A comparative study on the chemo-enzymatic upgrading of renewable biomass to 5-Hydroxymethylfurfural

5-hydroxymethylfurfural (HMF) obtained from renewable biomass-derived carbohydrates is a potential sustainable substitute to petroleum-based building blocks. In the present work, we constituted a comparative study on the production of HMF from two widely available real biomasses in India- Agave americana and Casuarina equisetifolia. In the initial hydrolysis studies for the production of reducing sugars, 649.5 mg/g of fructose was obtained from the hydrolysis of 5% (w/v) A. americana biomass by the enzyme inulinase in 3 h at 50°C. Similarly, upon hydrolysis of 15% (w/v) C. equisetifolia biomass by the lignocellulolytic enzymes (laccase, cellulase and xylanase) from Trichoderma atroviride, 456.65 mg/g of reducing sugars was released in 24 h at 30°C. Subsequently, the dehydration of the obtained reducing sugars to HMF was achieved with titanium dioxide as the catalyst. The dehydration of A. americana-derived fructose at 140°C led to a maximum HMF yield of 92.6% in 15 min with 10% catalyst load. Contrarily, upon optimizing the process parameters for dehydration of C. equisetifolia derived reducing sugars, the maximum HMF yield of 85.7% was obtained at 110°C in 25 min with a TiO₂ concentration of 10%. This study reports for the first time the utilization of C. equisetifolia biomass for HMF production and thus, by utilizing these inexpensive, abundantly available and highly functionalized polysaccharides, a strategical approach can be developed for the production of fine chemicals, eliminating the need of fossil-based chemicals. Implications: The catalytic upgrading of lignocellulosic biomass into high-valued platform chemicals like 5-Hydroxymethylfurfural (HMF) implies an extremely significant challenge to the attempts of establishing a green economy. Casuarina equisetifolia and Agave americana represents a sustainable feedstock for the production of HMF through catalytic integration. The present work describes a two-step reaction process where the initial depolymerization step comprises of an enzymatic hydrolysis followed by a chemical-catalyst mediated dehydration process. The utilization of a biocatalytic approach followed by mild chemical catalyst eliminates the need of hazardous chemical conversion processes. Thus, the HMF produced via sustainable can bridge the gap between carbohydrate chemistry and petroleum-based industrial chemistry because of the wide range of chemical intermediates and end-products that can be derived from this compound.

Recent Advances in the Microwave-Assisted Production of Hydroxymethylfurfural by Hydrolysis of Cellulose Derivatives-A Review

The concepts of sustainable development, bioeconomy, and circular economy are being increasingly applied for the synthesis of molecules of industrial interest. Among these molecules, hydroxymethylfurfural as a platform molecule is the subject of various research approaches to improve its synthesis and productivity, and extend its potential uses. Accordingly, this review paper aims essentially at outlining recent breakthroughs obtained in the field of hydroxymethylfurfural production from sugars and polysaccharide feedstocks under microwave-assisted technology. The review discusses advances obtained via microwave activation in major production pathways recently explored, split into the following categories: (i) use of various homogeneous catalysts like mineral or organic acids, metal salts, or ionic liquids; (ii) feedstock dehydration making use of various solid acid catalysts; and (iii) non-catalytic routes.

Hybrid Organic-Inorganic Anatase as a Bifunctional Catalyst for Enhanced Production of 5-Hydroxymethylfurfural from Glucose in Water

Hybrid organic-inorganic anatase (hybrid-TiO₂ ) is prepared by a facile hydrothermal synthesis method employing citric acid. The synthetic approach results in a high surface-area nanocrystalline anatase polymorph of TiO₂ . The uncalcined hybrid-TiO₂ is directly studied as a catalyst for the conversion of glucose into 5-hydroxymethylfurfural (HMF). In the presence of the hybrid-TiO₂ , HMF yields up to 45 % at glucose conversions up to 75 % were achieved in water at 130 °C in a monophasic batch reactor. As identified by Ti K-edge XANES, hybrid-TiO₂ contains a large fraction of fivefold coordinatively unsaturated Ti^IV sites, which act as the Lewis acid catalyst for the conversion of glucose into fructose. As citric acid is anchored in the structure of hybrid-TiO₂ , carboxylate groups seem to catalyze the sequential conversion of fructose into HMF. The fate of citric acid bound to anatase and the Ti^IV Lewis acid sites throughout recycling experiments is also investigated. In a broader context, this contribution outlines the importance of hydrothermal synthesis for the creation of water-resistant Lewis acid sites for the conversion of sugars. Importantly, the use of the hybrid-TiO₂ with no calcination step contributes to dramatically decreasing the energy consumption in the catalyst preparation.

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.

Furfural Synthesis from d-Xylose in the Presence of Sodium Chloride: Microwave versus Conventional Heating

We investigate the existence of specific/nonthermal microwave effects for the dehydration reaction of xylose to furfural in the presence of NaCl. Such effects are reported for sugars dehydration reactions in several literature reports. To this end, we adopted three approaches that compare microwave-assisted experiments with a) conventional heating experiments from the literature; b) simulated conventional heating experiments using microwave-irradiated silicon carbide (SiC) vials; and at c) different power levels but the same temperature by using forced cooling. No significant differences in the reaction kinetics are observed using any of these methods. However, microwave heating still proves advantageous as it requires 30 % less forward power compared to conventional heating (SiC vial) to achieve the same furfural yield at a laboratory scale.

