Palladium-type catalysts

Another appHcation for this type catalyst is ia the purification of styrene. Trace amounts (200—300 ppmw) of phenylacetylene can inhibit styrene polymerization and caimot easily be removed from styrene produced by dehydrogenation of ethylbenzene using the high activity catalysts introduced in the 1980s. Treatment of styrene with hydrogen over an inhibited supported palladium catalyst in a small post reactor lowers phenylacetylene concentrations to a tolerable level of <50 ppmw without significant loss of styrene. 

For the activation of a substrate such as 19a via coordination of the two carbonyl oxygen atoms to the metal, one should expect that a hard Lewis acid would be more suitable, since the carbonyl oxygens are hard Lewis bases. Nevertheless, Fu-rukawa et al. succeeded in applying the relative soft metal palladium as catalyst for the 1,3-dipolar cycloaddition reaction between 1 and 19a (Scheme 6.36) [79, 80]. They applied the dicationic Pd-BINAP 54 as the catalyst, and whereas this type of catalytic reactions is often carried out at rt or at 0°C, the reactions catalyzed by 54 required heating at 40 °C in order to proceed. In most cases mixtures of endo-21 and exo-21 were obtained, however, high enantioselectivity of up to 93% were obtained for reactions of some derivatives of 1. 

Johnson et al. (J4) investigated the hydrogenation of a-methylstyrene catalyzed by a palladium-alumina catalyst suspended in a stirred reactor. The experimental data have recently been reinterpreted in a paper by Polejes and Hougen (P4), in which the original treatment is extended to take account of variations in catalyst loading, variations in impeller type, and variations of gas-phase composition. Empirical correlations for liquid-side resistance to gas-liquid and liquid-solid mass transfer are presented. 

For the synthesis of heterocycles, an efficient strategy has been introduced utilizing the dual transition metal sequences (Scheme 6).11,lla The key issue is the compatibility of the two catalyst systems. Jeong et al. studied the one-pot preparation of bicyclopentenone 35 from propargylsulfonamide 33 and allylic acetate.11 This transformation includes two reactions the first palladium-catalyzed allylation of 33 generates an enyne 34 and the following Pauson-Khand type reaction (PKR) of 34 yields a bicyclopentenone 35. The success of this transformation reflects the right combination of catalysts which are compatible with each other because the allylic amination can be facilitated by the electron-rich palladium(O) catalyst and the PKR needs a Lewis-acidic catalyst. Trost et al. reported the one-pot enantioselective... 

Interestingly, if the cyclization was carried out in the presence of an aryl or allyl halide and a palladium(O) catalyst, an additional C-C coupling step via presumed intermediate 210 led to the formation of tetrasubstituted furans of type 211 (Scheme 2.64) [101]. 

Since 1981, three-way catalytic systems have been standard in new cars sold in North America.6,280 These systems consist of platinum, palladium, and rhodium catalysts dispersed on an activated alumina layer ( wash-coat ) on a ceramic honeycomb monolith the Pt and Pd serve primarily to catalyze oxidation of the CO and hydrocarbons, and the Rh to catalyze reduction of the NO. These converters operate with a near-stoichiometric air-fuel mix at 400-600 °C higher temperatures may cause the Rh to react with the washcoat. In some designs, the catalyst bed is electrically heated at start-up to avoid the problem of temporarily excessive CO emissions from a cold catalyst. Zeolite-type catalysts containing bound metal atoms or ions (e.g., Cu/ZSM-5) have been proposed as alternatives to systems based on precious metals. 

In this work the preparation of orthophenylenediamine (OPDA) from 4-chloro-2-nitroani1ine was studied on alumina supported palladium catalysts. High OPDA yields were obtained on catalysts containing stabilized ionic palladium. In the preparation of the given palladium containing catalysts anchoring type surface reactions were used. The existence of palladium in ionic form was evidenced by XPS and El R measurements. 

The catalytic oxycarbonylation of benzene and naphthalene to benzoic or naphthoic acid in the presence of Wacker-type catalysts has been reported in several patents,376,448 but difficulties in reoxidizing the reduced palladium have inhibited industrial use of this chemistry. 

Heck-type step-growth condensation polymerisation involves mainly palladium-based catalysts, although nickel-based catalysts are also effective. It is worth noting that this polycondensation requires a change in the oxidation state of the metal (e.g. Pd) [schemes (30) and (31)] [71], which is in contrast to chain growth polymerisation, such as ethylene/carbon monoxide alternating copolymerisation promoted by Pd-based catalysts [schemes (82) and (83) in Chapter 3], for which the preservation of the oxidation state of palladium, Pd(II), is typical [83-85] ... 

In this project, we make use of platinum-type catalyst on silica gel. Although this is less selective than more modem palladium-based catalysts, kinetic data are available in the literature as an LHHW model [2], better suited for flexible reactor design. The reaction rate equations are ... 

The catalyst plays a crucial role in technology. Previously, catalysts were based on palladium of 1 to 5 wt% impregnated on silica with alkali metal acetates as activators. Modern catalysts employ as enhancers noble metals, mostly gold. A typical Bayer-type catalyst consists of 0.15-1.5 wt% Pd, 0.2-1.5 wt% Au, 4-10 wt% KOAc on spherical silica particles of 5 mm diameter [14], The reaction is very fast and takes place mainly inside a thin layer on the particle surface (egg-shell catalyst). 

The hydrogenation of 2-ethyl-5,6,7,8-tetrahydroanthraqumone (THEAQ) at the oxygen in the presence of a palladium supported catalyst is a key step in the industrial production of hydrogen peroxide. In industrial plants, the performance of the catalyst slowly decreases because of deactivation. Two types of catalyst poisoning are operative, a reversible one, related to the presence of water, and a permanent one, probably due to the condensation of two or more anthraquinone molecules on the palladium surface. The kinetic data obtained from laboratory runs are used to simulate the performance in industrial plants. 

In this chapter, recent advances in our understanding of catalytic fluorination under heterogeneous conditions are surveyed from the standpoint of catalyst properties, including developments based on the use of mixed metal fluorides having different structural types, and reaction mechanisms. Much of the newer work has been the result of the need to replace chlorofluorocarbons (CFCs) by alternatives, hydrofluorocarbons (HFCs) or, more controversially, hydrochlorofluorocarbons (HCFCs), following adoption of the Montreal and successor Protocols [2,3]. Where relevant, aspects of catalytic hydrogenolysis, where fluorides have been used as replacement supports in the conventional palladium/carbon catalysts, and isomerization reactions are included. 

The concept of surfactant-type catalysts described above was also found to be applicable to catalytic systems other than Lewis acid-catalysed reactions. For example, we have developed palladium-catalysed allylic substitution reactions using a combination of Pd(PPh3)4 and a non-ionic surfactant, Triton X-100 [32]. 

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