Reaction kinetics catalyst characteristics

The ROTOBERTY internal recycle laboratory reactor was designed to produce experimental results that can be used for developing reaction kinetics and to test catalysts. These results are valid at the conditions of large-scale plant operations. Since internal flow rates contacting the catalyst are known, heat and mass transfer rates can be calculated between the catalyst and the recycling fluid. With these known, their influence on catalyst performance can be evaluated in the experiments as well as in production units. Operating conditions, some construction features, and performance characteristics are given next. 

V-Sb-oxide based catalysts show interesting catal)dic properties in the direct synthesis of acrylonitrile from propane [1,2], a new alternative option to the commercial process starting from propylene. However, further improvement of the selectivity to acrylonitrile would strengthen interest in the process. Optimization of the behavior of Sb-V-oxide catalysts requires a thorough analysis of the relationship between structural/surface characteristics and catalytic properties. Various studies have been reported on the analysis of this relationship [3-8] and on the reaction kinetics [9,10], but little attention has been given to the study of the surface reactivity of V-Sb-oxide in the transformation of possible intermediates and on the identification of the sxirface mechanism of reaction. 

In this paper we will first describe a fast-response infrared reactor system which is capable of operating at high temperatures and pressures. We will discuss the reactor cell, the feed system which allows concentration step changes or cycling, and the modifications necessary for converting a commercial infrared spectrophotometer to a high-speed instrument. This modified infrared spectroscopic reactor system was then used to study the dynamics of CO adsorption and desorption over a Pt-alumina catalyst at 723 K (450°C). The measured step responses were analyzed using a transient model which accounts for the kinetics of CO adsorption and desorption, extra- and intrapellet diffusion resistances, surface accumulation of CO, and the dynamics of the infrared cell. Finally, we will briefly discuss some of the transient response (i.e., step and cycled) characteristics of the catalyst under reaction conditions (i.e.,... 

Another important catalyst characteristic is porosity. Particularly when heavy feeds are processed, high pore volumes and pore diameters are required to reduce pore diffusion limitations. These limitations occur when the intrinsic rate of reaction is high compared with the rate of diffusion of the reactants through the catalyst particle to the active surface. The catalyst is then not used effectively, and reaction rates and selectivity become functions of particle size. If the kinetics of the reaction are known, it is possible to estimate from theory the reaction rate or threshold above which a catalyst of known size will begin to exhibit diffusion limitations. 

MIP catalyst 76 also performs enantioselective hydrolysis of non-activated d-and L-phenylalanine ethyl ester 78 at pH 7.4, with characteristic saturation kinetics (Scheme 13.19). While this catalyst does not have the precise shape complementarity to ester 78, a threefold enhancement compared to the blank reaction was observed. Furthermore, enantiodiscrimination was obtained, as compound 78 was hydrolyzed 1.44-fold faster than ent-78. Although the reported rate enhancement was modest compared to the standards in asymmetric synthesis, it can be considered as a first step in the development of imprinted polymers for enantioselective catalysts. 

In the above Da denotes the Damkohler number as the ratio of the characteristic process time H/V to the characteristic reaction time l/r0. The reaction rate r0 is a reference value at the system pressure and an arbitrary reference temperature, as the lowest or the highest boiling point. For catalytic reactions r0 includes a reference value of the catalyst amount. R is the dimensionless reaction rate R = r/r0. The kinetics of a homogeneous liquid-phase reaction is described in general as function of activities ... 

The classical method of investigation of effects of diffusion on reactions is typically to run a reaction with catalyst particles of various sizes. For zeolites, the resistance of intracrystalline diffusion is normally much larger than that characteristic of molecular diffusion or Knudsen diffusion that could occur in the spaces between the zeolite crystals in a catalyst particle. Thus, the crystal size of the zeolite has to be varied instead of the particle size to determine the effects of diffusion on zeolite-catalyzed reactions. Kinetics of the MTO reaction has been measured with SAPO-34 crystals with identical compositions and sizes of 0.25 and 2.5 pm 89). The methanol conversion was measured as a function of the coke content of the two SAPO-34 crystals in the TEOM reactor. 

In this highly exothermic reaction, kinetic control is a function of the catalyst characteristics and surface temperature, and the mass transport control and boundary layer thickness is based on the catalyst bed geometry and flow velocity. Both the gauze and ceramic foam have tortuous flow paths, but the extruded cordierite monolith has microchan-... 