Study of the Influence of Key Process Parameters on Furfural Production

The present work reports the influence of key process variables on the furfural formation from leached chestnut-wood chips in a pressurized reactor. Effect of temperature, pressure, type and concentration of the catalyst solution, the steam flow rate or stripping module, the moisture content of the wood particles and geometric characteristics such as size and type of the reactor, particle size and bed height were considered systematically. One stage process was only taken into consideration. Lab-scale and pilot-scale studies were performed. The results of the non-catalysed laboratory experiments were compared with an actual non-catalysed (auto-catalysed) industrial process and with experiments on the pilot scale, the latter with 28% higher furfural yield compared to the others. Application of sulphuric acid as catalyst, in an amount of 0.03-0.05 g (H2SO4 100%)/g d.m. (dry material), enables a higher production of furfural at lower temperature and pressure of steam in a shorter reaction time. Pilot scale catalysed experiments have revealed very good performance for furfural formation under less severe operating conditions, with a maximum furfural yield as much as 88% of the theoretical value.

Effects of Soluble Lignin on the Formic Acid-Catalyzed Formation of Furfural: A Case Study for the Upgrading of Hemicellulose

A comprehensive study is presented on the conversion of hemicellulose sugars in liquors obtained from the fractionation of Miscanthus, spruce bark, sawdust, and hemp by using formic acid. Experimental tests with varying temperature (130-170 °C), formic acid concentration (10-80 wt%), carbohydrate concentrations, and lignin separation were carried out, and experimental data were compared with predictions obtained by reaction kinetics developed in a previous study. The conversions of xylose and arabinose into furfural were inherently affected by the presence of polymeric soluble lignin, decreasing the maximum furfural yields observed experimentally by up to 24%. These results were also confirmed in synthetic mixtures of pentoses with Miscanthus and commercial alkali lignin. This observation was attributed to side reactions involving intermediate stable sugar species reacting with solubilized lignin during the conversion of xylose into furfural.

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.

Conversion of corn stalk into furfural using a novel heterogeneous strong acid catalyst in γ-valerolactone

A novel solid acid catalyst was prepared by the copolymerization of p-toluenesulfonic acid and paraformaldehyde and then characterized by FT-IR, TG/DTG, HRTEM and N2-BET. Furfural was successfully produced by the dehydration of xylose and xylan using the novel catalyst in γ-valerolactone. This investigation focused on effects of various reaction conditions including solvent, acid catalyst, reaction temperature, residence time, water concentration, xylose loading and catalyst dosage on the dehydration of xylose to furfural. It was found that the solid catalyst displayed extremely high activity for furfural production. 80.4% furfural yield with 98.8% xylose conversion was achieved at 170°C for 10 min. The catalyst could be recycled at least five times without significant loss of activity. Furthermore, 83.5% furfural yield and 19.5% HMF yield were obtained from raw corn stalk under more severe conditions (190°C for 100 min).

Direct Production of 5-Hydroxymethylfurfural via Catalytic Conversion of Simple and Complex Sugars over Phosphated TiO2

A water-THF biphasic system containing N-methyl-2-pyrrolidone (NMP) was found to enable the efficient synthesis of 5-hydroxymethylfurfural (HMF) from a variety of sugars (simple to complex) using phosphated TiO2 as a catalyst. Fructose and glucose were selectively converted to HMF resulting in 98 % and 90 % yield, respectively, at 175 °C. Cellobiose and sucrose also gave rise to high HMF yields of 94 % and 98 %, respectively, at 180 °C. Other sugar variants such as starch (potato and rice) and cellulose were also investigated. The yields of HMF from starch (80-85 %) were high, whereas cellulose resulted in a modest yield of 33 %. Direct transformation of cellulose to HMF in significant yield (86 %) was assisted by mechanocatalytic depolymerization-ball milling of acid-impregnated cellulose. This effectively reduced cellulose crystallinity and particle size, forming soluble cello-oligomers; this is responsible for the enhanced substrate-catalytic sites contact and subsequent rate of HMF formation. During catalyst recyclability, P-TiO2 was observed to be reusable for four cycles without any loss in activity. We also investigated the conversion of the cello-oligomers to HMF in a continuous flow reactor. Good HMF yield (53 %) was achieved using a water-methyl isobutyl ketone+NMP biphasic system.

From lignocellulosic biomass to furans via 5-acetoxymethylfurfural as an alternative to 5-hydroxymethylfurfural

A facile pathway to furan derivatives from lignocellulosic biomass via 5-acetoxymethylfurfural (AMF) was developed. AMF possesses advantageous properties due to its less-hydrophilic acetoxymethyl group relative to the hydroxymethyl group of 5-hydroxymethylfurfural (HMF). The hydrophobicity and chemical stability of AMF allowed practical isolation and purification to afford a highly pure product of up to 99.9 %. AMF was produced in good to excellent yields under mild conditions from 5-chloromethylfurfural (CMF) and alkylammonium acetates, both of which could be obtained directly from lignocellulosic biomass. Heterogeneous reactions with polymer-supported alkylammonium acetates were also established; this showed the feasibility of a continuous process for this pathway. AMF could be transformed into various promising furanic compounds, such as 2,5-furandicarboxylic acid (FDCA), 2,5-furandimethanol (FDM), and 5-hydroxymethyl-2-furanoic acid (HFA), in high yields.

