"Activated Borane": A Porous Borane Cluster Polymer as an Efficient Lewis Acid-Based Catalyst

Borane cluster-based porous covalent networks, named activated borane (ActB), were prepared by cothermolysis of decaborane(14) (nido-B10H14) and selected hydrocarbons (toluene, ActB-Tol; cyclohexane, ActB-cyHx; and n-hexane, ActB-nHx) under anaerobic conditions. These amorphous solid powders exhibit different textural and Lewis acid (LA) properties that vary depending on the nature of the constituent organic linker. For ActB-Tol, its LA strength even approaches that of the commonly used molecular LA, B(C6F5)3. Most notably, ActBs can act as heterogeneous LA catalysts in hydrosilylation/deoxygenation reactions with various carbonyl substrates as well as in the gas-phase dehydration of ethanol. These studies reveal the potential of ActBs in catalytic applications, showing (a) the possibility for tuning catalytic reaction outcomes (selectivity) in hydrosilylation/deoxygenation reactions by changing the material's composition and (b) the very high activity toward ethanol dehydration that exceeds the commonly used γ-Al2O3 by achieving a stable conversion of ∼93% with a selectivity for ethylene production of ∼78% during a 17 h continuous period on stream at 240 °C.

Review on supported metal catalysts with partial/porous overlayers for stabilization

Heterogeneous catalysts of supported metals are important for both liquid-phase and gas-phase chemical transformations which underpin the petrochemical sector and manufacture of bulk or fine chemicals and pharmaceuticals. Conventional supported metal catalysts (SMC) suffer from deactivation resulting from sintering, leaching, coking and so on. Besides the choice of active species (e.g. atoms, clusters, nanoparticles) to maximize catalytic performances, strategies to stabilize active species are imperative for rational design of catalysts, particularly for those catalysts that work under heated and corrosive reaction conditions. The complete encapsulation of metal active species within a matrix (e.g. zeolites, MOFs, carbon, etc.) or core-shell arrangements is popular. However, the use of partial/porous overlayers (PO) to preserve metals, which simultaneously ensures the accessibility of active sites through controlling the size/shape of diffusing reactants and products, has not been systematically reviewed. The present review identifies the key design principles for fabricating supported metal catalysts with partial/porous overlayers (SMCPO) and demonstrates their advantages versus conventional supported metals in catalytic reactions.

Improving the hydrodeoxygenation activity of vanillin and its homologous compounds by employing MoO3-incorporated Co-BTC MOF-derived MoCoOx@C

Lignin-derived aryl ethers and vanillin are essential platform chemicals that fulfil the demands for renewable aromatic compounds. Herein, an efficient heterogeneous catalyst is reported for reforming vanillin via a selective hydrodeoxygenation route to 2-methoxy-4-methyl phenol (MMP), a precursor to medicinal, food, and petrochemical industries. A series of MoCoO_x@C catalysts were synthesized by decorating the Co-BTC MOF with different contents of MoO₃ rods, followed by carbonization. Among these catalysts, MoCoO_x@C-2 afforded ∼99% vanillin conversion and ∼99% MMP selectivity at 150 °C in 1.5 h in an aqueous medium. In contrast, CoO_x@C afforded ∼75% vanillin conversion and ∼85% MMP selectivity. Detailed catalyst characterization revealed that CoO_x and Co₂Mo₃O₈ were the active species contributing to the higher activity of MoCoO_x@C-2. The excellent H₂-adsorption characteristics and acidity of MoCoO_x@C-2 were beneficial to the hydrodeoxygenation of vanillin and other homologous compounds. The DFT adsorption energy calculations suggested the favourable interactions of vanillin and vanillyl alcohol with the Co₂Mo₃O₈ sites in MoCoO_x@C-2. The catalyst could be efficiently recycled 5 times, with a negligible loss in activity after the 5th cycle. These findings provide a systematic explication of the active sites of the mixed metal oxide-based MoCoO_x@C-2 catalyst for the selective hydrodeoxygenation of vanillin to MMP, which is important for the academic and industrial catalysis community.

Hydrodeoxygenation of oleic acid over NiMo bimetallic catalysts supported on niobium phosphate

The properties of the support have profound effects on the catalytic performance of the supported catalyst.