The catalysts which have presented the most suitable characteristics for this oxidation are the metal oxides and metal oxides mixtures of transition elements of the V and VI groups, and the hterature reports information related to the formulation, preparation and evaluation of the catalysts (2 - 6), although very few data have been pubhshed related to the reaction kinetics. Gunduz and Akpolat (5) present experimental kinetic data of gas phase oxidation of toluene to benzaldehyde over V2O5 catalysts. Their results are based on the redox model and are restricted to the temperature of 430 C. Also, it is not found in the hterature enough data which allow to analyze the activity and behavior of V2O5 catalysts based only on their physical characteristics. 

The intrinsic activity depends on the chemical and physical properties of the active component. For unsupported catalysts, the most important properties are the composition and structure of the catalyst surface and the presence, or absence, of special sites such as Br0nsted or Lewis acid centers, anion or cation defects, and sites of high coordination. For supported catalysts, the size and morphology of the dispersed phase are of additional importance. If intraparticle transport of reactants occurs with a characteristic time that is short compared to that of the reaction, then the observed and intrinsic rates of reaction will be identical. When the characteristic time for intraparticle mass transport is less than that for reaction, the observed rate of reaction per unit mass of catalyst becomes less than the intrinsic value, and the reaction kinetics are dominated by the effects of intraparticle mass transport. The factors governing intraparticle transport are the diffusivities of the reactants and products and the characteristic distance for diffusion. 

In this chapter we also discuss heterogeneous catalytic adsorption and reaction kinetics. Catalysis has a significant impact on the United States economy and many important reactions employ catalysts. We describe the kinetic principles that are needed for rate studies and demonstrate how the concepts for homogeneous reactions apply to heterogeneously catalyzed reactions with the added constraint of surface- site conservation. The physical characteristics of catalysts are dis- cussed in Chapter 7. 

In contrast to the PEC system, catalysts are placed in the plasma zone in the PDC system, so a lot more complicated interactions are expected between the plasma and the surface of catalyst. Again, the PDC system has both characteristics of gas-phase NTP and catalytic process. Table 5 summarizes the difference between the NTP alone and the PDC system. One of the important advantages of the PDC system over the conventional NTP reactors is the high energy efficiency (see Fig.3). Another important characteristic of the PDC system is its kinetics. Determination of reaction kinetics is useful to understand the overall characteristics of the chemical reaction in question. The influence of gas residence time (i.e. GHSV) is not observed both the NTP alone and the PDC. 

Draw the dimensionless molar density profile of reactant A within a porous wafer catalyst for the following values of the intrapeUet Damkohler number. The reaction kinetics are zeroth-order and the characteristic length L is one-half of the wafer thickness, measured in the thinnest dimension. Put all five curves on the same set of axes and be as quantitative as possible on both axes. Dimensionless molar density I a is on the vertical axis and dimensional spatial coordinate rj is on the horizontal axis. 

The importance of the cocatalyst in metal-catalyzed polymerization processes can be appreciated as follows. First, to form active catalysts, catalyst precursors must be transformed into active catalysts by an effective and appropriate activating species. Second, a successful activation process requires many special cocatalyst features for constant catalyst precursor and kinetic/thermodynamic considerations of the reaction. Finally, the cocatalyst, which becomes an anion after the activation process, is the vital part of a catalytically active cation—anion ion pair and may significantly influence polymerization characteristics and polymer properties. Scheme 1 depicts the aforementioned relationships between catalyst and cocatalyst in metal-catalyzed olefin polymerization systems. 

It can be seen that the pulse decay strongly depends on cycle time (tperi d) and reactor bed length. If the periodic time is very long the system reaches a so-called quasisteady state in periodic operation and very short periods (see curve (a) in Fig. 4.9) lead to a merging of pulses. The choice of pulsing frequency can be influenced, e.g. by pulse attenuation characteristic, mass storage in catalyst particles and reaction kinetics. 

In order to elucidate this problem, Bukhavtsova and Ostrovskii [9] have studied experimentally the reaction kinetics on the catalysts with (and without) capillary condensation. The model catalysts Pt/Si02 with approximately the same characteristics except porous structure (Table 23.4) were used. Two modifications of support Si02 were used KCK-1 with relatively large pores, and KCM-5 with small pores (Figure 23.8). Except pore size distribution, the platinum distribution among the pores with different size was measured by adsorption method [53]. 

Adams catalyst (Pt02/H+) reductions of the substituted benzoic acids are generally rapid but usually give product ratios characteristic of kinetic controlled reactions. Reductions in strong base with Raney nickel catalyst appear to give the equilibrated (thermodynamic) product. 

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