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 Na59A159Si,330384 -xH,O treated with an aqueous solution of NiC1, so as to yield a homogeneous distribution of Ni, with Si/ Ni = 7) by recording the extended X-ray absorption fine structure (EXAFS) abov? the Ni edge and also its high-resolution diffraction pattern (at A = 1.5486 A). We have obtained unique insights into the microenvironment of the Ni'+ 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 Sn 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 (Cia 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 A1 EXAFS for the semi-quantitative 3;Jdy of the dealumination of faujasitic zeolites.

A hydration-controlled nano-valve in a zeolite?†

“…This has prevented 11 a definitive location of cation positions by Rietveld refinement of powder diffraction data as was possible in other studies. 12 However EXAFS spectroscopy is capable of yielding one-dimensional (angle-averaged) information on the local cation environment regardless of specimen crystallinity. Calcium and potassium K-edge EXAFS measurements (Daresbury SRS) on dehydrated Ca : K-clinoptilolite yield average nearest neighbour peaks 11 which are indeed consistent with the nano-valve model described here (the operative Ca 2+ cation coordinated to 3 framework oxygens ( ~2.35 Å distance) and K + to 4 framework oxygens ( ~3.0 Å); details and typical plots are available as electronic supplementary information; see http://www.rsc.org/suppdata/cc/ 1998/2527).…”

Modelling of catalysts and its relation to experimental problems

“…A study of George et al will show how detailed information may be obtained for a widely studied (Maddox et al 1988) material, namely Ni-zeolite Y, for which activation is known to involve migration of Ni2+ ions from the hexagonal prism (Sj) sites into supercage positions as shown schematically in figure la. In a preliminary study George et al (1992a) showed how the Sl5 Ni2+ ion is stabilized by an extensive relaxation (0.5-0.8 A|) of the surrounding oxygen ions: a result which nicely explains recent exa fs data (of Dooryhee et al 1990Dooryhee et al , 1991 on the dehydrated Ni-zeolite Y system. More detailed simulations were, however, able to follow the change in the energy of the Ni2+ as it moves out of the Sx site, through the sodalite cage and into the supercage, as illustrated in figure lb.…”

Applications of X-ray Powder Diffraction in Protein Crystallography and Drug Screening

“…The SCXD method for structure solution consolidated its applicability in macromolecules early on, with the first crystal structure of a protein, myoglobin, solved in 1960, while increasing numbers of large molecules and macromolecular assemblies have been determined by crystallographic methods during the following years. In the field of X-ray Powder Diffraction (XRPD), in 1947 the Philips 2 of 35 company introduced the first commercial powder diffractometer, while in the 1950-1960s this technique was used primarily by metallurgists and mineralogists for phase identification to study structural imperfections [7], and in some cases structure solution of inorganic materials and minerals [8,9]. The first protein crystal structure determined from high-resolution XRPD data was a variant of T 3 R 3 human insulin-zinc complex [9].…”