Research Article pubs.acs.org/journal/ascecg
Fabrication of Silver−Tungsten Wafer-like Nanoarchitectures for Selective Epoxidation of Alkenes Shilpi Ghosh,† Shankha S. Acharyya,† Takehiko Sasaki,‡ and Rajaram Bal*,† †
Catalytic Conversion & Processes Division, CSIR-Indian Institute of Petroleum, Dehradun 248005, India Department of Complexity Science and Engineering, Graduate School of Frontier Sciences, The University of Tokyo, Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan
‡
S Supporting Information *
ABSTRACT: Silver−tungsten oxide wafer-like nanoarchitectures were prepared for the first time for selective epoxidation of a wide range of alkenes to their corresponding epoxides with high yield. The wafer-like nanostructure provides sufficient contact between the substrate/catalyst and overcomes the steric influence of the bulkier substrates. The influence of various reaction parameters, such as temperature, substrate to oxidant molar ratio, reaction time, and so forth, were investigated in detail. The catalyst was characterized by XRD, XPS, EXAFS, ICP-AES, RAMAN, SEM, and TEM. Raman studies prove that the formation of peroxo tungsten species is responsible for epoxiation reaction. High stability and recyclability of the Ag−W catalyst is also observed under the investigated conditions. KEYWORDS: Silver−tungsten nanowafer, Alkene epoxidation reaction
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INTRODUCTION From fundamental studies through various technological applications, nanomaterials have garnered unprecedented attention due to their strikingly different properties at nano level dimensions compared to the nature of its bulk counterparts.1 Direct fabrication of complex nanoarchitectures is highly desirable and still remains a challenge in materials science.2 Because of their size- and shape-dependent properties, much effort has been made to control the morphologies of nanoparticles and to organize them into well-defined structures.3 Through the synergism between the composition and shape of the nanostructures, which ultimately defines the electron structure and surface atomic arrangement, the catalytic performance of the nano architectures can be finely tuned.4 Thus, the appropriate selection of precursors and growth parameters is vital for facile synthesis of nanomaterials with well-defined architectures. Alkene epoxidation reactions are an important class of reactions within the broad domain of oxidation chemistry because the epoxides are valuable chemical intermediates for the manufacture of commodity chemicals, such as drug intermediates, food additives, and agrochemicals as well as the fact that they undergo a wide range of reactions.5 Propylene oxide is one of the most important synthetic intermediates produced in the industry and, currently, its production exceeds 10 million tons per annum.6 Thus, catalytic epoxidation of olefins has been of considerable interest from both an academic and industrial point of view. Transition metal catalysts, such as Mo, W, Ti, V, Mn, and Re, have been © XXXX American Chemical Society
successfully employed to catalyze alkene epoxidation, but the processes are mostly homogeneous in nature and suffer from the production of alcohol as a byproduct.5,7,8 Epoxidation reactions of substituted alkenes were also achieved using stoichiometric amounts of peracetic acid and m-chlorobenzoic acid.9 However, the use of peracids is not regarded as an environmentally benign process because equivalent amounts of acid waste are produced. The safety issues associated with handling peracids are also a matter of concern. The employment of TS-1 as a heterogeneous catalyst for the epoxidation of alkenes is associated with diffusion limitations in the case of bulky cyclic alkenes such as cyclohexene and cyclooctene.10 Recently, gallium oxide nanorods reportedly catalyze epoxidation in an ethyl acetate medium.11 A layered metal−organic framework has been proven as an epoxidation catalyst with TBHP, which suffers from the production of alcohol as a byproduct.12 Ti-MCM-41 materials have been studied to catalyze epoxidation reactions, but due to the lower hydrophobicity, which insists water adsorption, the catalyst exhibits lower activity and also suffers from leaching of Ti species.13 Polyoxometalate-supported tungsten oxides are effective catalysts for epoxidation,14 but often the requirement of microwave irradiation or additives are major drawbacks of these processes.15 An epoxidation reaction of alkenes was also Received: July 23, 2015 Revised: September 8, 2015
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DOI: 10.1021/acssuschemeng.5b00743 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering reported over supported gold nanoparticle catalysts.16 However, to date, a Ag−W catalyst has not been synthesized in the form of nanowafers and applied in the epoxidation of a wide range of alkenes. Wafer-like porous NaTi2(PO4)3 was used for constructing high performance electrodes for aqueous rechargeable sodium batteries.17 In comparison with the spherical nanoparticles, understanding the growth parameters and reaction conditions that yield wafer-like nanoarchitectures is at the present time not well established. Herein, we report for the first time the fabrication of very small silver oxide nanoparticles (∼3 nm) anchored on tungsten oxide nanowafers and their growth parameters. The newly prepared Ag−W catalyst exhibited excellent catalytic properties for selective epoxidation of alkenes, including propylene, using H2O2 (50%) as an environmentally benign oxidant. The Ag−W catalyst, with multiple steplike edges, provides wider accessibility to the bulkier substrates, such as cyclooctene and norbornene, thus preventing steric influence and therefore also representing a more effective catalyst as compared to the previously reported TS-1 catalyst. Moreover, propylene and cyclohexene, which are very much prone to allylic oxidation, are here selectively transformed to their corresponding epoxides with high yield. Furthermore, the synthetic procedure is a facile one-step process that is easy to scale up, and large quantities of Ag−W can be easily prepared.