Production of furfural from xylose, water-insoluble hemicelluloses and water-soluble fraction of corncob via a tin-loaded montmorillonite solid acid catalyst

The conversion of xylose, water-insoluble hemicelluloses (WIH) and water-soluble fraction (WSF) of corncob to furfural was performed using montmorillonite with tin ions (Sn-MMT) containing double acid sites as a solid acid catalyst. The co-existence of Lewis acids and Brønsted acids in Sn-MMT was shown to improve the furfural yield and selectivity. 76.79% furfural yield and 82.45% furfural selectivity were obtained from xylose using Sn-MMT as a catalyst in a biphasic system with 2-s-butylphenol (SBP) as the organic extracting layer and dimethyl sulfoxide (DMSO) as the co-solvent in contact with an aqueous phase saturated with NaCl (SBP/NaCl-DMSO) at 180°C for 30min. Furthermore, Sn-MMT also demonstrated the excellent catalytic performance in the conversion of pentose-rich materials of corncob and 39.56% and 54.15% furfural yields can be directly obtained from WIH and WSF in the SBP/NaCl-DMSO system, respectively.

Production of 5-hydroxymethylfurfural from corn stalk catalyzed by corn stalk-derived carbonaceous solid acid catalyst

A carbonaceous solid acid was prepared by hydrothermal carbonization of corn stalk followed by sulfonation and was characterized by FT-IR, XRD, SEM and elemental analysis techniques. The as-prepared corn stalk-derived carbonaceous solid acid catalyst contained SO3H, COOH, and phenolic OH groups, and was used for the one-step conversion of intact corn stalk to 5-hydroxymethylfurfural (5-HMF) in the ionic liquid 1-butyl-3-methyl imidazolium chloride ([BMIM][Cl]), where a 5-HMF yield of 44.1% was achieved at 150 °C in 30 min reaction time. The catalytic system was applicable to initial corn stalk concentration of up to ca. 10 wt.% for the production of 5-HMF. The synthesized catalyst and the developed process of using corn stalk-derived carbon catalyst for corn stalk conversion provide a green and efficient strategy for crude biomass utilization.

Production of furfural from waste aqueous hemicellulose solution of hardwood over ZSM-5 zeolite

This study aimed to produce furfural from waste aqueous hemicellulose solution of a hardwood kraft-based dissolving pulp production processing in a green method. The maximum furfural yield of 82.4% and the xylose conversion of 96.8% were achieved at 463K, 1.0g ZSM-5, 1.05g NaCl and organic solvent-to-aqueous phase ratio of 30:15 (V/V) for 3h. The furfural yield was just 51.5% when the same concentration of pure xylose solution was used. Under the optimized condition, furfural yield was still up to 67.1% even after the fifth reused of catalyst. Catalyst recycling study showed that ZSM-5 has a certain stability and can be efficiently reused.

InCl3-catalyzed conversion of carbohydrates into 5-hydroxymethylfurfural in biphasic system

InCl3, a water-compatible Lewis acid, was used for the conversion of microcrystalline cellulose to produce 5-hydroxymethylfurfural (HMF) in a H2O/THF biphasic system. Addition of NaCl increased the HMF yield significantly but suppressed the levulinic acid (LA) formation. The HMF yield of 39.7% was obtained in 2h at 200°C in the NaCl-H2O/THF catalytic system catalyzed by InCl3. The catalytic system also showed effectiveness to convert other carbohydrates to HMF, including glucose, fructose, sucrose, starch, which demonstrated great potential towards different feedstocks.

From furfural to fuel: synthesis of furoins by organocatalysis and their hydrodeoxygenation by cascade catalysis

The synthesis of furoins from biomass-derived furfural and 2-methylfurfural is demonstrated in high yields in green and renewable solvents using N-heterocyclic carbene organocatalysts. The resulting furoin molecules are used as precursors for fuels using cascade catalysis, first by using Pd/C with acidic co-catalysts under very mild conditions to yield oxygenated C12 molecules. Two main products were formed, which we identified as 1,2-bis(5-methyltetrahydrofuran-2-yl)ethane and 1-(5-methyltetrahydrofuran-2-yl)heptanol. The use of a Pd/Zeolite-β catalyst under more extreme conditions resulted in the complete hydrodeoxygenation of 5,5'-dimethylfuroin to dodecanes in high yields (76%) and exceptional selectivity (94%) for n-dodecane.

Efficient solid acid catalyst containing Lewis and Brønsted Acid sites for the production of furfurals

Self-assembled nanoparticulates of porous sulfonated carbonaceous TiO2 material that contain Brønsted and Lewis acidic sites were prepared by a one-pot synthesis method. The material was characterized by XRD, FTIR spectroscopy, NH3 temperature-programmed desorption, pyridine FTIR spectroscopy, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, N2 -sorption, atomic absorbance spectroscopy, and inductively coupled plasma optical emission spectroscopy. The carbonaceous heterogeneous catalyst (Glu-TsOH-Ti) with a Brønsted-to-Lewis acid density ratio of 1.2 and more accessible acid sites was effective to produce 5-hydroxymethylfurfural and furfural from biomass-derived mono- and disaccharides and xylose in a biphasic solvent that comprised water and biorenewable methyltetrahydrofuran. The catalyst was recycled in four consecutive cycles with a total loss of only 3 % activity. Thus, Glu-TsOH-Ti, which contains isomerization and dehydration catalytic sites and is based on a cheap and biorenewable carbon support, is a sustainable catalyst for the production of furfurals, platform chemicals for biofuels and chemicals.

