Metal noncatalytic

Noncatalytic surfaces (e.g., nonconductors, noncatalytic metals, noncatalytic semiconductors) have to be activated (i.e., made catalytic) prior to electroless deposition. This activation is performed by generating catalytic nuclei on the surface of a noncatalytic material. Two major types of processes have been used to produce catalytic nuclei electrochemical and photochemical. 

Manufacture. Furan is produced commercially by decarbonylation of furfural in the presence of a noble metal catalyst (97—100). Nickel or cobalt catalysts have also been reported (101—103) as weU as noncatalytic pyrolysis at high temperature. Furan can also be prepared by decarboxylation of 2-furoic acid this method is usually considered a laboratory procedure. 

The metal parts of the injection molder, ie, the liner, torpedo, and nozzle, that contact the hot molten resin must be of the noncatalytic type to prevent accelerated decomposition of the polymer. In addition, they must be resistant to corrosion by HCl. Iron, copper, and zinc are catalytic to the decomposition and caimot be used, even as components of alloys. Magnesium is noncatalytic but is subject to corrosive attack, as is chromium when used as plating. Nickel alloys such as Duranickel, HasteUoy B, and HasteUoy C are recommended as constmction materials for injection-molding metal parts. These and pure nickel are noncatalytic and corrosion-resistant however, pure nickel is rather soft and is not recommended. 

The injection mold need not be made of noncatalytic metals any high grade tool steel may be used because the plastic cools in the mold and undergoes Httle decomposition. However, the mold requires good venting to allow the passage of small amounts of acid gas as well as air. Vents tend to become clogged by corrosion and must be cleaned periodically. 

Vinylidene Chloride Copolymer Foams. Low density, fine-celled VDC copolymer foams can be made by extmsion of a mixture of vinylidene chloride copolymer and a blowing agent at 120—150°C (190). The formulation must contain heat stabilizers, and the extmsion equipment must be made of noncatalytic metals to prevent accelerated decomposition of the polymer. The low melt viscosity of the VDC copolymer formulation limits the size of the foam sheet that can be extmded. 

Electroless reactions must be autocatalytic. Some metals are autocatalytic, such as iron, in electroless nickel. The initial deposition site on other surfaces serves as a catalyst, usually palladium on noncatalytic metals or a palladium—tin mixture on dielectrics, which is a good hydrogenation catalyst (20,21). The catalyst is quickly covered by a monolayer of electroless metal film which as a fresh, continuously renewed clean metal surface continues to function as a dehydrogenation catalyst. Silver is a borderline material, being so weakly catalytic that only very thin films form unless the surface is repeatedly cataly2ed newly developed baths are truly autocatalytic (22). In contrast, electroless copper is relatively easy to maintain in an active state commercial film thicknesses vary from <0.25 to 35 p.m or more. 

The best known of metal carbene reactions, cydopropanation reactions, have been used since the earliest days of diazo chemistry for addition reactions to the carbon-carbon double bond. Electron-donating groups (EDG) on the carbon-carbon double bond facilitate this catalytic reaction [37], whereas electron-withdrawing groups (EWG) inhibit addition while facilitating noncatalytic dipolar cycloaddition of the diazo compound [39] (Scheme 5). There are several reviews that describe the earlier synthetic approaches [1, 2,4, 5,40-43], and these will not be duplicated here. Focus will be given in this review to control of stereoselectivity. 

Various phosphines are valuable ligands for metal centers involved in catalytic and noncatalytic organometallic chemistry. Unfortunately, phosphines are readily oxidized to phosphine oxides and to prevent their oxidation they... 

In this chapter, we wiU review electrochemical electron transfer theory on metal electrodes, starting from the theories of Marcus [1956] and Hush [1958] and ending with the catalysis of bond-breaking reactions. On this route, we will explore the relation to ion transfer reactions, and also cover the earlier models for noncatalytic bond breaking. Obviously, this will be a tour de force, and many interesting side-issues win be left unexplored. However, we hope that the unifying view that we present, based on a framework of model Hamiltonians, will clarify the various aspects of this most important class of electrochemical reactions. 

Summary of literature data on methane decomposition catalysts and preferred temperature range. Catalysts 1 = nickel, 2 = iron, 3 = carbon, and 4 = other transition metals (Co, Pd, Pt, Cr, Ru, Mo, W). The dotted line arbitrarily separates heterogeneous (catalytic) and homogeneous (noncatalytic, gas phase) temperature regimes of the methane decomposition reaction. 

