高分辨率x射线粉末衍射和x射线吸收光谱相结合的高面积沸石吸附剂和催化剂的结构研究

E. Dooryhee, G. Greaves, A. T. Steel, R. P. Townsend, S. Carr, John Meurig Thomas, C. Catlow
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引用次数: 47

摘要

我们在高温下表征了一种模型均匀非均相催化剂,用于烃类的低聚化(一种初始成分为Na59Al59Si133O384·xH2O的镍交换沸石,经NiCl2水溶液处理,从而产生均匀分布的Ni)。当Si/Ni = 7时),通过记录Ni边缘以上的扩展x射线吸收精细结构(EXAFS)及其高分辨率衍射图(λ= 1.5486 A),我们对Ni2+离子在制备、脱水和还原状态下的微环境有了独特的认识;特别是得到了原子座标、占位和键长的值。高分辨率X射线衍射,使用特殊构造的环境电池,能够在室温1.75 bar的压力下,在钠交换沸石X吸附剂(每单元电池1.5个Xe原子)中吸附Xe原子的精确位置,通过粉末衍射模式的Rietveld细化来确定。直接证明了位于SΠ位置的强极化Na+离子在将Xe牢固地结合到超级笼内壁中的作用。本文还报道了其他沸石的x射线吸收近边结构(XANES)以及Al, Si和Ga边缘以上的平行EXAFS研究。结合(Ga边)XANES和EXAFS研究表明,与晶体(gallo)沸石X和Y中存在的网络相似的没水硅酸盐网络已经在前驱体凝胶中形成,并且也存在于与胶体-非晶材料接触的水溶液中,它们共同构成了凝胶。我们还报道了Al - EXAFS在破碎质沸石脱铝半定量研究中的第一个应用。
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Structural studies of high-area zeolitic adsorbents and catalysts by a combination of high-resolution X-ray powder diffraction and X-ray absorption spectroscopy
We have characterized at high temperature a model uniform heterogeneous catalyst for the oligomerization of hydrocarbons (a nickel-exchanged zeolite of initial composition Na59Al59Si133O384·xH2O treated with an aqueous solution of NiCl2 so as to yield a homogeneous distribution of Ni, with Si/Ni = 7) by recording the extended X-ray absorption fine structure (EXAFS) above the Ni edge and also its high-resolution diffraction pattern (at λ= 1.5486 A). We have obtained unique insights into the microenvironment of the Ni2+ ions in the as-prepared and the dehydrated as well as the reduced state of the catalyst; in particular, values of atomic coordinates, site-occupancy and bond lengths, have been obtained. High-resolution X-ray diffraction, using a specially constructed environmental cell, has enabled the precise location of sorbed Xe atoms in a sodium-exchanged zeolite X adsorbent (1.5 atoms of Xe per unit cell) under a pressure of 1.75 bar at room temperature to be determined from a Rietveld refinement of the powder diffraction pattern. Direct proof of the role of the strongly polarizing Na+ ion located at the SΠ site in firmly binding the Xe to the inner wall of the supercage is obtained.X-ray absorption near-edge structures (XANES) as well as parallel EXAFS studies above the Al, Si and Ga edges on a variety of other zeolites are also reported. Combined (Ga edge) XANES and EXAFS studies show that gallosilicate networks akin to those present in crystalline (gallo) zeolite X and Y are already formed in the precursor gel and are also present in the aqueous solution in contact with the colloidal-amorphous material, which together constitute the gel. We also report one of the first applications of Al EXAFS for the semi-quantitative study of the dealumination of faujasitic zeolites.
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