Active Oxygen Species Promoted Catalytic Oxidation of 5-Hydroxymethyl-2-furfural on Facet-Specific Pt Nanocrystals

Electron Structure Tuned Oxygen Vacancy-Rich AuPd/CeO2 for Enhancing 5-Hydroxymethylfurfural Oxidation

The design of high activity catalyst for the efficiently conversion of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) gains great interest. The rationally tailoring of electronic structure directly affects the interaction between catalysts and organic substrates, especially molecular oxygen as the oxidant. This work, the bimetallic catalysts AuPd/CeO2 were prepared by the combining method of chemical reduction and photo-deposition, effectively concerting charge between Au and Pd and forming the electron-rich state of Au. The increasing of oxygen vacancy concentration of CeO2 by acidic treatment can facilitate the adsorption of HMF for catalysts and enhance the yield of FDCA (99.0 %). Moreover, a series of experiment results combining with density functional theory calculation illustrated that the oxidation performance of catalyst in HMF conversion was strongly related to the electronic state of interfacial Au-Pd-CeO2. Furthermore, the electron-rich state sites strengthen the adsorption and activation of molecular oxygen, greatly promoting the elimination of β-hydride for the selective oxidation of 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) to FDCA, accompanied with an outgoing FDCA formation rate of 13.21 mmol ⋅ g-1 ⋅ min-1 at 80 °C. The perception exhibited in this research could be benefit to understanding the effects of electronic state for interfacial sites and designing excellent catalysts for the oxidation of HMF.

Hydroxy and surface oxygen effects on 5-hydroxymethylfurfural oxidation to 2,5-furandicarboxylic acid on β-MnO2: DFT, microkinetic and experiment studies

Manganese dioxide, β-MnO2, has shown potential in catalyzing the oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA), a monomer of bioplastic polyethylene furanoate (PEF). Herein, the insight into the hydroxy (OH) and surface oxygen effects on the HMF-to-FDCA reaction over β-MnO2 is clarified through a comprehensive investigation using density functional theory (DFT) calculations, microkinetic modeling, and experiment. Theoretical analyses revealed that both active surface oxygen and OH species (from either base or solvent) facilitate C-H bond breaking and OH insertion, promoting the catalytic activity of β-MnO2. Microkinetic modeling demonstrated that the FFCA-to-FDCA and DFF-to-FFCA steps are the rate-limiting steps of the hydroxylated and non-hydroxylated surfaces, respectively. These theoretical results agree well with the experiment when water and dimethyl sulfoxide (DMSO) were used as solvents. In addition, the synthesized β-MnO2 catalyst showed high stability and activity, maintaining stable HMF conversion (≥99 mol%) and high FDCA yield (85-92 mol%) during continuous flow oxidation for 72 hours at pO2 of 1 MPa, 393 K and LHSV of 1 h-1. Thus, considering both hydroxy and surface oxygen species is a new strategy for enhancing the catalytic activity of Mn oxides and other metal oxide catalysts for the HMF-to-FDCA reaction.

Attachment of facile synthesized NaCo2O4 nanodots to SiO2 nanoflakes for sodium-rich boosted Pt-dominated ambient HCHO oxidation

Surface alkali metal ions are typically utilized as available promoters for ambient HCHO oxidation. In this study, NaCo₂O₄ nanodots with two different preferential crystallographic orientations are synthesized by facile attachment to SiO₂ nanoflakes with varying degrees of lattice defects. A unique Na-rich environment is established through interlayer Na⁺ diffusion based on the small size effect. The optimized catalyst Pt/HNaCo₂O₄/T₂ can deal with HCHO below 5 ppm in the static measurement system with a sustained release background and produces approximately 40 ppm of CO₂ in 2 h. Combining the experimental analyses with density functional theory (DFT) calculations, the possible catalytic enhancing mechanism is proposed from the support promotion perspective, and the positive synergistic effect of Na-rich, oxygen vacancies and optimized facets for Pt-dominant ambient HCHO oxidation via both kinetic and thermodynamic processes is confirmed.

