Palladium complexes alumina

Hydrosilylation of butadiene using palladium complexes supported on inorganic materials such as silica and alumina has been carried out (77, 72) however, the supported catalyst is not stable and it is difficult to compare with the soluble catalysts. 

Acetylene hydrogenation. Selective hydrogenation of acetylene to ethylene is performed at 200°C over sulfided nickel catalysts or carbon-monoxide-poisoned palladium on alumina catalyst. Without the correct amount of poisoning, ethane would be the product. Continuous feed of sulfur or carbon monoxide must occur or too much hydrogen is chemisorbed on the catalyst surface. Complex control systems analyze the amount of acetylene in an ethylene cracker effluent and automatically adjust the poisoning level to prepare the catalyst surface for removing various quantities of acetylene with maximum selectivity. 

Different conditions (including additives and solvent) for the reaction have been reported,often focusing on the palladium catalyst itself," or the ligand." Catalysts have been developed for deactivated aryl chlorides," and nickel catalysts have been used." Modifications to the basic procedure include tethering the aryl triflate or the boronic acid to a polymer, allowing a polymer-supported Suzuki reaction. Polymer-bound palladium complexes have also been used." " The reaction has been done neat on alumina," and on alumina with microwave irradiation." Suzuki coupling has also been done in ionic liquids," in supercritical... 

Reactions of acetylacetonates with silica are not as general as with alumina. Cu(acac)2 is adsorbed on the silica in high concentrations while the platinum and palladium complexes do not interact with the silica at all and are easily removed by washing. No adequate explanation of these differences was proposed. 3 palladium acetylacetonate complex was also used for the... 

This approach applied to the description of the hydrogenation reactions has been described in detail in our previous paper [13]. For the hydrogenation of 2,4-DNT over palladium on alumina catalyst the complex consecutive-parallel reaction pathways are shown in Fig 2. The appropriate rate equations can be written as follows [13] ... 

Entrapment or intercalation of metal species in pores and cavities of solid supports has frequently been used for the immobilization of catalysts in inorganic materials such as zeolites, clays, charcoals, silicas, aluminas, and other solids. Though this review article focuses on the immobilization of palladium complexes on polymer supports via covalent and/or coordination bonds, recent novel approaches to polymer-supported palladium species (including palladium nanoparticles) via nonbonding immobilization, such as encapsulation and incarceration, are intriguing because of their high potential for utility. In this section, several representatives are introduced. 

Examples of sohd-bound Pd-catalyzed carbonylation of aryl and alkenyl halide, allyl alcohol, and derivatives are abundant in the literature. Polyketones have been obtained via carbonylation of ethylene and carbon monoxide catalyzed by palladium complexes of polysiloxane-bound phosphinet t or Pd(dppp) absorbed on alumina.f f Similar processes can also be carried out by catalyst formed simply by absorbing Pd(02CNEt2)2(NHEt2)2 onto silica geL Polyphosphine-bound palladium has been used to prepare ethyl hexanoate from 1-pentene, CO, and ethanol. Similar esterification of styrene has been achieved using a bimetallic system involving palladium and nickel immobilized on poly(Af-vinyl-2-pyrrolidone).f ... 

Moore and Vicic (2008) developed a new method to prepare 2,2 -bipyridines in short reaction times using microwave radiations and heterogeneous catalysts. It was found that Ni/Al203-Si02 afforded 2,2 -bipyridine products in upto 86% yields in 1 h. Palladium supported on alumina also provided comparable yields of 2,2 -bipyri-dines (72%) to that for homogeneous PEPPSI and tetrakis(triphenylphosphine) palladium complexes. 

In Eq. (8.75), Z is the adsorption site on palladium on alumina catalyst. On this site, acetophenone (A) is adsorbed and hydrogenated to J4-l-phenyl ethyl alcohol (B) and to S-1-phenyl ethyl alcohol (C). This is followed by acylation of B to J4-1-phenyl ethyl acetate (P) over an immobihsed Hpase. First the acyl donor, ethyl acetate (Q), is bound with the free enzyme (E) forming a noncovalent enzyme-acyl complex (EQ), releasing ethanol (I). The substrate in the acylation step, J4-1-phenyl ethyl alcohol (B), is thereafter combined with EQ, which subsequendy relinquished J4-phenyl ethyl acetate (P) and E. Styrene (S) is obtained by dehydration of B and C over Pd/AI2O3. The side product ethyl benzene (F) is formed as a result of fast hydrogenation of S (step 7 ) and de-acylation of P in the presence of H2 releasing acetic acid (AcOH) to the media. 

The most widely used method for adding the elements of hydrogen to carbon-carbon double bonds is catalytic hydrogenation. Except for very sterically hindered alkenes, this reaction usually proceeds rapidly and cleanly. The most common catalysts are various forms of transition metals, particularly platinum, palladium, rhodium, ruthenium, and nickel. Both the metals as finely dispersed solids or adsorbed on inert supports such as carbon or alumina (heterogeneous catalysts) and certain soluble complexes of these metals (homogeneous catalysts) exhibit catalytic activity. Depending upon conditions and catalyst, other functional groups are also subject to reduction under these conditions. 

Freund s group at the Fritz Flaber Institute have put much emphasis on linking surface science studies with applied catalysts through replicating the latter with model systems without having to resort to the complexity of the real system. A system they have studied in detail is that of nitric oxide chemisorption at a palladium-alumina model catalyst, where they isolated different... 

Besides the interaction with CO, Figure 9 shows the most recent results of the simplest unsaturated hydrocarbon (acetylene) with alumina supported palladium. Here, however, the very small peak intensities indicate that tunneling spectroscopy shows little promise for studying complex hydrocarbons on supported metals. 

The hydrogenation of but-2-yne in the gas phase has been investigated using alumina-supported Group VIII metals, other than palladium, and over copper—alumina [200,201], With the exception of copper, which was 100% stereoselective for c/s-but-2-ene formation, the distribution of the initial reaction products, as shown in Table 20, are more complex than was observed with palladium. 

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

Johnson et al. (J3) suggest the use of the hydrogenation reaction of a-methylstyrene with a suspended palladium-alumina catalyst as an alternative test system to establish the effect of agitation variables on liquid-phase mass-transfer coefficients. They found the over-all hydrogen transfer coefficient to vary in a complex manner with agitator speed, and to increase with the 0.6 power of the superficial gas velocity up to a point beyond which the transfer showed no further change with gas velocity. 

Considering that under reaction conditions identical to those stated just above the Pd(l 11) single-crystal surface remained metallic, palladium nanoparticles are apparently easier to oxidize than bulk palladium, possibly because of the higher abundance of surface defects. The palladium oxide phase may be located not only on the particle surface but also at the palladium/alumina interface (515). Partial oxidation of palladium particles has been observed previously for combustion reactions on technological catalysts and may lead to complex hysteresis phenomena (see Refs (514,516) and references therein). 

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