Robert Raja

Molecular-sieve catalysts for the selective oxidation of linear alkanes by molecular oxygen

Terminally oxidized hydrocarbons are of considerable interest as potential feedstocks for the chemical and pharmaceutical industry, but the selective oxidation of only the terminal methyl groups in alkanes remains a challenging task. It is accomplished with high efficiency and selectivity by some enzymes; but inorganic catalysts, although inferior in overall performance under benign conditions, offer significant advantages from a processing standpoint. Controlled partial oxidation is easier to achieve with ‘sacrificial’ oxidants, such as hydrogen peroxide, alkyl hydroperoxides oriodosylbenzene, than with molecular oxygen or air. These sacrificial oxidants, themselves the product of oxidation reactions, have been used in catalytic systems involving tailored transition-metal complexes in either a homogeneous state, encapsulated in molecular sieves or anchored to the inner surfaces of porous siliceous supports. Here we report the design and performance of two aluminophosphate molecular sieves containing isolated, four-coordinated Co(III) or Mn(III) ions that are substituted into the framework and act, in concert with the surrounding framework structure, as regioselective catalysts for the oxidation of linear alkanes by molecular oxygen. The catalysts operate at temperatures between 373 K and 403 K through a classical free-radical chain-autoxidation mechanism. They are thus able to use molecular oxygen as oxidant, which, in combination with their good overall performance, raises the prospect of using this type of selective inorganic catalyst for industrial oxidation processes.

Exploiting nanospace for asymmetric catalysis: Confinement of immobilized, single-site chiral catalysts enhances enantioselectivity

In the mid-1990s, it became possible to prepare high-area silicas having pore diameters controllably adjustable in the range ca. 20-200 Å. Moreover, the inner walls of these nanoporous solids could be functionalized to yield single-site, chiral, catalytically active organometallic centers, the precise structures of which could be determined using in situ X-ray absorption and FTIR and multinuclear magic angle spinning (MAS) NMR spectroscopy. This approach opened up the prospect of performing heterogeneous enantioselective conversions in a novel manner, under the spatial restrictions imposed by the nanocavities within which the reactions occur. In particular, it suggested an alternative method for preparing pharmaceutically and agrochemically useful asymmetric products by capitalizing on the notion, initially tentatively perceived, that spatial confinement of prochiral reactants (and transition states formed at the chiral active center) would provide an altogether new method of boosting the enantioselectivity of the anchored chiral catalyst. Initially, we anchored chiral single-site heterogeneous catalysts to nanopores covalently via a ligand attached to Pd(II) or Rh(I) centers. Later, we employed a more convenient and cheaper electrostatic method, relying in part on strong hydrogen bonding. This Account provides many examples of these processes, encompassing hydrogenations, oxidations, and aminations. Of particular note is the facile synthesis from methyl benzoylformate of methyl mandelate, which is a precursor in the synthesis of pemoline, a stimulant of the central nervous system; our procedure offers several viable methods for reducing ketocarboxylic acids. In addition to relying on earlier (synchrotron-based) in situ techniques for characterizing catalysts, we have constructed experimental procedures involving robotically controlled catalytic reactors that allow the kinetics of conversion and enantioselectivity to be monitored continually, and we have access to sophisticated, high-sensitivity chiral chromatographic facilities and automated high-throughput combinatorial test rigs so as to optimize the reaction conditions (e.g., H(2) pressure, temperature, time on-stream, pH, and choice of ligand and central metal ion) for high enantioselectivity. This Account reports our discoveries of selective hydrogenations and aminations of synthetic, pharmaceutical, and biological significance, and the findings of other researchers who have achieved similar success in oxidations, dehydrations, cyclopropanations, and hydroformylations. Although the practical advantages and broad general principles governing the enhancement of enantioselectivity through spatial confinement are clear, we require a deeper theoretical understanding of the details pertaining to the phenomenology involved, particularly through molecular dynamics simulations. Ample scope exists for the general exploitation of nanospace in asymmetric hydrogenations with transition metal complexes and for its deployment for the formation of C-N, C-C, C-O, C-S, and other bonds.

Nanotechnology in catalysis

Nanotechnology and Heterogeneous Catalysis.- Oxide-Supported Metal Thin-Film Catalysts: The How and Why.- Developing Catalytic Nanomotors.- Catalysis by Gold: Recent Advances in Oxidation Reactions.- Gold Catalysts Supported on Nanostructured Materials: Support Effects.- Highly Effective Nanocatalysts Prepared Through Sol-Gel Technique.- Dendrimer Templates for Supported Nanoparticle Catalysts.- Tungsten Oxide Nanorods: Synthesis, Characterization, and Application.- Catalysis by Metal and Oxide Nanoparticles, Single Metal Atoms and Di-Nuclear Oxo-Ions in Zeolites.- A Dual Catalytic Role of Co Nanoparticles in Bulk Synthesis of Si-Based Nanowires.- Influence of Particle Size and Interaction with the Support on Redox and Catalytic Properties of Metals, Metal Oxides, and Metal Complexes.- Thermo-Catalytic Oxidation of Dihydroxybenzenes in the Presence of Nanoparticle Iron Oxide.- Synthesis of Palladium-Based Supported Catalysts by Colloidal Oxide Chemistry.- Gold-Based Nanoparticle Catalysts for Fuel Cell Reactions.- Carbon-Supported Core-Shell PtSn Nanoparticles: Synthesis, Characterization and Performance as Anode Catalysts for Direct Ethanol Fuel Cell.