Probing and Leveraging the Structural Heterogeneity of Nanomaterials for Enhanced Catalysis

The marriage between nanoscience and heterogeneous catalysis has introduced transformative opportunities for accessing better nanocatalysts. However, the structural heterogeneity of nanoscale solids stemming from distinct atomic configurations makes it challenging to realize atomic-level engineering of nanocatalysts in the way that is attained for homogeneous catalysis. Here, we discuss recent efforts in unveiling and exploiting the structural heterogeneity of nanomaterials for enhanced catalysis. Size and facet control of nanoscale domains produce well-defined nanostructures that facilitate mechanistic studies. Differentiation of surface and bulk characteristics for ceria-based nanocatalysts guides new thoughts toward lattice oxygen activation. Manipulating the compositional and species heterogeneity between local and average structures allows regulation of catalytically active sites via the ensemble effect. Studies on catalyst restructurings further highlight the necessity to assess the reactivity and stability of nanocatalysts under reaction conditions. These advances promote the development of novel nanocatalysts with expanded functionalities and bring atomistic insights into heterogeneous catalysis.

Selective Oxidation of Biomass Molecules via ZnO Nanoparticles Modified Using Charge Mismatch of the Doped Co ions

A charge mismatch between transition-metal-ion dopants and metal oxide nanoparticles (MO NPs) within an engineered complex engenders a significant number of oxygen vacancies (V_O) on the surface of the MO NP construct. To elucidate in-depth the mechanism of this tendency, Co ions with different charge states (Co³⁺ and Co²⁺) were doped into ZnO NPs, and their atomic structural changes were correlated with their photocatalytic efficiency. We ascertained that the increase of the Zn-O bond distances was distinctly affected by Co³⁺-ion doping, and, subsequently, the number of V_O was noticeably increased. We further investigated the mechanistic pathways of the photocatalytic oxidation of 2,5-hydroxymethylfurfural (HMF), which have been widely investigated as biomass derivatives because of their potential use as precursors for the synthesis of sustainable alternatives to petrochemical substances. To identify the reaction products in each oxidation step, selective oxidation products obtained from HMF in the presence of pristine ZnO NPs, Co³⁺- and Co²⁺-ion-doped ZnO NPs were evaluated. We confirmed that Co³⁺-ion-doped ZnO NPs can efficiently and selectively oxidize HMF with a good conversion rate (∼40%) by converting HMF to 2,5-furandicarboxylic acid (FDCA). The present study demonstrates the feasibility of improving the production efficiency of FDCA (an alternative energy material) by using enhanced photocatalytic MO NPs with the help of the charge mismatch between MO and metal-ion dopants.

Advances in Catalytic Routes for the Homogeneous Green Conversion of the Bio-Based Platform 5-Hydroxymethylfurfural

5-Hydroxymethylfufural (HMF) is an intriguing platform molecule that can be obtained from biomasses and that can lead to the production of a wide range of products, intermediates, or monomers. The presence of different moieties in HMF (hydroxy, aldehyde, furan ring) allows to carry out different transformations such as selective oxidations and hydrogenations, reductive aminations, etherifications, decarbonylations, and acetalizations. This is a great chance in a biorefinery perspective but requires the development of active and highly selective catalysts. In this view, homogeneous catalysis can lead to efficient conversion of HMF at mild reaction conditions. This Review discussed the recent achievements in homogeneous catalysts development and application to HMF transformations. The effects of metal nature, ligands, solvents, and reaction conditions were reported and critically reviewed. Current issues and future chances have been presented to drive future studies toward more efficient and scalable processes.

