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VPO catalysts

Spin-echo mapping therefore provides valuable structural information on VPO catalysts, and can also be used, for example, to determine magnetic characteristics such as the Weiss temperature of pure phases. Additionally, standard MAS NMR techniques have successfully been employed to yield important information on VPO catalysts, in a similar manner as for supported catalysts discussed above [105, 106, 108, 112, 113]. [Pg.216]


Promoters are sometimes added to the vanadium phosphoms oxide (VPO) catalyst during synthesis (129,130) to increase its overall activity and/or selectivity. Promoters may be added during formation of the catalyst precursor (VOHPO O.5H2O), or impregnated onto the surface of the precursor before transformation into its activated phase. They ate thought to play a twofold stmctural role in the catalyst (130). First, promoters facilitate transformation of the catalyst precursor into the desired vanadium phosphoms oxide active phase, while decreasing the amount of nonselective VPO phases in the catalyst. The second role of promoters is to participate in formation of a soHd solution which controls the activity of the catalyst. [Pg.454]

Fluidized-bed reactor systems put other unique stresses on the VPO catalyst system. The mixing action inside the reactor creates an environment that is too harsh for the mechanical strength of a vanadium phosphoms oxide catalyst, and thus requires that the catalyst be attrition resistant (121,140,141). To achieve this goal, vanadium phosphoms oxide is usually spray dried with coUoidal siUca [7631-86-9] or polysiUcic acid [1343-98-2]. Vanadium phosphoms oxide catalysts made with coUoidal sUica are reported to have a loss of selectivity, while no loss in selectivity is reported for catalysts spray dried with polysUicic acid (140). [Pg.455]

Fresh butane mixed with recycled gas encounters freshly oxidized catalyst at the bottom of the transport-bed reactor and is oxidized to maleic anhydride and CO during its passage up the reactor. Catalyst densities (80 160 kg/m ) in the transport-bed reactor are substantially lower than the catalyst density in a typical fluidized-bed reactor (480 640 kg/m ) (109). The gas flow pattern in the riser is nearly plug flow which avoids the negative effect of backmixing on reaction selectivity. Reduced catalyst is separated from the reaction products by cyclones and is further stripped of products and reactants in a separate stripping vessel. The reduced catalyst is reoxidized in a separate fluidized-bed oxidizer where the exothermic heat of reaction is removed by steam cods. The rate of reoxidation of the VPO catalyst is slower than the rate of oxidation of butane, and consequently residence times are longer in the oxidizer than in the transport-bed reactor. [Pg.457]

Oxygen has also been shown to insert into butadiene over a VPO catalyst, producing furan [110-00-9] (94). Under electrochemical conditions butadiene and oxygen react at 100°C and 0.3 amps and 0.43 volts producing tetrahydrofuran [109-99-9]. The selectivity to THF was 90% at 18% conversion (95). THF can also be made via direct catalytic oxidation of butadiene with oxygen. Active catalysts are based on Pd in conjunction with polyacids (96), Se, Te, and Sb compounds in the presence of CU2CI2, LiCl2 (97), or Bi—Mo (98). [Pg.343]

Friability tests can be used for various purposes. They are widely used in quality control. Here, samples of produced material are subjected to a more or less arbitrary but well defined stress. The attrition extent is assessed by comparison with a standard value and a decision is reached whether the material meets the standard. Moreover, friability tests are often used for comparison of different materials to select the most attrition-resistant one. This is a usual procedure in the case of catalyst development. For example, Contractor et al. (1989) tested anew developed fluidized bed VPO-catalyst in a submerged-jet attrition test (described below). Furthermore, the specific attrition rate of a material in a certain process can be roughly estimated by friability tests. In this case the stress must be similar to that occurring in the process and the obtained degradation extent must be compared with those of other materials from which the process attrition rate is known. [Pg.448]

Both devices described above were developed in order to test the friability of fluid-cracking catalysts. Nowadays the application of these or similar tests is a common procedure in the development of fluidized bed catalysts. Contractor et al. (1989), for example, used a submerged-jet test to compare the attrition resistance of newly developed VPO catalysts. In fact, such tests can be applied to any type of fluidized bed processes. Sometimes they have to be slightly modified to adapt them to the process under consideration. The drilled plate may, for example, be substituted by... [Pg.451]

