Study On Continuous Flow Process Of Aldol Condensation

Aldol condensation is an important class of organic synthesis reactions that can grow carbon chains by forming new C-C bonds.It is one of the most important methods for the preparation of many valuable chemical products and intermediate products.Most of the current Aldol condensation use kettle reactors and intermittent operation processes.Since this is a non-stationary operation,there are problems such as poor stability and efficiency of heat and mass transfer,low productivity of equipment,long reaction time,difficult to control the end point of the reaction,low selectivity and yield of the target product.Continuous flow reaction technology has the advantages of stable reaction conditions,safe and controllable without leakage,stable and efficient mass and heat transfer process,precise control of process conditions and automatic control because of the steady-state operation process.It has attracted a lot of attention and achieved rapid development in the field of organic synthesis.In this paper,two typical Aldol condensation processes,2,2,4-trimethyl-1,3-pentanediol from isobutyraldehyde and 3-hydroxybutane from acetaldehyde,were studied to investigate the application of continuous flow operation method in these two Aldol condensation processes and explore the feasibility of the new process to replace the traditional intermittent process,include:1.2,2,4-trimethyl-1,3-pentanediol was prepared by using Isobutyraldehyde as raw material,sodium hydroxide solution as catalyst and continuous flow process,and the effect of each process condition on the reaction was investigated and optimized.The results showed that the process conditions of 50%concentration of sodium hydroxide,1.0 ratio of sodium hydroxide to isobutyraldehyde and 40°C reaction temperature.Only a short reaction time（10 min）was required for it to achieve 99.02%conversion of isobutyraldehyde,93.57%2,2,4-trimethyl-1,3-pentanediol selectivity and 92.65%yield.On this basis,the kinetic equations of the reaction were established,and the kinetic parameters such as equilibrium constants and reaction activation energy were obtained by fitting calculations.The results showed that it had the smallest reaction activation energy of 26.34 k J/mol at a concentration of 50%of sodium hydroxide.2.The reaction process for the continuous preparation of 3-hydroxybutanal using acetaldehyde as raw material and sodium hydroxide and sodium carbonate solution as catalysts was investigated.A microchannel reactor with high heat and mass transfer efficiency was used to address the problems of high exothermic effect of the reaction and thermal sensitivity of raw material and product.The effect of different process conditions on the reaction was investigated and optimized.The reaction time was 7 min at a catalyst concentration of 5 wt%,sodium hydroxide to sodium carbonate ratio of 0.67,catalyst to acetaldehyde ratio of 0.2.Meanwhile,the yield of3-hydroxybutyraldehyde was 35.15%at a reaction temperature of 20℃.A reduced-pressure distillation operation was proposed to achieve the separation in order to obtain a high purity of3-hydroxybutyraldehyde.3.The solid basic catalyst was developed to replace the homogeneous catalyst for the homogeneous catalytic process of Aldol condensation with high waste liquid volume,high catalyst and auxiliary consumption,difficult product separation and equipment corrosion.The solid basic catalyst re-Mg_xAl-LDHs with interlayer as OH^-was produced by roasting and hydration recovery process using Mg_xAl-LDHs as precursor and characterized by XRD,SEM,BET,TG and IR.The catalytic performance evaluation revealed that the re-Mg₄Al-LDHs solid basic catalyst exhibited excellent base catalytic performance for the acetaldehyde condensation reaction for the preparation of 3-hydroxybutyraldehyde with 71.88%acetaldehyde conversion,72.15%3-hydroxybutyraldehyde selectivity and 51.86%3-hydroxybutyraldehyde yield.This provides a theoretical basis for further development of solid basic catalysts suitable for hydroxyaldehyde condensation.