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Developing efficient renewable electrocatalytic processes for chemical manufacture is of commercial interest, especially from the biomass-derived feedstock. Selective electrocatalytic hydrogenation (ECH) of biomass-derived lignin monomers to high-value oxygen-functional compounds is promising to achieve this goal. However, ECH has to date lacked the satisfactory selectivity to upgrade lignin monomers to high-value oxygenated chemicals due to the reduction of vulnerable -OCH3 that exists in most lignin monomers. Herein we report carbon-felt supported ternary RhPtRu catalysts with a record faradaic efficiency (FE) of 62.8% and a selectivity of 91.2% to the methoxy-cyclohexanes (2-methoxy-cyclohexanol and 2-methoxy-cyclohexanone) from guaiacol, via a strong inhibition effect on the cleavage of methoxy group, representing the best performances compared to previous reports. We further conducted a brief TEA to demonstrate a profitable ECH of guaiacol to high-value methoxy-cyclohexanes using our designed RhPtRu ternary catalysts.

Light-driven catalysis on catalytically versatile group VIII metals, which has been widely used in thermal catalysis, holds great potential in solar-to-chemical conversion. We report a novel photocatalysis process for the selective hydrogenolysis of aryl ethers in lignin on a heterogeneous catalyst of non-precious Ni nanoparticles supported on ZrO2. Three aryl ether bonds in lignin were successfully cleaved under mild conditions with excellent conversion and good to excellent selectivity under visible light irradiation. We also used solar irradiation to demonstrate a significant reduction in the total energy consumption. The light irradiation excited interband transitions in Ni nanoparticles and the resultant energetic electrons enhanced the activity of reductive cleavage of the aryl ethers. Its application potential was illustrated by the depolymerization of dealkaline lignin to give a total monomer yield of 9.84 wt% with vanillin, guaiacol, and apocynin as the three major products.

Despite progress in the depolymerization of lignin, only a few studies convert the obtained monomers to value-added products. Here we introduce a strategy to synthesize branched benzene lubricant (BBL) and branched cyclic lubricant (BCL) base oils from lignin-derived monomers and aldehyde. We perform carbon–carbon coupling via Bronsted acid-catalyzed hydroxyalkylation/alkylation (HAA) then hydrodeoxygenation (HDO). Optimum HAA reaction conditions achieve up to 90% guaiacol conversion and an HAA product containing 76% BBL and 24% enal condensation product over a P-SiO2 catalyst. Subsequent HDO of HAA products over an Ir-ReOx/SiO2 catalyst produces a lubricant-ranged mixture of BCL (C24) up to yield (82%) and small fractions of dodecyl cyclohexane and C10 and C15 carbons alkanes. The kinematic viscosity, viscosity index, and Noack volatility of these base oils are comparable to commercial petroleum-derived poly α-olefin Group IV and refrigerant base oils. This approach provides a sustainable pathway for replacing petroleum-derived base oils.

The transformation of renewable lignin-based platform chemicals into value-added nitrogen-containing compounds is an emerging strategy for lignin utilization. However, multi-reactive sites on these platform chemicals make it challenging to control the product selectivity, thereby resulting in limited success. In this work, we developed the reductive-coupling of guaiacol, a typical lignin-based platform chemical, with amines and H2 to synthesize methoxy-functionalized cyclohexylamines. It was demonstrated that Pd/C was a very efficient catalyst for this kind of reaction, and high yields of the target products can be obtained. Notably, this methodology can be applied for the reductive-coupling of various guaiacol analogues with amines to synthesize alkoxyl-functionalized cyclohexylamines with high yields. A mechanism study revealed that the reaction occurred through the generation of 2-methoxycyclohexanone and its subsequent reductive amination.

Lignin represents the largest renewable feedstock of aromatic moieties in nature. However, its valorisation towards organic chemicals poses a challenging task. We herein report the selective electrochemical depolymerisation of several technically relevant organosolv lignins to aromatic fine chemicals with a simple two-electrode arrangement using caustic soda as electrolyte. Vanillin, syringaldehyde, the corresponding ketones and guaiacol were obtained with a combined maximum of 7.8 wt% isolated yield. Deeper insight in the process was provided by FT-IR, HSQC NMR and 31P NMR analyses of the lignins.

