Prueba de hipótesis

The organic molecules in the hydrothermal synthesis are not simple pH regulators. They also direct the formation of the open framework structures with respect to dense phases via non-bonding interactions, as shown by Petrovic et al.47 In some cases a close relationship exists between the structure and the organic moiety, with a strong non- bonding interaction.

The charge and size of the organic molecule strongly influences the structure and the composition. In the case of the zeolite ZK-448 (LTA), where tetramethylammonium cation (TMA) added into the synthesis remains occluded in the sodalite cage,49this must occur during synthesis since the organic cations are too large to be incorporated after crystallisation. It is said that these organic cations act as structure directing agents (SDAs) arranging the gel species around themselves to crystallise the framework. The term ‘template’ is therefore commonly used to indicate this.

The crystallisation process is very complex making it difficult to define precisely what ‘template’ means. It can be that one template forms many different structures, for example TMA forms 17 structures, or many different templates can lead to the formation of one single structure, as in the case of ZSM-5.50Davis et al. distinguish three possible roles for the organic guest molecules:51

 Space fillers, acting only to exclude water from the pores and helping to stabilise the growing framework. That is the case in the synthesis of zeolite ZSM-5 which can be achieved in the presence of numerous organic cations, where the organics are acting only as space fillers.

 Structure direction agents leading to a preferred structure via a combination of other gel parameters.

 True templates direct the framework to adopt a unique symmetry and geometry. As an example, the zeolite ZSM-18 (MEI) is templated by the triquaternary

octahydrohexamethyl benzotripyrrolium cation. Modelling (section 1.5) shows a very close fit between the cage and the organic molecule and energy minimisation indicates that the template is unable to rotate in the cage which has the same threefold symmetry as the tri-quat (Fig. 1.9).52

Fig. 1.9 The example of true template for ZSM-18 and the organic molecule tripyrrolium triquat [(C4H4N(CH3)2)3]3+, (courtesy of P. A. Wright).

Tetramethylammonium (TMA) was the first organic cation introduced in a zeolite synthesis in the 1960s by Barrer and Denny and the group of Kerr and Kokotailo.53,54 Since then, a wide range of quaternary ammonium ions, amines and cationic complexes has been applied in all kind of zeolite and zeotype synthesis, and is the most effective way to obtain new materials.

Normally these organic species are investigated experimentally by ‘trial and error’ and to determine their effectiveness as structure directing agents (SDAs). The flexibility of the organic species seems to be one factor that enables the formation of more than one product as the experimental conditions change (from the results of this thesis, for example, the tetraethylammonium cation (TEA) shows two distinct conformations when

templating two different types of cages, Chapter 3 and 4). Loket al.showed the necessity of working in the correct gel chemistry to observe the action of the SDA (this thesis present our results in Chapter 5).50 In addition, Corma et al. used the introduction of heteroatoms (Ge, Ti) into the gel as a route to synthesise new framework types.55

1.4.1 Templating Effect of Organic Species in Zeotypes

1.4.1.1 Alkyl Amines and Quaternary Ammonium Species

The synthesis of AlPO zeotypes requires the presence of organic amines or quaternary ammonium cations. Already in 1982, Flanigen et al.remarked on the ‘templating’ effect of such species, since without their presence dense AlPO phases were formed.

This early work included the formation of AlPO-17 (ERI) with three different organic species with similar sizes (quinuclidine, neopentylamine and cyclohexylamine) and AlPO-20 (SOD) only obtained using the small and spherical tetramethylammonium cation (TMA) accommodated in the sodalite cage. By contrast, AlPO-5 was obtained with 23 different organic species, where due to its cylindrical channel structure and wide aperture, the organic species act merely as void fillers.

Since then a large variety of organic templates have been applied in zeotype synthesis, Table 1.1 shows the organic species commonly applied in the AlPO regime.

In fact, there is a trend towards rationalisation of the use of particular templates for specific pore sizes,56 as in the case of the novel MgAPO STA-257 (SAT) using quinuclidinium ions of the form [(C7H13N)-(CH)n-(NC7H13)]2+(n = 4,5) which possesses an elongated cage where the template is occluded, figure 1.10 shows the good fit between cage and template.

Table 1.1Common organic species applied in the AlPO synthesis.50

Organic Species Typical Resulting AlPO structure

TMAOH AlPO-20

TEAOH AlPO-18, AlPO-5

TPAOH AlPO-5

Me3N AlPO-21

Et3N AlPO-5

Dipropylamine AlPO-11, AlPO-31 Diisopropylamine AlPO-11

Dicyclohexylamine AlPO-5

Isopropylamine AlPO-14

Tert-BuNH2 AlPO-14

Diethylethanolamine AlPO-5

Dimethylethanolamine AlPO-5, AlPO-21

Ethylene urea AlPO-12

Tetraethylethylene diamine AlPO-21

Quinuclidine AlPO-7, AlPO-16, AlPO-17

Fig. 1.10The experimentally-determined position of the diquaternary cation [(C7H13N)-(CH)4-(NC7H13)]2+

used as template within the elongated cages of the magnesioaluminophosphate STA-2. Template represented by a space filling model and framework by a ball and sticks model (courtesy of P. A. Wright).

1.1.4.2 Macrocycles, the ‘Co-Templating’ Effect and Gel Chemistry

Although traditionally primary, secondary and tertiary alkylamines or quaternary ammonium salts have been used, cyclic species can be alternative structure directing agents (SDAs) in the formation of frameworks containing cavities.

Such is the case for aza-crown ethers (the so-called kryptofix family, denoted with the nomenclature K22, K21, etc) and azamacrocycles (the cyclam family):

 Kryptofix 22 forms the AlPO analogue of MCM-61 Mu-13 (MSO)58and K222 AlPO-42,59which has the LTA topology. Moreover, as in the case of K21, it can be necessary to add another organic species to form the zeotype. In this specific case, K21 templates AlPO-42 in the presence of TMA. This approach is denoted ‘co-templating’ (K22, K21, K222 and TEA are described in Fig. 1.111,2and3).  Tetramethylcyclam (TMcyclam, Fig. 1.11 4) templates the novel MAPOs STA-6

(SAS) and STA-7 (SAV). In this example, the synthesis of pure SAPO STA-7 needs the addition of TEA (Fig. 1.115) as a ‘co-template’.

Fig. 1.11Organic SDAs,1n=1 K22 and n=0 K21,2K222,3TMA,4TMcyclam and5TEA.

The ‘co-templating’ effect is studied in detail in Chapter 3 and 4 but it is important to mention that the extra template stabilises the structures by fitting into the smaller cages, in these particular examples the SOD cage of LTA and the so called A-cage of SAV.59,60

As illustrated by these examples, the use of the correct inorganic gel regime is fundamental to the activity of these SDAs. As an example the substituted zeotypes DAF-

O N O O O N O O N + _N+ N N N N 3 4 2 5 1 n = 0, 1 O N O O O N

(

1 and several members of the STA (St Andrews) family have not been synthesised as pure AlPOs.61 It has been postulated that it is not possible to incorporate charged templates in the neutral frameworks. In the following chapters it can be seen that for the case of STA-7 this fact remains valid (Chapter 3) whereas in the case of STA-2 the formation of the pure AlPO is achievable (Chapter 6).