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Applied Catalysis A: General 283 (2005) 117–124 www.elsevier.com/locate/apcata
Selective epoxidation of allyl acetate with tert-butyl hydroperoxide over MoO3/MgO Kenta Shimuraa, Hiroyoshi Kanaia,*, Kazunori Utania, Kazuo Matsuyamab, Seiichiro Imamuraa a
Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan The Central Research Institute, NOF Corporation, Aza-Kitakomatsudani, Taketoyo-cho, Chita-gun, 470-2398, Japan
b
Received 14 September 2004; received in revised form 12 November 2004; accepted 30 December 2004 Available online 21 January 2005
Abstract Highly selective epoxidation of allyl acetate was carried out over MoO3/MgO using tert-butyl hydroperoxide as an oxidizing agent. The catalytically active species was identified as monolayer MgMoO4 on MgO. MgO with large pores is suitable for selective epoxide formation. The highest yield of glycidyl acetate was obtained over 7 wt.% MoO3/MgO, in which MgMoO4 species were highly dispersed on the surface of MgO. Trialkyl borates were good promoters for selective epoxide formation. Among them, triisopropyl borate afforded the highest yield of 92%. 1H and 11B NMR measurements of a homogeneous MoO2(acac)2-B(OBun)3-(t-C4H9OOH) system showed that alkyl groups of the borate exchange with t-butyl hydroperoxyl group, and that the boron atom interacts with the oxygen of Mo=O, promoting reactivity of an oxygen atom in the hydroperoxyl group to attack the double bond of allyl acetate. # 2005 Elsevier B.V. All rights reserved. Keywords: Epoxidation; Allyl acetate; tert-Butyl hydroperoxide; MoO3/MgO; DR–UV–vis; XANES; Trialkyl borate
1. Introduction Epoxides are important intermediates in organic syntheses and are also used as raw materials for various petrochemical products. The chlorohydrin process has been applied as one of the most effective methods to produce epoxides, but this method is not desirable from the environmental point of view because of its use of harmful chlorine [1]. Another disadvantage is that contamination of a very small amount of chloro compounds cannot be avoided even by elaborate rectification. Such chloro compounds in an epoxy-resin wafer damage electronic functions of integrated circuits. One promising process is an indirect epoxidation using hydrogen peroxide or alkyl hydroperoxides as oxidizing agents, since by-products are environmentally friendly or * Corresponding author. Tel.: +81 75 724 7555; fax: +81 75 724 7555. E-mail address: [email protected] (H. Kanai). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.12.042
can be easily reconverted to hydroperoxides through dehydration and re-oxidation [2]. The reaction between an olefin and ROOH is believed to be ionic, where the electron-deficient oxygen atom of ROOH coordinated to a metal ion catalyst tends to attack electron-donating olefins [3]. Allyl acetate, having an electron-deficient group, is one of the hard-to-make-epoxide olefins. We have tried to synthesize glycidyl acetate from allyl acetate with t-butyl hydroperoxide. We found two MoO3-based catalysts: one is MoO3/TiO2 [4] and the other is MoO3/a-Al2O3 modified with pyridine [5]. However, the yield was 55% at the highest. These results suggested that higher yield could be obtained by suppressing the ring opening of epoxide caused by acid sites of the catalysts [6]. We have succeeded in synthesizing glycidyl acetate in a 92% yield over MoO3/MgO in the presence of triisopropyl borate. We will report the selective epoxidaion of allyl acetate with t-butyl hydroperoxide over MoO3/MgO, together with the characterization of the highly selective catalysts.
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2. Experimental 2.1. Catalysts and reagents The supports were commercial MgO (Nacalai Tesque Co., abbreviated as MgO-n) and a series of MgO reference catalysts supplied by the Catalysis Society of Japan (abbreviated as MgO-100A, -500A, -1000A, -2000A). Their pore radii, surface areas and particle sizes are listed in Table 1. Other reagents were used as obtained commercially. An aqueous solution of t-butyl hydroperoxide (abbreviated as TBHP: 68.7 wt.%, NOF Corporation) was shaken with benzene, and the organic layer was separated and dried with anhydrous MgSO4 for two days. The TBHP-benzene solution was filtered and stored over molecular sieve 5A. The concentration of TBHP (3.8 mol l1) was determined by iodometry. A known amount of (NH4)6Mo7O244H2O was dissolved in deionized water and MgO was added to the solution. After stirring for 30 min, the solution was evaporated to dryness at 333 K. The solid portion was dried at 353 K overnight, followed by calcination at 823 K for 3 h in air. MoO3/aAl2O3(acac), where acac stands for acetylacetonate, was prepared from MoO2(acac)2 and a-Al2O3 in ethanol solution, followed by evaporation, drying at 353 K, and calcination at 823 K for 3 h in air.
Fig. 1. Epoxidation of allyl acetate over 7 wt.% MoO3/MgO-n at 383K. *: Conversion, &: selectivity, ~: yield.
reflux condenser and a septum rubber. The flask was immersed in an oil bath (383 K). A benzene solution of TBHP (0.3 ml, TBHP 1.5 mmol) was injected into the flask and the reaction was started by stirring the solution with a magnetic agitator. When an additive (trialkyl borate) was used, it was pre-mixed with allyl acetate. Glycidyl acetate and TBHP were analyzed by a Shimadzu GC-14A gas chromatograph (capillary column: DB-23 (J&W), 323– 423 K).
3. Results and discussion
2.2. Catalyst characterization
3.1. Epoxidation of allyl acetate with t-butyl hydroperoxide over MoO3/MgO catalyst
The diffuse reflectance technique was applied to record the ultraviolet spectra on a Shimadzu UVPC-3100 spectroreflectometer. XRD analyses were carried out with a Rigaku Denki Geigerflex 2012 X-ray analyzer, and XPS spectra were obtained with a JEOL JPS-9010MX X-ray photoelectron spectrometer. Pore radii of catalysts were measured by automatic gas and vapor adsorption measurements with a Nihon Bell BELSORP 18 PLUS meter. X-ray absorption measurements (Mo K-edge) were done at the Beam Line 10B station of Photon Factory, KEK (Tsukuba) by a transmission mode. 1H and 11B NMR measurements were carried out on a Bruker AV400M NMR spectrometer.
