Modulating the Host Nature by Coating Alumina: A Strategy to

Oct 20, 2010 - before modification with the base precursor, potassium nitrate. ... The majority of the potassium nitrate is decomposed at 690 °C on u...
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J. Phys. Chem. C 2010, 114, 18988–18995

Modulating the Host Nature by Coating Alumina: A Strategy to Promote Potassium Nitrate Decomposition and Superbasicity Generation on Mesoporous Silica SBA-15 Yuan-He Sun, Lin-Bing 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 ReceiVed: July 25, 2010; ReVised Manuscript ReceiVed: September 29, 2010

A new strategy was utilized to generate strong basicity on mesoporous silica SBA-15 by precoating alumina before modification with the base precursor, potassium nitrate. The nature of the mesoporous silica host was greatly modulated by the alumina interlayer. Such an alumina interlayer plays a double role by enhancing the guest-host interaction to promote the decomposition of potassium nitrate and by improving the alkali-resistance of the mesoporous silica host. The majority of the potassium nitrate is decomposed at 690 °C on unmodified SBA-15, while the temperature decreases to 460 °C after precoating an alumina layer. Moreover, the ordered mesoporous structure of the parent, SBA-15, is well-preserved even if the supported potassium nitrate was decomposed to strongly basic potassium oxide, which is quite different from the complete destruction of the mesostructure in the absence of alumina. As a result, materials possessing both a mesoporous structure and superbasicity with a high strength of 27.0 were successfully fabricated. We also demonstrated that both the amount and the location of aluminum in the samples are of great importance for the generation of strong basicity. The content of alumina should reach as high as 20 wt %, which is necessary for the formation of an intact interlayer; thus, the silica frameworks can be well-protected after introducing strongly basic species. Also, the location of aluminum on the pores rather than in the frameworks is demanded from the point of view of mesostructure protection. Introduction Solid superbasic materials are extremely desirable for applications in environmentally benign and economical catalytic processes because they can catalyze various reactions under mild conditions and reduce waste production.1-3 Among various porous hosts used for the preparation of solid superbases, mesoporous materials attract extensive attention. Such materials possess high surface areas and large pore openings, which can reduce mass transfer limitations and allow bulky reactant molecules to enter the pores. Since the discovery of mesoporous silica M41S, a series of ordered mesoporous materials have been synthesized using the surfactant templating method,4,5 which is of great interest for adsorption, sensing, and catalysis.6-10 In contrast to other candidates with a mesostructure, mesoporous silicas are easier to synthesize and have better stability. An incredible degree of control has been achieved on silica with various pore symmetries, such as hexagonal, cubic, and lamellar.11-15 Therefore, a variety of attempts have been made to generate basic sites on mesoporous silicas up to now. By treating mesoporous silicas, such as MCM-41 and SBA15, in the presence of ammonia, the oxygen in their frameworks was partially displaced by nitrogen. The oxynitride frameworks were thus created, which afforded new kinds of solid bases with an ordered mesostructure.16-18 However, high treatment temperatures (>900 °C) had to be employed in the process, and the strength of basic sites still needed further improvement. Grafting organic bases onto silanol groups provided an interesting approach to generate basic sites on mesoporous silicas, but the base strength of these organic-inorganic hybrid materials was * To whom correspondence should be addressed. Phone: +86-2583587177. Fax: +86-25-83587191. E-mail: [email protected] (L.-B.S.), [email protected] (X.-Q.L.).

relatively weak.19-21 Additionally, they can only be used at temperatures lower than 170 °C because of the degradation of organic molecules at elevated temperatures.22 To improve the base strength, alkaline metal oxides, which are strongly basic, were introduced to modify mesoporous silicas. Mesoporous solid strong bases can be prepared by impregnation of MCM-41 with cesium acetate solution and subsequent calcination.23 Nevertheless, the obtained bases showed poor stability because cesium oxide can react with the silica host and damage the mesoporous frameworks.24 A neutral salt, potassium nitrate, has been widely used as the guest to generate strong basicity on various porous hosts, such as alumina, zirconia, and zeolites.25-29 Aiming at forming strong basicity on mesoporous silicas, SBA-15 was introduced as the host to disperse potassium nitrate. Unfortunately, the obtained material exhibited weak basicity, and the mesostructure of SBA-15 was destroyed completely in the process of activation to decompose potassium nitrate.30 Hence, generation of strong basicity on mesoporous silicas is still an open question up until now. Two main factors are considered to hinder the generation of strong basicity on mesoporous silicas. The first factor is the weak host-guest interaction between silica and the base precursor (for example, potassium nitrate), which leads to the difficulty in the decomposition of the base precursor to strongly basic species (for example, potassium oxide). As reported previously, only a small amount of potassium nitrate can be decomposed on silica even if the sample was activated at the high temperature of 600 °C.31 The second factor is the poor alkali resistance of mesoporous silicas, which results in the collapse of the mesoporous structure after the formation of strongly basic species.32 Aiming at generating strong basicity on mesoporous silicas, both of these two shortcomings must be overcome. In the present study, we designed a new strategy

