Aqueous Phase Conversion of Hexoses into 5-Hydroxymethylfurfural and Levulinic Acid in the Presence of Hydrochloric Acid: Mechanism and Kinetics

Characterization, ultrafiltration, depolymerization and gel formulation of ulvans extracted via a novel ultrasound-enzyme assisted method

Sea lettuce, or Ulva spp., dominates global algal biomass and significantly contributes to "green tides.", representing a sustainable source for biomaterials. This study explores an innovative ultrasound-enzyme assisted extraction method with the novel Cellic® CTEC3 enzyme cocktail, applied for the first time in Ulva spp. succesfully enhancing ulvan release and extraction efficiency. Various processing methods, including ultrafiltration and dialysis, were employed to achieve higher ulvan purity. Dialyzation of ulvan resulted in a more purified product with a carbohydrate content up to 55.34 %, a sulfate content up to 21 %, and no glucose contamination. Liquid extracts were fractionated through ultrafiltration, with a 3 kDa MWCO yielding 93.51 % ulvan precipitate, representing 50.28 % of the total extractable ulvan. Sequential ultrafiltration concentrated ulvans but only partially modified their molecular weight distribution. Depolymerization using microwave and H2O2 shifted ulvans towards lower molecular weights, reducing high molecular weight residue. HPSEC confirmed pH-dependent aggregation behavior, with all isolated ulvans having molecular weights above 786 kDa. Hydrolysis methods were compared, with 2-hour 1 M TFA hydrolysis at 121 °C providing the best monosaccharide profile of ulvan. FTIR and NMR analyses showed preservation of sulfation. Rheology indicated biopolymeric behavior and stable gel formation. Ulvans demonstrated nutraceutical potential, being suitable for a low Na+ and high K+ diet, with a Na+:K+ ratio as low as 0.14, and were rich in Mg2+.

Efficient Conversion of Glucose to Hydroxymethylfurfural: One-pot Brønsted Base and Acid Promoted Selective Isomerization and Dehydration

Development of elegant, selective, and efficient strategies for the production of value-added platform chemicals from renewable feedstocks are in high demand to achieve the future needs and sustainable goals. In this context, an efficient acid-promoted synthesis of highly valuable hydroxymethylfurfural (HMF) has been demonstrated from glucose, a major constituent of lignocellulosic biomass. The major challenge in the conversion of glucose to HMF is the selective isomerization of glucose to ketose, which in the present work has been successfully addressed through the amine-mediated rearrangement of glucose to aminofructose under Amadori rearrangement. Importantly, subsequent dehydration step affords HMF and regenerates the amine employed in the first step, which could be readily recovered. In addition, scale-up and successful integration into one-pot synthesis of HMF proves the efficiency and applicability of the present transformation in large scale application. In addition, the method was also successfully extended to other monosaccharides and disaccharides to produce HMF.

Precursor-Driven Catalytic Performances of Al2O3-Supported Earth-Abundant Ni Catalysts in the Hydrogenation of Levulinic Acid and Hydroxymethylfurfural into Added-Value Chemicals

It has been shown that the nature of the metal precursor and the thermal effects during calcination determine the physicochemical properties of the catalysts and their catalytic activity in the levulinic acid (LA) and 5-hydroxymethylfurfural (HMF) hydrogenation reactions. The endothermic effect during calcination of the inorganic nickel precursor promoted higher metal dispersion and stronger interaction with the alumina surface. In contrast, the exothermic effects during the calcination of organic nickel precursors resulted in smaller metal dispersion and lower interaction with the support surface. A clear relationship was found between the size of the metal crystallites and the yield of LA hydrogenation reaction. The smaller crystallites were more active in the LA hydrogenation reaction. In turn, the size of the metal particles and their nature of interaction with the surface of the alumina influence the hydrogenation pathways of the HMF.

