Effect of temperature on catalytic

Figure 1 Effect of temperature on catalytic performance of Mg/Al/O catalyst in m-cresol methylation m-cresol conversion (u), selectivity to 3-MA (v), 2,3-DMP (X), 2,5-DMP (ct), 3,4-DMP (p), polyalkylates ( ).

Figure 3 Effect of temperature on catalytic performance of Mg/Fe/O catalyst in m-cresol methylation. Symbols as in Figure 1.

Figure 20.11 Effect of temperature on catalytic performance ofV/Mo/O catalyst in cyclohexane ammoxidation [128].

The effects of temperature on catalytic cracking of various plastics over FZ-W catalyst have been investigated [84] and are shown in Table 28.8. In the temperature range 300-350°C, the highest yield of oil products was achieved. When the temperature is below 300°C, yields of residue and noncondensed gases are low, large amount of feedstocks were left uncracked Above 350°C, the yields of residue and noncondensed gas increased, and the amount of oil products obtained decreased relatively. 

Comparing hydrogenation yields vs. temperature curve with hydrogen adsorption vs. temperature curve (Fig. 1) on vanadium oxide, a close similarity can be noticed. This parallelism led to a study of the effect of temperature on catalytic structure. 

Figure 8.62, Effect of temperature on the catalytic rates of C02, N2 and N20 formation and on the corresponding N2 selectivity, for open (unpromoted) and closed (NEMCA) circuit conditions on Rh/YSZ during NO reduction by C3H6.67,68 Reprinted from ref. 68 with permission from Elsevier Science.

Fig. 2. Effects of temperature on the catalytic performance in the internal reforming of CH4 by CO2 over ESC of SOFC system.

In order to obtain reasonable concentrations of CHj- radicals in the low pressure studies, reactions were carried out at temperatures > 700 C whereas, the conventional catalytic studies of Vannice and co-workers have been carried out at temperatures 700 °C. Therefore, for comparison with our low pressure results, the effect of temperature on the rate of Nj formation at a total pressure of 760 Torr has been investigated up to 875 °C for three gas compositions (i) 1 % NO, (ii) 1 % NO + 0.25 % CH4 and (iii) 1 % NO + 0.25% CH4 -f- 0.5% Oj. The results are summarized in Figure 7. At 700 °C, the trends with respect to the effect of gas composition are qualitatively the same as those reported by Vannice i.e., the addition of CH enhanced the rate of Nj formation, and the addition of O2 enhanced the rate even more. At 700 C the effect of adding CH4 was to increase the rate two-fold, while the effect of adding CH4 O2 was to increase the rate five-fold. 

The effect of temperature on the catalytic performance of Mg/Fe/O is reported in Figure 3. The behavior was quite different from that of the Mg/Al/O catalyst. The conversion of m-cresol with Mg/Fe/O was always lower than that with Mg/Al/O. The selectivity to 3-MA was almost negligible in the whole range of temperature. The selectivity to polyalkylates and to 3,4-DMP was also much lower than that observed with Mg/Al/O. Therefore, the catalyst was very selective to the products of ortho-C-methylation, 2,3-DMP and in particular 2,5-DMP. This behavior has to be attributed to specific surface features of Mg/Fe/O catalyst, that favor the ortho-C-methylation with respect to O-methylation. A different behavior of Mg/Al/O and Mg/Fe/O catalysts, having Mg/Me atomic ratio equal to 4, has also been recently reported by other authors for the reaction of phenol and o-cresol methylation [5], The effect was attributed to the different basic strength of catalysts. This explanation does not hold in our case, since a similar distribution of basic strength was obtained for Mg/Al/O and Mg/Fe/O catalysts [4],... 

When the rate of an enzyme catalyzed reaction is studied as a function of temperature, it is found that the rate passes through a maximum. The existence of an optimum temperature can be explained by considering the effect of temperature on the catalytic reaction itself and on the enzyme denaturation reaction. In the low temperature range (around room temperature) there is little denaturation, and increasing the temperature increases the rate of the catalytic reaction in the usual manner. As the temperature rises, deactivation arising from protein denaturation becomes more and more important, so the observed overall rate eventually will begin to fall off. At temperatures in excess of 50 to 60 °C, most enzymes are completely denatured, and the observed rates are essentially zero. 

