Synthesis of furfural from xylose, xylan, and biomass using AlCl3·6H2O in biphasic media via xylose isomerization to xylulose

Efficient separation of lignocellulose component and furfural production from hemicellulose using a γ-valerolactone/H2O system

This study investigates a one-step process for the simultaneous separation of cellulose, hemicellulose, and lignin from lignocellulosic biomass using γ-valerolactone (GVL)/H2O pretreatment, which also converts hemicellulose to furfural (FF). A response surface methodology (RSM) was applied to optimize the process parameters. Under optimal conditions (163 °C for 1.5 h), the maximum FF yield reached 85.36 %, and cellulose purity was 80.80 %. The extracted GVL-lignin was characterized as low-molecular-weight guaiacyl-syringyl (G-S) type nanospheres with significant antioxidant activity. The conversion of hemicellulose to FF was enhanced by the synergistic interaction of Lewis acid (MgCl2) and Brønsted acid (H2SO4), which promoted the isomerization of xylose and dehydration to FF. The high-purity cellulose yielded 18.87 g/L ethanol through separate hydrolysis and fermentation (SHF), 27 times higher than untreated solids. Additionally, GVL exhibited consistent performance in lignocellulose separation after six cycles. This method offers an efficient and sustainable approach to valorize lignocellulosic biomass, advancing the production of high-value products.

Roles of carbon dioxide in the conversion of biomass or waste plastics

Under the current trend of pursuing sustainable development and environmental protection, the important application of carbon dioxide (CO2) in the conversion process of biomass or waste plastics has become a research direction of concern. The goal of this conversion process is to achieve the efficient use of carbon dioxide, providing a process for the efficient use of biomass, and solving the environmental problems caused by plastics. Remarkable progress has been made in the study of the reaction of CO2 with other substances to produce methane, low-carbon hydrocarbons, methanol, formic acid, and its derivatives, as well as ethers, aldehydes, gasoline, low-carbon alcohols, and other chemicals. In this paper, the important role of CO2 in the conversion of alcohol, sugar, cellulose, and waste plastics was reviewed, with emphasis on the important applications of CO2 as a carbon source, reactant, reaction medium, enhancing agent, solvent, and carrier gas in the conversion of biomass or waste plastics and the basic insights of the reaction mechanism. The emerging CO2 new roles not only put forward the green application of CO2 but also have guiding significance for the utilization of biomass resources and the treatment of waste plastics.

Catalytic Production and Upgrading of Furfural: A Platform Compound

Furfural is a renewable platform compound that can be derived from lignocellulosic biomass. The highly functionalized molecular structure of furfural enables us to prepare a variety of high value-added chemicals, which will help realize biomass high-value utilization, and alleviate energy and environmental problems. This paper reviews the research progress on furfural production and upgrading to C5 chemicals from the catalyst perspective. The emphasis is placed on summarizing and refining the catalytic mechanism and in-depth analysis of available data. Specifically, the reaction mechanism of furfural production and upgrading is summarized firstly from the perspective of reaction pathways and reaction kinetics. Then, the available data are further processed to evaluate the actual reaction efficiency of different catalytic systems from multiple dimensions. Finally, based on statistical analysis, the challenges and opportunities of furfural-based research are proposed.

One-pot, microwave (MW)-assisted production of furfural from almond-, oil-, and wine-derived co-products through biorefinery-based approaches

This work outlines the first microwave (MW)-assisted protocol for the production of biofuel precursor furfural (FF) from the raw agricultural waste almond hull (AH), olive stone (OS), and the winemaking-derived grape stalk (GS), grape marc (GM) and exhausted grape marc (EGM) through a one-pot synthesis process. To enhance the overall yield, a catalytic process was firstly developed from xylose, major constituent of hemicellulose present in lignocellulosic biomass. This method afforded FF with 100 % selectivity, yielding over 85 % in isolated product when using H2SO4, as opposed to a 37 % yield with AlCl3·6H2O, at 150 °C in only 10 min. For both catalysts, the developed methodology was further validated, proving adaptable and efficient in producing the targeted FF from the aforementioned lignocellulosic raw materials. More specifically, the employment of AlCl3·6H2O resulted in the highest selectivity (up to 89 % from GM) and FF yield (42 % and 39 % molar from OS and AH, respectively), maintaining notable selectivity for the latter (61 and 48 % from AH and OS). At this regard, and considering the environmental factor of sustainability, it is important to point out the role of AlCl3·6H2O in contrast to H2SO4, thus mitigating detrimental substances. This study provides an important management of agricultural waste through sustainable practises for the development of potential bio-based chemicals, aligning with Green Chemistry and process intensification principles.

