Chromia-alumina catalyst activity data

Fig. 34. The dependence of the activation energy for electrical conductivity Ea on the chromium concentration for a series of chromia-alumina catalysts. The data are from 184).

Figure 1.15 Diffuse reflectance UV-Vis spectra from a series of chromia/alumina catalysts after various treatments [115], All these spectra display a shoulder at about 16,700 cm-1 corresponding to the first d-d transition of Cr3+, but the main feature seen in the hydrated and calcined samples at about 26,000 cm-1 due to a Cr6+ charge transition is absent in the data for the reduced sample. This points to a loss of the catalytically active Cr6+ phase upon reduction. (Reproduced with permission from Elsevier.)...

Those who are interested in chromia-alumina catalyst are referred to the detailed review article by Poole and Maclver (44). These authors also studied the sintering of chromia-alumina (45) and found that chromia in the catalyst sinters faster than alumina. Their data suggest that catalyst activity loss may be a serious problem at the temperature levels required for high propane conversions. 

Several authors have found a direct relationship between the y phase or Cr + concentration detected by electron spin resonance and the catalytic activity of chromia-alumina catalysts. This relationship is illustrated by the data of Bukanaeva et al, (133), shown in Fig. 20 for... 

Chapman et al. (184) studied the electric conductivity thermoelectric potential (g) of a series of coprecipitated chromia-alumina catalysts pretreated at 500°. Activation energies were calculated from the straight lines obtained from the temperature dependence of the conductivity data between 400 and 500° by means of the equation... 

Eischens and Selwood 176) have made a study of the activity of reduced chromia-on-alumina catalysts for the dehydrocyclization of n-heptane. The activity per unit weight of chromium was found to increase sharply at concentrations below about 5 wt. % Cr activities were measured down to 1.9 wt. % Cr concentration at which point the highest activity was observed. Selwood and Eischens concluded that this effect is due to the fact that the chromia is most dispersed at these low concentrations, in agreement with the present EPR data. However, if a two-site mechanism 177) is necessary for dehydrocyclization, the activity may drop at even lower chromium concentrations due to isolation of individual chiomium spins. 

Conversion of Isophorone to Metaxylenol is a well documented reaction in the patent literature. Activity and life-time of three eatalytic systems Alumina, Zirconia and Thoria with or without variable level of Chromia doping have been studied in the present work. For Zirconia and Thoria doping with Chromia results in remarkable enhancements in catalytic activities, but for alumina, the effect is less pronounced. In all these cases, there is cin optimum level of Chromia doping theat results in maximum efficiency. Although high initial activities are obsereved with all these catalysts, the rates of deactivation are fast in all cases. Spectroscopic and other physico-chemical measurements have been carried out on these catlysts. A nationalisation of the observed activities is offered on the basis of such physico-chemical data... 

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