Hydroxymethylfurfural and its Derivatives: Potential Key Reactants in Adhesives

Recent Advances in Bio-Based Adhesives and Formaldehyde-Free Technologies for Wood-Based Panel Manufacturing

Purpose of Review

Conventional formaldehyde-based adhesives for wood-based composite panels are subject to significant concerns due to their formaldehyde emissions. Over the past decade, the wood adhesive industry has undergone a considerable transformation that is characterized by a major push in bio-adhesive development. Various bio-based materials have been explored to create alternatives to conventional formaldehyde-based adhesives. Moreover, growing interest in circularity has led to increasingly exploiting industrial coproducts and by-products to find innovative solutions.

Recent Findings

Industrial production generates many coproducts that can serve as renewable resources to produce eco-friendly materials. These coproducts offer alternative supply sources for material production without encroaching on food production. Many bio-based compounds or coproducts, such as saccharides, proteins, tannins, and lignocellulosic biomass, can also be used to develop bio-based adhesives. As part of ongoing efforts to reduce formaldehyde emissions, new hardeners and crosslinkers are being developed to replace formaldehyde and bio-scavengers. Other alternatives, such as binderless panels, are also emerging.

Summary

This review focuses on sources of bio-based material derived from by-products of various industries, which have many advantages and disadvantages when incorporated into adhesives. Modification methods to enhance their properties and performance in wood-based panels are also discussed. Additionally, alternatives for developing low-emission or formaldehyde-free adhesives are addressed, including hardeners, bio-scavengers, and binderless options. Finally, the environmental impact of bio-based adhesives compared to that of synthetic alternatives is detailed.

Biotransamination of Furan-Based Aldehydes with Isopropylamine: Enzyme Screening and pH Influence

Furan-based amines are highly valuable compounds which can be directly obtained via reductive amination from easily accessible furfural, 5-(hydroxymethyl)furfural (HMF) and 2,5-diformylfuran (DFF). Herein the biocatalytic amination of these carbonyl derivatives is disclosed using amine transaminases (ATAs) and isopropylamine (IPA) as amine donors. Among the different biocatalysts tested, the ones from Chromobacterium violaceum (Cv-TA), Arthrobacter citreus (ArS-TA), and variants from Arthrobacter sp. (ArRmut11-TA) and Vibrio fluvialis (Vf-mut-TA), afforded high levels of product formation (>80 %) at 100-200 mM aldehyde concentration. The transformations were studied in terms of enzyme and IPA loading. The pH influence was found as a key factor and attributed to the imine/aldehyde equilibrium that can arise from the high reactivity of the carbonyl substrates with a nucleophilic amine such as IPA.

Chemoselective Lipase-Catalyzed Synthesis of Amido Derivatives from 5-Hydroxymethylfurfurylamine

The acylations of furfurylamine and 5-hydroxymethylfurfurylamine (HMFA) have been studied finding immobilized Candida antarctica lipase B (CALB) as an ideal biocatalyst. CALB was used immobilized on two different supports (Novozyme 435 and EziG-CALB), with the polymer-coated controlled porosity glass carrier material from EnginZyme being an excellent carrier to yield an active and stable enzymatic preparation for the acylation of the primary amine group. The amount of the acyl donor in the reaction was a key factor to achieve the mono- and chemoselective N-protection of HMFA with large excess of ethyl acetate leading to the formation of the N,O-diacetylated product. Thus, a series of 16 nonactivated esters were used to selectively modify the amine group of HMFA, obtaining 9 hydroxy amides under mild reaction conditions and with quantitative yields through chromatography-free transformations. The influence of substrate concentration was studied, resulting in complete conversions in all cases after 22 h (100-1000 mM). Excellent results were observed at 100 and 200 mM of HMFA, while higher concentrations led to longer reaction times and, to some extent, the formation of the diacetylated product (up to 7% after 22 h at 1 M). After this optimization, a metric analysis was performed to confirm the high sustainability of the presented process (E-factor of 1.1 excluding solvents) upon intensification of the biotransformation to 1 g at 200 mM HMFA concentration. The possibility of obtaining orthogonally protected HMFA-derived amido esters has been achieved through a clean and sequential one-pot process using EziG-CALB, which involved the use of ethyl methoxy acetate as the nonactivated ester for N-acylation and the activated vinyl acetate for O-protection.

