J. Phys. Chem. C 2008, 112, 17657–17663
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Formation of Ordered Mesoporous MgO with Tunable Pore Diameter and Its Application As Excellent Alkaline Catalyst in Baeyer-Villiger Oxidation Jingxia Li, Wei-Lin Dai,* and Kangnian Fan* Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and InnoVatiVe Materials, Fudan UniVersity, Shanghai 200433, People’s Republic of China ReceiVed: July 29, 2008; ReVised Manuscript ReceiVed: September 21, 2008
Highly ordered mesoporous MgO catalysts with tunable pore size distribution and fairly good alkaline property were successfully obtained by a double replication procedure. Small-angle XRD, TEM, and nitrogen adsorption and desorption results show that these mesoporous MgO materials possess a highly ordered 2D hexagonal mesostructure with tunable pore diameter that is obtained using different hydrothermal temperature derived SBA-15 and CNK-3 carbon as templates. The alkaline density was determined for all samples from the technique of temperature-programmed desorption of CO2 and FT-IR, and the mesoporous MgO with large pore diameter shows more basic sites than the reference bulk one. Such mesoporous MgO materials have been successfully used as base catalysts in the Baeyer-Villiger oxidation of cyclic ketones to the corresponding lactones. 100% adamantanone conversion and >99% lactone selectivity were obtained over mesoporous MgO373. The pore diameter has slight influence on the conversion of ketones, since the average pore diameters of mesoporous MgO are large enough to allow the reactants free access to the active sites. In terms of the proposed mechanism, the enhanced medium alkaline properties will be more favorable for the attack by hydrogen peroxide to form a hydroperoxide and percarbonic oxide, and in turn enhancing the catalytic activity. In addition, the higher surface area and large pore volume are attributed to the higher activity of mesoporous MgO-373 than the others, signifying the promising potential applications of these mesoporous MgO materials in alkaline catalysis. 1. Introduction Reports on the preparation of mesoporous metal oxides are becoming more numerous due to the wide range of properties and potential applications of these new materials.1 Among these applications, great interest has been paid on the potential as heterogeneous catalysts over porous materials with narrow pore size distribution, since they provide a combination of large specific surface area and a certain degree of size- and shapeselectivity (molecular sieve properties).2-4 As known, the surface property of catalysts, especially surface acidity and basicity, are important in acid-base catalysis. However, most of the porous materials ranging from micro- to mesoporous are either acidic or neutral to date.5-7 Reports on the formation of the corresponding basic materials are scarce. Thus, it is desirable to develop alternative methods to produce abundantly mesoporous materials with intrinsic basicity. MgO (periclase) as a conversional solid base catalyst or catalyst support is attractive for heterogeneous catalytic reactions including Baeyer-Villiger (BV) oxidation,8 flavanone synthesis,9 etc. MgO is also used as a good catalyst support due to its facilities on the selectivity and stabilities of the active metals in unusual electronic states, i.e., Pt/MgO10 and Ru-Cu/MgO.11 Nowadays, MgO is usually prepared by using conventional methods based on the decomposition of various magnesium salts and Mg(OH)2 (brucite),12 which often leads to a relatively small surface area and small micropores, thus restricting its wide applications in many fields. The disadvantage has been eliminated since mesoporous MgO was prepared with hard templating methods by Tiemann et al.13,14 Mesoporous MgO materials * To whom all correspondence should be addressed. E-mail: wldai@ fudan.edu.cn. Fax: (+86-21) 65642978.
