The Different Catalyst Systems

The four types of oxygen tolerance are exemplified with four experiments of the oxidation of methanol as shown in Figure 3. Table 2 summarizes the results of the oxidation of methanol and HMF for the different catalyst systems. 

In Ziegler-Natta catalysts, quantitative information about the number of active centers is very important. Together with kinetic data and polymer microstructural and stereochemical analyses, they contribute to the formulation of the most likely reaction mechanism and to the understanding of the roles the different catalyst system components play. With the advent of supported catalysts, this information has become... 

The manufacture of polybutadiene rubber (BR), which is carried out mainly in solution processes using butyllithium or the Ziegler-Natta-type catalysts mentioned in Table 1, has some general features in common, despite the different catalyst systems [83]. The feed requirements for polymerization using Ziegler-Natta or... 

As a rule, however, carbanion reactivities in the coordinative saturated y9-aminoethyl transition metal complexes are generally kinetically too strongly suppressed. One possible way of circumventing this barrier is shown in Scheme 3, along route ( ). If the transition metal is d-electron-rich and can receive the proton by oxidative addition, then the alkylamine can be eventually more easily generated by reductive elimination with formation of the starting transition metal complex. With these principles of catalytic activation as a guideline, the different catalyst systems described in the literature will now be discussed in some detail. 

Regarding the structure of the cyanoprene homopolymers (Table II), a trend becomes apparent when using the different catalyst systems. In a sample polymerized with diphenylphosphine lithium, 70% 1,2-linkages and about 30% 1,4-linkages were found, but the presence of traces of 3,4-adducts could not be excluded. There was no indication of cyclic structures although with stronger diphenylphosphine-potassium cyclic compounds were found. About 62% were 1,2-linkages while the remainder consisted of about equal parts of 1,4- and 3,4-adducts, the cyclic proportions included. 

In more comprehensive studies, the catalytic performance of the silica-supported catalyst based on the different ligands was found to be essentially uninfluenced by the type of ionic liquid in the gas-phase propene hydroformylation reactions, as no significant differences were observed with [BMIM][PF6] or [BMIM][n-C8Hi70S03] being the ionic liquids applied (except for the catalyst preformation which appeared to depend on the solubility of the ligand and precursor). Furthermore, this was also shown to be the case in a study performed with analogous catalysts based on alternative oxide supports such as titania, alumina and zirconia [97]. In Table 5.6-4 the most relevant results for the hydroformylation of propene with the different catalyst systems based on the ionic liquid [BMIMjfPFe] and pre-dried silica support are compiled. 

The results for the 12.4%Co/Si02 catalyst, the 0.2%Ru-10%TiO2 catalyst, the 0.5%Pt-15%Co/Al2O3 catalyst, and the 25%Co/Al203 catalyst are summarized in Tables 6 - 9 for comparative purposes among the different catalyst systems, and for comparison with the four case studies previously summarized. 

Product selectivities differ between the different catalyst systems. In all cases, reactions are regioselective giving anii-Markovnikov or 2,1-addition of the phosphine to the least hindered end of the unsaturated system (Scheme 12a). For example, in the hydrophosphination of styrenes this selectivity can be attributed to the organization of the transition state to P-C bond formation. In the case of the 2,1-insertion of styrene into the Ca-P bond the phenyl group may stabilize the adjacent anionic center. In the case of a 1,2-insertion no such stabilization exists [106]. 

Alternatively, butadiene can be oxidized in the presence of acetic acid to produce butenediol diacetate, a precursor to butanediol. The latter process has been commercialized (102—104). This reaction is performed in the Hquid phase at 80°C with a Pd—Te—C catalyst. A different catalyst system based on PdCl2(NCCgH )2 has been reported (105). Copper- (106) and rhodium- (107) based catalysts have also been studied. 

Reactions of the Side Chain. Benzyl chloride is hydrolyzed slowly by boiling water and more rapidly at elevated temperature and pressure in the presence of alkaHes (11). Reaction with aqueous sodium cyanide, preferably in the presence of a quaternary ammonium chloride, produces phenylacetonitrile [140-29-4] in high yield (12). The presence of a lower molecular-weight alcohol gives faster rates and higher yields. In the presence of suitable catalysts benzyl chloride reacts with carbon monoxide to produce phenylacetic acid [103-82-2] (13—15). With different catalyst systems in the presence of calcium hydroxide, double carbonylation to phenylpymvic acid [156-06-9] occurs (16). Benzyl esters are formed by heating benzyl chloride with the sodium salts of acids benzyl ethers by reaction with sodium alkoxides. The ease of ether formation is improved by the use of phase-transfer catalysts (17) (see Catalysis, phase-thansfer). 

