**1. Introduction**

258 Advances in Crystallization Processes

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In recent years, the threat of the potential shortage of oil becomes one of the difficult problems which not only influences the quality of human life but also triggers the regional conflicts or wars. Thus, the oil management becomes the hottest topic in the contemporary economical and political world. To well solve that problem, there are two different approaches, 1) to discover/develop the alternative energy sources such as: bio-energy (biogas, biofuel), sunlight energy (or power plants), energy of wind (or power plants), nuclear energy (or power plants), etc. and 2) to increase the efficiency of crude oil (fossil fuels) processing and quality of final product. Although the first one seems interesting and ambitious, the existing drawbacks such as the low (but increasing) efficiency of sunlight transformation and the potential risk of the leakage of radioactive materials from nuclear facilities, become the great obstacle for the fast and promising development of these types of 'New plants Energetic Strategy'. Compare with the first approach, the latter seems more mild, reliable and realizable. To achieve such goal, the increase of the efficiency of catalysts is the key. Most frequently, different types of zeolites are used as catalysts for crude oil processing – among them, zeolite ZSM-5 has most expressive role.

Zeolite ZSM-5, as a member of the family of pentasil zeolites, has aroused tremendous interest after its first discovery by the research group of Mobile Company in the year 1972 [1]. With its adjustable framework Al content (from 0 to about 8Al per unit cell), two dimensional micropore channels (0.55 nm × 0.54 nm; Fig. 1a), sinusoidal pore geometry along c axis (Fig.1b) and easy insertion of hetero-T atoms, this material plays an important role in many of crucial catalytic processes such as hydro-cracking, de-waxing, alkylation, etc., [2-5] as well as in separation of organic compounds with different sizes and shapes [6]. In the case when zeolite ZSM-5 was used as catalyst, most of reactions are 'diffusioncontrolled' [7]. This means that the product distribution largely depend on the nature and location of active sites in the crystalline framework of catalyst. Thus, the increase of the

<sup>\*</sup> Corresponding Author

Crystallization of Sub-Micrometer Sized ZSM-5 Zeolites in SDA-Free Systems 261

(SDA/SiO2 ratio normally exceeds 0.2) and expensive silica sources (normally TEOS), which

Thus, an expected ideal approach for the synthesis of small sized ZSM-5 zeolites is a compromise between heterogeneous and homogeneous crystallization systems. However, even in the case of successful compromise between heterogeneous and homogeneous crystallization systems, the problem of use of SDAs is still persisting [27, 32-36]. On the other hand, many attempts of the synthesis of zeolite ZSM-5 without organic templates [6,25,27,34-49] were encountered with another group of problems such as that zeolite ZSM-5 can be synthesized only in narrow ranges of SiO2/Al2O2 (∼ 40-70) and Na2O/SiO2 (∼ 0.13-0.20) [36], formation of impurity phases such as un-reacted amorphous solids [47], quartz [41], mordenite [42] and analcime [44], uncontrollable crystal size [25], long crystallization time [40] and low yield [40]. Most of the mentioned problems can, however, be overcome by addition of small amount of seed crystals (ZSM-5, silicalite-1) in the TPA+-free reaction mixture [6, 27, 35, 38, 40, 45, 50]. Seed induction synthesis is a well developed strategy which could not only shorten the duration of synthesis, but also control the product properties [51]; addition of seed crystals results in the formation of zeolite ZSM-5 with high degree of crystallinity and a narrow size distribution at short synthesis times [38,40]. Such method has been used for the synthesis of zeolites with various framework topologies [52]. Recently, small sized zeolites were obtained

fastly, using this approach [53]. This method, although old, is still under developing.

for their further industrial-scale production and use in various applications.

**2. Controllable synthesis of sub-micrometer sized ZSM-5 zeolites** 

crystals as can be seen in the corresponding SEM images (Fig. 4).

from clear solution and used as seeds for the further growth of ZSM-5 nanocrystals.

