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The preparation of vanadium-phosphate catalysts by Atomic Layer Deposition (ALD) was studied for application to selective oxidation of n-butane to maleic anhydride (MA). Modification of bulk V2O5 by ALD with trimethylphosphate significantly increased the selectivity to MA but the catalyst performance was still much worse than that observed for bulk vanadium phosphate (VPO). Silica-supported VPO was then prepared by ALD of P and V and compared to a VPO/SiO2 catalyst prepared by impregnation. Raman spectra showed that the ALD-prepared materials were more homogeneous, and temperature-programmed desorption of 2-propanol demonstrated that the VPO phase covered the silica more uniformly when deposited by ALD. However, the bulk VPO catalyst remained the most selective for MA. Possible reasons for why the supported catalysts showed lower performance are discussed and suggestions made for improving these catalysts. [ABSTRACT FROM AUTHOR]

Vanadium phosphorus oxide (VPO) was immobilized on SBA-15 via a facile impregnation process. The structure and VPO species on SBA-15 were characterized by XRD, TEM, BET, FTIR and XPS techniques. It was found the VPO/SBA-15 catalysts maintained the two-dimensional hexagonal structure of SBA-15, and the supported VPO species existed in a highly dispersed amorphous state. VPO/SBA-15 catalyst was tested for the ammoxidation of dichlorotoluenes (DCTs) to corresponding dichlorobenzonitriles (DCNs) in a lab-scale tube reactor. Compared with VPO/SiO2 (large-pore silica-supported VPO), VPO/SBA-15 exhibited appreciable enhancement in the catalytic performance due to the unique channel structure of SBA-15. The yield and selectivity toward 2,4-DCN increased by 7% and 6%, while toward the sterically hindered 2,6-DCN increased by 7% and 13%, respectively. [ABSTRACT FROM AUTHOR]

α II -V 1- x M x OPO 4 (M = W,Mo). • Solution combustion synthesis (SCS) leads to a variety of phase pure catalysts. • Doping with Mo and W tunes the oxidation state of V in VOPO 4 between 4+ and 5+. • α II -(V 1- x W x)OPO 4 reaches selectivities of maleic anhydride of more than 30%. • α II type solid solutions are a flexible platform for further research. The vanadyl pyrophosphate (VPP) based catalyst is unique in converting n -butane selectively (60–70%) into maleic anhydride (MAN), whereas a MAN selectivity of 20% may be regarded as high for structurally different catalyst systems. We present novel vanadium phosphorus oxides and mixed metal phosphate solid solutions tested for n -butane oxidation to MAN with a selectivity of >30%. The majority of the catalysts were prepared by solution combustion synthesis. (V 1- x W x)OPO 4 with α II structure was found to be more active and selective in the oxidation of n -butane compared to β -VOPO 4. By adjusting the tungsten content the oxidation state of vanadium in (V 1- x W x)OPO 4 can be tuned between 4.74 and 4.99, which is regarded as a key factor for MAN production. All catalysts were structurally stable, but the specific surface area increased during the reaction, as detected by X-ray diffraction and N 2 physisorption, respectively. (V 1- x Mo x)OPO 4 was also stable, but the MAN selectivity was lower compared to β -VOPO 4. Low conversions result from the low surface area of the screening samples, however, could be overcome by advanced synthesis protocols. [ABSTRACT FROM AUTHOR]

