From furfural to fuel: synthesis of furoins by organocatalysis and their hydrodeoxygenation by cascade catalysis

Production of Renewable Hydrocarbon Biofuels with Lignocellulose and Its Derivatives over Heterogeneous Catalysts

In recent years, the issues of global warming and CO2 emission reduction have garnered increasing global attention. In the 21st Conference of the Parties (convened in Paris in 2015), 179 nations and the European Union signed a pivotal agreement to limit the global temperature increase of this century to well below 2 K above preindustrial levels. To fulfill this objective, extensive research has been conducted to use renewable energy sources as potential replacements for traditional fossil fuels. Among them, the production of hydrocarbon transportation fuels from CO2-neutral and renewable biomass has proven to be a particularly promising solution due to its compatibility with existing infrastructure. This review systematically summarizes research progress in the synthesis of liquid hydrocarbon biofuels from lignocellulose during the past two decades. Based on the chemical structure (including n-paraffins, iso-paraffins, aromatics, and cycloalkanes) of hydrocarbon transportation fuels, the synthesis pathways of these biofuels are discussed in four separate sections. Furthermore, this review proposes three guiding principles for the design of practical hydrocarbon biofuels, providing insights into future directions for the development of viable biomass-derived liquid fuels.

An Alternative Method for the Synthesis of N-Pentafluorophenyl Triazolium Salts

A route for the synthesis of 1,2,4-triazolium salts via oxidation of a thione precursor is demonstrated. N-Pentafluorophenyl-substituted salts are produced in 20-63% overall yields. Isolation and purification of the azolium salts are simplified compared to the traditional synthetic route. Late-stage selection of the counterion allows the synthesis of a variety of salts from a parent thione. The salts have been compared in Stetter and cross-benzoin reactions with an appreciable counterion effect in both reactions.

Redesigning plant cell walls for the biomass-based bioeconomy

Lignocellulosic biomass-the lignin, cellulose, and hemicellulose that comprise major components of the plant cell well-is a sustainable resource that could be utilized in the United States to displace oil consumption from heavy vehicles, planes, and marine-going vessels and commodity chemicals. Biomass-derived sugars can also be supplied for microbial fermentative processing to fuels and chemicals or chemically deoxygenated to hydrocarbons. However, the economic value of biomass might be amplified by diversifying the range of target products that are synthesized in living plants. Genetic engineering of lignocellulosic biomass has previously focused on changing lignin content or composition to overcome recalcitrance, the intrinsic resistance of cell walls to deconstruction. New capabilities to remove lignin catalytically without denaturing the carbohydrate moiety have enabled the concept of the "lignin-first" biorefinery that includes high-value aromatic products. The structural complexity of plant cell-wall components also provides substrates for polymeric and functionalized target products, such as thermosets, thermoplastics, composites, cellulose nanocrystals, and nanofibers. With recent advances in the design of synthetic pathways, lignocellulosic biomass can be regarded as a substrate at various length scales for liquid hydrocarbon fuels, chemicals, and materials. In this review, we describe the architectures of plant cell walls and recent progress in overcoming recalcitrance and illustrate the potential for natural or engineered biomass to be used in the emerging bioeconomy.

Two-step catalytic conversion of lignocellulose to alkanes

Direct conversion of lignocellulose to alkanes is challenged by the complex and recalcitrant nature of the starting material. Generally, alkanes are obtained from one of the main lignocellulose constituents (cellulose, hemicellulose or lignin) after their separation, and platform chemicals derived therein. Here we describe a two-step methodology, which uses unprocessed lignocellulose directly, targeting a mixture of alkanes. The first step involves the near-complete conversion of lignocellulose to alcohols, using a copper doped porous metal oxide (Cu-PMO) catalyst in supercritical methanol. The second step comprises a novel solvent exchange procedure and the exhaustive hydrodeoxygenation (HDO) of the complex mixture of aliphatic alcohols, obtained upon depolymerization, to C₂–C₁₀ alkanes by either HZSM-5 or Nafion at 180 °C in conjunction with Pd/C in dodecane. This describes an unprecedented two-step process from lignocellulose to hydrocarbons, with an overall carbon yield of 50%.

