GB807149A - Dehydrogenation process - Google Patents

Abstract

Non-aromatic hydrocarbons having at least two carbon atoms are converted to hydrocarbons having a higher carbon-to-hydrogen ratio by heating with at least 0.2 mol. of bromine per mol. of hydrocarbon to at least 400 DEG C. for at least 0.001 second. The following conversions may be effected: compounds having an aliphatic chain of 2 to 5 non-quaternary carbon atoms or a ring of 5 carbon atoms are dehydrogenated to give compounds having a greater number of unsaturated linkages e.g. ethane to ethylene and acetylene, ethylene to acetylene, propane to propylene and methylacetylene or allene, n-butane to 1-butene, 2-butene and 1,3-butadiene; 1-butene or 2-butene to 1,3-butadiene; n-pentane to 1-pentene, 2-pentene and 1,3-pentadiene; 1-pentene or 2-pentene to 1,3-pentadiene; isopentane to 3-methyl-1-butene, 2 methyl-2-butene, 2-methyl-1-butene and isoprene; cyclopentane to cycopentene and cyclopentadiene-1,3; methylcyclopentane to 1-methylcyclopentene, 3-methylcyclopentene, 4-methylcyclopentene, 1-methylcyclopentadiene-1,3, 2-methylcyclopentadiene-1,3 and 5-methylcyclopentadiene-1,3. Compounds containing an aliphatic chain of 6 non-quaternary carbon atoms or cyclic compounds containing a chain or chains capable of forming a 6-membered ring can be dehydrocyclised to aromatics as well as forming olefines e.g. n-hexane forms n-hexenes and benzene; n-hexenes form benzene; n-heptane forms n-heptene, toluene and a small amount of benzene; n-heptenes form toluene; monomethylhexanes form monomethylhexenes and tolene; monomethylhexanes form toluene; n-octane yields octenes, ethylbenzene and o-xylene; n-octenes yield ethylbenzene and o-xylene; monomethylheptanes yield monomethylheptenes and chiefly xylenes; dimethyl (other than geminal) hexanes yield the corresponding mono-olefines and xylenes; n-decane yields decenes, n-butyl benzene, propytoluene, diethylbenzene, less substituted benzenes and some corresponding compounds with unsaturated side chains; 5-methylnonane yields olefines and n-butylbenzene, propyltoluene, 2-ethyl-p-xylene, and less-substituted benzenes and compounds with unsaturated side chains. Hydroaromatic naphthenes and cyclo-olefines give aromatics e.g. cyclohexane gives benzene; methylcyclohexane gives toluene; ethylcyclohexane give ethylbenzene, 1,2-dimethylcyclohexane gives o-xylene; 1,3-dimethylcyclohexane gives m-xylene; 1,4-dimethylcyclohexane gives p-xylene. The preferred molar amounts of bromine per mol. of hydrocarbon for certain reactions are as follows: saturated hydrocarbons to diolefines, 1 to 2; olefine to diolefine, 0.5 to 1; C6 + aliphatic hydrocarbons to aromatics, 2 to 4; cyclo-olefine to aromatic, 1 to 3; saturated hydrocarbon to mono-olefine, 0.5 to 0.75. The residence time is preferably below 10 seconds. Pressures of subatmospheric to superatmospheric (e.g. 35-70 kg./cm.2) can be used but atmospheric is generally suitable. Temperatures of 500-800 DEG C. are preferred. Mixtures of hydrocarbons can be converted and an inert diluent such as benzene or naphthalene can be added. The reaction zone may be an unobstructed tube. Bromine is preferably added as molecular bromine but compounds yielding bromine under reaction conditions, e.g. alkyl, 1,2-dibromides can be used. If the product is highly reactive, e.g. a diolefine, it may be desirable to separate the product quickly into inorganic bromine species and organic product mixture which may be worked up in conventional manner e.g. by fractionation, solvent extraction or adsorption. In a typical example a vaporous feed mixture of n-butane and free bromine in a molar ratio of 1 : 1.1 was passed through an unobstructed quartz tube, 1.2 cms. inside diameter and 43 ccs. in volume at one atmosphere pressure and 550 DEG C. with a residence time of 0.9 second. Hydrogen bromide and bromine were absorbed from the effluent in aqueous sodium hydroxide. It was found that 73 per cent of the butane was converted to butenes and butadiene-1,3 in 78 per cent yield.