Showing papers on "Dehydrogenation published in 2012"

Coking- and Sintering-Resistant Palladium Catalysts Achieved Through Atomic Layer Deposition

Abstract: We showed that alumina (Al(2)O(3)) overcoating of supported metal nanoparticles (NPs) effectively reduced deactivation by coking and sintering in high-temperature applications of heterogeneous catalysts We overcoated palladium NPs with 45 layers of alumina through an atomic layer deposition (ALD) process that alternated exposures of the catalysts to trimethylaluminum and water at 200°C When these catalysts were used for 1 hour in oxidative dehydrogenation of ethane to ethylene at 650°C, they were found by thermogravimetric analysis to contain less than 6% of the coke formed on the uncoated catalysts Scanning transmission electron microscopy showed no visible morphology changes after reaction at 675°C for 28 hours The yield of ethylene was improved on all ALD Al(2)O(3) overcoated Pd catalysts

Efficient Subnanometric Gold-Catalyzed Hydrogen Generation via Formic Acid Decomposition under Ambient Conditions

Abstract: Formic acid (FA) has tremendous potential as a safe and convenient source of hydrogen for sustainable chemical synthesis and renewable energy storage, but controlled and efficient dehydrogenation of FA by a robust solid catalyst under ambient conditions constitutes a major challenge. Here, we report that a previously unappreciated combination of subnanometric gold and an acid-tolerant oxide support facilitates the liberation of CO-free H2 from FA. Applying an ultradispersed gold catalyst comprising TEM-invisible gold subnanoclusters deposited on zirconia to a FA-amine mixture affords turnover frequencies (TOFs) up to 1590 per hour and a turnover number of more than 118 400 at 50 °C. The reaction was accelerated at higher temperatures, but even at room temperature, a significant H2 evolution (TOFs up to 252 h–1 after 20 min) can still be obtained. Preliminary mechanistic studies suggest that the reaction is unimolecular in nature and proceeds via a unique amine-assisted formate decomposition mechanism on A...

From Esters to Alcohols and Back with Ruthenium and Osmium Catalysts

Abstract: There and back again: hydrogenation of esters and the reverse reaction of dehydrogenative coupling of alcohols are efficiently catalyzed by dimeric complexes of Ru and Os under neutral conditions. The Os dimer is an outstanding catalyst for the hydrogenation of alkenoates and triglycerides, and allows production of fatty alcohols from olive oil. This complex converts ethanol into ethyl acetate and hydrogen under reflux.

First-Principles Calculations of Propane Dehydrogenation over PtSn Catalysts

Abstract: Density functional theory calculations have been performed to investigate the effect of Sn on the catalytic activity and selectivity of Pt catalyst in propane dehydrogenation. Five models with different Sn to Pt surface molar ratios are constructed to represent the PtSn surfaces. With the increase of the Sn content, the d-band of Pt is broadened, which gives rise to a downshift in the d-band center on the PtSn surfaces. Consequently, the bonding strength of propyl and propylene on the alloyed surfaces is lowered. With the decomposition of the adsorption energy, the change in the surface deformation energy is predicted to be the dominant factor that determines the variation in the adsorption energy on the surface alloys, while on the bulk alloys the change in the binding energy makes a major contribution. The introduction of Sn lowers the energy barrier for propylene desorption and simultaneously increases the activation energy for propylene dehydrogenation, which has a positive effect on the selectivity t...

Cooperative Catalysis by Iridium Complexes with a Bipyridonate Ligand: Versatile Dehydrogenative Oxidation of Alcohols and Reversible Dehydrogenation–Hydrogenation between 2‐Propanol and Acetone

Abstract: This dehydrogenation works with catalyst loadings as low as 0.002 mol% (turnover numbers up to 47,500).

