Article pubs.acs.org/IECR
Synthesis, Characterization, and Epoxidation Activity of TungstenIncorporated SBA-16 (W-SBA-16) Rajamanickam Maheswari,†,§ Muthusamy P. Pachamuthu,§ Anand Ramanathan,† and Bala Subramaniam*,†,‡ †
Center for Environmentally Beneficial Catalysis, The University of Kansas, Lawrence, Kansas 66047, United States Department of Chemical and Petroleum Engineering, The University of Kansas, Lawrence, Kansas 66045, United States § Department of Chemistry, Anna University, Chennai−600025, India ‡
ABSTRACT: Tungsten, in varying amounts, was incorporated into a SBA-16 structure via a one-pot direct synthesis method under an acidic medium using Pluronic F127 triblock co-polymer as a template and n-butanol as a co-surfactant. Tetraethyl orthosilicate (TEOS) and sodium tungstate were used as the Si and W sources, respectively. The resulting materials (denoted as W-SBA-16) are characterized for structural ordering, textural properties, and types of tungsten incorporation by techniques such as small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), N2 sorption, high-resolution transmission electron microscopy (HR-TEM), diffuse-reflectance ultraviolet−visible light (DR-UV-vis) microscopy, temperature-programmed reduction in a hydrogen atmosphere (H2-TPR), and temperature-programmed desorption of ammonia (NH3-TPD). The surface area (823− 354 m2/g) and pore volume (0.71−0.44 cm3/g) of the W-SBA-16 materials are found to decrease with an increase in tungsten loading (from 2.7 wt % to 30.4 wt % of the total sample). Isolated framework WO4 species and octahedrally coordinated polytungstate species are observed at all tungsten loadings, while bulk WO3 species are observed only at higher tungsten loadings. The W-SBA-16 materials display significant acidity that is tunable with tungsten loading, and they selectively catalyze the epoxidation of cyclohexene to cyclohexene oxide with H2O2 as an oxidant. The fact that bulk WO3 alone does not catalyze the reaction implies that the framework-incorporated W species and/or the polytungstate species are responsible for the observed catalysis. For this reaction, three-dimensional cubic mesostructured catalysts (W-SBA-16, W-KIT-6, and W-KIT-5) perform better than two-dimensional mesostructured (W-SBA-15) material. The problem of gradual tungsten leaching must be overcome for these catalysts to have practical utility. 516 without significant changes in the structural properties of the parent silicates. In these materials, tungsten exists as isolated WO4 species, polytungstate species, and bulk WO3 species. These materials show tunable acidity, depending on the extent of tungsten loading. For the metathesis of 1-butene and ethene to propene, it was recently reported that the catalytic performance of W-KIT-6 prepared via the direct incorporation method is superior to WO3/KIT-6 prepared via impregnation.17 In this manuscript, we report the direct incorporation of tungsten into SBA-1618-type mesoporous silicate, which possesses large cagelike mesoporous structure (corresponding to the Im3m space group) with good thermal and hydrothermal stability.19 Metal ions such as V, Cu, and Fe have previously been directly incorporated into SBA-16 matrix.20−22 Because of the narrow range of surfactant concentrations (3−5 wt %) required during SBA-16 synthesis under acidic conditions, these metal ions are incorporated by suitably adjusting the pH of the synthesis gel. In this work, we have extended this synthesis technique to incorporate tungsten into SBA-16 by a direct
1. INTRODUCTION Alkene epoxidation is an important reaction in both the fine and bulk chemical industries. 1 It is well-known that homogeneous tungsten complexes utilize W(VI) as the active center for the epoxidation reactions.2,3 Tungsten-based heterogeneous catalysts have also been reported to show excellent activity and selectivity for epoxidations with H2O2.1,4,5 In tungsten-incorporated silicate catalysts, the presence of isolated tetrahedral WO4 species has been shown to be active for epoxide formation. 6 Nanoparticles of WO 3, either unsupported17 or supported on MCM-48,7 have been reported to be active for olefin oxidation with H2O2. Tungsten incorporation into already-synthesized supports is achieved by either impregnation or grafting of tungsten species.8−10 These post-synthetic routes of metal incorporation suffer from disadvantages such as a need to use expensive tungsten precursors and reduced specific surface area and pore volume. Direct incorporation involves adding the metal precursors to the synthesis mixture used to prepare the support. In recent years, tungsten has been directly incorporated into high-surface-area mesoporous molecular sieves such as MCM-41,11,12 MCM-48,13 SBA-15,14 and HMS14 under either acidic or basic conditions. In general, the incorporation of metal ions into the framework of silicate network (Si−O−M) is a challenge under acidic conditions. We have recently reported direct incorporation of tungsten into cubic ordered mesoporous materials such as KIT-615 and KIT© 2014 American Chemical Society
Special Issue: Ganapati D. Yadav Festschrift Received: Revised: Accepted: Published: 18833
April 30, 2014 June 28, 2014 June 30, 2014 June 30, 2014 dx.doi.org/10.1021/ie501784c | Ind. Eng. Chem. Res. 2014, 53, 18833−18839
Industrial & Engineering Chemistry Research
Article
in water) were added dropwise to the reaction mixture with continuous stirring (∼600 rpm). Liquid samples (0.5 mL) were then withdrawn at the end of each run and filtered through a syringe filter. The products were further diluted with acetonitrile and quantified on a GC-17A Shimadzu gas chromatograph instrument equipped with a DB-5 column. The conversion and selectivity values have an experimental error of ±5%.