The epoxidation of allyl acetate with TBHP was carried out at 383 K over 7 wt.% MoO3/MgO-n (Fig. 1). The epoxidation reactions at temperatures lower than 383 K were not preferable since both their conversion and their selectivity were very low. Hereafter, reactions were carried out at 383 K. The selectivity of epoxide (glycidyl acetate) decreased with the elapse of time. The decrease of the selectivity was caused by the ring opening of the produced epoxide, as shown in Fig. 2. Glycidyl acetate reacted rapidly over MoO3/a-Al2O3(acac) as its acid sites attacked the epoxide [5,6]. The addition of pyridine moderated the acidity, causing less ring opening. Almost no ring opening
2.3. Reaction procedure Catalyst (0.1 g) and allyl acetate (5 ml) were charged under nitrogen in a 50 ml two-necked flask equipped with a Table 1 Properties of MgO MgO, symbol
Pore radius (nm)
Surface area (m2 g1)
Particle size (mm)
n 100A 500A 1000A 2000A
2.13 1.46 1.87 1.95 1.72
8.6 120 28.1 16.0 7.7
25.4 0.01 0.05 0.10 0.20
Fig. 2. Reactions of glycidyl acetate over MoO3-based catalysts at 383K. *: 7 wt.% MoO3/MgO-n. t-Butanol was added at the time indicated by the arrow. &: 7 wt.% MoO3/a-Al2O3(acac) + pyridine. ~: 2 wt.% MoO3/aAl2O3(acac).
K. Shimura et al. / Applied Catalysis A: General 283 (2005) 117–124
Fig. 3. Effects of the amount of MoO3 loading on the epoxidation of allyl acetate over MoO3/MgO-n. Reaction time: 6 h. *: Conversion, &: selectivity, ~: yield.
occurred over MoO3/MgO-n. The addition of t-butanol, which was formed from TBHP in the progress of the epoxidation, to slow ring opening (Fig. 2). There was a very weak basicity in MoO3/MgO-n when it was calcined at 823 K. Epoxides are ring-opened by acids or bases [6]. Glycidyl acetate was almost inert to the weak basic sites of MoO3/MgO-n at 383 K, as shown in Fig. 2. The effects of the amount of MoO3 loading on the epoxidation of allyl acetate are shown in Fig. 3. The conversion increased with the increase in loading up to 7– 10 wt.% of MoO3. Further increase in loading resulted in an initial decrease of conversion and then a gradual increase. The selectivity of epoxide decreased drastically with the increase in loading up to 30 wt.%. The maximum yield (53%) of epoxide was obtained over 7 wt.% MoO3/MgO-n after 6 h. 3.2. Effects of MgO having different pore sizes A series of MgO that have various pore sizes were used as the supports. Epoxidation of allyl acetate was carried out over 7 wt.% MoO3/MgO (Table 2). The conversion, selectivity, and yield after 6 h are plotted as a function of pore radius of MgO in Fig. 4. Though Fig. 4 shows some scatter in the plots of the conversion and the yield, the selectivity tends to increase with the increase in pore radius.
Table 2 Effects of MgO on the epoxidation of allyl acetate over 7 wt.% MoO3/MgO MgO
Pore radius (nm)
Yield (%)a
Maximum yield (%) (Loading)b
100A 500A 1000A 2000A n
1.46 1.87 1.95 1.72 2.13
1.2 6.4 24 26 53
15 21 43 46 53
a b
(40) (30) (15) (10) (7)
Yield after 6 h. Maximum yield at the loading indicated in the parentheses.
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Fig. 4. Effects of pore radius of MgO on the epoxidation of allyl acetate over 7 wt.% MoO3/MgO. Reaction time: 6 h. *: Conversion, &: selectivity, ~: yield.
Table 2 also gives the maximum yields at the loadings indicated in the parentheses. No MgO could be superior to MgO-n as the amount of loading was changed from 2 to 40 wt.%. 3.3. Characterization of MoO3/MgO catalyst Many papers on the preparation and structure of MoO3/ MgO catalysts have been reported: preparation [7–12], Raman spectra [13–16], DR–UV–vis spectra [17–21], XANES measurements [22–27]. Catalytic activity is dependent on the method of preparation and the amount of loading. We will discuss the factors of MoO3/MgO catalysts available to achieve selective epoxidation of allyl acetate. BET surface areas, particle sizes and pore radii of MgO are listed in Table 1. MgO-n is a commercial reagent and the rest (MgO-100A, 500A, 1000A, 2000A) are Reference Catalysts supplied by the Catalysis Society of Japan. Their precise composition lists are not available, but it becomes clear that their pore sizes are crucial factors in the selective epoxidation of allyl acetate (Fig. 4). 3.3.1. XRD The XRD profiles of MoO3/MgO-n are shown in Fig. 5. No crystal phase of MoO3 was detected in MoO3/MgO-n loaded with less than 30 wt.% MoO3, but b-MgMoO4 [28] was observed in the catalysts loaded with more than 5 wt.% MoO3. A considerable amount of MgO was hydrolyzed into Mg(OH)2 which reacted with Mo7O246 to give MgMoO4 [29]. The obtained MgMoO4 which was only dried at 353 K gave no crystalline signals in its XRD. Heating at 823 K resulted in the appearance of crystalline MgMoO4. When a mechanically ground mixture of 7 wt.% MoO3 and 93 wt.% MgO-n was heated at 823 K in air for 3 h, MgMoO4 was identified in its XRD pattern as was reported by Stampfl et al. [16]. Even if MoO3 was formed during the catalyst preparation, well-contacted MoO3-MgO mixture
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Fig. 5. XRD profiles of MoO3/MgO-n. The number in the figure is the amount of loading (wt.%). *: MgMoO4, ~: MgO.
