Nanoporous oxidic solids: the confluence of heterogeneous and homogeneous catalysis

Abstract

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.

Surface chromium single sites: open problems and recent advances

“…Cr sites grafted on amorphous silica or mesoporous silica or zeolitic materials have been recently investigated as catalysts in photocatalytic reactions such as CO oxidation in the presence of H 2 , NO decomposition, partial oxidation of propane, oxidation of propene or other selective oxidation reactions, such as cyclohexane to cyclohexanone (Yamashita et al 2001;Kamegawa et al 2006;Anpo et al 2009;Thomas et al 2009). Usually, the starting point is Cr VI in the form of chromate, and a charge transfer excited state is claimed as intermediate species, sometimes as Cr V and others as Cr IV .…”

The Coming of Advanced Materials: A Personal View of the Contributions by Cambridge Scientists

“…[53] A selection of some of the advanced solid catalysts suitable for industrially important reactions, and designed in my group, is given in Table 3. [54,55] Figure 6. illustrates graphically my approach to the design of single-site heterogeneous catalysts.…”

“…[56] The scale of complexity of catalysis is enormous, ranging from a single-atom active center (in a suitable structural environment) to complex molecular compounds to fiendishly complicated mixed-metal oxides. [56][57][58][59][60] Open-structure, nanoporous catalysts are to a considerable degree designable to order [50][51][52][53][54][55] partly because there are now reliable computational methods [61] for selecting organic template molecules that lead to the formation of a desired nanostructure, and partly because such solids can be readily functionalized so as to yield a given target catalyst. [52] It is not readily possible to design de novo, complex, close-packed solids that may be feasible heterogeneous catalysts for any generally required conversions.…”

“…These workers, using DFT procedures, have arrived at descriptors that help us identify new catalysts for simple reactions (such as the water-gas shift reaction or methanation) taking place on transition metals. In addition, inspired work by the Eindhoven group led by van Santen and coworkers [64] has brought forth many hybrid catalysts (amplified elsewhere [55] ) where there is a coalescence of heterogeneous and heterogeneous catalysts.…”

“…Insofar as developing new advanced catalysts to support sustainable chemistry, much effective progress (such as in taking full advantage of biomass) has already been achieved. [54,55,66,67] …”