Hydrothermal conversion of xylose, glucose, and cellulose under the catalysis of transition metal sulfates

Hydrothermal Carbonization of Biomass for Electrochemical Energy Storage: Parameters, Mechanisms, Electrochemical Performance, and the Incorporation of Transition Metal Dichalcogenide Nanoparticles

Given the pressing climate and sustainability challenges, shifting industrial processes towards environmentally friendly practices is imperative. Among various strategies, the generation of green, flexible materials combined with efficient reutilization of biomass stands out. This review provides a comprehensive analysis of the hydrothermal carbonization (HTC) process as a sustainable approach for developing carbonaceous materials from biomass. Key parameters influencing hydrochar preparation are examined, along with the mechanisms governing hydrochar formation and pore development. Then, this review explores the application of hydrochars in supercapacitors, offering a novel comparative analysis of the electrochemical performance of various biomass-based electrodes, considering parameters such as capacitance, stability, and textural properties. Biomass-based hydrochars emerge as a promising alternative to traditional carbonaceous materials, with potential for further enhancement through the incorporation of extrinsic nanoparticles like graphene, carbon nanotubes, nanodiamonds and metal oxides. Of particular interest is the relatively unexplored use of transition metal dichalcogenides (TMDCs), with preliminary findings demonstrating highly competitive capacitances of up to 360 F/g when combined with hydrochars. This exceptional electrochemical performance, coupled with unique material properties, positions these biomass-based hydrochars interesting candidates to advance the energy industry towards a greener and more sustainable future.

Production of value-added hydrochar from single-mode microwave hydrothermal carbonization of oil palm waste for de-chlorination of domestic water

A huge amount of palm waste generated daily represents a problematic high-moisture waste to be disposed of, yet it also represents a promising biomass resource to be transformed into a value-added product. A single-mode microwave hydrothermal carbonization process incorporating steam purging was developed and utilised to convert high-moisture palm waste into hydrochar over a range of process temperatures from 150 to 300 °C. The microwave hydrothermal carbonization recorded a shorter process duration (10 min) and prevented the occurrence of hot spots within the reactor. The resulting hydrochar showed up to 94.3 wt% of mass yield, 69.2 wt% of fixed carbon, and 412.3 m²/g of surface area. The subsequent application of the hydrochar in de-chlorination of domestic water demonstrated an impressive removal performance of up to 98.9% of free chlorine, exhibiting 435 min of breakthrough time, and 40.0 mg/g of bed capacity in continuous column operation. The results show great promise of microwave hydrothermal carbonization as a desirable approach to produce desirable hydrochar for de-chlorination application.

Liquefaction of Cellulose for Production of Advanced Porous Carbon Materials

Cellulose is a renewable resource for the production of advanced carbonaceous materials for various applications. In addition to direct carbonization, attention has recently been paid to the preparation of porous carbons from liquid cellulose-based precursors. Possible pathways of cellulose conversion to a liquid state suitable for the preparation of porous carbons are summarized in this review. Hydrothermal liquefaction leading to liquid mixtures of low-molecular-weight organics is described in detail together with less common decomposition techniques (microwave or ultrasound assisted liquefaction, decomposition in a strong gravitation field). We also focus on dissolution of cellulose without decomposition, with special attention paid to dissolution of nonderivatized cellulose. For this purpose, cold alkalines, hot acids, ionic liquids, or alcohols are commonly used.

