Ultra-Dilute SnCl4-Catalyzed Conversion of Concentrated Glucose to 5-Hydroxymethylfurfural in Aqueous Deep Eutectic Solvent

5-Hydroxymethylfurfural(HMF) is a versatile chemical synthesized from glucose dehydration catalyzed by metal chloride (MClx) in deep eutectic solvents (DESs). However, the low glucose concentration and high catalyst dosage hinder large-scale HMF production. Herein, we report an aqueous DES of tetraethylammonium bromide(TEAB)-glucose for converting concentrated glucose (40 wt %, relative to TEAB) using ultra-dilute SnCl4 (0.25 mol %), achieving a 62 % yield of HMF. Ultra-dilute MClx-catalyzed selective conversion of glucose is feasible only when combining SnCl4 with Br-based DES, which is elucidated by density functional theory and molecular dynamic calculations. Using SnCl4 is essential due to its higher glucose isomerization activity than AlCl3 and CrCl3, which can be attributed to its low-barrier coordination with glucose and its barrier-free separation from fructose. Halide anions in DESs strongly interact with glucose, hindering the MClx-glucose coordination and thereby reducing MClx's activity for glucose isomerization. Consequently, Br-based DESs facilitate higher activity of MClx than Cl-based DESs, due to the weaker interaction between halide anion and glucose. In addition, we elucidated the side reactions including condensation, polymerization, and isomerization, and proposed a reaction network. Our findings clarify the differential activity of MClx and the impact of halide anions in DESs on MClx's activity.

Tuning of MgO's base characteristics by blending it with amphoteric ZnO facilitating the selective glucose isomerization to fructose for bioenergy development

Fructose serves as an important intermediate in the preparation of liquid fuel compounds. Herein, we report its selective production via a chemical catalysis method over ZnO/MgO nanocomposite. The blending of an amphoteric ZnO with MgO reduced the latter's unfavorable moderate/strong basic sites that can influence the side reactions in the sugar interconversion, reducing fructose productivity. Of all the ZnO/MgO combinations, a 1 : 1 ratio of ZnO and MgO showed a 20% reduction in moderate/strong basic sites in MgO with ∼2-2.5 times increase in weak basic sites (overall), which is favorable for the reaction. The analytical characterizations affirmed that MgO settles on the surface of ZnO by blocking the pores. The amphoteric ZnO undertakes the neutralization of the strong basic sites and improves the weak basic sites (cumulative) by the Zn-MgO alloy formation. Therefore, the composite afforded as high as 36% fructose yield and 90% selectivity at 90 °C; especially, the improved selectivity can be accounted for by the effect of both basic and acidic sites. The favorable action of acidic sites in controlling the unwanted side reactions was maximum when an aqueous medium contained 1/5^th methanol. However, ZnO's presence regulated the glucose's degradation rate by up to 40% compared to the kinetics of pristine MgO. From the isotopic labelling experiments, the proton transfer pathway (or LdB-AvE mechanism by the formation of 1,2-enediolate) is dominant in the glucose-to-fructose transformation. The composite exhibited a long-lasting ability based on the good recycling efficiency of up to 5 cycles. The insights into the fine-tuning of the physicochemical characteristics of widely available metal oxides would help develop a robust catalyst for sustainable fructose production for biofuel production (via a cascade approach).

Conversion of Glucose to 5-Hydroxymethylfurfural Using Consortium Catalyst in a Biphasic System and Mechanistic Insights

We found an effective catalytic consortium capable of converting glucose to 5-hydroxymethylfurfural (HMF) in high yields (50%). The reaction consists of a consortium of a Lewis acid (NbCl5) and a Brønsted acid (p-sulfonic acid calix[4]arene (CX4SO3H)), in a microwave-assisted reactor and in a biphasic system. The best result for the conversion of glucose to HMF (yield of 50%) was obtained with CX4SO3H/NbCl5 (5 wt%/7.5 wt%), using water/NaCl and MIBK (1:3), at 150 °C, for 17.5 min. The consortium catalyst recycling was tested, allowing its reuse for up to seven times, while maintaining the HMF yield constant. Additionally, it proposed a catalytic cycle by converting glucose to HMF, highlighting the following two key points: the isomerization of glucose into fructose, in the presence of Lewis acid (NbCl5), and the conversion of fructose into HMF, in the presence of CX4SO3H/NbCl5. A mechanism for the conversion of glucose to HMF was proposed and validated.

