Organometallic Chemistry as a Basis for Understanding Heterogeneous Catalysis

Abstract

At the present time, our knowledge of the mechanism of action of most heterogeneous catalysts is limited. At the most primitive level, only the overall product distribution is known, while the nature of the catalyst-reactant interactions remains obscure. In a few cases, more detailed mechanistic proposals have been advanced, but they remain modest in scope compared to the detailed mechanisms that have been developed in molecular organic chemistry, and more recently, in organometallic chemistry. Indeed, even the apparently well-established catalytic mechanisms (for example, hydrogenation of olefins) are sometimes contested, indicating the fragility of the evidence on which these proposals are based. It must be recognized, however, that by comparison to strictly molecular systems, heterogeneous catalysts are intrinsically much more complicated. In addition, the application of physical methods to structural and mechanistic problems of heterogeneous systems, as well as interpretation of the results, can be difficult.A common problem encountered in heterogeneous catalysis is the complexity of the surfaces of solid catalysts. The so-called "active sites", a concept proposed by Sir H. S. Taylor [II. may be very small in number relative to the overall surface and, consequently, their structure is almost unknown at an atomic level. It is probably this complexity which has inspired the development of surface science applied to catalysis. New concepts have emerged slowly from this approach: surface reconstruction in the presence of adsorbed molecules, surface mobility and structural reorganization of molecular-like species. It has been demonstrated that the binding of chemisorbed molecules resembles organometallic ligation (e.g., YJ l, 11 2 and YJ3-adsorbed CO, Jt-bound ethylene and YJ l, YJ2 and YJ5-ethylidene species) 12]. Naturally, the concepts derived from surface science have been, for the most part, applied to the chemisorption process and not to the reactiv ity pattern, although the latter is, in fact, the key to catalytic mechanisms.At a fundamental level, we are interested in the elementary steps of heterogeneous catalytic mechanisms, that is, the succession of chemical events in which bonds of a substrate molecule are broken or created in the proximity of the surface. The number of examples in which this sllccession of events has been demonstrated on a well-defined surface is still limited. However, a growing awareness of this deficiency has spurred the recent development of surface organometallic chemistry 13]. In this relatively new field, the organometallic character of the "active site" is exploited in order to model reaction mechanisms.During a catalytic cycle, substrates which interact with the surface form one or several chemical bonds with one or several surface atoms. Hence the "active site" is a 39 R. W. Joyner and R. A. van Santen (eds.), Elementary Reaction Steps in Heterogeneous Catalysis,[39][40][41][42][43][44][45][46][47][48][49]

Kinetics and Mechanisms of Thermally Induced Alkane Eliminations from Silica-Supported Bis(alkyl)chromium(IV) and -vanadium(IV) Complexes

“…Predicting and designing the surface chemistry of complex organometallic fragments are goals with major implications for heterogeneous catalysis. , Reactivity studies on conventional solid catalysts rarely yield clear mechanistic conclusions due to the inherent heterogeneity of active sites. However, if the oxygen atoms of oxide surfaces can be considered as donor atoms of a supporting ligand , (since the bonds on covalent oxide surfaces such as that of silica are fairly localized), then useful analogies may be made with molecular organometallic chemistry …”

On the Origins and Development of “Surface Organometallic Chemistry”

“…(Figure 1.4 ). This is a major achievement, mainly due to Basset and his group, in surface organometallic chemistry because it has been thus possible to prepare " single site " catalysts for various known or new catalytic reactions [53] such as metathesis of olefi ns [54] , polymerization of olefi ns [55] , alkane metathesis [56] , coupling of methane to ethane and hydrogen [57] , cleavage of alkanes by methane [58] , hydrogenolysis of polyolefi ns [59] and alkanes [60] , direct transformation of ethylene into propylene [61] , etc. These topics are considered in detail in subsequent chapters.…”

Solid‐State NMR and DFT Studies on the Formation of Well‐Defined Silica‐Supported Tantallaaziridines: From Synthesis to Catalytic Application

“…Molecular metallaaziridine complexes have been documented for two decades, but have only recently been explored within the framework of surface organometallic chemistry. This field has a solid track record in isolating and identifying transition metal catalysts with unusual reactivities . Recent advances include the isolation of single‐site, well‐defined surface species such as [(≡Si‐O‐Ta(=CH 2 )Me 2 ] (an active catalyst for alkane metathesis) and [≡Si‐O‐Zr(HNMe 2 )(η 2 NMeCH 2 )(NMe 2 )] .…”