Acceptor-free dehydrogenation of secondary alcohols by heterogeneous cooperative catalysis between Ni nanoparticles and acid–base sites of alumina supports

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.

Reactivation of Ni-TiO2 catalysts in hydrogen flow and in supercritical 2-propanol-Comparative study by electron spin resonance in situ

Highly dispersed Ni-TiO2 catalyst has been studied in the process of preparation and under catalytic transfer hydrogenation reaction conditions in supercritical 2-propanol (250°C, 70 bar) using electron spin resonance in situ. Electron spin resonance in situ has been used to study the process of the catalyst passivation and subsequent reduction of the oxide layer in the gas flow. Reduction of the NiO layer on the surface of passivated Ni nanoparticles has been detected in supercritical 2-propanol, which is in agreement with kinetic modeling data. It has been found that the reduction of the nickel oxide layer in supercritical 2-propanol occurs at a lower temperature compared with the reduction in hydrogen flow, according to in situ electron spin resonance study.

A copper nitride nanocube catalyst for highly efficient hydroboration of alkynes

The hydroboration of alkynes with bis(pinacolato)diboron is a useful method for the synthesis of vinyl boronate esters, which are essential intermediates in organic syntheses. Copper catalysts have been used extensively in these reactions. However, previously reported Cu-catalyst systems inevitably require additives and elevated temperatures. Herein, we report, for the first time, a simple and efficient hydroboration of alkynes under additive-free and mild reaction conditions (i.e., at a temperature of 30 °C) using a copper nitride nanocube (Cu₃N NC) catalyst. A wide range of alkynes can be transformed into their corresponding boronate esters. Cu₃N NCs are also applicable in the hydroboration of alkynes with tetrahydroxydiboron to synthesize vinyl boronic acids. Moreover, the Cu₃N NCs were easily separated by simple filtration and could be reused several times without any loss of their original activity. Hence, these highly active and reusable Cu₃N NC catalysts offer an environmentally friendly route for the efficient production of vinyl boronates.

Acceptorless dehydrogenation of alcohols to carboxylic acids by palladium nanoparticles supported on NiO: delving into metal-support cooperation in catalysis

In this work, we have developed a simple NiO-supported Pd nanocatalyst (Pd@NiO) for oxidant-free dehydrogenative oxidation of primary alcohols to carboxylic acids along with hydrogen gas as a byproduct. The catalyst has been characterized by techniques like XRD, HRTEM, SEM-EDX, XPS and ICP-AES. The nanostructured Pd@NiO material showed excellent dehydrogenative oxidation activity and outperformed the activity of free NiO or Pd nanoparticles supported on silica/carbon as a catalyst, which could be attributed to synergistic effect of Pd and NiO. A diverse range of aromatic and aliphatic primary alcohols could be efficiently converted to their corresponding carboxylates in high yields with a catalyst loading as low as 0.08 mol%. Notably, highly challenging biomass derived heterocyclic alcohols such as furfuryl alcohol and piperonyl alcohol can also be efficiently converted to their corresponding acids. Moreover, our catalyst can convert benzyl alcohol to benzoic acid on a gram scale with 89% yield. Interestingly, the H₂ gas liberated in the reaction can also be used as a substrate for the hydrogenation of 3a to 4a in 65% yield. The nanostructured catalyst is highly reusable and no significant decrease in activity was observed after six reaction cycles. A kinetic study revealed that the reaction followed first-order kinetics with a rate constant of k = 1.47 × 10^-4 s^-1, under optimized conditions. The extent of reactivity of different functionalities towards dehydrogenation was also investigated using a Hammett plot showing good linearity.

Synthesis of N-Heterocycles via Oxidant-Free Dehydrocyclization of Alcohols Using Heterogeneous Catalysts

N-Heterocycles, such as pyrroles, pyrimidines, quinazolines, and quinoxalines, are important building blocks for organic chemistry and the fine-chemical industry. For their synthesis, catalytic borrowing hydrogen and acceptorless dehydrogenative coupling reactions of alcohols as sustainable reagents have received significant attention in recent years. To overcome the problems of product separation and catalyst reusability, several metal-based heterogeneous catalysts have been reported to achieve these transformations with good yields and selectivity. In this Minireview, we summarize recent developments using both noble and non-noble metal-based heterogeneous catalysts to synthesize N-heterocycles from alcohols and N-nucleophiles via acceptorless dehydrogenation or borrowing hydrogen methodologies. Furthermore, this Minireview introduces strategies for the preparation and functionalization of the corresponding heterogeneous catalysts, discusses the reaction mechanisms and the roles of metal electronic states, and the influence of support Lewis acid-base properties on these reactions.

