**2. Controlment of the Ti coordination states**

## **2.1 Tuning of synthesis conditions**

There are mainly three kinds of Ti species in TS-1. Tetrahedrally coordinated titanium, which was mentioned above, is usually considered to be the active center for the oxidation reactions. This Ti species can form a five-membered ring (5MR) structure with hydrogen peroxide and alcohol (see **Figure 1**) [22], which can oxidize propene to prepare propene oxide. The 5MR structure is very stable and is easy to be formed in the epoxidation of alkenes, according to density functional theory study. The β-O atom in the structure is partially positive due to the hydrogen bond between alcohol and O-O-H; thus, the C=C bond can react with the β-O atom by nucleophilic attack. The 5MR mechanism explains the effects of alcohol solvents

**145**

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

coordinated Ti is an intermediate in propene epoxidation [25].

Until now, tetrapropylammonium ions (TPA<sup>+</sup>

synthesize TS-1 because they have the same TPA+

coordination states of Ti qualitatively and quantitatively [28].

in propene epoxidation well; hence it is widely accepted. The other two types of Ti species in TS-1 are octahedrally coordinated Ti (usually called extra-framework Ti) and anatase TiO2. When the feeding amount of titanium source in the synthesis gel is more than 2.5 mol%, the excessive Ti will transform to these two Ti species. The anatase TiO2 can decompose hydrogen peroxide to water and O2; thus, the generation of anatase TiO2 should be avoided [23]. The octahedrally coordinated Ti can be generated from the tetrahedrally coordinated Ti coordinating with two water molecules. The function of octahedrally coordinated Ti is still controversial. Former researchers thought it was inert for oxidation, but recent works have reported that it was also active for selective oxidation [24]. Wang et al. found that octahedrally

It is well known that the hydrolysis rate of titanium sources is faster than that of silicon sources and the similar hydrolysis extent or crystallization rates of them benefit for the generation of more tetrahedrally coordinated Ti. Therefore, we hydrolyzed the two sources individually to complete hydrolysis simultaneously [26]. Tetrapropylammonium hydroxide (TPAOH) was used as template and base and was added to silicon and titanium sources. The silicon source was hydrolyzed at 313 K for 5 h, while the titanium source was at room temperature for 0.5 h. The hydrolysis of the two sources was completed at the same time. After that, the two hydrolysates were mixed together and crystallized at 443 K for 2 d. Under these conditions, the content of tetrahedrally coordinated Ti in TS-1 was ~1 wt%, while the total content of Ti was ~1.9 wt%. From this result, we know that the generation of tetrahedrally

TS-1 as the template. In consideration of the high price of TPAOH, many researchers tend to use tetrapropylammonium bromide (TPABr) to structure-directly

cheaper. However, due to the introduction of Br<sup>−</sup> and the reduction of basicity, the particle size of TS-1 enlarges obviously when TPABr is used. The TPAOH system often obtains nanosized TS-1, while the TPABr system usually gets microsized particles. We explored a method for synthesizing small-crystal TS-1 in the TPABr system [27], which will be presented in detail in Section 3.1. Herein, we only discuss the influence of molar ratio of Si/Ti (*n*(Si/Ti)) on the coordination states of Ti when synthesizing small-crystal TS-1 in the TPABr system. Small-crystal TS-1 with different feeding *n*(Si/Ti) (20, 50, and 80) was synthesized by adding different amounts of titanium source (TiCl4) to the synthesis gel. The weight content of silicon and titanium in the samples obtained by inductively coupled plasma-optical emission spectrometer (ICP-OES) shows that the actual *n*(Si/Ti) was slightly higher than the feeding one, except for sample with the feeding *n*(Si/Ti) of 20. Ultraviolet/ visible diffuse reflectance (UV/vis) spectroscopy, Raman spectroscopy, and X-ray adsorption near edge structure (XANES) spectroscopy were used to study the

