Biomass-derived Platform Molecules Research Articles

Investigating base-catalyzed aldol condensation reactions on alkali-free hydrotalcites and the role of thermal decomposition

Aldolization is a crucial carbon–carbon coupling reaction used in the conversion of biomass-derived platform molecules into valuable chemicals and fuels. In this work, we combine experiment and theory to explore the role thermal decomposition has in controlling the structure of alkali-free Mg-Al layered double hydroxides (LDHs) and their subsequent performance in the base-catalyzed aldol condensation of acetone, focusing on the constituent aldolization and dehydration steps. LDHs were synthesized via co-precipitation and subjected to calcination at temperatures up to 600 °C to produce mixed metal oxides of the form Mg3AlOz which were further examined by SEM, N2 physisorption, XRD, and CO2-TPD. Aldolization and dehydration behaviors were examined in a batch reactor and fit with kinetic models to capture the sensitivities of both reactions to calcination temperature. Additional experiments involving the selective blocking of base sites by CO2 poisoning and reversal at varying temperatures showed that both aldolization and dehydration are likely driven by the strongest base sites, yielding a site density (0.19 μmol m−2) in agreement with separate propionic acid titration studies. Density functional theory (DFT) simulations of Mg5Al2O8 surfaces were employed to gain microscopic insights into these materials, revealing the importance of Al substitution in creating cation vacancies which can drive aldol condensation through enolate intermediates stabilized in the oxyanion resonance form. High-coverage simulations of the surface show participation of the vacancy in both aldolization and dehydration, rationalizing the coupling of rate trends observed experimentally. Ultimately, this study provides new insights and approaches useful in the design of heterogeneous aldolization catalysts.

Highly efficient selective hydrogenation of furfural to tetrahydrofurfuryl alcohol over MOF-derived Co-Ni bimetallic catalysts: The effects of Co-Ni alloy and adsorption configuration

The targeted conversion of furfural (FA) over inexpensive catalyst is a critical and challenging project. Herein, CoNi alloy catalysts (xCoyNi/C) were prepared through one-step pyrolysis of metal–organic frameworks and used in furfural (FA) hydrogenation to tetrahydrofurfuryl alcohol (TFOL). The formation of CoNi alloy leads to a reconfiguration of the electronic structure on the metal surface, which affects the adsorption of functional groups on FA and furfuryl alcohol (FOL). In situ FT-IR analysis and DFT calculations verify that bimetallic CoNi alloy catalyst exhibits more suitable reactants adsorption and hydrogenation rate in comparison with the monometallic Co and monometallic Ni catalysts. Meanwhile, CoNi alloy significantly reduces the activation barrier from FOL to TFOL. Hence, the 1Co1Ni/C catalyst gives the highest TFOL yield of > 99 %. This study provides a simple strategy for the preparation of alloy catalysts for tuning product selectivity and presents practical potential for upgrading biomass-derived platform molecules.

Chemical Transformation of Biomass-Derived Furan Compounds into Polyols

Polyols such as 1,5-pentadiol, 1,6-hexanediol, and 1,2,6-hexanetriol are crucial chemicals, traditionally derived from non-renewable fossil sources. In the pursuit of sustainable development, exploring renewable and environmentally benign routes for their production becomes imperative. Furfural and 5-hydroxymethylfurfural are C5 and C6 biomass-derived platform molecules, which have potential in the synthesis of various polyols through hydrogenation and hydrogenolysis reactions. Currently, there is an extensive body of literature exploring the transformation of biomass-derived furan compounds. However, a comprehensive review of the transformation of furan compounds to polyols is lacking. We summarized the literature from recent years about the ring-opening reaction involved in converting furan compounds to polyols. This article reviews the research progress on the transformation of furfural, furfuryl alcohol, and 2-methylfuran to 1,2-pentadiol, 1,4-pentadiol, 1,5-pentadiol, and 1,2,5-pentanetriol, as well as the transformation of 5-hydroxymethylfurfural to 1,2-hexanediol, 1,6-hexanediol, and 1,2,6-hexanetriol. The effects of the supported Pd, Pt, Ru, Ni, Cu, Co, and bimetallic catalysts are discussed through examining the synergistic effects of the catalysts and the effects of metal sites, acidic/basic sites, hydrogen spillover, etc. Reaction parameters like temperature, hydrogen pressure, and solvent are considered. The ring opening catalytic reaction of furan rings is summarized, and the catalytic mechanisms of single-metal and bimetallic catalysts and their catalytic processes and reaction conditions are discussed and summarized. It is believed that this review will act as a key reference and inspiration for researchers in this field.