Complete Low-Temperature Transformation and Dissolution of the Three Main Components in Corn Straw

The conversion of biomass faces the challenge of mass and heat transfer, as well as the exertion of heterogeneous catalyst, because raw biomass exists usually in solid state. In this work, the simultaneous transformation and dissolution of the three main components (hemicellulose, cellulose, lignin) in corn straw were achieved in ethanol/ valerolactone (GVL)/H2 O (10 : 10 : 40, v/v/v) co-solvent system. With the assistance of AlCl3  ⋅ 6H2 O, the conversion of hemicellulose, lignin and cellulose was >96 % at 170 °C. The conversion of solid biomass into fluid, overcoming the mass transfer restrictions between solid biomass and solid catalysts, provides new raw materials to further upgrading. H2 O could penetrate inside the crystalline cellulose to swell even dissolve it, while ethanol and GVL acted as media to dissolve especially the G unit in lignin. The H+ derived from AlCl3  ⋅ 6H2 O hydrolysis could break the linkages of lignin-hemicellulose and glycosidic bond in saccharides, and aluminum chloride promoted the next degradation of polysaccharides to small molecules. Consequently, as high as 33.2 % yield of levulinic acid and 42.2 % yield of furfural were obtained. The cleavage of β-O-4 and Cβ -Cγ bonds in lignin produced large amounts of lignin-derived dimers and trimers. The total yield of monomeric phenols is up to 8 %.

Applications of biomass-derived solvents in biomass pretreatment - Strategies, challenges, and prospects

Biomass pretreatment is considered a key step in the 2nd generation biofuel production from lignocellulosic biomass. Research on conventional biomass pretreatment solvents has mainly been focused on carbohydrate conversion efficiency, while their hazardousness and/or carbon intensity were not comprehensively considered. Recent sustainability issues request further consideration for eco-friendly and sustainable alternatives like biomass-derived solvents. Carbohydrate and lignin-derived solvents have been proposed and investigated as green alternatives in many biomass processes. In this review, the applications of different types of biomass pretreatment solvents, including organic, ionic liquid, and deep eutectic solvents, are thoroughly discussed. The role of water as a co-solvent in these pretreatment processes is also reviewed. Finally, current research challenges and prospects of utilizing biomass-derived pretreatment solvents for pretreatment are discussed. Given bioethanol's market potential and increasing public awareness about environmental concerns, it will be a priority adopting sustainable and green biomass pretreatment solvents in biorefinery.

Towards efficient and greener processes for furfural production from biomass: A review of the recent trends

As mentioned in several recent reviews, biomass-based furfural is attracting increasing interest as a feasible alternative for the synthesis of a wide range of non-petroleum-derived compounds. However, the lack of environmentally friendly, cost-effective, and sustainable industrial procedures is still evident. This review describes the chemical and biological routes for furfural production. The mechanisms proposed for the chemical transformation of xylose to furfural are detailed, as are the current advances in the manufacture of furfural from biomass. The main goal is to overview the different ways of improving the furfural synthesis process. A pretreatment process, particularly chemical and physico-chemical, enhances the digestibility of biomass, leading to the production of >70 % of available sugars for the production of valuable products. The combination of heterogeneous (zeolite and polymeric solid) catalyst and biphasic solvent system (water/GVL and water/CPME) is regarded as an attractive approach, affording >75 % furfural yield for over 80 % of selectivity with the possibility of catalyst reuse. Microwave heating as an activation technique reduces reaction time at least tenfold, making the process more sustainable. The state of the art in industrial processes is also discussed. It shows that, when sulfuric acid is used, the furfural yields do not exceed 55 % for temperatures close to 180 °C. However, the MTC process recently achieved an 83 % yield by continuously removing furfural from the liquid phase. Finally, the CIMV process, using a formic acid/acetic acid mixture, has been developed. The economic aspects of furfural production are then addressed. Future research will be needed to investigate scaling-up and biological techniques that produce acceptable yields and productivities to become commercially viable and competitive in furfural production from biomass.

Chemoenzymatic catalytic synthesis of furfurylamine from hemicellulose in biomasses

Recently, efficient synthesis of furan-based chemicals from biomacromolecule via chemoenzymatic approaches have been widely recognized. In this work, an efficient conversion of biomacromolecule (e.g., xylan in biomass) to furfurylamine (FLA) was developed in a tandem reaction by bridging with chemocatalysis and biocatalysis. Various biomasses (e.g., corncob, bagasse, bamboo shoot shell, corn stalk, rice straw stalk, reed, water bamboo and sunflower stalk) could produce different titer of furfural due to the diverse xylan content in biomass. After being catalyzed by shrimp shell-supported solid acid catalyst (Sn-DAT-SS) in deep eutectic solvent choline chloride:ethylene glycol (ChCl:EG) - water (10:90, v/v) at 170 °C after 30 min, corncob gave the highest furfural yield of 52.4 %. The potential catalytic mechanism for Sn-DAT-SS-catalyzing the conversion of biomass into furfural in ChCl:EG - water was proposed. It was found that by-products (formic acid, levulinic acid, 5-hydroxymethylfurfural) and soluble sugars (glucose, xylose, arabinose, cellobiose) produced during the conversion of biomass to furfural had certain inhibition effects on the biotransamination of furfural to FLA. Biomass-derived furfural (36.7-92.3 mM) could be fully aminated to FLA by E. coli CCZU-XLS160 cells harboring ω-transaminase after 24-72 h. The established chemoenzymatic strategy for converting biomacromolecules into valuable furan-based products was successfully developed in an eco-friendly system.

