Vanadium-doped catalysts

The oxidative dehydration of isobutyric acid [79-31-2] to methacrylic acid is most often carried out over iron—phosphoms or molybdenum—phosphoms based catalysts similar to those used in the oxidation of methacrolein to methacrylic acid. Conversions in excess of 95% and selectivity to methacrylic acid of 75—85% have been attained, resulting in single-pass yields of nearly 80%. The use of cesium-, copper-, and vanadium-doped catalysts are reported to be beneficial (96), as is the use of cesium in conjunction with quinoline (97). Generally the iron—phosphoms catalysts require temperatures in the vicinity of 400°C, in contrast to the molybdenum-based catalysts that exhibit comparable reactivity at 300°C (98). 

Junior et al. [27] substituted Cr with V in the iron oxide catalysts. They synthesized the catalyst by heating vanadium-doped iron(III) hydroxoacetate (IHA) under nitrogen. The vanadium-doped catalysts exhibit better activity compared to pure iron oxide and commercial catalyst. This is due to both... 

Similar to molybdenum oxide catalyst the capability to emit singlet oxygen is inherent to Si02 doped by Cr ions as well. Similar to the case of vanadium oxide catalysts in this system the photogeneration occurs due to the triplet-triplet electron excitation transfer from a charge transfer complex to adsorbed oxygen. 

Very striking results on the interactions of molecules with a catalyst have been recently reported in zeolite catalysis because of the well ordered structure of these materials it is worth mentioning the subjects of zeolite design [10] and of acidic properties of metallosilicates [11]. In other areas where polycrystallinic or even amorphous materials arc applied, highly interesting results are now numerously emerging (such as hydrocarbon oxidation on vanadium-based catalysts [12] location of transition metal cations on Si(100) [13] CO molecules on MgO surfaces [14] CH4 and O2 interaction with sodium- and zinc-doped CaO surfaces [15] CO and NO on heavy metal surfaces [16]). An illustration of the computerized visualization of molecular dynamics of Pd clusters on MgO(lOO) and on a three-dimensional trajectory of Ar in Na mordenitc, is the recent publication of Miura et al. [17]. 

Mohamed, M.M. and M.M. Al-Esaimi (2006). Characterization, adsorption and photocatalytic activity of vanadium-doped Ti02 and sulfated Ti02 (rutile) catalysts Degradation of methylene blue dye. Journal of Molecular Catalysis A-Chemical, 255(1-2), 53-61. 

Further evidence for the catalytic importance of amorphous material comes from experiments carried out with cobalt-doped catalysts. Hutchings et al. (217) found that doping of the catalysts with cobalt improved their performance. Moreover, Sajip et al. (148) found that the cobalt-promoted catalysts are far more disordered than the undoped catalysts. In the doped catalysts, the promoter is dispersed in the amorphous phase, and cobalt is not found in the vanadyl pyrophosphate crystals. It is thought that one of the properties of the cobalt promoter is to stabilize the disordered phase and V -containing phases in the final catalysts, which leads to improved performance. This suggestion implies that the disordered material is the catalytically active vanadium phosphate phase. 

Zazhigalov et al. (209) investigated cobalt-doped vanadium phosphate catalysts prepared by coprecipitation and impregnation methods. The performance of catalysts prepared by both methods was improved as a consequence of the promotion. The cobalt is thought to have been present as cobalt phosphate, which is considered to stabilize excess phosphorus at the surface, which has previously been foimd to be an important characteristic of active catalysts. 

Dopant, optimal amount expressed as a ratio to vanadium Promotional effect on conversion C and MA selectivity S for the undoped and (doped) catalysts expressed as % Reasons for promotion Reference... 

A number of other groups have also found that zirconium enhances the activity of vanadium phosphate catalysts [11, 56, 146, 148, 150, 154, 156-163]. Zeyss and coworkers [158] investigated catalysts doped with 5 to 15% zirconium. Unlike the... 

Roncari. E. et al., Vanadium-doped TiOj catalysts. A unifying picture of powders and suspensions, Colloids Surf. A. 117, 267, 1996. 

The catalytic behavior of vanadium-based catalysts can be generally modified with the addition of a second element which acts as promoter. In this paper we present a comparative study on the catalytic properties in the ODH of propane of undoped and Me-doped Al203-supported vanadium catalysts, in which metal oxides with redox and/or acid-base properties (K, Bi, Mo, P) have been used as potential promoters. 

