Hydrogen-bonding and confinement-enabled one-step immobilization of functional ionic liquids in hierarchical silica for efficient methacrolein synthesis

Functionalized ionic liquids (FILs), as novel catalysts, offered a promising way for synthesizing methylacrolein (MAL) from formaldehyde (FA) and propionaldehyde (PA) via Mannich reaction. However, the high dosage and the difficulty in separating and recycling of FILs hindered their practical application in industry. Although immobilization of FILs onto solid supports can partially address these issues, it often leads to a significant loss of catalytic activity. To address these challenges, this work developed an immobilized functional ionic liquid catalyst ([HDEA]Ac/SG) with exceptional structural stability and high catalytic activity via sol–gel method. Taking advantage of the abundant surface hydroxyl groups of silica gel and hierarchical porosity, [HDEA]Ac was firmly immobilized within the support through interfacial hydrogen bonding and confinement effect. Benefiting from the reduced mass transfer resistance of reactants and products, the enhanced FIL dispersion in support, and the increased FIL site accessibility endowed by the hierarchical porous structure of silica gel, the [HDEA]Ac/SG catalyst achieved a PA conversion of 91.2 % and a MAL yield of 80.2 %, the highest values reported to date for heterogeneous systems. In situ FT-IR and Raman spectroscopy confirmed that the reaction mechanism over [HDEA]Ac/SG follows a typical Mannich pathway, analogous to that in homogeneous systems. The catalyst exhibited excellent recyclability and retained its catalytic activity after at least 10 cycles. This work provides a cost-effective and environmentally benign strategy for designing stable and efficient Mannich reaction catalysts, bridging the performance gap between homogeneous and heterogeneous systems in the synthesis of MAL from FA and PA.