Sodium sulfite-formaldehyde pretreatment of mixed hardwoods and its effect on enzymatic hydrolysis

Exploring how lignin structure influences the interaction between carbohydrate-binding module and lignin using AFM

Nonproductive adsorption of cellulase onto the residual lignin in substrate seriously hinders the enzymatic hydrolysis. To understand how lignin structure affects lignin-cellulase interaction, the carbohydrate-binding module (CBM) functionalized atomic force microscope tip was used to measure CBM-lignin interaction by single-molecule dynamic force spectroscopy in this work. The results showed that sulfonated lignin (SL) has the greatest adhesion force to CBM (4.74 nN), while those of masson pine milled wood lignin (MWL), poplar MWL and herbaceous MWLs were 2.85, 1.03 and 0.27-0.61 nN, respectively. It provides direct quantitative evidence for the significance of lignin structure on lignin-cellulase interaction. The CBM-MWLs interaction decreased sharply to 0.054-0.083 nN while SL was added, indicating the primary mechanism of SL promoting lignocellulose hydrolysis was significantly reducing the nonproductive adsorption of substrate lignin on cellulase. Finally, the "competitive adsorption" mechanism was proposed to interpret why SL effectively promotes the enzymatic hydrolysis of lignin-containing substrates.

Integrated biorefinery approaches for the industrialization of cellulosic ethanol fuel

Lignocellulosic biomass is an abundant and sustainable raw material, but its conversion into ethanol fuel has not yet achieved large-scale industrialization and economic benefits. Integrated biorefineries have been widely identified as the key to achieving this goal. Here, four promising routes were summarized to assemble the new industrial plants for cellulose-based fuels and chemicals, including 1) integration of cellulase production systems into current cellulosic ethanol processes; 2) combination of processes and facilities between cellulosic ethanol and first-generation ethanol; 3) application of enzyme-free saccharification processes and computational approaches to increase the bioethanol yield and optimize the integration process; 4) production of multiple products to maximize the value derived from the lignocellulosic biomass. Finally, the remaining challenges and perspectives of this field are also discussed.

Comparison of sulfomethylated lignin from poplar and masson pine on cellulase adsorption and the enzymatic hydrolysis of wheat straw

In this work, effects of sulfomethylated lignins (SLs) prepared from masson pine (SL_M) and poplar (SL_P) on enzymatic hydrolysis and cellulase-lignin interaction were comparatively investigated. The results showed that both SL_M and SL_P significantly promoted the substrate enzymatic digestibility. The total sugar yield increased from 38.6% to 74.4% and ∼ 100%, respectively at 10 FPU/g-cellulose of cellulase dosage. The protein content in hydrolysate linearly increased with the addition of SL (0 - 1.6 g/g-substrate lignin), which suggested the competitive adsorption of cellulase may occur to substrate lignin and SLs. Further structural analysis of lignin revealed the high S/(V + H) ratio was directly related to the high enzymatic saccharification efficiency. The strong interaction between SL and cellulase decreased the nonproductive adsorption of cellulase onto substrate lignin and increased the accessibility of cellulase to carbohydrate, which was considered to be the key factor for the improvement of substrate enzymatic digestibility.

The sequential Fenton oxidation and sulfomethylation pretreatment for alleviating the negative effects of lignin in enzymatic saccharification of sugarcane bagasse

The sugarcane bagasse (SCB) was pretreated by the sequential Fenton oxidation and sulfomethylation pretreatment (FSP) for reducing the lignin inhibition in enzymatic saccharification. The FSP showed the delignification of 67.76% and the glucan retention in pretreated solid of 97.81%. Compared with sulfomethylation pretreated SCB (SP-SCB), the lignin surface coverage and surface hydrophobicity of FSP-SCB decreased by 1.84 and 4.84 times, respectively, led to enzymatic accessibility increased by 20%. Using the cellulase loading of 20 FPU/g glucan, the 72 h yields of glucose and xylose were 76.24% and 64.83%, respectively, which was 1.3-2.0 times higher than that of sulfomethylation treated SCB alone. Fenton oxidative reaction as a pre-step will help sulfomethyl group to easily and more introduce on aromatic ring, thereby accelerating the delignification.