Molecular design and experimental study of cellulose conversion to 5-hydroxymethylfurfural catalyzed by different ratios of Brønsted/Lewis acid ionic liquids

Cellulose conversion into 5-hydroxymethylfurfural (5-HMF) is difficult because of the strong hydrogen bonding existed in cellulose chains. Brønsted/Lewis (B/L) biacidic functionalized ionic liquids (ILs) have great advantages in acid-catalyzed tandem reactions, but the catalytic effect of ILs differs considerably depending on B/L acid ratios. Therefore, this work designed a series of reactions with different proportions of biacidic ILs for the preparation of 5-HMF from cellulose. The tandem reaction is often performed in the presence of a solvent, and the activity of the catalyst is also affected by the solvent. Therefore, in this work, the solvation model density(SMD) model was introduced into the quantum chemical calculation method for molecular design to predict the catalytic effect and explore the catalytic mechanism. The calculation results and experiments jointly showed that [(HSO₃-P)₂im]Cl·ZnCl₂ had the highest efficiency, with a 5-HMF yield of 65.66%. This study facilitates the directional optimization design of the catalyst.

Co-hydrothermal carbonization of sewage sludge and model compounds of food waste: Influence of mutual interaction on nitrogen transformation

This study reports the transformation behavior of nitrogen during the co-hydrothermal carbonization of sewage sludge and model compounds (microcrystalline cellulose, starch, lignin, and xylan) of food waste at 220 °C, with a focus on the reaction routes between starch/xylan and NH₄⁺. Most of the nitrogen in the raw sludge was transformed into organic-N (44.6%) and NH₄⁺ (23.3%) in the aqueous product, and only 20.3% of nitrogen was retained in the hydrochar. The added model compounds could react with organic-N (i.e., amino acids and amines) and NH₄⁺ in aqueous products through Maillard and Mannich reactions, generating heterocyclic-N (especially pyrrole-N) which further polymerizes to form nitrogen-containing polyaromatic hydrochar. This leads to an increase in the retention rate of nitrogen to 36.8-50.9%, especially upon the addition of starch and xylan. During the hydrothermal carbonization of starch/xylan in the NH₄⁺ solution, the polymers are first hydrolyzed into monomers, followed by their further reaction with NH₄⁺ to generate pyrrole-N and pyridine-N in aqueous products (especially xylan), and the pyrrole-N can then polymerize with aromatic clusters to form hydrochar-N. The results show that the model compounds of food waste substantially affect the nitrogen transformation pathways during hydrothermal carbonization, mainly because of the structures of their monomers. These findings can guide the production of sludge-based hydrochar with the targeted regulation of nitrogen content and species.

Elemental migration and transformation during hydrothermal liquefaction of biomass

Over the past few decades, energy and environmental crises have worsened due to the excessive consumption of fossil fuels. Hydrothermal liquefaction (HTL) is a promising technology for sustainable biocrude production from biomass. However, elemental migration and transformation during HTL of biomass have only received scant attention to date. Understanding the transformation mechanism is beneficial for downstream biocrude upgrading and by-products utilization for the future industrialization of HTL. In this paper, biomass is grouped into six categories: microalgae, macroalgae, lignocellulose, food waste, manure, and sludge. The biochemical composition and HTL product distribution of six kinds of biomass are compared. The conversion process of the biomacromolecules (including lipids, proteins, cellulose, hemicellulose, and lignin) and the interactions between them are also reported. Furthermore, the distribution of carbon, nitrogen, sulfur, and inorganic elements (Na, K, Ca, Mg, Al, Fe, Zn, Cu, Pb, Cd, etc.) in the HTL products is summarized, and the transformation of the organic and inorganic elements during HTL of biomass is explored. Finally, outlooks for the HTL of biomass are proposed.

Hydrothermal liquefaction of lignocellulose for value-added products: Mechanism, parameter and production application

Abundant, environmentally friendly, and sustainable lignocellulose is a promising feedstock for replacing fossil fuels, and hydrothermal liquefaction is an effective technology to convert it into liquid fuels and high-value chemicals. This review summarizes and discusses the reaction mechanism, main influence factor and the production application of hydrothermal liquefaction. Particular attention has been paid to the reaction mechanism of the structural components of lignocellulose, i.e., cellulose, hemicellulose, and lignin. In addition, the influence factors including types of lignocellulose, temperature, heating rate, retention time, pressure, solid-to-liquid ratio, and catalyst are discussed in detail. The limitations in the hydrothermal liquefaction of lignocellulose and the prospects are proposed. This provides deep knowledge for understanding the process as well as the development of advanced products from lignocellulose.