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atoms in the planar surface of [WO6]6−.18 Nucleation of metal−surfactant molecules [CTA−WO6]2− took place in the anisotropic direction because of the CTA+ hydrophobic tails,18 which favors the formation of rodlike morphology. In a control experiment, without the addition of surfactant CTAB, we failed to observe any definite morphology (Figure S1, Supporting Information), reflecting the structure directing property of CTAB. It is worth mentioning that, before hydrothermal treatment, rodlike aggregated morphologies were generated (Figure S2). Hydrothermal treatment (HT) probably interferes with the surface energy and is responsible for the generation of the wafer-like structure. The growth evolution of the nanowafers was investigated as a function of time by taking aliquots at different stages of the hydrothermal treatment and characterizing them via SEM and TEM. Within 1 h of the HT process, we could observe a planar alignment of nanorods, which are serving as intermediates (Figure S3). Van der Waals attraction between the incoming nanorods promoted the lateral growth; besides, planar alignment of the nanorods ensures the largest side-to-side contact area between the nanorods and therefore effectively decreases the interfacial energy, thus fostering lateral growth, whereas the upper surface remains passivated by chemically saturated Ag atoms. Therefore, lateral growth occurs much faster than that of the vertical part. However, the involvement of Ostwald ripening, electrostatic, crystal-face interaction, and dipolar interactions cannot be excluded. In other words, growth anisotropy of the Ag−W is favored by its strong anisotropy in the layer structure. Finally, a large area assembly of WO3 nanowafers were obtained in a layer-by-layer stacking configuration with multiple steplike edges after a 16 h HT process (Figure 4). Further increasing the HT time (24 h), self-assembled architectures form agglomerated morphology (Figure S4). Namely, the formation of nanowafers is favored at a high Ag:CTAB ratio of 1:1; otherwise, irregular aggregates formed at much lower Ag:CTAB ratios of 1:0.15 (Figure S5). Catalyst Characterization. The X-ray diffraction pattern of the Ag−W catalyst showed typical peaks at 2θ values of 23.2, 23.5, 24.3, 33.2, and 34.2°, which corresponds to the monoclinic WO3 (JCPDS No. 43-1035, space group: P21/n) (Figure 1). In addition, the diffraction peaks at 2θ values of 33.0, 38.0, and 55.0° correspond to Ag2O crystal faces of (111), (200), and (220), respectively, which coincide well with the literature values (JCPDS No. 41-1104). Both silver and tungsten retain their phases after the reaction as confirmed by XRD (Figure 1e). XPS clearly showed the binding energies of Ag 3d5/2 at around ∼367.8 eV and Ag 3d3/2 at 373.9 eV, which can be attributed to the Ag (+1) state19 (Figure 2). After the reaction, the corresponding Ag 3d binding energy of 367.8 eV affirms that the oxidation state of silver oxide does not change after catalysis (Figure 2b). Furthermore, the W 4f5/2 and 4f7/2 spectrum corresponding to the binding energy values of 38.0 and 35.8 eV suggest that the tungsten in the tungsten oxide sample exists as W6+ (Figure S6).20 The topology of the catalyst was studied by SEM (Figure 3a−c), which showed that the Ag−W catalyst is built systematically in a layer-by-layer stacking configuration with multiple steplike edges. The elemental mapping demonstrated the homogeneous distribution of Ag(I) nanoparticles on WO3 (Figure 3d−f). A representative high resolution TEM (HRTEM) analysis as shown in Figure 3g and h revealed that the catalyst is composed of wafer-like morphology of WO3 in which very small silver(I) NPs of ∼3 nm in size were anchored, lending further support to
EXPERIMENTAL SECTION
