The advantages and future potential of single-site heterogeneous catalysts

Using open-structure (nanoporous) solids, advantage may be taken of single-site catalytically active centres to effect an enormous range of conversions of organic compounds where regio-selectivity and shape-selectivity loom large. When the single-site active centres are accommodated within a well-defined nanospace (typically pores of ca 30 Å diameter), uniquely powerful enantio-selective conversions may also be achieved; thus organometallic complexes, composed of cheap (nitrogen-based) chiral ligands anchored to the concave walls of the nanoporous solid, are especially powerful enantio-selective hydrogenation catalysts for pharmaceutically significant precursor species. The design, characterization and performance of single-site nanoporous catalysts are described and their application in (1) aerobic, shape-selective oxidation of cyclohexane to adipic acid; (2) region-specific oxidation of linear alkanes; (3) bromine-free synthesis of terephthalic acid from p-xylene using air as oxidant; (4) the in situ generation of hazardous reagents for Baeyer–Villiger oxidation of ketones to lactones and for the epoxidation of olefins; and (5) the single-step production of ε-caprolactam from cyclohexanone are outlined. Atomically engineered, multinuclear nanoparticle (bimetallic) catalysts are also described and their application in (1) one-step, solvent-free hydrogenation of polyenes; (2) asymmetric synthesis of pharmaceutical intermediates; (3) region-selective and stereo-selective allylic aminations; (4) single-step hydrogenation of dimethyl tererpthalate to 1,4-cyclohexane-dimethanol; and (5) renewable nylons from glucose and its biocatalytic derivatives is summarized. The key features of these solid catalysts (which facilitate separation of products from reactants) are that (a) they dispense with the use of corrosive reagents (and often of solvents); (b) permit the replacement of aggressive oxidants (e.g. HNO₃, CrO₃, etc) or even H₂O₂ and alkyl hydroperoxides, with molecular oxygen as the oxidant, thereby facilitating use of low-temperature and otherwise environmentally benign processes; (c) they are generally stable and recyclable and require no chemical initiators; and (d) they offer significant processing advantages in large scale and industrial operations when compared to homogeneous catalysts, especially in the generation of vital intermediates (surfactants, detergents, pharmaceuticals, polymers, perfumes and cosmetics). Typically, just one of the products (nylon-6), made by the “green”, environmentally benign bifunctional catalysts described herein, is in great demand industrially for the manufacture of (i) apparel and other textiles; (ii) floor coverings; (iii) industrial yarns; (iv) engineering plastics and (v) films for food and industrial packaging.