Exploring in Situ Functionalization Strategy in a Hard Template Process

ARTICLE pubs.acs.org/JPCC

Exploring in Situ Functionalization Strategy in a Hard Template Process: Preparation of Sodium-Modified Mesoporous Tetragonal Zirconia with Superbasicity Lu Gong, Lin-Bing Sun,* Yuan-He Sun, Tian-Tian Li, and Xiao-Qin Liu* State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China

bS Supporting Information ABSTRACT: An in situ functionalization strategy was developed in a hard template process to fabricate mesoporous solid superbases, and sodium-modified mesoporous zirconia (NaMZ) was successfully prepared. The obtained materials were characterized by various methods including XRD, TEM, N2 adsorption, IR spectroscopy, XRF, and CO2 TPD. The results show that the NaMZ materials exhibit well-defined mesostructure, tetragonal crystalline frameworks, and superbasicity with a high strength of 27.0. The NaOH solution plays a double role by removing the silica template SBA-15 and functioning as the guest. The sodium species is thus coated onto the mesoporous zirconia formed in situ. Hence, the fabrication and functionalization of mesoporous zirconia can be realized in one step, which avoids the possible structural damage of nonsiliceous mesoporous oxides in post-treatment. The stability of metastable tetragonal zirconia was found to be due to the confined space provided by the mesoporous silica template and the formation of Si
O
Zr linkages. It was also demonstrated that the NaMZ materials exhibited excellent basic catalytic performance. The turnover frequency (TOF) value on the material NaMZ-2 can reach 95.0 h
1 with 100% selectivity to the target product dimethyl carbonate, which is much higher than the value for the extensively used homogeneous catalyst CH3ONa (45.8 h
1).

’ INTRODUCTION Mesoporous superbases are highly attractive for applications in environmentally benign and economical catalytic processes, because they can catalyze diverse reactions under mild conditions.1,2 Their large pore openings are beneficial to the diffusion of substrates and can avoid deactivation due to coke formation that usually takes place in microporous catalysts.3,4 To date, great efforts have been directed toward the fabrication of mesoporous materials with superbasicity. Among various candidates with mesostructure, mesoporous silicas are easy to synthesize and have good stability. The use of mesoporous silicas as supports to generate basic sites has attracted much attention since the discovery of mesoporous silica M41S.5,6 Grafting organic bases onto silanol groups presents an interesting method to fabricate basic sites on mesoporous silicas.7
9 However, the base strength of the resultant materials needs further improvement. Also, such materials can be used only at low temperatures (900 °C) have to be employed in such processes, and the strength of basic sites is still relatively weak. To r 2011 American Chemical Society

improve the base strength, alkaline metal oxides, which are strongly basic, have been employed to modify mesoporous silicas. Mesoporous solid strong bases can be prepared by impregnation of MCM-41 with cesium acetate solution and subsequent calcination.14 However, the obtained bases exhibit poor stability because the formed basic species (cesium oxide) can react with silica and damage the framework of the host.15 The base precursor potassium nitrate has been widely used to generate strongly basic sites on various hosts.16
19 Aiming to form strong basicity on mesoporous silicas, SBA-15 has been employed as a host to disperse guest potassium nitrate. Unfortunately, the obtained material exhibited weak basicity, and the mesostructure of SBA-15 was destroyed completely during the activation to decompose potassium nitrate.20,21 Because of the reaction of strongly basic species with silica, it is difficult to generate strong basicity on mesoporous silicas. Although many attempts have been made, the synthesis of mesoporous superbases still remains a challenge. In contrast to silica, nonsiliceous oxide zirconia exhibits better alkali resistance and is a well-known host for many solid strong bases.22
24 The reaction of strongly basic species with the host Received: March 5, 2011 Revised: May 8, 2011 Published: May 18, 2011 11633

