Ageing process Alkyl

After a short induction period, the activity of polymerization increases as a function of the monomer concentration. After reaching a maximum, a continuous decrease is observed, due to fast aging processes as alkyl exchange, hydrogen transfer, and reduction of the transition-metal species occur in parallel with catalytic polymerization. There exist today a number of different conceptions concerning the nature of the center of polymerization activity. However, the formation of a complex between the titanium and the aluminum components is now accepted by all workers. 

Fink [23]. It was shown that, after a short induction period, the activity of polymerization reaches a maximum, then decreases continuously, due to fast aging processes such as alkyl exchange, hydrogen transfer, and reduction of the transition metal species. 

In the case of organophosphate poisoning, the effectiveness of 2-PAM is dependent on its early administration. The phosphorylated AChE tends to convert to a form that cannot be reactivated by the oximes. This phenomenon is known as aging process. It involves the removal of an alkyl group from the dialkyl phosphorylated enzyme, producing an anionic phosphate moiety that is insensitive to oximes and other nucleophiles (Figure 7.17) (O Brien, 1976). 

An analysis of the aging process in the alkylation of benzene with ethylene over a rare earth-exchanged X zeolite (REX) (43) provides insight into the physicochemical phenomena operative. In this reaction, a catalyst lifetime of over 792 hours was observed at 204°, benzene ethylene molar ratio of 5, and pressure of 500 psig. Under similar conditions, but at atmospheric pressure, a drastic increase in the rate of catalyst aging was observed, with catalytic activity lasting only 2 to 3 hours. Subsequent mechanistic studies showed that most of the catalyst aging in this reaction could be attributed to complex, temperature-dependent side reactions of ethylene (44,44a). 

The ageing process consists of the monodealkylation of the AChE-nerve agent complex. The loss of an alkyl group produces a conformational change that results in the formation of a very stable agent-enzyme complex which is then resistant to spontaneous hydrolysis and reactivation by oximes. The rate of ageing is dependent... 

IR spectroscopy was used to follow the ageing processes of two types of epoxy resins. Resin samples were exposed to ionising radiation and the effects at the molecular level were determined. Aromatic amine cured resins were oidy slightly affected by the radiation, but those cured by alkyl diamines were more sensitive and thus absorbed more water molecules (41). 

FIGURE 57.7 Aging of the DFP phosphorylated NTE protein. The aging process is a dealkylation reaction. In the case of AChE, the alkyl group is released to the medium as the corresponding alcohol. This represents the case of NTE with DFP in which the isopropyl group remains attached to the protein and is released to the medium only after an alkaline treatment. The nature of the "Z" site remains unknown. 

The major problem of the application of zeolites in alkane-alkene alkylation is their rapid deactivation by carbonaceous deposits. These either strongly adsorb on acidic sites or block the pores preventing the access of the reactants to the active sites. A further problem is that in addition to activity loss, the selectivity of the zeolite-catalyzed alkylation also decreases severely. Specifically, alkene formation through oligomerization becomes the dominant reaction. This is explained by decreasing ability of the aging catalyst to promote intermolecular hydride transfer. These are the main reasons why the developments of several commercial processes reached only the pilot plant stage.356 New observations with Y zeolites reconfirm the problems found in earlier studies.358,359... 

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