Production of 5-hydroxymethylfurfural from starch-rich food waste catalyzed by sulfonated biochar

Characterization of temperature-dependent subcritical water hydrolysis pattern of strong and floury rice cultivars and potential utilizations of their hydrolysates

This study investigated the effects of subcritical water (SW) temperatures on the hydrolysis pattern and characteristics of hydrolysates prepared with strong rice (SR) and floury rice (FR). The characteristics of the hydrolysates were generally dependent on the rice cultivar in the SW temperature range of 150-250 °C, while the cultivar dependence was diminished at temperatures greater than 300 °C. Based on brix and reducing sugar content, an optimal production of rice hydrolysates was obtained at a SW temperature range of 200-250 °C. However, thermal conversion of sugar into acids, 5-hydroxymethylfurfural (HMF) and furfural was manifested at 250 °C. The rice hydrolysates prepared at 250 ∼ 300 °C had the highest antioxidant activity with strong umami intensity, but they suppressed the growth of prebiotics. Therefore, the present study demonstrated that controlling the SW temperature is crucial to improve rice hydrolysis efficiency and to regulate the physiological activity of the hydrolysates.

Thermochemical Research on Furfurylamine and 5-Methylfurfurylamine: Experimental and Computational Insights

The need to transition from fossil fuels to renewables arises from factors such as depletion, price fluctuations, and environmental considerations. Lignocellulosic biomass, being abundant, and quickly renewable, and not interfering with food supplies, offers a standout alternative for chemical production. This paper explores the energetic characteristics of two derivatives of furfural-a versatile chemical obtained from biomass with great potential for commercial sustainable chemical and fuel production. The standard (p° = 0.1 MPa) molar enthalpies of formation of the liquids furfurylamine and 5-methylfurfurylamine were derived from the standard molar energies of combustion, determined in oxygen and at T = 298.15 K, by static bomb combustion calorimetry. Their standard molar enthalpies of vaporization were also determined at the same temperature using high-temperature Calvet microcalorimetry. By combining these data, the gas-phase enthalpies of formation at T = 298.15 K were calculated as -(43.5 ± 1.4) kJ·mol-1 for furfurylamine, and -(81.2 ± 1.7) kJ·mol-1 for 5-methylfurfurylamine. Furthermore, a theoretical analysis using G3 level calculations was performed, comparing the calculated enthalpies of formation with the experimental values to validate both results. This method has been successfully applied to similar molecules. The discussion looks into substituent effects in terms of stability and compares them with similar compounds.

Recent Advances in Zeolites-Catalyzed Biomass Conversion to Hydroxymethylfurfural: The Role of Porosity and Acidity

Biomass is an attractive raw material for the production of fuel oil and chemical intermediates due to its abundant reserves, low price, easy biodegradability, and renewable use. Hydroxymethylfurfural (5-HMF) is a valuable platform chemically derived from biomass that has gained significant research interest owing to its economic and environmental benefits. In this review, recent advances in biomass catalytic conversion systems for 5-HMF production were examined with a focus on the catalysts selection and feedstocks' impact on the 5-HMF selectivity and yield. Specifically, the potential of zeolite-based catalysts for efficient biomass catalysis was evaluated given their unique pore structure and tunable (Lewis and Brønsted) acidity. The benefits of hierarchical modifications and the interactions between porosity and acidity in zeolites, which are critical factors for the development of green catalytic systems to convert biomass to 5-HMF efficiently, were summarized and assessed. This Review suggests that zeolite-based catalysts hold significant promise in facilitating the sustainable utilization of biomass resources.

High Conversion of Concentrated Sugars to 5-Hydroxymethylfurfural over a Metal-free Carbon Catalyst: Role of Glucose-Fructose Dimers

To reduce the production cost of chemicals from renewable resources, the feedstock loading must be high and the catalyst must be of low cost and efficient. In this study, at a very short reaction time of 10 min at 125 °C, concentrated sugar solutions (20 wt %, 101 wt % on solvent) were converted to 5-hydroxymethylfurfural (HMF) over a cotton gin trash (CGT)-derived sulfonated carbon catalyst in a 1-butyl-3-methyl-imidazolium chloride ([BMIM]Cl) and 2-methyltetrahydrofuran (MeTHF) biphasic system. We report, for the first time, that the presence of glucose either as a covalently bonded monomer in sucrose or in a mixture with fructose achieved yields of HMF up to 62 mol % compared to a value of only 39 mol % obtained with fructose on its own. In the concentrated reaction medium, glucose, fructose, and sucrose molecules produce difructose anhydrides, dimers/reversion products, and sucrose isomers. The glucose-fructose dimers formed in sucrose and glucose/fructose reaction systems play a critical role in the transformation of the sugars to a higher-than-expected HMF yield. Thus, a strategy of using cellulosic glucose, where it is partially converted to fructose content and the high sugar concentration sugar mixture is then converted to HMF, should be exploited for future biorefineries.

Sodium ions removal by sulfuric acid-modified biochars

Sulfuric acid modifies the biochar derived from corn cobs, stalks, and reeds. Amongst the modified biochar, corn cobs-biochar has the highest BET (101.6 m2 g-1), followed by reeds-biochars (96.1 m2 g-1). The Na+ adsorption capacities for pristine biochars are corn cobs-pristine biochar: 24.2 mg g-1, corn stalks-pristine biochar: 7.6 mg g-1, and reeds-pristine biochar: 6.3 mg g-1, relatively low for field applications. The acid-modified corn cobs biochar has a superior Na+ adsorption capacity of up to 221.1 mg g-1, much higher than literature reports and the other two tested biochars. This corn cobs-modified biochar has also a satisfactory Na+ adsorption capacity (193.1 mg g-1) from actual water collected from a sodium-contaminated city, Daqing, China. The FT-IR spectroscopy and XPS spectrum reveal that the embedded surface -SO3H groups onto the biochar correlate with its superior Na + adsorption, attributable to the ion exchange mechanisms. The biochar surface accessible to sulfonic group grafting can generate a superior Na+ adsorbing surface, which is for the first time reported and has great application potential for the remediation of sodium-contaminated water.

