New Understandings of the Relationship and Initial Formation Mechanism for Pseudo-lignin, Humins, and Acid-Induced Hydrothermal Carbon

Research progress on organic acid pretreatment of lignocellulose

Lignocellulosic biomass is a naturally occurring, renewable resource that is utilized to produce a variety of high-value-added products, such as fuels, acids, and building block chemicals. The pretreatment of lignocellulosic biomass is a crucial step in the deconstruction and fractionation of its components. Organic acids, such as formic, acetic, lactic, and maleic acids, have been widely studied for their effectiveness in lignocellulose pretreatment. Organic acid-based pretreatment techniques are gaining increased attention due to their ability to selectively separate hemicellulose and cellulose, promote oligomer formation, and minimize byproducts. This paper presents a comprehensive review of the various advancements in the science and application of organic acids for the pretreatment of lignocellulose. Furthermore, the significant challenges of organic acid recovery after pretreatment are highlighted, and different recovery methods are discussed. The future challenges related to utilizing organic acids for lignocellulose pretreatment are summarized, with a strong emphasis on adopting a sustainable approach to converting valuable bioresources into renewable products.

Insights into hydrothermal deconstruction and humification of vegetable waste by non-catalytic steam explosion

Source minimization and valorization of vegetable waste has attracted considerable interests recently, but has been limited by its distinctive features of high humidity and perishability. To mitigate these challenges and draw upon its features, this study provides a novel non-catalytic hydrothermal process for rapid humification of broccoli waste by steam explosion (SE). The highest humic substance, fulvic acid (FA) and humic acid yields of 23.48 wt%, 18.70 wt% and 6.07 wt% were obtained within 30 min. Thermal-acidic condition and instantaneous decompression action proved to be favorable for substrate deconstruction, precursor production and humus formation. Potential pathways of hydrothermal humification of vegetable waste were revealed after clarifying the molecular level structure of FA by EA, FTIR, XPS, 2D-NMR and Py-GCMS. This work fills the knowledge gap in the mechanism of hydrothermal humification of vegetable waste and provides technical support for enhancing its agronomic value and efficient cycle in clean production.

Hydrothermal carbonization of Chinese medicine residues: Formation of humic acids and combustion performance of extracted hydrochar

Aiming at the sustainable management of high-moisture Chinese medicine residues (CMR), an alternative way integrating hydrothermal carbonization (HTC), humic acids (HAs) extraction and combustion of remained hydrochar has been proposed in this study. Effect of HTC temperature, HTC duration, and feedwater pH on the mass yield and properties of HAs was examined. The associated formation mechanism of HAs during HTC was proposed. The combustion performance of remained hydrochar after HAs extraction was evaluated. Results show that the positive correlation between hydrochar yield and HAs yield is observed. According to three-dimensional excitation emission matrix (3D EEM) fluorescence intensity, the best quality of HAs is achieved with a yield of 8.17 % at feedwater pH of 13 and HTC temperature of 200 °C. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses show abundant aromatic and aliphatic structure as well as oxygenated functional groups in HAs, which is like commercial HAs (HA-C). Besides, in terms of comprehensive combustion index (CCI), HTC can improve the combustion performance of CMR, while it becomes a bit worse after HAs extraction. Higher weighted mean apparent activation energy (Em) of hydrochar indicating its highly thermal stability. HAs extraction reduces Em and CCI of remained hydrochar. However, it can be regarded a potential renewable energy. This work confirms a more sustainable alternative way for CMR comprehensive utilization in near future.