Facile generation of unsaturated-coordinated and atomically-dispersed hafnium active sites for the highly efficient catalytic transfer hydrogenation of levulinic acid

The sluggish kinetic of the hydrogen transfer reaction pose a significant challenge in developing efficient catalytic transfer hydrogenation (CTH) system for upgrading biomass-derived platform molecules. In this contribution, for the first time, hafnium (Hf) species have been found to coordinate with the hydroxyl groups on halloysite nanotubes surface to construct a single-atom Hf catalyst (Hf@HNTs) with unsaturated-coordinated structure at room temperature. Especially, the Hf(1.7)@HNTs delivered an impressive catalytic performance for the CTH of levulinic acid (LvA) to gamma-valerolactone (GVL) with high turn-over frequency (TOF) of 43.1 h−1 at 160 °C, considerably exceeding that of coordination-saturated HfO2 and state-of-art Hf-based catalysts. Experimental and density functional theory (DFT) studies revealed that Hf(1.7)@HNTs possessed unsaturated-coordinated and atomically-dispersed Hf-O Lewis acid-base pairs, which favored the adsorption and activation of LvA. Moreover, DFT calculations explicitly demonstrated that the energy barriers for the hydrogen transfer between the LvA and hydrogen donor were significantly reduced on the Hf(1.7)@HNTs than that of HfO2.

Harnessing ultrasound-derived hydroxyl radicals for the selective oxidation of aldehyde functions.

Ultrasonic irradiation holds potential for the selective oxidation of non-volatile organic substrates in the aqueous phase by harnessing hydroxyl radicals as chemical initiators. Here, a mechanistic description of hydroxyl radical-initiated glyoxal oxidation is constructed by gleaning insights from photolysis and radiation chemistry to explain the yields and kinetic trends for oxidation products. The mechanistic description and kinetic measurements reported herein reveal that increasing the formation rate of hydroxyl radicals by changing the ultrasound frequency increases both the rates of glyoxal consumption and the selectivity towards C2acid products over those from C-C cleavage. Glyoxal consumption also occurs more rapidly and with greater selectivity towards C2acids under acidic conditions, which favor the protonation of carboxylate intermediates into their less reactive acidic forms. Leveraging such pH and frequency effects is crucial to mitigating product degradation by secondary reactions with hydroxyl radicals and oxidation products (specifically hydrogen peroxide and superoxide). These findings demonstrate the potential of ultrasound as a driver forthe selective oxidation of aldehyde functions to carboxylic acids, offering a sustainable route for valorizing biomass-derived platform molecules.

Rational Design of Transition-Metal-Based Catalysts for the Electrochemical 5-Hydroxymethylfurfural Reduction Reaction.