Research Progress of Highly Efficient Noble Metal Catalysts for the Oxidation of 5-Hydroxymethylfurfural

5-hydroxymethylfurfural (HMF) is considered to be one of the most pivotal multifunctional biomass platform chemicals. This Review discusses recent advances in catalytic oxidation of HMF towards high-value products. The reaction mechanism of different noble metals and the path of HMF oxidation to high-value products have been deeply investigated in the noble metal catalytic system. The reaction mechanisms of different noble metals and HMF conversion paths were compared in detail. Moreover, the factors affecting the performance of different noble metal catalysts were summarized. Finally, effective strategies were put forward to improve the catalytic performance of noble metal catalysts. The purpose is to provide a valuable reference for the academic research on the preparation of oxidation products from biomass-based HMF and the industrial application of noble metal catalysts.

Crystal Faces-Tailored Oxygen Vacancy in Au/CeO2 Catalysts for Efficient Oxidation of HMF to FDCA

Developing an efficient catalyst to upgrade 5-hydroxymethylfurfural (HMF) to high-value-added downstream chemicals is of great significance in biomass conversion. Nanorod (110)-, nanocube (100)-, and nanooctaheron (111)-CeO₂ -supported Au nanoparticles were prepared to investigate the intrinsic effect of CeO₂ crystal faces on the oxidation of HMF to 2,5-furandicarboxylic acid (FDCA). The experimental results and density functional theory calculation revealed that the concentration of oxygen vacancy (V_O ) for exposed specific crystal faces was crucial to the oxygen adsorption ability, and Au/nanorod-CeO₂ with the highest V_O concentration promoted the formation of more oxygen active species (superoxide radical) on CeO₂ (110) crystal face than (100) and (111) crystal faces. Besides, the higher V_O concentration could provide a strong adsorption ability of HMF, greatly boosting the activation of HMF. Thus, these results led to a superior catalytic activity for HMF oxidation over Au/nanorod-CeO₂ (FDCA yield of 96.5 %). In-situ Fourier-transform (FT)IR spectroscopy uncovered the HMF oxidation pathway, and the possible catalytic mechanism was proposed. The deep insight into the role of regulation for crystal faces provides a basis for the rational design of highly active facets for the oxidation of HMF and related reactions.

Application of aluminosilicate clay mineral-based composites in photocatalysis

Aluminosilicate clay mineral (ACM) is a kind of typical raw materials that used widely in manufacturing industry owing to the abundant reserve and low-cost exploring. In past two decades, in-depth understanding on unique layered structure and abundant surface properties endows ACM in the emerging research and application fields. In field of solar-chemical energy conversion, ACM has been widely used to support various semiconductor photocatalysts, forming the composites and achieving efficient conversion of reactants under sunlight irradiation. To date, classic ACM such as kaolinite and montmorillonite, loaded with semiconductor photocatalysts has been widely applied in photocatalysis. This review summaries the recent works on ACM-based composites in photocatalysis. Focusing on the properties of surface and layered structure, we elucidate the different features in the composition with various functional photocatalysts on two typical kinds of ACM, i.e., type 1:1 and type 2:1. Not only large surface area and active surface hydroxyl group assist the substrate adsorption, but also the layered structure provides more space to enlarge the application of ACM-based photocatalysts. Besides, we overview the modifications on ACM from both external surface and the inter-layer space that make the formation of composites more efficiently and boost the photo-chemical process. This review could inspire more upcoming design and synthesis for ACM-based photocatalysts, leading this kind of economic and eco-friendly materials for more practical application in the future.

