Hydrodeoxygenation of Oxygenates Derived from Biomass Pyrolysis Using Titanium Dioxide-Supported Cobalt Catalysts

Bio-oil upgrading to produce biofuels and chemicals has become an attractive topic over the past decade. However, the design of cost- and performance-effective catalysts for commercial-scale production remains a challenge. Herein, commercial titania (TiO2) was used as the support of cobalt (Co)-based catalysts (Co/TiO2) due to its low cost, high availability, and practicability for commercialization in the future. The Co/TiO2 catalysts were made with two different forms of TiO2 (anatase [TiO2-A] and rutile [TiO2-R]) and comparatively evaluated in the hydrodeoxygenation (HDO) of 4-propylguaicol (4PG), a lignin-derived model compound. Both Co/TiO2 catalysts promoted the HDO of 4PG following a similar pathway, but the Co/TiO2-R catalyst exhibited a higher activity in the early stages of the reaction due to the formation of abundant Ti3+ species, as detected by X-ray photoelectron spectroscopy (XPS) and hydrogen-temperature programed reduction (H2-TPR) analyses. On the other hand, the Co/TiO2-A catalyst possessed a higher acidity that enhanced propylcyclohexane production at prolonged reaction times. In terms of reusability, the Co/TiO2-A catalyst showed a higher stability (less Co leaching) and reusability compared to Co/TiO2-R, as confirmed by transmission electron microscopy (TEM) and inductively coupled plasma optical emission spectroscopy (ICP-OES) analyses. The HDO of the real bio-oil derived from pyrolysis of Leucaena leucocephala revealed that the Co/TiO2-A catalyst could convert high oxygenated aromatics (methoxyphenols, dimethoxyphenols, and benzenediols) to phenols and enhanced the phenols content, hinting at its potential to produce green chemicals from bio-feedstock.

Efficient mapping of lignin depolymerization product pools and quantification of specific monomers through rapid GPC-HPLC-UV/VIS analysis

BACKGROUND

Growing research on lignin depolymerization to functionalized bio-aromatics has necessitated dedicated analysis techniques. However, immense variability in molecular weight and functional groups of the depolymerization products impedes fast analysis of a large number of samples while remaining in-depth enough for catalyst screening or reaction condition optimization. While GPC-HPLC-UV/VIS has been a promising technique, up until now, the information it provides is largely qualitative. By enabling quantification of key monomeric products and through further reduction of overall analysis time, this study aims to increase the potential of GPC-HPLC-UV/VIS for fast and in-depth characterization of lignin depolymerization product pools.

RESULTS

Analysis of selected samples, isolated from GPC-HPLC-UV/VIS analyses of lignin depolymerization product pools, with gas chromatography (GC) equipped with an Orbitrap high-resolution accurate mass spectrometer (Orbitrap-HR/AM-MS) is successful in identifying the main low monomeric products. Moreover, these identifications are further substantiated through GPC-HPLC-UV/VIS analysis of standards. Furthermore, straight forward quantification of these products directly within GPC-HPLC-UV/VIS is successfully developed with limits of detection ≤0.05 mmol/L, which is at least on par with more complex analysis techniques. Additionally, several different reversed phase columns are assessed to reduce 2nd dimension (2D) time and, hence, overall analysis time while maintaining the possibility for quantification. A reduction in overall analysis time of about 30% as compared to the state-of-the-art is achieved by using a YMC Triart BIO C4 column as 2D.

SIGNIFICANCE

Through the enhancements introduced in this study, GPC-HPLC-UV/VIS emerges as a unique technique for the analysis of lignin depolymerization product pools, which is capable of fast yet sufficiently in-depth analysis of a large volume of samples. This capability is indispensable for catalyst screening and fine-tuning reaction conditions.

