Engineering S-Scheme Heterojunction via MOF-on-MOF for Photocatalytic Nitroarene Hydrogenation

Photocatalytic nitroarene reduction provides a promising strategy for the sustainable production of aniline. The construction of S-scheme heterostructures with a clear interfacial charge transfer mechanism is considered as an effective strategy to improve the photocatalytic performance of photocatalysts. The assembly of MOF-on-MOF might be used to construct S-scheme heterojunctions due to the rich structures, effective charge transport channels, and fast mass transfer of MOFs. Herein, 2D Pd-PPF-1 was coated on 3D Pd-PCN-222 through a presurface modification strategy, and the prepared Pd-PPF-1/Pd-PCN-222 with an S-scheme heterojunction displayed the morphology of a 2D nanoflower winding around a 3D rod. As for photocatalytic nitroarene hydrogenation, the as-obtained Pd-PPF-1/Pd-PCN-222 catalyst exhibited much higher photocatalytic performance than Pd-PPF-1, Pd-PCN-222, or a physical mixture of Pd-PPF-1 and Pd-PCN-222. The high catalytic performance of Pd-PPF-1/Pd-PCN-222 might be attributed to the formation of the S-scheme heterojunction, which not only retained the redox capability of the parent MOFs but also separated photogenerated carriers. This work presents a constructive route for designing 2D-on-3D MOF S-scheme heterojunction with controllable morphology and high photocatalytic ability.

Copper-decorated strategy based on defect-rich NH2-MIL-125(Ti) boosts efficient photocatalytic degradation of methyl mercaptan under sunlight

Photocatalysis has received significant attention as a technology that can solve environmental problems. Metal-organic frameworks are currently being used as novel photocatalysts but are still limited by the rapid recombination of photogenerated carriers, low photogenerated electron migration efficiency and poor solar light utilization rate. In this work, a novel photocatalyst was successfully constructed by introducing Cu species into thermal activated mixed-ligand NH2-MIL-125 (Ti) via defect engineering strategy. The constructed defect structure not only provided 3D-interconnected gas transfer channels, but also offered suitable space to accommodate introduced Cu species. For the most effective photocatalyst 0.2Cu/80%NH2-MIL-125 (300 °C) with optimized Cu content, the photocatalytic degradation rate of CH3SH achieved 4.65 times higher than that of pristine NH2-MIL-125 under visible light (λ > 420 nm). At the same time, it showed great degradation efficiency under natural sunlight, 100 ppm CH3SH was completely removed within 25 min in full solar light illumination. The improved catalytic efficiency is mainly due to the synergistic effect of the integrated Schottky junction and rich-defective NH2-MIL-125, which improved the bandgap and band position, and thus facilitated the separation and transfer of the photo-generated carriers. This work provided a facile way to integrate Schottky junctions and rich-defective MOFs with high stability. Due to its excellent degradation performance under sunlight, it also offered a prospective strategy for rational design of high-efficiency catalysts applied in environmental technologies.

Stabilization of transition metal heterojunctions inside porous materials for high-performance catalysis

Transition metal-based heterostructural materials are a class of very promising substitutes for noble metal-based catalysts for high-performance catalysis, due to their inherent internal electric field at the interface in the heterojunctions, which could induce electron relocalization and facilitate charge carrier migration between different metal sites at heterostructural boundaries. However, redox-active metal species suffer from reduction, oxidation, migration, aggregation, leaching and poisoning in catalysis, which results in heavy deterioration of the catalytic properties of transition metal-based heterojunctions and frustrates their practical applications. To improve the stability of transition metal-based heterojunctions and sufficiently expose redox-active sites at the heterosurfaces, many kinds of porous materials have been used as porous hosts for the stabilization of non-precious metal heterojunctions. This review article will discuss recently developed strategies for encapsulation and stabilization of transition metal heterojunctions inside porous materials, and highlight their improved stability and catalytic performance through the spatial confinement effect and synergistic interaction between the heterojunctions and the host matrices.