Integrated Preparation of Hollow Lignin Nanoparticles as a Drug Carrier and Levulinic Acid from the Poplar Wood Prehydrolysis Liquor

Prehydrolysis liquid (PHL) from dissolving pulp and biorefinery industries is rich in saccharides and lignin, being considered as a potential source of value-added materials and platform molecules. This study proposed an environmentally friendly and simple method to prepare morphologically controllable hollow lignin nanoparticles (LNPs) and levulinic acid (LA) from PHL. In the first step, after hydrothermal treatment of PHL with p-toluenesulfonic acid (p-TsOH), lignin with a uniform molecular weight was obtained to prepare LNPs. The prepared LNPs have an obvious hollow structure, with an average size of 490-660 nm, and exhibit good stability during 30 days of storage. When the as-obtained LNPs were used as a sustained-release agent for amikacin sulfate, the encapsulation efficiency reached over 70% and the release efficiency within 40 h reached 69.2% in a pH 5.5 buffer. Subsequently, the remaining PHL that contains saccharides was directly used for LA production under the catalysis of p-TsOH. At 150 °C for 1.5 h, the LA yield reached 58.4% and remained at 56% after 5 cycles of p-TsOH. It is worth noting that only p-TsOH was used as a reactive reagent throughout the entire preparation process. Overall, this study provided a novel pathway for the integrated utilization of PHL and showed the immense potential of the preparation and application of LNPs.

Integrated preparation of functional lignin nanoparticles and levulinic acid directly from the pre-hydrolysis liquor of poplar wood

The pre-hydrolysis liquor (PHL) produced during pulp dissolution and biomass refining is mainly composed of hemicellulose and lignin, and it is a potential source for production of value-added materials and platform chemicals; however, their utilization has been a serious challenge. In this study, we proposed a green and simple strategy to simultaneously prepare size-controlled functional lignin nanoparticles (LNPs) and levulinic acid (LA) from PHL as the raw material. The as-prepared LNPs exhibited remarkable stability thanks to the presence of saccharides with abundant oxygen-containing groups and surface charges, which prevented aggregation and maintained long-term storage stability. Trace amounts of the LNPs (≤ 0.2 wt%) could stabilize various Pickering emulsions, even with oil-to-water ratios as high as 5:5 (v/v). Subsequently, the remaining PHL was directly used to produce LA without adding a catalyst; under optimal conditions (160 °C and 1 h), the yield of LA was 56.3 % based on the dry saccharide content in the raw PHL. More importantly, p-toluenesulfonic acid (p-TsOH), the only reactive reagent used during the entire preparation process, including the two preparation steps of the LNPs and LA, was reusable, and the recovery rate was >70 % after five cycles. Overall, this green and simple strategy effectively and comprehensively utilized the PHL and showed potential for producing biobased nanomaterials and platform chemicals.

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.

Simultaneous quantification of total carbohydrate and protein amounts from aqueous solutions by the sulfuric acid ultraviolet absorption method (SA-UV method)

Based on the sulfuric acid-ultraviolet assay (SA-UV, developed by Albalasmeh et al., 2013), we have further expanded this method for the simultaneous quantification of saccharides (carbohydrates) and proteins by ultraviolet spectrophotometry. The absorbance of saccharides depends on the formation of furfurals by dehydration in the presence of concentrated sulfuric acid, whereas proteins are unaffected and can be quantified by UV active peptide bonds and aromatic amino acid residues. In saccharide/protein mixtures the SA-UV assay offers a good alternative and substitutes the need for two different methods, like the phenol-sulfuric acid (PSA, developed by DuBois et al., 1951) and bicinchoninic acid (BCA, developed by Smith et al., 1985) assays. For the development of this method, we used glucose and BSA as model substrates and performed a method validation in terms of linearity, LOD, LOQ, accuracy, and precision. Simultaneous quantification in glucose/BSA mixtures is possible down to 20 mg/L from 30 μL sample volumes, and even low content mixtures with concentrations down to 2 mg/L can appropriately be quantified from higher volumes by an evaporation technique.

