Investigation On The Preparation And Adsorption Performance Of MgO-based Inorganic Oxide Fibers By Electrospinning Method

Electrospinning is a facile and advanced method for nanofiber preparation.Combined with sol-gel method,oxide fibers can be prepared by electrospinning,and structure control can be realized by designing precursors and controlling heat treatment process,thus obtaining a variety of oxide fiber materials that meet expectations.Magnesium oxide(MgO)has the characteristics of high melting point,high activity and high specific surface area(SBET),and can be used as heat insulating material,catalyst and adsorbent.MgO fiber can be used as light refractory material,while MgO fiber with submicron-sized fine diameter has application value in new fields due to the larger SBET and other structural characteristics.This thesis focused on the development and utilization of MgO-based inorganic oxide fibers,and conducted a series of researches on electrospinning of MgO fibers and SiO2-MgO binary oxide fibers,and discussed their(liquid phase)adsorption properties.The work is based on the development of solution electrospinning strategy for aqueous solvent system,aiming at low cost,simplicity and environmentally sound,avoiding the pollution and safety problems of organic solvent while optimizing the fiber preparation method.The main contributions of this thesis include:solving the problem of high-temperature pulverization of fine diameter MgO fibers due to the brittle nature of MgO,successfully synthesizing the water-based Si-Mg sol,filling the gap in direct electrospinning preparation strategy of SiO2-MgO fibers.The main work of the dissertation is as follows:?.Improving MgO fiber quality through precursor optimization1.The preparation,characterization and analyses of evolution process of MgO fibers.Magnesium acetate tetrahydrate(MATH)and poly magnesium citrate(PMC)were used as double precursors,water was used as solvent,polyvinyl pyrrolidone(PVP)was used as spinning aid for electrospinning,and MgO fiber was obtained after heat-treatment.The Fourier transform infrared(FT-IR)spectrum,X-ray diffraction(XRD),thermogravimetry and differential scanning calorimetry(TG/DSC)analyses showed that the double precursor fibers have mild thermal decomposition process and large crystallization temperature range,while the thermal decomposition process of single precursor was more violent and the crystallization temperature range was narrow.The slow removal of volatiles and the slow heat loss were the two main characteristics of the mild decomposition process.The thermal decomposition process had a significant impact on the morphology and quality of fibers.According to the results of thermal analyses,a formation mechanism of polycrystalline MgO fiber was proposed.The appropriate ratio of MATH and PMC brought about a mild thermal decomposition process and a continuous crystallization process,thus forming MgO fiber with compact structure and excellent thermal stability.When the mass fraction of PMC in the precursor was 30%-50%,the fiber would not be pulverized after heat-treated at 1000?,and maintain a fluffy morphology with an average diameter distribution of 23 5±5nm.Therefore,the use of double precursors could solve the problem of high-temperature pulverization of MgO fibers,and the controlment of morphology and structure for MgO fibers could be realized by adjusting the ratio of precursors.?.Preparation of mesoporous MgO fibers by double precursors1.Propanoic acid took the place of glacial acetic acid as the ligand source.Magnesium oxypropionate(PMP)and PMC were used as the new double precursors for spinning,and the specific surface area of mesoporous MgO fibers was improved by adjusting the precursor ratio and heat-treatment.Since the heat-treatment process significantly affects the structure and composition of oxide fibers,the differences in properties of MgO fibers heat-reated in air and water vapor were carried out,compared and discussed.Scanning electron microscopy(SEM)and transmission electron microscopy(TEM)characterization results showed that air-treated fibers had a unique hemispherical secondary structure,while steam-treated fibers did not.The fibers treated in water vapor had regular morphology and uniform composition,but their SBET-and surface alkalinity were lower than those heat-treated in air.The air heat-trated fibers obtained from double precursor with the mass ratio of PMC:PMP of 1:1.76 had a SBET of 232.4 m2/g,a total pore volume of 0.560 cm3/g,and mesoporous pore diameters were concentrated at 2-10 nm.2.The adsorption kinetics,isotherm and interaction mechanism of MgO fiber for Pb(?)and Cd(?)were studied.The maximum adsorption capacities of MgO fiber for Pb(?)and Cd(?)were 2983.4 and 1824.0 mg/g,respectively.Compared with other reported powdered MgO adsorbents,mesoporous MgO fibers were easy to move in aqueous solution,easy to separate from solution,and have larger adsorption capacity for Pb(?)and Cd(?).The mechanism of adsorption of Pb(?)and Cd(?)by mesoporous MgO fibers included surface precipitation of heavy metal ions with OH-and ion exchange with Mg2+ or H+in acidic hydroxyl.On the one hand,the high adsorption capacity of mesoporous MgO fiber was attributed to its strong hydration and hydroxylation ability,which resulted in a hydroxyl-rich surface.On the other hand,the fiber morphology and surface hemispherical structure enhanced the fluidity and solid-liquid contact area of the