Steam explosion as sustainable biomass pretreatment technique for biofuel production: Characteristics and challenges

The biorefining process of lignocellulosic biomass has recently emerged as one of the most profitable biofuel production options. However, pretreatment is required to improve the recalcitrant lignocellulose's enzymatic conversion efficiency. Among biomass pretreatment methods, the steam explosion is an eco-friendly, inexpensive, and effective approach to pretreating biomass, significantly promoting biofuel production efficiency and yield. This review paper critically presents the steam explosion's reaction mechanism and technological characteristics for lignocellulosic biomass pretreatment. Indeed, the principles of steam explosion technology for lignocellulosic biomass pretreatment were scrutinized. Moreover, the impacts of process factors on pretreatment efficiency and sugar recovery for the following biofuel production were also discussed in detail. Finally, the limitations and prospects of steam explosion pretreatment were mentioned. Generally, steam explosion technology applications could bring great potential in pretreating biomass, although deeper studies are needed to deploy this method on industrial scales.

On the gelation of humins: from transient to covalent networks

Humins are a by-product of many acid-catalyzed biorefinery processes converting polysaccharides into platform chemicals. The valorization of humin residue to increase the profit of biorefinery operations and reduce waste is a field that is growing interest as the production of humins continues to increase. This includes their valorization in materials science. For successful processing of humin-based materials, this study aims to understand the thermal polymerization mechanisms of humins from a rheological perspective. Thermal crosslinking of raw humins leads to an increase in their molecular weight, which in turn leads to the formation of a gel. Humin's gels structure combines physical (thermally reversible) and chemical (thermally irreversible) crosslinks, and temperature plays an essential role in the crosslink density and the gel properties. High temperatures delay the formation of a gel due to the scission of physicochemical interactions, drastically decreasing their viscosity, whereas upon cooling a stronger gel is formed combining the recovered physicochemical bonds and the newly created chemical crosslinks. Thus, a transition from a supramolecular network to a covalently crosslinked network is observed, and properties such as the elasticity or reprocessability of humin gels are influenced by the stage of polymerization.

Investigation of the Formation, Characterization, and Oxidative Catalytic Valorization of Humins

The industrial use of biomass, e.g., for the production of platform chemicals such as levulinic acid, became increasingly important in recent years. However, the efficiency of these processes was reduced by the formation of insoluble solid waste products called humins. Herein, the formation of humins from various carbohydrates was investigated under different process conditions, in order to obtain information about the structure and the formation mechanism. During this process, new potential structural fragments of humins were identified. Subsequently, the produced humins were oxidatively converted to low-molecular-weight carboxylic acids with the use of polyoxometalate catalysts. The experiments showed that the use of sugars in acetic acid and ethanol only lead to the formation of a small amount of humins, which were also structurally most suitable for conversion to carboxylic acids. The main products of the oxidative valorisation of these humins were acetic acid, formic acid, and CO₂, respectively, and our results indicate that certain functional groups were converted preferentially. These findings will help to improve processes for the valorisation of biomass by enabling an overall more efficient use of thermo-sensitive feedstock such as carbohydrates.

Improved glucose yield and concentration of sugarcane bagasse by the pretreatment with ternary deep eutectic solvents and recovery of the pretreated liquid

In this study, a novel ternary deep eutectic solvents (DES) consisting of choline chloride/PEG/hydroxyethyl sulfonic acid (HSA) was developed to effectively improve glucose yield and concentration of sugarcane bagasse, and the conditions of the pretreatment were optimized by response surface method (RSM). Under the optimal conditions, the maximum glucose concentration (GC) could reach 12.39 g/L (HSA concentration 1.34 %, PEG400, 2.3 h, 150 °C), and the maximum glucose yield (GY) was 0.2497 g/g (HSA concentration 1.41 %, PEG400, 2.1 h, 150 °C). Hemicellulose was completely removed, and the maximum lignin removal rate was 86.89 %. After pretreatment, 95 % of the pretreated liquid can be recycled. Finally, the structural and morphological changes of bagasse before and after pretreatment were investigated by scanning electron microscopy (SEM), Fourier Transform infrared analyzer (FT-IR) and X-ray diffraction (XRD).

