5-Hydroxymethylfurfural and its Downstream Chemicals: A Review of Catalytic Routes

Biomass assumes an increasingly vital role in the realm of renewable energy and sustainable development due to its abundant availability, renewability, and minimal environmental impact. Within this context, 5-hydroxymethylfurfural (HMF), derived from sugar dehydration, stands out as a critical bio-derived product. It serves as a pivotal multifunctional platform compound, integral in synthesizing various vital chemicals, including furan-based polymers, fine chemicals, and biofuels. The high reactivity of HMF, attributed to its highly active aldehyde, hydroxyl, and furan ring, underscores the challenge of selectively regulating its conversion to obtain the desired products. This review highlights the research progress on efficient catalytic systems for HMF synthesis, oxidation, reduction, and etherification. Additionally, it outlines the techno-economic analysis (TEA) and prospective research directions for the production of furan-based chemicals. Despite significant progress in catalysis research, and certain process routes demonstrating substantial economics, with key indicators surpassing petroleum-based products, a gap persists between fundamental research and large-scale industrialization. This is due to the lack of comprehensive engineering research on bio-based chemicals, making the commercialization process a distant goal. These findings provide valuable insights for further development of this field.

Selective electrosynthesis of platform chemicals from the electrocatalytic reforming of biomass-derived hexanediol

6-Hydroxyhexanoic acid and adipic acid are platform chemicals and are widely used as building blocks for the synthesis of important polymers. Nevertheless, the industrial syntheses of these two chemicals are fossil fuel-based and involve the use of corrosive acid and emission of the NOx greenhouse gas. In this study, the electrosynthesis of 6-hydroxyhexanoic acid and adipic acid from the electrochemical oxidation of hexanediol at the nanoporous nickel oxyhydroxide modified electrode was explored as an environmentally-benign alternative to the industrial syntheses of 6-hydroxyhexanoic acid and adipic acid. The effects of electrolysis conditions, including the electrolyte pH and applied potentials, on faradaic efficiency and product distribution of the electrochemical oxidation of hexanediol, were thoroughly examined through a series of controlled-potential electrolyses. In addition, the scale-up electrosynthesis of 6-hydroxyhexanoic acid and adipic acid using a flow-type electrolyzer was also demonstrated.

Lignocellulosic biorefineries: A multiscale approach for resource exploitation

Biomass can become the source for chemicals towards a sustainable production system. However, the challenges it presents such as the variety of species, their widespread and sparse availability, and the expensive transportation claims for an integrated approach to design the novel production system. Multiscale approaches have not been properly extended to biorefineryes design and deployment, due to the comprehensive experimental and modelling work they require. A systems perspective provides the systematic framework to analyze the availability and composition of raw materials across regions, how that affects process design, the portfolio of products that can be obtained by evaluating the strong link between the biomass features and the process design. The use of lignocellulosic materials requires for a multidisciplinary work, that must lead to new process engineers with technical competences in biology, biotechnology but also process engineering, mathematics, computer science and social sciences towards a sustainable process/chemical industry.

Polymers without Petrochemicals: Sustainable Routes to Conventional Monomers

Access to a wide range of plastic materials has been rationalized by the increased demand from growing populations and the development of high-throughput production systems. Plastic materials at low costs with reliable properties have been utilized in many everyday products. Multibillion-dollar companies are established around these plastic materials, and each polymer takes years to optimize, secure intellectual property, comply with the regulatory bodies such as the Registration, Evaluation, Authorisation and Restriction of Chemicals and the Environmental Protection Agency and develop consumer confidence. Therefore, developing a fully sustainable new plastic material with even a slightly different chemical structure is a costly and long process. Hence, the production of the common plastic materials with exactly the same chemical structures that does not require any new registration processes better reflects the reality of how to address the critical future of sustainable plastics. In this review, we have highlighted the very recent examples on the synthesis of common monomers using chemicals from sustainable feedstocks that can be used as a like-for-like substitute to prepare conventional petrochemical-free thermoplastics.

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.

Simultaneous production of 1,6-hexanediol, furfural, and high-purity lignin from white birch: Process integration and techno-economic evaluation

An integrated strategy of multiple catalytic conversions was developed to completely utilize three major fractions of biomass, thereby increasing the revenue from lignocellulosic biomass (white birch). Cellulose was converted into 1,6-hexanediol (1,6-HDO) with a yield of 21.8% via a series of catalytic conversions, hemicellulose was converted into furfural with a yield of 87.2% via dehydration, and lignin was purified into high-purity lignin with a yield of 71.7% via two-step purification. Heat integration was performed to mitigate the challenges associated with the large energy requirements of the process. Additionally, a techno-economic analysis was conducted to investigate the feasibility of the proposed process. The minimum selling price (MSP) of 1,6-HDO is estimated to be $3,922/ton, meaning that the economics of the proposed process are favorable compared to petroleum-derived 1,6-HDO production ($4,400/ton). The effect of economic parameters on the MSP of 1,6-HDO was also investigated via a wide array of sensitivity analyses.

