Investigatioin On Electrospinning Preparation And Adsorbability Of Mesoporous Oxide Fibers

According to the International Union of Pure and Applied Chemistry(IUPAC),the pore size in 2-50 nm named as.mesoporous.In the 1990s,Mobil Oil firstly reported the M41S mesoporous silica material.With the development for almost 30 years,dozens of mesoporous silica materials have been prepared,but the preparation of non-silicon oxide mesoporous materials is still a challenge.Non-silicon-based mesoporous oxide materials have attracted more and more attention due to their unique ability that can provide more reactive sites for the application in catalyst/catalyst support,.adsorption/desorption,electrode materials and.sensors.As functional materials,mesoporous oxide fibers combine the properties of mesoporous oxide and one-dimensional fiber morphology,which not only have the advantages of uniform pore size,ordered pore structure and high specific surface area of mesoporous materials,but also have large aspect ratio of one-dimensional material,self-supporting and easy recovery and utilization.Among them,zirconia(ZrO2)has attracted a great deal of attention due to its both acid and base active centres on the surface as well as oxidizing and reducing sites.Moreover,abundant oxygel-containing functional groups as well as unsaturated Lewis acid-base Zr4+-O2-pairs make ZrO2 as a widely novel material for ion exchange membranes,oxygen sensors,adsorption-separation,catalyst and catalyst support,bio-medicine diagnostics and treatment etc.Magnesia(MgO)is a nontoxic,economic and environmental friendly functional metal oxide with high isoelectric point,which has been widely employed as a good bactericide and adsorbent for wastewater treatment.Electrospinning is a versatile,simple and cost-effective strategy to obtain a high quality one-dimensional material with tunabel microstructure.Based on the point,this dissertation focused on the preparation of mesoporous ZrO2 and MgO nanofibers by electrospinning,and systematically studied the influence of different heat treatment processes,structure regulation and surface modification on the mesopore properties of the nanofibers.The structure and surface chemistry of mesoporous nanofiber,the adsorption mechanism of organic dyes and fluoride ions were studied.The influences of adsorption temperature,pH value,background ions and competitive adsorption of various pollutants on adsorption efficiency were evaluated.The effects of mesoporous ZrO2 and MgO nanofibers on the adsorption of organic dyes and fluoride ions were investigated in detail.In addition,the structure-activity relationship between the mesoporous structure and adsorption capacity was summarized,which provided the theoretical guidance for the further preparation of other mesoporous oxide fibers,and laid the foundation for the practical application of mesoporous metal oxide fibers as adsorbents.The main contents of the dissertation are as follows:In chapter 1,the research status of mesoporous oxide fibers were briefly introduced at first,and the research progress and existing problems in the synthesis and application at domestic and overseas were summarized.And then the significance,research idea and the main research work of this dissertation were put forward.In chapter 2,the effects of two different heat treatment processes on the mesoporous properties of ZrO2 fibers were compared.The main work is as follows:(1)The non-ionic surfactant,polyethylene oxide-polypropylene oxide-polyethylene oxide,triblock copolymer,(PEO-PPO-PEO,abbreviated as P-123)was used as the template,polyethylene oxide(PEO)as the spinning aid,poly-acetylacetonato-zirconium(PAZ)as zirconium source and methanol-water as mixed solvent to prepare the spinning solution.The diameter distribution of the precursor fibers were 2.01 ± 0.24 ?m.(2)The precursor fibers were heat-treated in air and water steam,respectively.Comparing with the FT-IR,XRD and XPS results of the fibers after two different heat treatments,it was found that the water steam not only could efficiently promote hydrolysis of acetylacetone,but also could better use the released energy as activation energy to accelerate the crystallization of t-ZrO2.As the heat-treated temperature rising to 400 ? in air,the fibers were still amorphous state and the t-ZrO2 was formed until the temperature increased to 450 ?.While,the crystalline ZrO2 was formed with the heat-treamted temperature rising to 300 ? in water sheam.The statistical results obtained from the SEM images indicated that the diameter for fibers heat-treated in air was 0.89 ± 0.09 ?m,while that for fibers heat-treated with water steam was 1.29 ±0.12 p.m.Compared with the heat treatment in water sheam,the fibers heat-treated in air exhibited more evident volume shrinkage.The water stream is considered that can directly remove the internal organics and the shrinkage only occurs at the surface of fibers without inward diffusion.