Hollow Organic Polymer Solid Base With Free Space Around Base Site,Mesoporous Channels And Its Application In Knoevenagel Reaction

Solid base is one of the most widely used solid base catalysis in the industry,they are required for C-C bond formation reactions such as isomerization,esterification,alkylation,redox,aldol condensation,and Michael addition.At present,organic framework solid bases have been universally applied in catalytic reactions and adsorption chemistry.However,there are some shortcomings such as active sites embedding,difficult pore structure construction and lack of rigidity of the skeleton,which seriously affect the catalytic and adsorption activity of solid base.In order to improve the catalytic activity of organic polymer solid bases,hollow and porous organic polymer solid base nanospheres/nanobowls are designed in this paper,which have the following structural characteristics:(1)Organic polymer solid bases are swellable with organic solvents and can be exposed more catalytic sites;(2)Hollow structure make a difference in concentration of the reaction substrate,which can drive mass transfer;(3)Mesoporous shell provides a smooth mass transfer channels;(4)Free space around basic sites is beneficial for the reactive species to reach the catalytic active site.In this paper,polystyrene nanosphere(PS)is used as core template,and a layer of functionalized polymer is coated on the PS surface by emulsion polymerization.The PS core is removed by tetrahydrofuran(THF)to obtain hollow organic polymer nanospheres PS-NH₃/OR HOPNs;Through the sodium hydride solution,organic acid templates were washed to build free space around base sites;By a method for high-temperature swelling-low-temperature freezing-removing solid DMF,using N,N-dimethyl formamide(DMF)as an annealing agent,inter-connected mesoporous channels in the styrene backbone were constructed,and the hollow organic polymer solid bases#PS-NH₂ HMOPBs with mesoporous channels and free space around base site were prepared.At the same time,solid polymer microspheres PS-NH₂ OPNs,hollow polymer microspheres PS-NH₂ HOPNs,and samples annealed at different temperatures of 85?,115?and 145?were prepared,respectively corresponding to#PS-NH₂ HMOPBs(80),#PS-NH₂HMOPBs(115)and#PS-NH₂ HMOPBs(145).Finally,their catalytic differences in Knoevenagel reactions were studied.In SEM images,solid base PS-NH₂ HMOPBs(a)have regular nano-bowl morphology and good dispersibility,and TEM images also show they have an internal cavity structure.Infrared spectrum(IR)analysis shows that the characteristic peak at1453 cm^-1is bending vibration of NH₂,the stretching vibration peak of N-H bond also exists at 3500?3350 cm^-1,and the stretching vibration peak of C-N bond also appears at1070?1170 cm^-1.These prove that the amino groups were loaded on polymer styrene backbone.Combined with elemental analysis,it shows that sulfur content was significantly reduced after acid ions were removed,and-NH₂ content of PS-NH₂HMOPBs(a)is 0.82 mmol·g^-1 according to nitrogen element content.In Nitrogen adsorption-desorption analysis,the surface area of PS-NH₂ HMOPBs(a)was 41.2m²·g^-1,the pore size was uniformly distributed 4 nm.The solid base#PS-NH₂HMOPNs(80)annealed at 80?maintained the original mesoporous structure,while new pore channels were generated at 10-40 nm,and the specific surface area,pore volume increased to 56 m²·g^-1 and 0.41 cc·g^-1.In summary,we provide an effective strategy for effectively improving the mass transfer process of substrates by forming hollow porous shells,inter-connected pore channels,and free space around base sites.In this paper,solid base polymer spheres PS-NH₂ OPNs,hollow polymer spheres PS-NH₂ HOPNs,mesoporous hollow solid base nano-bowls PS-NH₂ HMOPBs(a),and#PS-NH₂ HMOPBs(80),#PS-NH₂ HMOPBs(115)and#PS-NH₂ HMOPBs(145)were used to catalyze the Knoevenagel condensation reaction,and their catalytic reaction kinetics were explored.Then using PS-NH₂ HMOPBs(a)as the solid base catalyst,the optimal Knoevenagel condensation reaction conditions are researched as follows:ethanol is used as the reaction solvent at 20?,and the amount of catalyst is 5 mmol%.Comparing the catalytic reaction kinetics of different samples,it was concluded that#PS-NH₂ HMOPBs(80)have the best catalytic activity,and the yield was 99%at 2 h.After ten cycles,the yield was 92%,and the catalytic activity was restored to 97%when they were repaired with HCl solution.