Catalytic total hydrodeoxygenation of biomass-derived polyfunctionalized substrates to alkanes

Catalytic Hydrodeoxygenation of Phenols and Cresols to Gasoline Range Biofuels

Unlike fossil fuels, biomass has oxygen amounts exceeding 10 wt%. Hydrodeoxygenation (HDO) is a crucial step in upgrading biomass to higher heating value liquid fuels. Oxygen removal has many challenges due to the complex chemistry and the high reactivity leading to irreversible catalyst deactivation. In this study, the focus is on the catalytic HDO of aromatic oxygen-containing model compounds in biomass: phenols and cresols. In the current work, literature on catalytic HDO of phenols using molecular hydrogen is reviewed, with a focus on non-nickel-based mono- and bi-metallic catalysts, as nickel-based catalysts were reviewed elsewhere. In addition, the catalytic HDO of m-cresol using molecular hydrogen is examined. This review also addresses the use of hydrogen donors for the HDO of phenols and cresols. The operating conditions, catalysts, products, and yields are summarized to find the catalyst with promising activity and high selectivity toward aromatics. A critical review of the reactions that successfully led to HDO is presented and research gaps related to the HDO of phenols and cresols are highlighted. The conclusions provide potential successful catalyst combinations that can be used for HDO of phenols, cresols, and liquid aromatic hydrocarbons.

Hydrodeoxygenation of Oxygen-Containing Aromatic Plastic Wastes into Cycloalkanes and Aromatics

Chemical recycling and upcycling offer promising approaches for the management of plastic wastes. Hydrodeoxygenation (HDO) is one of the appealing ways for conversion of oxygen-containing plastic wastes, including polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polycarbonate (PC), polyphenyl ether (PPO), and polyether ether ketone (PEEK), into cyclic alkanes and aromatics in high yields under mild reaction conditions. The challenge lies in achieving C-O activation while preserving C-C bonds. In this review, we highlight the recent advancements in catalytic strategies and catalysts for the conversion of these oxygen-containing plastic wastes into cycloalkanes and aromatics. The reaction systems, including multi-step routes, direct HDO and transfer HDO methods, are exemplified. The design and performance of HDO catalysts are systematically summarized and compared. We comprehensively discuss the functions of the catalysts' components, reaction pathway and mechanism to gain insights into the HDO process for efficient valorization of oxygen-containing plastic wastes. Finally, we provide perspectives for this field, with specific emphasis on the non-noble metal catalyst design, selectivity control, reaction network and mechanism studies, mixed plastic wastes management and product functionalization. We anticipate that this review will inspire innovations on the catalytic process development and rational catalyst design for the HDO of oxygen-containing aromatic plastics to establish a low-emission circular economy.

Highly Efficient Iridium-Iron-Molybdenum Catalysts Condensed on Boron Nitride for Biomass-Derived Diols' Hydrogenolysis to Secondary Monoalcohols

A trade-off of activity-selectivity exists in primary C-O hydrogenolysis of biomass-derived diols to secondary alcohols over bimetallic catalysts, especially the combination of noble metal and early transition metal in the metallic state and metal oxide state, respectively. Herein, the combination of high surface concentration of boron nitride (BN)-supported metals and the addition of Mo as third metal broke the trade-off. High yields (>50%) of secondary alcohols were obtained with robust productivity up to 25-fold based on Ir over Ir-Fe0.13-Mo0.08/BN (Ir = 20 wt %, Fe/Ir = 0.13, Mo/Ir = 0.08) than previously reported Ir-Fe catalysts. In contrast, simply increasing the loading amount of Ir-Fe catalysts or the addition of Mo species only enhanced the productivity by <2-4-fold. Various characterizations showed that large Ir loading enables the formation of condensed nanostructures (∼2 nm) on the BN support, which further alloy with Mo and Fe to form an face centred cubic (fcc)-type trimetallic alloy with surface enrichment of Fe. On the other hand, in Ir-Fe0.25-Mo0.08/BN with lower Ir (5 wt %) and lower Ir-based activity, the Mo species were rather bound on the support surface possibly as the MoBx state. These structures were formed by simple impregnation and reduction with H2 at the reaction temperature (453 K). The high activity of Ir-Fe0.13-Mo0.08/BN (20 wt % Ir) is derived from two aspects: (1) the formation of condensed nanostructures (∼2 nm) exposing more active sites and (2) alloying with Mo to modify the electronic state of Ir to enhance the H2 activation ability, as shown by the decreased Ea (82-84 → 67 kJ mol-1).

