Selective Heterogeneous Catalysts Research Articles

Heterogeneous catalytic aerobic oxidation of primary amines to amides using unsupported nanoporous palladium catalyst

An efficient and highly selective heterogeneous catalyst system for aerobic oxidation of primary amines was developed using unsupported palladium nanopores (PdNPore). A variety of aromatic, aliphatic, and heterocyclic primary amines were oxidized to the corresponding amides in satisfactory to excellent yields using ambient air as an oxidant. No palladium was leached from PdNPore during the oxidation reaction. The catalyst could be readily recovered and reused without a decrease in catalytic activity. The kinetic studies and control experiments demonstrated that nitrile acts as an intermediate, which undergo hydration to afford amide as the final product.

Scaling-Up Microwave-Assisted Synthesis of Highly Defective Pd@UiO-66-NH2 Catalysts for Selective Olefin Hydrogenation under Ambient Conditions.

The need to develop green and cost-effective industrial catalytic processes has led to growing interest in preparing more robust, efficient, and selective heterogeneous catalysts at a large scale. In this regard, microwave-assisted synthesis is a fast method for fabricating heterogeneous catalysts (including metal oxides, zeolites, metal-organic frameworks, and supported metal nanoparticles) with enhanced catalytic properties, enabling synthesis scale-up. Herein, the synthesis of nanosized UiO-66-NH2 was optimized via a microwave-assisted hydrothermal method to obtain defective matrices essential for the stabilization of metal nanoparticles, promoting catalytically active sites for hydrogenation reactions (760 kg·m-3·day-1 space time yield, STY). Then, this protocol was scaled up in a multimodal microwave reactor, reaching 86% yield (ca. 1 g, 1450 kg·m-3·day-1 STY) in only 30 min. Afterward, Pd nanoparticles were formed in situ decorating the nanoMOF by an effective and fast microwave-assisted hydrothermal method, resulting in the formation of Pd@UiO-66-NH2 composites. Both the localization and oxidation states of Pd nanoparticles (NPs) in the MOF were achieved using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and X-ray photoelectron spectroscopy (XPS), respectively. The optimal composite, loaded with 1.7 wt % Pd, exhibited an extraordinary catalytic activity (>95% yield, 100% selectivity) under mild conditions (1 bar H2, 25 °C, 1 h reaction time), not only in the selective hydrogenation of a variety of single alkenes (1-hexene, 1-octene, 1-tridecene, cyclohexene, and tetraphenyl ethylene) but also in the conversion of a complex mixture of alkenes (i.e., 1-hexene, 1-tridecene, and anethole). The results showed a powerful interaction and synergy between the active phase (Pd NPs) and the catalytic porous scaffold (UiO-66-NH2), which are essential for the selectivity and recyclability.

Chiral crystalline organic salts as supports of Pd nanoparticles forming selective heterogeneous catalysts for hydrogenation of nitroarenes

A novel catalyst based on Pd nanoparticles encapsulated in a chiral crystalline organic salt matrix is prepared from tetrasodium tetrakis(4-sulfonatophenyl)methane and tetrakis[4-(S)-prolinamidophenyl]methane in water at room temperature under hydrogen atmosphere. The hydrogenation of p-nitrobenzaldehyde over this catalyst stalls at the stage of p-aminobenzyl alcohol, whereas Pd/C brings the reduction further to p-toluidine.

КАТАЛИЗ ГИДРИРОВАНИЯ АРОМАТИЧЕСКИХ СОЕДИНЕНИЙ: ИСТОРИЯ И СОВРЕМЕННОЕ СОСТОЯНИЕ. ЧАСТЬ 2. ГИДРИРОВАНИЕ ПОЛИЯДЕРНЫХ АРОМАТИЧЕСКИХ УГЛЕВОДОРОДОВ, АЗОТ- И КИСЛОРОДЗАМЕЩЕННЫХ АРЕНОВ

The review is devoted to hydrogenation processes of aromatic substrates differing in reactivity using catalysts of different activity. The global trend in this area is the search for more active and selective heterogeneous catalysts for hydrogenation of the aromatic ring, both for industrial processes and in laboratory practice. An analysis of literature data on the hydrogenation of polynuclear aromatic hydrocarbons, phenol, aniline and their derivatives, as well as arenes substituted with a functional group, is presented. Various types of catalysts for the hydrogenation of an aromatic ring, the patterns of the process depending on the structure of the substrate and reaction conditions, the nature of the active phase and the catalyst support are considered.

