Benzene, carbon formation, hydrogen

Barnett et al. [AIChE J., 7 (211), 1961] have studied the catalytic dehydrogenation of cyclohexane to benzene over a platinum-on-alumina catalyst. A 4 to 1 mole ratio of hydrogen to cyclohexane was used to minimize carbon formation on the catalyst. Studies were made in an isothermal, continuous flow reactor. The results of one run on 0.32 cm diameter catalyst pellets are given below. 

Violently decomposed by water, with the formation of hydrogen chloride.2 Soluble in alcohol, ether, benzene, carbon tetrachloride, and chloroform.1... 

For a fresh catalyst, no carbon and hydrogen exist on its surface. With benzene as the reactant, hydrogen/carbon (molar) is close to unity. Under the same temperature and pressure, coke formation for the ethylene feeding is serious. With the Y-type zeolite, when the temperature increases in the gas phase alkylation, the coke is easily formed, with its concentration on the catalyst surface also high. The same result also appear for the LP alkylation after 14 hours, while the coke concentration in the SCFP alkylation is low even after 55 hours. 

In several cases, the in situ formation of hydrogen peroxide is the first step of the process. Thus, phenol can be obtained from benzene, carbon monoxide (5 atm) and oxygen (65 atm) at 70 °C in a benzene-water-methyl isobutyl ketone mixture, with TS-1 and a palladium complex as catalysts [26]. Despite a 91% selectivity to phenol, benzene conversion (3.2%) and productivity are still too low for industrial application. The palladium complex is required to promote hydrogen peroxide formation upon reaction of oxygen, carbon monoxide and water [27[. 

In this paper it is shown that the growth of carbon deposits can be maintained for long periods of time in the presence of hydrogen. The effect of hydrogen on the kinetics of carbon formation from benzene in hydrogen has been studied in experiments using nickel and iron foils. The results are presented below. 

We propose mechanisms for carbon formation in the presence of hydrogen on nickel and iron surfaces. Benzene is assumed to adsorb on the metal surface and to be hydrogenated to intermediates. These intermediates decompose to form atomic carbon which migrates through a metal crystallite to form carbon filaments. 

Mechanism of Surface Carbon Formation During the Pyrolysis of Benzene in the Presence of Hydrogen... 

The mechanism of carbon formation on polycrystalline Cu foils was studied for the pyrolysis of benzene in the presence of hydrogen. The study was carried out on a microbalance tubular flow reactor at temperatures rapging form 800 to 1050°C. The rate of surface carbon deposition was constant after an initial period during which no appreciable deposition was observed. 

The Cu sheets, after being washed in acetone, where reduced "in situ" with hydrogen at the reaction temperature of each run. The rate of carbon formation was measured as a function of temperature, benzene and hydrogen partial pressures, and gas phase reactor residence time. 

Calculations for this system show that carbon can always form before benzene has reached gasification equilibrium. Further, at atmospheric pressure, carbon formation can occur at very low benzene conversions, unless a very large excess of hydrogen is used. At a fixed hydrogen-to-benzene ratio, increasing the total pressure favors gasification and retards carbon deposition, based on equilibrium considerations. 

By-products from EDC pyrolysis typically include acetjiene, ethylene, methyl chloride, ethyl chloride, 1,3-butadiene, vinylacetylene, benzene, chloroprene, vinyUdene chloride, 1,1-dichloroethane, chloroform, carbon tetrachloride, 1,1,1-trichloroethane [71-55-6] and other chlorinated hydrocarbons (78). Most of these impurities remain with the unconverted EDC, and are subsequendy removed in EDC purification as light and heavy ends. The lightest compounds, ethylene and acetylene, are taken off with the HCl and end up in the oxychlorination reactor feed. The acetylene can be selectively hydrogenated to ethylene. The compounds that have boiling points near that of vinyl chloride, ie, methyl chloride and 1,3-butadiene, will codistiU with the vinyl chloride product. Chlorine or carbon tetrachloride addition to the pyrolysis reactor feed has been used to suppress methyl chloride formation, whereas 1,3-butadiene, which interferes with PVC polymerization, can be removed by treatment with chlorine or HCl, or by selective hydrogenation. 

A synthetically useful virtue of enol triflates is that they are amenable to palladium-catalyzed carbon-carbon bond-forming reactions under mild conditions. When a solution of enol triflate 21 and tetrakis(triphenylphosphine)palladium(o) in benzene is treated with a mixture of terminal alkyne 17, n-propylamine, and cuprous iodide,17 intermediate 22 is formed in 76-84% yield. Although a partial hydrogenation of the alkyne in 22 could conceivably secure the formation of the cis C1-C2 olefin, a chemoselective hydrobora-tion/protonation sequence was found to be a much more reliable and suitable alternative. Thus, sequential hydroboration of the alkyne 22 with dicyclohexylborane, protonolysis, oxidative workup, and hydrolysis of the oxabicyclo[2.2.2]octyl ester protecting group gives dienic carboxylic acid 15 in a yield of 86% from 22. 

---