Structure changes of lignin and their effects on enzymatic hydrolysis for bioethanol production: a focus on lignin modification

Enzymatic hydrolysis contributes to obtaining fermentable sugars using pretreated lignocellulose materials for bioethanol generation. Unfortunately, the pretreatment of lignocellulose causes low substrate enzymatic hydrolysis, which is due to the structure changes of lignin to produce main phenolic by-products and non-productive cellulase adsorption. It is reported that modified lignin enhances the speed of enzymatic hydrolysis through single means to decrease the negative effects of fermentation inhibitors or non-productive cellulase adsorption. However, a suitable modified lignin should be selected to simultaneously reduce the fermentation inhibitors concentration and non-productive cellulase adsorption for saving resources and maximizing the enzymatic hydrolysis productivity. Meanwhile, the adsorption micro-mechanisms of modified lignin with fermentation inhibitors and cellulase remain elusive. In this review, different pretreatment effects toward lignin structure, and their impacts on subsequent enzymatic hydrolysis are analyzed. The main modification methods for lignin are presented. Density functional theory is used to screen suitable modification methods for the simultaneous reduction of fermentation inhibitors and non-productive cellulase adsorption. Lignin-fermentation inhibitors and lignin-cellulase interaction mechanisms are discussed using different advanced analysis techniques. This article addresses the gap in previous reviews concerning the application of modified lignin in the enhancement of bioethanol production. For the first time, based on existing studies, this work posits the hypothesis of applying theoretical simulations to screen efficient modified lignin-based adsorbents, in order to achieve a dual optimization of the detoxification and saccharification processes. We aim to improve the integrated lignocellulose transformation procedure for the effective generation of cleaner bioethanol.

Recrystallization of Cellulose, Chitin and Starch in Their Individual and Native Forms

Semi-crystalline natural polymers are involved in many technological processes. Biopolymers having identical chemical compositions can differ in reactivity in heterogeneous transformations depending on their crystal structure (polymorphic modification). This paper compares the crystal structure recrystallization processes occurring in natural polysaccharides (cellulose, chitin, and starch) in the individual form and as a component of native biomass. Aqueous treatment of pre-amorphized semi-crystalline biopolymers was shown to result in swelling, thus alleviating the kinetic restrictions imposed on the restoration of crystalline regions and phase transition to the thermodynamically more stable polymorphic modification. During recrystallization, cellulose I in the individual form and within plant-based biomass undergoes a transition to the more stable cellulose II. A similar situation was demonstrated for α- and β-chitin, which recrystallize only into the α-polymorphic modification in the case of both individual polymers and native materials. Recrystallization of A-, B-, and C-type starch, both in the individual form and within plant-based flour, during aqueous treatment, results in a phase transition, predominantly to the B-type starch. The recrystallization process depends on the temperature of aqueous treatment; longer treatment duration has almost no effect on the recrystallization degree of polymers, both in the individual form and within native materials.

Understanding acid hydrolysis of corn stover during densification pretreatment for quantitative predictions of enzymatic hydrolysis efficiency using modified pretreatment severity factor

DLCA(sa) pretreatment (densifying lignocellulosic biomass with sulfuric acid followed by autoclave treatment), featured with low treatment temperature and densification, demonstrate high efficiency in biomass pretreatment. In this study, the effects of temperature, acid loading, time on the hydrolysis of xylan, cellulose and lignin during DLCA(sa) pretreatment were systematically investigated. It was shown that DLCA(sa) pretreatment can effectively solubilize xylan, achieving an 84% xylose recovery under mild conditions (130 °C, 30 min, and 0.125 g/g acid loading). The conventional pretreatment severity factor correlated and further modified to improve the accuracy in evaluating the xylan hydrolysis. Additionally, a mathematical model based on the xylan hydrolytic kinetics was proposed to predict the enzymatic hydrolysis. Kinetic model suggested that mechanical densification facilitates the penetration of acid into the biomass matrix, leading to increased accessibility of xylan to acid catalysis.

Spiers Memorial Lecture: Mechanochemistry, tribochemistry, mechanical alloying - retrospect, achievements and challenges

The paper presents a view on the achievements, challenges and prospects of mechanochemistry. The extensive reference list can serve as a good entry point to a plethora of mechanochemical literature.