Abstract The vapor pressure osmometry (VPO) of metal dialkyldithiophospate (MDDPs) association in simple hydrocarbon solvents [1] has been analyzed using the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) [2]. From this analysis, we obtain a fluid-state free energy theory of MDDPs, including zinc dialkyldithiophosphates (ZnDDPs), an important class of multifunctional lubricant additives present in most automotive and commercial vehicle formulations marketed worldwide [3]. As expected, the PC-SAFT parameters of the MDDP compounds are found to vary smoothly with molecular size, metal atom type (Zn, Cd, Hg, Pb), and alkyl chain structure. Analysis of benzene/MDDP VPO isotherms with PC-SAFT suggests that selected MDDP compounds cross-associate with benzene. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Catalytic single-lifetime reduction could be due to coke deposits and imbalanced in I V 4+/I V 5+ ratio. • Change of physical and chemical property caused by coking is the reason of the bad regenerated performance of VPO catalyst. • The I V 4+/I V 5+ ratio could return to normal value after reactivation. • The correlations of carbon deposit and time were fitted by empirical Voorhies equations. Abstract Improvement in activity for the Zr doped VPO catalyst appears to be a general observation in aldol condensation of methyl acetate and formaldehyde to methyl acrylate. The deactivation behavior significantly differs between VPO and VPO-Zr catalysts. In this work, the deactivation/reactivation behavior and carbon formation in aldol condensation on VPO and VPO-Zr catalysts were investigated. After partial deactivation of the conversion of methyl acetate and formaldehyde to methyl acrylate with a fixed-bed reactor, the deactivated catalysts were fractionated from the reactor. The samples were then analyzed by spectroscopic techniques, gas adsorption measurements and thermogravimetry. The VPO-Zr catalyst showed differences in deactivation/reactivation behavior from the VPO catalyst under same reaction conditions. This finding could be partially ascribed to different coke locations in the deactivated catalysts with varied porosity. The I V 4+/I V 5+ ratio of V atom remarkably differed among fresh, used, and refreshed catalysts, which could be a key factor affecting catalyst deactivation. The relations of carbon formation and reaction time were fitted by empirical Voorhies equation over VPO and VPO-Zr catalysts. 1 [ABSTRACT FROM AUTHOR]

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]

The synthesis of two novel catalytic materials based on vanadium mixed oxides has been described in present study. The first material was a Vanadium-Phosphorous mixed oxide (VPO) carbon-supported material with fibrous morphology and a very high surface area, and stable under oxidizing conditions up to 350 °C. The other material is a bulk mixed VPO oxide with fibrous structure obtained after optimizing the calcination of the supported carbon fiber catalyst, which presents a very high surface area. Both materials seem to be very attractive as catalysts in a wide range of partial oxidation processes in both liquid and gas phase based on the structural and chemical characterization data. [ABSTRACT FROM AUTHOR]

The effect of calcination temperature on the catalytic activity of VPO catalyst for one step synthesis acrylic acid with acetic acid and formalin was systematically studied using a fixed-bed tubular micro-reactor. The influence of calcination temperature on physicochemical properties especially crystal phase structure of VPO catalysts was obtained. The catalysts were analyzed and characterized with in situ high temperature XRD (HT-XRD), BET, TEM, TG/DTA, XPS, as well as NH 3 , CO 2 -TPD, and pyridine-FTIR methods. Larger special surface area and higher V 4+ concentration favored higher conversion and selectivity. The optimum special surface area and I V 4+ / I V 5+ ratio was obtained with controlling calcination temperature. The catalyst deactivation was also explored. [ABSTRACT FROM AUTHOR]

A precipitation-deposition method in an organic medium was used to prepare vanadium phosphorus oxide (VPO) catalysts supported on SBA-15 and TUD-1. Oxidation of cyclohexane has been examined over mesoporous silica supported VPO catalyst. The supported VPO catalyst has been characterized by XRD, N 2 physisorption, SEM, HRTEM, H 2 -TPR and NH 3 -TPD techniques. The characteristic X-ray diffraction patterns of bulk VPO catalyst and VPO catalysts supported on mesoporous SBA-15 and TUD-1 showed that each of them are having well-crystallized vanadyl pyrophosphate [(VO) 2 P 2 O 7 ] phase. The vanadium redox centers were easily accessible for supported VPO catalyst. The TUD-1 supported VPO catalyst had higher acidity as found in NH 3 -TPD studies. The conversion of cyclohexane increased in the case of VPO dispersed on TUD-1 mesoporous material. From SEM and HRTEM results it is found that the nature of the support and VPO loading had a profound influence on the crystallinity, morphology and chemical property of the dispersed VPO phase. The changes in all the property had a prominent role on its catalytic activity for the oxidation of the cyclohexane. [ABSTRACT FROM AUTHOR]

