Identification of Vicinal Silanols and Promotion of Their Formation on MCM-41 via Ultrasonic Assisted One-Step Room-Temperature Synthesis for Beckmann Rearrangement

Siliceous mesoporous MCM-41 materials were synthesized under ultrasonic treatment from 0.5 to 3 h. The obtained U-[Si]MCM-41 catalysts were applied in the Beckmann rearrangement of cyclohexanone oxime (CHO) to epsilon-caprolactam (CPL). The ultrasonic treatment can increase the pore wall thickness, while the density of surface SiOH groups increased slightly. Quantitative MAS NMR studies of the U-[Si]MCM-41 catalysts hint at the presence of isolated and vicinal silanols. After dehydration at 393 K, H-1 MAS NMR signals of poly-hydrogen-bonded vicinal silanols occurred, which are strongly reduced after dehydration at 473 K. Correspondingly, a much higher conversion of CHO to CPL was observed for U-[Si]MCM-41 dehydrated at 393 K in comparison with those dehydrated at 473 K. This finding indicates poly-hydrogen bonded SiOH groups can promote the reaction and improve the catalytic performance of siliceous catalytic materials in the Beckmann rearrangement of CHO in an energy economic way at a suitable activation temperature.