Keggin型钼钒磷杂多酸催化剂上丙烷选择氧化性能的研究

系统研究了不同数目V⁵⁺取代的钼钒磷杂多酸H_3+nPMo_12－nV_nO₄₀ (n＝0～4)催化剂上丙烷选择氧化反应性能. 通过BET, IR, TPR, 紫外-可见光谱等表征手段对催化剂的理化性质进行了考察, 并对催化剂的结构-性能关系进行了初步关联. 在杂多酸的一级结构中, V⁵⁺对Mo⁶⁺的取代不仅改变了杂多阴离子金属-氧桥的键强以及晶格氧的插入能力, 而且也相应地调变了样品的酸量. 催化剂活性随V⁵⁺取代数量的递增而增强; 适宜数量的V⁵⁺取代提高了含氧酸产物的选择性, 而过量的V⁵⁺取代则导致部分氧化产物的深度氧化. 考察了在Keggin型杂多酸二级结构上引入钒物种的影响, 也即将钒物种(VO)²⁺作为抗衡离子取代部分质子以调变催化剂的结构与性质. 实验表明, 处于一级结构和二级结构(VO)²⁺抗衡离子]中的V在反应中均可离析出少量V₂O₅物种. 适宜量的(VO)²⁺物种以及离析出来的少量V₂O₅物种可能均对催化剂的性能有贡献. 显然, 钒在不同位置的价态变化以及形态的不同, 会导致催化性能的相应改变.

Partial Oxidation of Propane over Keggin Type Molybdovanadophosphoric Acids

Partial oxidation of propane over Keggin type molybdovanadophosphoric acids was studied. The physico-chemical properties of the catalysts were investigated by means of BET, UV-visible diffuse reflectance spectra, IR and TPR, and the preliminary structure-performance relationship was established. The substitution of Mo^6＋ with V^5＋ in Keggin structure decreased the vibration strength of bridging M—O bonds and significantly increased the oxidation and oxygen insertion abilities of Keggin oxoanion, thus influencing the catalytic activity and selectivity to oxygenated products. Optimized vanadium atom substitution enhanced the selectivity to acid products and the optimal selectivity could be obtained on the H₅PMo₁₀V₂O₄₀ catalyst. However, excessive vanadium substitution led to deep oxidation of the acid products. It was thought that appropriate amount of (VO)^2＋and small amount of vanadic oxide species segregating from the primary and the secondary structures of the oxo-anions have an impact on the reaction performance.