**3. Improvement of the diffusion property**

## **3.1 Reducing of particle size in TPABr system**

Most reactions catalyzed by zeolites occur in their channels. A short channel means a short diffusion pathway for reactants from bulk to active centers (such as tetrahedrally coordinated Ti), therefore reducing the particle size benefits the diffusion. We have mentioned in Section 2.1 that the particle size of TS-1 obtained in the TPABr hydrothermal system is often at the micron scale, which is disadvantageous for diffusion. Hence, we tried to control the particle size of TS-1 in the TPABr system by adding different seeds. First, we used the mother liquor of nanosized TS-1 as the seed [27]. The synthesis process is illustrated in **Figure 6**. The mother liquor was prepared by crystallizing the synthesis gel at 443 K for 48 h, according to prior work [26]. The size of the obtained seed is ~100 nm. When using powdery TS-1 as the seed, microsized TS-1 was obtained, the size of which was 2 × 1 × 0.5 μm. However, when the seed was changed to the mother liquor, the size decreased significantly to 600 × 400 × 250 nm, so we called it small-crystal TS-1. Its catalytic performance was evaluated in the epoxidation of propene and hydroxylation of phenol. The conversion of H2O2 and selectivity of

**Figure 6.** *Synthesis process of TS-1 in TPABr system.*

PO on small-crystal TS-1 reached 92 and 98%, respectively. In the hydroxylation of phenol, using small-crystal TS-1 as a catalyst also resulted in a higher conversion of phenol.

The synthesis conditions were then systematically studied, including purification methods for small-crystal TS-1, the Si/Ti molar ratio in TS-1, the amount of the seed, and the crystallization period [34]. It was found that the size of TS-1 was not significantly affected by the synthesis conditions except for the amount of seed. The highest catalytic performance of the propene epoxidation was obtained when the TS-1 was purified three times by precipitation; the *n*(Si/ Ti) was 50, the weight ratio of seed/SiO2 was 0.06, and the crystallization time was 48 h.

After that, the seed was changed to the mother liquor of nanosized silicalite-1, the size of which was ~80 nm [35]. The particle size can be adjusted from 1200 to 200 nm by varying the seed amount from 0.05 to 12 wt%. The relationship between particle size and the content of seed is shown in **Figure 7**. The data fits well to a power function, with the fit equation of:

$$\text{Crystal size} = 468.7 \text{ (content of seed)}^{-0.3158} \tag{1}$$

**151**

*Coordination States and Catalytic Performance of Ti in Titanium Silicalite-1*

octahedrally coordinated Ti and anatase TiO2. The content of tetrahedrally coordi-

*Particle size of TS-1 plotted against the weight percent of seed using in synthesis. (data points represent experimental data, and the continuous curve represents a power law fit to the data with equation given on* 

In addition to reducing the particle size, synthesis of hollow materials can also enhance diffusion properties. Treating with TPAOH solution is one of the most commonly used methods for generating hollow spaces in TS-1. However, this method leads to the transformation of tetrahedrally coordinated Ti to extraframework Ti, which is harmful for the catalytic performance. Therefore, we provided a method for synthesizing a hollow core-shell material to prevent the generation of extra-framework Ti [36]. The hollow silicalite-1@titanium silicalite-1 (H-S-1@TS-1) core-shell material was synthesized in a TPAOH hydrothermal system with hollow silicalite-1 serving as the core (see **Figure 8**). The very small TS-1 particles grow along the external surface of hollow silicalite-1, thus generating hollow material. Since the hollow structure was given by silicalite-1, the tetrahedrally coordinated Ti in H-S-1@TS-1 was not converted to other coordination states. Thus, the extra-framework Ti was absent in H-S-1@TS-1. Due to the synergy function of pure tetrahedrally coordinated Ti species, higher Ti content on external surface, and enhanced diffusion properties, H-S-1@TS-1 showed better propene epoxidation activity (TOF = 13.61 mol/(mol∙h)) than traditional microporous TS-1 (TOF = 8.08 mol/(mol∙h)) and that posttreated with TPAOH solution

In the posttreatment with TPAOH solution, the "SiO4" in the crystals is dissolved and recrystallized on the external surface. When TS-1 was extruded with silica as the support and the obtained extrudate was treated with TPAOH solution, the silica support would crystallize and restrain the dissolution of "SiO4" in the crystals [37]. Hence, we introduced a titanium source to the posttreatment to make it crystallize

nated Ti achieved ~1.2 wt% when the feeding *n*(Si/Ti) was 50.

