Study On Supported Heteropoly Acids For Ethane And Propane Partial Oxidation

Heteropoly acids (HPA) are promising catalysts for ethane and propane partial oxidation. Silica supported heteropoly acids were adopted here to investigate the ethane oxidative dehydrogenation and propane partial oxidation with molecular oxygen as oxidant.A conventional acidification method incorporated with an ether extraction technique was used to prepare HPA with Keggin structure. An incipient wetness method was employed to prepare silica supported HPA. The supported HPAs were characterized by IR ,XRD , TPD , TPR , BET , TG , DTA , and their catalytic performance was examined in an atmospheric fixed-bed reactor at a feed ratio of C₂H₆/O₂ (C₃H₈/O₂)=2 and a space velocity of 1500h^-1. Catalyst samples after reaction were characterized by XRD , TPD , TPR and XPS. The reaction mechanism and pathways were also discussed.It has been found that HPAs with a Keggin structure can be prepared successfully by method employed. The support SiO₂ improves the thermal stability of HPAs. The calcination temperature, the amount of loading on the support and V, W substitution into the Keggin structure influence the acidity and redox property of supported HPAs.It was observed that there exists a light off temperature for ethane reaction. When the reaction was carried out at below this temperature, the conversion is very low, while over this temperature, the reaction was controlled by a free radical gas phase mechanism. Above the light off temperature, the main products of ethane oxidative dehydration were ethene, carbon monoxide and carbon dioxide. Gas phase radical reaction was the main pathway, but the surface reaction took place simultaneously. The acidity of HPA controls the ethane adsorption on the surface. The stronger the acidity was, the higher the light-off temperature of radical reaction was. The redox property influences also the catalytic performance. H₆PMo₉V₃O₄₀/SiO₂ with a suitable redox property gave the best result of ethane oxidative dehydrogenation.The partial oxidation of propane on supported HPA showed different catalytic performance at lower and higher reaction temperatures respectively. The conversion of propane was relatively low at low temperatures, but it is beneficial for the production of partial oxidation products. The conversion of propane was high at high temperatures, but the amount of CO_X in the product was relatively high. At low temperatures, only the surface reaction took place. It is proposed that propane be adsorbed in two manners: M-O—C₃H₈ and M—C₃H₈. The former one is beneficial for the production of partial oxidation products, while the latter one is for the production of CO_x At H₃PMo₍12_O40/SiO₂ (calcined at 350°C. 20wt% on SiO₂ ), propane was mainly adsorbed in a M-O—C₃H₈ manner, so it gives a good result. At high temperatures, there were two pathways: gas phase radical reaction and surface reaction. At this temperature, propane was mainly adsorbed on the surface in M—C₃H₈ manner, so it gives a lot of CO_x.