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J. Phys. Chem. C 2008, 112, 6132-6138
Improving the Hydrophobicity and Oxidation Activity of Ti-MWW by Reversible Structural Rearrangement Lingling Wang, Yueming Liu, Wei Xie, Haihong Wu, Xiaohong Li, Mingyuan He, and Peng Wu* Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal UniVersity, North Zhongshan Rd. 3663, Shanghai 200062, People’s Republic of China ReceiVed: December 29, 2007; In Final Form: February 20, 2008
The postsynthesis treatment of Ti-MWW having the three-dimensional (3D) MWW structure with aqueous amine solutions has been carried out with the purpose to improve its hydrophobicity and catalytic activity in the liquid-phase oxidations. The treatment with piperidine (PI) or hexamethyleneimine (HMI) converted the 3D MWW structure into the corresponding lamellar precursor, which returned reversibly to the 3D MWW structure by further calcination. The treatments with other amines, however, caused a structural collapse or crystalline transfer to other phases. In the case of PI treatment, the structural conversion from 3D MWW to the MWW lamellar precursor occurred readily at 443 K at a PI/SiO2 molar ratio of >0.1 within 1 day for the Ti-MWW samples with various Si/Ti ratios. The structural interchange did not alter the amount as well as the coordination states of the Ti active sites, but removed the internal silanols by ca. 40%, leading to a defectless Ti-MWW catalyst with a more rigid and hydrophobic framework. This kind of structural rearrangement improved the catalytic activity by up to 20% in the ammoximation of ketones and also in the epoxidation of a wide range of alkenes with various molecular dimensions.
Introduction Zeolites attract research attention because of their ever expanding applications in a number of chemical processes ranging from effective heterogeneous catalysts to useful ionexchanger and separation/sorption agents.1 The applicability and performance of the zeolites in real uses strongly depend on the hydrophobicity, an important characteristic in addition to porosity, specific surface area, crystallinity, and framework composition. The hydrophobicity becomes more important particularly in the case of crystalline titanosilicates and related Ti-containing mesoporous materials because they are usually used under aqueous conditions with hydrogen peroxide as an oxidant.2,3 TS-1, the titanosilicate with the MFI topology, is capable of catalyzing actively the oxidation of a number of organic compounds with H2O2, some of which already has found industrial application in the clean production of bulk chemicals.4 The catalytic ability of TS-1 is commonly considered to be contributed by a relatively high hydrophobicity.2 Al-free TiBeta with more hydrophobic interior pores is active for the oxidation of alkanes and alkenes with aqueous H2O2, while Al containing Ti-Beta is less efficient mainly owing to a higher hydrophilicity induced by the framework Al-related hydroxyl groups.5 On the other hand, Ti-containing mesoporous materials, characteristic of much more open porosity (2.5-10 nm mesopores), are expected to show higher oxidation activity than microporous titanosilicates.6,7 However, the reality is that the former catalysts are much less active for the oxidation with aqueous H2O2 when the shape selectivity issue is excluded.7 The low activity of mesoporous materials is partially due to their Ti species having an amorphous environment in the silica * Corresponding author. Phone/fax: +86-21-6223-2292. E-mail: pwu@ chem.ecnu.edu.cn.