10.1021/jp106939d  2010 American Chemical Society Published on Web 10/20/2010

Modulating the Host Nature by Coating Alumina to modulate the nature of mesoporous silica SBA-15 by precoating an alumina layer prior to potassium modification. By use of such a strategy, the enhancement of the host-guest interaction and the improvement of the host alkali resistance can be realized simultaneously. We demonstrated that the decomposition temperature of the base precursor, potassium nitrate, could be obviously lowered, and superbasicity was successfully created on mesoporous silica SBA-15. The structural and basic properties of the obtained materials were well characterized by various approaches. The effect of the amount and location of aluminum on basicity generation was clarified. On the basis of the experimental results, the possible mechanism was also proposed. Experimental Section Materials Synthesis. Mesoporous silica SBA-15 was synthesized according to the reported method as follows.33 Two grams of triblock copolymer P123 (EO20PO70EO20) was dissolved in 75 g of 1.6 M HCl aqueous solution with stirring at 40 °C. A 4.25 g portion of tetraethylorthosilicate (TEOS) was then added to the homogeneous solution and stirred at this temperature for 24 h. Finally, the temperature was heated to 100 °C and held at this temperature for 24 h under a static condition. The as-prepared sample was recovered by filtration, washed with water, and air-dried at room temperature. The removal of the template was carried out at 550 °C in air for 5 h. Alumina was introduced to SBA-15 by two approaches, namely, direct and postsynthesis. Wet impregnation was used to prepare the postsynthetic samples. A required amount of Al(NO3)3 · 9H2O was dissolved in deionized water, followed by addition of SBA-15. After stirring at room temperature for 24 h, the mixture was evaporated at 80 °C, dried at 100 °C for 4 h, and calcined at 550 °C for 5 h in air. The resulting samples, alumina-coated SBA-15, were denoted as AS(p)-n, where n is the mass percentage of alumina. The direct synthetic process was similar to that for SBA-15 except that a certain amount of Al(NO3)3 · 9H2O was added to the synthetic solution before the introduction of TEOS. Additionally, evaporation was used to replace the procedure of filtering to make the aluminum species remain in the resultant sample. The obtained sample was denoted as AS(d)-n, where n is the mass percentage of alumina. The base precursor, potassium nitrate, was introduced by wet impregnation. An identical amount of potassium nitrate (namely, 30 wt %) was used for all samples. Typically, 0.3 g of potassium nitrate was dissolved in 12 mL of deionized water, followed by addition of 0.7 g of host. After stirring at room temperature for 24 h, the mixture was evaporated at 80 °C and subsequently dried at 100 °C for 4 h. The obtained solid was calcined at 600 °C for 2 h to decompose potassium nitrate in a N2 flow. The final samples were denoted as KS, KAS(p)-n, or KAS(d)-n for potassium species supported on SBA-15, AS(p)-n, or AS(d)-n, respectively, where n is the mass percentage of alumina. The elemental analysis by inductively coupled plasma-atomic emission spectrometry (ICP-AES) shows that AS(p)-5, AS(p)-10, AS(p)-20, AS(p)-30, and AS(d)-20 samples contain 4.9, 10.4, 20.2, 29.9, and 20.0 wt % of alumina, respectively, which is in good agreement with the theoretical values (i.e., 5, 10, 20, 30, and 20 wt %, respectively). This means that aluminum is well recovered in both post- and direct synthetic processes. 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 0.7 to 8° and 5 to 80° at 40 kV and 40 mA. The N2 adsorption-desorption