Materials. W(OEt)6, AgNO3, cetyltrimethylammonium bromide (CTAB), hydrazine, HPLC grade (∼99.9%) cyclooctene, acetonitrile, and so forth were purchased from Sigma-Aldrich Co. Hydrogen peroxide (50 wt % in water) was purchased from Merck KGaA, Darmstadt, Germany. All the chemicals were used without further purification. Double distilled water was used during the preparation of catalyst. Preparation of a Ag−W Wafer-Like Nanoarchitecture. In a typical preparation method, 4.9 g of tungsten ethoxide (VI) was dissolved in 20 mL of ethanol. The pH was adjusted to ∼9 by dropwise addition of NH4OH solution followed by the addition of 0.15 g of AgNO3. Then, 0.50 g of CTAB solution was added, and the solution was stirred for 2 h. A solution of hydrazine monohydrate (80% solution) was added dropwise to the well-stirred mixture at RT. AgNO3, CTAB, and NH2NH2 were mixed maintaining a molar ratio of Ag:CTAB:NH2NH2 = 1:1:2. The mixture was stirred vigorously for 1 h and subsequently sealed in a Teflon-lined stainless steel autoclave. The autoclave was heated to and maintained at 180 °C for 16 h and then allowed to cool at room temperature. The samples were washed with ethanol prior to drying at 100 °C for 6 h. The resultant dry powder was treated by thermal heating at 550 °C for 6 h in the presence of oxygen (10 mL/min).
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RESULTS AND DISCUSSION Generation of Ag−W Wafer-Like Nanoarchitectures. The Ag (I) NPs supported on wafer-like WO3 was prepared by a surfactant-assisted method using tungsten ethoxide and silver nitrate as the precursors. The formation mechanism can be attributed to the hydrolysis of tungsten ethoxide, which leads to the formation of W(OH)6 followed by the reaction W(OEt)6 + H2O → W(OH)6 + HOH. The generation of the wafer-like morphology can be explained on the basis of surfactant (CTAB) assisted nucleation−growth rate of the seed, which actually determines the final morphology. In an alkaline medium, the formation of [WO6]6− octahedrons were favored. Driven by stereochemical effects, the positively charged cationic surfactant (CTA+) with a hydrophobic tail was attracted by electrostatic interaction of the four negatively charged oxygen B
DOI: 10.1021/acssuschemeng.5b00743 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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reduction temperature of support WO3 shifted to relatively lower values may be attributed to the nanosize effect that is easily reduced. Ag−K edge extended X-ray absorption fine structure (EXAFS) analysis of the Ag−W catalyst exhibited that no remarkable changes occurred in the oxidation state of the silver species of the fresh and the spent catalyst during the epoxidation reaction (Table 1 and Figure S9). It was observed that a Ag−O contribution at 0.2325 nm with a coordination number (C.N.) of 1.7 and the absence of Ag−Ag contribution indicates the formation of Ag2O in the fresh catalyst. For the spent catalyst, the Ag−O bond at 0.2313 nm with C.N. 1.8 was observed. The observation indicated that the oxidation state of Ag(I) remained unchanged after the catalysis (Table 1). Raman spectra of the Ag−W nanowafer are described in Figure 4. The bands in the range between 700 and 802 cm −1 correspond to the ν(W−O−W) stretching vibration, which are typical Raman peaks of crystalline WO3. The band at 939 cm−1 is attributed to the ν(WO) stretching mode of the symmetric terminal W O.23 The treatment of Ag−W with H2O2 resulted in new absorption band at 573 and 872 cm−1 in the Raman spectrum (Figure 4b). This band could be assigned as ν(W(O2))asymp and ν(O−O), respectively, which demonstrated that peroxo species were formed in the presence of H2O2.14 The presence of peroxo tungsten species was detected when we recovered and dried the catalyst after H2O2 addition. However, addition of cyclooctene caused the peroxo signal to disappear, thus proving the reaction of cyclooctene with this surface-bound peroxo species (Figure 4c). The peak at 760 cm −1 also corresponds to the ν(W−O−W) stretching vibration, which is visible in fresh and spent catalyst. However, in the case of the sample after H2O2 treatment, the peak may be of low intensity, which merges with the peak at 720 cm−1, and for that reason, it is not apparently visible. The Raman spectra of the fresh and spent catalyst were not significantly different, which further signifies the excellent