The structure of the ionic liquid choline lysinate ([Ch][Lys]) and its water mixtures, including a dissolved model lignin residue, guaiacol, is revealed at atomic and nanoscale resolution using neutron diffraction. The performance of [Ch][Lys] for biomass pretreatment depends on both its detailed H-bonding capacity and bicontinuous polar/apolar nanostructure, which is stabilised by lysinate adopting an H-bonded cyclic conformer that enhances its amphiphilicity. These features are heightened by the addition of water, which forms nanoscale pockets but also extends the H-bonding network within the IL polar domains while largely preserving its bicontinuous nanostructure, rendering the ionic liquid–water mixture an effective solvent for organic molecules bearing various functional groups. A model representative lignin compound, guaiacol, is found to H-bond through its phenol group to both the anion and water, destabilising the lysinate cyclic conformer and altering the liquid nanostructure.

This study explores the valorization of Kraft lignin by conversion into nylon precursors in a two-step process. First, lignin was depolymerized in dilute alkaline aqueous solution under atmospheric N2 at 200 °C to give guaiacol with high selectivity (>80%) with a total monomer production of 13% based on lignin input. Solution and solid state NMR analyses and reactions of model compounds indicated that depolymerization took place via cleavage of β-O-4 bonds in lignin. In the second step, lignin-derived guaiacol was selectively converted to the nylon precursors cyclohexanol/cyclohexanone (KA oil) using Ru/C catalyst under 1 bar H2 and 150 °C. This two-step process constitutes a low-temperature and low-pressure pathway for producing value-added chemicals from lignin using water as the reaction solvent.

Lignin is recognized as a renewable and rich source of aromatics, which could be an alternative to fossil fuel-based petrochemicals. However, the development of an efficient catalyst and industrially scalable process for lignin depolymerization remains a challenge. The present work is therefore dedicated towards a one-pot approach for depolymerization of commercial kraft lignin in a batch reactor with higher loading using the water–THF solvent mixture over the HZSM-5 and NaOH catalytic system. Process parameters such as time, temperature, lignin-to-solvent (w/v), water-to-THF (v/v), NaOH-to-lignin (w/w), catalyst-to-lignin (w/w), and Si/Al mole ratios of HZSM5 were chosen carefully and optimized using statistical analysis – response surface methodology (RSM). A significant yield of 61% for a particular single ring aromatic compound (i.e. guaiacols) was obtained under the optimized reaction conditions: reaction time = 2 h, temperature = 533 K, lignin-to-solvent ratio = 0.060 (1 : 16.6), water-to-THF ratio = 0.86 : 1, catalyst-to-lignin ratio = 1 : 6 (0.16), NaOH-to-lignin ratio = 0.1 and Si/Al ratio = 280. Around 77% of lignin was selectively converted to guaiacols (79%) with minimal char formation (7.7%). The Hansen solubility parameter and RSM studies indicate that the obtained results were significantly related to the solubility of lignin and guaiacol at different water–THF ratios. FT-IR and 1H-NMR studies were also performed to correlate the process outcome with the presence of functional groups in identified EtOAc soluble products. A plausible mechanism for catalytic depolymerization of lignin has been proposed considering the product distribution obtained under varying process parameters.

A magnetically separable bimetallic AgPd/Fe@CNX catalyst has been synthesized and utilized for upgrading of lignin-derived guaiacol to cyclohexanol via hydrodeoxygenation using formic acid as a source of hydrogen under visible light irradiation. A nitrogen-enriched carbonaceous material, carbon nitride (CNX), is obtained via calcination of chitosan, a marine-derived abundant waste material and subsequently utilized as a support for the immobilization of nano-ferrites and silver and palladium nanoparticles. Hydrogen generation from renewable sources such as formic acid and its ideal utilization for the upgrading of biomass-derived guaiacol are the main findings. The synergetic effect of Ag and Pd on the CNX support enhances the decomposition of formic acid which is aptly utilized for the selective defunctionalization of guaiacol at ambient pressure.