This work described a simple two-step process for the complete lignocellulose conversion to alkanes with high carbon yield.

Towards Improved Biorefinery Technologies: 5-Methylfurfural as a Versatile C6 Platform for Biofuels Development

Low chemical stability and high oxygen content limit utilization of the bio-based platform chemical 5-(hydroxymethyl)furfural (HMF) in biofuels development. In this work, Lewis-acid-catalyzed conversion of renewable 6-deoxy sugars leading to formation of more stable 5-methylfurfural (MF) is carried out with high selectivity. Besides its higher stability, MF is a deoxygenated analogue of HMF with increased C/O ratio. A highly selective synthesis of the innovative liquid biofuel 2,5-dimethylfuran starting from MF under mild conditions is described. The superior synthetic utility of MF against HMF in benzoin and aldol condensation reactions leading to long-chain alkane precursors is demonstrated.

Overcoming cellulose recalcitrance in woody biomass for the lignin-first biorefinery

BACKGROUND

Low-temperature swelling of cotton linter cellulose and subsequent gelatinization in trifluoroacetic acid (TFA) greatly enhance rates of enzymatic digestion or maleic acid-AlCl₃ catalyzed conversion to hydroxymethylfurfural (HMF) and levulinic acid (LA). However, lignin inhibits low-temperature swelling of TFA-treated intact wood particles from hybrid poplar (Populus tremula × P. alba) and results in greatly reduced yields of glucose or catalytic conversion compared to lignin-free cellulose. Previous studies have established that wood particles from transgenic lines of hybrid poplar with high syringyl (S) lignin content give greater glucose yields following enzymatic digestion.

RESULTS

Low-temperature (- 20 °C) treatment of S-lignin-rich poplar wood particles in TFA slightly increased yields of glucose from enzymatic digestions and HMF and LA from maleic acid-AlCl₃ catalysis. Subsequent gelatinization at 55 °C resulted in over 80% digestion of cellulose in only 3 to 6 h with high-S-lignin wood, compared to 20-60% digestion in the wild-type poplar hybrid and transgenic lines high in guaiacyl lignin or 5-hydroxy-G lignin. Disassembly of lignin in woody particles by Ni/C catalytic systems improved yields of glucose by enzymatic digestion or catalytic conversion to HMF and LA. Although lignin was completely removed by Ni/C-catalyzed delignification (CDL) treatment, recalcitrance to enzymatic digestion of cellulose from the high-S lines was reduced compared to other lignin variants. However, cellulose still exhibited considerable recalcitrance to complete enzymatic digestion or catalytic conversion after complete delignification. Low-temperature swelling of the CDL-treated wood particles in TFA resulted in nearly complete enzymatic hydrolysis, regardless of original lignin composition.

CONCLUSIONS

Genetic modification of lignin composition can enhance the portfolio of aromatic products obtained from lignocellulosic biomass while promoting disassembly into biofuel and bioproduct substrates. CDL enhances rates of enzymatic digestion and chemical conversion, but cellulose remains intrinsically recalcitrant. Cold TFA is sufficient to overcome this recalcitrance after CDL treatment. Our results inform a 'no carbon left behind' strategy to convert total woody biomass into lignin, cellulose, and hemicellulose value streams for the future biorefinery.

Catalytic coupling of biomass-derived aldehydes into intermediates for biofuels and materials

Catalytic upgrading of biomass-based aldehydes into chain-extended intermediates for downstream applications in biofuels, fine chemicals, and renewable materials, is reviewed.

