Conversion characteristics of sugars derived from Quercus mongolica to levulinic acid by two-step acid-catalyzed treatment

Abstract

The objective of the present study is to understand the conversion characteristics of C6 and C5 sugars during acid-catalyzed treatment. Based on this knowledge, this study also aimed to produce levulinic acid from lignocellulosic biomass (Quercus mongolica) via a two-step acid-catalyzed treatment, to utilize a by-product of this reaction for production of levulinic acid, and to evaluate the productivity of this process in comparison with conventional fermentable sugar production methods. The conversion of C6 and C5 sugars (i.e., glucose, galactose, mannose, xylose, and arabinose) by acid-catalyzed treatment using sulfuric acid (SA) was markedly affected by the reaction conditions (reaction temperature: 100-230°C, acid concentration: 0-2%), but differences among C6 or C5 sugars proved insignificant. In the absence of an acid catalyst, C6 sugar decomposition initiated at 160°C and was preferentially converted into 5-HMF, furfural, and humin, but not levulinic acid and formic acid, were generated. However, in the presence of an acid catalyst, C6 sugar was efficiently converted into levulinic acid via a 5-HMF intermediate at a lower reaction temperature (120-130°C), and the highest yield of levulinic acid (29.3 g/100 g C6 sugar at 200°C, 2% SA) was obtained by single-step acid-catalyzed treatment. However, the levulinic acid yield was slightly reduced due to the generation of humin at temperatures over 210°C. By contrast, C5 sugar was decomposed at 160°C and was almost immediately converted into furfural (28.1 g/100 g C5 sugar (230°C)) in the absence of SA. However, under acidic conditions, C5 sugars were efficiently converted into furfural at lower reaction temperatures (120°C)

the furfural yield was highest at 180°C in 1% SA, 37.6 g/100 g C5 sugar. Moreover, C5 sugar generated a large amount of an unidentified precipitate (37.3 g/100 g C5 sugar at 230°C, 2% SA) due to the strong condensation reaction caused by furfural, which might lead to the increased condensation of other products. Based on these results, Two-step acid-catalyzed treatment of Quercus mongolica under various reaction conditions was conducted for levulinic acid production. During the 1st step of acid-catalyzed treatment, most of the hemicellulosic C5 sugars (15.6 g/100 g biomass) were released into the liquid hydrolysate at 150°C in 1% SA

a solid fraction containing 53.5% of the C6 sugars remained. Subsequently, during the 2nd step of acid-catalyzed treatment of the solid fraction under the selected conditions (reaction temperature: 160-230°C

acid concentration: 1% or 2%), a higher levulinic acid yield (16.5 g/100 g biomass) was obtained at 200°C in 2% SA than in the single-step treatment. Resultant liquid hydrolysate obtained from 1st step acid-catalyzed treatment of Quercus mongolica at 150°C in 1% SA was additionally treated by a zeolite as a catalyst. The zeolite was alkaline treated with different NaOH concentration for using C5 sugars as levulinic acid source, and then it could possess sizeable pores and relatively higher portions of strong acid sites. Considering effects of other treatment factors, reaction temperature (150-210°C) and time (30-300 min), 3.5 g/100 g biomass of levulinic acid was produced through zeolite-catalyzed treatment at 190°C, 180 min, and zeolite treated by 0.25 M NaOH. In total, 20.0 g of levulinic acid could be produced from 100 g of Quercus mongolica via a multi-step treatment process in terms of biorefinery concept in this study. However, 31.5 g/100 g biomass of fermentable sugar (glucose) was produced from same solid fraction using a conventional process including enzymatic hydrolysis. Therefore, the proposed two-step acid-catalyzed treatment process is attractive due to the price competiveness of levulinic acid and the increased by-product utilization rate. In conclusion, the conversion characteristics of lignocellulosic biomass to levulinic acid under acid-catalyzed conditions were investigated. Based on this, levulinic acid was produced from lignocellulosic biomass via the two-step acid-catalyzed treatment process. Then, levulinic acid production via by-product application was performed in a biorefinery concept. Therefore, the levulinic acid yield was maximized using the total process, which used a novel lignocellulosic biomass biorefinery process to effectively produce levulinic acid.