Owing to the complexity of lignocellulosic biomass and its notorious resistance to chemical transformation, energy-efficient and cost-effective production of liquid fuels from lignocellulose remains a mammoth challenge. So far, two strategies have been reported to address this challenge: (i) separation of lignocellulose into isolated sugars and lignin followed by biological or chemical (hydrolysis) processing; (ii) thermochemical treatment of lignocellulose to produce upgradeable intermediates, such as bio-oils by pyrolysis or syngas by gasification, coupled with subsequent catalytic upgrading.

Thermochemical processes offer the total conversion of lignocellulose, but are often non-selective and intractable, and the resultant bio-oils or syngas need to be upgraded for further utilization. Although hydrolysis-based approaches offer selective production of liquid fuels, they are generally multistep and thus very energy-intensive. Moreover, the lignin by-products generated from the hydrolysis of lignocellulose are usually burned as a low-value fuel. Powerful drivers therefore exist to develop alternative efficient and selective strategies to directly convert raw lignocellulose into liquid fuels.

… Here we report that, by using a multifunctional Pt/NbOPO₄ catalyst, raw woody biomass can be directly converted into liquid alkanes in high yields in a single-phase medium (cyclohexane) with cellulose, hemicellulose and lignin fractions in solid woods being converted into hexane, pentane and alkylcyclohexanes, respectively, representing direct conversion of raw lignocellulose into liquid alkanes under mild conditions over a single catalyst. Importantly, no chemical pretreatment (for example, hydrolysis and separation) to the raw wood is required for this process, and thus, tremendous energy savings can be potentially gained in comparison with the existing thermochemical- and hydrolysis-based approaches.

—Xia et al.

The main reaction pathways. The reaction occurred by the direct hydrogenolysis of the β-1,4 linkage to D-glucose (2) and 1-deoxy-D-glucose (6) first, and then 6 was converted to hexane by sequential hydrogenolysis via 2-hydroxymethyl-tetrahydropyran (8), 2-methyltetrahydropyran (12) and hexanols (16) and (18), whereas the conversion of 2 has three main reaction pathways: (i) hydrogenated to sorbitol (3) and then dehydrated to sorbitan (4) and isosorbide (5), followed by sequential hydrogenolysis via 2-ethyl-THF (13) and hexanols (16) and (17). (ii) Isomerized to fructose and then dehydrated to 5-hydroxymethylfurfural followed by hydrogenation and sequential hydrogenolysis via 2,5-dimethylfuran (10), 2-hexanone (14), 5-methyl-THF-2-methanol (9), 2,5-dimethyl-THF (11) and 2-hexanol (18). (iii) Dehydrated to 1,6-anhydroglucose (7) followed by sequential hydrogenolysis via oxepane (15) and n-hexanol (16). Xia et al. Click to enlarge.

The researchers tested seven different types of wood sawdusts, including both softwoods and hardwoods, for direct hydrodeoxygenation over the catalyst. The reactions were conducted at 190 °C and 5 MPa H₂ for 20 h. More than 20 wt% total mass yield of liquid alkanes was achieved for all woods, among which birch wood gave the highest mass yield of 28.1 wt%.

As a point of comparison, the theoretical mass yield of alkanes from raw woody biomass is limited to ~50 wt%, because the removed oxygen accounts for almost half of the mass loss. The 28.1 wt% yield, the researchers said, was excellent.

In addition to C₁–C₆ alkane products, the team found “surprisingly appreciable” amounts of alkylcyclohexanes, indicating that not only the cellulose and hemicellulose but also the lignin fraction in sawdusts were converted into alkanes.

The type of wood had a significant influence on both mass and carbon yields of the alkane products. In general, the team found higher yields of hexanes and pentanes from softwoods; the carbon yields of hexanes and pentanes on the basis of cellulose and hemicellulose fractions reached 72.8 and 69.3% on average, respectively—comparable to the results when using pure cellulose as a feedstock

The yield of alkylcyclohexanes produced from hardwoods was much higher than that from softwoods, with an average carbon yield of 34.0% from hardwood. Although seemingly low, that yield is actually very high, the team explained, because there is a large proportion of C–C linkages in lignin structure (30–34% for hardwoods and 43–51% for softwoods on average) which are hardly cleaved under such mild reaction conditions. This limits the maximum theoretical carbon yield of monomer alkylcyclohexanes at 44–49% from hardwoods and 24–32% from softwoods.

Dr. Sihai Yang, lead author of the study, used the Science & Technology Facilities Council’s ISIS Neutron and Muon source to study the biomass and catalyst at the molecular level. Using an instrument called TOSCA, Dr. Yang and ISIS scientist Dr. Stewart Parker used neutrons to see how a model of lignocellulose interacted with the surface of the catalyst to produce useful fuel.

They determined that he superior efficiency of this catalyst for direct hydrodeoxygenation of lignocellulose originates from the synergistic effect between Pt, NbO_x species and acidic sites.

Resources

• Qineng Xia, Zongjia Chen, Yi Shao, Xueqing Gong, Haifeng Wang, Xiaohui Liu, Stewart F. Parker, Xue Han, Sihai Yang & Yanqin Wang (2016) “Direct hydrodeoxygenation of raw woody biomass into liquid alkanes” Nature Communications 7, Article number: 11162 doi: 10.1038/ncomms11162