Optimal catalyst loading

Finally, an important trade-off with obtaining high S/C ratios is that of time. In early development it is often more desirable to have reactions complete in a short and reproducible timeframe, rather than to optimize catalyst loading. For larger scale manufacture, especially for markets other than pharmaceuticals, obtaining the best S/C ratio will be of greater importance. 

Optimal catalyst loading, at uniform Nafion content, is depicted in Figure 4.35. As can be seen, optimization requests to increase catalyst loading at the membrane by a factor of 10 as compared to the reference uniform value (Figure 4.35a). However, the effect of this strongly nonuniform catalyst distribution is not large, the cell current density increases by 10% only (Figure 4.35b). 

A comparison of the optimal shapes of electrolyte content for PEFC and DMFC (Figures 4.34a and 4.36a) shows that these shapes are very similar. Thus, it is advisable to optimize catalyst loading by taking the optimal shape of electrolyte loading in Figure 4.36a. This procedure can be considered as a first iteration step in the process of simultaneous optimization of electrolyte and catalyst loadings. 

Nonetheless, the simple model above clearly indicates the trends it prescribes that both Nafion and catalyst loadings should grow toward the membrane surface. The optimal Nafion content is almost linear, while the optimal catalyst loading is parabolic-like (Figure 4.39). Calculations show that even non-optimal shapes of similar types improve the CL performance, that is, real shapes could always be chosen in accordance with structural limitations. 

Once the procedure for the preparation of thin catalyst film is optimized, one has to optimize catalyst loading. This is done by measuring ORR current for different Al and estimating mass activity of the catalyst (/k,mass) at a given potential (selection made using the same guidelines as forAESA) as ... 

Sigman et al. have optimized their system too [45]. A study of different solvents showed that the best solvent was f-BuOH instead of 1,2-dichloroethane, which increased the conversion and the ee. To ensure that the best conditions were selected, several other reaction variables were evaluated. Reducing the catalyst loading to 2.5 mol % led to a slower conversion, and varying temperature from 50 °C to 70 °C had little effect on the selectivity factor s. Overall, the optimal conditions for this oxidative kinetic resolution were 5 mol % of Pd[(-)-sparteine]Cl2, 20 mol % of (-)-sparteine, 0.25 M alcohol in f-BuOH, molecular sieves (3 A) at 65 °C under a balloon pressure of O2. 

Several catalysts were prepared and tested for their hyam activity. Nnmerons preparation methods were investigated. Catalysts prepared nsing the method that provided the most active catalysts were nsed in this stndy. The aim was to see how varying the carbon support, Pd loading and modifier addition would affect the activity, selectivity and filterability of the catalyst in hopes of identifying the optimal catalyst for the hyam reaction (activity greater than 25 g hyam/g Pd, selectivity > 90% and fast filtration rate). 

Allyl methylcarbonate reacts with norbornene following a ruthenium-catalyzed carbonylative cyclization under carbon monoxide pressure to give cyclopentenone derivatives 12 (Scheme 4).32 Catalyst loading, amine and CO pressure have been optimized to give the cyclopentenone compound in 80% yield and a total control of the stereoselectivity (exo 100%). Aromatic or bidentate amines inhibit the reaction certainly by a too strong interaction with ruthenium. A plausible mechanism is proposed. Stereoselective CM-carboruthenation of norbornene with allyl-ruthenium complex 13 followed by carbon monoxide insertion generates an acylruthenium intermediate 15. Intramolecular carboruthenation and /3-hydride elimination of 16 afford the -olefin 17. Isomerization of the double bond under experimental conditions allows formation of the cyclopentenone derivative 12. 

In addition to the W and Mo carbonyl complexes that have most commonly been used for the cycloisomerization of alkynols, an Rh-based catalyst system has recently been developed which uses substantially lower catalyst loadings (1.5-2.5 mol%) than have typically been required for the W and Mo systems (10-50 mol%).369 Among the various ligands studied, P(/>-F-C6H4)3 proved to be particularly effective. Interestingly, this ligand has also been found to be optimal for an Ru system that catalyzes the same type of cycloisomerization (Equation (104)).370,371... 

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