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

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

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

Crystallization of Sub-Micrometer Sized ZSM-5 Zeolites in SDA-Free Systems 269

morphology of the final product, as can be seen comparing of the samples prepared without (Figs. 9A, 9B, 9C and 9D) and with addition of sodium sulphate (Figs. 9A', 9B', 9C' and 9D')**.**  Also, the addition of sodium sulphate in the reaction mixture does not affect the phase composition (crystallinity) of products as revealed by XRD analysis of the product samples. Knowing the templating role of sodium ions in the TPA-free synthesis of MFI-type zeolites [13, 22, 56-58], these findings indicate that the rate-determining factor is concentration of "free" low-molecular weight silicate species, in the short synthesis duration of this seed-

Fig. 10. Particle size distributions by number (A, B, C and D) and by volume

(*B* = [Na+/SiO2]b) of Na ions 0.08 (dashed curves) 0.64 (solid curves). Alkalinity

(A', B', C' and D') of the products obtained from reaction mixtures having two batch content

(*A* = [Na2O/H2O]b) were: 0.001 for A and A', 0.002 for B and B', 0.002 for B and B', 0.003 for C and C', 0.003 for C and C', 0.004 for D and D'. *N*D is number percentage and *V*D is volume percentage of crystals having the sphere equivalent diameter *D*. (Adopted from Ref. [55]

Since the generation of these species is mainly caused by hydrolysis of silica precursors

[54], it is evident that the alkalinity of system, *A*, determines the rate of crystallization and not the concentration of "free" Na+ ions. In the other words, when A < 0.004, fully crystalline product cannot be obtained for *t*<sup>c</sup> ≤ 2 h regardless to the content of sodium ions in

ions and thus, on the batch alkalinity, *A*

induction system

with permission of Publisher)

the reaction mixture [54,55].

which depend on the concentration of "free" OH-

Fig. 9. SEM images of the products obtained by hydrothermal treatment (at 483 K for 2 h) of the reaction mixtures 0.001/0.08 (A), 0.001/0.64 (A'), 0.002/0.16 (B), 0.002/0.64 (B'), 0.003/0.24 (C), 0.003/0.64 (C'), 0.004/0.32 (D) and 0.004/0.64 (D'). The scale bars are of 1 μm. (Adopted and reproduced from Ref. [55] with permission of Publisher)

Fig. 9 shows the SEM images of the product obtained by hydrothermal treatment (at 483K for 2h) of the reaction mixtures having low alkalinities (*A* = 0.001 - 0.004) and different batch contents of sodium ions (*B* = 0.08 and 0.64). It can be observed that the increase of the batch content, *B*, of sodium ions at the same batch alkalinity, *A*, does not significantly influence the

Fig. 9. SEM images of the products obtained by hydrothermal treatment (at 483 K for 2 h) of the reaction mixtures 0.001/0.08 (A), 0.001/0.64 (A'), 0.002/0.16 (B), 0.002/0.64 (B'), 0.003/0.24 (C), 0.003/0.64 (C'), 0.004/0.32 (D) and 0.004/0.64 (D'). The scale bars are of

Fig. 9 shows the SEM images of the product obtained by hydrothermal treatment (at 483K for 2h) of the reaction mixtures having low alkalinities (*A* = 0.001 - 0.004) and different batch contents of sodium ions (*B* = 0.08 and 0.64). It can be observed that the increase of the batch content, *B*, of sodium ions at the same batch alkalinity, *A*, does not significantly influence the

1 μm. (Adopted and reproduced from Ref. [55] with permission of Publisher)

morphology of the final product, as can be seen comparing of the samples prepared without (Figs. 9A, 9B, 9C and 9D) and with addition of sodium sulphate (Figs. 9A', 9B', 9C' and 9D')**.**  Also, the addition of sodium sulphate in the reaction mixture does not affect the phase composition (crystallinity) of products as revealed by XRD analysis of the product samples.

