Showing papers on "Polyoxometalate published in 1991"

Catalysis by polyoxometalates. Part 1.—Supported polyoxoanions of the Keggin structure: spectroscopic study (IR, Raman, UV) of solutions used for impregnation

Abstract: Solutions of Keggin-structure polyoxometalates have been studied by IR, Raman and UV spectroscopies, with various solvents and counterions. For the tetraalkylammonium compounds, cation–anion aggregates are observed. This model of cation–anion aggregates can be applied to the solutions of the acids in DMF and DMSO. The different behaviour of the acids in water, acetonitrile and acetone can be explained in terms of anion–anion interactions, similar to those existing in the solid state. The results are discussed in connection with the use of polyoxometalate solutions for preparing supported catalysts by impregnation.

Polyoxometalate intercalated layered double hydroxides.

Abstract: This disclosure describes the synthesis of layered double hydroxide compositions interlayered by polyoxometalate anions with Keggin-type structures and having the formula [M1-x"Mx'''(OH)2]Ax/n-.yH2O, wherein, M'' is a divalent metal and M''' is a trivalent metal, A is an anionic polyoxometalate of negative charge n, x range between 0.12 to 0.8, and y is a positive number. The products exhibit well-defined XRD peaks corresponding to uniformly crystalline layered products with basal spacing values > 14 .ANG..

[H3KV12As3O39(AsO4)]6- and related topological and/or structural aspects of polyoxometalate chemistry

Abstract: On reduction of an aqueous solution of vanadate(V) with thiocyanide in the presence of As(V), the black-gray mixed-valence compound K6[H3KV12As3O39(AsO4)].8H2O (1) is formed. It crystallizes in the space group C2/c with a = 39.993 (15) angstrom, b = 13.467 (4) angstrom, c = 18.209 (6) angstrom, beta = 111.52 (3)-degrees, and Z = 8. Refinement of 686 variables with 8827 observed reflections yields R = 0.049 and R(w) = 0.058. The compound contains an anion with approximate C3 symmetry, which can be described as a fragment of a sphere capped with a potassium ion. The oxygen and/or metal atoms of polyoxometalates often span the vertices of topologically and/or geometrically interesting polyhedra. Related aspects of 1 are discussed.

Polyoxometatate intercalated layered double hydroxides

Abstract: This invention relates to an intercalated uniform crystalline layered double hydroxide clay composition interlayered by polyoxometalate anions with Keggin-type structures and having the formula Zn2Al(OH)2Ax/n .yH2O wherein A is an anionic polyoxometalate of negative charge n, x ranges between 0.12 and 0.8 and the x-ray diffraction basal spacing value is greater than about 14 A. It further relates to the use of such compositions as catalysts in the oxidation of methane, sulfur oxides and nitrogen oxides.

Systematic Preparation Of Polyoxometalate Pillared Layered Double Hydroxides Via Direct Aqueous Reaction

Abstract: The series of layered double hydroxides, Mg n Al(OH)2n+2 X (n = 2–5, X = NO3 − or Cl−) and Ni n Al(OH) 2n+2 X (n = 2–4, X = NO3 − or Cl−), were pillared with polyoxometalate ions of the Keggin structure, ranging from [PW12O40]3− to [PV4W8O40]7−, in aqueous solution. Full exchange of the Keggin ions into the layers is achieved as evidenced by X-ray powder diffraction, infrared spectroscopy, elemental analysis and thermogravimetric analysis. Also, partial exchange was observed for Ni5Al(OH)12NO3 The key factor that effects these reactions is the pre-swelling step. Thus, when the layered double hydroxides are thoroughly wet either by preparing wet solids or by soaking the dried products for an extended period, pillaring reactions proceed with great ease. Relations can be extracted between the water content and the separation between the layers, as well as between layer charge density and the charge of the Keggin ion. The pillared layered double hydroxides were found generally to have interlayer distances ranging from 11.0 to 13.5 A, which is smaller than the numerical sum of the Keggin ion diameter and the ideal thickness of the hydroxide layer. This fact and the low crystallinity are attributed to strong interaction between the basic layered double hydroxides and the acidic Keggin ions. A more crystalline phase with the expected 14.5 A interlayer spacing was also obtained as a mixture with the lower spaced products.