Identification of Crucial Intermediates in the Formation of Humins from Cellulose-Derived Platform Chemicals Under Brønsted Acid Catalyzed Reaction Conditions

Humins are one of the undesirable products formed during the dehydration of sugars as well as the conversion of 5-hydroxymethylfurfural (HMF) to value-added products. Thus, reducing the formation of humins is an important strategy for improving the yield of the aforementioned reactions. Even after a plethora of studies, the mechanism of formation and the structure of humins are still elusive. In this regard, we have employed density functional theory-based mechanistic studies and microkinetic analysis to identify crucial intermediates formed from glucose, fructose, and HMF that can initiate the polymerization reactions resulting in humins under Brønsted acid-catalyzed reaction conditions. This study brings light into crucial elementary reaction steps that can be targeted for controlling humins formation. Moreover, this work provides a rationale for the experimentally observed aliphatic chains and HMF condensation products in the humins structure. Different possible polymerization routes that could contribute to the structure of humins are also suggested based on the results. Importantly, the findings of this work indicate that increasing the rate of isomerization of glucose to fructose and reducing the rate of reaction between HMF molecules could be an efficient strategy for reducing humins formation.

Insight into the dehydration of high-concentration fructose to 5-hydroxymethylfurfural in oxygen-containing polar aprotic solvents

A high 5-HMF yield of 85.4% was achieved in polar aprotic oxygen-containing solvent with strong electrophilic maleic acid by quenching DHH.

Potential Hydrothermal-Humification of Vegetable Wastes by Steam Explosion and Structural Characteristics of Humified Fractions

In this work, steam explosion (SE) was exploited as a potential hydrothermal-humification process of vegetable wastes to deconstruct their structure and accelerate their decomposition to prepare humified substances. Results indicated that the SE process led to the removal of hemicellulose, re-condensation of lignin, degradation of the cellulosic amorphous region, and the enhancement of thermal stability of broccoli wastes, which provided transformable substrates and a thermal-acidic reaction environment for humification. After SE treatment, total humic substances (HS), humic acids (HA_s), and fulvic acids (FA_s) contents of broccoli samples accounted for up to 198.3 g/kg, 42.3 g/kg, and 166.6 g/kg, and their purification were also facilitated. With the increment of SE severity, structural characteristics of HA_s presented the loss of aliphatic compounds, carbohydrates, and carboxylic acids and the enrichment of aromatic structures and N-containing groups. Lignin substructures were proved to be the predominant aromatic structures and gluconoxylans were the main carbohydrates associated with lignin in HA_s, both of their signals were enhanced by SE. Above results suggested that SE could promote the decomposition of easily biodegradable matters and further polycondensation, aromatization, and nitrogen-fixation reactions during humification, which were conducive to the formation of HA_s.

Characterization of the Soluble Products Formed during the Hydrothermal Conversion of Biomass-Derived Furanic Compounds by Using LC-MS/MS

To reveal the hydrothermal conversion routes of the biomass-derived furanic compounds, the soluble products formed during the hydrothermal conversion of 5-hydroxymethylfurfural (HMF), furfural, and furfuryl alcohol were analyzed by liquid chromatography-mass spectrometry (LC-MS) and LC-MS/MS. Multiple carbocyclic compounds containing hydroxy group and carbonyl group were detected, with a molecular mass in the range of 154-272 Da and carbon chain of the length 8-15. The formation of these soluble carbocyclic compounds was proposed to involve hydrolytic ring-opening of the furanic ring, intermolecular aldol condensation, intramolecular aldol condensation, and C-C cleavage reaction. The C-C cleavage reaction was proposed to occur on the dicarbonyl structure of the formed primary polymers.

New Intensification Strategies for the Direct Conversion of Real Biomass into Platform and Fine Chemicals: What Are the Main Improvable Key Aspects?

