*3.1. E*ff*ect of Heating Time*

The crystallization of K-MER zeolite was first studied by varying the hydrothermal heating times at 180 ◦C. An amorphous solid product was formed at 0 h according to the XRD analysis where a strong broad XRD hump at 2θ = 22.3◦ was detected (Figure 1a: W-1). The XRD data was supported by FESEM observation of nanoparticles (ca. 58 nm) with coral-like structure (Figure 2a). With a heating time of 10 h, a significant drop in the Si/Al ratio from 7.81 to 3.73 was observed in the solids (Table 1). Yet, no crystalline phase was revealed by XRD technique at this time. Nevertheless, the amorphous hump became weaker and shifted to 2θ = 27.8◦ (Figure 1b: W-2). Both XRD and AAS elemental analyses thus revealed the occurrence of amorphous phase reorganization into secondary more reactive amorphous solid at 10 h [26]. This amorphous-amorphous phase transformation was also confirmed by FESEM study where bulkier amorphous entities (ca. 180 nm) were formed (Figure 2b). Interestingly, the appearance of particles with more well-defined nanorod morphology (ca. 33 <sup>×</sup> 4 nm2) was detected randomly in W-2 sample via microscopic investigation. This indicated that the nucleation process of K-MER zeolite had occurred. These nanorods were agglomerated into bundle-like secondary particles (ca. 650 nm) and were grown on the surface of an amorphous particle.

**Figure 1.** XRD patterns of (**a**) W-1, (**b**) W-2, (**c**) W-3 and (**d**) W-4 samples heated for 0, 10, 14 and 20 h, respectively.

**Figure 2.** FESEM images of (**a**) W-1, (**b**) W-2, (**c**) W-3 and (**d**) W-4 samples heated for 0, 10, 14 and 20 h, respectively.


**Table 1.** The chemical compositions of precursor hydrogels and their respective synthesis conditions.

<sup>a</sup> Am. = Amorphous.

When the heating time was prolonged to 14 h, the amorphous solids were completely consumed as nutrient and subsequently crystalline K-MER zeolite nanocrystals were produced (Figure 1c: W-3). At this time, the Si/Al ratio became nearly constant, with a value of 2.29. As shown in Figure 2c, the crystalline primary (ca. 40 nm) and secondary (ca. 1.3 μm) particles had grown to a larger size

due to simultaneous occurrence of nucleation and crystal growth processes. The XRD analysis also supported this conclusion, as no amorphous hump was seen in the XRD pattern (Figure 1c). Indeed, the pattern showed major peaks at 2θ = 8.96◦, 10.84◦, 12.46◦, 16.58◦, 17.80◦, 27.50◦, 28.10◦, 30.28◦ and 32.88◦ which were characteristics of the MER framework topology [27]. Further increasing the heating time to 20 h showed no change in the framework composition and framework type but the XRD peaks with higher intensity and narrower peaks were recorded (Figure 1d: W-4), indicating Ostwald ripening and crystal growth were dominating the crystallization process [28]. This was supported by the FESEM microscopy showing that the crystalline K-MER zeolite nanorods further agglomerated and transformed into larger MER crystals (>1 μm) with bullet-shape morphology.

### *3.2. E*ff*ect of Heating Temperature*

Heating temperature plays a very crucial role in zeolite crystallization process as it provides energy to overcome the activation energy of the reactions (polycondensation, induction, nucleation, crystal growth, etc.) [29,30]. Hence, the hydrogel with the same molar composition (1Al2O:7SiO2:3.5K2O:196H2O) was subjected to hydrothermal treatment at 120, 140, 160 and 180 ◦C for 14 h. The W-5 solid product appeared to be amorphous at 120 °C and the Si/Al ratio of the solid was 3.29 (Figures 3a and 4a, Table 1). The chemical composition reached nearly 2.60 when the synthesis temperature was raised to 140 °C. At this temperature, K-MER zeolite nanorods (ca. 28 nm) were obtained which tend to form secondary agglomerated particles of ca. 240 nm (Figures 3b and 4b: W-6). Upon increasing the temperature to 160 ◦C (W-7) and 180 ◦C (W-3), no change in the framework chemical composition (Si/Al ratio) and crystalline phase were detected but the size of primary and secondary particles became larger due to further crystals growth at higher temperature (Figure 3c,d and Figure 4c,d). The results indicate that the crystallization of MER zeolite is a thermally activated reaction.

**Figure 3.** XRD patterns of (**a**) W-5, (**b**) W-6, (**c**) W-7 and (**d**) W-3 samples upon heating at 120, 140, 160 and 180 ◦C for 14 h, respectively.

**Figure 4.** FESEM images of (**a**) W-5, (**b**) W-6, (**c**) W-7 and (**d**) W-3 samples upon heating at 120, 140, 160 and 180 ◦C for 14 h, respectively.
