Suppression Effect of Gamma-Valerolactone on the Mild Alkaline Pretreatment of Hybrid Pennisetum

Abstract

Gamma-valerolactone (GVL) assisted NaOH/water pretreatment for hybrid pennisetum delignification was first investigated. Contrary to the hypothesized results, the addition of GVL significantly suppressed the delignification and hemicellulose removal by mild alkaline pretreatment (37 °C for 24 h). An addition of GVL to GVL/NaOH molar ratio of 1/1 resulted in lignin removal at ∼23% compared with that at ∼75% without GVL. This suppression effect was further confirmed by scanning electron microscopy and Fourier-transform infrared spectroscopy. The extracted lignin was further characterized by two-dimensional heteronuclear single quantum coherence nuclear magnetic resonance spectroscopy, suggesting that the addition of GVL suppressed the decomposition of large lignin molecules by NaOH/water and the release of hemicellulose, resulting in extracted lignin with lower molecular weights and less hemicellulose. The decrease in the partial alkalinity (PA) of alkaline solutions caused by GVL addition was likely the main reason for the suppressed delignification, suggested by the good correlation between PA and delignification in both GVL/NaOH/water and NaOH/water (R2 = 0.967). This study suggested that solvent-assisted alkaline pretreatment can also result in an unwanted suppression effect, and the changes in PA could be referred to as an indicator while selecting organic solvents.