Nowadays, the solvothermal conversion of biomass has reached a good level of development, and now it is necessary to improve the process intensification, in order to boost its further growth on the industrial scale. Otherwise, most of these processes would be limited to the pilot scale or, even worse, to exclusive academic investigations, intended as isolated applications for the development of new catalysts. For this purpose, it is necessary to improve the work-up technologies, combining, where possible, reaction/purification unit operations, and enhancing the feedstock/liquid ratio, thus improving the final concentration of the target product and reducing the work-up costs. Furthermore, it becomes decisive to reconsider more critically the choice of biomass, solvent(s), and catalysts, pursuing the biomass fractionation in its components and promoting one-pot cascade conversion routes. Screening and process optimization activities on a laboratory scale must be fast and functional to the flexibility of these processes, exploiting efficient reaction systems such as microwaves and/or ultrasounds, and using multivariate analysis for an integrated evaluation of the data. These upstream choices, which are mainly of the chemist’s responsibility, are fundamental and deeply interconnected with downstream engineering, economic, and legislative aspects, which are decisive for the real development of the process. In this Editorial, all these key issues will be discussed, in particular those aimed at the intensification of solvothermal processes, taking into account some real case studies, already developed on the industrial scale.

Conversion of Xylose to Furfural over Lignin-Based Activated Carbon-Supported Iron Catalysts

In this study, conversion of xylose to furfural was studied using lignin-based activated carbon-supported iron catalysts. First, three activated carbon supports were prepared from hydrolysis lignin with different activation methods. The supports were modified with different metal precursors and metal concentrations into five iron catalysts. The prepared catalysts were studied in furfural production from xylose using different reaction temperatures and times. The best results were achieved with a 4 wt% iron-containing catalyst, 5Fe-ACs, which produced a 57% furfural yield, 92% xylose conversion and 65% reaction selectivity at 170 °C in 3 h. The amount of Fe in 5Fe-ACs was only 3.6 µmol and using this amount of homogeneous FeCl3 as a catalyst, reduced the furfural yield, xylose conversion and selectivity. Good catalytic activity of 5Fe-ACs could be associated with iron oxide and hydroxyl groups on the catalyst surface. Based on the recycling experiments, the prepared catalyst needs some improvements to increase its stability but it is a feasible alternative to homogeneous FeCl3.

A hydrothermal process to turn waste biomass into artificial fulvic and humic acids for soil remediation

A novel preparation protocol for synthetic, look-a-like humic substances (i.e., fulvic and humic acids) simulating geochemical processes through hydrothermal reaction is presented, with crude waste biomass as an omnipresent and universal precursor. The chemical nature of the organic scaffold and the type and abundance of oxygen-containing functional groups of the synthetic humic substances (A-FA and A-HA) are revealed by a series of examinations. Results from EA, XPS analyze, FTIR spectra and NMR technology matched well each other, suggesting high similarity on chemical structure (abundant aromatic frameworks) and contents (e.g. N and S elements) in both humic acids. Pyrolysis-gas-chromatography/mass spectrometry (Py-GC/MS) analysis is employed on the organic structure and is directly compared to extracted natural humic matter from black soils (Harbin, China). Dehydrated carbohydrates and their condensates with low molecular weight that are rich in oxygen are the main structural components of the artificial fulvic acids, while aromatic structures and aliphatic side chains are almost absent. Aromatics (7.43%) and in some cases long-chain aliphatics (7.15%) are more prominent in the A-HA sample. The combination of the diverse analytical techniques not only allows a better understanding of artificial fulvic and humic acids, but also supports the high similarity to natural humic substances in structure and morphology. As the technology can be easily scaled and is comparable cheap, the as obtained products can be discussed to rehabilitate used up farm land.