J. Phys. Chem. C, Vol. 114, No. 44, 2010 18989 isotherms were measured using a Belsorp II system at -196 °C. The samples were degassed at 300 °C for 4 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 by using the Barrett-Joyner-Halenda (BJH) method. Transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) analysis were performed on a JEM-2010 UHR electron microscope operated at 200 kV. Fourier transform infrared (IR) measurements were performed on a Nicolet Nexus 470 spectrometer by means of the KBr pellet technique. The elemental contents of the samples were measured by ICP-AES (Optima 2000DV, PerkinElmer). The samples were dissolved with HF and diluted to a prescribed concentration. The spectra were collected with a 2 cm-1 resolution. Solid-state 27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were recorded at 9.37 T on a Bruker Avance 400D spectrometer at a frequency of 104.17 MHz using a 4 mm zirconia rotor. The spin rate of the sample was 9.0 kHz, and a 4.5 µs pulse width was used with a pulse delay of 1.5 s. AlCl3 · 6H2O was used as the reference for chemical shifts. Thermogravimetry (TG) analysis was conducted on a thermobalance (STA-499C, NETZSCH). About 10 mg of sample was heated from 35 to 900 °C in a N2 flow (25 mL · min-1). Temperature-programmed desorption (TPD) of CO2 experiments were conducted on a BELSORP BEL-CAT-A apparatus. The sample was pretreated at 200 °C for 2 h prior to the adsorption of CO2 at 50 °C. After the physically adsorbed CO2 was purged by a He flow at 50 °C, the sample was heated to 800 °C at the rate of 8 °C · min-1, and the CO2 liberated was detected by an online OmniStar mass spectrometer. The base strength of materials was detected by using a series of Hammett indicators, for which about 100 mg of sample was activated in a N2 flow with 30 mL · min-1 at 600 °C for 2 h before the test. The indicators employed were phenolphthalein (H- ) 9.3), 2,4dinitroaniline (H- ) 15.0), 4-nitroaniline (H- ) 18.4), benzidine (H- ) 22.5), 4-chloroaniline (H- ) 26.5), and aniline (H- ) 27.0). To measure the amount of basic sites, 50 mg of sample was shaken in 10 mL of aqueous HCl (0.05 M) for 24 h and the slurry was separated by a centrifuge. The remaining acid in the liquid phase was titrated with aqueous NaOH (0.01 M). Results Coating Alumina on Mesoporous Silica SBA-15. Figure S1A (Supporting Information) shows the low-angle XRD patterns of SBA-15 and alumina-coated SBA-15 samples. The (100), (110), and (200) reflections indicate that the ordered mesostructure of SBA-15 is well-preserved, despite that alumina has been introduced by either direct or postsynthetic methods. In general, the full width at half-maximum (fwhm) of the (100) reflection should increase with the introduction of metal oxides because of the decreasing scatter contrast between pore walls and pore space. However, the fwhm of the (100) reflection of AS(p)-5 and AS(p)-10 samples (0.089 and 0.095°, respectively) is apparently smaller than that of the parent, SBA-15 (0.099°). The fwhm decrease can be ascribed to the formation of a smooth alumina layer on the internal walls of SBA-15, similar to what happened in magnesia- or yttria-modified SBA-15.34,35 For the sample derived from direct synthesis, however, the fwhm (0.097°) is smaller than that of SBA-15, even if the content of alumina reaches 20 wt %. This is evidently different from the sample originated from postsynthesis. Thus, the salt effect of

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Figure 1. IR spectra of (a) SBA-15, (b) AS(p)-5, (c) AS(p)-10, (d) AS(p)-20, (e) AS(p)-30, and (f) AS(d)-20 samples.