The electrochemical reduction reaction (HMFRR) of 5-hydroxymethylfurfural (HMF) has emerged as a promising avenue for the utilization and refinement of the biomass-derived platform molecule HMF into high-value chemicals, addressing energy sustainability challenges. Transition metal electrocatalysts (TMCs) have recently garnered attention as promising candidates for catalyzing HMFRR, capitalizing on the presence of vacant d orbitals and unpaired d electrons. TMCs play a pivotal role in facilitating the generation of intermediates through interactions with HMF, thereby lowering the activation energy of intricate reactions and significantly augmenting the catalytic reaction rate. In the absence of comprehensive and guiding reviews in this domain, this paper aims to comprehensively summarize the key advancements in the design of transition metal catalysts for HMFRR. It elucidates the mechanisms and pH dependency of various products generated during the electrochemical reduction of HMF, with a specific emphasis on the bond-cleavage angle. Additionally, it offers a detailed introduction to typical in-situ characterization techniques. Finally, the review explores engineering strategies and principles to enhance HMFRR activity using TMCs, particularly focusing on multiphase interface control, crystal face control, and defect engineering control. This review introduces novel concepts to guide the design of HMFRR electrocatalysts, especially TMCs, thus promoting advancements in biomass conversion.

Ultra-Dense Supported Ruthenium Oxide Clusters via Directed Ion Exchange for Efficient Valorization of 5-Hydroxymethylfurfural.

Maximizing the loadings of active centers without aggregation for a supported catalyst is a grand challenge but essential for achieving high gravimetric catalytic activity, especially toward multi-step reactions. The oxidation of 5-hydroxymethylfurfural (HMF), a key biomass-derived platform molecule, into 2,5-furandicarboxylic acid (FDCA), a promising alternative to polyester monomer, is such a multi-step reaction that involves 6 proton and electron transfers. This process often demands strong alkaline environment but also suffers from the alkali-driven polymerization side-reaction. Meanwhile, neutral media ameliorates the polymerization, but lacks efficient catalyst toward deep oxidation. Herein, we devised a strategy of creating ultra-dense supported Ru oxide clusters via directed ion exchange in a Co hydroxyanion (CoHA) support material. Pyrimidine ligands were first incorporated into the CoHA interlayers, and the subsequent evacuation of pyrimidines created porous channels for the directed ion exchange with the built-in anions in CoHA, which allowed the dense and mono-disperse functionalization of RuCl6 2- anions and their resulting Ru oxide clusters. These ultra-dense Ru oxide clusters not only enable high HMF electrooxidation currents under neutral conditions but also create microscopic channels in-between the clusters for the expedited re-adsorption and oxidation of intermediates toward highly oxidized product, such as 5-formyl-2-furoic acid (FFCA) and FDCA. A two-stage HMF oxidation process, consisting of ambient conversion of HMF into FFCA and FFCA oxidation into FDCA under 60 °C, was eventually developed to first achieve a high FDCA yield of 92.1 % under neutral media with significantly reduced polymerization.

Modulating acid sites in Y zeolite for valorisation of furfural to get γ-valerolactone.

Furfural is a biomass-derived platform molecule that can be converted into a variety of useful products. Catalysts having appropriate balance between Lewis and Brønsted acid sites are suitable for valorisation of furfural. Lewis acidic metal ion incorporated zeolites were studied for this purpose. However, incorporating Lewis acidic metal ions into an alumino-silicate framework of a zeolite is a cumbersome process. Hence, an attempt has been made in this work to modulate the acid sites of Y zeolite via thermal treatment to effect controlled dealumination and use it for valorisation of furfural using isopropyl alcohol, which is a cascade transformation. The thermal treatment of zeolites changed the distribution of acid sites and increased the weak plus moderate to strong acid site ratio. Among the thermally dealuminated Y, beta and mordenite zeolites, with SiO2/Al2O3 ratio 5.2, 25 and 20, only Y zeolite could yield γ-valerolactone, the final product of the aimed cascade transformation. Complete conversion of furfural and 52% γ-valerolactone yield could be achieved under the optimized conditions using NH4Y zeolite thermally dealuminated at 700 °C (TY700). The better catalytic activity of TY700 could be correlated to a combination different factors such as framework structure, suitable weak plus moderate to strong acid site ratio, presence of both penta-coordinated and octahedral Al sites and balance between Brønsted and Lewis acid sites.

2-Methylimidazole-modulated 2D Cu metal-organic framework for 5-hydroxymethylfurfural hydrodeoxygenation.

