Cocatalysts enhanced activity

Cocatalysts enhance overall average activity primarily by accelerating the development of polymerization rate and to a lesser degree by raising the maximum rate attained [52,684,686-688]. The induction time is shortened (and usually eliminated), and the polymerization rate then rises to its maximum more quickly. The more rapid development of activity in the presence of cocatalyst suggests that active sites are reduced, and even initiated, more quickly. The higher rate suggests that some sites are created that would not have become active otherwise. 

M0CI5 and WCl6 alone can induce the polymerization of various monosubstituted acetylenes. The use of a suitable organometallic cocatalyst enhances the catalytic activity. With these catalysts, polymer molecular weight is low or medium (<10 ) for 1-hexyne and phenylacetylene but reaches one million for sterically crowded monomers like t-butylacetylene and or /2o-substituted phenylacetylenes. 

Well defined high valent Schrock type carbene complexes of Mo or W with imido ligands are nowadays favoured as very active and selective catalysts for Ring opening Metathesis Polymerization (ROMP) and Acyclic Diene Metathesis Condensation (ADMET). Low valent Fischer type carbene complexes are less active and often need cocatalysts to enhance activities [1]. 

Molybdenum and rhenium oxide eatalysts based on siliceous mesoporous sieves and OMA, respectively, proved enhanced catalytic activity in comparison witii corresponding catal) using conventional supports. The origin of tiiis enhanced activity is not completely clear and is still a subject of discussions and continuous research. The better accessibility of catalysts site located in mesopores undoubtedly represents tire essential contribution to the increased catalytic activity. Rhenium (VII) oxide on organized mesoporous alumina preserves known tolerance of Re catalysts to the polar-substituted olefins. The presence of cocatalysts like Mc4Sn is essential similarly as for conventional systems. However, tire catalysts with higher pore size were found to deliver better results. 

Organic Lewis bases are widely used as electron donors in the third generation supported Ziegler-Natta catalysts for alpha-olefin polymerization to enhance activity and to improve stereospecificity. These catalysts comprise a solid catalyst, MgCl2/TiCl4/electron donor (-ED), and a cocatalyst, an aluminium alkyl usually complexed with an electron donor. 

In order to enhance the activity of coordination catalysts we typically add a cocatalyst. The cocatalyst works synergistically with the catalyst to allow us to tailor the tacticity and molecular weight of the product while also enhancing the rate of the reaction. An example of a commercially used cocatalyst is methylaluminoxane used in conjunction with metallocene catalysts. 

Large R2 substituents induce effective ion-separation between the cationic active species and an anionic cocatalyst, which allows more space for ethylene coordination to the metal and for its insertion into the carbon-metal bond. In addition, electronically, the ion separation increases the electrophilicity of the catalytically active species and hence enhances the reactivity toward ethylene. 

Often Lewis acids are added to the system as a cocatalyst. It could be envisaged that Lewis acids enhance the cationic nature of the nickel species and increase the rate of reductive elimination. Indeed, the Lewis acidity mainly determines the activity of the catalyst. It may influence the regioselectivity of the catalyst in such a way as to give more linear product, but this seems not to be the case. Lewis acids are particularly important in the addition of the second molecule of HCN to molecules 2 and 4. Stoichiometrically, Lewis acids (boron compounds, triethyl aluminium) accelerate reductive elimination of RCN (R=CH2Si(CH3)3) from palladium complexes P2Pd(R)(CN) (P2= e g. dppp) [7], This may involve complexation of the Lewis acid to the cyanide anion, thus decreasing the electron density at the metal and accelerating the reductive elimination. 

Palladium complexes exhibit even higher catalytic activity and produce branched acids preferentially.132 133 The selectivity, however, can be shifted to the formation of linear acids by increasing the phosphine concentration.134 Temperature, catalyst concentration, and solvent may also affect the isomer ratio.135 Marked increase in selectivity was achieved by the addition of Group IVB metal halides to palladium136 and platinum complexes.137 Linear acids may be prepared with selectivities up to 99% in this way. The formic acid-Pd(OAc)2-l,4-bis(diphe-nylphosphino)butane system has been found to exhibit similar regioselectivities.138 Significant enhancements of catalytic activity of palladium complexes in car-bomethoxylation by use of perfluoroalkanesulfonic acid resin cocatalysts was reported.139,140... 

In new studies heteropoly acids as cocatalysts were found to be very effective in combination with oxygen in the oxidation of ethylene.1311 Addition of phosphomo-lybdic acid to a chloride ion-free Pd(II)-Cu(II) catalyst system results in a great increase in catalytic activity and selectivity.1312 Aerobic oxidation of terminal alkenes to methy ketones can be performed with Pd(OAc)21313 or soluble palladium complexes. Modified cyclodextrins accelerates reaction rates and enhance selectivities in two-phase systems under mild conditions.1315 1316... 

The selectivity in the formation of 1,4-diacetoxy-2-butene (1,4-DAB) is considerably enhanced when tellurium compounds are used as cocatalysts. Thus a heterogeneous catalyst, prepared by impregnation of Pd(N03)2 and Te02 dissolved in HN03 over active charcoal (Pd/Te = 10), can be used for the oxidation of butadiene (by 02 in AcOH at 90 °C) to 63% trans-l,4-DAB, 25% cis-1,4-DAB and 12% 3,4-diacetoxy-l-butene. Conventional soluble catalysts such as Pd(OAc)2/Li(OAc) are much less selective in the formation of 1,4-DAB 429 The gas-phase 1,4-diacetoxylation of butadiene in the presence of Pd-Te catalysts is currently being industrially developed by Mitsubishi and BASF 430... 

The deposition of noble metals (e.g.. Ft, Rh) or metal oxides (e.g., NiO, RUO2) onto photocatalyst surfaces is an effective way of enhancing photocatalyst activity (Sato and White, 1980 Subramanian et al., 2001). The cocatalyst improves the efficiency of photocatalysts, as shovm in Figure 15, as a result of (i) the capture of CB electrons or VB holes from the photocatalysts (Maruthamuthu and Ashokkumar, 1988), thereby reducing the possibility of electron-hole recombination and (ii) the transference... 

The standard electrode potentials are far more anodic than that of one-electron transfer process, -0.284 V (SHE) and the visible-light photocatalytic activity of platinum-loaded tungsten(VI) oxide could be interpreted by enhanced multiple-electron transfer process by deposited platimun (45), since it is well known that platinum and the other noble metals catalyze such multiple-electron transfer processes. Similar phenomena, cocatalyst promoted visible-light photocatalytic activity, have been reported with palladium 46) and copper oxide (47). Thus, change of reaction process seems beneficial to realize visible-light photocatalytic activity. 

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