Atom‐by‐Atom Resolution of Structure–Function Relations over Low‐Nuclearity Metal Catalysts

Controlling the structure sensitivity of catalyzed reactions over metals is central to developing atom‐efficient chemical processes. Approaching the minimum ensemble size, the properties enter a non‐scalable regime in which each atom counts. Almost all trends in this ultra‐small frontier derive from surface science approaches using model systems, because of both synthetic and analytical challenges. Exploiting the unique coordination chemistry of carbon nitride, we discriminate through experiments and simulations the interplay between the geometry, electronic structure, and reactivity of palladium atoms, dimers, and trimers. Catalytic tests evidence application‐dependent requirements of the active ensemble. In the semi‐hydrogenation of alkynes, the nuclearity primarily impacts activity, whereas the selectivity and stability are affected in Suzuki coupling. This powerful approach will provide practical insights into the design of heterogeneous catalysts comprising well‐defined numbers of atoms.     

A Stable Single‐Site Palladium Catalyst for Hydrogenations

We report the preparation and hydrogenation performance of a single‐site palladium catalyst that was obtained by the anchoring of Pd atoms into the cavities of mesoporous polymeric graphitic carbon nitride. The characterization of the material confirmed the atomic dispersion of the palladium phase throughout the sample. The catalyst was applied for three‐phase hydrogenations of alkynes and nitroarenes in a continuous‐flow reactor, showing its high activity and product selectivity in comparison with benchmark catalysts based on nanoparticles. Density functional theory calculations provided fundamental insights into the material structure and attributed the high catalyst activity and selectivity to the facile hydrogen activation and hydrocarbon adsorption on atomically dispersed Pd sites.     

Polymer-supported 2,2′-dipyridylmethane: catalytic activity of transition metal complexes in hydrogenations and oligomerizations

The palladium(II) acetate complex of the chelating ligand 2,2′-dipyridylmethane supported on polystyrene-2% divinylbenzene is an efficient catalyst for hydrogenation of alkenes and alkynes. Cyclopentadiene can be reduced with high selectivity to cyclopentene, but no selectivity is observed for the non-conjugated diene 1,5-cyclooctadiene. In the hydrogenation of 3-methylcyclohex-2-en-1-ol only small amounts of ketone are formed as a by-product, in contrast to the reaction catalysed by palladium on charcoal. Nickel(II) complexes of the same ligand catalyze the trimerization of butadiene to 1,5,9-cyclododecatrienes.     

A General One-Pot Methodology for the Preparation of Mono- and Bimetallic Nanoparticles Supported on Carbon Nanotubes: Application in the Semi-hydrogenation of Alkynes and Acetylene

A facile and straightforward methodology for the preparation of monometallic (copper and palladium) and bimetallic nanocatalysts (NiCu and PdCu) stabilized by a N-heterocyclic carbene ligand is reported. Both colloidal and supported nanoparticles (NPs) on carbon nanotubes (CNTs) were prepared in a one-pot synthesis with outstanding control on their size, morphology and composition. These catalysts were evaluated in the selective hydrogenation of alkynes and alkynols. PdCu/CNTs revealed an efficient catalytic system providing high selectivity in the hydrogenation of terminal and internal alkynes. Moreover, this catalyst was tested in the semi-hydrogenation of acetylene in industrially relevant acetylene/ethylene-rich model gas feeds and showed excellent stability even after 40 h of reaction.     

Kinetics of the hydrogenation of 2-chloro-4-nitroaniline over skeletal nickel and supported palladium catalysts in an aqueous solution of 2-propanol

The kinetics of the liquid-phase hydrogenation of 2-chloro-4-nitroaniline in an aqueous solution of 2-propanol over skeletal nickel and supported palladium catalysts is studied. The selectivity of the reaction with respect to 2-chloro-1,4-phenylenediamine is determined. It is found that samples of supported palladium catalysts differ with respect to the amount of the active component and the nature of the support. Some of their structural characteristics are provided.     

The promoting effect of a dicyanamide based ionic liquid in the selective hydrogenation of citral

Conventional supported heterogeneous palladium catalysts in combination with a dicyanamide based ionic liquid are highly active with excellent selectivity in enabling the one-pot synthesis of citronellal through citral hydrogenation.     

Catalytic Selective Dihydrosilylation of Internal Alkynes Enabled by Rare‐Earth Ate Complex

Hydrosilylation of alkynes generally yield vinylsilanes, which are inert to the further hydrosilylation because of the steric effects. Reported here is the first successful dihydrosilylation of aryl‐ and silyl‐substituted internal alkynes enabled by a rare‐earth ate complex to yield geminal bis‐ and tris(silanes), respectively. The lanthanum bis(amido) ate complex supported by an ene‐diamido ligand proved to be the ideal catalyst for this unprecedented transformation, while the same series of yttrium and samarium alkyl and samarium bis(amido) ate complexes exhibited poor activity and selectivity, indicating significant effects of the ionic size and ate structure of the rare‐earth catalysts.     