Strong adsorption of phenolics on zeolite acid sites causes high trapping inside zeolite channels and low catalytic activity of zeolite-supported catalysts in atmospheric pressure hydrodeoxygenation (HDO) of lignin-derived phenolics. This adsorption is more severe at low temperatures, and restricts the atmospheric HDO of phenolics to high reaction temperatures. The purpose of this research was to develop a catalyst with low phenolic trapping potential and high HDO efficiency under mild reaction conditions. Among the tested catalysts (Fe/HBeta, FeReOx/HBeta, Fe/MCM-41, ReOx/MCM-41, FeReOx/MCM-41, Fe/ZrO2 and FeReOx/ZrO2), the novel catalyst of FeReOx/ZrO2 exhibited the highest catalytic efficiency for mild-condition (pressure: 1 atm and temperature x/ZrO2 displayed remarkably enhanced performance, and its catalytic activity for the HDO of m-cresol at 350 °C was almost twice higher than that of Fe/HBeta(38) at 500 °C. Importantly, FeReOx/ZrO2 revealed a high HDO efficiency (BTX yield of 50.5 wt% with phenolic trapping below 5 wt%) at a low temperature of 250 °C, while Fe/HBeta(38) almost lost its entire catalytic activity at this temperature, and gave a low BTX yield of 3.0 wt% with a high trapped phenolic yield of 83.1 wt%. The remarkable catalytic activity of FeReOx/ZrO2 in the HDO of phenolics at atmospheric pressure and temperatures as low as 250 °C is a result of its mesoporosity and oxophilicity as well as its well-balanced acidity induced by both rhenium oxide and zirconia support causing a high dehydration efficiency.

A two-step catalytic process for woody biomass fractionation and valorization is presented. Both lignin and carbohydrate are converted to useful chemicals. Oak sawdust is effectively delignified in methanol using different homogeneous acids (sulphuric acid, aluminium triflate, p-methylbenzenesulfonic acid, hydrochloride acid and phosphoric acid) in the first step, resulting in a lignin oil and cellulose pulp. In the second step, the lignin oil is upgraded to phenolic monomers in methanol in a H2 atmosphere by a Pd/C catalyst. The degree of delignification correlates inversely with the aromatic monomer yield. Characterization of the lignin oil by gel permeation chromatography and heteronuclear single quantum coherence NMR revealed that the cleavage of β-O-4 bonds during acidolysis is accompanied by repolymerization of lignin fragments. A model compound study shows that species containing unsaturated aldehyde groups are prone to form high molecular-weight products due to repolymerization. The use of Pd/C in combination with H2SO4 or aluminium triflate did not lead to noticeable leaching, whereas severe leaching and Pd particle size growth occurred when HCl and p-TsOH were used as acid catalysts. H3PO4 is the suitable acid catalyst for the present process since the lignin repolymerization is minimized in the first step (160 °C, 2 h, atmospheric N2 pressure). Up to 25.0 wt% yield of mainly 4-n-propyl syringol/guaiacol and their derivatives was obtained over the Pd/C catalyst in the second step (180 °C, 2 h, 30 bar H2). The Pd/C catalyst can be recycled with no significant loss of activity and selectivity after four runs. The cellulose pulp can be easily separated from the other products and shows enhanced susceptibility to enzymatic hydrolysis, resulting in glucose yields as a high as 66 wt%.

Lignin as the only natural resource of aromatics has the potential to be the feedstock of choice for the production of value-added chemicals. However, the recalcitrance of lignin has limited its valorization into value-added products, and it is often burned for heat. The acid-catalyzed organosolv extraction of lignin results in the formation of interunit carbon–carbon bonds, which limit its upgrading. In this work, three different solvent systems (methanol, acetone, and acetic acid) were evaluated for the extraction of native lignin from wild-type and genetically modified poplar species (wild type, high-S, and low-S). Over 68% of the original lignin in the biomass was isolated and subjected to further upgrading over a heterogeneous Ni/C catalyst (10 wt% of catalyst in methanol solvent, under 35 bar H2 pressure at 225 °C). Three major monomeric phenolic products, guaiacol, isoeugenol, and 4-propenyl syringol, were obtained. Methanol-extracted lignin gave the best yield of >60% of the said aromatic products. Methanol as a nucleophile reacted with the Cα benzylic carbocation formed during the organosolv extraction, minimizing the carbon–carbon bond formation. This protection by methanol was demonstrated by NMR spectroscopy. Scanning electron microscopy (SEM) images showed differences in the isolated lignin on the micron scale from the three different treatments.

