Other Catalyst Systems

Kinetic studies indicate that a phenoxyl radical is generated by the Fe(III) complex, which combines with 0 and subsequently with another 

The oxidative coupling of 2,6-disubstituted phenols occurs in the presence of an oxidizing agents like alkaline ferrocyanide [75,76], MnO [77], PbO [78], CrO [79], strong base [80], etc., but also takes 

Oxidative coupling of 2,6-disubstituted phenols is catalyzed by copper-amine complexes [66]. Endres et al. [70] have shown that the ratio of the nitrogen-containing ligand and copper can be used to control C-0 vs. C-C coupling. 

Copper chloride coordinated to aminated polystyrene is active in the oxidation of 2,6-dime thy Iphenol (2,6-DMP) to the DPQ [95]. In the mechanism proposed, a fi-peroxocopper(II) complex of the structure  

A similar macromolecular imidazole ligand has also been synthesized and tested in oxidative coupling. The activities are higher compared with low molecular analogs, and this was at least partly due to the enhanced rate of copper reoxidation [96]. 

In our initial studies on the [5+2] cycloaddition, several metal catalysts were screened. Rhodium(I) systems were found to provide the optimum yields and generality [26]. Since the introduction of this new reaction in 1995, our group and others have reported other catalyst systems that can effect the cycloaddition of tethered VCPs and systems. These new catalysts thus far include chlororhodium dicarbonyl dimer ( [RhCl(CO)2]2 ) [27], bidentate phosphine chlororhodium dimers such as [RhCl(dppb)]2 [28] and [RhCl(dppe)]2 [29], and arene-rhodium complexes [(arene)Rh(cod)] SbFs [30]. [Cp Ru(NCCH3)3] PFg has also been demonstrated to be effective in the case of tethered alkyne-VCPs [31], but has not yet been extended to intermolecular systems or other 2n -components. 

The reaction of VCP 79 illustrates the performance of the rhodium(I) dimer (Tab. 13.4). For reference, attempts to effect [5+2] cycloadditions with this substrate (79) and [RhCl(PPh3)3]/silver triflate resulted only in the formation of complex product mixtures. In remarkable contrast, when this same substrate was treated with 5 mol% [RhCl(CO)2]2 for 20 min in toluene at 110°C, the [5+2] cycloadduct 80 was obtained in 80% yield. Despite these significant advantages, tethered alkene-VCPs are not successfully converted with this catalyst. 

In 2000 Xumu Zhang and co-workers reported a bisphosphine complex of rhodium, [RhCl(dppb)]2, which, when used with silver hexafluoroantimonate, in some cases offers a rate advantage over the Wilkinson s catalyst/silver triflate system [28], allowing the reaction to be conducted at room temperature (Tab. 13.5). Scott Gilbertson and coworkers reported a related example, employing [Rh(dppe)(CH2Cl2)2] SbFg as a catalyst for the [5+2] reaction (Tab. 13.5, entry 5) [29]. 

Arene complexes of rhodium have also been proven to be effective pre-catalysts for the [5+2] reaction [30]. We had previously shown that intramolecular diene-alkyne (and 

An analogous complex 93 (](CioH8)Rh(COD)] SbFg) is prepared in a single step from commercially available ]RhCl(COD)]2 through a known procedure [35]. The complex is air-stable at room temperature for prolonged periods of time and retains its catalytic activity even after several months storage. 

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Dow catalysts have a high capabihty to copolymetize linear a-olefias with ethylene. As a result, when these catalysts are used in solution-type polymerisation reactions, they also copolymerise ethylene with polymer molecules containing vinyl double bonds at their ends. This autocopolymerisation reaction is able to produce LLDPE molecules with long-chain branches that exhibit some beneficial processing properties (1,2,38,39). Distinct from other catalyst systems, Dow catalysts can also copolymerise ethylene with styrene and hindered olefins (40). 

