8016
Ind. Eng. Chem. Res. 2008, 47, 8016–8024
Alkene Epoxidation with Ethylbenzene Hydroperoxides Using Molybdenum Heterogeneous Catalysts Laura Barrio,† Jose M. Campos-Martín,*,† M. Pilar de Frutos,‡ and Jose L. G. Fierro*,† Instituto de Cata´lisis y Petroleoquímica, CSIC, Marie Curie 2, Cantoblanco, 28049 Madrid, Spain, and Centro de Tecnología Repsol YPF, A-5, Km. 18, 28930 Mo´stoles, Madrid, Spain
Molybdenum-containing catalysts were prepared by anchoring Mo(VI) groups onto different aminofunctionalized silica surfaces and then tested in the epoxidation reaction of 1-octene with ethylbenzene hydroperoxide (EBHP) as an oxidant. Catalytic performance was found to depend on the chemical structure and nature of the functional group. EBHP conversion-time profiles exhibited an S-shaped behavior, suggesting that an induction period is required to develop the active site. Selectivity-conversion plots indicated that selectivities as high as 80% could be obtained at EBHP conversions of about 80% when the Mo complexes were immobilized on a silica substrate functionalized with a diamino-containing moiety. In contrast, surface functionalization with sCOOH was found to be detrimental, as acid groups catalyze EBHP decomposition. Experiments designed to determine possible leaching of molybdenum into the reaction medium during the tests indicated that the catalyst prepared on a triamine-containing substrate exhibited good stability with almost no change in performance after three consecutive runs. The high stability of the Mo(VI) complex in this catalyst was shown by UV-vis and photoelectron spectroscopic techniques. Introduction Oxidation is a core technology for converting petroleum-based materials to useful chemicals of higher oxidation states. Among these products, epoxides are chemical compounds of great importance as intermediate products in the preparation of a wide variety of chemical products ranging from petrochemical compounds to fine chemicals.1 Although 1,2-epoxiethane can be obtained by a direct reaction in the gas phase between ethylene and oxygen in the presence of silver catalysts, the epoxides of a larger carbon chain, such as propylene oxide (PO), must be obtained by reaction in the liquid phase. A universal method of epoxide production relies on the dehydrochlorination of chlorohydrins by aqueous solution of alkalis.1,2 This method yields an equimolar quantity of waste aqueous solution of alkali metal chlorides, as well as considerable amounts of waste chlororganic derivatives, which are the products of chlorine addition to the double bond. A safer environmental approach is the oxidation of olefin hydroperoxides in the presence of catalysts. Current worldwide PO production is approximately 6 million metric tons per year, and both technologies are used: hydroperoxides (employed in various forms by Lyondell, Shell, Sumitomo, and Repsol) and chlorohydrins (Dow).2–4 The core of the hydroperoxide-based PO process comprises the catalyzed epoxidation reaction of propylene with ethylbenzene hydroperoxide (EBHP) to propylene oxide and 1-phenylethanol. The active centers of the catalysts used in this process are transition metals in high oxidation states with Lewis acidity, such as Mo(IV), Ti(IV), V(V), and W(IV).2–6 Epoxide selectivity depends on the Lewis acidity and the oxidation state of the metals.7 Thus, molybdenum-containing catalysts are best for this process,8 and most of them are synthesized from dioxomolybdenum(VI) acetylacetonate, which itself shows good catalytic properties in oxidation reactions.9 Some studies have addressed * To whom correspondence should be addressed. E-mail: jlgfierro@ icp.csic.es (J.L.G.F.),
[email protected] (J.M.C.-M.). Tel.: +34915854796 (J.L.G.F.),+345854948 (J.M.C.-M.).Fax:+34915854760 (J.M.C.-M.). † Instituto de Cata´lisis y Petroleoquímica. ‡ Centro de Tecnología Repsol YPF.
