Magnetic Pickering Interfacial Catalyst: Design,Synthesis And Application In Liquid-Liquid Biphasic Reaction

Liquid-liquid?L-L?biphasic reaction refers to the mass transfer and reaction process between two immiscible phases.Many important reactions in fundamental research and industrial production,such as oxidation,transesterification,epoxidation,esterification and hydrogenation,belong to this system.However,the immiscibility of the biphasic system results in high mass transfer resistance and limited interfacial area,so the reactions suffer from limited efficiency.On the view of the mechanism of interfacial reaction,the interfacial area is one of key factors determining the reaction rate.Pickering emulsion with considerable interface area provides a promising method for accelerating catalytic reaction efficiency.Magnetic separation is considered as a representative of new strategy of direct capture technology in chemical separation because of high efficiency and energy saving.Magnetic nanoparticles with scale effect and special surface properties are served as a significant carrier for realizing“heterogeneous catalysis”to“homogeneous catalysis”.Therefore,a variety of Pickering interfacial catalysts?PICs?were synthesized via surface design and adjustment on the basis of magnetic nanoparticles,and constructed a highly active,easily separated,low energy-consuming catalytic system.This new concept achieve the combination of mass transfer intensification,efficient catalyst recovery and rapid product separation in L-L biphasic reactions.?1?The Fe₃O₄ nanoparticles were prepared through the reverse micelle approach under basic condition.Later on,the Fe₃O₄@SiO₂ with core-shell structure was fabricated based on the St?ber method.A sol-gel process was used to coat silica on Fe₃O₄.A mixture of tetraethyl orthosilicate?TEOS?and methacryloxy propyl trimethoxyl silane?MPS?were allowed to hydrolysis and co-deposited onto the surface of Fe₃O₄,affording reactive C=C double bonds for the next polymerization step.Then the nanohybrids Fe₃O₄@SiO₂@PTMA,bearing catalytic TEMPO functionalities,were synthesized through distillation-precipitation polymerization?DPP?of the precursor monomer 2,2,6,6-tetramethyl-4-piperidyl methacrylate?TMPM?and subsequent oxidation reaction.The TEMPO loading,particle size and surface wettability of the nanoparticles were regulated by varying the amount of monomers addition.The catalysts were applied to the Anelli oxidation system?NaClO/NaBr/TEMPO?,and stabilized water-in-oil?W/O?Pickering emulsion to accelerate biphasic reaction rate.The performance of resultant nanohybrids were then evaluated,and showed superior catalytic activity for selective oxidation of alcohols.Moreover,the PIC can be easily recycled in 30 min by applying magnetic field,and reused for the 4 cycles of oxidation without any loss on either conversion or selectivity.?2?Magnetic and CO₂-responsive PIC was developed to achieve the balance of high catalytic activity and fast recovery of nanocatalyst in biphasic system.The monomers of hydrophobic TMPM?catalyst precursor?and relative hydrophilic,CO₂-responsive DEA were copolymerized with Fe₃O₄@SiO₂-MPS to yield Fe₃O₄@SiO₂@P?TMPM-DEA?by DPP.Afterwards,H₂O₂ was used to oxidize precursor and TEMPO-immobilized PICs were received.The adjustment of TEMPO and DEA loading and the surface wettability of the particles were achieved with various addition of TMPM and DEA.CO₂ was introduced into the Pickering emulsions stabilized by Fe₃O₄@SiO₂@P?TMPM-DEA?,and the demulsification were accomplished within 20 min.Then the catalysts were recovered in 2 min by magnet.After 5 cycles,the recovered nanohybrids still exhibited great performance of emulsification and response demulsification.In the Anelli system for alcohol oxidation,the PIC exhibited threefold enhancement in catalytic efficiency in comparison with unemulsified two-phase.Besides,the green and sustainable catalyst can be reused for subsequent 5 cycles of alcohol oxidation with little loss in catalytic activity.?3?PICs were widely used in aqueous-oil biphasic reactions,but seldom employed in oil-oil biphasic system.Pickering interfacial catalysts were developed and used for highly effective and clean production of biodiesel.The catalyst was synthesized by immobilization of 1,1,3,3-tetramethylguanidine?TMG?onto the shell of Fe₃O₄@Chloromethyl PS nanohybrids.According to the measurement of contact angle,it was found that Fe₃O₄@PS-TMG has good wettability for both methanol and soybean oil.The resultant nanocatalyst can be adsorbed onto L-L interface and stabilized soybean oil-in-methanol Pickering emulsion under stirring.The stable Pickering emulsions provide larger contact area for biphasic reactions,thereby enhancing transesterification.When the temperature was 60°C,catalyst dosage was 5 wt%,the molar ratio of alcohol to oil was 15:1,the yield of biodiesel catalyzed by Fe₃O₄@PS-TMG was 98.6%,which was 1.7 times that of TMG and similar to KOH.Furthermore,the PIC can be facile separated from the reaction liquid in 15 min by magnet,and showed excellent stability over 5 recycles.?4?The PICs grafted organic base obtained by miniemulsion and post-modification method were suffered from low catalyst loading because of the limited active sites on the surface of nanoparticles.In order to gain the high loading of organic base catalysts,magnetic nanoparticles with polymer brushes were prepared by reversible addition fracture chain transfer?RAFT?polymerization.Firstly,two monomers with alkaline groups of 1,1,3,3-tetramethyl-2-?4-vinylbenzyl?guanidine?TVBMG?and 2-methyl-1-?4-vinylbenzyl?-1,4,5,6-tetrahydropyrimidine?SCm?were synthesized.The RAFT reagent benzyl 9H-carbazole-9-carbodithioate?BCBD?was anchored on the surface of Fe₃O₄@Chloromethyl PS to receive Fe₃O₄@PS-BCBD.Then,the monomer of TVBMG and SCm were grafted on the surface of Fe₃O₄@PS-BCBD via RAFT to afford Fe₃O₄@PS-PTVBMG(1.8 mmol·g^-1)and Fe₃O₄@PS-PSCm(2.1 mmol·g^-1).The PIC with controllable organic base loading were achieved with various monomer feeding.When the temperature was 60°C,catalyst dosage was 5 wt%and the molar ratio of alcohol to oil was 15:1,the yield of biodiesel catalyzed by Fe₃O₄@PS-PTVBMG and Fe₃O₄@PS-PSCm were 99.2%and 99%at 6 h,respectively.The catalytic activity of Fe₃O₄@PS-PTVBMG and Fe₃O₄@PS-PSCm were twice that of TMG.After 5 cycles,the catalysts showed excellent activity and stability.