Low-Temperature Fabrication of Mesoporous Solid Strong Bases by Using Multifunction of a Carbon Interlayer

Abstract

Mesoporous solid strong bases are highly promising for applications as environmentally benign catalysts in various reactions. Their preparation attracts increasing attention for the demand of sustainable chemistry. In the present study, a new strategy was designed to fabricate strong basicity on mesoporous silica by using multifunction of a carbon interlayer. A typical mesoporous silica, SBA-15, was precoated with a layer of carbon prior to the introduction of base precursor LiNO3. The carbon interlayer performs two functions by promoting the conversion of LiNO3 at low temperatures and by improving the alkali-resistant ability of siliceous host. Only a tiny amount of LiNO3 was decomposed on pristine SBA-15 at 400 °C; for the samples containing >8 wt % of carbon, however, LiNO3 can be entirely converted to strongly basic sites Li2O under the same conditions. The guest-host redox reaction was proven to be the answer for the conversion of LiNO3, which breaks the tradition of thermally induced decomposition. More importantly, the residual carbon layer can prevent the siliceous frameworks from corroding by the newly formed strongly basic species, which is different from the complete destruction of mesostructure in the absence of carbon. Therefore, materials possessing both ordered mesostructure and strong basicity were successfully fabricated, which is extremely desirable for catalysis and impossible to realize by conventional methods. We also demonstrated that the resultant mesoporous basic materials are active in heterogeneous synthesis of dimethyl carbonate (DMC) and the yield of DMC can reach 32.4%, which is apparently higher than that over the catalysts without a carbon interlayer (<12.9%) despite the same lithium content. The strong basicity, in combination with the uniform mesopores, is believed to be responsible for such a high activity.

Characterization for Li3PO4 catalysts toward elucidation of crystalline form and performance relationship

“…Also the olefinic species possibly existed in the used catalyst, for their characteristic stretching vibration frequencies appear at 1623, 1678, 3009, and 3035 cm À1 . [24] And the alkaline strengths of the medium and the strong basic sites are lower than the weak one. It can be found that there is a weak and broad desorption peak at about 350 8C on the surface of the FC800, while on the surface of the FC320 three desorption peaks of CO 2 centred at 193, 321, and 458 8C are detected corresponding to 3 basic sites which are weak, medium and strong, respectively.…”

Design and fabrication of mesoporous heterogeneous basic catalysts

“…Alkali metal oxides are very useful basic species and can catalyze a variety of organic reactions. [137][138][139][208][209][210][211] Mesoporous silica is precoated with an interlayer prior to the introduction of base precursors. Direct modification usually leads to the degradation of mesostructure and the formation of weakly basic sites.…”

“…In the presence of a carbon layer, nevertheless, the conversion of KNO 3 proceeded via a rather different way and the main products were NO and CO 2 . 208 It is known that carbon interlayers can be prepared through a range of organic precursors. 137 In addition to KNO 3 , other alkali metal nitrates such as LiNO 3 can also be converted to corresponding basic oxides through the redox reactions with carbon layers, leading to the fabrication of various mesoporous solid strong bases.…”

“…Consequently, the conversion of KNO 3 to basic species K 2 O on SBA-15 and carbon-containing supports can be tentatively described by eqn (1) and (2) in Scheme 3. 137,208 Actually, organic templates involved in the synthesis of mesoporous silica are also alternative carbon sources. 208 It is known that carbon interlayers can be prepared through a range of organic precursors.…”

“…137,208 Actually, organic templates involved in the synthesis of mesoporous silica are also alternative carbon sources. 208 Accordingly, strong basicity is constructed on mesoporous silica SBA-15 at low temperatures, and the ordered mesostructure is well maintained. 30).…”