molybdenum complexes whose ligands contain donor atoms such as oxygen, sulfur, and/or nitrogen.10–13 Since the early 1990s and the advent of mesoporous silicas, the development of new solid catalysts and the heterogenization of homogeneous systems for oxidation processes have become very attractive topics. A major drawback of chromium, vanadium, and molybdenum catalysts coordinated to monodentate ligands is that they usually undergo leaching of the active species into solution, especially in the presence of protic agents such as alcohols or organoperoxides.14 A simple way to minimize the leaching of the active ingredient is to anchor polydentate ligands onto the support surface, as this type of chelating system offers high coordinative stability for the catalytic ingredient. This general idea has been explored following two different approaches: (i) polymer-supported catalysts and (ii) tethering of molybdenum complexes onto inorganic solids, mainly silica. On the one hand, the immobilization of Mo(VI) catalysts on polymer substrates has been achieved employing boronic acid, aminated polystyrene, polymethacrylate, polybenzimidazole, and polysiloxane resins and then tested in the epoxidation of alkenes in the presence of tert-butyl hydroperoxide.15–21 On the other hand, Mo(VI) complexes have been tethered onto inorganic substrates such as sol-gel-derived silicates,22,23 and organofunctionalized MCM-4124–29 materials have been employed. Among these catalysts, polydentate aminated compounds performed best in the reaction and were found to be highly reusable. Against this background, this work was undertaken to investigate both the incorporation of molybdenum into a silica substrate functionalized with polydentate ligands and the effect of the nature of ligands on activity for the epoxidation of 1-octene with EBHP as an oxidant. Certain coordination and surface characteristics of the catalyst were revealed by UV-vis and photoelectron spectroscopy, respectively, and were related to catalyst performance in the target reaction. Experimental Section Catalyst Preparation. Several commercial functionalized silicas were employed as support (SiliaBond from SiliCycle).
10.1021/ie800262x CCC: $40.75 2008 American Chemical Society Published on Web 06/14/2008
Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8017 Scheme 1. Functionalized Commercial SiliaBond Supports Employed for Catalyst Preparation
Table 1. Catalyst Labeling and Molybdenum Contents of the Samples % Mo catalyst Mo1 Mo2 Mo3 Mo4 Mo5 Mo6
molybdenum precursor c
MoO2(acac)2 MoO2(acac)2c MoO2(acac)2c MoO2(acac)2c (NH4)6Mo7O24 H2O [(NH4)6Mo7O24 H2O] + H2O2
supporta
maximum loadingb
present
a b c d c c
11.6 8.6 11.8 12.8 11.8 11.8
7.3 3.2 9.2 0.6 9.8 10.6
a See Scheme 1. b For an incorporation of one molybdenum atom per functional group. c Bisoxomolybdenum(VI) acetilacetonate.
These supports have different functional groups (see Scheme 1), including amino, carbonyl, and thiourea chelating agents. The amount of functional groups was determined from the provider by carbon-nitrogen-sulfur analysis. The support was stirred in 10 mL of ethanol (Sigma-Aldrich) per gram of silica. An excess of bis(acetylacetonate) dioxomolybdenum(VI) (SigmaAldrich) of 1.5 mol per mole of functional group present in the support was added to the solution. This mixture was kept under reflux for 4 h. The solid was then filtered and washed twice with 50 mL of ethanol. The remaining adsorbed molybdenum precursor was eliminated by Soxhlet extraction with ethanol for 48 h. Finally, the solid was dried at room temperature. A second source of molybdenum was used: ammonium molybdate tetrahydrate (Sigma-Aldrich). This salt is not soluble in ethanol, so a mixture of ethanol/water (3:1) or ethanol/1% H2O2 in water (3:1) was used as solvent, following the same preparation procedure. Characterization. The molybdenum loadings of the catalysts were determined by inductively coupled plasma absorption spectrometry using a Perkin-Elmer Optima 3300 DV instrument.