structural stability of the Ag2O/WO3 (Ag−W) wafer-like nanoarchitecture under the reaction conditions. Catalytic Activity. Ag−W nanowafers were tested for oxidation of alkenes in a double neck round-bottom flask in an oil bath connected with a spiral condenser where H2O2 (50%) was added dropwise over a period of 10 min to avoid the immediate decomposition of H2O2. In the case of propylene, the catalytic reaction was performed in a 100 mL high-pressure Teflon-lined stainless steel closed reactor. Typically, 10 mL of acetonitrile solvent along with 0.1 g of catalyst were taken in the reactor, and the required amount of H2O2 was added. The closed reactor was pressurized with 10 bar of propylene, and the reactor was heated to the reaction temperature (80 °C) with constant stirring by an external magnetic stirrer (details in the Supporting Information). Reaction conditions, such as reaction temperature, H2O2 mole ratio, silver loading, and reaction time, were varied to obtain the optimum conditions for catalyst screening. At a substrate:H2O2 molar ratio of 1:3, linear alkenes, such as propene, 1-hexene, and allyl alcohol, and cyclic alkenes, such as cyclopentene, cyclohexene, cycloheptene, cyclooctene, norbornene, styrene, trans-stilbene, and so forth, transformed to their corresponding epoxides with very high yield (Table 2). Acetonitrile (MeCN) was employed as an environmentally acceptable and inexpensive solvent. It shows good solubility of the substrate, and the catalyst could be welldispersed. Moreover, the activation of H2O2 is facilitated in the presence of MeCN. Generally, the solvent acetonitrile in the presence of hydrogen peroxide generates peroxycarboxyimidic
Figure 1. XRD patterns of (a) W (VI) Oxide, (b) Ag(0), (c) Ag(I) oxide, (d) fresh Ag−W, (e) spent Ag−W.
Figure 2. Ag 3d core level spectra of (a) fresh and (b) spent Ag−W catalyst.
the SEM analysis. The spacing of the lattice fringes of ∼0.38 and 0.26 nm, due to the (020) plane of WO3 and (220) plane of silver oxide, respectively, was clearly visible (Figure 3i). The TEM image of the spent catalyst showed that the topology and particle size of the catalyst were hardly changed even after five reuses (Figure S7). These results are in good agreement with an excellent retention of activity of the catalyst. The before and after reaction particle size distributions suggest that the waferlike WO3 surface imparts a high structural and thermal stability to the 3 nm Ag(I) NPs due to the metal−support interaction, which also helps to resist the sintering of the Ag(I) NPs during catalysis. The strong metal support interaction can also be evidenced from the H2-TPR profile of Ag/WO3 nanowafers, as shown in Figure S8, which exhibit reduction peaks at 353, 401, 483, and 718 °C. The reduction of commercial Ag2O was at ∼147 °C.21 Commercial WO3 reduced progressively in three stages WO3 to W(0): WO3(VI) to WO2.9(V,VI) to WO2(IV) to W(0), exhibiting three main peaks at 457, 537, and 727 °C.22 The reduction peak at 353 °C in Ag−W catalyst can be attributed to the reduction of Ag(I) ions, the value is much higher than that of Ag2O, suggesting that Ag(I) ions available on the surface of WO3 interact strongly with the support and thus are more difficult to be reduced. Furthermore, the C
DOI: 10.1021/acssuschemeng.5b00743 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 3. SEM images of (a−c) Ag−W nanolayered catalyst, (d−f) SEM elemental mapping, and (g−i) TEM of Ag−W.
Table 1. EXAFS Curve-Fitting Parameters at AgK-Edge Fourier-Transformed k3-Weighted EXAFS Functionsa catalyst
shell
CN
R (1 × 10−1 nm)
DW (1 × 10−3 nm2)
Δr (1 × 10−1 nm)
ΔE0 (eV)
Rf (%)
fresh spent
Ag−O Ag−O
1.7 ± 0.4 1.8 ± 0.4
2.325 ± 0.019 2.313 ± 0.016
7.7 ± 2.7 9.3 ± 2.7
1.2−2.4 1.5−3.0
0.7 ± 2.5 3.0 ± 2.3
1.85 2.35
a EXAFS = extended X-ray absorption fine structure; CN = coordination number; R = bond length; DW: Debye−Waller factor; ΔK: the range of wavenumbers used in the fitting = 30−100 nm−1; Δr = the range of bond distances used in the fitting; S02: amplitude reducing factor = 0.95; ΔE0: shift of the edge-position; Rf: reliability factor.