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The Journal of Physical Chemistry C that occurs in silica-supported materials can thus be avoided. The successful synthesis of mesoporous zirconia offers a good chance to prepare mesoporous superbasic materials.25,26 However, two drawbacks hinder the application of mesoporous zirconia as a base host. First, nonsiliceous oxides are less stable than their silica analogues, which can result in the structural damage even if postmodification is carefully conducted during the introduction of basic species.27,28 Second, the monoclinic rather than tetragonal phase is preferentially formed in mesoporous frameworks, because of the metastability of the tetragonal phase, but monoclinic zirconia lacks the vacancies and active sites required for use as catalysts and catalyst supports.22,23 These drawbacks also obstruct the application of mesoporous zirconia in preparing functional materials other than solid bases. In the present study, we developed an in situ functionalization strategy in a hard template process for the first time by flexible utilization of mesoporous silica and basic solution and successfully prepared sodium-modified mesoporous zirconia (NaMZ) with well-defined mesostructure, tetragonal crystalline frameworks, and superbasicity. In addition to use as a hard template, mesoporous silica also provides a silicon source for the formation of Si
O
Zr linkages, which is essential to the stabilization of tetragonal zirconia and the generation of superbasicity. The basic solution plays a double role by removing the silica template and functioning as the guest. The sodium species is thus coated onto the mesoporous zirconia formed in situ. This strategy allows the fabrication and functionalization of mesoporous zirconia in one step, avoids post-treatment framework damage, and saves time and energy. We also demonstrate that the prepared NaMZ materials exhibit excellent basic catalytic performance (high conversion and 100% product selectivity) in the synthesis of dimethyl carbonate (DMC) that is superior to the performance of some typical solid bases and even to the extensively used homogeneous catalyst CH3ONa.

’ EXPERIMENTAL SECTION Materials Synthesis. Mesoporous silica SBA-15 for use as a hard template was synthesized according to the report of Zhao et al.29 Specifically, 2 g of triblock copolymer P123 (EO20PO70EO20) was dissolved in 75 g of 1.6 M HCl aqueous solution with stirring at 40 °C. Then, 4.25 g of tetraethylorthosilicate was added to the homogeneous solution and stirred at this temperature for 24 h. Finally, the mixture was heated to 100 °C and held at this temperature for 24 h under static conditions. The as-prepared sample was recovered by filtration, washed with water, and air-dried at room temperature. The removal of template was carried out at 550 °C in air for 5 h. Zirconium precursor was introduced into the silica template by impregnation. One gram of ZrOCl2 3 8H2O was dissolved in 10 mL of deionized water, followed by addition of 1 g of SBA-15. After being stirred at room temperature for 24 h, the mixture was evaporated at 80 °C and subsequently dried at 100 °C overnight. The obtained solid was calcined from room temperature to 600 °C at a rate of 2 °C min
1 and maintained at 600 °C for 3 h. The impregnation and calcination procedure was repeated, and the intermediate ZrO2/SBA-15 was formed. ZrO2/SBA-15 was then reacted with 1, 2, and 3 M aqueous NaOH solution for 24 h. After filtration and drying in a vacuum at 60 °C, the materials NaMZ-1, NaMZ-2, and NaMZ-3, respectively, were produced. The material MZ was prepared by thoroughly washing the product from the reaction of ZrO2/SBA-15 with 2 M NaOH

