Chemo-enzymatic conversion of glucose into 5-hydroxymethylfurfural in seawater

Study on the role of alkali halides on the mutarotation and dehydration of d-xylose in aqueous solution

Although the xylose mutarotation and transformation have been investigated largely separately, their relationship has been rarely systematically elaborated. The effect of several factors such as xylose concentration, temperature, and salt concentration, affecting the mutarotation of xylose are discussed. Nine alkali halides (LiCl, NaCl, KCl, LiBr, NaBr, KBr, LiI, NaI, and KI) are used to test salt effects. The relationship between xylose rotation rate constant (kM), specific optical rotation at equilibrium ([α]eqm), α/β ratio, H chemical shift difference (ΔΔδ), Gibbs free energy difference (ΔG), hydrogen ion or hydroxide ion concentration ([H+] or [OH-]), and xylose conversion is discussed. Different salts dissolved in water result in different pH of the solutions, which affect the mutarotation of xylose, with the nature of both cation and anion. Shortly, the smaller the cation radius is and the larger the anion radius is, the greater the mutarotation rate is. In the dehydration of xylose to furfural in salty solutions, xylose conversion is positively correlated to mutarotation rate, H+ or OH- concentration, and the energy difference between α-xylopyranose and β-xylopyranose. Although the [α]eqm of xylose is positively correlated with α/β configuration ratio, there is no obvious correlation with xylose dehydration. The conversion to furfural in chlorides is superior to that in bromines and iodides, which is due to the fact that the pH of chloride salts is smaller than that of the corresponding bromide and iodized salts. Higher H+ concentration prefers to accelerate the formation of furfural. In basic salt solutions, the xylulose selectivity is higher than that of furfural at the initial stage of reaction. The furfural selectivity and carbon balance are better in acidic condition rather than in basic condition. In H2O-MTHF (2-Methyltetrahydrofuran) biphasic system, the optimal furfural selectivity of 81.0 % is achieved at 190 °C in 1 h with the assistance of LiI and a little HCl (0.2 mmol, 8 mmol/L in aqueous phase). A high mutarotation rate represents rapid xylose conversion, but a high furfural selectivity prefers in acidic solutions, which would be perfect if organic solvents were available to form biphasic systems.

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

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

Recent developments in the biology and biotechnological applications of halotolerant yeasts

Natural hypersaline environments are inhabited by an abundance of prokaryotic and eukaryotic microorganisms capable of thriving under extreme saline conditions. Yeasts represent a substantial fraction of halotolerant eukaryotic microbiomes and are frequently isolated as food contaminants and from solar salterns. During the last years, a handful of new species has been discovered in moderate saline environments, including estuarine and deep-sea waters. Although Saccharomyces cerevisiae is considered the primary osmoadaptation model system for studies of hyperosmotic stress conditions, our increasing understanding of the physiology and molecular biology of halotolerant yeasts provides new insights into their distinct metabolic traits and provides novel and innovative opportunities for genome mining of biotechnologically relevant genes. Yeast species such as Debaryomyces hansenii, Zygosaccharomyces rouxii, Hortaea werneckii and Wallemia ichthyophaga show unique properties, which make them attractive for biotechnological applications. Select halotolerant yeasts are used in food processing and contribute to aromas and taste, while certain gene clusters are used in second generation biofuel production. Finally, both pharmaceutical and chemical industries benefit from applications of halotolerant yeasts as biocatalysts. This comprehensive review summarizes the most recent findings related to the biology of industrially-important halotolerant yeasts and provides a detailed and up-to-date description of modern halotolerant yeast-based biotechnological applications.

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

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

Conversion of Glucose to 5-Hydroxymethylfurfural, Levulinic Acid, and Formic Acid in 1,3-Dibutyl-2-(2-butoxyphenyl)-4,5-diphenylimidazolium Iodide-Based Ionic Liquid

The separation process between 5-hydroxymethylfurfural (HMF) and trace glucose in glucose conversion is important in the biphasic system (aqueous–organic phase), due to the partial solubility property of HMF in water. In addition, the yield of HMF via the dehydration reaction of glucose in water is low (under 50%) with the use of Brønsted acid as a catalyst. Therefore, this study was conducted to optimize the production and separation of products by using a new hydrophobic ionic liquid (IL), which is more selective than water. The new IL (1,3-dibutyl-2-(2-butoxyphenyl)-4,5-diphenyl imidazolium iodide) [DBDIm]I was used as a solvent and was optimized for the dehydration reaction of glucose to make a more selective separation of HMF, levulinic acid (LA), and formic acid (FA). [DBDIm]I showed high performance as a solvent for glucose conversion at 100 °C for 120 min, with a yield of 82.2% HMF, 14.9% LA, and 2.9% FA in the presence of sulfuric acid as the Brønsted acid catalyst.