Recent advancements and challenges in emerging applications of biochar-based catalysts

The sustainable utilization of biochar produced from biomass waste could substantially promote the development of carbon neutrality and a circular economy. Due to their cost-effectiveness, multiple functionalities, tailorable porous structure, and thermal stability, biochar-based catalysts play a vital role in sustainable biorefineries and environmental protection, contributing to a positive, planet-level impact. This review provides an overview of emerging synthesis routes for multifunctional biochar-based catalysts. It discusses recent advances in biorefinery and pollutant degradation in air, soil, and water, providing deeper and more comprehensive information of the catalysts, such as physicochemical properties and surface chemistry. The catalytic performance and deactivation mechanisms under different catalytic systems were critically reviewed, providing new insights into developing efficient and practical biochar-based catalysts for large-scale use in various applications. Machine learning (ML)-based predictions and inverse design have addressed the innovation of biochar-based catalysts with high-performance applications, as ML efficiently predicts the properties and performance of biochar, interprets the underlying mechanisms and complicated relationships, and guides biochar synthesis. Finally, environmental benefit and economic feasibility assessments are proposed for science-based guidelines for industries and policymakers. With concerted effort, upgrading biomass waste into high-performance catalysts for biorefinery and environmental protection could reduce environmental pollution, increase energy safety, and achieve sustainable biomass management, all of which are beneficial for attaining several of the United Nations Sustainable Development Goals (UN SDGs) and Environmental, Social and Governance (ESG).

Efficient production of hydroxymethyl-2-furfurylamine by chemoenzymatic cascade catalysis of bread waste in a sustainable approach

In this study, efficient and sustainable conversion of waste bread (WB) to 5-hydroxymethyl-2-furoamine (HMFA) was achieved in a cascade reaction in betaine:malonic acid (B:MA) - water. 5-HMF (30.3 wt% yield) was synthesized from WB (40.0 g/L) in B:MA - water (B:MA, 18 wt%) in 45 min at 190 °C. By using the newly created recombinant E. coli HNILGD-AlaDH cells expressing L-alanine dehydrogenase (AlaDH) and ω-transaminase mutant HNILGD as biocatalyst, the WB-valorized 5-HMF was biologically aminated into HMFA in a high yield (92.1%) at 35 °C for 12 h through in situ removal of the amino transfer by-products of the amine donor, greatly reducing amine donor dosage (from D-Ala/5-HMF = 16/1 to D-Ala/5-HMF = 2/1, mol/mol) and improving the productivity of HMFA (0.282 g HMFA per g WB). This two-step chemical-enzymatic cascade reaction strategy with B:MA and HNILGD-AlaDH whole-cell provides a new idea for the chemoenzymatic synthesis of valuable furan chemicals from waste biomass.

Effective one-pot chemoenzymatic cascade catalysis of biobased feedstock for synthesizing 2,5-diformylfuran in a sustainable reaction system

2,5-Diformylfuran, which can be prepared via the oxidation of biobased HMF, has received considerable attention because of its potential applications in producing furan-based chemicals and functional materials, such as biofuels, polymers, fluorescent material, vitrimers, surfactants, antifungal agents and medicines. This work aimed to develop an efficient one-pot process for chemoenzymatic transformation of biobased substrate to 2,5-diformylfuran with deep eutectic solvent (DES) Betaine:Lactic acid ([BA][LA]) catalyst and oxidase biocatalyst in [BA][LA]-H₂O. Using waste bread (50 g/L) and D-fructose (18.0 g/L) as feedstocks in [BA][LA]-H₂O (15:85, vol/vol), the yields of HMF were 32.8% (15 min) and 91.6% (90 min) at 150 °C, respectively. These prepared HMF could be biologically oxidized to 2,5-diformylfuran by Escherichia coli pRSFDuet-GOase, achieving a productivity of 0.631 g 2,5-diformylfuran/(g fructose) and 0.323 g 2,5-diformylfuran/(g bread) after 6 h under the mild performance condition. This bioresourced intermediate 2,5-diformylfuran was effectively synthesized from biobased feedstock in an environmentally-friendly system.

Transformation of bread waste into 2,5-furandimethanol via an efficient chemoenzymatic approach in a benign reaction system

Via combination catalysis with deep eutectic solvent lactic acid:betaine (chemocatalyst) and HMFOMUT cell (biocatalyst: E. coli HMFOMUT whole-cell), one-pot manufacture of 2,5-furandimethanol from waste bioresource was constructed in a chemoenzymatic approach. With bread waste (50 g/L) as substrate, the 5-hydroxymethylfuran yield reached 44.2 Cmol% (based on bread waste) by lactic acid:betaine (15 wt%) at 180 °C for 15 min. With glucose as co-substrate, HMFOMUT could transform 5-hydroxymethylfurfural (150 mM) to 2,5-furandimethanol (84.5 % yield) after 1 day at 37 °C and pH 7.0. In lactic acid:betaine-H₂O, HMFOMUT effectively converted bread-derived 5-hydroxymethylfurfural into 2,5-furandimethanol in a productivity of 700 kg 2,5-furandimethanol per kg 5-hydroxymethylfurfural (230 kg 2,5-furandimethanol per kg bread). In an eco-friendly lactic acid:betaine system, an effective one-pot chemoenzymatic strategy was firstly developed to convert bread waste into 2,5-furandimethanol, which would reduce the operation cost and has potential application value for valorizing waste food bioresource into value-added furan.