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solution with deionized water. A reference material with the same components as NaMZ-2 was prepared. After silica and zirconia with a Si/Zr ratio of 0.31 was ground manually for 30 min and calcined at 600 °C for 3 h, NaOH with a Na/Zr ratio of 0.67 was introduced by impregnation. The obtained material was denoted as Na/ZrO2
SiO2. Characterization. X-ray diffraction (XRD) patterns of the materials were recorded using a Bruker D8 Advance diffractometer with Cu KR radiation in the 2θ range from 0.5° to 8° and from 5° to 80° at 40 kV and 30 mA. The average crystallite size was calculated from the (111) diffraction peak using Scherrer’s equation.30 Transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) analyses were performed on a JEM-2010 UHR electron microscope operated at 200 kV. N2 adsorption
desorption isotherms were measured using a Belsorp II system at
196 °C. The samples were degassed at 150 °C for 3 h prior to analysis. The Brunauer
Emmett
Teller (BET) surface area was calculated using adsorption data in a relative pressure ranging from 0.04 to 0.20. The total pore volume was determined from the amount adsorbed at a relative pressure of about 0.99. The pore diameter was calculated from the adsorption branch using the Barrett
Joyner
Halenda (BJH) method. The elemental contents of the composites were measured with an ARL ADVANT’XP X-ray fluorescence (XRF) spectrometer. Fourier transform infrared (IR) measurements were performed on a Nicolet Nexus 470 spectrometer by means of the KBr pellet technique. The spectra were collected with a 2 cm
1 resolution. The base strengths of materials were determined using a series of Hammett indicators.21 The indicators employed included phenolphthalein (H
= 9.3), 2,4-dinitroaniline (H
= 15.0), 4-nitroaniline (H
= 18.4), benzidine (H
= 22.5), 4-chloroaniline (H
= 26.5), and aniline (H
= 27.0). Each indicator was dissolved in benzene to form a 0.1 wt % solution. About 100 mg of sample was activated in a N2 flow (99.999%) at 30 mL 3 min
1 at 700 °C for 2 h. After the sample had been transferred to cyclohexane under the protection of nitrogen, two drops of indicator solution were added to the suspension. The color change of the indicator at the solid surface was monitored. To measure the amount of basic sites, 100 mg of sample after activation was shaken in 10 mL of aqueous HCl (0.20 M) for 24 h, and the slurry was separated with a centrifuge. The remaining acid in the liquid phase was titrated with standard base (0.05 M aqueous NaOH), and phenolphthalein was employed as an indicator. The amount of HCl consumed was utilized to denote the amount of basic sites. Temperatureprogrammed desorption of CO2 (CO2 TPD) experiments were conducted on a BELSORP BEL-CAT-A apparatus. The sample was pretreated at 350 °C for 2 h prior to the adsorption of CO2 (99.999%) at 50 °C. After the physically adsorbed CO2 had been purged with a He flow (99.999%) at 50 °C, the sample was heated to 800 °C at a rate of 8 °C 3 min
1, and the CO2 liberated was detected with an online OmniStar mass spectrometer. Catalytic Tests. DMC was synthesized from the transesterification of ethylene carbonate and methanol. In a typical process, methanol (0.5 mol), ethylene carbonate (0.1 mol), and catalyst (0.5 wt % of methanol) were added to a 50 mL three-necked glass flask with a water-cooled condenser. All catalysts except CH3ONa were pretreated at 700 °C in a N2 flow (99.999%, 30 mL min
1) for 2 h prior to reaction. The reaction was conducted at 65 °C with stirring for 1 h under a nitrogen atmosphere. After the reaction had finished, the products and unreacted substrates were 11634

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Table 1. Physicochemical and Catalytic Properties of Materials elemental compositiona SBETb sample


1

Vpb

base strength amount of basic sites DMC yieldc DMC formation rate (H
)

(mmol g
1)

(%)

(mmol g
1 m
2)

TOFd (h
1)

0.300

9.3

0.249

27.0

2.0

2.1

0.13

13.0

5.4

28.2

2.56

103

0.228

62.4

27.0

8.3

66.5

9.97

0.31

73

95.0

0.184

27.0

10.2

75.4

15.95

0.67

0.31

85.4

5

0.016

15.0

8.1

12.8

39.53







18.3













68.6




45.8

2

Na/Zr

Si/Zr

MZ

0.12

0.30

248

NaMZ-1

0.38

0.32

170

NaMZ-2

0.67

0.31

NaMZ-3

0.86

Na/ZrO2
SiO2e CH3ONa

3


1

(m g ) (cm g )

a Weight ratio detected by XRF. b SBET,: BET specific surface area; Vp, pore volume. c Reaction conditions: methanol, 0.5 mol; ethylene carbonate, 0.1 mol; catalyst, 0.5 wt % of methanol; temperature, 65 °C; time, 1 h. d Turnover frequency per Naþ ion. e Reference material prepared by impregnating NaOH into the mixture of silica and zirconia.

