Liquid phase reactions, thermodynamic

Reactions in solution proceed in a similar manner, by elementary steps, to those in the gas phase. Many of the concepts, such as reaction coordinates and energy barriers, are the same. The two theories for elementary reactions have also been extended to liquid-phase reactions. The TST naturally extends to the liquid phase, since the transition state is treated as a thermodynamic entity. Features not present in gas-phase reactions, such as solvent effects and activity coefficients of ionic species in polar media, are treated as for stable species. Molecules in a liquid are in an almost constant state of collision so that the collision-based rate theories require modification to be used quantitatively. The energy distributions in the jostling motion in a liquid are similar to those in gas-phase collisions, but any reaction trajectory is modified by interaction with neighboring molecules. Furthermore, the frequency with which reaction partners approach each other is governed by diffusion rather than by random collisions, and, once together, multiple encounters between a reactant pair occur in this molecular traffic jam. This can modify the rate constants for individual reaction steps significantly. Thus, several aspects of reaction in a condensed phase differ from those in the gas phase ... 

The rapid initial absorption and reaction of ethylene forms polyethylbenzenes unselectively, and the slow liquid phase reaction of the polyethylbenzenes with unreacted benzene results in an approach towards thermodynamic equilibrium. That is, although the amounts of higher polyethylbenzene is negligible under thermodynamic control, they can be considerably under kinetic control. 

Since the development of nonequilibrium thermodynamics in the late 1940s, initiated by the work of Prigogine (7), numerous reports have appeared dealing with the possibility of oscillations in reaction systems far from equilibrium. Initially the main focus of these studies was the Belouzov-Zhabotinskii liquid-phase reaction (2), but since the discovery of oscillating reactions in heterogeneous catalysis in the late 1960s (3-7), over 300 publications have described research in this field as well. This review focuses on this emerging and important area of research. 

Since the stirred tank is used mainly for liquid phase reactions we have not introduced the pressure as a second thermodynamic variable. If this were necessary, the reaction would have to be considered as a function of composition, temperature, and pressure. In any case, we shall assume the pressure to be constant. 

It is not intended that the literature concerning the hydrogenation of alkenylalkynes and dialkynes shall be reviewed in detail. However, the hydrogenation of molecules as unsaturated as these provides further examples of the operation of the thermodynamic factor which are of interest. The palladium-, platinum-, and nickel-catalyzed hydrogenations of vinylacetylene (H2C=CH—C=CH) provides 1,3-butadiene as the major initial product butenes and butane are also produced (57). The product distributions are constant in liquid phase reactions until the parent hydrocarbon has been removed, showing that vinylacetylene is more strongly adsorbed than 1,3-butadiene and the butenes. The relative yields of butenes and butane resemble those obtained in 1,3-butadiene hydrogenation over these metals (see Section III, F, 6). 

Remarkable differences exist between the liquid-phase and gas-phase reactions under otherwise similar conditions. The selectivity towards the cis-alcohol is still above the thermodynamically expected value but significantly lower than under liquid-phase conditions. In contrast to the liquid-phase reactions, dehydration of the alcohols to the corresponding alkene is an important side-reaction. The oxidation of both the cis- and the trans-alcohol clearly showed that the olefin is exclusively formed from the cis-alcohol. Dehydration of the trans-alcohol is assumed to proceed by isomerisation via a MPVO mechanism to the corresponding cis-alcohol. 

Reactive distillation is used with thermodynamically limited reversible liquid-phase reactions and is particularly attractive when one of the products has a tower boiling point than the reactants. For reversible reactions of this type. 

TTie two reactors with recycle shown in (i) and (j) can be used for highly exothermic reactions. Here the recycle stream is cooled and returned to the reactor to dilute and cool the inlet stream thereby avoiding hot spots and runaway reactions. The PFR with recycle is used for gas-phase reactions, and the CSTR is used for liquid-phase reactions. The last (wo reactors, (k) and (I), are used for thermodynamically limited reactions where the equilibrium lies far to the left (reactant side)... 

