Catalyst tolerance

In contrast with the AFC, the PAFC can demonstrate reliable operation with 40 percent to 50 percent system efficiency even when operating on low quality fuels, such as waste residues. This fuel flexibility is enabled by higher temperature operation (200°C vs. 100°C for the AFC) since this raises electro-catalyst tolerance toward impurities. Flowever, the PAFC is still too heavy and lacks the rapid start-up that is nec-essaiy for vehicle applications because it needs preheating to 100°C before it can draw a current. This is unfortunate because the PAFC s operating temperature would allow it to thermally integrate better with a methanol reformer. 

The alumina content, the amount of rare-earth, and the type and amount of zeolite affect catalyst tolerance to vanadium poisoning. 

The order of reactivity of these three catalysts towards alkenes (but also towards oxygen) is 1 > 3 > 2. As illustrated by the examples in Table 3.18, these catalysts tolerate a broad spectrum of functional groups. Highly substituted and donor- or acceptor-substituted olefins can also be suitable substrates for RCM. It is indeed surprising that acceptor-substituted alkenes can be metathesized. As discussed in Section 3.2.2.3 such electron-poor alkenes can also be cyclopropanated by nucleophilic carbene complexes [34,678] or even quench metathesis reactions [34]. This seems, however, not to be true for catalysts 1 or 2. 

Thus, in order to determine the processability of petroleum a series of consistent and standardized characterization procedures are required (ASTM, 1995). These procedures can be used with a wide variety of feedstocks to develop a general approach to predict processability. The ability to predict the outcome of feedstock (especially heavy oils and residua) processing offers (1) the choice of processing sequences (2) the potential for coke lay-down on the catalyst (3) determining the catalyst tolerance to different feedstocks (4) predictability of product distribution and quality and (5) incompatibility during processing and incompatibility of the products on storage. 

The Schrock catalysts are more active and are useful in the conversion of sterically demanding substrates, while the Grubbs catalysts tolerate a wide variety of functional groups. 

Table V, Entries 5 and 6). The catalyst tolerates both sulfur and nitrogen substituents (Table V, Entries 2, 4, and 8). Protected (1-ami no-alcohols are smoothly converted into the corresponding aldehydes without detectable racemisation (Table V, Entries 4 and 8) (28). It is also noteworthy that the reaction conditions are sufficiently mild as to be compatible with the Boc protecting group (Table V, Entry 8). 

Catalysts for ethylene/carbon monoxide copolymerisation were initially obtained from Ni(II) derivatives, such as K2Ni(CN)4 and (w-Bu4N)2 Ni(CN)4, and Pd(II) derivatives, such as [(w-Bu3P)PdCl2]2, Pd(CN)2 and HPd(CN)3, often combined with alcohol or protonic acid as a cocatalyst [241]. It must be emphasised that, in contrast to titanium-, zirconium- or vanadium-based catalysts, nickel- and palladium-based catalysts tolerate polar functional groups (including hydroxyl, carboxylic and sulfonic groups)... 

Whereas these solid catalysts tolerate water to some extent, or even use aqueous H2O2 as the oxidant, the use of homogeneous Ti catalysts in epoxi-dation reactions often demands strictly anhydrous conditions. The homogeneous catalysts are often titanium alkoxides, possibly in combination with chiral modifiers, as in the Sharpless asymmetric epoxidation of allylic alcohols (15). There has recently been an increase in interest in supporting this enantioselective Ti catalyst. 

O Neill and coworkers recently introduced the Co(dpm)2 catalyst 319 to make the silyl peroxidation more effective (entry 7) [385]. It gives consistently higher yields (68-90%) than Co(acac)2 (5-80%). The catalyst tolerates free alcohol functions well. 

Methanol synthesis resembles that of ammonia in that high temperatures and pressures are used to obtain high conversions and rates. Improvements in catalysts allow operation at temperatures and pressures much lower than those of the initial commercial processes. Today, low-pressure Cu-Zn-Alminium oxide catalysts are operated at about 1500 psi and 250°C. These catalysts must be protected from trace impurities that the older high-pressure (5000 psi and 350°C) and medium-pressure (3000 psi and 250°C) catalysts tolerate better. Synthesis gas production technology has also evolved so that it is possible to maintain the required low levels of these trace impurities. 

