Carbide chain initiation

On the basis of the nature of CO adsorption and of the nature of chain initiator intermediates, popular mechanistic proposals include the carbide mechanism,1-2 wherein CO adsorbs dissociatively and the carbide (C ) is the chain initiator intermediate, and the enolic mechanism,3 involving molecular adsorption of CO and the formation of an oxygen intermediate, the enol (HC OH). 

Chain initiation. As illustrated by both the carbide mechanism and the CO insertion mechanism, the FTS reaction is initiated through CO activation. The divergence for two mechanisms is that the carbide mechanism requires CO to firstly dissociate, while the CO insertion mechanism proposes CO to be hydrogenated at the initial step. Some recent density functional studies based on periodic catalyst models have evaluated the possibility of these different activation pathways by calculating the activation energy barriers of the involved steps under different reaction conditions (temperatures, pressures and coverages). Here, we will discuss these results in both direct and H-assisted routes. 

The chains of hollow carbon may be initially chains consisting of Ni (or carbide) particles covered with graphitic carbon. The chains lying on the hot surface of the cathode are heated, and Ni atoms evaporate through defects of the outer graphitic carbon because the vapor pressure of Ni is much higher than carbon. Thus, the carbon left forms hollow graphitic layers. 

The catalytic oxidation of ethylene to ethylene oxide was apparently developed into a commercial process by Zimakov (460). Zimakov assumes that ethylene is first activated by migration of an electron, then converted to a peroxide radical which initiates a chain reaction. His use of silver on corundum promoted with BaCh and modified by dichloro-ethane is similar to that disclosed in patents of Carbide and Carbon Chemical Corporation. 

Recombination of CH. fragments is an essential step to initiate the chain-growth reaction according to the Sachtler-Biloen carbide mechanism. In a series of elegant papers, Cheng et al. (31-33) reported on the structure dependence as well as on the metal dependence of this class of reactions. Activation energies for CH. —CH recombination on flat and stepped surfaces of cobalt are listed in Table 4. 

Specihcally with regard to the pyrolysis of plastics, new patents have been filed recently containing variable degrees of process description and equipment detail. For example, a process is described for the microwave pyrolysis of polymers to their constituent monomers with particular emphasis on the decomposition of poly (methylmethacrylate) (PMMA). A comprehensive list is presented of possible microwave-absorbents, including carbon black, silicon carbide, ferrites, barium titanate and sodium oxide. Furthermore, detailed descriptions of apparatus to perform the process at different scales are presented [120]. Similarly, Patent US 6,184,427 presents a process for the microwave cracking of plastics with detailed descriptions of equipment. However, as with some earlier patents, this document claims that the process is initiated by the direct action of microwaves initiating free-radical reactions on the surface of catalysts or sensitizers (i.e. microwave-absorbents) [121]. Even though the catalytic pyrolysis of plastics does involve free-radical chain reaction on the surface of catalysts, it is unlikely that the microwaves on their own are responsible for their initiation. 

Davis26 pointed out in his review on FT synthesis mechanism that the reaction pathway is also dependent on catalyst compositions and operation conditions. Their experimental data on FT synthesis with iron catalysts at low temperatures suggested that instead of carbide mechanism, an oxygenate intermediate, similar as the formate species responsible of the WGS reaction, existed on the surface and initiated carbon chain propagation. The final chain termination step was accompanied by elimination of the oxygen atom. 

The added 1-alkenes reach the metal sites aided by the SCF and adsorb onto the active sites as alkyl radicals to initiate carbon chain growth the resulting chains are indistinguishable from other carbon chains formed directly from synthesis gas. These new alkyl radicals consume additional methylene units to initiate new carbon chain propagation processes. Thus the selectivity for methane, which is formed mainly from methylene hydrogenation, decreases. CO adsorption and cleavage of CO to carbide on the metal site, as well as hydrogenation of carbide to methylene species, are both accelerated. This is attributed to increased consumption of the adsorbed methylene species. Experimentally, the CO conversion increased with addition of 1-alkene. This acceleration may contribute to the suppressed CO2 selectivity as well in the alkene-added reaction, as CO2 is the byproduct from CO in the water-gas shift reaction. 

Modern GPPS is produced by continuous bulk and solution processes developed in the mid-1950s by major PS producers, BASF, Dow Chemical, Monsanto, Union Carbide, and others. In the modern continuous GPPS process, as the one shown in Figure 13.6, styrene monomer is continuously fed to a packed column (normally alumina, silica gel, or clay) to remove moisture, impurities, and inhibitor, blended with recycled styrene monomer, peroxide initiator (normally dialkyl or diacyl peroxides, such as di-fert-butyl peroxide, dicumyl peroxide, or fert-butyl peroxibenzoate utilized at low concentrations [I] <0.5% w/w in the feed), chain transfer agent (normally aliphatic... 

The first one is the carbide mechanism (Fig. 10), which was initially advanced by Fischer and Tropsch in 1925 and later further developed by Sachtler, Biloen and others. Therefore it is also known as Sachtler-Biloen mechanism. The main steps of this mechanism involve CO cleavage to a surface carbide (CHj as the monomer for chain propagation, hydrogenation (eventually to methane), and the formation and coupling of surface hydrocarbyl (C Hj species, from which 1-alkenes are produced by p-elimi nation. 

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