Characterisation of Variations in Vanadium Phosphate Catalyst Microstructure with Preparation Route (original) (raw)
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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.
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
Dependence of n-Butane Activation on Active Site of Vanadium Phosphate Catalysts
Catalysis Letters, 2009
The nature and the role of oxygen species and vanadium oxidation states on the activation of n-butane for selective oxidation to maleic anhydride were investigated. Bi-Fe doped and undoped vanadium phosphate catalysts were used a model catalyst. XRD revealed that Bi-Fe mixture dopants led to formation of a II -VOPO 4 phase together with (VO) 2 P 2 O 7 as a dominant phase when the materials were heated in n-butane/air to form the final catalysts. TPR analysis showed that the reduction behaviour of Bi-Fe doped catalysts was dominated by the reduction peak assigned to the reduction of V 5? species as compared to the undoped catalyst, which gave the reduction of V 4? as the major feature. An excess of the oxygen species (O 2-) associated with V 5? in Bi-Fe doped catalysts improved the maleic anhydride selectivity but significantly lowering the rate of n-butane conversion. The reactive pairing of V 4? -Owas shown to be the centre for n-butane activation. It is proposed that the availability and appearance of active oxygen species (O -) on the surface of vanadium phosphate catalyst is the rate determining step of the overall reaction.
Industrial & Engineering Chemistry Research, 2010
The vanadyl hydrogen phosphate hemihydrate (VOHPO 4 · 0.5H 2 O), with well-defined crystal size, has been successfully synthesized for the first time, using a simple one-step solvothermal process that was free of surfactants and water and had a short reaction time and low temperature. The synthesis was performed via the reaction of V 2 O 5 and H 3 PO 4 with an aliphatic alcohol (1-propanol or 1-butanol) at high temperatures (373, 393, and 423 K) in a high-pressure autoclave. The mixture of reactions directly gave the VOHPO 4 · 0.5H 2 O, which is a valuable commercial catalyst precursor for the selective oxidation of n-butane to maleic anhydride. The catalyst precursors were dried by microwave irradiation. The reaction conditions (by varying the reducing agent and reaction temperature) were used further for optimization of the crystallite size, surface area, morphology, and activity of the nanostructure of vanadium phosphate oxide [(VO) 2 P 2 O 7 ] catalyst. This new method significantly reduced the preparation time and lowered the production temperature (50%) of catalyst precursor (VOHPO 4 · 0.5H 2 O), when compared to conventional hydrothermal synthesis methods. The as-prepared (VO) 2 P 2 O 7 catalyst under various conditions exhibited remarkably different physical and chemical properties, indicating the potential of the suggested method in tuning the crystalline structure and surface area of (VO) 2 P 2 O 7 to improve its catalytic performance. It was found that the length of the carbon chain in an alcohol and reaction temperature in the solvothermal condition had a great impact on the chemical and physical properties of resulting catalysts. Interestingly, there was no trace of VO(H 2 PO 4 ) 2 , which is an impurity noted to be readily formed under solvothermal preparation conditions. The precursors and catalysts were characterized using a combination of powder X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) surface area measurement, scanning electron microscopy (SEM), and temperature-programmed reduction in hydrogen (H 2 -TPR). A correlation between the surface area of the catalyst and the activity was observed. Finally, the yield of maleic anhydride was significantly increased from 21% for conventional catalyst to 38% for the new solvothermal catalyst.
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. ...
The chemistry of catalysts based on vanadium-phosphorus oxides
1984
Vanadium-phosphorus mixed oxides, catalysts for C4 oxidation to maleic anhydride, were prepared in organic medium and tested in n-butane and butene-1 selective oxidation. These catalysts were compared with those prepared in aqueous medium. Before calcination, the two methods of preparation gave rise to the same phase, but the catalysts prepared in oraanic medium were characterized by higher surface area and lower oxidability in air. The latter characteristic makes it possible to obtain the selective compound of vanadium(IV) B-phase, (after calcination in air), in the full 0.95-1.15 P:V ratio range without the phases of vanadium(V) obtained in the case of catalysts prepared in aqueous medium. Unlike catalysts prepared in aqueous medium, an excess of phosphorus with respect to the ratio 1.0 led to a decrease both in activity and selectivity to maleic anhydride. The higher surface area of catalysts prepared in organic medium makes it possible to oxidize n-butane at lower temperatures than with catalysts prepared in aqueous medium.
Catalysis Today, 2008
The physico-chemical and catalytic properties of three ways of modified catalysts were studied, i.e. (i) the addition of both Bi and Fe (nitrate form) during the refluxing VOPO 4 Á2H 2 O with isobutanol (Catalyst A), (ii) the simultaneous addition of BiFe oxide powder in the course of the synthesis of precursor VOHPO 4 Á0.5H 2 O (Catalyst B) and (iii) the mechanochemical treatment of precursor VOHPO 4 Á0.5H 2 O and BiFe oxide in ethanol (Catalyst C). It was found that surface area of the modified catalysts has increased except Catalyst B. The reactivity of the oxygen species linked to V 5+ and V 4+ was studied by using H 2 -TPR, which also affected the catalytic performance of the catalyst. The conversion of n-butane decreases with an increment of oxygen species associated with V 5+ .
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.
Physico-chemicals and catalytic properties of manganese-promoted vanadium phosphate (VPO) catalyst
Reaction Kinetics and Catalysis Letters, 2007
The addition of 1% Mn promoter to vanadium phosphate catalyst led to doubling of the specific surface area from 20.3 (unpromoted) to 39.4 m2 g−1. The XRD pattern of the Mn-promoted catalyst gave only the characteristics of the (VO)2P2O7 phase, indicating that the Mn was incorporated into the crystal lattice of the catalyst. The Mn-promoted catalyst was also twice as active in removing the total amount of oxygen. However, since the only oxygen species related to V4+ being removed and no oxygen species associated with V5+ was observed, the n-butane conversion was not much improved as compared to the unpromoted counterpart. A necessary amount and distribution of the V5+ phase in a well crystalline V4+ phase is essential in order to enhance the catalytic performance in the mild oxidation of n-butane to maleic anhydride.