prepared with this technique exhibit much higher specific surface area, larger pore volume, and narrow pore size distribution; these parameters usually play important roles in catalytic reactions. However, since alkaline property is a crucial factor to the catalytic property, the relationship of the preparation method and the alkaline property of various MgO will be of great interest. In the present work, we have synthesized mesoporous MgO materials with tunable pore diameters using templates of SBA15 silica and CMK-3 carbon with various pore diameters. These mesoporous MgO materials as alkaline catalysts were investigated in the Baeyer-Villiger oxidation with different cycloketones. The BV reaction, first employed in 1899 by Baeyer and Villiger, has become a very important protocol to convert ketones into lactones in organic synthesis, and the corresponding lactones and esters are important synthetic intermediates in the chemical, agrochemical, and pharmaceutical industries.15,16 To date, the only suitable oxidants for the BV reaction were the peracids which suffered from not only safety problems but also a bad atomic economy that causes a lot of waste. In terms of the considerations of green chemistry, great efforts have been made to develop an alternative catalytic method, preferentially a heterogeneous one. Recently Corma17 and his colleagues reported new classes of tin-supported zeolite catalysts for the BV oxidation of ketones. Other tin-supported heterogeneous catalysts were also reported, such as Sn-MCM-4, Sn-HT,18 and Sn/palygorsk.19 In addition, other alkaline catalysts, including MgO, Mg(OH)2, and HT, also have been claimed as good catalysts in BV reaction by Ruiz and his co-workers.8,20,21 However, all of them have micropores, which limited their further applications in the bulky ketone oxidation and, most
10.1021/jp806703n CCC: $40.75 2008 American Chemical Society Published on Web 10/21/2008
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Figure 1. Powder X-ray diffraction patterns of (a) SBA-15 silica, CMK-3 carbon, and mesoporous MgO and (b) mesoporous MgO-373, MgO-403 and MgO-423. The inset in panel b shows wide-angle reflections assignable to MgO-373.
importantly, the relationship of the acidic/basic property and the catalytic behavior have not been well discussed. In the present work, we report the catalytic performance of the asprepared mesoporous MgO with different large pore diameter in the BV reaction with aqueous hydrogen peroxide as the oxygen donor. The effect of the pore size, the proposed mechanism, and the relationship between the catalytic activity and their alkaline properties are well correlated by using the analysis from X-ray powder diffraction (XRD), Fourier transform infrared spectra (FT-IR), N2 adsorption-desorption together with the transmission electron micrographs (TEM), and thermal programmed desorption of CO2 (CO2-TPD). 2. Experimental Section 2.1. Preparation of Mesoporous Metal Oxides. SBA-15 silica was synthesized following the procedure reported by Zhao et al. except for the modifications of the starting composition and stirring mode.22 The starting composition was 10 g of P123, 0.10 mol of TEOS, 0.60 mol of HCl, and 20 mol of H2O. Typically, 1.1 g of TEOS was added to 19.0 mL of 1.6 M HCl containing 0.5 g of P123 at 308 K. The mixture was stirred with magnetic stirring until TEOS was completely hydrolyzed. Subsequently, SBA-15 with tunable pore diameter was obtained by heating for 24 h at 373, 403, and423 K respectively under static conditions in a closed Teflon lined autoclave. The products were filtered, dried without washing, and calcined at 823 K. All of the SBA-15 samples were denoted as SBA-15-373, SBA15-403, and SBA-15-423 in terms of different hydrothermal temperature. CMK-3 carbon was prepared following a literature procedure.23 The CMK-3 materials were denoted as CMK-3-373, CMK-3-403, and CMK-3-423 in terms of different SBA-15 templates. The following wet impregnation procedures were used for the synthesis of mesoporous MgO with tunable pore size: a 0.25 g sample of CMK-3-X was dispersed in 2.8 mL of
aqueous solution (1.0 mol · L-1) of Mg(NO3)2, and the resulting solution was stirred for 2 h for the impregnation of Mg(NO3)2 on the mesoporous carbon support. After removal of the remaining solution and a subsequent drying step at 573 K, this process was repeated once, then white product was attained after calcination at 773 K for 6 h in a quartz tube under a dry air flow to remove the carbon aerogel templates. The as-prepared MgO materials from different SBA-15-X and CMK-3-X templates were also denoted as MgO-X. Reference MgO (R-MgO) was synthesized by calcined commercial Mg(OH)2 at 773 K for 6 h. 2.2. Characterizations. Small-angle X-ray scattering (SAXS) patterns were collected using Rigaku RU-V 200 model and Cu KR radiation. XRD patterns are recorded on a Bruker D8 Advance diffractometer using Cu KR radiation (λ ) 0.154 nm). The tube voltage was 40 kV, and the current was 40 mA. TEM micrographs are obtained on a Joel JEM 2010 microscope, which is equipped with an energy dispersive X-ray emission analyzer (EDX). Specific surface areas of the samples are measured by nitrogen adsorption at 77 K (Micromeritics Tristar ASAP 3000) using the Brunauer-Emmett-Teller (BET) method. The pore size distributions (average pore diameter and mean pore volume) were measured from the N2 desorption isotherm using the cylindrical pore model (Barrett-Joyner-Halenda method, BJH). Fourier transform infrared (FTIR) spectra were recorded on NEXUS 470 and the samples were finely grounded, dispersed in KBr, and pelletized. CO2-TPD experiment was carried out on a homemade apparatus. About 200 mg of sample was outgassed at 773 K in helium flow for 2 h. After the sample was cooled to 393 K under He, carbon dioxide was switched into the system. The amount of desorbed CO2 was determined by heating the sample at a ramping rate of 25 deg · min-1 from ambient temperature to 973 K in helium flow (40 mL · min-1) with a TCD detector.
