Preparation And Properties Of Magnesium Oxide

Magnesium oxide has been widely applied in various fields including semiconductor devices,refractory,high temperature superconducting products and catalysts,due to its typical wide band gap?7.8 eV?,thermal stability and surface basicity.In recent years,numerous studies have demonstrated that the performance of MgO is highly influenced by its morphology.Although several studies reported on the synthesis of MgO and its adsorption and catalytic properties,the preparation process was complicated,time-consuming.The prepared magnesium oxide is still unable to fully meet the needs of adsorption and catalysis.In order to resolve this problem,in this thesis,the following works were mainly carried out as described below:First of all,with Mg?NO₃?₂ 6H₂O and Na₂CO₃ as raw materials,the adsorption properties of magnesium oxide to Congo red in aqueous solution could be improved by adding trace amount of sodium silicate during the synthesis process of rod-like magnesium oxide.Its adsorption capacity is 3236 mg/g,which has been reported as the highest value of magnesium oxide for adsorption of Congo red to date.At the same time,it was found that the amount of sodium silicate,stirring time and calcination temperature affected the the strength of surface basicity of the magnesim oxide,so the adsorption property of the series of MgO has been changed according to different conditions.By systematically studying the adsorption process of Congo red on the as-synthesized MgO,it was found that the adsorption process fits well with pseudo-second-order equation and Langmuir adsorption model.It is noticeable that the adsorption performance of rod-like magnesium oxide can also be enhanced in the synthesis process by introducing the trace amount of sodium phosphate,sodium tripolyphosphate and sodium polyphosphate besides sodium silicate.On the contrary to these salts,the performance of the MgO from the systems of sodium fluoride and sodium oxalate presents a gradually decreasing trend with the increase of their involved amount which could be due to their different physico-chemical properties.Owing to the powerful capability of FT-IR spectrometry,it is clearly that with the increase of sodium silicate,sodium phosphate,sodium tripolyphosphate and sodium polyphosphate,the intensity of P-O stretching vibration and Si-O stretching vibration increases graduall.More interstingly,the intensity of Mg-O bending vibration peak from FT-IR spectra is closely related to the adsorption capability of the resulting particles from different amounts of sodium phosphate,sodium tripolyphosphate,sodium polyphosphate and sodium silicate.The weaker the absorption band for the Mg-O bending vibration,the stronger the adsorption ability of resulting MgO for Congo red.However,when sodium fluoride and sodium oxalate were used as an additive,an opposite phenomenon was observed.We also observed that the introduction of 0.1 g sodium silicate can not only enhance the adsorption performance of rod-like MgO,but also facilitate the capacity of trapezoid-like and sphere-like magnesium oxide.Different morphologies of MgO including trapezoidal,rod-like,flower-like,nest-like,sphere-like structures were prepared by a facile precipitation method.The as-obtained porous MgO was used as catalyst for the reaction of glycerol and dimethyl carbonate to produce glycerol carbonate.By carefully comparing the different morphologies of MgO,it was obvious that trapezoidal MgO exhibits the superior performance among others due to not only its weaker basicity,but also the molar ratio of Mg and O of trapezoidal MgO is closer to 1,which could make it easy to adsorb the glycerol and dimethyl carbonate in solution.These unique physical and chemical properties facilitate the contact between glycerol and dimethyl carbonate.In the reusing experiment,it was found that the second catalytic yield of the first catalytic magnesium oxide under 500 ^oC for roasting 3 h was very low,but when the MgO calcined under nitrogen condition at 400 ^oC within 3 h and then calcined in air at 500 ^oC within the same period of time,the yield of multiple catalysis is over 90%.By FT-IR and CO₂-TPD analysis,it is found that the two calcination methods affect the surface structure of the materials,and make their recycling performance different.