UV/vis spectroscopy is one of the first spectral techniques used for the detection

of Ti coordination states in titanium silicalites. Peak deconvolutions were performed using the PeakFit program with the Gaussian fitting method. In the spectra of small-crystal TS-1 (**Figure 2**), there are three major absorption bands centered at 200–210, 230–290, and 310–330 nm. The band at 200–210 nm is assigned to tetrahedrally coordinated Ti, while that at 310–330 nm belongs to anatase TiO2. There are more than one kind of Ti species between 230 and 290 nm in the UV/vis spectra of TS-1. The band at approximately 250–290 nm is attributed to the octahedrally coordinated Ti species, which is inactive for the oxidation reactions, and the band at 230–250 nm is an isolated Ti species with a lower coordination number of oxygen than octahedrally coordinated Ti (such as pentahedrally coordinated Ti). The

) are necessary for synthesizing

cation, but TPABr is much

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

coordinated Ti is difficult.

**Figure 1.** *Diagram of 5MR structure.*

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

*Stability and Applications of Coordination Compounds*

**2. Controlment of the Ti coordination states**

**2.1 Tuning of synthesis conditions**

Titanium silicalite-1 (TS-1) with MFI topology was first hydrothermally synthesized by Taramasso et al. [3]. After that, it has attracted much attention due to its excellent catalytic activity for selective oxidation with H2O2, such as alkene epoxidation [4–9], aromatics hydroxylation [10–12], ketone ammoximation [13–15], alkane oxidation [16, 17], and so on [18, 19]. Therefore, it is considered a milestone in the field of zeolitic catalysis. MFI topology contains two types of 10-membered ring channels, which are the straight channel (0.56 × 0.54 nm) and zigzag channel (0.55 × 0.51 nm). The substitution of titanium atoms for framework silicon or aluminum atoms generates a molecular sieve with tetrahedrally coordinated Ti. The isolated tetrahedrally coordinated titanium (also called framework Ti) in TS-1 is the main active center for catalytic oxidation. However, the amount of tetrahedrally coordinated Ti is limited (2.5 mol%), because the lattice expansion inhibits the insertion of Ti into the framework [20]. In the next 30 years, phenol hydroxylation to benzene diols, cyclohexanone ammoximation to cyclohexanone oxime, butanone ammoximation to diacetyl monoxime, and propene epoxidation to propene oxide catalyzed by TS-1 were commercialized successively. Nevertheless, there are still many problems in the synthesis and application of TS-1, such as the transformation of tetrahedrally coordinated Ti to octahedrally coordinated Ti or anatase TiO2 (loss of the active center) in the reaction, and the deactivation of the catalyst by blocking of channels [21]. Therefore, many researchers have made an effort to solve these problems, and so do we. To further improve the catalytic performance and expand the application of TS-1, it is necessary to summarize our current research

In this chapter, we describe our recent progress on controlling Ti coordination states, design of porosity, and applications of TS-1. We hope that this summary will help in understanding the developing process and our contribution to research

There are mainly three kinds of Ti species in TS-1. Tetrahedrally coordinated titanium, which was mentioned above, is usually considered to be the active center for the oxidation reactions. This Ti species can form a five-membered ring (5MR) structure with hydrogen peroxide and alcohol (see **Figure 1**) [22], which can oxidize propene to prepare propene oxide. The 5MR structure is very stable and is easy to be formed in the epoxidation of alkenes, according to density functional theory study. The β-O atom in the structure is partially positive due to the hydrogen bond between alcohol and O-O-H; thus, the C=C bond can react with the β-O atom by nucleophilic attack. The 5MR mechanism explains the effects of alcohol solvents

**144**

**Figure 1.**

*Diagram of 5MR structure.*

achievement.

on TS-1.

in propene epoxidation well; hence it is widely accepted. The other two types of Ti species in TS-1 are octahedrally coordinated Ti (usually called extra-framework Ti) and anatase TiO2. When the feeding amount of titanium source in the synthesis gel is more than 2.5 mol%, the excessive Ti will transform to these two Ti species. The anatase TiO2 can decompose hydrogen peroxide to water and O2; thus, the generation of anatase TiO2 should be avoided [23]. The octahedrally coordinated Ti can be generated from the tetrahedrally coordinated Ti coordinating with two water molecules. The function of octahedrally coordinated Ti is still controversial. Former researchers thought it was inert for oxidation, but recent works have reported that it was also active for selective oxidation [24]. Wang et al. found that octahedrally coordinated Ti is an intermediate in propene epoxidation [25].

