Copper Nanoparticles from Copper Aluminum Hydrotalcite: An Efficient Catalyst for Acceptor- and Oxidant-Free Dehydrogenation of Amines and Alcohols

Creation of a Highly Active Small Cu-Based Catalyst Derived from Copper Aluminium Layered Double Hydroxide Supported on α-Al2 O3 for Acceptorless Alcohol Dehydrogenation

A highly dispersed carbonate-intercalated Cu2+ -Al3+ layered double hydroxide (CuAl LDH) was created on an unreactive α-Al2 O3 surface (CuAl LDH@α-Al2 O3 ) via a simple coprecipitation method of Cu2+ and Al3+ under alkaline conditions in the presence of α-Al2 O3 . A highly reducible CuO nanoparticles was generated, accompanied by the formation of CuAl2 O4 on the surface of α-Al2 O3 (CuAlO@α-Al2 O3 ) after calcination at 1073 K in air, as confirmed by powder X-ray diffraction (XRD) and Cu K-edge X-ray absorption near edge structure (XANES). The structural changes during the progressive heating process were monitored by using in-situ temperature-programmed synchrotron XRD (tp-SXRD). The layered structure of CuAl LDH@α-Al2 O3 completely disappeared at 473 K, and CuO or CuAl2 O4 phases began to appear at 823 K or 1023 K, respectively. Our synthesised CuAlO@α-Al2 O3 catalyst was highly active for the acceptorless dehydrogenation of benzylic, aliphatic, or cyclic aliphatic alcohols; the TON based on the amount of Cu increased to 163 from 3.3 of unsupported CuAlO catalyst in 1-phenylethanol dehydrogenation. The results suggested that Cu0 was obtained from the reduction of CuO in the catalyst matrix during the reaction without separate reduction procedure and acted as a catalytically active species.

Fabrication and Characterization of Cu Nanoparticles Dispersed on ZnAl-Layered Double Hydroxide Nanocatalysts for the Oxidation of Cyclohexane

In the chemical industry, designing high-performance catalysts for the oxidation of cyclohexane into value-added products such as cyclohexanol and cyclohexanone (the combination is known as KA oil) is critical. The catalytic activity of copper nanoparticles supported on layered double hydroxide (LDH) for the liquid phase oxidation of cyclohexane was examined in this study. In this work, we have developed Cu nanoparticles supported on layered double hydroxide nanocatalysts, abbreviated as CuNPs@LDH, by the chemical reduction approach. Various physical methods were used to characterize the resulting material, including ICP-AES, XRD, FTIR, SEM, EDX, HRTEM, and BET surface area. The catalytic activity of copper nanoparticles supported on LDH was examined for the liquid phase oxidation of cyclohexane with tert-butyl hydroperoxide. CuNPs@LDH nanocatalysts with an excellent 52.3% conversion of cyclohexane with 97.2% selectivity of KA oil was obtained after 6 h at 353 K. The hot filtration test further indicated that CuNPs@LDH was a heterogeneous catalyst that could be recycled at least six times without suffering a substantial reduction in its catalytic activity.

Catalytic oxidative dehydrogenation of N-heterocycles with nitrogen/phosphorus co-doped porous carbon materials

A metal-free oxidative dehydrogenation of N-heterocycles utilizing a nitrogen/phosphorus co-doped porous carbon (NPCH) catalyst is reported. The optimal material is robust against traditional poisoning agents and shows high antioxidant resistance. It exhibits good catalytic performance for the synthesis of various quinoline, indole, isoquinoline, and quinoxaline ‘on-water’ under air atmosphere. The active sites in the NPCH catalyst are proposed to be phosphorus and nitrogen centers within the porous carbon network.

Green oxidations made easy. Metal-free dehydrogenation of N-heterocycles are possible in using N,P-co-doped porous carbon materials “on” water using air.

Copper–cobalt coordination polymers and catalytic applications on borrowing hydrogen reactions

A porous copper–cobalt polymer was synthesized and achieved applications for the N-alkylation of sulfonamides with alcohols, and carboxamides with alcohols.

