Overcoming Softwood Recalcitrance by Improved 2-Naphthol Addition to Steam Explosion Pretreatment
Christoph-M. Seidel, Simone Brethauer, Michael H. Studer, Philipp Rudolf von Rohr, Thomas Pielhop
Abstract
The prevention of lignin repolymerization/condensation in lignocellulose pretreatment can enhance both the bioconversion of cellulose and the quality of the obtained lignin. 2-Naphthol is a very effective pretreatment additive for suppressing lignin repolymerization, which enables overcoming the extraordinarily high recalcitrance of softwood to bioconversion. In this work, new approaches for adding 2-naphthol to the steam explosion pretreatment of softwood were studied. It was found that spray impregnation of spruce wood chips with 2-naphthol dissolved in acetone or ethanol is equally effective as the usually used soak-impregnation. However, spray-impregnation allows for an eight-fold reduction in solvent consumption. 2-Naphthol impregnation enabled practically quantitative cellulose conversion to glucose using an enzyme dosage of 30 FPU (filter paper units) g –1 cellulose, which corresponds to a digestibility enhancement of 89.1% compared to the control without additive. It was further disclosed that spraying of water-suspended 2-naphthol onto wood chips enabled a high glucose yield of 81.9% using an enzyme dosage of 30 FPU g –1 cellulose, which corresponds to an enhancement of 54.8% compared to the control and omits the use of solvents. An economic evaluation showed that spraying dissolved or water-suspended 2-naphthol is more cost-effective than soak-impregnation or admixing 2-naphthol flakes/powder to the biomass.