An efficient and sustainable furfurylamine production from biomass-derived furfural by a robust mutant ω-transaminase biocatalyst

Furfurylamine is a key furan-based compound for manufacturing perfumes, fibers, additives, medicines and agrochemicals. It can be obtained by amination of furfural by ω-transaminase (AtAT) from Aspergillus terreus. In this work, site-directed mutant of amino acid residues [Threonine (T) at AT130 was mutated to Methionine (M) and Glutamic acid (E) at AT133 was mutated to Phenylalanine (F)] was used to change in the flexible region of AtAT. The transamination activity and thermostability were significantly improved. In ChCl:MA (30 wt%), furfural (500 mM) was efficiently transformed into furfurylamine (92% yield) with TMEF after 12 h. 101.3 mM of biomass-derived furfural and 129.7 mM of D-xylose-derived furfural were wholly converted into furfurylamine within 5 h, achieving the productivity of 0.465 g furfurylamine/(g xylan in corncob) and 0.302 g furfurylamine/(g D-xylose). This established chemoenzymatic conversion strategy by bridging chemocatalysis and biocatalysis could be utilized in the valorisation of renewable biomass to valuable furans.

Novel Challenges on the Catalytic Synthesis of 5-Hydroxymethylfurfural (HMF) from Real Feedstocks

The depletion of fossil resources makes the transition towards renewable ones more urgent. For this purpose, the synthesis of strategic platform-chemicals, such as 5-hydroxymethylfurfural (HMF), represents a fundamental challenge for the development of a feasible bio-refinery. HMF perfectly deals with this necessity, because it can be obtained from the hexose fraction of biomass. Thanks to its high reactivity, it can be exploited for the synthesis of renewable monomers, solvents, and bio-fuels. Sustainable HMF synthesis requires the use of waste biomasses, rather than model compounds such as monosaccharides or polysaccharides, making its production more economically advantageous from an industrial perspective. However, the production of HMF from real feedstocks generally suffers from scarce selectivity, due to their complex chemical composition and HMF instability. On this basis, different strategies have been adopted to maximize the HMF yield. Under this perspective, the properties of the catalytic system, as well as the choice of a suitable solvent and the addition of an eventual pretreatment of the biomass, represent key aspects of the optimization of HMF synthesis. On this basis, the present review summarizes and critically discusses the most recent and attractive strategies for HMF production from real feedstocks, focusing on the smartest catalytic systems and the overall sustainability of the adopted reaction conditions.

Recent achievements in platform chemical production from food waste

Food waste conversion/valorization to produce bio-based chemicals plays a key role toward achieving carbon neutrality by 2050. Food waste valorization to renewable chemicals is thus an attractive and eco-friendly approach to handling food waste. The production of platform chemicals from food waste is crucial for making highly value-added renewable chemicals. However, earlier reviews dealing with food waste valorization to produce value-added chemicals have emphasized the enhancement of methane, hydrogen, and ethanol production. Along these lines, the existing methods of food waste to produce platform chemicals (e.g., volatile fatty acids, glucose, hydroxymethylfurfural, levulinic acid, lactic acid, and succinic acid) through physical, chemical, and enzymatic pretreatments, hydrolysis, fermentation, and hydrothermal conversion are extensively reviewed. Finally, the challenges faced under these methods are discussed, along with suggestions for future research on platform chemical production from food waste.

Bread Surplus: A Cumulative Waste or a Staple Material for High-Value Products?

Food waste has been widely valorized in the past years in order to develop eco-friendly materials. Among others, bread waste is currently of increasing interest, as it is considered a huge global issue with serious environmental impacts and significant economic losses that have become even greater in the post-pandemic years due to an increase in cereal prices, which has led to higher production costs and bread prices. Owing to its richness in polysaccharides, bread waste has been previously studied for its physico-chemical characteristics and its numerous biotechnological applications. The present review highlights the re-use of bread waste and its valorization as a valuable resource by making value-added products through numerous technological processes to increase efficiency at all stages. Many research studies reporting several transformation methods of surplus bread into ethanol, lactic acid, succinic acid, biohydrogen, hydroxymethylfurfural, proteins and pigments, glucose-fructose syrup, aroma compounds, and enzymes are widely discussed. The wide variety of suggested applications for recycling bread waste provides significant insights into the role of technology development in potentially maximizing resource recovery and consequently contributing to environmental performance by reducing the amount of bread waste in landfills.

Valorization of hexoses into 5-hydroxymethylfurfural and levulinic acid in acidic seawater under microwave hydrothermal conditions

Typical value-added platform chemicals 5-hydroxymethylfurfural (HMF) and levulinic acid (LA) can be obtained from hexoses under microwave hydrothermal (MHT) conditions. This study explored the detailed transformation process regarding the MHT products in acidic seawater obtained using glucose and fructose as raw materials. The facile conversion of fructose compared with glucose was mainly ascribed to their different activation energies (56.721 and 88.594 kJ mol-1, respectively). The HMF and LA product yields were strongly affected by the MHT temperature and holding time in two types of hexose solution. Undesirable humins were found to inevitably form under each set of reaction conditions. The carbon balance results for reactants and products showed that up to 60% of fructose carbon was converted into value-added chemicals, while 47% of glucose carbon underwent the same conversion in acidic seawater under the optimal MHT conditions. This study provides further knowledge regarding the role of microwave heating combined with acidic seawater in green chemistry and is a useful reference for the biorefinery industry.