Catalytic pyrolysis of biomass over zeolites for bio-oil and chemical production: A review on their structure, porosity and acidity co-relation

The oxygenated compounds found in bio-oil limit their application as a transportation fuel. Several studies were reported on eliminating the oxygenated components from bio-oil so as to improve its fuel properties. This work is dedicated to studying the shape selectivity, porosity, structure, acidity of zeolites and their effect in bio-oil and chemicals production. The unified pore size, specific structure, controlled Si/Al ratio, unique channels and circular entrances, mesoporosity, and acidity are the utmost discerning parameters for aromatics production and deoxygenation reaction. The conversion of biomass-derived oxygenates to aromatics using zeolite is subjected to the reactants entering the pore, conversion inside the pore, and diffusing out of the products from the zeolite pores. These approaches were considered for an in-depth understanding of zeolite properties, which will enhance the fundamental understanding of pyrolysis.

Transition Metal Phosphides (TMP) as a Versatile Class of Catalysts for the Hydrodeoxygenation Reaction (HDO) of Oil-Derived Compounds

Hydrodeoxygenation (HDO) reaction is a route with much to offer in the conversion and upgrading of bio-oils into fuels; the latter can potentially replace fossil fuels. The catalyst’s design and the feedstock play a critical role in the process metrics (activity, selectivity). Among the different classes of catalysts for the HDO reaction, the transition metal phosphides (TMP), e.g., binary (Ni2P, CoP, WP, MoP) and ternary Fe-Co-P, Fe-Ru-P, are chosen to be discussed in the present review article due to their chameleon type of structural and electronic features giving them superiority compared to the pure metals, apart from their cost advantage. Their active catalytic sites for the HDO reaction are discussed, while particular aspects of their structural, morphological, electronic, and bonding features are presented along with the corresponding characterization technique/tool. The HDO reaction is critically discussed for representative compounds on the TMP surfaces; model compounds from the lignin-derivatives, cellulose derivatives, and fatty acids, such as phenols and furans, are presented, and their reaction mechanisms are explained in terms of TMPs structure, stoichiometry, and reaction conditions. The deactivation of the TMP’s catalysts under HDO conditions is discussed. Insights of the HDO reaction from computational aspects over the TMPs are also presented. Future challenges and directions are proposed to understand the TMP-probe molecule interaction under HDO process conditions and advance the process to a mature level.

Template Guiding for the Encapsulation of Uniformly Subnanometric Platinum Clusters in Beta-Zeolites Enabling High Catalytic Activity and Stability

Subnanometric metal clusters have attracted extensive attention because of their unique properties as heterogeneous catalysts. However, it is challenging to obtain uniformly distributed metal clusters under synthesis and reaction conditions. Herein, we report a template-guidance protocol to synthesize subnanometric metal clusters uniformly encapsulated in beta-zeolite, with the metal ions anchored to the internal channels of the zeolite template via electrostatic interactions. Pt metal clusters with a narrow size range of 0.89 to 1.22 nm have been obtained on the intersectional sites of beta-zeolite (Pt@beta) with a broad range of Si/Al molar ratios (15-200). The uniformly distributed Pt clusters in Pt@H-beta are subject to strong electron withdrawal by the zeolite, which promotes transfer of active hydrogen, providing excellent activity and stability in hydrodeoxygenation reactions. A general strategy is thus proposed for the encapsulation of subnanometric metal clusters in zeolites with high thermal stability.

Improving the Conversion of Biomass in Catalytic Pyrolysis via Intensification of Biomass—Catalyst Contact by Co-Pressing

Biomass pyrolysis is a promising technology for fuel and chemical production from an abundant renewable source. It takes place usually in two stages; non-catalytic pyrolysis with further catalytic upgrading of the formed pyrolysis oil. The direct catalytic pyrolysis of biomass reduces the pyrolysis temperature, increase the yield to target products and improves their quality. However, in such one-stage process the contact between biomass and solid catalyst particles is poor leading to an excessively high degree of pure thermal pyrolysis reactions. The aim of this study was to enhance the catalyst-biomass contact via co-pressing of biomass and catalyst particles as a pre-treatment method. Catalytic pyrolysis of biomass components with HY and USY zeolites was studied using thermogravimetric analysis (TGA), as well as experiments in a pyrolysis reactor. The liquid and coke yields were characterized using gas chromatography, and TGA respectively. The TGA results showed that the degradation of the co-pressed cellulose occurred at lower temperatures compared to the pure thermal degradation, as well as catalytic degradation of non-pretreated cellulose. All biomass components produced better results using the co-pressing method, where the liquid yields increased while coke/char yields decreased. Bio-oil from catalytic pyrolysis of cellulose with HY catalyst mainly produced heavier fractions, while in the presence of USY catalyst medium fraction was mainly produced within the gasoline range. For hemicellulose catalytic pyrolysis, the catalysts had similar effects in enhancing the lighter fraction, but specifically, HY showed higher selectivity to middle fraction while USY has produced higher percentage of lighter fraction. Using with both catalysts, co-pressing had the best effect of eliminating the heavier fraction and improving the gasoline range fraction. Spent catalyst from co-pressed sample had lower concentrations of coke/char components due to the shorter residence times of volatiles, which suppresses the occurrence of secondary reactions leading to coke/char formations.

