Chemistry of hydrocarbon compounds – Adding hydrogen to unsaturated bond of hydrocarbon – i.e.,... – Hydrocarbon is contaminant in desired hydrocarbon
Patent
1994-04-08
1996-04-02
Mc Farlane, Anthony
Chemistry of hydrocarbon compounds
Adding hydrogen to unsaturated bond of hydrocarbon, i.e.,...
Hydrocarbon is contaminant in desired hydrocarbon
585269, C07C 510
Patent
active
055042686
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for the selective hydrogenation of aromatic acetylene compounds present as impurities in vinyl-aromatic compounds comprising contacting the aromatic acetylene compound with hydrogen in the presence of a selective hydrogenation catalyst.
Aromatic acetylene compounds are common impurities in vinyl-aromatic compounds. For example, acetylenic compounds result as by-product in the production of vinyl-aromatic compounds by the dehydrogenation of ethyl-aromatic compounds. Excessive dehydrogenation leads to the highly unsaturated aromatic acetylene compounds. For example, in the production of styrene by dehydrogenation of ethylbenzene, phenylacetylene may be formed in amounts up to about 1,000 ppm. These acetylenic compounds are detrimental in the polymerization or copolymerization of the vinyl-aromatic compounds. The acetylenic impurities may lead to deactivation of the polymerization catalyst and to the formation of gels, such as, for example in the production of polymers or copolymers of styrene produced according to anionic or free radical polymerization techniques. It is thus desired to reduce the amounts of acetylenic compounds present in vinyl-aromatic compounds.
The vinyl-aromatic compound, however, contains a vinyl group which competes for hydrogenation with the acetylene group of the aromatic acetylene compound. Non-selective and excessive hydrogenation will therefore result in an undesired conversion of the vinyl-aromatic compound to the corresponding ethyl-aromatic compound, and thus cause loss of the desired product.
Japanese Patent Application No. 62-72633 discloses a method for the selective hydrogenation of impurities with a high degree of unsaturation which are present in styrenes, by carrying out the hydrogenation in at least two steps under conditions that in each step the molar ratio of hydrogen supplied to the reaction system to phenylacetylene is 10 or less, and preferably 5 to 1.5. As follows from the examples, the styrene still contains 50 ppm phenylacetylene, whereas the styrene loss was 0.02 percent.
In Japanese Patent Application No. 62-72634 a method for the purification of styrenes is disclosed by selectively hydrogenating the phenylacetylene present in the styrenes while the partial pressure of the hydrogen which is supplied to the reaction system is kept at 2 kg/cm.sup.2 or less. From the examples it becomes clear that in order to achieve an acceptable phenylacetylene conversion a significant loss of styrene should be accepted.
According to the present invention aromatic acetylene compounds present as impurities in vinyl-aromatic compounds can be selectively hydrogenated to very low levels, and preferably to levels of about 10 ppm or less, without substantial loss of vinyl-aromatic compound.
The present invention provides a process for the selective hydrogenation of aromatic acetylene compounds present as impurities in vinyl-aromatic compounds comprising adding hydrogen and an inert gas to a liquid phase vinyl-aromatic compound containing an aromatic acetylene compound and contacting the aromatic acetylene compound with hydrogen in the presence of a selective hydrogenation catalyst, wherein the partial hydrogen pressure is from about 0.001 to about 0.05 bar.
It has been found that a highly selective hydrogenation can be achieved by having in the liquid phase a molar excess of the aromatic acetylene compound with respect to hydrogen. As the vinyl-aromatic compound concentration is much higher than that of the aromatic acetylene compound, whereas the latter compound adsorbs much stronger, both compounds compete for the active catalyst sites. The aromatic acetylene compound adsorbed on the active catalyst site is then hydrogenated to the vinyl-aromatic compound. It is important that the vinyl-aromatic compound before being released from the active catalyst site and being replaced by the much stronger adsorbing aromatic acetylene compound, has a low exposure to hydrogen in the liquid phase surrounding the catalyst. By limiting hydrogen availabil
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de Benito Fernando A.
van der Aalst Matheus J. M.
Mc Farlane Anthony
The Dow Chemical Company
Yildirim Bekir L.
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