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A set of vanadium phosphorous oxide (VPO) catalysts, mainly consisting of (VO) 2 P 2 O 7 , VO(PO 3 ) 2 or VOPO 4 ·2H 2 O bulk crystalline phases, has been investigated for the oxidative dehydrogenation (ODH) of ethane to ethylene, a key potential reaction for a sustainable industrial and socioeconomic development. The catalytic performance on these VPO catalysts has been explained on the basis of the main crystalline phases and the corresponding surface features found by XPS and LEISS at 400 °C, i.e. within the temperature range used for ODH reaction. The catalysts based on (VO) 2 P 2 O 7 phase presented the highest catalytic activity and productivity to ethylene. Nevertheless, the catalysts consisting of VO(PO 3 ) 2 structure showed higher selectivity to ethylene, reaching 90% selectivity at ca. 10% ethane conversion. To the best of our knowledge, this is the highest selectivity reported on a vanadium phosphorous oxide at similar conversions for the ethane ODH. In general, catalysts consisting of crystalline phases with vanadium present as V 4+ , i.e. (VO) 2 P 2 O 7 and VO(PO 3 ) 2, were found to be significantly more selective to ethylene than those containing V 5+ phases. The surface analysis by XPS showed an inverse correlation between the mean oxidation state of vanadium near surface and the selectivity to ethylene. The lower averaged oxidation states of vanadium appear to be favoured by the presence of V 3+ species near the surface, which was only found in the catalysts containing V 4+ phases. Among those catalysts the one based on VO(PO 3 ) 2 phase shows the highest selectivity, which could be related to the most isolated scenario of V species (the lowest V content relative to P) found at the outermost surface by low energy ion scattering spectroscopy (LEISS), a “true” surface technique only sensitive to the outermost atomic layer. [ABSTRACT FROM AUTHOR]

Abstract: The magnetic properties of [(VO)3(μ-PO4)2(2,2′-bpy)(μ-OH2)]1/3H2O (1) and (VO)2H4P2O9 (2), a tubular and a layered vanadium(IV) phosphates containing triply oxido bridged VIV dimers, are analyzed considering the Bleaney–Bowers S =1/2 dimer model. In compound 1 the presence of an additional VIV connected with the VIV dimers through μ1,2-PO4 3− bridges is described with a Curie–Weiss type correction. This model reproduces the magnetic properties of compound 1 with g =1.956, J dim =−102.1cm−1, θ =−0.4cm−1 and Nα =278×10−6 emumol−1. In compound 2, the presence of a small percentage of paramagnetic impurity has to be considered to account for the divergence of χm at low temperatures. This simple model reproduces the magnetic data of compound 2 with g =1.99, J =−62.4cm−1 and a 1.2% of monomeric impurity. The moderate antiferromagnetic coupling found in the triply oxido bridges VIV dimers is justified from the structural parameters of the bridge. These studies confirm that the coupling through –O–P–O– bridges is antiferromagnetic and relatively weak, as well as previous magneto-structural correlations in this kind of oxido bridges. DFT calculations on a dinuclear fragment model for the two systems gave the following values of J calc =−72.4cm−1 for 1 and J calc =−5.2cm−1 for 2. These values reproduce the antiferromagnetic nature of the superexchange interactions between the VIV centers. [Copyright &y& Elsevier]

The structural properties and oxidation state of vanadium in alumina-supported vanadium-phosphorous oxides at nearly monolayer coverage influence its catalytic performances. The population distribution VVO and VIVO species in the amorphous VPO phase strongly depends on the sample history. By using alumina-supported VPO instead of conventional bulk catalysts, we facilitate enhanced discrimination between active phase and bulk phase signals in the vanadium-phosphorous oxygen system. This brings light on the speciation of VPO catalysts in general. Our observations were achieved by using mainly in situ EPR operando Raman–GC spectroscopic techniques at relevant temperatures and gas conditions, showing that the (VivO)2P2O7 phase is formed during propane ammoxidation at 753K. Such transformation is less intense in inert atmosphere at 753K. Ammonia adsorption on the VPO catalyst surface leads to the formation of VO2+ sites, probably bearing Brønsted and/or Lewis acidity. Propane ammoxidation appears to promote the formation of mixed valence compounds. [ABSTRACT FROM AUTHOR]

Abstract: Cycling catalyst between a net oxidizing and reducing gaseous environment has been practiced commercially to produce maleic anhydride from n-butane over vanadium pyrophosphate. Typically, the oxidation period is less than 1min to minimize catalyst inventory. In this study, the effect of the oxidation period on maleic anhydride productivity was assessed in the range of 0.3–10min. Irrespective of the feed gas composition during the reduction period, the productivity increased linearly with the oxidation soak time in air. A full range of reducing conditions was examined from the pure redox mode (10% n-butane in argon) to highly oxidizing conditions typical of fixed bed operation (1.4% n-butane and 18.1% oxygen). On average, maleic anhydride yield increased by up to 50% when the oxidation time was extended from 0.3 to 10min. The maleic anhydride yield was lowest under redox mode and it was highest when the feed composition was close to equimolar in n-butane (∼6%) and oxygen. Our results show that industrial CFB reactor performance may be improved considerably by efficient regeneration of the catalyst and optimization of the reducing feed gas composition. [ABSTRACT FROM AUTHOR]

Abstract: The hybrid 2D compound [{Cu(bpy)}2(VO)3(PO4)2(HPO4)2]·2H2O (1), has been investigated due to its interesting magnetic and catalytic properties. Compound (1) acts as an efficient catalyst in the epoxidation of cyclohexene and styrene. The chemoselectivity towards the epoxidation of cyclohexene is notoriously higher than the one observed towards styrene. The bulk antiferromagnetic behaviour of [{Cu(bpy)}2(VO)3(PO4)2(HPO4)2]·2H2O (1) can be well described with a pentanuclear model, using five J values. Both antiferromagnetic and ferromagnetic interactions mediated by phosphate bridges are found to be present in this hybrid copper(II)–vanadium(IV) material. [Copyright &y& Elsevier]

Abstract: Cited advantages of circulating fluidized bed reactors (CFB) include higher selectivity and conversion together with the ability to optimize the process conditions of each vessel independently—temperature, gas partial pressure and residence time. DuPont commercialized a CFB process to produce maleic anhydride in which a vanadium pyrophosphate (VPO) was cycled between a fast bed riser and an air fed regenerator. Together with VPO, we examined two other redox catalyst systems—MoVSb (acrylic acid from propane) and FeMoO (methanol to formaldehyde). The lattice oxygen capacity of the FeMoO catalyst was about five times higher than either the VPO or MoVSb with little adsorbed carbon but a significant quantity of chemisorbed water. Above 350°C, carbon deposition was detected and increased with increasing temperature. Carbon deposition decreased with increasing temperature for the MoVSb system and its lattice oxygen capacity was slightly higher than for VPO. The carbon deposition pattern for VPO was the opposite of the MoVSb and increased with temperature. Based on a hydrogen and carbon mass balance during the catalyst re-oxidation treatment, the molecular composition of the adsorbed species were C4H6 and C3H3—like for the VPO and MoVSb, respectively. Based on the high lattice oxygen capacity, the formaldehyde reaction appears to be ideally suited for development in a CFB. Whereas the lattice oxygen contribution of the MoVSb is equivalent to VPO, less oxygen is required to produce acrylic acid (compared to maleic anhydride) so the incentive of developing a CFB process should be greater than for butane oxidation to maleic anhydride. [Copyright &y& Elsevier]