The societal significance of catalysis and the growing practical importance of single-site heterogeneous catalysts

Abstract

The concept of single-site heterogeneous catalysis, herein defined and extensively illustrated, offers a strategy for the design of new solid catalysts. By capitalizing on the opportunities presented by nanoporous materials to assemble a wide range of new, well-defined, catalytically active centres, it is possible to bring about numerous environmentally benign processes that can replace traditional methods of chemical production. The latter often employs aggressive, corrosive or hazardous reagents. By using both microporous (less than 20 Å diameter) and mesoporous solids (20-500 Å diameter), abundant scope exists for the construction and application of shape-selective, regio-selective and enantioselective catalysts.Keywords: nanoporous solids; selectivity; benign reagents; renewable resources; clean technology; sustainability BackgroundIt would be difficult to overestimate or exaggerate the practical importance of heterogeneous catalysis. Think of the products of petroleum and natural gas upon which civilized life now relies. All the following are manufactured through the agency of solid catalysts: fuels (for transport and heating), fabrics, flavours, fragrances, fertilizers, certain foodstuffs and most pharmaceuticals. Moreover, many of the molecular building blocks for the production of a wide range of the commodities used in everyday life are also generated catalytically: propylene, benzene, toluene, xylene, benzaldehyde, terephthalic acid, adipic acid, caprolactam (precursor of nylon 6) and a variety of monomers for polymeric composites.In fundamental, academic terms, the phenomenon of catalysis is endlessly fascinating and perennially new. Its study presents a multiplicity of intellectual and technological challenges. If we are to understand how and why certain molecules transform readily and others do not when they impinge upon an active site, it is necessary, continually, to devise, develop and deploy new techniques of investigation. This is equally true if our aim-and it is one of prime importanceis to know more about the nature of the active site itself. Without a detailed understanding of the atomic architecture of the catalytically active centre, we are unlikely to be in a position to improve existing catalysts or design superior new ones (Thomas 1997).Because they readily facilitate the separation of products from unreacted reagents, and also because they are amenable to the various processes of thermal reactivation, heterogeneous catalysts (which almost invariably involve solid, active phases) are much more convenient to use than homogeneous ones. Consequently, they now figure eminently in the drive towards green chemistry, clean technology and sustainability. Those scientists concerned with such topics are currently exploring new ways of generating both energy and novel materials and in this regard, solid catalysts are of pivotal importance.There is now an exigent need to design and develop catalysts that can cope with and transform readily available reactants in an environmentally benign manner. N...

Crossing the divide between homogeneous and heterogeneous catalysis in water oxidation

“…Phosphonate-derivatized catalysts and molecular assemblies provide a basis for sustained water oxidation on these surfaces in acidic solution but are unstable toward hydrolysis and loss from surfaces as the pH is increased. Here, we report enhanced surface binding stability of a phosphonate-derivatized water oxidation catalyst over a wide pH range (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12) by atomic layer deposition of an overlayer of TiO 2 . Increased stability of surface binding, and the reactivity of the bound catalyst, provides a hybrid approach to heterogeneous catalysis combining the advantages of systematic modifications possible by chemical synthesis with heterogeneous reactivity.…”

“…Examples include the Haber-Bosch process, steam reforming, Ziegler-Natta polymerization, and hydrocarbon cracking (1)(2)(3)(4)(5)(6)(7)(8). Research in heterogeneous catalysis continues to flourish (9-15) but iterative design and modification are restricted by limitations in materials preparation and experimental access to surface mechanisms.…”

Zeolite Catalysis for Biomass Conversion

“…Generally, there are two types of single-site heterogeneous catalysts (SSHC), as defi ned by Thomas [ 96 ]. The fi rst one, which comprises the immobilization of well-defi ned ensembles (metal nanoparticles, organic functionalities) onto not necessarily well-defi ned carrier materials fall beyond the scope of this chapter.…”

Microwave-assisted synthesis of Ag-doped MOFs-like organotitanium polymer with high activity in visible-light driven photocatalytic NO oxidization

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