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dehydration of ethanol aluminium oxide
When a liquid alcohol (like ethane) is heated with a strong acid catalyst (like concentrated sulfuric or phosphoric acid) then the alcohol dehydrates to form a gaseous alkene. The alkene loses hydrogen bonds and becomes more volatile to make it easier to distill or collect the vapor as a gas over water.
Its mechanism is very simple. The hydroxyl groups on the alcohol are broken off by the very polar acid to produce a carbocation that forms an alkene C=C bond. The alcohol then becomes free to react with more acids to produce even more alkenes.
This reaction is a common method of making gaseous alkenes such as ethene in the lab. It can be done simply by boiling some ethanol in a flask and passing the vapor over aluminum oxide that is heated in a long tube.
Some people use this as an alternative to using sulphuric acid because it produces a cleaner and less messy result. But the acid still oxidises some of the alcohol to carbon dioxide and is simultaneously reduced to sulphur dioxide so both gases have to be removed.
The resulting alkenes are quite messy to work with and it can be difficult to get rid of them. It also reacts with the alcohol to produce a mass of carbon and there are other side reactions as well.
A number of heterogeneous alumina supported nickel metal catalysts were screened for direct one-pot conversion of ethanol to 1-butanol at 250 degC and 70 bar. The best catalyst giving the highest conversion and selectivity was commercial Ni/Al2O3 (HTC-500).
