There are elaborate examples in literature describing the production of new generation biofuels from sugars, sugar alcohols or other platform molecules such as HMF and levulinic acid, but research on the direct route from low cost cellulose to alkanes is still in its infancy. Although high temperature hydropyrolytic routes from biomass towards mixtures of gasoline and other compounds are promising, there is room to improve the carbon efficiency to liquid alkanes. Due to its high natural abundance and uniform chemical structure with repeating C₆ sugar units, cellulose should be the ideal precursor for selectively making C₆ alkanes (and thus light naphtha) as C–C bond breaking and forming are not required. The main challenge is to selectively break C–O in presence of C–C bonds.

…This paper reports a direct, fast and selective conversion of cellulose into liquid straight-chain alkanes, mainly n-hexane, by tuning the hydrogenation selectivity of a commercial Ru catalyst in a biphasic liquid system. The surface modification steers the reaction via a novel pathway, forming liquid alkanes through intermediate HMF.

—de Beeck et al.

The catalytic reaction occurs under hydrogen pressure in the presence of tungstosilicic acid, dissolved in the aqueous phase, and modified Ru/C, suspended in the organic phase. Tungstosilicic acid is primarily responsible for cellulose hydrolysis and dehydration steps, while the modified Ru/C selectively hydrogenates intermediates en route to the liquid alkanes.

Overview of the process. de Beeck et al. Click to enlarge.

The dominant route to the liquid alkanes proceeds via 5-hydroxymethylfurfural (HMF), whereas the more common pathway via sorbitol appears to be less efficient, they found.

High liquid alkane yields were possible through:

1. selective conversion of cellulose to glucose and further to HMF by gradually heating the reactor;

2. hydrothermal modification of commercial Ru/C to tune its chemoselectivity to furan hydrogenation rather than glucose hydrogenation; and

3. the use of a biphasic reaction system with optimal partitioning of the intermediates and catalytic reactions.

In contrast to other recently reported hydroprocessing processes, the authors said, their biphasic liquid approach at moderate temperatures mainly produces straight- chain alkanes with n-hexane and n-pentane as the major components. Recuperation of alkanes—floating on top of a separate water phase—is easy, while hydrogen selectivity is high as almost no gaseous products are formed as the C-C bonds are not broken. (Breaking those bonds results in gaseous ethanes such as methane and ethane.)

The catalytic system proved appreciably reusable and was applicable on raw softwood sawdust (almost 40% n-hexane yield). Future improvement in n-hexane yield is envisioned through a more selective formation of 2,5-DMTHF (or HMMF and DMF) to circumvent the n-pentane production. Identification of the modifying role of TSA on Ru/C, optimization of the stability of the catalytic biphasic system and decreasing the carbon content in the water phase are several focus points for future research.

—de Beeck et al.

This work was carried out within the EU FP7 project BIOCORE supported by the European Commission through the Seventh Framework Program.

Resources

• Beau Op de Beeck, Michiel Dusselier, Jan Geboers, Jensen Holsbeek, Eline Morré, Steffen Oswald, Lars Giebeler and Bert F. Sels (2014) “Direct catalytic conversion of cellulose to liquid straight-chain alkanes” Energy Environ. Sci. doi: 10.1039/c4ee01523a