Construction Of Hierarchical Structures Of Nano Fe₃O₄ Hybrids As The Highly Efficient Heterogeneous Catalysts

Catalysts are widely used in daily life,especially in chemical,petrochemical,biochemical,environmental protection industries.Currently,there are many kinds of catalysts.Most homogeneous catalysts are difficult to separate from the reaction system.It greatly limits their application range.Therefore,heterogeneous catalysts have received extensive attention,such as in the fields of solid acid catalysts,organic alkali catalysts,metals and metal oxide catalysts,complex catalyst,rare earth catalysts,molecular sieve catalysts,bio-enzyme catalysts and so on.Phase transfer catalysts and noble metal catalysts are the most widely used.But there are still some problems need to be solved.In the case of phase transfer catalysts,the challenge is increasing catalyst reactive group content while improving the dispersibility and separation recovery performance.For noble metal catalysts,due to the limited reserves and high prices of noble metal,the problems are reviving and reduce the cost.In recent years,scientists have carried out lots of research works to solve these problems.Combining iron oxide nanoparticles with high specific surface area and superparamagnetic properties with catalysts is an effective strategy to realize recovery.However,realize the efficient combination of nano iron oxide and catalyst at the micro-nano level,ensure the composite structure does not affect the activity of the catalyst remain to be solved.In view of these common problems,this work selects two types of catalysts as research objects,including phase transfer catalysts and noble metal silver.The composite method of Fe₃O₄ nanoparticles and catalyst are discussed.The influence of factors such as composite structure,morphology,surface properties on catalyst performance have been in-depth studied We finally realize to increase the active group loading of phase transfer catalyst and endow catalyst with magnetic field separation performance at the same time.The problem of noble metal catalyst recovery is solved.The main research contents are as follows:Fe₃O₄ nanoparticles are chosen as cores.Glycidyl methacrylate（GMA）and ethylene glycol dimethacrylate（EGDMA）are used as monomer and crosslinking agent.Fe₃O₄@P（GMA-EGDMA）composite particles with core-shell structure are prepared by seed emulsion polymerization.A series of magnetic nanoparticles-supported quaternary ammonium phase transfer catalysts（MQPTCs）with different exchange capacities can be obtained by adjusting the amount of monomers,and the highest exchange capacity can reach up to 1.14 mmol/g.In order to evaluate the catalytic activity of MQPTCs,the efficiency of nucleophilic reaction of benzyl alcohol and benzyl bromide to dibenzyl ether is studied.It is found that the conversion rate of phase transfer reaction can reach 96.3%within 3.5 h.The prepared phase transfer catalyst can be separated and reused conveniently by an external magnetic field.The conversion rate can still be maintained to 94.1%after 8 times of recycling.This demonstrates that the designed catalyst has good stability.In order to further improve the dispersibility of the composite phase transfer catalyst in the reaction system,building Janus structure is propose.Firstly,P（GMA-AA-DVB）are prepared by using GMA and acrylic acid（AA）as monomers,divinylbenzene（DVB）as crosslinking agent.Then the magnetic Janus composite particles Fe₃O₄&P（GMA-AA-DVB）are prepared by solvothermal method.The magnetic Janus quaternary ammonium salt phase transfer catalysts（MJPTCs）are obtained by the quaternization of epoxy groups on the surface of Fe₃O₄&P（GMA-AA-DVB）with trimethylamine,and their structures and catalytic properties are systematically studied.A series of MJPTCs with different exchange capacity can be obtained by adjusting the amount of polymer,and the maximum exchange capacity can reach2.29 mmol/g.The catalytic activity of MJPTCs is evaluated by studying the efficiency of nucleophilic reaction of benzyl alcohol and benzyl bromide.The results show that the phase transfer reaction can be completely reacted within 2.5 h,and the final conversion rate can reach 99%.The catalytic efficiency of MJPTCs is comparable to that of small molecule catalysts.Moreover,the catalytic performance of the MJPTCs is significantly better than that