Deactivation Mechanism And Kinetic Studies Of Ionic Liquid Catalysts For Methacrolein Synthesis

Methacrolein(MAL)is an important intermediate and widely used in the production of fine chemicals such as methyl methacrylate.It is obtained by oxidation of isobutene or aldol reaction of formaldehyde and propanal.This study investigated catalyst deactivation mechanism in the ionic liquid-catalyzed synthesis of MAL from the aldol reaction of propanal and formaldehyde.The kinetics of diethylamine acetate deactivation and the aldol reaction were studied.Both sets of the kinetic date are essential for the industrial application of the MAL synthesis process.The main results are as follows:(1)Two kinds of secondary amine ionic liquids including diethylamine acetate and diethanolamine acetate were synthesized.The influences from temperature,atmosphere and reactants on catalyst deactivation were investigated.The proposed mechanism of catalyst deactivation was demonstrated using elemental analysis,FT-IR,ESI-MS/MS and NMR.Formaldehyde combines with diethylamine acetate to form the hemiaminal intermediate,which is then oxidized by active oxygen into N,N-diethylformamide.Radical scavengers could inhibit the catalyst deactivation.As the optimal radical scavenger,4-hydroxy-2,2,6,6-tetramethyl-piperidinooxy(TEMPO)reduced the yield of deactivation product from 1.83%to 1.03%.The deactivation mechanism of diethanolamine acetate is that under the action of formaldehyde,an imine structure is formed.The-OH of diethanolamine acetate connects with the carbon atom to form 2-(3-oxazoline)ethanol.It could then form N-methyldiethanolamine under the action of high temperature and formaldehyde.This process causes the catalyst to lose the active site.(2)The initial kinetic equation for diethylamine acetate deactivation was obtained.The high activation energy(115 kJ/mol)indicates that catalyst deactivation is serious during catalyst recovery under high temperature.These kinetic results would provide reference for catalyst addition,catalyst lifetime prediction and new technology prospects in the MAL industry.(3)Kinetics of the aldol reaction catalyzed by diethylamine acetate was investigated by monitoring propanal concentration in real time.Since this activation energy is only 42.16 kJ/mol,the reaction should be kept at low temperatures to minimize catalyst deactivation.These kinetic data are valuable for reactor design and scale-up experiments.