In Situ UV Raman Spectroscopic Study on the Reaction Intermediates

Article pubs.acs.org/JPCC

In Situ UV Raman Spectroscopic Study on the Reaction Intermediates for Propylene Epoxidation on TS-1 Longlong Wang, Guang Xiong,* Ji Su, Peng Li, and Hongchen Guo School of Chemical Engineering, State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, Liaoning, China ABSTRACT: The reaction intermediates for propylene epoxidation were investigated by in situ UV Raman spectroscopy. A feature at 837 cm−1, which has been assigned to the O−O stretching mode in the Ti−OOH (η2) species, was observed in Raman study for the first time. The physisorbed H2O2 and triangular Ti(O2) species are also present at different stages during the in situ measurement. The results obtained by gas chromatography-Raman spectrometry (GCRaman) during the propylene epoxidation process give direct evidence that the 6-coordinated Ti−OOH (η2) intermediates are important active sites for propylene epoxidation. After adding CH3OH to the TS-1/H2O2/H2O/system, the Raman spectra show the existence of a Ti−O−CH3 moiety and 6-coordinated Ti−OOH (η2) species. The five-numbered ring titanium hydroperoxo intermediate was not detected in this study. The results showed that the 6-coordinated Ti−OOH (η2) species are active sites in the methanol-included epoxidation reaction process. cm−1 (385 nm) corresponding to the yellow color is clearly observed. This band has been assigned to LMCT from an O− O moiety to the Ti center.11−13 Unfortunately, the fine configuration of the peroxo or hydroperoxo complexes cannot be identified by only this technique. For further investigation, X-ray absorption spectroscopies (both in the XANES2,3,14 and EXAFS12,15−17 regions) have been applied in investigating the structure of the species formed by contact between H2O2 and Ti centers inside the TS-1 framework. For the XANES spectrum, Bordiga et al.8 showed that the features characteristic of Ti(IV) in a tetrahedral environment are drastically modified, indicating the complete loss of the Td symmetry. At the same time, two prominent and well-defined features around 4984 and 4995 eV appear in the edge and postedge region. As for the EXAFS data, both the first and second coordination around the Ti atom are changed completely.8 Well interpreted in terms of the rupture of a Ti− O−Si bridge, EXAFS data showed that there are 2.8 first shell framework ligand and two O atoms from a new side-on O−O ligand located at 2.01 Å.12,18 The X-ray absorption spectroscopy data, accompanied by the band at 26000 cm−1 (385 nm) in the UV−vis spectrum, has been considered as the fingerprints of a side-on η2 Ti-peroxo complex.8 FT-infrared spectroscopy is another important technique for investigation of the TS-1/H2O2/H2O system.19,20 The Ti− OOH (η2) species, considered as the active oxidation site in the H2O2-loaded TS-1 sieve, was directly detected with the use of

1. INTRODUCTION The excellent catalytic performances of Ti-silicalite-1 (TS-1)1 have been found in selective oxidation of organic compounds2−5 over the past decades. In particular, TS-1 catalyst demonstrated markedly high selectivity in olefin epoxidation using H2O2 as oxidant at low temperature.2,4,6,7 Numerous efforts have been made to detect and identify intermediates for epoxidation reactions with H2O2 on TS-1. However, nearly 30 years after the discovery of this catalyst, the detailed structures of the active intermediates are still debated. One reason for the controversy is due to the presence of water, which may preclude the applications of IR, XANES, and EXAFS spectroscopies. The other important reason is ascribed to lability of the complexes formed by H2O2 on Ti(IV) centers in TS-1.8 In spite of all these problems, great efforts still have been made to detect and identify the intermediates for the reaction. By the titration experiments in aqueous medium with NaOH, Bonino et al.9 observed that the acidity of the TS-1/H2O2/H2O system is remarkably enhanced with respect to the acidity of the other four separate systems (H2O2/H2O, TS-1/H2O, SiO2/ H2O2/H2O, and S-1/H2O2/H2O). The increased acidity of the TS-1/H2O2/H2O system is ascribed to a peculiar interaction between hydrogen peroxide and Ti(IV) centers. These data can be simply explained by assuming that Ti−OOH species is formed after the addition of H2O2/H2O solution on dehydrated TS-1 samples.9 For a long time, UV−vis DRS spectroscopy has been the most-used technique to investigate the nature of the active species in the TS-1/H2O2/H2O system, as the presence of water has no influence on the UV−vis spectrum.10 Upon the addition of H2O2/H2O solution, a new band around 26000 © 2012 American Chemical Society

