Silica-alumina catalysts kinetics

Silica-alumina catalysts, kinetics of cumene cracking by, 8, 293 Sintering properties, of cracking catalysts, 4, 87... 

The Kinetics of the Cracking of Cumene by Silica-Alumina Catalysts Charles D. Prater and Rudolph M. Laqo... 

Column II of Table IV gives, for each particle size, the rate of cracking of cumene above which diffusion phenomena will influence the observed kinetics for the silica-alumina catalyst described. Column III of Table IV gives the temperature at which these rates are observed for very pure cumene. This temperature was determined from the experi-... 

The isomerization of butene-1 and butene-2 has been studied over conventional ion exchange resins 110 in> as well as over the sulfonated and phosponated polyphenyls 110>. A careful kinetic study showed that the reaction over these materials as well as over silica-alumina catalysts goes by way of a common intermediate, which can be understood best as the secondary butyl carbonium ion. 

This chemistry suggests a role for H2O as cocatalyst with the monovalent zeolites if the equilibrium is moved to the right in the presence of a base. In the present work, the effects of small deliberate replacements of Na by Ca ", of cation deficiency, and of H2O on the catalytic properties of Na-Y zeolite were studied. Also investigated was the possibility that carbonaceous residues form the catalytic sites, as was reported for the isomerization of n-butenes over silica-alumina catalysts (3, 8, 9). The isomerization of the n-butenes provided a useful tool for these studies because it follows first-order kinetics (10) and proceeds over Na-Y zeolite via the 5ec-butylcarbonium ion (11, 12). 

The possible effect of competing molecular components on the kinetics of a reaction can be illustrated by studies made on the cracking of cumene to propylene and benzene over silica-alumina catalyst in the presence of various diluents in the vapor phase. 

Example 11-7 The rate of isomerization of o-butane with a silica-alumina catalyst is measured at 5 atm and 50°C in a laboratory reactor with high turbulence in the gas phase surrounding the catalyst pellets. Turbulence ensures that external-diffusion resistances are negligible, and so Q = Q. Kinetic studies indicate that the rate is first order and reversible. At 50°C the equilibrium conversion is 85%. The effective diffusivity is 0.08 cm /sec at reaction conditions, and the density of the catalyst pellets is 1.0 g/cm, regardless of size. The measured, global rates when pure n-butane surrounds the pellets are as follows ... 

In the elucidation of the kinetics of the cracking of cumene on silica-alumina catalyst, the actions of inhibitors (poisons) on the reaction were studied. These inhibitors compete with cumene for cracking sites. Theoretical analysis leads to an expression from which the equilibrium constant for adsorption of inhibitors on cracking sites can be calculated. 

Cumene (isopropylbenzene) cracking by porous silica-alumina catalyst has been studied extensively. This includes studies with respect to coke production (I, M), the maximum depth of active centers (3), kinetics 4), and the effect of diffusion transport phenomena on the kinetics (5). 

Studies (4) made on the cracking of cumene by silica-alumina catalyst show that the kinetics is represented in the temperature range 300-500° by scheme I on top of the following page, where S is cumene, A is a catalytic site, SA is adsorbed cumene, m is benzene, mA is adsorbed benzene, n is propylene, P is an inhibitor, and PA is inhibitor adsorbed on a cracking site. 

At the start of the catalytic decomposition of methanol, the active surface constitutes 25-28% of the total surface for both catalysts studied. This is the case although the total surface area of the alumina is only half that of the silica-alumina catalyst. This would appear to support the idea that the active centers on which the decomposition of methanol takes place are qualitatively identical for the two materials. A more detailed discussion of this hypothesis will be undertaken later, when we shall consider the kinetics of dehydration of alcohol and ether on aluminum oxide and silica-alumina. 

We present below the results of our investigation of the kinetics of transformation of ethanol and diethyl ether on pure aluminum oxide and on silica-alumina catalysts of various compositions. These established some important facts concerning the nature of active centers and of the mechanism of this reaction. 

Another correlation is shown in Fig. 11 where activity of a number of silica-alumina catalysts for cracking isopropyl benzene is plotted against acidity by titration by anhydrous n-butylamine in nonaqueous medium (Tamele, 9b). Cracking was conducted at 500° at various space velocities. Since the conversion is not a linear function of activity, and the reaction constants could not be calculated from conversions without knowledge of the kinetics of the reaction, values of fc were calculated... 

