Organocatalytic Dehydration of Fructose-Based Carbohydrates into 5-Hydroxymethylfurfural in the Presence of a Neutral Inner Salt

A series of organic sulfonate inner salts, viz., aprotic imidazolium- and pyridinium-based zwitterions bearing sulfonate groups (-SO₃^-), were synthesized for the catalytic conversion of fructose-based carbohydrates into 5-hydroxymethylfurfural (HMF). The dramatic cooperation of both the cation and anion of inner salts played a crucial role in the HMF formation. The inner salts have excellent solvent compatibility, and 4-(pyridinium)butane sulfonate (PyBS) affords the highest catalytic activity with 88.2 and 95.1% HMF yields at almost full conversion of fructose in low-boiling-point protic solvent isopropanol (i-PrOH) and aprotic solvent dimethyl sulfoxide (DMSO), respectively. The substrate tolerance of aprotic inner salt was also studied through changing the substrate type, demonstrating its excellent specificity for catalytic valorization of fructose-moiety-containing C6 sugars, such as sucrose and inulin. Meanwhile, the neutral inner salt is structurally stable and reusable; after being recycled four times, the catalyst showed no appreciable loss of its catalytic activity. The plausible mechanism has been elucidated based on the dramatic cooperative effect of both the cation and sulfonate anion of inner salts. The noncorrosive, nonvolatile, and generally nonhazardous aprotic inner salt used in this study will benefit many biochemical-related applications.

Highly Efficient One-Step Conversion of Fructose to Biofuel 5-Ethoxymethylfurfural Using a UIO-66-SO3H Catalyst

In this study, a novel sulfonic acid-modified catalyst for MOFs (UIO-66-SO₃H) was synthesized using chlorosulfonic acid as a sulfonating reagent and first used as efficient heterogeneous catalysts for the one-pot conversion of fructose into biofuel 5-ethoxymethylfurfural (EMF) in a cosolvent free system. The physicochemical properties of this catalyst were characterized by Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), and powder X-ray diffraction (XRD). The characterization demonstrated that the sulfonic acid group was successfully grafted onto the MOF material and did not cause significant changes to its morphology and structure. Furthermore, the effects of catalyst acid amount, reaction temperature, reaction time, and catalyst dosage on reaction results were investigated. The results showed that the conversion of fructose was 99.7% within 1 h at 140°C, while the EMF yield reached 80.4%. This work provides a viable strategy by application of sulfonic acid-based MOFs for the efficient synthesis of potential liquid fuel EMF from renewable biomass.

Catalytic Upgrading of Lignocellulosic Biomass Sugars Toward Biofuel 5-Ethoxymethylfurfural

The conversion of biomass into high-value chemicals through biorefineries is a requirement for sustainable development. Lignocellulosic biomass (LCB) contains polysaccharides and aromatic polymers and is one of the important raw materials for biorefineries. Hexose and pentose sugars can be obtained from LCB by effective pretreatment methods, and further converted into high-value chemicals and biofuels, such as 5-hydroxymethylfurfural (HMF), levulinic acid (LA), γ-valerolactone (GVL), ethyl levulinate (EL), and 5-ethoxymethylfurfural (EMF). Among these biofuels, EMF has a high cetane number and superior oxidation stability. This mini-review summarizes the mechanism of several important processes of EMF production from LCB-derived sugars and the research progress of acid catalysts used in this reaction in recent years. The influence of the properties and structures of mono- and bi-functional acid catalysts on the selectivity of EMF from glucose were discussed, and the effect of reaction conditions on the yield of EMF was also introduced.

Catalytic Synthesis of the Biofuel 5-Ethoxymethylfurfural (EMF) from Biomass Sugars

A new generation of bioplatform molecule 5-ethoxymethylfurfural (EMF) has excellent energy density and combustion performance, which makes it a potential fuel additive. This article reviews the factors that affect the production of EMF from different feedstocks, including platform compounds, monosaccharides, polysaccharides, and raw lignocellulosic biomass. Focus is placed on discussing the catalytic efficiency with pros and cons of different acid catalysts, including homogeneous catalysts (i.e., liquid acids and metal salts), heterogeneous catalysts (i.e., zeolites, heteropolyacid-based hybrids, and SO3H-based catalysts), ionic liquids, mixed acid catalysts, and deep eutectic solvents (DESs). Except for the commonly used ethanol solvent, this review also summarizes the influence of the cosolvent system (e.g., ethanol/dimethylsulfoxide (DMSO), ethanol/tetrahydrofuran (THF), and ethanol/γ-valerolactone (GVL)) on the EMF yield.

