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Robert Raja

Publications and source records attributed to Robert Raja.

15 recordsLinked to original sources
PubMed2006

Highly efficient one-step conversion of cyclohexane to adipic acid using single-site heterogeneous catalysts.

A solid source of 'active' oxygen (acetylperoxyborate, APB), when dissolved in aqueous solution in the presence of a single-site microporous catalyst containing redox centres (Fe(III)AlPO-31, Mn(III)AlPO-5, Fe(III)AlPO-5), converts cyclohexane with high efficiency (ca. 88%) and exceptionally high selectivity (ca. 81%) to adipic acid at 383 K; this procedure is also effective in converting styrene to styrene oxide and -pinene and (+)-limonene to their corresponding epoxides.

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PubMed2005

Single-site heterogeneous catalysts.

Intellectually, the advantages that flow from the availability of single-site heterogeneous catalysts (SSHC) are many. They facilitate the determination of the kinetics and mechanism of catalytic turnover-both experimentally and computationally-and make accessible the energetics of various intermediates (including short-lived transition states). These facts in turn offer a rational strategic principle for the design of new catalysts and the improvement of existing ones. It is generally possible to prepare soluble molecular fragments that circumscribe the single-site, thus enabling a direct comparison to be made, experimentally, between the catalytic performance of the same active site when functioning as a heterogeneous (continuous solid) as well as a homogeneous (dispersed molecular) catalyst. This approach also makes it possible to modify the immediate atomic environment as well as the central atomic structure of the active site. From the practical standpoint, SSHC exhibit very high selectivities leading to the production of sharply defined molecular products, just as do their homogeneous analogues. Given that mesoporous silicas with very large internal surface areas are ideal supports for SSHC, and that more than a quarter of the elements of the Periodic Table may be grafted as active sites onto such silicas, there is abundant scope for creating new catalytic opportunities.

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PubMed2005

Design of a "green" one-step catalytic production of epsilon-caprolactam (precursor of nylon-6).

The ever-increasing industrial demand for nylon-6 (polycaprolactam) necessitates the development of environmentally benign methods of producing its precursor, epsilon-caprolactam, from cyclohexanone. It is currently manufactured in two popular double-step processes, each of which uses highly aggressive reagents, and each generates substantial quantities of largely unwanted ammonium sulfate as by-product. Here we describe a viable laboratory-scale, single-step, solvent-free process of producing epsilon-caprolactam using a family of designed bifunctional, heterogeneous, nanoporous catalysts containing isolated acidic and redox sites, which smoothly convert cyclohexanone to epsilon-caprolactam with selectivities in the range 65-78% in air and ammonia at 80 degrees C. The catalysts are microporous (pore diameter 7.3 A) aluminophosphates in which small fractions of the Al(III)O4(5-) and P(V)O4(3-) tetrahedra constituting the 4-connected open framework are replaced by Co(III)PO4(5-) and Si(IV)O4(4-) tetrahedra, which become the loci of the redox and acidic centers, respectively. The catalysts may be further optimized, and already may be so designed as to generate selectivities of approximately 80% for the intermediate oxime, formed from NH2OH, which is produced in situ within the pore system. The advantages of such designed heterogeneous catalysts, and their application to a range of other chemical conversions, are also adumbrated.

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PubMed2005

Novel, benign, solid catalysts for the oxidation of hydrocarbons.

The catalytic properties of two classes of solid catalysts for the oxidation of hydrocarbons in the liquid phase are discussed: (i) microporous solids, encapsulating transition metal complexes in their cavities and (ii) titanosilicate molecular sieves. Copper acetate dimers encapsulated in molecular sieves Y, MCM-22 and VPI-5 use dioxygen to regioselectively ortho-hydroxylate L-tyrosine to L-dopa, phenol to catechol and cresols to the corresponding o-dihydroxy and o-quinone compounds. Monomeric copper phthalocyanine and salen complexes entrapped in zeolite-Y oxidize methane to methanol, toluene to cresols, naphthalene to naphthols, xylene to xylenols and phenol to diphenols. Trimeric mu3-oxo-bridged Co/Mn cluster complexes, encapsulated inside Y-zeolite, oxidize para-xylene, almost quantitatively, to terephthalic acid. In almost all cases, the intrinsic catalytic activity (turnover frequency) of the metal complex is enhanced very significantly, upon encapsulation in the porous solids. Spectroscopic and electrochemical studies suggest that the geometric distortions of the complex on encapsulation change the electron density at the metal ion site and its redox behaviour, thereby influencing its catalytic activity and selectivity in oxidation reactions. Titanosilicate molecular sieves can oxidize hydrocarbons using dioxygen when loaded with transition metals like Pd, Au or Ag. The structure of surface Ti ions and the type of oxo-Ti species generated on contact with oxidants depend on several factors including the method of zeolite synthesis, zeolite structure, solvent, temperature and oxidant. Although, similar oxo-Ti species are present on all the titanosilicates, their relative concentrations vary among different structures and determine the product selectivity.