Advances in enzymatic conversion of biomass derived furfural and 5-hydroxymethylfurfural to value-added chemicals and solvents

The progress of versatile chemicals and bio-based fuels using renewable biomass has gained ample importance. Furfural and 5-hydroxymethylfurfural are biomass-derived compounds that serve as the cornerstone for high-value chemicals and have a myriad of industrial applications. Despite the significant research into several chemical processes for furanic platform chemicals conversion, the harsh reaction conditions and toxic by-products render their biological conversion an ideal alternative strategy. Although biological conversion confers an array of advantages, these processes have been reviewed less. This review explicates and evaluates notable improvements in the bioconversion of 5-hydroxymethylfurfural and furfural to comprehend the current developments in the biocatalytic transformation of furan. Enzymatic conversion of HMF and furfural to furanic derivative have been explored, while the latter has substantially overlooked a foretime. This discrepancy was reviewed along with the outlook on the potential usage of 5-hydroxymethylfurfural and furfural for the furan-based value-added products' synthesis.

5-(Chloromethyl)Furfural as a Potential Source for Continuous Hydrogenation of 5-(Hydroxymethyl)Furfural to 2,5-Bis(Hydroxymethyl)Furan

5-(Chloromethyl)furfural (CMF) is cheaper than sugars, because it can be obtained from biomass waste. Herein, the stepwise conversion of CMF to 2,5-bis(hydroxymethyl)furan (BHMF) via 5-(hydroxymethyl)furfural (HMF) was demonstrated for the first time. The purified CMF was hydrolyzed in continuous mode followed by extraction with ethyl acetate (EA), resulting in a HMF yield of 70 mol%. The following factors were assessed during continuous hydrogenation of the produced HMF: the presence of EA in the reaction solvent, HMF concentrations of up to 10 wt% in the feed, the mass production of mesoporous Cu-Al₂ O₃ (meso-CuA-kg), the shaping of meso-CuA-kg into cylindrical pellets, and the setup of the catalytic reactor. Through these efforts, the hydrogenation of HMF over meso-CuA-kg could be sustained for 100 h under the above optimized conditions, affording BHMF in 98 % yield. The approach described in this study can greatly contribute to the value-added transformation of CMF into HMF and BHMF.

Nature-Derived Okra Gel as Strong Hemostatic Bioadhesive in Human Blood, Liver, and Heart Trauma of Rabbits and Dogs

Bioadhesive performance can be compromised due to bleeding. Bleeding increases mortality. Adhesives with hemostatic function are of great significance. A sustainable and robust hemostatic bioadhesive from okra is reported. The adhesive strength reaches around three and six-fold higher than commercial fibrin on pigskin and glass, respectively. The okra gel presents high-pressure resistance and great underwater adhesive strength. In human blood experiments, the okra gel can activate platelets, enhance the adhesion of activated platelets, and release coagulation factors XI and XII. By forming a fast gel layer and closely adhering to the wound, it can quickly stop bleeding in the liver and heart of rabbits and dogs. Meanwhile, okra gel can cause platelet activation at the wound site and further strengthen its hemostatic performance. It is biocompatible, biodegradable, and can promote wound healing and shows potential as a sustainable bioadhesive, especially in the scenario of significant hemorrhage.

Formaldehyde-Free Resins for the Wood-Based Panel Industry: Alternatives to Formaldehyde and Novel Hardeners

Due to its carcinogenic properties, the presence of formaldehyde in resins and other industrial products has been a subject of great concern in recent years. The presented review focuses on modern alternatives for the production of wood-based panels; i.e., substitutes for formaldehyde in the production of amino and phenolic resins, as well as novel hardeners for formaldehyde-free wood adhesives. Solutions in which formaldehyde in completely replaced are presented in this review. Recent advances indicate that it is possible to develop new formaldehyde-free systems of resins with compatible hardeners. The formaldehyde substitutes that have primarily been tested are glyoxal, glutaraldehyde, furfural, 5-hydroxymethylfurfural, and dimethoxyethanal. The use of such substitutes eliminates the problem of free formaldehyde emission originating from the resin used in the production of wood-based panels. However, these alternatives are mostly characterized by worse reactivity, and, as a result, the use of formaldehyde-free resins may affect the mechanical and strength properties of wood-based panels. Nonetheless, there are still many substantial challenges for the complete replacement of formaldehyde and further research is needed, especially in the field of transferring the technology to industrial practice.