Solvent-free, low-temperature, selective hydrogenation of polyenes using a bimetallic nanoparticle Ru-Sn catalyst

The point of attachment of bimetallic Ru6Sn particles which are anchored to the pore walls of a highly dispersed high-area mesoporous silica is found to be the tin atom, as indicated by in situ and ex situ measurements. This catalyst displays high activity for the low-temperature, selective hydrogenation of cyclic polyenes under solvent-free conditions (see scheme).

Single-Step, Highly Active, and Highly Selective Nanoparticle Catalysts for the Hydrogenation of Key Organic Compounds

Pores for cluster catalysts: Nanoparticles of both Ru5Pt and Ru10Pt2, uniformly distributed along the inner walls of mesoporous silica, exhibit high catalytic performance in the single-step hydrogenation of dimethyl terephthalate (DMT, to 1,4-cyclohexanedimethanol (CHDM); see scheme), of benzoic acid (to cyclohexane carboxylic acid), and of naphthalene (in the presence of sulfur) to cisdecalin.

Preparation, characterisation and performance of encapsulated copper-ruthenium bimetallic catalysts derived from molecular cluster carbonyl precursors

High-performance, bimetallic nanoparticle catalysts (M1+M2) were obtained by gentle thermolysis of their precursor metal-cluster carbonylates physisorbed inside the mesoporous channels of silica (the hexagons in the figure). The Cu–Ru catalyst anchored on silica is stable in use and has been tested in the catalytic hydrogenation of hex-1-ene, diphenylacetylene, phenylacetylene, stilbene, cis-cyclooctene, and D-limonene.

Direct conversion of methane to methanol

Abstract Methane has been converted to a mixture of methanol and formaldehyde at ambient conditions with high activity (TON above 100) and selectivity (CO2 less than 5%) using phthalocyanine complexes of Fe and Cu encapsulated in zeolites as catalysts and O2/tert-butyl hydroperoxide as oxidants.

Redox solid catalysts for the selective oxidation of cyclohexane in air

The selective oxidation of cyclohexane to cyclohexanol, cyclohexanone and adipic acid using molecular oxygen as the oxidant and at moderate temperatures (403 K) has been investigated over four different cobalt‐containing aluminophosphate (AlPO) molecular sieves. There is a correlation between catalytic activity and the fraction of (framework) Co(II) ions that is first oxidised to Co(III) in air. CoAlPO‐36 (pore aperture 6.5 x 7.5 Å) exhibits significant activity for the oxidation of cyclohexane in contrast to CoAlPO‐18, which, although it has the highest fraction of oxidisable cobalt, does not show any activity chiefly because of its smaller pores.

Nanoporous oxidic solids: the confluence of heterogeneous and homogeneous catalysis

The several factors that render certain kinds of nanoporous oxidic solids valuable for the design of a wide range of new heterogeneous catalysts are outlined and exemplified. These factors include: (i), their relative ease of preparation, when both mesoporous siliceous frameworks (ca. 20 to 250 A diameter pores) and microporous framework-substituted aluminophosphates (ca. 4 to 14 A diameter pores) can be tailored to suit particular catalytic needs according to whether regiospecific or enantio- or shape-selective conversions are the goal; (ii), the enormous internal (three-dimensional) areas that these nanoporous solids possess (typically 10(3) m(2) g(-1)) and the consequential ease of access of reactants through the internal pores of the solids; (iii), the ability, by judicious solid-state preparative methods to assemble spatially isolated, single-site active centres at the internal surfaces of these open-structure solids, thereby making the heterogeneous catalyst simulate the characteristic features of homogenous and enzymatic catalysts; (iv), the wide variety of in situ, time-resolved and ex situ experimental techniques, coupled with computational methods, that can pin-point the precise structure of the active site under operating conditions and facilitate the formulation of reaction intermediates and mechanisms. Varieties of catalysts are described for the synthesis (often under environmentally benign and solvent-free conditions) of a wide range of organic materials including commodity chemicals (such as adipic and terephthalic acid), fine and pharmaceutical chemicals (e.g. vitamin B(3)), alkenes, epoxides, and for the photocatalytic preferential destruction of carbon monoxide in the presence of hydrogen. Nanoporous oxidic solids are ideal materials to investigate the phenomenology of catalysis because, in many of them, little distinction exists between a model and a real catalyst.

Solvent-free routes to clean technology.

A major aim for the chemical technology of the future is the avoidance of noxious and environmentally unacceptable organic solvents. In this concept article we discuss more environmentally friendly and highly selective alternatives which we have evolved for carrying out a number of important chemical conversions. These entail the use of porous heterogeneous catalysts in which the active sites have been atomically engineered and fully characterized. Such solid catalysts operate under solvent-free conditions and usually entail one-step processes.