(530e) Catalytic Activity of Unique Base Sites Formed Via Nitridation and Subsequent Methylation of Mesoporous Silica SBA-15

Ordered micro and meso porous silica-based materials (e.g., mesoporous silica and zeolite) have attracted wide attention because of their unique structures and attended high surface area, besides their low toxicity, and high chemical and thermal stabilities. Among their applications, their use as heterogeneous base catalysts is one of a challenge. Nitridation of silica materials (i.e., substitution of N-atom with framework O-atom) can create Lewis base sites. Although they have been known to catalyze basic reaction such as Knoevenagel condensation, their basicity is relatively lower than conventional homogeneous catalysts and applicable reactants have been limited.

Here, we demonstrate the preparation of methylated nitrogen substituted mesoporous silica SBA-15 (Me-NSBA-15), and confirmed that the basicity and also nucleophilicity were enhanced to catalyze several reactions.^1),2) Methylation of N-substituted mesoporous silica SBA-15 was achieved by reacting methyl iodide under reflux condition, and the functionalization was confirmed by elemental analysis and FTIR spectroscopy. Powder XRD showed the ordered porous structure was maintained. The catalytic test was performed with a batch reactor and formed products were analyzed with GC-FID. The Me-NSBA-15 catalyzed the Knoevenagel condensation of benzaldehyde and diethyl malonate (2 mmol C₇H₆O, 2 mmol C₇H₁₂O₄, 2.13 mL ethanol, 20 mg catalyst, 353 K for 24 h), which has never been catalyzed with simple N-substitution SBA-15. This suggests that the methylation enhanced the basicity of Lewis base sites, and allowed to deprotonate a higher pK_a reactant (i.e., pK_a of diethyl malonate is 16.4, whereas the pK_a of ethyl cyanoacetate is 13.1). Methylation also enhanced the nucleophilicity. In Morita-Baylis-Hillman reaction of formaldehyde and methyl acrylate (4mmol CH₂O, 2 mmol C₄H₆O₂, 2.0 mL 1,4-dioxane, 20 mg catalyst, 423 K for 48 h), the yield of the product (64% at 24 h) was comparable with that of conventional homogeneous catalyst, DABCO (69 % at 24 h), and higher than that of the catalyst without methylation, NSBA-15 (30% at 24 h).

1) Y. Furukawa, M. Ogura, J. Am. Chem. Soc. 136, 2014, 119.

2) K. Sugino, N. Oya, N. Yoshie, and M. Ogura, J. Am. Chem. Soc. 133, 2011, 20030.