Selective Hydrogenolysis of Erythritol over Ir-ReOx /Rutile-TiO2 Catalyst

Partial hydrogenolysis of erythritol, which can be produced at large scale by fermentation, to 1,4-butanediol (1,4-BuD) is investigated with Ir-ReO_x /SiO₂ and Ir-ReO_x /rutile-TiO₂ catalysts. In addition to the higher conversion rate over Ir-ReO_x /TiO₂ than over Ir-ReO_x /SiO₂ , which has been also reported for glycerol hydrogenolysis, Ir-ReO_x /TiO₂ showed higher selectivity to 1,4-BuD than Ir-ReO_x /SiO₂ , especially at low conversion levels, leading to high 1,4-BuD productivity of 20 mmol_1,4-BuD  g_Ir ^-1  h^-1 at 373 K (36 % conversion, 33 % selectivity). The productivity based on the noble metal amount is higher than those reported previously, although the maximum yield of 1,4-BuD (23 %) is not higher than the highest reported values. The reactions of various triols, diols and mono-ols are tested and the selectivity and the reaction rates are compared between catalysts and between substrates. The Ir-ReO_x /TiO₂ catalyst showed about twofold higher activity than Ir-ReO_x /SiO₂ in hydrogenolysis of the C-OH bond at the 2- or 3-positions in 1,2- and 1,3-diols, respectively, whereas the hydrogenolysis of C-OH at the 1-position is less promoted by the TiO₂ support. Lowering the loading amount of Ir on TiO₂ (from 4 wt % to 2 or 1 wt %) decreases the Ir-based activity and 1,4-BuD selectivity. Similarly, increasing the loading amount on SiO₂ from 4 wt % to 20 wt % increases the Ir-based activity and 1,4-BuD selectivity, although they remain lower than those for TiO₂ -supported catalyst with 4 wt % Ir. High metal loadings on the support seem to be important.

H2 -Free Re-Based Catalytic Dehydroxylation of Aldaric Acid to Muconic and Adipic Acid Esters

As one of the most demanded dicarboxylic acids, adipic acid can be directly produced from renewable sources. Hexoses from (hemi)cellulose are oxidized to aldaric acids and subsequently catalytically dehydroxylated. Hitherto performed homogeneously, we present the first heterogeneous catalytic process for converting an aldaric acid into muconic and adipic acid. The contribution of leached Re from the solid pre-reduced catalyst was also investigated with hot-filtration test and found to be inactive for dehydroxylation. Corrosive or hazardous (HBr/H2 ) reagents are avoided and simple alcohols and solid Re/C catalysts in an inert atmosphere are used. At 120 °C, the carboxylic groups are protected by esterification, which prevents lactonization in the absence of water or acidic sites. Dehydroxylation and partial hydrogenation yield monohexenoates (93 %). For complete hydrogenation to adipate, a 16 % higher activation barrier necessitates higher temperatures.

