氧化铝对Pt-SO4^2-／ZrO2-Al2O3固体酸催化剂结构与酸性的影响

通过混捏法制备了Pt-SO4^2-／ZrO2-Al2O3固体酸催化剂，考察了该催化剂在正构烷烃低温异构化反应中的催化稳定性．采用X射线衍射、差热分析、X射线光电子能谱、核磁共振、N2吸附-脱附和微量热等表征技术，对氧化铝的作用机理进行了研究．结果表明，氧化铝起到结构助催化剂作用，对四方相氧化锆具有稳定作用，并延迟了氧化锆的晶化，抑制了氧化锆的烧结，从而增大了催化剂的比表面积和孔容．XPS结果表明，Zr3d峰向低结合能方向移动，Al2p峰向高结合能方向移动，说明Al^3＋的电子向Zr^4＋偏移．微量热结果表明氧化铝的引入提高了催化剂的酸量和酸强度．^27Al固体核磁共振结果表明，Pt-SO4^2-／ZrO2-Al2O3中的六配位铝的结构发生了变化．氧化铝的引入还可能有助于氧化锆晶格缺陷的形成，焙烧过程中Al^3＋迁移进入氧化锆晶格并同晶取代Zr^4＋形成氧缺陷位，这些氧缺陷位在S=O键诱导下显示强酸性．

Effect of Alumina on Structure and Acidity of Solid Acid Catalyst Pt-SO4^2-/ZrO2-Al2O3

Solid acid catalyst Pt-SO~(2-)_4/ZrO_2-Al_2O_3 was prepared by a kneading method.Its stability for the isomerization of normal alkane at low temperature was evaluated in a flow microreactor-chromatogram system.The effect of alumina on physico-chemical properties of the catalyst was investigated by means of N_2 adsorption-desorption,X-ray diffraction,differential thermal analysis,X-ray photoelectron spectroscopy,magic angle spinning nuclear magnetic resonance spectroscopy and microcalorimetry.As a structure co-catalyst,alumina can retard the crystallization of zirconia and restrain zirconia particles from sintering and agglomerating,so that the specific surface area and pore volume of the catalyst are increased.Alumina also inhibits the transformation of zirconia from a metastable tetragonal phase to a monoclinic phase.The XPS results indicate that an electron transfer from Al~(3+) to Zr~(4+)occurs.The()~(27)Al MAS NMR results show that the structure of octahedral coordinated Al is changed.The number of acid sites is increased and the acid strength is enchanced in Pt-SO~(2-)_4/ZrO_2-Al_2O_3 compared with Pt-SO~(2-)_4/ZrO_2 according to the microcalorimetry results.A new acid site model is proposed,in which alumina also helps to form lattice defects in zirconia.When the catalyst is calcined,Al~(3+) ions move into the zirconia crystal lattice and replace Zr~(4+) ions so that the oxygen ion defects are generated.These oxygen ion defects show strong acidity under the induction of the(S=O) bond of SO~(2-)_4.