the precursor during the synthesis of the AS(d)-20 sample should also play an important role in the fwhm decrease besides the formation of a smooth layer.36,37 Only a broad peak with 2θ at 22° assigned to amorphous silica is detected in the wide-angle XRD pattern of SBA-15 (Figure S1B, Supporting Information). After the introduction of alumina, no new diffraction peak appears, indicating that alumina is well dispersed on SBA-15 by either direct or postsynthetic methods. As depicted in Figure S2A (Supporting Information), the isotherms of AS(p)-5 and AS(p)-10 samples are of type IV with an H1 hysteresis loop, quite similar to that of the parent, SBA15. The pore diameter of AS(p)-5 and AS(p)-10 (8.0 nm) is identical to that of SBA-15 (Figure S2B, Supporting Information). These results confirm the low-angle XRD patterns, indicating that alumina forms a smooth layer and results in lessblocked mesopores. Further increasing the alumina content to 20-30 wt %, the onset of inflections gradually shifts toward lower p/p0 values due to the formation of alumina nanoparticles. Correspondingly, the pore diameter of AS(p)-20 and AS(p)-30 samples declined to 7.1 and 6.2 nm, respectively. For the direct synthetic sample AS(d)-20, the shape of the isotherm is quite similar to that of SBA-15, which is consistent with low-angle XRD results. Figure 1 shows the IR spectra of different samples. An obvious band at 960 cm-1 ascribed to the bending vibration of silanols (Si-OH) is observed.38,39 The introduction of 5 wt % of alumina leads to a sharp decrease of the band. Moreover, the IR band of silanols declines gradually with the increase of the alumina content. Such a band becomes invisible in AS(p)20 and AS(p)-30 samples, indicating that the silanol groups are completely suppressed by coating alumina. These results imply the strong interaction between alumina and SBA-15, which leads to the consumption of silanol groups and subsequently forms a smooth layer on the surface of SBA-15.35,38 Interestingly, for the sample AS(d)-20 derived from direct synthesis, a considerable amount of silanol groups is preserved. It is inferred that only alumina located on the surface can consume silanol groups, whereas those in the frameworks are not probable to interact with surface silanol groups. As a result, a number of aluminum species should enter the frameworks of SBA-15 for the direct synthetic sample; however, most of the aluminum species may locate on the pores and form a smooth layer in the case of postsynthetic samples. To further clarify the local environment of aluminum, solidstate 27Al NMR spectra were recorded and are shown in Figure 2. Three distinct signals assigned to octahedral, pentahedral, and tetrahedral aluminum atoms are detected. It is agreed that

Sun et al.

Figure 2. Solid-state 27Al NMR spectra of (a) AS(p)-20 and (b) AS(d)20 samples.

Figure 3. Low-angle XRD patterns of (a) KS, (b) KAS(p)-5, (c) KAS(p)-10, (d) KAS(p)-20, (e) KAS(p)-30, and (f) KAS(d)-20 samples.

tetrahedral coordination indicates that aluminum is incorporated into the siliceous frameworks, whereas octahedral coordination corresponds to extraframework aluminum species.40,41 Pentahedral coordination originates from aluminum located at the interface between tetrahedral aluminosilicate frameworks and octahedral alumina phases.42 The presence of pentahedral aluminum suggests that the local arrangement of aluminum atoms is different from that in bulk γ-alumina, which excludes the formation of a large amount of a separate alumina phase.43 For the sample AS(d)-20, the framework aluminum species are predominant (45% from deconvolution analysis). In the case of AS(p)-20, however, aluminum located in the frameworks is only 17%, and extraframework aluminum species are principal. These results thus give further evidence of the different local environments of aluminum for the samples derived from direct and postsynthesis, being consistent with the above IR data. Structural Characterization of Materials Functionalized with Potassium Species. To obtain mesoporous solid bases, potassium nitrate was introduced to SBA-15 precoated with alumina. A strongly basic species, potassium oxide, would be generated from the decomposition of potassium nitrate during activation. Structural properties of the resulting materials were characterized by various methods. Figure 3 shows the low-angle XRD patterns of potassium-functionalized samples. In the absence of alumina, the mesoporous structure of pure SBA-15 is entirely destroyed by strongly basic potassium oxide formed in the process of activation. Likewise, introducing 5 and 10 wt % of alumina still fails to protect the siliceous host, and the mesostructure of SBA-15 rarely remained. When the amount of alumina rises to higher than 20 wt %, the main peak of the

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Figure 4. (A) N2 adsorption-desorption isotherms and (B) pore size distributions of (a) KS, (b) KAS(p)-5, (c) KAS(p)-10, (d) KAS(p)-20, (e) KAS(p)-30, and (f) KAS(d)-20 samples. Curves are offset for clarity.

