October 31, 2011
Back to the future: Oxidize C4 hydrocarbons to butadiene. The increased production of shale gas in the United States is stimulating interest in “on-purpose” methods for producing 1,3-butadiene. Low US ethane prices, driven by abundant supplies from shale gas, are causing flexible ethylene cracker operators to turn away from using higher priced naphtha as a cracking feedstock in favor of less expensive ethane.
The unintended consequence of this is a shortage of cracker byproduct butadiene, which has caused butadiene prices to skyrocket. High butadiene prices are spurring researchers worldwide to reexamine the idea of dehydrogenating or oxydehydrogenating n-butenes or even n-butane to produce butadiene “on purpose”. These processes were used years ago in the United States and more recently in Russia.
S. H. Oh and co-inventors disclose bismuth molybdate–based catalysts that mediate the oxidative dehydrogenation of C4 raffinate streams to butadiene in good yields. One of the most effective catalysts is Bi2Mo2O9. In one example, a C4 mixture with the composition shown in the table, along with air and steam, is fed into a Pyrex reactor that contains the Bi–Mo catalyst. The n-butene/air/steam ratio is set at 1:3.75:15. The amount of catalyst is adjusted so that the gaseous hourly space velocity (based on n-butene throughput) is 300 h–1. The reaction temperature is maintained at 475 °C.
The patent reports the results of the examples in terms of n-butenes, not the mixed C4 feed. It states that the conversion of n-butenes is 57.26%, and the selectivity to 1,3-butadiene is 91.83%. This gives an overall 1,3-butadiene yield of 52.58%. It is not clear whether these values are actually based on the C4 feed or the inventors somehow calculated them on the basis of the ≈73% of n-butenes in the feed. (SK Innovation, SK Global Chemical [Seoul]. US Patent 8,003,840, Aug. 23, 2011; Jeffrey S. Plotkin)