Synthesis of 5-hydroxymethylfurural from carbohydrates using large-pore mesoporous tin phosphate

A large-pore mesoporous tin phosphate (LPSnP-1) material has been synthesized hydrothermally by using Pluronic P123 as the structure-directing agent. The material is composed of aggregated nanoparticles of 10-15 nm in diameter and has a BET surface area of 216 m(2)  g(-1) with an average pore diameter of 10.4 nm. This pore diameter is twice as large as that of mesoporous tin phosphate materials synthesized through the surfactant-templating pathways reported previously. LPSnP-1 shows excellent catalytic activity for the conversion of fructose, glucose, sucrose, cellobiose, and cellulose to 5-hydroxymethylfurfural (HMF) in a water/methyl isobutyl ketone biphasic solvent to give maximum yields of HMF of 77, 50, 51, 39, and 32 mol %, respectively, under microwave-assisted heating at 423 K. Under comparable reaction conditions, LPSnP-1 gives 12 % more HMF yield than a small-pore mesoporous tin phosphate catalyst that has an identical framework composition. This confirms the beneficial role of large mesopores and nanoscale particle morphology in catalytic reactions that involve bulky natural carbohydrate molecules.

Efficient one-pot synthesis of 5-chloromethylfurfural (CMF) from carbohydrates in mild biphasic systems

5-Halomethylfurfurals can be considered as platform chemicals of high reactivity making them useful for the preparation of a variety of important compounds. In this study, a one-pot route for the conversion of carbohydrates into 5-chloromethylfurfural (CMF) in a simple and efficient (HCl-H3PO4/CHCl3) biphasic system has been investigated. Monosaccharides such as D-fructose, D-glucose and sorbose, disaccharides such as sucrose and cellobiose and polysaccharides such as cellulose were successfully converted into CMF in satisfactory yields under mild conditions. Our data shows that when using D-fructose the optimum yield of CMF was about 47%. This understanding allowed us to extent our work to biomaterials, such as wood powder and wood pulps with yields of CMF obtained being comparable to those seen with some of the enumerated mono and disaccharides. Overall, the proposed (HCl-H3PO4/CHCl3) optimized biphasic system provides a simple, mild, and cost-effective means to prepare CMF from renewable resources.

Furfural formation from the pre-hydrolysis liquor of a hardwood kraft-based dissolving pulp production process

This study aimed to produce furfural from the PHL. Results showed best furfural yield of 32.8% and the furfural selectivity of 37.7% in the monophase system (170 °C, 100 min), while they were 60.1% and 69.8%, respectively in the biphase system. The lower furfural selectivity in the monophase system was explained by more side reactions, such as fragmentation, condensation reactions, resinification and others. Model compounds such as: xylose, furfural, syringaldehyde, were used to confirm/identify these side reactions. The addition of dilute sulfuric acid/acetic acid in the system under the same conditions decreased the recovery of furfural. The addition of syringaldehyde into the PHL also led to a decrease in the furfural yield, supporting the conclusion that lignin structures in the PHL may also be involved in the side reactions, thus decreasing the furfural yield.

An efficient and heterogeneous recyclable silicotungstic acid with modified acid sites as a catalyst for conversion of fructose and sucrose into 5-hydroxymethylfurfural in superheated water

Acidity modified silver exchanged silicotungstic acid (AgSTA) catalyst was prepared and characterized by X-ray diffraction, FT-IR spectroscopy, Raman spectroscopy, FT-IR pyridine adsorption, SEM imaging, EDX mapping, and antimicrobial activity was also tested. The catalytic activity was evaluated for the dehydration of fructose and sucrose in superheated water. As a result, 98% conversion of fructose with 85.7% HMF yield and 87.4% HMF selectivity in 120 min reaction time at 120 °C reaction temperature using 10 wt.% of AgSTA catalyst was achieved. While, 92% sucrose conversion with 62.5% of HMF yield was obtained from sucrose at uniform condition in 160 min. The effect of reaction parameters, such as reaction temperature, time, catalyst dosage, and effect acidity on HMF yield was also investigated. The AgSTA catalyst was separated from the reaction mixture by filtration process at end of the reaction and reused eight times without loss of catalytic activity.

Conversion of cellulose to HMF in ionic liquid catalyzed by bifunctional ionic liquids

A new kind of bifunctional ionic liquid catalysts was synthesized to degrade microcrystalline cellulose in [BMIM]Cl at atmospheric pressure. The effects of reaction temperature, amount of catalysts, reaction time, ionic liquid purity and cellulose concentration on conversion were investigated. At low temperature cellulose can be degraded with being heated in [BMIM]Cl by oil bath. Among the as-synthesized catalysts, Cr([PSMIM]HSO4)3 exhibited the best performance. The HMF yield of 53% and TRS yield of 94% can be achieved at 120 °C in [BMIM]Cl for 5 h over 0.05 g Cr([PSMIM]HSO4)3/2.0 g [BMIM]Cl with 95% cellulose conversion.

