September 30, 2013
Produce bioethanol without fermentation. As bioethanol continues to be an important component of gasoline, a high–carbon efficiency, nonfermentative route becomes increasingly important. One idea under exploration is gasifying cellulosic feedstocks to biosynthesis gas (syngas) and then converting the gas to mixed alcohols. The second step, however, is problematic because the initial formation of methanol is equilibrium-limited, and very high pressures are required to obtain even modest yields of C2+ alcohols.
Inventors B. J. Daniel and B. P. Gracey disclose a technique for making ethanol from syngas with high carbon efficiency. Methanol is first made from syngas by using conventional process technology. (Whether the syngas is biobased, natural gas–based, or coal-based is irrelevant.) The methanol is then carbonylated to acetic acid, again with conventional technology.
The patent’s invention is the hydrogenation of acetic acid (HOAc) to a mixture of ethanol (EtOH) and ethyl acetate (EtOAc). The EtOAc is separated and hydrogenated in another reactor to give additional EtOH.
In the patent’s one example, hydrogen and HOAc in a 10:1 mol ratio are passed over a Pd–Ag–Rh–Fe catalyst in the primary reactor at 230 ºC, 2.0 MPa pressure, and a gaseous hourly space velocity (GHSV) of 4343 h–1. The HOAc conversion is 41.9%, of which 19.7% is EtOAc, 21.6% EtOH, 0.4% MeCHO, and 0.2% Me2O. The overall selectivity to ethyl groups that can be recovered as EtOH is 93.8%.
In the secondary reactor, hydrogen and the EtOAc from the primary reactor in a 10:1 mol ratio are passed over a copper-based catalyst at 200 ºC, 5.0 MPa, and a GHSV of 4491h-1. The conversion of EtOAc is 69.5%, and the selectivity to ethyl groups that can be recovered as EtOH is 99.9%. Overall, the selectivity to EtOH is a high 95.7% (BP PLC [London]. US Patent 8,502,001, Aug 6, 2013; Jeffrey S. Plotkin)
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