Coproduction of xylooligosaccharides and fermentable sugars from sugarcane bagasse by seawater hydrothermal pretreatment

In this study, natural seawater without additional chemicals was selected to treat sugarcane bagasse for the production of xylooligosaccharides and glucose. This pretreatment not only more effectively conserves freshwater resources than hydrothermal pretreatment and enzymatic hydrolysis, but also decreases corrosion of the equipment relative to techniques utilizing acid and alkaline pretreatment. The maximum yield of 67.12% xylooligosaccharides (of initial xylan), including 11.49% xylobiose, 16.23% xylotriose, 23.82% xylotetraose, and 15.58% xylopentaose was obtained under mild condition (175 °C for 30 min). Moreover, greater amounts of xylotetraose were generated during seawater hydrothermal pretreatment under all conditions, likely because NaCl in seawater cut the hydrogen bonds between xylo-oligomers. In addition, 94.69% cellulose digestibility and 78.58% xylan digestibility were achieved from the solid residue with an enzyme dosage of 30 FPU/g cellulose. Results indicated that seawater hydrothermal pretreatment is a more environmentally-friendly and sustainable technique for producing xylooligosaccharides and fermentable sugars than other methods.

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

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

Biocatalysis in seawater: Investigating a halotolerant ω-transaminase capable of converting furfural in a seawater reaction medium

The increasing demand for freshwater and the continued depletion of available resources has led to a deepening global water crisis. Significant water consumption required by many biotechnological processes contributes to both the environmental and economic cost of this problem. Relatively few biocatalytic processes have been developed to utilize the more abundant supply of seawater, with seawater composition and salinity limiting its use with many mesophilic enzymes. We recently reported a salt tolerant ω-transaminase enzyme, Ad2-TAm, isolated from the genome of a halophilic bacterium, Halomonas sp. CSM-2, from a Triassic period salt mine. In this study we aimed to demonstrate its applicability to biocatalytic reactions carried out in a seawater-based medium. Ad2-TAm was examined for its ability to aminate the industrially relevant substrate, furfural, in both seawater and freshwater-based reaction systems. Furfural was aminated with 53.6% conversion in a buffered seawater system, displaying improved function versus freshwater. Ad2-TAm outperformed the commonly employed commercial ω-TAms from Chromobacterium violaceum and Vibrio fluvialis, both of which showed decreased conversion in seawater. Given the increasingly precarious availability of global freshwater, such applications of enzymes from halophiles have the ability to reduce demand for freshwater in large-scale industrial processes, delivering considerable environmental and economic benefits.

Cyclopentyl Methyl Ether (CPME): A Versatile Eco-Friendly Solvent for Applications in Biotechnology and Biorefineries

The quest for sustainable solvents is currently a matter of intense research and development, as solvents significantly contribute heavily to the waste generated by chemical industries. Cyclopentyl methyl ether (CPME) is a promising eco-friendly solvent with valuable properties such as low peroxide formation rate, stability under basic and acidic conditions, and relatively high boiling point. This Review discusses the potential use of CPME for applications in biotechnology (e.g., biotransformations, as solvent or cosolvent), biorefineries, and bioeconomy (e.g., for furan synthesis or as an extractive agent in liquid-liquid separations), as well as for other purposes, such as chromatography or peptide synthesis. Although CPME is currently produced by petrochemical means with a remarkably high atom economy, its biogenic production can be envisaged from substrates such as cyclopentanol or cyclopentanone, which can be derived from furfural or from (bio-based) adipic acid, respectively. The combination of the promising properties of CPME as a (co)solvent with a future (economic) biogenic origin would be advantageous for setting strategies aligned with the sustainable chemistry principles.