Pore Blocking by Phenolates as Deactivation Path during the Cracking of 4-Propylphenol over ZSM-5

Cracking of propyl side chains from 4-propylphenol, a model compound for lignin monomers, is studied for a commercial ZSM-5 zeolite catalyst. The decline of 4-propylphenol conversion with time on stream can be delayed by co-feeding water. FTIR spectroscopy shows the formation of chemisorbed phenolates during reactions and significant amounts of phenolics are detected by GC-MS of the extract from the spent catalysts. Thus, chemisorbed phenolates are identified as the main reason for deactivation in the absence of water. Regardless of the amount of co-fed water, substituted monoaromatics and polyaromatic species are formed. Comprehensive characterization of the spent catalysts including Raman and solid-state 27Al NMR spectroscopy, and thermogravimetric analysis points to a combination of deactivation processes. First, phenolates bind to Lewis acid sites within the zeolite framework and hinder diffusion unless they are hydrolyzed by water. In addition, light olefins created during the cracking process react to form a polyaromatic coke that deactivates the catalyst more permanently.

Parametric Study of Pt/C-Catalysed Hydrothermal Decarboxylation of Butyric Acid as a Potential Route for Biopropane Production

Sustainable fuel-range hydrocarbons can be produced via the catalytic decarboxylation of biomass-derived carboxylic acids without the need for hydrogen addition. In this present study, 5 wt% platinum on carbon (Pt/C) has been found to be an effective catalyst for hydrothermally decarboxylating butyric acid in order to produce mainly propane and carbon dioxide. However, optimisation of the reaction conditions is required to minimise secondary reactions and increase hydrocarbon selectivity towards propane. To do this, reactions using the catalyst with varying parameters such as reaction temperatures, residence times, feedstock loading and bulk catalyst loading were carried out in a batch reactor. The highest yield of propane obtained was 47 wt% (close to the theoretical decarboxylation yield of 50 wt% on butyric acid basis), corresponding to a 96% hydrocarbon selectivity towards propane. The results showed that the optimum parameters to produce the highest yield of propane, from the range investigated, were 0.5 g butyric acid (0.57 M aqueous solution), 1.0 g Pt/C (50 mg Pt content) at 300 °C for 1 h. The reusability of the catalyst was also investigated, which showed little or no loss of catalytic activity after four cycles. This work has shown that Pt/C is a suitable and potentially hydrothermally stable heterogeneous catalyst for making biopropane, a major component of bioLPG, from aqueous butyric acid solutions, which can be sourced from bio-derived feedstocks via acetone-butanol-ethanol (ABE) fermentation.

One-Pot Synthesis of Ultra-Small Pt Dispersed on Hierarchical Zeolite Nanosheet Surfaces for Mild Hydrodeoxygenation of 4-Propylphenol

The rational design of ultra-small metal clusters dispersed on a solid is of crucial importance in modern nanotechnology and catalysis. In this contribution, the concept of catalyst fabrication with a very ultra-small size of platinum nanoparticles supported on a hierarchical zeolite surface via a one-pot hydrothermal system was demonstrated. Combining the zeolite gel with ethylenediaminetetraacetic acid (EDTA) as a ligand precursor during the crystallization process, it allows significant improvement of the metal dispersion on a zeolite support. To illustrate the beneficial effect of ultra-small metal nanoparticles on a hierarchical zeolite surface as a bifunctional catalyst, a very high catalytic performance of almost 100% of cycloalkane product yield can be achieved in the consecutive mild hydrodeoxygenation of 4-propylphenol, which is a lignin-derived model molecule. This instance opens up perspectives to improve the efficiency of a catalyst for the sustainable conversion of biomass-derived compounds to fuels.