Received: February 22, 2012 Revised: March 30, 2012 Published: April 6, 2012 9122

dx.doi.org/10.1021/jp3017425 | J. Phys. Chem. C 2012, 116, 9122−9131

The Journal of Physical Chemistry C

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in situ FT-infrared spectroscopy.20 An O−O stretch absorption at 837 cm−1 and a broad OH band at 3400 cm−1 are attributed to Ti−OOH (η2) species. After propylene loading, the Ti− OOH (η 2 ) feature at 837 cm −1 disappeared quickly; consequently, a conclusion was obtained that Ti−OOH (η2) is the active site of olefin epoxidation in TS-1. Besides experimental approaches, computer calculations8,16,21−23 have also been adopted to investigate the species formed upon dosing hydrogen peroxide on titanium-silicalite-1 zeolite. According to DFT calculations at the B3-LYP level performed on a HYDRap structure, Bordiga et al.8 concluded that the [HOO−Ti(OSi)3] moiety in the HYDRap is a reliable model of the Ti−OOH (η2) species which have been experimentally observed by XANES and EXAFX spectra in the TS-1/H2O2/H2O system.24 Ti−OOH (η2) species have also been predicted as the most stable Ti hydroperoxo species with ab initio and DFT cluster calculations.15,25,26 Accordingly, possible intermediates have been put forward (see Scheme 1).

bands are considered to be related to the [TiO4] units in the lattice.37−39 Recently, an enhanced Raman band at 700 cm−1 was assigned to the Ti−O−Ti linkages of the nonframework amorphous Ti species.36 In the spectra excited by 325 and 532 nm lines, the bands at 144, 395, and 637 cm−1 are ascribed to nonframework anatase TiO2.38 Furthermore, Bordiga et al. found a new band at 618 cm−1 with an application of a 442 nm laser source.11,31 The new feature is assigned to a Raman enhanced vibration mode of a side-on η2 Ti-peroxo complex in the TS-1/H2O2/H2O system. Unfortunately, further in situ Raman investigation has not been done since then; i.e., the peroxo or hydroperoxo complexes have not been studied either during the aging process or the epoxidation reactions by Raman spectroscopy. In this study, the formation of the reaction intermediates on TS-1 for propylene epoxidation is first explored by in situ UV Raman spectroscopy. The intermediates, i.e., the peroxo or hydroperoxo complexes, were detected after dropping H2O2/ H2O aqueous solution into the TS-1 sieve or introducing propylene into the TS-1/H2O2/H2O system. In particular, the new feature of a side-on Ti−OOH (η2) complex is observed by Raman spectroscopy. Accordingly, the most possible active oxygen-donating intermediates for propylene epoxidation on TS-1 are proposed.

Scheme 1. Possible Intermediates in the TS-1/H2O2/H2O System

2. EXPERIMENTAL SECTION Synthesis. The TS-1 molecular sieve was prepared according to the literature.32 Chemical reagents include tetrabutylorthotitanate (98%, Kefeng chemical reagent Co., Ltd., Shanghai), tetraethyl orthosilicate (AR, Westlong Chemical Plant, Shantou), isopropyl alcohol (AR, Fuyu Fine Chemical Co., Ltd., Tianjin), and tetrapropyl ammonium hydroxide (TPAOH, synthesized according to the literature, 1.55 M33). The titanium source and silicon source were hydrolyzed in TPAOH solution at 298 K. After being hydrolyzed for 6 h, the two solutions were mixed together and then the gel was crystallized at 443 K for 72 h. Finally, the sample was calcined at 813 K for 6 h to remove the template. The molar composition is 1.0 SiO2:0.02 TiO2:0.3 TPAOH:1.0 IPA:30 H2O. Elemental analysis shows that the Si/Ti ratio of the TS-1 sample is 53.7. For comparison, silicalite-1 zeolite (denoted as S-1) was synthesized according to the same procedure without adding the titanium source. Characterization. Elemental analysis has been carried out with an XRS-3400 X-ray fluorescence (XRF) spectrometer. X-ray powder diffraction patterns of the samples were taken on a Rigaku D/Max 2400 diffractometer (Shimadzu Co.) using a nickel-filtered Cu Ka X-ray source at a scanning rate of 0.02 over the range between 5.0 and 40.0°. The UV−vis spectra were recorded on a SHIMADZU UV240 spectrometer using BaSO4 as a reference. UV-Raman spectra were recorded on a DL-2 Raman spectrometer, with a collection time of 300 s. A 244 nm line of LEXEL LASER and a 325 nm line of He−Cd laser were used as the excitation sources. The laser power at the sample was less than 5 mw. An Acton triple monochromator was used as a spectrometer for Raman scattering. The spectra were collected by a Prinston CCD detector. Powder of the TS-1 sample (0.05 g) was pressed into a self-sustaining pellet. After dropping 50 μL of H2O2 aqueous solution (AR, 30%), the aging process of the TS-1/H2O2/H2O system was recorded by UV Raman spectrometer. Also, water was added to the pellet after hours of

