Mesoporous silica hard template

Zhao[266] demonstrated the successful synthesis of highly ordered mesoporous silicon carbides with unusually high surface areas (430-720 m2/g), uniform pore sizes (<3.5 nm), and extremely high thermal stabilities (up to 1400 °C) replicated by mesoporous silica hard templates via a one-step nanocasting process. Highly ordered 2-D hexagonal (p6m) and bicontinuous cubic (Ia3d) SiC nanowire arrays have been cast from the hard templates, mesoporous silica SBA-15 and KIT-6, respectively. 

Ordered mesoporous silica seems to be an ideal hard template, which can be used as a mold for other mesostructures with various compositions, such as ordered mesoporous carbon and metal oxides. Mesoporous silicas with various different structures are available, and silica is relatively easily dissolved in HF or NaOH. Alternatively, mesoporous carbons with a solid skeleton structure are also suitable choices as hard templates due to their excellent structural stability on thermal or hydrothermal and chemical treatment. A pronounced advantage of carbon is the fact that it is much easier to remove than silica by simple combustion. The nanocasting synthesis of mesoporous carbon by using mesoporous silica as template will be discussed in detail in the section on mesoporous carbon. In many cases, silica is unsuitable for synthesizing framework compositions other than carbon, since the leaching of the silica typically affects the material which is filled into the silica pore system. 

Ordered mesoporous nanocrystalline CeOi was obtained by using silica hard templates.CeCls was incorporated into the mesoporous silica and then converted to cerium hydroxides by reacting with ammonia vapour. Calcination of cerium silica composite and removal of the silica using NaOH solution produced mesoporous CeOi. 

The disadvantage of traditional hard template synthesis of porous carbon is that it usually involves several steps starting with the preparation of a mesoporous silica/surfactant mesophase followed by calcination to remove the surfactant (to generate the mesoporous sUica hard template), introduction of carbon precursor into the mesoporous siUca, carbonisation of the carbon precursor and finally silica etching (i.e. washing with HF or NaOH) to generate the mesoporous carbon. Therefore, recently there have been some attempts to prepare mesoporous carbon via more direct methods involving fewer steps. 

Supra-molecular self-assembly provide routes for the synthesis of ordered mesoporous carbon molecular sieves via a soft-template route as recently reported by Meng et al. (2006), Huang et al. (2007), Wan et al. (2007) and Xue et al. (2008). As an alternative to hard-template methods, researchers have focused on the synthesis of OMC by a single-step (soft-template) method, which can eliminate the pre-formed silica (hard) template and tedious infiltration steps. In this regard, commercially available amphiphilic triblock copolymers such as pluronics or symper-onics are used for the preparation of OMCs. The synthesis procedure involves the preparation... 

Fig. 4.8 Preparation of a porous protein assembly using mesoporous silica as a hard template.

Figure 2.15 General concept of the syntheses of mesoporous materials (a) mesoporous silica synthesis with soft template (b) mesoporous carbon synthesis with hard template.

Silver nanoparticles were also synthesized inside mesoporous silica by y-irradiationd These hard templates can also be used as nanoreactors to obtain nanomaterials of controlled shapes such as metal nanowires. Ichikawa and co-workers synthesized platinum and palladium nanowires in mesoporous silica However,... 

Two kinds of template, viz. hard template and soft template, are usually available for nanocasting processes. The true liquid crystal templating synthesis can be considered a soft-template process. In general, the hard template means an inorganic solid. For example, mesoporous silica as a template to replicate other materials, such as carbon or metal oxides, by which the pore structure of the parent can be transferred to the generated porous materials. A 3-D pore network in the template is necessary to create a stable replica. Mesoporous silica and carbon are commonly used templates for nanocasting synthesis. 

Zeolites were already employed as templates in the synthesis of microporous carbon with ordered structures.[247] The discovery of ordered mesoporous silica materials opened new opportunities in the synthesis of periodic carbon structures using the templating approach. By employing mesoporous silica structures as hard templates, ordered mesoporous carbon replicas have been synthesized from a nanocasting strategy. The synthesis is quite tedious and involves two main steps (i) Preparation and calcination of the silica mesophase, and (ii) filling the silica pore system by a carbon precursor, followed by the carbonization and selective removal of the silica framework. 

In the Pt-doped hexagonal mesophase formed from CPCI (cetyl pyridinium chloride), platinum ions are adsorbed at the surface of the surfactant cylinders. They are reduced radiolytically into a metal layer as a nanotube of around 10 nm diameter and a few hundred nm long (Fig. 3f). Extraction of all these nanostructures is achieved by dissolution of the soft template using alcohol. This possible easy extraction constitutes a marked advantage over the synthesis in hard templates, such as mesoporous silica or carbon nanotubes, the dissolution of which is more hazardous for the metal nanostructures. 

Hard templates, whose release requires acid or basic attack such as zeolites and mesoporous silica, used as templates for porous carbon preparation (Kim et al., 2003 Yang et al., 2005), can be taken as examples. 

In summary, highly cubic ordered cobalt oxides with crystalline walls have been synthesized by an accurately controlled incipient wetness approach using mesoporous silica as hard template and cobalt nitrate as the precursors. Compared with the previously reported solution impregnation processes, this technique is facile, time-saving and economical. This technique can be used to synthesize ordered mesoporous metal oxides, and we expect that rvith this approach various kinds of mesoporous oxides or other materials can be synthesized, which may benefits their applications in many areas such as in catalysis, sensors, clean energy and electronics. 

Ordered Mesoporous Carbons The first ordered mesoporous carbons were synthesized by hard templating methods (141). The use of porous inorganic silica templates allowed the preparation of series of ordered mesoporous carbon materials, designated as CMK-x materials (carbon molecular sieves Korean Advanced Institute of Science and Technology). The hrst one, CMK-1, was prepared using MCM-48 as a template and sucrose as the carbon source impregnated in the... 

For each of the following compositions, name what would be a preferred hard template (mesoporous silica or carbon) for preparing mesoporous materials ... 

The other method for preparing ordered mesoporous materials is the so-called hard template method using hard mesoporous silica or replicated carbon templates. The metal precursors are filled into hard templates. In this method, heat treatment can be performed at a higher temperature without structural collapse and highly crystallised materials can be obtainedl ° ] (Figure 3.6). 

A limitation of the hard template method is that the resulting materials must be stable in HF or NaOH solution and that the precursors must not react with the silica template at a high temperature. For example, in the formation of lithium containing transition metal oxides, it is necessary to first form the transition metal oxide as a mesoporous solid to prevent reaction of the alkali metal Li with the silica template and then to react the mesoporous transition metal solid with a lithium source, such as LiOH. 

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