Mild Catalytic Mechanism of the Mannich Reaction for Synthesizing Methylacrolein by sec-Amine Short-Chain Aliphatic Acid Ionic Liquid Catalysts

Abstract

By density functional theory (DFT), we report a detailed mechanistic study on the synthesis of methylacrolein (MAL) by a mild Mannich reaction of formaldehyde (FA) and propionaldehyde (PA) catalyzed by sec-amine short-chain aliphatic acid ionic liquids (ILs). ILs exhibit excellent catalytic activity and create mild reaction conditions (45 °C) by dramatically decreasing the reaction energy barrier (24.43 kcal mol −1 ) in the decomposition step of Mannich bases (MBs) comapred to without ILs. Three key intermediates observed by DFT calcutations were identified by electron spray ionization mass spectrometry (ESI-MS) analysis. We systematically investigated the catalytic effect of ILs with different sec-amines (HNR 2 , R = CH 3 , C 2 H 5 , etc.) on the activation energy and different short-chain aliphatic acids (RCOOH, R= H, CH 3 et al.) on the decomposition step of MBs. This work is intended to provide a thorough explanation of the synthesis mechanism of the mild Mannich reaction of MAL catalyzed by ILs from the theoretical aspect, which may give a favorable guidance for the practical application.

Continuous Synthesis of Methacrolein over Sulfonic Acid Resin Catalysts: Unraveling the Effect of Acid Strength and Solvent Permittivity

“…18 In addition to experimental studies, Yan et al investigated the detailed reaction mechanism over anionic liquid catalysts by the density functional theory method and revealed the cooperation effect between sec-amine and aliphatic acid in enhancing MAL synthesis. 19 Apart from linear alkyl amines, cyclic sec-amines, such as L-proline 20 and morpholine, 21 were discovered to exhibit significant catalytic efficiency. However, as most of the processes were catalyzed by aqueous sec-amine solutions and performed in batch reactors, the product separation and catalyst recovery are intractable.…”

“…Yan et al utilized isotopic tracing experiments to verify the reaction mechanism of MAL formation catalyzed by diethylamine acetate . In addition to experimental studies, Yan et al investigated the detailed reaction mechanism over anionic liquid catalysts by the density functional theory method and revealed the cooperation effect between sec-amine and aliphatic acid in enhancing MAL synthesis . Apart from linear alkyl amines, cyclic sec-amines, such as l -proline and morpholine, were discovered to exhibit significant catalytic efficiency.…”