- Traditional Syntheses of 1,3-Butadiene
The first industrial manufacturing processes for butadiene were based on coal conversion products such as acetylene, acetal-dehyde, ethanol, and formaldehyde. There are basically three synthetic routes characterized by formation of the C4 butadience chain either from C2 units or from C2 and C1 units, generally in multistep processes.
In former East Germany, a certain amount of butadiene is still being produced from acetylene in a four-step process. In this process, acetylene is initially converted into acetaldehyde and then aldolized to acetaldol. The acetaldol is reduced to 1,3-butanediol with a Ni catalyst at 110 °C and 300 bar. Finally, in the fourth step, the 1,3-butanediol is dehydrated in the gas phase at 270°C using a Na polyphosphate catalyst:
The selectivity to butadiene is about 70% (based on CH3CHO). One variation of the four-step process uses acetaldehyde from the dehydration of ethanol. This acetaldehyde is then converted over a Zr-/Ta-oxide/SiO2 catalyst at 300-350 °C with an overall yield of about 70%. This process is used commercially in India and China (two plants with a total capacity of 85000 tonnes per year in 1990).
Another method for butadiene manufacture based on ethanol is known as the Lebedew process. It was developed in the CIS, and is still employed commercially there, as well as in Poland and Brazil. In this process, ethanol is dehydrogenated, dehydrated, and dimerized in one step at 370-390°C over a MgO-SO2 catalyst:
The selectivity to butadiene reaches as high as 70%. Today, this process could be of interest to countries not pocessing a petrochemical base but having access to inexpensive ethanol from fermentation.
In the third traditional method, the Reppe process, acetylene and formaldehyde are initially converted into 2-butyne-1,4-diol from which 1,4-butanediol is manufactured. This product is still of great industrial significance today. Subsequently, a direct twofold dehydration ensues, but due to technical considerations this is usually a two-step process with tetrahydrofuran as the intermediate product. The Reppe process is totally uneconomical today.
Modern industrial processes for butadiene are based exclusively on petrochemicals. C4 cracking fractions or butane and butene mixtures from natural and refinery waste gases are economical feedstocks.
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