Thermodynamically stable and metastable solids: New approaches to the synthesis of anhydrous phosphates containing vanadium, molybdenum, and/or tungsten

Abstract

This thesis reports on syntheses, crystallization, phase relations and redox phenomena in the quasi-ternary oxide systems WO_2.5/VO_2.5/PO_2.5 and MoO_2.5/VO_2.5/PO_2.5. This work focused on finding new catalyst lead structures for the gas-phase oxidation of <i>n</i>-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). <br /><b>W^VOPO₄</b> belongs to the series of monophosphate tungsten bronzes (WO₃)_2<i>m</i>(PO₂)₄ (<i>m</i> = 2). The single crystal structure refinement of WOPO₄ {<i>P</i>2₁/<i>m</i>, Z = 4, <i>a</i> = 6.5538(4) Å, <i>b</i> = 5.2237(8) Å, <i>c</i> = 11.1866(8) Å, <i>β</i> = 90.332(7)°} as well as the XRPD pattern shows monoclinic instead of the reported orthorhombic symmetry. A significant improved structure model is provided. <br /><b>Phosphates (<i>M</i>^III_1/3W^VI_2/3)OPO₄</b> (<i>M</i>: V, Cr, Fe, Mo; W^VOPO₄ 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 (W^V,VIO₃)₈(PO₂)₄ (MBTB with <i>m</i> = 4) leading to (<i>M</i>^III_1/6W^VI_5/6O₃)₈(PO₂)₄ (<i>M</i>: V, Cr). <br /><b>In the system W/V/P/O</b> along the quasi binary line WOPO₄-VOPO₄ solid solutions (V_1-xW_x)OPO₄ with four different structure types have been identified: 0 ≤ x ≤ 0.01, <i>β</i>-VOPO₄ structure type; 0.04 ≤ x ≤ 0.26, <i> α</i>_II-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 <i>α</i>_II-(V_0.74W_0.26)OPO₄, <i>m</i>-(V^IVO)(PO₃)₂, a vanadium-doped mono-phosphate tungsten bronze of approximate composition (W_5/6V_1/6O₃)₈(PO₂)₄ (WO₃)₈(PO₂)₄ structure type and a lower phase boundary composition V(W_0.9V_0.1O₂)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 (V_1-xW_x)OPO₄ with <i>α</i>_II- and <i>β</i>-VOPO₄ structure type have been established by various physical measurements (EPR, XPS, UV/Vis/NIR spectroscopy and magnetic measurements). Accordingly, the formulation (V^IV_xV^V_1-2xW^VI_x)OPO₄ is appropiate. The solid solutions are Class II semiconductors (<i>E</i>_g ≈ 0.7 eV). In catalytic testing for <i>n</i>-butane oxidation to MA, (V_0.8W_0.2)OPO₄ (<i>α</i>_II-VOPO₄ structure type) showed promising behavior (Activity <i>X </i> _n-butane = 32%, Selectivity S_MA = 45%). <br /><b>The new polynary phosphate V^III(W^VIO₂)₂(P₂O₇)PO₄</b> was synthesized and characterized. Its composition corresponds to (V_1-xW_x)OPO₄ with x = 0.67. V^III(W^VIO₂)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). <br /> 2(WO)³⁺ + (VO)³⁺ + O^2- → 2(WO₂)²⁺ + V³⁺ (eq. 1) <br /> 2(PO₄)^3- → (P₂O₇)^4- + O^2- (eq. 2) <br /> A homogeneity range according to (V_1-xW_x)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. <br /> The V^III(W^VIO₂)₂(P₂O₇)PO₄ structure type shows a wide crystal chemical stability range allowing substitution of V³⁺ by several others trivalent ions (<i>M</i>³⁺: Sc, Cr, Fe, Mo, Ru, Rh, Ir, In). However the strong reductant Ti³⁺ yielded different redox product Ti^IVP₂O₇ and W^V,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 <i>M</i>^III(W^VIO₂)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 (V_xW_0.5-xP_0.5)O_2.5+δ (0.0 ≤ x ≤ 0.25) as well as multi-metal phosphate (Fe_0.1V_0.1W_0.3P_0.5)O_2.5+δ were obtained as single phase material. It was shown that depending on <i>p</i>(O₂) during the post-combustion annealing, the oxidation states of vanadium in the solid solution can be set. Most remarkably, (V_0.25W_0.25P_0.5)O_2.5+δ as well as (Fe_0.1V_0.1W_0.3P_0.5)O_2.5+δ show very promising catalytic behavior in selective oxidation of <i>n</i>-butane to MA ((V_0.25W_0.25P_0.5)O_2.5+δ: Activity <i>X_n-butane</i> = 37%, Selectivity S_MA = 32% and (Fe_0.1V_0.1W_0.3P_0.5)O_2.5+δ: Activity <i>X_n-butane</i>= 28%, Selectivity S_MA = 67%). <br /><b>In the system Mo/V/P/O</b> along the quasi-binary line of MoOPO₄-VOPO₄ solid solutions (V_1-xMo_x)OPO₄ with two different structure types have been obtained by SCS followed by heating in air: 0.0 ≤ x ≤ 0.30 thermodynamically stable <i>β</i>-VOPO₄ structure type; 0.0 ≤ x ≤ 0.20 thermodynamically metastable <i>γ</i>-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 (V^IV_xV^V_1-2xMo^VI_x)OPO₄ are suggested. <br /><b>Reduction of <i>β</i>-VOPO₄</b> 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 <i>β</i>-VOPO₄ and the other from <i>β</i>-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 Fe^IIFe^III(V^IVO)(P₂O₇)(PO₄). The DFT results suggest, however,vanadium adopts just the oxidation state III+. <br /><b>Reduction of V₂O₅ by moist H₂</b> led to the formation of two metastable phases V^IV,V₄O₉ (isotypic to (VO)₂P₂O₇) and V^IVO₂(B) and the thermodynamically stable phase V₂O₃ at 305, 315, and 450 °C, respectively.