Catalyst Loading and Activation

The main properties of the catalyst used are given in Table 10.11. Hundred milliliter of catalyst was loaded into the catalytic basket and activated in situ by sulfiding with gas oil containing 1.6wt% DMDS at the following conditions 2mLf J(vaLc , h), 28kg/cm2, 2000 std fF Hj/bbl of oU, at tanperatures of 260°C and 320°C for 3 and 12 h, respectively. 

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Some important features of these models are summarized in the next section. However, it should be noted that a comprehensive quantitative mathematical model for PTC, accounting for the intrinsic kinetics of ion-exchange and main organic reaction, mass conservation of species, overall mass conservation, interphase and intraphase mass transfer, catalyst loading and activity, equilibrium partitioning of catalyst, location of reaction (organic phase, aqueous or solid phase, or interface), and flow patterns for each phase, are yet to be developed. 

The results given in Table 1 are expressed in T50 namely the temperature at which 50% conversion is reached. Table 1 also presents the number of exposed noble metal on each catalyst expressed in mmol/g of catalyst. As shown by this table, very important differences of activity are evidenced. It was thus not possible to compare all the catalysts at a common reaction temperature. Moreover, the important differences of activity do not allow any reliable comparison of turn over frequencies in this simple set of experiments. Nevertheless, it is still possible to interpret the activity as a function of the amount of metal exposed. The differences of activity between catalysts cannot be attributed to the combination of loading and dispersion. Indeed, Pd/C DP is more active with 3.3 exposed Pd/g of catalyst than Pd/C El with 3.8 exposed Pd/g of catalyst. In contrast. Figure 1 shows that an obvious relationship exists between the catalyst loading and activity expressed in T50. This correlation indicates that the catalytic activity is directly proportional to the amount of Pd deposited on the support for loadings superior to 1 wt%. It also shows that the... 

Because the Group VIA and Group VIIIA metals are most conveniently prepared as oxides, a sulfiding step is necessary. That will be discussed in Section 7 (Catalyst Loading and Activation). 

Typical platinum catalyst loadings needed to support the anodic and cathodic reactions are currently 1 to 2 mg/cm" oi active cell area. Owing to the cost of platinum, substantial efforts have been made to reduce the catalyst loading, and some fuel cells have operated at a catalyst loading of 0.25 mg/cm". 

Preformed complexes of type 52, 53 and 59, and in situ catalyst systems based on IMesX (X = CO, MeS03H, HCl, HBr, HI) and IPrHCl, have been tested for telomerisation of butediene with primary and secondary amines. Under optimised conditions, and low catalyst loadings, excellent activities and selectivities were observed [82]. 

Addition of a strong acid snch as methanesnlfonic acid (MSA) to the reaction mixture has a positive impact on the reactivity, as shown in Figure 3.8. The induction time is shortened by 10 minutes and the reaction rate almost doubled. Due to the reaction rate increase from the acid addition, the catalyst loading could be lowered. In addition, the hydrogen pressnre conld be donbled to rednce the reaction time by half. However, improvements from addition of acid and pressure increase are not sufficient to make this process commercially viable because the catalyst loading and the TOF are significantly lower than the criteria listed in Table 3.n. Therefore, we initiated a search for catalysts more active than Et-DnPhos-Rh catalyst. 

The evolving structural characteristics of CLs are particularly important for further analysis of transport of protons, electrons, reactant molecules (O2), and water as well as for the distribution of electrocatalytic activity at Pt-water interfaces. In principle, the mesoscale simulations allow relating these properties to the choices of solvent, ionomer, carbon particles (sizes and wettability), catalyst loading, and hydration level. Explicit experimental data with which these results could be compared are still lacking. Versatile experimental techniques have to be employed to study particle-particle interactions, structural characteristics of phases and interfaces, and phase correlations of carbon, ionomer, and water in pores. 

As previously discussed, activation of the iridium-phosphoramidite catalyst before addition of the reagents allows less basic nitrogen nucleophiles to be used in iridium-catalyzed allylic substitution reactions [70, 88]. Arylamines, which do not react with allylic carbonates in the presence of the combination of LI and [Ir(COD)Cl]2 as catalyst, form allylic amination products in excellent yields and selectivities when catalyzed by complex la generated in sim (Scheme 15). The scope of the reactions of aromatic amines is broad. Electron-rich and electron-neutral aromatic amines react with allylic carbonates to form allylic amines in high yields and excellent regio- and enantioselectivities as do hindered orlAo-substituted aromatic amines. Electron-poor aromatic amines require higher catalyst loadings, and the products from reactions of these substrates are formed with lower yields and selectivities. 

Complex 2 efficiently catalyzes the cycloaddition reaction of methacrolein with the nitrones I-V. Table 1 lists some results obtained. The reactions were performed in CH2CI2 in the presence of 4 A molecular sieves, with 5 mol% of catalyst loading and a 1/140/20 catalyst/methacrolein/nitrone molar ratio. Typically, quantitative yields are obtained after a few hours at —25°C. The acyclic nitrone 11 generates the less active system but, even so, 78% conversion was achieved after 24 h at —10°C (entry 2). Enantiomeric excesses greater than 90% were achieved in most cases. A greater excess of methacrolein improves both rate and enantioseletivity (compare entry 6 with a catalyst/methacrolein/nitrone molar ratio 1/28/20 with entry 3 with a 1/140/20 molar ratio). To avoid undesired nitrone coordination, addition of the cyclic nitrones III-V was accomplished over 10 h. 

This chapter exemplifies how the development of highly active catalysts and creative reaction design have revitalized the interest in Stetter reactions as a valuable tool for efficient C-C-bond-forming reactions. The next challenges to be met in this field are the reduction of the catalyst loading and the catalytic asymmetric intermolecular Stetter reactions. 

It was concluded that the catalyst lifetime is a function of the catalyst loading and does not relate to its way of synthesis [321]. With larger loadings, catalysts are active for a long time before they need reactivation. With regard to lifetime and activity, the four catalysts were ranked as follows wet impregnation incipient wetness > UV decomposition of precursors > sputtering. In case of loss of performance, two... 

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