Catalyst in additions

Fluorides. Tantalum pentafluoride [7783-71-3] TaF, (mp = 96.8° C, bp = 229.5° C) is used in petrochemistry as an isomerization and alkalation catalyst. In addition, the fluoride can be utilized as a fluorination catalyst for the production of fluorinated hydrocarbons. The pentafluoride is produced by the direct fluorination of tantalum metal or by reacting anhydrous hydrogen fluoride with the corresponding pentoxide or oxychloride in the presence of a suitable dehydrating agent (71). The ability of TaF to act as a fluoride ion acceptor in anhydrous HF has been used in the preparation of salts of the AsH, H S, and PH ions (72). The oxyfluorides TaOF [20263-47-2] and Ta02F [13597-27-8] do not find any industrial appHcation. 

A significant problem is the dehydrocoupling reaction, which proceeds only at low yields per pass and is accompanied by rapid deactivation of the catalyst. The metathesis step, although chemically feasible, requires that polar contaminants resulting from partial oxidation be removed so that they will not deactivate the metathesis catalyst. In addition, apparendy both cis- and /ra/ j -stilbenes are obtained consequendy, a means of converting the unreactive i j -stilbene to the more reactive trans isomer must also be provided, thus complicating the process. 

One of the advantages of the thermal recuperative oxidizer is that it is possible to process organics that may be a poison or be detrimental to catalyst. In addition, if the organic concentration is very high, for example the organic level is of the 20 to 25% LEL, then thermal recuperative oxidation is appropriate. 

In 1990, Jacobsen and subsequently Katsuki independently communicated that chiral Mn(III)salen complexes are effective catalysts for the enantioselective epoxidation of unfunctionalized olefins. For the first time, high enantioselectivities were attainable for the epoxidation of unfunctionalized olefins using a readily available and inexpensive chiral catalyst. In addition, the reaction was one of the first transition metal-catalyzed... 

The wide applicability of the PK reaction is apparent in the synthesis of pyrroles, for example, 45, en route to novel chiral guanidine bases, levuglandin-derived pyrrole 46, lipoxygenase inhibitor precursors such as 47, pyrrole-containing zirconium complexesand iV-aminopyrroles 48 from 1,4-dicarbonyl compounds and hydrazine derivatives. The latter study also utilized Yb(OTf)3 and acetic acid as pyrrole-forming catalysts, in addition to pyridinium p-toluenesulfonate (PPTS). 

The benzylic position of an alkylbcnzene can be brominated by reaction with jV-bromosuccinimide, and the entire side chain can be degraded to a carboxyl group by oxidation with aqueous KMnCfy Although aromatic rings are less reactive than isolated alkene double bonds, they can be reduced to cyclohexanes by hydrogenation over a platinum or rhodium catalyst. In addition, aryl alkyl ketones are reduced to alkylbenzenes by hydrogenation over a platinum catalyst. 

Esters are usually prepared from carboxylic acids by the methods already discussed. Thus, carboxylic acids are converted directly into esters by SK2 reaction of a carboxyfate ion with a primary alkyl halide or by Fischer esterification of a carboxylic acid with an alcohol in the presence of a mineral acid catalyst. In addition, acid chlorides are converted into esters by treatment with an alcohol in the presence of base (Section 21.4). 

Sodium or potassium phenoxide can be carboxylated regioselectively in the para position in high yield by treatment with sodium or potassium carbonate and carbon monoxide. Carbon-14 labeling showed that it is the carbonate carbon that appears in the p-hydroxybenzoic acid product. The CO is converted to sodium or potassium formate. Carbon monoxide has also been used to carboxylate aromatic rings with palladium compoimds as catalysts. In addition, a palladium-catalyzed reaction has been used directly to prepare acyl fluorides ArH —> ArCOF. ... 

