Reduction Of VOPO4 Research Articles

Influence of the Ethylene Glycol, Water Treatment and Microwave Irradiation on the Characteristics and Performance of VPO Catalysts for n-Butane Oxidation to Maleic Anhydride

In this paper, the preparation of vanadium phosphate catalysts was shown to be improved by (1) using V2O5 and ethylene glycol as starting and reducing agent material, respectively for VOPO4 · 2H2O, (2) subsequent water treatment and (3) microwave irradiation. In particular, the preparation route, based on the reduction of VOPO4 · 2H2O with various alcohols, is described in detail and contrasted with other three established methods performed by using ethylene glycol and isobutyl alcohol as reductant and solvent for V2O5 or distilled water as a solvent material. The preparation of catalyst precursor is carried out by two different methods, namely conventional heating and microwave irradiation. With this new technique, catalysts derived from the reduction of VOPO4 · 2H2O by ethylene glycol exhibit substantially higher surface area (typically >40 m2 g−1) and activity. In fact, the surface area of the catalyst is significantly enhanced when the precursor is refluxed by distilled water and dried by microwave heating. The characterization of catalysts was carried out using X-ray diffraction (XRD), Brunauer–Emmer–Teller (BET) surface area measurement, temperature programmed reduction (H2-TPR), temperature-programmed reaction (TPRn) and scanning electron microscopy (SEM). This study shows that employing ethylene glycol as reducing agent, followed by adding the water treatment step to catalyst synthesis procedure, and using microwave irradiation would give rise to enhanced surface area, activity and selectivity of the catalyst. Moreover, it introduces a more energy efficient procedure for preparation of vanadium phosphate catalyst used in selective oxidation of n-butane process.

Reaction of vanadium phosphates with alcohols at elevated temperature and pressure

The reaction of VOPO4·2H2O, β-VOPO4 and VOHPO4·0.5H2O with alcohols in an autoclave at elevated temperatures (100–400 °C) and pressures (1–150 bar) is described and discussed. The reduction of VOPO4·2H2O with alcohols at ambient pressure is a standard method of preparation for VOHPO4·0.5H2O which is a valuable commercial catalyst precursor for the oxidation of butane to maleic anhydride. Surprisingly, the use of higher reaction pressures presents an unexplored region, and we show that primary alcohols at high temperature and pressure reduce both VOPO4·2H2O and β-VOPO4 to tetragonal VPO4·H2O, whereas VOHPO4·0.5H2O is reduced to form monoclinic VPO4·H2O. Previously these materials have been prepared by slow hydrothermal synthesis requiring the presence of templates, and hence we present new more facile synthetic pathways to these V(III) compounds. The catalytic performance for the selective oxidation of butane to maleic anhydride of these materials pretreated in situ with butane/air is also described and discussed in terms of the structures of the materials formed under the reaction conditions.

The Role of Brønsted and Lewis Acid Sites of Vanadyl Pyrophosphate Measured by Dimethylpyridine-temperature Programmed Desorption in the Selective Oxidation of Butane

Acidic properties of three types of (VO)2P2O7 catalysts were investigated by temperature programmed desorption (TPD) using 3, 5- and 2, 6-dimethylpyridine as probes, and the selective oxidation of butane to maleic anhydride (MA) was performed. VPO-org was prepared in organic solvent, VPO-redu was obtained by reduction of VOPO4·2H2O, and VPO-aq was prepared in aqueous medium. 3, 5-Dimethylpyridine (3, 5-DMP) is adsorbed on both Brønsted and Lewis acid sites, whereas 2, 6-dimethylpyridine (2, 6-DMP) is selectively adsorbed on Brønsted acid sites due to the steric hindrance of the two methyl groups, so the amounts and strengths of the Brønsted and Lewis acid sites could be determined separately. The (VO)2P2O7 catalysts had four types of acid sites: weak and strong Brønsted acid sites, and weak and strong Lewis acid sites. The acidic properties were greatly dependent on the preparation methods as follows: VPO-org had a larger amount of the strong Brønsted acid sites and these acid sites were relatively weak. VPO-redu had a larger amount of the strong Lewis acid sites and VPO-aq had fewer acid sites. The selectivity to MA at low conversion increased with the amount of strong Lewis acid sites, indicating that the strong Lewis acid sites are important for MA formation. The strong Brønsted acid sites may promote the consecutive oxidation of MA.

Comparison of vanadium phosphate catalysts derived from VOPO4·2H2O prepared from H3PO4 and H4P2O7 Electronic Supplementary Information available. See http://www.rsc.org/suppdata/cp/b1/b105304n/

Vanadium phosphate catalysts prepared from VOPO4 ·2H2O are described and discussed. The use of H4P2O7 as a phosphorus source, as compared with H3PO4, is shown to give an improved synthesis of VOPO4·2H2O. Reduction of VOPO4·2H2O with alcohols from both the materials leads to the formation of VOHPO4·0.5H2O. However, the material derived from H4P2O7 has a higher intrinsic activity (mol maleic anhydride formed per unit surface area) than that derived from H3PO4. The final catalysts are shown to comprise (VO)2P2O7 together with a range of V(V) VOPO4 phases and the origin of the enhanced activity is discussed in terms of the catalyst structure.

Characterisation of Variations in Vanadium Phosphate Catalyst Microstructure with Preparation Route

Three vanadium phosphate catalysts prepared by different methods are compared and contrasted for the selective oxidation ofn-butane to maleic anhydride. VOHPO4·0.5H2O catalyst precursors were prepared via three distinct routes: (a) using aqueous HCl as a reductant for V2O5, (b) using isobutanol as solvent and reductant for V2O5; and (c) using isobutanol as a reducing agent for VOPO4·2H2O. The precursors were transformed under identical conditions (385°C, 1.5% butane in air, 1000 h−1, 75 h) to three activated catalysts. The morphology of the precursors and the final catalysts have been assessed using a combination of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and31P NMR spin-echo mapping. Even though they exhibited very similarspecificactivities for maleic anhydride production, the three activated vanadium phosphate catalysts were found to have widely varying morphologies. The implications of this observation are discussed and a few suggestions are made as to the possible nature of the active site in this catalyst system. The material derived from the reduction of VOPO4·2H2O exhibited the highest catalyst activity by virtue of its optimised (VO)2P2O7morphology and high surface area.

Preparation and Characterization of Vanadyl Hydrogen Phosphate Hydrates; VO(HPO4)·1.5H2O and VO(HPO4)·0.5H2O

Abstract A new phase of vanadyl(IV) hydrogen phosphate sesquihydrate, VO(HPO4)·1.5H2O, has been obtained by the reduction of VOPO4·2H2O with 1-butanol. The unit cell is the orthorhombic system with lattice constants a = 7.43 Å, b = 9.62 Å, and c = 7.97 Å. in space group Pmmn.

Redox intercalation of alkali metals into vanadyl phosphate dihydrate

Alkali-metal-containing intercalates, of the general formula MxVOPO4 · 2H2O, where M = Li, Na, K, Rb and Cs, have been prepared by reduction of VOPO4· 2H2O in iodide solution. The results of both structural and morphological studies carried out upon these intercalation compounds are reported.

Control of the composition and morphology of vanadium phosphate catalyst precursors from alcohol treatment of VOPO4·2H2O

The composition and the morphology of vanadium phosphate catalyst precursors is shown to be controlled by the choice of the alcohol selected to carry out the dehydration and reduction of VOPO4·2H2O.