It is well known that the hydrolysis rate of titanium sources is faster than that of silicon sources and the similar hydrolysis extent or crystallization rates of them benefit for the generation of more tetrahedrally coordinated Ti. Therefore, we hydrolyzed the two sources individually to complete hydrolysis simultaneously [26]. Tetrapropylammonium hydroxide (TPAOH) was used as template and base and was added to silicon and titanium sources. The silicon source was hydrolyzed at 313 K for 5 h, while the titanium source was at room temperature for 0.5 h. The hydrolysis of the two sources was completed at the same time. After that, the two hydrolysates were mixed together and crystallized at 443 K for 2 d. Under these conditions, the content of tetrahedrally coordinated Ti in TS-1 was ~1 wt%, while the total content of Ti was ~1.9 wt%. From this result, we know that the generation of tetrahedrally coordinated Ti is difficult.

Until now, tetrapropylammonium ions (TPA<sup>+</sup> ) are necessary for synthesizing TS-1 as the template. In consideration of the high price of TPAOH, many researchers tend to use tetrapropylammonium bromide (TPABr) to structure-directly synthesize TS-1 because they have the same TPA+ cation, but TPABr is much cheaper. However, due to the introduction of Br<sup>−</sup> and the reduction of basicity, the particle size of TS-1 enlarges obviously when TPABr is used. The TPAOH system often obtains nanosized TS-1, while the TPABr system usually gets microsized particles. We explored a method for synthesizing small-crystal TS-1 in the TPABr system [27], which will be presented in detail in Section 3.1. Herein, we only discuss the influence of molar ratio of Si/Ti (*n*(Si/Ti)) on the coordination states of Ti when synthesizing small-crystal TS-1 in the TPABr system. Small-crystal TS-1 with different feeding *n*(Si/Ti) (20, 50, and 80) was synthesized by adding different amounts of titanium source (TiCl4) to the synthesis gel. The weight content of silicon and titanium in the samples obtained by inductively coupled plasma-optical emission spectrometer (ICP-OES) shows that the actual *n*(Si/Ti) was slightly higher than the feeding one, except for sample with the feeding *n*(Si/Ti) of 20. Ultraviolet/ visible diffuse reflectance (UV/vis) spectroscopy, Raman spectroscopy, and X-ray adsorption near edge structure (XANES) spectroscopy were used to study the coordination states of Ti qualitatively and quantitatively [28].

UV/vis spectroscopy is one of the first spectral techniques used for the detection of Ti coordination states in titanium silicalites. Peak deconvolutions were performed using the PeakFit program with the Gaussian fitting method. In the spectra of small-crystal TS-1 (**Figure 2**), there are three major absorption bands centered at 200–210, 230–290, and 310–330 nm. The band at 200–210 nm is assigned to tetrahedrally coordinated Ti, while that at 310–330 nm belongs to anatase TiO2. There are more than one kind of Ti species between 230 and 290 nm in the UV/vis spectra of TS-1. The band at approximately 250–290 nm is attributed to the octahedrally coordinated Ti species, which is inactive for the oxidation reactions, and the band at 230–250 nm is an isolated Ti species with a lower coordination number of oxygen than octahedrally coordinated Ti (such as pentahedrally coordinated Ti). The

undercoordinated Ti has a higher energy than the octahedral one, so its band shifts to a shorter wavelength. The content of tetrahedrally coordinated Ti, octahedrally coordinated Ti, and anatase TiO2 increases when *n*(Si/Ti) is decreased, and the increase of anatase TiO2 content is much stronger than that of other species, proving that the introduction of Ti in the framework is limited. It is notable that a new band appears at ~235 nm in the sample with *n*(Si/Ti) of 80. We consider it to belong to the pentahedrally coordinated Ti by combining these results with the results of XANES. This Ti state might be formed by one Ti atom with five "SiO4". Thus, a high *n*(Si/Ti) promotes more "SiO4" coordination with this Ti atom, which is beneficial to the generation of pentahedrally coordinated Ti.