Hydrothermal Dehydration of Monosaccharides Promoted by Seawater: Fundamentals on the Catalytic Role of Inorganic Salts

In biorefining, the conversion of carbohydrates under subcritical water conditions is a field of extensive studies. In particular, the hydrothermal decomposition of benchmark C6- and C5-monosaccharides, i.e., D-glucose and D-xylose, into furanics and/or organic acids is fully considered. Herein, we propose to establish the fundamentals of the decomposition of D-glucose and D-xylose under subcritical water conditions in the presence of specific salts (i.e., NaCl and KI) and in seawater. Our results demonstrated that the introduction of inorganic salts was found to modify sugars dehydration yields. Different NaCl concentrations from 0.21 to 1.63 mol L-1 promoted the conversion of D-xylose to 2-furfural (2-F) from 28 to 44% (molar yield). NaCl also improved 5-hydroxymethylfurfural (5-HMF) generation from D-glucose as well as rehydration of 5-HMF to levulinic and formic acid. KI favored other pathways toward formic acid production from D-glucose, reaching 20% in the upper concentration. Compared to a solution of equivalent NaCl concentration, seawater enhanced selectivity toward lactic acid which was raised by 10% for both monosaccharides, and sugars conversion, especially for D-glucose whose conversion was increased by 20%. 5-HMF molar yield around 30% were achieved from D-glucose in seawater at 211°C and 20 bars after 15 min.

Development of Betaine-Based Sustainable Catalysts for Green Conversion of Carbohydrates and Biomass into 5-Hydroxymethylfurfural

Renewable and sustainable betaine-based catalysts (BX) derived from the betaine sugar industry or ChCl were developed for the production of 5-hydroxymethylfurfural (HMF) from various carbohydrates. The HMF yields in the BX-based media reached up to 88 %, 66 %, 37 % and 53 %, for the conversion of fructose, glucose, cellulose, and lignocellulosic biomass, respectively. In addition, choline-O-sulfate was synthesized and demonstrated to be an efficient catalyst for the conversion of fructose to HMF. From the perspective of green and sustainable chemistry, this work demonstrates benefits not only in the preparation of sustainable catalysts but also the green production of HMF from biomass.

Microemulsion and Sol-Gel Synthesized ZrO₂-MgO Catalysts for the Liquid-Phase Dehydration of Xylose to Furfural

Two series of catalysts were prepared by sol-gel and microemulsion synthetic procedure (SG and ME, respectively). Each series includes both pure Mg and Zr solids as well as Mg-Zr mixed solids with 25%, 50% and 75% nominal Zr content. The whole set of catalysts was characterized from thermal, structural and surface chemical points of view and subsequently applied to the liquid-phase xylose dehydration to furfural. Reactions were carried out in either a high-pressure autoclave or in an atmospheric pressure multi-reactor under a biphasic (organic/water) reaction mixture. Butan-2-ol and toluene were essayed as organic solvents. Catalysts prepared by microemulsion retained part of the surfactant used in the synthetic procedure, mainly associated with the Zr part of the solid. The MgZr-SG solid presented the highest surface acidity while the Mg3Zr-SG one exhibited the highest surface basicity among mixed systems. Xylose dehydration in the high-pressure system and with toluene/water solvent mixture led to the highest furfural yield. Moreover, the yield of furfural increases with the Zr content of the catalyst. Therefore, the catalysts constituted of pure ZrO₂ (especially Zr-SG) are the most suitable to carry out the process under study although MgZr mixed solids could be also suitable for overall processes with additional reaction steps.

Heterogeneously Catalyzed Hydrothermal Processing of C5-C6 Sugars

Biomass has been long exploited as an anthropogenic energy source; however, the 21st century challenges of energy security and climate change are driving resurgence in its utilization both as a renewable alternative to fossil fuels and as a sustainable carbon feedstock for chemicals production. Deconstruction of cellulose and hemicellulose carbohydrate polymers into their constituent C₅ and C₆ sugars, and subsequent heterogeneously catalyzed transformations, offer the promise of unlocking diverse oxygenates such as furfural, 5-hydroxymethylfurfural, xylitol, sorbitol, mannitol, and gluconic acid as biorefinery platform chemicals. Here, we review recent advances in the design and development of catalysts and processes for C₅-C₆ sugar reforming into chemical intermediates and products, and highlight the challenges of aqueous phase operation and catalyst evaluation, in addition to process considerations such as solvent and reactor selection.