hexagonal lattice emerges in low-angle XRD patterns. Apparently, the existence of an alumina interlayer on SBA-15 avoids the contact of strongly basic potassium species with siliceous frameworks, preventing the mesostructure from corroding. However, introducing 20 wt % of alumina by direct synthesis fails to protect the mesostructure of SBA-15. These results reflect that the effect of alumina on the protection of the host structure is closely related to both the amount and the location of alumina. Figure 4A,B illustrates N2 adsorption-desorption isotherms and corresponding pore size distributions of different samples. No hysteresis loop is detected in the isotherm of the KS sample, and the adsorption amount of N2 is negligible. This means that the mesostructure of SBA-15 is completely destroyed after modification with potassium species. Coating 5-10 wt % of alumina results in a slight increase of N2 adsorption, but the destruction of the majority of mesostructure is still unavoidable. An obvious hysteresis loop is observed on the isotherm of the KAS(p)-20 sample, indicating the preservation of the mesoporous structure after precoating 20 wt % of alumina. The pore diameter of the KAS(p)-20 sample is 6.2 nm, as shown in Figure 4B. This value is somewhat smaller than the pore diameter of the AS(p)-20 sample (7.1 nm), which demonstrates the location of potassium species in the mesopores of host. It can also be found that the mesostructure of the KAS(d)-20 sample is destroyed from the results of N2 adsorption. Both low-angle XRD patterns and N2 adsorption results exhibit that direct modification with strongly basic potassium species leads to the complete destruction of the mesostructure of SBA-15. Coating alumina higher than 20 wt % via the postsynthetic method can prevent the host structure from corroding. However, the introduction of alumina with the same amount (20 wt %) via the direct synthetic method has no effect on the protection of the host structure. TEM provides another important technique to characterize the periodic ordering of the mesostructure. As displayed in Figure 5A,B, an ordered mesoporous structure can be observed for the KAS(p)-20 sample, being in good agreement with the results of XRD and N2 adsorption. EDX analysis was also employed to estimate the local elemental composition of the sample. As given in Figure 5C, potassium, aluminum, and silicon signals can be clearly identified, giving the evidence that potassium and aluminum are successfully introduced to the silica host. Although several random areas were selected for EDX analysis, the resulting K/Si and Al/Si atomic ratios were similar, reflecting a homogeneous distribution of potassium and aluminum species in the silica host.

Figure 5. (A, B) TEM images and (C) EDX spectrum of the sample KAS(p)-20.

Figure 6. Wide-angle XRD patterns of (a) KS, (b) KAS(p)-5, (c) KAS(p)-10, (d) KAS(p)-20, (e) KAS(p)-30, and (f) KAS(d)-20 samples (A) before and (B) after activation. O and H denote potassium nitrate of the orthorhombic and hexagonal phases, respectively.

Decomposition of Supported Potassium Nitrate and the Resulting Basicity. Figure 6A shows the wide-angle XRD patterns of potassium-containing samples before activation. The characteristic diffraction lines of orthorhombic phase potassium nitrate (JCPDS No. 74-2055) are detected on KAS(p) samples, which is the same as potassium nitrate dispersed on pure SBA15. Interestingly, hexagonal phase potassium nitrate (JCPDS No. 76-1693) is predominant on the KAS(d)-20 sample, which is quite different from KS and KAS(p) samples. After activation, the diffraction lines of potassium nitrate in the KS sample are still evident, indicating that the decomposition of potassium nitrate on mesoporous silica is quite difficult even at the high temperature of 600 °C. Similar results were also reported previously on potassium nitrate modified bulk silica.31 Coating 5 wt % of alumina leads to a sharp decrease of crystalline potassium nitrate. When the content of alumina is higher than 10 wt %, all of the supported potassium nitrate can be

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Figure 7. IR spectra of KS (a) before and (b) after activation, KAS(p)20 (c) before and (d) after activation, and KAS(d)-20 (e) before and (f) after activation.