The vanadium phosphorous oxide (VPO) catalysts, supported on Santa Barbara amorphous molecular sieve (SBA-15) with different vanadium loadings, were prepared by the precipitation method in an organic phase. Techniques such as X-ray diffraction, Bruner–Emmett–Teller method, scanning electron micrograph (SEM), and transmission electron micrograph (TEM) were used for characterization of the catalysts. The high surface area, large pore volume, and pore size of SBA-15 provide an excellent dispersion of (VO)2P2O7species on the support surface. Oxidation of various alcohols was studied in the liquid phase over the VPO/SBA-15 catalyst, usingtert-butylhydroperoxide as an oxidant. The activities of VPO/SBA-15 samples were considerably higher than that of the bulk VPO catalyst. At 90°C, the obtained activities were 0.567 andover the bulk VPO and 15 wt.% VPO/SBA-15 catalysts, respectively. The effects of substrates, oxidants, reaction time, oxidant/alcohol molar ratio, solvents, amount of catalyst, catalyst recycling, and leaching of VPO in the liquid phase were also investigated. [ABSTRACT FROM PUBLISHER]

We report here the results of density functional theory quantum mechanical (QM) studies of the detailed chemical mechanism underlying the n-butane selective oxidation to form maleic anhydride (MA) on vanadyl pyrophosphate [(VO)PO] and vanadyl phosphate [VOPO] surfaces. This QM-derived mechanism differs substantially from previous suggestions but is in excellent agreement with key experimental observations. We find that the O(1)=P bond of the oxidized X1 phase of the VOPO surface is the active site for initiating the VPO chemistry, by extracting the H from the n-butane C-H bond. This contrasts sharply with previous suggestions, all of which involved the V=O bonds. The ability of O(1)=P to cleave alkane C-H bonds arises from a new unique mechanism that decouples the proton transfer and electron transfer components of this H atom transfer reaction. We find that the juxtaposition of a highly reducible V next to the P=O bond but coupled via a bridging oxygen dramatically enhances the activity of the P=O bond to extract the proton from an alkane, while simultaneously transferring the electron to the V to form V. This Reduction-Coupled Oxo Activation (ROA) mechanism had not been known prior to these QM studies, but we believe that it may lead to a new strategy in designing selective catalysts for alkane activation and functionalization, and indeed it may be responsible for the selective oxidation by a number of known mixed metal oxide catalysts. To demonstrate the viability of this new ROA mechanism, we examine step by step the full sequence of reactions from n-butane to MA via two independent pathways. We that find that every step is plausible, with a highest reaction barrier of 21.7 kcal/mol. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, we conduct a theoretical study of the half-metallic properties of the bulk and the (110) and (001) surfaces of rocksalt VPo with an equilibrium lattice constant of 0.59nm. The calculations are done using the first-principles full-potential linearized augmented plane–wave method based on the density functional theory. We show that both the (110) and (001) surfaces of rocksalt VPo preserve the bulk half-metallicity, and the atomic magnetic moments at both the (110) and (001) surfaces are increased compared to those in bulk VPo. Moreover, by calculating the relaxed surface energies, we assess the surface stability, and find that both (110) and (001) surfaces are not stable in equilibrium conditions, and non-equilibrium growth techniques are required for the realization of rocksalt VPo thin films. [Copyright &y& Elsevier]

Abstract: In heterogeneous catalysis, catalyst precursor activation and subsequent initial operation (‘conditioning’) under reaction conditions are often highly dynamic steps which are important in delivering effective catalyst performance over time scales of years. In many cases however, the phenomena occurring in both steps are often poorly understood. A novel approach to assess the conditioning step, comprising advanced catalyst testing methods in micro-reactors, catalyst characterisation and detailed kinetic and activity modelling techniques is demonstrated for a model system: the selective oxidation of n-butane to maleic anhydride (MA) using a pre-activated vanadium phosphorus oxide (VPO) catalyst. A transient kinetic model for the conditioning step is presented which describes the decline of population of active sites for three reaction pathways on the catalyst surface over time. Two reaction pathways, n-butane --> MA and n-butane --> COx decay similarly with time whilst a third, MA --> COx declines very sharply. The approach used provides important mechanistic information on catalyst reaction kinetics whilst also providing understanding the impact of reaction conditions on the catalyst during conditioning. [Copyright &y& Elsevier]

