Classical, Heterogeneous Catalysts

The modeling of molybdenum-based classical catalysts supported on alumina was improved by the use of calculations with periodic boundary conditions [10, 49, 52], which better represent the alumina surface [67]. It became possible to describe the relative stabilities of the different surface sites, including the effect of temperature and water pressure. The more stable (110) and (100) alumina surfaces were considered, and the investigation focused on the structure of the potential initial molybdenum-oxide monomeric and dimeric species, as well as the corresponding methylidene species and their reactivity with ethene [10,49]. 

Stability Favorable on the minor (100) surface Favorable on the minor (100) surface Most abundant stmcture 

Reactivity Most active Most active Low activity 

As a consequence, under dehydrating conditions, the square-planar, monooxo species (42) supported on the (110) alumina surface is the most abundant molybdenum-oxide species, while the pentacoordinated dioxo species (40) is expected to be formed on the minor (100) surface. 

---

Most of the supports so far studied are conventional in the field of catalysis. Some new kinds of support have emerged including mesostructured materials,193 dendrimers, organic-inorganic hybrids, and natural polymers such as polysaccharides or polyaminoacids. Nevertheless, at the moment, supported catalysts still suffer from relatively poor stability when compared to classical heterogeneous catalysts, and from limited activity when compared to homogeneous catalysts. The driving force for this research is thus to make up some of these deficits. 

Note also that, in contrast to classical heterogeneous catalysts, the initiation step of [=SiORe(=CtBu)(=CHtBu)(CH2tBu)] is well defined and corresponds to the cross-metathesis of the alkene with the neopentyhdene ligand. In fact, in the metathesis of propene, 0.7 equiv of a 3 1 mixture of 3,3-dimethyl-l-butene and 4,4-dimethyl-2-pentene is formed (Figure 3.27) the nearly quantitative formation of cross-metathesis products is consistent with a real single-site catalyst. Moreover,... 

In nonalkylation systems, such as the classic heterogeneous catalysts [alumina-supported M0O3, Re207, and WtCOtg] or combinations such as WC16 + AICI3, the carbene must come from the alkene, which is a more difficult problem to explain. 

In addition to the classical heterogeneous catalysts such as zeolites, or metals on supports, there are now a host of other catalysts which are not truly homogeneous and where the catalyst, the reactants and the products are in different phases. Such catalysts can offer the advantages of high activity and selectivity associated with the more usual homogeneous catalysts, as well as... 

The aesthetically pleasing silica-supported alkyl, alkylidene, alkylidyne Re complex =SiORe[=C u][=CH Bu][CH2 Bu] (Scheme 3, Structures 15-17), prepared from the molecular precursor, has also been found to be highly reactive for the metathesis of propene [13]. Moreover, the evolution of roughly one equivalent of a 1 3 mixture of 3,3-dimethylbutene and 4,4-dimethyl-2-pentene is consistent with a cross-metathesis of the neopentylidene ligand of 1 and propene. The turnover obtained also exceeds those obtained with classical heterogeneous catalysts such as W03/Si02 used industrially for decades (Lummus process). 

Nevertheless, classical heterogeneous catalysts like particulate noble metals may be immobilized on the nanotube surface as well. Nanoparticles of platinum or rhodium, for instance, can be deposited on cup-stacked carbon nanotubes by reductive precipitation (Figure 3.114b). The catalysts obtained this way suit an application in fuel cells run on methanol. Electrodes made from the nanotube material exhibit twice the efficiency as compared to the classical material XC-72-carbon. The particles of noble metal on the nanotube surface catalyze the direct conversion of methanol into CO2 (MeOH -1- H2O CO2 -1- 6 H -1- 6e ). A material to be employed in such fuel cells has to meet some essential requirements, including a maximal specific surface, a defined porosity and a high degree of crystalhnity. Carbon nanotubes are endowed with exactly these characteristics, which is why they are the most suitable material for electrodes. Their high price, however, is still prohibitive to an industrial scale application. 

Finally, to complete the field of heterogenization, it should be mentioned that the catalyst class of synthetic polypeptide derivatives and catalytic antibodies do not belong to the classical heterogenized catalysts as they do not have a known homogeneous counterpart. They are macromolecular systems... 

Classical heterogeneous catalysts are the subject of four major chapters in the book. We elaborate in detail on our current understanding of the molecular events that underline their catalytic phenomena and attempt to deduce from these results important catalytic reactivity concepts. This detailed understanding provides a basis for the comparison of the mechanistic principles between heterogeneous, enzyme, and homogeneous organometaUic cluster catalysis. 

Schemes 6.3 and 6.4) [3-5, 7-9, 11, 35-44]. Other studies focused on the classical heterogeneous catalysts, such as M0O3 [6, 10, 45-54] and Re20y supported on silica or alumina [55-59]. We will first focus on the studies of well-defined complexes of general formula M(ER )(=CHR )(X)(Y) (M = Re, Mo, or W ER = alkylidyne, imido, or 0x0 X and/or Y = alkoxy, siloxy, alkyl, or pyrrolyl). Afterwards, we will summarize the contributions of classical heterogeneous catalysts with the aim of highlighting the differences and similarities with the well-defined catalysts. 

Figure 6.S SCILT, catalyst materials - schematic view on a classical heterogeneous catalyst coated with a thin layer of ionic liquid.

The main purpose of a SCILL system is not to immobilise an ionic liquid, but to modify the catalytic reactivity of a solid surface. For this purpose, the internal surface of a classical heterogeneous catalyst is coated with a thin film of ionic liquid. A schematic picture of a SCILL catalyst is shown in Figure 6.S. 

Classical heterogeneous catalysts, consisting of metal particles supptnted on a solid surface such as silica or carbon, are of great commercial importance. Tt has also proved possible to support a variety of organometallic species on silica so as to obtain mononuclear complexes covalently anchored to the silica surface. Silica has surf ace SiOH groups, often denoted sSi—O—H, which can form sSi—O—M links to the attached metal. The oxophilic early metals are particularly well suited to this approach. 

Many classical heterogeneous catalysts, e.g., oxides and related compounds with incorporated ruthenium, gold, palladium, or platinum were found to be effective for the aerobic oxidation of alcohols. Silver and cobalt were also included in this catalytic family and have raised expectations regarding the availabifity of the catalysts. 

---