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Equilibrium and Structure Studies of Aqueous Vanadophosphates and Molybdovanadophosphates

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Equilibrium and Structure Studies of Aqueous Vanadophosphates and Molybdovanadophosphates Equilibrium and structure studies of aqueous vanadophosphates and molybdovanadophosphates by Anna Selling <5 °< c/> ■O • V AKADEMISK AVHANDLING som med tillstånd av rektorsämbetet vid Umeå Universitet för erhållande av filosofie doktorsexamen framlägges till offentlig granskning vid Kemiska instutitionen, Sal C, LuO, Naturvetarhuset, fredagen den 7 juni 1996, kl. 13.00. Fakultetsopponent: Dr. Atsushi Yagasaki, Kwansei Gakuin University, Japan. U m eå 1996 Title: Equilibrium and structure studies of aqueous vanadophosphates and molybdovanadophosphates. Author: Anna Selling Address: Department of Inorganic Chemistry, Umeå University, S -901 87 Umeå, Sweden. Abstract: This thesis comprises a summary and discussion of four papers, where heteropolyanions containing phosphorus as heteroatom and molybdenum and/or vanadium as addenda atoms have been studied. The ultimate goal was to study the equilibrium spéciation in the reduced molybdovanadophosphate system. The latter is a good catalyst in the selective oxidation of olefins to carbonyls. The studies were been performed in 0.6 M Na(Cl) medium at 25 °C. The main techniques employed included potentiometry, NMR- and ESR spectrometry, complemented with FTIR, X-ray diffraction and cyclic voltammetry. Experimental data were treated with the computer program LAKE, which is capable of simultaneously handling multimethod data. Before the reduced molybdovanadophosphate system could be studied, the subsystems needed to be known. Since the spéciation in most of the oxidised subsystems has been reported earlier in the same ionic medium, only the binary phosphate and the ternary vanadophosphate systems had to be studied. These are reported in this thesis. All three pXa values for the phosphate system have been determined with high precision. In the vanadophosphate system, equilibrium species with the formula [HzPVi40 42](9 z) and the trans-bicapped Keggin structure were formed. Formation constants for the three stable species and pKa values for both stable and metastable species are reported. The oxidised molybdovanadophosphate system also had to be thoroughly studied and the study was focused on Keggin ratio (M o+V)/P = 12/1 solutions in the pH range 1.5-4.5. A complete assignment of 31P and51V NMR resonances arising from [M011VPO40]4’ and the [MoioX^PO^]5' isomers together with their corresponding pKa values are reported and a rough assignment of the [M09V3PO40]6’ isomers is made. An equilibrium model and formation constants for the oxidised molybdovanadophosphate system are also reported. In the reduced molybdovanadophosphate system six species have been identified and their formation constants determined. The [MonV^PO^]5' species and the mixed valance state species, [MoioVvVivP04o]6’, can both be monoprotonated. A doubly-reduced Keggin species, [^M oioV ^PO ^]5" and a lacunary [HtMoioV^PC^]5' species were formed as well. Key words: Molybdovanadophosphate, vanadophosphate, molybdate, vanadate, phosphate, heteropolyanions, Keggin, redox, equilibria, potentiometry, NMR (31P, 51V), ESR. ISBN 91-7191 -184-7 -55 pages and 4 appendices Equilibrium and structure studies of aqueous vanadophosphates and molybdovanadophosphates by Anna Selling -5 * <s> •< I Vii %%% AKADEMISK AVHANDLING som med tillstånd av rektorsämbetet vid Umeå Universitet för erhållande av filosofie doktorsexamen framlägges till offentlig granskning vid Kemiska instutitionen, Sal C, LuO, Naturvetarhuset, fredagen den 7 juni 1996, kl. 13.00. Fakultetsopponent: Dr. Atsushi Yagasaki, Kwansei Gakuin University, Japan. Um eå 1996 Title: Equilibrium and structure studies of aqueous vanadophosphates and molybdovanadophosphates. Author: Anna Selling Address: Department of Inorganic Chemistry, Umeå University, S -901 87 Umeå, Sweden. Abstract: This thesis comprises a summary and discussion of four papers, where heteropolyanions containing phosphorus as heteroatom and molybdenum and/or vanadium as addenda atoms have been studied. The ultimate goal was to study the equilibrium spéciation in the reduced molybdovanadophosphate system. The latter is a good catalyst in the selective oxidation of olefins to carbonyls. The studies were been performed in 0.6 M Na(Cl) medium at 25 °C. The main techniques employed included potentiometry, NMR- and ESR spectrometry, complemented with FTIR, X-ray diffraction and cyclic voltammetry. Experimental data were treated with the computer program LAKE, which is capable of simultaneously handling multimethod data. Before the reduced molybdovanadophosphate system could be studied, the subsystems needed to be known. Since the spéciation in most of the oxidised subsystems has been reported earlier