Cellulose nanocrytals (CNCs) are predominantly produced using the traditional strong acid hydrolysis process. In most reported studies, the typical CNC yield is low (approximately 30%) despite process optimization. This study investigated the hydrolysis of a bleached kraft eucalyptus pulp using sulfuric acid between 50 and 64 wt % at temperatures of 35–80 °C over time periods of up to 240 min for the production of CNCs. The experimental design captured the feature of the coexistence of a variety of reaction products, such as CNC, cellulosic solid residue (CSR), glucose, and xylose, in the product stream for accurate kinetic modeling to improve the CNC production yield. The kinetic model describing the solubilization of cellulose fibers used three phenomenological reactions, namely, hydrolysis of xylan to form xylose, depolymerization of cellulose to CNCs, and hydrolysis of cellulose to form glucose, each of which can be described by pseudohomogenous first-order kinetics. The concept of “degrees of hydrolyzable xylan or cellulose” to reflect the inhomogeneity of xylan or cellulose in hydrolysis was incorporated into the kinetic modeling to improve model accuracy. The developed model showed excellent predictability for CNC production. Both the experimental data and the model clearly indicate that CNC production was limited by cellulose depolymerization at low acid concentrations of below 58 wt %, but controlled by CNC degradation when the acid concentration was higher than 58 wt %. This work for the first time provides the most plausible description of CNC production kinetics, which is significant for the commercial production of CNCs.

The biomass recalcitrance of the lignocellulose cell wall constructed by its chemical components, especially hemicelluloses and lignin, has become a bottleneck for the efficient release of glucose. The presence of hemicelluloses has been considered as a major factor limiting the enzymatic digestibility of lignocellulose biomass. However, most of the reported works on the effect of hemicelluloses removal on cellulose hydrolysability were conducted via dilute acid pretreatment at high temperature (>160 °C), and inconsistent conclusions have been found. In the present work, we studied the effects of xylan content on enzymatic digestibility of wheat straw cellulose in the cases of high and low lignin contents. Particularly, xylan removal was achieved by sulfuric acid hydrolysis under mild conditions (120 °C) to minimize lignin melting and migration in the cell wall and lignin structure modification. As revealed by various structure characterizations, when no lignin was removed, xylan removal by dilute acid hydrolysis resulted in reduction of particle size, deformation of the cell shape, etching of the cell lumen surface, some fracture and slight delamination of cell wall, with associated great increase in porosity and specific surface area. These structural modifications greatly improved cellulose digestibility. However, the presence of residual lignin also showed significant negative impacts by physical blocking and nonproductive adsorption of cellulases. In the case of low lignin content (∼4%), cellulose fibers become liberated and significant etching, delamination, fracture and even disappearance of the walls were visualized with xylan removal, which remarkably increased the effective surface area for cellulase binding with cellulose. The finding of this work demonstrates that the limiting action of hemicelluloses seems to be not important to cellulose digestibility as that observed in high-temperature (>160 °C) dilute acid pretreatment. Delignification seems to be more efficient to improve cellulose accessibility for mild-condition (<120 °C) pretreatment. It indicates that the interaction effects between lignin and hemicelluloses as structural factors limiting cellulose digestibility should be considered for investigating the mechanisms of effects of structure features on cellulose accessibility.Keywords: Cellulase adsorption; Enzymatic digestibility; Enzyme activity; Hemicelluloses; Lignin; Lignocellulose; Substrate structures;

Several solvents and ionic liquids, namely formic acid (Formiline process), concentrated phosphoric acid (CPA), N-methylmorpholine-N-oxide (NMMO) and 1-allyl-3-methylimidazolium chloride ([AMIM]Cl), were used to pretreat wheat straw in order to increase cellulose digestibility for ethanol production. When being directly used to pretreat the raw wheat straw under corresponding optimized conditions, the improvement of cellulose hydrolyzability followed the order of CPA > Formiline > NMMO > [AMIM]Cl. However, when Formiline pretreated (delignified) substrates were further post-treated by the above cellulose solvents, the cellulose digestibility was significantly improved particularly with low cellulase loadings. Cellulose solvent post-treatment resulted in deconstruction of hydrogen-bond networking, alteration of cellulose polymorphs, decrease in crystallinity, depolymerization of cellulose chain with dramatic reduction in particle size, thus greatly increasing cellulose accessible surface area. CPA post-treatment showed the best efficacy. Semi-simultaneous scarification and fermentation (sSSF) of a CPA post-treated substrate obtained an ethanol concentration of 41.6 g L−1 with 91.2% of yield at a relatively low cellulase loading (5 FPU per g solid) within 24 h incubation. A biorefining process was proposed based on Formiline pretreatment coupled with CPA post-treatment to achieve a co-production of ethanol, furfural and high-purity lignin, which greatly increases the potential revenue of the process.

