Synthesis, characterization and application of MTS-9 with different content of titanium

Microporous and Mesoporous Materials 171 (2013) 179–184

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Synthesis, characterization and application of MTS-9 with different content of titanium Derun Hua a, Zheng Zhou b, Yulong Wu a,⇑, Yu Chen a, Liu Ji a, Quanhua Xie a, Mingde Yang a a b

Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, PR China Natural Gas Treating Plant, Sinopec Zhongyuan Oilfield Company, China

a r t i c l e

i n f o

Article history: Received 24 September 2012 Received in revised form 20 November 2012 Accepted 26 November 2012 Available online 21 December 2012 Keywords: TS-1 MTS-9 Oxidation

a b s t r a c t Heteroatom of Ti is effectively incorporated into the framework of ordered mesoporous silica materials in strong acidic media by a two-step procedure. The synthesized samples (designated as MTS-9) are characterized by many methods. Results indicate that MTS-9 synthesized maintains mesoporous structure, and MTS-9 (Si/Ti = 53) possesses the more tetrahedrally coordinated titanium species. The catalysts are applied for oxidation of propene and styrene in methanol with hydrogen peroxide as the oxidant. As the ratio of Si to Ti is below 53, the increase of titanium content benefits to the formation of benzaldehyde. Crown Copyright Ó 2012 Published by Elsevier Inc. All rights reserved.

1. Introduction Oxidation of organic substances is an important process in synthesis of chemical intermediates. Particular interest has been focused on the identification of suitable heterogeneous catalysts that can be used under mild conditions using hydrogen peroxide as an oxidant. Most interest has been attracted to Ti-based porous material, and titanium silicalite (TS-1) zeolite was discovered in 1983 [1]. Since its discovery, TS-1 zeolite has attracted more and more attention due to the high reactivity, mild conditions, hydrophobicity and excellent stability [2–7]. A great many of the efforts have been made to develop porous materials, and many microporous titanium silicates [8–10] have been reported. However, these materials are limited in the size range of the oxidized substrates due to the relatively small pore (ca. 5.1 A). Therefore, porous titanium silicates with larger pore size have always been sought, and progress in solving this problem has been made. Such as, M41S [11–13], Ti-SBA-15 [14], these materials have pore size of 3–20 nm and exhibit good catalytic performance. Unfortunately, these materials have many drawbacks [6,15–17]. First of all, they are poor in hydrothermal stability; in addition, during postsynthesis, metal oxides tend to appear in the channel or external surface of the synthesized samples, which will jam the channel or play a negative role in catalysis; finally, the ratio of silica to titanium is lower than 60 because the hydrolysis rate of organic titanium is faster than that of organic silica. The above drawbacks lead to the poor catalytic performance. Therefore, novel mesoporous tita⇑ Corresponding author. Tel.: +86 10 89796028. E-mail address: [email protected] (Y. Wu).

nium silicates with TS-1-like titanium species, high titanium content and high catalytic ability are still desirable. Mesoporous TS1 catalyst [18–20] was used in the oxidation of olefin. Compared with conventional TS-1, mesoporous TS-1 improves the catalytic performance without changing the product distribution. Recently, there had been great progress in the synthesis and application of an ordered mesoporous titanosilicate (MTS-9) [21,22]. Particularly, MTS-9 showed excellent hydrothermal stability and very high activity for the oxidation of the smaller molecules of phenol and the bulky molecule of trimethylphenol [23]. In our present work, we briefly report the synthesis of an ordered mesoporous titanium silicate (MTS-9) in the ice-water for retarding the hydrolysis rate of organic titanium and preventing forming the bulk TiO2. In the synthesis system, MTS-9 with the different ratios of silica to titanium is synthesized and used as the catalyst for oxidation of propene and styrene. The performance of TS1 and MTS-9 is compared to the oxidation.

2. Experiments 2.1. Synthesis The modified synthesis for MTS-9 [22] is as follows: (1) The zeolite precursor’s solution with zeolite TS-1 primary structure units were prepared by mixing 0.3 mL of Ti (OC4H9)4 and 5.6 mL of TEOS under stirring for 45 min in ice-bath, followed by 6 mL of TPAOH aqueous solution (25%) dropwise, and mixing 12 mL of H2O last (TiO2/SiO2/TPAOH/H2O molar ratios of 1.0/30/8/1500). The mixture was then aged at 45 °C for 72 h. The final product was also a clear