Effective alkaline metal-catalyzed oxidative delignification of hybrid poplar

BACKGROUND

Strategies to improve copper-catalyzed alkaline hydrogen peroxide (Cu-AHP) pretreatment of hybrid poplar were investigated. These improvements included a combination of increasing hydrolysis yields, while simultaneously decreasing process inputs through (i) more efficient utilization of H2O2 and (ii) the addition of an alkaline extraction step prior to the metal-catalyzed AHP pretreatment. We hypothesized that utilizing this improved process could substantially lower the chemical inputs needed during pretreatment.

RESULTS

Hybrid poplar was pretreated utilizing a modified process in which an alkaline extraction step was incorporated prior to the Cu-AHP treatment step and H2O2 was added batch-wise over the course of 10 h. Our results revealed that the alkaline pre-extraction step improved both lignin and xylan solubilization, which ultimately led to improved glucose (86 %) and xylose (95 %) yields following enzymatic hydrolysis. An increase in the lignin solubilization was also observed with fed-batch H2O2 addition relative to batch-only addition, which again resulted in increased glucose and xylose yields (77 and 93 % versus 63 and 74 %, respectively). Importantly, combining these strategies led to significantly improved sugar yields (96 % glucose and 94 % xylose) following enzymatic hydrolysis. In addition, we found that we could substantially lower the chemical inputs (enzyme, H2O2, and catalyst), while still maintaining high product yields utilizing the improved Cu-AHP process. This pretreatment also provided a relatively pure lignin stream consisting of ≥90 % Klason lignin and only 3 % xylan and 2 % ash following precipitation. Two-dimensional heteronuclear single-quantum coherence (2D HSQC) NMR and size-exclusion chromatography demonstrated that the solubilized lignin was high molecular weight (Mw ≈ 22,000 Da) and only slightly oxidized relative to lignin from untreated poplar.

CONCLUSIONS

This study demonstrated that the fed-batch, two-stage Cu-AHP pretreatment process was effective in pretreating hybrid poplar for its conversion into fermentable sugars. Results showed sugar yields near the theoretical maximum were achieved from enzymatically hydrolyzed hybrid poplar by incorporating an alkaline extraction step prior to pretreatment and by efficiently utilizing H2O2 during the Cu-AHP process. Significantly, this study reports high sugar yields from woody biomass treated with an AHP pretreatment under mild reaction conditions.

Coproduction of xylose, lignosulfonate and ethanol from wheat straw

A novel integrated process to coproduce xylose, lignosulfonate and ethanol from wheat straw was investigated. Firstly, wheat straw was treated by dilute sulfuric acid and xylose was recovered from its hydrolyzate. Its optimal conditions were 1.0wt% sulfuric acid, 10% (w/v) wheat straw loading, 100°C, and 2h. Then the acid treated wheat straw was treated by sulfomethylation reagent and its hydrolyzate containing lignosulfonate was directly recovered. Its optimal conditions were 150°C, 15% (w/v) acid treated wheat straw loading, and 5h. Finally, the two-step treated wheat straw was converted to ethanol through enzymatic hydrolysis and microbial fermentation. Under optimal conditions, 1kg wheat straw could produce 0.225kg xylose with 95% purity, 4.16kg hydrolyzate of sulfomethylation treatment containing 5.5% lignosulfonate, 0.183kg ethanol and 0.05kg lignin residue. Compared to present technology, this process is a potential economically profitable wheat straw biorefinery.

