**3. Influence of batch alkalinity**

Although the controllability of SSC approach has been proved by successful syntheses of sub-micrometer sized ZSM-5 zeolites by using different kinds of silicalite-1 seed crystals, another interesting aspect of this investigation is a study of the critical processes which occur during the crystallization. The materials with more controllable functionalities may be synthesized only with the thorough understanding of the crystallization mechanisms. In order to achieve this goal, the parameters influencing the crystallization pathway and properties of the products are adjusted. Since it is well known that the batch alkalinity play the key role in zeolite synthesis [13, 22, 30, 56-58], the influence of batch alkalinity, *A*=[Na2O/SiO2]b, of this SDA-free system, is in detail studied in this part. In this part of study, the reaction mixtures (hydrogels) having different batch alkalinities, A, were seeded with 4 wt. % of 260 nm silicalite-1 seed crystals. The samples were characterized by SEM, XRD and PSD analyzes.

Fig. 7 shows the SEM images of the crystalline end products (zeolite ZSM-5) synthesized at different batch alkalinities. It can be clearly observed that at low batch alkalinities (*A* ≤ 0.003), the ZSM-5 crystals are either surrounded or covered with amorphous species, indicating that the crystallization process is not completed. Increase of the batch alkalinity causes the morphological change of the samples from irregular particles (0.004 ≤ A ≤ 0.005) to cubic crystals (0.006 ≤ A ≤ 0.010). With further increase of the batch alkalinity (0.011 ≤ A ≤ 0.012), the morphologies of the samples again return to irregular shapes.

All the products are entirely crystalline and have adjustable size in the range from 270 nm to 1100 nm. This indicates that the SSC approach is a powerful tool for controlling the crystal size of the ZSM-5 zeolites. Besides the crystal size, the morphology of the product could also be controlled via changing the morphology of the seed crystals. The surface-stacking morphology of the silicalite-1 seed crystals (synthesized via microwave heating method) could be fairly well 'cloned' to the final ZSM-5 products (Figure 6). Based on the above phenomena, it could be found that the product properties could be adjusted via a 'seed-

Fig. 6. The silicalite-1 seed crystals with surface-stacking morphology (a) and the

corresponding ZSM-5 product (b). The scale bars is of 1 μm. (Adopted from Ref. [50] with

Although the controllability of SSC approach has been proved by successful syntheses of sub-micrometer sized ZSM-5 zeolites by using different kinds of silicalite-1 seed crystals, another interesting aspect of this investigation is a study of the critical processes which occur during the crystallization. The materials with more controllable functionalities may be synthesized only with the thorough understanding of the crystallization mechanisms. In order to achieve this goal, the parameters influencing the crystallization pathway and properties of the products are adjusted. Since it is well known that the batch alkalinity play the key role in zeolite synthesis [13, 22, 30, 56-58], the influence of batch alkalinity, *A*=[Na2O/SiO2]b, of this SDA-free system, is in detail studied in this part. In this part of study, the reaction mixtures (hydrogels) having different batch alkalinities, A, were seeded with 4 wt. % of 260 nm silicalite-1 seed crystals. The samples were characterized by SEM,

Fig. 7 shows the SEM images of the crystalline end products (zeolite ZSM-5) synthesized at different batch alkalinities. It can be clearly observed that at low batch alkalinities (*A* ≤ 0.003), the ZSM-5 crystals are either surrounded or covered with amorphous species, indicating that the crystallization process is not completed. Increase of the batch alkalinity causes the morphological change of the samples from irregular particles (0.004 ≤ A ≤ 0.005) to cubic crystals (0.006 ≤ A ≤ 0.010). With further increase of the batch alkalinity (0.011 ≤ A ≤

0.012), the morphologies of the samples again return to irregular shapes.

dependent' manner.

permission of Publisher.)

XRD and PSD analyzes.

**3. Influence of batch alkalinity** 

Fig. 7. SEM images of the products hydrothermally synthesized (at 438 K for 2 h) from the reaction mixtures (hydrogels) having: *A* = 0.001 (A), *A*= 0.002 (B), *A* = 0.003 (C), *A* = 0.004 (D), *A* = 0.005 (E), *A* = 0.006 (F), *A* = 0.007 (G), *A* = 0.008 (H), *A* = 0.009 (I), *A* = 0.010 (J), *A* = 0.011 (K), and *A* = 0.012 (L). The scale bars in all figures are of 1 μm. (Adapted from Ref. [54] with permission of Publisher)

XRD analysis of the products revealed that fully crystalline zeolite ZSM-5 are obtained within the alkalinity range of 0.006 ≤ A ≤ 0.010, which is in accordance with the observation in the corresponding SEM images (Figs. 7F – 7L).