Taking into consideration the background of ZSM-5 synthesis conditions, and accompanying this with the basic knowledge of seed induced synthesis, we have developed a seed surface crystallization (SSC) approach, in which the sub-micrometer sized ZSM-5 zeolites can be obtained in a controllable manner [50, 54, 55]. More importantly, the growth of crystalline end products is achieved in a heterogeneous, SDA-free crystallization system, which is good basis

In this chapter, SDA-free, SSC approach for the crystallization of sub-micrometer sized ZSM-5 zeolites including the influence of various synthesis parameters on the product properties and the crystallization mechanism will be discussed in detail. The relevant content is divided into five parts: (1) Controllable synthesis, (2) Influence of batch alkalinity, (3) Influence of sodium ions and gel ageing, (4) Crystallization mechanism, and (5)

Batch oxide molar chemical composition of the reaction mixture (hydrogel) for the synthesis was 1.0 Al2O3/100 SiO2/28 Na2O/4000 H2O. A series of silicalite-1 nanocrystals having different mean diameters (90, 180, 220, 260 and 690 nm; Fig. 2) were prepared by synthesis

Fig. 3 shows that the well crystalline ZSM-5 crystals are obtained after 2 h of hydrothermal treatment at 483K with addition of 4 wt.% of seeds (with respect to the total amount of silica in the reaction mixture). The crystal size of product increases with increasing size of seed

On the other hand, using different amount of the same seed crystals (260 nm in this case), the crystal size of the product decreases with increasing amount of added seed crystals (Fig. 5).

inevitably increases the production cost [31].

Modeling approach.

catalyst reactivity and thus, its efficiency can be achieved by increase of easily accessible active sites and by decrease of the length of diffusion path of reactants/intermediates/products and eliminating the probability of the occurrence of cokeformation side-reactions. Upon decreasing the crystal size, the diffusion paths of the reactant and product molecules inside the pores becomes shorter, and thus this can result in the reduction or elimination of undesired diffusion limitations of the reaction rate [8-10].

Fig. 1. The topological view of ZSM-5 crystals, (a) skeletal diagram of the (100) face and (b) channel structure.

To synthesize small-sized ZSM-5 zeolites and achieve the precise control of crystal size, the synthesis mechanism of ZSM-5 and the critical processes occurring during its crystallization should be understood thoroughly. From the well documented literature data, it is evident that, besides the framework constituents such as silica and alumina in different forms [1,11- 18] and different alkaline bearing cations [13,19-24], the presence of organic structure directing agent (SDA) is vital for the synthesis of zeolite ZSM-5 [1,11-13,15,18,21,25] and other high-silica types of zeolites.

Zeolite ZSM-5 is conventionally synthesized by hydrothermal treatment of the reactive gel containing aluminosilicate as well as the tetrapropylammonium ions (TPA+) as structure directing agent [1,11-13,15,18,25-27]. For the sake of industrial scale production, the synthesis of ZSM-5 zeolites could also be performed by using cheaper silica sources (fumed or precipitated silica) with reduced amount of SDAs. In such cases, the starting reaction mixture appears in the form of dense gel and the gelation phenomena can be observed at very short period of the mixing of the reactants; such system is also denoted as heterogeneous crystallization system [28,29]. However, the size of the products obtained from this route is normally in the range of several micrometers to tens micrometers, and the crystal size seems very difficult to be adjusted.

Using the known gel compositions for the synthesis of ZSM-5 zeolites, recently, a method called 'clear solution' synthesis has been proposed for the successful preparation of small sized ZSM-5 zeolites [30]. Since the 'clear solution' approach generally uses silica species in molecular form and thus a transparent solution appears at the initial stage of crystallization, such synthesis system is also called homogeneous crystallization system [28,29]. Although the crystal size could be facilely controlled by this method, such approach has obvious drawbacks such as low product yield (less than 5 wt. %), use of large amount of both SDAs

catalyst reactivity and thus, its efficiency can be achieved by increase of easily accessible active sites and by decrease of the length of diffusion path of reactants/intermediates/products and eliminating the probability of the occurrence of cokeformation side-reactions. Upon decreasing the crystal size, the diffusion paths of the reactant and product molecules inside the pores becomes shorter, and thus this can result in the reduction or elimination of undesired diffusion limitations of the reaction rate [8-10].

