氧化鎢與磷酸對ZrO2-TiO2固態酸表面酸性影響之探討

Abstract

本研究以低電負度的Zr4+與Ti4+金屬離子搭配模板法製備多孔性ZrO2-TiO2複合金屬氧化物作為固態酸基材，並以摻雜與含浸兩種手法將具有高電負度中心的氧化鎢與磷酸物種導入基材結構，以探討修飾物種對表面酸性的影響。結果顯示本研究成功地製備比表面積高達195 m2/g 且孔徑集中於6.3 nm的多孔材料，ZrO2與TiO2彼此間的相容性有效穩定基材無晶相結構直至600C，表面酸量則為859 µmol/g。磷酸 (PO4)與氧化鎢 (WO3) 以摻雜法導入結構中後可將基材表面酸量分別提高至1,547及972 µmol/g，產生最高酸量的表面P/M及W/M比例分別為0.44及0.16，反之以含浸法將修飾物種附載於表面對酸性提升較不顯著。晶格中W6+離子的高配位數與高電負度使基材表面形成氧空缺與強路易士酸 (Lewis acid)，另一方面，低配位的P5+離子則使表面生成大量氫氧官能基的布朗斯特酸 (Bronsted acid)。以結構穩定度而言，PO4相較於WO3更能抑制基材結晶達700C以上，因此磷酸修飾的基材可表現出270 m2/g的比表面積與3.5 nm的主要孔徑。經NH3-TPD重複測試，磷酸化ZrO2-TiO2固態酸具有良好氨氣吸脫附再現性，可供重複使用；而與商用沸石NAY (1,544 µmol/g) 與ZSM-5 (519 µmol/g) 比較，也顯示該樣品具有高酸性、酸密度與更優良的應用潛力。

Owing to the low electronegtivity of Zr4+ and Ti4+ ions, a porous ZrO2-TiO2 composite was prepared using a templating method and was further modified with tungstate and phosphate species, which contain high electrongativity centers, through incorporation or impregnation method to increase the surface aidity. The substrate exhibited a high surface area of 195 m2/g with a pore size of 6.3 nm. The quantity of the acidic sites was 859 µmol/g. Because of the great solubility of the ZrO2 in the TiO2 matrix, crystallization is inhibited and the porous structure is stabilized till 600C. Rather than impregnation, incorporation of the two modifiers enhanced the surface acidity to a higher degree. The incorporated phosphate and tungstate species increased the number of the acidic sites to 1,547 and 972 µmole/g, respectively. The optimal P/M and W/M raito which leads to the highest acidity was 0.44 and 0.16, respectively. The W6+ ions with a high coordination number and electronegativity within the surface lattice resulted in oxygen vacancies and strong Lewis acid sites. On the other hand, P5+ ions led to Bronsted acidity because their low coordination number creates substaintial anounts of hydroxyl groups on the Ti4+ or Zr4+ centers. Compared to the WO3 moieties, PO43- species is more capable of stabilizing the microstructure of the substrate to aginast thermally induced transformation. Therefore, the incorporated phospahe speices increased the surface area to 270 m2/g and maintained the small pore size of 3.5 nm. The repeated NH3-TPD test indicated that the modified solid acid peformed high recoverability. In addition, its acid amount and density was higher than those of the commercial zeolite, suggesting the phosphated ZrO2-TiO2 solid acids a promising adsorbent for removal of base gases.