A Highly Efficient Dimeric Manganese‐Catalyzed Selective Hydroarylation of Internal Alkynes

We have developed a general and site‐predictable manganese‐catalyzed hydroarylation of internal alkynes in the presence of water, under an air atmosphere without the involvement of ligand. The unique catalytic feature of this reaction is highlighted by comparison with other widely used transition metal catalysts including palladium, rhodium, nickel, or copper. The simple operation, high efficiency and excellent functional group compatibility make this protocol practical for more than 90 structurally diverse internal alkynes, overcoming the influence of both electronic and steric effect of alkynes. Its exclusive regio‐ and chemoselectivity originates from the unique reactivity of the manganese‐based catalyst towards an inherent double controlled strategy of sterically hindered propargyl alcohols without the installing of external directing groups. Its synthetic robustness and practicality have been illustrated by the concise synthesis of bervastatin, a hypolipidemic drug, and late‐stage modification of complex alkynes with precise regioselectivity.     

Metal Nanoparticles Supported on Biomass‐Derived Hierarchical Porous Heteroatom‐Doped Carbon from Bamboo Shoots: Design,Synthesis and Applications

Heteroatom‐doped porous carbon derived from biomass have recently received increasing attention due to their unique properties such as high electrical conductivity, large specific surface area, high porosity, and easy availability, which are appealing materials for versatile applications in catalysis, energy, separation and adsorption, and life sciences as well. On the basis of our previous work in this field, we summarized in this account our recent progress on design, synthesis of metal (e. g., Pd, Co) nanoparticles supported heteroatom‐doped hierarchical porous carbon material derived from bamboo shoots and their applications for important organic transformations, including chemoselective semihydrogenation of alkynes, hydrosilylation of alkynes, cascade synthesis of benzofurans from terminal alkynes and iodophenols, selective hydrogenation of functionalized nitroarenes to form anilines, imines, and formamides. Finally, the current state and future challenges in this field are discussed. We hope this account could shed light on the rational design of novel non‐noble metal based heterogeneous catalysts derived from biomass for efficient and sustainable organic transformations.     

Multiporous Carbon Encapsulated Ni Nanoparticles Promoting Glycerol Valorisation towards Hydrogenation against Rearrangement†

A dual‐templating method was used to synthesize a series of hierarchical carbon supports containing different proportions of spherical macropores (ca. 200 nm in diameter) and mesoporous channels (ca. 4 nm in diameter). These and some other conventional carbon materials were subsequently impregnated with Ni and tested for the conversion of glycerol. The hierarchical catalysts exhibited a significantly higher conversion (96%) and selectivity (77%) to 1,2‐propanediol, and the specificity selectivity coefficient (6.1) towards 1,2‐propanediol against lactic acid was three times higher than that observed over a conventional Ni/C_micro catalyst (2.1). The enhanced performance of these materials, compared with the Ni nanoparticles supported on conventional carbon supports, was attributed to their high surface areas (> 1110 m²?g^?1) and large pore volumes (ca. 0.4 cm³?g^?1) permitting greater accessibility of substrate and/or intermediates to Ni active sites. Given that the concentration of accessible Ni sites in these materials is higher, a competitive benzilic‐acid‐rearrangement reaction to produce lactic acid was suppressed, leading to an enhanced hydrogenation selectivity to 1,2‐propanediol. This study evidences the potential benefits, which can be established from utilizing hierarchical support materials in the valorization of biomass.     

Characterization of a Polymer-Bound Palladium Acetate Catalyst and Studies on the Selective Hydrogenation of Dienes and Alkynes

Abstract

Palladium(II) acetate has been anchored onto a copolymer support containing pyridyl and carboxyl groups. XPS studies showed the Pd 3d binding energies for the recovered catalyst to be less by 1 eV after being used in hydrogenation studies. However, x-ray studies and a chemical test based on KCN treatment failed to reveal any palladium oxide or palladium metal formation in the recovered catalyst. It is presumed that an acetate ligand is lost during hydrogenation, which could be the reason for the lowering of the palladium 3d binding energies in the recovered catalyst. Results of investigations of the hydrogenation of olefins and selectivity of the catalyst toward the hydrogenation of dienes and alkynes are presented. The loss of palladium due to leaching under the reaction conditions employed was found to be very low (<1%/cycle).     