Samples 1 and 2 correspond to VPO treated in steam for 92 and 312 h, respectively. Samples 3 and 4 are N2-treated and activated base VPO catalysts, respectively. MA capacities represent the total amount of MA liberated by reduction in 1.5% butane/N2 at the reaction temperature. Table II shows that the base and N2-treated catalyst have nearly equal activities in the presence of air in the reactant stream and continue to operate. [Pg.231]

Micro structures in heterogeneous catalysts are closely related to the catalytic properties. TEM and related microanalytic techniques are powerful tools in characterising catalysts at atomic level. The obtained structural information is essential to the understanding of correlations between microstructures and catalytic properties. In this lecture note, the general principle of characterization of catalysts by TEM is introduced and the applications on Pt/Si02 model system and on VPO catalysts are intensively described. [Pg.473]

Vanadium phosphorus oxides (VPO) are commercially used as catalysts for the s5mthesis of maleic anhydride from the partial oxidation of n-butane. The phase constitution and the morphology of the catalyst are found to be dependent on the preparation routes and the applied solvent [78]. Recently, a method to prepare VPO catalysts in aqueous solution at elevated temperature was reported [79]. In addition to the linear relationship between specific activity and surface area, a small group of catalysts exhibit enhanced activity, which could be due to the combination of a higher proportion of V phases in the bulk of vanadyl pyrophosphate (V0)2P207 catalyst [79, 80]. With high relevance to the catalytic properties, the microstructure characterisation of VPO therefore is of great importance. [Pg.482]

With the aim to study the characteristics of VPO catalysts in the course of butane oxidation to maleic anhydride together with a simultaneous evaluation of the catalytic performance, we have used Raman spectroscopy which is a very sensitive probe for determining the presence of V0P04-like entities together with (VO)2P207. An in situ Laser Raman Spectroscopy (LRS) cell was built in our laboratory (6). In the corresponding publication (6 ), the preparation and the characterization by XRD, Ip... [Pg.218]

In this communication, we compare VPO catalysts which differ by their conditions of preparation, considering both their LRS spectra registered under reaction conditions and the corresponding catalytic results. LRS data are discussed in relation with results for n-butane oxidation to maleic anhydride. [Pg.218]

It appears that the four VPO catalysts differ in the LRS spectra by the relative... [Pg.221]

Figure 2 LRS spectra of the VPO catalysts recorded at 420°C in the in situ cell under steady state conditions. Figure 2 LRS spectra of the VPO catalysts recorded at 420°C in the in situ cell under steady state conditions.
Figure 3 X-Ray diffraction patterns of the VPO catalysts at room temperature after the in situ Raman cell run. Figure 3 X-Ray diffraction patterns of the VPO catalysts at room temperature after the in situ Raman cell run.
This study has resulted in interesting informations concerning the active sites of the VPO catalysts for n-butane oxidation to maleic anhydride being obtained. The study of VPO catalysts in the course of n-butane oxidation by an in-situ Raman cell has shown... [Pg.228]

The behavior of ethane is different from the other alkanes. It is the only alkane that undergoes significant dehydrogenation on the VPO catalyst, as well as the only one for which combustion is the predominant reaction on VMgO. However, an ethyl species is too small to interact with two V ions simultaneously on any of the three catalysts. A phenomenological explanation of this behavior of ethane has been suggested [10]. In this explanation, the possible reactions of ethyl, propyl, and 2-methylpropyl species were compared by statistically counting the number of various types of bonds in each species ... [Pg.404]

For the VPO catalyst, for some unknown reasons, which may be related to the much lower reaction temperature, the C —H bond breaks much more... [Pg.404]

The procedures for synthesizing the catalysts have a marked effect on their activity. The active VPO catalyst phases are produced as follows. [Pg.113]

For high performance industrial catalysts capable of activating butane, organic media are used for the synthesis of VPO, which results in a large area of active (010) orientations. VPO catalysts synthesized in organic media are therefore used in the present chapter to elucidate dynamic butane catalysis by in situ ETEM. The other methods of preparation can be multi-phasic, which can result in the modification of the reactivity of the catalyst (Centi 1993, Kiely et al 1996). [Pg.113]

Precursor to active catalyst transformation VHPO to active VPO catalysts and dynamic electron diffraction... [Pg.113]

