The integration of dilute acid hydrolysis of xylan and fast pyrolysis of glucan to obtain fermentable sugars

Sustainable production of fuels and chemicals is becoming increasingly important due to a growing global demand for energy, uncertainty in the supply of petroleum resources and environmental concerns with petrochemicals processing [1, 2]. Lignocellulose is one of the key potential and attractive energy resources to overcome increasing energy needs while being environmentally benign [3]. As the important intermediates in the biological and chemical conversion of biomass, fermentable sugars can be converted to a series of products or biofuels via microbial fermentation [4, 5]. Nevertheless, access to sugars is hindered by the recalcitrance of plant cell walls. The majority of glucose in lignocellulose is highly locked into crystalline cellulose. Conversion of biomass into sugars suitable for fermentation is one of the leading challenges in developing biofuels.

Enzymatic saccharification is generally considered to be a sustainable approach to release fermentable sugars from lignocellulose [6]. Enzyme is utilized to breakdown long-chain polysaccharides into oligosaccharides and monosaccharides. Several structural features, such as the surface area and crystallinity, have been proposed as major hurdles to both the rate and extent of biomass saccharification. Pretreatment prior to enzymatic hydrolysis is an essential step for overcoming the structural and steric barriers to enzyme access for more efficient hydrolysis. Enzymatic hydrolysis has several advantages, such as lower energy consumption, mild operating conditions, lower capital of equipment and no inhibitors or toxic derivatives contained in enzymatic hydrolysate. Nevertheless, high pretreatment and enzyme cost (accounting for one-third of the ethanol production cost from cellulose), slow hydrolysis rate, low product concentration and sensitivity to contaminants originated from other biomass components restrict its economical feasibility and large scale applications [7]. Since the final ethanol concentration is directly proportional to initial sugar concentration, to reduce the cost in cellulosic ethanol production, high sugar concentrations are required to achieve high ethanol titers to reduce the energy required for distilling the ethanol from fermentation broths [8]. However, the sugar concentration resulting from enzymatic saccharification is too low (about 1 %) for practical fermentation. As the solid loading increases, the viscosity of the hydrolysate also increases, contributing to mixing and mass transfer problems that reduce sugar conversion. High solid loading also leads to unproductive binding of enzyme to substrate and product inhibition, which are stumbling blocks for converting biomass to high concentrations of fermentable sugars. The technical barriers and fundamental limitations in enzymatic saccharification processes have proven to be complex and difficult to overcome [9]. Acid hydrolysis is also a common approach to hydrolyze biomass. The concentrated acids play a dual role in biomass hydrolysis. By disrupting its network of intra- and inter-chain hydrogen bonds, strong acids decrystallize cellulose and make it accessible to reagents, and by catalyzing the hydrolysis of glycosidic bonds, strong acids cleave hemicellulose and cellulose into sugars. Nonetheless, the hazards of handling concentrated acids and the complexities of recycling them have limited the adoption of this technology. In dilute acid, hemicellulose and amorphous cellulose are easily and nearly completely hydrolyzed to fermentable sugars accessible to microorganisms for biofuels production. But the remained solid residue (mainly contained crystalline cellulose) is more difficult to be hydrolyzed [10].

Although enzymatic saccharification and acid hydrolysis have received most of the attention, fast pyrolysis is a little-explored alternative technology to release fermentable sugars from biomass [11]. Cellulose readily depolymerizes during pyrolysis at 500 °C in very short residence time to yield predominately levoglucosan (1,6-anhydro-β-d-glucopyranose). The utilization of anhydrosugars is critical to ensure the economic viability of biofuel production from the pyrolysis of biomass because of the significant concentration of levoglucosan in the pyrolysis oil. Levoglucosan is the intramolecular glucoside between C-1 and C-6 of d-glucopyranose. Biochemical studies have indicated that levoglucosan could be directly phosphorylated to glucose 6-phosphate with the Mg-ATP-dependent levoglucosan kinase, and then metabolized through the general glycolytic pathway [12]. It has been proven that levoglucosan can be utilized as fermentation substrate. Itaconic acid was produced from pure levoglucosan by Aspergillus terreus K26 with the same yield and at the same rate as produced from glucose [13]. Citric acid was produced from levoglucosan by Aspergillus niger CBX-209 [14]. Oleaginous yeasts Rhodosporidium toruloides and Rhodotorula glutinis could be good candidates for pyrolytic sugar utilization with high conversion yield of levoglucosan to lipid production (similar to that of glucose) [15]. Escherichia coli KO11 was genetically modified and could utilize levoglucosan as a sole carbon and energy source for ethanol production [16]. As an anhydrosugar, levoglucosan could be readily hydrolyzed by mild acid to glucose, thereby providing another potentially rapid and efficient route to the production of bio-ethanol [17]. The feasibility of a bio-refinery concept in which levoglucosan present in bio-oil was separated, hydrolyzed, detoxified and fermented to produce ethanol or lipids was demonstrated [18]. Depending on the type of biomass and the operating conditions used for pyrolysis, pyrolysis oil could contain upwards of 33 wt% of levoglucosan [19]. Additionally, the process of fast pyrolysis which utilizes short time without enzymes or catalysts to overcome biomass recalcitrance and liberates valuable fermentable sugar, has a lower capital investment than that of biochemical pathways and shows a significant advantage from the economic perspective [20, 21].

