Alkaline sulfonation and thermomechanical pulping pretreatment of softwood chips and pellets to enhance enzymatic hydrolysis

Effect of hornification on the isolation of anionic cellulose nanofibrils from Kraft pulp via maleic anhydride esterification

Cellulose nanofibrils (CNF) isolation based on a catalyst-free maleic anhydride esterification has proven to be effective, however, the effects of pulp hornification on CNF isolation by this strategy have yet to be explored, which could present significant impacts for CNF isolation. Herein, dried northern bleached softwood Kraft pulp (D-NBSK) and never-dried northern bleached softwood Kraft pulp (ND-NBSK) were selected as the substrates. After esterification with maleic anhydride (MA), the esterified ND-NBSK pulp (E-ND) shows a significantly smaller size and more fragmented structure than the esterified D-NBSK pulp (E-D). Meanwhile, higher degree of esterification can be realized for the never dried pulp as compared to the dried pulp, which is corroborated by the significantly stronger characteristic peaks of CO (1720 cm-1) and -COO- (1575 cm-1) from the FTIR spectra and the higher surface charge content (0.86 ± 0.04 mmol/g vs. 0.55 ± 0.05 mmol/g). A comparison of the characteristics of the resulting CNF similarly demonstrated the negative impact of hornification. Overall, this work indicates that hornification tends to reduce the accessibility of chemical reagents to the pulp, leading to insufficient deconstruction. Such negative impact of hornification should be considered when performing nanocellulose isolation, especially when using pulp as feedstock.

Comparison of Alkaline Sulfite Pretreatment and Acid Sulfite Pretreatment with Low Chemical Loading in Saccharification of Poplar

Sulfite pretreatment is a productive process for lignin dissolution in lignocelluloses and to reduce the hydrophobicity of lignin by sulfonation, thus promoting the hydrolyzability of the substrate. Previously, sulfite pretreatment needs high dosages of chemicals and thus results in the high cost of the pretreatment and the great pressure of environmental pollution. To overcome these problems, it was crucial to research whether alkaline sulfite pretreatment (ALS) and acid sulfite pretreatment (ACS) with low chemical loading could enhance the saccharification of poplar. In this work, the results indicated that with low loading of chemicals in sulfite pretreatment, ALS pretreatment (1.6% Na2SO3 and 0.5% NaOH) at 180 °C removed more lignin, resulted in lower hydrophobicity and higher cellulase adsorption capacity of poplar than ACS pretreatment (1.6% Na2SO3 and 0.5% H2SO4) at 180 °C. A satisfying glucose yield of 84.9% and a xylose yield of 76.0% were obtained from poplar after ALS pretreatment with 1.6% Na2SO3 and 0.5% NaOH at 180 °C for 1 h using 10 FPU cellulase/g dry matter, saving sodium sulfite by 60.0% compared to the loading of sulfite in traditional sulfite pretreatment. The strategy developed in this work reduced chemical loading and cellulase loading in alkali sulfite pretreatment for the saccharification of poplar.

In-situ lignin sulfonation for enhancing enzymatic hydrolysis of poplar using mild organic solvent pretreatment

Biomass pretreatment is an essential strategy to overcome biomass recalcitrance and promote lignocellulosic bioconversion. Here, a reusable organic solvent system (formic acid-methanesulfonic acid) was explored to pretreat poplar under a mild temperature (below 100 °C). The results showed that the co-solvent system could extract basically complete hemicelluloses and part of lignin with original cellulose retained in the pretreated substrates. Meanwhile, sulfonic acid groups were introduced into lignin structure remained in the substrates. The glucose conversion yield of the substrates with a higher concentration of sulfonic acid groups (13.2 mmol/kg) reached 45.9 % by reducing the hydrophobic interaction between lignin and cellulase, showing 89.3 % improvement compared with that of the substrates treated with single formic acid. This progressive study aimed to develop a new strategy to realize sulfonation and promote enzymatic hydrolysis of substrates by using mild organic solvent pretreatment.

Fermentative Lactic Acid Production From Lignocellulosic Feedstocks: From Source to Purified Product

The second (lignocellulosic biomass and industrial wastes) and third (algal biomass) generation feedstocks gained substantial interest as a source of various value-added chemicals, produced by fermentation. Lactic acid is a valuable platform chemical with both traditional and newer applications in many industries. The successful fractionation, separation, and hydrolysis of lignocellulosic biomass result in sugars’ rich raw material for lactic acid fermentation. This review paper aims to summarize the investigations and progress in the last 5 years in lactic acid production from inexpensive and renewable resources. Different aspects are discussed—the type of raw materials, pretreatment and detoxification methods, lactic acid-producers (bacteria, fungi, and yeasts), use of genetically manipulated microorganisms, separation techniques, different approaches of process organization, as well as main challenges, and possible solutions for process optimization.

The use of steam pretreatment to enhance pellet durability and the enzyme-mediated hydrolysis of pellets to fermentable sugars

Although densified wood pellets are an attractive biomass feedstock for bioenergy and biofuels production, partly due to their ease of transport, their friability and hygroscopic nature (attraction of moisture) have proven problematic in terms of storage and handling. Pre-steaming the biomass was shown to reduce the need for size reduction, significantly increasing pellet durability by relocating the plant cell wall lignin to the fibre surface and consequently enhancing binding between particles. Although steam pretreatment has been shown to facilitate enzyme-mediated hydrolysis of biomass, by increasing cellulose accessibility, drying and pelletization partially impeded enzymatic hydrolysis. However, the incorporation of alkaline deacetylation or neutral sulfonation step prior to pre-steaming was shown to mitigate many of the negative effects of drying. Although drying and pelletization did not significantly impact the redistribution of lignin, a mild mechanical refining step was shown to further enhance the hydrolysis of the cellulose component of the pelletized biomass.

