Immobilization of Phosphotungstic Acid (PTA) on Imidazole ...

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Immobilization of Phosphotungstic Acid (PTA) on Imidazole Functionalized Silica: Evidence for the Nature of PTA Binding by Solid State NMR and Reaction Studies L. T. Aany Sofia,† Asha Krishnan,† M. Sankar,‡,§ N. K. Kala Raj,*,‡ P. Manikandan,‡,⊥ P. R. Rajamohanan,† and T. G. Ajithkumar*,† Central NMR Facility, and Catalysis and Inorganic Chemistry DiVision, National Chemical Laboratory, Pune 411008, India ReceiVed: June 30, 2009; ReVised Manuscript ReceiVed: October 20, 2009

Phosphotungstic acid (PTA) immobilized onto imidazole functionalized fumed silica and was used as an efficient catalyst for epoxidation of a variety of olefins using aqueous H2O2 as an oxidant. Negligible leaching of PTA under the reaction conditions employed indicates a strong interaction between PTA and imidazole. The immobilized catalysts could be separated and reused after the catalytic cycle. Evidence for the heterogenization of PTA on the imidazole functionalized fumed silica has been inferred from different spectroscopic techniques like IR, UV-vis, and NMR. Importantly, the nature of binding of PTA on the support has been studied in detail by solid state NMR spectroscopy using 15N labeled imidazole support. It is clear from the NMR studies that the effective heterogenization of PTA is mainly due to imidazolium ion formation on the support by the acidic protons of PTA and the resultant ion pair. 1. Introduction Heteropolyacids (HPAs) are a large fundamental class of inorganic compounds having both acidic and redox properties. The first to be characterized and the best known among these are the Keggin heteropolyanions, having the general formula XM12O40x-8, where X is the central atom (Si4+, P5+, etc.) with an oxidation state x, and M is the metal ion (Mo6+ or W6+). The countercations present may be H+, H3O+, etc. HPAs exhibit high proton mobility and high thermal stability and are highly soluble in polar solvents. These properties give them the ability to catalyze several acid, redox, and bifunctional reactions in both homogeneous and heterogeneous systems.1-3 Although HPAs in their acidic form or transition metal ion substituted forms are versatile compounds, their main disadvantages are high solubility in polar solvents and low surface area ( titania > carbon > alumina. Apart from the usual impregnation method for supporting PTA on supports, other techniques like the sol-gel technique with or without using ionic liquids as the template17 and immobilization on chemically modified silica have been reported in the literature. Since physically adsorbed PTA on the support surface can leach out easily, immobilization by means of chemical bonding via functionalized silica is the most effective approach. Among the different silica supports that are used, fumed silica has the advantages of low cost of production and ease of availability. In addition, its physicochemical property, which can be modified easily, becomes a crucial factor in determining the property of the final grafted material. It has already been shown that immobilization of heteropolyacids on fumed silica is one of the effective approaches in designing catalyst systems that give high selectivity to the desired product, and the functionalized fumed silica has better affinity to anchor HPAs including PTA over other silica supports like MCM-41.18,19 Immobilization of HPAs on a functionalized support containing nitrogen donors gives more stability and enhanced catalytic activity.20 Kaleta et al.21 and Kala Raj et al.14 have reported the immobilization of HPA inside the channels of Si-MCM-41 and SBA-15, respectively. The HPA is bonded strongly to these functionalized mesoporous supports by means of ionic interaction with the strong σ-donor amine groups, which prevents/ reduces the leaching of the catalyst in polar solvents. In all the

10.1021/jp906108e CCC: $40.75  2009 American Chemical Society Published on Web 11/16/2009

