Silica kinetic profile

On the other hand, there is no doubt that diffusion through the silica does affect polymerization activity (although probably not the kinetic profile). This is discussed in Section IV. 

These same factors probably also determine the quite different kinetic profile of Cr/silica, whose sites are slower to form and slower to die. The reduction assumes more importance on silica, controlling the development of activity. Alkylmetals accelerate the reduction but do not create a vast new population of sites. 

When the Cr02Cl2 adsorbed as chromate, such as on silica that had been calcined at 400 °C, normal polymerization activity was observed at 100 °C and a concentration of ethylene of 1.0 mol L-1 in isobutane. Indeed, the activity was nearly identical to that of Cr03/silica activated at 400 °C. The kinetics profile of the polymerization reaction was also the same, as shown in Figure 7. The polymer FILMI, MW, and MW breadth were also almost the same, as was the UV-vis reflectance spectrum. In contrast, the chlorochromate catalysts were not active for ethylene polymerization under these conditions. Thus, the monochromate species... 

Another way of shortening the induction time, and thus increasing the overall reaction rate, is to reduce the catalyst before it is added to the reactor. Treatment of activated Cr(VI) /silica in CO at 350 °C, leading to Cr(II)/silica, brings on reaction immediately. In contrast to Cr(VI)/silica, these Cr(II)/silica catalysts can initiate polymerization at 25 °C or even lower temperatures [45,269]. Figure 16 shows how CO reduction affects the kinetics profile of Cr/silica. 

Cr/alumina exhibits a "fast" kinetics profile that is quite different from that of Cr/silica. The polymerization rate develops rapidly, especially when the alumina has been acidified by treatment with silica, fluoride, phosphate, or sulfate. Cr/alumina exhibits polymerization kinetics similar to that of Cr/AlP04, a topic that is discussed in Section 15. The polymerization rate rises quickly when ethylene is added, but later it tends to decay slowly. The rapid initial rise indicates that reduction of Cr(VI), or desorption of redox by-products, and/or alkylation of the chromium, may be more facile on alumina than on silica. Alumina is known as a strong adsorbent in its own right, so that adsorption of by-products from chromium onto the neighboring surface is one possible contributing cause of the rapid development of polymerization rate. 

The Cr/aluminophosphate catalysts exhibit a "fast kinetics profile relative to the profile of Cr/silica. That is, the polymerization rate of Cr/AIPO4 develops almost immediately with no induction time [637], The polymerization rate rises for about 10-20 min, and then declines during the rest of the 1-2-h run. Figure 168 shows an example of the reaction kinetics, when the catalyst had a P/Al atomic ratio of 0.8 and was activated at 700 °C. The rapid development of polymerization suggests that the initiation steps are faster on Cr/aluminophosphate catalysts than on Cr/silica catalysts. This difference could indicate faster reduction, or alkylation, or that the redox by-products, such as formaldehyde, are more quickly removed from the reaction diluent. These aluminophosphate supports are usually better adsorbents for polar compounds than silica activated at 700 °C, and this difference may contribute to the kinetics profile. 

Cr/aluminophosphate catalysts respond to activation temperature in many of the same ways that Cr/silica does, and there are some differences too. The activity of the catalyst is generally increased at higher activation temperatures. Figure 175 shows how the kinetics of polymerization with a 0.8 P/Al catalyst responded to the activation temperature of the catalyst. There was little change in the overall shape of the kinetics profile only the height varied. The average activity of the catalyst improved when the activation temperature was raised from 300 up to 700 °C. 

One of the most interesting results of this approach of using "two-valent" chromium species is the effect on catalyst activity. Yields as high as 16 kg g 1 h 1 were obtained, which was more than twice that of the chromium oxide parent, and many times more than that of the chromium alkyl when deposited on silica. The induction time of chromium oxide was eliminated, and also the declining kinetics profile of the chromium alkyl catalyst. That is, the hybrid catalyst seemed to have incorporated the best aspects of both parents to yield unusually high polymerization activity. 

Rh > Ir > Ni > Pd > Co > Ru > Fe A plot of the relation between the catalytic activity and the affinity of the metals for halide ion resulted in a volcano shape. The rate determining step of the reaction was discussed on the basis of this affinity and the reaction order with respect to methyl iodide. Methanol was first carbonylated to methyl acetate directly or via dimethyl ether, then carbonylated again to acetic anhydride and finally quickly hydrolyzed to acetic acid. Overall kinetics were explored to simulate variable product profiles based on the reaction network mentioned above. Carbon monoxide was adsorbed weakly and associatively on nickel-activated-carbon catalysts. Carbon monoxide was adsorbed on nickel-y-alumina or nickel-silica gel catalysts more strongly and, in part, dissociatively,... 

Metathesis activity. A quantitative comparison of metathesis activities was made in the gas phase homometathesis of propylene. The reaction kinetics are readily monitored since all olefins (propylene, ethylene, cis- and fra/3s-2-butylenes) are present in a single phase. Metathesis of 30 Torr propylene was monitored in a batch reactor thermostatted at 0 °C, in the presence of 10 mg catalyst. The disappearance of propylene over perrhenate/silica-alumina (0.83 wt% Re) activated with SnMe4 is shown in Figure 2a. The propylene-time profile is pseudo-first-order, with kob (1.11 + 0.04) X 10" slightly lower rate constant, (0.67 constants are linearly dependent on Re loading. Figure 3. The slope yields the second-order rate constant k = (13.2 + 0.2) s (g Re) at 0°C. 

Temperature measurements (corrected for radiation) were made with silica-coated Pt-Pt/10% Rh thermocouples, about 4 mils in diameter. The temperature and species concentration profile as a function of distance through the flame provided the basic data for the kinetic analyses. 

All the above problems can be applied to the upper part of the CFB gasifier. With an accurate fluid dynamic model the axial profiles of biomass, char, calcined dolomite and silica sand could be calculated, but the lack of accurate kinetic information on the effect of the concentration of these solids on the rate of all reactions involved avoids to calculate the axial profiles of the gaseous confounds (H2, CO, CO2... and tars). So, it... 

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. 

Wade et al. [46] have compared the experimental band profiles of p-nitrophenyl-ff-D-mannopyranoside on silica-bonded Concanavalin A, obtained in affinity chromatography, and the best fit parameters to their model. This model i.e., Thomas model) uses a Langmuir kinetic and neglects the axial dispersion. The best values of the parameters are calculated using a Simplex program to minimize the sum of the residuals of the predided and experimental band profiles. Figure 14.10 illustrates the results obtained and shows excellent agreement. 

Dijt et al. [41 ] also studied the desorption kinetics with reflectometry for PEO molecules adsorbed on a silica surface by replacing the polymer solution with solvent. For molar masses above 10 g mol no detectable decrease in the adsorbed amount takes place on the time scale of the experiment (hours to days). For lower molar mass some desorption is observed the decrease approaches about 15% for A/ = 7100 g mol. Owing to the high-affinity character of PEO on silica, complete detachment of the polymeric molecules is highly suppressed. With reflectometry one detects the change in the total mass present in flic surface layer. In order to check if changes in the volume fraction profile take place after replacing the polymer solution by solvent, measurements of the layer thickness are more relevant. 

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