In order to find more details of particulate properties, the Laser Light Scattering analysis of the obtained samples has been carried out. The particle size distribution (PSD) curves of the samples, measured by Mastersizer 2000 (Malvern) particle size analyzer, are shown in Fig. 8. The crystalline end products obtained under optimal alkalinity range (0.006 ≤ *A* ≤ 0.01) appear as a mixture of single crystals (50 – 60 % by number) having the size between 400

Crystallization of Sub-Micrometer Sized ZSM-5 Zeolites in SDA-Free Systems 267

Batch alkalinity*a* Phase*b* Si/Al ratio*c* Particle shape*<sup>d</sup>* 0.001 Amor*e*+ZSM-5 47 *f* 0.002 Amor*e*+ZSM-5 43 *f* 0.003 Amor*e*+ZSM-5 32 irregular 0.004 ZSM-5 28 irregular 0.005 ZSM-5 23 irregular+cubic 0.006 ZSM-5 18 cubic 0.007 ZSM-5 15 cubic 0.008 ZSM-5 12 cubic 0.009 ZSM-5 10 cubic 0.010 ZSM-5 9 cubic 0.011 Phillipsite+ZSM-5 +Amor*e* 11 Spherical+irregular 0.012 Phillipsite+ZSM-5 +Amor*e* 11 Spherical+irregular

f. Exact information cannot be provided because of the existence of large portion of amorphous

Table 1. Summary of structural, chemical and morphological properties of the products

It is expected that, besides the influence of alkalinity, sodium ions [13, 22, 56-58] and duration of room-temperature gel ageing [59] also influence the crystallization pathway and properties of zeolite ZSM-5. The sodium ion is recognized as the potential template ions which make the influences on the nucleation process of MFI zeolites especially in the SDA-

On the other hand, room temperature ageing of the reaction mixture shortens not only the duration of 'induction period' and the entire crystallization process, but also diminishes the differences in crystal size distributions of the final product [59, 60]. Moreover, in some cases, the ageing of the reaction mixture (hydrogel) influences also the morphology [61] and even phase composition of the final product [62]. From the above reasons, the influences of the mentioned parameters on the properties of crystallized zeolite ZSM-5 are described in this part. Since the increase of batch alkalinity (*A*=[Na2O/H2O]b) is accompanied with the increase of sodium content, the related studies are performed at different batch alkalinities with the addition of sodium sulphate as the source of sodium ions excess. The batch content of sodium ions is expressed as *B* = [Na+/SiO2]b. The batch molar composition of the reaction mixture for the synthesis of zeolite ZSM-5 is expressed as, 1.0Al2O3/100SiO2/xNa2O/4000H2O/yNa2SO4, where the values x and y are changed to adjust the batch alkalinity and batch content of sodium ions, respectively. The samples synthesized at different alkalinity and different sodium ion content are denoted as A/B

a. A=[Na2O/H2O]b

impurities

c. Calculated from XRF results d. Obtained from SEM observations e. Amor, abbreviation of amorphous

obtained at different alkalinities

free system [13, 22, 57, 58].

b. Determined from corresponding XRD patterns

(Adopted from Ref. [54] with permission of Publisher)

**4. Influence of sodium ions and gel ageing** 

(e.g., 0.003/0.24 for *A* = 0.003 and *B* = 0.24).

and 600 nm and aggregates of the single particles (40 – 50 % by number) having the size higher than 600 nm.

Fig. 8. Particle size distribution (by volume) of the products hydrothermally synthesized (at 438 K for 2 h) from the reaction mixtures (hydrogels) having: *A* = 0.001 (A), *A* = 0.002 (B), *A* = 0.003 (C), *A* = 0.004 (D), *A* = 0.005 (E), *A* = 0.006 (F), *A* = 0.007 (G), *A* = 0.008 (H), *A* = 0.009 (I), *A* = 0.010 (J), *A* = 0.011 (K), and *A* = 0.012 (L). Dashed curves represent the particle size distribution of silicalite-1 nanocrystals. *V*D is volume percentage of crystals having the sphere equivalent diameter *D.* (Adopted from Ref. [54] with permission of Publisher)

Besides the phase purity and particulate properties, the Si/Al ratio of final products also correlates very well with the batch alkalinity. Table 1 summarizes the influence of alkalinity on the product properties. It could be concluded that the well crystallized ZSM-5 zeolites with adjustable Si/Al ratio from 10 - 18 can be obtained in the alkalinity range of 0.006 ≤ A ≤ 0.010.


a. A=[Na2O/H2O]b

266 Advances in Crystallization Processes

and 600 nm and aggregates of the single particles (40 – 50 % by number) having the size

Fig. 8. Particle size distribution (by volume) of the products hydrothermally synthesized (at 438 K for 2 h) from the reaction mixtures (hydrogels) having: *A* = 0.001 (A), *A* = 0.002 (B), *A* = 0.003 (C), *A* = 0.004 (D), *A* = 0.005 (E), *A* = 0.006 (F), *A* = 0.007 (G), *A* = 0.008 (H), *A* = 0.009 (I), *A* = 0.010 (J), *A* = 0.011 (K), and *A* = 0.012 (L). Dashed curves represent the particle size distribution of silicalite-1 nanocrystals. *V*D is volume percentage of crystals having the sphere equivalent diameter *D.* (Adopted from Ref. [54] with permission

Besides the phase purity and particulate properties, the Si/Al ratio of final products also correlates very well with the batch alkalinity. Table 1 summarizes the influence of alkalinity on the product properties. It could be concluded that the well crystallized ZSM-5 zeolites with adjustable Si/Al ratio from 10 - 18 can be obtained in the alkalinity range of 0.006 ≤ A ≤ 0.010.

higher than 600 nm.

of Publisher)

b. Determined from corresponding XRD patterns

c. Calculated from XRF results

d. Obtained from SEM observations

e. Amor, abbreviation of amorphous

f. Exact information cannot be provided because of the existence of large portion of amorphous impurities

(Adopted from Ref. [54] with permission of Publisher)

Table 1. Summary of structural, chemical and morphological properties of the products obtained at different alkalinities