Identification of active catalysts for the acceptorless dehydrogenation of alcohols to carbonyls

Acceptorless dehydrogenation into carbonyls and molecular hydrogen is an attractive strategy to valorize (biobased) alcohols. Using 2-octanol dehydrogenation as benchmark reaction in a continuous reactor, a library of metal-supported catalysts is tested to validate the predictive level of catalytic activity for combined DFT and micro-kinetic modeling. Based on a series of transition metals, scaling relations are determined as a function of two descriptors, i.e. the surface binding energies of atomic carbon and oxygen. Then, a volcano-shape relation based on both descriptors is derived, paving the way to further optimization of active catalysts. Evaluation of 294 diluted alloys but also a series of carbides and nitrides with the volcano map identified 12 promising candidates with potentially improved activity for alcohol dehydrogenation, which provides useful guidance for experimental catalyst design. Further screening identifies β-Mo2N and γ-Mo2N exposing mostly (001) and (100) facets as potential candidates for alcohol dehydrogenation.

High performance of ultrasonic-assisted synthesis of two spherical polymers for enantioselective catalysis

Chiral polymers have aroused great attention in among chiral supramolecular materials based on their features. Herein, for the first time, the synthesis of chiral polymeric composites (CMNPs/1,4-Zbtb & 1,3-Zbtb) have been reported with entrapment through three strategies: ultrasonic irradiation, solvothermal, and mechanical stirring. According to the obtained results, it is found that ultrasound-assisted synthesis can be considered as an inexpensive and efficient method than the others, from the point ofviewof energy and time consuming. In this strategy, encapsulation of chiral magnetic nanoparticles (CMNPs) by using tetrazole-based polymers (Zbtbs) happens, in-situly. These chiral sphere-like inorganic-organic polymers can be considered as core and shell composites with catalytic activity due to their acidic (semi unsaturated Zn: open metal sites) and basic (abundant basic nitrogens) centers. In these structures, the unprecedented chirality induction can happen from the core to shell by non-covalent interaction, easily. They could catalyze symmetric oxidation and asymmetric henry condensation to give chiral β-nitroalkanol. Circular dichroism and chiral gas chromatography were used to characterize the produced enantiomers. These chiral polymeric materials can be considered as unique acid-base bifunctional catalysts with efficient properties such as high stability, enantiomeric excess, enantioselectivity to the main product, and protecting from CMNPs leaching.

Light-Driven Alcohol Splitting by Heterogeneous Photocatalysis: Recent Advances, Mechanism and Prospects

Splitting of alcohols into hydrogen and corresponding carbonyl compounds, also called acceptorless alcohol dehydrogenation, is of great significance for both synthetic chemistry and hydrogen production. Light-Driven Alcohol Splitting (LDAS) by heterogeneous photocatalysis is a promising route to achieve such transformations, and it possesses advantages including high selectivity of the carbonyl compounds, extremely mild reaction conditions (room temperature and irradiation of visible light) and easy separation of the photocatalysts from the reaction mixtures. Because a variety of alcohols can be derived from biomass, LDAS can also be regarded as one of the most sustainable approaches for hydrogen production. In this Review, recent advances in the LDAS catalyzed by the heterogeneous photocatalysts are summarized, focusing on the mechanistic insights for the LDAS and aspects that influence the performance of the photocatalysts from viewpoints of metallic co-catalysts, semiconductors, and metal/semiconductor interfaces. In addition, challenges and prospects have been discussed in order to present a complete picture of this field.

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.

Spinel copper–iron-oxide magnetic nanoparticles with cooperative Cu(i) and Cu(ii) sites for enhancing the catalytic transformation of 1,2-propanediol to lactic acid under anaerobic conditions

The synergy between Cu(i) and Cu(ii) sites in spinel CuFeO_x magnetic nanoparticles contributes to the significant enhancement in catalytic 1,2-propanediol transformation into lactic acid.

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.