**3.2 Design of hollow TS-1 materials**

**Figure 7.**

*the plot).*

(TOF = 9.75 mol/(mol∙h)).

**3.3 Insertion of Ti on the external surface**

*DOI: http://dx.doi.org/10.5772/intechopen.89864*

The degree of fitting measured by the coefficient of determination (R2 ) is 0.9968. Crystallization time plays a less important role than seed amount on the adjustment of the size. TS-1 with different crystal sizes was characterized and evaluated in the propene epoxidation. The catalytic activity and selectivity for PO are enhanced by decreasing the particle size from 1200 to 200 nm, due to gradually eliminated diffusion limitations. The seed is significant for this system, because eliminating the seed leads to poor crystallization and catalytic activity. The mechanism of the seed function was studied by simulating the transformation process of the seed in the TS-1 synthesis system. When the content of seed is lower than 1 wt%, it primarily performed as a nucleus for the growth of silicon and titanium sources. As the content increases, more seed will dissolute to secondary structural units first and then accelerate the crystallization.

Furthermore, the low pH of the TPABr system promotes the similarity of crystallization rates of silicon and titanium sources, inhibiting the generation of *Coordination States and Catalytic Performance of Ti in Titanium Silicalite-1 DOI: http://dx.doi.org/10.5772/intechopen.89864*

#### **Figure 7.**

*Stability and Applications of Coordination Compounds*

PO on small-crystal TS-1 reached 92 and 98%, respectively. In the hydroxylation of phenol, using small-crystal TS-1 as a catalyst also resulted in a higher conver-

The synthesis conditions were then systematically studied, including purification methods for small-crystal TS-1, the Si/Ti molar ratio in TS-1, the amount of the seed, and the crystallization period [34]. It was found that the size of TS-1 was not significantly affected by the synthesis conditions except for the amount of seed. The highest catalytic performance of the propene epoxidation was obtained when the TS-1 was purified three times by precipitation; the *n*(Si/ Ti) was 50, the weight ratio of seed/SiO2 was 0.06, and the crystallization time

After that, the seed was changed to the mother liquor of nanosized silicalite-1, the size of which was ~80 nm [35]. The particle size can be adjusted from 1200 to 200 nm by varying the seed amount from 0.05 to 12 wt%. The relationship between particle size and the content of seed is shown in **Figure 7**. The data fits well to a

The degree of fitting measured by the coefficient of determination (R2

0.9968. Crystallization time plays a less important role than seed amount on the adjustment of the size. TS-1 with different crystal sizes was characterized and evaluated in the propene epoxidation. The catalytic activity and selectivity for PO are enhanced by decreasing the particle size from 1200 to 200 nm, due to gradually eliminated diffusion limitations. The seed is significant for this system, because eliminating the seed leads to poor crystallization and catalytic activity. The mechanism of the seed function was studied by simulating the transformation process of the seed in the TS-1 synthesis system. When the content of seed is lower than 1 wt%, it primarily performed as a nucleus for the growth of silicon and titanium sources. As the content increases, more seed will dissolute to secondary structural

Furthermore, the low pH of the TPABr system promotes the similarity of crystallization rates of silicon and titanium sources, inhibiting the generation of

Crystal size = 468.7 (content of seed)−0.3158 (1)

) is

**150**

sion of phenol.

*Synthesis process of TS-1 in TPABr system.*

**Figure 6.**

was 48 h.

power function, with the fit equation of:

units first and then accelerate the crystallization.

*Particle size of TS-1 plotted against the weight percent of seed using in synthesis. (data points represent experimental data, and the continuous curve represents a power law fit to the data with equation given on the plot).*

octahedrally coordinated Ti and anatase TiO2. The content of tetrahedrally coordinated Ti achieved ~1.2 wt% when the feeding *n*(Si/Ti) was 50.