Electrochemical Activation. In the electrochemical method, catalytic nuclei of metal M on a noncatalytic surface S may be generated in an electrochemical oxidation-reduction reaction,... 

Another zinc-utilizing enzyme is carbonate/dehydratase C (Kannan et al., 1972). Here, the zinc is firmly bound by three histidyl side chains and a water molecule or a hydroxyl ion (Fig. 27). The coordination is that of a distorted tetrahedron. Metals such as Cu(II), Co(Il), and Mn(ll) bind at the same site as zinc. Hg(II) also binds near, but not precisely at, this site (Kannan et al., 1972). Horse liver alcohol dehydrogenase (Schneider et al., 1983) contains two zinc sites, one catalytic and one noncatalytic. X-Ray studies showed that the catalytic Zn(II), bound tetrahedrally to two cysteines, one histidine, and water (or hydroxyl), can be replaced by Co(II) and that the tetrahedral geometry is maintained. This is also true with Ni(Il). Insulin also binds zinc (Adams etai, 1969 Bordas etal., 1983) and forms rhombohedral 2Zn insulin crystals. The coordination of the zinc consists of three symmetry-related histidines (from BIO) and three symmetry-related water molecules. These give an octahedral complex... 

Biomimics of noncatalytic, or structural, zinc generally focus on zinc-thiolate clusters designed after the metal—thiolate clusters of metal-lothionein (see Section IV,C). Adamantoid anions of formula [ (jiz-SPh)6(ZnSPh)4] are targets of synthetic and structural study, where each metal ion is coordinated in tetrahedral fashion by bridging and terminal thiolate ligands (see Hencher et al., 1985 Dean and Vittal, 1987, and references cited therein). 

The rearrangement of 5-bromo-3-methoxy-2-phenylimidazolo[l,2-a] pyridine (70) into the 8-isomer 73 also undoubtedly involves a sequential metalation, transmetalation, and metal-halogen exchange pathway involving intermediates 71-75 and the noncatalytic generation of the 5,8-dibromo derivative 75 (Scheme 20) (83S987). 

Another chemical approach to the chemical conversion of methane involves organometallic reactions.85-89 Interesting work with iridium complexes and other transition metal insertion reactions (rhodium, osmium, rhenium, etc.) were carried out. Even iron organometallics were studied. These reactions take place in the coordination spheres of the metal complexes, but so far the reactions are stoichiometric and noncatalytic.77 In terms of synthetic hydrocarbon chemistry, these conversions are thus not yet practical, but eventually it is expected that catalytic reactions will be achieved. 

Noncatalytic oxidation to produce acetic acid can be carried out in the gas phase (350-400°C, 5-10 atm) or in the liquid phase (150-200°C). Liquid-phase catalytic oxidations are operated under similar mild conditions. Conditions for the oxidation of naphtha are usually more severe than those for n-butane, and the process gives more complex product mixtures.865-869 Cobalt and other transition-metal salts (Mn, Ni, Cr) are used as catalysts, although cobalt acetate is preferred. In the oxidation carried out in acetic acid solution at almost total conversion, carbon oxides, carboxylic acids and esters, and carbonyl compounds are the major byproducts. Acetic acid is produced in moderate yields (40-60%) and the economy of the process depends largely on the sale of the byproducts (propionic acid, 2-butanone). 

Although the noncatalytic thermal disproportionation of alkenes is symmetry-for-bidden, the formation of metal complex 1 with the four-centered cyclic intermediate makes the reaction symmetry-allowed.61... 

With manganese and cobalt acetate the reaction of peracetic acid with acetaldehyde is very fast, and AMP is not detected. By comparing our rates with literature values of k17, k.17, and k18 we cannot propose a mechanism in which the only role of the metal ion is to catalyze the decomposition of AMP. The experimental rates in the presence of either manganese or cobalt acetates are much faster than the noncatalytic rate of formation of AMP. Thus, AMP per se is probably not an intermediate in the presence of these catalysts. 

Two recent reports of noncatalytic metal complex-promoted reduction of C02 merit discussion here. Work by Floriani et al. details reactions of C02 with early transition metal complexes in toluene solution leading to deoxygenation or disproportionation and reduction (161a). The three reactions are shown as (80a)-(80c). 

Catalytic hydrodemetallation studies with vanadyl naphthenates have been reported by West (1984). The instability of these compounds makes them relatively poor models for mechanistic studies. At typical hydro-treating conditions, a homogeneous noncatalytic demetallation route and a heterogeneous catalytic route contribute to overall metal removal. 

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