Fabrication And Characterization Of Mesoporous Cordierite Materials

Cordierite(Mg2A14Si5O18)has become a commonly used carrier material for monolithic catalysts due to its low thermal expansion coefficient,high temperature resistance,good chemical stability and excellent thermal shock resistance.In this thesis,a new method is proposed to improve the structure of cordierite carrier,so as to enlarge its specific surface area.The glass with a major composition of cordierite stoichiometry(2MgO·2A12O3·5SiO2)is the precursor of this fabrication.Once the dendrite structure prepared,hydrofluoric acid is used as solvent to remove the residual glass phase filling in the crystal.After that,the cordierite material with mesoporous structure will be obtained.The differential scanning calorimeter(DSC),scanning electron microscope(SEM)and X-ray Diffraction Analysis(XRD)were used to characterize the samples performance.The effects of different crystalline TiO2 nucleating agents on the crystallization ability and crystal morphology of the glass were systematically studied.The effect of TiO2 content and heat treatment on the crystallization ability of the glass and the morphology of precipitated crystals were investigated.Under the etch conditions,the etching of glass-ceramics by hydrofluoric acid was demonstrated.And we also tested the compressive strength,thermal expansion coefficient,BET specific surface area and pore size analysis of the sample.The results show that,different types of TiO2 have different promoting effects on crystal growth in different directions.And to some extent,the crystallization performance of the sample with the nucleating agent is improved when compared with the sample without the introduction of the nucleating agent.The introduction of the nucleating agent significantly reduces the crystallization activation energy of the glass,meanwhile,according to the Kissinger method,the activation energy decreased from 513.7 kJ/mol to 421.6 kJ/mol.Under the same crystallizing conditions,the samples with rutile TiO2 as the nucleating agent have the best crystallization properties and the best dendrites are precipitated.When the TiO2 content is lower than 2.60%,the cordierite phase is the main crystalline phase in the glass-ceramics.Until the TiO2 content increases to 3.90%even larger,the main crystalline phase is transformed from cordierite to spinel phase,and the crystal morphology changes from"dendritic" to "particulate".The optimum micro-crystallization heat treatment system for nucleating agent TiO2 content of 2.60%was summarized as follows:768 ? nucleation treatment for 2 hours,949? crystallization treatment for 2 hours,heating rate 10 ?/min.If the concentration of hydrofluoric acid is overhigh,the crystal will be heavily eroded,otherwise,the etching efficiency will be low.By fitting the reaction series,the relationship between etching time and hydrofluoric acid concentration was deduced as tad=?m/S·ki·CHFni(CHF?6.55,i=1;CHF>6.55.i=2),where n1=0.93,k1=4.81,n2=1.71,k2=1.12 k2=1.12.The optimized HF concentration is 6.55 mol/L.The etch rate of l mol/LHF solution on amorphous phase in MAS is 218 times of pure cordierite crystal at room temperature.The activation energy values of cordierite and amorphous phase in HF solution are 52.5 and 30.6 kJ/mol respectively.The glass-ceramics were etched using 6 mol/L hydrofluoric acid for 40 min to obtain mesoporous cordierite with a compressive strength of 10.8 MPa,a thermal expansion coefficient of 1.2×10-6/K,and a specific surface area of 59.4 m2/g.The average pore size is 10.9 nm.Continue to increase the etching time will cause erosion of the cordierite structure.