#### **3.2 Design of hollow TS-1 materials**

In addition to reducing the particle size, synthesis of hollow materials can also enhance diffusion properties. Treating with TPAOH solution is one of the most commonly used methods for generating hollow spaces in TS-1. However, this method leads to the transformation of tetrahedrally coordinated Ti to extraframework Ti, which is harmful for the catalytic performance. Therefore, we provided a method for synthesizing a hollow core-shell material to prevent the generation of extra-framework Ti [36]. The hollow silicalite-1@titanium silicalite-1 (H-S-1@TS-1) core-shell material was synthesized in a TPAOH hydrothermal system with hollow silicalite-1 serving as the core (see **Figure 8**). The very small TS-1 particles grow along the external surface of hollow silicalite-1, thus generating hollow material. Since the hollow structure was given by silicalite-1, the tetrahedrally coordinated Ti in H-S-1@TS-1 was not converted to other coordination states. Thus, the extra-framework Ti was absent in H-S-1@TS-1. Due to the synergy function of pure tetrahedrally coordinated Ti species, higher Ti content on external surface, and enhanced diffusion properties, H-S-1@TS-1 showed better propene epoxidation activity (TOF = 13.61 mol/(mol∙h)) than traditional microporous TS-1 (TOF = 8.08 mol/(mol∙h)) and that posttreated with TPAOH solution (TOF = 9.75 mol/(mol∙h)).

#### **3.3 Insertion of Ti on the external surface**

In the posttreatment with TPAOH solution, the "SiO4" in the crystals is dissolved and recrystallized on the external surface. When TS-1 was extruded with silica as the support and the obtained extrudate was treated with TPAOH solution, the silica support would crystallize and restrain the dissolution of "SiO4" in the crystals [37]. Hence, we introduced a titanium source to the posttreatment to make it crystallize

### **Figure 8.**

*Preparation process of H-S-1@TS-1.*


#### **Table 2.**

*Elemental composition of the TS-1 treated with different Ti contentsa .*

with silica support and insert more Ti on the external surface [38]. Different amounts of the titanium source (tetrabutyl titanate hydrolysate) were added to the TPAOH solution. **Table 2** shows the elemental compositions of the samples both in the bulk and on the external surface. The *n*(Si/Ti) in the bulk is hardly affected by the individual TPAOH treatment. The Ti content increases gradually with increasing concentration of Ti in the postsynthesis solution. However, not all Ti in the samples transforms to tetrahedrally coordinated Ti. The X-ray photoelectron spectroscopy (XPS) results show that the content of Ti on the external surface reduced after the treatment with TPAOH solution, which is probably due to the crystallization of amorphous silica occurring at the external surface of TS-1 particles and covering more Ti species. The *n*(Si/Ti) on the external surface decreases as Ti concentration in the postsynthesis solution increases, indicating that Ti can be located at the external surface under these postsynthesis conditions, and the amount of Ti is restricted by the Ti concentration and the limitation of Ti in the MFI topology. The samples were evaluated in the hydroxylation of phenol. The conversion of phenol increases to a different extent after the postsynthesis. The hollow spaces generated in the crystals

**153**

*Coordination States and Catalytic Performance of Ti in Titanium Silicalite-1*

sion decreases as Ti concentration increases over 10 vol%.

**3.5 One-pot synthesis of meso−/microporous titanium silicalite**

In recent years, research on the synthesis of hierarchical molecular sieves has attracted much attention, because they have the advantages of microporous (good catalytic activity and hydrothermal stability) and mesoporous (excellent diffusion property) materials simultaneously. The preparation methods of hierarchical molecular sieves are mainly postsynthesis and one-pot synthesis. The postsynthesis method uses acid or base to treat micropores, which was introduced above. Onepot synthesis of hierarchical titanium silicalite was first reported by Jacobsen et al. using carbon black as a hard template [39]. After that, a series of hierarchical titanium silicalites were synthesized by using different carbon-based materials. However, the complexity of the synthesis procedure seriously limited the industrial applications of these hierarchical materials. An attractive method is to utilize suitable surfactants as soft mesoporous templates for the direct synthesis of hierarchical materials. Cetyltrimethyl ammonium bromide (CTAB) is one of the most commonly used surfactants. Nevertheless, the mesopores in the CTAB-directed materials are mostly intercrystals. Furthermore, the micropores and mesopores are often phase-separated from each other. We explored an easy and new route for synthesizing the meso−/microporous titanium silicalite with controllable pore diameter by using CTAB and TPAOH as mesoporous and microporous template, respectively [40]. The new route is adding CTAB to the hydrolysis reaction of the silicon source so it forms mesopores prior to the crystallization of microporous MFI topology and prevents the occurrence of phase separation. In other words, this porosity formation sequence makes the two kinds of channels in the materials, which are micropores with MFI topology and mesopores with worm-like morphology, distributed homogeneously. The pore diameter of the mesopores can be adjusted from the maximum