Recent advances in the transesterification of β-keto esters

The ability to selectively transesterify β-keto esters is a useful transformation in organic synthesis. The increasing interest in transesterification for the synthesis of compounds of pharmaceutical importance, as well as for biodiesel production, has led to the development of a variety of different approaches. This article aims to summarise recent advances in the transesterifications of β-keto esters. Particular interest has been paid to methodologies with minimal environmental impact.

This review summarises the many advances in the selective transesterification of β-keto esters, with a particular focus on the past 30 years.

Influence of the copper precursor on the catalytic transformation of oleylamine during Cu nanoparticle synthesis

Copper nanoparticles prepared from copper acetylacetonate complex or acetate complex show significant differences in terms of reaction mechanism and further catalytic transformation of oleylamine.

A decennary update on applications of metal nanoparticles (MNPs) in the synthesis of nitrogen- and oxygen-containing heterocyclic scaffolds

Heterocycles have been found to be of much importance as several nitrogen- and oxygen-containing heterocycle compounds exist amongst the various USFDA-approved drugs. Because of the advancement of nanotechnology, nanocatalysis has found abundant applications in the synthesis of heterocyclic compounds. Numerous nanoparticles (NPs) have been utilized for several organic transformations, which led us to make dedicated efforts for the complete coverage of applications of metal nanoparticles (MNPs) in the synthesis of heterocyclic scaffolds reported from 2010 to 2019. Our emphasize during the coverage of catalyzed reactions of the various MNPs such as Ag, Au, Co, Cu, Fe, Ni, Pd, Pt, Rh, Ru, Si, Ti, and Zn has not only been on nanoparticles catalyzed synthetic transformations for the synthesis of heterocyclic scaffolds, but also provide an inherent framework for the reader to select a suitable catalytic system of interest for the synthesis of desired heterocyclic scaffold.

On the importance of the crystalline surface structure on the catalytic activity and stability of tailored unsupported cobalt nanoparticles for the solvent-free acceptor-less alcohol dehydrogenation

Unsupported nanoparticles are now recognized as model catalysts to evaluate the intrinsic activity of metal particles, irrespectively of that of the support. Co nanoparticles with different morphologies, rods, diabolos and cubes have been prepared by the polyol process and tested for the acceptorless catalytic dehydrogenation of alcohols under solvent-free conditions. Rods crystallize with the pure hcp structure, diabolos with a mixture of hcp and fcc phases, while the cubes crystallize in a complex mixture of hcp, fcc and ε-Co phases. All the cobalt particles are found to be highly selective towards the oxidation of a model secondary alcohol, octan-2-ol, into the corresponding ketone while no significant activity is found with octan-1-ol. Our results show the strong influence of particle shape on the activity and catalytic stability of the catalysts: Co nanorods display the highest conversion (85%), selectivity (95%) and recyclability compared to Co diabolos and Co cubes. We correlate the nanorods excellent stability with a strong binding of carboxylate ligands on their {1 1 2¯ 0} facets, preserving their crystalline superficial structure, as evidenced by phase modulation infrared reflection absorption spectroscopy.

Supported Cobalt Catalysts for Acceptorless Alcohol Dehydrogenation

The acceptor-less dehydrogenation of 2-octanol was tested over cobalt supported on Al₂ O₃ , C, ZnO, ZrO₂ and various TiO₂ substrates. The catalysts were characterized by ICP, XRD and TGA-H₂ . For Co/TiO₂ P25, the effects of passivation, aging (storage at room temperature), and in situ activation under H₂ were investigated. The catalysts must be tested shortly after synthesis in order to prevent deactivation. Cobalt supported on TiO₂ P25 was the most active and 69 % yield of 2-octanone was obtained, using decane as a solvent. Selectivities for 2-octanone were observed in the range of 90 % to 99.9 %. Small amounts of C16 compounds were also formed due to aldol condensation/dehydration reactions. The catalysts exhibited higher conversion in the dehydrogenation of secondary alcohols (65-69 %), in comparison to primary alcohols (2-10 %). The dehydrogenation of 1,2-octanediol led to 1-hydroxy-2-octanone, with a selectivity of 90 % and 69 % for Co/TiO₂ P25 and Co/TiO₂ P90, respectively.

