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An efficient catalytic system for producing 5-hydroxymethylfurfural from high-concentration fructose has been built by creating micro-reactors with cetyltrimethylammonium bromide in a mixed solvent of 1,4-dioxane and water.

A universal understanding on the concentration-aggravated evolution of humins during fructose dehydration has been demonstrated, wherein difructose anhydrides act as the key intermediates for both 5-hydroxymethylfurfural and humin formations.

The high-efficiency conversion of fructose to 5-hydroxymethylfurfural (HMF) with approving yields is challenging in biorefinery. In this work, we develop an efficient method of preparing HMF from f...

The degradative condensation of hexose, which originates from the C-C cleavage of hexose and condensation of degraded hexose fragment, is one of the possible reaction pathways for the formation of humins in hexose dehydration to 5-hydroxymethylfurfural (HMF). Herein, the impacts of several polar aprotic solvents on the degradative condensation of fructose to small-molecule carboxylic acids and oligomers (possible precursors of humins) are reported. In particular, a close relationship between the tautomeric distribution of fructose in solvents and the mechanism of degradative condensation is demonstrated. Typically, α-fructofuranose in 1,4-dioxane and acyclic open-chain fructose in THF favor the conversion of fructose to formic acid and oligomers; α-fructopyranose in γ-valerolactone or N-methylpyrrolidone favors levulinic acid and oligomers, whereas β-fructopyranose in 4-methyl-2-pentanone favors acetic acid and corresponding oligomers. This close correlation highlights a general understanding of the solvent-controlled formation of oligomers, which represents an important step toward the rational design of effective solvent systems for HMF production.

We report a novel solid organocatalyst, a double-hydrogen-bonded sulfonated polymer catalyst (D-SPC), for the cascade conversion of fructose to 5-ethoxymethylfurfural (EMF) with a highest yield of 68.8 mol% and good catalyst recyclability. We demonstrate that the treatment of a sulfonated nitrogen-containing polymer catalyst (SPC) with dihydroxy acetone (DHA) molecules generates new double hydrogen-bonds (H-bonds) between the ring-attached sulfonic acid groups and DHA by deconstructing the existing H-bonds between the sulfonic acid groups and imine/amine nitrogens of the SPC, which is confirmed by FT-IR spectroscopy, elemental analysis, 1H MAS and 13C cross-polarization MAS NMR, X-ray photoelectron spectroscopy (XPS), and quantum chemical calculations. The 31P MAS NMR, pHsurface, and NH3-TPD of the catalysts reveal the weakening of acidity strength due to the stronger double H-bonding on the D-SPC. Notably, we figure out the inverse relationship between the acidity strength of the catalysts and the EMF yield in the one-pot fructose-to-EMF conversion, wherein the use of weaker acid catalysts accelerates the fructose dehydration while decelerating the side-reaction of furan ring opening, therefore avoiding the negative influence of water on the tandem reactions. Moreover, the existence of stable double H-bonds contributes to the good catalyst recyclability due to the stabilization of sulfonic acid groups. The acidic property of the solid catalysts could be adjusted by varying the DHA dosage and the preparation conditions of the SPC (i.e., monomer ratio and oxidant dosage), respectively. We show that the solid acid catalyst with weaker acidity strength and suitable acidity density (ca. 2.5 mmol g−1) is favorable for the one-pot synthesis of EMF directly from fructose. This work highlights the application of novel sulfonated polymer catalysts with adjustable acidic property for the highly selective production of EMF, a liquid biofuel candidate, from biomass-derived carbohydrates.

Biomass-derived hexose constitutes the main component of lignocellulosic biomass for producing value-added chemicals and biofuels. However, the reaction network of hexose is complicated, which makes the highly selective synthesis of one particular product challenging in biorefinery. This Review focuses on the selective production of 5-hydroxymethylfurfural (HMF) from glucose on account of its potential significance as an important platform molecule. The complex reaction network involved in glucose-to-HMF transformations is briefly summarized. Special emphasis is placed on analyzing the complexities of feedstocks, intermediates, (side-) products, catalysts, solvents, and their impacts on the reaction network. The strategies and representative examples for adjusting the reaction pathway toward HMF by developing multifunctional catalysts and promoters, taking advantage of solvent effects and process intensification, and synergizing all measures are comprehensively discussed. An outlook is provided to highlight the challenges and opportunities faced in this promising field. It is expected to provide guidance to design practical catalytic processes for advancing HMF biorefinery.