hydrothermal synthesis method utilizing the usual synthesis procedure for SBA-1618 while adding a suitable tungsten precursor solution. Detailed textural and structural characterizations, including the nature of tungsten incorporation in the SBA-16, and the acidity are presented for W-SBA-16 materials with various tungsten loadings. In addition, the catalytic activity of W-SBA-16 materials (for cyclohexene epoxidation using H2O2 as oxidant) is investigated and compared with those with other tungsten-incorporated mesoporous materials such as WSBA-15, W-KIT-6, and W-KIT-5.
3. RESULTS AND DISCUSSION 3.1. Characterization of W-SBA-16. SAXS patterns of WSBA-16 materials, compared with a pristine SBA-16 sample, are shown in Figure 1. The well-resolved peak with an intense
2. EXPERIMENTAL SECTION 2.1. Synthesis of W-SBA-16. The synthesis of W-SBA-16 materials was performed following the procedure reported for SBA-16 by Kleitz et al.,18 using Pluronic F127 as a nonionic template and n-butanol as a co-surfactant. In a typical synthesis, 3.5 g of triblock co-polymer Pluronic F127 (Sigma) was dissolved in 175 mL of 0.4 M HCl solution at 45 °C. To this mixture, 13 mL of n-butanol were added with stirring. Then, 16.7 g of tetraethyl orthosilicate (TEOS 98%, Acros Organics) and required amounts of sodium tungstate (Acros Organics) were added to the synthesis mixture, and the stirring was continued for 20 h. Subsequently, the reaction mixture was heated at 98 °C for 24 h under static conditions in a polypropylene bottle. The precipitated product was hot filtered without washing and dried overnight at 100 °C. The template was removed by calcination in a flow of air at 550 °C for 5 h. The resulting solids are denoted as W-SBA-16(X), where X represents the molar Si/W ratio used in the synthesis gel. 2.2. Characterization of W-SBA-16. Detailed characterization procedures may be found in previous publications.15,16 Briefly, small-angle X-ray scattering (SAXS) patterns were acquired with a Rigaku system equipped with a S-MAX 3000 instrument using a Bede Scientific microfocus tube source operating at 45 kV and 0.66 mA. Powder X-ray diffraction was performed with a Bruker D8 instrument using Cu Kα radiation (λ = 1.54 Å) operating at 30 kV and 15 mV. N2 sorption was performed at 77 K with a Quadrasorb SI automated surface area and pore size analyzer. The sample was degassed at 300 °C for 3 h prior to analysis. Diffuse-reflectance ultraviolet−visible spectroscopy (DR UV-vis) spectra were acquired using a Shimadzu Model UV-2450 UV−visible spectrometer and BaSO4 as reference. High-resolution transmission electron microscopy (HRTEM) images were acquired with a JEOL Model 3010 instrument equipped with an ultrahigh resonance (UHR) pole piece operated at an accelerating voltage of 300 kV. Temperature-programmed reduction (TPR) and desorption (NH3-TPD) studies were performed with a ChemBET TPD/TPR instrument using 5%H2/95%Ar and 5%NH3/95% He gas mixtures, respectively. Elemental compositions of WSBA-16 samples before and after reaction were acquired using inductively coupled plasma−optical emission spectroscopy (ICP-OES) technique with a Perkin−Elmer Model Optima 5300 DV equipped with concentric nebulizer and cyclonic spray chamber. 2.3. Oxidation of Cyclohexene. Cyclohexene oxidation with W-SBA-16 catalyst was carried out in a two-neck roundbottomed flask equipped with a water condenser, septum cap, and magnetic stirrer. The catalysts were pretreated for 2 h in air at 150 °C. In a typical run, 50 mg of W-SBA-16, 10 mmol of cyclohexene and 10 mL of acetonitrile were charged into the stirred flask. The reaction mixture was equilibrated to desired temperature in an oil bath and then 10 mmol of H2O2 (30 wt %
Figure 1. SAXS patterns of W-SBA-16 samples compared to that of pristine SBA-16.
