Vanadium loading effect

The three catalysts with different vanadium loadings gave different activities and selectivities and the catalyst with 6.4% by weight of V2O5 was the most active and selective. A similar trend has been also obtained in the runs performed at 400°C. These results demonstrate the sensitivity of ODH of propane to the structure of the vanadium clusters, but it seems that other factors must also be considered, such as the acidity of these clusters and the effect of titanium that, as we have demonstrated, can be amorphous, in this case, and not necessarily present as a homogeneous surface but simply surrounding vanadium oxide sites. 

Effect of vanadium loading in Vx-HMS catalysts on the selectivity to benzaldehyde at 400°C and to phenol at S00°C and on the conversion of toluene at450°C. 

The catalytic activity measurements were effected on samples treated at 380 (300 C for sample VNaTiP) and 600°C in He or air flow, at different reaction temperatures. The results are reported in Fig. 4-6. The parent material a-TiP exhibits low activity, NO conversions being lower than 5% up to 300°C either for sample treated at 380 C or at 600°C, and reaching 15% at 400°C for sample treated at 600°C. By contrast high activity is shown by vanadium modified phosphates even with low vanadium content. NO conversion increases with vanadium loading whatever the atmosphere and temperature of pretreatment. All samples, after treatment either at 380 or 600°C were found very selective towards the N2 formation. The conversion to N2O was negligible at low temperature, and reached values of about 1-3% at 300-400 C, suggesting the occurrence of ammonia oxidation reactions (3, 10) in low extent. 

Presently the catalytic selective NOx reduction by ammonia is efficient and widespread through the world for stationary sources. The remarkable beneficial effect of 02 for the complete reduction of NO into nitrogen is usually observed between 200 and 400°C. However, such a technology is not applicable for mobile sources due to the toxicity of ammonia and vanadium, which composes the active phase in vanadia-titania-based catalysts. Main drawbacks related to storing and handling of ammonia as well as changes in the load composition with subsequent ammonia slip considerably affect the reliability of such a process. On the other hand, the use of urea for heavy-duty vehicles is of interest with the in situ formation of ammonia. 

The effect of liquid loading was also studied by varying the V content at constant melt composition, i.e. constant ratios of K/V, Na/V, and Cs/V. An example of 1-2 mm large particles depicted in Fig. 11 shows an optimum vanadium content of about 3 wt% for this particular carrier and melt composition. 

On non-zeolitic particles in the absence of a vanadium passivator, vanadium (when present at the 0.4 wt% level) makes a greater contribution to contaminant coke and hydrogen yields than nickel at constant surface area and metals loading. Incorporation of a vanadium passivator into the catalyst matrix can greatly alter the selectivity effects of vanadium, and can essentially negate its effect on non-zeolitic particles as in the case of magnesium. 

The relative ease with which VpOr can be reduced to V(III) in aluminosilicates indicate the exiirence of weak metal-surface interactions and the inability of the surface to effectively passivate vanadium. Similarly, V on Kaolin (and metakaolin) exist mostly as the "free oxide and can (in part) be reduced to V(III) species. Therefore, DFCC systems containing metakaolin microspheres (or amorphous aluminosilicates (15)) should not be as effective as sepiolite in passivating metals TTke Ni and V. In fact, DCC mixtures loaded with 5000 ppm Ni-equivalents (that is 0.6% V + 0.38% Ni) are not metals resistant when metakaolin is used as a metals scavenger (1) ... 

This paper focuses on the metal deposition process during hydrodemetallisation (HDM) of vanadyl-tetraphenylporphyrin (VO-TPP) under industrial conditions. In catalyst pellets of a wide pore, low loaded molybdenum on silica, the vanadium deposition process was determined with EPMA and HREM. The effect of quinoline and HjS on the vanadium deposition profile is studied and an attempt is made to simulate the deposition profiles based on intrinsic reaction kinetics and percolation concepts. 

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