Nano BEA zeolite catalysts for the selective catalytic cracking of n-dodecane to light olefins

Nano BEA zeolite catalysts were synthesized and modified by desilication and then ion-exchanged with Co. The desilication was carried out using 0.1 M of NaOH. The synthesized and modified nano BEA catalysts were characterized via different characterization techniques. Ammonia temperature program desorption (NH₃-TPD) and the pyridine Fourier transform infrared (pyridine-FTIR) were utilized to investigate the acidity of catalysts. X-ray diffraction (XRD), ²⁷Al and ²⁹Si nuclear magnetic resonance (NMR) spectroscopy techniques were used to examine the structure of the catalysts. The XRD patterns of the as-synthesized nano BEA catalysts were identical to that of the reference, while the NMR analysis revealed the distribution of silicon and aluminum in the BEA structure. The scanning electron microscope (SEM) analysis confirmed that the fabricated catalysts were less than 100 nm. The desilication and Co ion-exchange altered the acidity of the catalyst. The catalysts were evaluated in the cracking of sssssss to light olefins in the temperature range from 400 °C to 600 °C. The conversion increased with the increase in the reaction temperature for both catalysts; the conversion was above 90% for the Co-BEA catalyst at a temperature above 450 °C. The yield of light olefins also increased at higher temperatures for both catalysts, while at a lower temperature the yield to light olefins was ca. 40% over that of Co-BEA.

Nano BEA zeolite catalysts were synthesized and modified by desilication and then ion-exchanged with Co. The desilication was carried out using 0.1 M of NaOH.

Deoxygenation of stearic acids using alkaline treated beta molecular sieves assisted by microwave irradiation

The deoxygenation reaction is important to produce bio-diesel from esters of fatty acids and/or fatty acids.

Bio-Based Cycloalkanes: The Missing Link to High-Performance Sustainable Jet Fuels

The development of sustainable energy solutions that reduce global carbon emissions, while maintaining high living standards, is one of the grand challenges of the current century. Transportation fuels are critical to economic development, globalization, and the advancement of society. Although ground vehicles and small aircraft are beginning a slow transition toward electric propulsion with energy sourced from solar radiation or wind, the extreme power requirements of jet aircraft require a more concentrated source of energy that is conveniently provided by liquid hydrocarbon fuels. This Review describes recent efforts to develop efficient routes for the conversion of crude biomass sources (e. g., lignocellulose) to cycloalkanes. These cycloalkanes impart advantageous properties to jet fuels, including increased density, higher volumetric heat of combustion, and enhanced operability. The combination of bio-based cycloalkanes and synthetic paraffinic kerosenes allows for the preparation of 100 % bio-based fuels that can outperform conventional petroleum-based fuels. In this Review methods are described that convert biomass-derived small molecules, including furfural, furfuryl alcohol, 5-hydroxymethylfurfural, cyclic ketones, phenolics, acyclic ketones, cyclic alcohols, furans, esters, and alkenes to high-density cycloalkanes. In addition to describing the chemical transformations and catalysts that have been developed to efficiently produce various cycloalkanes, this Review includes summaries of key fuel properties, which highlight the ability to generate fuels with customized performance metrics. This work is intended to inspire other researchers to study the conversion of sustainable feedstocks to full-performance aviation fuels. An acceleration of this research is critical to reducing the carbon footprint of commercial and military aviation on a timescale that will help blunt the impacts of global warming.

The Stabilization of Liquid Smoke through Hydrodeoxygenation Over Nickel Catalyst Loaded on Sarulla Natural Zeolite

Constituents of liquid smoke possess a huge potential to be converted as value-added chemicals, such as flavoring, antiseptics, antioxidants, or even fossil oil substitutes. However, liquid smoke instability, led by the presence of oxygenate compounds, is an obstacle for further utilization and processing. On the other hand, catalyst efficiency in hydrodeoxygenation (HDO) remains challenging. Sarulla natural zeolite (Z), with abundant availability, has not been comprehensively investigated in the catalytic performance of HDO. In this study, Sarulla natural zeolite with different Si/Al ratios, which are activated by several concentrations of hydrochloric acid and nickel supported by Z (Ni-Z) synthesized by wet impregnation, were evaluated for HDO of liquid smoke, particularly in reducing oxygenate compounds. Catalyst morphology, surface area, pores, and crystallinity are investigated. Catalytic performances were evaluated, particularly on reducing oxygenate compounds and the shifting of phenol and its derivatives. Furthermore, the liquid smoke product of HDO was analyzed by gas chromatography-mass spectrometry (GC-MS). The data obtained reveal that the HDO process of liquid smoke with the Z3 catalyst shows the best activity compared to Z5 and Z7, with phenol conversion of 62.39% and 11.93% of alkoxy reduction. Meanwhile, the best Ni metal catalyst system activity was given by the Ni-Z5 catalyst compared to Ni-Z3 and Ni-Z7, where phenol conversion and alkoxy reduction were at 60.06% and 11.49%, respectively.