Figure 1. Low-angle and wide-angle XRD patterns of (a) MZ, (b) NaMZ-1, (c) NaMZ-2, and (d) NaMZ-3 materials.

recovered from the flask and subjected to centrifugation. The obtained upper liquid was then analyzed by use of a Varian 3800 gas chromatography equipped with a flame ionization detector (FID).

’ RESULTS AND DISCUSSION Structural and Surface Properties. Table 1 presents the elemental compositions for MZ and NaMZ materials. A certain amount of sodium was detected in the MZ material (Na/Zr = 0.12), even though the material was thoroughly washed with water. This indicates that the sodium species remaining in the material might have a strong interaction with the host. The Na/ Zr ratio is 0.38 in NaMZ-1, and it rises with increasing concentration of NaOH solution. As can be seen from Table 1, the Na/ Zr ratio reaches 0.86 for NaMZ-3. These results show that our strategy is effective for the introduction of a basic guest with various contents. Interestingly, all materials exhibit quite similar Si/Zr ratios of about 0.31. We attempted to prolong the reaction time and use NaOH solution with different concentrations; however, the Si/Zr ratio remained constant. This suggests that the form of silicon existing in NaMZ materials is different from that in the SBA-15 template, which has a good alkali-resistant capacity and can survive in strongly basic solution. The representative low-angle XRD pattern of NaMZ materials (Figure 1) shows diffraction lines indexed as (100), (110), and

(200) reflections, corresponding to a two-dimensional hexagonal pore regularity of the p6mm space group. In comparison with the pattern for template SBA-15, the diffraction lines shift to slightly higher angles, implying some structural contraction during template removal. TEM provides another important technique to characterize the periodic ordering of mesostructure. Figure 2A presents the TEM image viewed along a direction perpendicular to the pore direction. An ordered mesostructure is observed, which confirms the low-angle XRD results. Figure S1 in the Supporting Information shows the TEM image of the mesoporous silica template. By comparing the TEM images of SBA-15 and NaMZ materials, it is safe to conclude that the structure of the silica template is well replicated. Figure 3A shows the N2 adsorption
desorption isotherms of MZ and NaMZ materials. A gradual increase in slope starts at a relative pressure of about 0.4 and is followed by a sharp increase ranging from a relative pressure of about 0.9 to 1.0 in both the adsorption and desorption branches. The upward slope from about 0.4 corresponds to capillary condensation with an apparent hysteresis loop, typical of mesoporous materials with uniform pore systems. The further increase at higher relative pressures reflects substantial interparticle porosity. The existence of such porosity is a common phenomenon in mesoporous materials prepared through the hard template method.31
33 As shown in Figure 3B, all materials exhibit narrow mesopore size distributions centered at 3
4 nm, indicative of the well-defined mesoporosity of the materials. The BET surface area of MZ can reach 248 m2 g
1 (Table 1), which is higher than that reported for mesoporous zirconia from self-assembly (137
150 m2 g
1).25,34 With increasing sodium content, both surface areas and pore volumes decline, suggesting the location of sodium species in the pores of the host. As shown in Table 1, the surface area of sample NaMZ-3 is 73 m2 g
1, which is obviously lower than that of MZ. Two factors are considered to be responsible for this phenomenon. The first factor is different sodium contents. As can be seen from Table 1, the Na/Zr ratio in sample NaMZ-3 (0.86) is much higher than that in sample MZ (0.12). This can leads to a decrease of the surface area. The second factor is the preferential location of sodium species in micropores. As displayed in Figure 3, sample MZ exhibits a large nitrogen uptake at low relative pressures (p/p0 < 0.1), whereas that for NaMZ-3 is quite low. It is known that the uptake at low relative pressures comes from micropores, and micropores make a larger contribution to surface area than mesopores. Therefore, the low surface area of 11635

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Figure 2. (A) TEM image and (D) SAED pattern of NaMZ-2 material.