UV-vis diffuse reflectance spectra measurements were recorded on a Cary 5000 spectrophotometer equipped with an integrating sphere. The samples were analyzed under ambient conditions. The percentage reflection was measured and presented by the normalized Kubelka-Munk function. X-ray photoelectron spectra were recorded on a VG Escalab 200R spectrometer equipped with a hemispherical electron analyzer and a Mg KR (hν ) 1253.6 eV) X-ray source (12 kV and 10 mA). The powder samples were packed into small aluminum cylinders and mounted on a sample rod in the pretreatment chamber and degassed at 773 K for 1 h before being moved into the analysis chamber. The base pressure of the ion-pumped analysis chamber was maintained below 3 × 10-9 mbar during data acquisition. Peak intensities were estimated by calculating the integral of each peak after smoothing and subtracting a Shirley-type background. All binding energies (BEs) were referenced to the Si 2p peak of the SiO2 substrate at 103.5 eV. This reference gave BE values with an accuracy of (0.1 eV. Atomic surface contents were estimated from the areas of the peaks, corrected using the corresponding sensitivity factors.30 Activity Testing. Catalytic epoxidation of 1-octene with ethylbenzene hydroperoxide (EBHP) was performed in a glass batch reactor equipped with a magnetic stirrer, a condenser, and a septum for withdrawing samples. In a typical run, 45 g of alkene (0.4 mol) and 33 g of a solution of EBHP (33 wt %) in ethylbenzene (0.08 mol of EBHP), kindly provided by RepsolYPF, were mixed in the reactor and heated to 393 K. Catalyst was then added. The amount of catalyst was selected to introduce 0.028 g of molybdenum into the reactor. 1-Octene was selected as the reactant because it is well-known that primary aliphatic alkenes are the most difficult to epoxidize and it is a common and particularly suitable alkene model for describing reactivity in the epoxidation of propene to propene
8018 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008
Figure 1. UV/vis spectra of (a) Mo samples prepared with different supports and (b) functionalized silica supports.
oxide (PO). An excess of alkene was used because EBHP is the most costly reactant in the PO process. The concentration of EBHP was measured by standard iodometric titration. The remaining organic compounds were analyzed by gas chromatography with a flame ionization detector (GC-FID) on an Agilent 6850 device equipped with an HP-WAX capillary column. These samples were pretreated with triphenylphosphine to decompose the EBHP quantitatively to 1-phenylethanol before GC analysis. Selectivity to epoxide was based on the amount of EBHP consumed. Epoxide selectivity was related to the hydroperoxide converted according to the equation S (%) ) 100 × [epoxide] / ([EBHP]0 - [EBHP])
(1)
where epoxide represents 1,2-epoxyoctane, the subscript 0 indicates initial values, and all concentrations are expressed on a molar basis. No other byproducts, such as diol or ether derivates from the octene, were detected. Used catalysts were filtered from the reaction mixture and then washed twice with 50 mL of ethanol before further characterization. Results and Discussion Characterization. Chemical analysis of the samples showed differing molybdenum incorporation, mainly depending on the type of functional group present in the silica (Table 1). Diaminofunctionalized support a was able to incorporate 63% of the maximum Mo loading. Support b (functionalized with triamine tetraacetate ligand) gave a much lower preparation yield, 37%. Interestingly, support c, which was functionalized with a chain containing three amino groups, gave a catalyst whose molybdenum loading approached the maximum loading (preparation yield ≈ 78%). This is a common trend for the two molybdenum sources used in this work, that is, bisoxomolybdenum acetylacetonate and ammonium heptamolybdate. The thioureafunctionalized substrate incorporated a very low molybdenum amount. Chemical analysis of the resulting catalyst (denoted Mo4) revealed a Mo loading of only 0.3%. This low preparation yield can be ascribed to the small size of the chelating agent, which might not be sufficient to trap Mo centers. In view of this very low Mo loading, no further tests were performed on this sample.
Figure 2. UV/vis spectra of samples prepared with different source of molybdenum.
UV/vis spectroscopy is a characterization technique that is particularly suited to the study of the molybdenum oxidation state and its coordination number. MoO2 complexes exhibit an absorption band around 300-400 nm assigned to a ligand-tometal Mo(dπ) f O(π) or Mo(dπ) f N(π) charge-transfer (LMCT) transition.31 However, the electronic spectra of these systems must be approached with caution and analyzed in detail, as the NH- (and/or NH2-) containing substrates exhibit absorption bands close to this region (Figure 1). Sample Mo2 has an absorption band around 325 nm that appears to be shifted slightly to higher wavelength with respect to that of the Mo(VI) species surrounded by oxygen atoms.32 This shift is due to the interaction of these Mo(VI) moieties with the nitrogen atoms of the functional group of the silica substrate. Samples Mo1 and Mo3 prepared with amino-functionalized silica substrate show slight but evident differences with respect their Mo-free support counterparts. The corresponding electronic spectra exhibit more intense absorption bands, and the absorption edge is shifted to higher wavelengths of 425 nm, which is clear evidence that the nitrogen groups are incorporated into the coordination sphere of the molybdenum center. The UV-vis
Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8019
Figure 3. XPS spectra of catalyst Mo3.