should be transformed into MgMoO4 by the heat-treatment in air. 3.3.2. XPS No difference was observed in the binding energies of Mo3d5/2,3/2 and O1s of 2–78 wt.% MoO3/MgO-n. The ratio of the (Mo3d5/2 + Mo3d3/2) peak-area to the Mg2p3/2 peakarea, IMo/IMg, is plotted versus MoO3 loading, together with the conversion and epoxide selectivity (Fig. 6). The ratio increases linearly with the increase in loading up to 7 wt.% and the slope in the range larger than 7 wt.% becomes smaller. This suggests that MgMoO4 monolayer was attained at 7 wt.% loading of MoO3 on MgO-n. The conversion increases with the increase in MgMoO4 monolayer. However, the selectivity decreases with the increase in loading, this effect is caused by the side-reaction of glycidyl acetate with t-butanol produced. The agreement of the inflection points between IMo/IMg and the epoxide yield suggests that monolayer MgMoO4 on MgO is responsible for the selective epoxidation. 3.3.3. Diffuse reflectance UV–vis spectra Diffuse reflectance UV–vis spectra of MoO3/MgO-n catalysts and standard molybdenum oxides are shown in Fig. 7. Tetrahedral molybdenum oxides (K2MoO4,
Fig. 6. Plots of XPS peak area ratio (IMo/IMg), conversion, and selectivity vs. MoO3 loading. Reaction time: 6 h. *: IMo/IMg, ~: conversion, &: selectivity.
Fig. 7. Diffuse reflectance UV–vis spectra of standard molybdenum oxides [I] and MoO3/MgO-n [II]. A: K2MoO4, B: Na2MoO4, C: MgMoO4, D: (NH4)6Mo7O244H2O, E: MoO3.
Na2MoO4, MgMoO4) give two peaks at 220 and 260– 270 nm. These peaks are assigned to the tetrahedral structure [17–21,30,31]. Octahedral molybdenum oxides ((NH4)6Mo2O24, MoO3) have two peaks at 220 and 320– 350 nm. Spectra of high-loading MoO3/MgO-n are similar to those of tetrahedral MoO42. However, 2 wt.% MoO3/ MgO-n lacks the peak at 260–270 nm. The peak gradually grows with the increase in loading. The positions of the absorption peaks of higher loaded MoO3/MgO-n coincide with those of crystalline MgMoO4. Thus, the peak at 260– 270 nm should be ascribed to the absorption of MgMoO4 which interacts with each other in the crystalline state. The appearance of the peak agrees with the saturation of the fixing sites of MgO-n with MgMoO4. 3.3.4. XANES spectra To clarify the structure of molybdenum oxide on MgO-n in detail, we performed X-ray absorption measurements at the Mo K edge. Their X-ray absorption near-edge structure (XANES) spectra are shown in Fig. 8. The intense pre-edge peak assigned to a 1s–4d transition is consistent with the tetrahedral environment [22–27]. The transition for regular octahedral Mo6+ is formally forbidden, but that for asymmetric tetrahedral one is allowed. The XANES spectra of the catalysts are similar to that of MgMoO4 with a tetrahedral configuration. A close examination shows a tendency that the pre-edge peak height increases with the increase in loading, while the shoulder peak in the post-edge is conversely weakened. More information was derived from their second derivative XANES spectra (Figs. 9 and 10). There are differences in the shape and position between tetrahedral
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Fig. 8. Normalized XANES spectra of 2–30 wt.% MoO3/MgO-n and standard molybdenum oxides.
Fig. 11. Pre-edge (A) and post-edge (B) XANES spectra of 7 wt.%MoO3 supported on MgO having various pore sizes.
Fig. 9. Second derivative XANES spectra of standard samples. A: MgMoO4, B: K2MoO4, C: BaMoO4, D: (NH4)2Mo2O7, E: (NH4)6Mo7O24, F: MoO3.
(K2MoO4, MgMoO4, BaMoO4) and octahedral standard Mo6+ species ((NH4)6Mo7O24, MoO3) (Fig. 9). The position and depth of the bottom of (NH4)2Mo2O7 in which the anion is a polymer of linked MoO6 octahedra and MoO4 tetrahedra [32] take an intermediate value. There are small but decisive differences between the second derivatives of low and high loading MoO3/MgO-n (Fig. 10). The bottom of the curve becomes deeper with the increase in loading up to 7 wt.%.
Fig. 10. The second derivative XANES spectra of 2–30 wt.% MoO3/MgOn. The number is the amount of loading (wt.%). A: MgMoO4.
The second derivative XANES spectrum of 7 wt.% MoO3/ MgO-n agrees very closely with that of MgMoO4. The bottom position shifts to higher energy for highly loaded MoO3/MgO-n. The inflection point is at 7 wt.% loading as is also observed in the XPS (Fig. 6) and DR–UV spectra (Fig. 7). The Mo K pre-edge and post-edge spectra in the XANES of 7 wt.% MoO3 supported on various kinds of MgO are shown in Fig. 11. The intensity of the pre-edge peak increases with increase in the pore size of MgO. The shoulder peak of the post-edge is conversely more intense for Mo oxide supported on smaller pore-sized MgO. The second derivatives of the pre-edge XANES spectra except for Mo on MgO-n are shown in Fig. 12. Their bottoms do not
Fig. 12. The second derivative XANES spectra of 7 wt.% MoO3 supported on MgO having various pore sizes.