1387-1811/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.11.035

D. Hua et al. / Microporous and Mesoporous Materials 171 (2013) 179–184

solution. (2) 0.8 g of EO20PO70EO20 (P123) was dissolved in 20 mL of H2O mixed with 5 mL of HCl (10 M/L), followed by 9.0 mL of precursor’s solution (containing 8 mmol of SiO2) obtained in step one. The mixture was stirred at 40 °C for 20 h, then transferred into an autoclave for additional reaction at 100 °C for 24 h. The product (MTS-9) prepared from the titanium silicate precursors was collected by filtration, washed several times to remove most of the template, dried in air, and calcined at 550 °C for 5 h to remove the residue template. Hydrothermal stability of MTS-9 was investigated by following textural changes. The water to sample ratio was fixed as 500 g/g, the hydrothermal treatment was carried out under reflux conditions at 110 °C. After hydrothermal treatment at given temperatures, sample was filtered, washed with distilled water, and immediately placed in an oven to dry for 5 h at 500 °C. 2.2. Characterization The Si/Ti ratio of the samples was measured by ICP-AES (PerkinElmer 3300 DV). X-ray powder diffraction patterns were recorded on a Bruker D8 ADVACE diffract meter with CuKa radiation of wavelength 1.5406 Å in the 2h range of 0.5–6° with scanning rate of 1° min1. N2 adsorption–desorption isotherms at 196 °C were recorded with a Micromeritics ASAP 2010 automatic sorption analyzer. The BET surface areas were calculated from the desorption isotherms in the relative pressure range of P/P0 = 0.011.0. The acidity of the catalyst was obtained via NH3-TPD. Each sample was first treated at 500 °C for 30 min at an air flow of 40 mL min1, and cooled to 110 °C, then exposed to 20% NH3/Ar for 30 min, and last purged by at 110 °C for 120 min in order to eliminate the physically adsorbed ammonia. Temperature programmed desorption (TPD) was conducted by ramping to 500 °C at 10 K min1, and NH3 (m/e is defined as desorption amount) in the effluent was detected and recorded as the function of temperature by a thermal conductivity detector (TCD) (BJGC, GSD301, China). The UV–Vis DRS (diffuse reflectance spectroscopy) of the catalyst was recorded over the range of 200–800 nm against the support as reference, on a Hitachi U-4100 spectrophotometer (Japan) equipped with an integration sphere diffuse reflectance attachment. UV-Raman spectra were obtained on a HR800UV-Raman spectrograph (Horiba Jobin Yvon Company, France). The 244.0 nm line from a He–Cd laser is used as the excitation source. The spectrum resolution is estimated to be 4.0 cm1. The mid-infrared spectra was collected on the Digilab FTS-3000 FT-IR spectrometer using KBr pellet technique. About 15 mg of the sample was pressed (under a pressure of 10 MPa) into a self-supported wafer of 13 mm diameter. This pellet was used to record the infrared spectra in the range 1300–400 cm1. 2.3. Catalysis test MTS-9 samples were tested as catalysts for oxidation of propene and styrene with aqueous hydrogen peroxide. Mixture of 0.2 g catalyst, 2 mL aqueous H2O2 and 50 mL CH3OH was transferred into the reactor with stir, and the reactor was sealed, then heated to 30 °C in the water bath. When the expected temperature reached, reactant (styrene 5 mL) was sequentially injected into the reactor. The reactor was rapidly cooled to terminate the reaction at the expected time. The gas was then released, and the pressure was slowly decreased. The mixture in the reactor was removed and separated from the catalyst via centrifugation. The consumption of H2O2 was monitored by volumetric titration as reported in Ref. [24], and production was analyzed using a GC-7890 (Shanghai Tianmei Technologies Co. Ltd., China) with a flame ionization detector (FID).

Table 1 Element analysis of MTS-9 with different Si/Ti ratios. Sample

Si/Ti

TiO2 (wt%)

1 2 3 4

71 53 37 26

1.85 2.45 3.49 4.91

The conversion and selectivity obtained for the batch experiments are calculated as follows:

