EXAFS studies of polyoxmetrallates and chromium pillared clays
EXAFS studies of polyoxmetrallates and chromium pillared clays
EXAFS (Extended X-ray Absorption Fine Structure) analysis of V2O5 gave structural parameters in good agreement with the crystal structure, with shells fitted to a distance of 5.2A. A good agreement was also found for (NH3)6V10O28 and K4V2W4O19, although the limitations of EXAFS analysis in fitting large, complex, structures was demonstrated. A multiple scattering contribution was included in the case of K4V2W4O19. The need for good quality data was indicated by the poorer agreement of Zn(H2O)6(NO3)2 and (NBu4)5HZnW12O40 with the crystal data. However, analysis of the polyoxanions, [VW5O19]3-, [Nb6O19]8- and [NbW5O19]3- produced a good fit of the theoretical model to the experimental data. For the latter, solution data, and both Nb K-edge and W L3-edge data, was available. EXAFS analysis was then applied to the chromium pillared clays, for which little structural information about the pillaring species was known. A solution of hydrolysed Cr3+ ions was the pillaring agent and this was shown to contain polymeric species. The EXAFS-derived distances support a model of edge-bridged CrO6 octahedra, in a zigzag array. EXAFS data of the chromium pillared clays, bentonite, beidellite and Laponite, was analysed with the same model. Structural changes in the chromium pillared clays after calcination and hydrogenation were monitored by EXAFS analysis. Upon calcination partial oxidation of Cr(III) to Cr(VI) occurs. Hydrogenation regenerates the octahedral CrO6 units, however, these may be arranged in clusters, rather than sheets as before. Methanol dehydration is catalysed by the chromium pillared clays. The most effective catalyst was chromium pillared beidellite after calcination in vacuo and hydrogenation. The major products were methane (5%), C2-C alkanes and dimethyl ether (35%) and C5+ products, including 2-methylbutane, 1,2-dimethylcyclopropane, dimethoxymethane and 2,2,3-trimethylpentane (60%). Gas chromatography and mass spectrometry of the reaction products formed from deuterated methanol suggest there are two routes for the reaction, which lead different percentages of hydrogen isotope exchange. Therefore it is possible that the reaction proceeds via active sites on the surface and in the interlayers.
University of Southampton
Rummey, Jacqueline Michelle
1990
Rummey, Jacqueline Michelle
Rummey, Jacqueline Michelle
(1990)
EXAFS studies of polyoxmetrallates and chromium pillared clays.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
EXAFS (Extended X-ray Absorption Fine Structure) analysis of V2O5 gave structural parameters in good agreement with the crystal structure, with shells fitted to a distance of 5.2A. A good agreement was also found for (NH3)6V10O28 and K4V2W4O19, although the limitations of EXAFS analysis in fitting large, complex, structures was demonstrated. A multiple scattering contribution was included in the case of K4V2W4O19. The need for good quality data was indicated by the poorer agreement of Zn(H2O)6(NO3)2 and (NBu4)5HZnW12O40 with the crystal data. However, analysis of the polyoxanions, [VW5O19]3-, [Nb6O19]8- and [NbW5O19]3- produced a good fit of the theoretical model to the experimental data. For the latter, solution data, and both Nb K-edge and W L3-edge data, was available. EXAFS analysis was then applied to the chromium pillared clays, for which little structural information about the pillaring species was known. A solution of hydrolysed Cr3+ ions was the pillaring agent and this was shown to contain polymeric species. The EXAFS-derived distances support a model of edge-bridged CrO6 octahedra, in a zigzag array. EXAFS data of the chromium pillared clays, bentonite, beidellite and Laponite, was analysed with the same model. Structural changes in the chromium pillared clays after calcination and hydrogenation were monitored by EXAFS analysis. Upon calcination partial oxidation of Cr(III) to Cr(VI) occurs. Hydrogenation regenerates the octahedral CrO6 units, however, these may be arranged in clusters, rather than sheets as before. Methanol dehydration is catalysed by the chromium pillared clays. The most effective catalyst was chromium pillared beidellite after calcination in vacuo and hydrogenation. The major products were methane (5%), C2-C alkanes and dimethyl ether (35%) and C5+ products, including 2-methylbutane, 1,2-dimethylcyclopropane, dimethoxymethane and 2,2,3-trimethylpentane (60%). Gas chromatography and mass spectrometry of the reaction products formed from deuterated methanol suggest there are two routes for the reaction, which lead different percentages of hydrogen isotope exchange. Therefore it is possible that the reaction proceeds via active sites on the surface and in the interlayers.
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Published date: 1990
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Local EPrints ID: 460843
URI: http://eprints.soton.ac.uk/id/eprint/460843
PURE UUID: 920d420f-3f97-4b98-bec7-677937b2f916
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Date deposited: 04 Jul 2022 18:30
Last modified: 04 Jul 2022 18:30
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Author:
Jacqueline Michelle Rummey
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