Encapsulation of Palladium Carbide Subnanometric Species in Zeolite Boosts Highly Selective Semihydrogenation of Alkynes

The selective hydrogenation of alkynes to alkenes is a crucial step in the synthesis of fine chemicals. However, the widely utilized palladium (Pd)-based catalysts often suffer from poor selectivity. In this work, we demonstrate a carbonization-reduction method to create palladium carbide subnanometric species within pure silicate MFI zeolite. The carbon species can modify the electronic and steric characteristics of Pd species by forming the predominant Pd−C₄ structure and, meanwhile, facilitate the desorption of alkenes by forming the Si−O−C structure with zeolite framework, as validated by the state-of-the-art characterizations and theoretical calculations. The developed catalyst shows superior performance in the selective hydrogenation of alkynes over mild conditions (298 K, 2 bar H₂), with 99 % selectivity to styrene at a complete conversion of phenylacetylene. In contrast, the zeolite-encapsulated carbon-free Pd catalyst and the commercial Lindlar catalyst show only 15 % and 14 % selectivity to styrene, respectively, under identical reaction conditions. The zeolite-confined Pd-carbide subnanoclusters promise their superior properties in semihydrogenation of alkynes.     

Dual Palladium/Scandium Catalysis toward Rotationally Hindered C3‐Naphthylated Indoles from β‐Alkynyl Ketones and o‐Alkynyl Anilines

A new dual palladium/scandium catalysis starting from β‐alkynyl ketones and o‐alkynyl anilines is reported for the first time, leading to the atom‐economic synthesis of rotationally hindered C3‐naphthylated indoles in moderate to good yields and high regioselectivity. This method can tolerate normal air conditions, and features the use of palladium/scandium cooperative catalysts without any ligand, facile double annulation involving various internal alkynes, and good functional group tolerance.     

Recyclable Porous Polymer‐Supported Copper Catalysts for Glaser and Huisgen 1,3‐Diolar Cycloaddition Reactions

A family of polymer‐attached phenanthrolines was prepared from solvothermal copolymerization of divinylbenzene with N‐(1,10‐phenanthroline‐5‐yl)acrylamide in different ratios. The polymer‐supported copper catalysts were obtained through typical impregnation with copper(II) salts. The polymers and supported copper catalysts have been characterized by N₂ adsortion, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric analysis (TG); they exhibit a high surface area, hierarchical porosity, large pore volume, and high thermal and chemical stabilities. The copper catalyst has proved to be highly active for Glaser homocoupling of alkynes and Huisgen 1,3‐diolar cycloaddition of alkynes with benzyl azide under mild conditions at low catalyst loading. The heterogeneous copper catalyst is more active than commonly used homogeneous and nonporous polystyrene‐supported copper catalysts. In particular, the catalyst is easily recovered and can be recycled at least ten times without any obvious loss in catalytic activity. Metal leaching was prevented due to the strong binding ability of phenanthroline and products were not contaminated with copper, as determined by ICP analysis.     

ZnAl‐Hydrotalcite‐Supported Au25 Nanoclusters as Precatalysts for Chemoselective Hydrogenation of 3‐Nitrostyrene

Chemoselective hydrogenation of 3‐nitrostyrene to 3‐vinylaniline is quite challenging because of competitive activation of the vinyl group and the nitro group over most supported precious‐metal catalysts. A precatalyst comprised of thiolated Au₂₅ nanoclusters supported on ZnAl‐hydrotalcite yielded gold catalysts of a well‐controlled size (ca. 2.0 nm)—even after calcination at 500 °C. The catalyst showed excellent selectivity (>98 %) with respect to 3‐vinylaniline, and complete conversion of 3‐nitrostyrene over broad reaction duration and temperature windows. This result is unprecedented for gold catalysts. In contrast to traditional catalysts, the gold catalyst is inert with respect to the vinyl group and is only active with regard to the nitro group, as demonstrated by the results of the control experiments and attenuated total reflection infrared spectra. The findings may extend to design of gold catalysts with excellent chemoselectivity for use in the synthesis of fine chemicals.     

Effect of the Structure of Carbon Nanofibers on the State of an Active Component and on the Catalytic Properties of Pd/C Catalysts in the Selective Hydrogenation of 1,3-Butadiene

The state of highly dispersed palladium particles supported on filamentous carbon was studied using high-resolution electron microscopy, XPS, and X-ray diffraction analysis. Three types of filamentous carbon were used, in which the basal planes of graphite were arranged along, across, and at an angle to the nanofiber axis. The amount of supported palladium was 0.25–5.8 wt %. The structure of the carbon support was found to affect the properties of the active component. Highly dispersed palladium particles exhibited the strongest interaction with a carbon surface formed by the butt ends of graphite (002) layers. This interaction resulted in electron transfer from the metal to the support and in the stabilization of palladium in the most dispersed state. A change in the properties of palladium particles caused a change in the catalytic properties of Pd/C catalysts in the reaction of selective 1,3-butadiene hydrogenation to butenes. The strong interaction of Pd²⁺ with the butt ends of graphite resulted in the stabilization of palladium in an ionic state. An increase in the fraction of Pd²⁺ in the catalysts was responsible for a decrease in both the overall activity and selectivity of Pd/C catalysts in the reaction of 1,3-butadiene hydrogenation to butenes.     