Immobilization of PTA above methods, the amine group was used as the anchoring agent. Recently, Mirkhani et al. reported that silica modified imidazole is a very good support for immobilization of Mn(III) salophen and plays an important axial ligand role in the oxidation reactions.22 Although there are many studies on the advantages of heterogenizing a homogeneous catalyst, the exact nature of bonding between the catalyst and the support is not well understood. Elemental analysis gives information about the average surface loading on the support materials. In order to gain information on the functional group moiety and also to understand the change in coordination environment of the compound, spectroscopic techniques such as IR, UV-vis, etc., are used. Solid state NMR is a powerful technique which can probe molecular level information, and this can be used to obtain complete information of the modified surfaces. The 29Si NMR studies of the surface and surface immobilized species were reported by Fyfe et al.23 on silica gel and activated high surface area glass beads. Caravajal et al.24 reported 29Si and 13C solid state NMR experiments of (3-aminopropyl)triethoxysilane modified silica. In a recent study, functionalization of imidazole on SBA-15 was confirmed by 13C solid state NMR.25 In another work, Bordoloi et al. have used 31P NMR to get clear evidence for the anchoring of heteropolyacids on the supports.26 The study reported in this paper was carried out with two main objectives. The primary objective was the immobilization of phosphotungstic acid (H3PW12O40, PTA) onto the surface of 3-(imidazolin-1-yl) propylsilane modified fumed silica to design a catalyst that shows high selectivity in the epoxidation reaction. Imidazolinyl based anchoring agent was chosen for many reasons: (a) relative easiness to synthesize from chloropropylsilane functional unit, (b) imidazole nitrogen is a better coordinating atom with its lone pair electron than the amine based ligand, and (c) availability of 15N labeled imidazole. Epoxidation of a variety of olefins using aqueous hydrogen peroxide as an oxidant was carried out to demonstrate the effective heterogenization of heteropolyacid in this catalyst. The second objective was to carry out a complete physicochemical characterization of this material using IR, UV, and an extensive solid state NMR study with 13C, 15N, 29Si, and 31P to extract detailed microscopic information on the catalyst system, especially to understand the nature of interaction between the functionalized units. Since the natural abundance of 15N is much less (≈0.3%), the 15N solid state measurements were carried out on the Si-Imid-PTA system prepared using 98% 15N enriched imidazole. As nitrogen atom is expected to play an important role in the anchoring, 15N NMR should help us to identify the changes in the bonding of C-NdC and H-N in the imidazole group. This in turn is expected to give valuable information about the structural features of the imidazole ring and its interaction with PTA catalyst. There are only a few detailed 15 N solid state NMR studies on the nitrogen functionalized silica surfaces.27-29 2. Experimental Methods 2.1. Chemicals. All the chemicals used were of analytical reagent grade. Fumed silica, chloropropyltriethoxysilane, 15N labeled imidazole, and imidazole were purchased from Aldrich Co. and phosphotungstic acid (H3[PW12O40].xH2O) from Loba Chemie and were used as received without further purification. Diethyl ether (Merck India) and acetonitrile (S.D. Fine) were of analytical grade and were used as received. The exact strength of hydrogen peroxide (Loba Chemie) was determined by redox titration with KMnO4 solution. The substrates, limonene, cis-

