protection gave a range of enantiomerically pure polycychc proline derivatives, as exemplified in... 

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