Study of the Co-VPO interaction in promoted n-butane oxidation catalysts (original) (raw)
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The beneficial effect of cobalt on VPO catalysts
Catalysis Today, 2003
The addition of Co to VPO formulations improves the yield of n-butane to maleic anhydride. In this work, different modes of impregnation and two different organic cobalt salts were used. The equilibrated catalysts were characterized using XRD, 31 P SEM NMR, FT-IR and acetonitrile adsorption to evaluate Lewis acidity. The best catalyst was obtained using Co acetyl acetonate for impregnation of the VOHPO 4 •0.5H 2 O precursor. This catalyst after equilibration had an optimum concentration of very strong Lewis acid sites, very low concentration of isolated V(V) centers, and no V(V) phases.
Effect of Mo on the Active Sites of VPO Catalysts upon the Selective Oxidation of n-Butane
Journal of Catalysis, 1999
The effect of the addition of Mo to VPO formulations on the physicochemical and catalytic properties of VPO solids was studied using X-ray diffraction (XRD), Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, Laser Raman spectroscopy (LRS), temperature-programmed reduction, and a flow reactor system. The addition of Mo to the oxides increases the activity and selectivity of the VPO catalysts. The promoting effect is a function of both the Mo loading and the way such cation was added to the VPO matrix. The best catalyst was obtained when 1% Mo was impregnated on the VOHPO 4 · 0.5H 2 O phase. At 400 • C 36% of molar yield to maleic anhydride was obtained in this catalyst against 12% of the unpromoted catalysts and only 3% of the solids where Mo was added during the phosphatation step. The impregnated 1%
The effect of the phase composition of model VPO catalysts for partial oxidation of n-butane
Catalysis Today, 1996
X-ray diffraction, Raman spectroscopy, 3'P MAS-NMR and spin-echo NMR indicated that model vanadium phosphorus oxide (VPO) precursors and catalysts contained various minor phases depending oxboth the synthetic approach and P/V ratios used. Raman spectroscopy revealed the presence of a number of micro-crystalline and amorphous V(W) and V(V) phases not evident by XRD. The presence of VOPO, phases was detrimental to the performance of the VP0 catalysts for KN-butane oxidation. The best model organic VP0 catalyst contained only vanadyl pyrophosphate with the highest degree of stacking order and virtually no VOPO, phase impurity. Raman spectroscopy detected vanadyl metaphosphate. VO(PO,),, in the catalysts derived from aqueous precursors possessing P/V ratios greater than I. Pure vanadyl metaphosphate catalyst was inactive in n-butane oxidation. s'P NMR demonstrated the absence of vanadyl metaphosphate and other impurity phases in the best catalyst derived from organic precursors at P/V = 1.18. The experimental data strongly indicate that the best VP0 catalysts for n-butane oxidation contain only vanadyl pyrophosphate with well-ordered stacking of the (200) planes.