It is generally accepted that the η2 titanium-hydroperoxo groups (Scheme 1b) are the active oxygen-donating intermediates for the epoxidation process of alkenes.2,11,13,18−21,24,27,28 Raman spectroscopy is considered as one of the most powerful tools for characterizing catalysts and reagents, particularly under reaction conditions.29 Unlike IR spectroscopy, there is no characterization problem caused by the presence of H2O when the Raman signal is acquired. Additionally, using continuous-wave ultraviolet laser beams, sample fluorescence can be simply avoided in the ultraviolet Raman spectrum. Ever since the early 1990s, Raman spectroscopy has been applied to the characterization of TS-1 catalysts. The feature band at 1125 cm−1, recognized to be a fingerprint of the insertion of Ti atoms in the zeolitic framework, was first detected by Scarano et al. in 1993.30 In particular, using a 244 nm laser source, three bands at 490, 530, and 1125 cm−1 are extremely enhanced. These 9123



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aging, and corresponding changes of the Raman spectra were recorded. During the in situ epoxidation experiments, the pellet of the TS-1 sample (0.05 g) was moved into a stainless steel cell equipped with a quartz window after dropping H2O2/H2O solution or H2O2/H2O/CH3OH solution. Propylene (3% propylene and 97% helium gas in volume fraction) was introduced into the cell at a flow rate of 40 mL/min. The reaction process was recorded with the UV Raman spectrometer. Meanwhile, the component analysis of the output gas was performed on a GC9790 gas chromatograph, using a flame ionization detector and a capillary column (PEG20M, 30 m). The concentration of propylene oxide was represented directly by its peak aera in the GC spectra.

3. RESULTS AND DISCUSSION The XRD patterns of TS-1 and S-1 are shown in Figure 1. Compared with S-1, the TS-1 sample shows fine MFI crystal

Figure 2. UV−vis spectrum of the TS-1 sample.

attributed to the bending and symmetric stretching vibrations of the framework Ti−O−Si species, while the band at 1125 cm−1 is assigned to the totally symmetric stretching mode of the [TiO4] units in the framework.37,38,39 The band at 960 cm−1, appearing in both IR and Raman spectra of TS-1, has been reckoned as a sign of access of Ti atoms into the framework.30,37 Ricchiardi et al. confirmed the assignment of the band at 960 cm−1 to the asymmetric stretching of the [TiO4] unit, which can equally be described as the out-of-phase antisymmetric stretching of the four connected Ti−O−Si oscillators, or as the out-of-phase stretching of the four Si−O bonds pointing toward Ti.37 Identification of the Ti−OOH (η2) Species. Figure 3b shows the Raman spectra of the TS-1 and S-1 mixed with a small amount of H2O2 aqueous solution. After dropping 10 μL of H2O2 aqueous solution (30%) into the pellets of the TS-1 sample (0.05 g), the color of TS-1 pellets quickly turned yellow and then the yellow colored pellets became cream colored within seconds. Raman spectra were obtained with both 244 and 325 nm laser lines (see Figures 3b and 4b, respectively). From the Raman spectra collected with a 244 nm laser line, the two bands at 960 and 1125 cm−1 shift to 990 and 1137 cm−1, and the 1125 cm−1 band is seriously quenched. The blue shift of the band at 960 cm−1 has been attributed to the expansion of the Ti coordination sphere.20 This is consistent with the blue shift of the same Raman band when H2O or NH3 was added into the TS-1 lattice, resulting in the formation of [Ti(H2O)2O4] or [Ti(NH3)2O4] complexes.11,37 Accordingly, the blue shift of the 960 cm−1 indicates that the coordination number of Ti atom in the framework increases from 4 to 6 after contacting H2O2 aqueous solution (30%). Once the Ti coordination sphere expands, the Td-like symmetry of Ti(IV) species is destroyed, and the symmetry of the vibrational modes is no longer the same as that of the LMCT. This well explains why the 1125 cm−1 band shifts to 1137 cm−1 and the resonance Raman is quenched. Moreover, a shoulder at 618 cm−1 has been assigned to the symmetric breathing mode of the Ti(O2) cycle, which has been discussed in the introduction part. Except the bands discussed above, a more important change in the spectra collected with the 244 nm line is the appearance of a new feature at 837 cm−1, which has never been reported in Raman spectra of the TS-1/H2O2/H2O system (see Figure 3b). This band has only been reported by Lin et al. in the in situ FTinfrared spectra,20 and was assigned to a Ti−OOH (η2) species.