spectra of samples Mo5 and Mo6 are similar to that of Mo3, although slight differences are observed (Figure 2). The edge of the absorption band shifts to higher wavelengths, which is characteristic of the presence of molybdenum polyoxoanions. The appearance of these polyoxoanions is not unexpected, as both the molybdenum source [(NH4)6Mo7O24] and the experimental conditions used in catalyst preparation favor either polyoxoanion formation in water in a broad range of pH values or dimeric peroxo compound structures in the presence of hydrogen peroxide.33,34 The UV-vis spectra of samples Mo5 and Mo6 and the reference (NH4)6Mo7O24 precursor were also recorded (Figure 2). If a comparison is made between the electronic spectra of (NH4)6Mo7O24 precursor and those of samples Mo5 and Mo6, a red shift in the absorption band of supported Mo5 and Mo6 is observed. Again, this behavior is a
clear indication of a change in the nature of the atoms surrounding the molybdenum center due to the presence of nitrogen atoms. UV/vis data indicate that catalyst preparation was successful and that the Mo centers were coordinated by the corresponding chelating agents. One of the more useful techniques for studying the chemical state of the atoms present in the surface layer region of solid materials is X-ray photoelectron spectroscopy (XPS). Accordingly, all of the samples employed in this work were studied by this technique, with the aim of revealing the molybdenum oxidation state and obtaining certain insights into its coordination sphere. First, we studied the Mo 3d energy region that shows the typical spin-orbit splitting of the Mo 3d doublet (3d5/2 and 3d3/2). As an illustration, Figure 3 displays the photoelectron spectrum in the range of the binding energy of the Mo 3d levels
8020 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 Table 2. Summary of XPS Data for Molybdenum-Containing Samples BE (eV) catalyst
Mo 3d5/2
Mo1
232.4
Mo2
232.4 (80) 231.2 (20)
Mo3
232.3
Mo5
232.5
Mo6
232.3
N 1s
Mo/N
Mo(232.6 eV)/N(397.8 eV)
397.7 (32) 399.5 (52) 401.2 (16) 398.2 (21) 399.8 (52) 401.3 (27) 397.6 (49) 399.6 (30) 401.3 (20) 397.7 (27) 399.7 (57) 401.6 (16) 397.8 (34) 399.7 (45) 401.1 (21)
0.21
1.02
0.11
0.20
0.17
0.56
0.16
0.61
0.19
0.56
Table 3. Catalytic Activity Data for the Samples in the Epoxidation of 1-Octene with EBHP at 100 °C
catalyst
amount of catalyst (g)
Mo1 Mo2 Mo3 Mo5 Mo6
0.4 0.9 0.3 0.3 0.3
conversion of EBHP (%) 2h
6h
selectivity to epoxide (%)
yield of epoxide (%)
99.1 42.7 72.1 60.7 32.2
82.4 98.3 95.4 90.0
85.4 50.4 81.0 85.5 84.4
84.6 41.5 79.6 81.6 76.0
Table 4. Chemical Analysis of the Samples after Reaction fresh
used
sample
% Mo
% Mo
% Mo lost
Mo1 Mo2 Mo3
7.3 3.2 9.2
5.7 3.2 9.2
27 none detected none detected
Table 5. Summary of XPS Data for the Used Catalysts BE (eV) catalyst
Mo 3d5/2
Mo1 used
232.5
Mo2 used
232.5 (79) 231.1 (21)
Mo3 used
232.6
N 1s
Mo/N
Mo(232.6 eV)/N(397.8 eV)
397.4 (32) 399.4 (52) 401.3 (17) 398.7 (24) 399.8(43) 401.0 (32) 397.6 (27) 399.7 (52) 401.3 (20)
0.25
0.98
0.11
0.10
0.22
0.78
of the representative Mo3 sample. It is emphasized here that the analysis of molybdenum focused solely on the most intense Mo 3d5/2 component because both the binding energies and quantitative measurements can be obtained more accurately. The Mo 3d spectra of catalysts tethered to amino-functionalized groups (Mo1, Mo3, Mo5, and Mo6) exhibit only one peak at 232.6 eV. This signal is often attributed to Mo(VI) species coordinated to nitrogen and oxygen groups13 (Table 2). Sample Mo2 presents an additional peak at 231.2 eV, in all probability due to surface impurities of MoO2(acac)2. Examination of the binding energy region of the N 1s spectra also provides valuable chemical information. The samples’ N 1s energy region has three components: one at ca. 401 eV, which is attributed to protonated N species in an organic chain; a second signal at 400.0 eV corresponding to neutral N atoms present in organo compounds; and finally, one close to 398.0 eV that can be attributed to nitrogen atoms coordinated with a
