Subcritical water hydrolysis of Phragmites for sugar extraction and catalytic conversion to platform chemicals

Abstract Phragmites karka, also known as common reed, is a perennial grass and a highly invasive crop species, which creates ecological problems by competing with native biodiversity and vegetation. This study involves subcritical water hydrolysis of Phragmites to produce monomeric sugars followed by the catalytic conversion of the sugar-rich hydrolysate to furfural and levulinic acid. Subcritical water hydrolysis was performed by the Central Composite Design method at variable temperatures (150–230 °C), reaction time (15–60 min) and feed concentration (2–5 wt%). The temperature was found to be the most prominent factor affecting biomass hydrolysis. The yield of total reducing sugars from biomass hydrolysis was in the range of 2.1–18.1% where the highest yield was obtained at the optimal temperature (190 °C), reaction time (37.5 min) and feed concentration (2 wt%). During subcritical water hydrolysis of Phragmites, two main degradation products obtained at a higher temperature (230 °C) and reaction time (37.5 min) were furfural (8.2%) and 5-hydroxymethylfurfural (11.7%). However, at 230 °C and a longer reaction time of 60 min, 5-hydroxymethylfurfural yield reduced to 5.1% owing to its conversion to humin while furfural yield elevated to 9.9%. Catalysts such as ZrO2, TiO2, Zr0.5Ti0.5O2, WO3–ZrO2, WO3–TiO2 and WO3–Zr0.5Ti0.5O2 were involved in the conversion of the sugar-rich hydrolysate obtained from subcritical water hydrolysis of Phragmites. The highest sugar conversion was found to be 92% with WO3–ZrO2 resulting in the yields of furfural (51%) and levulinic acid (34%). The activity of particular catalysts (e.g. WO3–ZrO2, WO3–TiO2 and WO3–Zr0.5Ti0.5O2) relied on the synergistic effects of Lewis and Bronsted acid sites.