Catalytic Transfer Hydrogenation of Biomass-Derived Carbonyls over Hafnium-Based Metal-Organic Frameworks

Construction of Silver Quantum Dot Immobilized Zn-MOF-8 Composite for Electrochemical Sensing of 2,4-Dinitrotoluene

In the present study, we report a highly effective electrochemical sensor for detecting 2,4-dinitrotoluene (2,4-DNT). The amperometric determination of 2,4-DNT was carried out using a gold electrode modified with zinc–metal organic framework-8 and silver quantum dot (Zn-MOF-8@AgQDs) composite. The synthesized nanomaterials were characterized by using transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and X-ray powder diffraction (XRD). The synthesized nanocomposite proved to be efficient in electro-catalysis thereby reducing the 2,4-DNT. The unique combination present in Zn-MOF-8@AgQDs composite offered an excellent conductivity and large surface area enabling the fabrication of a highly sensitive (−0.238 µA µM−1 cm−2), selective, rapid and stable 2,4-DNT sensor. The dynamic linear range and limit of detection (LOD) was about 0.0002 µM to 0.9 µM and 0.041 µM, respectively. A 2,4-DNT reduction was also observed during the linear sweep voltammetry (LSV) experiments with reduction peaks at −0.49 V and −0.68 V. This is an unprecedented report with metal organic framework (MOF) composite for sensing 2,4-DNT. In addition, the presence of other species such as thiourea, urea, ammonia, glucose, and ascorbic acid displayed no interference in the modified electrode suggesting its practicability in various environmental applications.

Interplay of Lewis and Brønsted Acid Sites in Zr-Based Metal-Organic Frameworks for Efficient Esterification of Biomass-Derived Levulinic Acid

We report the performance of UiO-66 and its Brønsted acid functionalized derivative, UiO-66-(COOH)₂, as heterogeneous catalysts for levulinic acid esterification with ethanol. Importantly, compared with UiO-66, UiO-66-(COOH)₂ displayed superior catalytic performance (up to 97.0 ± 1.1% yield of ethyl levulinate) attributed to the synergistic effect between Lewis acidic Zr clusters and Brønsted acidic -COOH groups. Furthermore, UiO-66-(COOH)₂ was stable and reusable without an appreciable loss in catalytic activity for at least five consecutive cycles. This study demonstrates that the interplay of Brønsted and Lewis acid sites in zirconium metal-organic frameworks leads to more efficient catalytic conversion of a biomass feedstock to biofuel, and with further hypothesis driven research, additional materials that show promise as candidates for catalytic conversion of biomass feedstocks to biofuels and valuable chemicals can be developed.

Heterogeneous Catalytic Upgrading of Biofuranic Aldehydes to Alcohols

Heterogeneous catalytic conversion of lignocellulosic components into valuable chemicals and biofuels is one of the promising ways for biomass valorization, which well meets green chemistry metrics, and can alleviate environmental and economic issues caused by the rapid depletion of fossil fuels. Among the identified biomass derivatives, furfural (FF) and 5-hydroxymethylfurfural (HMF) stand out as rich building blocks and can be directly produced from pentose and hexose sugars, respectively. In the past decades, much attention has been attracted to the selective hydrogenation of FF and 5-hydroxymethylfurfural using various heterogeneous catalysts. This review evaluates the recent progress of developing different heterogeneous catalytic materials, such as noble/non-noble metal particles, solid acids/bases, and alkali metal salts, for the efficient reduction of bio-based furanic aldehydes to alcohols. Emphasis is laid on the insights and challenges encountered in those biomass transformation processes, along with the focus on the understanding of reaction mechanisms to clarify the catalytic role of specific active species. Brief outlook is also made for further optimization of the catalytic systems and processes for the upgrading of biofuranic compounds.

Synergistic Catalysis of Ruthenium Nanoparticles and Polyoxometalate Integrated Within Single UiO-66 Microcrystals for Boosting the Efficiency of Methyl Levulinate to γ-Valerolactone

The synthesis of heterogeneous cooperative catalysts in which two or more catalytically active components are spatially separated within a single material has generated considerable research efforts. The multiple functionalities of catalysts can significantly improve the efficiency of existing organic chemical transformations. Herein, we introduce ruthenium (Ru) nanoparticles (NPs) on the surfaces of a metal–organic framework pre-encapsulated with polyoxometalate silicotungstic acid (SiW) UiO−66 (University of Oslo [UiO]) and prepared a 2.0% Ru/11.7% SiW@UiO−66 porous hybrid using the impregnation method. The close synergistic effect of metal Ru NPs, SiW, and UiO-66 endow 2.0% Ru/11.7% SiW@UiO-66 with increased activity and stability for complete methyl levulinate (ML) conversion and exclusive γ-valerolactone (GVL) selectivity at mild conditions of 80°C and at a H₂ pressure of 0.5 MPa. Effectively, this serves as a model reaction for the upgrading of biomass and outperforms the performances of the constituent parts and that of the physical mixture (SiW + Ru/UiO−66). The highly dispersed Ru NPs act as active centers for hydrogenation, while the SiW molecules possess Brønsted acidic sites that cooperatively promote the subsequent lactonization of MHV to generate GVL, and the UiO−66 crystal accelerates the mass transportation facilitated by its own porous structure with a large surface area.