n-Butane selective oxidation on vanadium-based oxides : Dependence on catalyst microstructure
1986
The oxidation of n-butane and of its intermediates to maleic anhydride has been studied on different vanadium-phosphorus oxides and on Ti02-and zeolites-supported vanadium oxides. On vanadium-phosphorus oxides the activity in n-butane selective oxidation depends strongly on the catalyst microstructure. On supported vanadium oxides n-butane is not selectively oxidized ; however, when the amount of vanadium deposed largely exceeds the monolayer amount, low yields of acetic acid are obtained. The analysis of the oxidation of some intermediates suggests that the mechanism of maleic anhydride formation from n-butane occurs through the successive formation of butadiene, 2,5-dihydrofuran and furan via successive cycles of oxygen insertion and allylic H-abstraction and that these properties are connected to the vanadium ions and not to a particular surface structure. On the contrary, the alkane activation requires a particular surface structure of vanadium deriving by straining of V-(OP) connections. A model of the possible mechanism for n-butane activation iS also given. INTRODUCTION Notwistanding the growing interest in the selective oxidation of n-butane to maleic anhydride (11, the nature of the active sites able to activate the paraffins is not clear and the only hypothesis made in the literature, involving the Presence Of D-species (21, is lacking the experimental support necessary in order to extraPol_ ate the results to real catalysts. Furthermore, due to the very complex and.multisteps reaction pattern, lacking knowledges are present in literature on the nature and structure of the active sites necessary for the successive steps from n-butane to maleic anhydride. Aim of this work was to analyze our data of the selective oxidation of n-butane and of its possible intermediates to maleic anhydride on different vanadium-based catalysts, for the Purpose of determining the reaction mechanism and the nature and ?roPerties of the active centers for the successive steps from n-butane to maleic 0166-9834/86/$03.50 0 1986 Elsevier Science Publishers B.V. anhydride. EXPERIMENTAL Vanadium-phosphorus oxides (VP): V205 was reduced in 37 % HCl, then o-H3P04 added to obtain a P:V atomic ratio of 1.0. The resulting solution was concentrated and then water added to obtain a blue precipitate which was dried at 150 C for 24 h (VP a*). ~205 was reduced in a mixture (3:2) of isobuthyllbenzyl alcohols, then o-H3P04 added to obtain a P:V atomic ratio of 1.0. The resulting slurry was filtered and dried at 150 C for 24 h (VP b*). Both precursor samples VP(a*) and VP(b*) were then activated in a flow of 1% n-butane/air at 400 C for 6 h, to give the VP(a) and VP(b) catalysts, respectively. Vanadium-supported oxides : VT1 18 and VTi 117 catalysts were prepared by impregnation with a NH4V03/oxalic acid/water solution, drying at 150 C for 24 h and calcination at 430 C for 3 h. Both Ti02 supports in the anatase form were Tioxide, CLDD 1587/ /2 (18.4 m2/g) and CLDD 1764/Z (117 m*/g) respectively. The amount of vanadium deposited in both cases was 10% wt of V205. Zeolites-supported vanadium oxides (VZ) were prepared by impregnation with a NH4V03 solution of Y-zeolite [ VZ(HY) 1, HZSM-5 or HZSM-11 [ VZ(ZSM5) and VZ(ZSM11) j and by ionic exchange with VOS04 solution of Y-zeolite [ V02+ Y ]. Amount of deposed vanadium is about 2 % wt of V205 and 3.5% wt in the case of V02+Y . Before catalytic tests, samples were activated in air at 430 C for 6 h. Further details on the preparation of all these catalysts and on their characterization have been reported previously (j-10). Catalytic tests Catalytic tests were performed in a flow reactor with analysis on-line of the reagent composition and products of reaction by means of two gas-chromatographs. Details of the reactor and method of analysis are reported elsewhere (8). One g of catalyst was used for each test. The reagent composition was hydrocarbon:oxygen:nitrogen 0.6:12.0:87. The total flow of the