reducing the diffusion limitation may be one of the reasons for the improvement of catalytic activity. The new generated tetrahedrally coordinated Ti on the external surface also provides more easily adsorbable active centers for reactants. At low Ti concentrations (≤10 vol%), increasing the Ti amount generates more active Ti centers on the external surface. Thus, the conversion of phenol increases as Ti concentration increases. However, at high Ti concentrations, the excessive Ti is converted to anatase TiO2 due to the limitation of titanium content in the MFI topology. The anatase TiO2 can block the channels and cover the active centers. Thus, the conver-

We have mentioned in Section 2.2 that adding gelatin to the synthesis gel of TS-1 could modify its morphology. Actually, it can adjust the thickness of the *b*-axis of MFI-type zeolites. The *b*-axis direction is parallel to the straight channel, so the short, straight, and open channels of zeolites with MFI topology are tailored for diffusion and catalysis. Ti-MFI, Al-MFI, Zr-MFI, Mn-MFI, Cu-MFI, and Fe-MFI plates were synthesized with *b*-axis lengths ranging from 40 to 200 nm by these means. The lengths of the other two dimensions are submicron-sized, which leads to an easy separation of zeolites from the mother liquor. The synergistic effects of the amino and carboxyl groups in gelatin lead to the generation of plate-like zeolites. The physical adsorption of cyclohexane indicates that TS-1 with a thickness of 40 nm has a faster diffusion rate than that of the traditional aggregated material. The TOF of cyclohexene epoxidation over TS-1 plates is about four times that of

*DOI: http://dx.doi.org/10.5772/intechopen.89864*

**3.4 Preparation of plate-like TS-1**

traditional nanosized TS-1.

*Coordination States and Catalytic Performance of Ti in Titanium Silicalite-1 DOI: http://dx.doi.org/10.5772/intechopen.89864*

reducing the diffusion limitation may be one of the reasons for the improvement of catalytic activity. The new generated tetrahedrally coordinated Ti on the external surface also provides more easily adsorbable active centers for reactants. At low Ti concentrations (≤10 vol%), increasing the Ti amount generates more active Ti centers on the external surface. Thus, the conversion of phenol increases as Ti concentration increases. However, at high Ti concentrations, the excessive Ti is converted to anatase TiO2 due to the limitation of titanium content in the MFI topology. The anatase TiO2 can block the channels and cover the active centers. Thus, the conversion decreases as Ti concentration increases over 10 vol%.

### **3.4 Preparation of plate-like TS-1**

*Stability and Applications of Coordination Compounds*

with silica support and insert more Ti on the external surface [38]. Different amounts of the titanium source (tetrabutyl titanate hydrolysate) were added to the TPAOH solution. **Table 2** shows the elemental compositions of the samples both in the bulk and on the external surface. The *n*(Si/Ti) in the bulk is hardly affected by the individual TPAOH treatment. The Ti content increases gradually with increasing concentration of Ti in the postsynthesis solution. However, not all Ti in the samples transforms to tetrahedrally coordinated Ti. The X-ray photoelectron spectroscopy (XPS) results show that the content of Ti on the external surface reduced after the treatment with TPAOH solution, which is probably due to the crystallization of amorphous silica occurring at the external surface of TS-1 particles and covering more Ti species. The *n*(Si/Ti) on the external surface decreases as Ti concentration in the postsynthesis solution increases, indicating that Ti can be located at the external surface under these postsynthesis conditions, and the amount of Ti is restricted by the Ti concentration and the limitation of Ti in the MFI topology. The samples were evaluated in the hydroxylation of phenol. The conversion of phenol increases to a different extent after the postsynthesis. The hollow spaces generated in the crystals