Importance of the decoration in shaped cobalt nanoparticles in the acceptor-less secondary alcohol dehydrogenation

Ligands matter for shaped decorated Co nanoparticles, at the frontier between homogeneous and heterogeneous catalysis.

A combined experimental and computational study of NHC-promoted desulfonylation of tosylated aldimines

NHC-catalyzed mild desulfonylation of tosylated aldimines provides efficient access to aryl nitriles with broad substrate scope. DFT calculations demonstrate that the reaction undergoes multiple stepwise processes.

Scandium(III) Triflate Catalyzed Direct Synthesis of N-Unprotected Ketimines

N-Unprotected ketimines are useful substrates and intermediates for synthesizing valuable nitrogen-containing compounds, but their potential applicability is limited by the available synthetic methods. To address this issue, we report a scandium(III) triflate catalyzed direct synthesis of N-unprotected ketimines. Using commercially available reagents and Lewis acid catalysts, ketones were directly transformed into the corresponding N-unprotected ketimines in high yields with broad functional group tolerance, even in multigram scales. The reactions were readily applicable for one-pot synthesis of important compounds such as a glycine Schiff base without isolation of N-unprotected ketimine intermediates. Preliminary mechanistic studies to clarify the reaction mechanism are also described.

Metal free biomimetic deaminative direct C-C coupling of unprotected primary amines with active methylene compounds

An unprecedented direct C-C coupling reaction of unprotected primary amines with active methylene compounds is reported. The reaction involves a biomimetic deamination of amines which was achieved under conditions free of metallic reagents and strong oxidizing agents. A wide range of primary amines was reacted with different active methylene compounds to provide structurally diverse trisubstituted alkenes and dihydropyridines. A kinetic study revealed an activation barrier of 10.1 kcal mol^-1 for the conversion of a key intermediate of the reaction.

Salt-Mediated Au-Cu Nanofoam and Au-Cu-Pd Porous Macrobeam Synthesis

Multi-metallic and alloy nanomaterials enable a broad range of catalytic applications with high surface area and tuning reaction specificity through the variation of metal composition. The ability to synthesize these materials as three-dimensional nanostructures enables control of surface area, pore size and mass transfer properties, electronic conductivity, and ultimately device integration. Au-Cu nanomaterials offer tunable optical and catalytic properties at reduced material cost. The synthesis methods for Au-Cu nanostructures, especially three-dimensional materials, has been limited. Here, we present Au-Cu nanofoams and Au-Cu-Pd macrobeams synthesized from salt precursors. Salt precursors formed from the precipitation of square planar ions resulted in short- and long-range ordered crystals that, when reduced in solution, form nanofoams or macrobeams that can be dried or pressed into freestanding monoliths or films. Metal composition was determined with X-ray diffraction and energy dispersive X-ray spectroscopy. Nitrogen gas adsorption indicated an Au-Cu nanofoam specific surface area of 19.4 m²/g. Specific capacitance determined with electrochemical impedance spectroscopy was 46.0 F/g and 52.5 F/g for Au-Cu nanofoams and Au-Cu-Pd macrobeams, respectively. The use of salt precursors is envisioned as a synthesis route to numerous metal and multi-metallic nanostructures for catalytic, energy storage, and sensing applications.

