There have been two types of approaches conventionally considered for the conversion of woody biomass to liquid fuels, Matson et al. note: (a) acid pretreatment and separation followed by fermentation or liquid phase processing, or (b) high temperature conversion such as gasification or pyrolysis to bio-oils. Each of these process has problems related to their efficiency.

• For (a), the lignin fraction, which comprises up to 30% of the carbon content, is often simply burned, although recent work is striving to improve this.

• For (b), a significant fraction of the feedstock is transformed into intractable carbonaceous waste (i.e., “char”), and refinery-scale infrastructure is usually needed for processing. This, in turn, makes forest-to-refinery transport a key economic consideration and therefore limits the size of processing units.

We offer a unique alternative, the methanol-mediated, catalytic conversion of woody biomass and of cellulose solids to liquids and gases. This process is quantitative (little or no char) and should be compatible with operation scales that match the transportation limitations of biomass feedstocks.

...Simple chemical conversion of lignocellulosic biomass is challenged by the complexity of this material; indeed, many of the studies of new biomass conversion methodologies have focused on simpler molecular models, especially for lignin. Effective utilization requires depolymerizing cellulose and lignin, the latter being nearly refractory toward hydrolysis of its phenolic ether linkages, and substantially reducing overall oxygen content without producing chars.

We have previously addressed the issue of lignin depolymerization by demonstrating that the simple model dihydrobenzofuran and the more complex organosolv lignin undergo catalytic hydrogenolysis of phenolic ether linkages in supercritical methanol in the presence of a copper-doped porous metal oxide (Cu20-PMO)...We now show that this single-stage process also depolymerizes and reduces cellulose and converts much more complex lignocellulose composites, various finely divided wood particles, to a mixture of alcohols potentially usable as liquid fuels.

...Conversion of lignin and cellulose to liquid fuels requires reduction of these highly functionalized biopolymers. The process described here uses catalytic methanol reforming to provide the necessary reducing equivalents R, although the introduction of syngas may also serve this purpose.

—Matson et al.

They applied the new procedure to several wood sawdusts, to powdered torrefied wood, and to cellulose fibers and identified and quantified the major components in the gas and liquid phases in each case. The liquid products fall largely into two groups of monomeric alcohols and ethers: HAE (higher alcohols and ethers), which are C₂–C₆ species, and CAE (substituted cyclohexyl alcohols and ethers), which are largely C₉–C₁₂ components.

The researchers envision that biomass conversion via this “UCSB process” would occur at moderate-sized facilities located where lignocellulose wastes or crops are generated, thereby ameliorating the biomass-to-refinery transportation cost. They describe three potential variations of the methodology:

• The “methanol option” involves utilizing MeOH both as the reaction medium and as a “liquid syngas” for reducing biomass components. This option should be the most easily scaled from the current technology, they suggest.

• The “methane option” uses CH₄ -to-syngas reforming to generate the necessary reducing equivalents. In the United States, an advantage of the latter option is that the methane distribution network is already extensive, including areas in the Midwest that would be major suppliers of biomass feedstocks. The disadvantage would be the addition of another process to the front end of this process.

• The “biomass-only option” uses a biomass-to-syngas conversion at the front end of the biomass-to-fuels process. The advantage would be an even lower CO₂ footprint combining an existing technology (biomass-to-syngas) with a new one (the UCSB process), and in the long run would clearly be a desirable goal, they suggest. With the latter two options, the methanol utilized as the reaction medium could be generated internally.

Successful utilization of renewable biomass solids via this process would have the potential of reducing the CO₂ burden from liquid fuel utilization without compromising food supply. Ongoing studies are focused on characterizing and optimizing the earth-abundant catalysts used in this process, on elucidating key mechanistic issues, and on evaluating how the reaction variables control yields and selectivity in product streams. For example, one approach may be to develop simple deoxygenation procedures to generate clean mixtures of alkanes.

—Matson et al.

Resources

• Theodore D. Matson, Katalin Barta, Alexei V. Iretskii, Peter C. Ford (2011) One-Pot Catalytic Conversion of Cellulose and of Woody Biomass Solids to Liquid Fuels. Journal of the American Chemical Society doi: 10.1021/ja205436c