Knowing the templating role of sodium ions in the TPA-free synthesis of MFI-type zeolites [13, 22, 56-58], these findings indicate that the rate-determining factor is concentration of "free" low-molecular weight silicate species, in the short synthesis duration of this seedinduction system

Fig. 10. Particle size distributions by number (A, B, C and D) and by volume (A', B', C' and D') of the products obtained from reaction mixtures having two batch content (*B* = [Na+/SiO2]b) of Na ions 0.08 (dashed curves) 0.64 (solid curves). Alkalinity (*A* = [Na2O/H2O]b) were: 0.001 for A and A', 0.002 for B and B', 0.002 for B and B', 0.003 for C and C', 0.003 for C and C', 0.004 for D and D'. *N*D is number percentage and *V*D is volume percentage of crystals having the sphere equivalent diameter *D*. (Adopted from Ref. [55] with permission of Publisher)

Since the generation of these species is mainly caused by hydrolysis of silica precursors which depend on the concentration of "free" OH- ions and thus, on the batch alkalinity, *A* [54], it is evident that the alkalinity of system, *A*, determines the rate of crystallization and not the concentration of "free" Na+ ions. In the other words, when A < 0.004, fully crystalline product cannot be obtained for *t*<sup>c</sup> ≤ 2 h regardless to the content of sodium ions in the reaction mixture [54,55].

Crystallization of Sub-Micrometer Sized ZSM-5 Zeolites in SDA-Free Systems 271

Fig. 12. Particle size distributions by volume of the products obtained by hydrothermal treatment (at 483 K for 2 h) of the reaction mixtures 0.007/0.56 (A) and 0.007/1.0 (B).

From the above results, it could be concluded that the addition of excess amount of sodium ions into the crystallization system has apparent effect on the particulate properties of the product. At low batch alkalinity, the additional sodium ions causes de-aggregation of the final products, rendering the particles with more uniform size distributions. At high batch alkalinity, the excess amount of sodium ions triggers the surface condensation reactions on the crystalline end products. However, the crystallization rate is not enhanced by the increase of batch sodium ion content, indicating that the determining factor of crystallization of ZSM-5 zeolites in SDA-free system is concentration of low molecular weight silicate

Fig. 13. SEM images of the products obtained by hydrothermal treatment (at 483 K for 2 h) of the reaction mixtures aged at room temperature for 0 (A), 3 (B) and 48 h (C) in the

presence of silicalite-1 seed crystals. The scale bars are of 1 μm. (Adopted from Ref. [55] with

(Adopted from Ref. [55] with permission of Publisher)

species, determined by batch alkalinity.

permission of Publisher)

Fig. 10 shows that the fraction of the small single particles (ZSM-5 crystals formed by growth of silicalite-1 seed crystals) having the size 400 – 600 nm increases with increasing alkalinity, *A*, of the reaction mixture and that, for a given batch alkalinity, the fraction of the single 400 – 600 nm particles increases with increasing value of *B*.

According to the principals of aggregation of zeolite particles, the tendency to aggregation decreases with the increase of the (negative) crystal surface charge (repulsion effect), and increases with the increase of the concentration of the "compensating" Na+ ions [55]. It is evident that at the simultaneous increase of OH ions and Na+ ions with increasing alkalinity, *A*, the repulsive force prevails the "compensating" effect of Na+ ions. In this context, addition of sodium sulfate additionally increases the negative charge of the crystal surface by oxy-anion effect of SO4 2- ions [63] and, at the same time, additionally reduces the "compensating" effect of Na+ ions. The result is that the aggregation is considerably reduced in the presence of sodium sulphate.

When the batch alkalinity further increases to *A* = 0.007, the perfect small-sized ZSM-5 crystals can be obtained, as described in previous section. An increase of the batch amount of sodium ions does not influence the phase purity of products as it was revealed by XRD analysis of the samples. However, the surface of the crystals become rough as it can be observed in the corresponding SEM images (Fig. 11). This phenomenon could be attributed to the effect of sodium ions on the cross-linking and polymerization of the silicate species in solution [64]; the species formed by cross-linking and polymerization are further deposited onto the crystalline surface of the final crystals, causing the roughing of the surface morphology of the product. This assumption is further supported by the increase of Si/Al ratio of the crystalline end product from 15 for [Na+/SiO2]b = 0.56 to 20 for [Na+/SiO2]b = 1.0 and simultaneous decrease of Si concentration in the liquid phase of the reaction mixture from 20.23 mg/ml for [Na+/SiO2]b = 0.56 to 14.00 mg/ml for [Na+/SiO2]b = 1.0, accompanying with the increase of the solid recovery yield from 42% to 50%. The surface roughing phenomena caused by the deposition of active species from liquid to solid phase (crystals) can also be evidenced by the increase of the average size of the crystalline end products (see PSD curves in Fig. 12). More interestingly, the particle size distribution also becomes narrower with the increased amount of sodium ions. This indicates that the addition of sodium sulphate into reaction mixture also prevents aggregation of the ZSM-5 crystals formed during hydrothermal treatment as was already shown in the cases of low batch alkalinities.

