Catalytic reaction equilibrium shift

At low temperatures the reaction is negatively affected by the lack of oxygen on the surface, while at higher temperatures the adsorption/desorption equilibrium of CO shifts towards the gas phase side, resulting in low coverages of CO. As discussed in Chapter 2, this type of non-Arrhenius-like behavior with temperature is generally the case for catalytic reactions. 

In the sorption-enhanced reforming (SER) process, one of the gaseous reaction products (C02) of the catalytic reforming reaction is separated from the reaction zone by sorption. As a result, the equilibrium of the reaction is shifted toward products according to the Le Chatelier s principle. Balasubramanian et al. [18] studied the SMR reaction in the presence of CaO as a C02 acceptor. Thus, in addition to reactions 2.4 and 2.6, the reaction of C02 with the C02 acceptor (CaO) takes place in the reaction zone ... 

At the beginning stage of dehydrogenation, the substrate organic hydride is adsorbed onto the catalyst surface from the liquid phase directly and easily. Catalytic reaction processes will succeed it, until the surface sites are filled with the adsorbed reactant and products. Once product desorption starts to form and grow a bubble, product readsorption becomes unfavorable due to the increment of translational entropy of the product molecule in the bubble, if compared with that in the solution, shifting the adsorption equilibrium for the product and suppressing its effect of rate retardation. 

Manipulation of chemical reactions shifting the equilibrium situation or manipulation of the conversion or selectivity of catalytic reactions are two possibilities. 

In this reaction, enynes having a terminal alkyne do not give a satisfactory result (Scheme 2). Presumably, the alkene part in product 3a further reacts with Ih to afford ruthenium carbene IV, which would be coordinated by the alkene part of the product. Thus, the catalytic activity would decrease (Scheme 2). It is thought that if this reaction is carried out under ethylene gas, Ih would be regenerated since the equilibrium reaction would shift to the left side because of a large amount of ethylene. When the reaction of 2a is carried out under ethylene gas, the catalytic activity is recovered, and 3a is obtained in 90% yield even with the use of 1 mol% of the ruthenium catalyst. ... 

Ardizzone et al. used the esterification of benzoic acid with methanol to test the catalytic performance of different SZ catalysts. " Water had to be continuously removed from the reaction medium to shift the reaction equilibrium to product formation and to avoid catalyst deactivation by sulfate leaching. According to these authors, catalysts with a higher density of acid sites with KdL values in the range —14.2 to -5.6 performed better. Acid sites with pKa. of... 

At low conversions (10-25%) high yields can be achieved (85-95%). 1,4-Dichloro-2-butenes formed may be used in the manufacture of other products or isomerized to give the desired 3,4-dichloro-l-butene. This is done in the presence of a catalytic amount of CuCl2 with the equilibrium shifted to the formation of 3,4-dichloro-1-butene by distilling it off the reaction mixture. Finally, 3,4-dichloro-l-butene undergoes alkaline dehydrochlorination to produce chloroprene (5-15% NaOH, 80-110°C). 

A physical sense of this limitation is simple. It implies that catalysis "does not shift the equilibrium , i.e. in the presence of a catalyst the equilibrium gas composition is in equilibrium itself. Generally speaking, it does not only hold for catalytic reactions. Limitations on the equilibrium constants of eqns. (72), (73), and (75) suggest that reactions do not "shift the equilibrium of the others since the equilibrium is detailed. 

A high CO pressure would shift equilibrium (4.3) to the left and the catalytic reaction would become slower. In this complex CO is a far better ligand than an alkene. On the other hand the reaction uses CO as a substrate, so it cannot be omitted. Furthermore, low pressures of CO may lead to decomposition of the cobalt carbonyl complexes to metallic cobalt and CO, which is also undesirable. Finally, the product alcohol may stabilize divalent cobalt species which are not active as a catalyst ... 

Whereas catalytic cracking is a useful reaction that requires high temperatures, a low reaction temperature is required in order to produce branched isomers at a high yield in reactions of the type shown in Fig. 4.71). The equilibrium shifts to the n-alkanes at high temperature. 

Many kinetic methods of analysis involve reactions whose rates depend on catalysts in solution (Section 15-7) most of these involve redox systems. A catalyst may be defined broadly as an agent that alters the rate of a reaction without shifting the position of equilibrium. The catalyst itself imdergoes no permanent change, although it may enter the reaction mechanism in a cyclic manner. The mechanisms and activation of catalytic reactions have been reviewed. ... 

The concept of combining membranes and reactors is being explored in various configurations, which can be classified into three groups, related to the role of the membrane in the process. As shown in Figure 25.12, the membrane can act as (a) an extractor, where the removal of the product(s) increases the reaction conversion by shifting the reaction equilibrium (b) a distributor, where the controlled addition of reactant(s) limits side reactions and (c) an active contactor, where the controlled diffusion of reactants to the catalyst can lead to an engineered catalytic reaction zone. In the first two cases, the membrane is usually catalytically inert and is coupled with a conventional fixed bed of catalyst placed on one of the membrane sides. 

Unfortunately, both excess sodium compound and chelated sodium compound catalyze the telomerization, even when the former is less effective, and it is difficult to separate these two reactions. With excess chelating agent, however, the equilibrium shift is clear. The data in Table III for excess chelating agent support the concept of a chelated sodium compound in equilibrium with free chelating agent and sodium compound. It is also possible, particularly below 25 °C, that catalytic species of stoichiometry chel2NaX or chel(NaX)2 are involved in addition to chel-NaX in the telomerization reaction. 

The catalytic reaction proceeds stagewise with interstage removal of the sulfur to shift the equilibrium. Interstage removal of water to shift the equilibrium even further is impractical because of plugging problems (3) with solid sulfur. 

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