Cracking wood gently

German scientists have combined ionic liquids and solid catalysts to gently break down the cellulose in wood and inedible plant material - easing the crucial first stage in converting waste biomass to fuels or feedstock chemicals. 

The linkages between sugar molecules in cellulose’s long chains are protected against chemical attack, because cellulose tangles itself into microfibrils which are hard to break through. Harsh conditions such as aggressive acid baths, or high temperatures and pressures, are usually required. 

But researchers at the Max Planck Institute for Coal Research at M?lheim, Germany, have now demonstrated that commercially available solid acidic resins can gently prise apart cellulose chains which have first been dissolved in ionic liquids (organic salts which are liquid at room temperature). The solid resins can be easily filtered out from the reaction mixture and recycled as catalysts, and the reaction also hydrolyses cellulose in untreated wood chip. 

Ferdi Sch?th and his coworkers used the ionic liquid 1-butyl-3-methylimidazolium chloride (BMIMCl) to dissolve cellulose fibrils, and then degraded the cellulose chains with a commercially available resin (Amberlyst 15DRY), which contains sulfonic groups (-SO₃H), at 100?C. Over the course of a few hours, the cellulose gradually depolymerised, forming first shorter chain molecules (oligomers) and finally sugars. 

Sch?th explains that it’s best to stop the process at the oligomer stage, because sugars are soluble in ionic liquids, so hard to separate. Longer oligomers, of around ten ahydroglucose units, can be separated from the ionic liquids by simply adding water. Sch?th suggests that these cellulose oligomers could then be treated with enzymes to give fuels or chemical feedstocks. The reaction is particularly selective: after five hours, only traces of sugars and sugar dehydration products (such as furfural) are formed - which is fortunate, as the furfurals would poison later enzymes.

The problem in scaling up this process, Sch?th admits, is that separating the ionic liquid solvent from added water requires a significant amount of energy. But he is confident that ’people in industry do not consider this an insurmountable obstacle.’ He has filed a patent on the technology and held exploratory talks with German chemical giants BASF. 

It’s been known for half a decade that ionic liquids can dissolve cellulose - even from solid wood; while researchers have also tried using solid acid resins to break down cellulose, but only in water and without much success. Sch?th suggests that cultural boundaries between research communities may explain why nobody has connected the two process before. 

Blake Simmons, a vice president at the Joint BioEnergy Institute in San Francisco - a US Department of Energy research centre studying cellulosic fuels - welcomes the new development: ’I think this work provides exciting new insight into the potential of combining an ionic liquid pretreatment with catalysts that hydrolyse cellulose, and represents a significant step forward in making this technology competitive with other established biomass pretreatment techniques,’ he told Chemistry World.   Mark Harmer, a research fellow at DuPont, also praised the work but added it was now critical to demonstrate that the ionic liquids could be recycled. 

Michael Gross

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