Catalytic transfer hydrogenation of furfural to furfuryl alcohol over a magnetic Fe3O4@C catalyst

Carbon-encapsulated Fe₃O₄ (Fe₃O₄@C) catalysts were prepared by a solvothermal method using glucose as the carbon source and their physicochemical properties were characterized via various analytical techniques.

Enhancement of light aromatics from catalytic fast pyrolysis of cellulose over bifunctional hierarchical HZSM-5 modified by hydrogen fluoride and nickel/hydrogen fluoride

Pore structure and accessible active sites of HZSM-5 (Z5) are the key factors for its catalysis. The bifunctional hierarchical Z5 were prepared with leaching agent HF and loading Ni, and their performance for catalytic fast pyrolysis (CFP) of cellulose was investigated in a drop tube quartz reactor. Z5 modified with 0.5 mol/L HF (0.5F-Z5) showed excellent light aromatics (LAs) yield, which can be attributed to the enhancement in the small mesopores (2-10 nm) and the decrease of Brønsted acid sites during dealumination. Simultaneously, the loading of a 1 wt% Ni produced more LAs than 0.5F-Z5, due to the improvement in deoxidation/hydrogenation reactions. The highest LAs yield (31.3%) was obtained over 1%Ni-0.5 mol/LHF-Z5, which increased by 44.9% compared to the parent Z5. In addition, the reaction routes over different active centers and acid-catalyzed reactions were analyzed, based upon the composition of bio-oils and catalyst characterization.

Hollow MFI Zeolite Supported Pt Catalysts for Highly Selective and Stable Hydrodeoxygenation of Guaiacol to Cycloalkanes

Hollow Silicalite-1 and ZSM-5 zeolites with hierarchical porous shells have been synthesized by using a dissolution-recrystallization method. The morphology, structure, and acidity of these zeolites supported Pt catalysts were characterized by XRD, FT-IR, MAS-SSNMR, FE-SEM, FE-TEM, N₂-BET, XPS, NH₃-TPD, and CO pulse chemisorption. Compared to the conventional ZSM-5 supported Pt catalyst, the special structure in hollow ZSM-5 zeolite significantly promotes the dispersion of metallic Pt and the synergistic effect between metal active sites and acid sites. These boost the catalytic activity, selectivity of guaiacol hydrodeoxygenation toward cycloalkanes and long-term stability over the Pt/hollow ZSM-5 catalyst combined with improved mass transfer of products and reactants derived from the hierarchical hollow porous structure. Moreover, the Pt/hollow ZSM-5 catalyst exhibits excellent low temperature catalytic activity to completely transform guaiacol into cycloalkanes with the cyclohexane selectivity of more than 93% at 220 °C, suggesting that hollow ZSM-5 zeolite is a promising support for upgrading of bio-oils.

Role of Sn promoter in Ni/Al2O3 catalyst for the deoxygenation of stearic acid and coke formation: experimental and theoretical studies

The effect of Sn promoter on a Ni/γ-Al₂O₃ catalyst towards the deoxygenation of stearic acid has been investigated.

The role of nanosized zeolite Y in the H2-free catalytic deoxygenation of triolein

The reduction in zeolite Y crystal size has improved the triolein conversion, yield of deoxygenated product and diesel range hydrocarbon selectivity.

High-silica zeolites for adsorption of organic micro-pollutants in water treatment: A review

High-silica zeolites have been found to be effective adsorbents for the removal of organic micro-pollutants (OMPs) from impaired water, including various pharmaceuticals, personal care products, industrial chemicals, etc. In this review, the properties and fundamentals of high-silica zeolites are summarised. Recent research on mechanisms and efficiencies of OMP adsorption by high-silica zeolites are reviewed to assess the potential opportunities and challenges for the application of high-silica zeolites for OMP adsorption in water treatment. It is concluded that the adsorption capacities are well-related to surface hydrophobicity/hydrophilicity and structural features, e.g. micropore volume and pore size of high-silica zeolites, as well as the properties of OMPs. By using high-silica zeolites, the undesired competitive adsorption of background organic matter (BOM) in natural water could potentially be prevented. In addition, oxidative regeneration could be applied on-site to restore the adsorption capacity of zeolites for OMPs and prevent the toxic residues from re-entering the environment.

