July 18, 2011
Take a biomass syngas route to ethanol. Ethanol (EtOH) is used increasingly as a gasoline component in the United States and other countries. One of the main concerns about this use of EtOH is whether farmland should be devoted to corn for EtOH production instead of food crops. To overcome this dilemma, so-called second-generation technologies designed to convert cellulosic biomass to EtOH are under intensive development.
There are two lines of attack to accomplish this objective. One is to genetically engineer organisms that ferment cellulose to EtOH. It is difficult, however, for these organisms to ferment biomass fast enough to make it economical.
In an alternative thermochemical method, biomass is gasified to synthesis gas (syngas), a mixture of carbon monoxide (CO) and hydrogen (H2), which is used to make EtOH. This is appealing because the gasification step is similar to widely used coal gasification. The stumbling block, however, is converting syngas to EtOH in high yields.
B. J. Daniel, B. P. Gracey, and J. G. Sunley disclose a multistep process for converting syngas to EtOH:
1. Generating syngas
2. Producing methanol (MeOH) and EtOH in an alcohol synthesis unit from syngas and methyl acetate (MeOAc) and/or ethyl acetate (EtOAc)
3. Separating MeOH and EtOH
4. Adding CO to the MeOH to make acetic acid (AcOH)
5. Feeding AcOH with MeOH or EtOH to an esterification unit to make MeOAc or EtOAc
6. Recycling the MeOAc or EtOAc to the alcohol synthesis unit
7. Recovering the EtOH from step 3
All of these steps except step 2 are well-known processes. The novelty of the inventors’ method is step 2, in which MeOH synthesis and ester hydrogenation occur in the same reactor with the same catalyst. The inventors give several examples to demonstrate the feasibility of this step.
In one example, a mixture of H2 (70 vol%), CO (10 vol%), MeOAc (4 vol%), and nitrogen (16 vol%) is passed over a commercial catalyst (33 wt% CuO and 66 wt% ZnO) at 240 °C, 7.6 MPa pressure, and a gaseous hourly space velocity of 6837 h–1. MeOAc conversion is 96.7%, and the selectivity to EtOH–EtOAc is 97.9%.
In this example, the mol ratio of produced MeOH to converted MeOAc is 2:24:1. This result demonstrates that MeOH is produced from MeOAc by hydrogenolysis and from the synthesis gas. (BP [London]. US Patent 7,947,746, May 24, 2011; Jeffrey S. Plotkin)