decomposed for the samples derived from both direct and postsynthesis. Apparently, the alumina layer modulates the surface nature of SBA-15 and subsequently promotes the decomposition of supported potassium nitrate. IR spectra of KS, KAS(p)-20, and KAS(d)-20 samples before and after activation at 600 °C were recorded to examine the decomposition of potassium nitrate. As shown in Figure 7, all samples before activation exhibit an obvious vibration band at 1384 cm-1 assigned to nitrate.44,45 Activation at 600 °C only slightly reduces the intensity of the 1384 cm-1 band in the KS sample, mirroring the existence of residual potassium nitrate in the sample. In the case of KAS(p)-20 and KAS(d)-20 samples, however, such a band of nitrate disappears after activation under the same conditions, indicating the complete decomposition of potassium nitrate. On the basis of IR results, the effect of the alumina layer on the promotion of potassium nitrate decomposition is confirmed. The elemental composition of samples before and after the decomposition of potassium nitrate was also monitored. The content of K and Al is 10.9 and 7.0 wt % respectively in KAS(p)-20 before activation, which is in good agreement with the theoretical value (K, 11.6 wt %; Al, 7.4 wt %). This means that no loss of K and Al takes place in the synthetic process. It is worth noting that the contents of K and Al simultaneously increase to 14.1 and 9.0 wt %, respectively, after activation, being consistent with the theoretical values (K, 13.8 wt %; Al, 8.8 wt %). Such an increase is reasonable, taking into account that the decomposition of potassium nitrate to potassium oxide leads to the decrease of the total weight. The constant K/Al ratio (1.57) confirms that aluminum is well preserved in the sample after activation. To gain a deep insight into the decomposition of potassium nitrate on alumina-coated SBA-15, the TG technique was employed. Figure 8A,B displays TG and DTG curves of KS, KAS(p)-20, and KAS(d)-20 samples before activation. All of the samples show a weight loss centered at around 100 °C assigned to the desorption of physically adsorbed water. The weight loss at this stage for KAS(p)-20 and KAS(d)-20 is obviously larger than that for KS, suggesting that alumina in the samples promotes the adsorption of water. The subsequent gradual weight loss up to 760 °C for the sample without alumina and up to 650 °C for aluminum-containing samples can be ascribed to the decomposition of potassium nitrate. The weight loss is calculated to be 17.4, 16.8, and 16.7% for KS, KAS(p)20, and KAS(d)-20 samples respectively, which is consistent with the theoretical value for the conversion of potassium nitrate to potassium oxide (16%). For potassium nitrate supported on

Sun et al.

Figure 8. (A) TG and (B) DTG curves of (a) KS, (b) KAS(p)-20, and (c) KAS(d)-20 samples before activation. Curves in panel B are offset for clarity.

SBA-15, the decomposition can be tentatively assigned to three stages centered at about 320, 520, and 690 °C. It is worth noting that potassium nitrate supported on alumina-coated SBA-15 decomposes via two stages centered at around 300 and 460 °C, which are obviously lower than that on pure SBA-15. Therefore, it is clear that the introduction of alumina to SBA-15 promotes the decomposition of potassium nitrate at much low temperatures. After the decomposition of supported potassium nitrate to potassium oxide, strongly basic materials are supposed to be produced. The basic properties of the materials were first characterized by measurement of the strength and amount of basic sites. The parent, SBA-15, exhibits a base strength (H-) of less than 9.3. Coating alumina on SBA-15 does not affect such a base strength. After introducing potassium species, the resulting materials show quite different base strengths. For samples KS, KAS(p)-5, and KAS(p)-10, the base strength of 9.3 is detected, indicating that no strongly basic sites are formed. It is noticeable that the basic sites with a high strength of 27.0 emerge on samples KAS(p)-20 and KAS(p)-30. According to the definition of Tanabe and Noyori,46 a solid material that has basic sites with a strength higher than 26.5 can be regarded as a solid superbase. This is the first evidence of the generation of superbasicity on potassium-functionalized SBA-15 precoated with alumina. Nevertheless, only a base strength of 9.3 is detected on the sample KAS(d)-20, in which alumina is introduced by direct synthesis. It should be stated that the content of extraframework aluminum is crucial to the generation of strong basicity. According to previous study, strongly basic species prefer to react with silica in materials containing both silicon and aluminum.47 As a result, the amount of extraframework aluminum should be large enough to form an intact interlayer between basic species and silica. That is, the alumina layer should cover the surface of silica; otherwise, the reaction of strongly basic species with silica would take place, leading to the yield of materials with weak basicity and a collapsed mesostructure. Although the sample KAS(d)-20 contains some extraframework aluminum, such an amount of aluminum is not sufficient to form an intact layer to protect the silica host, similar to what happened in KAS(p)-5 and KAS(p)-10 samples. Therefore, their mesostructure is destroyed, and only some weakly basic sites are generated. Further determination shows that the amount of basic sites on the parent SBA-15 is 0 mmol · g-1. Coating alumina leads to a slight increase of the amount of basic sites due to the amphoteric property of alumina.31 For example, the amount of basic sites for the sample AS(p)-20 is 0.19 mmol · g-1. For the sample KAS(p)-20, the amount of basic sites reaches 3.04 mmol · g-1, being in line with the theoretical value (2.99 mmol · g-1) that all supported

