Fractionation of yellow thatching grass (Hyparrhenia filipendula) for sugar production using combined alkaline and deep eutectic solvent pretreatment

Abstract

PURPOSE : Biomass pretreatment followed by enzymatic hydrolysis is one of the most viable ways to obtain sugars from biomass. In this work, the effect combined alkaline pretreatment and Deep Eutectic Solvent on enzyme hydrolysis of Hyparrhenia filipendula by cellulase is investigated. There is no previously reported literature on this substrate and the authors aim to establish baseline values for further research in the utilization of Hyparrhenia filipendula. METHODS : The yellow thatching grass (Hyparrhenia filipendula) was fractionated with a combination of alkaline and Deep Eutectic Solvent to increase sugar recovery. An alkaline solution of 10 wt % (w/v) of Sodium Hydroxide (NaOH) was used for the first stage of pretreatment at 100 °C for 4 h. Three DES, namely, Choline chloride (ChCl): urea; ChCl: glycerol; and Ethylene glycol: Citric acid at 1:2 molar ratio each, were heated to 80 °C until a clear solution was formed. The DESs were used for the second stage of pretreatment at 110 °C for 6 h in a Parr reactor. During the pretreatment, a solid: solvent ratio of 1:10 was used for the first and second stages of pre-treatment. Enzymatic hydrolysis was accomplished with a cellulase enzyme blend, Cellic CTec2, in a 50 mM sodium citrate buffer (pH 4.8) at 50 °C using a shaking incubator at a speed of 150 rpm. A solid loading of 2% and enzyme dosage of 50 g/100 g cellulose in the sample was used for all the experiments. Furthermore, samples were withdrawn every 24 h for 7 days and analyzed for glucose and xylose using High-Performance Liquid Chromatography (HPLC). RESULTS : A high delignification of 90% and hemicellulose removal of 70% was achieved with a combination of Alkali and ChCl: Urea pretreatment. Subsequently, the highest glucose and xylose conversion of 90% and 92% were observed, respectively, with the same sample. Additionally, the highest glucose yield achieved was 25 gL−1 from the combined alkaline/ChCl: Glycerol treated sample after 120 h. Moreover, the highest xylose yield was 3 gL−1 from the raw sample, the NaOH- pulped sample, and the ChCl: Glycerol-treated sample. CONCLUSION : The results from this study demonstrated that the solvents used for fractionating biomass have a significant effect on the sugar recovery during enzymatic hydrolysis. Also, the pretreatment with a combination of NaOH and DES of ChCl: Glycerol was the most effective for the recovery of glucose and total sugar. In conclusion, yellow thatching grass is a promising substrate for bio-refineries. However, the ideal conditions for enzyme hydrolysis should be investigated further to promote its utilization for value-added products.