Figure 3. (A) N2 adsorption
desorption isotherms and (B) pore size distributions of (a) MZ, (b) NaMZ-1, (c) NaMZ-2, and (d) NaMZ-3 materials. The ordinates of a
c in part A are offset by 80, 50, and 20 cm3 3 g
1, respectively, and the ordinates of a
c in part B are offset by 0.5, 0.2, and 0.1 cm3 3 g
1, respectively.

Figure 4. Wide-angle XRD patterns of ZrOCl2 3 8H2O calcined at (a) 400, (b) 500, (c) 600, and (d) 700 °C.

NaMZ-3 can be attributed to the high content of sodium and the preferential location of sodium species in micropores.

The wide-angle XRD pattern of the MZ material in Figure 1 shows the diffraction lines at 2θ = 30.2°, 35.3°, 50.3°, and 59.7°, which can be assigned to the (111), (200), (220), and (311) reflections, respectively, of tetragonal zirconia (JCPDS no. 170923). These diffraction lines are well preserved even when sodium species are introduced, revealing the stability of tetragonal zirconia in the mesoporous frameworks. As displayed in Figure 2B, the SAED pattern exhibits evident diffraction rings corresponding to (111), (200), (220), and (311) reflections, which verifies the presence of crystalline tetragonal zirconia in the mesoporous frameworks of NaMZ materials. The stabilization of the tetragonal phase at 600 °C is exciting, considering the usefulness of this crystalline phase but its instability at elevated temperatures.30 The crystalline phase transformation of unsupported ZrOCl2 3 8H2O was also investigated. As shown in Figure 4, intense diffraction lines of tetragonal zirconia can be observed for the material calcined at 400 °C. However, the monoclinic phase emerges at 500 °C and increases with increasing temperature. At the temperature of 700 °C, the monoclinic phase becomes predominant. This means that the formation of the monoclinic phase is unavoidable at temperatures higher than 500 °C for bulk zirconia. Nevertheless, only the tetragonal phase 11636

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Figure 5. Wide-angle XRD patterns of ZrOCl2 3 8H2O/SBA-15 calcined at (a) 400, (b) 500, (c) 600, and (d) 700 °C.

Figure 6. IR spectra of (a) SBA-15, (b) ZrO2/SBA-15, (c) MZ, (d) NaMZ-1, (e) NaMZ-2, and (f) NaMZ-3 materials.

appears for ZrOCl2 3 8H2O confined in the pores of SBA-15 (Figure 5). Moreover, the crystallite size gradually increases up to 600 °C and remains constant with a further increase of temperature. For the materials containing sodium species, a weak diffraction line at 33.9° is visible and increases gradually with increasing sodium content (Figure 1). The adsorption of atmospheric CO2 on strongly basic sites Na2O and subsequent formation of Na2CO3 (JCPDS no. 86-0312) should be responsible for such a diffraction line. The formation of carbonate provides evidence for the strong basicity of NaMZ materials and can be confirmed by IR results shown below. The wide-angle XRD patterns also show the synthetic process for NaMZ materials. Both SBA-15 and ZrO2/SBA-15 exhibit a broad diffraction peak centered at 22° corresponding to amorphous silica (Figure 5). Nonetheless, the diffraction peak disappears in NaMZ materials (Figure 1). This suggests that no bulk silica existed in the resulting materials even when a certain amount of silicon was determined by element analysis. Figure 6 depicts the IR spectra of various materials. The template SBA-15 shows bands at 1084, 800, and 460 cm
1, which correspond to Si
O
Si asymmetric stretching, Si
O
Si symmetric stretching, and O
Si
O bending vibrations. The

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Figure 7. CO2 TPD profiles of (a) MZ, (b) NaMZ-1, (c) NaMZ-2, and (d) NaMZ-3 materials.