Figure 1. XRD patterns of TS-1 and S-1.

structure and no other phase is detected. However, the enhancement of the peaks at 24.4 and 29.3° demonstrates that conversion occurs from a monoclinic symmetry of silicalite to an orthorhombic symmetry of titanium-silicalite-1.1,35 This phenomenon has been considered as evidence that Ti atoms are incorporated into the MFI framework. The UV−vis spectrum of the TS-1 sample is provided in Figure 2. The TS-1 sample shows the strong band at 212 nm, indicating the presence of framework Ti species. A relatively weak absorption band at 320 nm assigned to nonframework TiO2 (anatase) is also present.34 An amorphous nonframework Ti species corresponding to an absorption band at 260 nm is not found,36 suggesting no other Ti species are formed except framework Ti species and nonframework TiO2 (anatase). Figure 3a shows Raman spectra of TS-1 and S-1, collected with a 244 nm laser line. The Raman spectrum of S-1 exhibits the peaks at 380, 800, and 975 cm−1. The bands at 380 and 800 cm−1 are characteristic of S-1 zeolite,38 while the peak at 975 cm−1 is attributed to the Si−O stretching mode in Si−OH groups.37 In the spectrum of TS-1, several new peaks are observed by using a 244 nm line. The Raman peaks at 490, 530, and 1125 cm−1 are associated with the framework titanium species because they only appear when the laser line excites the charge-transfer transition of the titanium species in the framework of TS-1.38 The peaks at 490 and 530 cm−1 are 9124



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Figure 3. Raman spectra of TS-1 and S-1 collected with a 244 nm laser: (a) before contacting with H2O2/H2O solution; (b) after contacting with a small amount of H2O2 aqueous solution.

Figure 4. Raman spectra of TS-1 and TiO2 collected with a 325 nm laser: (a) before contacting with H2O2/H2O solution; (b) after contacting with a small amount of H2O2 aqueous solution.

The 837 cm−1 band, representing the Ti−OOH (η2) species, is also found in the Raman spectra collected with the 325 nm line (shown in Figure 4b). In contrast, the spectrum of S-1 shows only a new band at 875 cm−1, which is associated with the physisorbed H2O2 in TS-1.10,11,31 Figure 4a shows the Raman spectra of TS-1 and TiO2 excited by the 325 nm line. The 325 nm laser, though in the ultraviolet region, cannot completely avoid the fluorescence of TS-1. It is clear that the TS-1 sample contains nonframework anatase TiO2, which shows the Raman peaks at 144, 390, 515, and 637 cm−1. After dropping H2O2 on TS-1, the fluorescence background is greatly reduced. The weak bands at 837 and 875 cm−1, which are assigned to the Ti−OOH (η2) species and physisorbed H2O2 in TS-1, respectively, can be observed. To avoid the influence of TiO2 (anatase) and the fluorescence, the Raman spectra of further investigation were collected with only the 244 nm laser line. After dropping 50 μL of H2O2 aqueous solution (30%) to the pellet of the TS-1 sample (0.05 g), the aging process of the TS1/H2O2/H2O system was characterized by using a 244 nm laser line. The Raman spectra are shown in Figure 5. The addition of H2O2 aqueous solution partly quenches the bands at 490, 530, and 1125 cm−1 immediately. The band at 875 cm−1, which is attributed to H2O2/H2O solution physisorbed into the zeolite channels,10,11,31 is initially observed when liquid phase H2O2

aqueous solution is still present on the surface of the TS-1 sample pellet (Figure 5b and c). As the aging time increases, the