In situ utilization of photogenerated hydrogen for hydrogenation reaction over a covalent organic framework

A fully sp²-carbon conjugated COF (Py-FTP-COF) was designed and synthesized, exhibiting excellent hydrogen evolution rate of 5.22 mmol g^-1 h^-1. More importantly, in situ hydrogenation of nitroarenes under visible-light irradiation without any additional hydrogen source was successfully accomplished for the first time over COF-based materials.

Defined metal atom aggregates precisely incorporated into metal-organic frameworks

Nanosized metal aggregates (MAs), including metal nanoparticles (NPs) and nanoclusters (NCs), are often the active species in numerous applications. In order to maintain the active form of MAs in "use", they need to be anchored and stabilised, preventing agglomeration. In this context, metal-organic frameworks (MOFs), which exhibit a unique combination of properties, are of particular interest as a tunable and porous matrix to host MAs. A high degree of control in the synthesis towards atom-efficient and application-oriented MA@MOF composites is required to derive specific structure-property relationships and in turn to enable design of functions on the molecular level. Due to the versatility of MA@MOF (derived) materials, their applications are not limited to the obvious field of catalysis, but increasingly include 'out of the box' applications, for example medical diagnostics and theranostics, as well as specialised (bio-)sensoring techniques. This review focuses on recent advances in the controlled synthesis of MA@MOF materials en route to atom-precise MAs. The main synthetic strategies, namely 'ship-in-bottle', 'bottle-around-ship', and approaches to achieve novel hierarchical MA@MOF structures are highlighted and discussed while identifying their potential as well as their limitations. Hereby, an overview of standard characterisation methods that enable a systematic analysis procedure and state-of-art techniques that localise MA within MOF cavities are provided. While the perspectives of MA@MOF materials in general have been reviewed various times in the recent past, few atom-precise MAs inside MOFs have been reported so far, opening opportunities for future investigation.

Metal-organic frameworks as a good platform for the fabrication of multi-metal nanomaterials: design strategies, electrocatalytic applications and prospective

MOF-derived multi-metal nanomaterials are attracting numerous attentions in widespread applications such as catalysis, sensors, energy storage and conversion, and environmental remediation. Compared to the monometallic counterparts, the presence of foreign metal is expected to bring new physicochemical properties, thus exhibiting synergistic effect for enhanced performance. MOFs have been proved as a good platform for the fabrication of polymetallic nanomaterials with requisite features. Herein, various design strategies related to constructing multi-metallic nanomaterials from MOFs are summarized for the first time, involving metal nodal substitution, seed epitaxial growth, ion-exchange strategy, guest species encapsulation, solution impregnation and combination with extraneous substrate. Afterwards, the recent advances of multi-metallic nanomaterials for electrocatalytic applications, including oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), are systematically discussed. Finally, a personal outlook on the future trends and challenges are also presented with hope to enlighten deeper understanding and new thoughts for the development of multi-metal nanomaterials from MOFs.

A metal-organic framework with rich accessible nitrogen sites for rapid dye adsorption and highly efficient dehydrogenation of formic acid

