Template prevention

Further, encapsulation in the template prevents the nanoparticles from agglomeration, maintaining the nanocatalyst performance for long term which is crucial for commercial application. 

In order to get the pore system of zeolites available for adsorption and catalysis the template molecules have to be removed. This is generally done by calcination in air at temperatures up to 500 °C. A careful study (ref. 12) of the calcination of as-synthesized TPA-containing MFI-type single crystals by infrared spectroscopy and visible light microscopy showed that quat decomposition sets in around 350 °C. Sometimes special techniques are required, e.g. heating in an ammonia atmosphere (ref. 13) in the case of B-MFI (boron instead of aluminum present) to prevent loss of crystallinity of the zeolite during template quat removal. 

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. 

Formation from Template Surfaces Recently, a new method for the preparation of LUV was reported by Lasic et al. (1988). The method is based on a simple procedure which leads to the formation of homogeneous populations of LUV with a diameter of around L vim. Upon addition of solvent to a dry phospholipid film deposited on a template surface, vesicles are formed instantly without any chemical or physical treatment. The formation of multilamellar structures is prevented by inducing a surface charge on the bilayers. The size of the vesicles is controlled by the topography of the template surface on which the phospholipid film was deposited (Lasic, 1988). 

Scheme 2. Encapsulation of size- and shape-controlled Pt nanoparticles under neutral hydrothermal synthesis conditions of SBA-15. Silica templating block copolymers and silica precursors were added to PVP-protected Pt nanoparticle solutions and subjected to the standard SBA-15 silica synthesis conditions. Neutral, rather than acidic pH conditions were employed to prevent particle aggregation and amorphous silica formation [16j. (Reprinted from Ref. [16], 2006, with permission from American Chemical Society.)...

Transition-metal nanopartides are of fundamental interest and technological importance because of their applications to catalysis [22,104-107]. Synthetic routes to metal nanopartides include evaporation and condensation, and chemical or electrochemical reduction of metal salts in the presence of stabilizers [104,105,108-110]. The purpose of the stabilizers, which include polymers, ligands, and surfactants, is to control particle size and prevent agglomeration. However, stabilizers also passivate cluster surfaces. For some applications, such as catalysis, it is desirable to prepare small, stable, but not-fully-passivated, particles so that substrates can access the encapsulated clusters. Another promising method for preparing clusters and colloids involves the use of templates, such as reverse micelles [111,112] and porous membranes [106,113,114]. However, even this approach results in at least partial passivation and mass transfer limitations unless the template is removed. Unfortunately, removal of the template may re-... 

Building on earlier work of Osawa and co-workers [55], Oliver and Kowal [52] tested the feasibility of introducing a noncoded amino acid at an unassigned codon in M. luteus. DNA templates were prepared which coded for 19-mer polypeptides containing either the unassigned codon AGA(Arg) or the termination codon TAG at position 13 under the control of a T7 RNA polymerase promoter. The corresponding tRNAs, produced as described in Sect. 2, were based on tRNA and acylated with phenylalanine. The tRNA was modified to prevent recognition by the alanine aminoacyl-tRNA synthetase and to increase translational efficiency. 

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