Surfactant free RGO/Pd nanocomposites as highly active heterogeneous catalysts for the hydrolytic dehydrogenation of ammonia borane for chemical hydrogen storage

Abstract: In this study, monodisperse palladium (Pd) nanoparticles on reduced graphene oxide (RGO) surfaces were successfully prepared by a “wet” and “clean” method in aqueous solution. Without any surface treatment, Pd nanoparticles are firmly attached to the RGO sheets. These RGO/Pd nanocomposites exhibited catalytic activity in hydrogen generation from the hydrolysis of ammonia borane (AB). Their hydrolysis completion time and activation energy were 12.5 min and 51 ± 1 kJ mol−1, respectively, which were comparable to the best Pd-based catalyst reported. The TOF values (mol of H2 × (mol of catalyst × min)−1) of RGO/Pd is 6.25, which appears to be one of the best catalysts reported so far. We also obtained a 11B NMR spectrum to investigate the mechanism of this catalytic hydrolysis process. This simple and straightforward method is of significance for the facile preparation of metal nanocatalysts with high catalytic activity on proper supporting materials.

Synthesis of Ni–Ru Alloy Nanoparticles and Their High Catalytic Activity in Dehydrogenation of Ammonia Borane

Abstract: We report the synthesis and characterization of new Ni(x)Ru(1-x) (x = 0.56-0.74) alloy nanoparticles (NPs) and their catalytic activity for hydrogen release in the ammonia borane hydrolysis process. The alloy NPs were obtained by wet-chemistry method using a rapid lithium triethylborohydride reduction of Ni(2+) and Ru(3+) precursors in oleylamine. The nature of each alloy sample was fully characterized by TEM, XRD, energy dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). We found that the as-prepared Ni-Ru alloy NPs exhibited exceptional catalytic activity for the ammonia borane hydrolysis reaction for hydrogen release. All Ni-Ru alloy NPs, and in particular the Ni(0.74)Ru(0.26) sample, outperform the activity of similar size monometallic Ni and Ru NPs, and even of Ni@Ru core-shell NPs. The hydrolysis activation energy for the Ni(0.74)Ru(0.26) alloy catalyst was measured to be approximately 37 kJ mol(-1). This value is considerably lower than the values measured for monometallic Ni (≈70 kJ mol(-1)) and Ru NPs (≈49 kJ mol(-1)), and for Ni@Ru (≈44 kJ mol(-1)), and is also lower than the values of most noble-metal-containing bimetallic NPs reported in the literature. Thus, a remarkable improvement of catalytic activity of Ru in the dehydrogenation of ammonia borane was obtained by alloying Ru with a Ni, which is a relatively cheap metal.

Iron complex-catalyzed ammonia-borane dehydrogenation. A potential route toward B-N-containing polymer motifs using earth-abundant metal catalysts.

Abstract: Ammonia-borane (NH(3)BH(3), AB) has garnered interest as a hydrogen storage material due to its high weight percent hydrogen content and ease of H(2) release relative to metal hydrides. As a consequence of dehydrogenation, B-N-containing oligomeric/polymeric materials are formed. The ability to control this process and dictate the identity of the generated polymer opens up the possibility of the targeted synthesis of new materials. While precious metals have been used in this regard, the ability to construct such materials using earth-abundant metals such as Fe presents a more economical approach. Four Fe complexes containing amido and phosphine supporting ligands were synthesized, and their reactivity with AB was examined. Three-coordinate Fe(PCy(3))[N(SiMe(3))(2)](2) (1) and four-coordinate Fe(DEPE)[N(SiMe(3))(2)](2) (2) yield a mixture of (NH(2)BH(2))(n) and (NHBH)(n) products with up to 1.7 equiv of H(2) released per AB but cannot be recycled (DEPE = 1,2-bis(diethylphosphino)ethane). In contrast, Fe supported by a bidentate P-N ligand (4) can be used in a second cycle to afford a similar product mixture. Intriguingly, the symmetric analogue of 4 (Fe(N-N)(P-P), 3), only generates (NH(2)BH(2))(n) and does so in minutes at room temperature. This marked difference in reactivity may be the result of the chemistry of Fe(II) vs Fe(0).

DFT Study of the Water–Gas Shift Reaction and Coke Formation on Ni(111) and Ni(211) Surfaces

Abstract: We report for the first time results of a systematic density functional theory (DFT) study of the water-gas shift (WGS) reaction and coke formation pathways on Ni(111) and Ni(211) surfaces, consisting of 21 elementary-like steps and 12 surface species. Bronsted–Evans–Polanyi correlations are proposed for dehydrogenation and C–O bond breaking reactions on flat and stepped surfaces. The DFT results suggest that the flat surface is slightly more active for the WGS reaction, which occurs mainly via the carboxyl pathway with the CO* + OH* ⇌ COOH*+* as the rate determining step. On the stepped surface, beyond the carboxyl pathway, the DFT energetics indicates that a parallel route via formate and formyl intermediates is favored. Ni(111) has a much lower activity for C–O bond breaking, and thus, flat surfaces are less susceptible to deactivation by coke.