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Fig. 13. Effects of the amount of triisopropyl borate on the epoxidation of allyl acetate. (iso-PrO)3B/TBHP: *: 0, ~: 0.01, &: 0.03, 5: 0.06.
coincide with that of MgMoO4. They rather agree with those of MoO3/MgO-n loaded more than 10 wt.% (Figs. 10 and 12). Inspection of these pre-edge and post-edge peaks and second derivatives reveals that they are closely related with the results of the epoxidation activity. Though the compositions of MgO used are the same, the structures of molybdenum oxide species were affected by the microstructure of MgO supports; the small but decisive differences drastically reflected their epoxidation selectivity. 3.4. Effects of additives The addition of pyridine on MoO3/a-Al2O3 reduced its acidity and increased epoxide selectivity [5]. The coordination of pyridine to the acidic sites and Mo atoms of MoO3/ MgO-n retarded epoxidation of allyl acetate. Among 4substituted pyridines tested, 4-cyano- or 4-methoxycarbonyl-pyridine (py/Mo = 1/3), which bind weakly to the catalyst, improved selectivity in some degree. One of the authors found that the addition of trialkyl borates promoted the epoxidation of 1-butene [33]. We found that the addition of trialkyl borate promoted enormously the selective epoxidation of allyl acetate shown in Fig. 13, though it had no effect in the epoxidation over MoO3/a-Al2O3. The addition of 3–6 mol% to TBHP was appropriate in the improvement of both conversion and selectivity. The presence of any trialkyl borates is more effective than their absence. The efficiency of trialkyl borate is (MeO)3B < (EtO)3B < (n-PrO)3B (n-BuO)3B ((C18H37O)3B (iso-PrO)3B (Fig. 14). Triisopropyl
Fig. 14. Effects of trialkyl borates. : Conversion, : selectivity, : yield.
borate is the best promoter. However, there was no significant difference in the promoting effects of various trialkyl borates for the metathesis of allyl acetone and allyl chloride over Re2O7/Al2O3-Bu4Sn catalyst [34]. To elucidate the role of trialkyl borate, we ran 1H and 11B NMR measurements of homogeneous solutions composed of MoO2(acac)2, tri-n-butyl borate, t-butanol and TBHP in o-dichlorobenzene-d4 (Figs. 15 and 16). The appearance of free CH2OH group indicates the alkoxyl or alkyl peroxyl exchange between (n-BuO)3B and t-butanol or between (nBuO)3B and TBHP (Fig. 15). These were further confirmed by 11B NMR (Fig. 16). The chemical shifts are adjusted to that of B(OBun)3 in o-dichlorobenzene-d4. Based on the concentration dependence, the first equilibrium constant of the exchange between (n-BuO)3B and t-butanol was evaluated to be 0.12, and the second one 0.06. The first equilibrium constant of the exchange between (n-BuO)3B and TBHP was estimated to be 0.32. The rather large value for the latter exchange is due to the reduction of steric hindrance by one more neighboring oxygen atom in OOBut. 11 B NMR spectra (Fig. 16) supports the latter reaction, from which mono- and di-t-butyl peroxy derivatives of boron are presumed to be formed. The addition of MoO2(acac)2 to a
Fig. 15. 1H-NMR spectra of (n-BuO)3B in the presence of TBHP and t-butanol in o-dichlorobenzene-d4 (2.5 g).
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Scheme 1.
between Mo=O and the borate hooks in the hydroperoxy oxygen in the vicinity of Mo for easy coordination. Then the oxygen atom of the peroxy group attacks the double bond of allyl acetate to produce glycidyl acetate. The function of the trialkyl borate is to form a ternary transitional complex for easy epoxidation.
4. Conclusion Epoxidation of allyl acetate, which is one of the hard-tomake-epoxide olefins with TBHP, was successfully carried out over MoO3/MgO in the presence of trialkyl borate. The causes of successful epoxidation are as follows: (1) acid sites of MoO3 on acid supports cause ring opening unavoidably but MoO3/MgO underwent no ring opening of epoxide in the absence of alcohol; (2) monolayer tetrahedral MgMoO4 was attained at 7 wt.% MoO3 loading on commercially available MgO; (3) preferable MgO had large pores, (4) a small amount of triisopropyl borate (3 mol% to TBHP) interacted synergistically with TBHP and molybdenum catalyst and accelerated the epoxidation of allyl acetate.
Acknowledgements
Fig. 16. 11B NMR spectra of tri-n-butyl borates treated with MoO2(acac)2 and TBHP.
tri-n-butyl borate solution caused a shift of the peak to lower frequency, indicating the interaction between tri-n-butyl borate and O=Mo. Further addition of TBHP to the solution produced a new signal, which was deduced to be (tBuOO)(n-BuO)2B interacted with MoO2(acac)2. The first step of the epoxidation is the coordination of hydroperoxide to metal catalysts (e.g. Mo), allowing the peroxy oxygen of hydroperoxide electrophilic to attack the double bond of olefin [35]. Therefore, olefins with electronrich double bond are easily oxidized. Although allyl acetate has an electron-withdrawing group that makes its C=C bonds less likely to be attacked by oxygen atoms, the present catalyst system with trialkyl borates is remarkably effective. Here we discuss the action of trialkyl borate briefly (Scheme 1). Alkyl peroxy exchange between TBHP and trialkyl borate occurs, and the concurrent interaction
We thank Messrs S. Naito and S. Ichikawa of NOF Corporation for valuable discussions and for the supply of TBHP. We also thank Dr. Okano of Tateho Chemical Industries Co. Ltd. for the TEM image and data on the particle size of MgO-n. This work has been conducted under an entrustment contract between New Energy and Industrial Technology Development Organization (NEDO) and Japan Chemical Innovation Institute (JCII). The XAFS measurements were performed under the approval of the Program Advisory Committee of High Energy Acceleration Research Organization, Institute of Materials Structure Science, Photon Factory (Proposal No. 2003G287). We are grateful to Drs. Nomura and Usami for the XAFS measurements. We thank Mr. M. Sugiura, of NOF Corporation, for the 1H and 11 B NMR analyses.