J. Phys. Chem. C, Vol. 113, No. 50, 2009 21115 cyclooctene, 1-octene, norbornene, trans-2-octene, 1-methyl1-cyclohexene, etc., were procured from Aldrich and used as received. 2.2. Characterization. 2.2.1. Elemental Analysis and IR and UV Spectra. The elemental analysis of the samples was done with an EA 1108 CHNS element analyzer. Tungsten contents in the functionalized sample before and after catalytic reactions were estimated by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). FT-IR spectra of the powdered samples were recorded on a SHIMADZU FTIR-8300 spectrophotometer as KBr pellets. Diffuse reflectance UV-visible measurements were recorded at room temperature with BaSO4 as a reference on a Perkin-Elmer Lambda 650 Spectrometer. Nitrogen adsorption measurements were carried out on a Quantachrome Autosorb-1 at 77 K. First, the samples were activated at 423 K under vacuum, and then the adsorptiondesorption was conducted by passing N2 into the sample, which was kept under liquid nitrogen. The specific surface areas of the samples were calculated using the multiple-point BrunauerEmmett-Teller (BET) method in the relative pressure range P/P0 ) 0.05-0.3. 2.2.2. Multinuclear Solid State NMR. Solid state 31P, 13C, 29 Si, and 15N MAS (magic-angle spinning) and CP-MAS (crosspolarization-magic angle spinning) experiments were performed on a Bruker AV-300 NMR spectrometer equipped with a 7.05 T wide bore superconducting magnet, using a 4 mm BL MAS probe resonating at 121, 75.4, 59.6, and 30.3 MHz, respectively for 31P, 13C, 29Si, and 15N nuclei. The samples were packed in a 4 mm zirconia rotor and were spun at 10 kHz for all the experiments. A standard ramped-amplitude cross-polarization (RAMP-CPMAS) pulse sequence was used with a CP contact of 3, 4, and 4 ms for the 29Si, 13C, and 15N CPMAS, respectively. For the 29Si CPMAS spectra of the functionalized samples, 8000-10 000 scans were recorded with a repetition time of 4 s. For the 13C CPMAS spectrum of pure imidazole, 15 000 scans were recorded with a repetition time of 3 s, and for the functionalized samples, 10 000-15 000 scans were recorded with a repetition time of 5 s. For the 15N CPMAS spectra of 15 N labeled imidazole, 2000 scans were recorded with a recycle delay of 1 s, while for functionalized samples 4000-5000 scans were recorded with a recycle delay of 1 s. For the 31P MAS NMR spectra of dehydrated PTA 16 scans with a recycle delay of 1000 s and for the functionalized PTA, 120 scans with a recycle delay of 360 s were used. A single pulse excitation using high values for the recycle delay (>5*T1) ensured a complete relaxation of 31P nuclei. The 31P spin-lattice relaxation time (T1) was measured using standard saturation recovery pulse sequence and was 190 s for the dehydrated PTA and 70 s for the functionalized samples. Chemical shifts were referenced to the CH2 carbon of adamantane (38.48 ppm) for 13C, 2,2dimethyl-2-silapentane-5-sulfonate sodium salt (DSS) for 29Si, and 85% phosphoric acid for 31P, respectively. 15N chemical shifts are given with respect to glycine at 34.5 ppm. 2.3. Catalyst Preparation. The PTA anchored on 3-(imidazolin-1-yl)propylsilane was prepared using a two-step synthesis method as described below. 2.3.1. Preparation of 3-(Imidazolin-1-yl)propylsilane (SiImid). A schematic representation of the synthesis of 3-(imidazolin-1-yl)propylsilane (Si-Imid) is shown in Scheme 1. A mixture of fumed silica (1 g), 3-chlorotriethoxypropylsilane 0.241 g (1 mmol), and imidazole 0.0680 g (1 mmol) was refluxed for 24 h in p-xylene (40 mL) under nitrogen atmosphere. After refluxing for about 24 h, the mixture was cooled to room temperature, filtered, washed with xylene to remove

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SCHEME 1: Preparation of 3-(Imidazolin-1-yl)propylsilane (Si-Imid)

SCHEME 2: Anchoring of Phosphotungstic Acid onto 3-(Imidazolin-1-yl)propylsilane

Figure 1. FT-IR spectra of (a) fumed silica, (b) Si-Imid, (c) PTA, and (d) Si-Imid-PTA.

any unreacted 3-chlorotriethoxypropylsilane, and dried. The white product obtained is designated as Si-Imid. 2.3.2. Anchoring of PTA onto 3-(Imidazolin-1-yl)propylsilane. Scheme 2 represents the anchoring of PTA onto 3-(imidazolin-1-yl)propylsilane. Si-Imid (1 g) was added to an acetonitrile solution of PTA; 2.88 g (1 mmol) in 40 mL was taken in a round-bottom (RB) flask. The mixture was refluxed for 24 h under nitrogen atmosphere. After 24 h, the mixture was filtered, washed with acetonitrile and dichloromethane, and dried at room temperature. The final product was a white powder and is designated as Si-Imid-PTA. 15N labeled sample was prepared in a similar manner using 15N labeled imidazole and used for solid state NMR studies. The C/N molar ratio calculated from elemental analysis was 3.12, which is very close to the theoretically expected value 3, indicating that the chloro groups from 3-chlorotriethoxypropylsilane are replaced by imidazole completely. The tungsten content in the anchored PTA estimated by ICP-AES was 26.12 ppm. 2.4. Catalytic Activity. The expoxidations of alkenes (limonene, cis-cyclooctene, 1-octene, norbornene, trans-2-octene, 1-methyl-1-cyclohexene) were carried out in a two-necked RB flask with anhydrous CaCl2 guard tube. All reactions were carried out at 60 °C in acetonitrile solvent. Typically 5 mmol of substrate and 5 mmol of hydrogen peroxide (30%) were taken in 4.5 mL of solvent along with 0.05 g of Si-Imid-PTA as a catalyst. The reaction mixture was stirred constantly at the required temperature. Small aliquots of the samples were withdrawn at regular intervals and subjected to GC analysis to monitor the conversion of the substrate and selectivity to its products using chlorobenzene as an internal standard. The samples were analyzed using a HP-5890 gas chromatograph fitted with a fused megabore column SE-52, HP-% (cross-linked 5% PhMe silicone), 30 m in length, 0.53 mm i.d., 0.3 m film

