Surface organometallic chemistry is an area of heterogeneous catalysis which has recently emerged as a result of a comparative analysis of homogeneous and heterogeneous catalysis. The chemical industry has often favored heterogeneous catalysis, but the development of better catalysts has been hindered by the presence of numerous kinds of active sites and also by the low concentration of active sites. These factors have precluded a rational improvement of these systems, hence the empirical nature of heterogeneous catalysis. Catalysis is primarily a molecular phenomenon, and it must involve well-defined surface organometallic intermediates and/or transition states. Thus, one must be able to construct a well-defined active site, test its catalytic performance, and assess a structure-activity relationship, which will be used, in turn-as in homogeneous catalysis-to design better catalysts. By the transfer of the concepts and tools of molecular organometallic chemistry to surfaces, surface organometallic chemistry can generate well-defined surface species by understanding the reaction of organometallic complexes with the support, which can be considered as a rigid ligand. This new approach to heterogeneous catalysis can bring molecular insight to the design of new catalysts and even allow the discovery of new reactions (Ziegler-Natta depolymerization and alkane metathesis). After more than a century of existence, heterogeneous catalysis can still be improved and will play a crucial role in solving current problems. It offers an answer to economical and environmental problems faced by industry in the production of molecules (agrochemicals, petrochemicals, pharmaceuticals, polymers, basic chemicals).

Homogeneous and heterogeneous catalysis: bridging the gap through surface organometallic chemistry.

The borrowing hydrogen (BH) principle, also called hydrogen auto-transfer, is a powerful approach which combines transfer hydrogenation (avoiding the direct use of molecular hydrogen) with one or more intermediate reactions to synthesize more complex molecules without the need for tedious separation or isolation processes. The strategy which usually relies on three steps, (i) dehydrogenation, (ii) intermediate reaction, and (iii) hydrogenation, is an excellent and well-recognized process from the synthetic, economic, and environmental point of view. In this context, the objective of the present review is to give a global overview on the topic starting from those contributions published prior to the emergence of the BH concept to the most recent and current research under the term of BH catalysis. Two main subareas of the topic (homogeneous and heterogeneous catalysis) have been identified, from which three subheadings based on the source of the electrophile (alkanes, alcohols, and amines) have been consid...

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Advances in One-Pot Synthesis through Borrowing Hydrogen Catalysis

Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities Christophe Copeŕet,*,† Aleix Comas-Vives,† Matthew P. Conley,† Deven P. Estes,† Alexey Fedorov,† Victor Mougel,† Haruki Nagae,†,‡ Francisco Nuñ́ez-Zarur,† and Pavel A. Zhizhko†,§ †Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1−5, CH-8093 Zürich, Switzerland ‡Department of Chemistry, Graduate School of Engineering Science, Osaka University, CREST, Toyonaka, Osaka 560-8531, Japan A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov str. 28, 119991 Moscow, Russia

Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities

Publisher Summary This chapter focuses on spillover of sorbed species. The exchange of species from one position to another, either on the surface or through the bulk, has been well established. More unique is the mobilization of a sorbed species from one phase onto another phase where it does not directly adsorb. This has been defined as “spillover. Spillover may result in a spectrum of changes in the nonmetallic phase onto which it occurs. In the weakest sense, the spiltover species is transported across the surface of this phase as a two-dimensional gas. It may exchange with similar surface species.The spiltover species may react with the surface, which can result in the creation of surface defects and/or active sites. Further, the bulk of the solid may be transformed into a different structure. In each of these cases, the second phase is no longer an inert. It is not serving to promote the inherent activity on the first phase. The second phase is participating directly in the transport, exchange, and reaction with the spiltover species. In some cases it is able to become catalytically active on its own and thereby to participate directly in subsequent catalysis. The chapter discusses the types of phenomena associated with spillover, the spillover of species other than hydrogen, the aspect of spillover with the most significant catalytic implications, the implication of spillover to catalysis and other heterogeneous processes, the mechanism of spillover, and the nature of the surface and spiltover species.

Spillover of sorbed species

Many industrial catalysts contain isolated metal sites on the surface of oxide supports. While such catalysts have been used in a broad range of processes for more than 40 years, there is often a very limited understanding about the structure of the catalytically active sites. This review discusses how Surface Organometallic Chemistry (SOMC) engineers surface sites with well-defined structures and provides insight about the nature of active sites in industrial catalysts, focusing in particular on olefin production and conversion processes

/pdf/bridging-the-gap-between-industrial-and-well-defined-4f3dacemos.pdf

Bridging the Gap between Industrial and Well-Defined Supported Catalysts

Organometallic complexes of molybdenum and tungsten have been supported on silica or alumina and activated at temperatures at or above 373 K; the activities of the resulting substances as propene-disproportionation catalysts have been determined at ambient temperatures. Catalysts derived from alkyl and acetato-complexes show activities of the same order as catalysts made from supported Mo(CO)6. Tungsten-based catalysts have activities of the same order as molybdenum-derived catalysts, and catalysts derived from less thermally stable complexes are the more active. No propene disproportionation has been observed for chromium-based catalysts.

Organometallic complexes of molybdenum and tungsten as catalyst precursors in the disproportionation of propene

Treatment of silica or γ-alumina supports with a solution of hexamethyltungsten, followed by activation at temperatures of 373 K or above, yields alkene disproportionation catalysts with activities comparable to catalysts derived from molybdenum hexacarbonyl.

Disproportionation of propene by a heterogeneous catalyst derived from hexamethyltungsten

An examination of the exchange with deuterium of support-held hydrogen on a series of platinum–rhenium–alumina catalysts has shown that hydrogen spillover does not take place on catalysts where the rhenium content is in excess of 60% of the total metal.

Hydrogen spillover on alumina-supported platinum–rhenium catalysts

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Organometallic complexes of molybdenum and tungsten as catalyst precursors in the disproportionation of propene

Disproportionation of propene by a heterogeneous catalyst derived from hexamethyltungsten

Organometallic complexes of molybdenum and tungsten as catalyst precursors in the disproportionation of propene

Hydrogen spillover on alumina-supported platinum–rhenium catalysts

Disproportionation of propene by a heterogeneous catalyst derived from hexamethyltungsten