(3)The mesoporous features of the fibers treated with different processes were studied.As the dealing temperature rose to 300 ? in water steam,the surface area reached the highest value of 181.46 m2/g and the pore size stretched to 5.7 nm.The surface area for the fibers heated to 450 ? in air reached 120.95 m2/g,and the corresponding pore size was distributed in 1.3-3.1 nm.The result indicated that the water steam that could effectively remove the inside organic and prevent the pore structure shrink or collapse.The large hysteresis loops which were in-between typical H1 and H2 types indicated the well order cylinder type mesoporous structure in the fibers.The effects of spinning solution pH value of the fiber mesoporous properties were studied.When the pH value reduced to 2.58,the surface area of the ZrO2 fibers was reached the maximum 232.70 m2/g and the pore size was 6.27 nm.In an appropriate amount of HCl,the soft template P-123 can be protonated and the interaction with PAZ was enhanced for formation.of more stable micelle.Additionally,the formation mechanism of the mesoporous fibers fabricated by electrospinning was discussed.The micelle was first formed by the P-123 and PAZ oligomer composite where polypropylene oxide moieties acted as the core,and the polyethylene oxide associated PAZ as the shell at the initial aging time.Then the micelles were aggregated into rod-micelles by the cooperative formation mechanism.During the electrospinning progress,the rod-micelles were inserted into the fibers with evaporation of methanol and water,then the precursor was formed.Finally the organics was removed and the Zr02 was as the pore wall to form ordered mesoporous structure through the heat-treatment.In chapter 3,the main work focused on the preparation of mesoporous ZrO2 nanofibers by electrospinning and the adsorption properties for fluoride.(1)Based on the work of chapter 2,in this chapter,the polyaceticzirconium(PEZ)as the zirconium source,non-ionic surfactant P-123 was used as the template,polyethylene oxide(PVP)as spinning aid and methanol-water as mixed solvent to prepare the spinning solution.By the use of electrospinning,the diameter distribution of the precursor fibers were 0.509 ± 0.068 ?m and that of the mesoporous ZrO2 nanofibers obtained through treating the precursor fibers in water steam at 350 ?were 0.480 ± 0.075 ?m,(2)By adding different amounts of HCl,the effects of viscosity,pH value and conductivity of spinning solution on the mesoporous properties were studied.With the decrease of the pH value of the spinning solution,the conductivity was obviously enhanced and the surface area of the ZrO2 mesoporous fibers had a significantly improvement from 52.85 to 158.20 m2/g.While as the pH value reduced to 1.42,beads and slages appeared in the precursor fibers and fibers heat-treated at 350 ?,respectively,which may be due to that in the process of electrospinning,the high conductivity of the precursor gel caused the instability of the jet flow and aroused the electrospray.(3)The adsorption properties of the ZrO2 mesoporous fibers to fluoride and organic dyes were investigated.The residual fluoride concentration after adsorption equilibrium could be reduced to 1.41 mg/L with the initial fluoride concentration of 30 mg/L,which was below the permissible limit of fluoride in drinking water of 1.5 mg/L,according to the recommendation of WHO.The maximum adsorption capacity could be reached to 297.70 mg/g as initial fluoride concentration added to 300 mg/L.In addition,four types of organic dyes(Congo Red,Xylenol Orange,Methylene Blue and Methyl Orange,abbreviated as CR,XO,MB and MO,respectively)were used to test the organic dyes adsorption of as-synthesized Zr02 mesoporous fibers and the adsorption capacities.The adsorption capacity for CR was the highest with 102.41 mg/g.Besides,the ZrO2 fibers also had excellent adsorption for XO(adsorption capacity:96.85 mg/g)and MB(adsorption capacity:62.07 mg/g),while the sample had nearly no adsorption for MO.The results indicated that the as-synthesized ZrO2 mesoporous fibers had selective adsorption for organic dyes.In chapter 4,template-free electrospinning method was developed to synthesize MgO mesoporous nanofibers which exhibited excellent adsorption capacities for fluoride and CR.(1)Without the addition of any template,MgO precursor fibers are prepared using magnesium acetate and magnesium citrate as two magnesium sources,polyethylene oxide as spinning aid and water as the solvent.The diameter distribution of the precursor fibers were 250-500 nm.(2)After the treatment at different temperatures,the bump structure appeared on the surface of the MgO nanofibers and it became more obvious with the increasing of calcination temperature.The HRTEM displayed the nano-size bump and the nanocrystalline inside the nanofiber were all cubic MgO.The forming mechanism was investigated.The prepared mesoporous MgO nanofibers without using any template have high surface area of 194.17 m2/g,which still kept 40.78 m2/gas the temperature rose to 800 ?.(3)The adsorption characteristics of the MgO nanofibers to fluoride and organic dyes were studied,which showed that the obtained MgO nanofibers exhibited excellent adsorption for fluoride,and the maximum capacities for the organic dye CR was 4802.27 mg/g with removal rate over 95%.The adsorption process was simulated by isothermal adsorption and adsorption kinetics.The influence of the pH value on the adsorption process and the adsorption mechanism to fluoride were discussed.In chapter 5,the main conclusions and the innovation points of the thesis were summarized.The problems existing in the research work and some questions needing to be deeply researched and resolved were discussed,and the future work was prospected.