Sulfonic acid-functionalized PCP(Cr) catalysts with Cr3+ and -SO3H sites for 5-ethoxymethylfurfural production from glucose

5-Ethoxymethylfurfural (EMF) has been identified as a potential biofuel and fuel additive, for which the production from glucose (the most abundant and inexpensive monosaccharide) in a one-step process would be highly desirable. Here, the synthesis of sulfonic acid-functionalized porous coordination polymers (PCPs) and their application as catalysts for EMF synthesis are reported. PCP(Cr)-BA (PCP material with Cr³⁺ ions and H₂BDC-SO₃H linkers) and PCP(Cr)-NA (PCP material with Cr³⁺ ions and H₂NDC(SO₃H)₂ linkers) materials containing both Cr³⁺ sites and Brønsted-acidic –SO₃H sites were prepared. The morphology, pore structure, acidity, chemical composition, and thermal stability of the two functionalized PCP(Cr) catalysts were analyzed by systematic characterization. The catalysts featured a porous morphology and dual Cr³⁺ and –SO₃H sites, which enabled the cascade conversion of glucose to EMF. PCP(Cr)-BA exhibited higher performance than PCP(Cr)-NA with an EMF yield of 23.1% in the conversion of glucose at 140 °C after 22 h in an ethanol/water system. In addition, the as-prepared catalyst exhibited a high stability in the current catalytic system for EMF production from glucose with a constant catalytic activity in a four-run recycling test without an intermediate regeneration step.

The PCP(Cr)-BA catalysts featured porous morphology and dual Cr³⁺ and –SO₃H sites, which enabled the cascade conversion of glucose to EMF. In addition, the as-prepared catalyst exhibited a high stability in the current catalytic system.

Incorporation of Al3+ Sites on Brønsted Acid Metal-Organic Frameworks for Glucose-to-Hydroxylmethylfurfural Transformation

5-hydroxylmethylfurfural (HMF) is a bio-based chemical that can be prepared from natural abundant glucose by using combined Brønsted-Lewis acid catalysts. In this work, Al³⁺ catalytic site has been grafted on Brønsted metal-organic frameworks (MOFs) to enhance Brønsted-Lewis acidity of MOF catalysts for a one-pot glucose-to-HMF transformation. The uniform porous structure of zirconium-based MOFs allows the optimization of both acid strength and density of acid sites in MOF-based catalysts by incorporating the desired amount of Al³⁺ catalytic sites at the organic linker. Al³⁺ sites generated via a post-synthetic modification act as Lewis acid sites located adjacent to the Brønsted sulfonated sites of MOF structure. The local structure of the Al³⁺ sites incorporated in MOFs has been elucidated by X-ray absorption near-edge structure (XANES) combined with density functional theory (DFT) calculations. The cooperative effect from Brønsted and Lewis acids located in close proximity and the high acid density is demonstrated as an important factor to achieve high yield of HMF.

Efficient synthesis of 5-ethoxymethylfurfural from biomass-derived 5-hydroxymethylfurfural over sulfonated organic polymer catalyst

Herein, we investigated catalytic potential of a functionalized porous organic polymer bearing sulfonic acid groups (PDVTA-SO₃H) to the etherification of 5-hydroxymethylfurfural (HMF) to 5-ethoxymethylfurfural (EMF) under solvent-free conditions. The PDVTA-SO₃H material was synthesized via post-synthetic sulfonation of the porous co-polymer poly-divinylbenzene-co-triallylamine by chlorosulfonic acid. The physicochemical properties of the PDVTA-SO₃H were characterized by FT-IR, SEM, TG-DTG, and N₂ adsorption isotherm techniques. PDVTA-SO₃H had high specific surface area (591 m² g⁻¹) and high density of –SO₃H group (2.1 mmol g⁻¹). The reaction conditions were optimized via Box–Behnken response surface methodology. Under the optimized conditions, the PDVTA-SO₃H catalyst exhibited efficient catalytic activity with 99.8% HMF conversion and 87.5% EMF yield within 30 min at 110 °C. The used PDVTA-SO₃H catalyst was readily recovered by filtration and remained active in recycle runs.

The sulfonic acid grafted porous organic nitrogen-containing polymer (PDVTA-SO₃H) exhibited excellent catalytic activity in the synthesis of 5-ethoxymethylfurfural (EMF) from 5-hydroxymethylfurfural (HMF).

One-pot fructose conversion into 5-ethoxymethylfurfural using a sulfonated hydrophobic mesoporous organic polymer as a highly active and stable heterogeneous catalyst

We report a sulfonated hydrophobic mesoporous organic polymer (MOP-SO3H) as a highly efficient heterogeneous catalyst for one-pot 5-ethoxymethylfurfural (EMF) production from fructose in ethanol solvent.

One-Pot Synthesis of 2,5-Diformylfuran from Fructose by Bifunctional Polyaniline-Supported Heteropolyacid Hybrid Catalysts

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 “one-pot” and “one-step” 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 Brønsted acidic and redox characters. However, the anchoring of PMo12 on PAN reduces the Brønsted acidity by acid–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 “one-pot” 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–organic hybrid catalysts for producing valuable hexose-derived platform chemicals.