Sulfonic Derivatives as Recyclable Acid Catalysts in the Dehydration of Fructose to 5-Hydroxymethylfurfural in Biphasic Solvent Systems

Biphasic systems have received increasing attention for acid-catalyzed dehydration of hexoses to 5-hydroxymethylfurfural (HMF) because of their high efficiency in in situ extraction and stabilization of HMF. Different organic solvents and acid catalysts were applied in these systems, but their effects on the dehydration activity and HMF yield, and the recycling of homogeneous acid catalysts remain largely unexplored. Here, we tested different solvent systems containing a wide range of organic solvents with low boiling points to study the effects of their chemical structures on fructose dehydration and provided stable H₂O-dioxane and H₂O-acetonitrile biphasic systems with high HMF yields of 76-79% using water-soluble sulfonic derivatives as homogeneous acid catalysts under mild conditions (383 K). By analyzing the partition coefficients of HMF and sulfonic derivatives, 94.3% of HMF and 87.1% of NH₂SO₃H were, respectively, restrained in the dioxane phase and aqueous phase in the H₂O-dioxane biphasic system and easily divided by phase separation. The effects of the adjacent group in sulfonic derivatives and reaction temperature on fructose conversions and HMF yields suggest that in a specific biphasic system, the catalysts' acidity and reaction conditions significantly affect the fructose dehydration activity but hardly influence the optimal yield of HMF, and an almost constant amount of carbon loss was observed mainly due to the poor hydrothermal stability of fructose. Such developments offer a promising strategy to address the challenge in the separation and recycling of homogeneous acid catalysts in the practical HMF production.

Influence of delignification and reaction conditions in the aqueous phase transformation of lignocellulosic biomass to platform molecules

The effect of oxidative and reductive delignification processes on the hydrolysis of pine sawdust at mild conditions (200-1000 ppm of HCl and 140-220 °C) is studied in this work. Dimers and reduced sugars are the main products obtained with the fresh sawdust (>82%), reaching a maximum liquid phase yield of 17% after 8 h, at the strongest conditions. This conversion increases up to almost 40% with the pretreated sawdust, obtaining selectivities higher than 87% of levulinic acid and a well-defined distribution of the relevant platform molecules (sugars, HMF, furfural, levulinic acid) as function of the severity of the reaction, decreasing the humins formation and being possible to define different conditions to maximize each yield. These conclusions were corroborated by the kinetic analysis, obtaining a clear decrease in the energy activation for all the individual steps involved in this process.

Preparation of kapa carrageenan-based acidic heterogeneous catalyst for conversion of sugars to high-value added materials

A novel composite based on kappa-Carrageenan (κC) was prepared using N,N-methylene bisacrylamide (MBA) as the crosslinking agent. 5-Hydroxymethylfurfural (5-HMF) was produced by catalytic dehydration of fructose and glucose with MBA grafted κC (κC-g-MBA) as the solid acid catalyst due to sulfonic acid groups in biopolymer skeletons. Various reaction parameters such as optimization of the quantity of the catalyst, temperature, reaction time, and solvent were performed. It was established that for fructose dehydration, the best reaction conditions were the 160 °C as the optimized reaction temperature and 1 h reaction time, respectively. Under these conditions, the HMF yield and fructose conversion were 94.2% and 95.5%, respectively. Furthermore, 160 °C and 2 h were the best reaction temperature and reaction time for glucose dehydration, respectively. Under similar conditions, the HMF yield and glucose conversion are 47% and 93%, respectively. The catalyst was readily prepared from inexpensive materials with considerable reusability and reactivity.

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.

Current Situation of the Challenging Scale-Up Development of Hydroxymethylfurfural Production

Hydroxymethylfurfural (HMF) is a high-value platform chemical derived from renewable resources. In recent years, considerable efforts have been made to produce HMF also at industrial scale, which still faces some challenges regarding yield as well as sustainable and economic process designs. This critical Review evaluates the industrial process development of sustainable biomass conversion to HMF. Qualitative and quantitative guidelines are defined for the technological assessment of the processes described in patent literature. The formation of side products, difficulties in the separation and purification of HMF as well as catalyst regeneration were identified as major challenges in the HMF production. A first small-scale, commercial HMF production plant with a capacity of 300 tHMF per year has been operating in Switzerland since 2014.