Modulating the Host Nature by Coating Alumina

Figure 9. CO2-TPD profile of (a) KAS(p)-20 and (b) KAS(d)-20 samples.

potassium nitrate is converted to potassium oxide. The basic properties of potassium-functionalized SBA-15 precoated with alumina were further examined by CO2-TPD. As presented in Figure 9, a main desorption peak of CO2 at 160 °C with a shoulder at 290 °C are observed in the sample KAS(d)-20, and no peaks emerge at higher temperatures. This indicates that no strongly basic sites exist in KAS(d)-20, being consistent with the results of base strength. For the sample KAS(p)-20, however, three peaks of CO2 desorption at 184, 316, and 743 °C are observable. The desorption of CO2 at the high temperature of 743 °C demonstrates the existence of unusually strongly basic sites, which confirms the results of base strength, pointing out the successful generation of superbasicity. Discussion The introduction of basic species to porous hosts provides an uncomplicated approach to prepare solid strongly basic or even superbasic materials. However, two factors obstruct the generation of strong basicity on mesoporous silicas through direct modification with strongly basic species, according to our investigations. First, the decomposition of the base precursor on mesoporous silica is quite difficult due to the weak host-guest interaction. Only a small amount of potassium nitrate is decomposed even at the high temperature of 600 °C, as presented in wide-angle XRD patterns (Figure 6) and IR spectra (Figure 7). When the temperature is further increased to 690 °C, the majority of potassium nitrate can be converted to potassium oxide (TG results, Figure 8). Nonetheless, the reaction between strongly basic species with silica will be accelerated at such a high temperature, which is unfavorable to the preservation of the mesostructure, as discussed below. Second, the alkali-resistant ability of silica is poor, which is different from well-known hosts for solid strong bases, such as alumina and zirconia.31,48 Mesoporous frameworks would be destroyed completely owing to the reaction of strongly basic species with silica, even if only a small number of basic species (for example, potassium oxide from potassium nitrate decomposition) were produced (Scheme 1). Therefore, direct modification of mesoporous silica with strongly basic species to prepare solid bases seems unfeasible. Aiming at generating strong basicity on mesoporous silica, one should overcome both of the aforementioned drawbacks. In the present study, we designed a strategy to modulate the nature of mesoporous silica by precoating a layer of alumina before potassium nitrate modification. The alumina layer plays a double role by enhancing the host-guest interaction to promote the decomposition of the base precursor and by