Figure 5. In situ Raman spectra of the TS-1/H2O2/H2O system in the aging process, collected with a 244 nm laser: (a) TS-1 before contacting with H2O2 aqueous solution; (b) TS-1 after contacting with H2O2 aqueous solution for 6 min and (c) 15 min; (d) 30 min; (e) 45 min; (f) 60 min; (g) TS-1/H2O2/H2O system after adding H2O for 6 min and (h) 45 min. 9125



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Scheme 2. Possible Defective Ti Species in the TS-1 Latticea

875 cm−1 band gradually disappears and the pellet becomes cream colored. Meanwhile, another band at 837 cm−1 appears and its intensity increases with increasing aging time (see Figure 5d−f). To confirm the influence of water on the band at 837 cm−1, 10 μL of water was dropped to the cream colored pellet. The band at 837 cm−1 is clearly weakened because of the addition of water (Figure 5g). When evaporation occurs on the surface, the band at 837 cm−1 is recovered (Figure 5h). The intensity of the peak at 618 cm−1, which is assigned to the symmetric breathing mode of the Ti(O2) cycle, is relatively weak. The 837 cm−1 peak, which was initially found in the in situ IR spectra,20 has been assigned to the O−O stretching in the Ti−OOH (η2) species instead of the Ti−OOH (η1) species. Computational calculations have shown that, in the absence of solvent, the Ti−OOH (η2) species is ∼33 kJ/mol more stable than the η1 species.23,25,26 In addition, early DFT computation study showed that Ti atoms in the Ti−OOH (η2) species are 6coordinated, forming a distorted octahedral complex involving three oxygens of Si−O−Ti bridges, one oxygen atom of the adsorbed water, and two oxygen atoms from the hydroperoxy complex.15,22,27 Earlier literature also pointed out that the stability of the active 6-coordinated Ti−OOH (η2) species is likely due to the existence of the unfilled d-band of Ti and the physical nature of s−d hybridization,22 which differs from sp3 hybridization found in Si bounding. Recent experimental investigation9,24 together with computational calculations by the ONIOM approach have shown that an equilibrium does exist between the η2 Ti-hydroperoxo and the η2 Ti-peroxo,8,21 which is strongly influenced by the amount of water. It has been pointed out by Bordiga et al. that H2O molecules are responsible for the hydrolysis process of one Si−O−Ti bridge in the perfect tetrahedral [Ti(OSi)4] units.8 Both experiment40−44 and QM/MM investigation45,46 have shown that, when TS-1 is exposed to water, one Si−O−Ti bridge will be hydrolyzed and defective Ti moieties will dominate in hydrous conditions. Furthermore, it has been proposed that the defective Ti species are the actual catalytic sites in mesoporous Ti-silica catalysts.23 We propose three kinds of defective Ti species in this paper according to former literature,22,23,47 as shown in Scheme 2b−d. Scheme 2b shows the first kind of defective Ti species, resulting from the hydration of the perfect tetrahedral [Ti(OSi)4] units. However, using EXAFS and photoluminescence technology, Lamberti et al.42 indicated that, even for the samples dehydrated in a carefully controlled atmosphere at 400 K, a considerable fraction of Ti sites exhibits the substitution of a bridged oxygen with two OH groups. And this is the main type of defective Ti species in the TS-1/H2O2/H2O system. The second defective Ti species (Scheme 2c) has also been proved to coexist with the first one (Scheme 2b).10,48,49 Lamberti et al. gave direct evidence that both Ti atom and Si vacancy are most likely to be located in the same three preferential T sites (T7, T10, and T11). This means that a considerable fraction of Ti sites is located adjacent to a Si vacancy, resulting in [Ti(OSi)3OH] units adjacent to a hydroxylated nest.22,47 Consequently, beside regular [Ti(OSi)4] units, defective [Ti(OSi)3OH] units could also be significantly present in the TS-1 lattice10 (see Scheme 2c). The last kind of defective Ti species is shown in Scheme 2d. This kind of defective Ti species, proposed to be located only on the exterior termination surface of the TS-1 crystallites,47 occupies a very small fraction of all the defective Ti species.