Role of persistent free radicals and lewis acid sites in visible-light-driven wet peroxide activation by solid acid biochar catalysts - A mechanistic study

We report the synthesis of H₂SO₄-modified biochars (SBCs) as solid-acid catalysts to activate H₂O₂ at circumneutral pH under visible light radiation. Spent coffee grinds were pyrolyzed with TiO₂ at 300, 500 and 600 °C followed by steeping in 5 M H₂SO₄ and were used for the Fenton-like degradation of methyl orange (MO). The catalytic activity of SBC depended on the pyrolysis temperature and correlated well with the surface acidity and persistent free radical (PFR) concentration. Results showed that a complete MO removal and a TOC reduction of 70.2% can be achieved with SBC500 under photo-Fenton conditions. However, poisoning of the Lewis acid sites on SBC by PO₄^3- led to a dramatic decrease in the removal of MO with inhibition effects more pronounced than with radical scavengers, suggesting the key role played by acid-sites on the activation of H₂O₂. Finally, electron paramagnetic resonance (EPR) studies identified •OH as the key transient in the degradation followed by •O₂^- and ¹O₂. These findings suggest that H₂O₂ was likely adsorbed on the surface oxygenated functional groups before being decomposed by accepting electrons from the PFRs on the SBC surface.

Carbon-Based Nanocatalysts (CnCs) for Biomass Valorization and Hazardous Organics Remediation

The continuous increase of the demand in merchandise and fuels augments the need of modern approaches for the mass-production of renewable chemicals derived from abundant feedstocks, like biomass, as well as for the water and soil remediation pollution resulting from the anthropogenic discharge of organic compounds. Towards these directions and within the concept of circular (bio)economy, the development of efficient and sustainable catalytic processes is of paramount importance. Within this context, the design of novel catalysts play a key role, with carbon-based nanocatalysts (CnCs) representing one of the most promising class of materials. In this review, a wide range of CnCs utilized for biomass valorization towards valuable chemicals production, and for environmental remediation applications are summarized and discussed. Emphasis is given in particular on the catalytic production of 5-hydroxymethylfurfural (5-HMF) from cellulose or starch-rich food waste, the hydrogenolysis of lignin towards high bio-oil yields enriched predominately in alkyl and oxygenated phenolic monomers, the photocatalytic, sonocatalytic or sonophotocatalytic selective partial oxidation of 5-HMF to 2,5-diformylfuran (DFF) and the decomposition of organic pollutants in aqueous matrixes. The carbonaceous materials were utilized as stand-alone catalysts or as supports of (nano)metals are various types of activated micro/mesoporous carbons, graphene/graphite and the chemically modified counterparts like graphite oxide and reduced graphite oxide, carbon nanotubes, carbon quantum dots, graphitic carbon nitride, and fullerenes.

Preparation of biosorbent for the removal of organic dyes from aqueous solution via one-step alkaline ball milling of hickory wood

Biosorbent has attracted considerable attention recently for use in environment remediation and pollution control. Here, a simple and efficient method of one-step alkaline ball milling was designed to prepare porous hickory biosorbent without any thermal treatments. The products were characterized for their ability to remove methyl violet (MV) and titan yellow (TY) organic dyes from aqueous solutions. The one-step alkaline ball milled hickory (OABMH) biosorbent exhibited mesoporous microstructure, homogeneous morphology, and a diversity of oxygen-containing functional groups. Furthermore, OABMH could sorb 212.2 mg g^-1 MV and 5.6 mg g^-1 TY polar dyes, respectively, mainly through the surface complexation mechanism. Freundlich adsorption isotherm and intraparticle diffusion kinetic models best described MV adsorption by OABMH biosorbents. The results indicate that one-step alkaline ball milling technique is an efficient and economical approach for converting biomass into advanced biosorbents for environment remediation and water treatment.

Enhanced removal of ammonium from water using sulfonated reed waste biochar-A lab-scale investigation

The removal of excessive ammonium from water is vital for preventing eutrophication of surface water and ensuring drinking water safety. Several studies have explored the use of biochar for removing ammonium from water. However, the efficacy of pristine biochar is generally weak, and various biochar modification approaches have been proposed to enhance adsorption capacity. In this study, biochar obtained from giant reed stalks (300, 500, 700 °C) was modified by sulfonation, and the ammonium adsorption capabilities of both giant reed biochars (RBCs) and sulfonated reed biochars (SRBCs) were assessed. The ammonium adsorption rates of SRBCs were much faster than RBCs, with equilibrium times of ∼2 h and ∼8 h for SRBCs and RBCs, respectively. The Langmuir maximum adsorption capacities of SRBCs were 4.20-5.19 mg N/g for SRBCs, significantly greater than RBCs (1.09-1.92 mg N/g). Physical-chemical characterization methods confirmed the increased levels of carboxylic and sulfonic groups on sulfonated biochar. The reaction of ammonium with these O-containing functional groups was the primary mechanism for the enhancement of ammonium adsorption by SRBCs. To conclude, sulfonation significantly improved the adsorption performance of biochar, suggesting its potential application for ammonium mitigation in water.

A review on solid acid catalysis for sustainable production of levulinic acid and levulinate esters from biomass derivatives

Biomass is a kind of renewable and abundant resource that can be seen as an important candidate to solve the energy crisis. Levulinic acid (LA) and levulinate esters (LEs) have been widely researched as biomass-based platform compounds. In recent years, efficient, green, and environment-friendly solid acid catalysts have been developed for the fast production and resolution of the problems, such as low yield, high equipmental requirements, and difficulty in product separation, in the preparation of LA and LE from biomass. In this paper, the preparation routes of LA and LEs from various raw materials are introduced, and the solid acid catalysts involved in their production are emphatically reviewed. The challenges and prospects in LA and LE production from biomass are proposed to achieve a more economical and energy efficient process with the concept of sustainable development in the future.