Novel polymeric solid catalysts, Au@(polythiophene–polythiophene oxides) (Au@PTh–PThOx, x=1–2), are prepared by chemical over-oxidative polymerization of thiophene in the presence of Au nanoparticles. These organocatalysts show significantly higher activity than their liquid analogs (i.e., DMSO, a frequently-used solvent and catalyst) for the catalytic dehydration of fructose into 5-hydroxymethylfurfural (HMF) and exhibit a complete inhibiting effect on HMF rehydration to undesired levulinic acid in low-boiling solvents. We demonstrate that the sulfoxide (-SO) group in Au@PTh–PThOx is more active than the sulfone (-SO2) group, although the formation of the former could be maximized by lowering the concentration of surfactant or improving the concentration of oxidant used in catalyst preparation. The highest HMF yields of 72.6 % and 39.7 % are achieved over Au@PTh–PThO catalyst in 1,4-dioxane and water, respectively. This catalyst could be completely recycled for use in water without activity loss.

The performance of DMSO for the dehydration of fructose to 5-hydroxymethylfurfural (HMF) in the presence and absence of Bronsted acid has been theoretically investigated at G4 level in DMSO solution. The most stable form of fructose is β-D-fructofuranoses in DMSO solution. When Bronsted acid is present in the reaction system, H+ prefers to interact with DMSO other than fructose, forming [DMSOH]+ as the catalytically active species. For the dehydration of fructose to HMF, the catalytic performance of DMSO stems from the valence unsaturation of both S and O atoms and the unsaturated double bond of S═O, and the catalytic role of [DMSOH]+ originates from the valence unsaturation of both S and O atoms, as well as the H-mediated effect of −SOH group. For the initial and third water molecule release steps from fructose, both DMSO and [DMSOH]+ exhibit catalytic activity. Nevertheless, for the second water molecule release step from fructose, [DMSOH]+ displays catalytic activity, but DMSO does not. The active spec...

The transformation of readily available cellulosic biomass to small-molecule products is important in biorefining. To improve the economic viability, the formation of humins should be limited. Herein, we report the effective suppression of humin precursor (i.e., oligomer) formation in glucose-to-5-hydroxymethylfurfural (HMF) dehydration by the synergetic action of tetrahydrofuran (THF) and low-pressure CO2 (1.0 MPa). We show that four kinds of oligomers are formed in water, wherein the cross-condensation of HMF and levulinic acid (LA) generates oligomers (I) and (II), and the self-condensation of HMF generates oligomer (III), while the degradation of glucose/fructose generates oligomer (IV). In CO2-splitted triphasic H2O/THF (VH2O/VTHF = 1 : 1), the formation of oligomers (I) and (IV) is suppressed by CO2, while the formation of oligomers (I), (II), and (III) is inhibited by THF. This work highlights a strategy for increasing carbon utilization from glucose conversion by controlling the subreactions to favor the pathways towards the formation of small-molecule products.

The rehydration of 5-hydroxymethylfurfural (HMF), an important bio-based chemical building block, to levulinic acid (LA) and formic acid (FA) over Bronsted acid catalysts is the key block to the effective production of HMF from hexose. In this work, we develop a novel acidic solid organocatalyst, sulfonated polyaniline (SPAN), for the effective dehydration of fructose into HMF in the low-boiling water/1,4-dioxane cosolvent. The highest HMF yield of 71% is obtained from fructose with complete restriction of HMF rehydration to LA. We demonstrate that hydrogen bonds form between the ring-attached sulfonic acid group and the quinoid imine nitrogen as a result of internal doping, which confines the Bronsted acidity of the SPAN catalyst. The H-bonded sulfonic acid species is active for fructose-to-HMF dehydration and complete suppression on HMF rehydration. The chemical bonding of sulfonic acid groups on the backbone of the PAN chain allows stable recyclability of the polymer catalyst. This work highlights the potential importance of confining Bronsted acidity on a solid organocatalyst via H-bonding for transforming renewable carbohydrates into fine chemicals.