Fig. 11. SEM images of the products obtained by hydrothermal treatment (at 483 K for 2 h) of the reaction mixtures 0.007/0.56 (A) and 0.007/1.0 (B). The scale bars are of 1 μm. (Adopted from Ref. [55] with permission of Publisher)

Fig. 10 shows that the fraction of the small single particles (ZSM-5 crystals formed by growth of silicalite-1 seed crystals) having the size 400 – 600 nm increases with increasing alkalinity, *A*, of the reaction mixture and that, for a given batch alkalinity, the fraction of the

According to the principals of aggregation of zeolite particles, the tendency to aggregation decreases with the increase of the (negative) crystal surface charge (repulsion effect), and increases with the increase of the concentration of the "compensating" Na+ ions [55]. It is evident that at the simultaneous increase of OH- ions and Na+ ions with increasing alkalinity, *A*, the repulsive force prevails the "compensating" effect of Na+ ions. In this context, addition of sodium sulfate additionally increases the negative charge of the crystal

"compensating" effect of Na+ ions. The result is that the aggregation is considerably reduced

When the batch alkalinity further increases to *A* = 0.007, the perfect small-sized ZSM-5 crystals can be obtained, as described in previous section. An increase of the batch amount of sodium ions does not influence the phase purity of products as it was revealed by XRD analysis of the samples. However, the surface of the crystals become rough as it can be observed in the corresponding SEM images (Fig. 11). This phenomenon could be attributed to the effect of sodium ions on the cross-linking and polymerization of the silicate species in solution [64]; the species formed by cross-linking and polymerization are further deposited onto the crystalline surface of the final crystals, causing the roughing of the surface morphology of the product. This assumption is further supported by the increase of Si/Al ratio of the crystalline end product from 15 for [Na+/SiO2]b = 0.56 to 20 for [Na+/SiO2]b = 1.0 and simultaneous decrease of Si concentration in the liquid phase of the reaction mixture from 20.23 mg/ml for [Na+/SiO2]b = 0.56 to 14.00 mg/ml for [Na+/SiO2]b = 1.0, accompanying with the increase of the solid recovery yield from 42% to 50%. The surface roughing phenomena caused by the deposition of active species from liquid to solid phase (crystals) can also be evidenced by the increase of the average size of the crystalline end products (see PSD curves in Fig. 12). More interestingly, the particle size distribution also becomes narrower with the increased amount of sodium ions. This indicates that the addition of sodium sulphate into reaction mixture also prevents aggregation of the ZSM-5 crystals formed during hydrothermal treatment as was already shown in the cases of low

Fig. 11. SEM images of the products obtained by hydrothermal treatment (at 483 K for 2 h) of the reaction mixtures 0.007/0.56 (A) and 0.007/1.0 (B). The scale bars are of 1 μm.

(Adopted from Ref. [55] with permission of Publisher)

2- ions [63] and, at the same time, additionally reduces the

single 400 – 600 nm particles increases with increasing value of *B*.

surface by oxy-anion effect of SO4

in the presence of sodium sulphate.

batch alkalinities.

Fig. 12. Particle size distributions by volume of the products obtained by hydrothermal treatment (at 483 K for 2 h) of the reaction mixtures 0.007/0.56 (A) and 0.007/1.0 (B). (Adopted from Ref. [55] with permission of Publisher)

From the above results, it could be concluded that the addition of excess amount of sodium ions into the crystallization system has apparent effect on the particulate properties of the product. At low batch alkalinity, the additional sodium ions causes de-aggregation of the final products, rendering the particles with more uniform size distributions. At high batch alkalinity, the excess amount of sodium ions triggers the surface condensation reactions on the crystalline end products. However, the crystallization rate is not enhanced by the increase of batch sodium ion content, indicating that the determining factor of crystallization of ZSM-5 zeolites in SDA-free system is concentration of low molecular weight silicate species, determined by batch alkalinity.