Synthesis of a molecularly defined single-active site heterogeneous catalyst for selective oxidation of N-heterocycles

Generally, a homogeneous catalyst exhibits good activity and defined active sites but it is difficult to recycle. Meanwhile, a heterogeneous catalyst can easily be reused but its active site is difficult to reveal. It is interesting to bridge the gap between homogeneous and heterogeneous catalysis via controllable construction of a heterogeneous catalyst containing defined active sites. Here, we report that a molecularly defined, single-active site heterogeneous catalyst has been designed and prepared via the oxidative polymerization of maleimide derivatives. These polymaleimide derivatives can be active catalysts for the selective oxidation of heterocyclic compounds to quinoline and indole via the recycling of -C=O and -C-OH groups, which was confirmed by tracing the reaction with GC-MS using maleimide as the catalyst and by FT-IR analysis with polymaleimide as the catalyst. These results might promote the development of heterogeneous catalysts with molecularly defined single active sites exhibiting a comparable activity to homogeneous catalysts.

The recent development of efficient Earth-abundant transition-metal nanocatalysts

Whereas noble metal compounds have long been central in catalysis, Earth-abundant metal-based catalysts have in the same time remained undeveloped. Yet the efficacy of Earth-abundant metal catalysts was already shown at the very beginning of the 20th century with the Fe-catalyzed Haber-Bosch process of ammonia synthesis and later in the Fischer-Tropsch reaction. Nanoscience has revolutionized the world of catalysis since it was observed that very small Au nanoparticles (NPs) and other noble metal NPs are extraordinarily efficient. Therefore the development of Earth-abundant metals NPs is more recent, but it has appeared necessary due to their "greenness". This review highlights catalysis by NPs of Earth-abundant transition metals that include Mn, Fe, Co, Ni, Cu, early transition metals (Ti, V, Cr, Zr, Nb and W) and their nanocomposites with emphasis on basic principles and literature reported during the last 5 years. A very large spectrum of catalytic reactions has been successfully disclosed, and catalysis has been examined for each metal starting with zero-valent metal NPs followed by oxides and other nanocomposites. The last section highlights the catalytic activities of bi- and trimetallic NPs. Indeed this later family is very promising and simultaneously benefits from increased stability, efficiency and selectivity, compared to monometallic NPs, due to synergistic substrate activation.

Acceptorless dehydrogenation of alcohols using Cu–Fe catalysts prepared from Cu–Fe layered double hydroxides as precursors

Cu–Fe catalysts prepared from Cu–Fe layered double hydroxides (LDHs) exhibited high activity for dehydrogenation of alcohols under oxidant- and base-free conditions.

Dendrimer-Stabilized Metal Nanoparticles as Efficient Catalysts for Reversible Dehydrogenation/Hydrogenation of N-Heterocycles

Nanoparticles (Pd, Pt, Rh) stabilized by G4OH PAMAM dendrimers and supported in SBA-15 (MNPs/SBA-15 with M = Pd, Pt, Rh) were efficiently used as catalysts in the acceptorless dehydrogenation of tetrahydroquinoline/indoline derivatives in toluene (release of H₂) at 130 °C. These catalysts are air stable, very active, robust, and recyclable during the process. The reverse hydrogenation reaction of quinoline derivatives (H₂ storage) was also optimized and successfully performed in the presence of the same catalysts in toluene at 60 °C under only 1 atm of hydrogen gas. Such catalysts may be essential for the adoption of organic hydrogen-storage materials as an alternative to petroleum-derived fuels. Hot filtration test confirmed that the reaction follows a heterogeneous pathway. Moreover, PdNPs/SBA-15 was an excellent catalyst for the direct arylation at the C-2 position (via C-H activation) of indole in water in the presense of a hypervalent iodine oxidant. Thus, a one-pot dehydrogenation/direct arylation cascade reaction between indoline and an arylated agent was efficaciously performed in water, demonstrating the potential of the system to catalyze tandem heterogeneous/homogeneous processes by choice of the appropriate oxidant/reductant.

Concerted Functions of Surface Acid-Base Pairs and Supported Copper Catalysts for Dehydrogenative Synthesis of Esters from Primary Alcohols

Dehydrogenative synthesis of esters from primary alcohols proceeded efficiently over a ZrO₂-supported copper catalyst. A variety of esters were obtained from primary alcohols as well as diols in good to high yields. The key to the dehydrogenative synthesis of esters is the concerted effect of the acid-base pairs on ZrO₂ and metallic copper.