In the context of arenes generated from bioresources, a hydrothermally stable catalyst Ru/HZSM-5 is reported for the quantitative conversion of lignin-derived phenol, anisole, guaiacols, and syringols into bioaromatic hydrocarbons in a one-pot aqueous-phase process in moderate conditions (240 °C, 2 bar H2), with a high carbon balance of 98.6%. The selective hydrodeoxygenation reaction pathway from guaiacol proceeded via an initial hydrogenolysis route to phenol with a rate of 33 mmol g−1 h−1, and subsequently phenol was directly hydrogenolyzed to benzene at a rate of 10 mmol g−1 h−1 over Ru/HZSM-5 in selected conditions. Moreover, routes via the hydrogenation of guaiacol, as well as the hydrogenolysis of catechol and anisole, have been rationally excluded via comparative kinetic studies of the individual steps of the hydrodeoxygenation of catechol, anisole, and phenol over Ru/HZSM-5 in identical conditions. The selective cleavage of guaiacol at the C–O position of the Csp3–OAr group is influenced by space hindrance and the environment of the reactant (gas phase, liquid phase, or acidic aqueous phase). In the condensed liquid phase and gas phase, catechol rather than phenol is formed as the major product from guaiacol over Ru/HZSM-5. However, in the highly diluted aqueous phase, phenol alone (without detected catechol) is formed as the dominant product. In addition, the pathway for guaiacol conversion is sensitive to the hydrogen pressure and temperature, which demonstrates that a relatively high temperature and a low hydrogen pressure are crucial for manipulating the coupled tandem hydrogenolysis routes. Among various oxide-loaded Ru catalysts, Ru/HZSM-5 has been confirmed to be the most efficient catalyst for the target selective hydrodeoxygenation of guaiacol, and the metal sites and support were found to be highly stable for four successive catalytic runs in the near-critical aqueous phase that was used.

To produce basic chemicals from lignin, depolymerization and removal of oxygen from lignin through C–O cleavage and hydrodeoxygenation (HDO) are crucial steps. In this study, a novel, pseudo-homogeneous catalyst system, consisting of uniformly stabilized noble metal nanoparticles (NPs) in ionic liquids is developed for the selective reductive cleavage of C–O and HDO. Phenol and guaiacol as lignin monomer model compounds are investigated to gain an insight into the possible HDO pathway, meanwhile, dimeric model compounds such as diphenyl ether and benzyl phenyl ether are studied for the cleavage of C–O bonds between aromatic units. Four types of NPs including Pd, Pt, Rh and Ru were synthesized in situ and well distributed in ILs without aggregation. These catalytic systems displayed almost 100% conversion for various monomeric and dimeric lignin model compounds at 130 °C and were recycled several times without loss of activity. The catalytic selectivity of metals for HDO/C–O cleavage normally decreases in the order of Pt > Rh ∼ Ru ≫ Pd, which is similar to the order of NP size, Pd ≫ Pt > Rh ∼ Ru. With a mean diameter of 5.6 nm, Pt NPs in [Bmim]PF6 are identified as the best catalytic system for the transformation of lignin monomeric and dimeric model compounds with an almost 100% conversion and maximum 97% selectivity.

Lignocellulosic biomass is an important renewable resource that could substitute fossil feedstocks as a raw material for high value chemicals production. While the cellulosic fraction of biomass gives access to saccharides only, lignin could possibly give access to low molecular weight aromatic compounds. Strong alkalinity of the reaction medium is necessary in order to dissolve lignin and to obtain an electrically conducting solution. However, strong alkalinity is supposed to be one of the reasons for consecutive oxidations to undesired organic acids and carbon dioxide. The high pH also limits the use of only a few metals as electrode materials. We report the use of pure deep eutectic solvents (DES) in order to dissolve lignin in combination with electrochemical oxidative depolymerisation. We also present successful extraction of the products through liquid–liquid extraction and their characterization by size exclusion chromatography and GC-MS, proving that a successful depolymerisation of lignin was achieved. Guaiacol and vanillin were the two most abundant detected products.