Acid-Modified Clays for the Catalytic Obtention of 5-Hydroxymethylfurfural from Glucose

5-hydroxymethylfurfural (5-HMF) is an important platform molecule for the synthesis of high-added value products. Several synthesized clay materials, such as mesoporous hectorite and fluorohectorite, in addition to commercial montmorillonite K-10, have been acid modified by different methodologies to be applied as catalysts for the obtention of 5-HMF from glucose. The effects of the Brønsted and/or Lewis acidity, the reaction temperature and time, and the catalyst/glucose ratio on the conversion but especially on the selectivity to 5-HMF have been studied. By comparing the synthesized clays, the best selectivity to 5-HMF (36%) was obtained at 140 °C for 4 h with H-fluorohectorite because of the presence of strong Brønsted acid sites, although its conversion was the lowest (33%) due to its low amounts of Lewis acid sites. Different strategies, such as physical mixtures of montmorillonite K10, which contains high amounts of Lewis acid centers, with Amberlyst-15, which has high amounts of Brønsted acid sites, or the incorporation of rhenium compounds, were carried out. The best selectivity to 5-HMF (62%) was achieved with a mixture of 44 wt % Amberlyst-15 and 56 wt % of montmorillonite K10 for a 56% of conversion at 140 °C for 4 h. This proportion optimized the amount of Brønsted and Lewis acid sites in the catalyst under these reaction conditions.

Heterogeneous Catalysts for the Conversion of Glucose into 5-Hydroxymethyl Furfural

Lignocellulosic biomass, a cheap and plentiful resource, could play a key role in the production of sustainable chemicals. The simple sugars contained in the renewable lignocellulosic biomass can be converted into commercially valuable products such as 5-hydroxymethyl furfural (HMF). A platform molecule, HMF can be transformed into numerous chemical products with potential applications in a wide variety of industries. Of the hexoses contained in the lignocellulosic biomass, the successful production of HMF from glucose has been a challenge. Various heterogeneous catalysts have been proposed over the last decade, ranging from zeolites to metal organic frameworks. The reaction conditions vary in the reports in the literature, which makes it difficult to compare catalysts reported in different studies. In addition, the slight variations in the synthesis of the same material in different laboratories may affect the activity results, because the selectivity towards desired products in this transformation strongly depends on the nature of the active sites. This poses another difficulty for the comparison of different reports. Furthermore, over the last decade the new catalytic systems proposed have increased profoundly. In this article, we summarize the heterogeneous catalysts: Metal Organic Frameworks (MOFs), zeolites and conventional supported catalysts, that have been reported in the recent literature and provide an overview of the observed catalytic activity, in order to provide a comparison.

ZIF-8 Metal Organic Framework for the Conversion of Glucose to Fructose and 5-Hydroxymethyl Furfural

Herein, Zeolitic imidazolate framework-8 (ZIF-8) is considered as an easy and cheap to prepare alternative catalyst for the isomerization of glucose and production of 5-hydroxymethyl furfural (HMF). For the synthesis of the ZIF-8 catalysts two preparation methods were evaluated, being room temperature and hydrothermal synthesis at 140 °C. Of these, the hydrothermal synthesis method yields a material with exceptionally high surface area (1967 m2·g−1). As a catalyst, the ZIF-8 materials generated excellent fructose yields. Specifically, ZIF-8 prepared by hydrothermal synthesis yielded a fructose selectivity of 65% with a glucose conversion of 24% at 100 °C in aqueous reaction medium. However, this selectivity dropped dramatically when the reactions were repeated at higher temperatures (~140 °C). Interestingly, greater quantities of mannose were produced at higher temperatures too. The lack of strong Brønsted acidity in both ZIF-8 materials resulted in poor HMF yields. In order to improve HMF yields, reactions were performed at a lower pH of 1.0. At 140 °C the lower pH was found to drive the reaction towards HMF and double its yield. Despite the excellent performance of ZIF-8 catalysts in batch reactions, their activity did not translate well to the flow reactor over a continuous run of 8 h, which was operating with a residence time of 6 min. The activity of ZIF-8 halved in the flow reactor at 100 °C in ~3 h, which implies that the catalyst’s stability was not maintained in the long run.