Preparation of the high value-added chemical 2,5-dimethylfuran (2,5-DMF) from the biomass-derived platform molecule 5-hydroxymethylfurfural (HMF) is of great significance in the preparation of biofuels. Here, a bottom-up strategy was used to prepare a metal-organic framework (MOF) material with a two-dimensional nanosheet morphology, named CPM, in which an additive 2-methylimidazole was introduced into the hydrothermal process of Cu2+ ions and terephthalic acid. Subsequently, CPM-700 prepared by heat treatment under an inert atmosphere showed excellent catalytic performance in the reaction of HMF hydrodeoxygenation to 2,5-DMF. The materials before and after pyrogenation were characterized by PXRD, XPS, TEM, N2 adsorption and desorption and so on. It was confirmed that compared with the catalyst derived from the cubic MOF material self-assembled by Cu2+ and terephthalic acid, the morphology of 2D nanosheets was beneficial for the reaction of HMF to 2,5-DMF. Combined with the experimental data, the possible reaction path of 2,5-DMF preparation from HMF is that 2,5-dihydroxymethylfuran was formed by hydrogenation of the aldehyde group on the furan ring, and then 2,5-DMF was obtained by hydrogenolysis. This paper provides an effective route for 2D MOF-derived catalytic materials in the selective hydrogenation of HMF.

Selective hydrogenation of 5-hydroxymethylfurfural to 2,5-bis(hydroxymethyl)furan over Ni–Ga intermetallic catalysts and its kinetic studies

5-Hydroxymethylfurfural (HMF) has been listed as the “TOP 14” most important biomass-derived platform molecules, which can be converted into 2,5-bis(hydroxymethyl)furan (BHF) via selective hydrogenation of the carbonyl group on HMF.

Cooperative biomass-derived furfural synthesis and H2 evolution in one photoredox cycle over a MnxCd1-xS/Ti3C2 MXene Schottky junction structure

Selective and efficient photocatalytic transformation of biomass-derived platform molecules to high-value-added fuels or chemicals is a great challenge for green chemistry, especially utilizing visible light energy and earth-abundant catalytic materials. In this work, we report a 1D/2D MnxCd1-xS/Ti3C2 (MCSTC-X) Schottky junction structure to achieve efficient photocharge separation/transfer via in-situ interfacial self-assembly strategy. Under visible light illumination, the optimized MCSTC-7.5 catalyst enables the selective conversion of furfuryl alcohol into furfural with a production rate of 764.3 μmol·g−1·h−1, accompanied by water splitting into H2 with a generation rate of 756.5 μmol·g−1·h−1. Benefiting from the internal electric field in the Schottky junction structure, the efficient separation and transfer of photoinduced charge carriers are realized in one photoredox cycle. Moreover, a plausible reaction mechanism for the photocatalytic selective valorization of furfuryl alcohol to furfural and H2 evolution is proposed. Hopefully, this work would open a new avenue for sustainable and practical production of solar chemicals and fuels by rationally designing 1D/2D Schottky junction.

Synergistic Roles of the CoO/Co Heterostructure and Pt Single Atoms for High-Efficiency Electrocatalytic Hydrogenation of Lignin-Derived Bio-Oils.

Electrochemical hydrogeneration (ECH) of biomass-derived platform molecules, which avoids the disadvantages in utilizing fossil fuel and gaseous hydrogen, is a promising route toward value-added chemicals production. Herein, we reported a CoO/Co heterostructure-supported Pt single atoms electrocatalyst (Pt1-CoO/Co) that exhibited an outstanding performance with a high conversion (>99%), a high Faradaic efficiency (87.6%), and robust stability (24 recyclability) at -20 mA/cm2 for electrochemical phenol hydrogenation to high-valued KA oil (a mixture of cyclohexanol and cyclohexanone). Experimental results and the density functional theory calculations demonstrated that Pt1-CoO/Co presented strong adsorption of phenol and hydrogen on the catalyst surface simultaneously, which was conducive to the transfer of the adsorbed hydrogen generated on the single atom Pt sites to activated phenol, and then, ECH of phenol with high performance was achieved instead of the direct hydrogen evolution reaction. This work described that the multicomponent synergistic single atom catalysts could effectively accelerate the ECH of phenol, which could help the achievement of large-scale biomass upgrading.