stretching vibration of Si
OH (silanol groups) gives rise to the band at 966 cm
1. After the introduction of zirconia, all bands are well preserved except for that of silanol groups. The consumption of silanol groups by the interaction of the supported oxides and SBA-15 has been widely investigated. If the supported oxides (e.g., MgO and ZnO) show a strong interaction with the support, the consumption of silanol groups should take place and subsequently lead to the formation of Si
O
M linkages (M denotes a metal atom).35,36 Therefore, Si
O
Zr linkages should be produced according to the consumption of silanol groups. After the reaction with NaOH solution, all bands ascribed to silica disappear, which indicates the removal of template and the absence of any bulk silica surviving in the replicated materials, thus confirming the XRD results. The removal of the silica template also makes the Si
O
Zr vibration band emerge at 980 cm
1 in the MZ and NaMZ materials, whereas the bands at 600 and 510 cm
1 are derived from tetragonal zirconia.37,38 On the basis of abovementioned results, it can be tentatively concluded that the residual silicon exists in the form of Si
O
Zr linkages in MZ and NaMZ materials. Such a structure has a nice alkali-resistant ability and thus can survive in NaOH solution. For NaMZ materials, IR bands at 1571, 1467, 1368, 848, and 752 cm
1 are observable, which can be ascribed to the adsorption of atmospheric CO2 and H2O.39,40 This demonstrates the aforementioned XRD results, indicative of the formation of carbonate on NaMZ materials. Basicity and Catalytic Performance. As shown in Table 1, a base strength (H
) of only 9.3 was determined for the MZ material. However, all NaMZ materials exhibited base strengths as high as 27.0, which provides the first evidence for superbasicity according to Tanabe’s definition.41 To clarify whether the superbasicity of NaMZ materials results from some unusual oxides of sodium, the material NaMZ-2 was also activated in different atmospheres including H2, N2, or O2 prior to the measurement of base strength. Interestingly, all of the materials kept a high base strength of 27.0, which excludes the possibility that the superbasicity originates from some special oxides such as NamO (m > 2) or Na2On (n > 1). Otherwise, the base strength of the materials should be changed because the compounds NamO (m > 2) and Na2On (n > 1) would be destroyed during activation in oxidative and reductive atmospheres. Taking account of the results of titration, XRF, IR, and XRD measurements, the Na2O generated by high-temperature activation should be the main basic species on NaMZ materials, even though alkaline metal 11637

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The Journal of Physical Chemistry C oxides are difficult to detect, as reported previously.42,43 Figure 7 shows the CO2 TPD profiles of MZ and NaMZ materials. The MZ material exhibits a major desorption peak at about 180 °C, with a minor one at around 450 °C. After sodium has been introduced, the intensity of both desorption peaks increases, and a new peak at a high temperature of 700 °C appears. This means that both the amount and the strength of the basic sites are enhanced. The three desorption peaks centered at about 180, 450, and 700 °C can be tentatively ascribed to weakly, strongly, and unusually strongly basic sites, respectively. The CO2 desorption at 700 °C verifies the generation of superbasic sites in NaMZ materials. Moreover, the amounts of CO2 desorbed at 450 and 700 °C increase with increasing sodium content, indicating that the amount of strongly basic sites increases in the order of NaMZ-1 < NaMZ-2 < NaMZ-3. Quantitative analysis of the basic sites was also conducted. As shown in Table 1, the amount of basic sites was 2.0 mmol g
1 for sample MZ, which is in accordance with the theoretical value (2.1 mmol g
1) calculated from the sodium content. This indicates that the residual sodium species in sample MZ was soluble in acidic solution, even though it survived the process of water washing. The amount of basic sites in the NaMZ samples varied from 5.4 to 10.2 mmol g
1, which is in consistent with the theoretical values. These results thus provide further evidence of the successful introduction of sodium species; moreover, these sodium species act as basic sites and are catalytically active, as described below. Our materials were also used to catalyze the synthesis of DMC from the transesterification of ethylene carbonate and methanol.44 DMC is a versatile green chemical and has been proposed as a methylating agent in place of methyl halides and dimethyl sulfate, which are both toxic and corrosive.45,46 DMC is also used as a carbonylating agent, a polar solvent, an intermediate for polycarbonates, and a fuel additive. As listed in Table 1, only a small amount of DMC (2.1%) was produced over MZ. The DMC yield sharply increased to 28.2% upon catalysis with NaMZ-1. With increasing sodium content, the DMC yield continued rising and reached 75.4% with 100% selectivity over NaMZ-3. The activities of different materials correlate well with the amounts of strongly basic sites, as described above. This means that strongly basic sites are crucial to the synthesis of DMC. A reference material (Na/ ZrO2
SiO2) having the same composition as NaMZ-2 was also prepared. This reference material exhibited a much lower activity than NaMZ-2. The formation rate of DMC related to surface area was also calculated and is included in Table 1. Among the NaMZ materials, NaMZ-3 had the highest DMC formation rate. Also, the reference material (Na/ZrO2
SiO2) exhibited a high DMC formation rate because of its low surface area. Surprisingly, the activity of NaMZ-3 was found to be superior even to that of the conventional homogeneous catalyst CH3ONa. Further calculation showed that NaMZ materials exhibited large turnover frequency (TOF) values. The TOF value of NaMZ-2 reached 95.0 h
1, which is much higher than that of CH3ONa (45.8 h
1). This indicates the higher catalytic efficiency of sodium in NaMZ materials as compared with CH3ONa. In addition to the homogeneous catalyst CH3ONa, various typical solid base catalysts were also employed as a comparison. The catalytic activity of the conventional, well-known solid base MgO was examined. Under the same reaction conditions, the yield of DMC was only 2.7%, which is obviously lower than that of our NaMZ-2 material (66.5%). Two zeolites with strong basicity, namely, CsX and KL, showed DMC yields of 4.5% and 2.5%, respectively. The solid superbasic material sodium-modified alumina with the same