Hydro(deoxygenation) Reaction Network of Lignocellulosic Oxygenates

Hydrodeoxygenation (HDO) is a key transformation step to convert lignocellulosic oxygenates into drop-in and functional high-value hydrocarbons through controlled oxygen removal. Nevertheless, the mechanistic insights of HDO chemistry have been scarcely investigated as opposed to a significant extent of hydrodesulfurization chemistry. Current requirements emphasize certain underexplored events of HDO of oxygenates, which include 1) interactions of oxygenates of varied molecular size with active sites of the catalysts, 2) determining the conformation of oxygenates on the active site at the point of interaction, and 3) effects of oxygen contents of oxygenates on the reaction rate of HDO. It is realized that the molecular interactions of oxygenates with the surface of the catalyst dominates the degree and nature of deoxygenation to derive products with desired selectivity by overcoming complex separation processes in a biorefinery. Those oxygenates with high carbon numbers (>C10), multiple furan rings, and branched architectures are even more complex to understand. This article aims to focus on concise mechanistic analysis of biorefinery oxygenates (C_10-35 ) for their deoxygenation processes, with a special emphasis on their interactions with active sites in a complex chemical environment. This article also addresses differentiation of the mode of interactions based on the molecular size of oxygenates. Deoxygenation processes coupled with or without ring opening of furan-based oxygenates and site-substrate cooperativity dictate the formation of diverse value-added products. Oxygen removal has been the key step for microbial deoxygenation by the use of oxygen-removing decarbonylase enzymes. However, challenges to obtain branched and long-chain hydrocarbons remain, which require special attention, including the invention of newer techniques to upgrade the process for combined depolymerization-HDO from real biomass.

Highly efficient alloyed NiCu/Nb2O5 catalyst for the hydrodeoxygenation of biofuel precursors into liquid alkanes

Hydrodeoxygenation (HDO) is a crucial process for the synthesis of biofuels from renewable biomass.

Reduction of sugar derivatives to valuable chemicals: utilization of asymmetric carbons

Recent progress on non-furfural routes from sugar derivatives to valuable chemicals including chiral chemicals was reviewed.

Conversion of 5-Methyl-3-Heptanone to C8 Alkenes and Alkane over Bifunctional Catalysts

A one-step catalytic process was used to catalyze the hydrodeoxygenation of 5-methyl-3-heptanone (C8 ketone) to a mixture of 5-methyl-3-heptene, 5-methyl-2-heptene (C8 alkenes), and 3-methyl heptane (C8 alkane). High conversion of C8 ketone to the desired products was achieved over a single bed of a supported catalyst (bifunctional heterogeneous catalyst) consisting of one transition metal (copper (Cu) or platinum (Pt)) loaded on alumina (Al2O3) under mild operating conditions (reaction temperatures were varied between 180 °C to 260 °C, and the pressure was 1 atm). The C8 ketone was hydrogenated to 5-methyl-3-heptanol (C8 alcohol) over metal sites, followed by dehydration of the latter on acid sites on the support to obtain a mixture of C8 alkenes. These C8 alkenes can be further hydrogenated on metal sites to make a C8 alkane. The results showed that the main products over copper loaded on alumina (20 wt% Cu–Al2O3) were a mixture of C8 alkenes and C8 alkane in different amounts depending on the operating conditions (the highest selectivity for C8 alkenes (~82%) was obtained at 220 °C and a H2/C8 ketone molar ratio of 2). However, over platinum supported on alumina (1 wt% Pt–Al2O3), the major product was a C8 alkane with a selectivity up to 97% and a conversion of 99.9% at different temperatures and all H2/C8 ketone ratios.

Aqueous Dehydration, Hydrogenation and Hydrodeoxygenation Reactions of Bio-Based Mucic Acid over Ni, NiMo, Pt, Rh, and Ru on Neutral or Acidic Catalyst Supports

Hydrotreatment of mucic acid (also known as galactaric acid, an glucaric acid enantiomer), one of the most promising bio-based platform chemicals, was systematically investigated in aqueous media over alumina, silica, or carbon-supported transition (nickel and nickel-molybdenum) or noble (platinum, ruthenium and rhodium) metals. Mucic acid was only converted into mucic-1,4-lactone under non-catalytic reaction conditions in N2 atmosphere, while the 5 MPa gaseous H2 addition triggers hydrogenation in the bulk phase, resulting in formation of galacturonic and galactonic acid. However, dehydroxylation, hydrogenation, decarbonylation, decarboxylation, and cyclization occurred during catalytic hydrotreatment, forming various partially and completely deoxygenated products with a chain length of 3–6 C atoms. Characterization results of tested catalysts were correlated with their activity and selectivity. Insufficient pore diameter of microporous supports completely hindered the mass transfer of reactants to the active sites, resulting in negligible conversion of mucic acid. A comprehensive reaction pathway network was proposed and several industrially interesting compounds were formed, including levulinic acid, furoic acid, and adipic acid. However, selectivity towards adipic acid, a bio-based nylon 6,6 precursor, was low (up to 5 mol%) in aqueous media and elevated temperatures.

