Loss of Catalytic Activity

In selective poisoning or selective inhibition, a poison retards the rate of one catalysed reaction more than that of another or it may retard only one of the reactions. For example, there are poisons which retard the hydrogenation of olefins much more than the hydrogenation of acetylenes or dienes. Also, traces of sulphur compounds appear selectively to inhibit hydro-genolysis of hydrocarbons during catalytic reforming. 

A product of a reaction may cause poisoning or inhibition. The phenomenon is called selj-poisoning or autopoisoning. 

The conversion in a catalytic reaction performed under constant conditions of reaction often decreases with time of run or time on stream. This phenomenon is called catalyst deactivation or catalyst decay. If it is possible to determine the kinetic form of the reaction and, thus, to measure the rate constant for the catalytic reaction k, it is sometimes possible to express the rate of deactivation by an empirical equation such as 

Catalytic deactivation can sometimes be reversed and the original catalytic activity restored by some special operation called regeneration. For example, coked cracking catalyst is regenerated by burning off the coke (see 1.7.3, 1.9). 

If the catalytic reaction is a network of various processes, deactivation can lead to a change in the distribution of products. In such cases, the deactivation not only reduces the overall rate but it changes the selectivity. 

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Zeohtes have recendy been employed as soHd catalysts for the vapor-phase nitration of aromatics with nitric acid. Additional research is required to improve yields and to niinimi2e loss of catalytic activity as the nitration progresses (see Molecularsieves). 

The cationic aqua complexes prepared from traws-chelating tridentate ligand, R,R-DBFOX/Ph, and various transition metal(II) perchlorates induce absolute enantio-selectivity in the Diels-Alder reactions of cyclopentadiene with 3-alkenoyl-2-oxazoli-dinone dienophiles. Unlike other bisoxazoline type complex catalysts [38, 43-54], the J ,J -DBFOX/Ph complex of Ni(C104)2-6H20, which has an octahedral structure with three aqua ligands, is isolable and can be stored in air for months without loss of catalytic activity. Iron(II), cobalt(II), copper(II), and zinc(II) complexes are similarly active. 

Not only cationic, but also anionic, species can be retained without addition of specially designed ligands. The anionic active [FFPt(SnCl3)4] complex has been isolated from the [NEt4][SnCl3] solvent after hydrogenation of ethylene [27]. The PtCl2 precursor used in this reaction is stabilized by the ionic salt (liquid at the reaction temperature) since no metal deposition occurs at 160 °C and 100 bar. The catalytic solution can be used repeatedly without apparent loss of catalytic activity. 

Many other authors studied the catalytic activity of palladium in more complicated hydrogenation reactions because of being coupled with isomerization, hydrogenolysis, and dehydrogenation. In some cases the temperatures at which such reactions were investigated exceeded the critical temperature for coexistence of the (a + /3)-phases in the other case the hydrogen pressure was too low. Thus no hydride formation was possible and consequently no loss of catalytic activity due to this effect was observed. 

Reduction of trichloroethene to ethane took place in a modified fuel cell to which was introduced, although the loss of catalytic activity with time could present a serious limitation (Ju et al. 2006). 

Loss of catalytic activity resulting from internal displacements is not usually a serious problem below temperatures of about 100 C. However, highly active R-groups, such as benzyl, methyl and allyl, undergo internal displacement more readily, particularly in the presence of strong nucleopfiles. For instance, the presence phenolates and thiolates may lead to the formation of benzyl alcohol, ethers, or sulphides from benzyl-substituted quaternary ammonium salts. 

Kotschy et al. also reported a palladium/charcoal-catalyzed Sono-gashira reaction in aqueous media. In the presence of Pd/C, Cul, PPI13, and z -Pr2NH base, terminal alkynes smoothly reacted with aryl bromides or chlorides, such as 2-pyridyl chloride, 4-methylphenyl bromide, and so on, to give the expected alkyne products in dimethyl-acetamide (DMA)-H20 solvent. Wang et al. reported an efficient cross-coupling of terminal alkynes with aromatic iodides or bromides in the presence of palladium/charcoal, potassium fluoride, cuprous iodide, and triph-enylphosphine in aqueous media (THF/H20, v/v, 3/1) at 60°C.35 The palladium powder is easily recovered and is effective for six consecutive runs with no significant loss of catalytic activity. 