Cyclohexanol and alkyl cyclohexanol are important chemical intermediates. It is meaningful to prepare cyclohexanols from non-fossil-based biomass. Here we report Ru/ZrO2–La(OH)3, a metal-solid base bifunctional catalyst, to show its excellent performance on the partial hydrodeoxygenation of lignin-derived phenols. Guaiacol could be converted to cyclohexanol with a 91.6% yield in water. Alkyl phenols with one or two methoxy groups were converted into alkyl cyclohexanols with yields over 86.9%. The catalyst had good activity of removing a methoxy group and retaining a hydroxyl group. In this catalyst, Zr and La interacted with each other to form a mixed (hydr)oxide, thus making ZrO2–La(OH)3 a stable support. Ru was highly dispersed on the ZrLa support. The pathway from guaiacol to cyclohexanol was investigated and proposed as two parallel ways, demethoxylation followed by hydrogenation (I), the saturation of the aromatic ring through hydrogenation and then demethoxylation through direct hydrogenolysis (II).

The oxidative transformation of lignin into aromatic compounds is an attractive route for chemical utilization of lignocellulosic biomass. Unlike hydrogenolysis, no consumption of expensive hydrogen is required for the oxidative transformation. However, only limited success has been achieved for the oxidative conversion of lignin. Here, we report that cerium oxide-supported palladium nanoparticles (Pd/CeO2) can efficiently catalyze the one-pot oxidative conversion of 2-phenoxy-1-phenylethanol, a lignin model compound containing a β-O-4 bond and a Cα-hydroxyl group, in methanol in the presence of O2, producing phenol, acetophenone and methyl benzoate as the major products. Pd nanoparticles played a pivotal role in the oxidation of a Cα-hydroxyl group into a Cα-ketonic group, which was crucial for the transformation of the model compound. The presence of the Cα-ketonic group activated the β-O-4 bond, which was subsequently cleaved over the Pd/CeO2 catalyst, affording phenol and acetophenone. At the same time, the Cα–Cβ bond also underwent oxidative cleavage catalyzed by CeO2, producing benzoic acid and further methyl benzoate. The Pd/CeO2 catalyst could also catalyze the oxidative conversion of organosolv lignin under mild conditions (458 K), producing vanillin, guaiacol and 4-hydroxybenzaldehyde.

The cleavage of C–O bonds in phenol, catechol, and guaiacol has been explored with mono- and dual-functional catalysts containing Ni and/or HZSM-5 in the aqueous phase. The aromatic ring of phenol is hydrogenated in the first step, and the C–O bond of the resulting cyclohexanol is dehydrated in sequence. The initial turnover frequency (TOF) of phenol hydrodeoxygenation increases in parallel with the acid site concentration irrespective of the concentration of the accessible surface Ni atoms. For catechol and guaiacol conversion, Ni catalyzes the hydrogenolysis of the C–O bonds in addition to arene hydrogenation. For catechol, the hydrogenation of the aromatic ring and the hydrogenolysis of the phenolic –OH group occur in parallel with a ratio of 8:1. The saturated cyclohexane-1,2-diol can be further dehydrated over HZSM-5 or hydrogenolyzed on Ni to complete hydrodeoxygenation. Guaiacol undergoes primarily hydrogenolysis (75%) to phenol via demethoxylation, and the hydrogenation route accounts for only 25%. This is attributed to the steric effects arising from the adjacent sp3 hybrid O–CH3 group. 2-Methoxycyclohexanol (from guaiacol hydrogenation) reacts further either via hydrogenolysis by Ni to cyclohexanol or via acid catalyzed demethoxylation and rearrangement steps followed by the subsequent hydrogenation of the intermediately formed olefins. On Ni/HZSM-5, the hydrodeoxygenation activities are much higher for the phenolic monomers than for their respective saturated analogues, pointing to the importance of sp2 orbitals. The presence of proximal acid sites increases the activities of Ni in the presence of H2 by a synergistic action.

The use of calcined Mg–Al hydrotalcite as an efficient catalyst for the selective O-alkylation of catechol with dimethyl carbonate (DMC) to guaiacol in a continuous fixed bed flow reactor is described. A maximum guaiacol yield of 80% is obtained at 573 K under optimized conditions with a stable catalytic activity. A concerted mechanism involving acid–base pair sites in which a nucleophilic attack at the methyl carbon atom of DMC by the oxygen atom of the H-bonded phenolic compound or phenolate anion is proposed.