Designed Precursor for the Controlled Synthesis of Highly Active Atomic and Sub‐nanometric Platinum Catalysts on Mesoporous Silica

The development of new methods to synthesize nanometric metal catalysts has always been an important and prerequisite step in advanced catalysis. Herein, we design a stable nitrogen ligated Pt complex for the straightforward synthesis by carbonization of uniformly sized atomic and sub‐nanometric Pt catalysts supported on mesoporous silica. During the carbonization of the Pt precursor into active Pt species, the nitrogen‐containing ligand directed the decomposition in a controlled fashion to maintain uniform sizes of the Pt species. The nitrogen ligand had a key role to stabilize the single Pt atoms on a weak anchoring support like silica. The Pt catalysts exhibited remarkable activities in the hydrogenation of common organic functional groups with turnover frequencies higher than in previous studies. By a simple post‐synthetic treatment, we could selectively remove the Pt nanoparticles to obtain a mixture of single atoms and nanoclusters, extending the applicability of the present method.     

Changes in the course of reaction and regeneration of a Pd-Ag/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst for the selective hydrogenation of acetylene

The samples of Pd-Ag/Al₂O₃ catalysts for the selective hydrogenation of acetylene impurities in an ethane-ethylene mixture were studied using the IR spectroscopy of adsorbed CO, X-ray diffraction analysis, and thermogravimetry. In the course of reaction and regeneration, the total concentration of the supported metals (Pd and Ag) changed only slightly. The degree of accessibility of silver atoms to CO adsorption and the amount of these atoms in the nearest environment of palladium atoms decreased to result in an increase in the selectivity of acetylene hydrogenation to ethane. The decrease in the accessibility of silver was due to a change in the phase composition of the alumina support as a result of its rehydration. It was hypothesized that the resulting aluminum hydroxide with the boehmite morphology is a source of the strongest Lewis acid sites, which catalyze oligomerization processes on the catalyst surface.     

Role of polyaniline morphology in Pd particles dispersion. Hydrogenation of alkynes in the presence of Pd-polyaniline catalysts

Two polyaniline (PANI) samples of various molecular masses were used for the preparation of palladium catalysts (with 2 mass % of Pd). The physicochemical features of starting polyanilines were found to substantially affect the size and extent of palladium nanoparticles aggregation. Strongly aggregated large palladium particles appeared in the PANI sample of more compact morphology (PANI-H), higher crystallinity and lower specific surface area. Pd nanoparticles of a definitively smaller size were formed in the more amorphous PANI sample of looser morphology (PANI-L) and the extent of particles aggregation was markedly lower. The catalytic properties of Pd/PANI samples were studied in a liquid phase hydrogenation of unsaturated triple bond (C≡C) in alkynes reactants, phenylacetylene, and cyclohexylacetylene. The 2 mass % Pd/PANI-L catalyst prepared using polymer of less compact texture exhibited much higher activity in both reactions. In the presence of the 2 mass % Pd/PANI-L catalyst, alkene products were formed with a high selectivity (approximately 90 %) attained at the almost complete conversion of alkynes. This highly selective hydrogenation of the C≡C to the C=C bond was related to the presence of an electroactive polymer, PANI, in close proximity with Pd active sites. Polyaniline could have a role in a steric effect as well as in a modification of adsorptive properties of Pd centres.     

Microenvironment engineering of supported metal nanoparticles for chemoselective hydrogenation

Selective hydrogenation with supported metal catalysts widely used in the production of fine chemicals and pharmaceuticals often faces a trade-off between activity and selectivity, mainly due to the inability to adjust one factor of the active sites without affecting other factors. In order to solve this bottleneck problem, the modulation of the microenvironment of active sites has attracted more and more attention, inspired by the collaborative catalytic mode of enzymes. In this perspective, we aim to summarize recent advances in the regulation of the microenvironment surrounding supported metal nanoparticles (NPs) using porous materials enriched with organic functional groups. Insights on how the microenvironment induces the enrichment, oriented adsorption and activation of substrates through non-covalent interaction and thus determines the hydrogenation activity and selectivity will be particularly discussed. Finally, a brief summary will be provided, and challenges together with a perspective in microenvironment engineering will be proposed.

Insights on microenvironment engineering for metal nanoparticles using porous materials enriched with organic groups and how it determines the hydrogenation performance through non-covalent interaction are highlighted.