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: The formation of dispersed VPO phases on a lignocellulosic-based activated carbon results in a catalyst that is selective for the partial oxidation of propane, and stable under oxidizing conditions. The use of a stable activated carbon as a catalytic support for active VOx species during the partial oxidation of propane is described for the first time. [Copyright &y& Elsevier]

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]

Vanadium phosphorus oxide (VPO) catalysts synthesized via hydrothermal method were introduced with some different dopants, Cr, Ni, Fe, and Mn during the preparation of the precursors, VOHPO4 · 0.5H2O. All modified precursors were subsequently transformed under reaction conditions to give the active phase, (VO)2P2O7. Several techniques were used to characterize the physicochemical properties of the catalysts, such as X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), H2-temperature programmed reduction (TPR), laser Raman spectroscopy (LRS), and chemical analysis. The catalytic performance of the catalysts for selective oxidation of n-butane to maleic anhydride has been carried out by using a fixed bed microreactor (673 K, gas hourly space velocity [GHSV] = 2,400 hr-1). The results show that the addition of dopants into the VPO catalysts increased the surface area of the catalysts and induced the formation of V5+ phases as shown in XRD and LRS spectra that promoted the catalytic performance. A further water reflux treatment to all the precursors gave catalysts with only (VO)2P2O7 phase. However, the activity and selectivity of these catalysts were reduced markedly. [ABSTRACT FROM AUTHOR]

Abstract: The kind and percentage of point defects in commercial as well as laboratory prepared hemihydrate VPO precursors have been analysed using a combination of electron diffraction experiments and diffraction pattern simulation. Experimental patterns reveal the systematic existence of kinematically forbidden reflections that are attributed to vacant lattice sites in the VOHPO4·1/2H2O matrix. The vacancies are associated with vanadium ions and are distributed into the bulk and surface of hemihydrate in an ordered fashion. The amount of vacancies is in similar proportion between the two precursors studied. A considerable enhancement in the forbidden reflections intensity in doped hemihydrate samples has been experimentally observed, which is attributed to the partial substitution of dopant ions into V vacant sites. The beneficial effect of such a modified hemihydrate structure on the resulting VPO catalyst performance is demonstrated, since the facilitation of promoter incorporation in V-based vacant active sites brings upon an enhanced activity and selectivity for the catalyst. [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]

The molecular organization of N,N'-dimethyl-N,N'-dioctylhexylethoxymalonamide (DMDOHEMA), the current reference extractant for the DIAMEX (DIAMide EXtraction) process, is correlated with its water extraction properties from neutral media. The aggregation of DMDOHEMA in n-heptane was investigated by vapor pressure osmometry (VPO) and the aggregate speciation characterized by combined small-angle neutron and X-ray scattering (SANS and SAXS, respectively). Two approaches were taken to model the aggregation of the diamide and the water extraction as a function of the diamide concentration by taking into account a single aggregation equilibrium with an average aggregation number N equal to 4.28 ± 0.05; and a competition between two types of aggregates in the organic phase, namely, aggregates of the reverse micelle type with 4 diamides per aggregate, and an oligomeric structure composed of about 10 diamide molecules which appears at high extractant concentration (>1 mol/L). In both cases, the supramolecular speciation representing the monomers/aggregates distribution was determined, and for each supramolecular organization, a solubilization parameter was calculated using the Sergievskii-Dannus relationship. Thus, the correlation between the two types of micellization of the diamide and the extraction of water into the organic phase was demonstrated. The larger aggregates can extract about five times more water than monomers. [ABSTRACT FROM AUTHOR]

