Noble metal catalysts alkenes

Concerning consecutive reactions, a typical example is the hydrogenation of alkynes through alkenes to alkanes. Alkenes are more reactive alkynes, however, are much more strongly adsorbed, particularly on some group VIII noble metal catalysts. This situation is illustrated in Fig. 2 for a platinum catalyst, which was taken from the studies by Bond and Wells (45, 46) on hydrogenation of acetylene. The figure shows the decrease of... 

Isolated double bonds in alkenes and cycloalkenes are best reduced by catalytic hydrogenation. Addition of hydrogen is very easy and takes place at room temperature and atmospheric pressure over almost any noble metal catalyst, over Raney nickel, and over nickel catalysts prepared in special ways such as P-1 nickel [13] or complex catalysts, referred to as Nic [49]. 

The addition of hydrogen to alkenes is made possible only with noble metal catalysts. They allow for a multistep, low-energy reaction pathway. The noble metal catalyst may be soluble in the reaction mixture in that case we have a homogeneous hydrogenation (cf. Sec-... 

Alkenes without functional groups are difficult to hydrogenate enantioselec-tively with noble-metal catalysts. Titanium complexes with a C2 symmetric chiral ligand framework, as in the ansa-titanocene (22-XVI), reduce aryl-substituted C=C bonds in very high isolated and optical yields 19... 

Despite the haptophilic behavior of the hydroxy group towards both heterogeneous and homogeneous noble metal catalyst systems,a C—OH bond unactivated by adjacent alkenic or aromatic groups (see Chapter 4.7) is quite resistant to hydrogenolysis, and forcing conditions are usually required. 

All of the common hydrogenation catalysts can effect the complete saturation of the alkyne to alkane, but all catalysts are not equally effective in the selective hydrogenation to produce alkenes. Selectivities for cis 2-pentene formation from 2-pentyne decreased in the order Pd > Rh > Pt > Ru > Ir. Palladium is the most selective of the noble metal catalysts for alkyne semihydrogenation with respect... 

Common catalyst poisons, such as H2S and HgClj, also decrease disproportionately the reduction rate of alkenes and the working electrode potential under potentiometric operation 34). Although CO is a common poison of noble metal catalysts, its effect is associated with a one-to-one exclusion of H atoms from the surface 12, p. 115) and not with extensive blockage of neighboring sites, usual with strong poisons. Other catalyst poisons, such as traces of metals Al, Zn, Ti, Cu, Mo, and Co 248), that can exist in reactants or arise from gas lines and cell housings, have received only sporadic attention to date. 

A similar type of catalyst including a supported noble metal for regeneration was described extensively in a series of patents assigned to UOP (209-214). The catalysts were prepared by the sublimation of metal halides, especially aluminum chloride and boron trifluoride, onto an alumina carrier modified with alkali or rare earth-alkali metal ions. The noble metal was preferably deposited in an eggshell concentration profile. An earlier patent assigned to Texaco (215) describes the use of chlorinated alumina in the isobutane alkylation with higher alkenes, especially hexenes. TMPs were supposed to form via self-alkylation. Fluorinated alumina and silica samples were also tested in isobutane alkylation,... 

Alkene hydrogenation, alkane hydrogenolysis, and methanation of CO are used as test reactions for evaluating the catalytic activity of cluster-derived metal catalysts. Catalysts derived from noble metal carbonyl precursors such... 

Solid state ion exchange is a versatile tool for the fast and easy preparation of metal containing small pore (i. e., 8-membered ring) zeolites. Therefore it offers a valuable alternative to the crystallization inclusion method with its limited applicability. The introduction of noble metals into small pore zeolites via solid state ion exchange results in highly shape selective catalysts over which the hydrogenation of the linear alkene out of an equimolar mixture of hexene-(l) and 2,4,4-trimethylpentene-(l) is strongly preferred. This indicates that the major part of the metal is located in the intracrystalline voids of the zeolites. Preliminary fUrther experiments in our laboratory surest that the new method is not restricted to noble metal chlorides, but also works with other salts, e. g., oxides and nitrates. 

So far, catalyst design has aimed mainly at oxidant activation and little attention has been paid to the interaction between the metal center and the alkene. A requirement for a successful epoxidation system of wide scope for simple terminal alkenes would seem to be a new catalyst design focusing on activation of the substrate instead of the oxidant. This suggests noble metals as applicable catalytic centers, because of their affinity for terminal alkenes versus internal ones. This results in a change of role for the catalyst from electrophile to nucleophile in the system. 

Supported metal clusters play an important role in nanoscience and nanotechnology for a variety of reasons [1-6]. Yet, the most immediate applications are related to catalysis. The heterogeneous catalyst, installed in automobiles to reduce the amount of harmful car exhaust, is quite typical it consists of a monolithic backbone covered internally with a porous ceramic material like alumina. Small particles of noble metals such as palladium, platinum, and rhodium are deposited on the surface of the ceramic. Other pertinent examples are transition metal clusters and atomic species in zeolites which may react even with such inert compounds as saturated hydrocarbons activating their catalytic transformations [7-9]. Dehydrogenation of alkanes to the alkenes is an important initial step in the transformation of ethane or propane to aromatics [8-11]. This conversion via nonoxidative routes augments the type of feedstocks available for the synthesis of these valuable products. 

Aerobic oxidation is not going to be limited to alcohol oxidation. Apparently we are closer to finding suitable catalysts for alcohol oxidation, but oxidation of alkenes, aromatic hydrocarbons, alkylaromatics, imines, amines, sulfur compounds etc. will require much work in the forthcoming years. In some cases, the presence of radical initiators, even though in minor quantities, may serve to promote the oxidation catalyzed by noble metal nanopartides, and gold in particular [56, 108]. Thus, there is no doubt that the next years will witness exciting developments in the field of metal nanopartides for aerobic oxidation. 

A besetting problem with the study of alkene hydrogenation is deactivation of the catalyst by strongly-adsorbed carbonaceous deposits - or acetylenic residues that form even well below room temperature the base metals of Groups 8 and 10 are especially prone to this, but even the noble metals are not immune, and even the most fundamental studies are necessarily made on equilibrium surfaces, much of which is permanently inactivated. This raises the vexed question of whether such poisoned surfaces truly reflect the character of the metal, and indeed whether it is not on the carbonaceous overlayer that the catalysis occurs. Another idea that has been seriously suggested is that reaction actually occurs, in... 

This effect has some industrial relevance. Thus the hydrogenolysis activity of supported Ru/Os reforming catalysts can be reduced by adding small amounts of copper, so that more alkenes are formed. These high surface area catalysts (ca. 300 m /g) contain the metal in the form of mixed crystals, often less than 5 nm in diameter ( bimetallic clusters ). Here, too, the Cu is found exclusively on the surface of the noble metal Ru. 

---