a

(a) Perfect tetrahedral [Ti(OSi)4] units; (b) Defective Ti species resulting from the hydration of the perfect tetrahedral [Ti(OSi)4] units; (c) defective Ti species located adjacent to Si vacancies; (d) defective Ti species located only on the exterior termination surface of the TS-1 crystallites.

However, it has been proved to be a very active site for partial oxidation.23,47 Formation of 6-coordinated Ti−OOH (η2) complexes from all three kinds of defective Ti species discussed above is shown in Schemes 3c, 4b, and 5b, respectively. It has to be noticed that, for the 6-coordinated Ti−OOH (η2) complexes formed from a defective Ti species shown in Scheme 2b, an oxygen atom in the adjacent Si−OH but not a water oxygen bounds to the Ti center.8 This is because the oxygen atom in the adjacent Si−OH is in the first shell, and it could more easily bound to the Ti center, compared with a water ligand. With the influence of additional water molecules, equilibrium between the three different 6-coordinated Ti−OOH (η2) complexes and their corresponding 6-coordinated η2 Ti-peroxo complexes are also given in Schemes 3, 4, and 5, respectively. The 837 cm−1 band, observed in the in situ UV Raman spectra, is consequently assigned to the O−O stretching in the three kinds of 6-coordinated Ti−OOH (η2) complexes. For any type of Ti−OOH (η2) complexes, the quenching and recovery of the 837 cm−1 band confirms that water plays an important role in the formation process of Ti−OOH (η2) complexes. Role of the Ti−OOH (η2) Species in Partial Oxidation. To further investigate the role of the 6-coordinated η2 Tihydroperoxo complexes in partial oxidation, propylene was introduced into the in situ cell, after dropping 50 μL of H2O2 aqueous solution (30%) on the pellet of the TS-1 sample (0.05 g). Raman spectra were collected at different times of the 9126



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the concentration of propylene oxide provides the direct evidence that the 6-coordinated Ti−OOH (η2) complexes are the active oxygen-donating intermediates for the epoxidation process of propylenes. This result agrees well with the computational studies.15,21−23,27 With the disappearance of the 837 cm−1 feature, a band at 1640 cm−1 appears, and its intensity increases with increasing reaction time. The band corresponds to propylene physisorbed into the zeolite channels. Figure 6B shows the Raman spectra of the absorbed propylene on the bared TS-1 sample. The peaks at 1396 and 1640 cm−1 are assigned to CH and CC stretching of propylene, respectively. It is clear that propylene can be absorbed on the bare TS-1 in 5 min. The amounts of the absorbed propylene increase as the time continues. In contrast, the spectra of the absorbed propylene are not immediately observed when H2O2 solution is dropped onto the TS-1 in advance (see Figure 6A). It seems that the H2O and peroxo species on the surface prevents the absorption of propylene. Another possibility that cannot be entirely excluded is that propylene may be absorbed on the surface, but the reaction between the adsorbed propylene and 6-coordinated TiOOH complex is very fast. Therefore, the concentration of the stably absorbed propylene is very low, and they cannot be detected by Raman spectroscopy. Once the physisorbed H2O2 or TiOOH have been almost consumed, the stably absorbed propylene is present immediately in the spectra. Role of Methanol in the Epoxidation Process of Alkenes. Numerous studies have proved that methanol is the best solvent for liquid phase epoxidation of alkenes.2,3,7,14,27,35,40 A five-numbered ring titanium hydroperoxo intermediate (Scheme 6a) was proposed to be the oxygendonating species in solvent-involved epoxidation reactions,2,3,14,40 because the hydrogen bounding in the fivenumbered ring could stabilize both the hydrogen peroxide complex and the transition state for epoxide formation. However, later computational calculations argued that the activation barrier for epoxidation via a five-numbered ring is very high (210 kJ/mol),23 and that the five-numbered intermediate is not significantly more stable than other titanium hydroperoxo intermediates involving a single protic ligand on the Ti sites.16,27,28 A dual influence of methanol content in the solvent on the reaction rate was reported recently by Shin et al.35 They stated that the presence of methanol in the solvent would enhance the solubility of propylene, however, excessive methanol would inhibit the epoxidation reaction by competitive

