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 ap- plications 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 SBA- 15, 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 tem- peratures (>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 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 Neverthe- less, 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. Unfortu- nately, 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 * To whom correspondence should be addressed. Phone: +86-25- 83587177. Fax: +86-25-83587191. E-mail: lbsun@njut.edu.cn (L.-B.S.), liuxq@njut.edu.cn (X.-Q.L.). J. Phys. Chem. C 2010, 114, 18988–18995 18988 10.1021/jp106939d  2010 American Chemical Society Published on Web 10/20/2010