reactant was 70 cc/min. RESULTS Vanadium-phosphorus oxides The catalytic behaviors of VP(a) and VP(b) catalysts in n-butane and 1-butene selective oxidation are reported in Figure 1. Catalyst VP(b) is more active than catalyst VP(a) both in alkene and alkane oxidation, in agreement with the higher surface area (27 and 6 m2/g, respectively). However the difference in activity is much greater in the n-butane than in 1-butene oxidation. Furthermore, whereas the V -+(0-P) bond, the presence of medium-strong Lewis acidity due to Va atoms can be thus explained in the catalyst VP(a) surface. However the enhanced Lewis acidity of the catalyst VP(b) surface cannot be explained. The (020) planes are connected by pyrophosphate groups and thus disorder in the stacking-fold of the (020) planes induces the straining of the V~lr~~(O-Pl bonds, which can be schematically represented as follows : A SCHEME 2 The Lewis acidity of the Vb atoms is enhanced by this effect as compared to the Lewis acidity of the corresponding Va atoms. It has been shown on solid super acid (16) that the first step in the activation of n-butane is the extraction of an H-from the n-butane by very-strong Lewis sites. Similarly, it is possible to hypothesize that the very-strong Lewis sites observed on catalyst VP(b) and to a lesser extent on catalyst VP(a), are the sites responsible for the first step in alkane activation on vanadium-phosphorus oxides. It is thus possible to propose the mechanism of n-butane selective activation showed in Scheme 3. A coordinated attack of itrong Lewis sites ( Vb atoms ) and of a strong base ( OS-) activates the n-butane, giving the corresponding olefins which are further quickly oxidized due to their higher reactivity. Relationship between structure and mechanism of oxidation --In a previous work we have showed (9) that the general mechanism of maleic anhydride formation from n-butane can be written as follows : n-butane -Dbutenes --Obutadiene -4furan -+maleic anhydride 7 G.Centi, Z.Tvaruzkova, F.TrifirB, P.Jiru and L.Kubelkova, Appl. Catal., 13 (19841 69. 8 G.Centi, G.Fornasari and F.Trifir6, Ind. Eng. Chem. Prod. Res. Dev., 24 (1985) 32. 9 G.Centi, G.Fornasari and F.Trifird, J. Catal., 89 (1984) 44. 10 F.Cavani, G.Centi and F.Trifir6, Appl. Catal., 15 (1985) 151. 11 E.Bordes, P.Courtine and J.W.Johnson, J. Solid State Chem., 55 (1984) 270. 12 J.
Journal of the Taiwan Institute of Chemical Engineers, 2015
A series of cobalt-doped vanadium phosphorus oxides (VPO-Co) catalysts, as well as unpromoted sample (VPO) was prepared using classical organic method via VOHPO 4 •0.5H 2 O precursor followed by calcinations in 1.5% butane/air environment at 400°C for 24 h. Techniques such as XRD, BET surface area, chemical titration, SEM, TPR and FT-IR were used for characterization of the catalysts. The results showed that promoted VPO catalyst were contained crystalline form of V 4+ (vanadyl pyrophosphate, (VO) 2 P 2 O 7), CoPO 4 phase and a small amount of V 5+ (β-VOPO 4 phase). However, Co promoted VPO contained poorer crystallinity compared to the unpromoted catalysts. Co was found to increase the average oxidation number of vanadium from 4.28 to 4.43 due to increase of V 5+ oxidation state from 28 to 43%. Oxidation of cyclohexane, for the first time, was studied in the liquid phase over VPO and VPO-Co catalysts, using tert-butylhydroperoxide (TBHP) as an oxidant. The catalytic tests showed that cobalt doping significantly increased the overall activity for the oxidation of cyclohexane. At 90°C, the obtained activities were 0.076 and 0.491 g pro /g VPO /h over the VPO and VPO-Co (molar ratio Co/V = 0.1) catalysts, respectively. The effects of Co loading, TBHP/cyclohexane molar ratio, amount of the catalyst, solvents and catalyst recycling were investigated. The kinetic of cyclohexane oxidation was investigated at different temperatures using VPO-Co(0.1) and excess TBHP. The order of reaction with respect to cyclohexane was determined to be pseudo-first order. The value of the apparent activation energy was also determined.
