Thermodynamic and Kinetic Studies on Side Cyclization during Methylacrolein Production Catalyzed by Diethanolamine

Diethanolamine as a secondary amine can effectively catalyze mild synthesis of methylacrolein (MAL) from propionaldehyde and formaldehyde via the Mannich reaction. However, the undesired cyclization of the iminium intermediate inhibits MAL selectivity due to the inherently high reactivity of hydroxyl groups. In this work, thermodynamic and kinetic insights into the side cyclization occurred during the diethanolamine-catalyzed synthesis of MAL. Three types of cyclization compounds generated from diethanolamine and formaldehyde, propionaldehyde, and MAL were confirmed by FT-IR, GC–MS, ESI-MS, and NMR characterizations. Furthermore, thermodynamic analysis for these reversible cyclization reactions was performed using the enthalpy, entropy, and Gibbs free energy estimated by the group contribution method, revealing that the cyclization is exothermic and spontaneous over a broad range of reaction temperatures. With thermodynamic equilibrium constants in hand, detailed kinetic studies were carried out for the determination of reaction orders, pre-exponential factor, and activation energy. The obtained thermodynamic and kinetic information will be useful for the further optimization of the reaction process.