Efficient Reaction Systems for Lignocellulosic Biomass Conversion to Furan Derivatives: A Minireview

Lignocellulosic biomass as abundant, renewable, and sustainable carbon feedstock is an alternative to relieve the dependence on fossil fuels and satisfy the demands of chemicals and materials. Conversions of lignocellulosic biomass to high-value-added chemicals have drawn much attention recently due to the high availability of sustainable ways. This minireview surveys the recent trends in lignocellulosic biomass conversion into furan derivatives based on the following systems: (1) ionic liquids, (2) deep eutectic solvents, and (3) biphasic systems. Moreover, the current challenges and future perspectives in the development of efficient routes for lignocellulosic biomass conversion are provided.

Recent Advances in the Catalytic Conversion of Biomass to Furfural in Deep Eutectic Solvents

Lignocellulose is recognized as an ideal raw material for biorefinery as it may be converted into biofuels and value-added products through a series of chemical routes. Furfural, a bio-based platform chemical generated from lignocellulosic biomass, has been identified as a very versatile alternative to fossil fuels. Deep eutectic solvents (DES) are new “green” solvents, which have been employed as green and cheap alternatives to traditional organic solvents and ionic liquids (ILs), with the advantages of low cost, low toxicity, and biodegradability, and also have been proven to be effective media for the synthesis of biomass-derived chemicals. This review summarizes the recent advances in the conversion of carbohydrates to furfural in DES solvent systems, which mainly focus on the effect of adding different catalysts to the DES system, including metal halides, water, solid acid catalyst, and certain oxides, on the production of furfural. Moreover, the challenges and perspectives of DES-assisted furfural synthesis in biorefinery systems are also discussed in this review.

Continuous hydrothermal furfural production from xylose in a microreactor with dual-acid catalysts

An effective continuous furfural production from xylose in a microreactor over dual-acid catalysts was proposed. In this work, furfural was synthesized in an organic solvent-free system using formic acid and aluminum chloride as catalyst. The role of these catalysts in the consecutive reactions was examined and verified. The influence of operating conditions including xylose concentration, reaction temperature, residence time, total catalyst concentration, and catalyst ratio on the yield of furfural was investigated and optimized. The furfural yield of 92.2% was achieved at the reaction temperature of 180 °C, residence time of 15 min, catalyst molar ratio of 1 : 1, xylose concentration of 1 g L⁻¹, and total catalyst concentration of 16 mM. The superior production performance of our process was highlighted in terms of the low catalyst concentration and short residence time compared to those of other systems based on the literature. In addition, a continuous in situ catalyst removal (purification) was demonstrated, providing further insights into the practical development of continuous furfural production.

An effective continuous furfural production from xylose in a microreactor over dual-acid catalysts was proposed. In this work, furfural was synthesized in an organic solvent-free system using formic acid and aluminum chloride as catalyst.

Valorization of Waste Lignocellulose to Furfural by Sulfonated Biobased Heterogeneous Catalyst Using Ultrasonic-Treated Chestnut Shell Waste as Carrier

Recently, the highly efficient production of value-added biobased chemicals from available, inexpensive, and renewable biomass has gained more and more attention in a sustainable catalytic process. Furfural is a versatile biobased chemical, which has been widely used for making solvents, lubricants, inks, adhesives, antacids, polymers, plastics, fuels, fragrances, flavors, fungicides, fertilizers, nematicides, agrochemicals, and pharmaceuticals. In this work, ultrasonic-treated chestnut shell waste (UTS-CSW) was utilized as biobased support to prepare biomass-based heterogeneous catalyst (CSUTS-CSW) for transforming waste lignocellulosic materials into furfural. The pore and surface properties of CSUTS-CSW were characterized with BET, SEM, XRD, and FT-IR. In toluene–water (2:1, v:v; pH 1.0), CSUTS-CSW (3.6 wt%) converted corncob into furfural yield in the yield of 68.7% at 180 °C in 15 min. CSUTS-CSW had high activity and thermostability, which could be recycled and reused for seven batches. From first to seventh, the yields were obtained from 68.7 to 47.5%. Clearly, this biobased solid acid CSUTS-CSW could be used for the sustainable conversion of waste biomasses into furfural, which had potential application in future.

Enhanced Furfural Production in Deep Eutectic Solvents Comprising Alkali Metal Halides as Additives

The addition of alkali metal halide salts to acidic deep eutectic solvents is here reported as an effective way of boosting xylan conversion into furfural. These salts promote an increase in xylose dehydration due to the cation and anion interactions with the solvent being a promising alternative to the use of harsh operational conditions. Several alkali metal halides were used as additives in the DES composed of cholinium chloride and malic acid ([Ch]Cl:Mal) in a molar ratio of 1:3, with 5 wt.% of water. These mixtures were then used as both solvent and catalyst to produce furfural directly from xylan through microwave-assisted reactions. Preliminary assays were carried out at 150 and 130 °C to gauge the effect of the different salts in furfural yields. A Response Surface Methodology was then applied to optimize the operational conditions. After an optimization of the different operating conditions, a maximum furfural yield of 89.46 ± 0.33% was achieved using 8.19% of lithium bromide in [Ch]Cl:Mal, 1:3; 5 wt.% water, at 157.3 °C and 1.74 min of reaction time. The used deep eutectic solvent and salt were recovered and reused three times, with 79.7% yield in the third cycle, and the furfural and solvent integrity confirmed.