thickness, and a FID. The products were confirmed by GC-MS using a QP-5000 SHIMADZU mass spectrometer. 2.5. Leaching and Recycle Tests. The possible leaching of anchored PTA into the solution was studied by carrying out a reaction with cyclooctene as a representative substrate under the same reaction conditions. In a controlled reaction, the catalyst was filtered out from the reaction mixture after 1 h of reaction time, and the reaction was allowed to continue with the remaining filtrate with fresh addition of aq. H2O2. Aliquots of the samples were withdrawn at definite intervals and were analyzed by GC, and the results were compared with a reaction without removing the catalyst intermittently. Recycling tests with the catalyst were also carried out with the same substrate. A reaction was started using 5 mmol of cyclooctene and 5 mmol of aq. H2O2 (30%) in 4.5 mL of acetonitrile solvent and 0.05 g of Si-Imid-PTA catalyst. The reaction was carried out at 60 °C for 4 h. At the end of the reaction, the catalyst was filtered off, washed thoroughly with acetonitrile, and dried. This dried catalyst was used again as catalyst for a fresh reaction. It was reused for two more reaction cycles. 3. Results and Discussion 3.1. Characterization of the Catalyst. 3.1.1. FT-IR. FTIR spectra of fumed silica, Si-Imid, PTA, and Si-Imid-PTA are shown in Figure 1a, b, c, and d. Some of the important bands which are observed in the spectra are given in Table 1. In fumed silica, bands at 801 cm-1 (symmetric stretching frequency of Si-O-Si), 965 cm-1 (stretching frequency of Si-O-H), and 1000-1200 cm-1 (antisymmetric stretching of Si-O-Si)30 are observed which are also present in the spectra of Si-Imid and Si-Imid-PTA. Peaks in the region of 3500 cm-1 are observed which correspond to adsorbed water present in the system.31

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TABLE 1: Infrared Spectroscopic Data for Fumed Silica, Si-Imid, PTA, and Si-Imid-PTA before the Catalytic Reaction vibration mode assignment

fumed silica, cm-1

Si-Imid, cm-1

PTA, cm-1

Si-Imid-PTA, cm-1

Si-O Si-O-Si/W-O-W W-O-W Si-O-H W-O Si-O/P-O CdN

467 801 965 1094 -

467 802 931 1094 1447

801 889 983 1081 -

467 814 897 955 981 1095 1456

The bands observed in the range of 2800-2950 cm-1 are due to the propyl group present in Si-Imid and Si-Imid-PTA. The band at 1447 cm-1 for Si-Imid is assigned to the CdNs of the imidazole group, which is shifted toward 1456 cm-1 in Si-ImidPTA which could be due to the change in environment of the double-bonded nitrogen in imidazole. A small band between 1400-1410 cm-1 in Si-Imid and Si-Imid-PTA is the characteristic band of the C-N bond which can be attributed to anchoring of imidazole to the propyl group.25 PTA shows characteristic IR bands at 1081 cm-1 (stretching frequency of P-O in the central PO4 tetrahedron), 983 cm-1 (terminal bands for WdO in the exterior WO6 octahedron), 889 cm-1, and 801 cm-1 (W-O-W bands). In Si-Imid-PTA, bands are observed at 981 and 897 cm-1 which confirms the presence of PTA in SI-Imid-PTA.22,25,32-36 3.1.2. UV-Visible. Diffuse reflectance UV-visible spectra of fumed silica, imidazole, Si-Imid, PTA, and Si-Imid-PTA are shown in Figure 2a, b, c, d, and e. Pure fumed silica shows no absorption in the UV-vis region. Imidazole shows a UV absorption peak at 210 nm.37 This peak is shifted to 226 nm in Si-Imid due to the functionalization of imidazole to fumed silica. Neat PTA shows absorption bands at 254 and 315 nm which are attributed to oxygen-tungsten charge transfer absorption bands for the Keggin anion.11,38,39 On anchoring with Si-Imid, the 254 nm band is shifted to 266 nm and the 315 nm band appears as a shoulder band reconforming the presence of PTA in Si-Imid-PTA. Changes in intensity of these bands compared to the neat PTA are due to the presence of another component like imidazole. 3.1.3. Nitrogen Adsorption Studies. The nitrogen sorption isotherms for fumed silica, Si-Imid, and Si-Imid-PTA were carried out by the BET method at 77 K. The surface area of fumed silica has reduced from 502 to 139 m2/g on imidazole

Figure 2. UV-visible spectra of (a) fumed silica, (b) imidazole, (c) Si-Imid, (d) PTA, and (e) Si-Imid-PTA.

