## 2 Topical Research Areas Related to Ionic Liquids and the Contribution of Spiroammonium Compounds
### Organic Compounds as Structure-Directing Agents in Zeolite Synthesis  
#### 2.6.1 The State of the Art  
Due to their exceptional catalytic and selective adsorption properties in combination with thermal and chemical stability, zeolites have enormous use in oil refining and petrochemistry, with an increasing number of applications in fine chemical synthesis and environmental catalysis. Zeolites have come to dominate the world catalyst global market [170; 171]. The consideration of zeolites as a future material is due to the many facile synthesis methods that can be used to obtain different structures with variations in pore size, surface area, pore volume, and physical properties. Every year, there is an increase in the number of new zeolite structures; their database, created by the International Zeolite Association (IZA), contains more than 250 known topologies [172]. Synthetic zeolites are typically obtained via the crystallization of gels containing water, silica, and alumina sources (and other metals: B, Ga, Ge, and so forth) under hydrothermal conditions at basic pH with temperatures in the range 60-200 \({}^{\circ}\)C. Usually, the gel includes cations of the structure-directing agents (SDAs). SDAs could be alkali metal cations (Na\({}^{+}\), K\({}^{+}\), Li\({}^{+}\), etc.) or positively charged organic molecules and mineralizing agents (OH\({}^{-}\) or F\({}^{-}\)). The crystallization mechanism of zeolites is usually described by a classical combination of nucleation and crystal growth steps [173]. The use of the organic SDAs is associated with the concept of the host-guest chemistry of organic molecules and inorganic systems during zeolite synthesis [174]. The addition \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Entry** & **Chiral PTC** & **Solvent** & **Time, h** & **Yield, \%** & **ee, \%** \\ \hline
1 & 5 mol\%, & CH\({}_{2}\)Cl\({}_{2}\) & 2.0 & 90 & 0 \\
2 & 5 mol\%, **A** & CH\({}_{2}\)Cl\({}_{2}\) & 0.5 & 98 & +90 \\
3 & 5 mol\%, **B** & CH\({}_{2}\)Cl\({}_{2}\) & 1.5 & 90 & 0 \\
4 & 5 mol\%, **A** & toluene & 0.5 & 90 & +97 \\
5 & 5 mol\%, **A** & MTBE & 0.5 & 98 & +98 \\
6 & 1 mol\%, **A** & MTBE & 3.0 & 98 & +96 \\
7 & 1 mol\%, **A** & MTBE & 5.0 & 95 & +96 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The reaction conditions to determine the catalytic activity of the catalysts **A** and **B**. Reproduced from [167]. Copyright 2018 Royal Society of Chemistry. of these organic molecules to the zeolite reaction mixtures provokes a particular ordering of the inorganic units around them, which directs the crystallization pathway towards a unique zeolite framework; hence, they are called structure-directing agents. In recent years, thanks to the use of SDAs in the synthesis of zeolites, tremendous progress has been achieved; the discovery of an extremely large number of new zeolite frameworks has taken place. Several properties determine the structure-directing effect of these organic species, especially their molecular size and shape, hydrophobicity, rigidity vs. flexibility, and hydrothermal stability [175]. SDAs were first used as pore generators in the synthesis of high-silica zeolites but also produced an effect on the crystallization evolution of zeolite, the size of the crystal, and zeolite chemical composition [176]. The latest research attempts to transfer the asymmetric nature of organic chiral molecules used as structure-directing agents to the zeolite lattice to produce the chiral enantioselective frameworks, which is one of the biggest demands in the current material chemistry [177]. Organic SDAs must meet appropriate criteria for hydrophobicity/hydrophilicity due to the C/N+ ratio, which is typically between 11 and 16, which limits the number of organic SDA candidates that can be used. Zeolites may be classified according to their pore size as microporous (a diameter less than 2 nm), mesoporous (a diameter between 2 and 50 nm), and macroporous (up to 50 nm), as well as based on their surface area and physical properties [178].