Hydrochar derived from municipal sludge through hydrothermal processing: A critical review on its formation, characterization, and valorization

Additional options for the sustainable treatment of municipal sludge are required due to the significant amounts of sludge, high levels of nutrients (e.g., C, N, and P), and trace constituents it contains. Hydrothermal processing of municipal sludge has recently been recognized as a promising technology to efficiently reduce waste volume, recover bioenergy, destroy organic contaminants, and eliminate pathogens. However, a considerable amount of solid residue, called hydrochar, could remain after hydrothermal treatment. This hydrochar can contain abundant amounts of energy (with a higher heating value up to 24 MJ/kg, dry basis), nutrients, and trace elements, as well as surface functional groups. The valorization of sludge-derived hydrochar can facilitate the development and application of hydrothermal technologies. This review summarizes the formation pathways from municipal sludge to hydrochar, specifically, the impact of hydrothermal conditions on reaction mechanisms and product distribution. Moreover, this study comprehensively encapsulates the described characteristics of hydrochar produced under a wide range of conditions: Yield, energy density, physicochemical properties, elemental distribution, contaminants of concern, surface functionality, and morphology. More importantly, this review compares and evaluates the current state of applications of hydrochar: Energy production, agricultural application, adsorption, heterogeneous catalysis, and nutrient recovery. Ultimately, along with the identified challenges and prospects of valorization approaches for sludge-derived hydrochar, conceptual designs of sustainable municipal sludge management are proposed.

Formation of Formic Acid from Glucose with Simultaneous Conversion of Ag2O to Ag under Mild Hydrothermal Conditions

Formation of formic acid from renewable biomass resources is of great interest since formic acid is a widely used platform chemical and has recently been regarded as an important liquid hydrogen carrier. Herein, a novel approach is reported for the conversion of glucose, the constituent carbohydrate from the cellulose fraction of biomass, to formic acid under mild hydrothermal conditions with simultaneous reduction of Ag2O to Ag. Results showed that glucose was selectively converted to formic acid with an optimum yield of 40.7% and glycolic acid with a yield of 6.1% with 53.2% glucose converting to carbon dioxide (CO2) immediately at a mild reaction temperature of 135 °C for 30 min. In addition, Ag2O was used as a solid oxidant for glucose oxidation, which avoids the use of traditionally dangerous liquid oxidant H2O2. Furthermore, complete conversion of Ag2O to Ag can be achieved. This study not only developed a new method for value-added chemical production from renewable biomass but also explored an alternative low-carbon and energy-saving route for silver extraction and recovery.

Catalytic Conversion of Xylose to Furfural by p-Toluenesulfonic Acid (pTSA) and Chlorides: Process Optimization and Kinetic Modeling

Furfural is one of the most promising precursor chemicals with an extended range of downstream derivatives. In this work, conversion of xylose to produce furfural was performed by employing p-toluenesulfonic acid (pTSA) as a catalyst in DMSO medium at moderate temperature and atmospheric pressure. The production process was optimized based on kinetic modeling of xylose conversion to furfural alongwith simultaneous formation of humin from xylose and furfural. The synergetic effects of organic acids and Lewis acids were investigated. Results showed that the catalyst pTSA-CrCl₃·6H₂O was a promising combined catalyst due to the high furfural yield (53.10%) at a moderate temperature of 120 °C. Observed kinetic modeling illustrated that the condensation of furfural in the DMSO solvent medium actually could be neglected. The established model was found to be satisfactory and could be well applied for process simulation and optimization with adequate accuracy. The estimated values of activation energies for xylose dehydration, condensation of xylose, and furfural to humin were 81.80, 66.50, and 93.02 kJ/mol, respectively.