Valeric Biofuels from Biomass-Derived γ-Valerolactone: A Critical Overview of Production Processes.

This review analyzes critically the production of valeric biofuels from γ-valerolactone, a relevant biomass-derived platform molecule. Initially, the main properties of valeric esters as fuels for spark- and compression-ignition engines are summarized. Then, catalytic routes to valeric esters from γ-valerolactone are meticulously analyzed, describing the acid- and metal-catalyzed reactions taking part in the tandem catalysis. Only works focused on the production of the valeric biofuels were considered, excluding the cases where these esters were observed in minor amounts or as byproducts. The role of the appropriate selection of the support, catalytic species, catalyst preparation and experimental conditions on the valeric ester productivity are thoroughly commented. Finally, some concluding remarks and perspectives are given, mentioning the areas where additional efforts must be done in order to turn the dream of a massive and renewable valeric biofuel production into a reality.

Halide Adsorption Enhances Electrochemical Hydrogenolysis of 5-Hydroxymethylfurfural by Suppressing Hydrogenation.

Reductive upgrading of 5-hydroxymethylfurfural (HMF), a biomass-derived platform molecule, to 2,5-dimethylfuran (DMF), a biofuel with an energy density 40% greater than that of ethanol, involves hydrogenolysis of both the aldehyde (C═O) and the alcohol (C-OH) groups of HMF. It is known that when hydrogenation of the aldehyde occurs to form 2,5-bis(hydroxymethyl)furan (BHMF), BHMF cannot be further reduced to DMF. Thus, aldehyde hydrogenation must be suppressed to increase the selectivity for DMF production. Previously, it was shown that on a Cu electrode hydrogenolysis occurs mainly through proton-coupled electron transfer (PCET), where a proton from the solution and an electron from the electrode are transferred to the organic species. In contrast, hydrogenation occurs not only through PCET but also through hydrogen atom transfer (HAT), where a surface-adsorbed hydrogen atom (H*) is transferred to the organic species. This study shows that halide adsorption on Cu can effectively suppress HAT by decreasing the steady-state H* coverage on Cu during HMF reduction. As HAT enables only aldehyde hydrogenation, a striking suppression of BHMF is observed, thereby enhancing DMF production. We discuss how the identity and concentration of the halide, along with the reduction conditions (i.e., potential and pH), affect halide adsorption on Cu and identify when optimal halide coverages are achieved to maximize DMF selectivity. Our experimental results are presented alongside computational results that elucidate how halide adsorption affects the adsorption energy of hydrogen and the steady-state H* coverage on Cu, which provide an atomic-level understanding of all experimentally observed effects.

Mechanistic Changes with Applied Potential in Cu Fouling during Furfural Electrochemical Hydrogenation and Hydrogenolysis