MOFs with adequate free nitrogen sites have potential applications in dye adsorption and formic acid dehydrogenation. Here, we successfully synthesized a novel 3-D MOF 1 ([(CH₃)₂NH₂][Cd(L)DMA]·0.5DMA·1.5H₂O) with a special two-fold interpenetrating framework through a simple solvothermal reaction between CdCl₂·1.5H₂O and a nitrogen-rich triangular tricarboxylate-based linker (H₃L, 4,4',4''-s-triazine-2,4,6-tribenzoic acid). After removing the guest molecules of dimethylacetamide (DMA) and H₂O, including the coordinated DMA from 1 by vacuum activation at 423 K, a compound named 1' with a formula of [(CH₃)₂NH₂][Cd(L)] and a similar interpenetrating framework structure was obtained. In comparison with compound 1, the total void volume of 1' is nearly doubled, and thus may provide higher potential for the adsorption of other guest molecules. Notably, the pyridine N atoms located in the middle of the triangular tricarboxylate-based linker are not involved in the coordination with Cd²⁺, and are all uniformly dispersed throughout the whole framework of the 3-D MOFs. Due to its unique structural features, the 3-D MOF 1' could effectively adsorb the cationic dye MB⁺ for recycling purposes. The rapid adsorption rate (0.7 × 10^-2 g mg^-1 min^-1) and the relatively high capacity (900 mg g^-1) for MB⁺ demonstrate the potential of 1' in dye adsorption. In addition, 1' may also be used as an effective support to immobilize PdAu NPs via the double-solvent method. The resultant catalyst Pd0.8Au0.2/1' exhibits decent catalytic activity for the dehydrogenation of formic acid with a TOF value of 1854 h^-1 at 333 K. The existence of a large void volume and accessible pyridine N atoms provide a suitable environment for achieving a high dispersion of PdAu NPs, thereby leading to the formation of a catalytically active and stable supported noble-metal NP catalyst for H₂ generation from formic acid decomposition.

Cobalt nanoparticles supported on microporous nitrogen-doped carbon for efficient catalytic transfer hydrogenation reaction between nitroarenes and N-heterocycles

Catalytic transfer hydrogenation reaction between nitroarenes and saturated N-heterocycles to simultaneously synthesize value-added anilines and unsaturated N-heterocycles is attractive due to its low-cost, atomic economic, and environmental-friendly properties. Herein, we...

Thermo-enhanced photocatalytic oxidation of amines to imines over MIL-125-NH2@Ag@COF hybrids under visible light

Thermo-enhanced photocatalysis combines the advantages of thermocatalysis and photocatalysis and provides a very promising approach for the selective oxidation of organic compounds to value-added chemicals. In this work, the amino group in MIL-125-NH₂ first reacts with formaldehyde to form the reducing group (-NH-CH₂OH), which can in situ auto reduce the introduced Ag⁺ ions to Ag clusters/nanoparticles in the cavities. Then the formed MIL-125-NH-CH₂OH@Ag was further coated with a covalent organic framework (COF) through imine bonds to form a series of MIL-125-NH-CH₂OH@Ag@COF hybrids. Oxidative coupling of amines was selected to evaluate the photocatalytic performance of these materials under visible light at set temperatures (20-60 °C). With an optimized composition, MIL-125-NH-CH₂OH@Ag-0.5@COF-2 not only improves the optical properties, but also exhibits the highest conversion (almost 100%) of benzylamine under visible light at 60 °C and good stability for at least three cycles. Free radical capture experiments and electron spin resonance detection demonstrated that holes (h⁺), hydroxyl (˙OH) and superoxide radicals (O₂˙^-) were the active species. The results prove that the MIL-125-NH-CH₂OH@Ag@COF hybrid possessed higher photocatalytic performance than individual MIL-125-NH₂, Ag and COF on account of the efficient separation and transfer of photoinduced electrons and holes. Moreover, the promotion of the reaction temperature on the photocatalytic oxidation of amines has been reported, revealing that the conversion of benzylamine over MIL-125-NH-CH₂OH@Ag-0.5@COF-2 at 60 °C is nearly twice as high as that at 20 °C under visible light irradiation. Therefore, the thermo-enhanced photocatalytic oxidation performance of the MOF@Ag@COF hybrid demonstrates the great potential of thermal energy for further improving the photocatalytic selective oxidation performance.