Figure 3.

29

Si CP-MAS spectra of (a) Si-Imid and (b) Si-Imid-PTA.

functionalization, and it was further reduced to 77 m2 /g upon anchoring the PTA. This observation indirectly confirms the anchoring of PTA onto Si-Imid. The reduction in surface area may be due to the presence of the bulkier trisiloxypropyl imidazole group and the heteropolyacid anion on the surface of fumed silica. 3.1.4. Solid State NMR. Although the presence of PTA in Si-Imid-PTA is confirmed using elemental analysis, IR and UV-vis, and solid state NMR studies probed through 29Si, 13C, and 15N NMR can bring out another dimension of the structural aspects. The 29Si CP-MAS spectra of the Si-Imid and Si-ImidPTA samples are shown in Figure 3 in which prominent peaks are observed at -110, -102, -60, and -68 ppm. The peaks at δ ) -110 and -102 ppm can be assigned as the Q4 [Si(OSi)4] and Q3 [Si(OSi)3OH] sites of fumed silica. The spectrum of fumed silica has already been reported in the literature and is known to have peaks at -109, -100, and -91 ppm corresponding to the Q4, Q3, and Q2 [Si-(OSi)2(OH)2]40,41 sites. The Q4 structural units represent interconnected SiO4 tetrahedrons, while the Q3 and Q2 structural units represent the silanol groups associated with the surface of silica. The intensity of the Q2 site at -91 ppm is much less in the functionalized silica because most of the silanol groups associated with Q2 sites of the fumed silica are wiped out and get attached to 3-(imidazolin-1yl)propylsilane. During functionalization, new covalent Si-O-Si linkages are formed which are designated as the T3 structural unit [(-O-)3Si-CH2CH2CH2(C3H3N2)] and the T2 structural unit [(-O-)2Si-CH2CH2CH2(C3H3N2)]. The T3 structural unit which corresponds to the peak at δ ) -68 ppm indicates the formation of Si-O-Si linkage of the imidazolin-1-yl-propyl group on the surface of silicon atoms of fumed silica through three siloxane bonds. The T2 structural unit gives a peak at around -60 ppm, and the remaining OEt group of triethoxycholoropropylsilane that is not anchored onto the surface of silica may undergo hydrolysis to form a Si-OH species on the support. The relatively high intensity of the peak at -68 ppm (T3 structural unit) confirms that the 3-(imidazolin-1-yl)propylsilane groups have attached to the surface of fumed silica.23,42 Since the PTA is expected to attach with the imidazole molecule, we do not expect any change in the 29Si spectra of Si-ImidPTA from the Si-Imid, and that is seen in Figure 3b. The 13C CP-MAS spectra of neat imidazole, Si-Imid, and SiImid-PTA are shown in Figure 4. The numbering sequence for each element is given at the top of these figures. The spectrum of imidazole was also recorded for comparison. In the 13C spectrum of imidazole, peaks are observed in the aromatic region at 135 ppm (C1), 127 ppm (C2), and 115 ppm (C3). For Si-Imid (Figure 4b) and Si-Imid-PTA (Figure 4c), two sets of peaks

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Figure 4. 13C CPMAS spectra of (a) imidazole, (b) Si-Imid, and (c) Si-Imid-PTA.

Figure 5. 15N CPMAS NMR spectra of (a) imidazole, (b) Si-Imid, and (c) Si-Imid-PTA.

