Patent Watch

November 17, 2008

Produce butadiene from n-butane in two stages. Butadiene is a feedstock for producing many synthetic rubbers and polymer resins. In the case of synthetic rubbers, butadiene can be homopolymerized (to give polybutadiene rubber, PBR) or copolymerized with various monomers, including styrene (SBR, SBS) and acrylonitrile (NBR). Butadiene is used as a feedstock for engineering resins—notably acrylonitrile–butadiene–styrene (ABS)—and hexamethylenediamine and laurylactam for producing nylons. Butadiene is also used in the commercial production of 1,4-butanediol and THF. Recent developments in butadiene-based chemistry include conversion to 1-octene and 1-octanol by telomerization synthesis, conversion to styrene monomer by cyclodimerization, and new routes to ε-caprolactam.

Although butadiene has many value-added uses, it is not produced “on purpose”. It is an ethylene byproduct, subject to the vagaries of all the elements that drive ethylene profitability. Butadiene is obtained in only ~13–15% the yield of ethylene when naphtha is the cracking feedstock, and the yield sharply decreases with lighter cracking feedstocks. Because the price of crude oil was very high during most of 2008, gas-based feedstocks were preferred for many steam crackers, causing a scarcity of butadiene supply; accordingly, prices rose dramatically. These events stimulated interest in developing improved on-purpose butadiene processes based on butane dehydrogenation. In certain parts of the world—mostly the former Soviet Union—some butadiene production uses dehydrogenation technology. In the United States, Texas Petrochemicals runs its oxidative dehydrogenation plant Oxo-D on a campaign basis.

S. Crone and colleagues at BASF disclose a technique for converting n-butane to butadiene in high yield. Their invention is based on an autothermal, nonoxidative catalytic dehydrogenation of n-butane in one reactor zone, followed immediately by a second zone in which unreacted n-butane and normal butenes formed in the first zone are mixed with oxygen and oxidatively dehydrogenated to butadiene. The first-zone reaction is endothermic, and the heat to run it is supplied by selective hydrogen combustion.

Conversion of n-butane in the first zone is 49.5%, and selectivity to butenes and butadiene is 97.9%. Conversion in the second zone is 100% for 1-butene and 92.5% for 2-butene, with selectivity to butadiene of 95%. (BASF SE [Ludwigshafen, Germany]. U.S. Patent 7,435,860, Oct 14, 2008; Jeffrey S. Plotkin)

[See also this week’s Noteworthy Chemistry.—Ed.]

View patent information from CAS.

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