Fig. 13. SEM images of the products obtained by hydrothermal treatment (at 483 K for 2 h) of the reaction mixtures aged at room temperature for 0 (A), 3 (B) and 48 h (C) in the presence of silicalite-1 seed crystals. The scale bars are of 1 μm. (Adopted from Ref. [55] with permission of Publisher)

Crystallization of Sub-Micrometer Sized ZSM-5 Zeolites in SDA-Free Systems 273

Fig. 15. SEM images of the products obtained by hydrothermal treatment (at 483 K for 2 h) of the reaction mixture (A = 0.008; [Na+/SiO2]b = 0.64) aged at room temperature for 5 (A), 20 (B), 48 (C) and 120 h (D) before addition of seeds and heating. The scale bars are of 1 μm.

Similar like in the cases of the ageing in the presence of seeds, the results from the ageing of the seed-free reaction mixture also show the same trend (Figs. 15 and 16). The crystallinity of the product does not markedly change with ageing. On the other hand, the PSD curves in Fig. 16 show a gradual decrease of the fraction of large-size "particles" (10 – 500 µm) and simultaneous increase of the fraction of discrete particles (crystals) having the size in the range from 0.45 – 10 µm, with the increase of the ageing duration. This undoubtedly shows that the ageing of the seed-free reaction mixture prevents the agglomeration of growing

The independence of the properties of crystalline end products on the duration of room temperature ageing of reaction mixtures indicates that the formation of precursors for the subsequent growth of ZSM-5 zeolites does not occur during the room-temperature ageing in the current system and, at the same time, this confirms the hypothesis that, under such conditions, the active growth precursors can only be formed at reaction temperature. Since in SDA-free crystallization systems, the formation of new nuclei is greatly suppressed in the presence of seed crystals [51], the size of discrete crystals in the product depend only on the

(Adopted from Ref. [55] with permission of Publisher)

amount and size of the seeds present in the reaction mixture.

crystals of zeolite ZSM-5.

In addition, the influence of room temperature ageing of the reaction mixture on the product properties has also been investigated. Since in the investigated crystallization system, the seed crystals are added as one of the gradients of the gel precursors, the relevant studies are separated as ageing in the presence or absence of seed crystals.

Figs. 13 and 14 show the SEM images and PSD curves of the ZSM-5 zeolites crystallized from the reaction mixtures aged for different times in the presence of silicalite-1 seed crystals. Although the ageing does not change morphology of the crystals, the particle size distribution becomes narrower with increasing the ageing duration. Moreover, it can also be observed that the particle size of the product slightly decreases with ageing.

Fig. 14. Particle size distributions by volume of the products obtained by hydrothermal treatment (at 483 K for 2 h) of the reaction mixtures having aged at room temperature for 0 (A), 3 (B) and 48 h (C) in the presence of silicalite-1 seed crystals. (Adopted from Ref. [55] with permission of Publisher)

In addition, the influence of room temperature ageing of the reaction mixture on the product properties has also been investigated. Since in the investigated crystallization system, the seed crystals are added as one of the gradients of the gel precursors, the relevant studies are

Figs. 13 and 14 show the SEM images and PSD curves of the ZSM-5 zeolites crystallized from the reaction mixtures aged for different times in the presence of silicalite-1 seed crystals. Although the ageing does not change morphology of the crystals, the particle size distribution becomes narrower with increasing the ageing duration. Moreover, it can also be

Fig. 14. Particle size distributions by volume of the products obtained by hydrothermal treatment (at 483 K for 2 h) of the reaction mixtures having aged at room temperature for 0 (A), 3 (B) and 48 h (C) in the presence of silicalite-1 seed crystals. (Adopted from Ref. [55]

with permission of Publisher)

separated as ageing in the presence or absence of seed crystals.

observed that the particle size of the product slightly decreases with ageing.