Performance Analysis. In order to determine the effect of hydrotreating on catalytic cracking performance, the above feedstocks were evaluated at a low severity cracking condition (catalyst-to-oil ratio of 6.0 and reactor temperature of 910° F) and a high severity cracking condition (catalyst-to-oil of 8.0 and reactor temperature of 1010° F). The results from the catalytic cracking of these feedstocks (shown in Tables I and II) are shown in Tables III through V. The results presented in these tables are... 

The equations describing the concentration and temperature within the catalyst particles and the reactor are usually non-linear coupled ordinary differential equations and have to be solved numerically. However, it is unusual for experimental data to be of sufficient precision and extent to justify the application of such sophisticated reactor models. Uncertainties in the knowledge of effective thermal conductivities and heat transfer between gas and solid make the calculation of temperature distribution in the catalyst bed susceptible to inaccuracies, particularly in view of the pronounced effect of temperature on reaction rate. A useful approach to the preliminary design of a non-isothermal fixed bed catalytic reactor is to assume that all the resistance to heat transfer is in a thin layer of gas near the tube wall. This is a fair approximation because radial temperature profiles in packed beds are parabolic with most of the resistance to heat transfer near the tube wall. With this assumption, a one-dimensional model, which becomes quite accurate for small diameter tubes, is satisfactory for the preliminary design of reactors. Provided the ratio of the catlayst particle radius to tube length is small, dispersion of mass in the longitudinal direction may also be neglected. Finally, if heat transfer between solid cmd gas phases is accounted for implicitly by the catalyst effectiveness factor, the mass and heat conservation equations for the reactor reduce to [eqn. (62)]... 

To sum up, special attention should be given to the effect of temperature on the process during the design and control of commercial catalytic reactors. Moreover, the same size of catalyst particles should be used at any scale so that the catalyst effectiveness factor also remains the same. The available catalytic surface per reactor volume and the space velocity (when the rate is not controlled by mass transfer phenomena) should also be left unchanged at any scale. 

Fig. 39.—The Effect of Temperature on the Oxidation of Ammonia with Platinum as Catalytic Agent.

If the reaction rate is a function of pressure, then the momentum balance is considered along with the mass and energy balance equations. Both Equations 6-105 and 6-106 are coupled and highly nonlinear because of the effect of temperature on the reaction rate. Numerical methods of solution involving the use of finite difference are generally adopted. A review of the partial differential equation employing the finite difference method is illustrated in Appendix D. Figures 6-16 and 6-17, respectively, show typical profiles of an exothermic catalytic reaction. 

The other factor that can show the influence of kinetic, catalytic, and adsorption effects on a diffusion-controlled process is the temperature coefficient.10 The effect of temperature on a diffusion current can be described by differentiating the Ilkovic equation [Eq. (3.11)] with respect to temperature. The resulting coefficient is described as [In (id,2/id,iV(T2 — T,)], which has a value of. +0.013 deg-1. Thus, the diffusion current increases about 1.3% for a one-degree rise in temperature. Values that range from 1.1 to 1.6% °C 1, have been observed experimentally. If the current is controlled by a chemical reaction the values of the temperature coefficient can be much higher (the Arrhenius equation predicts a two- to threefold increase in the reaction rate for a 10-degree rise in temperature). If the temperature coefficient is much larger than 2% °C-1, the current is probably limited by kinetic or catalytic processes. 

At temperatures greater than 45 C, thermal denaturation of the enzyme becomes increasingly significant. At about 55 C, rapid denaturation completely destroys the catalytic function of the enzyme protein. The effect of temperature on the rates of enzyme-catalysed reactions is illustrated in the figure above. 

