Generally, there are several major strategies to convert lignocellulose into liquid hydrocarbon fuels: fast pyrolysis, liquefaction, and gasification followed by Fischer-Tropsch synthesis. These routes usually deal with whole lignocellulose leading to upgradeable platforms such as bio-oil or syngas. Another important route involves depolymerization of lignocellulose to yield platform molecules such as furfural, hydroxymethylfurfural (HMF), or levulinic acid, etc. Catalytic transformation of platform molecules for the production of liquid hydrocarbon fuels can be obtained by oxygen removal process (e.g. dehydration, hydrogenation, hydrogenolysis, decarbonylation) or in some cases along with increase the length of carbon chain via C-C coupling reactions (e.g. aldol condensation, hydroxyalkylation, ketonization, oligomerization) with reactive intermediates.

The latter method is a very promising way to go, because it offers pathways for the desired chain extensions to meet the number of carbon atoms for the aimed fuel range, such as diesel and jet fuels, which are two types of fuel in greatest demand and currently sourced mainly from petroleum. Various reactive intermediates such as acetone, 2-hexanone, hydroxyacetone or dihydroxyacetone, mesityl oxide, methyl isobutyl ketone (MIBK), propanal, butanal, and levulinic acid38 have been employed as synthons for the synthesis of liquid hydrocarbon fuels. This route is especially important and meaningful when the synthon is derived from biomass.

On the other hand, recent advances in metabolic engineering have enabled the biological production of transportation fuels via non-fermentative pathway, however, these processes usually suffer from low yields and titers. Alternatively, sugar molecules derived from cellulose or hemicellulose can be fermented into hydrocarbons by microorganisms or genetically altered microorganisms. For example, acetone-butanol-ethanol (ABE) fermentation is one of the oldest known industrial processes with a history of more than 100 years. … However, the economics of this process was hampered by a number of bottlenecks such as co-production of low value product acetone and high cost of separation. Moreover, high acetone levels in the fermentation broth result in serious mass loss of acetone due to its high volatility, thus leading to a lower solvent yield.

—Zhu et al.

The Nanjing Tech team had earlier used metabolic engineering to acetone with the C₄ compound acetoin (3-hydroxy-2-butanone) during ABE fermentation with good yield.

Acetoin is amenable to C-C bond formation with other platform molecules; the team was interested in exploring the use of acetoin as a potential bio-based C₄ building block for the synthesis of renewable liquid hydrocarbon fuels. In the study, they synthesized C₉-C₁₄ straight, branched, or cyclic alkanes from the reaction of acetoin with lignocellulose-derived platform molecules using different catalysts.

They found that TFA-ZrO₂, the ionic liquid EAIL, and aluminium phosphate were efficient and recyclable catalysts for the HAA (HydroxyAlkylation–Alkylation) reaction and aldol condensation, respectively—each the first step in different reaction pathways converting acetoin to different hydrocarbon products. The subsequent hydrodeoxygenation (HDO) step could be achieved by using a Pd/C + H-beta zeolite—found to have high activity in the conversion of various furan- or aromatic- based oxygenates.

This work not only provides a new bio-based platform chemical for the direct synthesis of liquid alkanes, but also offers new opportunities to upgrade biomass to clean fuels and chemicals by bridging the biological and chemical catalysis gap.

—Zhu et al.

Resources

• Chenjie Zhu, Tao Shen, Dong Liu, Jinglan Wu, Yong Chen, Linfeng Wang, Kai Guo, Hanjie Ying and Pingkai Ouyang (2016) “Production of liquid hydrocarbon fuels with acetoin and platform molecules derived from lignocellulose” Green Chem., 18, 2165-2174 doi: 10.1039/C5GC02414E