Impact of Design on the Activity of ZrO2 Catalysts in Cellulose Hydrolysis-Dehydration to Glucose and 5-Hydroxymethylfurfural

The one-pot hydrolysis-dehydration of activated microcrystalline cellulose was studied in pure hydrothermal water at 453 K over ZrO2 catalysts produced by thermodegradation, microwave treatment, mechanical activation, and sol–gel methods and spent without any co-catalyst. ZrO2 prepared by microwave treatment was more active compared to ones derived by other methods. The catalyst calcination temperature also impacted reactivity. The cellulose conversion increased simultaneously with acidity and SBET, which in turn were set by the preparation method and calcination temperature. Phase composition did not affect the activity. Yields of glucose and 5-HMF reaching 18 and 15%, respectively, were over the most promising ZrO2 prepared by microwave treatment at 593 K. To our knowledge, this ZrO2 sample provided the highest activity in terms of TOF values (15.1 mmol g−1 h−1) compared to the pure ZrO2 systems reported elsewhere. High stability of ZrO2 derived by microwave irradiation was shown in five reaction runs.

Activation of biochar through exoenzymes prompted by earthworms for vermibiochar production: A viable resource recovery option for heavy metal contaminated soils and water

The industrial revolution and indiscriminate usage of a wide spectrum of agrochemicals account for the dumping of heavy metals in the environment. In-situ/ex-situ physical, chemical, and bioremediation strategies with pros and cons have been adopted for recovering metal contaminated soils and water. Therefore, there is an urgent requirement for a cost-effective and environment-friendly technique to combat metal pollution. Biochar combined with earthworms and vermifiltration is a suitable emerging technique for the remediation of metal-polluted soils and water. The chemical substances (e.g., sodium hydroxide, zinc chloride, potassium hydroxide, and phosphoric acid) have been used to activate biochar, which also faces several shortcomings. Studies reveal that extracellular enzymes have been used to activate biochar which is produced by earthworms and microbes that can alter the surface of the biochar. The present review focuses on the global scenario of metal pollution and its remediation through biochar activation using earthworms. The earthworms and biochar can produce "vermibiochar" which is capable of reducing the metal ions from contaminated water and soils. The vermifiltration can be a suitable technology for metal removal from wastewater/effluent. Thus, the biochar has a trick of producing entirely new options at a time when vermifiltration and other technologies are least expected. Further attention to the biochar-assisted vermifiltration of different sources of wastewater is required to be explored for the large-scale utilization of the process.

A review on nitrogen transformation in hydrochar during hydrothermal carbonization of biomass containing nitrogen

Biomass is a type of renewable and sustainable resource that can be used to produce various fuels, chemicals, and materials. Nitrogen (N) in biomass such as microalgae should be reduced if it is used to produce fuels, while the retention of N is favorable if the biomass is processed to yield chemicals or materials with N-containing functional groups. The engineering of the removal and retention of N in hydrochar during hydrothermal carbonization (HTC) of biomass rich in protein is a research hot spot in the past decade. However, the N transformation during HTC has not yet been fully understood. In order to mediate the migration and transformation of N in hydrochar, the present review overviewed i) the characteristics of hydrochar and the original feedstock, ii) the possible N transformation behavior and mechanisms, and iii) the effect of factors such as feedstock and pyrolysis parameters such as temperature on hydrochar N. The high temperature and high protein content promote the dehydration, decarboxylation, and deamination of biomass to produce hydrochar solid fuel with reduced N content, while the Millard and Mannich reactions for lignocellulosic biomass rich in carbohydrate (cellulose, hemicellulose, and lignin) at medium temperatures (e.g., 180-240 °C) significantly promote the enrichment of N in hydrochar. The prediction models can be built based on properties of biomass and the processing parameters for the estimation of the yield and the content of N in hydrochar.

The formation of 5-hydroxymethylfurfural and hydrochar during the valorization of biomass using a microwave hydrothermal method

5-Hydroxymethylfurfural (HMF) and levulinic acid (LA) are regarded as value-added platform chemicals that can be derived from biomass waste. However, humins are inevitably produced during valorization processes, reducing the product yields. Previous studies indicated that microwave heating combined with acidic seawater as a reaction medium promotes HMF formation. The present work therefore investigated the relationship between the production of HMF and LA in the liquid phase and that of insoluble humins (that is, hydrochar) under microwave heating in acidic seawater. The selectivities for HMF and LA were found to decrease as the reaction time was increased, as a result of hydrochar formation, and both dehydration and decarboxylation evidently dominated the production of hydrochar in succession. HMF evidently played the most important role in hydrochar formation, and was consumed approximately seven times more rapidly than either fructose or LA. The hydrochar formation mechanism reported herein may be applicable to other similar hydrothermal processes.

A two-step process for energy-efficient conversion of food waste via supercritical water gasification: Process design, products analysis, and electricity evaluation

The huge amount of food waste (FW), containing high organic matter content and moisture, is difficult to be well treated. Supercritical water gasification (SCWG) can efficiently convert FW to H₂-rich syngas. However, it requires high energy input due to the high temperature and high pressure. This study provided an innovative "two-steps heating process" for the SCWG of FW, which firstly utilized hydrothermal (HT) pretreatment to shorter time of SCWG. The effects of different HT temperature (200 °C, 250 °C, 300 °C, 30 min) to SCWG temperature (480 °C, 30 min) and the different residence time (20 min HT - 40 min SCWG, 30 min HT - 30 min SCWG, and 40 min HT - 20 min SCWG) on total syngas yield, carbon conversion efficiency (CE), cold gas efficiency (CGE), and hydrogen conversion efficiency (HE) were studied. Moreover, the energy input by means of electricity consumption in each experiment was measured to determine the energy saving rate. The optimal condition (200 °C, 20 min HT - 40 min SCWG), obtaining the gas yield (17.22 mol/kg), CE (20.10%), CGE (22.13%), and HE (41.54%), was higher than the gas yield (16.53 mol/kg), CE (19.98%), CGE (20%), and HE (38.08%) of directly SCWG (60 min, 0 °C-480 °C). Moreover, the TOC of derived liquid and the pyrolysis characteristics of solid residues were analyzed. Additionally, it was also observed the HT pretreatment helped to reduce the electricity consumption. The highest energy saving rate was 15.58%.