Hydrophilic and organophilic pervaporation of industrially important α,β and α,ω-diols

The distillation-based purification of α,β and α,ω-diols is energy and resource intensive, as well as time consuming. Pervaporation separation is considered to be a remarkable energy efficient membrane technology for purification of diols. Thus, as a core pervaporation process, hydrophilic polyvinyl alcohol (PVA) membranes for the removal of water from 1,2-hexanediol (1,2-HDO) and organophilic polydimethylsiloxane-polysulfone (PDMS-PSF) membranes for the removal of isopropanol from 1,5 pentanediol (1,5-PDO) were employed. For 1,2-HDO/water separation using a feed having a 1 : 4 weight ratio of 1,2-HDO/water, the membrane prepared using 4 vol% glutaraldehyde (GA4) showed the best performance, yielding a flux of 0.59 kg m-2 h-1 and a separation factor of 175 at 40 °C. In the organophilic pervaporation separation of the 1,5-PDO/IPA feed having a 9 : 1 weight ratio of components, the PDMS membrane prepared with a molar ratio of TEOS alkoxy groups to PDMS hydroxyl groups of 70 yielded a flux of 0.12 kg m-2 h-1 and separation factor of 17 638 at 40 °C. Long term stability analysis found that both hydrophilic (PVA) and organophilic (PDMS) membranes retained excellent pervaporation output over 18 days' continuous exposure to the feed. Both the hydrophilic and organophilic membranes exhibited promising separation performance at elevated operating conditions, showing their great potential for purification of α,β and α,ω-diols.

Ruthenium triphos complexes [Ru(X(CH2PPh2)3-κ3-P)(NCCH3)3](OTf)2; X = H3C-C, N) as catalysts for the conversion of furfuryl acetate to 1,4-pentanediol and cyclopentanol in aqueous medium

The ruthenium complexes [Ru(H3CC(CH2PPh2)3-κ3-P)(NCCH3)3](OTf)2 (1, (H3CC(CH2PPh2)3 = triphos) and [Ru(N(CH2PPh2)3-κ3-P)(NCCH3)3](OTf)2 (2, N(CH2PPh2)3 = N-triphos) have been evaluated as homogeneous ionic hydrogenation catalysts for the catalytic hydrodeoxygenation of furfuryl alcohol and furfuryl acetate to 1,4-pentanediol and cyclopentanol in aqueous media reaction mixtures. For furfuryl alcohol, only marginal yields of 1,4-pentanediol could be achieved with mass balance deficiencies due to humin formation ranging from 67% to 90%. Attempts to improve the catalytic activity of 2 by enhancing its water solubility by nitrogen protonation and (or) methylation failed. Employing the less self-reactive furfuryl acetate as the substrate substantially diminishes humin formation, yielding up to 43% of 1,4-pentanediol and 19% of cyclopentanol (via Piancatelli rearrangement) with 1 and up to 33% of 1,4-pentanediol and 5% of cyclopentanol with 2. A design of experiments study was used to determine and compare the yield responses of the multiple parallel reaction channels with 1,4-pentanediol, cyclopentanol, and humins as a function of reaction temperature, time, catalyst load, and substrate concentration. This explores the correlations between these parameters and their impact on the reaction outcome and suggests an extremely complex overall reaction cascade of interdependent pathways of both acid- and metal-catalyzed steps with some significant differences emerging between the two catalysts.

Catalytic strategy for conversion of fructose to organic dyes, polymers, and liquid fuels

We report a process to produce a versatile platform chemical from biomass-derived fructose for organic dye, polymer, and liquid fuel industries. An aldol-condensed chemical (HAH) is synthesized as a platform chemical from fructose by catalytic reactions in acetone/water solvent with non-noble metal catalysts (e.g., HCl, NaOH). Then, selective reactions (e.g., etherification, reduction, dimerization) of the functional groups, such as enone and hydroxyl groups, in the HAH molecule enable applications in organic dyes and polyether precursors. High yields of target products, such as 5-(hydroxymethyl) furfural (HMF) (85.9% from fructose) and HAH (86.3% from HMF) are achieved by sequential dehydration and aldol-condensation with a simple purification process (>99% HAH purity). The use of non-noble metal catalysts, the high yield of each reaction, and the simple purification of the target product allow for beneficial economics of the process. Techno-economic analysis indicates that the process produces HAH at minimum selling price (MSP) of $1958/ton. The MSP of HAH product allows the economic viability of applications in organic dye and polyether markets by replacing its counterparts, such as anthraquinone ($3200-$3900/ton) and bisphenol-A ($1360-$1720/ton).

Synthesis of Dicarboxylic Acids from Aqueous Solutions of Diols with Hydrogen Evolution Catalyzed by an Iridium Complex

A catalytic system for the synthesis of dicarboxylic acids from aqueous solutions of diols accompanied by the evolution of hydrogen was developed. An iridium complex bearing a functional bipyridonate ligand with N,N-dimethylamino substituents exhibited a high catalytic performance for this type of dehydrogenative reaction. For example, adipic acid was synthesized from an aqueous solution of 1,6-hexanediol in 97 % yield accompanied by the evolution of four equivalents of hydrogen by the present catalytic system. It should be noted that the simultaneous production of industrially important dicarboxylic acids and hydrogen, which is useful as an energy carrier, was achieved. In addition, the selective dehydrogenative oxidation of vicinal diols to give α-hydroxycarboxylic acids was also accomplished.