Production of Low-Freezing-Point Highly Branched Alkanes through Michael Addition

A new approach for the production of low-freezing-point, high-quality fuels from lignocellulose-derived molecules was developed with Michael addition as the key step. Among the investigated catalysts, CoCl₂ ⋅6 H₂ O was found most active for the Michael addition of 2,4-pentanedione with FA (single aldol adduct of furfural and acetone, 4-(2-furanyl)-3-butene-2-one). Over CoCl₂ ⋅6 H₂ O, a high carbon yield of C₁₃ oxygenates (about 75 %) can be achieved under mild conditions (353 K, 20 h). After hydrodeoxygenation, low-freezing-point (<223 K) branched alkanes with 13 carbons within jet fuel ranges were obtained over a Pd/NbOPO₄ catalyst. Furthermore, C_18,23 fuel precursors could be easily synthesized through Michael addition of 2,4-pentanedione with DFA (double-condensation product of furfural and acetone) under mild conditions and the molar ratio of C₁₈ /C₂₃ is dependent on the reaction conditions of Michael addition. After hydrodeoxygenation, high density (0.8415 g mL^-1 ) and low-freezing-point (<223 K) branched alkanes with 18, 23 carbons within lubricant range were also obtained over a Pd/NbOPO₄ catalyst. These highly branched alkanes can be directly used as transportation fuels or additives. This work opens a new strategy for the synthesis of highly branched alkanes with low freezing point from renewable biomass.

Iridium Clusters Encapsulated in Carbon Nanospheres as Nanocatalysts for Methylation of (Bio)Alcohols

C-H methylation is an attractive chemical transformation for C-C bonds construction in organic chemistry, yet efficient methylation of readily available (bio)alcohols in water using methanol as sustainable C1 feedstock is limited. Herein, iridium nanocatalysts encapsulated in yolk-shell-structured mesoporous carbon nanospheres (Ir@YSMCNs) were synthesized for this transformation. Monodispersed Ir clusters (ca. 1.0 nm) were encapsulated in situ and spatially isolated within YSMCNs by a silica-assisted sol-gel emulsion strategy. A selection of (bio)alcohols (19 examples) was selectively methylated in aqueous phase with good-to-high yields over the developed Ir@YSMCNs. The improved catalytic efficiencies in terms of activity and selectivity together with the good stability and recyclability were contributable to the ultrasmall Ir clusters with oxidation chemical state as a consequence of the confinement effect of YSMCNs with interconnected nanostructures.

Synthesis of Renewable Lubricant Alkanes from Biomass-Derived Platform Chemicals

The catalytic synthesis of liquid alkanes from renewable biomass has received tremendous attention in recent years. However, bio-based platform chemicals have not to date been exploited for the synthesis of highly branched lubricant alkanes, which are currently produced by hydrocracking and hydroisomerization of long-chain n-paraffins. A selective catalytic synthetic route has been developed for the production of highly branched C₂₃ alkanes as lubricant base oil components from biomass-derived furfural and acetone through a sequential four-step process, including aldol condensation of furfural with acetone to produce a C₁₃ double adduct, selective hydrogenation of the adduct to a C₁₃ ketone, followed by a second condensation of the C₁₃ ketone with furfural to generate a C₂₃ aldol adduct, and finally hydrodeoxygenation to give highly branched C₂₃ alkanes in 50.6 % overall yield from furfural. This work opens a general strategy for the synthesis of high-quality lubricant alkanes from renewable biomass.

Catalytic Hydrodeoxygenation of High Carbon Furylmethanes to Renewable Jet-fuel Ranged Alkanes over a Rhenium-Modified Iridium Catalyst

Renewable jet-fuel-range alkanes are synthesized by hydrodeoxygenation of lignocellulose-derived high-carbon furylmethanes over ReO_x -modified Ir/SiO₂ catalysts under mild reaction conditions. Ir-ReO_x /SiO₂ with a Re/Ir molar ratio of 2:1 exhibits the best performance, achieving a combined alkanes yield of 82-99 % from C₁₂ -C₁₅ furylmethanes. The catalyst can be regenerated in three consecutive cycles with only about 12 % loss in the combined alkanes yield. Mechanistically, the furan moieties of furylmethanes undergo simultaneous ring saturation and ring opening to form a mixture of complex oxygenates consisting of saturated furan rings, mono-keto groups, and mono-hydroxy groups. Then, these oxygenates undergo a cascade of hydrogenolysis reactions to alkanes. The high activity of Ir-ReO_x /SiO₂ arises from a synergy between Ir and ReO_x , whereby the acidic sites of partially reduced ReO_x activate the C-O bonds of the saturated furans and alcoholic groups while the Ir sites are responsible for hydrogenation with H₂ .

