Catalyst Pair Converts Cellulose Directly To Hexane

Researchers report a one-pot catalytic reaction to convert cellulose to n-hexane in high yield (ACS Sustainable Chem. Eng. 2014, DOI: 10.1021/sc5001463).

A direct path from cellulose to hexane has the potential to simplify routes for turning biomass into aromatic compounds like benzene or into isohexane, a component of gasoline, says Bert Sels of Catholic University of Leuven, in Belgium, who was not involved with the research. Such a process could effectively replace petroleum as a starting material for those chemicals.

Some scientists have worked on catalysts to break cellulose into molecules of glucose, which can then be transformed into other chemicals. However, converting cellulose to glucose and then to hexane requires multiple reactions, some of which may not produce enough for industrial applications. What’s more, some metal-based catalysts break up the cellulose into carbon chains smaller than hexane, meaning chemists would have to add in other reactions to rebuild the longer chain.

In 2013, Yoshinao Nakagawa and Keiichi Tomishige, together with their colleagues at Tohoku University, in Japan, used a catalyst to convert glucose directly to hexane (ChemSusChem, DOI: 10.1002/cssc.201200940). They wondered if this catalyst, made from iridium metal and rhenium oxide supported on silica, also could yield hexane directly from cellulose.

The researchers combined microcrystalline cellulose, their catalyst, and the zeolite HZSM-5 in water and heated the mixture to 210 °C under an atmosphere of hydrogen gas. Acid in the pores of the zeolite hydrolyzes cellulose into glucose. Next, glucose and hydrogen react, with the help of the metal catalyst, to produce the six-carbon alcohol sorbitol. The zeolite and metal catalysts then convert sorbitol to hexane, which floats into a layer of dodecane above the water mixture, making it easy to recover the product, Nakagawa says. The overall process, starting from cellulose, produces hexane in 78% yield after 24 hours.

Typically, to help push the conversion of cellulose into other chemicals, researchers grind cellulose crystals into small pieces. The process enhances the reactivity of the cellulose but requires a lot of energy. So Atsushi Fukuoka of Hokkaido University, in Japan, is excited to see the high yields of hexane starting directly from microcrystalline cellulose, because that suggests it might be possible to eliminate the grinding step. The researchers, he says, next should test this new reaction using raw biomass instead of processed cellulose.

Bert M. Weckhuysen of Utrecht University, in the Netherlands, likes the idea of a one-pot process to convert cellulose to hexane but wonders about the lifetimes of the two catalysts. Also, he thinks it would be interesting to include a reaction that forms branched hydrocarbons for transportation fuels.