A mechano-biocatalytic one-pot approach to release sugars from lignocellulosic materials

Highlights

• The mechno-biocatalytic one-pot process is environmentally friendly.

• High titer and good yield of monosaccharide production can be achieved.

• Corn stover had the best titer and conversion among four feedstocks.

• The one-pot process demonstrated better energy performance than the conventional processes.

• The one-pot process had higher exergy efficiency than the conventional processes.

Abstract

A novel, mechano-biocatalytic one-pot process was developed by this study to efficiently release monosaccharides from lignocellulosic materials in an environmentally-friendly manner. The process synergistically integrates ball milling and enzymatic hydrolysis to complete pretreatment and hydrolysis of lignocellulosic materials in a single step without chemical supplements. High sugar titer and conversion from lignocellulosic materials were simultaneously achieved. Among four studied feedstocks (solid digestate, corn stover, switchgrass, and miscanthus), corn stover demonstrated much better sugar concentration and conversion. Under the preferred reaction condition, the glucose concentration reached 55.20 g/L with a glucose conversion of 88.63%. The corresponding xylose concentration was 20.06 g/L with a xylose conversion of 67.34%. The energy and exergy analyses further indicate that the studied process had better energy and exergy profiles than the conventional combined hydrolysis process. The average energy consumption of the mechano-biocatalytic process for four feedstocks was 1.05 kWh-e/kg dry biomass that was 56% lower than the average energy consumption (2.37 kWh-e/kg dry biomass) of the conventional process. The corresponding average exergy efficiency of the mechano-biocatalytic process was 67% that was much higher than the average efficiency (52%) of the conventional process. These results show that the mechano-biocatalytic one-pot process as an environmentally friendly approach can significantly simplify the pretreatment and hydrolysis and enhance their efficiencies for advanced fuel and chemical production.

Introduction

More than 140 billion metric tons of biomass, including energy crops, forestry and agricultural residues and wastes, are produced annually in the world [1]. The biomass has an energy potential as high as 1500 exajoules, which is approximately 1.8 times more than the projected world primary energy demand in 2040 [2]. However, the reality is that less than 70 exajoules per year of the world energy consumption is currently from biomass. One of the primary barriers hindering extensive use of the biomass by the biorefining industry is the recalcitrant structure of lignocellulosic feedstock, which requires either energy- and chemical-intensive processes [[3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]] or considerably long biological processes [14,15] to pretreat and hydrolyze them. New technologies must be developed to enable efficient biomass pretreatment and hydrolysis to release monosaccharides for biofuel and chemical production in an environmentally friendly manner.

Considering the physical and chemical steps of generating monosaccharides from lignocellulosic feedstock (size reduction of biomass, biomass decrystallization, and cellulose and hemicellulose hydrolysis), many studies have focused on mechano-chemical pretreatment and enzymatic hydrolysis (Table 1). Loustau-Cazalet et al. developed a vibro-ball-milling (VBM) process with and without chemical (H₂O₂ Urea, NaOH, H₃PO₄, and Betaine Cl) supplement to pretreat corn stover [16]. Enzymatic hydrolysis of the biomass pretreated by the VBM process with chemical supplement converted 98% of cellulose into glucose, which is much higher than the process without chemical supplement (56% glucose conversion). De Vrije et al. studied an extrusion-NaOH pretreatment coupled with enzymatic hydrolysis to release monosaccharides from Miscanthus [17], which had 69% and 38% of glucose and xylose conversions, respectively. Vandenbossche et al. designed a thermo-mechano-chemical-biological extrusion that combines thermos-mechano-chemical action and biocatalytic action in a twin-screw extruder to produce monosaccharides from six biomass feedstocks [18]. The extrusion process solubilized 7–13% of the cellulose and 12–75% of the hemicellulose in original biomass feedstocks. Mais et al. applied a combined ball milling and enzymatic hydrolysis on steam explosion treated Douglas-fir wood chips to generate monosaccharides [19]. A glucose conversion of 85% was reached at an enzyme loading of 25 filter paper unit (FPU)/g cellulose during a 48-h hydrolysis. Lee et al. used a planetary ball mill with alumina balls to prepare corn stover for the following enzymatic hydrolysis [20]. High monosaccharide conversion of 92% and relatively high sugar concentration of 35 g/L were achieved. Falls et al. applied NaOH augmented ball milling followed by enzymatic hydrolysis and obtained 91% of monosaccharide conversion and 33 g/L monosaccharide in the hydrolysate from switchgrass [21]. Balch et al. recently developed a bacterial fermentation process in a ball-milling reactor to realize a one-step conversion of switchgrass into biofuel [22]. Total fractional carbohydrate solubilization was 88% after 5 days cultivation with an initial carbohydrate concentration of 8.5 g/L (including glucan, xylan and arabinan) in the culture broth. However, these approaches either use corrosive chemicals, high-temperature thermal process, complicated treatment steps, low sugar titer and yield, or still require multiple steps to generate mono-saccharides form biomass. A consolidated pretreatment and hydrolysis process has yet to be achieved.

Therefore, the objective of this study is to develop a novel, mechano-biocatalytic one-pot process that only applies physical and biological treatment to directly release monosaccharides with high titer and yield from cellulose and hemicellulose in lignocellulosic materials. Agate ball mills were used to carry out the one-pot process where ball milling pretreatment and enzymatic hydrolysis of lignocellulosic biomass are simultaneously and synergistically occurred. The process was optimized for four representative agricultural residues and energy crops, solid digestate from anaerobic digestion, corn stover, miscanthus, and switchgrass. Energy and exergy analyses were then conducted to compare the preferred one-pot process with the conventional pretreatment and hydrolysis processes (acid or alkali pretreatment followed by enzymatic hydrolysis).