Tetramethylammonium template

It is interesting to note that although the first examples of template effects were observed in nitrogen macrocycles (see chapter 2) no template effect appears to operate in the synthesis of 72. Richman and Atkins note this in their original report . The authors replaced the sodium cation with tetramethylammonium cations and still obtained greater than 50% yield of tetra-N-tosyl-72. Shaw considered this problem and suggested that because of the bulky N-tosyl groups, .. . the loss of internal entropy on cyclization is small He offered this as an explanation for the apparent lack of a template effect in the cyclization. 

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 zeolite synthesis, large cations such as tetramethylammonium (NMe4" ) and tetrapropylammonium (N(C3H7)4" ) can be used as a template around which the aluminosilicate framework crystallizes with large cavities to accommodate the ion. On subsequent heating the cation is pyrolysed, but the structure retains the cavities. Such structures formed around a single molecule template, with pore sizes between 200 and 2000 pm, are known as microporous. 

The formation of novel silicon-rich synthetic zeolites has been facilitated by the use of templates, such as large quaternary ammonium cations instead of Na+. For instance, the tetramethylammonium cation, [(CH3)4N], is used in the synthesis of ZK-4. The aluminosilicate framework condenses around this large cation, which can subsequently be removed by chemical or thermal decomposition. ZSM-5 is produced in a similar way using the tetra-.n-propyl ammonium ion. Only a limited number of large cations can fit into the zeolite framework, and this severely reduces the number of [AIO4] tetrahedra that can be present, producing a silicon-rich structure. 

Unimolecular capsules have also been prepared around a single resorcin[4] arene core, such as 80, held together by hydrogen bonds [ 179]. These capsules form in the presence of tetramethylammonium chloride, whereby the cation exists within the cavity and the chloride anion exists outside. Evidence for this arrangement was obtained from both solution studies ( H-NiMR) and X-ray crystal structures (Fig. 64). Studies with either larger or smaller cationic partners to the chloride formed weaker, less stable complexes, indicating that the size of the cation may template the capsule formation. 

In the search for zeolites with new pore dimensions and channel structure, researchers employed the use of templating molecules to direct the growth of the aluminosilicates. Tetramethylammonium was the first of these templates used to prepare sodalite and others soon followed [61]. Recently, spectroscopic studies have... 

The use of organic cations has opened a new field in the synthesis of new zeolites (28-30). These cations are thought to increase the solubility of the silicate and aluminate ions and to arrange the water molecules in a template-like fashion. Using tetramethylammonium cations and Y type zeolites were... 

It is interesting to note that replacement of the sodium cations by tetra-methylammonium ion in the Richman-Atkins procedure did not inhibit the reaction as one might expect in a template cyclization but only decreased the yield of the tetraaza-crown to about 50%. Gokel and coworkers suggested that the increase in the yield from 50% to 80% when tetramethylammonium was replaced by sodium does indicate a small template effect by the sodium ion (Gokel et al., 1982). Bogatskii and coworkers used phase-transfer con-... 

Synthesis methods are described below for four very different but important zeolite structures, A,2 Y,3 tetramethylammonium (TMA) offretite4 and tetra-propylammonium (TPA) ZSM-5.5,6 These four were selected because they span the composition range from 1 1 Si Al to a potentially aluminum-free zeolite structure (A to ZSM-5). In addition, these syntheses provide examples of fundamental concepts in crystallization such as templating (TMA offretite and ZSM-5), low-temperature nucleation (Y), and variable reactant (silica) sources. 

Raman spectra of single crystals of ZSM-39 were obtained by Vergilov and Valtchev [863]. These spectra exhibited a highly polarized line at 3610 cm which was ascribed to a structure-bound hydroxy group. Also, vibrations of the framework and template cations (tetramethylammonium and trimethylammo-nium) were discussed. 

The dynamic character of the template-zeolite interactions has only been studied recently [39]. Experimental data (solid state and NMR between -120 C and 20°C), as well as molecular dynamic (MD) calculations at -150 C, -120 C and 0 C, indicate that the tetramethylammonium (TMA) ion occluded in the beta cage of sodalite has a considerable degree of rotational freedom. Figure 10, even at low temperature. MD shows that this rotational freedom originates from the dynamics of the zeolite lattice the distance between... 

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