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.

Lignocellulose Fractionation Using Recyclable Phosphoric Acid: Lignin, Cellulose, and Furfural Production

The conversion of lignocellulose into its building blocks and their further transformation into valuable platform chemicals (e. g., furfural) are key technologies to move towards the use of renewable resources. This paper explored the disentanglement of lignocellulose into hemicellulose-derived sugars, cellulose, and lignin in a biphasic solvent system (water/2-methyltetrahydrofuran) using phosphoric acid as recyclable catalyst. Integrated with the biomass fractionation, in a second step hemicellulose-derived sugars (mainly xylose) were converted to furfural, which was in situ extracted into 2-methyltetrahydrofuran with high selectivity (70 %) and yield (56 wt %). To further increase the economic feasibility of the process, a downstream and recycling strategy enabled recovery of phosphoric acid without loss of process efficiency over four consecutive cycles. This outlines a more efficient and sustainable use of phosphoric acid as catalyst, as its inherent costs can be significantly lowered.

Optimization with Response Surface Methodology of Microwave-Assisted Conversion of Xylose to Furfural

The production of furfural from renewable sources, such as lignocellulosic biomass, has gained great interest within the concept of biorefineries. In lignocellulosic materials, xylose is the most abundant pentose, which forms the hemicellulosic part. One of the key steps in the production of furfural from biomass is the dehydration reaction of the pentoses. The objective of this work was to assess the conditions under which the concentration of furfural is maximized from a synthetic, monophasic, and homogeneous xylose medium. The experiments were carried out in a microwave reactor. FeCl3 in different proportions and sulfuric acid were used as catalysts. A two-level, three-factor experimental design was developed for this purpose. The results were further analyzed through a second experimental design and optimization was performed by response surface methodology. The best operational conditions for the highest furfural yield (57%) turned out to be 210 °C, 0.5 min, and 0.05 M FeCl3.

Novel Routes in Transformation of Lignocellulosic Biomass to Furan Platform Chemicals: From Pretreatment to Enzyme Catalysis

The constant depletion of fossil fuels along with the increasing need for novel materials, necessitate the development of alternative routes for polymer synthesis. Lignocellulosic biomass, the most abundant carbon source on the planet, can serve as a renewable starting material for the design of environmentally-friendly processes for the synthesis of polyesters, polyamides and other polymers with significant value. The present review provides an overview of the main processes that have been reported throughout the literature for the production of bio-based monomers from lignocellulose, focusing on physicochemical procedures and biocatalysis. An extensive description of all different stages for the production of furans is presented, starting from physicochemical pretreatment of biomass and biocatalytic decomposition to monomeric sugars, coupled with isomerization by enzymes prior to chemical dehydration by acid Lewis catalysts. A summary of all biotransformations of furans carried out by enzymes is also described, focusing on galactose, glyoxal and aryl-alcohol oxidases, monooxygenases and transaminases for the production of oxidized derivatives and amines. The increased interest in these products in polymer chemistry can lead to a redirection of biomass valorization from second generation biofuels to chemical synthesis, by creating novel pathways to produce bio-based polymers.

The effect of sodium chloride concentration on the mutarotation and structure of d-xylose in water: Experimental and theoretical investigation

The effect of NaCl concentration on the structure of d-xylose in H₂O was studied. It was found that NaCl could prolong the equilibrium time between the two main configurations, α-xylopyranose and β-xylopyranose. The proportion of α-xylopyranose was slightly increased in NaCl-H₂O solution than that in H₂O, and the alteration of NaCl on α-xylopyranose and β-xylopyranose was different. Theoretical calculations demonstrated that NaCl was more favorable to stabilize the structure of α-xylopyranose. Na⁺ had attraction with O atoms (α: O₆; β: O₆ and O₁), with the outflow of electron from C atom to O atom on the C₁-O₆ bond, which was beneficial to the transformation between chain form and pyran forms. Cl^- had interaction with the hydroxyl groups of xylose. The interaction between xylose and NaCl, was also evidenced by the variation of ³⁵Cl and ²³Na NMR spectra. The findings could provide guidance for understanding the conformational change and design of xylose conversion ways. It also provided valuable information for making efficient use of hemicellulose.

Enhanced Conversion of Xylan into Furfural using Acidic Deep Eutectic Solvents with Dual Solvent and Catalyst Behavior

An efficient process for the production of furfural from xylan by using acidic deep eutectic solvents (DESs), which act both as solvents and catalysts, is developed. DESs composed of cholinium chloride ([Ch]Cl) and malic acid or glycolic acid at different molar ratios, and the effects of water and γ-valerolactone (GVL) contents, solid/liquid (S/L) ratio, and microwave heating are investigated. The best furfural yields are obtained with the DES [Ch]Cl:malic acid (1:3 molar ratio)+5 wt % water, under microwave heating for 2.5 min at 150 °C, a S/L ratio of 0.050, and GVL at a weight ratio of 2:1. Under these conditions, a remarkable furfural yield (75 %) is obtained. Direct distillation of furfural from the DES/GVL solvent and distillation from 2-methyltetrahydrofuran (2-MeTHF) after a back-extraction step enable 89 % furfural recovery from 2-MeTHF. This strategy allows recycling of the DES/GVL for at least three times with only small losses in furfural yield (>69 %). This is the fastest and highest-yielding process reported for furfural production using bio-based DESs as solvents and catalysts, paving the way for scale-up of the process.

