ChemCatChem

The cover picture shows a filmstrip illustrating the bright future of cellulosic biomass as a sustainable feedstock for producing valuable chemicals and biofuels. We are, however, still far away from cost-effective, large-scale catalytic applications and the state-of-the-art is still frequently hampered by the notorious recalcitrance of the polymeric biomolecules. In their Minireview on p. 82 ff., Van de Vyver et al. make an attempt to capture the wide range of recent developments in this area, and they speculate on possible future directions with a primary focus on the role of heterogeneous catalysis.

Oxidation into CO₂ is a major solution to CO abatement in air depollution treatments. Due to the increasing price of noble metals and to remarkable progresses in oxide syntheses, catalytic oxidation of carbon monoxide over oxide catalysts has recently gained in interest. In this Review, the kinetics and mechanism of CO oxidation on single and mixed oxides are examined, alongside the catalyst structures.

Industrial zeolite and magic: The accessibility of zeolites (see figure) is of paramount importance to capitalize on their effectiveness in industrial catalysis. The variety of synthesis strategies proposed today for the preparation of ‘hierarchical’ zeolite materials combining micro- and mesoporosity is discussed.

Shift happens: Concerns about the depletion of fossil fuel reserves and the environmental problems related to its use have encouraged the development of new catalytic approaches for producing carbon-based chemicals from cellulosic biomass. Here, recent progress in the implementation of (mainly) heterogeneous acid, bifunctional, and multistep catalysis is reviewed.

Clusterophilia: Iridium offers a range of catalytic properties and forms numerous compounds with small, stable frameworks, including those with a tetrahedral Ir₄ framework (see figure) and those with an octahedral Ir₆ framework. Supported iridium clusters show catalytic activity in reactions such as ethylene hydrogenation. This Minireview discusses the synthesis, chemistry, and catalytic properties of Ir₄ and Ir₆ clusters.

Amine for the top: A biocatalyst able to perform transamination of less hindered ketones is modified to transform a sterically demanding ketone substrate to the corresponding optically pure amine. This goal is achieved by rational analysis and design of the catalyst's structure as well as other techniques of directed evolution. The biocatalyst is successfully adapted to the process and not vice versa.

Size matters: Particle size is an important factor in determining catalytic activity. The low sintering rate of ceria-supported transition metal particles maintains smaller particle sizes and thereby also catalytic performance. This sintering resistance was recently found to result from strong metal–support interactions with defects and terraces on the ceria surface.

You can call me alkyne: The tri(tert-butoxy)silanolate-supported tungsten benzylidyne complex [PhCW{OSi(OtBu)₃}₃] efficiently catalyzes alkyne cross-metathesis and ring-closing alkyne metathesis under vacuum-driven or molecular sieve-promoted conditions. The addition of 5 Å molecular sieves boosts the performance of this catalytic system.

Supports illustrated: Supported gold catalysts are shown to be active and selective for the alkylation of aromatics using benzyl alcohol for the first time. The catalysts are prepared using an ionic liquid deposition method which leads to alkylation reactions, whereas the conventional wet impregnation leads predominantly to oxidation products.

The magnificent neutral: Secondary amines are cyclized into the corresponding scalemic pyrrolidines and piperidines with in situ-prepared chiral alkylamido ate and neutral yttrium and ytterbium catalysts. The enantiomeric excess of 83 % ee is the highest reported value for a rare-earth-promoted hydroamination of a secondary amine.

Everything's gone green: The selective oxidation of aromatic alcohols to aldehydes has been achieved by use of a green energy source and solvent combination. Solar energy with a recyclable Pt/TiO₂ photocatalyst in water at ambient temperature brought about the desired oxidation with high selectivity and relatively long-term stability.

Immobilization for enantioselectivity: Covalent tethering of a rhodium(I) phosphine complex on the surface of highly ordered mesoporous silica nanoparticles (MSN) gives rise to the RhPMSN/(−)- cinchonidine catalytic system, which gives 50 % ee in the hydrogenation of ethyl pyruvate. In contrast, no enantioselectivity is detected in the hydrogenation of ethyl pyruvate catalyzed by the very similar homogeneous system [RhCl(PPh₃)₃]/(−)-cinchonidine.

Direct and to the point: The simple precatalyst system FeCl₃/TMEDA has been effectively applied to cross-coupling reactions of various arylmagnesium bromides and alkenyl bromides. With activated aryl bromides and in the absence of polar functional groups, the method can be modified to allow for domino iron-catalyzed magnesiation–cross-coupling reactions.

Raman knows: UV resonance Raman spectroscopy, performed under a controlled atmosphere, is able to highlight extra-framework iron species, both before and after interaction with NO probe molecule. Furthermore, this work demonstrates that resonance Raman spectra are complementary to those obtained by IR spectroscopy.

Nitrogen, Inc.: A novel nitrogen-incorporated TS-1 zeolite is prepared by direct calcination of the as-synthesized TS-1 powder in NH₃ flow at high temperature. By introducing nitrogen into the framework of TS-1 zeolite, the acidic sites of TS-1 are reduced effectively and the material shows high catalytic activity and stability for epoxidation of propylene with dilute hydrogen peroxide.

