A comparative study of VPO catalysts in the oxidation of butane to maleic anhydride (original) (raw)
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Journal of Catalysis, 1991
In order to try to control the V4+/V 5+ ratio of VPO catalysts for butane oxidation to maleic anhydride, a new method of preparation of these catalysts has been developed: it consists of the reaction of VCI3 (V 3 ÷) with V20 5 (V 5 +) for the preparation of the precursor. Two series of catalysts have been prepared in aqueous and organic media. The V3+/V 5+ ratio has been varied and its influence on the physicochemical features and on the catalytic properties of the catalysts has been studied. The best catalysts in both preparation media correspond to V3+/V s+ = 1 in the starting material. Catalysts have been characterized using X-ray diffraction, infrared spectroscopy, UV diffuse reflectance, Raman spectroscopy, 3~p MAS NMR, and XPS techniques. The combination of all these techniques led us to conclude that the best catalyst consisted of an oxidized surface (y-VOPO4) (V 5+) in interaction with reduced matrix ((VO)2P2OT) (V4+).
Catalysis Letters, 2006
Two vanadium phosphate catalysts (VPH1 and VPH2) prepared via hydrothermal method are described and discussed. Both catalysts exhibited only highly crystalline pyrophosphate phase. SEM showed that the morphologies of these catalysts are in plate-like shape and not in the normal rosette-type clusters. Temperature-programmed reduction in H2 resulted two reduction peaks at high temperature in the range of 600–1100 K. The second reduction peak appeared at 1074 K occurred as a sharp peak indicated that the oxygen species originated from V4+ phase are having difficulty to be removed and their nature are less reactive compared to other methods of preparation. Modified VPH2 gave better catalytic performance for n-butane oxidation to maleic anhydride contributed by a higher BET surface area, high mobility and reactivity of the lattice oxygen associated to the V4+ which involved in the hydrocarbon’s activation. A slight increased of the V5+ phase also enhanced the activity of the VPH2 catalyst.
2010
I would like to begin with by thanking Allah the almighty, for his bounties upon us and for his assistance in my studies and w ithout him, nothing is possible. I am deeply grateful to my supervisor, Professor Graham Hutchings, for his guidance, teachings and constant support. I wish to thankfully acknowledge Dr. Jonathan Bartley for his advice and unlimited support on resolving technical problem s and discussing experimental data. I am also very thankful to Dr. N icholas Dum m er for his suggestions and corrections during the writing o f this thesis. Thanks are due to my employer, King A bdulaziz City for Science and Technology (KACST) in Saudi Arabia for financial support. Special thanks to my Friend Salem Bawaked and all my friends in lab 1.88 and 1.96 for their help during my study in Cardiff. Meanwhile I have to thank the Leigh University, USA for getting the TEM images for m y study. To my beloved parents, you know how special you are how much you are loved. Thanks for your prays for me and thanks for being there at the other end o f the p h o n e... Finally, I express my deep thanks to my wife for being here with me during my study period, without you I do not think I could have made it.
Selective oxidation of n-butane to maleic anhydride on vanadyl pyrophosphate
Journal of Catalysis - J CATAL, 1998
In a previous publication (J. Catal. 171, 383 (1997)) we have shown that the oxidation of a pure and well-crystallized (VO) 2 P 2 O 7 catalyst at 500 • C for different times improves the catalytic performance in the n-butane selective oxidation to maleic anhydride. This has been explained by a proper density of selective V V species associated with structural defects. In the present work we bring additional information on the nature of the V (V) species formed during oxidation. By using electrical conductivity, Raman, XPS, and 31 P NMR (spin echo mapping and MAS), it is concluded that: (i) Upon oxygen exposure, isolated V V species appear at the surface but also in the bulk of (VO) 2 P 2 O 7 (V IV phase) within some depth without the formation of any definite VOPO 4 (V V phase) phase. (ii) A suitable V (V) /V (IV) ratio around 0.25 is suggested by XPS analysis for the best catalytic performance. For longer oxidation treatments, the development of amorphous V (V) microdomains occurs. The formation of such domains is detrimental to n-butane selective oxidation.