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Selective epoxidation of allyl acetate with tert-butyl hydroperoxide over MoO3/MgO Kenta Shimuraa, Hiroyoshi Kanaia,*, Kazunori Utania, Kazuo Matsuyamab, Seiichiro Imamuraa a
Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan The Central Research Institute, NOF Corporation, Aza-Kitakomatsudani, Taketoyo-cho, Chita-gun, 470-2398, Japan
b
Received 14 September 2004; received in revised form 12 November 2004; accepted 30 December 2004 Available online 21 January 2005
Abstract Highly selective epoxidation of allyl acetate was carried out over MoO3/MgO using tert-butyl hydroperoxide as an oxidizing agent. The catalytically active species was identified as monolayer MgMoO4 on MgO. MgO with large pores is suitable for selective epoxide formation. The highest yield of glycidyl acetate was obtained over 7 wt.% MoO3/MgO, in which MgMoO4 species were highly dispersed on the surface of MgO. Trialkyl borates were good promoters for selective epoxide formation. Among them, triisopropyl borate afforded the highest yield of 92%. 1H and 11B NMR measurements of a homogeneous MoO2(acac)2-B(OBun)3-(t-C4H9OOH) system showed that alkyl groups of the borate exchange with t-butyl hydroperoxyl group, and that the boron atom interacts with the oxygen of Mo=O, promoting reactivity of an oxygen atom in the hydroperoxyl group to attack the double bond of allyl acetate. # 2005 Elsevier B.V. All rights reserved. Keywords: Epoxidation; Allyl acetate; tert-Butyl hydroperoxide; MoO3/MgO; DR–UV–vis; XANES; Trialkyl borate
1. Introduction Epoxides are important intermediates in organic syntheses and are also used as raw materials for various petrochemical products. The chlorohydrin process has been applied as one of the most effective methods to produce epoxides, but this method is not desirable from the environmental point of view because of its use of harmful chlorine [1]. Another disadvantage is that contamination of a very small amount of chloro compounds cannot be avoided even by elaborate rectification. Such chloro compounds in an epoxy-resin wafer damage electronic functions of integrated circuits. One promising process is an indirect epoxidation using hydrogen peroxide or alkyl hydroperoxides as oxidizing agents, since by-products are environmentally friendly or * Corresponding author. Tel.: +81 75 724 7555; fax: +81 75 724 7555. E-mail address: [email protected] (H. Kanai). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.12.042
can be easily reconverted to hydroperoxides through dehydration and re-oxidation [2]. The reaction between an olefin and ROOH is believed to be ionic, where the electron-deficient oxygen atom of ROOH coordinated to a metal ion catalyst tends to attack electron-donating olefins [3]. Allyl acetate, having an electron-deficient group, is one of the hard-to-make-epoxide olefins. We have tried to synthesize glycidyl acetate from allyl acetate with t-butyl hydroperoxide. We found two MoO3-based catalysts: one is MoO3/TiO2 [4] and the other is MoO3/a-Al2O3 modified with pyridine [5]. However, the yield was 55% at the highest. These results suggested that higher yield could be obtained by suppressing the ring opening of epoxide caused by acid sites of the catalysts [6]. We have succeeded in synthesizing glycidyl acetate in a 92% yield over MoO3/MgO in the presence of triisopropyl borate. We will report the selective epoxidaion of allyl acetate with t-butyl hydroperoxide over MoO3/MgO, together with the characterization of the highly selective catalysts.
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2. Experimental 2.1. Catalysts and reagents The supports were commercial MgO (Nacalai Tesque Co., abbreviated as MgO-n) and a series of MgO reference catalysts supplied by the Catalysis Society of Japan (abbreviated as MgO-100A, -500A, -1000A, -2000A). Their pore radii, surface areas and particle sizes are listed in Table 1. Other reagents were used as obtained commercially. An aqueous solution of t-butyl hydroperoxide (abbreviated as TBHP: 68.7 wt.%, NOF Corporation) was shaken with benzene, and the organic layer was separated and dried with anhydrous MgSO4 for two days. The TBHP-benzene solution was filtered and stored over molecular sieve 5A. The concentration of TBHP (3.8 mol l1) was determined by iodometry. A known amount of (NH4)6Mo7O244H2O was dissolved in deionized water and MgO was added to the solution. After stirring for 30 min, the solution was evaporated to dryness at 333 K. The solid portion was dried at 353 K overnight, followed by calcination at 823 K for 3 h in air. MoO3/aAl2O3(acac), where acac stands for acetylacetonate, was prepared from MoO2(acac)2 and a-Al2O3 in ethanol solution, followed by evaporation, drying at 353 K, and calcination at 823 K for 3 h in air.
Fig. 1. Epoxidation of allyl acetate over 7 wt.% MoO3/MgO-n at 383K. *: Conversion, &: selectivity, ~: yield.
reflux condenser and a septum rubber. The flask was immersed in an oil bath (383 K). A benzene solution of TBHP (0.3 ml, TBHP 1.5 mmol) was injected into the flask and the reaction was started by stirring the solution with a magnetic agitator. When an additive (trialkyl borate) was used, it was pre-mixed with allyl acetate. Glycidyl acetate and TBHP were analyzed by a Shimadzu GC-14A gas chromatograph (capillary column: DB-23 (J&W), 323– 423 K).
3. Results and discussion
2.2. Catalyst characterization
3.1. Epoxidation of allyl acetate with t-butyl hydroperoxide over MoO3/MgO catalyst
The diffuse reflectance technique was applied to record the ultraviolet spectra on a Shimadzu UVPC-3100 spectroreflectometer. XRD analyses were carried out with a Rigaku Denki Geigerflex 2012 X-ray analyzer, and XPS spectra were obtained with a JEOL JPS-9010MX X-ray photoelectron spectrometer. Pore radii of catalysts were measured by automatic gas and vapor adsorption measurements with a Nihon Bell BELSORP 18 PLUS meter. X-ray absorption measurements (Mo K-edge) were done at the Beam Line 10B station of Photon Factory, KEK (Tsukuba) by a transmission mode. 1H and 11B NMR measurements were carried out on a Bruker AV400M NMR spectrometer.