Figure 3.22. Dynamic electron diffraction (ED) image of the topotactic transformation of the VHPO precursor to active VPO catalyst in N2 (a) (010) VHPO at room temperature (b) physical mixture of VHPO and VPO at 425 °C (c) final VPO in the (010) active plane and (d) VPO microcrystals (V) and cracks (arrowed) on the precursor surface. Figure 3.22. Dynamic electron diffraction (ED) image of the topotactic transformation of the VHPO precursor to active VPO catalyst in N2 (a) (010) VHPO at room temperature (b) physical mixture of VHPO and VPO at 425 °C (c) final VPO in the (010) active plane and (d) VPO microcrystals (V) and cracks (arrowed) on the precursor surface.
An SEM image of a rosette-shaped, well-calcined and activated VPO catalyst is shown in in figure 3.24(a). VPO catalyst structure at room temperature and the corresponding ED patterns are shown in figures 3.24(b) and (c), respectively. [Pg.115]

Figure 3.24. (a) An SEM image of rose-shaped VPO catalysts (b) in situ... [Pg.116]

Figure 3.25. In situ catalysis (a) fresh VPO catalyst (b) dynamic real-time formation of atomic scale catalyst restructuring in butane after 2 min at 400 °C (c) enlarged image of (b) showing two sets of partial dislocations and (d) dynamic image of two sets of extended defects along symmetry-related (201) in (010) VPO after reduction in butane for several hours (diffraction contrast). The inset shows the defect nucleation near the surface. Careful defect analysis shows them to be formed by novel glide shear, (e) One set of the defects in high resolution (f) and (g) show diffraction contrast images of defects in 201 and 201. (After Gai et al, Science, 1995 and 1997 Acta Cryst. B 53 346.)... Figure 3.25. In situ catalysis (a) fresh VPO catalyst (b) dynamic real-time formation of atomic scale catalyst restructuring in butane after 2 min at 400 °C (c) enlarged image of (b) showing two sets of partial dislocations and (d) dynamic image of two sets of extended defects along symmetry-related (201) in (010) VPO after reduction in butane for several hours (diffraction contrast). The inset shows the defect nucleation near the surface. Careful defect analysis shows them to be formed by novel glide shear, (e) One set of the defects in high resolution (f) and (g) show diffraction contrast images of defects in 201 and 201. (After Gai et al, Science, 1995 and 1997 Acta Cryst. B 53 346.)...
The EM studies show that the novel glide shear mechanism in the solid state heterogeneous catalytic process preserves active acid sites, accommodates non-stoichiometry without collapsing the catalyst bulk structure and allows oxide catalysts to continue to operate in selective oxidation reactions (Gai 1997, Gai et al 1995). This understanding of which defects make catalysts function may lead to the development of novel catalysts. Thus electron microscopy of VPO catalysts has provided new insights into the reaction mechanism of the butane oxidation catalysis, catalyst aging and regeneration. [Pg.122]

Reactivity of VPO catalysts and design of promoted catalyst systems... [Pg.122]

VPO catalyst selectivity is tested by both fixed-bed microreactor measurements and by pulsed microreactor measurements. In the former, the rate constants are measured in a microreactor on about 1 g of catalyst at temperature between 360 and 390 °C in a 1.5% butane/air environment. The pulsed microreactor evaluations are carried out by injecting 0.05 ml pulses of butane using a gas-sampling valve over about 0.5 g of catalyst in a microreactor heated to about 380 °C. /i-butane conversion and selectivity to maleic anhydride (MA)... [Pg.122]

VPO catalyst, they are assumed to be V2Ok units made up of pairs of distorted edge-sharing V05 square pyramids. The assumption of these active sites, especially for the VPO catalyst, was discussed in detail in Ref. 56, in view of the fact that the most selective VPO catalyst for butane oxidation to maleic anhydride contained a slight excess of phosphorus over the stoichiometric ratio for vanadyl pyrophosphate, the phosphorus was concentrated on the surface (57-61), and the average vanadium valence of the catalyst under reaction conditions was about 4.1 (57, 58). [Pg.29]


See other pages where VPO catalysts is mentioned: [Pg.456]    [Pg.23]    [Pg.225]    [Pg.369]    [Pg.59]    [Pg.482]    [Pg.484]    [Pg.97]    [Pg.97]    [Pg.215]    [Pg.218]    [Pg.221]    [Pg.225]    [Pg.111]    [Pg.113]    [Pg.123]    [Pg.517]    [Pg.522]    [Pg.29]   
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See also in sourсe #XX -- [ Pg.333 ]

See also in sourсe #XX -- [ Pg.25 ]

See also in sourсe #XX -- [ Pg.503 , Pg.508 ]




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Synthesis and characterization of VPO catalysts

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