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While zeolites have included primarily aluminium and silicon the heteroatomic substitution of such zeolites has occurred frequently, as many seek to explore the properties that the addition of such atoms could yield.

1.5.3.1 Aluminium

The incorporation of aluminium into zeolites has a long and varied history with many frameworks producing a large array of Si/Al ratios. Examples like zeolites X/Y can exist in their high silicate and high aluminate forms.1 _{The inclusion of aluminium into the zeolites}

structure has two major effects.

1.5.3.1.1 Defects or Lewis Acid Sites

One of the interesting properties of aluminosilicates is the formation of defects in the framework. SiO4 units are neutral while AlO4 units are negative, which results in the

zeolite framework being negatively charged. This negative charge needs to be balanced. The presence of extra-framework species, such as the organic SDA cations, usually aids this. Without the presence of a charge balancing agent in the synthesis this often leads to

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the formation of defects in the zeolite, as the zeolite structure compensates for the negative charge. This leads to the formation of Lewis acid sites (Figure 1.11), which can help to add to the catalytic ability of the zeolite.55

Figure 1.11 Shows a schematic representation of a Lewis acid site, where the negative charge of the AlO4 unit

results in the formation of a positive charge on the silicon, resulting in a defect site. 1.5.3.1.2 Brønsted Acid Sites

The presence of this net negative charge into the zeolite framework can also be compensated for by the incorporation of protons through ion exchange. The binding these protons to the lone pairs of bridging oxygens neutralises this negative charge and forms Brønsted acid sites (Figure 1.12).

Figure 1.12 Shows a schematic representation of Brønsted acid sites, where the negative charge of the AlO4 is

counteracted by the addition of a proton to the lone pair of a bridging oxygen.

These acidic sites play a major role in the zeolite’s catalytic activity and the framework becomes more hydrophilic due to this increase in charge. Another interesting feature of aluminium based zeolites frameworks is the lack of Al-O-Al bonds in the framework. This is explained by Löwenstein’s avoidance rules, which explains that the oxygen bridge between two aluminium ions only has stability when at least one of the aluminium atoms has the enhanced coordination of five or six rather than 4. Therefore, in a zeolite framework consisting of tetrahedra no two aluminium ions can occupy the centres of the tetrahedra linked by one oxygen bridge. This further explains the fact that in all known cases when substituting silicon with aluminium the maximum substitution is only 50%.56

Although recent literature has called into question Löwenstein’s avoidance rules in some zeolite frameworks.57

1.5.3.2 Phosphorus in Zeolites

The presence of phosphorus in zeolite frameworks was first achieved through the formation of the first AlPOs. AlPOs have a similar framework structure to traditional

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zeolites, with tetrahedral AlO4− and PO4+units bound together through corner sharing

oxygens. The need for charge balancing leads to the formation of a zeolite framework with a 1:1 ratio of Al to P. There are over 50 phosphate containing structures recognised by the IZA, some of which are not known in their zeolite analogue forms.58

SAPOs consist of Al, P, and Si tetrahedra together in the same framework. This means that they have the properties of both zeolites and AlPOs. The structure can be thought of as and an AlPO with Si substituted into the framework. The substitution of Si into the framework avoids the formation of unfavourable bonds like P-O-P, P-O-Si and Al-O-Al. Si is therefore only incorporated into the SAPO structure in two ways: either into a hypothetical P site or five Si are incorporated into one hypothetical Al site and four hypothetical P sites. This can alternatively be described as a combination of the replacement of an Al-P atom pair and the three P atoms surrounding the replaced Al.15,59– 61 _{It is believed that the ability to easily make and break bonds during hydrothermal}

synthesis means that unfavourable P-O-Si bonds are avoided.

The distribution of Si in SAPOs has an impact on the number of acid sites and the catalytic activity of the framework. It is assumed that by increasing the amount of Si the number of acid sites will increase. However, these acid sites are weak as Si with more Si nearest neighbours will have a greater electronegativity and stronger acid sites. The inability to control the distribution of Si in SAPOs also leads to the formation of larger silica islands, where Si surround the internal Si and so no acid sites are formed. To maximise the acidity and catalytic activity, it is therefore desirable to increase the amount of Si while also minimizing the size of the Si islands.62 _{This has been achieved with the use of mineralising}

agents to slowly release silicate into the synthesis gel during crystallisation.63 1.5.3.3 Germanium in Zeolites

The use of germanium has yielded the formation of several new framework structures. The fact that Ge and Si are in the same group on the periodic table suggests that they have similar chemistry and so Ge can be substituted into the framework for Si. However, the subtle differences between Ge and Si lead to certain nuances that makes Ge advantageous for the synthesis of new frameworks. The larger size of the Ge atom leads to a smaller Ge- O-Ge angles (≈130˚) compared to Si-O-Si angles (≈146˚). This allows for Ge to compensate for the additional strains present in d4r and d3r thereby stabilising frameworks that contain these SBUs.53,64 _{The use of GeO}

2 in syntheses has led to the formation of many

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and large pore openings. The use of GeO2 has also led to the formation of the first zeolites

with d3r, which have even more strain present (ITQ-49 and ITQ-44).65,66

The presence of high levels of Ge also leads to increased thermal instability, due to the hydrolytically unstable Ge-O-Ge bonds, so some germanosilicates are unable to undergo calcination without prior modification. Also, the susceptibility of Ge to hydrolysis means that most of these structures, even uncalcined, are unstable in air and will breakdown over time. Such disadvantages, along with the higher price of GeO2, means that the use of

germanosilicates (or materials derived there from) in industrial processes is severely limited. Interest has increased in post-synthesis isomorphous substitution of the Ge in the framework structures for other elements like Si and Al.67,68