Conversion of xylan, d-xylose and lignocellulosic biomass into furfural using AlCl3 as catalyst in ionic liquid

In order to define a new green catalytic pathway for the production of furfural, the catalyzed conversion of xylan into furfural in 1-butyl-3-methylimidazolium chloride was studied by using mineral acids and metal chlorides as catalysts under microwave irradiation. Amongst these catalysts, AlCl(3) resulted in the highest furfural yield of 84.8% at 170°C for 10s. The effect of AlCl(3) on the conversion efficiency of d-xylose and untreated lignocellulosic biomass was also investigated, the yields of furfural from corncob, grass and pine wood catalyzed by AlCl(3) in [BMIM]Cl were in the range of 16-33%. [BMIM]Cl and AlCl(3) could be recycled for four runs with stable catalytic activity. AlCl(3) is less corrosive than mineral acids, and the use of ionic liquid as reaction medium will no longer generate toxic wastewater, thus this reaction system is more ecologically viable.

One-pot synthesis of 5-hydroxymethylfurfural directly from starch over SO(4)(2-)/ZrO2-Al2O3 solid catalyst

The synthesis of 5-hydroxymethylfurfural (HMF) directly from starch was studied in dimethyl sulfoxide-water. The effects of catalyst variation, reaction time, water content, catalyst loading and temperature on the reaction were investigated. The SO(4)(2-)/ZrO(2)-Al(2)O(3) catalyst was found to act as a bifunctional catalyst with high activity for both hydrolysis and dehydration of starch. HMF yield of 55% was obtained after 6h at 423K for the reaction of starch (the molar ratio of water to glucose in starch is 44/1) over the SO(4)(2-)/ZrO(2)-Al(2)O(3) catalyst, which bears high acidity and moderate basicity with Zr/Al molar ratio of 1:1.

The optimization of formic acid hydrolysis of xylose in furfural production

Formic acid, a byproduct of furfural process, can be an effective catalyst for dehydration of xylose into furfural. Due to the low corrosion resistance, easy to be separated and reused, there is a growing interest in the use of formic acid as catalyst. In this study, response surface methodology (RSM) was used to optimize the hydrolysis process in order to obtain high furfural yield and selectivity. Three important parameters, initial xylose concentration (40-120 g/L), temperature (170-190 °C), formic acid concentration (5-15 g/L) were optimized. The optimum initial xylose concentration, formic concentration, reaction temperature were 40 g/L, 10 g/L, and 180 °C, respectively. Under these conditions, the maximum furfural yield of 74% and selectivity of 78% were achieved.

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

Furfural was prepared in high yields (75 %) from the reaction of xylose in a water-tetrahydrofuran biphasic medium containing AlCl(3)·6H2O and NaCl under microwave heating at 140 °C. The reaction profile revealed the formation of xylulose as an intermediate en route to the dehydration product (furfural). The reaction under these conditions reached completion in 45 min. The aqueous phase containing AlCl(3)·6H(2)O and NaCl could be recycled multiple times (>5) without any loss of activity or selectivity for furfural. Extension of this biphasic reaction system to include xylan as the starting material afforded furfural in 64 % yield. The use of corn stover, pinewood, switchgrass, and poplar gave furfural in 55, 38, 56, and 64 % yield, respectively, at 160 °C. Even though AlCl(3)·6H(2)O did not affect the conversion of crystalline cellulose, moderate yields of the by-product 5-hydroxymethylfurfural (HMF) were noted. The highest HMF yield of 42 % was obtained from pinewood. The coproduction of HMF and furfural from biomass was attributed to the weakening of the cellulose network in the biomass, as a result of hemicellulose hydrolysis. The multifunctional capacity of AlCl(3)·6H(2)O (hemicellulose hydrolysis, xylose isomerization, and xylulose dehydration) in combination with its ease of recyclability make it an attractive candidate/catalyst for the selective synthesis of furfural from various biomass feedstocks.

Biorefining: heterogeneously catalyzed reactions of carbohydrates for the production of furfural and hydroxymethylfurfural

Furfurals are important intermediates in the chemical industry. They are typically produced by homogeneous catalysis in aqueous solutions. However, heterogeneously catalyzed processes would be beneficial in view of the principles of green chemistry: the elimination of homogeneous mineral acids makes the reaction mixtures less corrosive, produces less waste, and facilitates easy separation and recovery of the catalyst. Finding an active and stable water-tolerant solid acid catalyst still poses a challenge for the production of furfural (furan-2-carbaldehyde) and 5-(hydroxymethyl)-2-furaldehyde (HMF). Furfural is produced in the dehydration of xylose, and HMF is formed from glucose and fructose in the presence of an acidic catalyst. Bases are not active in dehydration reaction but do catalyze the isomerization of monosaccharides, which is favorable when using glucose as a raw material. In addition to the desired dehydration of monosaccharides, many undesired side reactions take place, reducing the selectivity and deactivating the catalyst. In addition, the catalyst properties play an important role in the selectivity. In this Review, catalytic conversion approaches are summarized, focusing on the heterogeneously catalyzed formation of furfural. The attractiveness of catalytic concepts is evaluated, keeping in mind productivity, sustainability, and environmental footprint.

