Template synthesis limitations

Zeolites have ordered micropores smaller than 2nm in diameter and are widely used as catalysts and supports in many practical reactions. Some zeolites have solid acidity and show shape-selectivity, which gives crucial effects in the processes of oil refining and petrochemistry. Metal nanoclusters and complexes can be synthesized in zeolites by the ship-in-a-bottle technique (Figure 1) [1,2], and the composite materials have also been applied to catalytic reactions. However, the decline of catalytic activity was often observed due to the diffusion-limitation of substrates or products in the micropores of zeolites. To overcome this drawback, newly developed mesoporous silicas such as FSM-16 [3,4], MCM-41 [5], and SBA-15 [6] have been used as catalyst supports, because they have large pores (2-10 nm) and high surface area (500-1000 m g ) [7,8]. The internal surface of the channels accounts for more than 90% of the surface area of mesoporous silicas. With the help of the new incredible materials, template synthesis of metal nanoclusters inside mesoporous channels is achieved and the nanoclusters give stupendous performances in various applications [9]. In this chapter, nanoclusters include nanoparticles and nanowires, and we focus on the synthesis and catalytic application of noble-metal nanoclusters in mesoporous silicas. 

The template synthesis by the method of Mann et al. is limited to large objects because of the nanoparticle dimension. There is also an additional restriction on its widespread use caused by the nanoparticle being stable only in deionized water with pH between 10 and 11. 

As seen above, the randomness of the Mobius strip approach and numerous difficult steps in Schill s directed approach were highly limiting factors in a trefoil knot synthesis. Both these major obstacles might be circumvented by the use of an unambiguous templated synthesis procedure. 

The following two synthetic techniques are applied for obtaining complexes of aliphatic, aromatic, and heterocyclic imines in the limits of the discussed variant of the template synthesis interaction of coordinated mono- and diamines with mono- and dicarbonyl derivatives (method A) and reactions of complexes of o-hydroxyalde-hydes (ketones) with amines and diamines (method B). The simplest transformation of this type is represented by the reaction (3.176) [53,54] ... 

Since the first demonstration that DNA can act as a scaffold for metallization to produce conducting wires [270], the proposal to create circuits on a nanometre size scale has appeared achievable. With CMOS technology predicted to reach a miniaturization limit by 2012 [271], and the high cost and low throughput associated with nanometre scale scanning probe and electron beam lithographic top-down techniques, the DNA-templated synthesis of wires with widths typically less than 50 nm may provide a scheme to cir-... 

DNA-Templated Organic Synthesis. An exciting recent concept developed largely by Liu and co-workers is DNA-templated organic synthesis. DNA-templated synthesis generates products individually linked to ODNs that encode and direct their synthesis. DNA-templated synthesis is limited by the fact that DNA-linked reagents need to be prepared, reactions need to be... 

In comparison to the zeolite synthesis approach there are many disadvantages associated with the preparation of intrazeolite complexes by the flexible ligand and template synthesis methods. The complexes are difficult to characterize, especially if the ligand has multiple coordination modes available and some of the target metal ions may remain uncomplexed which will complicate any reactivity studies. Additionally, there are limitations to the types of metal complexes that might be encapsulated in a zeolite. The only criteria for incorporating metal complexes... 

There are several techniques that allow to produce polymeric nanofibers, such as drawing, template synthesis, phase separation, self-assembly, solution blow spinning, and electrospinning. The drawing process requires a polymer with appropriate viscoelastic properties that is able to be deformed and kept connected by cohesive forces. Besides being simple and inexpensive, this technique is very limited for conjugate polymers, since most of them have lower solubility and form solutions with a small viscous modulus. 

However, two evident limitations of the template synthesis are that (1) the guest species has to be stable under the synthesis conditions of the host material (which, especially in the case of zeolites, can be relatively harsh in terms of pH or... 

Dissolution of the membrane material exposes arrays of cylinders that are attached at one end to a conductive electrode. Template synthesis can be used to prepare size-monodisperse particles or cyhndrical wires that are up to 50 pm in length and 15 ran to 5.0 pm in diameter. Template synthesis is the most popular method for preparing nanostructures of various materials using electrodeposition. Its primary disadvantage is that commercial templates are usually less than 50 pm in thickness and this places an upper limit on the length of the nanowires that can be obtained. 

The template synthetic method is based on the in situ hydrothermal crystallization of clay mineral layers (from a gel) using selected water-soluble polymers as templates [61]. Template synthesis is essentially limited to water-soluble polymers, and the synthetic clay mineral formed under the conditions described by the authors is a poorly ordered fluorohectorite. On the other hand, the method is potentially capable of promoting the dispersion of silicate layers in a one-step process. 

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