New Classes of Catalysts

Benefiting from the assistance of oxygen to simultaneously remove CO by the Prox reaction, oxygen-assisted WGS on bimetallic Cu-Pd/Ce02 reported by Bickford et al.33 have achieved an increased CO conversion from 95% (without oxygen) to more than 99.7% (with oxygen). The results were reported for a feed composition of 4 vol% CO, 2% 02, 40% H20, 10% C02, and 42% H2 at 210 °C and a GHSV of 17,760 IT1. 

A number of promising new catalysts have been reviewed recently by Ghenciu,6 Song,2 and Farrauto et al.7 Some of the most recent developments will be reviewed in the discussion below with special emphasis on catalyst performance compared with the classical HTS and LTS catalysts. 

There are some recent literature reports about new commercial catalysts designed to overcome some of the drawbacks associated with the traditional WGS catalysts. 

For example, a nonpyrophoric and proprietary catalyst has been developed by Engelhard as a replacement for the CuZn-based LTS catalyst. This catalyst also possesses features such as direct activation under process gas conditions, similar activity as the traditional catalyst, no safety and stability concerns upon air exposure, and good stability against temperature cycling and aging.39 

While some stability issues have been identified on the supported Au catalysts, a major progress was made by the discovery that Au/Ce02 showed remarkable performance stability.50 Among the catalysts with several Au loadings (Fig. 6.4), it was found that 3% Au showed the most stable operation over a 3-week period of tests due to an optimal ratio of surface Au and ceria sites. Evidence of strong metal-support interaction was correlated with the enhanced reducibility of ceria in the presence of Au nanoparticles. Further research into different catalyst preparation methods for Au/Ce02 showed that Au dispersion and the WGS activity are extremely sensitive to minor variations in the preparation procedure.51 

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Very recently, well-defined complexes with general formula [PdCl(T -Cp) (NHC)] were synthesised and tested for the homocoupling of non-electrodeficient arylboronic acids at room temperature with good results (Scheme 7.7) [51]- This new class of catalysts were synthesised from commercially available NHC palladium(II) chloride dimers and are air-stable. 

Liu YM, Cong PJ, Doolen RD, Guan SH, Markov V, Woo L, Zeyss S, Dingerdissen U. 2003. Discovery from combinatorial heterogeneous catalysis—a new class of catalyst for ethane oxidative dehydrogenation at low temperatures. Appl Catal A Gen 254 59 -66. 

A New Class of Catalyst for Cationic Polymerisations, W.R. Longworth and P.H. Plesch, Proceedings of the Chemical Society, 1958, 117. 

Most recently, Grubbs group demonstrated that some neutral salicylaldiminato nickel(II) complexes, whose skeleton structure appears as lb in Figure 1, show catalytic activities rivaling those of the bisimine complexes [9], This potentially opens the door to a new class of catalysts as the active sites derived from these nickel complexes are neutral, thus reducing the ion-pairing problems encountered in the current catalysts. 

C. Li, Sulfur-substituted and zinc-doped In(OH)3 A new class of catalyst for photocatalytic H2 production from water under visible light illumination, J. Catal. 237 (2006) 322-329. 

Based on the CCM system described above, an enantioselective version thereof was presented by Togni and coworkers a decade later [9]. The use of a new class of catalyst precursors led to substantially improved reactivity, and the system now was truly catalytic, with turnover frequency (TOP) values reaching up to 3.37 h at 348 K (Equation 6.5 Table 6.1). The absolute configuration of the resultant amine was determined by internal comparison of the X-ray crystal structure of the... 

Science Daily. New Class of Catalyst for Fuel Cells Beats Pure Platinum by a Mile. News release, October 24, 2007. Available online. URL http //www.sciencedaily.eom/releases/2007/l 0/071023164031. htm. Accessed May 28, 2009. The researchers have discovered an alloy of copper, cobalt, and platinum that exceeds pure platinum s catalytic activity for the reduction of oxygen. 

Group 10 transition metal catalysts including Ni and Pd are known as a new class of catalysts for the polymerization of substituted acetylenes, but the reports treating these catalysts are still not many. Some of the reports in an early stage displayed that the group 10 catalysts rather induce cyclic and linear oligomerizations of acetylene monomers. Thus, only fragmental information is available in some of the papers. 