Identification of Crucial Intermediates in the Formation of Humins from Cellulose-Derived Platform Chemicals Under Brønsted Acid Catalyzed Reaction Conditions

Humins are one of the undesirable products formed during the dehydration of sugars as well as the conversion of 5-hydroxymethylfurfural (HMF) to value-added products. Thus, reducing the formation of humins is an important strategy for improving the yield of the aforementioned reactions. Even after a plethora of studies, the mechanism of formation and the structure of humins are still elusive. In this regard, we have employed density functional theory-based mechanistic studies and microkinetic analysis to identify crucial intermediates formed from glucose, fructose, and HMF that can initiate the polymerization reactions resulting in humins under Brønsted acid-catalyzed reaction conditions. This study brings light into crucial elementary reaction steps that can be targeted for controlling humins formation. Moreover, this work provides a rationale for the experimentally observed aliphatic chains and HMF condensation products in the humins structure. Different possible polymerization routes that could contribute to the structure of humins are also suggested based on the results. Importantly, the findings of this work indicate that increasing the rate of isomerization of glucose to fructose and reducing the rate of reaction between HMF molecules could be an efficient strategy for reducing humins formation.

Au-Pd separation enhances bimetallic catalysis of alcohol oxidation

In oxidation reactions catalysed by supported metal nanoparticles with oxygen as the terminal oxidant, the rate of the oxygen reduction can be a limiting factor. This is exemplified by the oxidative dehydrogenation of alcohols, an important class of reactions with modern commercial applications^1-3. Supported gold nanoparticles are highly active for the dehydrogenation of the alcohol to an aldehyde⁴ but are less effective for oxygen reduction^5,6. By contrast, supported palladium nanoparticles offer high efficacy for oxygen reduction^5,6. This imbalance can be overcome by alloying gold with palladium, which gives enhanced activity to both reactions^7,8,9; however, the electrochemical potential of the alloy is a compromise between that of the two metals, meaning that although the oxygen reduction can be improved in the alloy, the dehydrogenation activity is often limited. Here we show that by separating the gold and palladium components in bimetallic carbon-supported catalysts, we can almost double the reaction rate compared with that achieved with the corresponding alloy catalyst. We demonstrate this using physical mixtures of carbon-supported monometallic gold and palladium catalysts and a bimetallic catalyst comprising separated gold and palladium regions. Furthermore, we demonstrate electrochemically that this enhancement is attributable to the coupling of separate redox processes occurring at isolated gold and palladium sites. The discovery of this catalytic effect-a cooperative redox enhancement-offers an approach to the design of multicomponent heterogeneous catalysts.

Sustainable Routes for the Synthesis of Renewable Adipic Acid from Biomass Derivatives

Adipic acid (AA) is a key industrial dicarboxylic acid intermediate used in nylon manufacturing. Unfortunately, the traditional process technology is accompanied by serious environmental pollution. Given the growing demand for adipic acid and the desire to reduce its negative impact on the environment, considerable efforts have been devoted to developing more green and friendly routes. This Review is focused on the latest advances in the sustainable preparation of AA from biomass-based platform molecules, including 5-hydroxymethylfufural, glucose, γ-valerolactone, and phenolic compounds, through biocatalysis, chemocatalysis, and the combination of both. Additionally, the development of state-of-the-art catalysts for different catalytic systems systematically is discussed and summarized, as well as their reaction mechanisms. Finally, the prospects for all preparation routes are critically evaluated and key technical challenges in the development of green and sustainable processes for the manufacture of AA are highlighted. It is hoped that the green adipic acid synthesis pathways presented can provide insights and guidance for further research into other industrial processes for the production of nylon precursors in the future.

Mechanistic insights into CoOx–Ag/CeO2 catalysts for the aerobic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid

CoOx–Ag/CeO2 catalysts achieve satisfactory FDCA yield from HMF, and a fundamental understanding about the reaction mechanism is provided.