Abstract: A series of silver-doped vanadium phosphorus oxides (Ag-VPO) catalysts were prepared by in situ synthesis, the catalytic tests showed that silver component significantly increased the overall activity for the oxidation of styrene by tert-butyl hydroperoxide (TBHP). It is interesting that Ag-VPO catalysts show no epoxidation selectivity but good selectivity for benzaldehyde in this study. To investigate the origin of the silver effect, the solids were characterized using X-ray diffraction (XRD), Fourier transformed-infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and other instruments, the (VO)2P2O7, β-VOPO4, Ag(V2P2O10) and Ag2(VO2)(PO4) phases were detected in all the Ag-VPO catalysts. This study also confirms that ratios of P/V play a crucial role in the activity and the selectivity of the catalyst, meanwhile, the other factors, such as solvent, oxidant concentration, reaction time, also have been investigated and reaction conditions are optimized. [Copyright &y& Elsevier]

Abstract: The lactam groups of dipyrrinones avidly engage in amide–amide hydrogen bonding to form dimeric association complexes in non-polar solvents (in CHCl3, K D ∼25,000M−1 at 22°C). The corresponding thioamides (dipyrrinthiones), prepared from dipyrrinones by reaction with Lawesson''s reagent, also form intermolecularly hydrogen-bonded dimers in non-polar solvents, albeit with much weaker association constants (in CHCl3, K D ∼200M−1 at 22°C). When a carboxylic acid group is tethered to C(9) of the dipyrrinone, as in the hexanoic acid of [6]-semirubin, tight intramolecular hydrogen bonding between the carboxylic acid group and the lactam moiety (intramolecular K assoc ≫25,000) is found in CHCl3 with no evidence of dimers. In contrast, the analogous dipyrrinthione, [6]-thiosemirubin, eschews intramolecular hydrogen bonds, as determined using NMR spectroscopy and vapor pressure osmometry, preferring to form intermolecularly hydrogen-bonded dimers of the thioamide–thioamide type. [Copyright &y& Elsevier]

The present article describes the synthesis, spectral, coordination and thermal aspects of N,N′-polymethylene bis(1-phenyl-3-methyl-4-trifluoroacetylimino-2-pyrazoline-5-ol)oxovanadium(IV) or copper(II) Schiff base complexes with alkyl backbones ranging from two to four carbons have been characterized on the basis of elemental analysis, magnetic moments, molar conductivity measurements, spectra (FTIR, ESR, UV-Visible, MS), VPO and thermal studies. The vapour pressure osmometry (VPO) and mass spectral studies indicate that the complexes are monomeric. An ESR study of all these complexes of VO(IV) and Cu(II) are consistent with the square pyramidal and square planar geometries of these metal ions, respectively. In addition, the kinetics and thermodynamic parameters for the different thermal decomposition steps of the complexes have been studied employing Horowitz-Metzger and Freemen-Carroll methods. [ABSTRACT FROM AUTHOR]

This paper describes the development of a new technique, atomic beam deposition (ABD), for the fabrication of catalysts with different active sites capable of catalyzing different chemical reactions. Using ABD, submonolayer amounts of Te and Cu atoms were deposited on vanadyl pyrophosphate (VPO) catalyst particles, and precise non-steady-state kinetic characterization was performed using the temporal analysis of products (TAP) reactor immediately following deposition. Results from TAP Knudsen pulse response experiments determine if small changes in the metal surface composition would produce detectable changes in catalytic properties. Partial oxidation of 1-butene to furan was used as the test reaction. Furthermore, this paper also demonstrates the sensitivity of the TAP reactor by measuring oxygen uptake on a single 350 μm diameter polycrystalline Pt particle packed in a bed with 100,000 inert quartz particles of similar dimensions. The Pt experiments demonstrate the precise manipulation of surface oxygen content. [ABSTRACT FROM AUTHOR]