Wet-impregnated niobosilicate catalysts for glycerol conversion into solketal

Novel mesoporous niobosilicate materials with a homogeneous dispersion of niobium species were synthesized via impregnation of (NH4)3[Nb(O2)2(edtaO2)]·H2O·H2O2 on different hydrophilic surfaces of porous siliceous supports: extra small silica nanoparticles (XS-SiO2) and silica nanotubes (NTs-SiO2) bearing three different Si/Nb molar ratios of 74, 37 and 18. The Nb-silica catalysts obtained were calcined at 600 °C and then fully characterized by different physico-chemical techniques. The niobium precursor was found to be successfully loaded on the two silica support surfaces without noticeable modification of the silicate structural integrity. The Nb-containing catalysts were applied as active and selective heterogeneous catalysts for acid-catalyzed condensation of glycerol with acetone yielding solketal (2,2-dimethyl-1,3-dioxolane-4-methanol). The catalytic performance of these materials is ascribed to the enhanced accessibility of their active sites given by their morphology and to the suitable combination of acid sites. Moreover, no leaching of active sites was evidenced and the catalyst reusability studies indicated that the Nb-XS-74 and Nb-NTs-74 catalysts were successfully recyclable and highly stable in the acetalization of glycerol. The robustness and stability of the mesoporous Nb-silicate materials were also supported via characterization of the spent catalysts after the fifth recycling.

A cage-on-MOF strategy to coordinatively functionalize mesoporous MOFs for manipulating selectivity in adsorption and catalysis

Functionalizing porous materials with capping agents generates hybrid materials with enhanced properties, while the challenge is how to improve the selectivity and maintain the porosity of the parent framework. Herein, we developed a “Cage-on-MOF” strategy to tune the recognition and catalytic properties of MOFs without impairing their porosity. Two types of porous coordination cages (PCCs) of opposite charges containing secondary binding groups were developed to coordinatively functionalize two distinct porous MOFs, namely MOF@PCC nanocomposites. We demonstrated that the surface-capped PCCs can act as “modulators” to effectively tune the surface charge, stability, and adsorption behavior of different host MOF particles. More importantly, the MOF@PCCs can serve as selective heterogeneous catalysts for condensation reactions to achieve reversed product selectivity and excellent recyclability. This work sets the foundation for using molecular cages as porous surface-capping agents to functionalize and manipulate another porous material, without affecting the intrinsic properties of the parent framework.

Ce-Zr UiO-66 MOF as recyclable heterogeneous catalyst for selective N-methylation

Metal-organic frameworks (MOFs) are proven heterogeneous recoverable catalysts in a number of organic transformations. However, their role in the reaction of amines with dialkyl carbonates has only been partially investigated. In this paper, the catalytic potential of Ce-Zr UiO-66 MOF towards the N-alkylation of secondary amines with dimethyl carbonate has been investigated. The desired MOF catalyst was prepared by the solvothermal method and its morphology, texture, porosity, functionality, and crystallinity were examined before its catalytic investigation. The MOF consisted of sphere-like particles with a surface area of 272 m2/g and an average pore size of 3.4 nm. Under the optimized conditions, the aromatic and heterocyclic amines yielded the N-methylated derivatives as the sole or the major product in the presence of Ce-Zr UiO-66 MOF. The maximum methylation percentage achieved was 100%. The reaction followed the pseudo-first-order rate kinetics and the heterogeneous bimetallic Ce-Zr UiO-66 MOF was reused four times without any significant activity loss. Thus, the Ce-Zr UiO-66 MOF serves as an efficient, selective, and reusable heterogeneous catalyst in N-methylation reactions.

High Catalytic Activity, Recyclable and Magnetically Separable Nanocatalyst Fe3O4@SiO2-Schiff base-Pd(II) for Synthesis of 12H-Benzo[5,6]Chromeno[2,3-b]Pyridine-10-Carbonitriles and Evaluation of Antibacterial Activity