walls, and is more possible because of a high hydrophilicity related to the abundant silanols as a result of incomplete condensation in synthesis. To find effective ways for improving the hydrophobicity and then the catalytic activity of zeolites and mesoporous materials is thus an attractive research subject. The hydrophobicity is effectively increased by introducing hydrophobic organic groups into the porous materials through either secondary modification or direct incorporation. After removal of hydrophilic silanols by trimethylsilylation, mesoporous Ti-MCM-41, Ti-MCM-48,8,9 and Ti-SBA-1510 greatly increased the oxidation activity in the epoxidation of various alkenes. This technique has been further applied to microporous TS-1 as well as amorphous SiO2/TiO2 aerogels.11 The silylation agents reported hitherto include trimethylsilyl chloride, hexamethyldisiloxane, hexamethyldisilazne, N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA), or a combination of two silanes, all donating the thimethyl moiety with a bulky molecular dimension. This kind of silylation is favorable for the removal of external silanols and those near pore entrance, while has limitations such as pore blocking and untouching the internal silanols especially located inside the micropores of zeolites.12 In addition to the above liquid-phase trimethylsilylation with guest silane agents, a unique posttreatment method has been reported recently.13 It makes possible the silylation of not only surface silanols but also those intracrystal ones. Through a combination of heating and ultraviolet-assisted curing of assynthesized silicate-1, the structure-directing agent (SDA) molecules of tetrapropylammonium (TPA) are evacuated effectively, and simultaneously their fragments can react with the silicate matrix to induce methylation. In contrast to the above organic functionalization leading to the pendent silylation groups, the organic groups are also incorporated to be embedded in the framework via direct synthesis.14a Various organic-inorganic zeolites containing
10.1021/jp712155k CCC: $40.75 © 2008 American Chemical Society Published on Web 03/27/2008
Improving the Hydrophobicity of Ti-MWW hydrophobic methylene groups have been hydrothermally synthesized with use of organosilane as a silicon source, and they are shown to adsorb preferentially the organic molecules.14b Nevertheless, the use of organosilane retards greatly the crystallization process of the zeolites, which presumably raise a difficulty for introducing the transition metal ions into the framework. The surfactant-assisted assembly of various silanes, on the other hand, synthesizes readily the highly ordered organic-inorganic mesoporous materials the walls of which have more structural tolerance than the silica-based framework.15 Nevertheless, how to connect the hydrophobic properties of these mesostructured materials with useful catalysis needs more research. Despite the well-established techniques for the hydrophobicity enhancement by incorporating the organic species into the porous materials, particularly through trimethylsilylation, the loss of organic groups and then the decrease of hydrophobicity are unavoidable when the catalysts are subjected to calciantion, for example, for the purpose of necessary regeneration. Moreover, in addition to conventional incorporation of organic groups, there is still much room for the search for new modification techniques that are practical and applicable to the titanosilicates with different structures and porosities. It is preferred to remove the internal silanols selectively but without damaging the active sites and porosity. We have recently developed a new titanosilicate with the MWW structure, Ti-MWW.16-18 Possessing a unique pore structure comprised of two independent 10-membered-ring (MR) channels and 12-MR cups on the crystal exterior, Ti-MWW has proven to be a highly active catalyst in oxidation reactions such as epoxidation of various alkenes and ammoximation of ketones.19-21 Ti-MWW has also been transformed to the titanosilicates with open porosity, such as Del-Ti-MWW,22 Ti-YNU-1,23 and Ti-MCM-3624 through delamination, interlayer expanding, and pillaring, respectively. The 3D Ti-MWW originates from the corresponding Ti-containing lamellar precursor, which is hydrothermally synthesized by using boric acid as a crystallization-supporting agent. Incomplete interlayer dehydroxylation and deboronation allow Ti-MWW to contain a number of defect sites such as hydroxyl nests, making it less hydrophilic in comparison to TS-1.19 Thus, it is still possible to achieve more active Ti-MWW catalysts by enhancing its hydrophobicity. In this study, according to the structural feature of MWW zeolite, we have carried out the reversible transformation between the 3D crystalline structure and the lamellar precursor of Ti-MWW by amine treatment. A