Formation of Ordered Mesoporous MgO
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2.3. Activity Test. BV oxidation was conducted in a 10 mL glass reactor. A typical procedure for the BV oxidation is as follows: 0.1 mL of cyclohexanone or 0.145 g of adamantanone, 0.35 mL of benzonitrile, 0.5 mL of 1,4-dioxane, 0.7 mL of hydrogen peroxide (50%, V/V, aqueous solution), and 0.025 g of catalyst were added to the reactor, and then the mixture was heated to 343 K and stirred for a certain time. The reaction products were identified by GC-MS analysis. 3. Results of Discussion 3.1. Crystalline Phase. Figure 1a shows the small powder XRD patterns of mesoporous MgO-373. The as-prepared SBA15-373 silica and CMK-3-373 carbon are also shown, which served as the parent templates. All the samples show diffraction peaks in the small-angle scattering regime (2θ < 4 °C) that are indexed as (100), (110), and (200) reflections associated with P6mm hexagonal symmetry.13,14 It is found that the thermalstably ordered mesoporous MgO was successfully obtained with this method. The resulting MgO material can be considered as the positive replica of SBA-15 silica and as the negative replica of CMK-3 carbon, since they are two templates in the preparation process of mesoporous MgO. However, to distinguish only the (100) reflection in the final product of MgO, a certain degree of broadening and an obvious decreasing of the reflections signify partial loss in structural order of MgO compared to SBA15 and CMK-3. In addition, the first replication process (from silica to carbon) resulted in a slight shrinkage of the periodic distance, thus, compared to SBA-15, the observed diffraction peak of the CMK-3 shifted to a higher degree can be easily understood. However, the second replication process did not lead to the further shift of diffraction peaks, indicating the periodic distance maintained in the process from the CMK-3 to MgO. The SAXS spectra of mesoporous MgO samples with different pore diameters were also shown in Figure 2. The lowangle reflections, corresponding to the two-dimensional hexagonal periodicity of the pore systems, are visible in all three samples. However, only the (100) peak can be observed in all of the samples and the intensity decreased from MgO-373 to MgO-423. The weak intensity of MgO-423 compared to MgO373 and MgO-403 can be partly interpreted from the consideration that the number of micropores is drastically reduced in the case of SBA-15-423,24 which was used as a template for CMK-3-423 and MgO-423. The partial loss of the periodicity of the MgO-423 can also be manifested in terms of the following TEM photos. In addition, the wide-angle powder XRD pattern of mesoporous MgO-373 (Figure 2, inset) exhibited reflections attributed to the crystalline structure of magnesium oxide. The combination of the small- and wide-angle XRD results showed that the ordered mesoporous MgO with crystalline walls are synthesized in the present process. The intense (100) peak reflected a d spacing value, and the corresponding large unit cell parameters a [a) d100 × 23] of all of the samples are given in Table 1. 3.2. Morphology. Figure 2 shows TEM photos of mesoporous MgO-373, MgO-403, and MgO-423, and the results of selected area electron diffraction (SAED) and EDX are also included. The highly ordered hexagonal arrangement of pores along the [001] direction and the alignment of cylindrical pores along the [110] direction are observed in Figure 2, parts a and b, indicating a long-range periodic order with hexagonal symmetry was formed in the mesoporous MgO-373 material. Obviously, the as-prepared MgO-373 can duplicate the morphology of the SBA-15-373 template and can present a typical
Figure 2. TEM images of different mesoporous MgO: (a and b), MgO373; (c and d), MgO-403; and (e and f), MgO-423. The EDX and SAED patterns are shown in the insets in a and b, respectively.