In this work, an effective and selective heterogeneous catalyst was produced by immobilization of palladium Schiff base-complexes on ferrite magnetite nanoparticles (MNP). The catalysts Fe3O4@SiO2-Schiff base-Pd(II) were synthesized using Fe3O4@SiO2 core–shell nanoparticles and functionalized with 3-chloropropyltrimethoxysilane (CPTMS) as Fe3O4@SiO2@Cl. The Schiff base ligand was synthesized by reaction of 2-hydroxy-1-naphthaldehyde with 4-aminophenol to produce of 1-(((4-hydroxyphenyl)imino)methyl)naphthalen-2-ol, then palladium acetate was added to produce of Schiff-base-Pd(II) complex. Finally, Fe3O4@SiO2-Schiff base-Pd(II) was synthesized by the reaction of Fe3O4@SiO2@Cl core–shell nanoparticles with Schiff-base-Pd(II). The catalyst was characterized by several techniques, such as FT-IR, TEM, EDX, XRD, TGA and VSM. An efficient tandem reaction approach is described to prepare novel 12H-benzo[5,6]chromeno[2,3-b]pyridine-10-carbonitrileusing 1 mol% catalyst in methanol. The described one-pot three-component reaction is characterized by short reaction time, high product yield, mild reaction conditions, simple workup procedure, simple purification and catalyst can be easily recovered and reused in at least seven sequential cycles without considerable leaching and loss of reactivity

Single-Site Carbon-Supported Metal-Oxo Complexes in Heterogeneous Catalysis: Structure, Reactivity, and Mechanism.

When early transition metal complexes are molecularly grafted onto catalyst supports, well-defined, surface-bound species are created, which are highly active and selective single-site heterogeneous catalysts (SSHCs) for diverse chemical transformations. In this minireview, we analyze and summarize a less conventional type of SSHC in which molybdenum dioxo species are grafted onto unusual carbon-unsaturated scaffolds, such as activated carbon, reduced graphene oxide, and carbon nanohorns. The choice of earth-abundant, low-toxicity, versatile metal constituents, and various carbon supports illustrates "catalyst by design" principles and yields insights into new catalytic systems of both academic and technological interest. Here, we summarize experimental and computational investigations of the bonding, electronic structure, reaction scope, and mechanistic pathways of these unusual catalysts.

Development of Silicon Nanowire Array–Metal Hybrid Catalysts for Batch and Flow Organic Reactions

AbstractThe development of highly efficient and reusable supported metal catalysts is important for academic and industrial synthetic organic chemistry; however, their widespread application remains a challenge because supported Pd, Rh, and Pt catalysts are expensive. To overcome these problems, we have developed novel, highly stable, reusable, and selective heterogeneous catalysts consisting of silicon nanowire arrays (SiNAs) and metal nanoparticle composites. Metal nanoparticles on SiNA have been applied as heterogeneous catalysts in the Mizoroki–Heck reaction, C–H arylation, hydrosilylation, hydrogenation, reductive alkylation of amines, and hydrogenative decarboxylation of fatty acids. The catalysts used in this study showed high catalytic activity in batch and microflow conditions. Their structural investigation using X-ray Photoelectron Spectroscopy (XPS) suggests that strong metallic bonding (alloy/agglomeration) between the metal and silicon (metal silicide bond formation) is key to the high catalyst stability.1 Introduction2 Development of Silicon Nanowire Array (SiNA) Hybrid Catalysts and Silicon Nanostructure (SiNS) Hybrid Catalysts3 Application of SiNA-Pd to Organic Synthesis4 SINA-Supported Mono- and Bimetallic Nanoparticles for Hydrogenation Reactions5 Application of SiNA-Pd to Microflow Reductive Alkylation Reactions6 Application of SiNA-Rh to Hydrogenative Decarboxylation Reactions using Microwave Irradiation7 Conclusions

Regulating the crystal phase of Pd/Nb2O5 for vanillin selective HDO at room temperature

Constructing heterogeneous catalysts with high efficiency and stability to achieve biomass HDO under comparatively mild conditions is an arduous challenge. Crystal phase of metal oxides supports could affect the geometric and electronic properties, causing the variation of catalytic performance. Herein, a series of Pd/Nb2O5 catalysts of pseudohexagonal crystal phase (Nb2O5-TT), orthorhombic phase (Nb2O5-T) and monoclinic phase (Nb2O5-H) are prepared through regulating the annealing temperature. Impressively, Pd/Nb2O5-T catalyst showed amazing catalytic performance of 100% conversion and 98.9% selectivity of 2-methoxy-4-methylphenol (MMP) at room temperature (25℃) within 30 min, superior than Pd/Nb2O5-TT, Pd/Nb2O5-H and other Pd-based catalysts that had been reported. Besides, the Pd/Nb2O5-T perform robust stability and maintain its high activity after ten times recovery experiment. This catalytic system is also suitable for different kinds of aldehydes compounds at room temperature, exhibiting the high activity and selectivity towards deoxygenated products. Based on the characterizations and experimental validations, it has confirmed that the strong metal-support interaction (SMSI) effect generates via electron transfer between Pd species and Nb2O5-T support, where in situ reduction of PdO occurred during the reaction, generating active interfacial Pd-O-Nb sites and accelerating the breaking of C-O bonds. The insights of this work provide a guiding direction for the future design of highly selective heterogeneous catalysts by crystal phase modulation engineering.