Fig. 1. The topological view of ZSM-5 crystals, (a) skeletal diagram of the (100) face and

To synthesize small-sized ZSM-5 zeolites and achieve the precise control of crystal size, the synthesis mechanism of ZSM-5 and the critical processes occurring during its crystallization should be understood thoroughly. From the well documented literature data, it is evident that, besides the framework constituents such as silica and alumina in different forms [1,11- 18] and different alkaline bearing cations [13,19-24], the presence of organic structure directing agent (SDA) is vital for the synthesis of zeolite ZSM-5 [1,11-13,15,18,21,25] and

Zeolite ZSM-5 is conventionally synthesized by hydrothermal treatment of the reactive gel containing aluminosilicate as well as the tetrapropylammonium ions (TPA+) as structure directing agent [1,11-13,15,18,25-27]. For the sake of industrial scale production, the synthesis of ZSM-5 zeolites could also be performed by using cheaper silica sources (fumed or precipitated silica) with reduced amount of SDAs. In such cases, the starting reaction mixture appears in the form of dense gel and the gelation phenomena can be observed at very short period of the mixing of the reactants; such system is also denoted as heterogeneous crystallization system [28,29]. However, the size of the products obtained from this route is normally in the range of several micrometers to tens micrometers, and the

Using the known gel compositions for the synthesis of ZSM-5 zeolites, recently, a method called 'clear solution' synthesis has been proposed for the successful preparation of small sized ZSM-5 zeolites [30]. Since the 'clear solution' approach generally uses silica species in molecular form and thus a transparent solution appears at the initial stage of crystallization, such synthesis system is also called homogeneous crystallization system [28,29]. Although the crystal size could be facilely controlled by this method, such approach has obvious drawbacks such as low product yield (less than 5 wt. %), use of large amount of both SDAs

(b) channel structure.

other high-silica types of zeolites.

crystal size seems very difficult to be adjusted.

(SDA/SiO2 ratio normally exceeds 0.2) and expensive silica sources (normally TEOS), which inevitably increases the production cost [31].

Thus, an expected ideal approach for the synthesis of small sized ZSM-5 zeolites is a compromise between heterogeneous and homogeneous crystallization systems. However, even in the case of successful compromise between heterogeneous and homogeneous crystallization systems, the problem of use of SDAs is still persisting [27, 32-36]. On the other hand, many attempts of the synthesis of zeolite ZSM-5 without organic templates [6,25,27,34-49] were encountered with another group of problems such as that zeolite ZSM-5 can be synthesized only in narrow ranges of SiO2/Al2O2 (∼ 40-70) and Na2O/SiO2 (∼ 0.13-0.20) [36], formation of impurity phases such as un-reacted amorphous solids [47], quartz [41], mordenite [42] and analcime [44], uncontrollable crystal size [25], long crystallization time [40] and low yield [40].

Most of the mentioned problems can, however, be overcome by addition of small amount of seed crystals (ZSM-5, silicalite-1) in the TPA+-free reaction mixture [6, 27, 35, 38, 40, 45, 50]. Seed induction synthesis is a well developed strategy which could not only shorten the duration of synthesis, but also control the product properties [51]; addition of seed crystals results in the formation of zeolite ZSM-5 with high degree of crystallinity and a narrow size distribution at short synthesis times [38,40]. Such method has been used for the synthesis of zeolites with various framework topologies [52]. Recently, small sized zeolites were obtained fastly, using this approach [53]. This method, although old, is still under developing.

Taking into consideration the background of ZSM-5 synthesis conditions, and accompanying this with the basic knowledge of seed induced synthesis, we have developed a seed surface crystallization (SSC) approach, in which the sub-micrometer sized ZSM-5 zeolites can be obtained in a controllable manner [50, 54, 55]. More importantly, the growth of crystalline end products is achieved in a heterogeneous, SDA-free crystallization system, which is good basis for their further industrial-scale production and use in various applications.

In this chapter, SDA-free, SSC approach for the crystallization of sub-micrometer sized ZSM-5 zeolites including the influence of various synthesis parameters on the product properties and the crystallization mechanism will be discussed in detail. The relevant content is divided into five parts: (1) Controllable synthesis, (2) Influence of batch alkalinity, (3) Influence of sodium ions and gel ageing, (4) Crystallization mechanism, and (5) Modeling approach.