We performed a systematic experimental kinetics study on AlCl3-catalyzed conversion of glucose to 5-hydroxymethylfurfural (HMF) in NaCl–H2O/tetrahydrofuran (THF) biphasic solvent. The kinetics model covers an extensive reaction network including the parallel and tandem reactions of isomerization, dehydration, decomposition, and polymerization from glucose. The accuracy of the model was verified by a parity plot and statistical significance analysis of the kinetic parameters. A deliberate insight into the intrinsic kinetic properties (reaction rate constant and apparent activation energy) of each subreaction elaborates the regulatory role of THF and NaCl on reaction pathways within the network. That is, THF suppresses the rehydration, degradation, and polymerization of HMF to unwanted byproducts, inhibits fructose-to-HMF dehydration and fructose-to-humins polymerization, but promotes the generation of formic acid (FA) from the direct degradation of both glucose and fructose by facilitating the generation o...

We report the preparation of bifunctional hybrid catalysts by supporting H3PMo12O40 (PMo12) heteropolyacid (HPA) on polyaniline (PAN) or formyl-functionalized PAN (F-PAN) for the &ldquo, one-pot&rdquo, and &ldquo, one-step&rdquo, synthesis of 2,5-diformylfuran (DFF) from fructose via 5-hydroxymethylfurfural (HMF) intermediate. We show that the PMo12 HPA is the main active species for both fructose dehydration and HMF oxidation owing to its Br&oslash, nsted acidic and redox characters. However, the anchoring of PMo12 on PAN reduces the Br&oslash, nsted acidity by acid&ndash, base interaction between protons in HPA and quinoid diimine structure in PAN, thereby reducing the dehydration performance. We demonstrate that the catalytic dehydration performance of the hybrid catalyst could be strengthened by grafting formyl groups on PAN before HPA anchoring. The highest DFF yield of 76.7% is obtained by conducting the &ldquo, reaction over the 40-PMo12/F3-PAN catalyst at 413 K for 7 h in air, wherein the side-reactions of fructose or HMF degradation and HMF rehydration have been significantly reduced. This hybrid catalyst is reusable without significant activity loss, highlighting the designing of stable inorganic&ndash, organic hybrid catalysts for producing valuable hexose-derived platform chemicals.

The selective removal of oxygen from 5-hydroxymethylfurfural (HMF) is challenging for the effective utilization of biomass. The catalytic mechanisms of palladium acetate toward the conversion of HMF to furfuryl alcohol (FFA), 5-methylfurfural (5-MF) and 2,5-dihydroxymethyl furan (DHMF) have been theoretically investigated. The decarbonylation of HMF to FFA includes (i) migratory extrusion, (ii) metal-acetate-co-assisted deprotonation, (iii) decarbonylation, (iv) metal-assisted deprotonation, and (v) migratory extrusion and catalyst regeneration. Both hydrogenation and deoxidation of HMF with HCOOH as the H-source involve (i) migratory extrusion, (ii) oxidative addition, (iii) reductive elimination, (iv) metal-assisted deprotonation, and (v) migratory extrusion and catalyst regeneration. The C–H bond cleavage is the crucial reaction step, in which the metal-acetate-co-assisted deprotonation is kinetically more preferable than the oxidative addition. Both FFA and DHMF are kinetically superior to 5-MF. In terms of selectivity, increasing the temperature is beneficial to decarbonylation and decreasing the temperature is advantageous to hydrogenation. The present finding provides molecular-level insight into the functions of both the metal-center and coordinated-ligand in the Pd(OAc)2 catalyst, which may drive the novel design of catalytic systems toward both decarbonylation and hydrogenation reactions.