Our theory explains liquid phase reaction dynamics in terms of an interplay between the thermodynamic restoring forces and the forces deriving from the solute gas phase potential U x). The latter propel the... 

The second approach is to formulate rules for the correlation of the enthalpies and entropies of liquids and gases so that by the use of established correlations of the parameters for the gases, those for the corresponding liquids may be estimated. A recent paper by Patrick [398] has considerably clarified this approach. Assuming ideal thermodynamic behaviour of the solution components, it was concluded that for reactions involving no change in the number of molecules, i.e. transfer reactions of the present type, the ratio of equilibrium constants of the gas and liquid phase reactions is unity. This would appear to be most simply explained if the forward and reverse rate coefficients (fef(liq.), kf(gas), k (liq.) and fer(gas)) were equal, i.e. fef(liq.) = fef(gas) and fer(hq-) = fer(gas), but this remains to be confirmed experimentjdly. 

The liquid-phase reaction kinetics of this family of tertiary alkyl ethyl ethers can be consistently described within the LHHW formalism [1], with the TTST being applied to the elementary mechanistic steps. The systematicity of reaction kinetics is discussed here based on ETC, which describe relation between kinetics and thermodynamics. Further, it is shown that the rate expressions for this nonideal liquid-phase reaction system should be written in terms of activities. 

From a theoretical standpoint, it is obvious that only thermodynamic consistent models of chemical and phase equilibria should be applied in RD modeling. We will discuss the subject from a more practical standpoint here, using typical liquid-phase reactions as examples, for which the thermodynamic consistent chemical equilibrium constant is given by equations (4.12) and (4.13). 

Organic reactions, particularly those constituting a synthetic scheme for a fine chemical, usually involve molecules reacting in the liquid phase. The effects of reactant structure and of the solvent (medium) in which the reaction occurs (the solvation effects) are not included in the conventional macroscopic approach to thermodynamics. Therefore, the treatment of liquid-phase reactions tends to be less exact than that of gas-phase reactions involving simpler molecules without these influences. 

TST has also been widely used to treat reactions in condensed phases. Wigner s dynamical perspective has particularly had an impact on the extension of TST to reactions in liquids. Most applications to liquid-phase reactions have used the thermodynamic formulation of TST [60], which includes the effects of the condensed phase on reaction free energies in an approximate manner. Chandler [61] provided a more rigorous formulation of classical TST for liquids. The new element introduced by the liquid phase is collisions of solvent molecules with the reacting species that can lead to recrossings of the dividing surface and a breakdown of the fundamental assumption. A recent review [16] documents many more advances in the extension of TST to the kinetics of condensed-phase processes. 

It is sometimes easier to calculate the equilibrium of a liquid phase reaction based on the thermodynamic data of the corresponding gas-phase reaction (Example 4.2.6). To derive the respective equations, we assume ideal gas as well as ideal liquid phase behavior and a simple reversible A to B reaction as example. The standard Gibb s enthalpy of the gas-phase reaction ArG is ... 

As a partial conclusion, by considering a thermodynamic selectivity, which is the difference between ionization energies of reactant and product, various reactions of oxidation can be ranked. AI represents the electron acceptor power of the gas (liquid) phase reaction and is the analog of A, which represents the electron donor power of the selective solid catalyst. 

For the non-oxidative activation of light alkanes, the direct alkylation of toluene with ethane was chosen as an industrially relevant model reaction. The catalytic performance of ZSM-5 zeolites, which are good catalysts for this model reaction, was compared to the one of zeolite MCM-22, which is used in industry for the alkylation of aromatics with alkenes in the liquid phase. The catalytic experiments were carried out in a fixed-bed reactor and in a batch reactor. The results show that the shape-selective properties of zeolite ZSM-5 are more appropriate to favor the dehydroalkylation reaction, whereas on zeolite MCM-22 with its large cavities in the pore system and half-cavities on the external surface the thermodynamically favored side reaction with its large transition state, the disproportionation of toluene, prevails. 

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