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sodium content as NaMZ-2 exhibited a yield of only 22.5% and is apparently inferior to our materials. The catalytic performance of the NaMZ materials implies that they are promising candidates for DMC synthesis in heterogeneous systems. Comparison of Catalyst Stability and Activity. Tetragonal zirconia is widely used as a catalyst and catalyst support for various applications. The existence of vacant sites on the surface is crucial to the dispersion of active species.47 By the insertion of potassium ions into the vacant sites, potassium species can be dispersed on the surface of tetragonal zirconia, which leads to the generation of superbasicity.22,23 However, tetragonal zirconia is a metastable phase and tends to transform to the monoclinic phase at elevated temperatures. Much work has thus focused on stabilizing tetragonal zirconia. In the present study, stable tetragonal zirconia was successfully obtained, and two factors should be taken into consideration. The first factor is the confined space of mesopores in the silica template. The critical size for tetragonal phase stabilization in nanocrystalline zirconia is reported to be about 6 nm.48 Further increase of crystallite size will result in the generation of the monoclinic phase. The template SBA-15 has a pore diameter of 8 nm, which might provide a confined space hindering the formation of large crystallites. In other words, the resultant zirconia particles should be less than 8 nm. Calculation using Scherrer’s equation implies a crystallite size of about 3.9 nm for MZ and NaMZ materials, thus confirming the effect of a confined space on the stabilization of tetragonal zirconia. The second factor is the formation of Si
O
Zr linkages. The substitution of zirconium with impurities, such as yttrium, magnesium, and calcium, is reported to be effective in stabilizing tetragonal zirconia.49 The impurities induce decreased kinetics of zirconia crystallite growth, which corresponds to an increase of the transformation temperature from the tetragonal to the monoclinic phase. The Si
O
Zr linkages can be viewed as chemical impurities that are able to stabilize the tetragonal phase, similarly to what happens in well-mixed zirconia
silica oxides.50 Increased stability of tetragonal zirconia upon the introduction of silica has also been reported by other groups. By use of a sol
gel method, mesoporous mixed oxides SiO2
ZrO2 with different silica contents were prepared.51 When the Si/Zr weight ratio was higher than 6.61, no crystalline phase was formed after calcination at 500 °C. Upon a decrease of the Si/Zr ratio to 2.48 and 0.41, zirconia with tetragonal crystallites appeared. Tetragonal zirconia was stable at 500 °C, but no results at higher temperatures were reported. Seo et al.52 synthesized a series of SiO2
ZrO2 supports by grafting a zirconium precursor onto silica. For the samples with Si/Zr weight ratios higher than 0.57, amorphous or tetragonal zirconia was formed. However, some monoclinic zirconia emerged for the sample with low silica content (Si/Zr = 0.23) at 700 °C. The use of basic reflux to stabilize zirconia was investigated by Nahas et al.53 Their results showed that an amount of monoclinic zirconia was formed for a sample with a Si/Zr weight ratio of 0.06 even at 500 °C. These results indicate that the stability of tetragonal zirconia is dependent on the preparation method, as well as the content of silica. As a result, the confined space and the formation of Si
O
Zr linkages should be responsible for the stability of mesoporous tetragonal zirconia frameworks. The formation of Si
O
Zr linkages is also essential to the generation of strong basicity because of their good alkali resistance. For comparison, a reference material having the same composition as NaMZ-2 (Na/ZrO2
SiO2) was prepared by impregnating NaOH into the mixture of silica and zirconia. Because no Si
O
Zr linkages formed, a reaction between 11638

