Investigations On Synthesis And Structure-Properties Of Mixed Oxides And Its KF-Supported Solid Base Catalysts From Layered Double Hydroxides

With strengthen of people's environment protection consciousness and the development of green chemistry,employment of eco-friendly solid bases catalysis in the fine chemical industry is of topical interest.In present paper,mixed oxides and KF-supported mixed oxides solid base catalysts were synthesized based on layered double hydroxides(LDHs) precursors.The structure and physico-chemical properties of the prepared catalysts were studied systematically.The novelty and major content of the thesis were detailed as follows:A kind of mixed metal oxide type solid base catalysts were obtained through calcining precursors of MgAlFe-CO₃-LDHs,CoMgAlFe -CO₃-LDH in which Fe³⁺species were chosen as a tailoring element controlling acid-base property.The structural,thermal,textural and bases properties of the mixed oxids were examined by using XRD,FT-IR,in situ high temperature XRD,TG-MS,CO₂-TPD,phenol adsorption and Knoevenagel condensation reactivity.The results manifest that the compositions and calcined temperature significantly affect the catalytic activity.Fe-500 derived from MgAlFe-CO₃-LDHs have the highest activity.And the catalytic activity is majorly dependent on the basic strength and the quantity of the surface basic sites,but less on the surface area and pore size.The middle strength basic site is critical for Knoevenagel condensation.Moreover,the Fe content significantly affect the catalytic activity.With Fe molar fraction range of 0.02≤Fe/(Mg+Al+Fe)＜0.06,catalytic activity of MgAlFe oxides is higher than that of MgAl mixed oxides from MgAl-CO₃-LDH,probably because that surface segregation of Fe in the samples with lower Fe content moderately increases the number of Lewis acidic sites and thus activates the carbonyl group of benzaldehyde.When Fe/(Mg+Al+Fe)beyond 0.06, the number of acidic site is enhanced with increasing content of Fe, whereas the reduced number of base sites result in a lower catalytic activity.A series of KF-supported MgAl mixed oxides was synthesized from the layered precursors for the first time.Their basicity character is revealed mainly by in situ FT-IR after adsorption of CO₂ and transesterification reaction of ethylene carbonate(EC)and methanol.The results manifest that the higher basicity and activity of catalysts is correlated to the highly dispersed F^-.The loading content of KF affects the catalytic activity.When the content of KF was 11.8 wt%,the EC conversion reaches 94%and the selectivity to dimethyl carbonate(DMC) is 81%.The corresponding space time yield(STY)is 0.75 DMC mmol/(g cat·min),which was 38 times to the as-prepared MgAl mixed oxides,and 7 times to Mg_2.5Al-OH-LDHs reported in literature.The present eco-friendly catalyst is a potential alternative to soluble bases in the fine chemical industry.