Scheme 3. Possible Formation Process of 6-Coordinated Ti−OOH (η2) Complexes from Defective Ti Species Resulting from the Hydration of the Perfect Tetrahedral [Ti(OSi)4] Units

reaction (see Figure 6A). A GC was connected to detect the concentration of propylene oxide after the reaction. Corresponding GC results together with the intensity of the 837 cm−1 band are shown in Figure 7. The band at 837 cm−1 is obviously influenced by the continuous introduction of propylene. What’s more, the intensity of this band is directly related to the concentration of propylene oxide. During 13−40 min, the intensity of the band at 837 cm−1 increases; meanwhile, the peak area of propylene oxide in GC spectra shows the same trend. After 46 min, both of the intensities drop sharply. The close relationship between the 837 cm−1 peak and

Scheme 4. Possible Formation Process of 6-Coordinated Ti−OOH (η2) Complexes from Defective Ti Species Which Are Located Adjacent to Si Vacancies

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Scheme 5. Possible Formation Process of 6-Coordinated Ti−OOH (η2) Complexes from Defective Ti Species Which Are Located Only on the Exterior Termination Surface of the TS-1 Crystallites

Figure 6. (A) In situ Raman spectra of the TS-1/H2O2/H2O system obtained at different times during the epoxidation reaction: (a) Raman spectrum obtained at the 6th minute, (b) 13th minute, (c) 20th minute, (d) 27th minute, (e) 33rd minute, (f) 40th minute, (g) 46th minute, (h) 53rd minute, (i) 65th minute, and (j) 85th minute. (B) In situ Raman spectra of the bare TS-1 obtained at different times with the flow of propylene: (a) TS-1 before contacting with propylene and (b) after introducing propylene for 5 min, (c) 10 min, (d) 25 min, and (e) 35 min.

peaks at 1033, 1124, and 1464 cm−1 are characteristic of CH3OH50 (Figure 9b and c). In the Raman spectra TS-1/ CH3OH system (Figure 9b), a new band at 600 cm−1 is attributed to the Ti−O stretching mode in the Ti−O−CH3 moiety,51 and its appearance gives direct evidence that the solvent molecule (CH3OH) bonds directly to the Ti center.2,14,27,40 In the TS-1/H2O2/H2O/CH3OH system, the 600 cm−1 band also appears together with a new feature at 836 cm−1, which has been assigned to the O−O stretching in the Ti−OOH (η2) intermediates. This indicates that Ti sites in the TS-1/H2O2/H2O/CH3OH system bond directly to the oxygen atom of the adsorbed methanol (or H2O), two oxygen atoms from the hydroperoxy complex, and three oxygen atoms from Si−O−Ti bridges, forming distorted octahedral 6-coordinated Ti−OOH (η2) species. The detailed configuration of the possible species is shown in Scheme 6b and c. After dropping 50 μL of H2O2/H2O/CH3OH solution (the wt % of CH3OH in the solvent is 42%) on the TS-1 sample pellet (0.05 g), the Raman spectra of the TS-1/H2O2/H2O/ CH3OH system under a continuous flow of propylene are collected and shown in Figure 10. The corresponding GC results and intensity of the 837 cm−1 peak are shown in Figure 11. It is found that the feature at 837 cm−1 is more conspicuous but disappears more quickly. The peak obviously appeared from the very beginning to the 18th minute, corresponding to a higher concentration of propylene oxide from the 5th minute to the 18th minute. The excellent consistency between the intensity of the band at 837 cm−1 and the concentration of

Figure 7. (a) Peak area of propylene oxide in the GC spectra obtained at different times during the epoxidation reaction. (b) Raman intensity of the 837 cm−1 band at different times during the epoxidation reaction.

adsorption. Solvents with different methanol concentrations were used in our experiment, and the concentration curves of propylene oxide were recorded with GC (see Figure 8). The formation rate reached the maximum value when the wt % of methanol was 42%. This data agrees well with that in ref 35. For further Raman investigation, the wt % of methanol in the solvent was adopted for 42%. Figure 9 shows the Raman spectra of the TS-1/H2O2/H2O, TS-1/CH3OH, and TS-1/H2O2/H2O/CH3OH systems collected after a soaking procedure of TS-1 pellets (0.05 g). The 9128



