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Catalytic significance of organometallic compounds immobilized on mesoporous silica: economically and environmentally important examples
No full textBecause of the availability of well-defined nanoporous silicas, possessing (controllable) pore diameters of 30 Å to ca. 500 Å, bulky organometallic species may be readily introduced on to the inner walls of these thermally stable, solid supports. Four kinds of highly active catalysts that may be derived from the combination of these two materials are illustrated. First, starting from titanocene dichloride, single-site TiIV-centered epoxidation catalysts of exceptional activity may be formed. These are now used in the preparation of epoxy polymers and other products from naturally occurring methyl esters of fatty acids, a novel example of sustainable development. Second, asymmetric organometallic species that exhibit good enantioselectivity as homogeneous catalysts (in the hydrogenation of keto-esters, for example) may be significantly improved in their catalytic performance when they are anchored to a concave silica surface, their ee values are boosted by the spatial restrictions imposed by the mesopores within which they are anchored. Examples of three distinct diamino-complexes of Rh(I) are illustrated for the selective hydrogenations of methylbenzoylformate and E-?-phenylcinnamic acid. In the third kind of catalyst immobilized on nanoporous silica, several mixed-metal carbonylates are used as precursors. They are firmly anchored to the inner walls and may, by gentle calcining, be converted to bimetallic catalysts [e.g., Ru6Pd6, Ru5Pt and Ru10Pt2] that exhibit very high activity, under mild conditions, in the single-step, solvent-free hydrogenation of a range of economically important unsaturated compounds. The fourth category of immobilized, organometallics illustrated here is of the "ship-in-bottle" kind, where, typically, a chlorinated copper-phthalocyanine incarcerated within the cages of zeolite-X efficiently converts methane to methanol and also smoothly oxyhalogenates a range of aromatic compounds (e.g., benzene, phenol, toluene, aniline, anisole and resorcinol) in air, again under mild conditions. The unconventional techniques, both in situ and ex situ, required to characterize all these immobilized catalysts are outlined
Exploiting nanospace for asymmetric catalysis: Confinement of immobilized, single-site chiral catalysts enhances enantioselectivity
No full textIn 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., H2 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
Catalytically active centres in porous oxides: design and performance of highly selective new catalysts
No full textActive centres have been designed on high-area, molecular sieve catalysts which, inter alia, can convert n-alkanes to n-alkanols and n-alkanoic acids, cyclohexane to cyclohexanol, cyclohexanone and adipic acid, and n-hexane to adipic acid all using either oxygen or air as oxidants. A number of one-step processes and solvent-free chemical conversions, of paramount importance in the development of clean technology, are also described with catalysts designed (i) to oxidise p-xylene to terephthalic acid aerobically, (ii) to effect Baeyer–Villiger reactions with oxygen, and (iii) for the conversion of cyclohexanone to ?-caprolactam under mild conditions. The inner surfaces of mesoporous silicas may also be atomically engineered so as to yield high-performance epoxidation of alkenes at TiIV-centred active sites, as well as enantioselective hydrogenations of organic species using constrained chiral catalysts
Nanoporous solids as receptacles and catalysts for unusual conversions of organic compounds
No full textSolid-state chemical principles, allied to a degree of chemical intuition, enables one to design open-structure solids on to the inner surfaces of which isolated catalytically active sites of different kinds may be placed. With such solids, which act simultaneously both as permeable catalysts and reaction vessels, a number of highly desirable chemical conversions—many of paramount importance in the context of “green” chemistry and clean technology—may be smoothly effected under environmentally benign conditions. Typical examples, illustrated here, include the selective oxidation of toluene to benzaldehyde, current methods of producing alcohols, aldehydes and acids, and the synthesis of -caprolactam in a by-product-free manner. Such open-structure solids, which house single-site active centres, are also readily amenable to detailed and precise structural elucidation.<br/
Design of a "green" one-step catalytic production of epsilon-caprolactam (precursor of nylon-6)
No full textThe ever-increasing industrial demand for nylon-6 (polycaprolactam) necessitates the development of environmentally benign methods of producing its precursor, ?-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 ?-caprolactam using a family of designed bifunctional, heterogeneous, nanoporous catalysts containing isolated acidic and redox sites, which smoothly convert cyclohexanone to ?-caprolactam with selectivities in the range 65–78% in air and ammonia at 80°C. The catalysts are microporous (pore diameter 7.3 Å) aluminophosphates in which small fractions of the Formula and Formula tetrahedra constituting the 4-connected open framework are replaced by Formula and Formula 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 ?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
Nanopore and nanoparticle catalysts
No full textThe design, atomic characterization, performance, and relevance to clean technology of two distinct categories of new nanocatalysts are described and interpreted. Exceptional molecular selectivity and high activity are exhibited by these catalysts. The first category consists of extended, crystallographically ordered inorganic solids possessing nanopores (apertures, cages, and channels), the diameters of which fall in the range of about 0.4 to about 1.5 nm, and the second of discrete bimetallic nanoparticles of diameter 1 to 2 nm, distributed more or less uniformly along the inner walls of mesoporous (ca. 3 to 10 nm diameter) silica supports. Using the principles and practices of solid-state and organometallic chemistry and advanced physico-chemical techniques for in situ and ex situ characterization, a variety of powerful new catalysts has been evolved. Apart from those that, inter alia, simulate the behavior of enzymes in their specificity, shape selectivity, regio-selectivity, and ability to function under ambient conditions, many of these new nanocatalysts are also viable as agents for effecting commercially significant processes in a clean, benign, solvent-free, single-step fashion. In particular, a bifunctional, molecular sieve nanopore catalyst is described that converts cyclohexanone in air and ammonia to its oxime and caprolactam, and a bimetallic nanoparticle catalyst that selectively converts cyclic polyenes into desirable intermediates. Nanocatalysts in the first category are especially effective in facilitating highly selective oxidations in air, and those in the second are well suited to effecting rapid and selective hydrogenations of a range of organic compounds
The role of the Jilin-DFRL collaboration in the emergence of open-structure single-site solid catalysts
No full textThe fruitful interaction between the Jilin group (under Ruren Xu) and that of one of the authors (JMT) at the Davy Faraday Research Laboratory (DFRL) is adumbrated, and the highlights achieved briefly assessed. These have led the authors to design and prepare a range of novel single-site solid catalysts that open up new (and benign) routes for the production of important commodity- and fine-chemicals and pharmaceuticals.<br/
Engineering active sites in AlPOs materials for enhancing synergy in heterogeneous catalytic reactions
No full textThe Sanctuary of Bel in perspective: selective destructions, selective memories, selective realities
Full text linkIn this chapter, we trace how the actions of Western archaeologists created a classical vision of the Temple of Bel in Palmyra at the expense of other histories. We draw on archival documents and photographs to demonstrate how the lenses and pens of the archaeologists working at the site since its ‘discovery’ in the eighteenth century transformed the complexity of the sanctuary into a single narrative that paid insufficient attention to the living population of Tadmor-Palmyra. We argue that a consistent practice of heritagizing the sanctuary, emptying the sanctuary, and divorcing it from its local community have led to actions in the present, both by Da’esh, but also by Western institutions, that continue that process of emptying and the disenfranchisement of local communities
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