Dependence of n-Butane Activation on Active Site of Vanadium Phosphate Catalysts (original) (raw)
Related papers
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
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+ .
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.
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. ...
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.
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.
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.