Rhenium-molybdenum catalysts

The metathetic reaction occurs in the gas phase at relatively high temperatures (150°-350°C) with molybdenum or tungsten supported catalysts or at low temperature (=50°C) with rhenium-based catalyst in either liquid or gas-phase. The liquid-phase process gives a better conversion. Equilibrium conversion in the range of 55-65% could be realized, depending on the reaction temperature. ... 

Solid catalysts for the metathesis reaction are mainly transition metal oxides, carbonyls, or sulfides deposited on high surface area supports (oxides and phosphates). After activation, a wide variety of solid catalysts is effective, for the metathesis of alkenes. Table I (1, 34 38) gives a survey of the more efficient catalysts which have been reported to convert propene into ethene and linear butenes. The most active ones contain rhenium, molybdenum, or tungsten. An outstanding catalyst is rhenium oxide on alumina, which is active under very mild conditions, viz. room temperature and atmospheric pressure, yielding exclusively the primary metathesis products. 

Recently, norbornene was reported to undergo ring-opening polymerization in the presence of molybdenum, tungsten, and rhenium halide catalysts (33). In each of these studies (33-35), the catalysis was interpreted by mechanisms involving the scission of the carbon-carbon single bond adjacent to the double bond, e.g., Eq. (12). 

A large number of styrenic monomers have been investigated in metal-catalyzed radical polymerizations. Polymerization of styrene (M-19) can be controlled with copper,28,84,85 152 176 ruthenium,57 60 62 66 86,205 iron,71 75 rhodium,86 140 rhenium,141 and molybdenum catalysts.144 The polymerizations have actively been studied with the copper-based systems, among which precisely controlled molecular weights and very narrow MWDs (MJMn =1.1) were obtained in a homogeneous system consisting of 1-13 (X = Br), CuBr, and L-3 in the bulk at 130 °C.85 Similar well-controlled polymerizations are feasible with several ruthenium (Ru-5)60 and iron (Fe-2,72 Fe-3,73 and Fe-471) complexes in conjunction with a bromide or iodide initiator. Even a chloride initiator (1-25, X = Cl) can afford narrow MWDs (MJMn =1.1) when coupled... 

The initial observation of a metal carbene that reacted with an alkene to give a metallacyclobutane complex was reported by Osborn and coworkers for the reaction shown in equation (10). This reaction was observed by NMR spectroscopy at low temperature (—70°C). When this reaction mixture was allowed to warm to higher temperature, polynorbornene was produced in high yield. Shortly after this discovery, the titanocene complex (4) was shown to be an efficient catalyst for the synthesis of monodisperse polynorbornenes. These discoveries, along with the synthesis of a new family of tungsten (5a), molybdenum (5b), and rhenium (6) catalysts,shown in Figure 1, have opened a new era of ROMP chemistry in which the polymer synthesis is guided by the selection of a catalyst... 

Cyclobutene, cyclopentene, and norbomene also give their polyalken-amers (64). The molecular weights are all high, and the stereochemistries are largely cis. Of these, cw-polypentenamer has also been made with molybdenum, tungsten, and rhenium halide catalysts, and cii-polynor-bomenamer (of lower molecular weight) with a molybdenum chloride catalyst, but polybutenamer made with metal halide catalysts (the metals tried were Ti, Mo, W, V, Cr, and Ru) has never been found to have more than 60% of its double bonds cis (64). 

Supported rhenium oxide catalysts have attracted much attention because of practical applications (see Ch. 17). One of the potential advantages of rhenium catalysts is that they are more tolerant of functional groups than tungsten or molybdenum catalysts (see Ch. 7). 

A timeline for the development of olefin metathesis, adapted from a review by Grubbs, is shown in Figure 21.3. Olefin metathesis is more than 50 years old. " It was first conducted with ill-defined rhenium, molybdenum, and tungsten systems generated from perrhenate, aluminum oxide, - and tetraethyl lead as additive, from molybdenum oxide on p-TiO and tetramethyltin as additive, ° or from tungsten phenoxides supported on niobium oxide and silicon oxide activated with alkylaluminum reagents. The temperatures for these processes are hi, but the catalysts are relatively inexpensive and can be long lived. These are the types of catalysts that have been used for the synthesis of commodity chemicals by olefin metathesis. 

Although rhenium is a precious metal, promoted Re207-based catalysts are to be preferred over molybdenum ones their activation is simpler and regenerability is much better. Rhenium oxide catalysts can be regenerated many times, while the rhenium can be recovered relatively easily from these catalysts after total deactivation. 

Various metal carbene complexes are known to catalyze this reaction, including those based on metals such as rhenium, molybdenum, tungsten, tantalum, titanium, and ruthenium (F ure 2A) A limited selection of these is discussed here. Well-defined and very active molybdenum catalysts such as Mol and Mo2 are known, but require careful handling in a glove... 

The most successful metathesis catalysts are those based on rhenium, molybdenum, or tungsten. An account of these catalyst systems is given in Section 16.3.1 to Section 16.3.3. 

These molybdenum catalysts deactivate faster than the rhenium catalysts due to an intrinsic deactivation mechanism [69],... 

When highly pure propene is not available commercially, it can be prepared by the reverse metathesis reaction of ethene and 2-butene [Eq. (1)]. The process is performed either at high temperatures (150-350°C) in the gas phase, over molybdenum or tungsten catalysts (Phillips triolefin process) [4], or at low temperatures (50°C) in the liquid phase, in the presence of rhenium-based catalysts (IFP-CPC process) [13], The raw material may be either ethene and the C4 fraction available from the hydroisomerization unit (previously submitted to an isomerization step to maximize its 2-butene content) or ethene alone, which, before admission to the metathesis unit, is partly dimerized to 1-butene, then isomerized to 2-butene in separate units. The process is useful in the event of a high demand for propene, since the C4 fraction is readily available from a cracking unit. 

Another facet of surface organometallic chemistry involves modelling of the mechanisms of surface reactions on the basis of the reactivity of molecular models. For example, the reactivity of metal-imine complexes of molybdenum is considered by CHAN, who proposes elementary steps constituting the catalytic cycle of the surface-catalyzed alkene ammoxidation reaction, which is of great industrial importance. HERRMANN provides some very fine examples of molecular models of the rhenium oxide catalysts used commercially in the alkene metathesis reaction. 

Propylene oxide is also produced in Hquid-phase homogeneous oxidation reactions using various molybdenum-containing catalysts (209,210), cuprous oxide (211), rhenium compounds (212), or an organomonovalent gold(I) complex (213). Whereas gas-phase oxidation of propylene on silver catalysts results primarily in propylene oxide, water, and carbon dioxide as products, the Hquid-phase oxidation of propylene results in an array of oxidation products, such as propylene oxide, acrolein, propylene glycol, acetone, acetaldehyde, and others. 

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