Isomeric reactions adsorption

This is the same case with which in Eqs. (2)-(4) we demonstrated the elimination of the time variable, and it may occur in practice when all the reactions of the system are taking place on the same number of identical active centers. Wei and Prater and their co-workers applied this method with success to the treatment of experimental data on the reversible isomerization reactions of n-butenes and xylenes on alumina or on silica-alumina, proceeding according to a triangular network (28, 31). The problems of more complicated catalytic kinetics were treated by Smith and Prater (32) who demonstrated the difficulties arising in an attempt at a complete solution of the kinetics of the cyclohexane-cyclohexene-benzene interconversion on Pt/Al203 catalyst, including adsorption-desorption steps. 

In addition to charge transfer processes, calculations of adsorption free energy and of isomerization reaction equilibrium and dynam-... 

Of course, certain features of overall kinetics are inaccessible via a cluster model method, such as the influence of pore structure on reactivity. The cluster model method cannot integrate reaction rates with concepts such as shape selectivity, and an alternative method of probing overall kinetics is needed. This has recently been illustrated by a study of the kinetics of the hydroisomerization of hexane catalyzed by Pt-loaded acidic mordenite and ZSM-5 (211). The intrinsic acidities of the two catalysts were the same, and differences in catalyst performance were shown to be completely understood on the basis of differences in the heat of adsorption of hexene, an intermediate in the isomerization reaction. Heats of adsorption are strongly dependent on the zeolite pore diameter, as shown earlier in this review (Fig. 11). 

This can be explained by the fact that, if the isomerization reaction is very fast, then the isomer concentration will approach the thermodynamic equilibrium at any point inside the cristallite. Thus, the isomer distribution in the surrounding fluid is controlled only by the adsorption of the various species. 

Fluorinated chromia used to catalyze the isomerization reaction of CHF2CHF2 to CF3CH2F, also exhibited evidence for the importance of chromium in higher oxidation states. FTIR spectroscopic measurements of CO adsorption confirmed the occurrence of Cr4+ and Cr5+ on the surface of chromia catalysts before being used [52]. During the activation, Cr4+ and Cr5+ sites were reduced and enhanced activity of the catalyst was observed. The reaction pathway proposed for isomerization involves the formation of hydrogen fluoride due to the degradation reaction of the fluoroalkane. 

Related to their similar pore diameter and pore structure, unsurprisingly the Henry adsorption constants for linear alkanes are very close to each other on zeolite ZSM-22 and ZSM-23 (Table I). Somewhat higher constants are obtained for 2- and 3-methylbranched alkanes on ZSM-23 compared to zeolite ZSM-22. The adsorption constants of linear alkanes are obviously hi er than branched alkanes on the two cases. The separation power of a zeolite between a linear and a branched hydrocarbon may be given by the separation factor (a), which is the ratio of Henry consteints of linear and branched molecules at a certain temperature, a values at 523 K are given for both zeolites in Table 1. For comparison, values for ZSM-5 are also included, which is one of the most popular shape selective catalyst used in isomerization reactions. From this table it can be seen that both ZSM-22 and ZSM-23 have higher separation constants compared to ZSM-5. The zeolites can be listed in the following order with respect to their separation capacity between linear and 2- and 3-methylbranched alkanes ZSM-22 > ZSM-23 > ZSM-5. In narrow pore structures such as zeolites ZSM-22 and ZSM-23 it is very probable that linear alkanes with smaller kinetic diameters have more access to the available adsorption sites compared to the more bulky branched molecules. This may be regarded as the first... 

The comparison includes elemental reactions, as chain-growth, and -branching, alternative olefin- or paraffin product desorption and olefin reactions of hydrogenation, isomerization, re-adsorption on growth sites and CO-methanation. 

Pronounced minima of interfacial tension at pH = 3 (Fig. 6) may reflect the isomerization reaction of albumin and the presence of a Fast isomeric form (F). It can be seen (Fig. 6), in agreement with other authors 16,38 that N-F transition is sensitive to the salt concentration in the bulk solution. Once again similar influence of the apolar air phase, in our case, and of a hydrophobic polyethylene on albumin adsorption at this pH is demonstrated. 

Molecular heats of adsorption play a role in many catalytic reactions. Figure 6.23 illustrates this for an isomerization reaction catalyzed by a solid acid. As explained in Chapter 3, the hydroisomerization of alkanes on a zeolite-supported metal proceeds through a bifunctional reaction mechanism, in which the metal has the function of activating C-H bonds and H2 at a low reaction temperature. The alkane-alkene equilibrium is established by metal catalysis, and the alkene is protonated and isomerized by the acidic protons of the zeolite... 

Similarly as in (6.48), the coverage of the alkene on the acid sites is controlled by the equilibrium constant of adsorption. Therefore, at low coverage the effective activation energy of the isomerization reaction becomes (6.55)... 

From the results of this kinetic study and from the values of the adsorption coefficients listed in Table IX, it can be judged that both reactions of crotonaldehyde as well as the reaction of butyraldehyde proceed on identical sites of the catalytic surface. The hydrogenation of crotyl alcohol and its isomerization, which follow different kinetics, most likely proceed on other sites of the surface. From the form of the integral experimental dependences in Fig. 9 it may be assumed, for similar reasons as in the hy-drodemethylation of xylenes (p. 31) or in the hydrogenation of phenol, that the adsorption or desorption of the reaction components are most likely faster processes than surface reactions. 

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