The Roles of H2O/Tetrahydrofuran System in Lignocellulose Valorization

Lignocellulosic biomass as a potential alternative to fossil resource for the production of valuable chemicals and fuels has attracted substantial attention, while reducing the recalcitrance of lignocellulosic biomass is still challenging due to the complex and cross-linking structure of biomass. Solvent system plays important roles in the pretreatment of lignocellulose, enabling the transformation of solid biomass to liquid fluid with better mass and heat transfer, as well as in the selective formation of target products. In particular, H2O/tetrahydrofuran (H2O/THF) system has recently been widely applied in lignocellulose valorization, which has been proved to exhibit outstanding efficiency for the conversion of lignocellulose, solubilization of the intermediates and products, and shifting reaction equilibrium, thereby significantly improving the yield and selectivity of target products, as well as the full utilization of lignocellulose. In addition, THF shows low toxicity, and is known as a renewable solvent which can be produced from bio-derived chemicals. Herein, this review concentrates on the advances of H2O/THF system in lignocellulose valorization in recent years. Several aspects relative to the roles of H2O/THF system are discussed as follows: the pretreatment of lignin, conversion of hemicellulose and cellulose components in lignocelluloses, and the promoting formation of valuable chemicals like furfural, 5-hydroxymethyl furfural (HMF), levulinic acid, and so on, as well as the inhibiting role in humins formation. This review might provide useful information for the design of effective solvent system in full utilization of lignocellulosic biomass.

Catalytic valorization of hardwood for enhanced xylose-hydrolysate recovery and cellulose enzymatic efficiency via synergistic effect of Fe3+ and acetic acid

BACKGROUND

Poplars are considered suitable dedicated energy crops, with abundant cellulose and hemicellulose, and huge surplus biomass potential in China. Xylan, the major hemicellulosic component, contributes to the structural stability of wood and represents a tremendous quantity of biobased chemicals for fuel production. Monomeric xylose conversion to value-added chemicals such as furfural, xylitol, and xylonic acid could greatly improve the economics of pulp-paper industry and biorefinery. Acetic acid (HAc) is used as a friendly and recyclable selective catalyst amenable to xylan degradation and xylooligosaccharides production from lignocellulosic materials. However, HAc catalyst usually works much feebly at inert woods than agricultural straws. In this study, effects of different iron species in HAc media on poplar xylan degradation were systematically compared, and a preferable Fe³⁺-assisted HAc hydrolysis process was proposed for comparable xylose-hydrolysate recovery (XHR) and enzymatic saccharification of cellulose.

RESULTS

In presence of 6.5% HAc with 0.17-0.25 wt% Fe³⁺, xylose yield ranged between 72.5 and 73.9%. Additionally, pretreatment was effective in poplar delignification, with a lignin yield falling between 38.6 and 42.5%. Under similar conditions, saccharification efficiency varied between 60.3 and 65.9%. Starting with 100 g poplar biomass, a total amount of 12.7-12.8 g of xylose and 18.8-22.8 g of glucose were harvested from liquid streams during the whole process of Fe³⁺-HAc hydrolysis coupled with enzymatic saccharification. Furthermore, the enhancement mechanism of Fe³⁺ coupled with HAc was investigated after proof-of-concept experiments. Beechwood xylan and xylose were treated under the same condition as poplar sawdust fractionation, giving understanding of the effect of catalysts on the hydrolysis pathway from wood xylan to xylose and furfural by Fe³⁺-HAc.

CONCLUSIONS

The Fe³⁺-assisted HAc hydrolysis process was demonstrated as an effective approach to the wood xylose and other monosaccharides production. Synergistic effect of Lewis acid site and aqueous acetic acid provided a promising strategy for catalytic valorization of poplar biomass.

Simultaneous Conversion of C5 and C6 Sugars into Methyl Levulinate with the Addition of 1,3,5-Trioxane

The simultaneous conversion of C₅ and C₆ mixed sugars into methyl levulinate (MLE) has emerged as a versatile strategy to eliminate costly separation steps. However, the traditional upgrading of C₅ sugars into MLE is very complex as it requires both acid-catalyzed and hydrogenation processes. This study concerns the development of a one-pot, hydrogenation-free conversion of C₅ sugars into MLE over different acid catalysts at near-critical methanol conditions with the help of 1,3,5-trioxane. For the conversion of C₅ sugars over zeolites without the addition of 1,3,5-trioxane, the MLE yield is quite low, owing to low hydrogenation activity. The addition of 1,3,5-trioxane significantly boosts the MLE yield by providing an alternative conversion pathway that does not include the hydrogenation step. A direct comparison of the catalytic performance of five different zeolites reveals that Hβ zeolite, which has high densities of both Lewis and Brønsted acid sites, affords the highest MLE yield. With the addition of 1,3,5-trioxane, the hydroxymethylation of furfural derivative and formaldehyde is a key step. Notably, the simultaneous conversion of C₅ and C₆ sugars catalyzed by Hβ zeolite can attain an MLE yield as high as 50.4 % when the reaction conditions are fully optimized. Moreover, the Hβ zeolite catalyst can be reused at least five times without significant change in performance.