Diverse Applications of Biomass-Derived 5-Hydroxymethylfurfural and Derivatives as Renewable Starting Materials

This Review summarizes recent efforts to capitalize on 5-hydroxymethylfurfural (HMF) and related furans as emerging building blocks for the synthesis of fine chemicals and materials, with a focus on advanced applications within medicinal and polymer chemistry, as well as nanomaterials. As with all chemical industries, these fields have historically relied heavily on petroleum-derived starting materials, an unsustainable and polluting feedstock. Encouragingly, the emergent chemical versatility of biomass-derived furans has been shown to facilitate derivatization towards valuable targets. Continued work on the synthetic manipulation of HMF, and related derivatives, for access to a wide range of target compounds and materials is crucial for further development. Increasingly, biomass-derived furans are being utilized for a wide range of chemical applications, the continuation of which is paramount to accelerate the paradigm shift towards a sustainable chemical industry.

Sustainable Catalytic Transformation of Biomass-Derived 5-Hydroxymethylfurfural to 2,5-Bis(hydroxymethyl)tetrahydrofuran

5-Hydroxymethylfurfural (5-HMF), one of the most important platform molecules in biorefinery, can be directly obtained from a vast diversity of biomass materials. Owing to the reactive functional groups (-CHO and -CH₂ OH) in the structure, this versatile building block undertakes several transformations to provide a wealth of high value-added products. Among numerous well-established paradigms, the catalytic hydrogenation of 5-HMF towards 2,5-bis(hydroxymethyl)tetrahydrofuran (BHMTHF) is of great interest because this downstream diol can be exploited in a wide range of industrial applications. Not surprisingly, incessant endeavors from both academia and industry to upgrade this catalytic process have been established over the years. The main aim of this Review was to provide a comprehensive overview on the development of heterogeneous metal catalysts for the 5-HMF-to-BHMTHF transformation. Herein, the rational design and utility of hydrogenating catalysts were elaborated in many aspects including metal types (Ni, Co, Pd, Ru, Pt, and bimetals), solid supports, preparation method, recyclability, operating conditions, and reaction regime (batch and continuous flow). In addition, the assessment of cooperative catalysts to convert carbohydrates into BHMTHF under one-pot cascade, tentative mechanism, as well as prospects and challenges for the chemo-selective hydrogenation of 5-HMF were also highlighted.

Integrated Cascade Process for the Catalytic Conversion of 5-Hydroxymethylfurfural to Furanic and TetrahydrofuranicDiethers as Potential Biofuels

The depletion of fossil resources is driving the research towards alternative renewable ones. Under this perspective, 5-hydroxymethylfurfural (HMF) represents a key molecule deriving from biomass characterized by remarkable potential as platform chemical. In this work, for the first time, the hydrogenation of HMF in ethanol was selectively addressed towards 2,5-bis(hydroxymethyl)furan (BHMF) or 2,5-bis(hydroxymethyl)tetrahydrofuran (BHMTHF) by properly tuning the reaction conditions in the presence of the same commercial catalyst (Ru/C), reaching the highest yields of 80 and 93 mol%, respectively. These diols represent not only interesting monomers but strategic precursors for two scarcely investigated ethoxylated biofuels, 2,5-bis(ethoxymethyl)furan (BEMF) and 2,5-bis(ethoxymethyl)tetrahydrofuran (BEMTHF). Therefore, the etherification with ethanol of pure BHMF and BHMTHF and of crude BHMF, as obtained from hydrogenation step, substrates scarcely investigated in the literature, was performed with several commercial heterogeneous acid catalysts. Among them, the zeolite HZSM-5 (Si/Al=25) was the most promising system, achieving the highest BEMF yield of 74 mol%. In particular, for the first time, the synthesis of the fully hydrogenated diether BEMTHF was thoroughly studied, and a novel cascade process for the tailored conversion of HMF to the diethyl ethers BEMF and BEMTHF was proposed.