in the same ionic medium, only the binary phosphate and the ternary vanadophosphate systems had to be studied. These are reported in this thesis. All three pXa values for the phosphate system have been determined with high precision. In the vanadophosphate system, equilibrium species with the formula [H2PVi4042](9‘z)’ and the trans-bicapped Keggin structure were formed. Formation constants for the three stable species and p Kz values for both stable and metastable species are reported. The oxidised molybdovanadophosphate system also had to be thoroughly studied and the study was focused on Keggin ratio (Mo+V)/P = 12/1 solutions in the pH range 1.5-4.5. A complete assignment of 31P and 51V NMR resonances arising from [MonVPO^]4- and the [M010V2PO40]5" isomers together with their corresponding pK& values are reported and a rough assignment of the [M0 9V3PO40 ]6' isomers is made. An equilibrium model and formation constants for the oxidised molybdovanadophosphate system are also reported. In the reduced molybdovanadophosphate system six species have been identified and their formation constants determined. The [MonV^PO^]5" species and the mixed valance state species, [MoioV'V^PO^]6-, can both be monoprotonated. A doubly-reduced Keggin species, [H2 Moi0VIV2P04o]5" and a lacunary [ILtMoioV^PC^]5* species were formed as well. Key words: Molybdovanadophosphate, vanadophosphate, molybdate, vanadate, phosphate, heteropolyanions, Keggin, redox, equilibria, potentiometry, NMR (31P, 51V), ESR. ISBN 91-7191 - 184-7 55 pages and 4 appendices mydaughter, husBand and parents Printed in Sweden by Solfjädern Offset AB Umeå 1996 Equilibrium and structure studies of aqueous vanadophosphates and molybdovanadophosphates Anna Selling Department of Inorganic Chemistry Umeå University S-901 87 Umeå, Sweden This thesis is a summary and discussion of the following papers, which are referred to by their Roman numerals I-IV in the text. I. Multicomponent Polyanions. 47. The Aqueous Vanadophosphate System. Anna Selling, Ingegärd Andersson, Lage Pettersson, Charles M. Schramm, Shannan L. Downey, and John H. Grate. Inorganic Chemistry, 1994, 33, 3141-3150. ü. Multicomponent Polyanions. 46. Characterization of the Isomeric Keggin Decamolybdodivanadophoshate Ions in Aqueous Solutions by 31P and 51V NMR. Lage Pettersson, Ingegärd Andersson, Anna Selling, and John H. Grate. Inorganic Chemistry, 1994,33, 982-993. El. Multicomponent Polyanions. 49. A Potentiometrie and (31P, 51V) NMR Study of the Aqueous Molybdovanadophosphate System. Anna Selling, Ingegärd Andersson, John H. Grate, and Lage Pettersson. Manuscript to be submitted to Inorganic Chemistry. IV. Multicomponent Polyanions. 50. Reduced Molybdovanadophosphates. Anna Selling, Ingegärd Andersson, John H. Grate, and Lage Pettersson. Manuscript. CONTENTS 1. Introduction ......................................................................................................... 1 1.1 Background ............................................................................................. 1 1.1.1 Vanadium ................................................................................. 1 1.1.2 Molybdenum .............................................................................1 1.1.3 Phosphorus..............................................................................2 1.2 Heteropolyanions .................................................................................... 2 1.2.1 Keggin anions, [Mi 2XC>4 o]y' ....................................................3 1.3 Methods of studying heteropolyanions in solution .................................5 1.4 Applications ............................................................................................ 6 1.5 Aim of the present work ..........................................................................7 2. Experimental....................................................................................................... 9 2.1 Ionic medium ........................................................................................ 9 2.2 EMF measurements .................................................................................10 2.3 NMR measurements .............................................................................. 12 2.3.131P and 5,V ................................................................................12 2.4 ESR measurements ............................................................................... 15 2.5 F lik measurements .............................................................................. 16 2.6 X-ray measurements ................................................................................16 2.7 CV measurements ....................................................................................16 3. Data treatment....................................................................................................
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