Influence of lignin addition on the enzymatic digestibility of pretreated lignocellulosic biomasses

The presence of lignin in lignocellulosic biomass is correlated with its enzymatic digestibility. Their correlation and mechanism have been investigated widely but have not been elucidated clearly. In this study, hydrophilic sulfonated lignin and hydrophobic kraft lignin were introduced into the enzymatic hydrolysis process to investigate their effects on the enzymatic digestibility of different pretreated lignocellulose. The influence of lignin addition on the enzymatic digestibility varied with both introduced lignin type and the pretreatment methods of substrates. Slight enhancement of enzymatic hydrolysis was observed for all substrates by adding kraft lignin. The addition of sulfonated lignin could effectively improve the enzymatic digestibility of green liquor and acidic bisulfite pretreated materials, but had little effect on sulfite-formaldehyde pretreated samples. The enzymatic digestibility of green liquor pretreated masson pine increased from 42% without lignin addition to 75% with 0.3g/g-substrate sulfonated lignin addition.

Enhancement in the enzymatic digestibility of hybrid poplar with poor residual hemicelluloses after Na2SO3 pretreatment

The aim of this work was to illustrate the contributions of delignification and the introduced sulfonic groups on the enzymatic digestibility of the Na2SO3-pretreated hybrid poplar with poor residual hemicelluloses (HPPRH). The higher the content of the introduced sulfonic group in the pretreated HPPRH was, the higher its enzymatic digestibility could be achieved. Delignification was favorable to increasing the content of sulfonic group in the pretreated HPPRH. The introduced sulfonic group contributed much more to the total glucose yield at low level of residual lignin. The introduced sulfonic groups could contribute 17.30% of total glucose yield (92.70%) and delignification could do 38.43% of it. Meanwhile, the delignification rate and the sulfonic group content in the pretreated HPRH were 59.88% and 283.51mmolkg(-1) lignin, respectively. Therefore, the sulfonic group introduced on the pretreated lignocellulosics could improve the enzymatic digestibility and make the sulfite process effective.

Sulfite-formaldehyde pretreatment on rice straw for the improvement of enzymatic saccharification

Rice straw is one of the most abundant agricultural residues in China. It is considered as a promising raw material for bioethanol production. In this work, rice straw was pretreated by sodium sulfite-formaldehyde (SF) for improving enzymatic saccharification. The SF pretreatment, using proven technology and industrialized equipment, showed efficient delignification selectivity and high carbohydrates retention in pretreated solid. The highest sugar yields of 79.0%, 88.8% and 71.1% for total sugar, glucan and xylan, respectively were obtained at an enzyme loading of 40 FPU/g-substrate after the raw material pretreated with 12% sodium sulfite at 160°C. About 75% of lignin was dissolved in pretreatment spent liquor and 78% of silica was retained in the residue of enzymatic hydrolysis. The results proved sulfite-formaldehyde as a promising pretreatment for the production of bioethanol as well as potential high value added by-products of silica nanoparticles and lignosulfonate.

Chemical Pretreatment Methods for the Production of Cellulosic Ethanol: Technologies and Innovations

Pretreatment of lignocellulose has received considerable research globally due to its influence on the technical, economic and environmental sustainability of cellulosic ethanol production. Some of the most promising pretreatment methods require the application of chemicals such as acids, alkali, salts, oxidants, and solvents. Thus, advances in research have enabled the development and integration of chemical-based pretreatment into proprietary ethanol production technologies in several pilot and demonstration plants globally, with potential to scale-up to commercial levels. This paper reviews known and emerging chemical pretreatment methods, highlighting recent findings and process innovations developed to offset inherent challenges via a range of interventions, notably, the combination of chemical pretreatment with other methods to improve carbohydrate preservation, reduce formation of degradation products, achieve high sugar yields at mild reaction conditions, reduce solvent loads and enzyme dose, reduce waste generation, and improve recovery of biomass components in pure forms. The use of chemicals such as ionic liquids, NMMO, and sulphite are promising once challenges in solvent recovery are overcome. For developing countries, alkali-based methods are relatively easy to deploy in decentralized, low-tech systems owing to advantages such as the requirement of simple reactors and the ease of operation.