Efficient and Selective Catalytic Conversion of Hemicellulose in Rice Straw by Metal Catalyst under Mild Conditions

Rice straw is an abundant material with the potential to be converted into a sustainable energy resource. Transition-metal catalysis activated the C–O bond in the hemicellulose of raw rice straw, cleaving it to form monosaccharides. The mechanism of rice straw catalytic conversion had a synergistic effect due to in situ acid catalysis and metal catalysis. The conditions for the hydrogenation of hemicellulose from rice straw were optimized: catalyst to rice straw solid/solid ratio of 3:10, stirring speed of 600 r/min, temperature of 160 °C, time of 3 h, solid/liquid ratio of 1:15, and H2 gas pressure of 1.5 MPa. An excellent hemicellulose conversion of 97.3% with the yields of xylose and arabinose at 53.0% and 17.3%, respectively, were obtained. The results from FTIR and SEM experiments also confirmed the destruction of the rigidity and reticulate structure of rice straw after the catalytic reaction.

Production of Levulinic Acid from Cellulose and Cellulosic Biomass in Different Catalytic Systems

The reasonable and effective use of lignocellulosic biomass is an important way to solve the current energy crisis. Cellulose is abundant in nature and can be hydrolyzed to a variety of important energy substances and platform compounds—for instance, glucose, 5-hydroxymethylfurfural (HMF), levulinic acid (LA), etc. As a chemical linker between biomass and petroleum processing, LA has become an ideal feedstock for the formation of liquid fuels. At present, some problems such as low yield, high equipment requirements, difficult separation, and serious environmental pollution in the production of LA from cellulose have still not been solved. Thus, a more efficient and green catalytic system of this process for industrial production is highly desired. Herein, we focus on the reaction mechanism, pretreatment, and catalytic systems of LA from cellulose and cellulosic biomass, and a series of existing technologies for producing LA are reviewed. On the other hand, the industrial production of LA is discussed in depth to improve the yield of LA and make the process economical and energy efficient. Additionally, practical suggestions for the enhancement of the stability and efficiency of the catalysts are also proposed. The use of cellulose to produce LA is consistent with the concept of sustainable development, and the dependence on fossil resources will be greatly reduced through the realization of this process route.

Identification of the Soluble Byproducts Formed during the Hydrothermal Conversion of Cellulose Catalyzed by Solid Tungstated Alumina

The soluble byproducts formed during the hydrothermal conversion of cellulose catalyzed by solid tungstated alumina (AlW) were analyzed by LC-MS and LC-MS² to determine their formulas and possible structures. These identified soluble compounds could be roughly divided into four species of carboxylic acids, α-carbonyl aldehydes, carbocyclic compounds, and furanic compounds with molecular mass in the range of 90-220 Da. Compared with the noncatalytic condition, the addition of AlW could increase the selectivity of carboxylic acids (especially α-hydroxy acid) from cellulose and suppress the formation of furanic compounds, carbocyclic compounds, and hydrochar. Based on the product distribution, the hydrothermal conversion route of glucose was proposed by regarding the formed α-carbonyl aldehydes as the key intermediates for formation of carboxylic acids, carbocyclic compounds, and furanic compounds.

MCM-41 Supported Co-Based Bimetallic Catalysts for Aqueous Phase Transformation of Glucose to Biochemicals

The transformation of glucose into valuable biochemicals was carried out on different MCM-41-supported metallic and bimetallic (Co, Co-Fe, Co-Mn, Co-Mo) catalysts and under different reaction conditions (150 °C, 3 h; 200 °C, 0.5 h; 250 °C, 0.5 h). All catalysts were characterized using N2 physisorption, Temperature Programmed Reduction (TPR), Raman, X-ray Diffraction (XRD) and Temperature Programmed Desorption (TPD) techniques. According to the N2-physisorption results, a high surface area and mesoporous structure of the support were appropriate for metal dispersion, reactant diffusion and the formation of bioproducts. Reaction conditions, bimetals synergetic effects and the amount and strength of catalyst acid sites were the key factors affecting the catalytic activity and biochemical selectivity. Sever reaction conditions including high temperature and high catalyst acidity led to the formation mainly of solid humins. The NH3-TPD results demonstrated the alteration of acidity in different bimetallic catalysts. The 10Fe10CoSiO2 catalyst (MCM-41 supported 10 wt.%Fe, 10 wt.%Co) possessing weak acid sites displayed the best catalytic activity with the highest carbon balance and desired product selectivity in mild reaction condition. Valuable biochemicals such as fructose, levulinic acid, ethanol and hydroxyacetone were formed over this catalyst.