Catalysis↗
PubMed2003

Constraining asymmetric organometallic catalysts within mesoporous supports boosts their enantioselectivity.

By constraining tethered asymmetric organometallic catalysts within the nanopores of silica supports so as to increase the interaction between the pore wall and the active center (and hence to restrict access of the reactant to the catalyst), a significant improvement in enantioselectivity is achieved. A schematic illustration of a cationic chiral, organometallic catalyst, [Rh(I)(COD)PMP] {(S)-(+)-1-(2-pyrrolidinylmethyl)-pyrrolidine and cyclooctadiene}, which is noncovalently anchored via a N-H...F hydrogen bond with the triflate ion, CF3SO3-, to the curved inner surface of a 38 A diameter pore of a silica support for the asymmetric hydrogenation of the C=O bond in methyl benzoylformate to the corresponding methyl mandelate is shown. Rh (purple); N (blue); H (white); F (green); C (gray); S (yellow); O (red).

Journal Article↗
PubMed2003

Bimetallic nanocatalysts for the conversion of muconic acid to adipic acid.

Adipic acid (2) production currently entails use and generation of environmentally harmful materials: an efficient catalyst, consisting of nanoparticles of Ru10Pt2 anchored within the pores of mesoporous silica, facilitates the production of (2) by hydrogenating muconic acid, that may be derived biocatalytically from D-glucose.

Adipates↗
PubMed2003

High-performance nanocatalysts for single-step hydrogenations.

Bimetallic nanoparticles (Ru(6)Pd(6), Ru(6)Sn, Ru(10)Pt(2), Ru(5)Pt, Ru(12)Cu(4), and Ru(12)Ag(4)) anchored within silica nanopores exhibit high activities and frequently high selectivities, depending upon the composition of the nanocatalyst, in a number of single-step (and often solvent-free) hydrogenations at low temperatures (333-373 K). The selective hydrogenations of polyenes (such as 1,5,9-cyclododecatriene and 2,5-norbornadiene) are especially efficient. Good performance is found with these nanoparticle catalysts in the hydrogenation of dimethyl terephthalate to 1,4 cyclohexanedimethanol and of benzoic acid to cyclohexanecarboxylic acid or to cyclohexene-1-carboxylic acid, and also in the conversion of benzene to cyclohexene (or cyclohexane), the latter being an increasingly important reaction in the context of the production of Nylon. Isolated atoms of noble metals (Pd, Rh, and Pt) in low oxidation states, appropriately complexed and tethered to the inner walls of nanoporous (ca. 3 nm diameter) silica, are very promising enantioselective hydrogenation catalysts. Nanoporous carbons, as well as other nanoporous oxides, may also be used to anchor and tether the kind of catalysts described here.

Journal Article↗
PubMed2001

Solvent-Free, Low-Temperature, Selective Hydrogenation of Polyenes using a Bimetallic Nanoparticle Ru-Sn Catalyst We thank Dr. R. G. Bell for assistance with the computer graphics, Drs. P. A. Midgeley, V. Keast, and M. Weyland for help with STEM, and gratefully acknowledge the support (via a rolling grant to J.M.T. and an award to B.F.G.J.) of EPSRC and the award of a research fellowship (for G.S.) from the Leverhulme Foundation and a Marie Curie Fellowship within the TMR Programme of the European Commission (for S.H.).

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Journal Article↗