Heterogeneous Catalytic Upgrading of Biofuranic Aldehydes to Alcohols

Heterogeneous catalytic conversion of lignocellulosic components into valuable chemicals and biofuels is one of the promising ways for biomass valorization, which well meets green chemistry metrics, and can alleviate environmental and economic issues caused by the rapid depletion of fossil fuels. Among the identified biomass derivatives, furfural (FF) and 5-hydroxymethylfurfural (HMF) stand out as rich building blocks and can be directly produced from pentose and hexose sugars, respectively. In the past decades, much attention has been attracted to the selective hydrogenation of FF and 5-hydroxymethylfurfural using various heterogeneous catalysts. This review evaluates the recent progress of developing different heterogeneous catalytic materials, such as noble/non-noble metal particles, solid acids/bases, and alkali metal salts, for the efficient reduction of bio-based furanic aldehydes to alcohols. Emphasis is laid on the insights and challenges encountered in those biomass transformation processes, along with the focus on the understanding of reaction mechanisms to clarify the catalytic role of specific active species. Brief outlook is also made for further optimization of the catalytic systems and processes for the upgrading of biofuranic compounds.

Efficient Synthesis of Furfural from Biomass Using SnCl₄ as Catalyst in Ionic Liquid

Furfural is a versatile platform molecule for the synthesis of various chemicals and fuels, and it can be produced by acid-catalyzed dehydration of xylose derived from renewable biomass resources. A series of metal salts and ionic liquids were investigated to obtain the best combination of catalyst and solvent for the conversion of xylose into furfural. A furfural yield of 71.1% was obtained at high xylose loading (20 wt%) from the single-phasic reaction system whereby SnCl₄ was used as catalyst and ionic liquid 1-ethyl-3-methylimidazolium bromide (EMIMBr) was used as reaction medium. Moreover, the combined catalyst consisting of 5 mol% SnCl₄ and 5 mol% MgCl₂ also produced a high furfural yield (68.8%), which was comparable to the furfural yield obtained with 10 mol% SnCl₄. The water⁻organic solvent biphasic systems could improve the furfural yield compared with the single aqueous phase. Although these organic solvents could form biphasic systems with ionic liquid EMIMBr, the furfural yield decreased remarkably compared with the single EMIMBr phase. Besides, the EMIMBr/SnCl₄ system with appropriate water was also efficient to convert xylan and lignocellulosic biomass corn stalk into furfural, obtaining furfural yields as high as 57.3% and 54.5%, respectively.

Efficient catalytic conversion of corn stalk and xylose into furfural over sulfonated graphene in γ-valerolactone

Sulfonated graphene (SG) was prepared and employed to convert corn stalk and xylose into furfural. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) were used to characterize SG. The effects of reaction time, temperature, substrate loading, catalyst dosage and solvents on the reaction were researched and optimized. SG exhibited high catalytic activity in the conversion of xylose and corn stalk to furfural. A fairly high furfural yield of 96% was achieved at 150 °C from xylose and a 71.9% furfural yield was obtained when using a 10.7 ratio (mass ratio: xylose to SG) at 140 °C. While a 48% furfural yield was obtained from corn stalk (based on the starting combined moles of xylan and glucan in corn stalk; yield was >100%, if based on only xylan) using a substrate loading (corn stalk to catalyst mass ratio) of 2.14 and a 19% 5-hydroxymethylfurfural (5-HMF) yield was obtained. What's more, a 43.9% yield of furfural was obtained in only 20 min. In addition, the reusability of SG was also investigated and shown to have good stability for xylose dehydration.

Catalytic conversion of corn stalk over sulfonated graphene.

The role of the molecular formula of ZnCl2·nH2O on its catalyst activity: a systematic study of zinc chloride hydrates in the catalytic valorisation of cellulosic biomass

We show the efficient and direct transformation of a range of low value cellulosic substrates such as lignocellulose and algal biomass, into higher value chemicals, including low molecular weight reducing saccharides and furanoid products.

Low-Temperature Continuous-Flow Dehydration of Xylose Over Water-Tolerant Niobia-Titania Heterogeneous Catalysts

The sustainable conversion of vegetable biomass-derived feeds to useful chemicals requires innovative routes meeting environmental and economical criteria. The approach herein pursued is the synthesis of water-tolerant, unconventional solid acid monolithic catalysts based on a mixed niobia-titania skeleton building up a hierarchical open-cell network of meso- and macropores, and tailored for use under continuous-flow conditions. The materials were characterized by spectroscopic, microscopy, and diffraction techniques, showing a reproducible isotropic structure and an increasing Lewis/Brønsted acid sites ratio with increasing Nb content. The catalytic dehydration reaction of xylose to furfural was investigated as a representative application. The efficiency of the catalyst was found to be dramatically affected by the niobia content in the titania lattice. The presence of as low as 2 wt % niobium resulted in the highest furfural yield at 140 °C under continuous-flow conditions, by using H₂ O/γ-valerolactone as a safe monophasic solvent system. The interception of a transient 2,5-anhydroxylose species suggested the dehydration process occurs via a cyclic intermediates mechanism. The catalytic activity and the formation of the anhydro intermediate were related to the Lewis acid sites (LAS)/Brønsted acid sites (BAS) ratio and indicated a significant contribution of xylose-xylulose isomerization. No significant catalyst deactivation was observed over 4 days usage.

