*3.6. Morphological E*ff*ects on the Catalytic Behavior of K-MER Zeolite*

By changing the synthesis parameters, K-MER zeolites with four distinct morphologies, namely nanorod (W-3), bullet-like (W-4), prismatic (W-9) and wheatsheaf-like (W-16) shapes, were obtained. Nevertheless, from the elemental analysis, all the samples exhibited nearly similar Si/Al ratio (ca. 2.29), which was close to the theoretical one (2.12) (Table 2) [36]. Furthermore, the K/Al ratio of all the samples was found to be near unity due to the fact that the positive charge of each K<sup>+</sup> non-framework cation has to be counter-balanced by a negative charge contributed by an Al atom in the [Si-O-Al]− form [25]. On the other hand, the total surface area of the samples was measured with the N2 adsorption isotherm analysis. Note that the total surface area determined was actually contributed only by the external surfaces because the size of N2 molecules is too large to probe the micropores of the K-MER zeolite [27,37]. The results indicated that the external surface area had positive correlation with the crystallite size of K-MER zeolite. For instance, K-MER zeolite with nanorod shape had the highest external surface area (39.57 m2/g) and the external surface area generally reduced as the crystallite size increased.

The surface basicity of these four zeolite samples was also characterized by using CO2-TPD. Upon CO2 adsorption and desorption from all samples, four deconvoluted signals with different intensities were observed indicating that the morphology had considerable effects on the basic strengths (weak basic sites: ca. 105 and 175 ◦C; medium basic sites: ca. 260 ◦C, medium-strong: 330 ◦C) (Figure 11). The number of active sites with different basic strengths was also quantified based on the amount of CO2 sorbed per gram of zeolite (Table 2). It was found that the number of total active sites (weak,

medium and medium-strong basic strengths) was linearly proportional to the surface area of K-MER zeolite (R<sup>2</sup> = 0.954). In fact, this is not surprising because most of the accessible basic sites, namely [Si-O-Al]−K+, are located at the external surfaces of the zeolite particles. As a result, K-MER zeolite with smaller crystallite size exhibited a larger number of basic sites [38]. In addition, the low Si/Al ratio of the zeolite framework also contributed to the basicity of K-MER zeolite because when the Si/Al ratio is low, more K<sup>+</sup> cations are needed by the zeolite for surface charge counter-balance, which leads to the enhancement of zeolite basicity. Nanorod-shaped K-MER zeolite appeared to have the largest number of medium-strong basic sites (2.03 mmol/g) followed by prismatic (0.94 mmol/g), wheatsheaf-like (0.65 mmol/g) and bullet-like (0.27 mmol/g) K-MER zeolite.

**Figure 11.** TPD-CO2 profiles of (**a**) nanorod (W-3), (**b**) bullet-like (W-4), (**c**) prismatic (W-9), and (**d**) wheatsheaf-like K-MER zeolites.


**Table 2.** TPD-CO2 basicity of K-MER zeolites with various morphologies.

<sup>a</sup> Equivalent to external surface area because micropore surface area was not measureable due to small micropore size of K-MER zeolite.

To study the morphological influences on the catalytic properties, the K-MER zeolites were tested in a model base-catalyzed reaction, i.e., cyanoethylation of methanol. The cyanoethylation of methanol with acrylonitrile was carried out under non-microwave instant heating where K-MER zeolites with different morphologies (W-3, W-4, W-9 and W-16) were used as the base catalysts (Figure 12). In general, the catalytic reactivity had a strong correlation with the morphology of zeolite catalysts. Remarkably, K-MER zeolite nanorods (W-3) exhibited superior catalytic activity with 94.1% of methanol conversion (100% selectivity) within 45 min of reaction at 140 ◦C, which could be explained by the largest number of accessible basic sites (particularly medium-strong basic sites) at its external surface. In contrast, bullet-like K-MER zeolite catalyst (W-4), which had the largest crystallite size and the lowest number of basic sites, showed the lowest catalytic conversion (44.2%) among the four K-MER zeolites studied. Hence, the results showed that the morphological properties had a direct influence on the catalytic activity of a zeolite whereby the morphology is directly associated with the number of accessible catalytic active sites [39]. Comprehensive work on the aspects of molecular diffusion on K-MER zeolites with different morphologies is in progress.

**Figure 12.** Catalytic performance of (**a**) nanorod (W-3), (**b**) bullet-like (W-4), (**c**) prismatic (W-9), and (**d**) wheatsheaf-like K-MER zeolites on cyanoethylation of methanol at 140 ◦C.
