Olefins catalyst preparation

Ziegler catalysts Complex catalysts prepared by interaction between an organometallic derivative and a transition metal derivative. A typical catalyst is the product of the interaction of TiCU and AIBU3. These catalysts polymerize olefins, particularly ethylene, to polyolefins, the polymerization generally being in a siereoregular manner. 

The isomerization of an allylic amine to an enamine by means of a formal 1,3-hydrogen shift constitutes a relatively small structural change. However, this transformation could be extremely valuable if it could be rendered stereoselective. In important early studies, Otsuka and Tani showed that a chiral cobalt catalyst, prepared in situ from a Co(ii) salt, a chiral phosphine, and diisobutylaluminum hydride (Dibal-H), can bring about the conversion of certain pro-chiral olefins to chiral, isomeric olefins by double bond migra-... 

According to detailed XRD analyses, the two catalyst preparation procedures under study formed solid solutions. The application of sol-gel method led to improved selectivity to olefins in the reaction of propane ODH, compared to the simple procedure of evaporation and decomposition. However, the propane conversion on the sol-gel catalysts was lower at the same experimental conditions, while the catalysts surface area was higher. Moreover, the sol-gel samples presented higher basicity as shown by C02 TPD. It could be explained by a better incorporation of Nd into the AEO lattice, creating cationic vacancies for attaining electroneutrality and thus rendering the nearby oxide anions coordinatively unsaturated and more basic. 

In catalyst preparation, one can use this knowledge to determine the relative contributions of various hydroxyl groups before and after application of the active phase onto the support. In this way Sibeijn etal. [31] established that rhenium oxide attached to acidic sites of the alumina support exhibits higher activity for the metathesis of olefins than rhenium oxide on neutral or basic sites. As, however, rhenium species preferentially exchange with basic hydroxyls, one needs to increase the loading above a certain value (6 wt% for an alumina of 200 m2/gram) before the catalyst exhibits appreciable activity [31]. 

This procedure describes the preparation and use of an effective chiral catalyst for the asymmetric allylation of aldehydes. A previous synthesis of optically pure 1-(phenylmethoxy)-4-penten 2-ol requires seven steps from D-mannitol.4 This procedure has been employed successfully with other aldehydes,5 and also with methallyltributylstannane5 (see Table). Catalysts prepared from (R)- or (S)-BINOL and Ti(0-i-Pr)4 at 2 1 stoichiometry have also proven useful in these reactions.The olefinic products may be regarded as latent aldol products between aldehydes and the enolate of actetaldehyde or acetone. In all cases examined thus far, enantioselectivity... 

For practical hydrogenation of olefins four classes of metal complexes are preferred (a) Rh complexes, the RhCl(PPh3)3, the so-called Wilkinson catalyst and the [Rh(diene)-(PR3)2]+ complexes, (b) a mixture of Pt and Sn chlorides, (c) anionic cyanocobalt complexes and (d) Ziegler catalysts, prepared from a transition metal salt and an alkylaluminum compound. 

In the results presented in Table 13.5, the addition of tin affects the kinetic selectivity r differently, depending on the catalyst preparation method. When compared to the monometallic PdO catalyst, r slightly decreases for the coimpregnated PdSn catalyst, but it sharply increases for the PdOSn catalyst prepared via the colloidal oxide synthesis. As the intrinsic kinetic constant rates k do not show significant discrepancies between the different catalysts, the main contribution of the variation of the kinetic selectivity is ascribed to the adsorption constant ratio fBo/ Butenes- In the case of the PdOSn catalyst, formation of but-l-ene is favored compared to its consumption because the X Bo/ Butenes ratio increases, indicating that olefin adsorption is much more destabilized than diene adsorption. Thus, the olefin easily desorbs before being hydrogenated into butane. 

Double-bond isomerization reaction of simple olefins requires strong basic catalysts. Various catalyst systems have been reported for this reaction. They include sodium-organosodium catalysts prepared in situ by reacting an excess of sodium with a reactive organic compound, such as o-chlorotoluene or anthracene as reported by Pines and co-workers 5-8). 

Catalysts and reaction conditions used are generally similar to those used for olefin isomerization. Catalysts reported are sodium-organosodium catalysts prepared in situ by reaction of a promoter such as o-chloro-toluene or anthracene with sodium 19-24), alkali metal hydrides 20,21), alkali metals 22), benzylsodium 26), and potassium-graphite 26). These catalysts are strong bases that can react with alkylaromatics to replace a benzylic hydrogen [Reaction (2)]. 

More pragmatically, one can attempt to address the question of the benefits of nanostructured catalysts by comparison against the properties of similar catalysts prepared by traditional methods. There are a couple of well-known reactions that are thought to be catalyzed by atomically dispersed metals on supports [95]. Epoxi-dation of olefins by titanium on silica is one [96, 97]. 

An ex-carbonyl K-promoted alumina-supported catalyst prepared from Ru3(CO),2 and decarbonylated under H2 at 450°C was more dispersed and more active and selective for C2-C5 olefins in the Fischer-Tropsch reaction than conventionally prepared samples [108]. 

