Roy, Subrata Chandra: Thermodynamically stable and metastable solids: New approaches to the synthesis of anhydrous phosphates containing vanadium, molybdenum, and/or tungsten. - Bonn, 2015. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
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author = {{Subrata Chandra Roy}},
title = {Thermodynamically stable and metastable solids: New approaches to the synthesis of anhydrous phosphates containing vanadium, molybdenum, and/or tungsten},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2015,
month = dec,

note = {This thesis reports on syntheses, crystallization, phase relations and redox phenomena in the quasi-ternary oxide systems WO2.5/VO2.5/PO2.5 and MoO2.5/VO2.5/PO2.5. This work focused on finding new catalyst lead structures for the gas-phase oxidation of n-butane to maleic anhydride. Synthesis of the new anhydrous phosphates was performed in three different techniques, namely solution combustion synthesis, classical solid state reaction and vapor phase moderated solid state reaction. Crystallization was achieved mainly by CVT and vapor phase moderated solid state reaction. Characterization of the newly obtained phases was accomplished by x-ray single crystal analysis, electron diffraction analysis, HRTEM analysis, magnetic and conductivity measurement and several spectroscopic techniques (XPS, EPR, UV/Vis/NIR, 31P-MAS-NMR).
WVOPO4 belongs to the series of monophosphate tungsten bronzes (WO3)2m(PO2)4 (m = 2). The single crystal structure refinement of WOPO4 {P21/m, Z = 4, a = 6.5538(4) Å, b = 5.2237(8) Å, c = 11.1866(8) Å, β = 90.332(7)°} as well as the XRPD pattern shows monoclinic instead of the reported orthorhombic symmetry. A significant improved structure model is provided.
Phosphates (MIII1/3WVI2/3)OPO4 (M: V, Cr, Fe, Mo; WVOPO4 structure type) have been synthesized by various methods (SCS, CVT, vapor phase moderated solid state reactions) and characterized. These are the first hetero-metallic examples of phases with MPTB structure type. W5+ is substituted by M3+1/3W6+2/3. Similar substitution was also observed for (WV,VIO3)8(PO2)4 (MBTB with m = 4) leading to (MIII1/6WVI5/6O3)8(PO2)4 (M: V, Cr).
In the system W/V/P/O along the quasi binary line WOPO4-VOPO4 solid solutions (V1-xWx)OPO4 with four different structure types have been identified: 0 ≤ x ≤ 0.01, β-VOPO4 structure type; 0.04 ≤ x ≤ 0.26, αII-VOPO4 structure type; 0.60 ≤ x ≤ 0.67, V(WO2)2(P2O7)(PO4) structure type; and 0.67 ≤ x ≤ 1 WOPO4 structure type. The latter is probably thermo-dynamically metastable. For x = 0.50 the formation of a four-phase equilibrium mixture of αII-(V0.74W0.26)OPO4, m-(VIVO)(PO3)2, a vanadium-doped mono-phosphate tungsten bronze of approximate composition (W5/6V1/6O3)8(PO2)4 (WO3)8(PO2)4 structure type and a lower phase boundary composition V(W0.9V0.1O2)2(P2O7)PO4 which is extended up to x = 0.67, V(WO2)2(P2O7)PO4 was observed. The oxidation states in the solid solutions (V1-xWx)OPO4 with αII- and β-VOPO4 structure type have been established by various physical measurements (EPR, XPS, UV/Vis/NIR spectroscopy and magnetic measurements). Accordingly, the formulation (VIVxVV1-2xWVIx)OPO4 is appropiate. The solid solutions are Class II semiconductors (Eg ≈ 0.7 eV). In catalytic testing for n-butane oxidation to MA, (V0.8W0.2)OPO4 (αII-VOPO4 structure type) showed promising behavior (Activity X n-butane = 32%, Selectivity SMA = 45%).
The new polynary phosphate VIII(WVIO2)2(P2O7)PO4 was synthesized and characterized. Its composition corresponds to (V1-xWx)OPO4 with x = 0.67. VIII(WVIO2)2 (P2O7)PO4 is the first mixed transition metal phosphate combining oxidation states III+ and VI+. These are in agreement with the results of XPS, magnetic measurements and UV/Vis/NIR spectroscopy. Formation of new phase from VOPO4 and WOPO4 can be interpreted as a combined redox (eq. 1) and Lux-Flood acid-base reaction (eq. 2).