Variation of the Procatalyst (Metal Component) and the Acetylenic Substrate. The in situ catalysts Co(acac)3-Et2AlCl-phosphine have proven to be well-suited for the synthesis of 4-aryl- and 4-alkyl-substituted deltacyclenes. The catalysts tolerate remote oxygen functionalities in the acetylenic substrate. However, they could not be used with functionalized acetylenes such as propargylic acid derivatives. 

Platinum compounds and complexes are the most important and commonly used catalysts for hydrosilylation processes [7 - 9]. Platinum catalysts tolerate a variety of functional groups, but some impurities may interact with them leading to catalyst poisoning [10]. This has stimulated much research aimed at employing other transition metal compounds as potential catalysts. For example, Rh(I) complexes are selective and active hydrosilylation catalysts [11] and more resistant to poisoning than the platinum ones [12]. 

A. Furstner and co-workers also showed that RCAM is indeed a very mild method, because during their stereoselective total synthesis of prostaglandin E2-1,15-lactone, the Mo[N-(f-Bu)(Ar)3]-derived catalyst tolerated a preexisting double bond and a ketone functionality. Chromatographic inspection of the reaction mixture revealed that no racemization took place before or after the ring closure, and the ee of the substrate and the product were virtually identical. 

Rare-earth metal catalysts tolerate a variety of aprotic functional groups for example, sterically encumbered mesityl sulfonamides sufficiently shield the oxygen from the metal center in order to allow hydrosilylation/carbocyclization, albeit at a reduced rate (20) [72],... 

Chloroarenes were efficiently hydrodechlorinated with a [RhCl2HL2] (L = PCy3 or Pz Pr3) catalyst in biphasic systems under mild conditions [267], The catalyst tolerates the presence of a variety of functional groups (R, OR, CF3, COAr, COOH, NH2). Some chloro heterocycles (e.g. 5-chloro-l-ethyl-2-methylimidazole) can be readily dehalogenated, but 2-chlorotiophene does not react at all. 

The catalysts tolerate carboxyl functionality, so a variety of acrylates have been copolymerized with ethylene using the a-diimine catalysts these acrylates include CH2=CHC02R (where R = H, Me, Et, f-Bu, CH2CH2OH, CH2CH2(CF2)9CF3, OCH2(CF2)e-CF3, or —(CH2)2SiCl3). Comonomers such as... 

Two years later, Marko et al. reported an improved catalytic system which only required 0.25 equivalent of potassium carbonate instead of 2 equivalents (89). The oxidation reaction described above is dramatically influenced by the nature of the solvent. Thus, if the reaction was performed in fluorobenzene, total conversion of undecan-2-ol to undecan-2-one could be reached with 0.25 equivalent K2CO3, whereas 2 equivalents of base were necessary in toluene to convert 90% of this secondary aliphatic alcohol (Table VI). These optimized conditions were applied to a variety of functionalized alcohols and the results are reported in Table VII. The catalyst tolerates both sulphur and nitrogen substituents on the substrate. Indeed, (thiophen-2-yl)methanol, N-protected (S)-valinol or (S)-prolinol could be oxidized to the corresponding aldehydes with very good yields. In addition, no racemization was detected for the two P-amino alcohols as well as for (2S,5i )-2-isopropyl-5-methylcyclohexanol. The hindered endo- and exo-borneol are both converted to camphor with similar reaction rates, despite their distinctly different steric properties. 

The cyclization-dimerisation of allenylketones 29 to furans 28 and/or 2,4-disubstituted furans 30 is well known. By making the choice of either PdCl2(MeCN)2 or rac-31 as catalyst, the allenylketones 29 could be preferentially converted into 28 or 30. Since the catalyst tolerates several functional groups (terminal alkynes, a-halogenketones, alkyl halides) this catalyst is an important improvement of Marshall s Ag(l)-catalyzed isomerisation of 29 to 28 <97CB1449 97CB1457>. 

An electronically and sterically diverse array of arylboronic acids serve as useful reaction partners (Table 2, entries 1-4). In addition, vinylboronic acids can be cross-coupled in good yield (entry 5), although reactions of alkylboronic acids proceed with somewhat lower efficiency (entry 6). The catalyst tolerates a range of functional groups, including esters, thioethers, and cyanides. 

---