Fine-Tuning of Ni/NiO over H-NbOx for Enhanced Eugenol Hydrogenation through Enhanced Oxygen Vacancies and Synergistic Participation of Active Sites

The hydrogenation of lignin-derived phenolics to produce valuable chemicals is a promising but challenging task. This study successfully demonstrates the use of sustainable transition metal-based catalysts to hydrogenate lignin-derived phenolics. A defect-induced oxygen vacancy containing H-NbOx prepared from commercial Nb2O5 was employed as a catalyst. H-NbOx exhibited higher oxygen vacancies (158.21 μmol/g) than commercial Nb2O5 (39.01 μmol/g), evaluated from O2-TPD. Upon supporting 10 wt % Ni, a Ni/NiO interface was formed over H-NbOx, which was intrinsically active for the hydrogenation of phenolics. 10Ni/H-NbOx exhibited a two-fold increase in activity than 10Ni/Nb2O5, achieving >99% eugenol conversion and affording ∼94% 4-propyl cyclohexanol selectivity, wherein ∼63% eugenol conversion and ∼73% 4-propyl cyclohexanol selectivity were obtained over 10Ni/Nb2O5. The Ni/NiO formation was confirmed by X-ray photoelectron spectroscopy, HR-TEM, and H2-TPR analysis, while the oxygen vacancies were verified by Raman spectroscopy and O2-TPD analysis. The resulting interface enhanced the synergy between Ni and H-NbOx and facilitated hydrogen dissociation, confirmed by H2-TPD. Remarkably, 10Ni/H-NbOx maintained its catalytic activity for five tested cycles and demonstrated exceptional activity with various phenolics. Our findings highlight the potential of a sustainable transition metal catalyst for the hydrogenation of lignin-derived phenolic compounds, which could pave the path to producing valuable chemicals in an environmentally friendly manner.

The catalytic hydrodeoxygenation of bio-oil for upgradation from lignocellulosic biomass

The increasing depletion of oil resources and the environmental problems caused by using much fossil energy in the rapid development of society. The bio-oil becomes a promising alternative energy source to fossil. However, bio-oil cannot be directly utilized, owing to its high proportion of oxygenated compounds with low calorific value and poor thermal stability. Catalytic hydrodeoxygenation (HDO) is one of the most effective methods for refining oxygenated compounds from bio-oil. HDO catalysts play a crucial role in the HDO reaction. This review emphasizes the description of the main processing of HDO and various catalytic systems for bio-oil, including noble/non-noble metal catalysts, porous organic polymer catalysts, and polar solvents. A discussion based on recent studies and evaluations of different catalytic materials and mechanisms is considered. Finally, the challenges and future opportunities for the development of catalytic hydrodeoxygenation for bio-oil upgradation are looked forward.

Selective Hydrodeoxygenation of Lignin-Derived Phenols to Aromatics Catalyzed by Nb2O5-Supported Iridium

The dominating catalytic approach to aromatic hydrocarbons from renewables, deoxygenation of phenol-rich depolymerized lignin bio-oils, is hard to achieve: hydrodeoxygenation (HDO) of phenols typically leads to the loss of aromaticity and to non-negligible fractions of cyclohexanones and cyclohexanols. Here, we report a catalyst, niobia-supported iridium nanoparticles (Ir@Nb₂O₅), which combines full conversion in the HDO of lignin-derived phenols with appreciable and tunable selectivity for aromatics (25-95%) under mild conditions (200-300 °C, 2.5-10 bar of H₂). A simple approach to the removal of Brønsted-acidic sites via Hünig's base prevents coking and allows reaction conditions (T > 225 °C, 2.5 bar of H₂), promoting high yields of aromatic hydrocarbons.

Effect of Physicochemical Properties of Co and Mo Modified Natural Sourced Hierarchical ZSM-5 Zeolite Catalysts on Vanillin and Phenol Production from Diphenyl Ether

The conversion of lignocellulose biomass to value-added chemicals is challenging. In this paper, the conversion process of diphenyl ether (DPE) as a model lignin compound to phenol and vanillin compounds involved a bifunctional catalyst in reaching the simultaneous one-pot reaction in mild conditions with a high yield product. The catalysts used in this conversion are hierarchical ZSM-5 zeolites and their cobalt oxide and molybdenum oxide impregnated derivate. The ZSM-5 zeolites were synthesized using alternative precursors from natural resources, i.e., Indonesian natural zeolite and kaolin. The physicochemical properties of the catalysts were determined with various characterization methods, such as: X-ray Diffraction (XRD), Fourier Transform Infra Red (FT-IR), Scanning Electron Microscope - Energy Dispersive X-ray (SEM-EDX), X-ray Fluorescence (XRF), Surface Area Analyzer (SAA), and NH3-Temperature Programmed Desorption (NH3-TPD). The catalytic activity on conversion of DPE substrates showed that the MoOx/HZSM-5 produced the highest %yield for phenol and vanillin products; 31.96% at 250 °C and 7.63% at 200 °C, respectively. The correlation study between the physicochemical properties and the catalytic activity shows that the dominance of weak acid (>40%) and mesoporosity contribution (pore size of ~ 9 nm) play roles in giving the best catalytic activity at low temperatures. Copyright © 2022 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0). 