TABLE 1: Textural Properties of Mesoporous MgO and the Parent Materials CMK-3 Carbon and SBA-15 Silica
sample SBA-15-373 SBA-15-403 SBA-15-423 CMK-3-373 CMK-3-403 CMK-3-423 MgO-373 MgO-403 MgO-423 R-MgO
wall thickness,d BET surface pore vol, pore diam dp, nm a, nm area, m2 g-1 cm3 g-1 nm 745 519 346 1150 1328 1484 256 150 111 18
1.25 1.32 1.10 1.20 1.57 1.69 0.49 0.33 0.18 0.15
6.6 9.1 10.2 3.0 4.6 6.2 6.4 7.1 9.5 33
10.3 10.5 11.4 10.1 10.5 10.8 10.1 10.6 11.2
3.7 1.4 1.2 7.5 5.9 4.6 3.7 3.5 1.7
a Calculated by the BET method. b Calculated by the BJH method from the desorption isotherm. c Calcinated at 873 K. d Calculated in terms of the difference between cell constants and pore diameters.
mesoporous structure. EDX analysis in Figure 3a confirms the absence of significant amounts of silica and carbon, and the SAED pattern in Figure 3b exhibits concentric diffraction rings with single, weakly resolved spots on the top of them, signifying that the mesoporous wall is well crystallined with the MgO phase. With the samples synthesized from different SBA-15 and CMK-3 templates, the mesoporous MgO samples with tunable pore diameter were also obtained. In panels c and d of Figure 3, the mesoporous MgO-403 material with a pore diameter around 9-10 nm also exhibits a periodic hexagonal pore arrangement as MgO-373. However, because of the partial loss
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Figure 5. CO2-TPD profiles of MgO-373, MgO-403, MgO-423, and R-MgO.
TABLE 2: Surface Basicity of the Catalysts Determined by CO2-TPD samples chemical adsorbance carbon dioxide amount, mmol/g weak acidic site, mmol/g (593 K)
Figure 3. Nitrogen physisorption isotherms and pore size distributions of MgO-373, MgO-403, and MgO-423.
Figure 4. FT-IR spectra of MgO-373, MgO-403, MgO-423, and R-MgO.
of the periodicity of MgO-423 as illustrated in the above S-XRD patterns, the TEM photo of MgO-423 has immerged with a worm-like structure as shown in Figure 2e,f. For the potential application of mesoporous MgO materials in the heterogeneous catalysis, the benefit of a mesoporous structure in the heterogeneous catalytic reaction was discussed intensively by a group led by Sachdev.25 Since all the samples
R-MgO MgO-373 MgO-403 MgO-423 0.38
1.08
0.78
0.56
0.36
0.30
0.23
0.21
0.10
0.71
0.47
0.31
0.02
0.07
0.08
0.04
with mesoporous structure show much higher surface area, they can be used as catalysts or supports in many reactions. The MgO materials with mesoporous structure may adsorb more reactant molecules on the surface than the normal commercial bulky MgO. The ordered pore system also can play an important role in the catalyst design for its ability to improve the molecular transport of reactants and products. Therefore, very high specific surface area and the ordered structural arrangement of the mesoporous MgO material in the present investigation may be responsible for its high catalytic activity. 3.3. Textual Property. We also investigated in detail the effects of different templates on the pore structure and BET surface area of the as-prepared mesoporous MgO samples based on nitrogen adsorption and desorption measurements. Table 1 shows the physicochemical properties of the MgO series and their SBA-15 silica and CMK-3 carbon templates with different pore diameters. With increasing hydrothermal temperature, the mean pore diameter calculated from the desorption branch of the nitrogen isotherms using the BJH model is virtually increased from 6.6 to 10.2 nm, while the specific pore volume and the specific surface area decrease, which may be traced back to the absence of the secondary micropore system when SBA-15 was synthesized at above 373 K.24 CMK-3 carbon with different pore diameters prepared from SBA-15 also shows high surface area. It also can be seen in Table 1 that all the mesoporous MgO samples show specific surface area above 110 m2 · g-1, much larger than the reference commercial MgO counterpart (18 m2 · g-1). The lower surface area of mesoporous MgO-373 sample (256 m2 · g-1) as compared to its positive replica SBA15-373 silica (745 m2 · g-1) can partially be attributed to the discrepancies of their densities. In addition, two replication steps (from silica to carbon and from carbon to magnesium oxide) will also result in the partial loss of structural order compared to SBA-15 as showen by XRD. Compared with different mesoporous MgO, the surface area of MgO-423 (111 m2 · g-1) is much lower than that of MgO-373 (256 m2 · g-1). The drastic
Formation of Ordered Mesoporous MgO
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TABLE 3: Baeyer-Villiger Oxidation of Different Ketones to Lactones over Various Catalystsa entry
substrate
catalyst
pore diameter, nm
T, K
time, h
ra,b h-1g-1
conversion,c %
1 2 3 4 5 6 7 8 9
cyclohexanone cyclohexanone cyclohexanone cyclohexanone adamantanone adamantanone adamantanone adamantanone adamantanone
MgO-373 MgO-403 MgO-423 R-MgO MgO-373 MgO-403 MgO-423 R-MgO no catalyst
6.4 7.1 9.5 33 6.4 7.1 9.5 33
343 343 343 343 343 343 343 343 343
8 8 8 8 6 6 6 6 12
40 37 36 35 60 54 50 25 0
90.5 83.9 81.1 79.0 100 95.1 84.1 42.3 0
a Reaction conditions: substrate 0.145 g of adamantanone, 0.1 mL of cyclohexanone, 0.33 mL of benzonitrile, and 0.7 mL of 50% hydrogen oxide. b ra ) moles of ketone converted per hour per gram of catalysts. c Conversion to lactone (selectivity is 100% in all cases).