We report an unprecedented solid organic-base catalyst, formyl-modified polyaniline (FS-PAN), for the dehydration of fructose to 5-hydroxymethylfurfural (HMF) with a high yield of 90.4 mol %. We demonstrate that the nitrogen atoms incorporated between the phenyl rings in the backbone of the polyaniline chain contribute to the basicity of the catalyst. The grafting of electron-withdrawing formyl groups to the imine nitrogen atoms leads to a significant increase of basicity of the polymer catalyst owing to the greater localization of electrons at the amide nitrogen atom formed. A linear dependence of the yield of HMF on the grafting level of formyl groups in FS-PAN indicates that the amide acts as the active phase. A possible reaction mechanism for this organic-base-catalyzed dehydration reaction is proposed. The side-reaction of HMF rehydration is inhibited thoroughly, and the condensation of any reaction intermediates to undesirable oligomers is restrained by this base catalyst. This organic-base catalyst can be recycled completely without loss of activity. This research highlights the first application of a highly effective and stable solid base catalyst for the transformation of renewable carbohydrates into fine chemicals.

Fructose was deoxygenated to furfuryl alcohol (FFA) by tandem dehydration and decarbonylation in one pot over the AlCl3⋅6 H2O/Pd(OAc)2 catalyst combination to give a high FFA yield of 40.6 %. AlCl3⋅6 H2O behaves as an effective Lewis acid to catalyze the dehydration of fructose to 5-hydroxymethylfurfural (HMF), and subsequently, Pd(OAc)2 catalyzes the removal of a CO moiety from HMF to produce FFA selectively. The hydroxyl group on the HMF intermediate was stabilized by poly(1-vinyl-2-pyrrolidinone) through an intermolecular hydrogen bond, which accelerated the dehydration of fructose and inhibited the hydrogenation of HMF by formic acid. The decarbonylation of HMF was promoted and the unwanted decomposition of fructose was inhibited through the use of 4 A molecular sieves. This research highlights a “one-pot” catalytic system to transform renewable carbohydrates into fine chemicals by tandem dehydration and decarbonylation reactions without the separation or purification of HMF.

The nature of the active aluminum species and their interaction with glucose in water are studied to establish a detailed mechanism for understanding AlCl3-catalyzed glucose-to-fructose isomerization. The combination of activity results with electrospray ionization tandem mass spectrometry (ESI-MS/MS) reveal that [Al(OH)2(aq)]+ species contribute a lot to the isomerization. Attenuated total reflection infrared spectroscopy (ATR-IR) results show that glucose undergoes a ring-opening process which is accelerated by the [Al(OH)2(aq)]+ species. The binding of acyclic glucose with [Al(OH)2(aq)]+ species occurs at the C1–O and C2–O positions of glucose, which initiates the hydride shift of the aldose-to-ketose isomerization. The in situ 27Al NMR data elucidate the maintenance of the hexa-coordinated form of Al species throughout the reaction. An obvious kinetic isotope effect occurs with the C2 deuterium-labeled glucose, confirming that the intramolecular hydride shift from the C2 to C1 positions of glucose is ...

Co-pyrolysis is regarded as an effective approach to upgrade the quality of pyrolysis products. In this work the activity of KF-846 was evaluated by co-pyrolysis of pubescens and low density polyethylene under different experimental conditions including catalytic mode, pyrolytic atmosphere and temperature, etc. The results showed that the fresh KF-846 exerted strong effects of cyclization, aromatization, hydrogen transfer and vapor-catalytic reforming reactions on the primary intermediates from the co-pyrolysis. The hydrogen-rich gases indicated a synergistic effect between Ni and Mo over KF-846 on producing hydrogen. More importantly, the reforming reactions might be inhibited to some extent by H2 atmosphere, the low temperature and the decrease of acidity over catalyst. Furthermore, it was deduced that the oxygen over the lattice of catalyst or some intermediates might transfer into other intermediates, possibly resulting in more products with high oxygen content, but it was presumed that the low temperature, co-pyrolysis process and N2 atmosphere could repress the trend to a certain degree. The mass and energy balance of co-pyrolysis were analyzed, and the main reaction pathways were also proposed. The interference in pyrolysis by regulating the catalytic mode, pyrolytic atmosphere and temperature, acidity over catalyst might posses a certain guiding significance for the pyrolytic technology and the design/selection of catalysts employed.