Upgrading bio-oil model compound over bifunctional Ru/HZSM-5 catalysts in biphasic system: Complete hydrodeoxygenation of vanillin

A complete hydrodeoxygenation(HDO) of vanillin to yield cycloalkanes was performed using bifunctional Ru loaded HZSM-5 catalysts with different metal loadings (0.1, 0.5, 1, 3, and 5 wt%) and Si/Al₂ ratios (Si/Al₂ = 23,300) in n-octane/water biphasic system. Both the reaction pathway and product distribution were influenced by the metal/acid balance of the catalysts. Higher metal/acid ratio promoted C_aryl-C cleavage reaction, resulting in the increased yield of cyclohexane. Synergetic effect of metal and acid sites was observed in the bifunctional catalyst, attaining as high as 40-fold increase of metal efficiency in the ring hydrogenation reaction, compared to lone metal site catalyst. The effect of solvent composition was evaluated, revealing that the presence of water promoted the overall HDO reaction. By balancing metal/acid and introducing appropriate solvent system, efficient catalytic system that minimized carbon loss and improved metal efficiency for vanillin HDO was obtained.

More than a support: the unique role of Nb2O5 in supported metal catalysts for lignin hydrodeoxygenation

How does Nb2O5 in supported catalysts affect the hydrodeoxygenation of lignin? This article discusses the effects of Nb2O5 in detail, including the promotion of C–O bond cleavage, the improvement of water resistance and the enhancement of durability.

Ab initio molecular dynamics with enhanced sampling in heterogeneous catalysis

Enhanced sampling ab initio simulations enable to study chemical phenomena in catalytic systems including thermal effects & anharmonicity, & collective dynamics describing enthalpic & entropic contributions, which can significantly impact on reaction free energy landscapes.

Reductive Catalytic Depolymerization of Semi-industrial Wood-Based Lignin

The current work studies the reductive catalytic depolymerization (RCD) of lignin from a novel semi-industrial process. The aim was to obtain aromatic mono-, di-, tri-, and tetramers for further valorization. The substrate and products were characterized by multiple analytical methods, including high pressure size-exclusion chromatography (HPSEC), gas chromatography–mass spectrometry, GC-flame ionization detector (FID), GC-FID/thermal conductivity detector (TCD), and NMR. The RCD was studied by exploring the influence of different parameters, such as lignin solubility, reaction time, hydrogen pressure, reaction temperature, pH, type and loading of the catalyst, as well as type and composition of the organic/aqueous solvent. The results show that an elevated temperature, a redox catalyst, and a hydrogen atmosphere are essential for the depolymerization and stability of the products, while the reaction medium also plays an important role. The highest obtained mono- to tetramers yield was 98% and mono- to dimers yield over 85% in the liquid phase products. The reaction mechanisms influenced the structure of the aliphatic chain in the monomers, but left the phenolic structure along with the methoxy groups largely unaltered. The current work contributes to the development and debottlenecking of the novel and sustainable overall process, which utilizes efficiently all the fractions of wood, in line with the principles of green engineering and chemistry.