are observed: one from the aromatic and the other from the aliphatic region. Though a set of three peaks from the aromatic region at 136 ppm (C1), 126 ppm (C2), and 120 ppm (C3) are seen for Si-Imid, only two peaks at 134 and 121 ppm are observed for Si-Imid-PTA in the same region. The second set of peaks from propyl carbons (aliphatic region) are at 49.32 ppm (C4), 24.1 ppm (C5), and 9.7 ppm (C6) for Si-Imid and 51.38 ppm (C4), 23.38 ppm (C5), and 9.2 ppm (C6) for Si-ImidPTA. The 15N CP-MAS spectra of neat imidazole, Si-Imid, and SiImid-PTA, are shown in Figure 5. 15N labeled imidazole and functionalized materials were used for 15N NMR studies. In the 15 N spectrum of imidazole (Figure 5a), peaks at 172 and 244 ppm are observed. The peak at 172 ppm is assigned as the protonated (pyrrole) type nitrogen atom43 labeled as N1, and the peak at 244 ppm is assigned as the nonprotonated (pyridine) type nitrogen atom labeled as N2 present in imidazole. In the 15 N spectrum of Si-Imid (Figure 5b), the N2 peak (244 ppm) appears to be broad and reduced in intensity, and the chemical shift value of the N1 peak has changed from 172 to 182 ppm with a shoulder at 171 ppm. There is an overall broadening in all the peaks which could be due to the chemical shift dispersion or dynamics when attached to the silica surface. The 15N spectrum of Si-Imid-PTA (Figure 5c) shows that the N2 peak is absent, a prominent peak at 182 ppm, and a small peak at 171 ppm are observed. The peak at 171 ppm is more intense than the similar one observed in Si-Imid. The anchoring of imidazole to silica surface through the propyl group of triethoxypropylsilane by means of chemical bond will change the environment of the N1 of the imidazole because the anchoring takes place at the N1 position leading to a downfield shift of the N1 peak from 172 to 182 ppm in the 15 N spectrum. This also causes a downfield shift for C4 which is attached to N1, resulting in a peak at 49.32 ppm in the 13C

Sofia et al.

Figure 6. 31P MAS NMR spectra of (a) dehydrated PTA, (b) Si-ImidPTA, and (c) Imid-PTA.

spectrum.44 The carbon atom attached to the electropositive Si group (C6) is more shielded and therefore shows an upfield shift resulting in the peak at 9.20 ppm. No protonation is expected at the N2 position in the Si-Imid sample. However, a decreased intensity of the N2 peak (244 ppm) in the 15N spectrum of SiImid indicates possible protonation at this nitrogen which may be due to the H+ ions from HCl25 remaining in the system. It may be noted here that HCl is a byproduct during the immobilization of imidazole which is normally eliminated by repeated washing but traces of which could remain in the system. The peak at 171 ppm in the 15N spectrum is also due to the protonated N2 nitrogen. The shift of about 1 ppm from the neat imidazole N1 peak is likely due to the change in environment. The evidence for protonation at N2 is also reflected in the 13C spectrum of Si-Imid. The decrease in intensity and a downfield shift of the C2 peak position compared to the imidazole peaks could be due to the protonation of N2 in Si-imid which causes the two carbon atoms (C2 and C3) to become nearly equivalent due to the equal probability for the proton being on N1 and N2. It is well-known that the acid form of imidazole, imidazolium ion, exists as tautomers of two equally contributing forms in which the proton is on either N1 or N2 which makes the carbon atoms (C2 and C3) indistinguishable.27,45-48 The 15N spectrum of Si-Imid-PTA (Figure 5c) shows that the N2 peak is absent. However, the peaks at 182 and 171 ppm became prominent and more intense in comparison to that in Si-Imid. This suggests that there is a complete replacement of chloride by PTA resulting in a complete protonation at N2, making the peak at 244 ppm vanish and showing an increase in the intensity of the 171 ppm peak. This is also supported by the 13C CP-MAS results of SiImid-PTA in which there are only two carbon peaks observed at 134 and 121 ppm. The peak at 121 ppm is due to the merging of C2 and C3 on complete protonation of the imidazole ring. The protonation of imidazole due to PTA also results in the downfield shift of C4 to 51.38 and an upfield shift of C6 to 9.2 ppm in the 13C spectrum. These observations from 13C and 15N NMR measurements clearly demonstrate that PTA interacts/ binds with Si-Imid by forming an ion pair, i.e., [imidazolium]+[H2PW12O40]- (refer to Scheme 2). In order to further substantiate the above points, additional experiments with 31P MAS NMR were carried out on neat PTA and Si-Imid-PTA. In addition, a comparison has been made with a sample that was prepared by interacting PTA with neat imidazole. The 31P MAS spectra of PTA (dehydrated), PTA anchored on Si-Imid-PTA, and PTA reacted with neat imidazole named as Imid-PTA are shown in Figure 6. In the neat dehydrated PTA, a sharp peak at -15.6 ppm (line width ≈ 30 Hz) is observed. It can be recalled here that PTA has a cubic structure of symmetry with a central P atom. For Si-Imid-PTA,