Selective One-Pot Production of High-Grade Diesel-Range Alkanes from Furfural and 2-Methylfuran over Pd/NbOPO4

A one-pot method for the selective production of high-grade diesel-range alkanes from biomass-derived furfural and 2-methylfuran (2-MF) was developed by combining the hydroxyalkylation/alkylation (HAA) condensation of furfural with 2-MF and the subsequent hydrodeoxygenation (HDO) over a multifunctional Pd/NbOPO₄ catalyst. The effects of various reaction conditions as well as a variety of solid-acid catalysts and metal-loaded NbOPO₄ catalysts were systematically investigated to optimize the reaction conditions for both reactions. Under the optimal reaction conditions up to 89.1 % total yield of diesel-range alkanes was obtained from furfural and 2-MF by this one-pot method.

Highly Selective Upgrading of Biomass-Derived Alcohol Mixtures for Jet/Diesel-Fuel Components

In light of the increasing concern about the energy and environmental problems caused by the combustion of petroleum-based fuels (e.g., jet and diesel fuels), the development of new procedures for their sustainable production from renewable biomass-derived platform compounds has attracted tremendous attention recently. Long-chain ketones/alcohols are promising fuel components owing to the fuel properties that closely resemble those of traditional fuels. The focus of this report is the production of long-chain ketones/alcohols by direct upgrading of biomass-derived short-chain alcohol mixtures (e.g., isopropanol-butanol-ethanol mixtures) in pure water. An efficient Pd catalyst system was developed for these highly selective transformations. Long-chain ketones/alcohols (C₈ -C₁₉ ), which can be used as precursors for renewable jet/diesel fuel, were obtained in good-to-high selectivity (>90 %) by using the developed Pd catalyst.

The Conversion of Starch and Sugars into Branched C10 and C11 Hydrocarbons

Oligosaccharides, such as starch, cellulose, and hemicelluloses, are abundant and easily obtainable bio-derived materials that can potentially be used as precursors for fuels and chemical feedstocks. To access the pertinent molecular building blocks (i.e., 5- or 6-carbon containing sugar units) located within these biopolymers and transform them into useful fuel precursors, oligosaccharide depolymerization followed by chain extension is required. This chain extension can readily be performed via a Garcia-Gonzalez-like approach using β-diketones under mild conditions to provide fuel precursors containing an increased carbon atom content that meets fuel requirements. In a subsequent step, ring opening and hydrodeoxygenation chemistry of these species allows for the preparation of branched alkanes under relatively mild conditions. This approach can be applied to monomeric sugars (glucose and xylose), oligosaccharides (starch), and potentially to hydrolyzed dedicated energy crops to allow the conversion of real biomass into fuel type molecules.

Oxidative Furan-to-Indole Rearrangement. Synthesis of 2-(2-Acylvinyl)indoles and Flinderole C Analogues

Oxidative rearrangement of 2-(2-aminobenzyl)furans affording 2-(2-acylvinyl)indoles in a stereocontrolled manner in good-to-excellent yields has been developed. Thus, (2-aminobenzyl)furans with electron-releasing alkoxy substituents in the phenyl group form only E-isomers of 2-(2-acylvinyl)indoles. Conversely, substrates without such substituents produce target products as Z-isomers exclusively. A short diastereoselective method for the transformation of the obtained 2-(2-acylvinyl)indoles into antimalarial bisindole alkaloid flinderole A-C analogues has been developed.

Potential and challenges of zeolite chemistry in the catalytic conversion of biomass

This review emphasizes the progress, potential and future challenges in zeolite catalysed biomass conversions and relates these to concepts established in existing petrochemical processes.