Clean Production of Levulinic Acid from Fructose and Glucose in Salt Water by Heterogeneous Catalytic Dehydration

Levulinic acid (LA) is considered to be one of the promising organic bio-platform chemicals and intermediates for the synthesis of fuels, chemicals, and polymers. In the present study, heterogeneous catalytic dehydration of hexose sugars, fructose and glucose, using a strong cation exchange resin (hydrogen form) as an acid catalyst, was performed to produce LA in an aqueous medium. The effect of salts such as NaCl, KCl, CaCl₂, Na₂CO₃, and Na₂SO₄ in the medium on the rate of sugar conversion and LA yield was evaluated. Under optimum reaction conditions, 10% (w/w) fructose was dehydrated to LA (with 74.6% yield) in 10% (w/w) NaCl aqueous solution in 24 h at 110 °C using the catalyst at 30% (w/w sugar). Even 10% (w/w) glucose monohydrate was directly dehydrated to LA (with 70.7% yield) under similar conditions but at 145 °C. This study shows that the salts enhance the rate of catalytic dehydration in the order of Cl^- > CO₃ ^2- > SO₄ ^2-. Thus, the combination of high sugar concentration and heterogeneous catalysis in an aqueous system under relatively mild conditions could provide a high-yielding and sustainable process for bio-based LA production.

Heparin Immobilized on Multiwalled Carbon Nanotubes for Catalytic Conversion of Fructose in Water with High Yield and Selectivity

Being a member of the glycosaminoglycan family of carbohydrates, native heparin is a highly sulfated polysaccharide. Herein, heparin was grafted onto polydopamine (PDA)- and poly(ethylene imine) (PEI)-coated multiwalled carbon nanotubes (MWCNTs) (heparin-PEI@PDA@MWCNT). The immobilized heparin consists of a sulfated repeating disaccharide unit, conferring a unique microenvironment when catalyzing fructose dehydration into 5-hydroxymethylfurfural (HMF). The hydrogen bonding interactions naturally occur between the disaccharide unit of heparin and the monosaccharide fructose, and the adjacent sulfonic acid groups catalyze the fructose dehydration. The reactions were performed in water, and heparin-PEI@PDA@MWCNT achieved an HMF yield of 46.2% and an HMF selectivity of 82.2%. For the dehydration of fructose in water, heparin-PEI@PDA@MWCNT exhibits advantages over published heterogeneous catalysts on the basis of HMF yield and HMF selectivity. Three aspects contribute to the environmentally benign processing: (1) the catalyst heparin is a natural sulfated polysaccharide; (2) the catalysis is carried out in water and not in organic solvents; and (3) fructose can be produced from a biomass resource.

Carbon Materials as Phase-Transfer Promoters for Obtaining 5-Hydroxymethylfurfural from Cellulose in a Biphasic System

Different carbonaceous materials were tested as mass-transfer promoters for increasing the yield of 5-hydroxymethylfurfural (5-HMF) in biphasic cellulose hydrolysis. The benefits of working with a biphasic system (water/methyl isobutyl ketone) under soft acid conditions were taken as starting point (no humins or levulinic acid production), with slow extraction kinetics as the weakest point of this approach. Carbon nanotubes (CNTs) and activated carbon (AC) were proposed to improve 5-HMF liquid-liquid mass transfer. A kinetic analysis of the extraction process indicated the competition between 5-HMF and glucose adsorption as the main cause of the poor results obtained with AC. In contrast, very promising results were obtained with CNTs, mainly at 1.5 wt % loading, with complete transfer of HMF and a high global mass-transfer coefficient. The use of CNTs improved the amount of 5-HMF in the organic phase by more than 270 %.