reduction can be interpreted from the S-XRD and TEM results as illustrated in Figures 2 and 3 presenting the slight disorder of the pore structure in MgO-423 and the absence of micropores in its parent SBA-15-423 template. In addition, the wall thicknesses calculated from the difference between cell constants a of the hexagonal pore arrangement and pore diameters are shown in Table 1. We find that, for SBA15, the enhanced hydrothermal temperature resulted in an increased pore diameter, while the corresponding wall thickness decreases abruptly. In the first replication step, the pores in SBA15 are not completely filled with carbon since the wall of CMK-3 is thinner than the pore diameter of SBA-15; however, the second step yields a much more efficient filling of the CMK-3 carbon pores with MgO and the wall thickness of different mesoporous MgO materials as well as their SBA-15 templates decreased from MgO-373 to MgO-423. The nitrogen adsorption-desorption isotherms of the different as-prepared mesoporous MgO samples are shown in Figure 3a. It is found that the isotherms of MgO-373, MgO-403, and MgO423 all exhibit type IV curves with H1-shaped hysteresis loops, in close resemblance to those of the “parent” SBA-15 and CMK-3 materials (not shown). The hysteresis loops with the closed points shift to higher relative pressures of 0.50-0.70 in the isotherms from MgO-373, MgO-403, to MgO-423, indicating an increase of the corresponding pore size of the samples.25-27 Moreover, Figure 3b shows the pore-size distribution plots calculated using the BJH equation from the desorption branch of the isotherm. The pore size distribution measurement indicates that the as-prepared MgO samples have abundant mesoporosity with a narrow pore-size distribution from 6.5 to 9.5 nm. This result illustrates that the mesoporous MgO with tunable pore size distribution was successfully synthesized by using SBA15 silica and CMK-3 carbon as templates. However, considering the dramatic reduced number of microspores in the case of SBA15-423, which is used as a template for CMK-3-423 and MgO423, the pore size distribution of the MgO-423 sample broadens considerably in terms of the results from S-XRD and TEM photos. It is interesting to find that such mesoporous structure can be maintained after 773 K calcinations. It is commonly accepted that the hydroxyl and carbonate groups play important roles in the catalytic performance of MgO as alkaline catalysts, so the FTIR spectra for different mesoporous magnesium oxides samples and the reference MgO are all collected in Figure 4 to unveil the discrepancies among them. All the catalysts present similar bands in two different regions. For wave -numbers over 3000 cm-1, both isolated hydroxyl groups (above 3700 cm-1) and hydrogen-bridging OH (below 3700 cm-1) are observed.28,29 Kirlin et al.30 also found up to eight bands in this region, assignable to the different types of OH groups present on the surface of the magnesium oxide,
TABLE 4: Recycling Results of MgO-373 Catalyst in Baeyer-Villiger Oxidation of Adamantanone recycle number
T, K
time, h
conversion, %
MgO-373fresh 1 2 3
343 343 343 343
6 6 6 6
100 100 100 98
which will not disappear completely at calcination temperatures below 900 °C. In the region below 2000 cm-1, both Mg-O stretching (1400-1441 cm-1)31 and bands due to adsorbed carbonate32 appear, and it was also observed by. Aramendı´a that MgO samples were easily carbonated under such conditions.33 The main carbonate species in our cases are unidentate carbonates that exhibit two characteristic bands at 1378 and 1516 cm-1, corresponding to the symmetric and asymmetric O-C-O stretching bands. The enhanced band intensity in the region below 2000 cm-1 for the mesoporous MgO compared to the reference one can partially contribute to its high alkaline property. 3.4. Density and Strength Distribution of Basic Sites. To achieve a complete characterization of the alkaline properties of mesoporous MgO samples, the total numbers, density, and strength distribution of alkaline sites were identified by TPD of preadsorbed carbon dioxide. Figure 5 shows typical CO2TPD profiles obtained for different mesoporous MgO prepared in the present work and the reference MgO after calcination at 773 K from commercial Mg(OH)2. The CO2 uptakes by various catalysts with different alkaline strengths are listed in Table 2. The alkaline sites can be divided into three types in terms of their different strengths (different carbon dioxide desorption temperature, TD), ca. low (TD below 493 K), medium (TD between 493 and 593 K), and high strength alkaline sites (TD over 593 K). Since the cleaning step prior to CO2 saturation was carried out at 893 K, those alkaline sites at TD > 893 K were not considered for the discussion in the present work. In Figure 5, the TPD profiles show similar shape for all of the samples. In the range of 393-693 K, only one desorption peak can be observed. The mesoporous MgO and the reference one show desorption peaks at about 450 or 520 K, respectively. Table 2 lists the numbers of weak, medium, and strong alkaline sites, expressed in micromoles of CO2 desorbed per gram of catalyst over different temperatures. According to the classification, the commercial MgO evidently corresponded to the weak alkaline material while the mesoporous MgO corresponded to the medium one. From these data, we can establish a total basicity scale of all the MgO samples. In terms of the CO2 desorbed at TD < 693 K, the order is the following: MgO-373 > MgO-403 > MgO-423 > R-MgO. Since alkaline sites had a close relationship to the specific surface area and pore volume, the increased intensity of alkaline sites in mesoporous MgO
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SCHEME 1: Proposed Mechanism for the Baeyer-Villiger Oxidation of Cyclohexanone with Hydrogen Peroxide/ Benzonitrile As Oxidant
compared with that of the reference can be partly ascribed to the discrepancy of the physicochemical parameters among these samples. Aside from the above finding, in the FTIR spectra, the high density of adsorbed carbonate species on the surface of mesoporous catalysts will also contribute to their higher alkaline density of mesoporous MgO than that of the commercial one. 3.5. Catalytic Test in Baeyer-Villiger Oxidation. The BV oxidations of different ketones to lactones were conducted at 343 K over a series of MgO catalysts with H2O2 as oxidant in benzonitrile medium. The observed yield and selectivity as well as the calculated activity (mmol/g) were given in Table 3. On the basis of these results, all the catalysts can provide 100% selectivity to lactones within a relatively short time. When cyclohexanone was used as the substrate, which is small enough to enter the 6 nm pore of mesoporous MgO, all the mesoporous MgO show higher activities than the reference commercial one, signifying the promising potential use of these ordered mesoporous MgO catalysts in the BV reaction. In terms of the characterizations mentioned above, the high activity of the mesoporous MgO samples can be easily ascribed to their high specific surface area, large pore volume, and special mesoporous pore structure. In addition, according to the CO2-TPD results, the increased amount of moderate alkaline sites of mesoporous MgO will also contribute to the enhancement of catalytic activity. It is also very interesting to find that among all the mesoporous MgO materials, MgO-373 shows the best catalytic performance with substrate conversion of 90.5%, whereas MgO403 and MgO-423 exhibit 83.9% and 81.1%, respectively. The low activity observed for MgO-403 and MgO-423 is probably due to their smaller surface area and lower pore volume. As was previously illustrated, MgO-423 possesses a broad pore size distribution centered at 10 nm, while MgO-403 and MgO-423 exhibit much larger pores than the size of the product molecule. Since the accessibility of product molecules to the catalyst surface is not the rate-determining step, the adsorption amount of product on the catalyst is crucial, which is controlled by the surface area and the pore volume of the catalyst. Therefore, MgO-403 and MgO-423 show a lower adsorption capacity as compared to MgO-373. Moreover, the slight decrease of numbers of alkaline sites will also result in the lower activity of MgO-403 and MgO-423 than that of MgO-373. The large pore diameter of mesoporous MgO can allow the oxidation of adamantanone more easily, which exhibits much larger molecular size than cyclohexanone. In an additional experiment, adamantanone was tested using various MgO