The epoxidation of allyl acetate with TBHP was carried out at 383 K over 7 wt.% MoO3/MgO-n (Fig. 1). The epoxidation reactions at temperatures lower than 383 K were not preferable since both their conversion and their selectivity were very low. Hereafter, reactions were carried out at 383 K. The selectivity of epoxide (glycidyl acetate) decreased with the elapse of time. The decrease of the selectivity was caused by the ring opening of the produced epoxide, as shown in Fig. 2. Glycidyl acetate reacted rapidly over MoO3/a-Al2O3(acac) as its acid sites attacked the epoxide [5,6]. The addition of pyridine moderated the acidity, causing less ring opening. Almost no ring opening
2.3. Reaction procedure Catalyst (0.1 g) and allyl acetate (5 ml) were charged under nitrogen in a 50 ml two-necked flask equipped with a Table 1 Properties of MgO MgO, symbol
Pore radius (nm)
Surface area (m2 g1)
Particle size (mm)
n 100A 500A 1000A 2000A
2.13 1.46 1.87 1.95 1.72
8.6 120 28.1 16.0 7.7
25.4 0.01 0.05 0.10 0.20
Fig. 2. Reactions of glycidyl acetate over MoO3-based catalysts at 383K. *: 7 wt.% MoO3/MgO-n. t-Butanol was added at the time indicated by the arrow. &: 7 wt.% MoO3/a-Al2O3(acac) + pyridine. ~: 2 wt.% MoO3/aAl2O3(acac).
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Fig. 3. Effects of the amount of MoO3 loading on the epoxidation of allyl acetate over MoO3/MgO-n. Reaction time: 6 h. *: Conversion, &: selectivity, ~: yield.
occurred over MoO3/MgO-n. The addition of t-butanol, which was formed from TBHP in the progress of the epoxidation, to slow ring opening (Fig. 2). There was a very weak basicity in MoO3/MgO-n when it was calcined at 823 K. Epoxides are ring-opened by acids or bases [6]. Glycidyl acetate was almost inert to the weak basic sites of MoO3/MgO-n at 383 K, as shown in Fig. 2. The effects of the amount of MoO3 loading on the epoxidation of allyl acetate are shown in Fig. 3. The conversion increased with the increase in loading up to 7– 10 wt.% of MoO3. Further increase in loading resulted in an initial decrease of conversion and then a gradual increase. The selectivity of epoxide decreased drastically with the increase in loading up to 30 wt.%. The maximum yield (53%) of epoxide was obtained over 7 wt.% MoO3/MgO-n after 6 h. 3.2. Effects of MgO having different pore sizes A series of MgO that have various pore sizes were used as the supports. Epoxidation of allyl acetate was carried out over 7 wt.% MoO3/MgO (Table 2). The conversion, selectivity, and yield after 6 h are plotted as a function of pore radius of MgO in Fig. 4. Though Fig. 4 shows some scatter in the plots of the conversion and the yield, the selectivity tends to increase with the increase in pore radius.
Table 2 Effects of MgO on the epoxidation of allyl acetate over 7 wt.% MoO3/MgO MgO
Pore radius (nm)
Yield (%)a
Maximum yield (%) (Loading)b
100A 500A 1000A 2000A n
1.46 1.87 1.95 1.72 2.13
1.2 6.4 24 26 53
15 21 43 46 53
a b
(40) (30) (15) (10) (7)
Yield after 6 h. Maximum yield at the loading indicated in the parentheses.
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Fig. 4. Effects of pore radius of MgO on the epoxidation of allyl acetate over 7 wt.% MoO3/MgO. Reaction time: 6 h. *: Conversion, &: selectivity, ~: yield.
Table 2 also gives the maximum yields at the loadings indicated in the parentheses. No MgO could be superior to MgO-n as the amount of loading was changed from 2 to 40 wt.%. 3.3. Characterization of MoO3/MgO catalyst Many papers on the preparation and structure of MoO3/ MgO catalysts have been reported: preparation [7–12], Raman spectra [13–16], DR–UV–vis spectra [17–21], XANES measurements [22–27]. Catalytic activity is dependent on the method of preparation and the amount of loading. We will discuss the factors of MoO3/MgO catalysts available to achieve selective epoxidation of allyl acetate. BET surface areas, particle sizes and pore radii of MgO are listed in Table 1. MgO-n is a commercial reagent and the rest (MgO-100A, 500A, 1000A, 2000A) are Reference Catalysts supplied by the Catalysis Society of Japan. Their precise composition lists are not available, but it becomes clear that their pore sizes are crucial factors in the selective epoxidation of allyl acetate (Fig. 4). 3.3.1. XRD The XRD profiles of MoO3/MgO-n are shown in Fig. 5. No crystal phase of MoO3 was detected in MoO3/MgO-n loaded with less than 30 wt.% MoO3, but b-MgMoO4 [28] was observed in the catalysts loaded with more than 5 wt.% MoO3. A considerable amount of MgO was hydrolyzed into Mg(OH)2 which reacted with Mo7O246 to give MgMoO4 [29]. The obtained MgMoO4 which was only dried at 353 K gave no crystalline signals in its XRD. Heating at 823 K resulted in the appearance of crystalline MgMoO4. When a mechanically ground mixture of 7 wt.% MoO3 and 93 wt.% MgO-n was heated at 823 K in air for 3 h, MgMoO4 was identified in its XRD pattern as was reported by Stampfl et al. [16]. Even if MoO3 was formed during the catalyst preparation, well-contacted MoO3-MgO mixture
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Fig. 5. XRD profiles of MoO3/MgO-n. The number in the figure is the amount of loading (wt.%). *: MgMoO4, ~: MgO.
