Studies On Oxidative Dehydrogenation Of Propane Over Polyoxometalates Catalysts And Synthesis Of H₂O₂ Under Normal Pressure Over Polyoxometalates-Supported Palladium Heterogeneous Catalysts

This dissertation first studied the oxidative dehydrogenation of propane at a mildtemperature (653 K) over two series of Keggin-type polyoxometalates catalysts (i.e.,Cs_xH_3-xPMo₁₂O₄₀ and Cs_xH_4-xPVMo₁₁O₄₀) with different cesium contents.Theconversion of propane decreased while the selectivity to propylene formationincreased with the increase of cesium content in these samples,and the yield ofpropylene reaches a maximum at appropriate cesium content for both series ofcatalysts.The best yield of propylene (9.3%) was achieved over theCs_2.56H_0.44PMo₁₂O₄₀ catalyst and the vanadium substitution did not enhance the yieldof propylene.The correlation between catalytic behaviors and physicochemicalproperties of these catalysts suggests that the acidity plays crucial roles for theoxidative dehydrogenation of propane.It is clarified that the rate of propaneconversion increased proportionally to the concentration of Br(?)nsted acid sites,whereas the selectivity to propylene formation decreased with the increase of thesurface acidity.This dissertation also studied the direct synthesis of H₂O₂ from H₂ and O₂ undernormal pressure over polyoxometalates-supported Pd catalysts in the absence of anyliquid acid or halide additives in the liquid phase.It was confirmed that theCs-substituted 12-tungstophospharates could be used as solid acid catalyst whichbenefited for the direct synthesis of H₂O₂ from H₂ and O₂.The combination of thecharacterizations and the catalytic reactions shows that both the acidity of thepolyoxometalate and the size of Pd nanoparticles may determine the catalyticbehaviors of the catalyst.Over 1.0 wt% Pd/Cs_1.5H_1.5PW₁₂O₄₀ catalyst with Pdnanoparticles of 3.8 nm and a proper acidity,the concentration of H₂O₂ could reach upto 1.1% in ethanol after 4 h of reaction at 283 K.This result was close to that obtainedin the reaction systems containing both acid and halide additives under the samecondition.This work provided a green route for the direct synthesis of H₂O₂ from H₂and O₂ under normal pressure.