transition metal (molybdenum). Figure 3 also shows the N 1s region of the sample Mo3. Another interesting finding comes from the O 1s line profile analysis. For the samples with the highest Mo loading, the O 1s line profile can be fitted in two components: a very intense peak at 532.6 eV due to the oxygen present in the silica substrate (Si-O-Si bonds) and a shoulder that can be resolved into a clear peak at 530.3 eV by applying standard fitting procedures. This latter peak at 530.3 eV is due to the oxygen present in the bisoxomolybdenum moiety coordinated to ligands (MoO2L) (see the representative spectrum of sample Mo3 in Figure 3). A summary of the more relevant qualitative (binding energies) and quantitative (atomic ratios) results derived from photoelectron spectroscopic analysis is compiled in Table 2. In this table, the atomic surface contents were estimated from the areas of the peaks, corrected using the corresponding atomic sensitivity factors.30 The Mo/N atomic ratio is very small, indicating that some of the nitrogen is not coordinated with molybdenum. Because of the presence of several species, mainly nitrogen, correct analysis of these data is not straightforward. To avoid this problem, an additional column is included in the table. This column gives the atomic ratio between the molybdenum species with a binding energy of 232.6 (coordinated molybdenum species) and the nitrogen species coordinated to Mo (BE ) 397.8 eV). It is evident from the atomic ratios that the molybdenum ratio is the highest for the sample prepared with the silica substrate functionalized with amino groups. It can also be observed that, in support a, containing two amino groups (sample Mo1), molybdenum interacts with only one nitrogen atom, but in sample Mo3, prepared using a silica substrate functionalized with three amino groups, molybdenum interacts with two nitrogen atoms. These observations indicate that molybdenum is incorporated as a monodentate species in sample Mo1, whereas the molybdenum complex adopts a bidentate structure in sample Mo3, so that it does not correspond to the number of nitrogen atoms present in the functionalized support. This finding could not be easily derived from samples Mo5 and Mo6, as molybdenum forms polyoxoanion species in these samples, and hence, the identification of the molybdenum complex is a difficult, if not impossible, task. Nonetheless, quantitative data provide very valuable information: The interaction between nitrogen (397.8 eV) and molybdenum involves two nitrogen atoms of the functional group (atomic ratio 0.5) (Table 2). Activity Testing. Considering that the molybdenum loading differs from sample to sample, it was decided to adjust the amount of catalyst in each experiment with the objective being to load the reactor with the same amount of molybdenum (Table 3). This approach obviously means that the amount of catalyst varies, whereas the number of moles of Mo remains constant in all runs. All samples showed catalytic activity; however, their activities were markedly lower than those previously reported for homogeneous samples.12,13 Indeed, reaction times as long as 6 h are required to reach high conversion values, as opposed to the much lower times, typically 1 h, needed for the reaction in the presence of a homogeneous Mo catalyst. Catalyst Mo1 displays the highest conversion values among all of the catalysts, reaching values close to that of a standard homogeneous catalyst12,13 (Figure 4). A careful inspection of the conversion-time curve reveals the appearance of an inflection point at short reaction times (5-15 min), indicating a change in catalytic behavior that is associated with a change in the active site responsible for the reaction. A different profile is displayed by catalyst Mo2, for which the conversion increases
Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008 8021
Figure 4. Activity results in the epoxidation of 1-octene with EBHP for the catalyst prepared with bisoxomolybdenum acetylacetonate.