The catalytic performance of the small-crystal TS-1 with different *n*(Si/Ti) was evaluated in the epoxidation of propene. The conversion of H2O2 over TS-1 with *n*(Si/Ti) of 20 is the lowest, indicating that the Ti coordination state is more important than its content for the catalytic activity. The highest turnover frequency (TOF) was obtained from the TS-1 with *n*(Si/Ti) of 80, demonstrating that the pentahedrally coordinated Ti was the most active species of all the Ti coordination types. The lowest TOF was obtained over the TS-1 with *n*(Si/Ti) of 20, indicating that octahedrally coordinated Ti was inert or had negative effects on the epoxidation. Based on the XANES and TOF data, we calculated the contents of differently coordinated Ti in the three samples (see **Table 1**). The content of pentahedrally coordinated Ti decreases and the content of octahedrally coordinated Ti increases with the decrease of feeding *n*(Si/Ti) from 80 to 50, suggesting that the insertion of Ti into the framework is at nearly the maximum at the *n*(Si/Ti) of 80. Continuing to add Ti leads to a sharp increase of octahedrally coordinated Ti. The TOFs of pure tetrahedrally, pentahedrally, and octahedrally coordinated Ti in small-crystal TS-1, which were calculated according to the total TOFs and contents of differently coordinated Ti, are 373.3, 1434.3, and 0 mol H2O2/(h·mol Ti), respectively. These results confirm that pentahedrally coordinated Ti is the most active species among the three coordination structures.

**Figure 2.** *UV/Vis spectra of the TS-1 with different* n*(Si/Ti).*


**147**

**Figure 3.**

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

unstabilization means high energy and thus high catalytic activity.

Nevertheless, the controllable synthesis of TS-1 with a large amount of pentahedrally coordinated Ti is still impossible. When more Ti was added to the synthesis gel, the pentahedrally coordinated Ti would transform to tetrahedrally and octahedrally coordinated Ti due to the loss of pentahedrally coordinated Ti generation

In addition, the stability sequence of differently coordinated Ti is anatase TiO2 > octahedrally coordinated Ti > tetrahedrally coordinated Ti > pentahedrally coordinated Ti. The catalytic activity sequence is opposite to the stability, because

From the above introduction, we know that matching the crystallization rates of silicon and titanium sources benefits the generation of tetrahedrally coordinated Ti. Therefore, some researchers tried to control the crystallization process by adding some modifiers. Fan et al. used different ammonium salts as the crystallization-mediating agents to synthesize TS-1 [29]. They found that the ammonium salts could not only drastically decrease the pH of the synthesis gel and slow down crystallization, but they could also modify the crystallization mechanism and make the incorporation of titanium into the framework match well with that of silicon. As a result, the formation of octahedrally coordinated Ti and anatase TiO2 was eliminated successfully. It was reported that the anionic polyelectrolyte poly(acrylic acid) was also able to facilitate the insertion of Ti to the framework via a liquid-phase and solid-phase transformation mechanism [30]. Some researchers used sucrose as the modifier [31], which would carbonize during the crystallization of TS-1 and release hydrogen ions. Therefore, the pH of the hydrothermal system was reduced, and the sucrose played a similar role to the

We adopted two natural macromolecular additives to adjust the coordination states of Ti, which were starch and gelatin [32]. When they were introduced to the synthesis gel, they also released hydrogen ions as the sucrose did. Hence, they can

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

conditions mentioned previously.

**2.2 Usage of additives**

ammonium salts.

also tune the coordination states of Ti.

*UV/Vis spectra of the TS-1 synthesized with different amounts of starch.*

#### **Table 1.**

*Contents of differently coordinated Ti.*

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

Nevertheless, the controllable synthesis of TS-1 with a large amount of pentahedrally coordinated Ti is still impossible. When more Ti was added to the synthesis gel, the pentahedrally coordinated Ti would transform to tetrahedrally and octahedrally coordinated Ti due to the loss of pentahedrally coordinated Ti generation conditions mentioned previously.

In addition, the stability sequence of differently coordinated Ti is anatase TiO2 > octahedrally coordinated Ti > tetrahedrally coordinated Ti > pentahedrally coordinated Ti. The catalytic activity sequence is opposite to the stability, because unstabilization means high energy and thus high catalytic activity.