2016
Vanadium phosphate catalysts were prepared by calcining VOHPO4·1.5H2O for different duration (24 and 48 hours) under pure nitrogen flow, in order to create anaerobic atmosphere. The synthesis of sesquihydrate precursor involved a two-step procedure in which VOPO4·2H2O acted as an intermediate before obtaining the precursor. Interestingly, it enhanced the formation of V phase in the catalysts. Results from XRD analysis had shown the crystalline sizes decreased under prolong calcination duration, which lead to increment in specific surface area. Scanning electron microscopy clearly showed that catalysts exhibited plate-like crystallites with folded edges, which were similar to petals of flowers that sandwiched together in layered structure. For EDX and ICP, both results presented similar trend, in which the P/V atomic ratio decreased as calcination duration increased. Prolong the duration of N2 calcination also resulted in an increment in the amount of oxygen desorbed from V species. ...
1998
V-P-O catalysts supported on the surface of silica and titania particles were studied and compared with bulk V-P-O. The catalytic performance was tested in the n-butane oxidation reaction to maleic anhydride, and the structure of the equilibrated catalysts was characterised with X-ray absorption spectroscopy (EXAFS) and (low-temperature) ESR spectroscopy. Our results show considerable differences in catalytic performance between VPO/TiO 2 on the one hand, and VPO/SiO 2 and VPO/bulk on the other hand, the yield to maleic anhydride being comparable for VPO/bulk and VPO/SiO 2 . The differences in catalytic behaviour are attributed to differences in the local structure around vanadium (EXAFS). Furthermore, different spin exchange interactions between vanadium atoms in the three samples have been observed (ESR). The combination of characterisation methods suggests that the structure of the supported V-P-O phase is amorphous and differs considerably from that of bulk crystalline vanadylpyrophosphate. We therefore propose that the oxidation of n-butane to maleic anhydride takes place over an amorphous surface V-P-O phase. This finding has high relevance for our understanding of the catalytic activity of bulk crystalline V-P-O catalysts as well.
2007
Oxidation of n-butane to maleic anhydride catalyzed by vanadium phosphate catalyst is one of significant worldwide commercial interest since decades. Introductions of dopants and/or mechanochemical treatment are the most promising approach for the improvement of the catalytic performance of vanadium phosphate catalyst. Tellurium doped vanadium phosphate catalyst (VPDTe) was prepared via VOPO 4 ·2H 2 O phase after calcinating the tellurium doped precursor, VOHPO 4 •0.5H 2 O at 733 K in a flowing of n-butane/air for 18 h. VPDTe catalyst gave very high for n-butane conversion, 80% compared to only 47% for the undoped catalytst. The crystallite size, morphology, surface reactivity and reducibility of the catalyst have been affected by the addition of tellurium. VPDTe catalyst has result a higher existence of V 5+ phase in the catalyst bulk with having nearly the optimum amount of V 5+ /V 4+ ratio, 0.23. The SEM micrographs showed that the tellurium altered the arrangement of the platelets from "rose-like" clusters to layer with irregular shape. The sizes of platelets are even thicker and
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.
The Journal of Physical Chemistry B, 2003
In situ X-ray absorption spectroscopy (XAS) and in situ X-ray photoelectron spectroscopy (XPS) have been applied to study the active surface of vanadium phosphorus oxide (VPO) catalysts in the course of the oxidation of n-butane to maleic anhydride (MA). The V L 3 near edge X-ray absorption fine structure (NEXAFS) of VPO is related to the details of the bonding between the central vanadium atom and the surrounding oxygen atoms. Reversible changes of the NEXAFS were observed when going from room temperature to the reaction conditions. These changes are interpreted as dynamic rearrangements of the VPO surface, and the structural rearrangements are related to the catalytic activity of the material that was verified by proton-transfer reaction mass spectrometry (PTR-MS). The physical origin of the variation of the NEXAFS is discussed and a tentative assignment to specific V-O bonds in the VPO structure is given. In situ XPS investigations were used to elucidate the surface electronic conductivity and to probe the ground state of the NEXAFS spectra.