Comprehensive Studies on Ionic Liquid-Catalyzed Continuous Methacrolein Synthesis via Aldol Reaction

Methacrolein (MAL) as an intermediate of methyl methacrylate is produced from C4 selective oxidation, which faces the challenging problems of selectivity control and petroleum depletion; therefore, the aldol reaction of formaldehyde with propionaldehyde receives great attention. In this work, we developed an effective plug flow reaction process for continuous methacrolein synthesis catalyzed by an efficient sec-amine-based ionic liquid with low dosage at enhanced temperature (100–160 °C) and pressure (1–5 MPa). The effects of the cation and anion structure, as well as reaction conditions, on the catalytic performance were systematically investigated for further optimization. Main side reactions including methylation of sec-amine group in cation, self-condensation of propionaldehyde, and dimerization of MAL were identified by (GC–MS) for the construction of reaction networks. It was found that the combination of N-methylcyclohexylamine and benzoic acid for ionic liquid preparation achieved the highest catalytic performance of 99.8% propionaldehyde conversion and 98.1% MAL selectivity under optimal condition. Reaction orders and activation barriers of both main and side reactions were determined, which enabled the establishment of four-step coupled kinetic model with regression deviation within 5%. Recycling tests revealed that the catalyst deactivation was mainly derived from irreversible N,N-dimethylcyclohexylamine formation; however, it could be economically recovered to partially compensate catalyst loss.