½Si  ½Sf  100 ½Si ½P Selectivity : Sð%Þ ¼ a  100 ½P

conversion : Cð%Þ ¼

where [S]i and [S]f are the number of molar of reaction before and after the reaction, respectively, [P]a and [P] are denoted as the number of molar of the expected production and all productions, respectively. 3. Results and discussion Element analysis of titanium content in the samples prepared is given. The corresponding Si/Ti ratios in the gels are equivalent to weight listed in Table 1. The effect of titanium content on acidity of MTS-9 catalyst is shown in Fig. 1. As shown in Fig. 1, for all catalysts, one ammonia desorption peak is found at 170 °C corresponding to the weak acid peak. Thus, it is confirmed that catalysts do not possess the strong acid sites. The acid amount of MTS-9 with different titanium content is listed in Table 2, the acid amount of MTS-9 unhappens with the increase of titanium content. Fig. 2A shows the small-angle X-ray diffraction pattern for a typical as-synthesized MTS-9. Samples show three well-resolved peaks that can be indexed as (1 0 0), (1 1 0) and (2 0 0) reflections associated with the hexagonal symmetry. The (1 0 0) peak shift slightly to lower angle with the increase of Ti content, suggesting pore size became larger. The d100 values are given in Table 3 with the corresponding unit cell parameter (a0) of different MTS-9

170

Si/Ti 26

Intensity (a.u)

180

37

53

71

150

200

250

300

350

400

450

Temperature (oC) Fig. 1. NH3-TPD of MTS-9 with different titanium content.

Table 2 The acid amount of MTS-9 with different titanium content. Sample (Si/Ti) Support (mmol/g)

71 0.1201

53 0.1189

37 0.1191

26 0.1210

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A

(110)

Intensity (a.u)

(100) Si/Ti 26

37

53

71 1.2

1.4

1.6

1.8

2.0

2.2

Intensity(a.u)

(100)

Intensity(a.u)

Si/Ti 26

B

Si/Ti 26

(200)

2.4

2θ

37

37

53

53 71

71

2

4

1

6

2

3

4

5

6

2θ

2θ Fig. 2. Small angle XRD of MTS-9.

Table 3 Results of d100 spacing and unit cell parameter (a0) of MTS-9. Si/Ti

Volume@(cc/g)(a.u)

26

37

Sample

Si/Ti

d100 (Å)

a0 (Å)

1 2 3 4

71 53 37 26

44.48 44.54 44.52 44.94

51.36 51.43 51.40 51.89

53

71

0.2

0.4

0.6

0.8

1.0

P/Po Fig. 3. N2 adsorption–desorption isotherm of MTS-9 with different titanium content.

samples, calculated from the peak (1 0 0) by the equation a0 = 2d100(3)1/2. The d-values and unit cell parameters increase slightly with incorporation of Ti. After hydrothermal treatment, the XRD patterns of MTS-9 (Fig. 2b) also show characteristic peaks assigned with hexagonal symmetry, suggesting that MTS-9 holds marked hydrothermal stability. Good hydrothermal stability of MTS-9 is ascribed to the following factors: (1) thicker mesoporous wall, (2) incorporation of Ti into silicon sites, and (3) zeolite-like primary and secondary building units in mesoporous wall. These factors are well confirmed by results of XRD, FT-IR and UV-Raman. The increase of a unit cell parameter with Ti incorporation is probably due to the larger size of Ti4+ compared to that of Si4+. Therefore, it may be inferred that Ti is partly incorporated into the framework position and/or walls of the silica network of MTS-9. Fig. 3 shows N2 adsorption–desorption isotherms of MTS-9 with the different ratios of Si/Ti. All the isotherms are type IV in the IUPAC classification and exhibit H1 hysteresis loops, which are typical of mesoporous solids [25,26]. The adsorption isotherms show a sharp inflection at relative pressure between 0.45 and 0.85, which is a characteristic of capillary condensation within mesopore. The increase of titanium causes the decrease of the surface area and the increase of pore volume and pore diameter, as is listed in Table 4. The increase in pore size may be attributed to the larger size of Ti4+ (0.64 Å) than Si4+ (0.40 Å) as well as to the Ti–O bond length being longer than that of Si–O. After hydrothermal treatment, the textural data of samples are listed in Table 4. According