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TABLE 2: Oxidation of a Few Representative Alkenes with Aqueous Hydrogen Peroxide Using Si-Imid-PTAa

a Experimental conditions: catalyst ) 0.05 g; substrate ) 5 mmol; aqueous H2O2 (30%) ) 5 mmol, temperature ) 60° C, time ) 4 h, solvent ) acetonitrile, a ) cyclooctanol and cyclooctanone, b and c ) please see the text, d ) thrice recycled catalyst.

peaks (Figure 6b) at -15.3 ppm (line width ≈ 90 Hz) and -13.6 ppm (line width ≈ 120 Hz) and for Imid-PTA two peaks (Figure 6c) at -15.6 and -13.7 ppm are observed. These results can be rationalized as follows. PTA on reacting with imidazole makes a new environment which may be due to the distortion of the cubic structure of PTA upon interaction with imidazole during functionalization. The peak at -13.6 ppm for Si-ImidPTA and for Imid-PTA at -13.70 ppm can be assigned to the phosphorus atom in the new environment. Even after functionalization, the peak due to the symmetric central P atom of PTA is present with significant intensity. This is due to the fact that there are eight molecules of phosphotungstic acids present in a

unit cube, the centers of the PTA anions being arranged in positions corresponding to the diamond structure.49 These molecules may remain as clusters which are strongly attached to the functionalized silica. The broad peaks at -15.3 ppm for Si-Imid-PTA and at -15.6 ppm for Imid-PTA may be due to this type of phosphorus present in the system.50-52 Thus, 31P NMR data clearly support the above model of PTA anchoring on imidazole of Si-Imid by forming an ion pair complex. 3.2. Catalytic Activity. The catalytic activity of Si-ImidPTA was studied for the oxidation of different alkenes such as limonene, cis-cyclooctene, 1-octene, norbornene, trans-2-octene,

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TABLE 3: Oxidation of cis-Cyclooctene with Aqueous Hydrogen Peroxide Using Si-NH2-PTA and PTA Impregnated on Fumed Silicaa

a

Experimental conditions same as Table 1 and the products formed given as Others* could not be identified.

1-methyl-1-cyclohexene, etc., using aqueous hydrogen peroxide as oxidant at 60 °C in acetonitrile solvent. These experiments were carried out mainly to demonstrate the efficiency and heterogeneous nature of the present catalyst. Additionally, controlled experiments with neat phosphotungstic acid and SiImid were also carried out under identical reaction conditions. The resulting data of substrate conversion and selectivity to the main products at the end of 4 h of reaction time are summarized in Table 2. As seen in the table, this catalyst system is active for epoxidation reaction and also selectively produced the desired epoxide compounds in most cases. Oxidation of ciscyclooctene, 1-octene, and norbornene gave corresponding epoxide with more than 99% selectivity with substrate conversion of 90, 34, and 85 mol %, respectively, and no efforts were made to optimize the conversion. With limonene, limonene diepoxide (46 mol %) and limonene epoxide (29 mol %) were formed with moderate conversion of 76 mol %. A certain amount of carvone (17 mol %) along with products like carveol, glycols, perillyl alcohol, etc. (included in Table 2 as Othersb and amounting to a total of 8 mol %) were also formed during the epoxidation of limonene. The oxidation of trans-2-octene and 1-methyl-1-cyclohexene also gave the respective epoxides as the major product with 91 mol % conversion. trans-2-Octene gave mixtures of 2, 3, and 4-octanones (23 mol %, marked as Othersc in Table 2), and 1-methyl-1-cyclohexene gave ketocarboxylic acid (29 mol %) and L-methyl-trans-1,2-cyclohexanediol (11 mol %). Formation of dihydroxy compounds is formed mainly due to the acidity arising from aq. H2O2/protons of PTA.53 As noticed, the selectivity to the epoxide product was always higher with the heterogeneous Si-Imid-PTA catalyst. It was observed that the conversion of the substrate as well as selectivity to the epoxide had reduced when neat PTA was used as catalyst (Table 2). This may be due to the fast decomposition of aq. H2O2 upon addition of neat PTA. When PTA impregnated on fumed silica was used as the catalyst, cis-cyclooctene, a conversion of 92% could be observed with a selectivity of 75% to the epoxide (Table 3). A comparative study using Si-NH2PTA as catalyst for the oxidation of cis-cyclooctene was also carried out, and the results are incorporated in Table 3. With the controlled experiments using Si-Imid as the catalyst under

