Metal formation

The most common catalysts are sodium hydroxide and calcium hydroxide, generally used at a modest excess over the nominal stoichiometric amount to avoid formaldehyde-only addition reactions. Calcium hydroxide is cheaper than NaOH, but the latter yields a more facile reaction and separation of the product does not require initial precipitation and filtration of the metal formate (57). 

The exact order of the production steps may vary widely in addition, some parts of the process may also vary. Metal formate removal may occur immediately after the reaction (62) following formaldehyde and water removal, or by separation from the mother Hquor of the first-stage crystallization (63). The metal formate may be recovered to hydroxide and/or formic acid by ion exchange or used as is for deicing or other commercial appHcations. Similarly, crystallization may include sophisticated techniques such as multistage fractional crystallization, which allows a wider choice of composition of the final product(s) (64,65). 

Lunker, n. Metal.) shrinkage cavity, shrink-hole, pipe, -bildtmg, /, Metal.) formation of cavities, piping. 

Kinetic studies of the decomposition of metal formates have occasionally been undertaken in conjunction with investigations of the mechanisms of the heterogeneous decomposition of formic acid on the metal concerned. These comparative measurements have been expected to give information concerning the role of surface formate [522] (dissociatively adsorbed formic acid) in reactions of both types. Great care is required,... 

Decompositions of oxalates containing the strongly electropositive metals yield an oxide product but the more noble elements yield the metal. Discussion of the mechanisms of these reactions and, in particular, whether metal formation necessarily involves the intermediate production of oxide which is subsequently reduced by CO has been extended to consideration of the kinetics of pyrolysis of the mixed oxalates [32]. 

The hydrogenation activity of the isolated hydrides 3 and 6 towards cyclooctene or 1-octene was much lower than the Wilkinson s complex, [RhCKPPhj) ], under the same conditions [2] furthermore, isomerisation of the terminal to internal alkenes competed with the hydrogenation reaction. The reduced activity may be related to the high stability of the Rh(III) hydrides, while displacement of a coordinated NHC by alkene may lead to decomposition and Rh metal formation. 

A composition for dissolving filter-cake deposits left by drilling mud in wellbores is composed of an aqueous solution of citric acid and potassium chloride, alkali metal formate, acid tetraphosphate, alkaline earth chloride, and alkali metal thiophosphate [1012]. 

This sequence emphasizes the fact that the oxide is first converted to the carbide and metal formation occurs subsequently by a reaction between the oxide and the carbide. 

The principles of tantalum metal formation by the carbothermic reduction of tantalum pentoxide and the technology of tantalum metal production by this method are similar to those pertaining to niobium metal production by carbothermy. 

Tungsten metal can be obtained by exposing tungsten oxides to dry hydrogen in a flow system maintained above about 500 °C. An increase in the temperature shifts the reaction towards metal formation. For the first two stages of reduction, corresponding to the forma-... 

Synthesis and characterization of an Al69 cluster which can be regarded as a slightly modified Al77 cluster show the extreme sensitivity of the cluster geometry— even with the same ligand—to reaction conditions and number of ligands. Many more clusters will have to be synthesized to get a consistent picture of their formation and, consequently, the mechanism of metal formation which represents, as already mentioned, one of the oldest chemical processes in history. 

Temperature Limits for Decay of Metal Formates and Oxalates [t 6]... 

No evidence of ruthenium metal formation was found in catalytic reactions until temperatures above about 265°C (at 340 atm) were reached. The presence of Ru metal in such runs could be easily characterized by its visual appearance on glass liners and by the formation of hydrocarbon products (J/1J) The actual catalyst involved in methyl and glycol acetate formation is therefore almost certainly a soluble ruthenium species. In addition, the observation of predominantly a mononuclear complex under reaction conditions in combination with a first-order reaction rate dependence on ruthenium concentration (e.g., see reactions 1 and 3 in Table I) strongly suggests that the catalytically active species is mononuclear. 

Carbon dioxide is known to readily insert into a metal-hydride bond to give a metal formate [57, 58] this forms the first step in insertion mechanisms of C02 hydrogenation (Scheme 17.2). Both this insertion step and the return path from the formate complex to the hydride, generating formic acid, have a number of possible variations. 

The Sonogashira reaction of 2-chloropyrazine 1-oxide gave only recovered starting material. Pentylation and octylation of 2-chloropyrazine 1-oxide also failed [9]. Possible explanations for these results were either catalyst agglomeration or metal formation from pyrazinylpalladium... 

As briefly discussed in section 1.1, and shown in Figure 1, the accepted mechanism for the catalytic cycle of hydrogenation of C02 to formic add starts with the insertion of C02 into a metal-hydride bond. Then, there are two possible continuations. The first possibility is the reductive elimination of formic acid followed by the oxidative addition of dihydrogen molecule to the metal center. The second possible path goes through the a-bond metathesis of a metal formate complex with a dihydrogen molecule. In this section, we will review theoretical investigations on each of these elementary processes, with the exception of oxidative addition of H2 to the metal center, which has already been discussed in many reviews. 

Though only an alkyl complex is possible in the ethylene insertion, there are several possible products of the C02 insertion, such as p -OCOH, p2-02CH, and p -COOH species, as shown in Figure 4. Thus, the following issues were investigated for the C02 insertion reactions (1) Which species is more easily formed, the metal-formate (M-OCOH) or the metal-carboxylic acid (M-COOH) (2) What are the most important interactions in the C02 insertion (3) How different is the C02 insertion from the C2H4 insertion ... 

We recently investigated [40] the reason why C02 is inserted into the Rh(I)-H bond with a significantly lower barrier than into the Rh(III)-H bond, as shown in Table 2. As discussed above, charge-transfer from the metal-hydride moiety to the K orbital of CO2 is very important in the CO2 insertion reaction, and, at the same time, the metal-formate moiety is very much stabilized by the donation of electrons from the metal fragment. Since the Rh(I) center is more electron-rich than Rh(III), the charge-transfer from the Rh(I)-H moiety to the k orbital of C02 is favored, and the formate moiety is provided with sufficient electrons. Consequently, CO2 is more easily inserted into the Rh(I)-H bond than into the Rh(III)-H bond. 

Reductive Elimination of Formic Acid from Transition-Metal Formate Complexes... 

As shown in Figure 1, the next step in the catalytic cycle of carbon dioxide hydrogenation is either reductive elimination of formic acid from the transition-metal formate hydride complex or CT-bond metathesis between the transition-metal formate complex and dihydrogen molecule. In this section, we will discuss the reductive elimination process. Activation barriers and reaction energies for different reactions of this type are collected in Table 3. 

Metathesis of a Transition-Metal Formate Complex with a Dihydrogen Molecule ... 

The other reaction path to obtain formic acid from the transition metal formate complex is metathesis with a dihydrogen molecule. This reaction course has been proposed experimentally, but no clear evidence has been reported so far. Energetics of this reaction from different complexes and with a variety of methods are collected in Table 4. 

Table 4. Computed activation barrier (E kcal/mol) and reaction energy (AE, kcal/mol) of the o-bond metathesis reactions of metal formate complexes with the dihydrogen...

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