detailed investigation on the process of rearrangement found that this novel secondary modification allowed Ti-MWW to become a defectless, more hydrophobic, and active catalyst. Experimental Section Preparation of Materials. The lamellar precursors used for preparing 3D Ti-MWW were hydrothermally synthesized in the presence of boric acid with use of piperidine (PI) as a structure-directing agent (SDA). Following the procedures reported previously,16 the gels with the molar compositions of 1.0SiO2:(0.01-0.05)TiO2:1.4PI:0.67B2O3:19H2O were crystallized under rotation (100 rpm) at 443 K for 7 days. The Ticontaining lamellar precursors thus obtained were refluxed with 2 M HNO3 solution and further calcined at 823 K to obtain Ti-MWW, which had the 3D MWW structure. The 3D Ti-MWW was postmodified in aqueous solution containing various secondary amines and quaternary ammonium
J. Phys. Chem. C, Vol. 112, No. 15, 2008 6133 hydroxides such as piperidine (PI), hexamethyleneimine (HMI), octyltrimethylamonium hydroxide (OCTMAOH), tetraethylammonium hydroxide (TEAOH), tetrapropylammonium hydroxide (TPAOH), piperazine, or 2,6-dimethylpiperidine. The treatment was carried out at an amine/SiO2 ratio of 0.1-1.0 and an H2O/ SiO2 ratio of 10 at 443 K for 1-7 days. The resulting solid was washed with deionized water, gathered by filtration, and subsequently dried at 373 K overnight. The organic species occluded were burned off in air at 803 K for 6 h, which led to structurally rearranged sample, denoted as Re-Ti-MWW. Characterization Methods. X-ray powder diffraction (XRD) patterns were measured on a Bruker D8 ADVANCE diffractometer with Cu KR radiation. UV-visible diffuse reflectance spectra were recorded on a Shimadzu UV-2400PC spectrophotometer with BaSO4 as a reference. Scanning electron microscopy (SEM) was performed on a Hitachi-4800 instrument after suspending the sample in ethanol. Specific surface area was measured by N2 adsorption at 77 K on an Autosorb Quancachrome 02108-KR-1 analyzer after the evacuation at 573 K for 5 h. IR spectra were collected at room temperature on a Shimadzu FTIR-8100 spectrometer at a spectral resolution of 2 cm-1 after the self-supported wafer (30 mg of 20 mm Ø) was outgassed at 773 K for 3 h. Inductively coupled plasma (ICP) analysis was performed on a Thermo IRIS Intrepid II XSP atomic emission spectrometer. TG-DTA and CHN elemental analyses were carried out with a METTLER TOLEDO TGA/SDTA851e instrument and an Elementar Vario EL III analyzer, respectively. Catalytic Reactions. The ammoximation runs were performed batchwise in a 50 mL three-necked flask equipped with a magnetic stirrer and a condenser. In a typical run, Ti-MWW catalyst (0.07-0.15 g), 10 mmol of ketone, 5 mL of water solvent, and NH3 aqueous solution (25%) (12 mmol for cyclohexanone and 15 mmol for methyl ethyl ketone) were charged into the flask and stirred at 338 K. The reaction was then initiated at 338 K by adding dilute H2O2 (10%, 120 mmol) at a constant rate continuously with a micropump within 1 h. After the addition of H2O2 was finished, the mixture was further stirred for 0.5 h. The epoxidation of various alkenes (1-hexene, cyclohexene, cycloheptene, and cyclooctene) with H2O2 was also carried out bathwise. The mixture containing Ti-MWW catalyst, 10 mL of MeCN, 10 mmol of alkene, and 10 mmol of H2O2 was stirred vigorously at 333 K for 2 h. The epoxidation of propylene was carried out in a 50 mL Teflon-lined stainless reactor that was connected to a propylene tank. In a typical run, 0.15 g of catalyst, 30 mmol of 30 wt % H2O2, and 10 mL of MeCN were vigorously stirred while propylene was charged continuously at a constant pressure of 0.25 MPa. After the catalyst’s powder was removed, the products for all reactions were analyzed on a gas chromatograph (Shimadzu 14B, FID detector) equipped with a 30 m DB-WAX or DB-1 capillary column. The amount of unconverted H2O2 was quantified by standard titration method with a 0.1 M Ce(SO4)2 solution. The products formed were determined by using authentic chemicals on a GC-MS (Agilent 6890 series GC system, 5937 network mass selective detector). Results and Discussion Postmodification of Ti-MWW. To improve its catalytic activity, Ti-MWW with the 3D MWW structure has been treated in an aqueous solution containing PI, HMI, OCTMAOH, TEAOH, TPAOH, piperazine, or 2,6-dimethylpiperidine. The parent Ti-MWW sample with a Si/Ti molar ratio of 37 was prepared from a lamellar precursor (Si/Ti ) 20) by acid washing and calcination. The treatment was carried out at a molar ratio
6134 J. Phys. Chem. C, Vol. 112, No. 15, 2008
Figure 1. XRD patterns of parent Ti-MWW (a) and as-treated samples with PI (b), HMI (c), OCTMAOH (d), TEAOH (e), TPAOH (f), piperazine (g), and 2,6-dimethylpiperidine (h). The treatment was performed at an amine/SiO2 ratio of 1.0 (except 0.1 for TPAOH) at 443 K for 1 day.