Tuning the Selective Adsorption Site of Biomass on Co3 O4 by Ir Single Atoms for Electrosynthesis

The electrosynthesis from 5-hydroxymethylfurfural (HMF) is considered a green strategy to achieve biomass-derived high-value chemicals. As the molecular structure of HMF is relatively complicated, understanding the HMF adsorption/catalysis behavior on electrocatalysts is vital for biomass-based electrosynthesis. The electrocatalysis behavior can be modulated by tuning the adsorption energy of the reactive molecules. In this work, the HMF adsorption behavior on spinel oxide, Co₃ O₄ is discovered. Correspondingly, the adsorption energy of HMF on Co₃ O₄ is successfully tuned by decorating with single-atom Ir. It is observed that compared with bare Co₃ O₄ , single-atom-Ir-loaded Co₃ O₄ (Ir-Co₃ O₄ ) can enhance adsorption with the CC groups of HMF. The synergetic adsorption can enhance the overall conversion of HMF on electrocatalysts. With the modulated HMF adsorption, the as-designed Ir-Co₃ O₄ exhibits a record performance (with an onset potential of 1.15 V_RHE ) for the electrosynthesis from HMF.

The Increasing Value of Biomass: Moving From C6 Carbohydrates to Multifunctionalized Building Blocks via 5-(hydroxymethyl)furfural

Recent decades have been marked by enormous progress in the field of synthesis and chemistry of 5-(hydroxymethyl)furfural (HMF), an important platform chemical widely recognized as the "sleeping giant" of sustainable chemistry. This multifunctional furanic compound is viewed as a strong link for the transition from the current fossil-based industry to a sustainable one. However, the low chemical stability of HMF significantly undermines its synthetic potential. A possible solution to this problem is synthetic diversification of HMF by modifying it into more stable multifunctional building blocks for further synthetic purposes.

Advances in catalytic routes for the production of carboxylic acids from biomass: a step forward for sustainable polymers

Polymers are ubiquitously present in our daily life because they can meet a wide range of needs and fields of applications. This success, based on an irresponsible linear consumption of plastics and the access to cheap oil, is creating serious environmental problems. Two lines of actions are needed to cope with them: to adopt a circular consumption of plastics and to produce renewable carbon-neutral monomers. This review analyses the recent advances in the chemocatalytic processes for producing biomass-derived carboxylic acids. These renewable carboxylic acids are involved in the synthesis of relevant general purpose and specialty polyesters and polyamides; some of them are currently derived from oil, while others can become surrogates of petrochemical polymers due to their excellent performance properties. Polyesters and polyamides are very suitable to be depolymerised to other valuable chemicals or to their constituent monomers, what facilitates the circular reutilisation of these monomers. Different types of carboxylic acids have been included in this review: monocarboxylic acids (like glycolic, lactic, hydroxypropanoic, methyl vinyl glycolic, methyl-4-methoxy-2-hydroxybutanoic, 2,5-dihydroxypent-3-enoic, 2,5,6-trihydroxyhex-3-enoic acids, diphenolic, acrylic and δ-amino levulinic acids), dicarboxylic acids (2,5-furandicarboxylic, maleic, succinic, adipic and terephthalic acids) and sugar acids (like gluconic and glucaric acids). The review evaluates the technology status and the advantages and drawbacks of each route in terms of feedstock, reaction pathways, catalysts and economic and environmental evaluation. The prospects and the new research that should be undertaken to overcome the main problems threatening their economic viability or the weaknesses that prevent their commercial implementation have also been underlined.

Adsorption-Free Growth of Ultra-Thin Molybdenum Membranes with a Low-Symmetry Rectangular Lattice Structure

Although low-symmetry lattice structure of 2D transition metals is highly anticipated for both fundamental research and potentially distinctive application, it still has not been experimentally realized, which greatly hinders the exploration of the unique properties. Here, ultra-thin body-centered-cubic (bcc) phase molybdenum (Mo) membranes are successfully synthesized with a low-symmetry rectangular (110) crystal face via an adsorption-free reaction. Through experimental and density functional theory studies, no foreign atoms being adsorbed is shown to be a key factor for the successful preparation of the bcc phase 2D transition metal with (110) faces. The realization of 2D Mo(110) with a low-symmetric rectangular lattice structure extends the scope of 2D structures and is also beneficial for the exploration and development of low-symmetry rectangular lattice-structured materials with unique properties.