Abstract: Vanadium phosphate catalysts were synthesized via VOPO4·2H2O and were calcined in two different hydrocarbon reaction environments, i.e. n-butane/air and propane/air. Both catalysts are denoted VPDB and VPDP, respectively. Both catalysts exhibited a good crystalline with characteristic peaks of pyrophosphate phase. However, the peaks for VPDP are shown to be more prominent than those of VPDB. BET surface area showed that VPDB gave higher surface area (23m2 g−1) compared to VPDP (18m2 g−1). The average V valence state for VPDP is 4.08 and the higher V valence state for VPDB is 4.26 due to higher amount of VV for VPDB. Furthermore 14.2% of VIII was found for VPDP but none for VPDB. SEM micrographs clearly revealed that the morphologies of both catalysts composed of plate-like crystallite that was arranged into the characteristic of rosette cluster. However, the catalyst calcined in n-butane/air environment (VPDB) resulted in an increment of the amount of plate-like crystal formed in the rosette rosebud agglomerates. TPR in H2 profiles of both catalysts gave two reduction peaks corresponding to two kinetically different oxygen species which were associated with VV and VIV phases, respectively. VPDB removed larger amount of active oxygen species linked to VIV phase which eventually caused a higher conversion rate in the selective oxidation of n-butane and propane to maleic anhydride and acrylic acid, respectively. [Copyright &y& Elsevier]

Abstract: The hybrid organic/inorganic copper oxovanadium diphosphonates of formula [Cu(bipy)(VO2)2(O3PCH2PO3)H2O] 1, which crystallizes in the orthorhombic symmetry, and [Cu(bipy)(VO2)(O3P(CH2)2PO2OH)]·1.5H2O 2, which crystallizes in the monoclinic symmetry, were obtained by the soft hydrothermal route. The layered VPOC structure of 1 may be described as a undulating (VO2)2(O3PCH2PO3)(H2O)2− network formed by the link between the V2PCO10 rings via the diphosphonate fragments, creating 18-membered {V4P5C2O7} cavities. The corrugated lamellar structure of 2 is obtained by the interconnection of four oxovanadium phosphorus V4P8C8O8 anionic rings with diphosphonate fragments, forming 28-atom cavities. In both compounds the copper diimine cation groups are coordinated to the chelating oxygen atoms of the PO3 and oxovanadium groups, and do not participate in building the 2D-structure. [Copyright &y& Elsevier]

In unsupported V-containing catalysts, V4+ sites are frequently present in amorphous oxidic clusters and/or crystalline paramagnetic bulk phases in which they are coupled by effective spin–spin exchange interactions. This work presents two EPR procedures for evaluating these interactions and relating their strength to the degree of disorder and catalytic performance, namely (1) evaluating permanent structural disorder by calculating exchange energies and integrals and (2) monitoring transient electronic disorder caused by catalyst–reactant interaction in working catalysts using the ratio of the fourth and the square of the second moment of the EPR signal. Selected application examples comprise VPO catalysts such as (i) (VO)2P2O7, in which a certain degree of structural disorder revealed to be essential for high catalytic performance in the selective oxidation of n-butane to maleic anhydride, (ii) (NH4)2(VO)3(P2O7)2 in which transient perturbation of spin–spin exchange sensitively reflects the strength of reactant adsorption during ammoxidation of toluene and (iii) (NH4)2VOP2O7 in which an amorphous VO2+-containing phase could be identified as active component besides inactive crystalline phases. Based on operando-EPR measurements, reasons for activity differences in MoVTeNb oxide catalysts could be suggested. [ABSTRACT FROM AUTHOR]

Abstract: The present work addresses the issue of the promoter effect produced by group VIB elements added to the basic VPO formulation using different methods and loads in order to obtain information linking the catalytic behavior of the solids with their physicochemical properties. The unpromoted VPO precursor (VOHPO4·1/2H2O) was prepared from V2O5 reduced with an alcoholic mixture and 100% H3PO4. Cr, Mo or W were either impregnated on the VPO precursor or co-precipitated during the precursor synthesis. The precursors containing varying loads of the promoter were activated following a standard procedure in the same flow micro-reactor where the catalytic performance was evaluated. The precursors and the catalysts were characterized by X-ray diffraction (XRD) and laser Raman spectroscopy (LRS). Acetonitrile was used as a probe molecule to test the strength of the Lewis acid sites monitored by FTIR. The amount of chemisorbed acetonitrile was measured in a standard high vacuum system. The addition of the promoters invariably increased the catalytic activity but in every case there was an optimum load to achieve the best selectivity. The origin of this maximum could be ascribed to the right balance between the presence of very strong Lewis acid sites and the development of V5+ isolated sites in the matrix of the vanadyl pyrophosphate, by far the major crystalline phase present in the catalyst. The three group VIB promoters affect the balance between acid and redox properties but increase the overall activity in all cases. [Copyright &y& Elsevier]