acid intermediate (CH3−C(NH)−O−O−H), which is a very good oxygen donor species.24 The relative rates of epoxidation of several cycloalkenes at the conditions stated above are listed in Table 2. From this data, it was observed that epoxide formation took place very selectively, irrespective of the steric hindrance of the bulkier cyclic olefins like cyclohexene, cyclooctene, and norbornene (Table 2, entries 3−6). Even the alkenes containing allylic hydrogen atoms, such as propylene and cyclohexene, transformed to their corresponding epoxides with high selectivity (Table 2, entries 1 and 3). Although the literature indicates that the steric hindrance of the bulky molecules determines the fate of the reaction, the Ag−W nanowafer clearly overcomes such an influence. The open structure of the WO3 overcome the steric influence and allows efficient epoxidation of the larger substrates. The catalytic performance of our catalyst was compared to that of previously reported catalysts (Tables S1 and S2). The catalytic activity of the Ag−W wafer-like catalyst was investigated subjecting the oxidation of cyclooctene (Cn) to cyclooctene oxide as a model reaction. The Ag−W catalyst shows poor activity in highly hydrophobic solvent such as n-octane and highly hydrophilic solvent such as N,N-dimethylformamide (DMF) (entries 9, 10 Table 3). The reason may be attributed to the fact that n-octane is immiscible in aqueous H2O2, exhibiting a barrier between the
Figure 4. Raman spectra of Ag−W nanowafers with (a) fresh catalyst, (b) after H2O2 treatment, and (c) with spent catalyst.
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ACS Sustainable Chemistry & Engineering Table 2. Oxidation of Various Substrates Catalyzed by Ag−Wa
Reaction conditions: solvent (MeCN) = 10 mL, substrate = 1 g, catalyst weight = 0.10 g, 25 °C, rotation speed = 700 rpm, 3.5 wt % Ag loading, 1:3 substrate:H2O2(50%) mole ratio. bCs = conversion of substrates. cSP = selectivity of product. dYA = Yield (%); other products: cyclopentenol, cyclopentenone from cyclopentene oxidation, cyclohexenol, cyclohexenone from cyclohexene oxidation, norborneols and 2-norbornanone from norbornene oxidation, and benzaldehyde as the main side product of both styrene and trans-stilbene oxidation. eTurn over frequency (TOF) = [moles of epoxide formed/one mole of Ag in the catalyst]/reaction time (h). fReaction was conducted in a high-pressure Teflon-lined stainless steel closed reactor at 10 bar pressure. a
catalyst surface, the reactant and oxidant molecules, thereby inhibiting the entire catalytic process. Also, the strong coordination between the DMF and the tungsten center may be a cause for catalytic inactiveness.14 Using a polar protic solvent like methanol, the catalytic activity being low may be due to the lower polarity of methanol (Table 3, entry 11). In general, the polarity of acetonitrile (dielectric constant ε/ε0 = 37.5) is higher than that of methanol (32.7).25 The polarity and aprotic nature of the acetonitrile solvent might have played the key role in enhancing the catalytic activity of the Ag−W nanoarchitecture in acetonitrile medium (Table 3, entry 7). We also believe that acetonitrile activates H2O2 by forming a peroxycarboximidic acid intermediate, which is a good oxygen transfer agent. Other polar solvents like ethyl acetate were found to be less active due to rapid hydrolysis under the reaction conditions, whereas acetic acid, although showing high conversion, had a selectivity to the epoxide that is comparatively less. (Table 3, entries 12 and 13). The high conversion in acetic acid solvent may be attributed to the fact that, in the presence of hydrogen peroxide, acetic acid forms peroxy acetic acid, which is responsible for epoxide formation, but because acetic acid is acidic in nature, it can protonate the epoxide quite easily, resulting in poor selectivity. Table 2 demonstrates that high conversion of Cn and selectivity to epoxy cyclooctane was achieved after 3 h. We also noticed that temperature played a crucial role in the epoxidation reaction (Figure 5). At room temperature, comparatively less conversion (22%) was observed with cyclooctene oxide selectivity of 99%