Efficient catalytic conversion of dilute-oxalic acid pretreated bagasse hydrolysate to furfural using recyclable ironic phosphates catalysts

Efficient conversion of dilute-oxalic acid pretreated bagasse hydrolysate to furfural was developed using recyclable ironic phosphates (FePO₄) catalysts in the modified heterogeneous system. The effects of reaction conditions on the furfural yields were investigated, and the stability and water solubility of catalysts were evaluated. Results showed that the maximum furfural yield of 88.7% was obtained in the modified biphasic system by FePO₄ catalysts at 190 °C for 120 min. The catalyst could be recycled and reused in conversion of the xylose-rich hydrolysate into furfural due to the unique feature that the catalyst showed solid state at room temperature and could be gradually dissolved into the aqueous phase upon increasing the reaction temperature and time. The experiments of five-time recycles showed that the FePO₄ catalyst exhibited excellent stability and catalytic performances.

One-pot selective conversion of lignocellulosic biomass into furfural and co-products using aqueous choline chloride/methyl isobutyl ketone biphasic solvent system

This study investigated simultaneous lignocellulose fractionation and conversion in a one-pot reaction using an aqueous choline chloride/methyl isobutyl ketone (ChCl/MIBK) biphasic solvent system. Under the optimized condition (170 °C, 60 min, 0.6 wt% H₂SO₄, 10.7 wt% solid loading), the biphasic solvent solubilized 96% xylan in raw switchgrass, which was simultaneously converted to furfural with a yield of 84.04%. The biphasic solvent was also able to selectively extract lignin, which had a high purity (93.1%), and uncondensed moieties (i.e., Hibbert's ketone), as well as decreased molecular weight and polydispersity index. The resultant pulp was enriched with cellulose (73.3%), which can be completely hydrolyzed into glucose within 48 h via enzymatic hydrolysis. Aqueous ChCl was successfully recycled and reused for atleast three cycles with similar performance in switchgrass fractionation. This study demonstrated that aqueous ChCl/MIBK biphasic system was an effective solvent system for co-production of furfural, high quality technical lignin and digestible cellulose for further upgrading.

How Pseudo-lignin Is Generated during Dilute Sulfuric Acid Pretreatment

Pseudo-lignin is generated from lignocellulose biomass during pretreatment with dilute sulfuric acid and has a significant inhibitory effect on cellulase. However, the mechanism of pseudo-lignin generation remains unclear. The following main points have been addressed to help elucidate the pseudo-lignin generation pathway. Cellulose and xylan were pretreated with sulfuric acid at different concentrations; aliquots were periodically collected; and the changes in the byproducts of the prehydrolysate were quantified. Milled wood lignin (MWL) mixed with cellulose and xylan was pretreated to evaluate the impact of lignin on pseudo-lignin generation. Furfural, 5-hydroxymethylfurfural, and MWL were pretreated as model compounds to investigate pseudo-lignin generation. The result indicated that the increasing acid concentration significantly promoted the generation of pseudo-lignin. When the acid concentration was increased from 0 to 1.00 wt %, pseudo-lignin was increased from 1.36 to 4.05 g. In addition, lignin promoted the pseudo-lignin generation through the condensation between lignin and the generated intermediates.

Enhanced bioconversion of hemicellulosic biomass by microbial consortium for biobutanol production with bioaugmentation strategy

As a renewable and sustainable source for next-generation biofuel production, lignocellulosic biomass can be effectively utilized in environmentally friendly manner. In this study, a stable, xylan-utilizing, anaerobic microbial consortium MC1 enriched from mangrove sediments was established, and it was taxonomically identified that the genera Ruminococcus and Clostridium from this community played a crucial role in the substrate utilization. In addition, a butanol-producing Clostridium sp. strain WST was introduced via the bioaugmentation process, which resulted in the conversion of xylan to biobutanol up to 10.8 g/L, significantly improving the butanol yield up to 0.54 g/g by 98-fold. When this system was further applied to other xylan-rich biomass, 1.09 g/L of butanol could be achieved from 20 g/L of corn cob. These results provide another new method to efficiently convert xylan, the main hemicellulose from lignocellulosic biomass into biofuels through a low-cost and eco-friendly manner.