of the core-shell structure phase transfer catalyst.The prepared phase transfer catalyst can be easily and quickly separated and reused by applying an external magnetic field,and the conversion rate can be maintained at 97%after 8 times recycling,manifesting that catalyst has excellent cycle life.For the purpose of reducing the overall density of the catalyst and preparing light catalyst carrier,it is necessary to decrease the size of inorganic component.Therefore,Fe₃O₄nanoparticles with small particle size are prepared by thermal decomposition using ferrocene as iron source.Since the Fe₃O₄ nanoparticles have catalytic activity,they are hydrophilically modified and used for heterogeneous Fenton reaction to catalyze the degradation of organic dye methyl orange.The experimental results of the catalytic reaction show that the degradation rate of methyl orange increases first and then decreases with the decrease of p H value.Increasing the reaction temperature is beneficial to the degradation of methyl orange.Gradually increasing the amount of hydrogen peroxide（H₂O₂）,the degradation rate of methyl orange shows a trend of increasing first,then decreasing and then increasing.The degradation rate of methyl orange with the amount of Fe₃O₄ is consistent with the trend of H₂O₂ dosage.After 3 min,the degradation rate of methyl orange can reach 99%.When the concentration of methyl orange is increased from 20 mg/L to 60 mg/L,the catalytic system still has excellent catalytic effect.The prepared hydrophilic Fe₃O₄ nanoparticles can be easily and quickly separated and reused by applying an external magnetic field.After 8 cycles,the degradation rate of methyl orange remains at 98.7%.Aiming to solve the problem of catalyst agglomeration and high density,we have designed a litchi-like catalyst with magnetic properties.In this part,small Fe₃O₄ nanoparticles are combined with polymer particles.It can not only avoid agglomeration of small sized Fe₃O₄nanoparticles,but also increase the specific surface area of carrier materials.P（MMA-AA-DVB）particles prepared by soap-free emulsion polymerization are used as carrier to grow Fe₃O₄ nanoparticles in situ on their surface by thermal decomposition to form litchi-like magnetic composite nanoparticles.Subsequently,P（MMA-AA-DVB）@Fe₃O₄@Ag magnetic catalysts are obtained by loading silver（Ag）nanoparticles on their surface.By simply adjusting the ratio of polymer dosage to iron source dosage,the coating amount of Fe₃O₄ nanoparticles on the surface of the polymer can be effectively controlled,and composite particles with different magnetic contents can be obtained.The reduction reaction of organic dye methyl orange is used as the template reaction,and the influence of different factors on the reaction is discussed.The results show that,the prepared magnetic catalysts exhibit good catalytic performance,and the reduction reaction can be completely completed within 10 min.The catalyst can be easily and quickly separated and reused through an external magnetic field,and the catalytic efficiency is maintained at 98%after 5 cycles.In order to further reduce the density and improve the catalytic performance,a new Ag composite catalyst with silicon dioxide（Si O₂）isolation layer on the surface of litchi-like magnetic composite structure is designed.P（St-AA-DVB）polymer particles are used as carrier to grow Fe₃O₄ nanoparticles in situ on their surface to form litchi-like magnetic composite nanoparticles.Subsequently,the surface is coated with a Si O₂ layer and then loaded with Ag nanoparticles.After removing the polymer carrier,a hollow structure Fe₃O₄@Si O₂@Ag magnetic composite catalyst is finally obtained for catalytic reduction of the organic dye methyl orange.By simply adjusting the ratio of the amount of polymer to the amount of iron source,the coating amount of Fe₃O₄ nanoparticles on the polymer surface can be effectively controlled,and finally hollow composite particles with different Fe₃O₄ content can be gained.The prepared Fe₃O₄@Si O₂@Ag magnetic catalyst has excellent catalytic activity for the reduction reaction of methyl orange,and the reduction reaction can be completely finished within 10 s.The catalyst can be easily and quickly separated and reused by an external magnetic field.After 5 cycles,the catalytic efficiency remains at 98.8%.It displays excellent cycleability and stability.