Synopsis by Wim Thielemans, Sep 23, 2011
Long, J, B Guo, X Li, Y Jiang, F Wang, SC Tsang, L Wang and KMK Yu. 2011. One step catalytic conversion of cellulose to sustainable chemicals utilizing cooperative ionic liquid pairs. Green Chemistry http://dx.doi.org/10.1039/c1gc15597k.
A new one-step process blends a pair of specially selected solvents with cellulose, overcoming a big hurdle in the race to use the plant-based material as a reliable source of chemicals and fuels.
Pairing up a unique blend of specialized chemical solvents with plant cellulose has solved a looming problem for chemists grappling to find an efficient method to break apart naturally abundant cellulose.
The one-step process uses two ionic liquids to break cellulose down into its smaller chemical pieces and convert these pieces into useful chemicals. This is an important feat since cellulose is targeted as an alternative source for the chemicals and fuels currently derived from fossil fuels.
The results show for the first time that combinations of ionic liquids can be very useful in guiding more efficient chemical reactions that create less waste and more product. The findings are interesting because they show it is possible to combine several processing steps just by choosing the correct mixture of solvents.
Cellulose is a major part of plant cells and is the most abundant renewable material on Earth. Every year, plants, algae and some bacteria produce in excess of an estimated 100 billion metric tons. Cellulose is not a food and is a waste product of agriculture.
An enormous research effort is underway to understand how to use cellulose as a starting material for biofuel and chemical production in order to replace crude oil. In this process, cellulose – a long chain of identical chemical units – needs to be separated into the individual units.
The key challenge to convert cellulose into chemicals is its poor solubility – that is, it does not break up easily in liquids. Chemical reactions, though, require close contact between all the parts to work well. In 2002, researchers reported that some ionic liquids were very good at dissolving cellulose.
Ionic liquids are salts – a designation for chemicals made up of both a positively and a negatively charged component– that are liquid at low temperatures, unlike common salts such as table salt. Bulky positive and negative groups that make up the ionic salts hinder their packing into a solid crystal. Thus, they stay liquid to much lower temperatures – even room temperature.
In this work, Chinese researchers used a mixture of two ionic liquids. The first was chosen to dissolve cellulose. The second was chosen because it increases cellulose breakdown into its individual units, and further, into useful chemicals.
Under the right conditions, all the cellulose was broken down. The reaction products were also removed from the ionic liquid mixture by simply adding an insoluble solvent such as methanol or hexane. While there were a variety of different reaction products, up to 48.5 percent of the cellulose could be converted to a single product: 2-(diethoxymethyl)furan. This chemical, in turn, can be easily converted into a variety of other products useful for the chemical and pharmaceutical industries.
The choice of the solvent to extract the reaction products from the ionic liquid also allows researchers to select the reaction products that are recovered.
The ionic liquid mixtures were also reused. No real difference in performance and composition was noticed over 10 repeated reactions.
The researchers chose to dissolve cellulose first followed by a reaction to convert cellulose into smaller, more useful products. However, the results suggest the system is versatile and – depending on which ionic liquids are selected – could be extended to several reactions in a row. Ionic liquids will certainly hold some surprises in the future.
Read more science at Environmental Health News.