Table 3. Comparative Study of Cyclooctene Epoxidation Based on Different Catalytic Systemsa entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14
catalyst
CSb (%)
Spc (%)
E0d (%)
9 11 45 33 56 99 97 12 7 34 31 95 5
34 17 56 45 52 97 96 85 95 93 87 78
1.0 0.62 8.4 4.9 9.7 32.0 31.0 3.4 2.2 10.5 9.0 22.1
com
Ag2O WO3com Ag2Ous WO3us Ag−W imp Ag−W imp/ WO3us Ag−We Ag−Wf Ag−Wg Ag−Wh Ag−Wi Ag−Wj Ag−Wk no catalyst
a
Reaction conditions: solvent (acetonitrile) = 10 mL, cyclooctene = 1 g, catalyst weight = 0.10 g, 3.5 wt % Ag loading, 1:3 substrate:H2O2 mole ratio, 80 °C, 3 h. bCS = conversion of substrates cSP = selectivity of cyclooctene oxide; dEo = [moles of epoxide formed/total moles of H2O2 added] × 100; com = commercial; us = catalyst prepared by surfactant; imp = impregnation method. Ag−W imp/WO3us = impregnated Ag−Wimp catalyst on WO3us support. eAg2O nanoparticles supported on WO3 wafer-like nanoarchitectures. fCatalyst after 5 runs. gn-Octane as solvent. hDMF as solvent. iMethanol as solvent. jEthyl acetate as solvent. kAcetic acid as solvent.
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DOI: 10.1021/acssuschemeng.5b00743 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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Figure 5. Effect of temperature on cyclooctene oxidation. [black] Conversion of cyclooctene; [red] selectivity to cyclooctene oxide; [green] yield. Reaction conditions: solvent = acetonitrile; cyclooctene = 1 g; weight of catalyst = 0.10 g; 3.5 wt % silver loading; 1:3 cyclooctene:H2O2 mole ratio; 3 h.
Figure 7. Effect of silver loading on cyclooctene oxidation. [black] Conversion of cyclooctene; [red] selectivity to cyclooctene oxide; [green] yield. Reaction conditions: solvent = acetonitrile; cyclooctene = 1 g; weight of catalyst = 0.10 g; 1:3 cyclooctene:H2O2 mole ratio; 80 °C; 3 h.
within 3 h of reaction. A temperature of 80 °C was found to be optimal. Upon further increasing the temperature to 100 °C, the cyclooctene oxide yield was decreased due to higher decomposition of H2O2 at 100 °C and also to the formation of cyclooctenol and cyclooctenone as side products. The effect of the substrate:H2O2 mole ratio was investigated and is shown in Figure 6. When the cyclooctene:H2O2 mole ratio was adjusted
that 3.5 wt % of silver loading is optimal because it showed the best catalytic performance toward olefin epoxidation. Using the same experimental conditions (catalyst wt % = 0.1 g, substrate = 1 g, 1:3 substrate:H2O2 mole ratio, 80 °C, in the case of cyclooctene, 3 h reaction time), 2 wt % of silver loaded catalyst exhibited cyclooctene conversion of 47% with 99% cyclooctene oxide selectivity. Whereas 3.5 wt % of silver loading gives 99% cyclooctene conversion with 97% cyclooctene oxide selectivity. Upon further increasing the silver loading to 6 wt %, we found that the conversion of cyclooctene remains the same (99%) but that the selectivity of cyclooctene oxide decreases to 87% due to the formation of cyclooctenol and cyclooctenone as major side products. Experiments were also carried out to understand the variation of Cn conversion as a function of reaction time (Figure 8). The conversion of cyclooctene increased from 45 to
Figure 6. Effect of cyclooctene:H2O2 mole ratio on cyclooctene oxidation. [black] Conversion of cyclooctene; [red] selectivity to cyclooctene oxide; [green] yield. Reaction conditions: solvent = acetonitrile; cyclooctene = 1 g; weight of catalyst = 0.10 g; 3.5 wt % silver loading; 80 °C; 3 h.
