Study On The Mannich Condensation Of Formaldehyde And Propionaldehyde To Produce Methacrolein

Methacrolein(MAL)is an important organic synthesis intermediate,which can be used to synthesize basic chemicals such as methacrylic acid(MAA),methyl methacrylate(MMA)and methallyl alcohol(MAO).At present,the isobutylene oxidation method is mainly adopted to produce the MAL,which derive from oil and natural gas.However,oil and natural gas resources in China are in short supply,so the development of the MAL clean production process with independent intellectual property rights for the coal-based route has a good competitive advantage.Firstly,for the target reaction of formaldehyde and propionaldehyde by Mannich condensation to produce methacrolein,the influence of reaction process conditions was investigated by using the kettle device,the analysis method of raw materials and products was established,and the characteristics and rules of the reaction were mastered.It was realized that there are many shortcomings such as low efficiency in the batch reaction of kettle type.On this basis,a tubular reaction device was built and the continuous reaction process research was carried out.In addition,good experimental results were obtained,which not only greatly reduced the catalyst dosage,but also greatly improved the propionaldehyde conversion rate and MAL selectivity.The optimized process conditions: material residence time 45 min,reaction temperature 110 °C,reaction pressure 0.8-1.0 MPa,and the raw material ratio is n(formaldehyde): n(propionaldehyde)=1.05,catalyst dosage 0.8wt%,catalyst molar composition n(acetic acid): n(diethylamine)= 1.0.Under the above optimized conditions,the continuous tube reaction results show that propionaldehyde single-pass conversion and MAL selectivity are both above 97 mol%.Secondly,based on a lot of experimental data,the chemical simulation software Aspen Plus were used to carry out thermodynamic calculation and rectification process simulation calculation for the synthesis of MAL from formaldehyde and propionaldehyde.Thermodynamic data show that the main reaction and the side reactions are exothermic reactions and can proceed spontaneously.In the range of50-120 °C,the effect of temperature on the equilibrium of chemical reactions is extremely limited.Then,using the NRTL-RK physical property method and the DSTWU model,the operating parameters and material composition of the reaction product system under different pressures were simulated by Aspen Plus,and the atmospheric distillation test was carried out to verify the results.The results showed that a mass of aldehydes would be polycondensed due to high temperature in atmospheric distillation column,resulting in a low yield of the product MAL.In order to better realize the separation of the methacrolein product system,it is necessary to operate under negative pressure and low temperature.Finally,the acid-base synergy of organic amines and organic acids in the composite catalysts was studied,and new catalyst combinations with high activity,high selectivity and high stability were screened out.Among them,the catalytic effect of secondary amine is the best.Furthermore,among the secondary amines,straight-chain amines have the best effect.And the organic acid mainly acts as an auxiliary catalysis,providing protons for the Mannich reaction,and the organic weak acid will make the combined catalytic effect better.Through the in-depth study of the reaction mechanism,it can provide reference for other Mannich reaction systems and expand the application of Mannich reaction.