The TPR profiles of doped and undoped alumina supported vanadium oxide catalysts are shown in Figure 1, while the temperatures of the maximum hydrogen consumption, T, are shown in Table 1. One peak with a maximum at 465°C is observed in the undoped sample (Fig. la) in agreement with previous results [5]. 

In the case of Bi-doped catalysts the peaks appear at 360°C and 520°C, respectively (Fig. 1g). Alumina-supported Bi203, free of vanadium, shows two peaks with maxima 310°C and 435°C (which correspond to a reduction from Bi3+ to BiO) (Fig. 1h). Thus, the peak at 360°C probably corresponds to the reduction of B 3+ in BiV04 (with a reducibility lower than in Bi203), whereas the peak at 520°C is related to the reduction of both Bin+ and V5+ species. In addition, it appears that the presence of bismuth reduces the reducibility of the v5+ species in the catalyst, as the temperature of the maximum hydrogen consumption related with V5+ reduction shifts from 465°C to 520°C. 

Basic sites, in addition to acid sites, can be also proposed in Bi-doped catalysts. In addition, it has been observed that V-Bi-0 catalysts are selective in the ODH of propane [16]. However, in our case, Bi-doped catalysts are not selective. This apparent contradiction can be explained when considering the presence of Bi -sites, probably vanadium free, with a high reducibility which can favor the deep oxidation reactions. 

Acrolein conversion takes place in the presence of catalysts based on mixed oxides of molybdenum and vanadium, doped by compounds of tungsten, copper, chromium, tellurium. arsenic, etc., and designed te improve low-temperature activity, productivity, and mechanical performance. They operate aroud 250 to 280 0, at between 0.L and 0.2.10 Pa absolute, also in the presence of steam (water to acrolein molar ratio about... 

Herten and Froment (1968) studied the reaction on a doped vanadium pentoxide catalyst in a quasi-isothermal laboratory fixed bed reactor. Kinetic measurements were made in the temperature range 325-402 C for a wide range of feed compositions and varying residence times. The reaction scheme proposed is the same as that shown in Figure 3.13 with the exception that no maleic anhydride was isolated in the reaction products. They found no evidence of significant oxidation of phthalic anhydride to carbon oxides. The conversion of o-tolualdehyde to phthaiide is considered to be a relatively unimportant step in their model. 

Partial oxidation of toluene to benzaldehyde over vanadium antimonate catalysts doped with titanium. The influence of the antimony content over the deactivation process. 

Procedures leading to preparation of vanadium-doped alumina- and/or titania-pillared montmorillonites are described and physicochemical characterization (chemical analysis, XRD, BET, ESR) of the products is provided. Results show that introduction of vanadium into the pillared montmorillonites leads to a rigid association of the dopant with pillars, irrespective of the method of preparation. The mode of vanadyl attachment in alumina-pillared samples does not depend on the mode of preparation, while in titania-pillared montmorillonite it does. Certain degree of delocalization of the unpaired electron into ligands and increased in-plane 7t-covalent bonding is observed for vanadyl ions present in the co-pillared (V-Ti)-PILC samples which also show particularly high activity in catalytic ammoxidation of m-xylene to nitrile product, as monitored by IR. A hypothesis is advanced that this effect is due to the unique character of vanadyl species present in these catalysts. 

The application of vanadium oxide catalysts in partial oxidation reactions is widely practiced. Several studies were carried out to investigate the promotional effect of alkali compounds or basic oxides to V2O5 on the catalytic performance in different reactions [58,84-88]. Generally, the promotion of V2O5 bulk and supported catalysts with alkali compounds improves the selectivity to partial oxidation products. The doping effect of mainly potassium compounds has been explained (i) by lowering of the acid sites, and/or (ii) by changing the redox properties of the catalysts. Furthermore, the addition of potassium influences the... 

Since little is known about crystal structures of the low-temperature mked oxides of niobia, it is difficult to explain from a theoretical point of view the good results obtained for doping with vanadium or chromium. It appears that the vanadium and chromium are present in the catalyst in a unique environment, which makes... 

Understanding the role of cafalysf promofers is not a simple matter. Confusion in the interpretation of promofer effects has resulted because different groups have reported contrasting results for the same promoters. The effects of promofers on vanadium phosphate performance was summarized by Ballarini ef al. (8) (Table 1). As illustrated by the work by Sananes-Schulz et al. (196), the catalyst preparation method can alter the effect of the promoter, as can the method of doping. Hutchings and... 

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