Figure 6. Catalytic behavior of sample Mo3 after several reuses.
increasing slowly over the first hour, but then rising sharply. This behavior is similar to that observed for catalyst Mo1 at longer reaction times.
Figure 5. UV/vis spectra of used catalysts.
very slowly with time (12% in 1 h), with no inflection point being detected at any reaction time. The low catalytic activity exhibited by catalyst Mo2 can be related to the nature of the active center. As the functionalized support employed in the preparation of catalyst Mo2 contains a chelating ligand, there is steric hindrance for the reactants (hydroperoxide and alkene) to reach the molybdenum site. Catalyst Mo3 exhibits behavior intermediate between those of Mo1 and Mo2, with conversion
The selectivity of EBHP to epoxide varies significantly for the different catalysts (Table 3). Catalysts Mo1 and Mo3 have selectivities higher than 80%, which is typical in the epoxidation of alkenes with EBHP on molybdenum catalysts. Indeed, the selectivity is somewhat higher for Mo1 than for Mo3, and this can be related to the different conversion-time paths for the two catalysts. The selectivity of EBHP to epoxide for catalyst Mo2, at around 50%, appears to be rather low. The low selectivity obtained for this catalyst can be linked to the nature of the functionalized support and to the low molybdenum loading. Particularly, the low Mo loading of this catalyst keeps
8022 Ind. Eng. Chem. Res., Vol. 47, No. 21, 2008
Figure 7. Activity test of Mo3 catalysts: standard test and test with filtering of the solid.
a number of free acid groups (sCOOH) on the functionalized substrate, which is known to catalyze the decomposition of EBHP. The S-shaped conversion curves obtained suggest that the active sites change during reaction and that an induction period is needed to achieve catalytic activity. It has been suggested that Mo sites might be fully oxidized2 in the presence of hydroperoxide prior to becoming active. Another possibility is that the leaching of Mo centers into the reaction mixture takes place over time, giving rise to a homogeneous mechanism. Upon completion of the catalytic tests, the molybdenum contents of the three catalysts (Mo1, Mo2, and Mo3) were determined. The data collected in Table 4 indicate that catalyst Mo1 lost around 27% of its initial Mo loading, whereas the
Figure 8. EBHP conversion and epoxide selectivity for Mo-loaded catalysts.
other two catalysts (Mo2 and Mo3) showed no significant differences. Thus, the fraction of molybdenum leached by catalyst Mo1 during reaction (about 110 ppm in solution) acts as a homogeneous catalyst that helps to improve its performance. For catalyst Mo1, the electronic spectra revealed that the N-containing groups of the functional groups incorporated on the surface of the commercial substrate are included within the coordination sphere of the molybdenum complex. In addition, quantitative data obtained by photoelectron spectroscopy revealed a Mo(232.6)/N(397.8)atomic ratio of 1.02, which is much higher than that in the other catalysts. As this ratio represents the interaction of Mo(VI) with the terminal sNH2 of the functional group, it is clear that the Mo(VI) · · · H2N interaction is weak enough to release a fraction of the molybdenum complex into the bulk liquid phase during the reaction. XPS analysis of used catalysts revealed no significant changes in the types of species present on the solid surfaces after the reaction has taken place. However, differences were observed when comparing atomic surface contents between fresh and used samples (Table 5). Sample Mo1 had a Mo/N ratio similar to that of the fresh catalyst, indicating that the molybdenum remaining on the catalysts was still coordinated by one N atom, whereas sample Mo2 exhibited a reduction of the signal due to molybdenum coordinated to nitrogen. This reduction could be ascribed to the deposition of organic compounds in the proximity of active sites. By contrast, catalyst Mo3 showed an increase in the amount of molybdenum coordinated by nitrogen. This could be due to a change in the coordination sphere of the active center during reaction. The Mo centers seem to be coordinated by fewer N atoms after reaction, whereas the fresh sample was considered to have two N atoms coordinating each active site. These changes in coordination show that the support functionalized with three amino groups has the optimal ligand properties for the heterogenization of molybdenum. The interaction between the organic chain and Mo is strong enough to avoid leaching, but not so strong as to hinder changes in coordination and hence in catalytic activity. Used catalysts were also analyzed by UV/vis spectroscopy (Figure 5). The electronic absorption band shows significant
