Aromatic hydrocarbons, acetylene formation from

Pyrolysis of acetylene to a mixture of aromatic hydrocarbons has been the subject of many studies, commencing with the work of Berthelot in 1866 (1866a, 1866b). The proposed mechanisms have ranged from formation of CH fragments by fission of acetylene (Bone and Coward, 1908) to free-radical chain reactions initiated by excitation of acetylene to its lowest-lying triplet state (Palmer and Dormisch, 1964 Palmer et al., 1966) and polymerization of monomeric or dimeric acetylene biradicals (Minkoff, 1959 see also Cullis et al., 1962). Photosensitized polymerization of acetylene and acetylene-d2 and isotopic analysis of the benzene produced indicated involvement of both free-radical and excited state mechanisms (Tsukuda and Shida, 1966). 

High quality MWNT were obtained from acetylene on Co3C>4/MgO catalyst [3], However, commercial acetylene often contains impurities which sometimes have harmful influence on the process. For instance, we observed poisoning of a catalyst, probably by phosphine, when obtaining CNF from commercial acetylene on Fe/C/Si02 catalyst. After careful purifying of acetylene the process proceeded normally. Further, sometimes aromatic hydrocarbons form if the catalyst used is not sufficiently selective. In our experiments formation of aromatics was observed frequently in case of acetylene. This creates ecological problems. 

Based on earlier studies on the total synthesis of benzene (2), which he called the keystone of the total aromatic edifice , Berthelot in 1867 carried out a remarkable experiment heating acetylene (1) - which he had prepared from the elements - in a bent bell-jar at a temperature where the glass began to soften , he noticed the formation of polymeric substances . When these were subjected to fractional distillation, benzene, styrene, and other aromatic hydrocarbons could be isolated, with 2 constituting approximately half of the product mixture (Scheme 1) [1]. 

They noted that in dimethoxyethane or in iso-octane (path a), the major product was dicarbonylcyclopentadienylcobalt (2) which must arise as a result of a retro Diels-Alder reaction of the norbornadiene (which would lead to the formation of acetylene and cyclopentadiene). When the solvent was changed to an aromatic hydrocarbon such as benzene or toluene (path b), the major cobalt-containing product was shown to be a complex derived from Co4(CO)i2, with three CO ligands on an apical cobalt being replaced by a molecule of the aromatic solvent (3). The group noted that they were also obtaining hydrocarbon and ketonic products derived from norbornadiene, acetylene and carbon monoxide .1,2... 

Aromatic hydrocarbons are known to be important in soot formation in flames. The aromatic structure may abet molecular growth leading to PAH and soot formation through its ability to stabilize radicals formed from addition of aromatic radicals to unsaturated aliphatics such as acetylenic species (jL>2.). Accordingly, both aromatics and unsaturated aliphatics would be important for growth processes. Both types of species are prevalent in the flame zone where growth occurs. Aromatic structures with unsaturated side chains also are observed there (1 >3). 

The sub-stoichiometric combustion involves the risk of soot formation as a result of pyrolysis reactions with acetylene and polycyclic aromatic hydrocarbons as soot precursors [107] [465]. The soot formation will start below a certain steam-to-carbon ratio depending on pressure and other operating parameters [111]. However, the data in Table 1.8 shows results from a soot-free pilot test (100 Nm NG/h) at a low steam-to-carbon ratio of 0.21 [111]. 

This review deals with metal-hydrocarbon complexes under the following headings (1) the nature of the metal-olefin and -acetylene bond (2) olefin complexes (3) acetylene complexes (4) rr-allylic complexes and (5) complexes in which the ligand is not the original olefin or acetylene, but a molecule produced from it during complex formation. ir-Cyclopentadienyl complexes, formed by reaction of cyclopentadiene or its derivatives with metal salts or carbonyls (78, 217), are not discussed in this review, neither are complexes derived from aromatic systems, e.g., benzene, the cyclo-pentadienyl anion, and the cycloheptatrienyl cation (74, 78, 217), and from acetylides (169, 170), which have been reviewed elsewhere. 

In Tables 10 to 12 we show the heats of formation calculated by the various methods, together with their deviation from the experimentally observed values for alkanes and cycloalkanes, alkenes and cydoalkenes, and acetylenes and aromatic compounds. Table 13 shows a comparison of heats of formation of hydrocarbon radicals calculated by the MINDO methods. Finally, in Tables 14 and 15 we show the results of MINDO/1 calculations on a selection of oxygen- and nitrogen-containing compounds. 

In the steam cracking of hydrocarbons, a small portion of the hydrocarbon feed gases decomposes to produce coke that accumulates on the interior walls of the coils in the radiant zone and on the inner surfaces of the transferline exchanger (TLX). Albright et identified three mechanisms for coke formation. Mechanism 1 involves metal-catalyzed reactions in which metal carbides are intermediate compounds and for which iron and nickel are catalysts. The resulting filamentous coke often contains iron or nickel positioned primarily at the tips of the filaments. This filamenteous coke acts as excellent collection sites for coke formed by mechanisms 2 and 3. Mechanism 2 results in the formation of tar droplets in the gas phase, often from aromatics. These aromatics are often produced by trimerization and other reactions involving acetylene. Some, but not all, of these droplets collect... 

Contact angles were measured on plasma polymers deposited from numerous hydrocarbon monomers of different structure containing triple bond, olefinic double bonds, aromatic and aliphatic structures. The results of contact angle measurements and evaluated surface energy properties for these polymers are summarized in Table II, column A. The data for plasma polymers from acetylene, ethylene, and hexane indicate that monomer unsaturation does not change substantially the dispersion component but increases the polar component to a considerable extent as in the case of acetylene. This, undoubtedly, is due to the high concentration of radicals in PP-AC and resulting rapid formation of carbonyls and... 

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