Catalytic Upgrading of 5-Hydroxymethylfurfural to Drop-in Biofuels by Solid Base and Bifunctional Metal-Acid Catalysts

Design and synthesis of effective heterogeneous catalysts for the conversion of biomass intermediates into long chain hydrocarbon precursors and their subsequent deoxygenation to hydrocarbons is a viable strategy for upgrading lignocellulose into distillate range drop-in biofuels. Herein, we report a two-step process for upgrading 5-hydroxymethylfurfural (HMF) to C9 and C11 fuels with high yield and selectivity. The first step involves aldol condensation of HMF and acetone with a water tolerant solid base catalyst, zirconium carbonate (Zr(CO3 )x ), which gave 92 % C9 -aldol product with high selectivity at nearly 100 % HMF conversion. The as-synthesised Zr(CO3 )x was analysed by several analytical methods for elucidating its structural properties. Recyclability studies of Zr(CO3 )x revealed a negligible loss of its activity after five consecutive cycles over 120 h of operation. Isolated aldol product from the first step was hydrodeoxygenated with a bifunctional Pd/Zeolite-β catalyst in ethanol, which showed quantitative conversion of the aldol product to n-nonane and 1-ethoxynonane with 40 and 56 % selectivity, respectively. 1-Ethoxynonane, a low oxygenate diesel range fuel, which we report for the first time in this paper, is believed to form through etherification of the hydroxymethyl group of the aldol product with ethanol followed by opening of the furan ring and hydrodeoxygenation of the ether intermediate.

Thiazolylidene-catalyzed cleavage of methyl oleate-derived α-hydroxy ketone to the corresponding free aldehydes

The thiazolylidene-catalyzed cleavage of the α-hydroxy ketone derived from methyl oleate gave the corresponding aldehydes under nonoxidative conditions through a retro-benzoin process. The aldehydes produced are in equilibrium with their corresponding acyloins. To illustrate the synthetic utility of this protocol, the aldehydes were recovered by distillation.

Biomass recalcitrance: a multi-scale, multi-factor, and conversion-specific property

Recalcitrance of plant biomass to enzymatic hydrolysis for biofuel production is thought to be a property conferred by lignin or lignin-carbohydrate complexes. However, chemical catalytic and thermochemical conversion pathways, either alone or in combination with biochemical and fermentative pathways, now provide avenues to utilize lignin and to expand the product range beyond ethanol or butanol. To capture all of the carbon in renewable biomass, both lignin-derived aromatics and polysaccharide-derived sugars need to be transformed by catalysts to liquid hydrocarbons and high-value co-products. We offer a new definition of recalcitrance as those features of biomass which disproportionately increase energy requirements in conversion processes, increase the cost and complexity of operations in the biorefinery, and/or reduce the recovery of biomass carbon into desired products. The application of novel processing technologies applied to biomass reveal new determinants of recalcitrance that comprise a broad range of molecular, nanoscale, and macroscale factors. Sampling natural genetic diversity within a species, transgenic approaches, and synthetic biology approaches are all strategies that can be used to select biomass for reduced recalcitrance in various pretreatments and conversion pathways.

Catalytic total hydrodeoxygenation of biomass-derived polyfunctionalized substrates to alkanes

The total hydrodeoxygenation of carbohydrate-derived molecules to alkanes, a key reaction in the production of biofuel, was reviewed from the aspect of catalysis. Noble metals (or Ni) and acid are the main components of the catalysts, and group 6 or 7 metals such as Re are sometimes added as modifiers of the noble metal. The main reaction route is acid-catalyzed dehydration plus metal-catalyzed hydrogenation, and in some systems metal-catalyzed direct CO dissociation is involved. The appropriate active metal, acid strength, and reaction conditions depend strongly on the reactivity of the substrate. Reactions that use Pt or Pd catalysts supported on Nb-based acids or relatively weak acids are suitable for furanic substrates. Carbohydrates themselves and sugar alcohols undergo CC dissociation easily. The systems that use metal-catalyzed direct CO dissociations can give a higher yield of the corresponding alkane from carbohydrates and sugar alcohols.