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The Journal of Physical Chemistry C strongly basic species and bulk silica occurred, leading to a small surface area of 5 m2 g
1 for the reference material (Table 1). Moreover, the reference material had a base strength of 15.0, which is much lower than that of NaMZ-2 (27.0). The yield of DMC was only 12.8%, and such catalytic performance is obviously worse than that of NaMZ-2 (66.5%) under the same reaction conditions. These results thus provide evidence of the importance of Si
O
Zr linkages in generating superbasicity.

’ CONCLUSIONS Sodium-modified mesoporous zirconia materials were fabriacted by use of an in situ functionalization strategy in a hard template system. Such materials have well-defined mesostructures with narrow pore size distributions. The mesoporous frameworks exhibit well-crystallized tetragonal zirconia that is quite stable even at the high temperatue of 700 °C. The base strength of the NaMZ materials can reach as high as 27.0. The materials exhibit excellent basic catalytic performance. The TOF value of the material NaMZ2 was found to be 95.0 h
1 with 100% selectivity to the target product DMC, which is much higher than the value for the extensively used homogeneous catalyst CH3ONa (45.8 h
1). The success of our strategy is due to the double role of the silica template and the flexible utilization of basic solution. In addition to its function as a template, mesoporous silica also presents a silicon source to react with zirconium to form Si
O
Zr linkages, which are of great significance in the stabilization of tetragonal zirconia. The bulk silica is removed by NaOH solution and forms pores in mesoporous zirconia, whereas the Si
O
Zr linkages can survive. Moreover, the NaOH solution also acts as a guest and is coated onto the newly formed mesopores. Hence, the fabrication and functionalization of mesoporous zirconia can be realized in one step, which avoids the possible destruction of nonsiliceous mesoporous frameworks in post-treatment and saves time and energy. Our strategy might open up an avenue for the design and synthesis of new functional materials. ’ ASSOCIATED CONTENT

bS

Supporting Information. TEM image of mesoporous silica SBA-15. This information is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ86-25-83587177. Fax: þ86-25-83587191. E-mail: lbsun@ njut.edu.cn (L.-B.S.), [email protected] (X.-Q.L.).

’ ACKNOWLEDGMENT The National Science Foundation of China (Nos. 21006048 and 20976082), the specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20093221120001), and the Natural Science Foundation of Jiangsu Province Colleges (Nos. 09KJB530004 and 08KJA530001) are acknowledged for their financial support of this research. ’ REFERENCES (1) Ono, Y. J. Catal. 2003, 216, 406. (2) Calvino-Casilda, V.; Perez-Mayoral, E.; Martin-Aranda, R. M.; Zienkiewicz, Z.; Sobczak, I.; Ziolek, M. Top. Catal. 2010, 53, 179.

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