Theoretical characterization of zeolite encapsulated platinum clusters in the presence of water molecules

Zeolite encapsulated metal clusters have shown high catalytic activity and superior stability due to confinement effects, the synergy between acidic and metal active sites, and strong metal-zeolite interactions. In the present work, density functional theory calculations were employed to study the stability of encapsulated Pt_n (n = 1-6) clusters in the zeolitic frameworks including Silicalite-1 and H-MFI. It has been found that the metal-zeolite interaction becomes stronger with the increasing Pt_n cluster size for both zeolitic frameworks. The encapsulated Pt_n clusters in the vicinity of the Brønsted acid site (BAS) of H-MFI form more stable Pt_nH_x (x = 1, 2) clusters. The presence of water molecules around the encapsulated Pt₆ cluster further enhances its stability, while the oxidation states of the encapsulated Pt_n cluster are largely affected by the BAS site and the surrounding water molecules. As the water concentration increases, water dissociation becomes more facile on the Pt₆@Silicalite-1 cluster while an opposite trend is found over the Pt₆H₂@H-MFI cluster. The proton of the BAS site can be transferred to the encapsulated Pt₆ cluster via a hydronium cluster H⁺(H₂O)_n, leading to the formation of the Pt₆H₂@H-MFI cluster.

Recovery of Arenes from Polyethylene Terephthalate (PET) over a Co/TiO2 Catalyst

Upcycling of spent plastics has become a more emergent topic than ever before due to the rapid generation of plastic waste associated with the change of lifestyles of the human society. Polyethylene terephthalate (PET) is a major aromatic plastic and herein, the conversion of PET back into arenes was demonstrated in a one-pot reaction combining depolymerization and hydrodeoxygenation (HDO) over a Co/TiO₂ catalyst. The effectiveness of the Co/TiO₂ catalyst in HDO and the underlining reaction pathway were established using the PET monomer terephthalic acid (TPA) as the substrate. Quantitative TPA conversion together with 75.2 mol% xylene and toluene selectivity under 30 bar initial H₂ pressure at 340 °C was achieved after 4 h reaction. More encouragingly, the catalyst induced both depolymerization and HDO reaction via C-O bond cleavage when PET was used as a substrate. 78.9 mol% arenes (toluene and xylene) was obtained under optimized conditions.

Phosphoric Acid Modification of Hβ Zeolite for Guaiacol Hydrodeoxygenation

Regulating the acid property of zeolite is an effective strategy to improve dehydration of intermediate alcohol, which is the rate-determining step in hydrodeoxygenation of lignin-based phenolic compounds. Herein, a commercial Hβ (SiO2/Al2O3 = 25) was modified by phosphoric acid, and evaluated in the catalytic performance of guaiacol to cyclohexane, combined with Ni/SiO2 prepared by the ammonia evaporation hydrothermal (AEH) method. Incorporating a small amount of phosphorus had little impact on the morphology, texture properties of Hβ, but led to dramatic variations in acid property, including the amount of acid sites and the ratio of Brønsted acid sites to Lewis acid sites, as confirmed by NH3-TPD, Py-IR, FT-IR and 27Al MAS NMR. Phosphorus modification on Hβ could effectively balance competitive adsorption of guaiacol on Lewis acid sites and intermediate alcohol dehydration on Brønsted acid sites, and then enhanced the catalytic performance of guaiacol hydrodeoxygenation to cyclohexane. By comparison, Hβ containing 2 wt.% phosphorus reached the highest activity and cyclohexane selectivity.

2G waste lignin to fuel and high value-added chemicals: Approaches, challenges and future outlook for sustainable development

Lignin is produced as a byproduct in cellulosic biorefinery as well in pulp and paper industries and has the potential for the synthesis of a variety of phenolics chemicals, biodegradable polymers, and high value-added chemicals surrogate to conventional petro-based fuels. Therefore, in this critical review, we emphasize the possible scenario for lignin isolation, transformation into value addition chemicals/materials for the economic viability of current biorefineries. Additionally, this review covers the chemical structure of lignocellulosic biomass/lignin, worldwide availability of lignin and describe various thermochemical (homogeneous/heterogeneous base/acid-catalyzed depolymerization, oxidative, hydrogenolysis etc.) and biotechnological developments for the production of bio-based low molecular weight phenolics, i.e. polyhydroxyalkanoates, vanillin, adipic acid, lipids etc. Besides, some functional chemicals applications, lignin-formaldehyde ion exchange resin, electrochemical and production of few targeted chemicals are also elaborated. Finally, we examine the challenges, opportunities and prospects way forward related to lignin valorization.