Acceptorless Alcohol Dehydrogenation Catalysed by Pd/C

Although the selective oxidation of alcohols to carbonyl compounds is a critical reaction, it is often plagued by several challenges related to sustainability. Here, the continuous, acceptorless dehydrogenation of alcohols to carbonyl compounds over heterogeneous catalysts was demonstrated, in the absence of oxidants, bases or acceptor molecules. In addition to improving selectivity and atom efficiency, the absence of an acceptor resulted in the co-production of molecular H₂ , a clean energy source, and permitted dehydrogenation to proceed at >98 % selectivity at turnover frequency values amongst the highest in the literature. Moreover, excellent durability was observed during continuous operation over 48 h, reaching space-time yields of 0.683 g_(product)  mL^-1  h^-1 , better than the state of the art by over two orders of magnitude. Alongside these breakthroughs, the basic kinetic parameters of the reaction were also determined, allowing some of the elementary reaction steps to be identified.

Oxidant free conversion of alcohols to nitriles over Ni-based catalysts

Ni-Based catalysts converting various primary alcohols to nitriles in high yields under oxidant-free, low temperature conditions.

Heterogeneous Catalytic Synthesis of 2-Methylbenzimidazole from 2-Nitroaniline and Ethanol Over Mg Modified Cu-Pd/γ-Al2O3

The direct synthesis of benzimidazoles from 2-nitroaniline and ethanol over Cu-Pd/γ-Al2O3 catalysts has the advantages of requiring easily available starting materials, having high efficiency, and a simple procedure. The modification by Mg of the Cu-Pd/γ-Al2O3 catalyst could improve the catalytic activity significantly. The addition of Mg to the Cu-Pd/γ-Al2O3 catalyst could maintain and promote the formation of CuPd alloy active sites. Meanwhile, the basicity of the support was enhanced appropriately by Mg, which generated more basic sites (Al-Oδ−) to accelerate the dehydrogenation of alcohol and increased the rate of the whole coupled reaction. The 2-nitroaniline was completely converted over Cu-Pd/(Mg)γ-Al2O3 after reacting for six hours, and the yield of 2-methylbenzimidazole was 98.8%. The results of this work provide a simple method to develop a more efficient catalyst for the “alcohol-dehydrogenation, hydrogen transfer and hydrogenation” coupled reaction system.

A Two-Step Oxidative Cleavage of 1,2-Diol Fatty Esters into Acids or Nitriles by a Dehydrogenation-Oxidative Cleavage Sequence

Dehydrogenative oxidation of vicinal alcohols catalyzed by a commercially 64 wt.% Ni/SiO₂ catalyst leads to the formation of α-hydroxyketone. This first step was developed without additional solvent according to two protocols: "under vacuum" or "with an olefin scavenger". The synthesis of ketols was carried out with good conversions and selectivities. The recyclability of the supported nickel was also studied. Acyloin was then cleaved with oxidative reagent "formic acid/hydrogen peroxide", which is cheap and can be used on a large scale for industrial oxidation processes. The global yield of this sequential system was up to 80 % to pelargonic acid and azelaic acid monomethyl ester without intermediate purification. By treating the acyloin intermediate with hydroxylamine, nitriles were obtained with a good selectivity.

Unsupported shaped cobalt nanoparticles as efficient and recyclable catalysts for the solvent-free acceptorless dehydrogenation of alcohols

Unsupported cobalt nanoparticles are active and chemoselective catalysts for the solvent-free acceptorless dehydrogenation of alcohols.

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.

A silver NP-dispersed water extract of fly ash as a green and efficient medium for oxidant-free dehydrogenation of benzyl alcohols

Herein, a green, efficient, and new catalytic system for dehydrogenative oxidation of benzyl alcohols using Ag nanoparticles (NPs) dispersed in water extract of fly ash (WEFA) has been developed. Various characterization techniques were performed to authenticate the formation of Ag@WEFA. The as-prepared Ag NPs (10–20 nm) were found to be dispersed in WEFA, as indicated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images. With Ag@WEFA, a variety of substituted benzyl alcohols were efficiently converted to carbonyl compounds in high yields. All the reactions were deliberately carried out without using any ligand or hazardous organic solvent. This catalytic system involving WEFA is a genuinely new concept. It is, therefore, expected to attract attention from researchers working in the areas of sustainable chemistry.

A new catalytic system for dehydrogenative oxidation of benzyl alcohols using Ag nanoparticles dispersed in WEFA is developed.