Biofuel Synthesis from Sorbitol by Aqueous Phase Hydrodeoxygenation over Bifunctional Catalysts: In-depth Study of the Ru–Pt/SiO2–Al2O3 Catalytic System

The catalytic performances of Ru–Pt/SiO2–Al2O3 catalysts synthetized by three methods (co-impregnation (CI), successive impregnations (SI) and redox deposition (CR)) were compared for their sorbitol transformation to hexane under hydrothermal conditions. The existence of Pt–Ru interaction was demonstrated by TEM-EDX only on SI and CR samples, with a PtRu alloy suspected by XRD and XPS. The chemical nature of the Ru species differed according to the synthesis method with the presence of Ru4+ species on SI–(Ru–Pt) and CR catalysts. The SI–(Ru–Pt)/SiO2–Al2O3 system displayed the best metal–acid function balance leading to the highest selectivity to hexane. The study of the reactivity of isosorbide and 2,5-dimethylfuran intermediates highlighted that the first one was poorly reactive compared to the second one, and the latter was selectively convertible to hexane. The synergy effect on SI– (Ru–Pt)/SiO2–Al2O3 catalyst was attributed to the presence of small-sized bimetallic particles favoring an electronic exchange from Ru to Pt, and increasing the formation of 2,5-dimethylfuran.

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.

Route for Conversion of Furfural to Ethylcyclopentane

A method for conversion of furfural, widely available platform chemical from biomass, to ethylcyclopentane, is reported. Ethylcyclopentane is a cyclic alkane with a relatively high octane number (RON 67, bp 103 °C) and could potentially serve as a drop-in biofuel. The synthetic route includes a transformation of furfural to 1-(furan-2-yl)propan-1-ol that is further subjected to Piancatelli rearrangement to give 5-ethyl-4-hydroxycyclopent-2-en-1-one. The subsequent hydrodeoxygenation affords ethylcyclopentane in 48% isolated yield.

Cleave and couple: toward fully sustainable catalytic conversion of lignocellulose to value added building blocks and fuels

The structural complexity of lignocellulose offers unique opportunities for the development of entirely new, energy efficient and waste-free pathways in order to obtain valuable bio-based building blocks. Such sustainable catalytic methods - specifically tailored to address the efficient conversion of abundant renewable starting materials - are necessary to successfully compete, in the future, with fossil-based multi-step processes. In this contribution we give a summary of recent developments in this field and describe our "cleave and couple" strategy, where "cleave" refers to the catalytic deconstruction of lignocellulose to aromatic and aliphatic alcohol intermediates, and "couple" involves the development of novel, sustainable transformations for the formation of C-C and C-N bonds in order to obtain a range of attractive products from lignocellulose.

A Durable Nickel Single-Atom Catalyst for Hydrogenation Reactions and Cellulose Valorization under Harsh Conditions

Hydrothermally stable, acid-resistant nickel catalysts are highly desired in hydrogenation reactions, but such a catalyst remains absent owing to the inherent vulnerability of nickel under acidic conditions. An ultra-durable Ni-N-C single-atom catalyst (SAC) has now been developed that possesses a remarkable Ni content (7.5 wt %) required for practical usage. This SAC shows not only high activities for hydrogenation of various unsaturated substrates but also unprecedented durability for the one-pot conversion of cellulose under very harsh conditions (245 °C, 60 bar H₂ , presence of tungstic acid in hot water). Using integrated spectroscopy characterization and computational modeling, the active site structure is identified as (Ni-N4)⋅⋅⋅N, where significantly distorted octahedral coordination and pyridinic N constitute a frustrated Lewis pair for the heterolytic dissociation of dihydrogen, and the robust covalent chemical bonding between Ni and N atoms accounts for its ultrastability.