Mechanochemical Transformations of Polysaccharides: A Systematic Review

Taking into consideration the items of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA), this study reviews application of mechanochemical approaches to the modification of polysaccharides. The ability to avoid toxic solvents, initiators, or catalysts during processes is an important characteristic of the considered approach and is in line with current trends in the world. The mechanisms of chemical transformations in solid reactive systems during mechanical activation, the structure and physicochemical properties of the obtained products, their ability to dissolve and swell in different media, to form films and fibers, to self-organize in solution and stabilize nanodispersed inorganic particles and biologically active substances are considered using a number of polysaccharides and their derivatives as examples.

Kinetic Model for Enzymatic Hydrolysis of Cellulose from Pre-Treated Rice Husks

Rice husks contain cellulose as a raw material for manufacturing second-generation bioethanol. Cellulose from pre-treated rice husks was converted into reducing sugars through enzymatic hydrolysis using enzymes derived from Aspergillus niger. This study aims to determine the kinetics of enzymatic hydrolysis at enzyme concentrations of 10, 15, and 20% (v/w) and hydrolysis times of 5, 10, 15, 20, and 25 h. The results showed that cellulose was hydrolyzed to form reducing sugars. The CMCase activity and FPase activity reached 548.940 and 314.892 U mL−1, respectively, much higher than most previous reports on this genus. From the calculation of the reaction rate using the Michaelis–Menten kinetic model, the value of the Michaelis constant ranges from 0.001 to 0.0007, and the maximum rate is 1.3 × 10−7 to 2.7 × 10−7 Mol L−1 s−1. The highest reducing sugar concentration was obtained (1.80 g L−1) at an enzyme concentration of 20% (v/w) and a hydrolysis time of 25 h.

Mechanical pretreatment of lignocellulosic biomass toward enzymatic/fermentative valorization

Lignocellulosic biomass (LCB) has the potential to replace fossil fuels, thanks to the concept of biorefinery. This material is formed mainly by cellulose, lignin, and hemicellulose. To maximize the valorization potential of this material, LCB needs to be pretreated. Milling is always performed before any other treatments. It does not produce chemical change and improves the efficiency of the upcoming processes.

Additionally, it makes LCB easier to handle and increases bulk density and transfer phenomena of the next pretreatment step. However, this treatment is energy consuming, so it needs to be optimized. Several mills can be used, and the equipment selection depends on the characteristics of the material, the final size required, and the operational regime: continuous or batch. Among them, ball, knife, and hammer mills are the most used at the laboratory scale, especially before enzymatic or fermentative treatments. The continuous operational regime (knife and hammer mill) allows us to work with high volumes of raw material and can continuously reduce particle size, unlike the batch operating regime (ball mill).

This review recollects the information about the application of these machines, the effect on particle size, and subsequent treatments. On the one hand, ball milling reduced particle size the most; on the other hand, hammer and knife milling consumed less energy. Furthermore, the latter reached a small final particle size (units of millimeters) suitable for valorization.

Characterization of Organosolv Lignins and Their Application in the Preparation of Aerogels

The production of novel materials and value-added chemicals from lignin has received considerable attention in recent years. Due to its abundant occurrence in nature, there is a growing interest in utilizing lignin as a feedstock for functional materials production, for example aerogels. Much like in the synthesis of phenol-based resins, the vacant ortho positions of the aromatic rings in lignin can crosslink with formaldehyde and form polymeric gels. After drying the hydrogels with supercritical CO₂, highly porous aerogels are obtained. Current study focuses on the preparation and thorough parametrization of organosolv lignins from different types of lignocellulosic biomass (aspen, pine, and barley straw) as well as their utilization for the preparation of lignin-5-methylresorcinol-formaldehyde aerogels. The thorough structural characterization of the obtained aerogels was carried out by gas adsorption, IR spectroscopy, and scanning electron microscopy. The obtained lignin-based monolithic mesoporous aerogels had specific surface areas and total pore volumes in the upward ranges of 450 m²/g and 1.4 cm³/g, respectively.

Mechanochemical Transformations of Biomass into Functional Materials

Biomass is one of the promising alternatives to petroleum-derived materials and plays a major role in our fight against climate change by providing renewable sources of chemicals and materials. Owing to its chemical and structural complexity, the transformation of biomass into value-added products requires a profound understanding of its composition at different scales and innovative methods such as combining physical and chemical processes. In this context, the use of mechanochemistry in biomass valorization is currently growing owing to its potentials as an efficient, sustainable, and environmentally friendly approach. This review highlights the latest advances in the transformation of biomass (i. e., chitin, cellulose, hemicellulose, lignin, and starch) to functional materials using mechanochemical-assisted methods. We focused here on the methodology of biomass processing, influencing factors, and resulting properties with an emphasis on achieving functional materials rather than breaking down the biopolymer chains into smaller molecules. Opportunities and limitations associated this methodology were discussed accordingly for future directions.