Fully Biobased Adhesive from Glucose and Citric Acid for Plywood with High Performance

Biomass-based adhesives have attracted much attention due to their eco-friendly, sustainable characteristics compared to formaldehyde-based adhesives; however, their low bonding strength and water resistance restrict their application. Thus, developing a high-performance biomass-based adhesive with excellent bonding strength and water resistance is necessary. In this work, a fully biomass-based citric acid-glucose (CAG) adhesive was produced by the esterification reaction of glucose and citric acid, which was validated by Fourier transform infrared (FT-IR), 13C nuclear magnetic resonance (13C NMR), and liquid chromatography-mass spectrometry (LC-MS). Furthermore, the properties of the CAG adhesive were tuned considering the effects of reaction time and molar ratio of citric acid/glucose (CA/G). It was revealed that increasing the molar ratio of CA/G is more advantageous to improve the shear strength and water resistance of plywood than the reaction time. The dry and wet strengths of plywood bonded by the CAG adhesive can reach the standard requirement (≥0.7 MPa) when the molar ratios of CA/G were more than 0.6 and the reaction time was 1 h. These results were better than those bonded by the urea-formaldehyde (UF) resin. Therefore, this green adhesive shows great potential to replace the existing industrial UF resin adhesives.

Atom-economic Approach to the Synthesis of α-(Hetero)aryl-substituted Furan Derivatives from Biomass

An atom-economic ring construction approach to the synthesis of α-(hetero)arylfurans based on renewable furanic platform chemicals has been developed. Corresponding compounds have been prepared in good to excellent yields via [2+2+2] and [4+2] cycloaddition reactions using metal-catalyzed or photoredox protocols. Easily available HMF-based 2-hydroxymethyl-5-ethynylfuran and 2-hydroxymethyl-5-cyanofuran were used as starting materials. A synthetic route with an improved carbon economy factor has been implemented to achieve sustainability aim. The possible application of arylfurans as molecular conductors has been investigated by DFT calculations, which revealed excellent charge transfer properties. As a future perspective, integration of biomass processing strategy into manufacturing of molecular electronics was pointed out to achieve the aim of sustainability.

Sulfuric Acid-Catalyzed Dehydratization of Carbohydrates for the Production of Adhesive Precursors

Carbohydrates and hexose-derived 5-hydroxymethylfurfural (5-HMF) are platform chemicals for the synthesis of sustainable binders. New, greener approaches aim at the development of production systems, which minimize process steps and avoid organic solvents or other auxiliaries that could interfere with subsequent resin synthesis. In our work, carbohydrate solutions rich in 5-hydroxymethylfurfural (5-HMF) were produced using a continuous-flow microreactor and diluted H₂SO₄ as the catalyst. After optimization of the process conditions (temperature, reaction time, catalyst content), a 5-HMF yield of 49% was obtained at a low reaction time of 0.6 min and a catalyst concentration of 1% at 175 °C and 17 bar pressure. Extensive rehydration of the product was avoided by efficient immediate cooling of the reaction solution. The stability of the reaction system was improved by increasing the inner diameter of the capillary in the flow reactor to 2 mm. Advantageously, the obtained reaction mixtures are used directly as precursors in the development of sustainable binder systems, without the need of additional purification, filtration, or extraction steps.

Value-added chemicals from biomass-derived furans: radical functionalisations of 5-chloromethylfurfural (CMF) by metal-free ATRA reactions

Biomass-derived 5-chloromethylfurfural (CMF), a congener of the well-known carbohydrate-based platform chemical 5-hydroxymethylfurfural (HMF), can efficiently be functionalised by radical transformations of its benzylic chloromethyl group. We report here the first examples of these radical reactions by way of metal-free, triethylborane/oxygen-induced atom transfer radical addition (ATRA) reactions between CMF and styrenes, which proceed with high yield and selectivity. The key intermediate, the 2-formyl-5-furfuryl radical derived from CMF, and its radical addition reactions were studied with regard to its electronic structure, i.e. spin density distribution and frontier molecular orbitals based on the NBO ansatz and activation barriers of the addition step using DFT and post-HF methods.

Catalytic C-H Functionalization of Unreactive Furan Cores in Bio-Derived Platform Chemicals

C-H functionalization is one of the most convenient and powerful tools in the arsenal of modern chemistry, deservedly nominated as the "Holy Grail" of organic synthesis. A frequent disadvantage of this method is the need for harsh reaction conditions to carry out transformations of inert C-H bonds, which limits the possibility of its use for modifying less stable substrates. Biomass-derived furan platform chemicals, which have a relatively unstable aromatic furan core and highly reactive side chain substituents, are extremely promising and valuable organic molecules that are currently widely used in a variety of research and industrial fields. The high sensitivity of furan derivatives to acids, strong oxidants, and high temperatures significantly limits the use of classical methods of C-H functionalization for their modification. New methods of catalytic functionalization of non-reactive furan cores are urgently required to obtain a new generation of materials with controlled properties and potentially bioactive substances.