One-step synthesis of biomass-based sulfonated carbon catalyst by direct carbonization-sulfonation for organosolv delignification

Biomass-based sulfonated carbon catalyst (SCC) was prepared from corncob via direct sulfuric acid carbonization-sulfonation treatment. Central composite design was used to evaluate temperature and time for optimizing SCC yield and sulfonic acid (SO₃H) density. The SO₃H groups were successfully introduced to the SCC as evidenced by FTIR and sulfur analysis. Numerical optimization results showed that 100 °C and 5.78 h are the optimal conditions for maximizing yield (61.24%) and SO₃H density (1.1408 mmol/g). The highest ethanol organosolv lignin (EOL) yield of 63.56% with a substrate yield of 39.08% was achieved at 20% SCC loading in the ethanol organosolv delignification of lignocellulosic biomass. The FTIR spectra of the isolated lignin revealed typical features of G-lignin, indicating that no drastic changes took place in the lignin structure during the process. This study developed a simple one-step preparation method of SCC, which was successfully used as a catalyst in an organosolv delignification of biomass.

Ball milling as a mechanochemical technology for fabrication of novel biochar nanomaterials

Mechanochemical synthesis of nano-biochar by ball-milling technology is gaining attention for the sake of its low-cost and eco-friendly nature. Ball milling as a non-/less-solvent technology can propel environmental sustainability and waste valorization into engineered biochar for advanced applications. Scalable production of biochar nanomaterials with superior properties (e.g., 400-500 m² g^-1 surface area and 0.5-1000 nm pore sizes) enables diverse applications in the field of energy and environment. This review critically evaluates the synthesis, characterization, and application of ball-milled biochar nanomaterials based on the latest findings. Limitations such as feedstock selection, process optimization, product homogeneity and reusability, environmental risks, and sustainability assessment remain challenging for further studies. This work highlights the recent advances on mechanochemical biochar technology and offer insights into opportunities and future prospects related to sustainable and facile synthesis of biochar-based novel materials for achieving sustainable development goals.

Microwave-assisted synthesis of bifunctional magnetic solid acid for hydrolyzing cellulose to prepare nanocellulose

The conventional studies on the preparation of nanocellulose used a high concentration of sulfuric acid that is difficult to remove and recover. A biochar-based solid acid with magnetic properties was developed to hydrolyze cellulose to prepare nanocellulose in this work. Two different methods were selected to investigate the properties of the synthesized magnetic carbon-based solid acids. The synthesized catalysts were characterized by SEM, TEM, XRD, NH3-TPD and FT-IR. The experimental results showed that two solid acids by the microwave-assisted synthesis had good magnetic properties by a magnet adsorption. Analysis by SEM and TEM showed that the two solid acids had rich pore structures. According to mineral element analysis, both solid acids contained high sulfur content. The solid acid was an amorphous carbon structural material with a surface rich in active groups. The catalytic activity of the biochar-based solid acids in cellulose hydrolysis to prepare nano-scale cellulosic material was evaluated. It was found that magnetic biochar-based solid acid (MBC-SA1) could achieve a high yield, which produced up to 57.68% for hydrolyzing cellulose into nanometers.

Hydochar and biochar: Production, physicochemical properties and techno-economic analysis

Utilization of natural, abundant, and renewable resources for the production of carbon materials with simple and energy-efficient processes is an upsurge interest. The production processes and resultant biochar can address widespread concerns such as climate change, energy crisis, and environmental pollution. The properties of produced chars (biochar/hydrochar) depend on the production methods, feedstock, and operating parameters, which significantly affect their use for various applications. In this review, production, physiochemical properties, and techno-economic analysis of chars are summarized. This review provides the fundamentals and reaction mechanism of char production methodologies. Physicochemical properties based on chemical composition, functional groups, structure, porosity, and shapes have been compared. The effects of operating parameters on the physicochemical properties of chars are discussed. In addition, this review offers insights on new directions for char production and research in the future, based on the updated and detailed investigation of energy balance with economy of char production methodologies.

Applications of Modified Biochar-Based Materials for the Removal of Environment Pollutants: A Mini Review

The biochar treated through several processes can be modified and utilized as catalyst or catalyst support due to specific properties with various available functional groups on the surface. The functional groups attached to the biochar surface can initiate active radical species to play an important role, which lead to the destruction of contaminants as a catalyst and the removal of adsorbent by involving electron transfer or redox processes. Centering on the high potential to be developed in field applications, this paper reviews more feasible and sustainable biochar-based materials resulting in efficient removals of environmental pollutants as catalyst or support rather than describing them according to the technology category. This review addresses biochar-based materials for utilization as catalysts, metal catalyst supports of iron/iron oxides, and titanium dioxide because the advanced oxidation process using iron/iron oxides or titanium dioxides is more effective for the removal of contaminants. Biochar-based materials can be used for the removal of inorganic contaminants such as heavy meals and nitrate or phosphate to cause eutrophication of water. The biochar-based materials available for the remediation of eutrophic water by the release of N- or P-containing compounds is also reviewed.

Biorenewable hydrogen production through biomass gasification: A review and future prospects

Hydrogen is recognized as one of the cleanest energy carriers, which can be produced from renewable biomass as a promising feedstock to achieve sustainable bioeconomy. Thermochemical technologies (e.g., gasification and pyrolysis) are the main routes for hydrogen production from biomass. Although biomass gasification, including steam gasification and supercritical water gasification, shows a high potential in field-scale applications, the selectivity and efficiency of hydrogen production need improvement to secure cost-effective industrial applications with high atom economy. This article reviews the two main-stream biomass-to-hydrogen technologies and discusses the significance of operating conditions and considerations in the catalytic system design. Challenges and prospects of hydrogen production via biomass gasification are explored to advise on the critical information gaps that require future investigations.