Ultra magnets: Magnetic CuFe₂O₄ nanoparticles have been simply prepared and applied as a catalyst in the CO cross-coupling of phenols and aryl halides to form diaryl ethers, an important structural motif in many products, industrial polymers, and biologically active compounds. The catalyst is recycled without a significant loss of catalytic activity.

How bizarre: There are two different reaction paths towards formation of ethylene oxide (EO); in addition to that via an oxometallacycle (OMC) intermediate, silver oxide-catalyzed epoxidation of ethylene with surface oxygen, which can occur on an oxide phase as long as no oxygen vacancies are present, is possible. This path is established computationally, by using density functional theory.

Fullerenes (C₆₀and C₇₀) were recently shown to be active in the hydrogenation of nitrobenzene, after activation to form the fullerene anion. However, the catalyst in this reaction is shown to contain residual nickel from the activation of the catalyst, which, as indicated by a series of control experiments, is most likely responsible for the catalytic conversion.

Länder the Fried, home of the MOF: Basolite C 300, a metal–organic framework exhibiting a hard Lewis acid character, is found to be the first efficient MOF catalyst for the Friedländer reaction between 2-aminobenzophenones and acetylacetone under mild reaction conditions, leading to the corresponding quinolines with excellent yields, opening the door towards its utilization for analogous condensations leading to useful biologically active compounds.

Enhancing the dispersion: Strong Au–C₆₀ interaction with the charge transfer from Au nanoparticles to C₆₀ in Au/C₆₀/SiO₂ has been proven to suppress the agglomeration of supported Au nanoparticles and thus enhance their dispersion on SiO₂ (see figure). and, consequently, their catalytic activity for CO oxidation has been improved.

A break from the norm: Abnormally C4-bound N-heterocyclic carbene palladium complexes (see picture) are effective precursors for the catalytic hydrogenation of a variety of olefins. Detailed mechanistic investigations indicate that catalyst activation involves oxidative H₂ addition and subsequent imidazolium reductive elimination as pictured, thus providing a heterogeneously operating catalyst.

Three is a magic number: Propylene oxide (PO) is an important and versatile intermediate used in the production of a large variety of valuable consumer products. The trimetallic catalyst RuO₂–CuO_x–NaCl/SiO₂ gives PO selectivities of 40–50 % at propylene conversions of 10–20 % in the temperature range 240–270 °C and at atmospheric pressure.

Cymene says: The structural characteristics, stability, and activity of (VO)₂P₂O₇ and VO(PO₃)₂ catalysts under liquid phase p-cymene oxidation reaction are elucidated. The structure of VO(PO₃)₂ catalyst remains unchanged whereas (VO)₂P₂O₇ undergoes structural transformation during the extended recycling usage, which affects its catalytic performance.

A novel continuous method for the preparation of a ternary Cu/ZnO/Al₂O₃ catalyst for use in methanol synthesis has been developed, and is based on a cascade of micromixers and a tubular aging reactor (see figure). Advantages over the conventional batch process led to an area-related activity about 50 % higher than that of a commercial catalyst.

Fables of the reconstruction: The nature and structure of highly-dispersed metallic clusters on oxide supports strongly depends on the environment. First-principles calculations coupled with thermodynamics show that Pt₁₃ clusters deposited on γ-alumina are very sensitive to H₂ pressure. High pressure induces a reconstruction from a biplanar to a cuboctahedral morphology, with formation of a surface hydride.

For methanol-to-olefin conversion in H-ZSM-5, theoretical simulations provide evidence that the ‘alkene cycle’ offers a viable path to the production of both propene and ethene, in contrast to the failing direct mechanisms. Combined with earlier work on polymethylbenzenes as active hydrocarbon pool molecules, it is clear that, in H-ZSM-5, multiple parallel and interlinked routes operate on a competitive basis.

Gene genius: Congeners of lipase from Thermoanaerobacter thermohydrosulfuricus containing globally substituted methionine, proline, phenylalanine, and tyrosine with related synthetic analogues (see examples in figure) have special features not found in nature. These features include changes in substrate accessibility and tolerance, as well as optimal temperature, pH, and enzymatic activity profiles.

Plastic fantastic: The possibility of attaining 100 % selectivity in the phenylacetylene hydrogenation reaction (at room temperature) on palladium nanoparticles supported on a porous poly(4-ethylstyrene-co-divinylbenzene) polymer is demonstrated by using in situ FTIR spectroscopy, facilitated by the infrared transparency of the porous polymer support.

Caught in a trap: The heteropolyacids H₃[P(M₃O₁₀)₄] (PMA; M=Mo, W) have been successfully entrapped within a metallic silver matrix. They represent the first inorganic-metal PMA@Ag composite catalysts that have been fully characterized by PXRD, SEM, TEM and EDX measurements. The systems show an intriguingly high activity and regioselectivity for the catalytic alkylation of arenes using 1-bromo- and 1-chloroadamantane.

Springer, New York, 2010. 526 pp., hardcover, € 171.15.—ISBN 978-0-387-98041-6

Wiley-VCH, Weinheim, 2010. 490 pp., hardcover, € 119.00.—ISBN 978-0-470-48759-4