adsorbent.?.Preparation of mesoporous SiO2-MgO hybrid fibers(SMHFs)by one-step sol electrospinning and heavy metal adsorption study1.Mesoporous SMHFs were prepared by one-step sol electrospinning to overcome the defect of poor mechanical strength of MgO and improve the recyclability of MgO-based fiber adsorbents.Spinnable Si-Mg sol precursor spinning solution was successfully synthesized by using strong acid magnesium salt MgCl2·6H2O,Mg(NO3)2·6H2O as magnesium source,tetraethyl orthosilicate(TEOS)as silicon source,water/ethanol as cosolvent,cetyltrimethyl ammonium bromide(CTAB)as pore expanding agent and polyvinyl alcohol(PVA)as spinning aid.Three heat-treatment systems,including linear heating,step heating and high pressure steam(HPS)pretreatment heating,were used to heat-treat fibers of Si/Mg ratio of 2:1.HPS pretreatment heating significantly increased the SBET of the fiber to 485.3 m2/g and the pore volume to 0.621 cm3/g..TEM results showed that the fibers obtained by the first two heat-treatment methods(air treatment)had a relatively dense shell structure,while HPS pretreatment heating removed pore-forming inhibiting factors and gave the fibers a uniform porous structure.The fibers prepared with MgC12-6H2O as magnesium source could expose MgO(111)polar surface,and its SBET and surface alkalinity were higher than those prepared with Mg(NO3)2 ·6H2O.2.The affinity order of mesoporous SMHFs to five common divalent metal ions was Pb(?)>Cu(?)>Ni(?)>Cd(?)>Zn(?).Adsorption kinetics showed that the adsorption process of Pb(?)and Cu(?)conformed to pseudo second-order(PSO)kinetic equation.The adsorption isotherms of Pb(?)and Cu(?)conformed to Langmuir model.The Langmuir adsorption capacities(QmL)of Pb(?)and Cu(?)were 787.9 and 493.0 mg/g respectively at 298 K.The Gibbs free energy change ?G0 of SiO2-MgO fiber adsorbing Pb(?)and Cu(II)ranged from-33.33 to-27.52 kJ/mol,and the absolute value of ?G10 increased with temperature rise,which indicated that the adsorption process was a physical adsorption and chemical adsorption coexisted process.Compared with MgO fiber and SiO2 fiber,SMHFs had ideal recyclability and recycling performance.After three adsorption cycles,the fiber morphology did not change significantly.Moreover,SMHFs neutralized the acidic solution without alkalizing it,as the solution equilibrium pH value after SiO2-MgO fiber absorbing heavy metal ions was 6.1-6.6.?.Preparation of mesoporous MgO-SiO2 fibers by citric-acid-assisted sol-gel method1.A novel,simple and template-free mesoporous MgO-SiO2 fiber synthesis strategy that met the green chemistry requirement was developed.Citric acid was used as an important multi purpose raw material which could be used for promoting TEOS hydrolysis,synthesizing Mg sol and reaming.In this part,water/ethanol was still used as co-solvent,TEOS hydrolysate,magnesium citrate sol and PVA spinning aid were used to synthesize spinnable Si-Mg sol,and appropriate amount of H2O2 was added to the sol in-situ to improve fiber quality.The whole spinning solution preparation process did not contain any template agent and harmful chemicals.H2O2 addition could decolorize the fiber,enhance the hydrophilicity,improve the surface alkalinity but reduce the SBET of the fiber.MgO-SiO2 fibers obtained by heat-treatment in air were cylindrical long fiber with smooth surface and slit-like pore structure of 3-17 nm mesopores.2.Fiber MS6H0.5-600 with Si:Mg molar ratio of 1.5:1 and SBET of 142.61 m2/g and strong surface alkalinity was selected as adsorbent.The adsorption capacity of MS6H0.5-600 for metal ions(Pb(?)/Cu(?))were better than those for organic pollutants,such as methylene blue(MB),fulvic acid(FA).The adsorption capacities for Pb(?),Cu(?),MB and FA were 753.12,480.96,315.6 and 24.04 mg/g,respectively.The adsorption isotherms and kinetics of Pb(II)and Cu(II)conformed to Langmuir model and PSO kinetics model,while FA adsorption conformed to Freundlich isotherm model and pseudo first-order(PFO)kinetics model.MB adsorption conformed to Freundlich isotherm model,but its adsorption kinetics process was special that the initial adsorption followed PSO model and then PFO model.The main adsorption mechanism of MgO-SiO2 fiber was ion exchange and micro-precipitation with Pb(II),Cu(?),and physical adsorption of MB and FA.Fiber showed good recycling ability to three pollutants except FA.Compared with the work in the previous chapter,MgO-SiO2 fiber prepared by citric acid-assisted sol-gel method had better morphology but lower SBET,and its adsorption performance was acceptable.This work simplified the raw materials and process steps required for preparing fibers and met green chemistry requirement.?.Characterization of magnesium silicate(MgSiO3)fiber1.Magnesium silicate fibers were obtained and characterized by high temperature heat-treatment with the help of the preparation methods of MgO-SiO2 fibers.The thermal decomposition process,the difference morphology and structure for two kinds of MgSiO3 fibers were investigated.Compared with the fibers after heat-treatment at 1000?,the average diameter of MgSiO3 fibers prepared with MgCl2·6H2O as magnesium source was 0.62 ?m,which was curled or helical and had a core-shell layered structure.The average diameter of the fibers prepared by citrate was 1.16 ?m with the morphology of long straight fibers,and the fiber structure was uniform.The fiber tablets pressed by the two fibers had low thermal conductivities,and which at 400-800? were 0.200-0.603 W/(m·K).