#### **2.2 Usage of additives**

*Stability and Applications of Coordination Compounds*

to the generation of pentahedrally coordinated Ti.

the three coordination structures.

*UV/Vis spectra of the TS-1 with different* n*(Si/Ti).*

*Contents of differently coordinated Ti.*

*n***(Si/Ti) Percentage of differently coordinated Ti/%**

20 42 0 58 50 76 21 3 80 50 50 0

**Tetrahedral Ti Pentahedral Ti Octahedral Ti**

undercoordinated Ti has a higher energy than the octahedral one, so its band shifts to a shorter wavelength. The content of tetrahedrally coordinated Ti, octahedrally coordinated Ti, and anatase TiO2 increases when *n*(Si/Ti) is decreased, and the increase of anatase TiO2 content is much stronger than that of other species, proving that the introduction of Ti in the framework is limited. It is notable that a new band appears at ~235 nm in the sample with *n*(Si/Ti) of 80. We consider it to belong to the pentahedrally coordinated Ti by combining these results with the results of XANES. This Ti state might be formed by one Ti atom with five "SiO4". Thus, a high *n*(Si/Ti) promotes more "SiO4" coordination with this Ti atom, which is beneficial

The catalytic performance of the small-crystal TS-1 with different *n*(Si/Ti) was evaluated in the epoxidation of propene. The conversion of H2O2 over TS-1 with *n*(Si/Ti) of 20 is the lowest, indicating that the Ti coordination state is more important than its content for the catalytic activity. The highest turnover frequency (TOF) was obtained from the TS-1 with *n*(Si/Ti) of 80, demonstrating that the pentahedrally coordinated Ti was the most active species of all the Ti coordination types. The lowest TOF was obtained over the TS-1 with *n*(Si/Ti) of 20, indicating that octahedrally coordinated Ti was inert or had negative effects on the epoxidation. Based on the XANES and TOF data, we calculated the contents of differently coordinated Ti in the three samples (see **Table 1**). The content of pentahedrally coordinated Ti decreases and the content of octahedrally coordinated Ti increases with the decrease of feeding *n*(Si/Ti) from 80 to 50, suggesting that the insertion of Ti into the framework is at nearly the maximum at the *n*(Si/Ti) of 80. Continuing to add Ti leads to a sharp increase of octahedrally coordinated Ti. The TOFs of pure tetrahedrally, pentahedrally, and octahedrally coordinated Ti in small-crystal TS-1, which were calculated according to the total TOFs and contents of differently coordinated Ti, are 373.3, 1434.3, and 0 mol H2O2/(h·mol Ti), respectively. These results confirm that pentahedrally coordinated Ti is the most active species among

**146**

**Table 1.**

**Figure 2.**

From the above introduction, we know that matching the crystallization rates of silicon and titanium sources benefits the generation of tetrahedrally coordinated Ti. Therefore, some researchers tried to control the crystallization process by adding some modifiers. Fan et al. used different ammonium salts as the crystallization-mediating agents to synthesize TS-1 [29]. They found that the ammonium salts could not only drastically decrease the pH of the synthesis gel and slow down crystallization, but they could also modify the crystallization mechanism and make the incorporation of titanium into the framework match well with that of silicon. As a result, the formation of octahedrally coordinated Ti and anatase TiO2 was eliminated successfully. It was reported that the anionic polyelectrolyte poly(acrylic acid) was also able to facilitate the insertion of Ti to the framework via a liquid-phase and solid-phase transformation mechanism [30]. Some researchers used sucrose as the modifier [31], which would carbonize during the crystallization of TS-1 and release hydrogen ions. Therefore, the pH of the hydrothermal system was reduced, and the sucrose played a similar role to the ammonium salts.

We adopted two natural macromolecular additives to adjust the coordination states of Ti, which were starch and gelatin [32]. When they were introduced to the synthesis gel, they also released hydrogen ions as the sucrose did. Hence, they can also tune the coordination states of Ti.