Microwaves in the Catalytic Valorisation of Biomass Derivatives

The application of microwave irradiation in the transformation of biomass has been receiving particular interest in recent years due to the use of polar media in such processes and it is now well-known that for biomass conversion, and particularly for lignocellulose hydrolysis, microwave irradiation can dramatically increase reaction rates with no negative consequences on product selectivity. However, it is only in the last ten years that the utilisation of microwaves has been coupled with catalysis aiming towards valorising biomass components or their derivatives via a range of reactions where high selectivity is required in addition to enhanced conversions. The reduced reaction times and superior yields are particularly attractive as they might facilitate the transition towards flow reactors and intensified production. As a consequence, several reports now describe the catalytic transformation of biomass derivatives via hydrogenation, oxidation, dehydration, esterification and transesterification using microwaves. Clearly, this technology has a huge potential for biomass conversion towards chemicals and fuels and will be an important tool within the biorefinery toolkit. The aim of this chapter is to give the reader an overview of the exciting scientific work carried out to date where microwave reactors and catalysis are combined in the transformation of biomass and its derivatives to higher value molecules and products.

Hydrolysis of Hemicellulose and Derivatives-A Review of Recent Advances in the Production of Furfural

Biobased production of furfural has been known for decades. Nevertheless, bioeconomy and circular economy concepts is much more recent and has motivated a regain of interest of dedicated research to improve production modes and expand potential uses. Accordingly, this review paper aims essentially at outlining recent breakthroughs obtained in the field of furfural production from sugars and polysaccharides feedstocks. The review discusses advances obtained in major production pathways recently explored splitting in the following categories: (i) non-catalytic routes like use of critical solvents or hot water pretreatment, (ii) use of various homogeneous catalysts like mineral or organic acids, metal salts or ionic liquids, (iii) feedstock dehydration making use of various solid acid catalysts; (iv) feedstock dehydration making use of supported catalysts, (v) other heterogeneous catalytic routes. The paper also briefly overviews current understanding of furfural chemical synthesis and its underpinning mechanism as well as safety issues pertaining to the substance. Eventually, some remaining research topics are put in perspective for further optimization of biobased furfural production.

Controlling the cleavage of the inter- and intra-molecular linkages in lignocellulosic biomass for further biorefining: A review

The abundant intermolecular linkages among cellulose, hemicellulose and lignin significantly limit the utilization of the most promising renewable biomass. Process control with solvents, catalysts and temperature is of significant importance providing ways to break the above linkages, and benefiting to the further conversion of the main biomass components to small molecular products. This article discusses the effect of catalyst under hydrothermal and organosolv treatment emphasizing the cleavage of the intermolecular linkage. Acidic catalysts show good performance on cleaving the linkages between carbohydrates and lignin. Basic catalysts promoted the dissolution of lignin component. Hydrogenolysis assisted conversion of lignin can efficiently break the intermolecular linkages to yield lignin-derived bio-oil, especially in co-solvent reaction system. Besides, the effects of single solvent and co-solvent systems, as well as the cleavage of the intramolecular linkages to yield target chemicals are also included. Several further study strategies are proposed.

Furfural production from biomass pretreatment hydrolysate using vapor-releasing reactor system

Biomass hydrolysate from autohydrolysis pretreatment was used for furfural production considering it is in rich of xylose, xylo-oligomers, and other decomposition products from hemicellulose structure. By using the vapor-releasing reactor system, furfural was protected from degradation by separating it from the reaction media. The maximum furfural yield of 73% was achieved at 200 °C for biomass hydrolysate without the use of the catalyst. This is because the presence of organic acids such as acetic acid in hydrolysate functioned as a catalyst. According to the results in this study, biomass hydrolysate with a vapor-releasing system proves to be efficient for furfural production. The biorefinery process which allows the separation of xylose-rich autohydrolysate from other parts from biomass feedstock also improves the overall application of the biomass.

Acid-Catalyzed Conversion of Carbohydrates into Value-Added Small Molecules in Aqueous Media and Ionic Liquids

Biomass is the only realistic major alternative source (to crude oil) of hydrocarbon substrates for the commercial synthesis of bulk and fine chemicals. Within biomass, terrestrial sources are the most accessible, and therein lignocellulosic materials are most abundant. Although lignin shows promise for the delivery of certain types of organic molecules, cellulose is a biopolymer with significant potential for conversion into high-volume and high-value chemicals. This review covers the acid-catalyzed conversion of lower value (poly)carbohydrates into valorized organic building-block chemicals (platform molecules). It focuses on those conversions performed in aqueous media or ionic liquids to provide the reader with a perspective on what can be considered a best case scenario, that is, that the overall process is as sustainable as possible.