Efficient utilization of renewable feedstocks: the role of catalysis and process design

Renewable carbon feedstocks such as biomass and CO₂ present an important element of future circular economy. Especially biomass as highly functionalized feedstock provides manifold opportunities for the transformation into attractive platform chemicals. However, this change of the resources requires a paradigm shift in refinery design. Fossil feedstocks are processed in gas phase at elevated temperature. In contrast, biorefineries are based on processes in polar solvents at moderate conditions to selectively deoxygenate the polar, often thermally instable and high-boiling molecules. Here, challenges of catalytic deoxygenation, novel strategies for separation and opportunities provided at the interface to biotechnology are discussed in form of showcases.This article is part of a discussion meeting issue 'Providing sustainable catalytic solutions for a rapidly changing world'.

New Routes for Refinery of Biogenic Platform Chemicals Catalyzed by Cerium Oxide-supported Ruthenium Nanoparticles in Water

Highly selective hydrogenative carbon-carbon bond scission of biomass-derived platform oxygenates was achieved with a cerium oxide-supported ruthenium nanoparticle catalyst in water. The present catalyst enabled the selective cleavage of carbon-carbon σ bonds adjacent to carboxyl, ester, and hydroxymethyl groups, opening new eight synthetic routes to valuable chemicals from biomass derivatives. The high selectivity for such carbon-carbon bond scission over carbon-oxygen bonds was attributed to the multiple catalytic roles of the Ru nanoparticles assisted by the in situ formed Ce(OH)₃.

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 .

Direct Synthesis of Renewable Dodecanol and Dodecane with Methyl Isobutyl Ketone over Dual-Bed Catalyst Systems

For the first time, we demonstrated two integrated processes for the direct synthesis of dodecanol or 2,4,8-trimethylnonane (a jet fuel range C₁₂ -branched alkane) using methyl isobutyl ketone (MIBK) that can be derived from lignocellulose. The reactions were carried out in dual-bed continuous flow reactors. In the first bed, MIBK was selectively converted to a mixture of C₁₂ alcohol and ketone. Over the Pd-modified magnesium- aluminium hydrotalcite (Pd-MgAl-HT) catalyst, a high total carbon yield (73.0 %) of C₁₂ oxygenates can be achieved under mild conditions. In the second bed, the C₁₂ oxygenates generated in the first bed were hydrogenated to dodecanol over a Ru/C catalyst or hydrodeoxygenated to 2,4,8-trimethylnonane over a Cu/SiO₂ catalyst. The as-obtained dodecanol can be used as feedstock in the production of sodium dodecylsulfate (SDS) and sodium dodecyl benzene sulfonate (SDBS), which are widely used as surfactants or detergents. The asobtained 2,4,8-trimethylnonane can be blended into conventional jet fuel without hydroisomerization.

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.

Regioselectivity and Reaction Mechanism of Ru-Catalyzed Hydrogenolysis of Squalane and Model Alkanes

The dependence of the C-C hydrogenolysis activity on reaction parameters and the structure of the substrate alkanes was investigated for Ru/CeO₂ catalyst with very small (dispersion: H/Ru=0.89) Ru particles. The substrate concentration and reaction temperature did not have a significant effect on the selectivity pattern, except that methane production was promoted at high temperatures. However, the hydrogen pressure had a marked effect on the selectivity pattern. C_tertiary -C bond dissociation, terminal C_secondary -C_primary bond dissociation, and fragmentation to form excess methane had negative reaction order with respect to hydrogen partial pressure, whereas C_secondary -C_secondary bond dissociation had an approximately zero reaction order. Therefore, a high hydrogen pressure is essential for the regioselective hydrogenolysis of C_secondary -C_secondary bonds in squalane. Ru/SiO₂ catalyst with larger Ru particles showed similar changes in the product distribution during the change in hydrogen pressure. The reaction mechanism for each type of C-C bond dissociation is proposed based on reactivity trends and DFT calculations. The proposed intermediate species for the internal C_secondary -C_secondary dissociation, terminal C_secondary -C_primary dissociation, and C_tertiary -C dissociation is alkyls, alkylidynes, and alkenes, respectively.