Kraft Lignin Ethanolysis over Zeolites with Different Acidity and Pore Structures for Aromatics Production

To utilize its rich aromatics, lignin, a high-volume waste and environmental hazard, was depolymerized in supercritical ethanol over various zeolites types with different acidity and pore structures. Targeting at high yield/selectivity of aromatics such as phenols, microporous Beta, Y, and ZSM-5 zeolites were first examined in lignin ethanolysis, followed by zeolites with similar micropore size but different acidity. Further comparisons were made between zeolites with fin-like and worm-like mesoporous structures and their microporous counterparts. Despite depolymerization complexity and diversified ethanolysis products, strong acidity was found effective to cleave both C–O–C and C–C linkages of lignin while mild acidity works mainly in ether bond breakdown. However, when diffusion of gigantic molecules is severe, pore size, particularly mesopores, becomes more decisive on phenol selectivity. These findings provide important guidelines on future selection and design of zeolites with appropriate acidity and pore structure to promote lignin ethanolysis or other hydrocarbon cracking processes.

The solvent determines the product in the hydrogenation of aromatic ketones using unligated RhCl3 as catalyst precursor

RhCl3-catalysed hydrogenation/hydrodeoxygenation of aromatic ketones produced alkylcyclohexanes in TFE and cyclohexyl alkyl alcohols in water at moderate temperatures. Rh-nanoparticles were found to be the true catalysts.

Recent advances in the synthesis and applications of mordenite zeolite - review

Among the many industrially important zeolites, mordenite is found to be interesting because of its unique and exceptional physical and chemical properties. Mordenite (high silica zeolite) is generally prepared by the hydrothermal method using TEA+ cations. TEA+ cations are the best templating agent, though they can create a number of issues, for instance, generating poison and high manufacturing cost, wastewater contamination, and environmental pollution. Hence, it is necessary to find a mordenite synthesis method without using an organic template or low-cost template. In this review, a number of unique sources were used in the preparation of mordenite zeolite, for instance, silica sources (rice husk ash, silica gel, silica fumes), alumina sources (metakaolin, faujasite zeolite) and sources containing both silica and alumina (waste coal fly ash). These synthesis approaches are also based on the absence of a template or low-cost mixed organic templates (for instance, glycerol (GL), ethylene glycol (EG), and polyethylene glycol 200 (PEG)) or pyrrolidine-based mesoporogen (N-cetyl-N-methylpyrrolidinium) modifying the mordenite framework which can create unique properties. The framework properties and optical properties (indium-exchanged mordenite zeolite) have been discussed. Mordenite is generally used in alkylation, dewaxing, reforming, hydrocracking, catalysis, separation, and purification reactions because of its large pore size, strong acidity, and high thermal and chemical stability, although the applications are not limited for mordenite zeolite. Recently, several applications such as electrochemical detection, isomerization, carbonylation, hydrodeoxygenation, adsorption, biomass conversion, biological applications (antibacterial activity), photocatalysis, fuel cells and polymerization reactions using mordenite zeolite were explored which have been described in detail in this review.

Effects of Crystallite Sizes of Pt/HZSM-5 Zeolite Catalysts on the Hydrodeoxygenation of Guaiacol

Herein, Pt/HZSM-5 zeolite catalysts with different crystallite sizes ranging from nanosheet (~2 nm) to bulk crystals (~1.5 μm) have been prepared for the hydrodeoxygenation of guaiacol, and their effects on the reaction pathway and product selectivity were explored. HZSM-5 zeolites prepared by seeding (Pt/Z-40: ~40 nm) or templating (Pt/NS-2: ~2 nm) fabricated intra-crystalline mesopores and thus enhanced the reaction rate by promoting the diffusion of various molecules, especially the bulky ones such as guaiacol and 2-methoxycyclohexanol, leading to a higher cyclohexane selectivity of up to 80 wt % (both for Pt/Z-40 and Pt/NS-2) compared to 70 wt % for bulky HZSM-5 (Pt/CZ: ~1.5 μm) at 250 °C and 120 min. Furthermore, decreased crystallite sizes more effectively promoted the dispersion of Pt particles than bulky HZSM-5 (Pt/Z-400: ~400 nm and Pt/CZ). The relatively low distance between Pt and acidic sites on the Pt/Z-40 catalyst enhanced the metal/support interaction and induced the reaction between the guaiacol molecules adsorbed on the acidic sites and the metal-activated hydrogen species, which was found more favorable for deoxygenation than for hydrogenation of oxygen-containing molecules. In addition, Pt/NS-2 catalyst with a highly exposed surface facilitated more diverse reaction pathways such as alkyl transfer and isomerization.