Catalysts prepared from iridium neutral binary carbonyl compounds and several supports have been studied extensively. Small Ir (x = 4, 6) clusters supported on several oxides and caged in zeolite, and their characterization by EXAFS, have been prepared [159, 179, 180, 194-196]. The nuclearity of the resulting metallic clusters has been related with their catalytic behavior in olefin hydrogenation reactions [197]. This reaction is structure insensitive, which means that the rate of the reac-hon does not depend on the size of the metallic particle. Usually, the metallic parhcles are larger than 1 nm and consequently they have bulk-like metallic behavior. However, if the size of the particles is small enough to lose their bulk-like metallic behavior, the rate of the catalytic reaction can depend on the size of the metal cluster frame used as catalyst. 

TABLE 2. Summary of Olefin Metathesis Catalysts Prepared by Reacting a Selected Schiff Bases with RuCI2( p-cymeneb and then Postreacting with Alkyl or Aromatic Lithium Salt"... 

Only one olefin dimerization, namely that of ethylene to butene, has been attempted using a supported metal catalyst (58) (Table V). The catalyst, prepared as in the preceeding reaction (34), had nearly the same activity as its homogeneous analog. However, the reaction clearly took place on the catalyst surface since no evidence could be found for any nickel in solution. The presence of a solvent (toluene) was important for the selectivity of the reaction since in its absence considerable amounts of hexenes and octenes were formed. 

A unique titanium(IV)-silica catalyst prepared by impregnating silica with TiCLt or organotitanium compounds exhibits excellent properties with selectivities comparable to the best homogeneous molybdenum catalysts.285 The new zeolite-like catalyst titanium silicalite (TS-1) featuring isomorphous substitution of Si(IV) with Ti(IV) is a very efficient heterogeneous catalyst for selective oxidations with H2C>2.184,185 It exhibits remarkable activities and selectivities in epoxidation of simple olefins.188,304-306 Propylene, for instance, was epoxidized304 with 97% selectivity at 90% conversion at 40°C. Shape-selective epoxidation of 1- and 2-hexenes was observed with this system that failed to catalyze the transformation of cyclohexene.306 Surface peroxotitanate 13 is suggested to be the active spe-... 

The hydrogenation of 7 over the catalyst prepared by the reduction of 3 in the absence of any added ligand gave a cis/trans product ratio of 4.0. In addition, a rather large amount of the endo olefin, 10, was also formed. When the catalyst was prepared by the reduction of 3 in the presence of triphenylphosphine (6), a saturated isomer ratio of 2.0 was observed with no double bond isomerization. The same product stereochemistry and lack of double bond isomerization was also found using pre-hydrogenated 1 in the reduction of 7 (3). Thus, one might assume that the catalytic species in these latter two reactions are quite similar, if not identical. 

The hydrogenation of 2 occurred reasonably well in benzene-ethanol to give a rather poor catalyst for the hydrogenation of 1-heptene. The reduction of 2 in the presence of 6 gave a catalyst which was almost twice as reactive for this hydrogenation (Table II). In both cases though, double bond isomerization occurred almost as rapidly as did hydrogenation of 7 over these reduced 2 catalysts. But in this case, the catalyst prepared from 2 in the absence of 6 was almost completely unreactive, presumably because of the increased bulk of the olefin (Table III). 

Previous work has shown that the electronic characteristics of the benzene substituent in triarylphosphine chlororhodium complexes have a marked influence on the rate of olefin hydrogenation catalyzed by these compounds. Thus, in the hydrogenation of cyclohexene using L3RhCl the rate decreased as L = tri-p-methoxyphenylphosphine > triphenylphosphine > tri-p-fluorophenylphosphine (14). In the hydrogenation of 1-hexene with catalysts prepared by treating dicyclooctene rhodium chloride with 2.2-2.5 equivalents of substituted triarylphosphines, the substituent effect on the rate was p-methoxy > p-methyl >> p-chloro (15). No mention could be found of any product stereochemistry studies using this type of catalyst. 

The only case of stereoelective polymerization of vinyl monomers so far known is the polymerization of some racemic a-olefins with the aid of catalysts prepared from TiCl4 or TiCls and bis-[(S)-2-methyl-butyl]-zinc [104,107). 

With modified Aratani catalysts (2, R = Ph and A = CH2Ph), Reissig observed moderate enantioselectivities (30-40% ee for the trans cyclopropane isomer) for reactions between trimethylsilyl vinyl ethers and methyl diazoacetate [26], but vinyl ethers are the most reactive olefins towards cyclopropanation and also the least selective [30,31]. Other chiral Schiff bases have been examined for enantio-selection by using the in situ method for catalyst preparation that was pioneered by Brunner, but enantioselectivities were generally low [32]. 

In die last 10 years or so an exciting new strategy has emerged for the formation of carbon-carbon double bonds, namely olefin metathesis. This work grew out of the development of Ziegler-Natta catalysts for die polymerizarion of cyclic olefins. It was found that when 2-pentene was treated with a catalyst prepared from tungsten hexachloride and ethylaluminum dichloride, a mixture of 2-pentene, 2-butene, and 3-hexene was produced in minutes at room temperature (rt) ... 

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