A study of the acid-catalyzed hydrolysis of cellulose dissolved in ionic liquids and the factors influencing the dehydration of glucose and the formation of humins

An investigation was carried out into the hydrolysis of cellulose dissolved in 1-ethyl-3-methylimidazolium chloride ([Emim][Cl]) and 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]) catalyzed by mineral acids. Glucose, cellobiose, and 5-hydroxymethylfurfural (5-HMF) were observed as the primary reaction products. The initial rate of glucose formation was determined to be of first order in the concentrations of dissolved glucan and protons and of zero order in the concentration of water. The absence of a dependence on water concentration suggests that cleavage of the β-1,4-glycosidic linkages near chain ends is irreversible. The apparent activation energy for glucose formation is 96 kJ mol(-1). The absence of oligosaccharides longer than cellobiose suggests that cleavage of interior glycosidic bonds is reversible due to the slow diffusional separation of cleaved chains in the highly viscous glucan/ionic liquid solution. Progressive addition of water during the course of glucan hydrolysis inhibited the rate of glucose dehydration to 5-HMF and the formation of humins. The inhibition of glucose dehydration is attributed to stronger interaction of protons with water than the 2-OH atom of the pyranose ring of glucose, the critical step in the proposed mechanism for the formation of 5-HMF. The reduction in humin formation associated with water addition is ascribed to the lowered concentration of 5-HMF, since the formation of humins is suggested to proceed through the condensation polymerization of 5-HMF with glucose.

Acid-catalyzed conversion of xylose, xylan and straw into furfural by microwave-assisted reaction

Furfural is a biomass derived-chemical that can be used to replace petrochemicals. In this study, the acid-catalyzed conversion of xylose and xylan to furfural by microwave-assisted reaction was investigated at selected ranges of temperature (140-190°C), time (1-30 min), substrate concentration (1:5-1:200 solid:liquid ratio), and pH (2-0.13). We found that a temperature of 180°C, a solid:liquid ratio of 1:200, a residence time of 20 min, and a pH of 1.12 gave the best furfural yields. The effect of different Brønsted acids on the conversion efficiency of xylose and xylan was also evaluated, with hydrochloric acid being found to be the most effective catalyst. The microwave-assisted process provides highly efficient conversion: furfural yields obtained from wheat straw, triticale straw, and flax shives were 48.4%, 45.7%, and 72.1%, respectively.

Furfural production from xylose using sulfonic ion-exchange resins (Amberlyst) and simultaneous stripping with nitrogen

The aim of this work deals with the development of new approaches to the production of furfural from xylose. It combines relatively cheap heterogeneous catalysts (Amberlyst 70) with simultaneous furfural stripping using nitrogen under semi-batch conditions. Nitrogen, compared to steam, does not dilute the vapor phase stream when condensed. This system allowed stripping 65% of the furfural converted from xylose and almost 100% of selectivity in the condensate. Moreover, high initial xylose loadings led to the formation of two water-furfural phases, which could reduce further purification costs. Constant liquid-vapor equilibrium along stripping could be maintained for different xylose loadings. The modeling of the experimental data was carried out in order to obtain a liquid-vapor mass-transfer coefficient. This value could be used for future studies under steady-state continuous conditions in similar reaction-systems.

Synthesis of furfural from xylose by heterogeneous and reusable nafion catalysts

Nafion 117 has been proven as a robust and reusable heterogeneous catalyst for the dehydration of 9.1 % (w/w) xylose in dimethyl sulfoxide (DMSO) to yield 60 % furfural in 2 h at 150 °C. The catalytic high activity promoted shorter reaction times to limit the formation of side-products which otherwise would lead to decreased yields. Within the allowable operating temperature range of Nafion (125 to 175 °C), the reaction was kinetically controlled. In corroboration with AFM and SEM imaging, ATR-FTIR confirmed that the Nafion catalytic activity remained unchanged after 15 repeated uses. With excellent chemical and thermal stability under the conditions for xylose dehydration compared to existing solid acid catalysts, this reusable Nafion system could be a step towards the more economical production of furfural from renewable biomass, an intermediate chemical for the preparation of value-added chemicals.

Synthesis of 5-(hydroxymethyl)furfural in ionic liquids: paving the way to renewable chemicals

The synthesis of 5-(hydroxymethyl)furfural (HMF) in ionic liquids is a field that has grown rapidly in recent years. Unique dissolving properties for crude biomass in combination with a high selectivity for HMF formation from hexose sugars make ionic liquids attractive reaction media for the production of chemicals from renewable resources. A wide range of new catalytic systems that are unique for the transformation of glucose and fructose to HMF in ionic liquids has been found. However, literature examples of scale-up and process development are still scarce, and future research needs to complement the new chemistry with studies on larger scales in order to find economically and environmentally feasible processes for HMF production in ionic liquids. This Minireview surveys important progress made in catalyst development for the synthesis of HMF in ionic liquids, and proposes future research directions in process technology.

Catalytic hydrolysis of lignocellulosic biomass into 5-hydroxymethylfurfural in ionic liquid

Production of 5-hydroxymethylfurfural (HMF) from cellulose catalyzed by solid acids and metal chlorides was studied in the 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) under microwave irradiation. Among the applied catalysts, the use of CrCl(3)/LiCl resulted in the highest yield of HMF. The effects of catalyst dosage (mole ratio of catalyst to glucose units in the feedstock) and reaction temperature on HMF yields were investigated to obtain optimal process conditions. With the 1:1 mol ratio of catalyst to glucose unit, the HMF yield reached 62.3% at 160°C for 10 min. Untreated wheat straw was also investigated as feedstock to produce HMF for the practical use of raw biomass, in which the HMF yield was comparable to that from pure cellulose. After the extraction of HMF, [BMIM]Cl and CrCl(3)/LiCl could be reused and exhibited no activity loss after three successive runs.