One-pot catalytic conversion of carbohydrates into furfural and 5-hydroxymethylfurfural

Recently, there has been growing interest in the transformation of renewable biomass into value-added chemicals and biofuels.

Seawater as Alternative to Freshwater in Pretreatment of Date Palm Residues for Bioethanol Production in Coastal and/or Arid Areas

The large water consumption (1.9-5.9 m(3) water per m(3) of biofuel) required by biomass processing plants has become an emerging concern, which is particularly critical in arid/semiarid regions. Seawater, as a widely available water source, could be an interesting option. This work was to study the technical feasibility of using seawater to replace freshwater in the pretreatment of date palm leaflets, a lignocellulosic biomass from arid regions, for bioethanol production. It was shown that leaflets pretreated with seawater exhibited lower cellulose crystallinity than those pretreated with freshwater. Pretreatment with seawater produced comparably digestible and fermentable solids to those obtained with freshwater. Moreover, no significant difference of inhibition to Saccharomyces cerevisiae was observed between liquids from pretreatment with seawater and freshwater. The results showed that seawater could be a promising alternative to freshwater for lignocellulose biorefineries in coastal and/or arid/semiarid areas.

Orthogonal tandem catalysis

Tandem catalysis is a growing field that is beginning to yield important scientific and technological advances toward new and more efficient catalytic processes. 'One-pot' tandem reactions, where multiple catalysts and reagents, combined in a single reaction vessel undergo a sequence of precisely staged catalytic steps, are highly attractive from the standpoint of reducing both waste and time. Orthogonal tandem catalysis is a subset of one-pot reactions in which more than one catalyst is used to promote two or more mechanistically distinct reaction steps. This Perspective summarizes and analyses some of the recent developments and successes in orthogonal tandem catalysis, with particular focus on recent strategies to address catalyst incompatibility. We also highlight the concept of thermodynamic leveraging by coupling multiple catalyst cycles to effect challenging transformations not observed in single-step processes, and to encourage application of this technique to energetically unfavourable or demanding reactions.

Production of 5-hydroxymethylfurfural from fructose by a thermo-regulated and recyclable Brønsted acidic ionic liquid catalyst

A thermo-regulated recyclable ionic liquid catalyst bearing acidic functional group for fructose dehydration to produce HMF in 91.2% yield.

Dehydration of different ketoses and aldoses to 5-hydroxymethylfurfural

5-Hydroxymethylfurfural (HMF) is considered an important building block for future bio-based chemicals. Here, we present an experimental study using different ketoses (fructose, sorbose, tagatose) and aldoses (glucose, mannose, galactose) under aqueous acidic conditions (65 g L(-1) substrate, 100-160 °C, 33-300 mM H2 SO4 ) to gain insights into reaction pathways for hexose dehydration to HMF. Both reaction rates and HMF selectivities were significantly higher for ketoses than for aldoses, which is in line with literature. Screening and kinetic experiments showed that the reactivity of the different ketoses is a function of the hydroxyl group orientation at the C3 and C4 positions. These results, in combination with DFT calculations, point to a dehydration mechanism involving cyclic intermediates. For aldoses, no influence of the hydroxyl group orientation was observed, indicating a different rate-determining step. The combination of the knowledge from the literature and the findings in this work indicates that aldoses require an isomerization to ketose prior to dehydration to obtain high HMF yields.

Electrocatalytic hydrogenation of 5-hydroxymethylfurfural in the absence and presence of glucose

