Homogeneous catalysts, thermal discussion

The mechanisms and rates discussed in this section apply primarily to homogeneous, noncatalytic thermal igas-phase reactions. However, the effect of homogeneous and heterogeneous catalysts and of irradiation will be mentioned in some cases. Many of the ideas discussed apply also to liquid-phase reactions, depending on the solvent. 

Novel and innovative approaches for the selective catalytic homogeneous oxidation of alkanes and alkenes to produce industrially important alcohols, aldehydes, ketones, and epoxides aie still urgently needed. Homogeneous catalytic oxidation is a process that is more selective to products and, as well, the reactions aie carried out at lower temperatures in comparison to heterogeneous catalytic oxidation reactions. However, the separation of the homogeneous catalyst from the oxidation products is energy intensive for example, distillation, as well as possible thermal decomposition of the catalyst during distillation. We will discuss... 

More interest, however, has been focused on the photochemistry of phosphine complexes of the second- and third-row transition metals in their lower oxidation states. This interest is primarily the result of the fact that such complexes are widely used as homogeneous catalysts in the solution phase, and it is theorized that photochemical techniques can be used to generate reactive excited states, or at least to generate reactive, coordinately unsaturated species. A primary goal of such work is the generation of a photocatalytic system whereby the photoproduct is a thermal catalyst, thereby making the transformation catalytic in the number of incident photons. Many of these ideas that have been pursued with tertiary phosphine complexes have also been followed for transition metal carbonyl complexes, with this latter photochemistry being discussed in Chapter 6. 

In order to exemplify the potential of micro-channel reactors for thermal control, consider the oxidation of citraconic anhydride, which, for a specific catalyst material, has a pseudo-homogeneous reaction rate of 1.62 s at a temperature of 300 °C, corresponding to a reaction time-scale of 0.61 s. In a micro channel of 300 pm diameter filled with a mixture composed of N2/02/anhydride (79.9 20 0.1), the characteristic time-scale for heat exchange is 1.4 lO" s. In spite of an adiabatic temperature rise of 60 K related to such a reaction, the temperature increases by less than 0.5 K in the micro channel. Examples such as this show that micro reactors allow one to define temperature conditions very precisely due to fast removal and, in the case of endothermic reactions, addition of heat. On the one hand, this results in an increase in process safety, as discussed above. On the other hand, it allows a better definition of reaction conditions than with macroscopic equipment, thus allowing for a higher selectivity in chemical processes. 

As discussed above, the level of sensitivity of most gas-phase oxidation catalysts is high with regard to the reaction temperature. Therefore, any Stage II-screening tool should operate under isothermal conditions for all active materials, and it is a given prerequisite that thermal equilibrium of the reactor and a homogeneous temperature distribution are essential. 

The catalysts which operate by means of an E2 mechanism give a high proportion of reaction products which are formed by the anti-elimination. This fact has been discussed in Sect. 2.1 and only few remarks need to be added here. Quantum chemical calculations [73] on the transition state of the dehydrochlorination of chloroethane, initiated by an attack of a basic species, confirmed the preference of the anti-elimination over the syn-mode. On the contrary, calculations on the transition state for non-catalytic (homogeneous) thermal elimination [201,202] confirmed the syn-elimination path. 

The coupling of heterogeneous reactions on the catalyst surface and homogeneous gas-phase reactions, as discussed in the previous section, is important for the design and operation of a catalytic combustor with maximum temperatures over 900 C, which is the case for gas turbine combustors. It is worth pointing out that the ignition of the fuel-air mixture over the catalyst at much lower temperatures than possible for homogeneous gas-phase combustion is the reason why catalytic combustors can operate at flame temperatures as low as 1100 C [53]. Hence, the formation of thermal NO, which is the most important type of NOx for gas turbine combustors, is practically avoided. 

A to products by considering mass transfer across the external surface of the catalyst. In the presence of multiple chemical reactions, where each iRy depends only on Ca, stoichiometry is not required. Furthermore, the thermal energy balance is not required when = 0 for each chemical reaction. In the presence of multiple chemical reactions where thermal energy effects must be considered becanse each AH j is not insignificant, methodologies beyond those discussed in this chapter must be employed to generate temperature and molar density profiles within catalytic pellets (see Aris, 1975, Chap. 5). In the absence of any complications associated with 0, one manipulates the steady-state mass transfer equation for reactant A with pseudo-homogeneous one-dimensional diffusion and multiple chemical reactions under isothermal conditions (see equation 27-14) ... 

Enzyme-like reactions have been discussed by Jacobs. Phtalocyanine has been introduced in the cavities of Y zeolite. The resulting catalyst mimics the catalytic activity of cythochrome P-450. Turnover numbers above 10,000 have been reached. The catalyst is active at room temperature, and it is shape selective. It is also thermally and chemically stable. Metallo-complexes incapsulated in zeolites can combine the advantages of homogeneous enzyme reactions and heterogeneous catalysis. 

It was established that catalysts present in CNTs also strongly affect thermal stability of CNTs in air. Active metal particles present in the nanotube samples catalyze carbon oxidation, so the amount of metal impurity in the sample can have a considerable influence on the thermal stability. For example, Zhou et al. (2001) found that if the oxidation of as-synthesized CNTs, which contained traces of catalyst (Fe), was quite rapid and homogeneous at 350 °C due to the catalytic effect, the purified CNTs had negligible weight loss, even after annealing at 460 °C. Furthermore, the presence of Fe obscured the dependence of oxidative stability on tube diameter as discussed earlier. After removing the Fe, all tubes were more appropriate for observing diameter-dependent oxidative stability. Li et al. (2011) have found that the presence of cobalt catalysts dramatically decreases the thermal stability of CNT/peroxide-curable methyl phenyl silicone gum composites as well. This means that the presence of uncontrolled impurities in CNTs can be one of the reasons for reduced reproducibility of sensor parameters. This conclusion is confirmed by results obtained by Boccaleri et al. (2006) and Zhou et al. (2001) (see Fig. 21.5). 

The effective reaction rate tm eff (related to the mass of catalyst/solid) already considers all extra- and intraparticle mass and heat transfer effects (Sections 4.5-4.7). Thus the pseudo-homogeneous model does not distinguish between the conditions in the fluid and in the sohd phase, as more sophisticated heterogeneous models do, as discussed, for example, in Baerns ef al. (2006), Froment and Bischoff (1990), and Westerterp, van Swaaij, and Beenackers (1998). Thus, gradients of temperature and concentration within the particle and in the thermal and diffusive boundary layers are combined by the use of an overall effectiveness factor that enables the system of four equations (mass and heat balances for solid and fluid phase) to be replaced by just two equations, Eqs. (4.10.125) and (4.10.126). 

In this chapter, the water-soluble carboxylic acids are divided into two homologous series, because aliphatic mono- and dicarboxylic acids behave quite differently with respect to thermal decomposition. Specifically, dicarboxylic acids readily undergo thermally induced homogeneous decarboxylation whereas, in all experiments conducted to date, unimolecular decarboxylation of monocarboxylic acids effectively would not occur under relevant hydrothermal conditions in the absence of a heterogeneous catalyst. Evidence for this duality of mechanism will be presented in the forthcoming discussion, but will be restricted primarily to acetic, oxalic, and malonic acids for which extensive experimental information is available. Moreover, a critical discussion will be presented of the current state of experimental... 

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