Figure 2. Scanning electron micrographes of parent 3D Ti-MWW (a), PI-treated Ti-MWW (b), HMI-treated Ti-MWW (c), and TPAOHtreated Ti-MWW (d).
of amine/SiO2 of 1.0 except for 0.1 in the case of TPAOH. Figure 1 shows the XRD patterns of Ti-MWW before and after the amine treatment at 443 K for 1 day but without calcination. The pattern of the parent sample showed the characteristic diffractions due to the 100, 101, and 102 planes of the MWW structure but with the absence of the 001 and 002 peaks due to the layered structure, which was fully consistent with that of the well-known 3D MWW structure.16 The SEM image indicated that the 3D Ti-MWW sample had a morphology of thin platelets, approximately 0.2-0.5 µm in length and 0.050.1 µm in thickness (Figure 2a). The treatment of 3D Ti-MWW with PI led to a sample showing the 001 and 002 peaks in the 2θ region of 3-7°, characteristic of a layered structure along the c-direction (Figure 1b). The posttreated sample with PI was structurally the same as the MWW lamellar precursor, which is usually obtained by direct hydrothermal synthesis. In addition, as-synthesized ReTi-MWW still showed well-resolved diffractions due to the 100, 101, and 102 planes of well-ordered MWW sheets. From the SEM image of Re-Ti-MWW shown in Figure 2b, the crystals also appeared to be thin platelets with the same crystal size as the 3D Ti-MWW parent. Thus, the PI treatment altered greatly the microscopic structure of 3D Ti-MWW, but it exerted no influence on its macroscopic morphology.
Wang et al.
Figure 3. XRD patterns of Ti-MWW samples as-treated at various PI/Si ratios (A) and for a different time (B). For panel A, the parent Ti-MWW (a) was treated at a PI/Si ratio of 0.1 (b), 0.3 (c), 0.5 (d), 0.7 (e), and 1.0 (f) at 443 K for 1 day. For panel B, the parent TiMWW was treated at a PI/Si ratio of 1.0 at 443 K for 1 day (b), 2 days (c), 3 days (d), 5 days (e), and 7 days (f).
3D Ti-MWW was also treated with other amines. Similar to the PI treatment, the HMI treatment also resulted in a structure consistent with that of the MWW lamellar precursor (Figure 1c), the crystals of which appeared to be thin platelets with the same crystal size as the parent (Figure 2c). However, the OCTMAOH treatment developed a pronounced peak at 2θ of 3.1° in the XRD pattern, which made the peaks corresponding to the MWW structure disappear completely (Figure 1d). It seemed that the MWW structure was destroyed totally in this strong basic media, and the dissolved fragments were subsequently assembled into a mesostructure because of the templating effect of the OCTMA surfactant. The sample obtained by the TEAOH treatment became an amorphous material as its XRD pattern showed that the MWW structure no longer existed and other crystalline phases were not formed (Figure 1e). The treatment with TPAOH also made the MWW structure degrade greatly with only a very weak 101 diffraction observed in the XRD pattern (Figure 1f). In contrast, new diffractions were developed at 2θ )7.8°, 8.8°, 23.2°, 23.8°, and 24.3° which are in agreement with the characteristic reflection peaks due to the MFI structure.25 With the assistance of TPA cations having a strong structure-directing effect for the formation of the MFI structure, Ti-MWW was recrystallized into a TS-1-like sample. The SEM image confirmed that the rectangular particles of 3-5 µm were formed with the surface covered by a small amount of thin platelets (Figure 2d). These larger particles showed the representative macroscopic morphology of the MFI phase, while the thin platelets adhering to the MFI-type crystals were those in the unconverted MWW phase during the TPAOH treatment at 443 K for 1 day. When the treatment was prolonged for 3 days, a pure MFI phase with a higher crystallinity was obtained (not shown). When Ti-MWW was treated with piperazine or 2,6-dimethylpiperidine, the structure did not transform from 3D MWW to lamellar precursor but degraded to a certain degree as evidenced by much less intensive XRD patterns (Figure 1g,h). These two secondary amines also have a 6-membered-ring molecular shape, but they showed an obviously different effect from PI and HMI in the postmodification of Ti-MWW. Using the same Ti-MWW parent and PI, we have further investigated in detail the effect of the amount of amine and treatment time on the process of secondary modification. As shown in Figure 3A, the 001 and 002 diffractions due to the MWW lamellar precursor were developed by the PI treatment in the PI/SiO2 molar ratio range of 0.1-1.0. The intensity of the 002 diffraction increased slightly with an increasing PI/SiO2 ratio from 0.1 to 0.3, whereas it became comparable at PI/SiO2 ratios of 0.5-1.0. The TG analyses indicated that the weight
Improving the Hydrophobicity of Ti-MWW
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TABLE 1: Physicochemical Properties of Ti-MWW before and after Structural Rearrangement after rearrangementa
parent no.