The transformation of n-butane over the Ga and Zn modified ZSM-5 catalysts produced similar aromatic hydrocarbons and gaseous products as over H-ZSM-5. Ethyl benzene was the only aromatic which was not formed with the proton form catalyst. Ga-H-ZSM-5 and Zn-H-ZSM-5 exhibited higher catalytic activity and selectivity to aromatics than the H-ZSM-5 catalyst The amount of cracking products formed for Ga- and Zn- modified catalysts were smaller than for ZSM-5 in its proton form. Toluene constituted almost 50 % of the aromatics formed while benzene, xylenes and ethylbenzene formed the rest The conversion of n-butane and selectivity to aromatic hydrocarbons increased with increasing temperature. The effect of temperature on n-butane conversion and aromatic selectivity over the catalysts is given in Figures 4 and 5. The product selectivity obtained from the transformation of n-butane over the H-ZSM-5, Ga-H-ZSM-5 and Zn-H-ZSM-5 catalysts at 803 K is given in Table 1. 

Figure 8 Continuous injection method of contacting the catalytic species fa) heat evolution versus time recorded during successive and continuous injection of the Cl for the anionic polymerization ofL6 at 105 °C (b) effect of temperature on the overall heat evolution (the Cl was delivered 7 h at a rate of 60 pi h )...

Fig. 5 shows the effect of various supports of nickel-loaded catalysts and reaction temperature on the methane conversion, in the partial oxidation of methane. At methane to oxygen ratio of 5 1, the maximum conversion of methane is 40 %, when reaction (5) proceeded, and 10% when complete oxidation proceeded. Only the oxidized diamond-supported Ni catalyst exceeded 10% conversion above 550 C, indicating that the synthesis gas formation proceeded. Ni-loaded LazOz catalyst afforded considerable methane conversion above 450 °C, but the product is mainly COz. Other supports to nickel showed no or only slight catalytic activity in the partial oxidation of methane. These results clearly show that oxidized diamond has excellent properties in the partial oxidation of methane at a low temperature, giving synthesis gas. Fig. 6 shows the effect of temperature on the product distribution, in the partial oxidation of methane. Above 550 °C, Hz and CO were produced, and below 500 °C, only complete oxidation occurred. The Hz to CO ratio should be 2 according to the stoichiometry. However, 3.2 and 2.8 were obtained at 550 and 600 °C, respectively. 

Walter et al. [84] discussed several common experimental methods to estimate the influence of internal mass transfer resistances on the observed rates of heterogeneous catalytic reactions. For example, when the reaction temperature is varied, because the intrinsic reaction rate increases more strongly with temperature than the rate of diffusion, the influence of mass transfer becomes more important and the observed apparent activation energy decreases as the temperature increases effects of temperature on selectivity may be more complex. 

The effect of temperature on the catalytic reaction is most easily determined with Arrhenius plots. Such plots of the turnover rate, or of the rate of individual steps in the catalytic reaction (e.g., kacyiation, kdegiycosyiation) can be very useful in comparisons between the catalytic reaction in the cryosolvent at subzero temperature and the reaction under normal conditions. 

The effect of temperature on enzymatic reactions is very complex. If the enzyme structure would remain unchanged as the temperature is increa.sed, the rate would probably follow the Arrhenius temperature dependence. However, as the temperature increases, the enzyme can unfold and/or become denatured and lose its catalytic activity. Consequently, as the temperature increase.s, the reaction rate, -fj, increases up to a maximum with increasing temperature and then decreases as the temperature is increased further. The descending part of this curve is called temperature inactivation or thermal denaiurizing. Figure 7-9 shows an example of this optimum in enzyme activity. ... 

Fig. 1 The effect of support on catalytic activity at the density of added vanadium equal to 0.25 (a) and 0.5 (b) monolayer. Drying temperature of the support, 393K.

Fig. 1 Effect of temperature on the cyclization activity ( Effect of temperature on the cyclization activity (P-CD formation rate) of the free and sol el encapsulated CGTase at pH 6.0. The maximum catalytic activity in each case was normalized to 100%...

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