Tubular morphology preservation and doping engineering of Sn/P-codoped hematite for photoelectrochemical water oxidation

Tubular hematite with high-concentration, uniform doping is regarded as a promising material for photoelectrochemical water oxidation. However, the high-temperature annealing commonly used for activating doped hematite inevitably causes deformation of the tubular structure and an increase in the trap states. In the present work, Sn-doped tubular hematite on fluorine-doped tin oxide (FTO) is successfully obtained at 750 °C from a Sn-coated FeOOH tube precursor. Sn/P codoping, which is rarely considered for hematite, is also achieved via a gas phase reaction in phosphide atmosphere. The tubular morphology allows the dopant to diffuse from both the inner and outer surfaces, thus decreasing the doping profile in the radial direction. The even distribution of Sn and P synergetically increases the carrier density of hematite by one order of magnitude, which shortens the width of the depletion layer to ca. 2.3 nm (compared with 19.3 nm for the pristine sample) and leads to prolonged carrier lifetime and efficient charge separation. In addition, this codoping protocol does not introduce additional surface trap states, as evidenced by the increased charge injection efficiency and surface kinetic analysis using intensity modulated photocurrent spectroscopy (IMPS). As a result, the morphology- and doping-engineered hematite exhibits photocurrents of 0.9 mA cm-2 at 1.23 V and 3.8 mA cm-2 at 2.0 V vs. RHE under AM 1.5 G illumination (100 mW cm-2) in 1.0 M NaOH, representing 4.5-fold and 4.8-fold enhancements, respectively, compared with the photocurrents of undoped hematite. The present method is shown to be effective for preparing multi-element-doped hematite nanotubes and may find broad application in the development of other nanotubular photoelectrodes with or without doping for efficient and robust water oxidation.

Catalytic Transfer Hydrogenation of Biomass-Derived Substrates to Value-Added Chemicals on Dual-Function Catalysts: Opportunities and Challenges

Aqueous-phase hydrodeoxygenation (APH) of bioderived feedstocks into useful chemical building blocks is one the most important processes for biomass conversion. However, several technological challenges, such as elevated reaction temperature (220-280 °C), high H₂ pressure (4-10 MPa), uncontrollable side reactions, and intensive capital investment, have resulted in a bottleneck for the further development of existing APH processes. Catalytic transfer hydrogenation (CTH) under much milder conditions with non-fossil-based H₂ has attracted extensive interest as a result of several advantageous features, including high atom efficiency (≈100 %), low energy intensity, and green H₂ obtained from renewable sources. Typically, CTH can be categorized as internal H₂ transfer (sacrificing small amounts of feedstocks for H₂ generation) and external H₂ transfer from H₂ donors (e.g., alcohols, formic acid). Although the last decade has witnessed a few successful applications of conventional APH technologies, CTH is still relatively new for biomass conversion. Very limited attempts have been made in both academia and industry. Understanding the fundamentals for precise control of catalyst structures is key for tunable dual functionality to combine simultaneous H₂ generation and hydrogenation. Therefore, this Review focuses on the rational design of dual-functionalized catalysts for synchronous H₂ generation and hydrogenation of bio-feedstocks into value-added chemicals through CTH technologies. Most recent studies, published from 2015 to 2018, on the transformation of selected model compounds, including glycerol, xylitol, sorbitol, levulinic acid, hydroxymethylfurfural, furfural, cresol, phenol, and guaiacol, are critically reviewed herein. The relationship between the nanostructures of heterogeneous catalysts and the catalytic activity and selectivity for C-O, C-H, C-C, and O-H bond cleavage are discussed to provide insights into future designs for the atom-economical conversion of biomass into fuels and chemicals.

Zr-MOF-808@MCM-41 catalyzed phosgene-free synthesis of polyurethane precursors

Zr-MOF-808@MCM-41 exhibited high catalytic activity, selectivity and stability for the synthesis of aromatic carbamates from aromatic amines and dimethyl carbonate.