should be transformed into MgMoO4 by the heat-treatment in air. 3.3.2. XPS No difference was observed in the binding energies of Mo3d5/2,3/2 and O1s of 2–78 wt.% MoO3/MgO-n. The ratio of the (Mo3d5/2 + Mo3d3/2) peak-area to the Mg2p3/2 peakarea, IMo/IMg, is plotted versus MoO3 loading, together with the conversion and epoxide selectivity (Fig. 6). The ratio increases linearly with the increase in loading up to 7 wt.% and the slope in the range larger than 7 wt.% becomes smaller. This suggests that MgMoO4 monolayer was attained at 7 wt.% loading of MoO3 on MgO-n. The conversion increases with the increase in MgMoO4 monolayer. However, the selectivity decreases with the increase in loading, this effect is caused by the side-reaction of glycidyl acetate with t-butanol produced. The agreement of the inflection points between IMo/IMg and the epoxide yield suggests that monolayer MgMoO4 on MgO is responsible for the selective epoxidation. 3.3.3. Diffuse reflectance UV–vis spectra Diffuse reflectance UV–vis spectra of MoO3/MgO-n catalysts and standard molybdenum oxides are shown in Fig. 7. Tetrahedral molybdenum oxides (K2MoO4,
Fig. 6. Plots of XPS peak area ratio (IMo/IMg), conversion, and selectivity vs. MoO3 loading. Reaction time: 6 h. *: IMo/IMg, ~: conversion, &: selectivity.
Fig. 7. Diffuse reflectance UV–vis spectra of standard molybdenum oxides [I] and MoO3/MgO-n [II]. A: K2MoO4, B: Na2MoO4, C: MgMoO4, D: (NH4)6Mo7O244H2O, E: MoO3.
Na2MoO4, MgMoO4) give two peaks at 220 and 260– 270 nm. These peaks are assigned to the tetrahedral structure [17–21,30,31]. Octahedral molybdenum oxides ((NH4)6Mo2O24, MoO3) have two peaks at 220 and 320– 350 nm. Spectra of high-loading MoO3/MgO-n are similar to those of tetrahedral MoO42. However, 2 wt.% MoO3/ MgO-n lacks the peak at 260–270 nm. The peak gradually grows with the increase in loading. The positions of the absorption peaks of higher loaded MoO3/MgO-n coincide with those of crystalline MgMoO4. Thus, the peak at 260– 270 nm should be ascribed to the absorption of MgMoO4 which interacts with each other in the crystalline state. The appearance of the peak agrees with the saturation of the fixing sites of MgO-n with MgMoO4. 3.3.4. XANES spectra To clarify the structure of molybdenum oxide on MgO-n in detail, we performed X-ray absorption measurements at the Mo K edge. Their X-ray absorption near-edge structure (XANES) spectra are shown in Fig. 8. The intense pre-edge peak assigned to a 1s–4d transition is consistent with the tetrahedral environment [22–27]. The transition for regular octahedral Mo6+ is formally forbidden, but that for asymmetric tetrahedral one is allowed. The XANES spectra of the catalysts are similar to that of MgMoO4 with a tetrahedral configuration. A close examination shows a tendency that the pre-edge peak height increases with the increase in loading, while the shoulder peak in the post-edge is conversely weakened. More information was derived from their second derivative XANES spectra (Figs. 9 and 10). There are differences in the shape and position between tetrahedral
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Fig. 8. Normalized XANES spectra of 2–30 wt.% MoO3/MgO-n and standard molybdenum oxides.
Fig. 11. Pre-edge (A) and post-edge (B) XANES spectra of 7 wt.%MoO3 supported on MgO having various pore sizes.
Fig. 9. Second derivative XANES spectra of standard samples. A: MgMoO4, B: K2MoO4, C: BaMoO4, D: (NH4)2Mo2O7, E: (NH4)6Mo7O24, F: MoO3.
(K2MoO4, MgMoO4, BaMoO4) and octahedral standard Mo6+ species ((NH4)6Mo7O24, MoO3) (Fig. 9). The position and depth of the bottom of (NH4)2Mo2O7 in which the anion is a polymer of linked MoO6 octahedra and MoO4 tetrahedra [32] take an intermediate value. There are small but decisive differences between the second derivatives of low and high loading MoO3/MgO-n (Fig. 10). The bottom of the curve becomes deeper with the increase in loading up to 7 wt.%.
Fig. 10. The second derivative XANES spectra of 2–30 wt.% MoO3/MgOn. The number is the amount of loading (wt.%). A: MgMoO4.
The second derivative XANES spectrum of 7 wt.% MoO3/ MgO-n agrees very closely with that of MgMoO4. The bottom position shifts to higher energy for highly loaded MoO3/MgO-n. The inflection point is at 7 wt.% loading as is also observed in the XPS (Fig. 6) and DR–UV spectra (Fig. 7). The Mo K pre-edge and post-edge spectra in the XANES of 7 wt.% MoO3 supported on various kinds of MgO are shown in Fig. 11. The intensity of the pre-edge peak increases with increase in the pore size of MgO. The shoulder peak of the post-edge is conversely more intense for Mo oxide supported on smaller pore-sized MgO. The second derivatives of the pre-edge XANES spectra except for Mo on MgO-n are shown in Fig. 12. Their bottoms do not
Fig. 12. The second derivative XANES spectra of 7 wt.% MoO3 supported on MgO having various pore sizes.
