Chromia conversion rates

Figure 1 shows plots of -ln(l-XH2) versus t for some of the monoalcohols tested, in comparison with ethanolamine, using the chromia-promoted catalyst. The plots are reasonably linear, but with some upward curvature indicating a deviation from first order behaviour at high conversion, in all cases the slopes (i.e. the rate) are less than that for the control ethanolamine. The structures and the first order rate constants, knz, calculated from the slopes in Figure 1 are listed in Table 2. 

The dehydrogenation rate was measured at reaction temperatures between 525 and 600°C and butene-l partial pressures of 0.05 to 0.25 bars. The results obtained showed that at temperatures of 575 and 600°C and high partial pressures of butene-1, a rapid decrease in conversion occurred within the first 20 minutes, followed subsequently by a much smaller rate of decrease. Initial reaction rates were measured and these rates were found to increase with reactant concentration up to a maximum of about 0.2 bars butene-l pressure and then to decrease on further increase of reactant partial pressure. These results agree with the data obtained by Carra and Forni (5) for the dehydrogenation of n-butane over a chromia alumina catalyst. 

On the other hand, H2S did not prevent cis-trans isomerization and double bond shift reactions from going to completion at 300° or 350°, and the olefin mixtures analyzed were always found to be close to thermodynamic equilibrium, whether they had been formed from thiophene, butadiene, or one of the olefins themselves. Nor did H2S prevent the total conversion of butadiene to butene, even when 10- to 20-cc. samples were used at reaction temperatures down to 200° C. and flow rates up to 10 liters per hour. This may be the explanation of the absence of butadiene from the thiophene reaction products over cobalt molybdate—that if it had been formed as it was over chromia, it would have reacted further too rapidly to survive. 

The conversion of cyclohexanes to aromatics is a classical dehydrogenation reaction which will readily take place on many transition metals and metal oxides. On chromia-alumina Herington and Eideal (S) have demonstrated the occurrence of cyclo-olefin intermediate products. Weisz and Swegler 25) have demonstrated the effect on benzene yield of allowing early diffusional escape of cyclo-olefin from the porous catalyst particle. Prater et al. 26) have developed evidence that cyclohexene occurs as a quasi-intermediate in aromatization catalysis over platinum catalyst also, although at a smaller concentration, because of a larger ratio of effective rate constants fe/Zci in the scheme... 

The effect of various activations of chromia in hydrogen flowing at about 25 ml/minute was tested by the reactions between 1-hexene and hydrogen (hydrogenation and isomerization). At low conversions the isomeric products are cis- and [Pg.29]

Both on the microcrystalline chromia mentioned above and on its amorphous predecessor, the rate of exchange of benzene at 110° was several times faster than the rate of exchange of toluene. From the data of a few runs at large conversions using deuterium and toluene on a catalyst activated in nitrogen at 470°, we earlier thought that toluene probably exchanged more rapidly than benzene (12). However, we had no runs with toluene and benzene on the same sample of catalyst. 

An equimolar mixture of hydrogen and deuterium was passed at a total flow rate of 256 mmoles per hour per gram of Cr203 over a fresh sample of chromia. After flushing for 3 hours at room temperature, the temperature was raised at the rate of 50° per hour and the effluent stream was analyzed periodically for HD by mass spectroscopy. The first sign of conversion was observed at 194°, conversion was about 80% of the equilibrium conversion at 248°, and essentially 100% at 273°. When the catalyst was cooled to 110°, conversion was 86% of the equilibrium conversion. A sample activated above 300° gave an equilibrium conversion at 25°. 

Some experiments on methanol decomposition over zinc oxide by Teller et al. (120) seem to demonstrate a remarkable transient activation, while similar ones over chromia show a similar marked deactivation. Several samples of each catalyst were studied in a flow system between about 300° and 430°, with irradiation during the reaction at a maximum dose rate of 8 x 10 ev gm i sec-i. The flow rate was such that a 10% conversion of the methanol would correspond to about 3 X 10 molecules decomposed per second per gram of catalyst. 

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... 

M. Tinkle, J. A. Dumesic, Isotopic exchange measurements of the rates of adsorption/desorp-tion and inter conversion of CO and CO2 over chromia-promoted magnetite implications for water-gas shift, J. Catal. 103 (1987) 65-78. 

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