Fig. 15. SEM images of the products obtained by hydrothermal treatment (at 483 K for 2 h) of the reaction mixture (A = 0.008; [Na+/SiO2]b = 0.64) aged at room temperature for 5 (A), 20 (B), 48 (C) and 120 h (D) before addition of seeds and heating. The scale bars are of 1 μm. (Adopted from Ref. [55] with permission of Publisher)

Similar like in the cases of the ageing in the presence of seeds, the results from the ageing of the seed-free reaction mixture also show the same trend (Figs. 15 and 16). The crystallinity of the product does not markedly change with ageing. On the other hand, the PSD curves in Fig. 16 show a gradual decrease of the fraction of large-size "particles" (10 – 500 µm) and simultaneous increase of the fraction of discrete particles (crystals) having the size in the range from 0.45 – 10 µm, with the increase of the ageing duration. This undoubtedly shows that the ageing of the seed-free reaction mixture prevents the agglomeration of growing crystals of zeolite ZSM-5.

The independence of the properties of crystalline end products on the duration of room temperature ageing of reaction mixtures indicates that the formation of precursors for the subsequent growth of ZSM-5 zeolites does not occur during the room-temperature ageing in the current system and, at the same time, this confirms the hypothesis that, under such conditions, the active growth precursors can only be formed at reaction temperature. Since in SDA-free crystallization systems, the formation of new nuclei is greatly suppressed in the presence of seed crystals [51], the size of discrete crystals in the product depend only on the amount and size of the seeds present in the reaction mixture.

Crystallization of Sub-Micrometer Sized ZSM-5 Zeolites in SDA-Free Systems 275

Fig. 17. Influence of the room-temperature ageing of the seed-free reaction mixture (A = 0.008; [Na+/SiO2]b = 0.64) on: (A) changes in the concentrations of silicon () and aluminum () in the liquid phase. (Adopted and reproduced from Ref. [55] with

morphology Seed amountBatch

(structural property) - - + - -

(size and distribution) + + + + +

(Si/Al in framework) - + + +/- -

a. The influences are expressed as pronounced influence (+), less influence (-), and influence depends on

Table 2. Summary of the influences of synthetic parameters on the properties of ZSM-5

alkalinity

Batch sodium ion content

Gel ageing effect

With the above studies, the influence of various parameters on the three most important properties (phase purity, particulate properties and chemical composition) of ZSM-5 zeolites can be clearly concluded in Table 2. From this summary, the most pronounced influence is attributed to the batch alkalinity, which changes all the relevant properties of the crystallineend product. In the large-scale synthesis of ZSM-5 zeolites for application, the batch alkalinity should be strictly controlled to obtain the qualified product. On the other hand, among the mentioned three properties of ZSM-5 zeolites, the particulate properties are most sensitive to the change of the environment of crystallization. Thus, the synthesis of ZSM-5 zeolites with controllable uniform crystal sizes is always a hot, attractive and enduring topic

permission of Publisher)

in the field of zeolites.

Particulate property

Chemical composition

the detailed condition

product. *a*

Phase purity

**5. Crystallization mechanism** 

Seed size &

Fig. 16. PSDs by volume of the products obtained by hydrothermal treatment (at 483 K for 2 h) of the reaction mixture (A = 0.008; [Na+/SiO2]b = 0.64) aged at room temperature for 5 (A), 20 (B), 48 (C) and 120 h (D) before addition of seeds and heating. (Adopted from Ref. [55] with permission of Publisher)

On the other hand, the influence of the hydrogel ageing on the PSDs of the crystalline end products is probably connected with the change (increase) of the concentration of Si (most probably in the form of low-molecular weight silicate species) in the liquid phase of the reaction mixture with prolonging ageing duration (Fig. 17). Although the increase of the concentration of Si in the liquid phase during ageing (Fig. 17) does not cause the formation of the growth precursor species at the ageing (room) temperature, it certainly facilitates the formation of the growth precursor species during heating of the reaction mixture, i.e., higher starting concentration of low-molecular weight silicate species causes higher rate of formation of the growth precursor species and thus, higher rate of growth of zeolite ZSM-5 on the silicalite-1 crystals. From this reason, particle (crystal) size at given crystallization time increases with increasing time of hydrogel ageing (Fig. 16). Since, on the other hand (i) crystal growth and agglomeration take place simultaneously and (ii), the rate of agglomeration increases with decreasing particle size [65], tendency of agglomeration decreases with increasing time of hydrogel ageing (Fig. 16).

Fig. 17. Influence of the room-temperature ageing of the seed-free reaction mixture (A = 0.008; [Na+/SiO2]b = 0.64) on: (A) changes in the concentrations of silicon () and aluminum () in the liquid phase. (Adopted and reproduced from Ref. [55] with permission of Publisher)
