Selectivity solid-liquid reaction

Many solid-liquid reactions are likely to benefit from the careful selection of the reactant (particle size, method of manufacture, etc.). Many reaction-crystallization systems may benefit from the use of seed crystals (see Section 5.4). 

One goal of our experimental program with the bench-scale unit was to develop the necessary correlations for use in the ultimate design of large commercial plants. Because of the complexity inherent in the three-phase gas-liquid-solid reaction systems, many models can be postulated. In order to provide a background for the final selection of the reaction model, we shall first review briefly the three-phase system. 

A very important part of such an undertaking is to be clear about what stages of a chemical process generate the most waste. Often this is found to be the separation stage, after the transformation of reactants to products, where all the various components of the final mixture are separated and purified. Approaches to chemical reactions which help to simplify this step are particularly powerful. Such an approach is exemplified by heterogeneous catalysis. This is an area of chemistry where the catalysts used are typically solids, and the reactants are all in the hquid or gas phase. The catalyst can speed up the reaction, increase the selectivity of the reaction, and then be easily recovered by filtration from the liquid, and reused. 

As was stated above, the very strong acidity (and probably together with the organophilicity of the pore wall) makes these salts very active catalysts in liquid-solid organic reaction systems. We wish to emphasize that this is the first example for the shape selective catalysis of heteropolyacids at least to our knowledge. 

Reactions accelerated by microwaves 363 Reactions not accelerated by microwaves 363 Superheating of liquid reaction mixture 3 64 Localized superheating in the solid phase 365 Selective heating 365 Hot spots 366... 

The unique ability of crown ethers to form stable complexes with various cations has been used to advantage in such diverse processes as isotope separations (Jepson and De Witt, 1976), the transport of ions through artificial and natural membranes (Tosteson, 1968) and the construction of ion-selective electrodes (Ryba and Petranek, 1973). On account of their lipophilic exterior, crown ether complexes are often soluble even in apolar solvents. This property has been successfully exploited in liquid-liquid and solid-liquid phase-transfer reactions. Extensive reviews deal with the synthetic aspects of the use of crown ethers as phase-transfer catalysts (Gokel and Dupont Durst, 1976 Liotta, 1978 Weber and Gokel, 1977 Starks and Liotta, 1978). Several studies have been devoted to the identification of the factors affecting the formation and stability of crown-ether complexes, and many aspects of this subject have been discussed in reviews (Christensen et al., 1971, 1974 Pedersen and Frensdorf, 1972 Izatt et al., 1973 Kappenstein, 1974). 

Monomers may be in the solid, liquid or gaseous state. In the first phase, blending and comminution is generally carried out to reduce inhomogeneities. For the same reason, polymer particle size can also influence the reaction. For example, in the grafting of vinyl chloride on polycaprolactam, the Cl content of the resultant polymer was 3.71 % and 2.16% when 0.05-0.09 mm and 0.4-0.63 mm diameter polycaprolactam particles were employed (31). To obtain polymer in the desired powder form, the polymer was pulverized and selectively precipitated. 

Substrate selectivity effects were investigated with polystyrene-supported polyethylene glycol) catalysts 51 (n = 3,61% RS) and 56 (Z = phenyl, n = 30, 8 % RS) under solid/solid/liquid conditions 180). The difference in rates of reaction of 1-bromobutane and 1-bromooctane with solid potassium phenoxide was a factor of about 3 (51 was more active). Measurements of the distribution of both bromides between... 

Sensitivity and complexity represent challenges for ATR spectroscopy of catalytic solid liquid interfaces. The spectra of the solid liquid interface recorded by ATR can comprise signals from dissolved species, adsorbed species, reactants, reaction intermediates, products, and spectators. It is difficult to discriminate between the various species, and it is therefore often necessary to apply additional specialized techniques. If the system under investigation responds reversibly to a periodic stimulation such as a concentration modulation, then a PSD can be applied, which markedly enhances sensitivity. Furthermore, the method discriminates between species that are affected by the stimulation and those that are not, and it therefore introduces some selectivity. This capability is useful for discrimination between spectator species and those relevant to the catalysis. As with any vibrational spectroscopy, the task of identification of a species on the basis of its vibrational spectrum can be difficult, possibly requiring an assist from quantum chemical calculations. 

Chemicals and the containment materials for chemical reaction do not interact equally with the commonly used microwave frequencies for dielectric heating and consequently selective heating may be achieved. Specifically, it is possible to cool the outside of the vessel with a coolant that is transparent to microwaves (solid C02 or liquid N2) and thereby have cold walls that still allowthe microwave energy to penetrate and heat the reactants, which are microwave active, in the vessel. Also for solid-state reactions contamination from the crucible walls may be minimised. 

In dealing with chemical process engineering, conducting chemical reactions in a tubular reactor and in a packed bed reactor (solid-catalyzed reactions) is discussed. In consecutive-competitive reactions between two liquid partners, a maximum possible selectivity is only achievable in a tubular reactor under the condition that back-mixing of educts and products is completely prevented. The scale-up for such a process is presented. Finally, the dimensional-analytical framework is presented for the reaction rate of a fast chemical reaction in the gas/liquid system, which is to a certain degree limited by mass transfer. 

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