similar experimental conditions, no olefin conversion could be observed. The above experiment indicates that the epoxidation reaction is catalyzed purely by PTA of Si-Imid-PTA and no contribution from Si-Imid, as expected, for the epoxidation of these alkenes under the reaction conditions. Further, the catalytic activity shown by the immobilized catalyst proves the effective anchoring of phosphotungstic acid successfully onto Si-Imid. The higher conversion and selectivity of the selected alkenes to the desired products using Si-Imid-PTA catalyst is attributed to the formation of tungsten-peroxo species for the formation of the selected products.54 Turnover frequency (TOF, h-1), defined as moles of product formed per mole of PTA per hour, were calculated for Si-ImidPTA and neat PTA (Table 2), and the results indicate that SiImid-PTA is a better catalyst than neat PTA for oxidation reactions when aq. H2O2 is used as the oxidant. 3.3. Catalyst Reuse and Leaching Studies. The stability of Si-Imid-PTA was studied in repeated epoxidation reactions under same reaction conditions using cyclooctene as a model substrate. After 4 h of cyclooctene epoxidation reaction, the Si-Imid-PTA catalyst was filtered from the reaction mixture and washed with acetonitrile and is referred here as recovered catalyst. The recovered catalyst was dried and used as a catalyst for the fresh reaction as above under identical experimental conditions, and the cycle was repeated thrice. It was found that the recycled catalyst was stable and active with selectivity of >99 mol % to epoxide although the substrate conversion was slightly low (see Table 2, entry 7). Even after three reaction cycles using the same catalyst, not much loss in activity could be observed. Both conversion as well as selectivity to the desired product remained almost similar to that of the fresh catalyst. Leaching experiments were performed with cyclooctene to confirm that the catalyst is heterogeneous in nature. The tungsten content present in the catalyst after final recycling studies was 16.853 ppm. The solid catalyst was separated from the reaction mixture at the end of 2 h, and the reaction was continued further only with the filtrate. Only traces of products were observed, showing that there is negligible leaching of the active species into the solution. There was no marked difference in the diffuse reflectance UV-visible spectra and 13C CPMAS (Figure 7 and


J. Phys. Chem. C, Vol. 113, No. 50, 2009 21121 interaction between PTA and imidazole. While IR, UV-vis, and other routine NMR studies provide evidence for the heterogenization of PTA on the imidazole functionalized fumed silica, the nature of binding of PTA on the support has been obtained from solid state NMR studies using 15N labeled imidazole support. Effective heterogenization of PTA is mainly due to imidazolium ion formation on the support due to protons of PTA, and they form an ion pair compound.

Figure 7. UV-visible spectra of (a) Si-Imid-PTA before catalytic reaction and (b) Si-Imid-PTA after catalytic reaction.

Acknowledgment. N.K.K.R. thanks the Department of Science & Technology (DST, New Delhi) for financial support. M.S. thanks the Council of Scientific and Industrial Research (CSIR, New Delhi) for a Senior Research Fellowship. The authors thank Dr. R. Nandini Devi, Dr. Maya Devi, Dr. Sujatha Mandal, Dr. R. H. Ingle, and Mr. Renny Mathew for useful discussions. References and Notes

Figure 8. 13C CPMAS NMR spectra of (a) Si-Imid-PTA before catalytic reaction and (b) Si-Imid-PTA after catalytic reaction.

Figure 9. 31P MAS NMR spectra of (a) Si-Imid-PTA before catalytic reaction and (b) Si-Imid-PTA after catalytic reaction.

8) of both used and fresh catalyst, whereas in the case of 31P MAS spectra (Figure 9), it is observed that an additional peak appears at around 8 ppm and there are broad peaks from 8 ppm continuing until about -16 ppm for the used catalyst. These peaks can be attributed to the presence of phosphorus of the distorted Keggin structure of PTA which is formed after the reaction.55,56 The distorted PW11O397- species could have been formed by the interaction of leached PTA with the support material. UV-visible spectra and 13C CPMAS spectra indicate that the catalyst is intact even after catalytic activity. 4. Conclusions The PTA anchored silica-imidazole system was used for epoxidation of a variety of symmetric and asymmetric olefins. The high olefin conversion and high selectivity to epoxide demonstrate that the present system is an efficient catalyst for such type of reactions. A negligible amount of leaching of PTA under the present reaction condition indicates the strong

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