Conversion of biomass into 5-hydroxymethylfurfural using solid acid catalyst

5-Hydroxymethylfurfural (HMF) was produced from monosaccharide (fructose and glucose), polysaccharide (inulin) and the Jerusalem artichoke juice by a simple one-pot reaction including hydrolysis and dehydration using solid acid under mild condition. Hydrated niobium pentoxide (Nb(2)O(5)·nH(2)O(2)) after pretreatment showed high catalytic activities for dehydration of mono- and polysaccharide to HMF at 433 K in water-2-butanol (2:3 v/v) biphasic system, giving high HMF yield of 89% and 54% from fructose and inulin, respectively. The HMF yield was up to 74% and 65% when inulin and Jerusalem artichoke juice were hydrolyzed by exoinulinase. The solid acid made the process environment-friendly and energy-efficient to convert carbohydrates into bio-fuels and platform chemicals.

Fast transformation of glucose and di-/polysaccharides into 5-hydroxymethylfurfural by microwave heating in an ionic liquid/catalyst system

An efficient method for converting glucose into 5-hydroxymethylfurfural (5-HMF), in the presence of CrCl3 catalyst, is developed by using the ionic liquid 1-butyl-3-methyl imidazolium chloride as solvent. A 5-HMF yield of 71 % is achieved in 30 s for 96 % glucose conversion with microwave heating at 140 °C. The activation energy of glucose conversion is determined to be 114.6 kJ mol(-1), with a pre-exponential factor of 3.5 x 10(14) min(-1). Fructose, sucrose, cellobiose, and cellulose are studied and 5-HMF yields of 54 % are obtained for cellulose conversion at 150 °C during 10 min of reaction time. Recycling of the ionic liquid and CrCl3 is demonstrated with six cycles of use.

Acid-catalysed conversion of saccharides into furanic aldehydes in the presence of three-dimensional mesoporous Al-TUD-1

The one-pot acid-catalysed conversion of mono/di/polysaccharides (inulin, xylan, cellobiose, sucrose, glucose, fructose, xylose) into 2-furfuraldehyde (FUR) or 5-hydroxymethylfurfural (HMF) in the presence of aluminium-containing mesoporous TUD-1 (denoted as Al-TUD-1, Si/Al = 21), at 170 ºC was investigated. Xylose gave 60% FUR yield after 6 h reaction; hexose-based mono/disaccharides gave less than 20% HMF yield; polysaccharides gave less than 20 wt % FUR or HMF yields after 6 h. For four consecutive 6 h batches of the xylose reaction in the presence of Al-TUD-1, the FUR yields achieved were similar, without significant changes in Si/Al ratio.

Direct, high-yield conversions of cellulose into biofuel and platform chemicals-on the way to a sustainable biobased economy

Steering away from alcohol: Fermentation of carbohydrates to ethanol might not be the best way to utilize biomass for the production of fuels and platform chemicals. Two different new remarkable approaches lead to polyols or furfural derivatives.

Isotope labeling studies on the formation of 5-(hydroxymethyl)-2-furaldehyde (HMF) from sucrose by pyrolysis-GC/MS

Although it is generally assumed that the reactivity of sucrose, a nonreducing sugar, in the Maillard reaction is due to its hydrolysis into free glucose and fructose, however, no direct evidence has been provided for this pathway, especially in dry and high temperature systems. Using specifically (13)C-labeled sucrose at C-1 of the fructose moiety, HMF formation was studied at different temperatures. Under dry pyrolytic conditions and at temperatures above 250 degrees C, 90% of HMF originated from fructose moiety and only 10% originated from glucose. Alternatively, when sucrose was refluxed in acidic methanol at 65 degrees C, 100% of HMF was generated from the glucose moiety. Moreover, the relative efficiency of the known HMF precursor 3-deoxyglucosone to generate HMF was compared to that of glucose, fructose and sucrose. Glucose exhibited a much lower conversion rate than 3-deoxyglucosone, however, both fructose and sucrose showed much higher conversion rates than 3-deoxyglucosone thus precluding it as a major precursor of HMF in fructose and sucrose solutions. Based on the data generated, a mechanism of HMF formation from sucrose is proposed. According to this proposal sucrose degrades into glucose and a very reactive fructofuranosyl cation. In dry systems this cation can be effectively converted directly into HMF.