Electrocatalytic hydrogenation of 5-hydroxymethylfurfural (HMF) to 2,5-dihydroxymethylfuran (DHMF) or other species, such as 2,5-dimethylfuran, on solid metal electrodes in neutral media is addressed, both in the absence and in the presence of glucose. The reaction is studied by combining voltammetry with on-line product analysis by using HPLC, which provides both qualitative and quantitative information about the reaction products as a function of electrode potential. Three groups of catalysts show different selectivity towards: (1) DHMF (Fe, Ni, Ag, Zn, Cd, and In), (2) DHMF and other products (Pd, Al, Bi, and Pb), depending on the applied potential, and (3) other products (Co, Au, Cu, Sn, and Sb) through HMF hydrogenolysis. The rate of electrocatalytic HMF hydrogenation is not strongly catalyst-dependent because all catalysts show similar onset potentials (-0.5 ± 0.2 V) in the presence of HMF. However, the intrinsic properties of the catalysts determine the reaction pathway towards DHMF or other products. Ag showed the highest activity towards DHMF formation (up to 13.1 mM cm(-2) with high selectivity> 85%). HMF hydrogenation is faster than glucose hydrogenation on all metals. For transition metals, the presence of glucose enhances the formation of DHMF and suppresses the hydrogenolysis of HMF. On poor metals such as Zn, Cd, and In, glucose enhances DHMF formation; however, its contribution in the presence of Bi, Pb, Sn, and Sb is limited. Remarkably, in the presence of HMF, glucose hydrogenation itself is largely suppressed or even absent. The first electron-transfer step during HMF reduction is not metal-dependent, suggesting a non-catalytic reaction with proton transfer directly from water in the electrolyte.

Integrated chemo-enzymatic production of 5-hydroxymethylfurfural from glucose

Sweets for my sweet: The production and isolation of 5-hydroxymethylfurfural (HMF) in high yield and purity is demonstrated by using a combination of glucose-fructose isomerization with sweetzyme in wet tetraethylammonium bromide (TEAB) and clean fructose dehydration to HMF catalyzed by using HNO₃ under moderate conditions, which allow the reuse of any unreacted glucose and TEAB.

Lipase-mediated selective oxidation of furfural and 5-hydroxymethylfurfural

Furfural and 5-hydroxymethylfurfural (HMF) are important biomass-derived platform chemicals that can be obtained from the dehydration of lignocellulosic sugars. A possible route for the derivatization of furanics is their oxidation to afford a broad range of chemicals with promising applications (e.g., diacids, hydroxyl acids, aldehyde acids, monomers for novel polymers). Herein we explore the organic peracid-assisted oxidation of furanics under mild reaction conditions. Using lipases as biocatalysts, alkyl esters as acyl donors, and aqueous solutions of hydrogen peroxide (30 % v/v) added stepwise, peracids are formed in situ, which subsequently oxidize the aldehyde groups to afford carboxylic acids with high yields and excellent selectivities. Furthermore, the use of an immobilized silica-based 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) affords the selective oxidation of the hydroxymethyl group of HMF to afford 2,5-diformylfuran. That product can be subsequently oxidized using again lipases for the in situ peracid formation to yield 2,5-furandicarboxylic acid, which is considered to be a key building block for biorefineries. These lipase-mediated reactions proceeded efficiently even with high substrate loadings under still non-optimized conditions. Overall, a proof-of-concept for the oxidation of furanics (based on in situ formed organic peracids as oxidants) is provided.

Lipase-catalyzed (trans)esterification of 5-hydroxy- methylfurfural and separation from HMF esters using deep-eutectic solvents

5-Hydroxymethylfurfural (HMF) is a valuable biomass-derived building block. Among possible HMF valorization products, a broad range of HMF esters can be synthesized. These HMF esters have found some promising applications, such as monomers, fuels, additives, surfactants, and fungicides, and thus several catalytic approaches for HMF (trans)esterifications have been reported. The intrinsic reactivity of HMF is challenging, forcing the use of mild reaction conditions to avoid by-product formation. This paper explores the lipase-catalyzed (trans)esterification of HMF with different acyl donors (carboxylic acids and methyl- and ethyl esters) mostly in solvent-free conditions. The results demonstrate that lipases may be promising alternatives for the synthesis of HMF esters-with high productivities and reactions at high substrate loadings-provided that robust systems for lipase immobilization are applied to assure an adequate reusability of the enzymes. Once (trans)esterifications have been conducted, the separation of unreacted HMF and HMF esters is performed by using deep-eutectic solvents (DES) as separation agents. DES are able to dissolve hydrogen-bond donors (e.g., HMF), whereas non-hydrogen-bond donors (in this case HMF esters) form a second phase. By using this approach, high ester purities (>99 %) and efficiencies (up to >90 % HMF ester recovery) in separations were obtained by using choline chloride-based DES.