Calcination template removal

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. 

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. 

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.)... 

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. 

Template removal was achieved by solvent extraction and calcination. The solvent extraction was performed by stirring 1 g of the air-dried product in 80ml ethanol (EtOH) and water mixture (1/1 V/V) for 2h. Then, it was filtered and washed with another 50ml EtOH and water. This extraction procedure was repeated four times (the final washing with water). The extracted product was finally air-dried and calcined in air at 973K. for 6h. 

Figure 41. The SEM micrograph images of an Er 1 Ti()2 inverse-opal structure templated using a colloidal crystal of 466-nm polystyrene beads by filling the interstitial volumes with colloidal 50-nm diameter Lr 1 Ti()2 nanocrystals followed by calcination to remove the poylystyrene. (a) Low magnification. (b) High magnification. [Adapted from (187).]...

A third possibility for the synthesis of nanomaterials in constrained volumes is the use of molds (Figure 3.1c). Advantages of this method include its simplicity, versatility, and precise control over the shape of the solid, even with intricate forms. An elegant example of this strategy is the preparation of zeolites which precisely replicate the complex microstructure of wood. To do this, Dong et al. [43] infiltrated a zeolite synthesis solution into a wood sample. After the necessary hydrothermal treatment, and subsequent calcination to remove the template as well as the wood, a zeolitic structure was obtained that reproduced with full detail and fidelity the wooden sample used as a mold. 

Thermal Stability. Most MeAPO molecular sieves exhibit good thermal stability, retaining crystallinity after a 400-600°C air calcination to remove the organic template. The ultimate thermal stability depends on structure-type, metal type, and metal concentration. 

Corma et al. have recently used EXAFS and XANES to assist in the characterisation of Ti-MCM-41 structures [61]. Figure 22 shows Ti XANES spectra for the Ti-MCM-41 material after calcination to remove the surfactant template and exposure to air (i.e.hydrated), and following subsequent dehydration. The dehydrated sample shows an intense preedge peak whose position and intensity are consistent with the presence of tetrahedrally coordinated Ti +. This peak is reduced in intensity and slightly shifted in the hydrated sample there are also some changes in the absorption edge profile. Corma et al. attribute these changes to a transition to distorted octahedral symmetry on hydration. 

The chemical compositions of the samples were determined by atomic adsorption spectroscopy (AAS). X-ray powder diffraction patterns were recorded after synthesis and template removal on a Siemens D5000 diffractometer using CuKa radiation. After calcination, nitrogen adsorption and desorption isotherms were measured on a Micromeritics ASAP 2010 sorption analyzer. Al MAS NMR spectra were recorded on a Bruker MSL 400 spectrometer using single pulse excitation with standard 4 mm rotors The resonance frequency was (Oo/271 = 104.31 MHz for Al using a 7t/20 pulse and a 0.5 s recycle delay. A 0.1 M solution of aluminum nitrate in water was employed as the chemical shift reference... 

Cobalt containing ZSM5 zeolite was synthesized hydrothermally in a stirred autoclave in 48 h at 743 K according to refs. (4, 5) using C0CI2.6H2O as cobalt source. After synthesis the sample was calcined to remove the template. 

In 2002, Chiang and coworkers[124] developed a new scheme for the confined synthesis of TPA-silicalite nanocrystals. The surfactant cetyltinmethylammonium bromide (CTAMeBr) (in ethanol solution) was added to the single- and double-heated TPA-silicalite precursor sols (SHPS and DHPS), and the mixture was flocculated at a certain pH value to collect the nano-size silicate species in the precursors, and then dried. The dried precursor/surfactant hybrid was pressed into pellets and then steamed in a stainless steel autoclave at 110 150°C for 7 36 h. Finally, the product was calcined to remove the surfactants and TPA. The particle sizes of silicalite-1 produced in this method are about 30 nm. The study indicates that the nanoparticles collected by surfactants already exhibit the structural features of MFI. They crystallize entirely to form silicalite-1 nanocrystals after steam treatment at 110 150 °C. This new solid-phase approach provides a way to synthesize MFI nanocrystals without the problem of separation and collecting nanocrystals from suspension, and it also avoids the large consumption and cost of special mesoporous templates used in the confined-synthesis methods. 

Self-assembly of molecules and nanoparticles to build well-defined structures, constitutes another approach to make model catalysts [33,34]. Here, nano-structured surfaces are made from nanoscale building blocks that are synthesized from atoms and molecules by chemical means. There has been a tremendous development in this field during the past decade, which includes a number of different strategies, including microemulsions [33], (micellar) block copolymers [35,36], and template CVD growth [37]. Relatively little work has, however, so far been directed toward heterogeneous catalysis in the sense described in this chapter, i.e., to make supported catalysts [38]. There are many reports on preparations but relatively much fewer on evaluations of catal3dic activity, trends, or reactivity versus particle size, etc. A main issue for model catalysts prepared by self-assembly is whether they maintain the well-defined character after, e.g., template removal and calcinations and other pretreatment steps, before they can be used as model catalysts. 

The most stable, adhesive and hydrophobic silicalite-1 films can be obtained by in situ crystallization on the silicon substrate, followed by calcination to remove the organic template.[105] Their elastic modulus reaches 30-40 GPa, but the dielectric constant was measured to be 2.7-3.3. These findings suggest an inverse relationship between the mechanical strength of the films and the lowest achievable dielectric constant. 

Preformed metal oxide nanoparticles have been successfully coated on polymer spheres by the use of the layer-by-layer method. This involves the coating of the template spheres with polyelectrolyte layers, which are oppositely charged to the metal oxide nanoparticles to be deposited. Alternating the polyelectrolyte and nanoparticle deposition has led to the successful formation of silica [67,68] and titania [69] coated PS spheres. Using this approach preformed crystalline nanoparticles can be deposited on the organic spheres and crystalline hollow spheres can be obtained without the need of calcination. On removal of the template and the polymer interlayers by heating, hollow spheres of the inorganic material can be obtained [68-70]. This procedure is described in detail in the chapter by Dr Frank Caruso. 

Interestingly, along with the pore volumes, an increase is observed for Au-SG-XX < Ag-SG-XX < Ag-IW-XX, as can be seen from the isotherms (fig. 1) and table 1. It should be pointed out that besides the different template contents the pore volumes are supposed to be influenced by the method of template removal (aqueous extraction vs. high-temperature calcination for 8 h). However, this fact needs to be explored in further experiments. 

These mesoporous molecular sieves are prepared using a liquid crystal templating mechanism in which micelles, which are assemblies of cationic alkyl trimethylammonium surfactants [CH3(CH2) N+(CH3)3] X , act as a template for the formation of the silicaceous material (Figure 6.2). In the silicate-rich aqueous solution, the hydrophobic tails of the surfactant cluster together, leaving the positively charged heads to form the outside of the rod-like liquid crystal micelles. The silicate anions are attracted to, and surround the micelles, aggregating into an open-framework amorphous solid, which precipitates. The solid is filtered off, and heated in air at up to 700 °C (calcination), which removes the surfactant and leaves the... 

It was shown that the pores can be modified without destruction of the periodic pore structure. However, for template removal calcination cannot be used because the organic groups would be destroyed as well. Thus, a solvent extraction process has to be applied. 

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