to 1:1, the conversion of Cn was low (∼47%); when the Cn:H2O2 mole ratio was increased to 1:3, the conversion increases to 99% with 97% cyclooctene oxide selectivity. Further increasing the ratio to 1:5 results in the selectivity dropping down to 93% without affecting the conversion of Cn. The selectivity drops on account of the formation of cyclooctenol and cyclooctenone. A further increase in the mole ratio to 1:10 results in low conversion because of water produced in the medium due to the decomposition of H2O2, which hinders the reactant molecules from coming into close contact with the catalyst. The lower cyclooctene conversion at higher molar ratio (1:10) may also be attributed to the higher substrate dilution and higher H2O concentration in the reaction medium, because 50% aqueous hydrogen peroxide is used here. The influence of silver loading toward the performance of epoxide formation was also studied in Figure 7. We have found
Figure 8. Effect of time on cyclooctene oxidation. [black] Conversion of cyclooctene; [red] selectivity to cyclooctene oxide; [green] yield. Reaction conditions: solvent = acetonitrile; cyclooctene = 1 g; weight of catalyst = 0.10 g; 3.5 wt % silver loading; 1:3 cyclooctene:H2O2 mole ratio; 80 °C.
99% after 3 h, but the selectivity of epoxy cyclooctene slightly drops from 99% to 97% due to the formation cyclooctenone and cyclooctenol as the major side products. A series of catalytic experiments were conducted to exploit the individual role of silver and tungsten oxide (Table 3). Commercial Ag2O and WO3 showed very poor activity. Bare WO3us prepared by our surfactant-assisted method catalyzed the oxidation reaction less selectively, moreover, severe leaching F
DOI: 10.1021/acssuschemeng.5b00743 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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was caused during the reaction. Alone, Ag2Ous prepared by our method showed very little activity. Moreover, Ag−W prepared by the impregnation method exhibited lesser activity (Table 3, entries 1−5). We have also presented the performance of an impregnated Ag−Wimp catalyst on WO3us support, which showed much lower catalytic activity (56% conversion with 52% epoxide selectivity; Table 3, entry 6) than our prepared Ag/WO3 wafer-like nanoarchitectures (Table 3, entry 7). This is attributed to the fact that Ag size in Ag−Wimp is larger and non-uniform and, therefore, unable to promote the rate of transfer oxygen species to the alkene system. The oxidation reaction barely proceeded in the absence of the catalyst (Table 3, entry 14). It is supposed that the support tungsten oxide was likely to play a role in activating H2O2. Our experimental findings using Raman analysis confirmed the evolution of peroxo tungsten species after treating the catalyst with H2O2 (Figure 4b). Because the peroxidic oxygen atom is electrophilic in nature, it can easily facilitate the formation of the epoxide with the transfer of an oxygen atom to the olefinic double bond. The d10 electronic configuration of Ag(I) favored interactions of Ag(I) with unsaturated alkene systems having low-lying empty orbitals.26 The small Ag(I) ions (∼3 nm) are supposed to activate the reactants (alkenes) and promote the rate of oxygen transfer from peroxo tungsten species to the activated alkenes to form the desired epoxide products. The combination of both Ag(I) and WO3 operates in a cooperative catalysis fashion, resulting in increased conversion and selectivity for the epoxidation of alkenes. The oxidant H2O2 was used in an excess amount (1:3 substrate:H2O2 molar ratio). Generally, H2O2 decomposes spontaneously over a catalytic surface. Hence, we used an excess of H2O2 so that the active oxygen species needed for the epoxidation of the alkenes could be available during the reaction. Furthermore, we took the reaction mixture after the end of cyclooctene epoxidation reaction (after 3 h) and performed permanganometric titrations to detect H2O2, but no H2O2 was detected in the reaction mixture, indicating that the unreacted H2O2 molecules have been completely decomposed. However, in the case of cyclopentene, we could detect H2O2 during the KMnO 4 titration. We have plotted H 2 O 2 consumption in terms of its efficiency (Eo) in Table 3. Reusability Test. The recyclability of the catalyst for the epoxidation of cyclooctene was carried out with the reused catalyst under the same reaction conditions. Activity of the recovered catalyst after five consecutive runs did not show any significant activity loss in terms of conversion or selectivity. After completion of the reaction, the solid catalyst was removed from the reaction mixture by filtration during hot conditions, and the reaction was allowed to proceed with the filtrate under the same conditions. The decanted liquid showed no further reaction in the absence of the catalyst, and therefore, any contribution from homogeneous catalysis can be neglected. Estimation of Ag and W for the spent catalyst was carried out by inductively coupled plasma (ICP) and atomic absorption spectroscopy analysis, and it was found that after five runs the spent catalyst showed trace amounts of Ag metal loss (