Algae as potential feedstock for the production of biofuels and value-added products: Opportunities and challenges

The current review explores the potential application of algal biomass for the production of biofuels and bio-based products. The variety of processes and pathways through which bio-valorization of algal biomass can be performed are described in this review. Various lipid extraction techniques from algal biomass along with transesterification reactions for biodiesel production are briefly discussed. Processes such as the pretreatment and saccharification of algal biomass, fermentation, gasification, pyrolysis, hydrothermal liquefaction, and anaerobic digestion for the production of biohydrogen, bio-oils, biomethane, biochar (BC), and various bio-based products are reviewed in detail. The biorefinery model and its collaborative approach with various processes are highlighted for the production of eco-friendly, sustainable, and cost-effective biofuels and value-added products. The authors also discuss opportunities and challenges related to bio-valorization of algal biomass and use their own perspective regarding the processes involved in production and the feasibility to make algal research a reality for the production of biofuels and bio-based products in a sustainable manner.

Rational production of highly acidic sulfonated carbons from kraft lignins employing a fractionation process combined with acid-assisted hydrothermal carbonization

Highly acidic lignin-derived sulfonated carbons (LDSCs) were produced from hardwood and softwood kraft lignins under mild conditions by applying fractionation and/or pre-carbonization treatments combined with acid-assisted hydrothermal carbonization. The use of lignin fraction with higher amount oxygen, obtained from the fractionation process, resulted in carbon with the highest density of surface acid groups and improved catalytic activity. The LDSCs were successful tested in the dehydration reaction of fructose to obtain 5-hydroxymethylfurfural, and the best catalyst can be recycled without loss in its catalytic activity after perform a simple regeneration process. In contrast, the pre-carbonization step, commonly performed in several works, resulted in LDSCs with low acidity. A simple and optimized methodology for obtaining LDSCs under mild conditions was developed, and the correlations between the preparation method and the physicochemical and catalytic properties established in this work may be extendible to other starting materials for rational sulfonated carbons production.

Acidic seawater improved 5-hydroxymethylfurfural yield from sugarcane bagasse under microwave hydrothermal liquefaction

5-Hydroxymethylfurfural (HMF) as value-added platform chemical can be derived from biomass. This study used microwave hydrothermal liquefaction (MHTL) to obtain HMF from sugarcane bagasse in acidic seawater conditions. The key processing parameters including temperature, reaction time, and liquid-to-solid ratio (L/S) were evaluated and optimized. The highest HMF yield of 8.1 wt% was obtained at 149 °C with a reaction time of 4 min and a L/S value of 12:1, respectively. This yield is considerable and even higher than the yield derived from sugarcane molasses under similar microwave conditions in the absence of seawater. Hence, acidic seawater was found to promote the hydrolysis of sugarcane bagasse to give HMF precursor (i.e. fructose and glucose), while simultaneously inhibiting the conversion of HMF to levulinic acid under MHTL conditions, possibly explaining the high HMF yield. This method presents a new and sustainable means of transforming waste biomass to valuable substances using seawater or brine wastewater.

Advances in design strategies for preparation of biochar based catalytic system for production of high value chemicals

The aim of this review is to provide the comprehensive and mechanistic information of biochar based catalytic systems for the production of fuels and fine chemicals with a concept of integrated biorefinery. The review presents an in-depth assessment of relationships between physico-chemical properties and catalytic performances of biochar based catalytic systems during the production of targeted compounds at the molecular/fundamental level. The catalytic performance of the biochar is associated with its unique physico-chemical properties (surface area/surface functionality/pores/mechanical strength/inorganic species) which provide a distinct catalytic route. The review also discusses the preparation methods and significance of the activation process for tuning of physico-chemical properties of biochar.

Tailoring acidity and porosity of alumina catalysts via transition metal doping for glucose conversion in biorefinery

Efficient conversion of food waste to value-added products necessitates the development of high-performance heterogeneous catalysts. This study evaluated the use of Al₂O₃ as a low-cost and abundant support material for fabricating Lewis acid catalysts, i.e., through the in-situ doping of Cu, Ni, Co, and Zr into Al₂O₃ followed by calcination. The characterisation results show that all catalysts were mainly amorphous. In particular, adding the transition metals to the Al₂O₃ matrix resulted in the increase of acidity and meso-/micro-pores. The catalysts were evaluated in the conversion of glucose, which can be easily derived from starch-rich food waste (e.g., bread waste) via hydrolysis, to fructose in biorefinery. The results indicate that the Ni-doped Al₂O₃ (Al-Ni-C) achieved the highest fructose yield (19 mol%) and selectivity (59 mol%) under heating at 170 °C for 20 min, of which the performance falls into the range reported in literature. In contrast, the Zr-doped Al₂O₃ (Al-Zr-C) presented the lowest fructose selectivity despite the highest glucose conversion, meaning that the catalyst was relatively active towards the side reactions of glucose and intermediates. The porosity and acidity, modified via metal impregnation, were deduced as the determinants of the catalytic performance. It is noteworthy that the importance of these parameters may vary in a relative sense and the limiting factor could shift from one parameter to another. Therefore, evaluating physicochemical properties as a whole, instead of the unilateral improvement of a single parameter, is encouraged to leverage each functionality for cost-effectiveness. This study provides insights into the structure-performance relationships to promote advance in catalyst design serving a sustainable food waste biorefinery.