Ultrastructural change in lignocellulosic biomass during hydrothermal pretreatment

In recent years, visualization and characterization of lignocellulose at different scales elucidate the modifications of its ultrastructural and chemical features during hydrothermal pretreatment which include degradation and dissolving of hemicelluloses, swelling and partial hydrolysis of cellulose, melting and redepositing a part of lignin in the surface. As a result, cell walls are swollen, deformed and de-laminated from the adjacent layer, lead to a range of revealed droplets that appear on and within cell walls. Moreover, the certain extent morphological changes significantly promote the downstream processing steps, especially for enzymatic hydrolysis and anaerobic fermentation to bioethanol by increasing the contact area with enzymes. However, the formation of pseudo-lignin hinders the accessibility of cellulase to cellulose, which decreases the efficiency of enzymatic hydrolysis. This review is intended to bridge the gap between the microstructure studies and value-added applications of lignocellulose while inspiring more research prospects to enhance the hydrothermal pretreatment process.

Tribochemistry, Mechanical Alloying, Mechanochemistry: What is in a Name?

Over the decades, the application of mechanical force to influence chemical reactions has been called by various names: mechanochemistry, tribochemistry, mechanical alloying, to name but a few. The evolution of these terms has largely mirrored the understanding of the field. But what is meant by these terms, why have they evolved, and does it really matter how a process is called? Which parameters should be defined to describe unambiguously the experimental conditions such that others can reproduce the results, or to allow a meaningful comparison between processes explored under different conditions? Can the information on the process be encoded in a clear, concise, and self-explanatory way? We address these questions in this Opinion contribution, which we hope will spark timely and constructive discussion across the international mechanochemical community.

Mechanochemistry as an Alternative Method of Green Synthesis of Silver Nanoparticles with Antibacterial Activity: A Comparative Study

This study shows mechanochemical synthesis as an alternative method to the traditional green synthesis of silver nanoparticles in a comparative manner by comparing the products obtained using both methodologies and different characterization methods. As a silver precursor, the most commonly used silver nitrate was applied and the easily accessible lavender (Lavandula angustofolia L.) plant was used as a reducing agent. Both syntheses were performed using 7 different lavender:AgNO₃ mass ratios. The synthesis time was limited to 8 and 15 min in the case of green and mechanochemical synthesis, respectively, although a significant amount of unreacted silver nitrate was detected in both crude reaction mixtures at low lavender:AgNO₃ ratios. This finding is of particular interest mainly for green synthesis, as the potential presence of silver nitrate in the produced nanosuspension is often overlooked. Unreacted AgNO₃ has been removed from the mechanochemically synthesized samples by washing. The nanocrystalline character of the products has been confirmed by both X-ray diffraction (Rietveld refinement) and transmission electron microscopy. The latter has shown bimodal size distribution with larger particles in tens of nanometers and the smaller ones below 10 nm in size. In the case of green synthesis, the used lavender:AgNO₃ ratio was found to have a decisive role on the crystallite size. Silver chloride has been detected as a side-product, mainly at high lavender:AgNO₃ ratios. Both products have shown a strong antibacterial activity, being higher in the case of green synthesis, but this can be ascribed to the presence of unreacted AgNO₃. Thus, one-step mechanochemical synthesis (without the need to prepare extract and performing the synthesis as separate steps) can be applied as a sustainable alternative to the traditional green synthesis of Ag nanoparticles using plants.

Enzymatic conversion of pretreated lignocellulosic biomass: A review on influence of structural changes of lignin

The demands of energy sustainability drive efforts to bio-chemical conversion of biomass into biofuels through pretreatment, enzymatic hydrolysis, and microbial fermentation. Pretreatment leads to significant structural changes of the complex lignin polymer that affect yield and productivity of the enzymatic conversion of lignocellulosic biomass. Structural changes of lignin after pretreatment include functional groups, inter unit linkages and compositions. These changes influence non-productive adsorption of enzyme on lignin through hydrophobic interaction and electrostatic interaction as well as hydrogen bonding. This paper reviews the relationships between structural changes of lignin and enzymatic hydrolysis of pretreated lignocellulosic biomass. The formation of pseudo-lignin during dilute acid pretreatment is revealed, and their negative effect on enzymatic hydrolysis is discussed.