Synthesis of Branched Biolubricant Base Oil from Oleic Acid

The mature manufacturing of synthetic lubricants (poly-α-olefins, PAO) proceeds through oligomerization, polymerization, and hydrogenation reactions of petrochemical ethylene. In this work, we utilize the inexpensive bio-derived oleic acid as raw material to synthesize a crotch-type C₄₅ biolubricant base oil via a full-carbon chain synthesis without carbon loss. It contains several cascade chemical processes: oxidation of oleic acid to azelaic acid (further esterification to dimethyl azelate) and nonanoic acid (both C₉ chains). The latter is then selectively hydrogenated to nonanol and brominated to the bromo-Grignard reagent. In a next step, a C₄₅ biolubricant base oil is formed by nucleophilic addition (NPA) of excessive C₉ bromo-Grignard reagent with dimethyl azelate, followed by subsequent hydrodeoxygenation. The specific properties of the prepared biolubricant base oil are almost equivalent to those of the commercial lubricant PAO6 (ExxonMobil). This process provides a new promising route for the production of value-added biolubricant base oils.

Metal-Acid Synergy: Hydrodeoxygenation of Anisole over Pt/Al-SBA-15

Hydrodeoxygenation (HDO) is a promising technology to upgrade fast pyrolysis bio-oils but it requires active and selective catalysts. Here we explore the synergy between the metal and acid sites in the HDO of anisole, a model pyrolysis bio-oil compound, over mono- and bi-functional Pt/(Al)-SBA-15 catalysts. Ring hydrogenation of anisole to methoxycyclohexane occurs over metal sites and is structure sensitive; it is favored over small (4 nm) Pt nanoparticles, which confer a turnover frequency (TOF) of approximately 2000 h^-1 and a methoxycyclohexane selectivity of approximately 90 % at 200 °C and 20 bar H₂ ; in contrast, the formation of benzene and the desired cyclohexane product appears to be structure insensitive. The introduction of acidity to the SBA-15 support promotes the demethyoxylation of the methoxycyclohexane intermediate, which increases the selectivity to cyclohexane from 15 to 92 % and the cyclohexane productivity by two orders of magnitude (from 15 to 6500 mmol g_Pt ^-1  h^-1 ). Optimization of the metal-acid synergy confers an 865-fold increase in the cyclohexane production per gram of Pt and a 28-fold reduction in precious metal loading. These findings demonstrate that tuning the metal-acid synergy provides a strategy to direct complex catalytic reaction networks and minimize precious metal use in the production of bio-fuels.

Downstream processing of lignin derived feedstock into end products

This review provides critical analysis on various downstream processes to convert lignin derived feedstock into fuels, chemicals and materials.

Tandem Hydrogenolysis-Hydrogenation of Lignin-Derived Oxygenates over Integrated Dual Catalysts with Optimized Interoperations

The efficient hydrodeoxygenation (HDO) of lignin-derived oxygenates is essential but challenging owing to the inherent complexity of feedstock and the lack of effective catalytic approaches. A catalytic strategy has been developed that separates C-O hydrogenolysis and aromatic hydrogenation on different active catalysts with interoperation that can achieve high oxygen removal in lignin-derived oxygenates. The flexible use of tungsten carbide for C-O bond cleavage and a nickel catalyst with controlled particle size for arene hydrogenation enables the tunable production of cyclohexane and cyclohexanol with almost full conversion of guaiacol. Such integration of dual catalysts in close proximity enables superior HDO of bio-oils into liquid alkanes with high mass and carbon yields of 27.9 and 45.0 wt %, respectively. This finding provides a new effective strategy for practical applications.

