Search

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]

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]

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]

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]

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]

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]