2(WO)3+ + (VO)3+ + O2- → 2(WO2)2+ + V3+ (eq. 1)
2(PO4)3- → (P2O7)4- + O2- (eq. 2)
A homogeneity range according to (V1-xWx)OPO4 with 0.60 ≤ x ≤ 0.67 (V(WO2)2(P2O7)PO4 structure type) was detected. Crystallization was obtained during vapour phase moderated solid state reactions using Cl2 as mineralizer.
The VIII(WVIO2)2(P2O7)PO4 structure type shows a wide crystal chemical stability range allowing substitution of V3+ by several others trivalent ions (M3+: Sc, Cr, Fe, Mo, Ru, Rh, Ir, In). However the strong reductant Ti3+ yielded different redox product TiIVP2O7 and WV,VI8P4O32. 31P-MAS-NMR studies of Sc, In, and Ir compound shows the presence of P2O7 and PO4 units. Monitoring the phase formation during the synthesis of MIII(WVIO2)2(P2O7)PO4 via SCS and subsequent heating revealed the presence of various metastable reaction intermediates with structures related to the ReO3 and WOPO4 types. In further experiments it was shown that solution combustion synthesis is a versatile method for obtaining various mixed metal phosphates with WO3-related structures. Thus the solid solution (VxW0.5-xP0.5)O2.5+δ (0.0 ≤ x ≤ 0.25) as well as multi-metal phosphate (Fe0.1V0.1W0.3P0.5)O2.5+δ were obtained as single phase material. It was shown that depending on p(O2) during the post-combustion annealing, the oxidation states of vanadium in the solid solution can be set. Most remarkably, (V0.25W0.25P0.5)O2.5+δ as well as (Fe0.1V0.1W0.3P0.5)O2.5+δ show very promising catalytic behavior in selective oxidation of n-butane to MA ((V0.25W0.25P0.5)O2.5+δ: Activity Xn-butane = 37%, Selectivity SMA = 32% and (Fe0.1V0.1W0.3P0.5)O2.5+δ: Activity Xn-butane= 28%, Selectivity SMA = 67%).
In the system Mo/V/P/O along the quasi-binary line of MoOPO4-VOPO4 solid solutions (V1-xMox)OPO4 with two different structure types have been obtained by SCS followed by heating in air: 0.0 ≤ x ≤ 0.30 thermodynamically stable β-VOPO4 structure type; 0.0 ≤ x ≤ 0.20 thermodynamically metastable γ-VOPO4 structure type. With increasing Mo concentration the temperature required to obtain the thermodynamically stable phase decreased (725 °C for x = 0.0 and 600 °C for x = 0.30) and the color of the material changes from bright yellow to olive green. The EPR, 31P-MAS-NMR, UV/Vis/NIR spectra as well as the magnetic and conducting behavior of these solid solutions are quite similar to the tungsten substituted solid solutions. Therefore the oxidation states according to (VIVxVV1-2xMoVIx)OPO4 are suggested.
Reduction of β-VOPO4 by moist hydrogen led to the two new metastable phases VPO4-m1 (at 500 °C) and VPO4-m2 (at 700 °C). For VPO4-m1 two structure models are proposed. One is derived from β-VOPO4 and the other from β-V2OPO4. Both structure models lead to square-planar VO4 units which are sharing corners with PO4 tetrahedra. For none of the two structure models of VPO4-m1via DFT calculation was successful. These calculations suggested a completely different structure built from VO4 tetrahedra and PO4 tetrahedra. For the second metastable polymorph VPO4-m2 relaxation of the structure model via DFT confirmed its relationship to FeIIFeIII(VIVO)(P2O7)(PO4). The DFT results suggest, however,vanadium adopts just the oxidation state III+.
Reduction of V2O5 by moist H2 led to the formation of two metastable phases VIV,V4O9 (isotypic to (VO)2P2O7) and VIVO2(B) and the thermodynamically stable phase V2O3 at 305, 315, and 450 °C, respectively.},

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