reflection at ∼0.77° (2θ) is attributed to the (110) plane and the weak reflections at ∼1.3° and 1.5° (2θ) are assigned to the (200) and (211) planes of the cubic Im3m structure, respectively.18 These reflections are diminished at the highest tungsten loading (Si/W = 10, 30 wt % W, as shown in Table 1), suggesting disruption of the Im3m structure at such tungsten loadings. The unit-cell parameter (a0) of W-SBA-16 was found to be lower than that of SBA-16 suggesting significant W incorporation in the extraframework position. Interestingly, an additional broad peak is also observed for the W-SBA-16 (20) sample (12.2 wt % W), suggesting a decrease in long-range structural ordering. As inferred from Table 1, elemental analyses of W-SBA-16 samples reveal close matches of the measured Si/W values with those used in the synthesis gel mixture. This indicates that most of the tungsten is successfully incorporated into the final material. XRD spectra of W-SBA-16 samples (Figure 2) in the high-angle region (2θ = 10°−80°) reveal a broad peak between 2θ = 15°−30° that is characteristic of mesoporous silica. No crystalline peaks are observed in samples with a tungsten loading of SBA-16 ≈ KIT-6 > SBA-15. This indicates that, for this reaction, three-dimensional (3D) cubic mesostructured catalysts (KIT-6, KIT-5, and SBA-16) perform better than the two-dimensional (2D) mesostructured SBA-15. However, the product distribution was found to be similar suggesting the nature and distribution of the W species are quite similar in these catalysts. The heterogeneous nature of the reaction was also tested by hot-filtering the reaction mixture after 3 h (cyclohexene conversion ∼13%) and continuing the reaction without the catalyst for 8 h. A small increase in cyclohexene conversion (from 13% at 3 h to 16% at 8 h) was found. This could be due to the tungsten species detected in the flitrate (measured by ICP-OES technique), corresponding to ∼5 wt % leaching from the catalyst. To assess catalyst stability, the W-SBA-16 (20) catalyst was filtered after each batch run, washed with acetone, dried and calcined at 300 °C for 3 h, and evaluated in three successive cycles. Such testing showed a decrease in cyclohexane conversion with each reaction cycle (Figure 10), attributed to leaching of active W species. Overall, there is an approximately 30% decrease in conversion after four cycles. Indeed, ∼30%−40% of the W species in the original catalyst is leached at the end of the four reaction cycles. Hot-filtration experiments confirm minimal activity of any leached tungsten species. Furthermore, experiments with Na 2WO 4 and WO3 compounds also reveal very little epoxidation activity. These results suggest that the epoxidation activity is mainly due to framework-incorporated W species. This explanation is further strengthened by a recent report that direct tungsten incorporation into KIT-6 framework provides superior catalytic activity during olefin metathesis reaction, compared to tungsten-doped KIT-6 catalyst by impregnation.17
4. CONCLUSIONS Direct incorporation of tungsten species into SBA-16 was successfully achieved by a one-pot synthesis procedure. The structural integrity of the W-SBA-16 materials and the tungsten incorporation therein were verified by SAXS, N2 sorption, XRD, and ICP-OES techniques. Tungsten is found to exist as either isolated WO4 species or as oligomeric species, even at lower tungsten loading. Crystalline WO3 species are formed at higher tungsten loadings, as inferred from XRD and UV-vis spectra. Isolated WO4 species and strong interaction of reducible W species with the support are confirmed from H2TPR results. NH3-TPD reveals the presence of acid sites with low-to-medium-type acid strength that can be tuned with tungsten loading. The W-SBA-16 samples are shown to be catalytically active for cyclohexene epoxidation with appreciable selectivity toward the epoxide. The problem of tungsten leaching from these catalysts must be overcome for these catalysts to have practical utility.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +1-785-864-2903. Fax: +1-785-864-6051. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was partly supported with funds from United States Department of Agriculture (USDA/NIFA Award No. 2011-10006-30362). The author M.P. is thankful to DST-FIST, UGC for instrument facility and financial support. The authors thank Dr. Brian P. Grady, University of Oklahoma, for SAXS analysis.
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DEDICATION We are extremely pleased to submit this manuscript as part of the Festschrift to honor Professor Ganapati D. Yadav, with whom one of the authors (B.S.) has enjoyed a special 18838
dx.doi.org/10.1021/ie501784c | Ind. Eng. Chem. Res. 2014, 53, 18833−18839
Industrial & Engineering Chemistry Research
Article
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friendship. Professor Yadav has made significant contributions in the synthesis and characterization of novel solid acid catalysts, and we hope that this contribution is a fitting tribute in that regard. In addition to leading an active catalysis research program, Professor G. D. Yadav has also provided exceptional leadership and service to advance the field of chemical engineering in India and to promote international collaborations. We wish him the best of health and happiness in the years to come.
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