Solid-liquid phase-transfer catalyst.1 The reagent represents a new class of catalysts, acyclic cryptands or tridents. It is singled out of a group as the best compromise of efficiency/price/toxicity. It solubilizes salts of alkali metals as well as of transition metals such as RuC13 and PdCl2, probably because of the flexibility of the molecule. In addition the trident is sensitive to the nature of the anion, but anionic activation is less than that obtained with cryptands. 

The Ln-BINOL derivative complexes are efficient asymmetric catalysts for Michael reactions and the epoxidations of enones. However, as was mentioned above, almost racemic products are obtained in the case of the asymmetric nitroaldol reaction of 2 with 12. For this transformation, a new class of catalysts, heterobimetallic species, have been developed. 

Chemical modification of the ALPOs is required to create a new class of catalysts. In the metalloaluminophosphates (MeALPOs), the framework contains metal (Me), aluminium and phosphorus. Thus, it becomes possible to produce a wide range of active catalysts with Lewis and Bronsted acid sites and redox properties by the partial replacement of Al3+ by Me2+ ions (e.g. Co, Cu, Mg, Zn, etc.) in an ALPO framework (Thomas, 1995 Martens etal, 1997). 

The integration of Lewis acidity with a catalytically active transition metal complex might lead to the development of a new class of catalysts with the possibility that these catalysts could outperform catalysts comprising only Lewis acids or only transition metal complexes. Thus, one of the key issues in successful catalytic transformation has been the generation of highly Lewis acidic metal centers that are catalytically active so that they can easily bind with a nucleophilic monomer to initiate the catalytic process. 

The use of thiazolium salts enables the benzoin condensation to proceed at room temperature. It can also be performed in dipolar aptotic solvents or under phase transfer conditions. Thiazolium salts such as vitamin Bi, thiazolium salts attached to y-cyclodextrin, macrobicyclic thiazolium salts, thiazolium carboxylate, ° naphtho[2,l-d]thiazolium and benzothiazolium salts catalyze the benzoin condensation and quaternary salts of 1-methylbenzimidazole and 4-(4-chlorophenyl)-4//-1,2,4-triazole are reported to have similar catalytic activity. Alkylation of 2-hydroxyethyl-4-methyl-l,3-thiazole with benzyl chloride, methyl iodide, ethyl bromide and 2-ethoxyethyl bromide yields useful salts for catalyzing 1,4-addition of aldehydes to activated double bonds. Insoluble polymer-supported thiazolium salts are catalysts for the benzoin condensation and for Michael addition of aldehydes. Electron rich al-kenes such as bis(l,3-dialkylimidazolidin-2-ylidenes) bearing primary alkyl substituents at the nitrogen atoms or bis(thiazolin-2-ylidene) bearing benzyl groups at the nitrogen atoms are examples of a new class of catalyst for the conversion of ArCHO into ArCHOHCOAr. 

Zeolite-supported metal clusters are a new class of catalyst made possible by syntheses involving organometallic chemistry and by precisely controlled treatment of metal complexes in zeolite cages. Elucidation of the preparation chemistry would not have been possible without the guidance of EXAFS spectroscopy. Clusters such as Ir4, Ir, and Pt (where n is about 6) are small enough to be considered quasi molecular rather than metallic. Their catalytic properties are distinct from those of metallic particles, even for structure-insensitive reactions. The zeolite pores seem to confer some properties on the clusters that are not yet well understood. 

With metallocene catalysts, not only homopolymers such as polyethylene or polypropylene can be synthesized but also many kinds of copolymers and elastomers, copolymers of cyclic olefins, polyolefin covered metal powders and inorganic fillers, oligomeric optically active hydrocarbons [20-25]. In addition, metallocene complexes represent a new class of catalysts for the cyclopolymerization of 1,5- and 1,6-dienes [26]. The enantio-selective cyclopolymerization of 1,5-hexadiene yields an optically active polymer whose chirality derives from its main chain stereochemistry. 

C. Venturello, R. D Alolslo, Quaternary ammonium tetrakis(diperoxotungsto)phosphates(3-) as a new class of catalysts for efficient alkene epoxidation with hydrogen peroxide, J. Org. Chem. 53 (1988) 1553. 

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