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Scheme 6. (a) Five-Membered Ring Ti-Hydroperoxo Intermediates Proposed in the TS-1/H2O2/H2O/CH3OH System; (b and c) Methanol-Involved 6-Coordinated Ti− OOH (η2) Species

Figure 9. Raman spectra of the different systems: (a) TS-1/H2O2/ H2O system; (b) TS-1/CH3OH system; (c) TS-1/H2O2/H2O/ CH3OH system.

Figure 10. In situ Raman spectra of the TS-1/H2O2/H2O/CH3OH system obtained at different times during the methanol-included epoxidation reaction, collected with a 244 nm laser: (a) Raman spectrum obtained at the 6th minute, (b) 12th minute, (c) 18th minute, (d) 25th minute, (e) 32nd minute, and (f) 38th minute.

Figure 8. Comparison of peak areas of propylene oxide in the GC spectra obtained at different times during the methanol-included epoxidation reaction.

propylene oxide shows direct experimental evidence that the 6coordinated Ti−OOH (η2) species (Schemes 5c and 6c) are the active sites in the methanol-included epoxidation reaction process. With the exhaustion of solvent-included oxygendonating intermediates, the less distorted 6-coordinated Ti sites (corresponding to the 992 and 1137 cm−1 bands) are formed, and physisorbed propylene appears (corresponding to the 1640 cm−1 peak). The 6-coordinated solvent-included Ti−OOH (η2) species has been first reported by Sinclair and Catlow,23 and the corresponding solvent-included epoxidation reaction scheme is available in ref 23. However, a five-numbered ring titanium hydroperoxo intermediate cannot be detected in this study. By DFT study, Sever and Root found that hydrogen bonding of the hydroperoxo intermediates with a protic solvent ligand via a

Figure 11. (a) Peak aera of propylene epoxidation in the GC spectra obtained at different times during the methanol-included epoxidation reaction. The wt % of CH3OH in the solvent is 42%. (b) Raman intensity of the 837 cm−1 band at different times during the methanolincluded epoxidation reaction. 9129



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monodentate five-numbered ring structure neither confer significant stabilization to the titanium hydroperoxo intermediate nor enhance the reactivity of the titanium hydroperoxo complex. A more reasonable explanation for the increase of reaction rate in the solvent-included epoxidation process is that methanol could increase both the intraporous concentration of alkene substrate and hydrogen peroxide oxidant available at the Ti sites.27,28

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CONCLUSIONS A gas chromatography-Raman spectrometry (GC-Raman) technique is used to identify the reaction intermediates for propylene epoxidation on TS-1. A band at 837 cm−1, corresponding to the O−O stretching mode in the 6coordinated Ti−OOH (η2) species, is found using in situ UV Raman spectroscopy. The band intensity is directly related to the formation rate of propylene oxide when the epoxidation reaction of propylene is performed. This provides direct evidence that the 6-coordinated Ti−OOH (η2) intermediates are the active sites for propylene epoxidation. With the presence of methanol, a band at 600 cm−1 assigned to the Ti−O stretching mode in the Ti−O−CH3 moiety is also found in Raman spectra. This indicates that Ti sites in the TS-1/ H2O2/H2O/CH3OH system bound directly to one oxygen atom of adsorbed methanol (or H2O), two oxygen atoms from the hydroperoxy complex, and three oxygen atoms of the Si− O−Ti bridges, forming a distorted octahedral 6-coordinated Ti−OOH (η2) species. In situ UV Raman spectra of the TS-1/ H2O2/H2O/CH3OH system in propylene epoxidation also give direct experimental evidence that the 6-coordinated Ti−OOH (η2) species are active sites in the solvent-included epoxidation reaction process. The five-numbered ring titanium hydroperoxo intermediate was not detected in this study.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Dalian University of technology-Qiwangda Research Center for Catalytic technology, the National “973” Project of China (2011CB201301), and the National Science Foundation of China (NSFC, Grant 20603004, 20773019).

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REFERENCES

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In Situ UV Raman Spectroscopic Study on the Reaction Intermediates

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