Multiple catalyst systems

In batch reactors, heat transfer will also limit the rate of heat-up to the required temperatures for initiation of polymerization. Use of a multiple catalyst system to provide lower temperature initiation has been proposed to minimize the time and energy required in heating. 

As Upases are able both to esterify free fatty acids and catalyze the alcoholysis of acylglycerols, they can in theory eliminate the need for multiple catalyst systems during biodiesel production from waste greases. Several groups have investigated... 

Figure 2. Comparison of gasification rates for non-, singleand multiple-catalyst systems...

Another procedure for preparing polymer blends involves in-situ polymerization. In many cases, the polymerization of one polymer is conducted in the presence of the other polymer. Impact polystyrene and ABS are typically prepared by polymerization of styrene or styrene-acrylonitrile in the presence of a high molecular weight rubber. Polyolefin blends can be made by the sequential polymerization of compositionally differing polyolefins (e.g., PP/EPR) or by the simultaneous polymerization of polyolefin variants with multiple catalyst systems. 

In a carbene-based multi-catalyst promoted multicomponent reaction, except for the cascade reaction (Scheme 43.7) [16], it is a challenge that each catalyst with different activation modes should be compatible with all reagents, intermediates, and other catalysts present from the onset of the multiple catalyst system for the one-pot domino reaction. 

Oxidation Catalysis. The multiple oxidation states available in molybdenum oxide species make these exceUent catalysts in oxidation reactions. The oxidation of methanol (qv) to formaldehyde (qv) is generally carried out commercially on mixed ferric molybdate—molybdenum trioxide catalysts. The oxidation of propylene (qv) to acrolein (77) and the ammoxidation of propylene to acrylonitrile (qv) (78) are each carried out over bismuth—molybdenum oxide catalyst systems. The latter (Sohio) process produces in excess of 3.6 x 10 t/yr of acrylonitrile, which finds use in the production of fibers (qv), elastomers (qv), and water-soluble polymers. 

In two processes under development as of 1997, the sulfur dioxide stream reacts with reduciag gas over a proprietary catalyst to form elemental sulfur. Both processes have achieved a sulfur recovery of 96% ia a single reactor. Multiple reactor systems are expected to achieve 99+% recovery of the feed sulfur. The direct sulfur recovery process (DSRP), under development at Research Triangle Institute, operates at high temperature and pressure. A similar process being developed at Lawrence Berkeley Laboratory is expected to operate near atmospheric pressure. 

The fourth chapter gives a comprehensive review about catalyzed hydroamina-tions of carbon carbon multiple bond systems from the beginning of this century to the state-of-the-art today. As was mentioned above, the direct - and whenever possible stereoselective - addition of amines to unsaturated hydrocarbons is one of the shortest routes to produce (chiral) amines. Provided that a catalyst of sufficient activity and stabihty can be found, this heterofunctionalization reaction could compete with classical substitution chemistry and is of high industrial interest. As the authors J. J. Bmnet and D. Neibecker show in their contribution, almost any transition metal salt has been subjected to this reaction and numerous reaction conditions were tested. However, although considerable progress has been made and enantios-electivites of 95% could be reached, all catalytic systems known to date suffer from low activity (TOP < 500 h ) or/and low stability. The most effective systems are represented by some iridium phosphine or cyclopentadienyl samarium complexes. 

Transition metal NPs dispersed in ILs are recognized as suitable catalyst systems for many reaction types with both typical homogeneous and heterogeneous catalysis (see Sections 1.3.1-1.3.5). The most investigated reactions are the hydrogenation of multiple bonds and arenes [1, 5, 12, 13, 43, 44, 54, 80, 89, 92], carbon-carbon cross-... 

Even considering that the recovery of an expensive catalyst system offers strong economic benefits, the option of a multiple use of a two- or three-component solvent system should be shown. Otherwise separation of the solvent mix into its components would compensate for potential benefits. 

As previously mentioned, Chung and co-workers have also developed a highly efficient coordinatively unsaturated catalyst system, which has general applicability to multiple heteroatom tethers (Scheme 12.8) [7]. This was the first example of a single rhodium catalyst system capable of facilitating [4-1-2] annulations for all three of the most common tethers (O, N, and C), including some intermolecular examples. 

An ionic liquid was fully immobilized, rather than merely supported, on the surface of silica through a multiple-step synthesis as shown in Fig. 15 (97). A ligand tri(m-sulfonyl)triphenyl phosphine tris(l-butyl-3-methyl-imidazolium) salt (tppti) was prepared so that the catalyst, formed from dicarbonylacetylacetonate rhodium and the ligand (P/Rh = 10), could be soluble in both [BMIMJBFq and [BMIM]PF6. The supported ionic liquid-catalyst systems showed nearly three times higher rate of reaction (rate constant = 65 min ) that a biphasic system for the hydroformylation of 1-hexene at 100°C and 1500 psi in a batch reactor, but the n/i selectivity was nearly constant the same for the two ( 2.4). Unfortunately, both the supported and the biphasic ionic liquid systems exhibited similar metal leaching behavior. 

The catalyst has very low activity during warmup until the catalyst temperature attains -200°C. Since the oxidation reactions are exothermic, the catalyst provides heat, which further heats the catalyst and increases the rate. Therefore, the ACC can exhibit multiple steady states, where the catalyst has essentially no activity until a certain temperature where the catalyst ignites. This is described as lightojf, and a good catalyst system has a low lightoff temperature. Lightoff characteristics are sketched in Figure 7-18. 

The (co)polymerization of dienes can be a good method for the preparation of polymers with reactive vinyl groups, a method that enables the preparation of polymers possessing plural vinyl groups per polymer chain. A fluorinated bis(phenoxy-imine) Ti complex was shown by Coates and co-workers to convert 1,5-hexadiene to poly(methylene-l,3-cyclopentane-fti-3-vinyl tetramethylene), which contained multiple vinyl groups. As already discussed, Saito et al. and others revealed that bis(phenoxy-imine) Ti complexes favored secondary insertion. " This is probably responsible for the formation of 3-vinyl tetramethylene units. Likewise, the same catalyst system can form sPP-/ -poly(methylene-l,3-cyclopentane-z -3-vinyl tetramethylene) from propylene and 1,5-hexadiene. Very recently. 

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