State-of-the-Art of Eggshell Waste in Materials Science: Recent Advances in Catalysis, Pharmaceutical Applications, and Mechanochemistry

Eggshell waste is among the most abundant waste materials coming from food processing technologies. Despite the unique properties that both its components (eggshell, ES, and eggshell membrane, ESM) possess, it is very often discarded without further use. This review article aims to summarize the recent reports utilizing eggshell waste for very diverse purposes, stressing the need to use a mechanochemical approach to broaden its applications. The most studied field with regards to the potential use of eggshell waste is catalysis. Upon proper treatment, it can be used for turning waste oils into biodiesel and moreover, the catalytic effect of eggshell-based material in organic synthesis is also very beneficial. In inorganic chemistry, the eggshell membrane is very often used as a templating agent for nanoparticles production. Such composites are suitable for application in photocatalysis. These bionanocomposites are also capable of heavy metal ions reduction and can be also used for the ozonation process. The eggshell and its membrane are applicable in electrochemistry as well. Due to the high protein content and the presence of functional groups on the surface, ESM can be easily converted to a high-performance electrode material. Finally, both ES and ESM are suitable for medical applications, as the former can be used as an inexpensive Ca2+ source for the development of medications, particles for drug delivery, organic matrix/mineral nanocomposites as potential tissue scaffolds, food supplements and the latter for the treatment of joint diseases, in reparative medicine and vascular graft producing. For the majority of the above-mentioned applications, the pretreatment of the eggshell waste is necessary. Among other options, the mechanochemical pretreatment has found an inevitable place. Since the publication of the last review paper devoted to the mechanochemical treatment of eggshell waste, a few new works have appeared, which are reviewed here to underline the sustainable character of the proposed methodology. The mechanochemical treatment of eggshell is capable of producing the nanoscale material which can be further used for bioceramics synthesis, dehalogenation processes, wastewater treatment, preparation of hydrophobic filters, lithium-ion batteries, dental materials, and in the building industry as cement.

Mechanochemical and Size Reduction Machines for Biorefining

In recent years, we have witnessed an increasing interest in the application of mechanochemical methods for processing materials in biomass refining techniques. Grinding and mechanical pretreatment are very popular methods utilized to enhance the reactivity of polymers and plant raw materials; however, the choice of devices and their modes of action is often performed through trial and error. An inadequate choice of equipment often results in inefficient grinding, low reactivity of the product, excess energy expenditure, and significant wear of the equipment. In the present review, modern equipment employing various types of mechanical impacts, which show the highest promise for mechanochemical pretreatment of plant raw materials, is examined and compared—disc mills, attritors and bead mills, ball mills, planetary mills, vibration and vibrocentrifugal mills, roller and centrifugal roller mills, extruders, hammer mills, knife mills, pin mills, disintegrators, and jet mills. The properly chosen type of mechanochemical activation (and equipment) allows an energetically and economically sound enhancement of the reactivity of solid-phase polymers by increasing the effective surface area accessible to reagents, reducing the amount of crystalline regions and the diffusion coefficient, disordering the supramolecular structure of the material, and mechanochemically reacting with the target substances.

A Brief Overview on Antioxidant Activity Determination of Silver Nanoparticles

Our objective in this review article is to find out relevant information about methods of determination of antioxidant activity of silver nanoparticles. There are many studies dealing with mentioned problem and herein we summarize the knowledge about methods evaluating the antioxidant activity of silver nanoparticles reported so far. Many authors declare better antioxidant activity of silver nanoparticles compared to the extract used for synthesis of them. In this review, we focused on methods of antioxidant activity determination in detail to find out novel and perspective techniques to solve the general problems associated with the determination of antioxidant activity of silver nanoparticles.

Unbound water in mechanochemical reactions of brown coal

Mechanochemical activation of coal is commonly employed in industry. However, even the simplest solid-phase reactions, such as neutralization of humic acids in brown coal, remain insufficiently studied. The hypothesis regarding the occurrence of mechanochemical neutralization under local hydrothermal conditions for humic acids in brown coal has been tested in this study. 3D modelling of the “block–interlayer” system (where coal particles are separated by air interlayers saturated with water vapor) was used. The 3D model showed that the permittivity is expected to rise from 14 to 16% as the moisture content in the system increases from 12 to 15%. The actual permittivities of coal with different moisture contents have been measured by dielectric spectroscopy. In the real system, the permittivity increases more than threefold as the moisture content rises from 12 to 15%. This increase is much greater than the calculated one, demonstrating that the phase containing unbound water appears in the system at a moisture content of ∼12–13% and may exert various effects on the solid-phase reaction. There is a correlation between the moisture content, permittivity, and predominant mechanisms of the reaction between the organic matter in brown coal and sodium percarbonate (a reagent simultaneously containing the alkaline and peroxidic components). The reactions between brown coal and alkaline reagents proceed under local hydrothermal conditions. Both the alkaline and peroxidic components of sodium percarbonate participate in the solid-phase reaction between brown coal and sodium percarbonate. The emergence of unbound water in coal significantly inhibits the oxidation reaction.