Waste-derived compost and biochar amendments for stormwater treatment in bioretention column: Co-transport of metals and colloids

Bioretention systems, as one of the most practical management operations for low impact development of water recovery, utilize different soil amendments to remove contaminants from stormwater. For the sake of urban sustainability, the utilization of amendments derived from waste materials has a potential to reduce waste disposal at landfill while improving the quality of stormwater discharge. This study investigated the efficiency of food waste compost and wood waste biochar for metal removal from synthetic stormwater runoff under intermittent flow and co-presence of colloids. Throughout intermittent infiltration of 84 pore volumes of stormwater, columns amended with compost and biochar removed more than 50-70% of influent metals, whereas iron-oxide coated sand was much less effective. Only a small portion of metals adsorbed on the compost (< 0.74%) was reactivated during the drainage of urban pipelines that do not flow frequently, owing to abundant oxygen-containing functional groups in compost. In comparison, co-existing kaolinite enhanced metal removal by biochar owing to the abundance of active sites, whereas co-existing humic acid facilitated mobilization via metal-humate complexation. The results suggest that both waste-derived compost and biochar show promising potential for stormwater harvesting, while biochar is expected to be more recalcitrant and desirable in field-scale bioretention systems.

Nitrogen containing functional groups of biochar: An overview

Biochar is a carbonaceous material produced by thermal treatment, e.g., pyrolysis, of biomass in oxygen-deficient or oxygen-free environment. Nitrogen containing functional groups of biochar have a wide range of applications, such as adsorption of pollutants, catalysis, and energy storage. To date, many methods have been developed and used to strengthen the function of N-containing biochar to promote its application and commercialization. However, there is no review available specifically on the development of biochar technologies concerning nitrogen-containing functional groups. This paper aims to present a review on fractionation, analysis, formation, engineering, and application of N-functional groups of biochar. The effect of influencing factors on biochar N-functional groups, including biomass feedstock, pyrolysis parameters (e.g., temperature), and additional treatment (e.g., N-doping) were discussed in detail to reveal the formation mechanisms and performance of the N-functional groups. Future prospective investigation directions on the analysis and engineering of biochar N-functional groups were also proposed.

Synthesis and characterization of sulfonated activated carbon as a catalyst for bio-jet fuel production from biomass and waste plastics

Sulfonated activated carbon-based catalysts were prepared by microwaved-assisted carbonization of phosphoric acid activated corncob followed by sulfonation using concentrated sulfuric acid. Sulfonation at different temperatures and times resulted in varied SO₃H group density of the SAC catalysts. Sulfonation temperature showed a significant effect on the introduction of SO₃H on the AC precursor while time had minor role. The SAC catalysts were characterized by means of N2 sorption analysis (specific surface area, pore-volume, average pore width), FTIR spectroscopy, SEM imaging, and sulfur analysis. The impact of catalysts SO₃H density on the product distribution and bio-oil composition from the catalytic co-pyrolysis of Douglas fir and LDPE was evaluated. The highest bio-jet fuels (aromatics and C9-16 alkanes) obtained was 97.51% using the SAC catalyst sulfonated at 100 °C for 5 h. Results showed that SAC has great potential as catalyst in the co-pyrolysis of biomass and plastics for the production of jet-fuel range hydrocarbons.

Development of biochar-based nanocatalysts for tar cracking/reforming during biomass pyrolysis and gasification

Gasification and pyrolysis have been considered as potential routes for the production of fuel gas from abundant, low-cost and renewable biomass materials. Tar is the undesirable but inevitable byproduct during these processes, which not only lowers the overall energy conversion efficiency but also leads to potential contamination and clogging problems. Catalytic reforming has been seen as one of the most promising methods for tar elimination, and biochar-based nanocatalysts (BBNs) show promise as an efficient tar reforming catalyst. Extensive research efforts have been devoted to the development of BBNs to reduce biomass tar below tolerable limits. This current study reviews the recent progress in the preparation and application of BBNs for converting tar into fuel gas during biomass gasification or pyrolysis. The formation and chemical characteristics of the BBNs are introduced for a better understanding of the catalytic mechanism; prospects for success, as well as challenges, are also discussed.

Comparison of hydrochar- and pyrochar-based solid acid catalysts from cornstalk: Physiochemical properties, catalytic activity and deactivation behavior

Biochar made from biowaste provides renewable carbon precursors for catalysts preparation. Here, solid acid catalysts were prepared through functionalizing biochars produced via hydrothermal and pyrolytic carbonization of cornstalk with -SO₃H groups. Hydrochar-based catalysts (HAC) and pyrochar-based catalysts (PAC) exhibited significantly different physiochemical properties, catalytic activities and deactivation behaviors. The test of catalytic effects on cellulose degradation uncovered that HAC had a higher density of -SO₃H but lower surface special area than PAC. Specifically, PAC prepared at 400 °C resulted in the maximum increase of cellulose conversion by 16.00-50.50%. In comparison, the highest yields of glucose (11.14%) and 5-hydroxymethylfurfural (29.54%) were achieved catalyzed via HAC prepared at 240 °C. The results of catalyst deactivation behavior further revealed that used catalysts had an obvious reduction of -SO₃H density. Interestingly, used HAC-240 catalysts showed similar patterns of weight loss to fresh ones due to its high stability.

Biochar facilitated bioprocessing and biorefinery for productions of biofuel and chemicals: A review

Biochar is traditionally used to improve soil properties in arable land and as adsorbent or precursor of activated carbon in wastewater treatment. Recent advances have shown biochar potentials in enhancing productions of biofuels and chemicals such as bio-ethanol, butanol, methane, hydrogen, bio-diesel, hydrocarbons and carboxylic acids. The properties of biochar such as high levels of porosity, functional groups, cation exchange capacity, pH buffering capacity, electron conductivity, and macro-/micro- nutrients (Na, K, Ca, Mg, P, S, Fe, etc.) provide appropriate conditions to relieve physicochemical stresses on microorganisms through pH buffering, detoxification, nutrients supply, serving as electron carrier and supportive microbial habitats. This paper critically reviewed biochar production and characteristics, biochar utilization in anaerobic digestion, composting, microbial fermentation, hydrolysate detoxification, catalysis in biomass refinery and biodiesel synthesis. This review provides novel vision of biochar application, which could guide future research towards cleaner and more economic production of renewable fuels and bio-based chemicals.