Heterogeneous Catalyst Design Principles for the Conversion of Lignin into High-Value Commodity Fuels and Chemicals

Lignin valorization has risen as a promising pathway to supplant the use of petrochemicals for chemical commodities and fuels. However, the challenges of separating and breaking down lignin from lignocellulosic biomass are the primary barriers to success. Integrated biorefinery systems that incorporate both homo- and heterogeneous catalysis for the upgrading of lignin intermediates have emerged as a viable solution. Homogeneous catalysis can perform selected chemistries, such as the hydrolysis and dehydration of ester or ether bonds, that are more suitable for the pretreatment and fractionation of biomass. Heterogeneous catalysis, however, offers a tunable platform for the conversion of extracted lignin into chemicals, fuels, and materials. Tremendous effort has been invested in elucidating the necessary factors for the valorization of lignin by using heterogeneous catalysts, with efforts to explore more robust methods to drive down costs. Current progress in lignin conversion has fostered numerous advances, but understanding the key catalyst design principles is important for advancing the field. This Minireview aims to provide a summary on the fundamental design principles for the selective conversion of lignin by using heterogeneous catalysts, including the pairing of catalyst metals, supports, and solvents. The review puts a particular focus on the use of bimetallic catalysts on porous supports as a strategy for the selective conversion of lignin. Finally, future research on the valorization of lignin is proposed on the basis of recent progress.

The Roles of H2O/Tetrahydrofuran System in Lignocellulose Valorization

Lignocellulosic biomass as a potential alternative to fossil resource for the production of valuable chemicals and fuels has attracted substantial attention, while reducing the recalcitrance of lignocellulosic biomass is still challenging due to the complex and cross-linking structure of biomass. Solvent system plays important roles in the pretreatment of lignocellulose, enabling the transformation of solid biomass to liquid fluid with better mass and heat transfer, as well as in the selective formation of target products. In particular, H2O/tetrahydrofuran (H2O/THF) system has recently been widely applied in lignocellulose valorization, which has been proved to exhibit outstanding efficiency for the conversion of lignocellulose, solubilization of the intermediates and products, and shifting reaction equilibrium, thereby significantly improving the yield and selectivity of target products, as well as the full utilization of lignocellulose. In addition, THF shows low toxicity, and is known as a renewable solvent which can be produced from bio-derived chemicals. Herein, this review concentrates on the advances of H2O/THF system in lignocellulose valorization in recent years. Several aspects relative to the roles of H2O/THF system are discussed as follows: the pretreatment of lignin, conversion of hemicellulose and cellulose components in lignocelluloses, and the promoting formation of valuable chemicals like furfural, 5-hydroxymethyl furfural (HMF), levulinic acid, and so on, as well as the inhibiting role in humins formation. This review might provide useful information for the design of effective solvent system in full utilization of lignocellulosic biomass.

Palladium nanoparticles supported on nanosheet-like graphitic carbon nitride for catalytic transfer hydrogenation reaction

Pd/g-C₃N₄ catalysts with well-dispersed, electron-enriched Pd nanoparticles immobilized on pyridinic N atoms of g-C₃N₄ are active for catalytic transfer hydrogenation under ambient conditions.

Aerobic oxidation of C4–C6 α,ω-diols to the diacids in base-free medium over zirconia-supported (bi)metallic catalysts

Aerobic oxidation of (C₄–C₆) α,ω-diols in water produces the corresponding α,ω-diacids in high 83–96% yields over a Au–Pt/ZrO₂ catalyst.

Enzymatic synthesis and characterization of novel terpolymers from renewable sources

2,5-Furandicarboxylic acid and itaconic acid are both important biobased platform chemicals and their terpolymer with 1,6-hexanediol (HDO) can be the starting point for a new class of reactive polyesters, with important applications. The green synthetic route developed in this study involves a biocatalytic condensation polymerization reaction of dimethyl furan-2,5-dicarboxylate (DMFDC) and dimethyl itaconate (DMI) with HDO in toluene at 80°C, using commercial immobilized lipases from Candida antarctica B. In the best conditions, the formed polymer product was isolated with more than 80% yield, containing about 85% terpolymer with average molecular mass of about 1200 (Mn, calculated from MALDI-TOF MS data) and 15% DMFDC_HDO copolymer. Considering the higher reactivity of DMFDC, the composition of the synthesized polymer can be directed by adjusting the molar ratio of DMFDC and DMI, as well as by extending the reaction time. Structural analysis by NMR demonstrated the regioselective preference for the carbonyl group from DMI adjacent to the methylene group. The biocatalyst was successfully reused in multiple reaction cycles.