A hydrothermally stable ytterbium metal-organic framework as a bifunctional solid-acid catalyst for glucose conversion

Yb6(BDC)7(OH)4(H2O)4 contains both bridging hydroxyls and metal-coordinated waters, possessing Brønsted and Lewis acid sites. The material crystallises from water at 200 °C. Using the solid as a heterogenous catalyst, glucose is converted into 5-hydroxymethylfurfural, via fructose, with a total selectivity of ∼70% after 24 hours at 140 °C in water alone: the material is recyclable with no loss of crystallinity.

Theoretical investigation on the mechanism of glucose-to-fructose isomerization synergistically catalyzed by MnCl2 and [C4SO3HMIM][CH3SO3] in [BMIM]Cl

The mechanism of glucose-to-fructose isomerization catalyzed by manganese chloride (MnCl₂) and 1-methyl-3-(3-sulfobutyl)-imidazolium methylsulfonate ([C₄SO₃HMIM][CH₃SO₃]) in a 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) ionic liquid (IL) was investigated computationally.

Effect of metal chlorides on glucose mutarotation and possible implications on humin formation

An in situ Raman spectroscopic kinetic study of the glucose mutarotation reaction in the presence of Lewis acids is presented herein. The effect of Lewis acids on humin formation reactions is also discussed.

Mechanistic Studies of the Cu(OH)+ -Catalyzed Isomerization of Glucose into Fructose in Water

The isomerization of glucose to fructose is a crucial interim step in the processing of biomass to renewable fuels and chemicals. This study investigates the copper-catalyzed glucose-fructose isomerization in water, focusing on insights into the roles of the dissolved copper species. Depending on the pH, the thermodynamic equilibrium shifted towards one or a few copper species, namely Cu²⁺ , Cu(OH)⁺ , and Cu(OH)₂ . According to thermodynamics, the highest concentration of Cu(OH)⁺ is at pH 5.3, at which the highest fructose yield of 16 % is achieved. The obtained fructose yields strongly correlate with the concentration of Cu(OH)⁺ . A pH decrease of 2-3 units was observed during the reaction, resulting in the deactivation of the catalyst through hydrolysis in acidic media. Based on the results of the catalytic experiments, as well as spectroscopic and spectrometric studies, we propose Cu(OH)⁺ as an active Lewis-acidic species following an intramolecular 1,2-hydride shift.

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.

Synthesis of different structured FePO4 for the enhanced conversion of methyl cellulose to 5-hydroxymethylfurfural

Catalytic mechanism of FePO₄ for the conversion of methyl cellulose into 5-HMF.

Interactions of D-cellobiose with selected chloride salts: A ¹³C NMR and FT-IR study

The interactions of cellulose model compound d-cellobiose with chloride salts of Zn(2+), Ca(2+), Li(+), Sn(2+), La(3+), Mg(2+), K(+) and NH4(+) were evaluated by measuring the (13)C NMR chemical shift changes (Δδ) of the disaccharide due to the addition of salts in D2O. The KCl and NH4Cl showed similar Δδ changes due to interactions only with the Cl(-) anion. Whereas other chloride salts showed interactions with both cation and anion. Among these salts the total interactions are in the order: Zn(2+)>Sn(2+)>Li(+)>Ca(2+)~La(3+)>Mg(2+). The FT-IR spectra of D-cellobiose-chloride salt 1:2 mixtures also indicate that KCl and NH4Cl interacts similarly with D-cellobiose in the solid state.

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.

Highly efficient conversion of carbohydrates into 5-hydroxymethylfurfural using the bi-functional CrPO4 catalyst

The highly efficient synthesis of 5-hydroxymethylfurfural (HMF) from carbohydrates was achieved using the inexpensive and bi-functional CrPO₄ catalyst in a biphasic system.

Genesis of a bi-functional acid–base site on a Cr-supported layered double hydroxide catalyst surface for one-pot synthesis of furfurals from xylose with a solid acid catalyst

The cross boundary between Cr³⁺ oxide and Mg–Al LDH generates highly active bi-functional acid–base sites for xylose isomerization.