Permittivity analysis for detection of unbound water in coal and analysis of its effect on mechanochemical reactions and technology realization.

Changes in the Crystallinity Degree of Starch Having Different Types of Crystal Structure after Mechanical Pretreatment

This paper examines the effect of mechanical activation on the amorphization of starch having different types of crystalline structure (A-type corn starch; B-type potato starch; and C-type tapioca starch). Structural properties of the starches were studied by X-ray diffraction analysis. Mechanical activation in a planetary ball mill reduces the degree of crystallinity in proportion to pretreatment duration. C-type tapioca starch was found to have the highest degree of crystallinity. Energy consumed to achieve complete amorphization of the starches having different types of crystalline structure was measured. The kinetic parameters of the process (the effective rate constants) were determined. The rate constant and the strongest decline in the crystallinity degree after mechanical activation change in the following series: C-type starch, A-type starch, and B-type starch.

The Effect of Thermomechanical Pretreatment on the Structure and Properties of Lignin-Rich Plant Biomass

The cooperative thermomechanical properties of plant-derived polymers have been studied insufficiently, although this feedstock has a very high potential. In the present paper, we analyzed the changes in the structure and physicochemical properties of lignin-rich biomass induced by thermomechanical pretreatment. Low-temperature treatment allows one to retain the original supramolecular structure of the cell walls, while an appreciably high disintegration degree is reached. This increases the reactivity of the material in the subsequent heterogeneous reactions. Mechanical pretreatment at medium temperatures (10 °C), when almost all cell wall polymers except for low-molecular-weight lignin are in the glassy state, enhances the mobility of cell wall polymers and causes sufficient cellulose disordering, while the specific surface area is not significantly increased. High-temperature pretreatment of reed biomass is accompanied by pore formation and lignin release from the cell wall structure, which opens up new prospects for using this biomass as a matrix to produce core–shell-structured sorbents of heavy metals. The energy consumed by mechanochemical equipment for the activation of reed biomass was determined.

The byproduct-organic acids strengthened pretreatment of cassava straw: Optimization and kinetic study

The subcritical liquid hot water (SLHW) pretreatment could be strengthened by its byproduct-organic acids, such as acetic acid (AA), lactic acid (LA) and formic acid (FA). The effects of these three acids on the pretreatment were investigated by the yield of fermentable sugars. The results showed that the addition of acids could effectively catalyze the hydrolysis of hemicellulose to C5 sugars and contribute to the subsequent enzymatic hydrolysis of cellulose. It was found that all three organic acids promote xylose production, and the copresence of AA + LA could limit the content of the fermentation inhibitor. The optimum proportion of three organic acids were 0.33 wt%AA + 0.45 wt%LA + 0.20 wt%FA, and the yield of C5 sugars after pretreatment and C6 sugar after enzymatic hydrolysis were 89.06% and 78.56%, respectively. The kinetic studies proved that byproduct-organic acids could promote xylose production and inhibit its further degradation and explained that xylose would accumulate at lower temperatures.

Determination of Surface Accessibility of the Cellulose Substrate According to Enzyme Sorption

As a heterogeneous process, enzymatic hydrolysis depends on the contact area between enzymes and the cellulose substrate. The surface area of a substrate is typically evaluated through the sorption of gases (nitrogen, argon, or water vapor) or sorption of high-molecular-weight pigments or proteins. However, lignocellulosic biomass uninvolved in the reaction because of inefficient binding or even the complete inhibition of the enzymes on the surface consisting of lignin or inorganic compounds is erroneously taken into account under these conditions. The initial rate of enzymatic hydrolysis will directly depend on the number of enzymes efficiently sorbed onto cellulose. In this study, the sorption of cellulolytic enzymes was used to evaluate the surface accessibility of the cellulose substrate and its changes during mechanical pretreatment. It was demonstrated that for pure cellulose, mechanical activation did not alter the chemical composition of the surface and the initial rate of hydrolysis increased, which was inconsistent with the data on the thermal desorption of nitrogen. New active cellulose sorption sites were shown to be formed upon. the mechanical activation of plant biomass (wheat straw), and the ultimate initial rate of hydrolysis corresponding to saturation of the accessible surface area with enzyme molecules was determined.