Still, important challenge remains for implementation of biomass fast pyrolysis process to get fermentable sugar. Levoglucosan yields from cellulose can be as high as 59 % [22]. Lignocellulose fast pyrolysis produces a very low amount of levoglucosan when compared with the theoretical yield based on cellulose fraction [23]. Whole pyrolysis oil contains 3 % of levoglucosan when untreated wood biomass is pyrolyzed. The mineral content of biomass has significant catalytic effects on the pyrolysis process. Even trace level of certain ash components can alter both the thermal degradation rate and chemical pathways during pyrolysis. Alkali and alkaline earth metals (AAEM) strongly catalyze pyranose ring scission by forming coordinate bonds with the oxygen atoms of vicinal hydroxyl groups of the glucose ring. This leads to homolytic scission of the ring during pyrolysis resulting formation of light oxygenates, rather than levoglucosan. A negative correlation between total ash content and bio-oil yield has been demonstrated [24]. The catalytic effect of inorganic dopants on cellulose to produce levoglucosan was examined [25, 26]. Pure cellulose was doped with varying concentrations of AAEM (NaCl, KCl, MgCl₂, CaCl₂). Char formation was strongly enhanced by AAEM addition compared with untreated cellulose. Anion additives produced more levoglucosan than the AAEMs, but all were lower than the pure cellulose. The authors postulated that inorganic metals reduce the activation energy for reactions that form glycolaldehyde, formic acid and acetol directly from cellulose. Levoglucosan was, therefore, not a reactant to form these products, but rather the glucan reacted preferentially through alternate pathways as a result of inorganic addition, thereby reducing yields of levoglucosan. In terms of levoglucosan yield, the influence of the inorganic metal ions was in the order of K⁺ > Na⁺ > Ca²⁺ > Mg²⁺ [25, 26]. Ash content up to a certain critical degree significantly impacts the levoglucosan yield. It is essential to remove the ash below such critical concentration from biomass prior to fast pyrolysis to achieve a greater levoglucosan yield. A number of efforts have been made for demineralization to achieve a high yield of levoglucosan from lignocellulose such as hot water washing and glycerol pretreatment [23, 27, 28]. Levoglucosan yield can be greatly increased if acid pretreatment is applied to demineralize the feedstock prior to pyrolysis. Previous research showed that infusion of certain mineral acids (sulfuric acid, phosphoric acid, hydrochloric acid, nitric acid, acetic acid and formic acid) into biomass converted AAEM into thermally stable salts allowing cellulose to more readily thermally depolymerize to levoglucosan and the effect of sulfuric acid was better than others [22]. Although acid pretreatment has previously been identified as influencing the outcome of biomass pyrolysis, differences among dilute sulfuric acid concentration has not been investigated and the utilization of hemicellulose was ignored.

Herein, the effect of sulfuric acid was further investigated in this study. The objective of this work was to develop a method to obtain fermentable sugars by dilute acid hydrolysis integrated with a fast pyrolysis step. Sugarcane bagasse and corncob were chosen as the biomass, and several concentrations of dilute sulfuric acid were utilized. Sugarcane bagasse and corncob were first hydrolyzed by dilute sulfuric acid to remove the main component of hemicellulose (xylan) as sugars. The remained cellulose (glucan) in the solid residue was further fast pyrolyzed to get levoglucosan.

The process to get fermentable sugars by integration of dilute acid hydrolysis and fast pyrolysis of sugarcane bagasse and corncob was evaluated. Dilute acid hydrolysis could remove more than 95.1 and 93.4 % of xylan obtaining 89.9 and 85.4 % yield of xylose from sugarcane bagasse and corncob, respectively. On the other hand, dilute acid hydrolysis prior to fast pyrolysis was also an effective pretreatment to enhance levoglucosan yield from acid pretreated biomass. Further increase in H₂SO₄ concentration (from 0 to 2 %) in dilute acid pretreatment increased the levoglucosan yield in fast pyrolysis. Compared with untreated feedstocks, the levoglucosan yields were increased from pretreated sugarcane bagasse (43.8 %) and corncob (35.2 %). The promotion was mainly attributed to the demineralization and accumulation of crystalline cellulose by acid pretreatment. The strategy in this work seemed a promising method to make full utilization of xylan and glucan, and get cost effective fermentable sugars from biomass for bio-refinery.