Beyond a solvent: the roles of 1-butyl-3-methylimidazolium chloride in the acid-catalysis for cellulose depolymerisation

In this report, 1-butyl-3-methylimidazolium chloride ([C4C1im]Cl) is demonstrated to enhance the kinetics of acid-catalysed hydrolysis of 1,4-β-glucans in binary solvent mixtures. [C4C1im]Cl plays other roles in the reaction beyond acting as a solvent for cellulose, as currently accepted. In fact, the presence of the IL increases the Hammett acidity of the catalyst dissolved in the reaction medium. The kinetic data from cellobiose and cellulose hydrolysis directly correlate with the acid strength found for p-toluenesulfonic acid in the different reaction media studied here. The current report identifies neglected, but yet very important phenomena occurring in cellulose depolymerisation.

Effect of organic solvent and Brønsted acid on 5-hydroxymethylfurfural preparation from glucose over CrCl3

An investigation of the effect of solvent and Brønsted acid on the mechanism of 5-hydroxymethylfurfural preparation from glucose over CrCl₃by experimental and computational study.

Enhanced conversion of carbohydrates to the platform chemical 5-hydroxymethylfurfural using designer ionic liquids

5-Hydroxymethylfurfural (HMF) is a key platform chemical that may be obtained from various cellulosic (biomass) derivatives. Previously, it has been shown that ionic liquids (ILs) facilitate the catalytic conversion of glucose into HMF. Herein, we demonstrate that the careful design of the IL cation leads to new ionic solvents that enhance the transformation of glucose and more complex carbohydrates into HMF significantly. In Situ NMR spectroscopy and computational modeling pinpoint the key interactions between the IL, catalyst, and substrate that account for the enhanced reactivities observed.

One-pot synthesis of furfural derivatives from pentoses using solid acid and base catalysts

One-pot synthesis of (2-furanylmethylene)malononitrile, a Knoevenagel product of furfural with malononitrile, from xylose efficiently proceeded by combined use of acid Amberlyst-15 and acid-base Cr/hydrotalcites in 44% yield.

Insights into the Cr(iii) catalyzed isomerization mechanism of glucose to fructose in the presence of water using ab initio molecular dynamics

In water, the partially hydrolyzed Cr complex [Cr(H₂O)₅OH]⁺²is the active species for glucose isomerization and not the unhydrolyzed, hexahydrated Cr⁺³.

Comparison of homogeneous and heterogeneous catalysts for glucose-to-fructose isomerization in aqueous media

Herein, the first comparison of the mechanisms of glucose-to-fructose isomerization in aqueous media enabled by homogeneous (CrCl3 and AlCl3 ) and heterogeneous catalysts (Sn-beta) by using isotopic-labeling studies is reported. A pronounced kinetic isotope effect (KIE) was observed if the deuterium label was at the C2 position, thus suggesting that a hydrogen shift from the C2 to C1 positions was the rate-limiting step with the three catalysts. (13) C and (1) H NMR spectroscopic investigations confirmed that an intra-hydride-transfer reaction pathway was the predominant reaction channel for all three catalysts in aqueous media. Furthermore, the deuterium atom in the labeled glucose could be mapped onto hydroxymethylfurfural and formic acid through reactions that followed the isomerization step in the presence of Brønsted acids. In all three catalysts, the active site appeared to be a bifunctional Lewis-acidic/Brønsted-basic site, based on a speciation model and first-principles calculations. For the first time, a mechanistic similarities between the homogeneous and heterogeneous catalysis of aldose-to-ketose isomerization is established and it is suggested that learning from homogeneous catalysis could assist in the development of improved heterogeneous catalysts.

The mechanism of glucose isomerization to fructose over Sn-BEA zeolite: a periodic density functional theory study

The isomerization of glucose to fructose in the presence of Sn-containing zeolite BEA (beta polymorph A) was studied by periodic DFT calculations. Focus was placed on the nature of the active site and the reaction mechanism. The reactivities of the perfect lattice Sn(IV) site and the hydroxylated SnOH species are predicted to be similar. The isomerization activity of the latter can be enhanced by creating an extended silanol nest in its vicinity. Besides the increased Lewis acidity and coordination flexibility of the Sn center, the enhanced reactivity in this case is ascribed to the reaction environment that promotes activation of the confined sugar intermediates through hydrogen bonding. The resulting multidentate activation of the substrate favors the rate-determining hydrogen-shift reaction. These findings suggest the important role of defect lattice sites in Sn-BEA for catalytic glucose isomerization.