Various Catalysts

By analogy with hydroformylation, dicobalt octacarbonyl has been examined as a hydrosilylation catalyst. Various silanes and a-olefins react, often exothermically. Thermal deactivation occurs above 60° C hence, large exotherms and high temperatures must be avoided (56, 57,130). Isomerization is more pronounced than for the bridged olefin complexes of Pt(II) and Rh(I) (see below) it even occurs with trialkoxysilanes (57). Though isomerization is faster than hydrosilylation, little variation in the relative rates of these two processes with the nature of the silane is observed this is in marked contrast to the bridged systems (55). 

Electro-catalysts which have various metal contents have been applied to the polymer electrolyte membrane fuel cell(PEMFC). For the PEMFCs, Pt based noble metals have been widely used. In case the pure hydrogen is supplied as anode fuel, the platinum only electrocatalysts show the best activity in PEMFC. But the severe activity degradation can occur even by ppm level CO containing fuels, i.e. hydrocarbon reformates[l-3]. To enhance the resistivity to the CO poison of electro-catalysts, various kinds of alloy catalysts have been suggested. Among them, Pt-Ru alloy catalyst has been considered one of the best catalyst in the aspect of CO tolerance[l-3]. 

In real systems (hydrocarbon-02-catalyst), various oxidation products, such as alcohols, aldehydes, ketones, bifunctional compounds, are formed in the course of oxidation. Many of them readily react with ion-oxidants in oxidative reactions. Therefore, radicals are generated via several routes in the developed oxidative process, and the ratio of rates of these processes changes with the development of the process [5], The products of hydrocarbon oxidation interact with the catalyst and change the ligand sphere around the transition metal ion. This phenomenon was studied for the decomposition of sec-decyl hydroperoxide to free radicals catalyzed by cupric stearate in the presence of alcohol, ketone, and carbon acid [70-74], The addition of all these compounds was found to lower the effective rate constant of catalytic hydroperoxide decomposition. The experimental data are in agreement with the following scheme of the parallel equilibrium reactions with the formation of Cu-hydroperoxide complexes with a lower activity. 

One of the most important difficulties in the use of homogeneous catalysis for technical applications is the recovery and reuse of the expensive metal catalyst. Various concepts for catalyst recychng are apphed to overcome this problem in current technical applications. A simple method is to perform the catalytic reaction in a single phase and to separate the product and the catalyst afterwards by distillation of the product, as reahzed, for... 

Double-bond isomerization reaction of simple olefins requires strong basic catalysts. Various catalyst systems have been reported for this reaction. They include sodium-organosodium catalysts prepared in situ by reacting an excess of sodium with a reactive organic compound, such as o-chlorotoluene or anthracene as reported by Pines and co-workers 5-8). 

In contrast to aryl trichloromethyl ethers (see Section 12.1.2.), the replacement of chlorine atoms by fluorine in aryl trichloromethyl sulfides using antimony(III) fluoride (or hydrogen fluoride) proceeds under milder conditions, without a catalyst. Various substituents in the aromatic ring have little effect on the halogen exchange. The yields of aryl trifluoromethyl sulfides are 60 to 90% (see Table 1). 

Ru and Pt complexes also catalyse the enyne metathesis. Ru and Pt catalysts were found to be more versatile catalysts than the Pd catalysts. PtCF is the most active and versatile catalyst without ligands [148], With this simple catalyst, various 1,6-enynes... 

The low reaction rates usually associated with the MBH reaction can be increased either by pressure [15a, 22, 34], by the use of ultrasound [35] and micro-wave radiation [14a], or by the addition of co-catalysts. Various intra- or inter-molecular Lewis acid co-catalysts have been tested [26, 36, 37] in particular, mild Bronsted acids such as methanol [36, 57d], formamide [38], diarylureas and thioureas [39] and water [27a, 40] were examined and found to provide an additional acceleration of the MBH reaction rate (Table 5.1). 

The transesterification with secondary alcohols was also reported by the same group [72], By utilizing l,3-dicydohexyl-imidazolin-2-ylidene (ICy, 11) as catalyst, various alcohols 87 (including aliphatic cyclic and aromatic alcohols) reacted with diverse esters 88 to yield the desired esters 89 (Scheme 9.26). The steric bulk at the a position of the alcohol reduces the reaction rates, and longer reaction times are required hence, the reaction proceeds slowly (5 days) with tertiary alcohols (1-adamantanol) and requires higher catalyst loadings (20 mol%) to yield moderate yields (54%). 

Some of the polybutadienes obtained with transition metal-based coordination catalysts have practical significance the most important is cA-1,4-polybutadiene, which exhibits excellent elastomeric properties. As regards isoprene polymers, two highly stereoregular polyisoprenes, a cA-1,4 polymer (very similar to natural rubber) and a trans- 1,4-polymer (of equal structure to that of gutta percha or balata) have been obtained with coordination catalysts. Various polymers of mixed 3,4 structure, amorphous by X-ray, were also obtained [7]. 

Microreactor technology offers the possibility to combine synthesis and analysis on one microfluidic chip. A combination of enantioselective biocatalysis and on-chip analysis has recently been reported by Beider et al. [424]. The combination of very fast separations (<1 s) of enantiomers using microchip electrophoresis with enantioselective catalysis allows high-throughput screening of enantioselective catalysts. Various epoxide-hydrolase mutants were screened for the hydrolysis of a specific epoxide to the diol product with direct on-chip analysis of the enantiomeric excess (Scheme 4.112). 

Besides Urushibara Ni and Ni boride catalysts, various finely divided nickel particles have been prepared by reaction of nickel salts with other reducing agents, such as sodium phosphinate 20,85 alkali metal/liquid NH3 21 NaH-f-AmOH (designated Nic) 22,86Na, Mg, and Zn in THF or Mg in EtOH 24 or C8K(potassium graphite)/THF-HMPTA (designated Ni-Grl) 23,87 Some of these have been reported to compare with Raney Ni or Ni borides in their activity and/or selectivity. 

The catalytic activities of TiSiWijO iOj in synthesizing n-amyl cinnamate are reported. It has been demonstrated that TiSiWi204i/Ti02 is an excellent catalyst. Various factors concerned in this reaction have been investigated. The optimum conditions have been found. That is, the molar ratio of alcohol to acid is 4 1 the mass ratio of the catalyst used to the reactants is 2.0 %, the reaction temperature 122-126 °C, and the reaction time 1.5 h. Under this condition, the yield of n-amyl cinnamate is 93.3 %. 

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