CO2 emissions are generated in the chemical industry by factors such as the generation of reactants, including hydrogen, as well as in heating of the reactors. Electrochemical methods can avoid the need for externally provided hydrogen by generating required protons in-situ from the electrolyte, and can be more easily coupled with renewable electricity than their thermocatalytic counterparts. Therefore, to assist in the mitigation of CO2 emissions, electrochemical processes can be used. Furfural is a biomass-derived platform molecule that can be electrochemically reacted to form several species of interest including fine chemicals and fuels. Furfuryl alcohol, the hydrogenation product, is one fine chemical of interest used to make furanic molds or resins. The hydrogenolysis product of furfural, 2-methylfuran, has been identified as a fuel additive. The electrochemical hydrogenation and hydrogenolysis over Cu catalysts in acidic media allows for the parallel production of furfuryl alcohol and 2-methylfuran. During the electrochemical hydrogenation and hydrogenolysis of furfural over Cu in acidic electrolyte, the Cu catalyst will become covered in carbonaceous material with use. This fouling changes with applied potential, with a polymeric material being formed at -560 mV RHE while at -700mV a non-polymeric carbonaceous material is formed on the Cu. In both cases, the fouling leads to deactivation of the catalyst. In this work, Cu metal foils were intentionally fouled to study the mechanisms that led to catalyst fouling to provide insights into possible mitigation techniques.A semi batch reactor was used in which the Cu working electrode was recycled to further foul the catalyst. The catholyte had 200 mM furfural and 0.5 M H2SO4 in a 80:20 vol% water:acetonitrile co-solvent. The Cu electrode was recycled thrice with fresh electrolytes each time. After use at -560 mV RHE a polymer formed on the surface; FTIR of the fouled surface showed consistent spectra as poly(furfuryl alcohol). At -700 mV RHE, no polymer was found, however a carbonaceous layer was found. XPS was used to better distinguish between the carbonaceous fouling that occurred at different applied potentials.

Synergy between Ni3Sn2 alloy and Lewis acidic ReOx enables selectivity control of furfural hydrogenation to cyclopentanone

Selectivity controlling is a crucial and challenging issue for the hydrogenation of furfural (FAL) to cyclopentanone (CPO). Herein, Ni3Sn2-ReOx/TiO2 is synthesized via successive impregnation and exhibits full conversion and 92.5 % CPO selectivity under 3.0 MPa H2 at 140 °C, which are much higher than those of Ni/TiO2 and Ni3Sn2/TiO2. Characterizations show that Ni3Sn2 is the main active phase that remarkably restrains the over-hydrogenation of the furan ring, while ReOx plays an electrophile or Lewis acid site to activate C-OH of furfuryl alcohol and induces its rearrangement. DFT calculations verify that Sn doping weakens the furan ring adsorption on Ni3Sn2 and shifts its adsorption configuration, which consequently inhibits side reactions and favors the metal-acid (Ni3Sn2-ReOx) synergy. The catalyst is stable, recyclable and also active at even 0.5 MPa H2 and 80 °C. This study provides an advanced strategy for the rational design of superior catalysts for tuning product selectivity, with practical potential for upgrading biomass-derived platform molecules.

Coordination-Promoted Bio-Catechol Electro-Reforming toward Sustainable Polymer Production.

Sustainable polymer production is essential for a carbon-neutral society. cis,cis-Muconic acid is attracting growing interest as a biomass-derived platform molecule with direct access to adipic acid and terephthalic acid, prominent monomers of commercial polymers. Here, a sustainable route of electro-reforming biorenewable catechol to cis,cis-muconic acid with concurrent H2 production has been proposed. By using a CuO foam electrode, a high cis,cis-muconate yield of 90% and a high faradaic efficiency of 87% can be achieved under ambient conditions without external oxidant. Zn2+ coordination with the catechol is central to the yield and selectivity. In a combinatory analysis via steady-state electrochemical kinetics, in situ spectroscopy, and theoretical calculation, we revealed that the reaction ensemble of catechol electrooxidation involves three major processes of polymerization, ring cleavage, and depolymerization, in which Zn2+ coordination is highly effective in delaying polymerization and promoting ring cleavage toward cis,cis-muconate. The catecholate coordinated to the Zn2+ cations reallocated its electron density with partial structural deformation to accelerate the electron transfer and facilitate the OH- nucleophilic attack. A practical two-electrode system was eventually demonstrated to efficiently and stably electro-reform catechol into isolable cis,cis-muconic acid and hydrogen, providing solutions for polymer sustainability via utilizing alternative biomass resources and electrified processes.

