Single‐Site Heterogeneous Catalysts

Abstract

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.

Design of a “green” one-step catalytic production of ε-caprolactam (precursor of nylon-6)

“…SSHCs have been designed by using an extension of the practices that we have evolved in recent years to effect other environmentally desirable or chemically demanding conversions [such as the oxyfunctionalization of terminal methyl groups by air or oxygen (3,26)]. A number of important other reactions, including the epoxidation of replenishable terpenes and esters of naturally occurring unsaturated fatty acids as well as the hydrogenation of polyenes and unsaturated ketoesters, may also be effected by using appropriately designed SSHCs (6,32).…”

“…SSHCs are those in which the active centers are spatially isolated from one another [and uniformly distributed through the solid (5)] such that each site has the same energy of interaction between it and the incoming reactant (6). Moreover, that energy remains constant when all of the sites participate in catalytic turnover.…”

Spectroscopic and Computational Insights on Catalytic Synergy in Bimetallic Aluminophosphate Catalysts

“…To quantify such interactions at the molecular level requires a detailed understanding of the nature of the active sites, and it is necessary to employ a range of physico-chemical, operando and spectroscopic characterisation techniques, that are best complemented when integrated with atomic level modelling studies. [9][10][11][12][13][14][15] In our recent work 16 we extended the family of transition-metal doped aluminophosphate (AlPO) frameworks, 10,17,18 to obtain isomorphous incorporation of bimetallic active centres, that display superior catalytic activity in oxidation reactions ( Figure S1 and Table S1). 16 Through a rational selection of appropriate metal combinations and synthetic strategy, it is possible to engineer and exploit synergic interactions between individual metal sites, deliberately placed within sufficiently close proximity such that their local geometry and electronic structure is modified to facilitate catalytic improvements.…”

Investigating site-specific interactions and probing their role in modifying the acid-strength in framework architectures

“…[1][2][3] Understanding the nature of the active site at the molecular level is therefore a fundamental prerequisite, for facilitating structure-property correlations, that enable the design of catalytically active entities 25 for enhancing rates and selectivity. [4,5] It is well-known that a diverse array of synthetic approaches [6][7][8][9][10] can be adopted for heterogenizing a wide-variety of molecular precursors on high-area supports for generating multimetallic metal nanoparticle catalysts that lead to enhanced catalytic 30 turnovers.…”