Template molecules

Figure B3.3.14. Template molecule in a zeolite cage. The CFIA stmcture (periodic in the calculation but only a fragment shown here) is drawn by omitting the oxygens which are positioned approximately halfway along the lines shown coimecting the tetrahedral silicon atoms. The molecule shown is 4-piperidinopiperidine, which was generated from the dicyclohexane motif suggested by computer. Thanks are due to D W Lewis and C R A Catlow for this figure. For fiirther details see [225].

Fig. 10. The relationship between a chiral template molecule and the polymeric matrix formed ia the presence of the template molecule.

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. 

For microporous compounds with special compositions, calcination effects are even more severe. As compared with zeolites, these compounds have lower thermal stability. Strictly speaking, most of them are nonporous since removal of the occluded guest molecules by calcination usually results in collapse. This is due to strong H-bonds with the framework, coordination bonds, and sometimes the templating molecule is shared with the inorganic polyhedra. Relevant examples of low-stability microporous compounds with interesting structural features are zeolitic open-framework phosphates made of Ga [178], In [179], Zn [180], Fe [181],... 

Zeolites are prepared by the linking of basic structural units around a template molecule. The structural units are typically based on oxides of silicon and aluminium, and the templates are usually individual small molecules. Under the right conditions, the silicon and aluminium oxide precursors will link up around the template to form a crystalline three-dimensional matrix containing the template molecules. The template... 

The pore size of most zeolites is <1.5nm. This microporosity limits their utility in most areas of chemistry, where the molecules used are much larger, and for which mesoporous materials would be necessary. Unfortunately, attempts to use larger template molecules in the zeolite synthesis, an approach which should in theory lead to larger pore size zeolites, have met with very little success. Indeed, some zeolitic materials have been prepared which have mesopores - none of these has ever displayed any real stability and most collapse on attempts to use them. A new methodology was thus required. 

The same periodic structures can also be formed from alternating AIO4 and PO4 tetrahedra the resulting aluminophosphates are not called zeolites but AlPOs. Zeolites are made by hydrothermal synthesis under pressure in autoclaves, in the presence of template molecules such as tetramethylammonium, which act as structure directing agents. 

In this work a new approach is desribed, which can help to understand ED over heterogeneous catalysts We also hope that this approach can be used to find new modifiers for enantioselective heterogeneous catalytic reactions. The basis for this approach is the steric shielding known in organic chemistry [7,8]. A chiral template molecule can induce shielding effect (SE) in such a way that it preferentially interacts with one of the prochiral sites of the substrate. If a substrate is preferentially shielded its further reaction can take place only fi"om its unshielded site resulting in ED. 

The similarities of above experimental results inspired us to investigate the role of SE in heterogeneous catalytic enantioselective hydrogenation reactions. In heterogeneous catalytic reaction the SE means that a given template molecule interacts with the prochiral substrate in the liquid phase in such a way that one of the prochiral sites is preferentially shielded. If the substrate is shielded then its adsorption onto the metal can take place with its unshielded site resulting in ED. 

The presence of triethylenetetramine in the hydrothermal synthesis of open-framework zinc phosphates results in a number of frameworks with one- to three-dimensional structures. The structures include one-dimensional ladders, two-dimensional layer structures, and one structure where the tetramine is bound to the zinc center. The structural type was highly sensitive to the relative concentration of the amine and phosphoric acid.411 Piperazine and 2-methylpiperazine can be used as templating molecules in solvothermal syntheses of zinc phosphates. The crystallization processes of the zinc compounds were investigated by real time in situ measurements of synchrotron X-ray powder diffraction patterns.412... 

In fact, such biomimetic molecules demonstrate the ability to tailor the growth of silica nanoparticles in a way that is very similar to diatom-extracted species. However, they demonstrate the same limitations in terms of morphological control of nanoparticle assembly. This is because the diatom shell architecture results not only from interactions of silica precursors with templating molecules but also benefits from a cell-driven molding of the vesicular compartment where silicification occurs [29]. Thus, it is very likely that diatom-like synthetic silica will only be achieved when such confinement/molding effects are taken into account in the design of biomimetic experiments [30]. 

Template Weight loss % 20-130 °C Total weight loss % 130-800 °C No of template molecules/UC... 

X-ray structure analysis showed that each of the cavities in the chabazite-type host structure contains two disordered SDA molecules. The lack of a superstructure suggests a random layered arrangement of the template molecules (Figure 2) similar to the crystal structure of the related compound CAL-1 [5],... 

The SSZ-33 zeolite was synthesized using the template molecule derived from 8-keto tricycle [5.2.1.0] decane. The boron containing SSZ-33 was converted to Al-SSZ-33 by a one-step reflux in 1 M aluminum nitrate. The SSZ-35 zeolite was prepared with the template camphorquinone. SSZ-35 zeolite was then subjected to three-fold ion exchange in ammonium nitrate. 

A recent discovery that RNA will act as a self-catalyst, called a ribozyme, leads to a simple three-step model for self-replication - this might include a surface. In the model (Figure 8.18), the template molecule T is self-complementary and is able to act as an autocatalyst. In the first step, it reversibly binds with its constituents A and B, forming the termolecular complex M. The termolecular complex undergoes irreversible polymerisation and becomes the duplex molecule D. Reversible dissociation of D gives two template molecules T, which can initiate new replication. The model preserves the order of the moieties on the template (the direction of the arrow) and the backbone, which may be on the surface... 

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