Recent Advances in the Efficient Synthesis of Useful Amines from Biomass-Based Furan Compounds and Their Derivatives over Heterogeneous Catalysts

Bio-based furanic oxygenates represent a well-known class of lignocellulosic biomass-derived platform molecules. In the presence of H2 and different nitrogen sources, these versatile building blocks can be transformed into valuable amine compounds via reductive amination or hydrogen-borrowing amination mechanisms, yet they still face many challenges due to the co-existence of many side-reactions, such as direct hydrogenation, polymerization and cyclization. Hence, catalysts with specific structures and functions are required to achieve satisfactory yields of target amines. In recent years, heterogeneous catalytic synthesis of amines from bio-based furanic oxygenates has received extensive attention. In this review, we summarize and discuss the recent significant progress in the generation of useful amines from bio-based furanic oxygenates with H2 and different nitrogen sources over heterogeneous catalysts, according to various raw materials and reaction pathways. The key factors affecting catalytic performances, such as active metals, supports, promoters, reaction solvents and conditions, as well as the possible reaction routes and catalytic reaction mechanisms are studied and discussed in depth. Special attention is paid to the structure–activity relationship, which would be helpful for the development of more efficient and stable heterogeneous catalysts. Moreover, the future research direction and development trend of the efficient synthesis for bio-based amines are prospected.

Selective electroreduction of levulinic acid by controlling the crystalline structure of PbO on electrodes

Electrocatalysis of biomass-derived platform molecules is an effective way to utilize biomass. Electrodes for the electrocatalytic hydrogenation of levulinic acid (LA) to selectively produce valeric acid (VA) have been extensively studied. Improving the electrocatalytic activity of electrodes for LA conversion is a meaningful and challenging task. This paper prepared β-PbO/Pb electrodes by laser processing. The higher adsorption of LA on the β-PbO surface was confirmed by in situ infrared and Raman spectra and density functional theory (DFT) calculations. The β-PbO/Pb electrode exhibited higher electrocatalytic activity than the α-PbO/Pb and Pb electrodes. Complete conversion of LA was achieved at a much lower electrode potential of −1.4 V (vs RHE) with an 85 % yield of VA and greater than 90 % selectivity. In addition, laser processing technology provides a strategy for the rapid processing of electrodes to enable large-scale continuous production in industry.

Hydrothermal Reactions of Biomass-Derived Platform Molecules: Mechanistic Insights into 5-Hydroxymethylfurfural (5-HMF) Formation during Glucose and Fructose Decomposition

The study investigates the primary reaction mechanisms of hydrothermal decomposition of glucose and fructose at 150–225 °C under various initial pH conditions (i.e., with an initial pH of 3–7) to reveal the formation mechanism of 5-hydroxymethylfurfural (5-HMF) from glucose and fructose. The results clearly show that dehydration to produce 5-HMF is not a primary reaction during hydrothermal decomposition of both glucose and fructose under noncatalytic conditions, indicating that 5-HMF cannot be directly produced from either glucose or fructose in water (with an initial pH of ∼7). In the presence of formic acid, dehydration to produce 5-HMF becomes a primary reaction during the hydrothermal decomposition of both glucose and fructose. The selectivity of dehydration reaction to produce 5-HMF increases with reducing the initial pH, i.e., up to ∼40% for glucose decomposition at 175 °C and ∼49% for fructose decomposition at 150 °C when the initial pH reduces to 3. Further kinetic analyses show that the rate constants of isomerization and retro-aldol condensation reactions almost remain unchanged under all initial pH conditions, while the rate constants of dehydration reactions to produce 5-HMF and levoglucosan increase almost linearly with the hydrogen ion concentration at the reaction temperature. This leads to increases in the selectivities of dehydration reactions, at the expense of other primary reactions. Our data also clearly indicate that dehydration reactions are acid-catalyzed reactions, while organic acid has almost no effect on isomerization and retro-aldol condensation reactions. Since organic acids can be formed at the early stage of sugar decomposition, this study provides the first experimental evidence in the field to clarify the formation mechanism of 5-HMF during sugar hydrothermal decomposition.