**Figure 3.** *UV/Vis spectra of the TS-1 synthesized with different amounts of starch.*

The addition of starch hardly influences the morphology of TS-1 but can eliminate the formation of extra-framework Ti. **Figure 3** shows the UV/vis spectra of TS-1 synthesized with different amounts of starch. An obvious absorption band at ~330 nm appears in the TS-1 synthesized without starch, proving the existence of anatase TiO2. As the content of starch and gelatin increases, the band of anatase TiO2 disappears gradually. The content of octahedrally coordinated Ti decreases with the increase of starch until the weight ratio of starch/SiO2 (*m*(St/ SiO2)) reaches 0.6. Then, the content of octahedrally coordinated Ti increases slightly, probably due to the introduction of starch promoting the coordination saturation of titanium ions. Compared to the TS-1 synthesized without starch, those obtained with starch have a higher content of tetrahedrally coordinated Ti, a quite low content of octahedrally coordinated Ti, and are free of anatase TiO2. Therefore, they show a much higher catalytic activity for the epoxidation of 1-butene.

Gelatin is similar to starch in its effect on Ti coordination states. However, it contains both amino and carboxyl groups. Thus, it has the ability to tune the morphology of MFI-typed zeolites. This will be further discussed in Section 3.4.

#### **2.3 Posttreatment with organic bases**

Many studies focused on the treatment of zeolites with organic bases, especially for the quaternary ammonium bases, because the treatment could improve the catalytic performance significantly. The treatment leads to the dissolution of "SiO4" in the TS-1 crystals and recrystallization on the external surface of crystals, generating hollow zeolites, which decreases the diffusion resistance. When the "SiO4" was dissolved, the coordination states of the neighbored Ti ions would be changed accordingly. Two Si-O bonds near to the tetrahedrally coordinated Ti may be broken, and the tetrahedrally coordinated Ti may transform to octahedrally coordinated Ti after combining with two water molecules (**Figure 4**).

We studied the treatment of small-crystal TS-1 with different organic bases, including ethylamine (EA), diethylamine (DEA), tetramethylammonium hydroxide (TMAOH), and tetrapropylammonium hydroxide (TPAOH) solutions [33]. The catalytic performances of phenol hydroxylation over the treated samples were improved to different extents. The TS-1 treated with TPAOH has the highest catalytic activity in the treated samples.

To understand the reason for this result, we characterized the treated samples with Ti *L*-edge XANES spectroscopy, the spectra of which are shown in **Figure 5**. The spectra consist of two sets of doublets, which correspond to the 2*p*1/2 and 2*p*3/2 transitions of the 3*d*<sup>0</sup> to 2*p*<sup>5</sup> 3*d*<sup>1</sup> states. The *L*2 edge is at a higher energy (462–470 eV), and the *L*3 edge is at a lower energy (455–462 eV). The splitting of each edge is attributed to the *t*eg and *e*g symmetry of the *d* orbital.

**Figure 4.**

*Structures of octahedrally coordinated Ti (a), pentahedrally coordinated Ti (b), and tetrahedrally coordinated Ti (c).*

**149**

**Figure 5.**

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

The higher energy peak of the *L*3 edge (459–462 eV) consists of two peaks in some samples, a main peak and a shoulder one, which is on the lower energy side (~460 eV) for rutile TiO2 and the higher energy side (461 eV) for anatase TiO2. The intensities of the two peaks are reversed for the substances that they represent. The tetrahedrally coordinated Ti is characterized by the absence of splitting of the peak at 459–462 eV and a relatively weaker intensity of the peaks at lower energies of both the *L*2 and *L*3 edges than those at higher energies. The pentahedrally coordinated Ti is characterized by a slight shift to higher energy and a drastic decrease of the lower energy peak of *L*3, a shift of the higher energy peak of *L*2 to lower energy, and the appearance of a shoulder peak on the lower energy peak of *L*2. We found that pentahedrally coordinated Ti existed in the TPAOH-treated TS-1, but it was absent in the other samples. Therefore, the generation of pentahedrally coordinated Ti is another reason for the increasing activity of TPAOH-treated TS-1. The possible structure of pentahedrally coordinated Ti is shown in **Figure 4**, the stable form of which is tetragonal pyramid.

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

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

**3. Improvement of the diffusion property**

*Ti L-edge XANES spectra of the TS-1 treated with different bases.*

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

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

*Stability and Applications of Coordination Compounds*

**2.3 Posttreatment with organic bases**

combining with two water molecules (**Figure 4**).

catalytic activity in the treated samples.