Si/Tic
Si/Bc
OHd (%)
SAe (m2 g-1)
Si/Tic
Si/Bc
CNHf (wt %)
OHd (%)
SAe (m2 g-1)
rel hydrophobicityb
1 2 3 4
37 74 99 140
39 42 39 47
100
520 556 548 543
38 76 100 142
45 49 51 56
13.8 14.0 14.1 14.1
65
513 560 540 536
0.85 0.82 0.83 0.81
100
63
a
The parent samples were treated at 1.0SiO2:1.0PI:10H2O at 443 K for 1 day, and were further calcined at 803 K unless indicated. b Expressed in the ratio of water adsorbed of structurally rearranged Ti-MWW to parent (according to ref 33). The amount of water adsorbed was determined by TG analyses after the calcined samples were put in a desiccator over the saturated solution of NH4Cl overnight. c Given by ICP. d The relative amount of OH groups was determined from the OH stretchings in IR spectra (3900-3000 cm-1). The parent samples was assumed to contain 100% OH groups. e Specific surface area given by N2 adsorption for calcined samples. f Given by TG analyses for uncalcined samples. The CHN elemental analyses indicated that the C/N atomic ratio was 5.2-5.4 (vs 5.0 of PI).
TABLE 2: A Comparison of Liquid-Phase Ketone Ammoximation between Parent Ti-MWW and with Structurally Rearranged Ti-MWWa cyclohexanone no. 1 2 3
catalystb Ti-MWWb Ti-MWWb Re-Ti-MWWc
methy ethyl ketone
Si/Ti
cat. amount (g)
conv (%)
sel. (%)
cat. amount (g)
conv (%)
sel. (%)
37 37 38
0.07 0.10 0.07
93.2 99.5 99.8
99.2 99.5 99.7
0.10 0.15 0.10
90.3 99.2 99.3
99.5 99.7 99.8
a
Reaction conditions: ketone, 10 mmol; H2O2, 12 mmol; NH3 (25 wt %), 12 mmol for cyclohexanone and 15 mmol for MEK; water solvent, 5 mL; temp, 338 K; time, 1.5 h. H2O2 was added dropwise at a constant rate over 1 h. b Parent Ti-MWW (Si/Ti ) 37) without PI treatment. c Structurally rearranged Ti-MWW (Si/Ti ) 38) prepared by PI treatment at 443 K for 1 day and further calcination.
percentage of organic amine species incorporated into the sample was almost the same (13.5-14.2 wt %) irrespective of the PI/ SiO2 ratio (Table 1). This means that the PI molecules are enough for interlayer expansion and structural transformation even at a PI/SiO2 of 0.1. On the other hand, the treatment time did not show any obvious influence on the intensity of 001 and 002 diffractions from 1 to 7 days (Figure 3B), indicating the structural change from 3D MWW structure to lamellar precursor was almost completed within 1 day and the structural transformation to other phases hardly took place. Figure 4 shows the XRD patterns of the Ti-MWW samples with various Si/Ti molar ratios before and after the PI treatment. The parent samples with a comparable crystallinity were converted readily to corresponding lamellar precursors as featured by the development of the 001 and 002 diffractions (Figure 4A,B). This kind of structural interchange was slightly Ti content-dependent since the samples with a higher Si/Ti ratio showed relatively stronger 001 and 002 diffractions. A further calcination at 803 K in air burned off the organic species occluded in the channels and caused an interlayer dehydroxylation. The 3D MWW structure was then restored, which made the 001 and 002 diffractions due to the layered structure disappear (Figure 4C). The XRD patterns of the calcined ReTi-MWW samples showed a negligible distinction from those of the 3D Ti-MWW parents in diffraction intensity. Thus, with a combination of hydrothermal treatment with PI and calcination, it is possible to make a reversible structural interchange between 3D MWW and lamellar precursor. Characterizations of Re-Ti-MWW. The physicochemical properties of the parent Ti-MWW and corresponding Re-TiMWW with various Si/Ti molar ratios have been characterized by various techniques. As shown by the Si/Ti and Si/B ratios given by ICP analyses, the PI treatment affected the amount of Ti negligibly but deboronated slightly independent of the Si/Ti molar ratio (Table 1). This is because the framework boron, having a relatively smaller ionic radius than the lattice silicon, is less stable in the silica matrix and tends to leach out of the crystalline structure under hydrothermal conditions.
