Hydrogen bonds turn sweetener into lignin bodyguard, boosting biomass yield 42%

Acid baths that strip lignin from wood also spark a nuisance reaction: reactive carbocations snap together, forming rock-solid C–C bonds that lock away valuable aromatics and sabotage downstream enzymatic saccharification. Instead of adding toxic blocking agents, researchers have turned to the candy aisle.
Writing in the Journal of Bioresources and Bioproducts, the group compared four commodity polyols—salicylic acid, mannitol, 2-naphthol and glycolic acid—at only 10 wt% dosage during 160 °C dilute-sulfuric pretreatment of eucalyptus. While all candidates slowed re-polymerisation, mannitol performed best, cutting weight-average molecular weight from 4 600 Da to 1 280 Da and raising subsequent hydrogenolysis monomer yield to 42 %, nearly double the 23 % obtained without additive.
NMR fingerprints showed why. Mannitol’s six staggered hydroxyls wrap around lignin side chains, forming an average of 28 hydrogen bonds per simulation box versus 14 for 2-naphthol. Density-functional calculations put the strongest bridge at -80 kJ mol-1 between mannitol and the Cα–OH of a β-O-4 ether, effectively capping the site where carbocations normally form. The protective cloak keeps β-O-4 linkages at 47 per 100 aromatic units—triple the level seen in additive-free controls—while condensed phenolic hydroxyls plummet from 21 % to below 1 %.
Molecular-dynamics movies reveal the sugar-alcohol molecules cluster within 0.35 nm of lignin within the first 5 ns of reaction, long before acid can protonate benzylic carbons. The additive acts like a reversible seat-belt: it fastens itself through hydrogen bonds, prevents the passenger from flying out of the window, then unbuckles when you dilute the slurry.
Economic eye-candy backs the science. Bulk mannitol sells for roughly $2 kg-1, one-fiftieth the price of ionic liquids already piloted for lignin protection. Because it is food-grade, the polyol requires no special solvent recovery loops and can be rinsed into downstream fermentation as an extra carbon source.
The team estimates integrating mannitol into a 50 t d-1 biorefinery would add less than $0.03 to the cost of a litre of bio-ethanol while boosting lignin-derived aromatic revenue by 60 %. They caution the work was done on hardwood; softwood rich in guaiacyl units may need tweaked loadings. Still, the principle—cheap polyols as sacrificial hydrogen-bond donors—offers an immediately deployable lever for the lignin-first biorefinery movement and a data-rich training set for machine-learning models hunting next-generation green additives.
With funding agencies in China already underwriting pilot-scale trials, the sweet solution could soon graduate from laboratory curiosity to commercial reality, turning sawdust into both sugars and high-purity aromatics without the usual tarry mess.