Enzymatic digestibility of sugar cane bagasse could be greatly enhanced by Formiline pretreatment, which comprises a formic acid (FA) delignification followed by an alkaline deformylation. The FA can be easily recovered and recycled for delignification, indicating that this pretreatment is a green process for biomass fractionation. It was found that removing hemicelluloses and lignin during pretreatment contributed to the increase of cellulose accessibility; however, delignification seemed to be more important for exposing cellulose fibers. The compact cell wall structure of raw bagasse was destroyed by removing considerable parts of lignin and hemicelluloses with liberation of cellulose fibers, and the specific surface area of the pretreated substrates increased by more than 2-fold. However, formylation of cellulose took place during FA delignification, which showed significant negative impact on the initial enzymatic hydrolysis rate and enzymatic polysaccharide conversion at 120 h. Removing formyl groups by alkaline post-treatment could well recover the cellulose digestibility but without significant alteration of the substrate structure.Keywords: Alkaline deformylation; Enzymatic digestibility; Formic acid delignification; Lignocellulosic biomass; Pretreatment; Structural feature

Pretreatment of lignocellulosic biomass by aqueous formic acid pertains to a biomass fractionation process to obtain multi-products for biorefining. In the present work, wheat straw was pretreated by aqueous formic acid under atmospheric pressure. The kinetics of delignification and polysaccharide solubilization were investigated using novel “potential degree of reaction” models, which were developed based on the multilayered structure of plant cell wall and reaction severity. Parameters termed as “potential degree of delignification (dD)” and “potential degree of solubilization (dS)” were introduced into the kinetic models for delignification and polysaccharide solubilization, respectively. The models fitted the experimental results very well. These models were also applicable as general models for describing the kinetics of various chemical pretreatments of different biomass feedstocks.

The biological production of 1,3-propanediol (1,3-PD) by microbial fermentation is promising because by-product glycerol produced in biodiesel production can be used as a carbon source. However, the salts present in the fermentation broth are negative to the downstream processing, particularly for the product colority. In the present work, we first studied the effects of several salts on the increase of colority and analyzed the possible mechanism. Ammonium salt ((NH4)2SO4) showed the most negative effect, which was probably due to the decrease of pH caused by the hydrolysis of ammonium salt thus facilitating the chromophoric reaction. A novel strategy was thus made by adjusting the initial pH of the feeding liquid for distillation. It was found that high pH (alkali condition) indeed reduced the distillate colority but showed no negative effects on the recovery yields of the main product 1,3-PD and major by-product 2,3-butanediol (2,3-BD). Scraped thin-film evaporation was greatly effective for desalination and recovering 1,3-PD and 2,3-BD with high recovery yields. High pH was also found to be beneficial to reduce the concentration of the impurity, acetic acid, in the distillate, which was of great importance for producing qualified 1,3-PD for polymerization. The novel strategy is thus very promising for recovering 1,3-PD from fermentation broth, particularly for the downstream processing at an industrial scale.

A process for co-production of ethanol and biodiesel from wheat straw was proposed. Dilute acid pre-hydrolysis of hemicellulose followed by enzymatic hydrolysis of cellulose were optimized to maximize recovery of total sugars. It was found that xylose yield obtained by super-dilute acid (0–0.1%) pretreatment under the experimental conditions was too low. By using moderate conditions (140–160 °C) with higher sulfuric acid concentration (0.3–0.6%), xylose recovery could be greatly increased to 60–70%. The relatively optimum conditions for dilute acid pretreatment were 0.5% H2SO4 at 140 °C for 1 h. 15.1 g L−1 ethanol with approximately 58% of theoretical yield obtained by SSF of the pretreated solid. The hydrolyzate was directly converted to microbial lipid using a mutagenized Rhodosporidium toruloides. The extracted lipid was well converted to biodiesel with 90% conversion ratio under the catalysis of immobilized lipase. Mass balance showed that 0.80 g biodiesel and 10.1 g ethanol were produced from 100 g of wheat straw. This work thus can provide a novel idea for biological production of biofuels from lignocellulosic biomass.