It should be first noted that no obvious band around 330 nm was observed for all parent samples (Figure 5A), indicating that the anatase-like phase was not formed during the process of hydrothermal synthesis and subsequent acid-reflux treatment. The 220 nm band, resulting from the charge transfer from O2to Ti4+, has been widely reported for the Ti-substituted zeolites and is characteristic of the tetrahedrally coordinated Ti highly dispersed in the framework.16 After the PI treatment and further calcination, the Re-Ti-MWW samples still showed a similar band predominately at 215 nm (Figure 5B), indicating the Tiactive sites remained in the framework to have a tetrahedral coordination. A slight blue shift was observed for the UV band of the Re-Ti-MWW samples, which is probably due to less water adsorption on these more hydrophobic samples (shown below), since the adsorption of water may cause a red shift.26 The IR spectra in the framework region showed the characteristic band at ca. 960 cm-1 (Figure 6), which is tentatively attributed to the stretching vibration of the framework Ti species.27 The above results verified that the Ti states did not change with the postmodification with PI. The IR spectra in the region of the hydroxyl stretching vibration gave us more substantial information concerning the structural rearrangement by the PI treatment (Figure 7). The parent Ti-MWW samples with the Si/Ti molar ratios of 37 and 74 showed the band at 3745 cm-1 attributed the external or terminal silanols on the crystal surface, the band at 3720 cm-1 probably due to asymmetric hydrogen-bonded silanols located inside the crystals, the band at 3690 cm-1 assigned to vicinal silanols, as well as the band at 3500 cm-1 due to hydrogen-bonded silanol nests.28 The internal silanols of TiMWW should originate from the defect sites as a result of deboronation and incomplete interlayer dehydroxylation. After the PI treatment, the OH stretching vibrations, particularly the 3720- and 3500-cm-1 bands, decreased clearly in intensity (Figure 7b,d). From the band area integrated in the region of 3000-3900 cm-1, the hydroxyl groups have been reduced by 35-37% after the PI treatment (Table 1). Considering the fact that the PI treatment caused a partial deboronation, which may
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Wang et al.
Figure 6. IR spectra in the framework vibration region of parent TiMWW (a) and PI-treated and further calcined Ti-MWW (b).
Figure 7. IR spectra in the region of hydroxyl stretching vibration of the parent Ti-MWW with a Si/Ti molar ratio of 37 (a), its corresponding PI-treated and further calcined sample (b), the parent TiMWW with a Si/Ti molar ratio of 99 (c), and its corresponding PItreated and further calcined sample (d).
Figure 4. XRD patterns of parent Ti-MWW (A), PI-treated TiMWW without calcination (B), and further calcined samples (C) with a Si/Ti molar ratio of 37 (a), 74 (b), 99 (c), and 140 (d). The treatment was performed at a PI/Si molar ratio of 1.0 at 443 K for 1 day.
Figure 8. 29Si MAS NMR spectra of (a) the parent Ti-MWW (Si/Ti ) 37) (a) and PI-treated and further calcined sample (b).
Figure 5. UV-visible spectra of parent Ti-MWW (A) and PI-treated and further calcined Ti-MWW (B) with a Si/Ti molar ratio of 37 (a), 74 (b), 99 (c), and 140 (d).