Au/CeO2-ZnO/Al2O3 as Versatile Catalysts for Oxidation Reactions: Application in Gas/Liquid Environmental Processes

The present work showcases the versatility of nanogold systems supported on Zn-doped ceria when applied in two important environmental processes, the total CO oxidation, and the liquid phase oxidation of glucose to gluconic acid. In the CO oxidation the suitability of these materials is clearly demonstrated achieving full conversions even at sub-ambient conditions. Regarding the glucose oxidation our materials display high conversion values (always over 50%) and very importantly full or almost full selectivity toward gluconic acid-an added value platform chemical in the context of biomass upgrading routes. The key factors controlling the successful performance on both reactions are carefully discussed and compared to previous studies in literature. To our knowledge this is one of the very few works in catalysis by gold combining liquid and gas phase reactions and represents a step forward in the flexible behavior of nano gold catalysts.

Recent progress in homogeneous Lewis acid catalysts for the transformation of hemicellulose and cellulose into valuable chemicals, fuels, and nanocellulose

The evolution from petroleum-based products to the bio-based era by using renewable resources is one of the main research challenges in the coming years. Lignocellulosic biomass, consisting of inedible plant material, has emerged as a potential alternative for the production of biofuels, biochemicals, and nanocellulose-based advanced materials. The lignocellulosic biomass, which consists mainly of carbohydrate-based polysaccharides (hemicellulose and cellulose), is a green intermediate for the synthesis of bio-based products. In recent years, the re-engineering of biomass into a variety of commodity chemicals and liquid fuels by using Lewis acid catalysts has attracted much attention. Much research has been focused on developing new chemical strategies for the valorization of different biomass components. Homogeneous Lewis acid catalysts seem to be one of the most promising catalysts due to their astonishing features such as being less corrosive to equipment and being friendlier to the environment, as well as having the ability to disrupt the bonding system effectively and having high selectivity. Thus, these catalysts have emerged as important tools for the highly selective transformation of biomass components into valuable chemicals and fuels. This review provides an insightful overview of the most important recent developments in homogeneous Lewis acid catalysis toward the production and upgrading of biomass. The chemical valorization of the main components of lignocellulosic biomass (hemicellulose and cellulose), the reaction conditions, and process mechanisms are reviewed.

Kinetics study of levulinic acid production from corncobs by tin tetrachloride as catalyst

Levulinic acid (LA) is an ideal platform chemical that can be produced through acid-catalyzed dehydration and hydrolysis of hexose sugars obtained from lignocellulosic materials. In this study, SnCl₄ was identified as an efficient catalyst for LA production and the reaction kinetics was investigated in a single water phase under different reaction conditions. The Box-Behnken design response surface methodology (RSM) was applied to determine the optimized reaction conditions and three individual variables including reaction temperature, duration, and catalyst concentration were evaluated. An appealing LA yield of 76.0% was achieved at 193 °C and 17 min with 82 mM SnCl₄ catalyst. A kinetics model was developed to predict the yields of glucose, HMF, and LA, which are tally with the experimental results. The analysis of the related kinetic parameters and the results of the RSM experiment helped to provide insights into the interplay between various reaction steps with SnCl₄ as catalysts.