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changes upon reaction. For all of the samples considered, the absorption band shifted toward shorter wavelengths. This finding indicates a change in coordination of Mo due to partial substitution of N ligands by oxidant peroxo groups during reaction. Additional experiments were performed to determine the reusability of these catalysts. Catalyst Mo3 was selected for this purpose because it has the highest catalytic activity per molybdenum center, as well as the maximum efficiency in the use of the oxidant. Catalyst Mo3 was reused three times more in reaction without changes in its catalytic behavior; both EBHP conversion and selectivity to epoxide remain almost constant after three reuses (Figure 6). However, the chemical analysis of the reused catalyst showed a decrease in the molybdenum content from 9.2% to 8.0%. Nevertheless, the remaining molybdenum coordinated to the functional groups of the silica substrate maintained a high activity in the epoxidation reaction. To evaluate the possible contribution of the dissolved molybdenum (homogeneous reaction) to the overall performance, a new experiment was performed. Once a catalytic test was initiated following standard reaction conditions, it was stopped after 150 min, and the hot liquid medium was removed with a syringe equipped with a filter and immediately transferred to another reactor already at the same temperature. The changes in composition of the liquid mixture brought about by solubilized molybdenum were monitored periodically35,36 (Figure 7). As the analysis of the liquid phase gave only marginal activity, it can be concluded that the epoxidation reaction occurs essentially in the heterogeneous phase for catalyst Mo3. Nevertheless, additional catalyst life-cycle experiments have to be carried out prior to its selection as a candidate for industrial application. Finally, the catalysts prepared with ammonium heptamolybdate (Mo5 and Mo6) were also tested in the target reaction. Both catalysts exhibited lower EBHP conversion levels than their counterpart prepared with bisoxomolybdenum acetylacetonate (Mo3) (Figure 8). This effect was even more pronounced for catalyst Mo6, which was prepared with hydrogen peroxide and therefore contained a peroxomolybdenum moiety. Catalysts Mo5 and Mo6 displayed different EBHP selectivities to epoxide, as they had higher selectivity values than Mo3. This activity pattern indicates that polyoxoanions of molybdenum are less active than isolated molybdenum species, whereas they are more selective to epoxide. The epoxide yield was slightly higher for Mo5 than for the other catalysts (Table 3); thus, Mo5 can be considered as the best catalyst of this series. Conclusions The best conversion and selectivity results in the epoxidation reaction were achieved using a polymer with two or three amine groups (supports a and c). The low catalytic activity obtained using the dicarboxylic group (support b) is due to the acid properties of the carboxylic group and also to the steric hindrance for the reactants to reach the molybdenum site. The polymer functionalized with diamino groups (support a) is able to load a high molybdenum content, although the majority of this metal is lost in the reaction medium. Leaching is very high on this support, and the reaction mechanism is ascribed to homocatalysis and not heterocatalysis. It has been shown that the polymer functionalized with triamine groups (support b) produces catalyst without a significant leaching effect in the reaction medium. It is possible to recycle this catalyst up to three times, maintaining its activity.