J. Phys. Chem. C, Vol. 114, No. 44, 2010 18993 preventing the host frameworks from corroding by the formed strongly basic species. The decomposition temperature of potassium nitrate was lowered noticeably by precoating alumina, and all of the supported potassium nitrate can be converted to potassium oxide after activation, as can be seen from wideangle XRD patterns, IR spectra, and TG analysis (Figures 6, 7, and 8). More important, the mesoporous structure of the resultant materials is well-preserved (low-angle XRD, N2 adsorption, and TEM results in Figures 3, 4, and 5) because the alumina interlayer avoids the direct contact of strongly basic species with the silica host. As a result, the materials possessing both an ordered mesostructure and superbasicity with the strength as high as 27.0 were successfully prepared. It is interesting to state that both the amount and the location of alumina are essential to the preparation of mesoporous solid bases. When the content of alumina is lower than 20 wt %, it is impossible to form an intact layer on the surface of SBA-15. That means that there are some exposed silica frameworks that are not covered by alumina. Hence, the reaction of strongly basic species with silica is still unavoidable, leading to the collapse of the mesoporous structure. When the content of alumina is higher than 20 wt %, an intact interlayer between basic species and silica is formed. The mesoporous structure of the host can thus be well-preserved. Aiming at forming a layer to effectively protect the host structure, the amount of alumina should reach 20 wt % for postsynthetic samples. The investigation concerning the direct synthetic sample was also conducted. The mesostructure was destroyed completely even if 20 wt % of alumina was introduced, which is quite different from postsynthetic samples (Scheme 1). As described above, the framework aluminum is predominant for the direct synthetic sample, whereas the majority of aluminum locates on the pores in the case of the postsynthetic samples. We believe that only extraframework aluminum contributes to the formation of the interlayer, which can prevent the silica framework from corroding. Thus it is conclusive that both the amount and the location of alumina is of great importance for subsequent generation of strong basicity. In comparison with the protection of the host structure, the host-guest interaction can be enhanced by coating alumina despite its content. Even coating 5 wt % of alumina has an effect on the improvement of host-guest interaction; thus, the decomposition of supported potassium nitrate is promoted. When the content of alumina reaches 10 wt %, all of the supported potassium nitrate can be decomposed. Also, the introduction of alumina via direct synthesis has a similar impact on the decomposition of potassium nitrate as that derived from postsynthesis (see TG results, Figure 8). This means that it is unnecessary to form an intact alumina layer for the enhancement of the host-guest interaction, which differs from the protection of host structure. In the present study, alumina was selected as the promoter to enhance the host-guest interaction, taking account of its electronegativity. According to the report of Tanaka and Ozaki,49 the electronegativity of cations in alumina and silica can be calculated to be 11.27 and 17.10, respectively. With the decrease of electronegativity, the metal-oxygen (or silicon-oxygen) interaction becomes weaker and the coordination ability of lattice oxygen increases.50 Because the electronegativity of the aluminum ion is obviously larger as compared with that of the silicon ion, the coordination ability of oxygen in aluminum with the guest is higher. As a result, coating mesoporous silica with alumina enables the surface to provide an enhanced host-guest interaction, weakening the bond of K+ and NO3- and promoting the decomposition of potassium

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SCHEME 1: Comparison of Potassium Species Loaded on SBA-15 as Well as on Alumina-Coated SBA-15 Derived from Direct and Postsynthesis

nitrate.27 Hence, potassium nitrate can be decomposed at lower temperatures on SBA-15 precoated with alumina. Despite the fact that great efforts have been dedicated, the generation of strong basicity on mesoporous silica remains a challenge up to now. In the present study, we disclosed the inherent factors that obstruct the formation of strongly basic sites on mesoporous silica. Furthermore, a strategy of a precoating alumina interlayer was designed in view of the inherent shortcomings of mesoporous silica. The alumina interlayer performs two functions by enhancing the host-guest interaction and protecting host frameworks simultaneously. The ordered, mesoporous, superbasic materials were thus successfully fabricated. In contrast with conventional methods for preparing mesoporous basic materials, such as organic base grafting and nitrogen incorporation, our strategy provides an effective approach to create much stronger basicity on mesoporous silica. Such a strategy may open up an avenue for the design and synthesis of new functional materials. Conclusions (1) Two reasons are demonstrated to be responsible for the difficulty in generating strong basicity on mesoporous silica, namely, the weak host-guest interaction that is unfavorable for the decomposition of the base precursor, potassium nitrate, and the poor alkali resistance of the host that results in the corrosion of siliceous frameworks by formed strongly basic species. (2) Precoating alumina on mesoporous silica provides an effective approach to modulate the host nature. The host-guest interaction is enhanced obviously. The majority of potassium nitrate is decomposed at 690 °C on unmodified SBA-15, while the temperature decreases to 460 °C after precoating an alumina

interlayer. Also, the alkali-resistant capacity is greatly improved by precoating alumina, and the ordered mesoporous structure can be well-preserved after the introduction of strongly basic potassium oxide. Thus, materials possessing both a mesostructure and superbasicity with a high strength of 27.0 were obtained. (3) Both the amount and the location of alumina play important roles in the fabrication of mesoporous solid strong bases. Coating various amounts of alumina can promote the decomposition of potassium nitrate to a corresponding degree. However, alumina with a content higher than 20 wt % is necessary to form an intact interlayer to protect the silica frameworks effectively. From the point of view of improving the alkali resistance, aluminum located on the pores rather than in the frameworks of mesoporous silica is required. 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. Supporting Information Available: XRD patterns and N2 adsorption-desorption isotherms of SBA-15, AS(p), and AS(d) samples. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ono, Y. J. Catal. 2003, 216, 406. (2) Shanbhag, G. V.; Choi, M.; Kim, J.; Ryoo, R. J. Catal. 2009, 264, 88.

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