A comparative study for the organic byproducts from hydrothermal carbonizations of sugarcane bagasse and its bio-refined components cellulose and lignin

Sugarcane bagasse was refined into cellulose, hemicellulose, and lignin using an ethanol-based organosolv technique. The hydrothermal carbonization (HTC) reactions were applied for bagasse and its two components cellulose and lignin. Based on GC-MS analysis, 32 (13+19) organic byproducts were derived from cellulose and lignin, more than the 22 byproducts from bagasse. Particularly, more valuable catechol products were obtained from lignin with 56.8% share in the total GC-MS integral area, much higher than the 2.263% share in the GC-MS integral areas of bagasse. The organic byproducts from lignin make up more than half of the total mass of lignin, indicating that lignin is a chemical treasure storage. In general, bio-refinery and HTC are two effective techniques for the valorization of bagasse and other biomass materials from agriculture and forest industry. HTC could convert the inferior biomass to superior biofuel with higher energy quantity of combustion, at the same time many valuable organic byproducts are produced. Bio-refinery could promote the HTC reaction of biomass more effective. With the help of bio-refinery and HTC, bagasse and other biomass materials are not only the sustainable energy resource, but also the renewable and environment friendly chemical materials, the best alternatives for petroleum, coal and natural gas.

Hydrothermal conversion of glucose to levulinic acid using multifunctional ionic liquids: effects of metal ion co-catalysts on the product yield

An efficient catalytic system comprising Bronsted acidic ionic liquids and Lewis acidic metal salts for hydrothermal glucose conversion to platform chemicals.

Cr(VI) reduction and immobilization by novel carbonaceous modified magnetic Fe3O4/halloysite nanohybrid

In this work, a novel "Dumbbell-like" magnetic Fe3O4/Halloysite nanohybrid (Fe3O4/HNTs@C) with oxygen-containing organic group grafting on the surface of natural halloysite nanotubes (HNTs) and homogeneous Fe3O4 nanospheres selectively aggregating at the tips of modified halloysite nanotubes was successfully synthesized. XRD, TEM, IR spectroscopy, XPS and VSM were used to characterize this newly halloysite nanohybrid and its formation mechanism was discussed. Cr(VI) ions adsorption experiments showed that the Fe3O4/halloysite nanohybrid exhibited higher adsorption ability with a maximum adsorption capacity of 132 mg/L at 303K, which is about 100 times higher than that of unmodified halloysite nanotubes. More importantly, with the reduction of Fe3O4 and electron-donor effect of oxygen-containing organic groups, Cr(VI) ions were easily reduced into low toxicity Cr(III) and then adsorbed onto the surface of halloysite nanohybrid. In addition, appreciable magnetization was observed due to the aggregation of magnetite nanoparticles, which make adsorbent facility separated from aqueous solutions after Cr pollution adsorption.

Organic Carbonates: Efficient Extraction Solvents for the Synthesis of HMF in Aqueous Media with Cerium Phosphates as Catalysts

We describe a process for the selective conversion of C6 -polyols into 5-hydroxymethylfurfural (5-HMF) in biphasic systems of organic carbonate/water (OC/W), with cerium(IV) phosphates as catalysts. Different reaction parameters such as the OC/W ratio, catalyst loading, reaction time, and temperature, were investigated for the dehydration of fructose. Under the best reaction conditions, a yield of 67.7 % with a selectivity of 93.2 % was achieved at 423 K after 6 h of reaction using [(Ce(PO4)1.5 (H2 O)(H3 O)0.5 (H2 O)0.5)] as the catalyst. A maximum yield of 70 % with the same selectivity was achieved after 12 h. At the end of the reaction, the catalyst was removed by centrifugation, the organic phase was separated from water and evaporated in vacuo (with solvent recovery), and solid 5-HMF was isolated (purity >99 %). The recovery and reuse of the catalyst and the relationship between the structure of the OC and the efficiency of the extraction are discussed. The OC/W system influences the lifetime of the catalysts positively compared to only water.