Structural studies of high-area zeolitic adsorbents and catalysts by a combination of high-resolution X-ray powder diffraction and X-ray absorption spectroscopy

Abstract

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 Å). 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 S_Π 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.