Only trace amounts of side-chain chlorinated products are formed with suitably active catalysts. It is usually desirable to remove reactive chlorides prior to fractionation in order to niinimi2e the risk of equipment corrosion. The separation of o- and -chlorotoluenes by fractionation requires a high efficiency, isomer-separation column. The small amount of y -chlorotoluene formed in the chlorination cannot be separated by fractionation and remains in the -isomer fraction. The toluene feed should be essentially free of paraffinic impurities that may produce high boiling residues that foul heat-transfer surfaces. Trace water contamination has no effect on product composition. Steel can be used as constmction material for catalyst systems containing iron. However, glass-lined equipment is usually preferred and must be used with other catalyst systems. 

Several other catalyst systems have been suggested, including boron fluoride and both crystalline and noncrystalline siUcas and alurninosihcates. Although no commercial faciUty exists, the concept of using a crystalline siUca or alurninosihcate catalyst in an integral reaction and distillation apparatus has been proposed (9). 

The catalyst should be reduced and sulfided during the initial stages of operation before use. Other catalyst systems used in HDS are NiO/MoOs and NiOAVOs. Because mass transfer has a significant influence on the reaction rates, catalyst performance is significantly affected by the particle size and pore diameter. 

Other catalyst systems such as iron V2O5-P2O5 over silica alumina are used for the oxidation. In the Monsanto process (Figure 6-4), n-butane and air are fed to a multitube fixed-bed reactor, which is cooled with molten salt. The catalyst used is a proprietary modified vanadium oxide. The exit gas stream is cooled, and crude maleic anhydride is absorbed then recovered from the solvent in the stripper. Maleic anhydride is further purified using a proprietary solvent purification system. ... 

Similar observations were reported by Wallace and Schrock18a) who utilized a tantalum based catalyst in their studies. In fact, studies of other catalyst systems are presently being persued vigorously 18 b). 

Crabtree s catalyst is an efficient catalyst precursor for the selective hydrogenation of olefin resident within nitrile butadiene rubber (NBR). Its activity is favorably comparable to those of other catalyst systems used for this process. Under the conditions studied the process is essentially first order with respect to [Ir] and hydrogen pressure, implying that the active complex is mononuclear. Nitrile reduces the catalyst activity, by coordination to the metal center. At higher reaction pressures a tendency towards zero order behavior with respect to catalyst concentration was noted. This indicated the likelihood of further complexity in the system which can lead to possible formation of a multinuclear complex that causes loss of catalyst activity. 

The need for higher product specificity and milder reaction conditions (see also Section IX) has led to extensive research in hydroformylation technology. This research, as reported in technical journals, patent literature, and commercial practice has been primarily concerned with catalysis by rhodium, in addition to the traditional cobalt, and with catalyst modification by trialkyl or triaryl phosphines. These catalyst systems form the basis for the major portion of the discussion in this chapter some other catalyst systems are discussed in Section VIII. 

Although Pd-based catalyst systems form only the desirable trans-1,4-hexadiene, their industrial value is very doubtful because of the reported poor yield of the C6 diene and the very low activity of the dimerization reactions compared to other catalyst systems. 

A compound with a highly temperatine-dependent property, such as solubility, is said to be thermomorphic. There are a variety of other catalyst systems that can be recovered on the basis of temperature-dependent solubilities, usually involving polymers [13-18]. Attention has also been given to nonthermal solubility switches . These include photochemically and chemically triggered precipitons [19-25], and the use of CO2 pressure to regulate solubilities [26-28]. The latter protocol is particularly suited to liuorous compounds. 