Electrocatalytic hydrogenation (ECH) is an option for stabilizing or upgrading bio-oil that employs mild conditions (≤80 °C and ambient pressure) compared to hydrotreatment. In this study, phenol, guaiacol (2-methoxyphenol), and syringol (2,6-dimethoxyphenol) were chosen as model lignin-like substrates because of their abundance in bio-oil and their high resistance to hydrogenation relative to the carbonyl compounds. Cathodic reduction was catalyzed by ruthenium supported on activated carbon cloth (Ru/ACC), a novel electrocatalyst. Incipient wetness impregnation and cation exchange methods were employed to prepare the electrocatalyst using three different ruthenium precursors. Scanning electron microscopy revealed that ruthenium nanoparticles within the range of 10 to 20 nm were deposited on the support by both catalyst synthesis methods. Catalysts prepared by cation exchange were more active than those prepared using incipient wetness impregnation, presumably because of support surface functionalization by the oxidation pretreatment. When using incipient wetness impregnation, catalysts synthesized with precursor hexaammineruthenium(III) chloride showed the best activity and electrochemical efficiency, followed by catalysts prepared with ruthenium(III) chloride and ruthenium(III) nitrosyl nitrate. The Ru/ACC electrocatalyst reduced guaiacol, phenol and syringol with similar electrochemical efficiencies, but temperature was an important variable; the electrochemical efficiency for guaiacol reduction increased from 8% at 25 °C to 17% at 50 °C, but then dropped back to 10% at 80 °C. Solution pH also affected catalyst activity and product selectivity, with acidic conditions favoring guaiacol conversion, electrochemical efficiency and cyclohexanol selectivity.

A cobalt Schiff base catalyst with ionic substituents on the ligand, N,N′-ethylenebis(acetylacetoniminato)-cobalt(II) hexafluorophosphoric pyridinium (Co-[Salen-Py][PF6]2), was synthesized. It displayed an excellent catalytic performance for the oxidation of 4-methyl guaiacol to vanillin (conversion = 100%, selectivity = 90%). Tentative reaction mechanism research indicated that the electron-withdrawing pyridinium substituent on the ligand of Co(acacen) is responsible for the high selectivity of vanillin. Meanwhile, utilizing ethylene glycol and water as solvent, vanillin can be isolated by simple crystallization in the form of a sodium salt, and the mother liquid of the crystallization, with a large amount of NaOH (the mole ratio of NaOH/4-methyl guaiacol = 2.38/1), can be successfully recycled at least three times, thereby decreasing the mole ratio of base/substrate from 3.3 : 1 to 1.05 : 1 when the mother liquid of crystallization was recycled. This strategy provides a potentially greener alternative for the synthesis of vanillin in industry.

Lignin is a component of lignocellulosic biomass from which important aromatic compounds can potentially be obtained. In the present work, Alcell and soda lignin were dissolved in the ionic liquid 1-ethyl-3-methylimidazolium diethylphosphate (EMIM DEP) and subsequently oxidized using several transition metal catalysts and molecular oxygen under mild conditions. CoCl2·6H2O in EMIM DEP proved particularly effective for the oxidation. The catalyst rapidly oxidized benzyl and other alcohol functionalities in lignin, but left phenolic functionality and 5–5′, β-O-4 and phenylcoumaran linkages intact, as determined by analysis of various lignin model compounds and ATR-IR spectroscopy. The catalyst system oxidized the alcohol functionality contained in cinnamyl alcohol to form cinnamaldehyde or cinnamic acid or disrupted the double bond to form benzoic acid or an epoxide. The benzyl functionality in veratryl alcohol, a simple non-phenolic lignin model compound, was selectively oxidized to form veratraldehyde at a maximum turnover frequency of 1440 h−1, compared to 10–15 h−1 reported for earlier systems. Phenolic functional groups contained in guaiacol, syringol, and vanillyl alcohol remained intact, although the benzyl alcohol group in the latter was oxidized to form vanillin. Incorporation of strongly bound tetradentate ligands to the catalyst yielded reduced activity relative to those derived from simple metal salts in EMIM DEP. The influence of reaction conditions, such as temperature, oxygen pressure and NaOH loading, were also investigated. The system represents a potential method in a biorefinery scheme to increase the oxygen functionality in lignin prior to depolymerization or additional functionalization of already depolymerized lignin.