Liquefaction of lignocellulose: Do basic and acidic additives help out?

• Lignocellulose is liquefied with high liquid yield (∼90 C%) in batch experiments.
• Use of base lowers the yield of undesired heavy product while acid increases it.
• Formation of heavy product is found dependent on pH of the liquid product.
• Base get partly neutralized and eventually favor the degradation of the bio-oil to heavy product under recycle conditions.
• Batch experiments are insufficient to evaluate acid/base additives.

Lignocellulosic feedstock can be converted to bio-oil by direct liquefaction in a phenolic solvent such as guaiacol. The bio-oil could then be further upgraded to transportation fuel using conventional oil refining process. The production of heavy components (molecular weight >1000 Da) was found to be a major hurdle in this process as it hinders the efficiency of recycling the bio-oil as liquefaction medium. This paper studies the effect of bases and acids on the liquefaction of biomass and their effectiveness in reducing the formation of heavy components. Acid was found to have a negative effect while all the bases showed a positive effect in reducing the formation of heavy components. Production of heavy components was found to depend on the pH of the medium; it decreases with increasing pH of the product measured after the reaction. Both acid and base additives got partly neutralized during the reaction and, therefore, do not operate as true catalysts. Further refill runs with KHCO3 were carried out to check the possibility to recycle the additive and the bio-oil as liquefaction medium. The additive appeared to perform poorly under recycle conditions. The base got neutralized and addition of fresh base catalyzed the degradation of the solvent (guaiacol) and the bio-oil upon oil recycling. This behavior may explain the recycling challenge faced in the PERC process. This study shows that single run experiments are insufficient to evaluate additives as they do not unravel acid/base deactivation and oil degradation. Refill runs are necessary for that.