to Table 4, the mesoporous structure (ca.15%) is partly lost; mesoporous structure (85%) is maintained after refluxing in boiling water for 12 h. It indicates that the Ti incorporation can enhance the hydrothermal stability of mesoporous materials MTS-9. UV–visible spectroscopy often is used to characterize the coordination environment of transition metal in the zeolite framework [27,28]. Many researchers have proposed a correlation between the position of the UV–visible absorption band and the nature and coordination of titanium species in silicates. The spectra of samples are shown in Fig. 4. Samples have the obvious adsorption band at 210 nm, which is similar to that of TS-1. According to the literatures [21,29,30], an absorption band in the wavelength region of 200–260 nm is attributed to the ligand-to-metal charge transfer (LMCT) band of the highly dispersed tetrahedrally coordinated titanium. As the ratio of Si to Ti is below 53, width of peak increases with the increase of titanium content, which indicates that octahedral coordinate titanium is present [28,31]. And sample (Si/Ti = 26) displays an absorption band centered at 320 nm, suggesting the presence of another kind of titanium oxide species, i.e., an aggregated bulk titanium oxide species [2,31,32]. The incorporation of titanium in the MFI framework is also confirmed by FT-IR. Fig. 5 shows IR spectra of MTS-9. There are a few of strong absorption bands at 458, 561, 803, 960 cm1 respectively. IR spectrum of MTS-9 shows a broad band at 458 cm1, which is marked with amorphous material. The absorption band at 561 cm1 is assigned to five-membered rings of T–O–T in zeolite [33]. The peaks at 803 and 960 cm1 are attributed to (Si–O–Si) vibration and (Si–OH) or (Si–O–Ti) [34,35], respectively. As shown in Fig. 4, it is found the intensity of the peak at 960 cm1 increases with the increase of the content of titanium, so it has been confirmed that the peak at 960 cm1 can be assigned to Ti–O–Si bond. UV-Raman spectroscopy is a powerful technique to the identification of incorporated framework transition-metal ions in zeolites [36,37]. UV-Raman spectroscopy can be used to identify the state of titanium species based on resonance Raman effects. Fig. 6 shows the UV-Raman spectra of samples with different titanium content. The UV-Raman spectrum of silica is different from that of the MTS9 sample. Several Raman bands at 1095, 975, 804 and 482 cm1 are

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Table 4 Structural parameters of the MTS-9 with different ratios of silica to titanium. Sample

Si/Ti

1 2 3 4

26 37 53 71

Pore volume (cc/g)

Surface area (m2/g)

Average pore size (nm)

Refluxed before

Refluxed after

Refluxed before

Refluxed after

Refluxed before

Refluxed after

1.12 1.07 1.02 1.01

1.06 0.98 0.95 0.86

5.57 5.41 4.58 4.31

5.91 5.87 5.42 5.57

650 763 859 910

591 682 710 704

Table 5 The catalytic performance of catalysts with different Si/Ti ratios on the oxidation of propene.

Si/Ti

Intensity (a.u)

71

53

Sample

Si/Ti

C (H2O2) (%)

C (propene) (%)

S (PO) (%)

TS-1 MTS-9 MTS-9 MTS-9 MTS-9

71 71 53 37 26

95.0 94.6 97.3 93.7 90.2

83.1 83.5 85.3 84.2 82.2

94.0 93.2 92.9 92.6 93.1

37 Reaction conditions: TR = 30 °C, and tR = 1 h.

26

200

250

300

350

400

450

500

550

600

Wavelength (nm) Fig. 4. UV–visible spectra for MTS-9 with different titanium content.

Si/Ti

960

Intensity(a.u)

26

37

53

71

500

1000

1500

-1 Wavelength(cm )

1095

975

804

482

Fig. 5. IR spectra of MTS-9 with different titanium content.

Intensity (a.u)

Si/Ti 26

37

53 71 SiO2

200 400 600 800 1000 1200 1400 1600 1800 2000

Raman shift (cm-1) Fig. 6. Raman spectra of MTS-9 with different titanium content.

common to MTS-9 and SiO2, suggesting the structure of MTS-9 remains the main feature of SiO2 [36]. There are differences, the relative intensity of bands at 975 and 804 cm1 keeps unchanged, while the relative intensity of others increases with titanium contents. Considering the results of the UV–Vis spectra, the trend is attributed to the amount of change in the titanium species in the framework. This is further confirmed that the bands are lonely associated with the framework titanium atoms of MTS-9. Raman bands at 1095 and 482 cm1 are observed in the UV-Raman spectra of MTS-9 samples, which are attributed to framework titanium species located mainly in the framework in an isolated state [36], whereas there are the characteristic Raman bands of TiO2 (anatase), at 144, 390, 516 and 637 cm1 for sample (Si/ Ti < 26), and these bands overlap band centered at 482 cm1. In this work, the catalytic performance of MTS-9 samples is investigated by oxidation of olefin compounds. Comparison of the results is made, and experimental data are listed in Table 5 and 6. TS-1(71) and MTS-9(71) show the similar activity for oxidation of propene, and the MTS-9 (Si/Ti = 53) shows the highest activity for propene oxidation with H2O2 as an oxidant, whereas MTS-9 (Si/Ti = 26) is the lowest active (Table 5). It has been suggested that the isolate, tetrahedral Ti(IV) in the –Si–O–Ti–O–Si– zeolite matrix are the active sites for the reaction, while octahedrally coordinated Ti(IV) is inactive [38–41]. TS-1(71) and MTS-9(71) were prepared by the same precursor, and all possess MFI primary structure, which is verified by IR band at 560 cm1, so TS-1 and MTS-9 hold the same activity. The catalytic activity increases with the decrease of the Si/Ti ratio, and the activity is the highest for MTS-9 with Si/ Ti = 53. Then, with further increase of titanium content, the catalytic activity decreases. In according to UV-DRS, the ratio of Si/Ti is above 53, the amount of isolate, tetrahedral Ti(IV) ions increases with the increase of titanium content, so the catalytic activity increases. With further increase of titanium content, the titanium species tend to aggregate and form non-framework titanium species which are inactive for the oxidation of olefin, therefore, the activity decreases. Diffusion of bulky molecules through the narrow zeolite pore channel (0.51 nm) to the active sites is subject to the pore size of the catalyst. So styrene is chosen as the reactant to study the effect of pore size on the activity. For the oxidation of styrene, the primary product expected from the oxidation of styrene is styrene oxide (SO). In contrast, phenylacetaldehyde (PADH) and

