Recent Catalyst Development

In recent years, much work has been devoted to the search for alternative oxide catalysts for NHs CR, and from the characterization work included in these studies, conclusions regarding the active sites have often been drawn. The most promising systems are based on the elements manganese and cerium, which are often combined with other elements or deposited on supports (see Sect. 2.4). 

The extremely high activity of bulk manganese oxides, which provide very large reaction rates at temperatures below 470 K, can apparently arise from several oxidation states of Mn [144—146]. It was shown that Mn02 is the most active phase, however, its labile oxygen favors the oxidation of ammonia to N2O [146] while Mn203 offers significantiy better selectivity for N2 at lower reaction rates [145, 146]. The sizeable N2O formation over these catalysts and their low thermal stability has prompted efforts to stabilize their active sites on supports or by creation of mixed oxide phases. 

Carbon-supported Mn oxides show similar activities as unsupported samples. On the basis of XPS evidence, Yoshikawa et al. concluded that the Mn oxidation state on their surface is +3 [147]. At the same time, Grzybek et al. [148] were 

Mixed oxides of manganese with other elements, e.g., with Fe [162] often exhibit much better thermal stability and selectivity to N2 at only slightly diminished activity (compared with pure Mn oxides). The improved properties have been attributed to Mn-O-M (M=Cr, Fe, Ce) linkages [163-165], and thermally stable stoichiometric compounds (CrMni.504 [163], Fe3Mn30g [164]) have been proposed to be active in these mixed-oxide catalysts. The redox chemistry would then be distributed between two redox elements. 

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This survey focuses on recent catalyst developments in phosphoric acid fuel cells (PAFC), proton exchange membrane fuel cells (PEMFC), and the previously mentioned direct methanol fuel cell (DMFC). A PAFC operating at 160-220 °C uses orthophosphoric acid as the electrolyte the anode catalyst is Pt and the cathode can... 

The following Tables 14.1-14.5 present a list of recent catalyst developments for the Heck, Suzuki, Suzuki-Miyaura, Sonogashira, Negishi, and Kumada reactions. 

A process has been introduced (88) based specifically on improved selection and control of operating variables. Potentially the most important new developments have been in the area of reforming catalysts. Up to a few years ago, work in this area centered on metal distribution studies (95) and general optimization (51). This work has been covered in reviews (14, 94, 135). Very little has been published on more recent catalyst development although some papers have been presented (53, 81, 111, 128, 144). Available information on the use of such new catalysts points to greatly improved stability, particularly at severe conditions, and also to increased yields by reduction of hydrocracking. There is no indication that the use of such catalysts involves any basic changes in... 

Allylic substitution was initially conducted with palladium complexes lacking any added dative ligands or with palladium complexes containing classic aromatic phosphine ligands. More recently, catalyst development for this reaction has focused on the design and synthesis of new ligands for enantioselective allylic substitution processes. These ligands were presented as part of Section 18.6 on enantioselective allylic substitution. 

Similar to the first edition of this Handbook, contributions have been arranged into three volumes. Volume I (Anna Wenzel, coeditor) emphasizes recent catalyst developments and mechanism and is intended to provide a foundation for the applications discussed throughout the rest of the Handbook. Volume 11 (Dan O Leary, coeditor) covers synthetic applications of the olefin metathesis reaction, and polymer chemistry is the topic of Volume III (Ezat Khosravi, coeditor). Chapter topics have been selected to provide comprehensive coverage of these... 

This value, assumed by Zalc and Loffler, was rather low. Recent catalyst development work revealed operation of water-gas shift catalysts at 180 L (h gcat) and thus only 350 g catalyst would be required. However, catalyst coatings need to be applied to achieve such improved performance, which increases the utilisation of the porous catalyst structure. 

More recent catalyst developments have identified additional catalyst compositions that are not based on a metallocene complex or the Dow constrained geometry system. Nova Chemicals Inc. has most likely... 

An important recent catalyst development is related to the nondestructive framework substitution of Si for A1 in the Y zeolite, described above. Using this recently disclosed mild aqueous chemistry, silicon-enriched forms of the Y zeolite structure (called LZ-210) have been synthesized, greatly improving the thermal and hydrothermal stability of the Y crystal. Cracking catalysts prepared from these silicon-rich zeolites show substantially increased cracking activity retention, following severe steaming pretreatments. In addition. 

Today the sulphonation route is somewhat uneconomic and largely replaced by newer routes. Processes involving chlorination, such as the Raschig process, are used on a large scale commercially. A vapour phase reaction between benzene and hydrocholoric acid is carried out in the presence of catalysts such as an aluminium hydroxide-copper salt complex. Monochlorobenzene is formed and this is hydrolysed to phenol with water in the presence of catalysts at about 450°C, at the same time regenerating the hydrochloric acid. The phenol formed is extracted with benzene, separated from the latter by fractional distillation and purified by vacuum distillation. In recent years developments in this process have reduced the amount of by-product dichlorobenzene formed and also considerably increased the output rates. 

More recently, further developments have shown that the reaction outlined in Scheme 4.33 can also proceed for other alkenes, such as silyl-enol ethers of acetophenone [48 b], which gives the endo diastereomer in up to 99% ee. It was also shown that / -ethyl-/ -methyl-substituted acyl phosphonate also can undergo a dia-stereo- and enantioselective cycloaddition reaction with ethyl vinyl ether catalyzed by the chiral Ph-BOX-copper(ll) catalyst. The preparative use of the cycloaddition reaction was demonstrated by performing reactions on the gram scale and showing that no special measures are required for the reaction and that the dihydro-pyrans can be obtained in high yield and with very high diastereo- and enantioselective excess. 