Journal of Catalysis, 1997
The oxidation of n-butane to maleic anhydride was investigated over a series of model-supported vanadia catalysts where the vanadia phase was present as a two-dimensional metal oxide overlayer on the different oxide supports (TiO 2 , ZrO 2 , CeO 2 , Nb 2 O 5 , Al 2 O 3 , and SiO 2 ). No correlation was found between the properties of the terminal V= =O bond and the butane oxidation turnover frequency (TOF) during in situ Raman spectroscopy study. Furthermore, neither the n-butane oxidation TOF nor maleic anhydride selectivity was related to the extent of reduction of the surface vanadia species. The n-butane oxidation TOF was essentially independent of the surface vanadia coverage, suggesting that the n-butane activation requires only one surface vanadia site. The maleic anhydride TOF, however, increased by a factor of 2-3 as the surface vanadia coverage was increased to monolayer coverage. The higher maleic anhydride TOF at near monolayer coverages suggests that a pair of adjacent vanadia sites may efficiently oxidize n-butane to maleic anhydride, but other factors may also play a contributing role (increase in surface Brønsted acidity and decrease in the number of exposed support cation sites). Varying the specific oxide support changed the n-butane oxidation TOF by ca. 50 (Ti > Ce > Zr ∼ Nb > Al > Si) as well as the maleic anhydride selectivity. The maleic anhydride selectivity closely followed the Lewis acid strength of the oxide support cations, Al > Nb > Ti > Si > Zr > Ce. The addition of acidic surface metal oxides (W, Nb, and P) to the surface vanadia layer was found to have a beneficial effect on the n-butane oxidation TOF and the maleic anhydride selectivity. The creation of bridging V-O-P bonds had an especially positive effect on the maleic anhydride selectivity.
Identification of vanadium species involved in sequential redox operation of VPO catalysts
Applied Catalysis A-general, 2000
A vanadyl pyrophosphate (VPO) catalyst was investigated for its sequential reduction and oxidation ability for butane oxidation, using a novel oscillating microbalance reactor. Correlation of the redox kinetics with the identities of the vanadium species obtained by potentiometric titration methods suggests that only the near-surface oxygen is readily available for the oxidation of butane; effective redox cycles can only occur between V 5+ and V 4+ ; accumulation of V 3+ species represents one mode of catalyst deactivation.
Catalysis Today, 1998
Surface characterization of V±P±O catalysts used in the oxidation of n-butane and n-pentane to maleic and phthalic anhydrides is reported. Two different states for the same catalyst are selected: after a short time under reaction conditions (100 h), the`non-equilibrated' system, and after a much longer time-on-stream (1000 h), the`equilibrated' catalyst. Characterization has been carried out by following the desorption of molecules with a mass spectrometer after catalysts had been in contact with the hydrocarbons and/or O 2 mixtures at 613 K. Valence state of vanadium under environmental conditions similar to those used in the desorption studies was obtained by XPS. It was concluded that both V V and V IV are capable of interacting with the hydrocarbon and that not all the V IV present at the (VO) 2 P 2 O 7 surface can interact. On the other hand, desorption of molecules resulting from such interaction occurs at lower temperatures from V IV -rich surfaces (`equilibrated' catalyst), than from the V V richer surface (`non-equilibrated' catalyst). # 1998 Elsevier Science B.V. 0920-5861/98/$32.00 # 1998 Elsevier Science B.V. All rights reserved. P I I S 0 9 2 0 -5 8 6 1 ( 9 8 ) 0 0 0 1 3 -3
XPS investigations of VPO catalysts under reaction conditions
Surface Science, 2005
The surface of vanadium phosphorus oxide (VPO) catalysts was investigated by (in situ) X-ray photoelectron spectroscopy (XPS) under reaction conditions. Two differently prepared VPO samples with similar catalytic activities showed different spectral behavior while the catalytic conditions were changed. The vanadium surface oxidation state of both catalysts was found to have the same value close to 4 under reaction conditions, while the oxidation state of vanadium in deeper layers differed significantly. The experimental results suggest that in VPO the catalytically active species located in the topmost surface layers (up to 1nm depth) are only weakly related to the structure of deeper layers. Based on our results we suggest that the deeper layers act as a substrate material only and can be different from the surface.
Journal of Catalysis, 2017
A set of vanadium phosphorous oxide (VPO) catalysts, mainly consisting of (VO)2P2O7, VO(PO3)2 or VOPO4•2H2O 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 ºC, i.e. within the temperature range used for ODH reaction. The catalysts based on (VO)2P2O7 phase presented the highest catalytic activity and productivity to ethylene. Nevertheless, the catalysts consisting of VO(PO3)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)2P2O7 and VO(PO3)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(PO3)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.