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D. Hua et al. / Microporous and Mesoporous Materials 171 (2013) 179–184 Table 6 The catalytic performance of catalysts with different Si/Ti ratios on the oxidation of styrene. Sample

Si/Ti

C (H2O2) (%)

TOF (styrene) (h1)

S (PA) (%)

S (BA) (%)

Othera

TS-1 MTS-9 MTS-9 MTS-9 MTS-9

71 71 53 37 26

11.7 21.2 29.5 21.0 19.1

9.59 20.85 22.99 11.69 8.31

78.1 79.9 80.2 78.7 76.8

18.5 18.7 13.8 17.5 21.0

3.4 2.4 2.0 3.8 2.2

Reaction conditions: TR = 30 °C, and tR = 5 h. TOF: gram of styrene converted per gram of Ti on the catalyst per hour. a S (PA) and S (BA) are denoted as mixture of 2-methoxy 2-phenyl ethanol, styrenediol and some unidentified compounds, selectivity to phenylacetaldehyde and benzaldehyde, respectively.

(Si/Ti = 53), which possesses more isolated titanium species, holds the highest activity. For oxidation of propene, the selectivity of the product has nothing to do with the catalyst. However, for oxidation of styrene, phenylacetaldehyde and benzaldehyde are major the products, and titanium content affects the distribution of the product, the increase of titanium content favors to the formation of benzaldehyde. Acknowledgements This work is supported by National Natural Science Foundation of China (Nos. 21176142, 20506011), Independent Research Programs of Tsinghua University (No. 2011Z08141). References Fig. 7. Reaction paths for the oxidation of styrene.

benzaldehyde (BADH) are major products, similar observations were also reported by earlier workers [4], which forms by rearrangement of styrene oxide and cleavage of C@C double bond [22,42], respectively. The reaction scheme is shown in Fig. 7. The dynamic diameter of styrene is bigger than the pore size of TS-1, it is difficult for the bulk reactant molecule to access the active sites inside the pore of TS-1, the oxidative reaction is diffusion-limited, and reactants only access the active sites on the extra-surface of catalyst, the active centers in micropore are nearly noneffective, which results in the low oxidation ability. As is listed in Table 6, TS-1 catalyst exhibits the low activity in oxidation of styrene. For MTS-9 catalyst, the activity of MTS-9 (Si/Ti = 53) is the highest, even for MTS-9 (Si/Ti = 26), the activity of that is higher than that of TS-1, which was attributed to the fact that the MTS-9 was synthesized with TS-1 precursor as titanium sources, the zeolite-like active sites and corresponding micropore can be introduced into mesoporous walls. Therefore, catalysts with large channel are preferent to bulk molecules. In addition, it is found that S(PADH) increases with the increase of titanium content, while the S(BADH) experiences a converse trend with the increase of the titanium content. The non-framework titanium species, such as the TiOx nano-particles in the sample (Si/Ti < 53), are present, which is verified by the results of UV-DRS and Raman. The non-framework titanium species favor the carbon–carbon bond cleavage [4,43]. Therefore, the catalyst with Si/Ti ratio below 42, the increase in the amount of TiOx nano-particles will promote the bond scission reaction, leading to formation of more benzaldehyde. 4. Conclusions In the work, samples with the different ratio of Si to Ti have been successfully synthesized, and are applicable to the oxidation of propene and styrene. The effect of titanium content and comparison of catalytic performance for MTS-9 and TS-1 were investigated with hydrogen peroxide as an oxidant. Among all catalysts, catalyst

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