D. N. Miller and R. S. Kirk [AIChE J., 8, 183 (1962)] studied the kinetics of the catalytic dehydration of primary alcohols to produce the respective olefins. These investigators employed a TCC silica-alumina catalyst in a fixed bed reactor operating at 1 atm and temperatures from 400 to 700° R The catalyst is characterized by a specific surface area of 350 rc /g and a porosity of ca. 0.5. Within the bed the apparent density of the catalyst is 1.15 g/cm. The density of the nonporous bulk solid silica-alumina is 2.30 g/cm. The catalyst received from the vendor was sieved to obtain five sets of particles with apparent particle diameters equal to 0.40, 2.30, 3.05, 4.06, and 5.11 mm. 

This is the same case with which in Eqs. (2)-(4) we demonstrated the elimination of the time variable, and it may occur in practice when all the reactions of the system are taking place on the same number of identical active centers. Wei and Prater and their co-workers applied this method with success to the treatment of experimental data on the reversible isomerization reactions of n-butenes and xylenes on alumina or on silica-alumina, proceeding according to a triangular network (28, 31). The problems of more complicated catalytic kinetics were treated by Smith and Prater (32) who demonstrated the difficulties arising in an attempt at a complete solution of the kinetics of the cyclohexane-cyclohexene-benzene interconversion on Pt/Al203 catalyst, including adsorption-desorption steps. 

A West Texas gas oil is cracked in a tubular reactor packed with silica-alumina cracking catalyst. The liquid feed mw = 0.255) is vaporized, heated, enters the reactor at 630°C and 1 atm, and with adequate temperature control stays close to this temperature within the reactor. The cracking reaction follows first-order kinetics and gives a variety of products with mean molecular weight mw = 0.070. Half the feed is cracked for a feed rate of 60 m liquid/m reactor hr. In the industry this measure of feed rate is called the liquid hourly space velocity. Thus LHSV = 60 hr Find the first-order rate constants k and k " for this cracking reaction. 

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. 

Figure 2 Kinetics of gas-phase propylene homometathesis at 0°C, catalyzed by (a) perrhenate/silica-alumina activated by SnMe4 (10 mg, 0.83 wt % Re) and (b) MeReOs on HMDS-capped silica-alumina (10 mg, 1.4 wt % Re). Solid lines are curve-fits to the first-order integrated rate equation. Solid squares first addition solid circles second addition open circles third addition of propylene (30 Torr) to the catalyst.

The metal-catalysed hydrogenation of cyclopropane has been extensively studied. Although the reaction was first reported in 1907 [242], it was not until some 50 years later that the first kinetic studies were reported by Bond et al. [26,243—245] who used pumice-supported nickel, rhodium, palladium, iridium and platinum, by Hayes and Taylor [246] who used K20-promoted iron catalysts, and by Benson and Kwan [247] who used nickel on silica—alumina. From these studies, it was concluded that the behaviour of cyclopropane was intermediate between that of alkenes and alkanes. With iron and nickel catalysts, the initial rate law is... 

In a detailed kinetic study, Sridhar and Ruthven [256], using nickel supported on Kieselghur (58% Ni), alumina (14% and 40% Ni) and silica-alumina (5% Ni), showed that over all four catalysts the rates of both hydrogenation and hydrocracking could be correlated according to the power rate law equation... 

Typical acidic catalysts are silica—alumina, transition metal sulphates or chlorides, calcium phosphate etc. They are characterised by low deuterium kinetic isotope effects and low stereoselectivity (see Tables 8,11 and 12). These results correspond to the E2cA or El mechanisms, between which a transition may be observed due to the influence of the structure of the reactants, i.e. according to the polarity of the Ca—X and Cp—H bonds. Again, the reactions of 1,2-dibromoethane and 1,1,2,2-tetrachloroethane yielded the evidence. The deuterium kinetic isotope effect on silica—alumina was 1.0 for the dibromo-derivative, which indicates a pure El mechanism, whereas for the tetrachloro-derivative, the value of 1.5 was found. 

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