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Fig. 13. Effects of the amount of triisopropyl borate on the epoxidation of allyl acetate. (iso-PrO)3B/TBHP: *: 0, ~: 0.01, &: 0.03, 5: 0.06.
coincide with that of MgMoO4. They rather agree with those of MoO3/MgO-n loaded more than 10 wt.% (Figs. 10 and 12). Inspection of these pre-edge and post-edge peaks and second derivatives reveals that they are closely related with the results of the epoxidation activity. Though the compositions of MgO used are the same, the structures of molybdenum oxide species were affected by the microstructure of MgO supports; the small but decisive differences drastically reflected their epoxidation selectivity. 3.4. Effects of additives The addition of pyridine on MoO3/a-Al2O3 reduced its acidity and increased epoxide selectivity [5]. The coordination of pyridine to the acidic sites and Mo atoms of MoO3/ MgO-n retarded epoxidation of allyl acetate. Among 4substituted pyridines tested, 4-cyano- or 4-methoxycarbonyl-pyridine (py/Mo = 1/3), which bind weakly to the catalyst, improved selectivity in some degree. One of the authors found that the addition of trialkyl borates promoted the epoxidation of 1-butene [33]. We found that the addition of trialkyl borate promoted enormously the selective epoxidation of allyl acetate shown in Fig. 13, though it had no effect in the epoxidation over MoO3/a-Al2O3. The addition of 3–6 mol% to TBHP was appropriate in the improvement of both conversion and selectivity. The presence of any trialkyl borates is more effective than their absence. The efficiency of trialkyl borate is (MeO)3B < (EtO)3B < (n-PrO)3B (n-BuO)3B ((C18H37O)3B (iso-PrO)3B (Fig. 14). Triisopropyl
Fig. 14. Effects of trialkyl borates. : Conversion, : selectivity, : yield.
borate is the best promoter. However, there was no significant difference in the promoting effects of various trialkyl borates for the metathesis of allyl acetone and allyl chloride over Re2O7/Al2O3-Bu4Sn catalyst [34]. To elucidate the role of trialkyl borate, we ran 1H and 11B NMR measurements of homogeneous solutions composed of MoO2(acac)2, tri-n-butyl borate, t-butanol and TBHP in o-dichlorobenzene-d4 (Figs. 15 and 16). The appearance of free CH2OH group indicates the alkoxyl or alkyl peroxyl exchange between (n-BuO)3B and t-butanol or between (nBuO)3B and TBHP (Fig. 15). These were further confirmed by 11B NMR (Fig. 16). The chemical shifts are adjusted to that of B(OBun)3 in o-dichlorobenzene-d4. Based on the concentration dependence, the first equilibrium constant of the exchange between (n-BuO)3B and t-butanol was evaluated to be 0.12, and the second one 0.06. The first equilibrium constant of the exchange between (n-BuO)3B and TBHP was estimated to be 0.32. The rather large value for the latter exchange is due to the reduction of steric hindrance by one more neighboring oxygen atom in OOBut. 11 B NMR spectra (Fig. 16) supports the latter reaction, from which mono- and di-t-butyl peroxy derivatives of boron are presumed to be formed. The addition of MoO2(acac)2 to a
Fig. 15. 1H-NMR spectra of (n-BuO)3B in the presence of TBHP and t-butanol in o-dichlorobenzene-d4 (2.5 g).
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Scheme 1.
between Mo=O and the borate hooks in the hydroperoxy oxygen in the vicinity of Mo for easy coordination. Then the oxygen atom of the peroxy group attacks the double bond of allyl acetate to produce glycidyl acetate. The function of the trialkyl borate is to form a ternary transitional complex for easy epoxidation.
4. Conclusion Epoxidation of allyl acetate, which is one of the hard-tomake-epoxide olefins with TBHP, was successfully carried out over MoO3/MgO in the presence of trialkyl borate. The causes of successful epoxidation are as follows: (1) acid sites of MoO3 on acid supports cause ring opening unavoidably but MoO3/MgO underwent no ring opening of epoxide in the absence of alcohol; (2) monolayer tetrahedral MgMoO4 was attained at 7 wt.% MoO3 loading on commercially available MgO; (3) preferable MgO had large pores, (4) a small amount of triisopropyl borate (3 mol% to TBHP) interacted synergistically with TBHP and molybdenum catalyst and accelerated the epoxidation of allyl acetate.
Acknowledgements
Fig. 16. 11B NMR spectra of tri-n-butyl borates treated with MoO2(acac)2 and TBHP.
tri-n-butyl borate solution caused a shift of the peak to lower frequency, indicating the interaction between tri-n-butyl borate and O=Mo. Further addition of TBHP to the solution produced a new signal, which was deduced to be (tBuOO)(n-BuO)2B interacted with MoO2(acac)2. The first step of the epoxidation is the coordination of hydroperoxide to metal catalysts (e.g. Mo), allowing the peroxy oxygen of hydroperoxide electrophilic to attack the double bond of olefin [35]. Therefore, olefins with electronrich double bond are easily oxidized. Although allyl acetate has an electron-withdrawing group that makes its C=C bonds less likely to be attacked by oxygen atoms, the present catalyst system with trialkyl borates is remarkably effective. Here we discuss the action of trialkyl borate briefly (Scheme 1). Alkyl peroxy exchange between TBHP and trialkyl borate occurs, and the concurrent interaction
We thank Messrs S. Naito and S. Ichikawa of NOF Corporation for valuable discussions and for the supply of TBHP. We also thank Dr. Okano of Tateho Chemical Industries Co. Ltd. for the TEM image and data on the particle size of MgO-n. This work has been conducted under an entrustment contract between New Energy and Industrial Technology Development Organization (NEDO) and Japan Chemical Innovation Institute (JCII). The XAFS measurements were performed under the approval of the Program Advisory Committee of High Energy Acceleration Research Organization, Institute of Materials Structure Science, Photon Factory (Proposal No. 2003G287). We are grateful to Drs. Nomura and Usami for the XAFS measurements. We thank Mr. M. Sugiura, of NOF Corporation, for the 1H and 11 B NMR analyses.
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