Monocyclic aromatic hydrocarbons production from catalytic cracking of pine wood-derived pyrolytic vapors over Ce-Mo2N/HZSM-5 catalyst

A series of Mo₂N/HZSM-5 and transition metal modified Mo₂N/HZSM-5 catalysts were prepared for the catalytic upgrading of pine wood-derived pyrolytic vapors for the selective production of monocyclic aromatic hydrocarbons (MAHs), while restraining the formation of polycyclic aromatic hydrocarbons (PAHs). Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) experiments were performed to determine the effects of several factors on selective MAHs production, including Mo₂N loading on HZSM-5, transition metal (Fe, Ce, La, Cu, Cr) modification of Mo₂N/HZSM-5, pyrolysis temperature, and catalyst-to-biomass ratio. In addition, quantitative experiments were conducted to determine the actual yields of major aromatic hydrocarbons and the source of aromatic hydrocarbons from basic biomass components. Results indicated that among the various catalysts, the Ce-10%Mo₂N/HZSM-5 exhibited the best performance on promoting the formation of MAHs and restraining the generation of PAHs. Under the optimal conditions, the actual yields of MAHs and PAHs from Ce-10%Mo₂N/HZSM-5 catalytic process were 99.8mg/g and 7.5mg/g, while those from HZSM catalyst were only 77.2mg/g and 23.7mg/g respectively. Furthermore, the possible catalytic mechanism of the Ce-Mo₂N/HZSM-5 catalyst was proposed based on the catalyst characterization.

Construction of Bifunctional Co/H-ZSM-5 Catalysts for the Hydrodeoxygenation of Stearic Acid to Diesel-Range Alkanes

Bifunctional Co/H-ZSM-5 zeolites were prepared by a surface organometallic chemistry grafting route, namely, by the stoichiometric reaction between cobaltocene and the Brønsted acid sites in zeolites. The catalyst was applied to a model reaction of the catalytic hydrodeoxygenation of stearic acid (SA). The cobalt species existed in the form of isolated Co²⁺ ions at the exchange positions after grafting, transformed to CoO species on the surface of the zeolite, stabilized inside the zeolite channels upon calcination in air, and finally reduced by hydrogen to homogeneous clusters of metallic cobalt species approximately 1.5 nm in size. During this process, the Brønsted acid sites of the H-ZSM-5 zeolites were preserved with a slightly reduced acid strength. The as-prepared bifunctional catalyst exhibited an approximately 16 times higher activity for the hydrodeoxygenation of SA (2.11 g_SA  g_cat^-1  h^-1 ) than the reference catalyst (0.13 g_SA  g_cat^-1  h^-1 ) prepared by solid-state ion exchange and a high C₁₈ /C₁₇ ratio of approximately 24. The remarkable hydrodeoxygenation performance of the bifunctional Co/H-ZSM-5 was owed to the effective synergy between the uniformed metallic cobalt clusters and the Brønsted acid sites in H-ZSM-5. The simplified reaction network and kinetics of the SA hydrodeoxygenation catalyzed by the as-prepared bifunctional Co/H-ZSM-5 zeolites were also investigated.

Hydrotreatment Upgrading of Bio-oil from Torrefaction of Pubescens in Alcohol over Pd/NbOPO4

Pd/NbOPO₄ multifunctional catalyst was prepared and used for catalytic hydrotreatment upgrading of bio-oil, which was obtained from low-temperature torrefaction (LTT) of pubescens and contained mainly lignin oligomers. The upgrading temperatures were investigated at 220-280 °C. Pd/NbOPO₄ exhibited good performance for the effective depolymerization of oligomers with increasing temperature. It was revealed that both Brönsted acid sites and Lewis acid sites existed on NbO _x , which contributed to the cleavage of the C-O-C bond. Furthermore, esterification, hydrogenation, and O-alkylation of monomeric derivatives also occurred simultaneously during the depolymerization process. After hydrotreatment upgrading, the average molecular weight of bio-oil decreased from M _w = 320 Da (M _n = 298 Da) to M _w = 273 Da (M _n = 254 Da) and the bio-oil quality was improved dramatically. The oxygen content decreased from 29.53 to 9.78 wt %. The upgraded bio-oil obtained at 280 °C had a heating value of 40.48 MJ kg^-1, which was much higher than that of the original bio-oil (26.96 MJ kg^-1).