Figure 4: Scheme for zeolite crystallization adapted from [59]. The process is divided into the following stages: (1) formation of hydrogen bonds between the structure directing agent (SDA) and the silicates present in the synthesis solution; (2) oligomerisation of silicates; (3) condensation of the silicate-SDA species to give the first stable crystalline nuclei; (4) crystal growth. Note the tetrapropylammonium ions circled in red which act as SDA in this case.