Gas-phase heterogeneous catalytic

For gas phase heterogeneous catalytic reactions, the continuous-flow integral catalytic reactors with packed catalyst bed have been exclusively used [61-91]. Continuous or short pulsed-radiation (milliseconds) was applied in catalytic studies (see Sect. 10.3.2). To avoid the creation of temperature gradients in the catalyst bed, a single-mode radiation system can be recommended. A typical example of the most advanced laboratory-scale microwave, continuous single-mode catalytic reactor has been described by Roussy et al. [79] and is shown in Figs. 10.4 and... 

For a number of liquid-phase reactions, the proper choice of a solvent can enhance selectivity, See, for example, IncL Eng. Chem., 62(9), 16 (1970). In gas-phase heterogeneous catalytic reactions, selectivity is an important parameter of any particular cataly.st. 

Example 7.4 Design a packed-bed reactor for the gas-phase, heterogeneous catalytic cracking reaction... 

Fixed bed reactors are the most common reactors for the study of gas-phase heterogeneous catalytic reactions. They are easy to use and catalysts in powder form can be readily used (usually after sieving the proper particle size fraction). Semi-empirical correlations are used to describe the physical phenomena occurring in these types of reactors [21,22]. However, in the case of very last and for exo- or endothermic reactions, it becomes difficult to measure the intrinsic kinetics with these reactors. To avoid external heat and mass transfer limitations as well as internal diffusion limitations, high flow rates and small particles are necessary. This quickly will lead to an excessive pressure drop over the reactor. 

In the case of three phase heterogeneous catalytic reactions, the rate of the process and its selectivity can be determined either by intrinsic reaction kinetics or by external diffusion (on the gas-liquid and gas-solid interface) as well as by internal diffusion through the catalyst pores. Careful analysis of mass transfer is important for the elucidation of intrinsic catalytic properties, for the design of catalysts, and for the scale up of processes. 

Heterogeneous catalysis takes place in multiphase systems. If a new phase, the supercritical phase, is selectively introduced, remarkable change can be expected. The effects caused by the introduction of a supercritical phase depend on many parameters, such as fluid properties, reaction conditions, and affinity. Successful supercritical phase heterogeneous catalytic reactions can be realized, as long as these parameters are carefully controlled. With greater activities, catalyst lifetimes, or selectivities than for gas phase or liquid phase reactions, some supercritical phase reactions have industrial potentid. 

The single CSTR has been used for many years in the laboratory for the study of kinetics of liquid phase reactions, and is now increasingly being employed for the measurement of gas/solid heterogeneous catalytic kinetics as well [early developments in the latter application are described by J.J. Car berry, Ind. Eng. Chem., 56, 39 (1964) D.J. Tajbl, J.B. Simons and J.J. Carberry, Ind. Eng. Chem. Eundls., 5, 171 (1966). A number of related designs based on internal recirculation of the reaction mixture through a small fixed bed of catalysts may also be treated conceptually as... 

Advanced learners should then also study Section 4.11.4 (on transport limitations in experimental catalytic reactors) and Sections 4.11.5.2-4.11.5.4, where some more complex examples are given (heterogeneously catalyzed gas-phase reaction, catalytic multiphase reaction, and non-isothermal oxidation of carbon). 

Catalytic processes may be homogeneous in the liquid or gas phase (for instance, nitrogen oxides in the Chamber process for sulfuric acid), but iudustrial examples are most often heterogeneous with a... 

A heterogeneous catalyst is a catalyst present in a phase different from that of the reactants. The most common heterogeneous catalysts are finely divided or porous solids used in gas-phase or liquid-phase reactions. They are finely divided or porous so that they will provide a large surface area for the elementary reactions that provide the catalytic pathway. One example is the iron catalyst used in the... 

Steps 1 through 9 constitute a model for heterogeneous catalysis in a fixed-bed reactor. There are many variations, particularly for Steps 4 through 6. For example, the Eley-Rideal mechanism described in Problem 10.4 envisions an adsorbed molecule reacting directly with a molecule in the gas phase. Other models contemplate a mixture of surface sites that can have different catalytic activity. For example, the platinum and the alumina used for hydrocarbon reforming may catalyze different reactions. Alternative models lead to rate expressions that differ in the details, but the functional forms for the rate expressions are usually similar. 

It is a misconception that most chemicals are manufactured in organic solvents. Most high-volume bulk chemicals are actually produced in solvent-free processes, or at least ones in which one of the reactants also acts as a solvent. Typical examples of such large-scale processes include the manufacture of benzene, methanol, MTBE, phenol and polypropylene. In addition, some heterogeneous gas-phase catalytic reactions, a class of solvent-free processes, are discussed in Chapter 4. 

Pioneering works in this area started in the 1960s by polymer scientists like D. Ballard at ICI and Y. Yermakov at the Novosibirsk Institute of Catalysis [54,55]. The need for polymers with better properties and for better technologies (gas phase processes) has led to the development of various strategies to obtain supported catalytic systems [56-58]. Here, we will concentrate on reactions leading to basic chemicals, and a comparison between homogeneous and heterogeneous catalysts will be performed when possible. 

The rate of an electrochemical reaction depends, not only on given system parameters (composition of the catalyst and electrolyte, temperature, state of the catalytic electrode surface) but also on electrode potential. The latter parameter has no analog in heterogeneous catalytic gas-phase reactions. Thus, in a given system, the potential can be varied by a few tenths of a volt, while as a result, the reaction rate will change by several orders of magnitude. 

Catalytic oxidation of CO heterogeneously in the gas phase and electrochemically in solution are important reactions for the removal of CO from reactant stock gases in... 

High throughput screening is one of the hot topics in heterogeneous catalysis. Advanced experimental techniques have been developed to screen and develop solid catalysts for gas-phase systems. However, for catalytic three-phase systems, rapid screening has got much less attention [1-6]. Three-phase catalysis is applied in numerous industrial processes, from synthesis of fine chemicals to refining of crade oil. 

---