Catalytic Oxidation Of Volatile Organic Compounds Over Palygorskite Synthesized Catalysts

VOCs (Volatile Organic Compounds) is one of major gaseous pollution emissions in many industrial processes. It is a mostly promising approach for catalytic oxidation, by which VOCs can be degraded into CO2and H2O at low temperatures. However, the undefined typical pollutants removal mechanism restrict the high active and patience catalyst to the industrial applications of catalytic oxidation. To solve the problem, toluene and formaldehyde are taken as the target pollutants, while pretreatment palygorskite(PG) as catalyst support, loaded metal oxides(MOC),, spinel-type oxide(STC) or perovskite-type oxide(POC) as the activity components(AC). A study has been carried on catalytic oxidation process of toluene and formaldehyde as well as the catalyst optimition. Thus the kinetic mechanism of the optimized catalytic oxidation is also going to be suggested.PG is purified to get high purity catalyst support. It shows no obvious impurities, but PG crystal beams with effectively dispersion exist characterized by transmission electron microscope and X-ray diffractions. A series catalysts loaded on the Cu, Mn MOC, SOC and POC respectively were prepared by impregnation、hydrolysis coprecipitation and mechanical mixing methods. The catalysts were characterized by means of XRD, BET, TEM, TPR and compressive strength test. TEM results show that AC loaded by impregnation and coprecipitation are uniformly dispersed in the surface of PG. Othwise, the particle size of mechanical mixed AC in the POC is50～200nm. It is indicated PG-supported MOC, STC and POC are synthesized by calcination under different temperatures from XRD patterns. STC and POC play important effects on the removal of toluene and formaldehyde. The compressive strength test simultaneously demonstrates that the particle strength of POC prepared by coprecipitation is more than16N, while the POC prepared by mechanical mixing tend to weaken with the increase of AC. When the loaded wt%ratio reached40%, the compressive strength decreased to8.6N. It is through hydrolysis coprecipitation method that MOC can be made with nano structure in PG surface. Obviously MOC have promising catalytic oxidation for toluene, moreover the addition of Ce will promote the formation of copper manganese spinel’s structure and also improve the conversion of toluene. The results prove that when Ce/Cu=0.3, the catalyst annealed at500℃has the highest removal rate of toluene,99%toluene can be removed by6%CuMn2Ce0.3/PG-500at288℃. Coprecipitation can be used to get the nano-structral La1-xSrxMnO3/PG POC after calcination at700℃, which exhibits amorphous phenomenon on PG, but remains the rod morphology when doped amount x=0～0.3and La1-xSrxMnO3is up to3%～11%. The conversion of toluene increases with the increase of doped strontium and amount of capacity,9%La0.7Sr0.3MnO3/PG catalyst transformations toluene completely with the temperature (T99) of285.3℃, and catalystic activity changes unconspicuously when a further increase of doped strontium and AC. In addition, the oxygen content in the catalytic oxidation reaction system should be kept above5vol%, in order to maintain the amount of oxygen required by the reaction system. In the stability experiments of9%La0.7Sr0.3MnO3/PG catalyst for100h, a relatively stable95%toluene conversion can be obtained. Micrometer-level La1-xSrxMnO3/PG made by mechanically mixing is established to be dispersed in PG surface when x=0.3. The temperature(T99) of30%La0.7Sr0.3MnO3/PG catalyst for a complete toluene conversion is302.7℃, with a promotion of17℃for that of the coprecipitated catalysts..Several steps are used in the kinetic study of toluene catalytic oxidation with. a POC(9%La0.7Sr0.3MnO3/PG) and a STC(6%CuMn2Ce0.3/PG-500). The kinetic results show that:the simple series Model is not suitable for describing toluene catalytic oxidation reaction kinetics, while Mars and Van Krevelen Model (MVK) Model is more suitable. The catalytic oxidation reaction of toluene in both catalysts is based on oxidation-reduction mechanism. The activation energy (surface reduction activation energy17.26kJ/mol and surface oxidation activation energy23.62kJ/mol) of toluene in the STC is less than that (22.77kJ/mol and30.54kJ/mol) of the POC, which proves that the reaction is more likely to occur in the STC.MOC, STC and POC have substantial advantage on catalytic oxidation of formaldehyde as well. A99%conversion rate of formaldehyde is performed at179.3℃for6%CuMn2Ce0.5/PG-500, the effect of doped Ce also has been proved in the promotion in formaldehyde conversion rate. However the favorable doped ratio of Ce and Cu is0.5, which is different to the toluene catalytic oxidation. A0.14ratio of La0.7Sr0.3MnO3perovskite to PG can substantially improve the conversion of formaldehyde to99%at195.7℃. In spite of this, experimental results demonstrate that a further increase of AC ratio will bring little promotions.