Rhodium nanoparticles confined in titania nanotubes for efficient Hydrogen evolution from Ammonia Borane

Designing efficient catalysts for hydrogen evolution from hydrolysis of ammonia borane (AB) have attracted considerable attention. Rhodium (Rh) based catalysts with rational design present remarkable catalytic performance for the reaction. Herein, we report the confined Rh@TiO₂ catalysts synthesized by atomic layer deposition combining with the sacrificial template approach, in which the Rh nanoparticles are uniformly confined on the inner surface of the porous titania nanotubes. The optimized catalysts show high catalytic activity with a turnover frequency value of 334.1 mol_H2·mol_Rh^-1·min^-1 and better durability. Mechanistic investigation demonstrates that the cleavage of OH bands in water should be the rate determining step, and the appropriate concentration of NaOH can further enhance the hydrogen evolution activity. The catalysts can also achieve the hydrogenation of various organic substrates using AB as the hydrogen source. In addition, our present strategy is general and can be extended to the synthesis of other confined catalysts for various catalytic reactions.

The Hydrogen-Storage Challenge: Nanoparticles for Metal-Catalyzed Ammonia Borane Dehydrogenation

Dihydrogen is one of the sustainable energy vectors envisioned for the future. However, the rapidly reversible and secure storage of large quantities of hydrogen is still a technological and scientific challenge. In this context, this review proposes a recent state-of-the-art on H₂ production capacities from the dehydrogenation reaction of ammonia borane (and selected related amine-boranes) as a safer solid source of H₂ by hydrolysis (or solvolysis), catalyzed by nanoparticle-based systems. The review groups the results according to the transition metals constituting the catalyst with a mention to their current cost and availability. This includes the noble metals Rh, Pd, Pt, Ru, Ag, as well as cheaper Co, Ni, Cu, and Fe. For each element, the monometallic and polymetallic structures are presented and the performances are described in terms of turnover frequency and recyclability. The structure-property links are highlighted whenever possible. It appears from all these works that the mastery of the preparation of catalysts remains a crucial point both in terms of process, and control and understanding of the electronic structures of the elaborated nanomaterials. A particular effort of the scientific community remains to be made in this multidisciplinary field with major societal stakes.

Catalytic Behavior of Iron-Containing Cubic Spinel in the Hydrolysis and Hydrothermolysis of Ammonia Borane

The paper presents a comparative study of the activity of magnetite (Fe₃O₄) and copper and cobalt ferrites with the structure of a cubic spinel synthesized by combustion of glycine-nitrate precursors in the reactions of ammonia borane (NH₃BH₃) hydrolysis and hydrothermolysis. It was shown that the use of copper ferrite in the studied reactions of NH₃BH₃ dehydrogenation has the advantages of a high catalytic activity and the absence of an induction period in the H₂ generation curve due to the activating action of copper on the reduction of iron. Two methods have been proposed to improve catalytic activity of Fe₃O₄-based systems: (1) replacement of a portion of Fe²⁺ cations in the spinel by active cations including Cu²⁺ and (2) preparation of highly dispersed multiphase oxide systems, involving oxide of copper.

Facile preparation of Cu-Fe oxide nanoplates for ammonia borane decomposition and tandem nitroarene hydrogenation

A facile substrate involved strategy was used to prepare Cu-Fe LDO (layered double oxide) nanoplates. The material exhibited good-efficiency for decomposition of ammonia borane (AB) in alkaline methanol solution. Significantly, the material also demonstrated excellent catalytic performance in the reduction of various nitroarenes by coupling with AB hydrolysis in a one pot tandem reaction, and gave excellent yields of the corresponding amine products.

Copper-Based Integral Catalytic Impeller for the Rapid Catalytic Reduction of 4-Nitrophenol

The integral catalytic impeller can simultaneously improve reaction efficiency and avoid the problem of catalyst separation, which has great potential in applying heterogeneous catalysis. This paper introduced a strategy of combining electroless copper plating with 3D printing technology to construct a pluggable copper-based integral catalytic agitating impeller (Cu-ICAI) and applied it to the catalytic reduction of 4-nitrophenol (4-NP). The obtained Cu-ICAI exhibits very excellent catalytic activity. The 4-NP conversion rate reaches almost 100% within 90 s. Furthermore, the Cu-ICAI can be easily pulled out from the reactor to be repeatedly used more than 15 times with high performance. Energy-dispersive spectrometry, X-ray diffraction, and X-ray photoelectron spectroscopy characterizations show that the catalyst obtained by electroless copper plating is a ternary Cu-Cu₂O-CuO composite catalyst, which is conducive to the electron transfer process. This low-cost, facile, and versatile strategy, combining electroless plating and 3D printing, may provide a new idea for the preparation of the integral impeller with other metal catalytic activities.