create new defect sites like hydroxyl nests in the framework, the amount of hydroxyl removed by the PI treatment is considerable. The removal of silanols by the structural arrangement was further investigated by 29Si MAS NMR spectroscopy (Figure 8). It has been reported that MWW-type materials have at least eight crystallographically inequivalent T sites,29 which has been supported by 29Si MAS NMR spectroscopy.30 The spectrum of Ti-MWW matches well with those reported previously.31,32 The Q4 sites in the region of -105 to 130 ppm could be deconvoluted into five lines which should be assigned to several
distinctive crystallographic sites in Ti-MWW but with overlapping resonances. In addition, the parent Ti-MWW showed also an obvious resonance at -103 ppm attributed to the Q3 site, Si(OH)(OSi)3 or Si(OH)(OTi)(OSi)2. Compared with the parent Ti-MWW sample, Re-Ti-MWW showed very similar resonances due to the Q4 sites but a less intensive -103 ppm resonance of the Q3 site. This is in agreement with the above IR spectra to verify that a portion of the defect sites disappeared definitely in the PI treatment. The quantitative data of the spectra simulation indicated that the Q3 sites were decreased by ca. 40%. Moreover, the relative hydrophobicity of Re-Ti-MWW was measured according to the procedures given by Anderson and Klinowski.33 The samples were pretreated by calcination at 773 K to remove any adsorbed water. They were then equilibrated at room temperature with the water vapor supplied by the saturated solution of NH4Cl. The amount of adsorbed water was then determined by thermogravimetry. The weight ratio of adsorbed water of Re-Ti-MWW to parent Ti-MWW was 0.81-0.85 for the samples with different Si/Ti ratios (Table 1). This indicates the PI treatment increased the hydrophobicity
Improving the Hydrophobicity of Ti-MWW
Figure 9. A possible mechanism for the structural rearrangement of Ti-MWW with the assistant of PI or HMI.
of MWW-type titanosilicate. This is presumably relative to partial removal of the hydroxyl groups at defect sites. Possible Formation Mechanism of Reversible Structural Rearrangement. The reversible interconversion between 3D MWW and lamellar structure took place only when using PI or HMI as a guest molecule (Figure 1). PI and HMI are two typical SDAs used for the nucleation and crystallization of the MWW structure. This means the posttreatment-induced structural conversion shares the same host-guest chemistry with the direct hydrothermal synthesis. The 3D MWW structure seems to recognize and accommodate only PI or HMI molecules at a molecular level, which leads to a structural conversion into the lamellar precursor. In this process, the internal hydroxyls and corresponding defect sites are partially removed to result in a more hydrophobic framework, while with the framework Ti species almost intact. The processes of structural rearrangement are shown graphically in Figure 9. The parent sample had the MWW structure showing a closed interlayer pore entrance of 10-MR. In the framework, it contained the tetrahedral Ti and B species along with a number of defect sites such as hydroxyl nests formed as a result of deboronation and incomplete interlayer dehydroxylation. After treated with PI (or HMI), the PI molecules were inserted into the interlayer space of Ti-MWW. The TG and CHN elemental analysis indicated that the amount of the organic species incorporated accounted for ca. 13.8 wt % with a C/N molar ratio of 5.3 (Table 1). This amount of organic species would be enough to intercalate the MWW sheets to form the layered structure. A further calcination burned off the organic species occluded between the layers and caused an interlayer dehydroxylation to reconstruct the 3D MWW structure. Thus, a reversible structural interchange between 3D MWW and lamellar precursor occurred during these treatment sequences. The intercalation of PI or HMI molecules was the key guest molecule essential for an interlayer expanding. The reversible structural change may induce a framework rearrangement, which made the framework Si species migrate into the neighboring hydroxyl nests or dehydroxylated partially the vicinal silanols to form the Si-O-Si linkage. This would mend the disconnection and increase the periodicity of the framework effectively. This kind of rearrangement has been realized usually through so-called framework recrystallization by steaming in the case of aluminosilicates such as Y and mordenite zeolites,34,35 in which the dealmination vacancy can be removed to a certain degree. Nevertheless, since the framework Ti species are more sensitive and delicate than the tetrahedral Al from the viewpoint of demetalation and change of the coordination state, the thermal
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Figure 10. A comparison of 1-hexene conversion (A) and propylene yield (B) between the parent Ti-MWW (a) and Re-Ti-MWW. 1-Hexene epoxidation conditions: 0.05 g of catalyst, 10 mmol of 1-hexene, 10 mmol of H2O2 (30 wt %), 10 mL of MeCN solvent; 333 K, 2 h. Propylene epoxidation conditions: 0.15 g of catalyst, 30 mmol of H2O2 (30 wt %), 10 mL of MeCN solvent, 313 K, 1 h. Propylene was charged continuously into the reactor at a constant pressure of 0.25 MPa.