Liquid hot water as sustainable biomass pretreatment technique for bioenergy production: A review

In recent years, lignocellulosic biomass has emerged as one of the most versatile energy sources among the research community for the production of biofuels and value-added chemicals. However, biomass pretreatment plays an important role in reducing the recalcitrant properties of lignocellulose, leading to superior quality of target products in bioenergy production. Among existing pretreatment techniques, liquid hot water (LHW) pretreatment has several outstanding advantages compared to others including minimum formation of monomeric sugars, significant removal of hemicellulose, and positive environmental impacts; however, several constraints of LHW pretreatment should be clarified. This contribution aims to provide a comprehensive analysis of reaction mechanism, reactor characteristics, influencing factors, techno-economic aspects, challenges, and prospects for LHW-based biomass pretreatment. Generally, LHW pretreatment could be widely employed in bioenergy processing from biomass, but circular economy-based advanced pretreatment techniques should be further studied in the future to achieve maximum efficiency, and minimum cost and drawbacks.

Current breakthroughs in the hardwood biorefineries: Hydrothermal processing for the co-production of xylooligosaccharides and bioethanol

The development of lignocellulosic biorefineries requires a first stage of pretreatment which enables the efficient valorization of all fractions present in this renewable material. In this sense, this review aims to show the main advantages of hydrothermal treatment as a first step of a biorefinery infrastructure using hardwood as raw material, as well as, main drawback to overcome. Hydrothermal treatment of hardwood highlights for its high selectivity for hemicelluloses solubilization as xylooligosaccharides (XOS). Nevertheless, the suitable conditions for XOS production are inadequate to achieve an elevate cellulose to glucose conversion. Hence, several strategies namely the combination of hydrothermal treatment with delignification process, in situ modification of lignin and the mixture with another renewable resources (concretely, seaweeds, and by-products generated in the food industry with high sugar content) were pinpointed as promising alternative to increase the final ethanol concentration coupled with XOS recovery in the hydrolysate.

The production of lactic acid from chemi-thermomechanical pulps using a chemo-catalytic approach

Previous work has shown that sulfonation and oxidation of chemi-thermomechanical pulps (CTMPs) significantly enhanced enzyme accessibility to cellulose while recovering the majority of carbohydrates in the water-insoluble component. In the work reported here, modified (sulfonated and oxidized) CTMPs derived from hard-and-softwoods were used to produce a DL-mix of lactic acid via a chemo-catalytic approach using lanthanide triflate (Ln (OTf)₃) catalysts (Ln = La, Nd, Er, and Yb). It was apparent that sulfonation and oxidation of chemi-thermomechanical pulps (CTMPs) also enhanced Ln(OTf)₃ catalyst accessibility to the carbohydrate components of the pulps, with the Er(OTf)₃ catalysts resulting in significant lactic acid production. Under optimum conditions (250 °C, 60 min, 0.5 mmol catalyst g^-1 biomass), 72% and 67% of the respective total carbohydrate present in the hard-and-softwood CTMPs could be converted to lactic acid compared to the respective 59% and 51% yields obtained after energy-intensive ball milling.

Enhancing Enzyme-Mediated Cellulose Hydrolysis by Incorporating Acid Groups Onto the Lignin During Biomass Pretreatment

Lignin is known to limit the enzyme-mediated hydrolysis of biomass by both restricting substrate swelling and binding to the enzymes. Pretreated mechanical pulp (MP) made from Aspen wood chips was incubated with either 16% sodium sulfite or 32% sodium percarbonate to incorporate similar amounts of sulfonic and carboxylic acid groups onto the lignin (60 mmol/kg substrate) present in the pulp without resulting in significant delignification. When Simon's stain was used to assess potential enzyme accessibility to the cellulose, it was apparent that both post-treatments enhanced accessibility and cellulose hydrolysis. To further elucidate how acid group addition might influence potential enzyme binding to lignin, Protease Treated Lignin (PTL) was isolated from the original and modified mechanical pulps and added to a cellulose rich, delignified Kraft pulp. As anticipated, the PTLs from both the oxidized and sulfonated substrates proved less inhibitory and adsorbed less enzymes than did the PTL derived from the original pulp. Subsequent analyses indicated that both the sulfonated and oxidized lignin samples contained less phenolic hydroxyl groups, resulting in enhanced hydrophilicity and a more negative charge which decreased the non-productive binding of the cellulase enzymes to the lignin.

A Whole-Slurry Fermentation Approach to High-Solid Loading for Bioethanol Production from Corn Stover

Corn stover is the most produced byproduct from maize worldwide. Since it is generated as a residue from maize harvesting, it is an inexpensive and interesting crop residue to be used as a feedstock. An ecologically friendly pretreatment such as autohydrolysis was selected for the manufacture of second-generation bioethanol from corn stover via whole-slurry fermentation at high-solid loadings. Temperatures from 200 to 240 °C were set for the autohydrolysis process, and the solid and liquid phases were analyzed. Additionally, the enzymatic susceptibility of the solid phases was assessed to test the suitability of the pretreatment. Afterward, the production of bioethanol from autohydrolyzed corn stover was carried out, mixing the solid with different percentages of the autohydrolysis liquor (25%, 50%, 75%, and 100%) and water (0% of liquor), from a total whole slurry fermentation (saving energy and water in the liquid–solid separation and subsequent washing of the solid phase) to employing water as only liquid medium. In spite of the challenging scenario of using the liquor fraction as liquid phase in the fermentation, values between 32.2 and 41.9 g ethanol/L and ethanol conversions up to 80% were achieved. This work exhibits the feasibility of corn stover for the production of bioethanol via a whole-slurry fermentation process.