Eggshell powder as an efficient recyclable catalyst generates H2O2 prompted radicals for selective oxidative mineralization of crystal violet dye at room temperature

Eggshell powder (ESP) was explored as a selective heterogeneous catalyst for oxidative degradation of crystal violet (CV) dye in aqueous media in presence of H2O2 at room temperature. Natural but throw-away material ESP as a carbonate (CO32−) source facilitated the formation of carbonate anion radicals in presence of H2O2. Carbonate anion radicals (CO3•−), though a weaker oxidant than ∙OH radical, efficiently served as a selective oxidant. Selective degradation of CV dye in aqueous solution was confirmed from the quenching experiments. Synergistic effects of both the radicals (CO3•− and ∙OH) in the dye degradation process was experimentally verified. At a dose of 10 g/L of ESP, 91% removal of CV (initial concentration: 10 mg/L) was achieved in 60 min. The reaction kinetics followed first order rate model. Intermediate products formed during CV degradation and the extent of mineralization were confirmed from MALDI-TOF-MS and TOC analysis, respectively. The turnover number (TON) and turnover frequency (TOF) were obtained as 2.45 × 10−6 mol per g and 6.81 × 10−10 mol per g per second. A promising reusability was achieved up to seven cycles. The ESP catalyst used in this study was characterized by SEM, FTIR, XRD, XPS, BET surface area analysis, UV–visible DRS, XRF, and pHZPC analyses. The present work offers a novel facile but selective route for CV degradation and opens up newer possibilities for eggshell waste utilization as catalyst.

Reprint of : Recent strategies for synthesis of metallosilicate zeolites

Incorporation of catalytically active metallic sites into the framework of silica-based zeolites is highly efficient for design of active, selective, and stable heterogeneous catalysts, but there are still many challenges for this kind of preparation such as the presence of aggregated metallic sites due to the alkaline media in the synthesis, relatively low concentration of these metallic sites in the zeolite frameworks, and difficulty for morphological control of metallosilicate zeolites. To overcome these issues, it has been developed new strategies for synthesis of metallosilicate zeolites such as introduction of additives to adjust crystallization rate, addition of chelating agents to form heteroatom complexes, growth of zeolite crystals along with targeted directions, zeolite transformation, and heteroatom replacements. In this review, the recent progresses for syntheses of metallosilicate zeolites are concisely summarized. In addition, the fundamental principles are briefly discussed for the formation of the metallosilicate zeolites. This review should be helpful for design and preparation of the heteroatom-substituted silica-based zeolites as highly efficient heterogeneous catalysts.

Palladium Nanoparticles Entrapped In a Hydrogen Bonded Crystalline Organic Salt Matrix as a Selective Heterogeneous Reduction Catalyst

Hydrogen bonded crystalline organic salt matrix encapsulates Pd nanoparticles and makes them selective non-pyrophoric catalysts towards reduction of carbon – carbon triple bonds, inhibiting simultaneous C=O reduction. The comparison of the catalytic performance with Pd/C was carried out to show similar catalytic activity of both catalysts towards C=C bonds.

Integrated Capture and Conversion of CO2 to Methanol in a Post‐Combustion Capture Solvent: Heterogeneous Catalysts for Selective CN Bond Cleavage

AbstractAn efficient and selective heterogeneous catalyst is identified for the condensed‐phase hydrogenation of captured CO2 in the presence of an advanced water‐lean post‐combustion capture solvent, (N‐(2‐EthoxyEthyl)‐3‐MorpholinoPropan‐1‐Amine), 2‐EEMPA. The catalysts commonly used for gas‐phase CO2 hydrogenation (e.g., Cu/Zn/Al2O3) cause deactivation of amine promoters via N‐methylation by CO cleavage of formamide intermediates. A heterogeneous catalyst system that suppresses N‐methylation of amine solvents is identified, demonstrating how Pt, supported by reducible metal oxides CeO2 or TiO2, can be selective for CN cleavage to produce methanol. This is the first known demonstration of integrated low‐temperature thermocatalytic capture and conversion of CO2 to methanol in an economically viable CO2 capture solvent. Technoeconomic analyses performed on the state‐of‐technology suggest that methanol can be produced with a minimum selling price of $4.4/gallon ($1,460/metric ton) when using CO2 captured from a 650 MW natural gas combined cycle plant. Ultimately, a road map of how realistic and achievable improvements to space velocity and methanol selectivity of this integrated process can enable near cost parity to fossil‐derived methanol, with a selling price of ≈$1.4/gal ($470/metric ton), is presented.