and hydrothermal treatments are unsuitable for reducing the internal defect sites in the titanosilicates. The present method of amine-assisted framework rearrangement influences little the Ti active sites but mends the MWW structure, leading to a defectless and more rigid and hydrophobic Ti-MWW catalyst. Catalytic Properties of Re-Ti-MWW. Ti-MWW has been reported to be a promising catalyst for the ammoximation of ketones20,36 and the epoxidation of various alkenes with H2O2 oxidant.17,19 We thus have investigated the catalytic properties of structurally rearranged Ti-MWW to check whether the method is practical and useful. Table 2 compares the results of liquid-phase ammoximation of cyclohexanone and methyl ethyl ketone (MEK) between the original Ti-MWW and the PI-treated one. The oxime selectivity reached 99% for both reactions. The more catalyst used, the higher was the conversion of the ketones. When the reactions were carried out with the same weight ratio of catalyst to substrate, the ketone conversion over Re-Ti-MWW was 6.6% and 9% higher than parent Ti-MWW for cyclohexanone and MEK, respectively. Thus, more hydrophobic Re-Ti-MWW shows superior catalytic activity to Ti-MWW in the ammoximation of ketones. To further ensure the effect of the structural rearrangement, we have prepared a series of Ti-MWW with different Ti content and carried out the PI treatment on these samples. The catalytic activity of the resultant catalysts has been checked in the epoxidation of 1-hexene and propylene with H2O2 (Figure 10). As expected the 1-hexene conversion and propylene yield increased with increasing amount of Ti active sites for both ReTi-MWW and parent Ti-MWW, but the former obviously showed a higher conversion than the latter. The structural rearrangement enhanced the catalytic activity by 10-20%. In addition to linear alkenes, Re-Ti-MWW has also been found to show a higher activity in the epoxidation of bulky substrates such as cycloalkenes. The turnover number (TON) for the oxidation of different cycloalkenes over the two titanosilicates is depicted in Figure 11. The bulky substrates needing an open reaction space are considered to be catalyzed mainly by the Ti species within the side pockets on the crystal surface and those inside the supercages of Ti-MWW.17 Reasonably, the larger substrate showed a lower activity. However, Re-Ti-MWW exhibited a higher TON than parent Ti-MWW for cyclohexene, cycloheptene, and cyclooctene. The structural rearrangement enhanced greatly the catalytic activity of Ti-MWW in the ammoximation of ketones and epoxidation of alkenes with different molecular dimensions. This
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Figure 11. Turnover number for the epoxidation of different clycloalkenes over parent Ti-MWW and Re-Ti-MWW. Reaction conditions: 0.05 g of catalyst, 10 mmol of alkene, 10 mmol of H2O2 (30 wt %), 10 mL of MeCN solvent, 333 K, 2 h.
can be ascribed to the increase of hydrophobicity following the partial removal of the internal silanols located on the defect sites. The enhancement has been observed for both linear and cyclic alkenes, the epoxidation of which is catalyzed by all the Ti active sites and by those located mainly in open space, respectively. This implies that the posttreatment with PI has made the framework defect-less uniformly throughout the crystals. Conclusions The postsynthesis treatment of 3D Ti-MWW zeolite with the solution of amines such as piperidine and hexamethyleneimine leads to a reversible structural conversion and rearrangement of the MWW framework. The structural rearrangement is amine-dependent, which can be achieved only with the assistance of SDAs used in the hydrothermal synthesis of TiMWW. Compared with conventional 3D Ti-MWW, Re-TiMWW contains a smaller amount of silanols and fewer defect sites, showing a higher hydrophobicity. The structural rearrangement improves effectively the catalytic activity of TiMWW in the liquid-phase ammoximation of ketones and the epoxidation of a wide range of alkenes with both small and large molecular dimensions. Acknowledgment. We thank the financial support by the National Natural Science Foundation of China (20673038), Science and Technology Commission of Shanghai Municipality (06SR07101, 07QA14017), and 973 Project (2006CB202508), Shanghai Leading Academic Discipline Project (B409). L.W. thanks the Ph.D. Program Scholarship Fund of ECNU 2007. References and Notes (1) Davis, M. E. Nature 2002, 417, 813. (2) Khouw, C. B.; Dartt, C. B.; Labinger, J. A.; Davis, M. E. J. Catal. 1994, 149, 195. (3) Tatsumi, T.; Koyano, K. A.; Tanaka, Y.; Nakata, S. Chem. Lett. 1997, 469.
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