catalysts with different pore diameters. When MgO-373 was used, after 6 h no substrate could be detected and the corresponding lactone was the only product. MgO-403 also shows as high as 95.1% adamentanone conversion, signifying a large pore diameter is favorable for BV oxidation reaction with large-sized substrates. Due to the slight loss of ordered pore structure and the broad pore size distribution, however, MgO-423 with 9.5 nm pore diameter shows much lower activity. Nevertheless, all the mesoporous MgO materials show much higher activity than the commercial one in the BV oxidation of adamentanone irrespective of their discrepancies in activity. The high activity observed for MgO-373 relative to MgO-403 and MgO-423 is probably due to its high BET surface area, proper pore diameter, and high pore volume. We also investigated the reusability of mesoporous MgO373 catalyst by filtering off the catalyst from one batch and washed, dried for another batch of experiment under identical conditions. As shown in Table 4, the used catalyst almost keeps its initial excellent adamentanone conversion as well as 100% selectivity to lactones after being reacted 4 times, suggesting the good stability and reusability of the as-prepared mesoporous MgO catalysts. These good results may show a promising potential for further industrial applications considering its excellent activity, 100% selectivity, and good reusability. 3.6. Discussions. The proposed mechanism of BV oxidation of ketones to lactones when using benzonitrile as solvent was shown in Scheme 1, which is analogous to that proposed by Llamas on the catalyst of MgO and Mg(OH)2.21 First, the Bronsted alkaline sites like OH- and CO32- on the surface of the mesoporous MgO catalysts are attacked by hydrogen peroxide to form a hydroperoxide and percarbonic oxide species that subsequently attacks benzonitrile to give a peroxycarboximidic intermediate. Second, the previous intermediate attacks cyclohexanone adsorbed on the acidic sites of the catalyst to form an intermediate equivalent to the Criegee adduct in homogeneous catalysis. Finally, the intermediates rearrange to caprolactone and benzamide. With the aid of the above proposed mechanism and the results in the present work, several conclusions on these unusual catalysts can be obtained: (1) Mesoporous MgO can be regarded as an effective catalyst in BV oxidation. (2) The pore diameter has a slight influence on the conversion of ketones with a small molecule, because the average pore diameters of all mesoporous samples are large enough to allow the reactants free access to the active sites and the products to diffuse freely. Thus the mesoporous MgO with about 6 nm pore diameter shows the
Formation of Ordered Mesoporous MgO highest activity among all the mesoporous MgO samples. (3) Higher surface area and special mesoporous pores will lead to the adsorption of more reactant molecules on the surface of the catalysts and facilitate the diffusion of the reactants and products. (4) The alkaline sites on the MgO catalysts are active sites in the title reaction, and more alkaline sites will result in higher activity of the BV reaction. In general, the mesoporous MgO catalysts with more alkaline sites will be more favorable for attack by hydrogen peroxide to form hydroperoxide and percarbonic oxide, and in turn, enhance the catalytic activity. As far as we are aware of, these materials have not been used in the BV oxidation with aqueous hydrogen peroxide as the oxidant, which opens a new avenue for designing new catalysts and green processes in fine organic chemistry. 4. Conclusions In summary, mesoporous MgO with different pore diameters can be obtained by using SBA-15 silica and CMK-3 carbon with various pore diameters as templates. Highly ordered 2D hexagonal mesostructure was attained in different mesoporous MgO materials in terms of results from TEM, SXRD, and nitrogen adsorption-desorption. Considering the CO2-TPD desorbed at MgO-403 > MgO-423 > R-MgO. The influence of the alkaline density and the pore size on the activity of the mesoporous MgO in the BV oxidation reaction was investigated and a mechanism was proposed. It is found that the pore diameter has a slight influence on the conversion of ketones with small molecules because the pores of all mesoporous samples are large enough to allow the reactants free access to the active sites and the products to diffuse easily. More alkaline sites on the MgO-373 samples will be favorable for the adsorption of ketones and benzonitrile molecules on the surface of the catalysts, thus leading to the highest activity among all the MgO materials in BV oxidations. Acknowledgment. This work was financially supported by the Major State Basic Resource Development Program (Grant No. 2003CB615807), NSFC (Project 20573024), and the Natural Science Foundation of Shanghai Science & Technology Committee (06JC14004).
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