Formation of furan and methylfuran by maillard-type reactions in model systems and food

The formation of furan and 2-methylfuran was studied in model systems based on sugars and selected amino acids. Both compounds were preferably formed under roasting conditions in closed systems yielding up to 330 micromol of furan and 260 micromol of 2-methylfuran per mol of precursor. The amounts obtained under pressure cooking conditions were much lower, usually below 20 micromol/mol, except for 2-furaldehyde, which yielded 70-100 micromol/mol of furan. Labeling studies indicated two major formation pathways for both furans: (i) from the intact sugar skeleton and (ii) by recombination of reactive C(2) and/or C(3) fragments. Under roasting conditions in the absence of amino acids, furan was mainly formed from the intact sugar skeleton. Formic and acetic acid were identified as byproducts of sugar degradation, indicating the split off of C(1) and/or C(2) units from hexoses. The presence of alanine, threonine, or serine promoted furan formation by the recombination of C(2) fragments, such as acetaldehyde and glycolaldehyde, which may originate from both sugars and amino acids. In aqueous solution, about half of furan was generated by the recombination of sugar fragments. 2-Methylfuran was preferably formed in the presence of amino acids by aldol-type reactions of C(2) and C(3) fragments with lactaldehyde as a key intermediate, the Strecker aldehyde of threonine. The total furan levels in cooked vegetables were increased by spiking with hexoses. However, in pumpkin puree, only about 20% of furan was formed from sugars, preferably from the intact carbon skeleton.

Browning of furfural and amino acids, and a novel yellow compound, furpipate, formed from lysine and furfural

Furfural is an important intermediate compound of the Maillard reaction of pentose or ascorbic acid. We examined the browning of furfural and lysine by heating and found a yellow compound, called furpipate, (E)-3-(2-furylmethylidene)-3H, 4H, 5H, 6H-pyridine-2-carboxylic acid. Furpipate is a novel pipecolic acid derivative and shows absorption maxima at 375 nm and 310 nm under acidic and alkaline conditions, respectively. This compound was the major colored compound of the heated solution containing lysine and furfural.

Acidic cesium salts of 12-tungstophosphoric acid as catalysts for the dehydration of xylose into furfural

Cesium salts of 12-tungstophosphoric acid, Cs(x)H(3-x)PW(12)O(40) (Cs(x)PW), in the bulk form or supported on medium-pore MCM-41 (3.7 nm) or large-pore (9.6 nm) micelle-templated silicas are active solid acid catalysts for the cyclodehydration of xylose into furfural, in a toluene/water solvent system (T/W) or in dimethyl sulfoxide (DMSO). The catalytic results are comparable to those obtained using sulfuric acid, under similar reaction conditions. The initial activities increase in the order H(3)PW(12)O(40)<Cs(2.0)PW<Cs(2.5)PW<silica-supported CsPW catalysts (15, 34wt% PW). Higher HPA loadings, larger pore diameter of the parent silica support, and higher reaction temperatures lead to higher furfural yields. The stability and reusability of the MCM-41-supported CsPW is higher in DMSO than in T/W.

Glucose reactions with acid and base catalysts in hot compressed water at 473 K

The effects of the homogeneous catalysts (H(2)SO(4) and NaOH) and heterogeneous catalysts (TiO(2) and ZrO(2)) on glucose reactions were examined in hot compressed water (473 K) by a batch-type reactor. From the homogeneous catalyst studies, we confirmed that the acid catalyst promoted dehydration, while isomerization of glucose to fructose was catalyzed by alkali. Anatase TiO(2) was found to act as an acid catalyst to promote formation of 5-hydroxymethylfuraldehyde (HMF). Zirconia (ZrO(2)) was a base catalyst to promote the isomerization of glucose. The effects of the additives were also confirmed through fructose reactions.

Design and synthesis of a series of melamine-based nitroheterocycles with activity against Trypanosomatid parasites

The parasites that give rise to human African trypanosomiasis (HAT) are auxotrophs for various nutrients from the human host, including purines. They have specialist nucleoside transporters to import these metabolites. In addition to uptake of purine nucleobases and purine nucleosides, one of these transporters, the P2 transporter, can carry melamine derivatives; these derivatives are not substrates for the corresponding mammalian transporters. In this paper, we report the coupling of the melamine moiety to selected nitro heterocycles with the aim of selectively delivering these compounds to the parasites. Some compounds prepared have similar in vitro trypanocidal activities as melarsoprol, the principal drug used against late-stage HAT, with 50% growth inhibitory concentrations in the submicromolar range. Selected compounds were also evaluated in vivo in rodent models infected with Trypanosoma brucei brucei and T. brucei rhodesiense and showed pronounced activity and in two cases were curative without overt signs of toxicity. Compounds were also tested against other trypanosomatid pathogens, Leishmania donovani and Trypanosoma cruzi, and significant activity in vitro was noted for T. cruzi against which various nitro heterocycles are already registered for use.

Synthesis, structure and biological activity of nickel(II) complexes of 5-methyl 2-furfural thiosemicarbazone

5-Methyl 2-furfuraldehyde thiosemicarbazone (M5HFTSC) with nickel(II) leads to three types of complexes: [Ni(M5HFTSC)(2)X(2)], [Ni(M5FTSC)(2)] and [Ni(M5FTSC)(2)] x 2DMF. In the first type the ligand remains in thione form, while in the two other, the anionic thiolato form is involved. The species [Ni(M5HFTSC)(2)X(2)] has been characterized spectroscopically. The structures of [Ni(M5FTSC)(2)] x 2DMF and [Ni(M5FTSC)(2)] have been solved using X-ray diffraction. Biological studies of [Ni(M5HFTSC)(2)Cl(2)] have been carried out in vitro for antifungal activity on human pathogenic fungi, Aspergillus fumigatus and Candida albicans, and in vivo for toxicity on mice. The results are compared to those of the ligand, the metal salt and a similar copper complex [Cu(M5HFTSC)Cl(2)].