By applying fumed SiO2 and the deposition–precipitation method based on organic medium, the composite VPO/fumed SiO2 catalysts were first prepared and tried for partial oxidation of n-butane to maleic anhydride. In the temperature range of 653–693 K the fumed SiO2-based catalysts not only showed good activity but also maintained sufficiently high MA selectivity in comparison to some conventional supported VPO catalysts. As an example, the catalyst with 30% VPO content showed butane conversion of 60% and MA selectivity of 58 mol% at 673 K. The turnover rates of low loading samples are found to be higher than that of high loading sample as well as unsupported catalyst. Besides the unique interaction which may exist between VPO component and the fumed SiO2 material, co-existence of dominant (VO)2P2O7 and minor VOPO4 in these non-equilibrated catalysts may be favorable for MA formation. Moreover, introducing the additive of polyethylene glycol (PEG) in the preparation medium can obviously enhance the dispersion of VPO component and hence lead to a more selective catalyst. [ABSTRACT FROM AUTHOR]

Time resolved in situ X-ray diffraction was used to study the formation of vanadyl hydrogen phosphate hemihydrate (VOHPO4·0.5H2O) in organic media. This technique has identified a new phase at early synthesis times (less than 300 s) at a d-spacing of 7.5 and 3.1 Å. The feature at 7.5 Å shifted to 6.7 Å during the first 20 min of synthesis and this phase collapsed as VOHPO4·0.5H2O formed. Thin symmetrical platelets of 10 μm×10 μm dimensions were observed when samples were recovered after short synthesis times. Energy dispersive X-ray analysis indicated that these platelets contained vanadium and phosphorus. The platelets appeared to delaminate possibly associated with the strain generated when the d-spacing shifted from 7.5 to 6.7 Å, from which growth of the VOHPO4·0.5H2O into the familiar rosette morphology occurs. [Copyright &y& Elsevier]

A novel reactor configuration is proposed to study the kinetics of heterogeneous catalytic reactions. It is based on the bubbling fluidization regime; gas velocities are sufficiently low to minimize bypassing but high enough to ensure solids backmixing and a uniform temperature gradient. The reactor is equipped with a gas manifold designed to operate the reactor either under steady-state feed conditions or with cyclical feeds. By cycling VPO catalyst between a reducing environment with butane and an oxidizing environment, we quantify the lattice oxygen contribution to the reaction. Furthermore, we show that carbon can build up on the VPO surface and this phenomenon increases with an increase in the severity of the reducing environment. We propose a simple kinetic model involving three sites—V5+, V4+ and VC4. The model adequately characterizes the transient partial oxidation of butane but further research is required to create a generalized model that fits all reaction environments. [Copyright &y& Elsevier]

In situ analysis of the surface of a working selective oxidation catalyst is an essential yet rarely conducted experiment in attempts to derive structure–function relationships. The case study of n-butane oxidation over vanadyl pyrophosphate (VPP) is used to develop a general working hypothesis and to illustrate that the molecular properties of the substrate sets boundary conditions on the surface chemical properties of the catalyst. Experiments using in situ X-ray photoelectron spectroscopy (XPS) and in situ low-energy X-ray absorption spectroscopy are used to derive compositional, electronic, and geometric structural information of the surface of the working VPP. These data allow the conclusion that a surface phase different from VPP must be present covering at least part of the active material. The recent data together with literature observations are used to derive a scenario explaining the function of VPP as a unique catalytic system. [ABSTRACT FROM AUTHOR]

