Irreversible deactivation

Deactivation of zeolite catalysts occurs due to coke formation and to poisoning by heavy metals. In general, there are two types of catalyst deactivation that occur in a FCC system, reversible and irreversible. Reversible deactivation occurs due to coke deposition. This is reversed by burning coke in the regenerator. Irreversible deactivation results as a combination of four separate but interrelated mechanisms zeolite dealu-mination, zeolite decomposition, matrix surface collapse, and contamination by metals such as vanadium and sodium. 

Enzyme activity generally passes through a maximum as the pH of the system in question is varied. However, the optimum pH varies with substrate concentration and temperature. Provided that the pH is not changed too far from the optimum value corresponding to the maximum rate, the changes of rate with pH are reversible and reproducible. However, if the solutions are made too acid or too alkaline, the activity of the enzyme may be irreversibly destroyed. Irreversible deactivation is usually attributed to denaturation of the proteinaceous enzyme. The range of pH in which reversible behavior is observed is generally small and this... 

However, residuum hydrotreating catalysts themselves are susceptible to irreversible deactivation caused by the accumulation of sulfided metal impurities. The gradual buildup of these impurities in the pores of a hydroprocessing catalyst causes plugging and deactivation. 

In this context the lipase was immobilized on a support which also adsorbed water and propionic acid. During the reaction, the water caused a decrease of the reaction rate. While the water adsorption on the catalyst results in a reversible decrease of the enzyme activity, an excessive accumulation of water in the bulk mobile phase resulted in rapid irreversible deactivation of the enzyme. 

Natural gas feedstock is very dependent of the source location in some cases it has high levels of H2S, CO2 and hydrocarbons. Organic sulfur compounds must be removed because they will irreversibly deactivate both reforming and WGS catalysts. Hence a preliminary feed desulfurization step is necessary. This process consists in a medium-pressure hydrogenation (usually on a cobalt-molybdenum catalyst at 290-370 °C), which reduces sulfur compounds to H2S, followed by H2S separation through ZnO adsorption (at 340-390 °C) or amine absorption [9]. 

It was previously thought that 5-FU inhibits the enzyme by classical competitive inhibition. However, it was found that 5-FU is a transition-state substrate, and it forms a covalent complex with tetrahydrofolate and the enzyme in the same way that the natural substrate does. The reaction, however, will not go to completion, since the fluoro-uridine derived from the antimetabolite remains attached to the enzyme, and the latter becomes irreversibly deactivated. Recovery can occur only through the synthesis of new enzyme. Fluorouracil is used in the treatment of breast cancer and has found limited use in some intestinal carcinomas. Unfortunately, this drug has the side effects usually associated with antimetabolites. Its prodrug, fluorocytosine (8.35, which is also an antifungal agent) is better tolerated. 

In this review the reactions terminating a growing chain are denoted as chain-terminating processes. We refer to the chain-terminating process with a reinitiation reaction as chain transfer, and to the process with an irreversible deactivation of propagating centers as chain termination. 

The two-state model does not capture irreversible deactivation effects which lead to decrease of enzyme activity over time. To capture irreversible decrease in enzyme activity, we build on the two-state model and include an irreversible first-order deactivation term (U — D) from U to a new state, the (irreversibly) inactivated state, designated D [Eq. (17.12)]. 

In a classic study on bovine pancreatic ribonuclease A at 90°C and pH conditions relevant for catalysis, irreversible deactivation behavior was found to be a function of pH (Zale, 1986) at pH 4, enzyme inactivation is caused mainly by hydrolysis of peptide bonds at aspartic acid residues as well as deamidation of asparagine and/or glutamine residues, whereas at pH 6-8, enzyme inactivation is caused mainly by thiol-disulfide interchange but also by fi-elimination of cystine residues, and deamidation of asparagine and/or glutamine residues. 

Nexium, A.85) is a single enantiomer of the chiral sulfoxide. Compounds A.83 through A.87 are prodrugs and rearrange to form an active species that irreversibly deactivates proton pumps. 

Introduction of external chemical contamination from the sampling devices into the sample should be avoided because binding of metals to biological molecules is possible in vitro, and this can change the distribution of metal-containing species in the sample. Heavy metal contaminants could cause protein precipitation as well as irreversible deactivation of enzymes. 

The problem associated with zeolites as nitration catalysts will be a reversible deactivation by coke deposition, and an irreversible deactivation by framework A1 removal (acid leaching). Optimization of zeolite activity, selectivity and life will be controlled by density of acid sites, crystalline size and hydrophobic/hydrophilic surface properties. 

Irreversible deactivation can have a similar effect on the hydrothermal deactivation by deteriorating coke selectivity (for instance for nickel poisoning). The hydrothermal deactivation on its turn will now also have an effect on the catalyst poisons, as for instance on the mobility of vanadium and on the deactivation of vanadium and nickel as dehydrogenation catalysts [2]. 

The reductive conditions during catalyst pretreatment may cause an irreversible deactivation as a result of particle growth and site coverage especially at high pH. 

For a sufficiently reducing reactant like ethanol both reversible and irreversible deactivation can be neglected. For MGP however unable to keep the catalyst in a low oxidation state particle growth occurs most probably via an Ostwald-ripening mechanism resulting in an irreversible decrease of the platinum surface area exposed. 

---