Condensation of α-Carbonyl Aldehydes Leads to the Formation of Solid Humins during the Hydrothermal Degradation of Carbohydrates

Catalytic hydrothermal conversion of carbohydrates could provide a series of versatile valuable platform chemicals, but the formation of solid humins greatly decreased the efficiency of the process. Herein, by studying the hydrothermal degradation behavior and analyzing the degradation paths of kinds of model compounds including carbohydrates, furan compounds, cyclic ketone derivatives, and some simple short carbon-chain oxy-organics, we demonstrate that α-carbonyl aldehydes and α-carbonyl acids are the key primary precursors for humin formation during the hydrothermal conversion process. Then, we analyzed the hydrothermal degradation paths of two simple α-carbonyl aldehydes including glyoxal and pyruvaldehyde and found that the α-carbonyl aldehydes could undergo aldol condensation followed by acetal cyclization and dehydration to form solid humins rich of furan ring structure or undergo Cannizaro route (hydration followed by 1,2-hydride shift) to form corresponding α-hydroxy acids. On the basis of the hydrothermal behavior of the α-carbonyl aldehydes, we mapped the hydrothermal degradation routes of carbohydrates (glucose, fructose, and xylose) and illuminated the formation details of α-carbonyl aldehydes, α-hydroxy acids, γ-lactones, furfural derivatives, and humins. Finally, we deduced the typical structure fragments of humins from three α-carbonyl aldehydes of pyruvaldehyde, 2,5-dioxo-6-hydroxy-hexanal, and 3-deoxyglucosone, all of which could be formed during the hydrothermal degradation of hexose.

Effect of severity factor on the hydrothermal pretreatment of sugarcane straw

The recalcitrant structures of sugarcane straw and related lignocellulosic biomasses require a pretreatment step to enable a better enzymatic attack during the hydrolysis. Factors like the energy consumption and the formation of inhibitors require the optimization of the pretreatment step. Thus, the influence of different severity factors (SF) on hydrothermal (also called liquid hot water, LHW) pretreatment was evaluated using a factorial design 2² with central point. The obtained results showed that low values of SF (<3.39) did not promote reasonable alteration in the sugarcane straw structures, whereas high SF values (>4.70) resulted in loss of hydrolyzed sugars, generation of inhibitors such as furfural, and formation of pseudo-lignin structures, despite high hemicellulose removal (∼97%). The residence time exhibited low influence on LHW. An optimum condition was found for the process (10 min and 195 °C) with low cellulose solubilization (9.80%) and a reasonable hemicellulose removal (85.45%).

A new approach to recycle oxalic acid during lignocellulose pretreatment for xylose production

BACKGROUND

Dilute oxalic acid pretreatment has drawn much attention because it could selectively hydrolyse the hemicellulose fraction during lignocellulose pretreatment. However, there are few studies focusing on the recovery of oxalic acid. Here, we reported a new approach to recycle oxalic acid used in pretreatment via ethanol extraction.

RESULTS

The highest xylose content in hydrolysate was 266.70 mg xylose per 1 g corncob (85.0% yield), which was achieved using 150 mmol/L oxalic acid under the optimized treatment condition (140 °C, 2.5 h). These pretreatment conditions were employed to the subsequent pretreatment using recycled oxalic acid. Oxalic acid in the hydrolysate could be recycled according to the following steps: (1) water was removed via evaporation and vacuum drying, (2) ethanol was used to extract oxalic acid in the remaining mixture, and (3) oxalic acid and ethanol were separated by reduced pressure evaporation. The total xylose yields could be stabilized by intermittent adding oxalic acid, and the yields were in range of 46.7-64.3% in this experiment.

CONCLUSIONS

This sustainable approach of recycling and reuse of oxalic acid has a significant potential application for replacing traditional dilute mineral acid pretreatment of lignocellulose, which could contribute to reduce CO₂ emissions and the cost of the pretreatment.