Vanadium reaction temperature effect

Vanadium phosphoms oxide-based catalysts ate unstable in that they tend to lose phosphoms over time at reaction temperatures. Hot spots in fixed-bed reactors tend to accelerate this loss of phosphoms. This loss of phosphoms also produces a decrease in selectivity (70,136). Many steps have been taken, however, to aHeviate these problems and create an environment where the catalyst can operate at lower temperatures. For example, volatile organophosphoms compounds are fed to the reactor to mitigate the problem of phosphoms loss by the catalyst (137). The phosphoms feed also has the effect of controlling catalyst activity and thus improving catalyst selectivity in the reactor. The catalyst pack in the reactor may be stratified with an inert material (138,139). Stratification has the effect of reducing the extent of reaction pet unit volume and thus reducing the observed catalyst temperature (hot... 

The introduction of microwave pr ents an excellent new option for the synthesis of VOPc from vanadium oxide, dicyanobenzene, and ethylene glycol. In the present study, the effectiveness of synthesizing crude VOPc liom vanadium oxide and dicyanobenzene rmder the two synthetic methods was investigate by comparing reaction temperatures. Also, the preparation of fine crystal VOPc was investigated from the crude VOPc synthesized at... 

Fig. 45. Effect of hydrogen partial pressure on vanadium deposition for an Arabian Heavy atmospheric residuum at a reaction temperature of 371°C (700°F) using a -in. extrudate catalyst (Tamm et at., 1981).

The oxidation of benzene to phenol can also be achieved using nitrous oxide as an oxidant in the presence of a catalytic system such as vanadium, molybdenum or tungsten oxides at 550 °C, and after addition of 30% of water to afford phenol in 10% yield . More effective catalytic systems have been investigated and zeolites show promise to be good catalysts for the oxidation of benzene to phenol with nitrous oxide . The use of zeolite catalysts has led to a reduction in the reaction temperature to 300-400°C, to the exclusion of water addition to the reaction mixture and to an increase in the yields up to 25-30% . Recently, direct oxidation of benzene to phenol by nitrous oxide has been commercialized . 

In order to evaluate the relative role of these two factors, the present results were compared with those obtained over powders in the absence of SO2 and H2O in the feed stream. Tests over powders were thus relative to the simplified case of negligible role of mass transfer limitations and absence of surface sulfates. A promoting effect of sintering on DeNOx activity was observed and it was interpreted as the effect of vanadium agglomeration, leading to the formation of V polymeric species with enhanced reactivity [5], The estimated values of specific intrinsic activity of the sulfates-ffee powders at the reaction temperature of 350°C are reported for comparison in Figure 3. The DeNOx activity of powders was always lower than that of the sulfate-containing monoliths, by a factor of 4 for the sample calcined at 500°C and a factor of 2.4 and 1.4 for the samples calcined at 750 and 800°C respectively. 

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. 

These metals, when deposited on the E-cat catalyst, increase coke and gas-making tendencies of the catalyst. They cause dehydrogenation reactions, which increase hydrogen production and decrease gasoline yields. Vanadium can also destroy the zeolite activity and thus lead to lower conversion. The deleterious effects of these metals also depend on the regenerator temperature the rate of deactivation of a metal-laden catalyst increases as the regenerator temperature increases. 

The replacement of vanadia-based catalysts in the reduction of NOx with ammonia is of interest due to the toxicity of vanadium. Tentative investigations on the use of noble metals in the NO + NH3 reaction have been nicely reviewed by Bosch and Janssen [85], More recently, Seker et al. [86] did not completely succeed on Pt/Al203 with a significant formation of N20 according to the temperature and the water composition. Moreover, 25 ppm S02 has a detrimental effect on the selectivity with selectivity towards the oxidation of NH3 into NO enhanced above 300°C. Supported copper-based catalysts have shown to exhibit excellent activity for NOx abatement. Recently Suarez et al and Blanco et al. [87,88] reported high performances of Cu0/Ni0-Al203 monolithic catalysts with NO/NOz = 1 at low temperature. Different oxidic copper species have been previously identified in those catalytic systems with Cu2+, copper aluminate and CuO species [89], Subsequent additions of Ni2+ in octahedral sites of subsurface layers induce a redistribution of Cu2+ with a surface copper enrichment. Such redistribution... 

Impurities with catalytic effects—Impurities that act as catalysts, reducing the activation energy of a process, may increase the rate of reaction significantly, even when present in small quantities. The presence of sulfuric acid, for example, increases the rate of decomposition and decreases the observed onset temperature of various isomers of ni-trobenzoic acid [28]. Also, other substances such as NaCl, FeCl3, platinum, vanadium chloride, and molybdenum chloride show catalytic effects. As a result, the decomposition temperature can be lowered as much as 100°C. Catalysts, such as rust, may also be present inadvertently. Some decomposition reactions are autocatalyzed, which means that one of more of the decomposition products will accelerate the decomposition rate of the original substance. 

It is now considered, by most groups working in this area, that vanadyl pyrophosphate (VO)2P207 is the central phase of the Vanadium Phosphate system for butane oxidation to maleic anhydride (7 ). However the local structure of the catalytic sites is still a subject of discussion since, up to now, it has not been possible to study the characteristics of the catalyst under reaction conditions. Correlations have been attempted between catalytic performances obtained at variable temperature (380-430 C) in steady state conditions and physicochemical characterization obtained at room temperature after the catalytic test, sometimes after some deactivation of the catalyst. As a consequence, this has led to some confusion as to the nature of the active phase and of the effective sites. (VO)2P207, V (IV) is mainly detected by X-Ray Diffraction. 

The fluorination of polyfluorocyclohexenes with antimony(V) fluoride proceeds under much more severe temperature conditions ( 150 C) than with vanadium(V) fluoride (see Section 12.4.2). In these reactions antimony(V) fluoride effects both the oxidative addition of fluorine and the substitution of vinyl chlorine atoms by fluorine.48... 

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