Kinetic and mechanistic insights into the roles of protonated morpholine species and weak acidic environment in methylacrolein production catalyzed by morpholine/acid

Synthesis of methylacrolein (MAL) from formaldehyde and propanal via Mannich reaction receives significant attention considering the gradual depletion of petroleum resource. Herein, series of morpholine/acid catalyst with different acid type and molar ratio were prepared to catalyze the propanal-formaldehyde reactions for MAL production. The catalytic performance of morpholine/acid was significantly affected by the morpholine/acid ratio, acid type, and initial pH value of reactant mixture. While the addition sequence of reactants and formaldehyde source would influence the yield and selectivity of MAL. As a result, the propanal conversion and MAL yield could reach 99.8 % and 89.7 % with catalysis of morpholine/acetic acid having the molar ratio of 1/3 under weak acidic environment (pH = 5.30) at 50 °C. It was revealed that the protonation of morpholine under weak acidic environment could effectively inhibit the deactivation of morpholine/acetic acid catalyst and promote the production of MAL, however, both basic and strong acidic environment would suppress the formation of MAL. Mechanistic DFT calculations also indicated the protonated morpholine species could not only interact with formaldehyde in the form of morpholine but also act as proton donor. The kinetic studies showed that the formation rate of MAL was first-order dependent on the concentrations of both propanal and formaldehyde with the activation barrier of 30.1 kJ/mol. Undesirably, the optimal morpholine/acetic acid catalyst would be gradually deactivated due to the side reactions of morpholine with formaldehyde, propanal and MAL.