This study presents information about crystal imperfections in the main phase of industrial vanadium phosphorous oxide catalysts that are used to catalyze the oxidation of n-butane to maleic anhydride, being an important intermediate in the chemical industry. The mechanism of this reaction is still debated, and the catalytically active and selective surface centers have not yet been identified. The results presented are based on X-ray diffraction data obtained by both laboratory-scale and synchrotron powder diffraction experiments, as well as laboratory-scale single-crystal diffraction experiments. It has been proven that pronounced Bragg reflection broadening effects found in laboratory-scale powder diffraction patterns of industrial VPO catalysts are real and not due to an insufficient 2-θ resolution of the apparatus. In the framework of this work, a powder diffraction full profile fitting strategy was developed using the TOPAS software, which was applied to analyze the X-ray diffraction data of four differently activated industrial catalyst samples, originating from one batch after they had been catalytically tested. It was found that the reflection broadening is mainly caused by an anisotropic crystal size, which results in platelet-shaped crystallites of vanadyl pyrophosphate. A further contribution to the reflex broadening, especially for (111), was found to be a result of stacking faults perpendicular to the a direction in the crystal structure of vanadyl pyrophosphate. These results were used to elaborate on possible correlations between structural proxies and catalytic performance. A direct correlation between the extension of coherently scattering domains in the z direction and the catalyst's selectivity could be proven, whereas the activity turned out to be dependent on the crystallite shape. Regarding the phase contents, it could be shown that sample catalysts containing a higher amount of β-VO(PO3)2 showed increased catalytic activity. [ABSTRACT FROM AUTHOR]

In this paper, a heteropolyacid catalyst VPO-Cr-PEG/TiO 2 was developed and the low temperature de-NO x performance of catalyst was studied. The experimental results showed that the de-NO x efficiency reached over 98% at the low temperature of 150–350 °C. The catalyst exhibited excellent sulfur and water resistance in the presence of 1000 ppm SO 2 and 8 vol% H 2 O. In the presence of polyethylene glycol (PEG), the crystal surface energy of the catalyst decreased, and (VO) 2 P 2 O 7 was the main active crystal phase of the catalyst. The selective catalytic reduction of NO x by NH 3 over VP(1/5)O-Cr-PEG/TiO 2 catalyst simultaneously followed the Langmuir-Hinshelwood and Eley-Rideal mechanisms. [Display omitted] • The heteropolyacid catalyst VPO-Cr-PEG/TiO 2 for NH 3 -SCR was developed. • The addition of PEG increased the crystallinity of VPO. • VP(1/5)O-Cr-PEG/TiO 2 showed good resistance to SO 2 & H 2 O. [ABSTRACT FROM AUTHOR]

Prepared unique mesh-hole structure of VPO catalyst by POM-ILs anions-cations synergetic Interaction for promoting C H bond activation. • Design a single-component additive POM-ILs with dual-function to enhance the activity of VPO catalyst. • The synergistic interaction of anions and cations of POM-ILs was illuminated. • POM-ILs@VPO shows unique mesh-hole structure and forms more oxygen vacancies and crystal defects. • POM-ILs@VPO exhibits high activation ability of C–H bond. • Finely divide the poly-acids (α , β , γ -acid) sites to describe the acidic properties. Vanadyl pyrophosphate (VPO) catalyst exhibits a unique ability to activate C H bond and selectively oxidize n- butane to maleic anhydride (MA). Here, a novel "function-oriented design" strategy was proposed to design and develop single-component additive polyoxometalate-ionic liquids (POM-ILs) with dual-function to enhance the activity of VPO catalyst. And the synergistic interaction of anions and cations of POM-ILs was illuminated combined with characterizations. The IL-cations guide the growth of the catalyst precursor to ordered nanolamellar structure rather than three-dimensional disordered rose-like. Correspondingly, it makes the catalyst exhibit a unique mesh-hole structure and benefit exposure of the active surface (200) of (VO) 2 P 2 O 7. The POM-anions decompose into the defective O P Mo compound and cause more oxygen vacancies and crystal defects formed on the surface of VPO after activation. Besides, we established a specific method to divide the poly-acids (α , β , γ -acid) sites to describe the acidic properties on the surface of VPO by NH 3 -TPD in qualitatively and quantitatively and related with catalytic performance. [ABSTRACT FROM AUTHOR]