Studies On Pickering Emulsion Catalytic System And Its Application In The Synthesis Of Methacrolein

Methacrolein(MAL)is an important intermediate in the production of methyl methacrylate.It is obtained by the condensation reaction of formaldehyde and propionaldehyde under the action of primary or secondary amine catalyst.The reaction process is carried out in the batch reactor,and the high temperature in the separation and recovery process will result in the deactivation and loss of the catalyst.In this paper,a fixed-bed reaction process of formaldehyde and propionaldehyde to synthesize MAL based on Pickering emulsion was established to solve the series of issues.The catalyst was immobilized in the micro-sized Pickering droplets stabilized by SiO2 particles,and then filled in the fixed-bed reactor,realizing the "immobilization" of liquid catalytic system.The process provides a new form for the separation process of catalyst from product,and solves the difficulty of recycling liquid catalyst.The specific research contents are as follows:(1)A series of SiO2 particle stabilizers with different wettability were prepared,and hydrophobic alkyl chains were introduced onto SiO2 particles by modification with silane reagents,giving oil-water interface activity for SiO2 particles.The effects of the different alkyl chain and the amount of silane reagents on the surface wettability and the emulsion-forming ability of the particles were investigated.Based on the characterizations of elemental analysis,wettability and interfacial tension,the correlation between the surface properties and wettability of particle stabilizer was obtained,and the construction law of W/O Pickering emulsion was obtained.(2)The Pickering emulsion stabilized by 3.0 MTMS-SiO2 was constructed.The effects of the dosage of particle stabilizer on the formation of Pickering emulsion and the stability of the system were investigated,and the regulation rules of Pickering droplet size and interface properties were revealed.The Pickering emulsion was used in the reaction process of continuous condensation of formaldehyde and propionaldehyde to produce MAL in a fixed-bed reactor.The structure-activity relationship between the interface properties of the reaction system and the catalytic efficiency was obtained by controlling the size of Pickering droplets.With an optimized procedure,the reaction could run continuously and stably for at least 150 h,and the conversion of propionaldehyde was 100%.(3)Pickering emulsion stabilized by both surfactant(MG)and particle(3.0 MTMS-SiO2)was developed based on the reaction rules of Pickering emulsion catalytic system.The interfacial tension between oil and water phases,as well as the size of Pickering droplets,were effectively reduced.The reaction interface area was further increased,and the catalytic performance of emulsion liquid system was improved.In addition,the thermal stability and rheological properties of Pickering emulsion were significantly improved.The system could flow steadily at the maximum flow rate for at least 100 h.When the reaction reached stability,the conversion of propionaldehyde was 100%and the MAL yield was greater than 95%.(4)The cross-linked Pickering droplets were designed and prepared using Pickering emulsion as a template.The immobilization of the ionic liquid catalyst was further enhanced by the porous network structure formed by the internal cross-link of the droplets,improving catalytic efficiency of the reaction.By means of elemental analysis and BET characterization,the transformation mechanism of reactants and products with different pore structures was revealed.(5)The complete process flow of batch reaction and Pickering emulsion continuous reaction were established respectively,and the benefits of Pickering emulsion catalytic system from the reaction process to the separation process were illustrated.In addition,by analyzing the relationship between interfacial tension and the surface wettability of particle stabilizer,the emulsification rules of various two-phase systems were explored,and a suitable approach for Pickering emulsion construction in a certain range was established.