Randomly distributed catalysts

Zeidan et al. [16] incorporated arylsulfonic acid and alkylthiol groups into mes-oporous silica by a cocondensation method, where 2-(4-chlorosulfonylphenyl)-ethyl-trimethoxysilane and 3-mercaptotrimethoxysilane were added to tetraethyl orthosilicate, forming a mesoporous silica (SBA-15) material via a sol-gel process (Fig. 23.4a). The resulting catalysts exhibited far greater activity and selectivity than materials containing only the acid. These randomly distributed catalysts achieved in the best case a per site yield (defined as moles of product per mole of sulfonic acid) of 82 and a 95% selectivity for bisphenol A, compared to a per site yield of 18 and 58% selectivity for bisphenol A when using homogeneous sulfonic acid alone. 

Dining the last couple of years CdS-containing Nafion membranes have been apphed for the photocleavage of H2S . They are not comparable with the monograin membranes because the CdS particles are at randomly distributed in a rather thick Nafion membrane. This technique is attractive for some applications because the semiconductor particles are immobilized . On the other hand, problems may arise because of diffusion problems in the nafion membrane. Mainly the photoassistol Hj-formation at CdS was investigated in the presence of a Pt-catalyst and with coprecipitated ZnS CdS without a catalyst . 

In atactic polymers, side groups are irregularly positioned on either side of the chain, as illustrated schematically in Fig. 1.8 c). A truly atactic polymer would comprise a random distribution of steric centers. In practice, atactic polymers typically show some preference for either meso or racemic placement The tendency towards stereoregularity is due to the fact that polymerization catalysts often contain steric centers, which tend to direct the incoming monomers and the growing chain into preferred configurations. 

This (R)-polyBINAP support, when treated with [Rh(cod)2]BF4 or [ RuC12(C6H6) 2]. afforded a heterogeneous asymmetric catalyst for hydrogenation of dehydroaminoacid derivatives and ketones.100 As an extension, a copolymer with BINOL and BINAP units randomly distributed... 

However, the technique suffers the drawback that in real catalyst systems, particles are randomly distributed over the support and thus will not all be in the correct orientation for diffraction into the angular collection range subtended by the objective aperture, and that the smaller clusters diffract too weakly to be detected against the support. A further difficulty is that random superposition of atoms in amorphous support materials, such as charcoal or silica can give rise to "speckle" which may be easily confused with small catalyst clusters. (16,13). 

For typical catalyst layers impregnated with ionomer, sizes of hydrated ionomer domains that form during self-organization are of the order of 10 nm. The random distribution and tortuosity of ionomer domains and pores in catalyst layers require more complex approaches to account properly for bulk water transport and interfacial vaporization exchange. A useful approach for studying vaporization exchange in catalyst layers could be to exploit the analogy to electrical random resistor networks of... 

Double alkoxides with alkali metals A[M(OR)6] (Table 8) were formed from the reaction of the pentaalkoxides with alkali metal alkoxides. The double alkoxides of Mg, Ca, Sr and Ba with Nb and Ta have been synthesized in the presence of MgCl2 as a catalyst.169 Refluxing M(OPri)5 and A(OPri)3 (A = A1 or Ga) in isopropyl alcohol afforded double isopropoxides of the type [MA(OPr )8J and [MA2(OPr1)n]170. [NbTa(OMe)10] appears to be the first mixed transition metal alkoxide isolated.171 NMR showed it to be in dynamic equilibrium with the symmetrical M2(OMe)i0 dimers in solution, with close to random distribution of the three species. 

Studies with sulfated zirconia promoted with Pt309 and industrial chlorinated Pt on AI2O3 isomerization catalysts310 led to the same conclusion, namely, the intermolecular mechanism operative for M-butane isomerization. A significant difference, however, is that on the industrial catalysts extensive hydride and methyl shifts taking place in the intermediates prior to P scission do not lead to a random distribution of the labels. Instead, a binomial distribution with one and three 13C atoms is observed.310 This is indicative of the involvement of the 31 carbocationic intermediate. 

Assuming a random distribution of the catalyst in a matrix, the bimolecular decomposition distance was estimated to be almost contact distance between molecules. 

When two catalyst molecules are needed for the 4-electron water oxidation, the catalytic activity shows an optimum catalyst concentration in the matrix since bimolecular decomposition still takes place, and cooperative distance between catalyst can be determined also by assuming a random distribution. 

When Ru-red was used as a catalyst in the presence of a large excess of Ce(IV) oxidant (Scheme 19.1), the rate of 02 evolution was first order with respect to the catalyst concentration, showing that Ru-red is capable of 4-electron oxidation of water. By the decomposition of the Ru-red, N2 was formed its formation rate was second order with respect to the catalyst concentration, showing that the decomposition is bimolecular. The decomposition distance in a polymer (Nafion) matrix was estimated by assuming the random distribution of the catalyst molecule in the matrix. The probability density P(r) of the distance between the nearest neighbor molecules (r nm) is represented by Eq. (19.5) according to the Poisson statistics... 

Secondary metathesis reactions are sometimes encountered during metathesis copolymerization, leading to a reshuffling of the units in the chain and eventually to a random distribution for example in the copolymerization of 248 and 258 using RUCI3 as catalyst, statistical copolymers are produced no matter whether the monomers are mixed initially or added sequentially576. See also the copolymers of 128 Section Vm.B.6. 

Enantioselective hydrolysis of p-nitrophenyl-N-(benzylocycarbonyl)-L-leucinate compared to its D-isomer could also be performed with a MIP catalyst built up with a racemic template [55]. In that case, the enantiodiscrimination is insured by the presence of r-His monomer during the polymerization process, performed under finely tuned conditions. The random distribution of quaternary trimethyl-ammonium groups through the polymer framework makes this MIP very soluble in water. The MIP-catalyzed ester hydrolysis was performed in a mixture of... 

Figure 8.4 Typical variation of the standard NMR parameter, %c, as a function of the variable of percolation e the different states of gelation were obtained by quenching the polymer in ice, at different times during the kinetics of synthesis of the network which occurs at 150 °C. The polymer is a slightly modified silicone chain which bears randomly distributed vinyl groups as comonomeric units (the mean number of vinyl groups along one chain is Cvi = 2 x 10"3). Links are created between vinyl and methyl groups the three symbols correspond to different catalyst concentrations (redrawn from [18])...

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