Unsupported Nanoporous Platinum-Iron Bimetallic Catalyst for the Chemoselective Hydrogenation of Halonitrobenzenes to Haloanilines

An unsupported nanoporous platinum-iron bimetallic catalyst (PtFeNPore) was prepared with an electrochemical dealloying technique. Its structure and composition were characterized through various measurement methods, such as X-ray absorption fine structure (XAFS) and X-ray photoelectron spectroscopy (XPS). An intermetallic compound and iron oxide species were both found in the PtFeNPore catalyst. The nanoporous structure and Lewis acidity (caused by iron oxide species) of the PtFeNPore catalyst resulted in superior catalytic activity and high selectivity. The PtFeNPore-catalyzed hydrogenation of various halonitrobenzenes proceeded successfully under mild reaction conditions and produced good to excellent yields of the corresponding haloanilines with high selectivity. PtFeNPore can be recycled through magnetic separation easily and reused five times without significant deactivation.

Impregnating Subnanometer Metallic Nanocatalysts into Self-Pillared Zeolite Nanosheets

Impregnation is the most commonly used approach to prepare supported metal catalysts in industry. However, this method suffers from the formation of large metal particles with uneven dispersion, poor thermal stability, and thus unsatisfied catalytic performance. Here, we demonstrate that the self-pillared MFI zeolite (silicalite-1 and ZSM-5) nanosheets with larger surface area and abundant Si-OH groups are ideal supports to immobilize ultrasmall monometallic (e.g., Rh and Ru) and various bimetallic clusters via simple incipient wetness impregnation method. The loaded subnanometric metal clusters are uniformly dispersed within sinusoidal five-membered rings of MFI and remain stable at high temperatures. The Rh/SP-S-1 is highly efficient in ammonia borane (AB) hydrolysis, showing a TOF value of 430 mol_H2 mol_Rh^-1 min^-1 at 298 K, which is more than 6-fold improvement over that of nanosized zeolite-supported Rh catalyst and even comparable with that of zeolite-supported Rh single-atom catalyst. Because of the synergistic effect between bimetallic Rh-Ru clusters and zeolite acidity, the H₂ generation rate from AB hydrolysis over Rh_0.8Ru_0.2/SP-ZSM-5-100 reaches up to 1006 mol_H2 mol_metal^-1 min^-1 at 298 K, and also shows record activities in cascade hydrogenation of various nitroarenes by coupling with the hydrolysis of AB. This work demonstrates that zeolite nanosheets are excellent supports to anchor diverse ultrasmall metallic species via the simple impregnation method, and the obtained nanocatalysts can be applied in various industrially important catalytic reactions.

Metal–organic framework (MOF)-derived catalysts for chemoselective hydrogenation of nitroarenes

The MOFs derived catalysts conducted in the chemoselective hydrogenation of substituted nitroarenes, including pyrolysis and confined catalyst, are reviewed.

Recent developments of nanocatalyzed liquid-phase hydrogen generation

Hydrogen is the most effective and sustainable carrier of clean energy, and liquid-phase hydrogen storage materials with high hydrogen content, reversibility and good dehydrogenation kinetics are promising in view of "hydrogen economy". Efficient, low-cost, safe and selective hydrogen generation from chemical storage materials remains challenging, however. In this Review article, an overview of the recent achievements is provided, addressing the topic of nanocatalysis of hydrogen production from liquid-phase hydrogen storage materials including metal-boron hydrides, borane-nitrogen compounds, and liquid organic hydrides. The state-of-the-art catalysts range from high-performance nanocatalysts based on noble and non-noble metal nanoparticles (NPs) to emerging single-atom catalysts. Key aspects that are discussed include insights into the dehydrogenation mechanisms, regenerations from the spent liquid chemical hydrides, and tandem reactions using the in situ generated hydrogen. Finally, challenges, perspectives, and research directions for this area are envisaged.