Although generalization of the present results to other catalyst systems is tempting, the evidence currently available is undoubtedly insufficient to justify such an extrapolation. It is clear, however, from both the present and previous work in this laboratory, that the chlorine atoms interact with the surface of the catalyst. There is also strong evidence to support the contention that the effect produced by the introduction of TCM into the feedstream can be primarily attributed to a modification of the catalyst surface and not to a gas phase process. The present work demonstrates that, at least with praseodymium oxide, the oxychloride is produced on addition of TCM and further, the oxychloride is largely responsible for the beneficial effects. Since the enhancements observed with TCM have been shown (2, 4-7) to be related to the nature of the catalyst, it is conceivable that these effects, while dependent on the formation of the oxychloride, are also a function of the thermodynamic stability of the oxychloride. Further work is in progress. 

The NSR technology has been also applied to diesel engines, and is most reliable and attractive method for lean-burn combustion vehicles. Diesel particulate-NOx reduction system (DPNR) method is used to realize the simultaneous and continuous reduction of particulate and NOx is also recommended. This catalyst system is DPF combined with NSR catalyst. Soot on catalyst is removed during NOx reduction by occasional rich engine modification. Many other catalyst systems with NSR catalyst have been also developed. With decreasing S content in fuel and successive development of... 

Mixed catalysts have the titanium in the oxidation states four and three together with an organic aluminum compound. The molar ratio of Ti[V to Tini is preferably 2.6 1 (4). Such a catalyst, preactivated with triethylaluminum exhibits a low tendency to form deposits. Other catalyst systems are based on organic zirconium or hafnium compounds. 

Using this approach to study heterogeneous catalysis on the atomic scale, we have investigated the mechanism of hydrocarbon catalysis by platinum surfaces. We shall describe in detail the results of these studies, which are pertinent in determining the nature of the active sites on the surface of this metal. We shall show how the results obtained for platinum may be extrapolated to other catalyst systems. Finally, we shall present a model of metal catalysis that has been emerging from our studies of platinum surfaces. 

In comparison to the bismuth molybdate and cuprous oxide catalyst systems, data on other catalyst systems are much more sparse. However, by the use of similar labeling techniques, the allylic species has been identified as an intermediate in the selective oxidation of propylene over uranium antimonate catalysts (20), tin oxide-antimony oxide catalysts (21), and supported rhodium, ruthenium (22), and gold (23) catalysts. A direct observation of the allylic species has been made on zinc oxide by means of infrared spectroscopy (24-26). In this system, however, only adsorbed acrolein is detected because the temperature cannot be raised sufficiently to cause desorption of acrolein without initiating reactions which yield primarily oxides of carbon and water. 

From a comparison of the coupling of alkyl bromides 1 with aryl Grignard reagents 2 with different catalysts it can be concluded that all provided essentially the same result, although the required reaction temperatures and times varied (Table 1, entries 1-3, 5, 6, 8, and 9). The catalytic system using 4 is the fastest of all. That the most electron-rich complex is the best electron donor to generate radicals is a likely explanation. An Fe(-II)/Fe(-I) manifold (17A-D) accounts for the observed results (Fig. 3, Table 1, entry 2). Whether other catalyst systems can reach this redox state under the reaction conditions remains open. However, other low-valent redox manifolds, such as the Fe(0)/Fe(I) or Fe(I)/Fe(II) manifolds (18A-D, 19A-D), are also viable and may account for the reactivity differences. 

The molecular weights of the products obtained with the metal sulfate-sulfuric acid catalyst were high but not higher than those obtained with other catalyst systems (27,28). 

Clearly, the level of sophistication involved in the TS-1 catalyst is greater than that involved with the other catalyst systems listed in Table 1. The generation of 2.5 mol% titanium in solid solution in silicalite makes for a very dilute system with a limited number of active sites per unit volume.8 However, this approach seems to be necessary to expand the range and applicability of selective oxidation catalysis. 

Polymerization. Figures 1 and 2 show how activating /%TiCl3 with various amounts and types of alkylaluminum compounds affects polymerization of butadiene and isoprene (2, 3). Tables I and II give additional information on the polymerization with the best of these catalysts and with a few other catalyst systems. 

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