Solid acids are shown to be effective catalysts for breaking down cellulose, a potentially renewable chemical feedstock
Chemical feedstocks currently rely heavily on fossil fuels, but efforts are being made to use biomass, in the form of simple sugars, instead. Unfortunately, most sugar in biomass is locked up in long cellulose molecules that are effectively protected against chemical processing by tight packing. Unlocking cellulose to provide renewable chemical feedstocks can be achieved by dissolving it in ionic liquids, and now Ferdi Schüth and colleagues at the Max-Planck-Institut für Kohlenforschung in Mülheim, Germany, have shown that solid acids are powerful catalysts1 for hydrolysing cellulose.
Amberlyst, a commercially available sulfonated resin, was able to split even microcrystalline cellulose into sugars. By comparing the resin to other solid acid catalysts, such as Nafion or alumina, Schüth and co-workers discovered that the combination of Amberlyst's high surface area and quite big pores was the key. Both of these properties need to be large enough to enable the long cellulose chains to approach and react. This also shows that the action of solid acids is not by simply releasing protons into the solvent.
Analysis of the products after five hours showed that the cellulose polymers had been broken down into oligomers averaging ten glucose units, and that the catalysts showed a preference for cleaving the longer chains. This results in the distribution of chain lengths, or polydispersity, narrowing over time. Oligomers with a controlled length and good polydispersity could thus be produced as feedstocks in biorefineries.
Rinaldi, R., Palkovits, R. & Schüth, F. Depolymerization of cellulose using solid catalysts in ionic liquids. Angew. Chem. Int. Ed. 10.1002/anie.200802879 (2008).
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Withers, N. Cellulose breakdown. Nature Chem (2008). https://doi.org/10.1038/nchem.71