Organosolv pretreatment of lignocellulose pertains to a biomass fractionation process to obtain cellulosic pulp, high-purity lignin, and hemicellulosic syrup. In the present work, sugarcane bagasse was delignified by aqueous acetic acid (AcH) under atmospheric pressure with addition of sulfuric acid (SA) as a catalyst. Based on the multilayered structure of plant cell wall and the inhibitive effect of dissolved lignin on delignification rate, a novel pseudo-homogeneous kinetic model was proposed by introducing the concept of “potential degree of delignification (dD)” into the model. It was found that delignification rate was a first-order reaction with respect to SA concentration, while AcH concentration showed a high reaction order to delignification rate. The activation energy for delignification was determined to be 64.41 kJ/mol. The relationships of kinetic constants and dD with reaction temperature, AcH, and SA concentrations were determined according to experimental data. Mechanism analysis indicated that cleavage of α-aryl ethers bonds were mainly responsible for the formation of lignin fragments. AcH concentration affected the solubility parameter (δ value) of AcH solution and the ability to form hydrogen bonds with lignin fragments. Therefore, the driving force for solubilizing lignin fragments increased with AcH concentration, and thus AcH concentration had a very significant influence on delignification rate.

Microbial lipid produced using yeast fermentation with inexpensive carbon sources such as lignocellulosic hydrolyzate can be an alternative feedstock for biodiesel production. Several inhibitors that can be generated during acid hydrolysis of lignocellulose were added solely or together into the culture medium to study their individual inhibitory actions and their synergistic effects on the growth and lipid accumulation of oleaginous yeast Rhodosporidium toruloides. When the inhibitors were present in isolation in the medium, to obtain a high cell biomass accumulation, the concentrations of formic acid, acetic acid, furfural and vanillin should be lower than 2, 5, 0.5 and 1.5 g/L, respectively. However, the synergistic effects of these compounds could dramatically decrease the minimum critical inhibitory concentrations leading to significant growth and lipid production inhibitions. Unlike the above-cited inhibitors, sodium lignosulphonate had no negative influence on biomass accumulation when its concentration was in the range of 0.5–2.0 g/L; in effect, it was found to facilitate cell growth and sugar-to-lipid conversion. The fatty acid compositional profile of the yeast lipid was in the compositional range of various plant oils and animal tallow. Finally, the crude yeast lipid from bagasse hydrolyzate could be well converted into fatty acid methyl ester (FAME, biodiesel) by enzymatic transesterification in a tert-butanol system with biodiesel yield of 67.2% and lipid-to-biodiesel conversion of 88.4%.

•Increasing biomass digestibility by phosphomolybdic acid pretreatments.•Good hydrolyzability and fermentability were obtained.•Re-oxidation of reduced phosphomolybdic acid for electricity generation.•Coupling bioethanol production and electricity generation from biomass.•Iron (III) as an efficient mediator for electron transfer in cathode reaction.Phosphomolybdic acid (PMo12) was used as an electron mediator and proton carrier to mediate biomass pretreatment for ethanol production and electricity generation from wheat straw. In the pretreatment, lignin was oxidized anaerobically by PMo12 with solubilization of a fraction of hemicelluloses, and the PMo12 was simultaneously reduced. In an external liquid flow cell, the reduced PMo12 was re-oxidized with generation of electricity. The effects of several factors on pretreatment were investigated for optimizing the conditions. Enzymatic conversion of cellulose and xylan were about 80% and 45%, respectively, after pretreatment of wheat straw with 0.25 M PMo12, at 95 °C for 45 min. FeCl3 was found to be an effective liquid mediator to transfer electrons to air, the terminal electron acceptor. By investigating the effects of various operation parameters and cell structural factors, the highest output power density of about 11 mW/cm2 was obtained for discharging of the reduced PMo12.Download high-res image (174KB)Download full-size image