Under the conditions used in this study, the catalytic activities were stable for NO reduction for all catalysts. However, in NOj reduction, deactivation was observed. For catalyst 1-7, there was a rapid, reversible deactivation that was more noticeable at lower temperatures. The activity could be restored by removing propene from the feed. Therefore, it was likely due to carbonaceous deposits on the catalyst. In addition, there was slow deactivation. For example, afto the experiment in Table 2 and cleaning in a flow ofN0/O2/H20 (0. l%/4.7%/1.5%, balance He) at SOOT, the catalyst showed an NO conversion of 33% and propene conversion of 42% at 450°C, versus 53 and 99%, respectively, before deactivation. For catalyst 1-5, only slow deactivation was observed. 

Abstract The use of A-heterocyclic carbene (NHC) complexes as homogeneous catalysts in addition reactions across carbon-carbon double and triple bonds and carbon-heteroatom double bonds is described. The discussion is focused on the description of the catalytic systems, their current mechanistic understanding and occasionally the relevant organometallic chemistry. The reaction types covered include hydrogenation, transfer hydrogenation, hydrosilylation, hydroboration and diboration, hydroamination, hydrothiolation, hydration, hydroarylation, allylic substitution, addition, chloroesterification and chloroacylation. 

The separation challenge is often thought of as simply separating the product from the catalyst. In addition, one must separate the catalyst from the product. Precious metal losses with product are desirably in the low parts per billion (ppb). High value specialty products may economically permit significantly higher metal losses. 

The aim of this work was to apply combined temperature-programmed reduction (TPR)/x-ray absorption fine-structure (XAFS) spectroscopy to provide clear evidence regarding the manner in which common promoters (e.g., Cu and alkali, like K) operate during the activation of iron-based Fischer-Tropsch synthesis catalysts. In addition, it was of interest to compare results obtained by EXAFS with earlier ones obtained by Mossbauer spectroscopy to shed light on the possible types of iron carbides formed. To that end, model spectra were generated based on the existing crystallography literature for four carbide compounds of... 

Not only phosphines or phosphites but also phosphoric acid trisdialkyl-amides (40), sulfoxides (41), etc. have been used as electron donors in the preparation of the catalyst. In addition, the catalytic activity of tetra-methylcyclobutadienenickel dichloride and alkylaluminum halides has been studied in detail (42, 43). 

These telomerization reactions of butadiene with nucleophiles are also catalyzed by nickel complexes. For example, amines (18-23), active methylene compounds (23, 24), alcohols (25, 26), and phenol (27) react with butadiene. However, the selectivity and catalytic activity of nickel catalysts are lower than those of palladium catalysts. In addition, a mixture of monomeric and dimeric telomers is usually formed with nickel catalysts ... 

Considering that these two transition-metal complexes are the only ones reported for the electrochemical cofactor reduction, the results are quite promising and show the need for further research in this field to identify new catalysts. In addition to the use of soluble redox mediators in electrochemical cofactor regeneration, modified electrodes have also been used. Details on these systems can also be found in the above-mentioned reviews [31, 32]. 

The examples illustrate the strong points of XRD for catalyst studies XRD identifies crystallographic phases, if desired under in situ conditions, and can be used to monitor the kinetics of solid state reactions such as reduction, oxidation, sulfidation, carburization or nitridation that are used in the activation of catalysts. In addition, careful analysis of diffraction line shapes or - more common but less accurate-simple determination of the line broadening gives information on particle size. 

Infrared spectroscopy can be considered as the first important modem spectroscopic technique that has found general acceptance in catalysis. The most common application of infrared spectroscopy in catalysis is to identify adsorbed species and to study the way in which these species are chemisorbed on the surface of the catalyst. In addition, the technique is useful in identifying phases that are present in precursor stages of the catalyst during its preparation. Sometimes the infrared spectra of adsorbed probe molecules such as CO and NO give valuable information on the adsorption sites that are present on a catalyst. 

The catalyst, the way it is operated, is about 25% faster than the Monsanto rhodium catalyst. In addition, it was assumed that the oxidative addition is no longer rate-determining and that now the migration of the methyl group to the co-ordinated carbon monoxide is rate-determining (Figure 6.3). 

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