Construction,Microstructural Control And Application Of Hollow Mesoporous Organic Polymeric Micro Bowls-supported Solid Acid

Solid sulfonic acid is one of the most widely used,green,and environmentally friendly solid acid catalysts in industry,and has many advantages,such as high selectivity of catalytic reactions,easy separation from raw materials and products,easy purification of products,and small corrosion to equipment.The solid sulfonic acid has been successfully applied to organic synthesis reactions,including hydrolysis,esterification,isomerization,and alkylation.In particular,polystyrene is an important carrier in polymer-supported sulfonic acid.At present,polystyrene-supported solid acids have some disadvantages,including small specific surface area and lack of controllable pores.Therefore,this results in the difficult accessibility of substrates to the catalytic sites owing to the limitation of mass transfer,reducing the catalytic efficiency of polymer-supported solid sulfonic acids.Based on the above-mentioned reasons,the microstructural control of pore structure and enhancement of mass transfer of reactants in carrier framework have important theoretical significances and potential practical applications.In view of these problems of polystyrene-supported sulfonic acid,we designed and prepared a class of polystyrene nanobowl-supported sulfonic acids with a hollow interior,mesoporous shell,and fully exposed acid sites.The as-fabricated polystyrene nanobowl-supported sulfonic acids had the following architectural features:(1)a hollow interior,producing concentration-driven mass transfer between internal and external surfaces of catalyst,(2)mesoporous shell,providing a transmission channel for reactants,(3)molecular-imprinted free space around acid sites,facilitating the easy accessibility of reactants to catalytic sites,(4)nanobowl with a well-shaped morphology,improving catalytic efficiency of solid acid.In this study,styrene and p-ethylbenzene sulfonate sodium derived from barium chloride,ammonium sulfate and sodium p-ethylbenzene sulfonate were used as monomers to prepare DVB-cross-linked hollow mesoporous organic polymeric nanobowl-supported solid acid[Poly(St-co-DVB-co-Vbsa)HFSNb]by coating a layer of polystyrene copolymer on the PS/AA nanosphere surface through emulsion polymerization,followed by etching of organic amine template using diluted HCl and removing PS/AA core using THF.The as-fabricated solid acid had a hollow interior,mesoporous shell,and free space around the acid sites.Elemental analysis showed that the solid acid had a high acid exchange capacity(0.92 mmol g^-1).To investigate their differences in catalytic reactions,solid polystyrene nanosphere-supported acid Poly(St-co-DVB-co-Vbsa)SFSNs with free space around the acid sites and hollow polystyrene nanosphere-supported acid Poly(St-co-DVB-co-Vbsa)HNs with no free space around the acid sites were also prepared as contrast solid acids.The hollow interior in nanobowls Poly(St-co-DVB-co-Vbsa)HFSNb was observed from SEM and TEM images.The mesoporous shell was verified by N₂ adsorption-desorption isotherms.Compared with contrast Poly(St-co-DVB-co-Vbsa)SFSNs and Poly(St-co-DVB-co-Vbsa)HNs,Poly(St-co-DVB-co-Vbsa)HFSNb had the highest specific surface area(97.9 m² g^-1)and the biggest pore volume(0.45 cc g^-1).Mac Millan chiral organocatalyst(MMIOC)was loaded on Poly(St-co-DVB-co-Vbsa)HFSNb,Poly(St-co-DVB-co-Vbsa)SFSNs,and Poly(St-co-DVB-co-Vbsa)HNs through acid-base reaction between sulfonic acid and amine to prepare corresponding supported organocatalysts,named as Poly(St-co-DVB-co-Vbsa-MMIOC)HFSNb?Poly(St-co-DVB-co-Vbsa-MMIOC)SFSNs,and Poly(St-co-DVB-co-Vbsa-MMIOC)HNs,respectively.The differences among them in adsorption kinetic behaviors and catalytic performances in the D-A reaction and Michael addition were comparatively studied.Experimental results showed that Poly(St-co-DVB-co-Vbsa-MMIOC)HFSNb had the highest adsorption capacity of MMIOC(0.45 mmol g^-1).In the D-A reaction of cinnamaldehyde with cyclopentadiene and Michael addition reaction of 2-carbomethoxycyclopentanone with trans-?-nitrostyrene,the catalytic results(46%yield,1.3:1 Dr,87%ee;78%yield,20:1 Dr,15%ee,respectively)were obtained,respectively.However,the yield and stereoselectivity in Michael addition reaction,promoted by 5^th-reused Poly(St-co-DVB-co-Vbsa-MMIOC)HFSNb,decreased to 56%yield,5:1 Dr,and5%ee,owing to the sharp loss of MMIOC organocatalyst.In view of this phenomenon,the loss of MMIOC in Poly(St-co-DVB-co-Vbsa-MMIOC)was investigated in different solvents.It was found that the loss of MMIOC was effectively prevented in toluene medium.Unfortunately,the Michael addition reaction could not successfully carry out in toluene.Owing to the sharp loss of organocatalyst MMIOC attached to solid acid through weak interaction via anion pair,we designed an available covalent immobilization strategy to prevent organocatalyst from losing in catalytic process.First,organocatalyst QNNH₂ was loaded in Poly(St-co-DVB-co-Vbsa)HFSNb through acid-base reaction similar as MMIOC.Subsequently,the double carbon-carbon bond in QNNH₂ were polymerized with the residual double carbon-carbon bond in polystyrene framework to fabricate corresponding nanobowl-supported organocatalysts with enhanced immobilization of QNNH₂ in mesopores via covalent bond,namedasPoly(St-co-DVB-co-Vbsa-QNNH₂)HFSNb,Poly(St-co-DVB-co-Vbsa-QNNH₂)SFSNs,and Poly(St-co-DVB-co-Vbsa-QNNH₂)HNs,respectively.The kinetic adsorption behaviors of QNNH₂ on various solid acids and desorption behaviors of QNNH₂in acid medium out from polymer-supported organocatalysts were comparatively investigated.Elemental analysis showed that Poly(St-co-DVB-co-Vbsa-QNNH₂)HFSNb had the highest adsorption capacity of QNNH₂(0.42 mmol g^-1).After being stirring in toluene for 48 h,the adsorption capacity of QNNH₂ was 0.40 mmol g^-1 in Poly(St-co-DVB-co-Vbsa-QNNH₂)HFSNb.In 10 m L of strong acidic ethanol solution(0.05 mol L^-1HCl,V _C2H5OH/V _H2O=1:4)for48 h,the adsorption capacity of QNNH₂ was 0.11 mmol g^-1,indicating that 27.5%of QNNH₂was anchored in mesopores via covalent bond.It was concluded that the loss of QNNH₂ was effectively inhibited through direct copolymerization.Fortunately,Poly(St-co-DVB-co-Vbsa-QNNH₂)HFSNb promoted asymmetric aldol reaction in high yield(95%)with excellent stereosselectivity(97%ee,89:11 Dr).The 5^th-reused Poly(St-co-DVB-co-Vbsa-QNNH₂)HFSNb possessed a good mechanical stability.The well-shaped morphology and a hollow interior were observed from the SEM and TEM images.In addition,the 5^th-reused Poly(St-co-DVB-co-Vbsa-QNNH₂)HFSNb showed good catalytic performances with high 91%yield,95%ee,and 85:15 Dr).