The general picture of the relative merits of homogeneous and heterogeneous processes has not yet emerged clearly. The homogeneous catalyst system may offer advantages in chemical efficiency but lead to difficulties of catalyst separation and recovery, or catalysts may tend to plate out in the reactor due to slight instability. Materials of construction may have to be different for the two rival plants. All these factors will have to be considered in an economic assessment and detailed studies made of the complete process networks in both cases. 

Figures lb and Ic report the conversions of CO and H2 as functions of reaction temperature for the different catalysts. In line >vith general literature indications over all the investigated systems CO and H2 oxidize at markedly lower temperatures than CH4 Tio% ranging from 393 to 523 K and from 393 to 573 K have been observed for CO and H2 respectively, to be compared with 773-823 K required by CH4.

A wide variety of solid materials are used in catalytic processes. Generally, the (surface) structure of metal and supported metal catalysts is relatively simple. For that reason, we will first focus on metal catalysts. Supported metal catalysts are produced in many forms. Often, their preparation involves impregnation or ion exchange, followed by calcination and reduction. Depending on the conditions quite different catalyst systems are produced. When crystalline sizes are not very small, typically > 5 nm, the metal crystals behave like bulk crystals with similar crystal faces. However, in catalysis smaller particles are often used. They are referred to as crystallites , aggregates , or clusters . When the dimensions are not known we will refer to them as particles . In principle, the structure of oxidic catalysts is more complex than that of metal catalysts. The surface often contains different types of active sites a combination of acid and basic sites on one catalyst is quite common. 

Figure 1. Dependence of polybutadiene cistacticity (%) on the nature of halogen ligand for different catalyst systems. The data of the first four lines are taken from Ref. 18. cis content by IR from Ref. 34.

The GPC trace is dramatically different when a CSA is added in the mixed catalyst system in that a simple composite GPC is not obtained (Fig. 13). Inclusion of either TEA or DEZ in the polymerization produces a single peak in the GPC,... 

The above results indicate that the monoalkylaluminum chloride rather than the dialkylaluminum chloride is the more effective cocatalyst. The relatively high activity of Kealy s catalyst can be attributed to the in situ formation of RA1C12 as a result of reduction of Ni11 by the dialkylaluminum chloride. However, such reduction is not possible in the R2A1C1/Ni° catalyst system. Thus, the differences between RA1C12 and R2A1C1 can be readily compared with the Ni° catalyst systems. 

The isomer distribution of the nickel catalyst system in general is similar qualitatively to that of the Rh catalyst system described earlier. However, quantitatively it is quite different. In the Rh system the 1,2-adduct, i.e., 3-methyl-1,4-hexadiene is about 1-3% of the total C6 products formed, while in the Ni system it varies from 6 to 17% depending on the phosphine used. There is a distinct trend that the amount of this isomer increases with increasing donor property of the phosphine ligands (see Table X). The quantity of 3-methyl-1,4-pentadiene produced is not affected by butadiene conversion. On the other hand the formation of 2,4-hexadienes which consists of three geometric isomers—trans-trans, trans-cis, and cis-cis—is controlled by butadiene conversion. However, the double-bond isomerization reaction of 1,4-hexadiene to 2,4-hexadiene by the nickel catalyst is significantly slower than that by the Rh catalyst. Thus at the same level of butadiene conversion, the nickel catalyst produces significantly less 2,4-hexadiene (see Fig. 2). 

In the literature there are many reports of the formation of active catalyst for the 1 1 codimerization or synthesis of 1,4-hexadiene employing a large variety of Co or Fe salts, in conjunction with different kinds of ligands and organometallic cocatalysts. There must have been many structures, all of which are active for the codimerization reaction to one degree or another. The scope of the catalyst compositions claimed to be active as the codimerization catalysts is shown in Table XV (69-82). As with the nickel catalyst system discussed earlier, the preferred Co or Fe catalyst system requires the presence of phosphine ligands and an alkylaluminum cocatalyst. The catalytic property can be optimized by structural control of these two components. 

Fischer-Tropsch A process for converting synthesis gas (a mixture of carbon monoxide and hydrogen) to liquid fuels. Modified versions were known as the Synol and Synthol processes. The process is operated under pressure at 200 to 350°C, over a catalyst. Several different catalyst systems have been used at different periods, notably iron-zinc oxide, nickel-thoria on kieselgtihr, cobalt-thoria on kieselgiihr, and cemented iron oxide. The main products are C5-Cn aliphatic hydrocarbons the aromatics content can be varied by varying the process conditions. The basic reaction was discovered in 1923 by F. Fischer and... 

HC Platforming [Hydrocracking] A version of the Platforming process which uses different catalyst systems before the reforming catalyst in order to partially hydrocrack the feed before converting it to aromatic hydrocarbons. 

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