2*p*1/2 and 2*p*3/2 transitions of the 3*d*<sup>0</sup>

The addition of starch hardly influences the morphology of TS-1 but can eliminate the formation of extra-framework Ti. **Figure 3** shows the UV/vis spectra of TS-1 synthesized with different amounts of starch. An obvious absorption band at ~330 nm appears in the TS-1 synthesized without starch, proving the existence of anatase TiO2. As the content of starch and gelatin increases, the band of anatase TiO2 disappears gradually. The content of octahedrally coordinated Ti decreases with the increase of starch until the weight ratio of starch/SiO2 (*m*(St/ SiO2)) reaches 0.6. Then, the content of octahedrally coordinated Ti increases slightly, probably due to the introduction of starch promoting the coordination saturation of titanium ions. Compared to the TS-1 synthesized without starch, those obtained with starch have a higher content of tetrahedrally coordinated Ti, a quite low content of octahedrally coordinated Ti, and are free of anatase TiO2. Therefore, they show a much higher catalytic activity for the epoxidation of

Gelatin is similar to starch in its effect on Ti coordination states. However, it contains both amino and carboxyl groups. Thus, it has the ability to tune the morphol-

Many studies focused on the treatment of zeolites with organic bases, especially for the quaternary ammonium bases, because the treatment could improve the catalytic performance significantly. The treatment leads to the dissolution of "SiO4" in the TS-1 crystals and recrystallization on the external surface of crystals, generating hollow zeolites, which decreases the diffusion resistance. When the "SiO4" was dissolved, the coordination states of the neighbored Ti ions would be changed accordingly. Two Si-O bonds near to the tetrahedrally coordinated Ti may be broken, and the tetrahedrally coordinated Ti may transform to octahedrally coordinated Ti after

We studied the treatment of small-crystal TS-1 with different organic bases, including ethylamine (EA), diethylamine (DEA), tetramethylammonium hydroxide (TMAOH), and tetrapropylammonium hydroxide (TPAOH) solutions [33]. The catalytic performances of phenol hydroxylation over the treated samples were improved to different extents. The TS-1 treated with TPAOH has the highest

To understand the reason for this result, we characterized the treated samples with Ti *L*-edge XANES spectroscopy, the spectra of which are shown in **Figure 5**. The spectra consist of two sets of doublets, which correspond to the

to 2*p*<sup>5</sup>

energy (462–470 eV), and the *L*3 edge is at a lower energy (455–462 eV). The splitting of each edge is attributed to the *t*eg and *e*g symmetry of the *d* orbital.

*Structures of octahedrally coordinated Ti (a), pentahedrally coordinated Ti (b), and tetrahedrally coordinated* 

3*d*<sup>1</sup>

states. The *L*2 edge is at a higher

ogy of MFI-typed zeolites. This will be further discussed in Section 3.4.

**148**

**Figure 4.**

*Ti (c).*

1-butene.

**Figure 5.** *Ti L-edge XANES spectra of the TS-1 treated with different bases.*

The higher energy peak of the *L*3 edge (459–462 eV) consists of two peaks in some samples, a main peak and a shoulder one, which is on the lower energy side (~460 eV) for rutile TiO2 and the higher energy side (461 eV) for anatase TiO2. The intensities of the two peaks are reversed for the substances that they represent. The tetrahedrally coordinated Ti is characterized by the absence of splitting of the peak at 459–462 eV and a relatively weaker intensity of the peaks at lower energies of both the *L*2 and *L*3 edges than those at higher energies. The pentahedrally coordinated Ti is characterized by a slight shift to higher energy and a drastic decrease of the lower energy peak of *L*3, a shift of the higher energy peak of *L*2 to lower energy, and the appearance of a shoulder peak on the lower energy peak of *L*2. We found that pentahedrally coordinated Ti existed in the TPAOH-treated TS-1, but it was absent in the other samples. Therefore, the generation of pentahedrally coordinated Ti is another reason for the increasing activity of TPAOH-treated TS-1. The possible structure of pentahedrally coordinated Ti is shown in **Figure 4**, the stable form of which is tetragonal pyramid.