Atom-Precise Ag Clusters as Precursors for Selective Bimetallic AgPd Heterogeneous Catalysts

Atom-Precise Ag Clusters as Precursors for Selective Bimetallic AgPd Heterogeneous Catalysts

Metal–Organic Frameworks as Powerful Heterogeneous Catalysts in Advanced Oxidation Processes for Wastewater Treatment

Nowadays, the contamination of wastewater by organic persistent pollutants is a reality. These pollutants are difficult to remove from wastewater with conventional techniques; hence, it is necessary to go on the hunt for new, innovative and environmentally sustainable ones. In this context, advanced oxidation processes have attracted great attention and have developed rapidly in recent years as promising technologies. The cornerstone of advanced oxidation processes is the selection of heterogeneous catalysts. In this sense, the possibility of using metal–organic frameworks as catalysts has been opened up given their countless physical–chemical characteristics, which can overcome several disadvantages of traditional catalysts. Thus, this review provides a brief review of recent progress in the research and practical application of metal–organic frameworks to advanced oxidation processes, with a special emphasis on the potential of Fe-based metal–organic frameworks to reduce the pollutants present in wastewater or to render them harmless. To do that, the work starts with a brief overview of the different types and pathways of synthesis. Moreover, the mechanisms of the generation of radicals, as well as their action on the organic pollutants and stability, are analysed. Finally, the challenges of this technology to open up new avenues of wastewater treatment in the future are sketched out.

Potassium catalysts supported on γ-Al2O3 to produce biodiesel


 Biodiesel production is carried out through homogeneous catalysis that forms complex reaction mixtures and has high operating costs. The aim of this work is to synthesize basic, active and selective heterogeneous catalysts for the transesterification reaction with vegetable oils employing g-alumina as support and potassium as active species. The catalysts were characterized by X-ray diffraction and infrared spectroscopy. The amount of catalyst used in the reaction was scanned in a range of 1-10% w, obtaining a maximum conversion of 85% at 55 °C with 6.5% w/w of catalyst with respect to the reagent load and with a methanol-oil molar ratio of 6: 1, reaching equilibrium conversion at six hours. The catalysts were tested at 40, 45, 50 and 55 °C in the transesterification reaction using refined sunflower oil and anhydrous methanol as reagents in a 6: 1 ratio. The 40% K catalyst obtained the best conversion at 55 °C with 85% and showed a selectivity of 45%.

CO2 methanation boosted by support-size-dependent strong metal-support interaction and B–O–Ti component

Strong metal-support interaction (SMSI) has a great impact on the activity and selectivity of heterogeneous catalysts, which was usually adjusted by changing reduction temperature or processing catalyst in different atmosphere. However, few researches concentrate on modulating SMSI through regulating the structure of the support. Herein, we show how changing the surface environment of the anatase TiO2 (B–TiO2) can be used to modulate the SMSI. The moderate TiOx overlayer makes the Ni metal highly dispersed on the high specific surface area of support, resulting in a substantially enhanced CO2 methanation rate. Besides, a novel phenomenon was observed that boron dopants promote the formation of the B–O–Ti interface site, enhancing the catalytic performance of CO2 hydrogenation. DFT calculations confirm that the B–O–Ti structure facilitates the activation of CO2 and further hydrogenation to methane.

Measuring and directing charge transfer in heterogenous catalysts

Precise control of charge transfer between catalyst nanoparticles and supports presents a unique opportunity to enhance the stability, activity, and selectivity of heterogeneous catalysts. While charge transfer is tunable using the atomic structure and chemistry of the catalyst-support interface, direct experimental evidence is missing for three-dimensional catalyst nanoparticles, primarily due to the lack of a high-resolution method that can probe and correlate both the charge distribution and atomic structure of catalyst/support interfaces in these structures. We demonstrate a robust scanning transmission electron microscopy (STEM) method that simultaneously visualizes the atomic-scale structure and sub-nanometer-scale charge distribution in heterogeneous catalysts using a model Au-catalyst/SrTiO3-support system. Using this method, we further reveal the atomic-scale mechanisms responsible for the highly active perimeter sites and demonstrate that the charge transfer behavior can be readily controlled using post-synthesis treatments. This methodology provides a blueprint for better understanding the role of charge transfer in catalyst stability and performance and facilitates the future development of highly active advanced catalysts.