*3.3. E*ff*ect of K2O Content*

Mineralizer is an essential chemical component in the synthesis of zeolites as it increases the overall solubility of inorganic species in the precursor hydrogels by providing accessibility to useful species at a level needed for the nucleation and crystal growth of zeolites. Hence, a precursor hydrogel with a composition of 7SiO2: 1Al2O3: *x*K2O: 196H2O (*x* = 3.5, 5.0 and 7.0) was heated at 180 ◦C for 14 h to study the effect of K2O mineralizer. From the XRD data, the framework chemical composition and the purity of crystallized products remained intact with no other competing phases detected in the K2O range studied (Table 1, Figure 5). However, a change in the crystal morphology was observed upon varying the K2O content. At *x* = 3.5 (W-3), the obtained crystals were in nanorod morphology, while a mixing of nanorods and prismatic crystals were observed at *x* = 5.0 (W-8), demonstrating the significant effect of mineralizer content on the morphological properties of K-MER zeolite (Figure 6a). A similar observation was also reported by Zhang et al. where the addition of a higher amount of NaOH as mineralizer promotes the crystal growth at a specific axis, resulting in the crystallization of silicalite-1 zeolite with spherical shape instead of common coffin shape [31]. Further increasing the K2O content (*x* = 7.0: W-9) led to the formation of K-MER zeolite with only pure prismatic-shape crystals (Figure 6c). As seen, the average size of prismatic shape crystals also reduced from <sup>125</sup> <sup>×</sup> 167 nm<sup>2</sup> to <sup>114</sup> <sup>×</sup> 160 nm2 when the K2O content increased due to the enhancement of the solubility of aluminosilicate species as a result of higher alkalinity in the hydrogel solution [32].

**Figure 5.** XRD patterns of samples prepared from an aluminosilicate gel precursor with a composition of 7SiO2: 1Al2O3: *x*K2O: 196H2O with *x* = (**a**) 3.5 (W-3), (**b**) 5 (W-8) and (**c**) 7 (W-9). All samples were heated at 180 ◦C for 14 h.

**Figure 6.** FESEM images of (**a**) W-3, (**b**) W-8 and (**c**) W-9 samples prepared from an aluminosilicate gel precursor with the composition of 7SiO2: 1Al2O3: *x*K2O: 196H2O with *x* = 3.5, 5 and 7, respectively. The arrows shown in (**b**) indicate the existence of K-W nanorod crystals in midst of K-W prismatic crystals.