A kinetic model which accounts for a multiplicity of active centres on supported catalysts has recently been developed. Computer simulations have been used to mechanistically validate the model and examine the effects on Its parameters by varying the nature of the distrlbultons, the order of deactivation, and the number of site types. The model adequately represents both first and second order deactivating polymerizations. Simulation results have been used to assist the interpretation of experimental results for the MgCl /EB/TlCl /TEA catalyst suggesting that... 

The methodologies described above lead to NHC precnrsors rather limited in terms of substitution at the four- and five-positions as their access is restricted to the accessibility of the appropriate diimine. As snch snbstitntions are of great interest in particular for the design of asymmetric catalysts, rontes to the synthesis of the NHC precursors have more recently been developed. Some of these approaches are described in Scheme 1.4. 

Effective catalysts have recently been developed for the addition of trialkyl-aluminum reagents to alkenes (carboalumination). 6 -(Pentamethylcyclopentadienyl) zirconium dimethylide activated by fra-(pentafluorophenyl)boron promotes the addition of trimethylaluminum to terminal alkenes.221... 

Considerable progress has been made on C02 fixation in photochemical reduction. The use of Re complexes as photosensitizers gave the best results the reduction product was CO or HCOOH. The catalysts developed in this field are applicable to both the electrochemical and photoelectrochemical reduction of C02. Basic concepts developed in the gas phase reduction of C02 with H2 can also be used. Furthermore, electrochemical carboxyla-tion of organic molecules such as olefins, aromatic hydrocarbons, and alkyl halides in the presence of C02 is also an attractive research subject. Photoinduced and thermal insertion of C02 using organometallic complexes has also been extensively examined in recent years. 

When a chiral ansa-type zirconocene/MAO system was used as the catalyst precursor for polymerization of 1,5-hexadiene, an main-chain optically active polymer (68% trans rings) was obtained84-86. The enantioselectivity for this cyclopolymerization can be explained by the fact that the same prochiral face of the olefins was selected by the chiral zirconium center (Eq. 12) [209-211]. Asymmetric hydrogenation, as well as C-C bond formation catalyzed by chiral ansa-metallocene 144, has recently been developed to achieve high enantioselectivity88-90. This parallels to the high stereoselectivity in the polymerization. 

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... 

Intermolecular enyne metathesis has recently been developed using ethylene gas as the alkene [20]. The plan is shown in Scheme 10. In this reaction,benzyli-dene carbene complex 52b, which is commercially available [16b], reacts with ethylene to give ruthenacyclobutane 73. This then converts into methylene ruthenium complex 57, which is the real catalyst in this reaction. It reacts with the alkyne intermolecularly to produce ruthenacyclobutene 74, which is converted into vinyl ruthenium carbene complex 75. It must react with ethylene, not with the alkyne, to produce ruthenacyclobutane 76 via [2+2] cycloaddition. Then it gives diene 72, and methylene ruthenium complex 57 would be regenerated. If the methylene ruthenium complex 57 reacts with ethylene, ruthenacyclobutane 77 would be formed. However, this process is a so-called non-productive process, and it returns to ethylene and 57. The reaction was carried out in CH2Cl2 un-... 

Chiral dirhodium(II) catalysts with carboxylate or carboxamidate ligands have recently been developed to take advantage of their versatility in metal carbene transformation, and these have now become the catalysts of choice for cyclopropanation. Chiral carboxylate ligands 195,103 196,104 and 197105 have been used for tetrasubstitution around a dirhodium(II) core. However, the enantioselectivity in intermolecular reactions with simple ketenes is marginal. 

This contribution is an in-depth review of chemical and technological aspects of the alkylation of isobutane with lightalkenes, focused on the mechanisms operative with both liquid and solid acid catalysts. The differences in importance of the individual mechanistic steps are discussed in terms of the physical-chemical properties of specific catalysts. The impact of important process parameters on alkylation performance is deduced from the mechanism. The established industrial processes based on the application of liquid acids and recent process developments involving solid acid catalysts are described briefly. 2004 Elsevier Inc. 

Finally, the catalytic enantioselective 1,3-dipolar cycloaddition reaction has recently been developed to be a highly selective reaction of nitrones with electron-deficient alkenes activated by chiral Lewis acids. High levels of regio-, diastereo-, and enantioselectivities can now be reached using catalysts 89 <2000JOC9080>, 90 <2002JA4968>, or 91 <2005JA13386> (Scheme 29). 

The recent enormous development in metathesis catalyst activity and stability has made ring-closing metathesis (RCM, Fig. 5) one of the most popular macrocyclization methods [38]. The enormous potential of this method lies in... 

At present, one of the most successful catalysts for enamine activation has been proline (2). Proline is a cheap, widely and commercially available amino acid that can be found in both enantiomeric forms and, as such, represents a remarkable synthetic alternative to many established asymmetric catalysts. Given such attractive features, it has become the catalyst of choice for many enamine-catalyzed processes. However, various more recent studies have demonstrated that proline is not a universal catalyst for transformations that involve the a-functionalization of ketone or aldehyde carbonyls. Indeed, these studies have demonstrated that the iminium catalysts developed by MacMillan (imidazolidinones) and Jprgensen (pyrrolidines) are also highly effective for enamine activation with respect to... 

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