Monomolecular deactivation

P-Hydride elimination Transfer to hydrogen Monomolecular deactivation... 

Singlet excited molecules are usually relatively short-lived and, therefore, are not very likely to undergo bimolecular reactions. In many cases, however, chemical bond cleavage competes with physical monomolecular deactivation paths. For example, singlet excited carbonyl groups contained in a polyethylene chain can undergo the Norrish type I reaction, resulting in a free radical couple [see Eq. (1-17)]. 

In the process of radical polymerization a monomolecular short stop of the kinetic chain arises from the delocalization of the unpaired electron along the conjugated chain and from the competition of the developing polyconjugated system with the monomer for the delivery of rr-electrons to the nf-orbitals of a transition metal catalyst in the ionic coordination process. Such a deactivation of the active center may also be due to an interaction with the conjugated bonds of systems which have already been formed. 

By absorption of light a molecule is promoted to a higher electronic state. The monomolecular physical processes for the dissipation of the excess energy are outlined in Fig. 5 in a so called Jablonski diagramm. In principle one has to differentiate between radiative and non-radiative deactivation on the one side and on the other side one has to consider if the multiplicity of the system is conserved or not. Radiative deactivation, i.e. deactivation accompanied by emission of light, is termed fluorescence if the transition occurs with spin conservation and phosphorescence, if spin inversion occurs. 

MCM-22, with a larger pore volume than ZSM-5, revealed behavior intermediate between what was observed for large- and medium-pore zeolites (126). Unverricht et al. (141) also examined MCM-22 at 353 and 393 K, it was found to produce mainly cracked products and dimethylhexanes and to deactivate rapidly. MCM-36 gained considerable interest that is evidenced by the patent literature (171-174). MCM-36 is a pillared zeolite based on the structure of MCM-22. Ideally, it should contain mesopores between layers of MCM-22 crystallites. This structure was found to be much more active and stable than MCM-22 (175). Alkane cracking experiments with zeolites having various pore dimensions evidenced the preference of monomolecular over sterically more demanding bimolecular pathways, such as hydride transfer, in small- and medium-pore zeolites (146). 

Excited-state relaxation can proceed spontaneously in monomolecular processes or can be stimulated by a molecular entity (quencher) that deactivates (quenches) an excited state of another molecular entity, by energy transfer, electron transfer, or a chemical mechanism [lj.The quenching is mostly a bimolecular radiationless process (the exception is a quencher built into the reactant molecule), which either regenerates the reactant molecule dissipating an energy excess or generates a photochemical reaction product (Figure 4.1). 

The situation described by the above considerations in all probability corresponds to that responsible for the second-order kinetics of catalyst decay observed in the cracking of small molecules on most catalysts. The ions formed in such reactions are probably too small and too simple to allow a significant rate of monomolecular elimination of saturated fragments to form the unsaturated site poisoning species. Rather, pairs of adjacent small ions seem to disproportionate and produce di-ions which stick to the surface and irreversibly deactivate two sites per event... 

From Eq. (3.19) it is apparent that the DF intensity varies quadratically with the triplet concentration and hence with excitation light intensity as long as the singlet state is deactivated by monomolecular decay. The DF intensity decays exponentially with a lifetime half of the phosphorescence lifetime rphos. 

Therefore immobilization of active centers on the supports is perhaps one possibility of diminishing the prevailing role of side reactions 4 and 5, and thereby of enhancing the efficiency of metal complex catalysts for polymerization of olefins. It was expected that spatial isolation of MX (as immobilization of enzymes prevented their deactivation) would lead to a decrease in bimolecular deactivation of active centers and in turn, to a cooperative stabilization preventing monomolecular termination. Instead, as earlier studies have shown (Fig. 12-6) [69] polymer-immobilized complexes are stable over time. Macromolecular metal complexes for polymerization processes can be used as powders, films, fiber... 

Heyden et al. suggested that hydrated and dehydrated monomolecular iron sites in Fe-ZSM-5 are responsible for N2O decomposition. They proposed that Z [FeO]+ is a key intermediate. Furthermore, water strongly adsorbs to give Z Fe(OH)2 "h This deactivates the Z [FeO]+ site. The activation energy for N2O decomposition in the presence of water increases steeply compared with the anhydrous situation, because water has to desorb from Z Fe(OH)2 in order for N2O reduction to occur. Hydration and subsequent dehydration of the oxy-iron complex may provide an alternative explanation for the oscillatory reaction found by El-Malki et al. shown in Fig. 4.28. If the reaction is not isothermal, the temperature fluctuations arising from the exothermic N2O decomposition reaction may lead to fluctuation in the water adsorption. This may provide an alternative explanation of the oscillatory kinetic behavior in the Fe +-ZSM-5 system. 

All the balances have accumulation, convection, axial dispersion, and reaction terms. The equations include liquid holdup, Bi, and superficial liquid velocity, w. Langmuir-type rate equation, for the main reaction, Equation 15.4, included also an activity correction term a. Kst and in Equations 15.5-15.7 indicate the adsorption parameters for stearic acid and heptadecene, respectively. Equation 15.4 corresponds to a monomolecular transformation of stearic acid via the adsorption of the reactant to the main product. Adsorption terms for stearic acid and heptadecene were used, since both of these compounds contain functional groups enabling adsorption on the active sites of the catalyst Reaction rates were assumed not to be limited by heptadecane adsorp-UoiL Thus, the adsorption term of heptadecane was n ected. In line with the experimental observations indicating catalyst deactivation. Equation 15.4 (Table 15.2) was modified to incorporate the decrease in catalyst activity. In particular, the activity was assumed... 

Next the activity of the catalyst has to be related to the amount of deactivating agent. This is done by means of exponential deactivation functions. Three deactivating functions were selected one for the effect of alkylation, one for the monomolecular steps and one for the bimolecular steps, as illustrated in Fig. 5.3.3.D-2. The deactivation parameter in the exponentials was derived from the experimental data. The evolution of the deactivating functions with the coke content is also shown. 

Likewise, monomolecular and bimolecular deactivation reactions have been described with the simple kinetic schemes described by Eqs. (12)-(14). 

In addition to monomolecular processes such as emission and radiationless deactivation there are very important bimolecular deactivation mechanisms... 

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