Hydrothermal carbonization of sewage sludge: A critical analysis of process severity, hydrochar properties and environmental implications

Hydrothermal carbonization (HTC) of sewage sludge reduces the waste volume and can be source of energy and valuable products. Furthermore, HTC offers several advantages over conventional dry-thermal pre-treatments, as no prior drying is requested, and the high quality of the char produced promotes applications as energy production and storage, wastewater remediation, and soil amendment. Relationships between char yields, physicochemical properties and process parameters are here analysed, with the aim to provide insight into the choice of the process severity required to fit the desired application. Moreover, presence and fate of heavy metals and organic contaminants are discussed. The highest reaction temperature is the main parameter affecting the physicochemical characteristics of the char produced, while the heating rate governs the heat mass transfer and the rate of intermediates formation. Depolymerization of the biomass results in a reduction of the oxygen to carbon ratio and, therefore, in augmented high heating values, further increased by deposition of 5-(hydroxymethyl)furfural. Recirculation of process water may enhance dehydration reactions and the deposition of degraded polymers, increasing dewaterability and yield, but field trials are recommended to assess the feasibility of this option. An overuse of chars for energy generation purposes would be deleterious for the environmental life cycle. Further research is encouraged to assess the pollutants abatement and their degradation pathways when incorporated in the carbonaceous product, to promote the application of hydrochars as soil amendment, as well as for environmental remediation purposes.

Dehydration of fructose, sucrose and inulin to 5-hydroxymethylfurfural over yeast-derived carbonaceous microspheres at low temperatures

This work prepared carbonaceous microspheres by hydrothermal carbonization of yeast cells followed by sulfonation with concentrated sulphuric acid (98%) at room temperature. The obtained carbonaceous product (CM-SO₃H) had a high acid density (1.80 mmol g⁻¹). We evaluated CM-SO₃H as a solid catalyst for the dehydration of fructose-based carbohydrates to 5-hydroxymethylfurfural (5-HMF) in the ionic liquid 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]). The effects of the catalyst and substrate loadings as well as the reaction temperature and time on the yield of 5-HMF were investigated. Under the optimum conditions, a 5-HMF yield of up to 83.5% was obtained from fructose with a reaction temperature of 80 °C for 30 min. Furthermore, 44.8% and 59.2% 5-HMF yields were obtained from sucrose (80 °C for 30 min) and inulin (80 °C for 60 min), respectively. CM-SO₃H and [BMIM][Cl] showed high stability and could be recycled between five and eight times without significant loss of catalytic activity. More importantly, the catalytic system could be applied to high substrate concentrations. CM-SO₃H combined with [BMIM][Cl] is a promising system for transforming fructose-based carbohydrates into 5-HMF.

This work prepared carbonaceous microspheres by hydrothermal carbonization of yeast cells followed by sulfonation with concentrated sulphuric acid (98%) at room temperature.

Starbon/High-Amylose Corn Starch-Supported N-Heterocyclic Carbene-Iron(III) Catalyst for Conversion of Fructose into 5-Hydroxymethylfurfural

Iron-N-heterocyclic carbene complexes (Fe-NHCs) have come to prominence because of their applicability in diverse catalytic reactions, ranging from C-C cross-coupling and C-X bond formation to substitution, reduction, polymerization, and dehydration reactions. The detailed synthesis, characterization, and application of novel heterogeneous Fe-NHC catalysts immobilized on mesoporous expanded high-amylose corn starch (HACS) and Starbon 350 (S350) for facile fructose conversion into 5-hydroxymethylfurfural (HMF) is reported. Both catalyst types showed good performance for the dehydration of fructose to HMF when the reaction was tested at 100 °C with varying time (10 min, 20 min, 0.5 h, 1 h, 3 h and 6 h). For Fe-NHC/S350, the highest HMF yield was 81.7 % (t=0.5 h), with a TOF of 169 h^-1 , fructose conversion of 95 %, and HMF selectivity of 85.7 %, whereas for Fe-NHC/expanded HACS, the highest yield was 86 % (t=0.5 h), with a TOF of 206 h^-1 , fructose conversion of 87 %, and HMF selectivity of 99 %. Iron loadings of 0.26 and 0.30 mmol g^-1 were achieved for Fe-NHC/expanded starch and Fe-NHC/S350, respectively.

Valorization of an underused sugar derived from hemicellulose: efficient synthesis of 5-hydroxymethylfurfural from mannose with aluminum salt catalyst in dimethyl sulfoxide/water mixed solvent

Mannose, an underused sugar derived from hemicellulose, can be effectively converted into 5-hydroxymethylfurfural (5-HMF) with aluminum chloride catalyst under mild conditions, offering useful reference for the refinery of biomass in the future.