Effect of Tetrahydrofuran on the Solubilization and Depolymerization of Cellulose in a Biphasic System

The dissolution of cellulose from biomass is a crucial but complicated issue for maximizing the utilization of biomass resources to produce valuable chemicals, because of the extreme insolubility of cellulose. A biphasic NaCl-H₂ O-tetrahydrofuran (THF) system was studied, in which most of the pure microcrystalline cellulose (M-cellulose, 96.6 % conversion at 220 °C) and that contained in actual biomass were converted. Nearly half of the O6-H⋅⋅⋅O3 intermolecular hydrogen bonds could be broken by THF in the H₂ O-THF co-solvent system, whereas the cleavage of O2-H⋅⋅⋅O6 intramolecular hydrogen bonds by H₂ O was significantly inhibited. In the NaCl-H₂ O-THF system, THF could significantly promote the effects of both H₂ O and NaCl on the disruption of O2-H⋅⋅⋅O6 and O3-H⋅⋅⋅O5 intramolecular hydrogen bonds, respectively. In addition, THF could protect and transfer the cellulose-derived products to the organic phase by forming hydrogen bonds between the oxygen atom in THF and the hydrogen atom of C4-OH in the glucose or aldehyde group in 5-hydroxymethylfurfural (HMF), which can lead more NaCl to combine with the -OH of M-cellulose and further disrupt hydrogen bonding in M-cellulose, thereby improving the yield of small molecular weight products (especially HMF) and further promoting the dissolution of cellulose. As a cheap and reusable system, NaCl-H₂ O-THF system may be a promising approach for the dissolution and further conversion of cellulose in lignocellulosic biomass without any enzymes, ionic liquids, or conventional catalysts.

Polar aprotic solvent-water mixture as the medium for catalytic production of hydroxymethylfurfural (HMF) from bread waste

Valorisation of bread waste for hydroxymethylfurfural (HMF) synthesis was examined in dimethyl sulfoxide (DMSO)-, tetrahydrofuran (THF)-, acetonitrile (ACN)-, and acetone-water (1:1v/v), under heating at 140°C with SnCl₄ as the catalyst. The overall rate of the process was the fastest in ACN/H₂O and acetone/H₂O, followed by DMSO/H₂O and THF/H₂O due to the rate-limiting glucose isomerisation. However, the formation of levulinic acid (via rehydration) and humins (via polymerisation) was more significant in ACN/H₂O and acetone/H₂O. The constant HMF maxima (26-27mol%) in ACN/H₂O, acetone/H₂O, and DMSO/H₂O indicated that the rates of desirable reactions (starch hydrolysis, glucose isomerisation, and fructose dehydration) relative to undesirable pathways (HMF rehydration and polymerisation) were comparable among these mediums. They also demonstrated higher selectivity towards HMF production over the side reactions than THF/H₂O. This study differentiated the effects of polar aprotic solvent-water mediums on simultaneous pathways during biomass conversion.

Insight into Aluminum Sulfate-Catalyzed Xylan Conversion into Furfural in a γ-Valerolactone/Water Biphasic Solvent under Microwave Conditions

A simple and efficient biphasic system with an earth-abundant metal salt catalyst was used to produce furfural from xylan with a high yield of up to 87.8 % under microwave conditions. Strikingly, the metal salt Al₂ (SO₄ )₃ exhibited excellent catalytic activity for xylan conversion, owing to a combination of Lewis and Brønsted acidity and its ability to promote good phase separation. The critical role of the SO₄^2- anion was first analyzed, which resulted in the aforementioned characteristics when combined with the Al³⁺ cation. The mixed solvent system with γ-valerolactone (GVL) as the organic phase provided the highest furfural yield, resulting from its good dielectric properties and dissolving capacity, which facilitated the absorption of microwave energy and promoted mass transfer. Mechanistic studies suggested that the xylan-to-furfural conversion proceeded mainly through a hydrolysis-isomerization-dehydration pathway and the hexa-coordinated Lewis acidic [Al(OH)₂ (aq)]⁺ species were the active sites for xylose-xylulose isomerization. Detailed kinetic studies of the subreaction for the xylan conversion revealed that GVL regulates the reaction rates and pathways by promoting the rates of the key steps involved for furfural production and suppressing the side reactions for humin production. Finally, the Al₂ (SO₄ )₃ catalyst was used for the production of furfural from several lignocellulosic feedstocks, revealing its great potential for other biomass conversions.

Efficient utilization of potash alum as a green catalyst for production of furfural, 5-hydroxymethylfurfural and levulinic acid from mono-sugars

Potash alum (PA) as an inexpensive, efficient and green catalyst for production of high value platform chemicals such as 5-hydroxymethylfurfural (HMF), levulinic acid and furfural from bio-renewable feedstocks, have been explored.

Production of furfural from xylose and corn stover catalyzed by a novel porous carbon solid acid in γ-valerolactone

An efficient catalytic system using S-RFC as catalyst was developed to produce furfural from xylose and corn stover in GVL.