Later, Chung et al. successfully developed an intramolecular Pauson-Khand reaction in water without any cosolvent by using aqueous colloidal cobalt nanoparticles as catalysts. The catalyst was prepared by reducing an aqueous solution of cobalt acetate containing sodium dode-cyl sulfate (SDS) surfactant. The cobalt nanoparticle could be reused eight times without any loss of catalytic activity (Eq. 4.57).107... 

Partial hydrolysis of nitrile gives amides. Conventionally, such reactions occur under strongly basic or acidic conditions.42 A broad range of amides are accessed in excellent yields by hydration of the corresponding nitriles in water and in the presence of the supported ruthenium catalyst Ru(0H)x/A1203 (Eq. 9.19).43 The conversion of acrylonitrile into acrylamide has been achieved in a quantitative yield with better than 99% selectivity. The catalyst was reused without loss of catalytic activity and selectivity. This conversion has important industrial applications. 

Bimetallic zinc complexes formed with hexaazamacrocycles were studied in the hydrolysis of activated carboxyesters. Potentiometric titration demonstrated the dominant presence of a dinuclear hydroxo bridged species at pH >7. /)-Nitrophenyl acetate is hydrolyzed with no loss of catalytic activity for at least 2.7 catalytic cycles 4... 

Importantly, 1 can be heterogenized on NH2-functionalized supports such as xerogels, fibers, etc., and in this form used repeatedly without significant loss of catalytic activity (Figure 2). The supported Ce-POM can be separated from the reaction mixture by simple filtration. 

Thiopurine S-methyltransferase deficiency two nucleotide transitions define the most prevalent mutant allele associated with loss of catalytic activity in Caucasians. Am J Hum Genet 1996 58 694-702. 

Krynetski EY, Schuetz JD, Galpin AJ, Pui CH, Relling MV, Evans WE. A single point mutation leading to loss of catalytic activity in human thiopurine S-methyltransferase. Proc Natl Acad Sci USA 1995 92 949-953. 

Alkylated diphosphines (R,R)-(92) and (93) were used as chiral ligands in the Pt-catalyzed hydroformylations of some alkeneic substrates. These ligands bring about a loss of catalytic activity with respect to the corresponding diphenylphosphine homolog, particularly in the case of the platinum systems. The regioselectivity favors the straight-chain (or less branched) isomer in the case of terminal alkenes with the exception of styrene the enantioselectivity is very low in all cases.320... 

V.W.F. Chan, M.J. Bjerrum, and C.F. Borders, Jr, Evidence that chemical modification of a positively charged residue at position 189 causes the loss of catalytic activity of iron-containing and manganese-containing superoxide dismutases. Arch. Biochem. Biophys. 279,195-201 (1990). 

A high tube wall temperature also affects the performance of the catalyst. Higher temperatures lead to increased carbon lay-down on the catalyst and a resultant loss of catalytic activity, as well as potential catalyst breakage. Both... 

An effective catalyst recycling with no loss of catalytic activity was accomplished by removing the liquid phase via the liquid sampling valve and re-charging the autoclave with a solution containing the substrate. In all cases, no rhodium leaching occurred. Remarkably, the hydrogenation activity of the 1,3-bis-... 

Catalyst deactivation refers to the loss of catalytic activity and/or product selectivity over time and is a result of a number of unwanted chemical and physical changes to the catalyst leading to a decrease in number of active sites on the catalyst surface. It is usually an inevitable and slow phenomenon, and occurs in almost all the heterogeneous catalytic systems.111 Three major categories of deactivation mechanisms are known and they are catalyst sintering, poisoning, and coke formation or catalyst fouling. They can occur either individually or in combination, but the net effect is always the removal of active sites from the catalyst surface. 

Liu, Ruettinger, Farrauto, and coworkers—loss of catalytic activity during shutdown/restart cycles linked to formation of carbonates. Liu and coworkers435 at Engelhard reported on the loss of catalytic activity when Pt/ceria catalysts were exposed to shutdown/restart cycles. Utilizing DRIFTS spectroscopy, the authors... 

The complex can be stored under argon without noticeable loss of catalytic activity. It is weighed under an argon atmosphere. 

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