*Elemental composition of the TS-1 treated with different Ti contentsa*

**Ti content/vol% In the bulk On the surface**

Untreated 97.06 2.79 46.4 98.67 1.18 111.9 97.10 2.76 46.9 98.99 0.86 153.5 96.96 2.89 44.8 98.51 1.34 98.1 96.81 3.02 42.7 98.29 1.54 85.3 96.73 3.12 41.4 98.28 1.57 83.6 96.65 3.20 40.3 98.25 1.60 81.7

*The elemental compositions in the bulk and on the surface were determined by ICP-OES and XPS, respectively.*

**SiO2/wt% TiO2/wt%** *n***(Si/Ti) SiO2/wt% TiO2/wt%** *n***(Si/Ti)**

*.*

**152**

**Figure 8.**

*a*

**Table 2.**

*Preparation process of H-S-1@TS-1.*

We have mentioned in Section 2.2 that adding gelatin to the synthesis gel of TS-1 could modify its morphology. Actually, it can adjust the thickness of the *b*-axis of MFI-type zeolites. The *b*-axis direction is parallel to the straight channel, so the short, straight, and open channels of zeolites with MFI topology are tailored for diffusion and catalysis. Ti-MFI, Al-MFI, Zr-MFI, Mn-MFI, Cu-MFI, and Fe-MFI plates were synthesized with *b*-axis lengths ranging from 40 to 200 nm by these means. The lengths of the other two dimensions are submicron-sized, which leads to an easy separation of zeolites from the mother liquor. The synergistic effects of the amino and carboxyl groups in gelatin lead to the generation of plate-like zeolites. The physical adsorption of cyclohexane indicates that TS-1 with a thickness of 40 nm has a faster diffusion rate than that of the traditional aggregated material. The TOF of cyclohexene epoxidation over TS-1 plates is about four times that of traditional nanosized TS-1.

#### **3.5 One-pot synthesis of meso−/microporous titanium silicalite**

In recent years, research on the synthesis of hierarchical molecular sieves has attracted much attention, because they have the advantages of microporous (good catalytic activity and hydrothermal stability) and mesoporous (excellent diffusion property) materials simultaneously. The preparation methods of hierarchical molecular sieves are mainly postsynthesis and one-pot synthesis. The postsynthesis method uses acid or base to treat micropores, which was introduced above. Onepot synthesis of hierarchical titanium silicalite was first reported by Jacobsen et al. using carbon black as a hard template [39]. After that, a series of hierarchical titanium silicalites were synthesized by using different carbon-based materials. However, the complexity of the synthesis procedure seriously limited the industrial applications of these hierarchical materials. An attractive method is to utilize suitable surfactants as soft mesoporous templates for the direct synthesis of hierarchical materials. Cetyltrimethyl ammonium bromide (CTAB) is one of the most commonly used surfactants. Nevertheless, the mesopores in the CTAB-directed materials are mostly intercrystals. Furthermore, the micropores and mesopores are often phase-separated from each other. We explored an easy and new route for synthesizing the meso−/microporous titanium silicalite with controllable pore diameter by using CTAB and TPAOH as mesoporous and microporous template, respectively [40]. The new route is adding CTAB to the hydrolysis reaction of the silicon source so it forms mesopores prior to the crystallization of microporous MFI topology and prevents the occurrence of phase separation. In other words, this porosity formation sequence makes the two kinds of channels in the materials, which are micropores with MFI topology and mesopores with worm-like morphology, distributed homogeneously. The pore diameter of the mesopores can be adjusted from the maximum

**Figure 9.**

*Synthesis process of meso−/microporous titanium silicalite.*

center of 2.6 nm to that of 6.9 nm by tuning the molar ratio of CTAB to silicon from 0.125 to 0.20. The introduction of CTAB also causes the variation in coordination states and location of Ti ions in the materials. More CTAB leads to a higher content of octahedrally coordinated Ti and a lower content of tetrahedrally coordinated Ti. Furthermore, more Ti is located near the external surface of TS-1 crystals, when adding more CTAB to the synthesis gel.

The meso−/microporous titanium silicalite catalysts were evaluated in the epoxidation of cyclohexene and showed excellent catalytic activity with respect to the conventional microporous TS-1, due to the enhanced diffusion properties in the mesopores and higher titanium content near the external surface of the former (**Figure 9**).