Nanopore-Supported Metal Nanocatalysts for Efficient Hydrogen Generation from Liquid-Phase Chemical Hydrogen Storage Materials

Hydrogen has emerged as an environmentally attractive fuel and a promising energy carrier for future applications to meet the ever-increasing energy challenges. The safe and efficient storage and release of hydrogen remain a bottleneck for realizing the upcoming hydrogen economy. Hydrogen storage based on liquid-phase chemical hydrogen storage materials is one of the most promising hydrogen storage techniques, which offers considerable potential for large-scale practical applications for its excellent safety, great convenience, and high efficiency. Recently, nanopore-supported metal nanocatalysts have stood out remarkably in boosting the field of liquid-phase chemical hydrogen storage. Herein, the latest research progress in catalytic hydrogen production is summarized, from liquid-phase chemical hydrogen storage materials, such as formic acid, ammonia borane, hydrous hydrazine, and sodium borohydride, by using metal nanocatalysts confined within diverse nanoporous materials, such as metal-organic frameworks, porous carbons, zeolites, mesoporous silica, and porous organic polymers. The state-of-the-art synthetic strategies and advanced characterizations for these nanocatalysts, as well as their catalytic performances in hydrogen generation, are presented. The limitation of each hydrogen storage system and future challenges and opportunities on this subject are also discussed. References in related fields are provided, and more developments and applications to achieve hydrogen energy will be inspired.

The function of metal-organic frameworks in the application of MOF-based composites

In the last two decades, metal-organic frameworks (MOFs), as a class of porous crystalline materials formed by organic linkers coordinated-metal ions, have attracted increasing attention due to their unique structures and wide applications. Compared to single components, various well-designed MOF-based composites combining MOFs with other functional materials, such as nanoparticles, quantum dots, natural enzymes and polymers with remarkably enhanced or novel properties have recently been reported. To efficiently and directionally synthesize high-performance MOF-based composites for specific applications, it is vital to understand the structural-functional relationships and role of MOFs. In this review, preparation methods of MOF-based composites are first summarized and then the relationship between the structure and performance is determined. The functions of MOFs in practical use are classified and discussed through various examples, which may help chemists to understand the structural-functional relationship in MOF-based composites from a new perspective.

Regulating Hydrogenation Chemoselectivity of α,β-Unsaturated Aldehydes by Combination of Transfer and Catalytic Hydrogenation

Two hydrogenation mechanisms, transfer and catalytic hydrogenation, were combined to achieve higher regulation of hydrogenation chemoselectivity of cinnamyl aldehydes. Transfer hydrogenation with ammonia borane exclusively reduced C=O bonds to get cinnamyl alcohol, and Pt-loaded metal-organic layers efficiently hydrogenated C=C bonds to synthesize phenyl propanol with almost 100 % conversion rate. The hydrogenation could be performed under mild conditions without external high-pressure hydrogen and was applicable to various α,β-unsaturated aldehydes.

Zeolitic imidazolate frameworks for use in electrochemical and optical chemical sensing and biosensing: a review