The activity and selectivity of this catalyst are high, making it a potential candidate for industrial use. The differences between the catalysts prepared with molybdenum acetylacetonate and ammonium heptamolybdate (lower activity and higher selectivity in the case of ammonium heptamolybdate) can be attributed to the different structure of the catalytic center; in the case of molybdenum acetylacetonate, the molybdenum species are isolated, and in the case of ammonium heptamolybdate, the molybdenum has polyoxoanion structures. Acknowledgment The authors acknowledge financial support from Repsol-YPF (Spain) and the Spanish Ministry of Science and Education in the projects PSE-310200-2006-2 and FIT-320100-2006-88. LBP gratefully acknowledges a fellowship granted by Repsol-YPF. Literature Cited (1) Nijhuis, T. A.; Makkee, M.; Moulijn, J. A.; Weckhuysen, B. M. The production of propene oxide: Catalytic processes and recent developments. Ind. Eng. Chem. Res. 2006, 45, 3447, and references therein. (2) Kobe J. M.; Evans, W. E.; June, R. L.; Lemanski, M. F. EpoxidationsIndustrial in Encyclopedia of Catalysis; Horva´th, I. T., Ed.; Wiley-VCH: Weinheim, Germany, 2003; Vol. 3, p 246. (3) Merlau, M. L.; Borg-Breen, C. C.; Nguyen, S. B. T. EpoxidationHomogeneous in Encyclopedia of Catalysis; Horva´th, I. T., Ed.; WileyVCH: Weinheim, Germany, 2003; Vol. 3, p 155. (4) Buijzfnink, J. K. F.; van Vlaanderen, J. J. M.; Crocker, M.; Niele, F. G. M. Propylene epoxidation over titanium-on-silica catalystsThe heart of the SMPO process. Catal. Today 2004, 93, 95–199. (5) De Vos, D. E.; Sels, B. F.; Jacobs, P. A. Practical heterogeneous catalysts for epoxide production. AdV. Synth. Catal. 2003, 345 (4), 457. (6) Sheldon, R. A.; Kochi, J. K. In Metal-Catalyzed Oxidations of Organic Compounds; Academic Press: New York, 1981. (7) Kollar, J. (Halcon International Inc.). Epoxidation Process, U.S. Patent 3,351,635, 1967. (8) Sheldon, R. A. Synthetic and mechanistic aspects of metal-catalyzed epoxidations with hydroperoxides. J. Mol. Catal. 1980, 7, 107. (9) Sheldon, R. A.; Van Doorn, J. A. Metal-catalyzed epoxidation of olefins with organic hydroperoxides. 1. Comparison of various metal catalysts. J. Catal. 1973, 31, 427. (10) Thiel, R. W.; Eppinger, J. Molybdenum-catalyzed olefin epoxidation: Ligand effects. Chem. Eur. J. 1997, 3 (5), 696. (11) Mitchell, M. J.; Finney, S. N. J. Am. Chem. Soc. 2001, 123, 862. (12) Martos Calvente, R.; Campos-Martin, J. M.; Fierro, J. L. G. Effective homogeneous molybdenum catalyst for linear terminal alkenes epoxidation with organic hydroperoxide. Catal. Commun. 2002, 3, 247. (13) Martos-Calvente, R.; de la Pen˜a O’Shea, V. A.; Campos-Martin, J. M.; Fierro, J. L. G.; Gutie´rrez-Puebla, E. Synthesis of bis[N,O-{2′-pyridylmethanolate}]dioxomolybdenum(VI) epoxidation catalyst and novel crystal structure derived from X-ray diffraction and DFT calculations. J. Mol. Catal. A: Chem. 2004, 214, 269. (14) Thiel, W. R.; Priermeier, T. The first olefin-substituted peroxomolybdenum complexsInsight into a new mechanism for the molybdenumcatalyzed epoxidation of olefins. Angew. Chem., Int. Ed. Engl. 1995, 34, 1737–1738. (15) Tempesti, E.; Giuffre, L.; Renzo, F. D.; Mazzachia, C.; Modica, G. Heterogenized boron(III)-molybdenum(VI) mixed oxo derivatives as new bimetallic catalysts for cyclohexene liquid-phase epoxidation. J. Mol. Catal. 1988, 45, 255. (16) Sherrington, D. C.; Simpson, S. Polymer-supported Mo and V cyclohexene epoxidation catalystssActivation, activity, and stability. J. Catal. 1991, 131, 115. (17) Miller, M. M.; Sherrington, D. C.; Simpson, S. Alkene epoxidations catalyzed by molybdenum(VI) supported on imidazole-containing polymers. 3. Epoxidation of oct-1-ene and propene. J. Chem. Soc., Perkin Trans. 1994, 2, 2091. (18) Miller, M. M.; Sherrington, D. C. Alkene epoxidations catalyzed by Mo(VI) supported on imidazole-containing polymers. 1. Synthesis, characterization, and activity of catalysts in the epoxidation of cyclohexene. J. Catal. 1995, 152, 368. (19) Miller, M. M.; Sherrington, D. C. Alkene epoxidations catalyzed by Mo(VI) supported on imidazole-containing polymers. 2. Recycling of
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ReceiVed for reView February 14, 2008 ReVised manuscript receiVed April 2, 2008 Accepted April 3, 2008 IE800262X