Heterogeneous Catalyzed Thermochemical Conversion of Lignin Model Compounds: An Overview

Thermochemical lignin conversion processes can be described as complex reaction networks involving not only de-polymerization and re-polymerization reactions, but also chemical transformations of the depolymerized mono-, di-, and oligomeric compounds. They typically result in a product mixture consisting of a gaseous, liquid (i.e., mono-, di-, and oligomeric products), and solid phase. Consequently, researchers have developed a common strategy to simplify this issue by replacing lignin with simpler, but still representative, lignin model compounds. This strategy is typically applied to the elucidation of reaction mechanisms and the exploration of novel lignin conversion approaches. In this review, we present a general overview of the latest advances in the principal thermochemical processes applied for the conversion of lignin model compounds using heterogeneous catalysts. This review focuses on the most representative lignin conversion methods, i.e., reductive, oxidative, pyrolytic, and hydrolytic processes. An additional subchapter on the reforming of pyrolysis oil model compounds has also been included. Special attention will be given to those research papers using "green" reactants (i.e., H₂ or renewable hydrogen donor molecules in reductive processes or air/O₂ in oxidative processes) and solvents, although less environmentally friendly chemicals will be also considered. Moreover, the scope of the review is limited to those most representative lignin model compounds and to those reaction products that are typically targeted in lignin valorization.

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.

Highly active and robust Ni-MoS2 supported on mesoporous carbon: a nanocatalyst for hydrodeoxygenation reactions

NiMoS2 nanoparticles supported on carbon, synthesized by a microemulsion method were used as a nanocatalyst for hydrodeoxygenation (HDO) of a lignin model compound - guaiacol. Two types of carbon supports - mesoporous carbon (CMK-3) and activated carbon (AC) with a predominantly microporous structure, were studied to investigate the role of porosity and nature of the porous structure in catalyst activity. The activity of NiMoS2/AC resulted in the complete guaiacol conversion at 13 h of reaction time to produce phenol (31.5 mol%) and cyclohexane (35.7 mol%) as the two main products. Contrastingly, NiMoS2/CMK-3 needed a much lesser reaction time (6 h) to attain a similar conversion of guaiacol but gave different selectivities of phenol (25 mol%) and cyclohexane (55.5 mol%). Increased cyclohexane production with NiMoS2/CMK-3 implied better deoxygenation of MoS2 and enhanced hydrogenation capacity of Ni since phenol is a partially deoxygenated product of guaiacol while cyclohexane is a completely deoxygenated and hydrogenated product. The superior catalytic activity and deoxygenating behavior of NiMoS2/CMK-3 catalysts could be attributed to the organized mesoporosity of the CMK-3 support in relation to the improved active phase distribution and access to active sites that facilitate the conversion of the reaction's product. Recyclability study implied NiMoS2/CMK-3 was more stable without significant changes in the catalytic activity even after three reaction cycles.

One-Pot Synthesis of Active Carbon-Supported Size-Tunable Ni2P Nanoparticle Catalysts for the Pyrolysis Bio-Oil Upgrade

Catalytic hydrodeoxygenation (HDO) over Ni₂P-based catalysts is a promising technology for the pyrolysis bio-oil upgrading. However, substantial challenges still remain in the realization of the size effect for phosphide catalysts in catalyzing this reaction, and the precise size engineering of these catalysts is difficult. In this work, the Ni₂P/active carbon (AC) catalysts with varying nickel phosphide nanoparticle sizes were one-pot prepared via the modified organic liquid chemical reaction method. The Ni₂P-based catalysts were tested for HDO of the pyrolysis oil model compound (salicylaldehyde), and the conversion of salicylaldehyde first increases and then decreases with the increase of Ni₂P nanoparticle size, demonstrating that the activity for HDO of salicylaldehyde can be controlled by using nickel phosphides of varying nanoparticle sizes. The Ni₂P-2/AC catalyst with approximately 5.49 nm Ni₂P nanoparticle size exhibited the highest activity with conversion of salicylaldehyde reaching over 99% within 180 min under 220 °C, 2 MPa H₂ pressure, and the corresponding yield toward o-cresol was over 97%.

Molecular behaviour of phenol in zeolite Beta catalysts as a function of acid site presence: a quasielastic neutron scattering and molecular dynamics simulation study

The dynamic behaviour of phenol in zeolite Beta is strongly influenced by the presence of Brønsted acid sites.