This review (with 145 refs.) summarizes the progress that has been made in the use of zeolitic imidazolate frameworks in chemical sensing and biosensing. Zeolitic imidazolate frameworks (ZIFs) are a type of porous material with zeolite topological structure that combine the advantages of zeolite and traditional metal-organic frameworks. Owing to the structural flexibility of ZIFs, their pore sizes and surface functionalization can be reasonably designed. Following an introduction into the field of metal-organic frameworks and the zeolitic imidazolate framework (ZIF) subclass, a first large section covers the various kinds and properties of ZIFs. The next large section covers electrochemical sensors and assays (with subsections on methods for gases, electrochemiluminescence, electrochemical biomolecules). This is followed by main sections on ZIF-based colorimetric and luminescent sensors, with subsections on sensors for metal ions and anions, for gases, and for organic biomolecules. The last section covers SERS-based assays. Several tables are presented that give an overview on the wealth of methods and materials. A concluding section summarizes the current status, addresses current challenges, and gives an outlook on potential future trends. Graphical abstract In recent years, ZIFs and their composites have been widely used as probes in chemical sensing, and these probes have shown great advantages over other materials. This review describes the current progress on ZIFs toward electrochemical, luminescence, colorimetric, and SERS-based sensing applications, highlighting the different strategies for designing ZIFs and their composites and potential challenges in this field.

One-Pot Fabrication of Pd Nanoparticles@Covalent-Organic-Framework-Derived Hollow Polyamine Spheres as a Synergistic Catalyst for Tandem Catalysis

Facile fabrication of nanocatalysts consisting of metal nanoparticles (NPs) anchored on a functional support is highly desirable, yet remains challenging. Covalent organic frameworks (COFs) provide an emerging materials platform for structural control and functional design. Here, a facile one-pot in situ reduction approach is demonstrated for the encapsulation of small Pd NPs into the shell of COF-derived hollow polyamine spheres (Pd@H-PPA). In the one-pot synthetic process, the nucleation and growth of Pd NPs in the cavities of the porous shell take place simultaneously with the reduction of imine linkages to secondary amine groups. Pd@H-PPA shows a significantly enhanced catalytic activity and recyclability in the tandem dehydrogenation of ammonia borane and selective hydrogenation of nitroarenes through an adsorption-activation-reaction mechanism. The strong interactions of the secondary amine linkage with borane and nitroarene molecules afford a positive synergy to promote the catalytic reaction. Moreover, the hierarchical structure of Pd@H-PPA allows the accessibility of active Pd NPs to reactants.

Noble-metal-free nanocatalysts for hydrogen generation from boron- and nitrogen-based hydrides

We focus on the recent advances in non-noble metal catalyst design, synthesis and applications in dehydrogenation of chemical hydrides (e.g. NaBH₄, NH₃BH₃, NH₃, N₂H₄, N₂H₄BH₃) due to their high hydrogen contents and CO-free H₂ production.

Isolated Iron Single-Atomic Site-Catalyzed Chemoselective Transfer Hydrogenation of Nitroarenes to Arylamines

Selective hydrogenation of nitroarenes to arylamines is a great challenge because of the complicated mechanism and competitive hydrogenation of reducible functional groups. Isolated single-atomic site catalysts, benefitting from their uniform and well-defined catalytic sites, are promising to achieve high activity and selectivity. Herein, we prepared an isolated iron single-atomic catalyst supported on ordered mesoporous nitrogen-doped carbon (Fe₁/N-C). The as-prepared Fe₁/N-C showed excellent activity and tolerance for functional groups in the transfer hydrogenation of nitroarenes over hydrazine hydrate. Density functional theory calculations revealed that the single atomically dispersed, partially positively charged Fe atoms and the lowered energy barrier collectively contribute to the superior hydrogenation performances for nitroarenes.

Encapsulation of Metal Nanoparticles within Metal-Organic Frameworks for the Reduction of Nitro Compounds

Nitro group reduction is a reaction of a considerable importance for the preparation of bulk chemicals and in organic synthesis. There are reports in the literature showing that incorporation of metal nanoparticles (MNPs) inside metal–organic frameworks (MOFs) is a suitable strategy to develop catalysts for these reactions. Some of the examples reported in the literature have shown activity data confirming the superior performance of MNPs inside MOFs. In the present review, the existing literature reports have been grouped depending on whether these MNPs correspond to a single metal or they are alloys. The final section of this review summarizes the state of the art and forecasts future developments in the field.

A nano-reactor based on PtNi@metal-organic framework composites loaded with polyoxometalates for hydrogenation-esterification tandem reactions

Tandem catalysis (i.e., a process in which a desirable product is synthesized by a one-step process consisting of sequential reactions) has attracted intensive attention owing to its sustainable green and atom-economical characteristics. In this process, the utilization of a high-efficiency multifunctional catalyst is key. However, different functional sites integrated within the catalyst are required to be rationally designed and precisely engineered to guarantee the synergy between the catalytic reactions. Herein, a novel kind of hydrogenation-esterification tandem catalyst with metal/acid (alloy/polyoxometalates) active sites integrated within the metal-organic frameworks (MOFs) was prepared by a facile self-sacrificial template route. In this tandem catalyst, the MOF cavities served as tandem reactors, the PtNi alloy sites encapsulated within the MOF material acted as hydrogenation sites, and the solid phosphotungstic acid embedded in the MOF cavities provided esterification sites. This well-designed tandem catalyst showed outstanding activity and selectivity towards the one-step synthesis of amino-ester-type anesthetics (e.g., benzocaine) owing to the synergistic catalysis of the metal and acid sites. Clearly, this novel tandem catalyst simplifies the traditional industry process and provides a new method to rationally construct new tandem catalysts.

General Immobilization of Ultrafine Alloyed Nanoparticles within Metal-Organic Frameworks with High Loadings for Advanced Synergetic Catalysis

The development of a general synthesis approach for creating fine alloyed nanoparticles (NPs) in the pores of metal-organic frameworks (MOFs) shows great promise for advanced synergetic catalysis but has not been realized so far. Herein, for the first time we proposed a facile and general strategy to immobilize ultrafine alloyed NPs within the pores of an MOF by the galvanic replacement of transition-metal NPs (e.g., Cu, Co, and Ni) with noble-metal ions (e.g., Pd, Ru, and Pt) under high-intensity ultrasound irradiation. Nine types of bimetallic alloyed NPs of base and noble metals were successfully prepared and immobilized in the pores of MIL-101 as a model host, which showed highly dispersed and well-alloyed properties with average particle sizes ranging from 1.1 to 2.2 nm and high loadings of up to 10.4 wt %. Benefiting from the ultrafine particle size and high dispersity of Cu-Pd NPs and especially the positive synergy between Cu and Pd metals, the optimized Cu-Pd@MIL-101 exhibited an extremely high activity for the homocoupling reaction of phenylacetylene under unprecedented base- and additive-free conditions and room temperature, affording at least 19 times higher yield (98%) of 1,4-diphenylbuta-1,3-diyne than its monometallic counterparts. This general strategy for preparing various MOF-immobilized alloyed NPs potentially paves the way for the development of highly active metal catalysts for a variety of reactions.

Three-Shell Cu@Co@Ni Nanoparticles Stabilized with a Metal-Organic Framework for Enhanced Tandem Catalysis

Transition-metal catalysts, particularly featuring a triple-layered core-shell structure, are very promising for practical application; however, reports on their synthesis and catalytic application for a cascade reaction were very rare. In this work, tiny Cu@Co@Ni core-shell nanoparticles (∼3.3 nm) containing Cu core, Co middle shell, and Ni outer shell stabilized by metal-organic framework (MOF) were successfully synthesized to give a quadruple-layered Cu@Co@Ni/MOF at moderate conditions. The catalyst exhibited superior catalytic performance toward the in situ hydrogenation of nitroarenes using the H₂ generated from the hydrolysis of ammonia borane (NH₃BH₃) under mild conditions. Interestingly, the Cu@Co@Ni/MOF also showed excellent catalytic activity toward CO oxidation reaction, which outperforms those of noble-metal catalysts. To our knowledge, this is the first report on transition-metal nanoparticles with a three-layered core-shell structure stabilized by MOF as a cooperative catalyst for cascade reaction and CO oxidation.