In situ synthesis of magnetically recyclable graphene-supported Pd@Co core–shell nanoparticles as efficient catalysts for hydrolytic dehydrogenation of ammonia borane

Abstract: Graphene supported Pd@Co core–shell nanocatalysts with magnetically recyclability were synthesized via the in situ synthesis strategy utilizing the distinction in reduction potentials of the two precursors with appropriate reductant. The as-synthesized catalysts exerted satisfied catalytic activity (916 L mol−1 min−1) and recycle stability for hydrolytic dehydrogenation of ammonia borane.

Preparation of Highly Dispersed Chromium Oxide Catalysts Supported on Mesoporous Silica for the Oxidative Dehydrogenation of Propane Using CO2: Insight into the Nature of Catalytically Active Chromium Sites

Abstract: Highly dispersed chromium oxide catalysts supported on mesoporous silica (Cr-MSU-x) were prepared via a (N0Mn+)I0 pathway with the goal of achieving the high performance oxidative dehydrogenation of propane (ODHP) reaction. The resulting materials exhibited a mesopore structure resembling 3D wormhole-like holes, as characterized by N2 adsorption–desorption isotherms and HR-TEM. Catalytic experimental results revealed that the catalyst with a 0.028 Cr/Si molar ratio showed the highest catalytic activity among the catalysts studied. Two types of chromium species, isolated Cr(VI) and polymeric Cr(VI) species, were observed, as evidenced by H2-temperature-programmed reduction. They were designated as “hard Cr(VI)” and “soft Cr(VI)” sites, respectively. The initial composition of the soft Cr(VI) in the total Cr(VI) is a major determinant factor in the ODHP performance.

General palladium-catalyzed aerobic dehydrogenation to generate double bonds

Abstract: We describe a general dehydrogenation procedure to form α,β-unsaturated aldehydes, ketones, esters and azobenzenes under very mild conditions, requiring catalytic commercial Pd(OAc)2, a catalytic weak inorganic base and air as the sole oxidant. In the presence of a diazafluorenone ligand, this process converts aliphatic aldehydes to α,β-unsaturated aldehydes in an open-flask fashion at ambient pressure and temperature. A broad spectrum of substrates, including aldehydes, ketones, esters, alcohols and hydrazines, were conveniently dehydrogenated under a relatively uniformed protocol. A mechanism involving β-elimination-driven enolization equilibrium shift was proposed.

Development of Dehydrogenation Catalysts and Processes

Abstract: Catalytic dehydrogenation plays an important role in production of light (C3–C4 carbon range), detergent range (C10–C13 carbon range) olefins and for ethylbenzene dehydrogenation to styrene. During the World War II, catalytic dehydrogenation of butane over a chromia–alumina catalyst was practiced for the production of butenes that were dimerized to octenes and hydrogenated to octanes to yield high-octane aviation fuels. The earlier catalyst development employed chromia–alumina catalyst and more recent catalytic developments use platinum or modified platinum catalysts. Dehydrogenation is a highly endothermic process and as such is an equilibrium limited reaction. Thus important aspects in dehydrogenation entail approaching equilibrium or near-equilibrium conversion while minimizing side reactions and coke formation.

Direct aerobic α,β-dehydrogenation of aldehydes and ketones with a Pd(TFA)2/4,5-diazafluorenone catalyst

Abstract: The direct α,β-dehydrogenation of aldehydes and ketones represents an efficient alternative to stepwise methods to prepare enal and enone products. Here, we describe a new Pd(TFA)2/4,5-diazafluorenone dehydrogenation catalyst that overcomes key limitations of previous catalyst systems. The scope includes successful reactivity with pharmaceutically important cyclopentanone and flavanone substrates, as well as acyclic ketones. Preliminary mechanistic studies compare the reactivity of this catalyst to previously reported dehydrogenation catalysts and reveal that cleavage of the α-C–H bond of the ketone is the turnover-limiting step of the catalytic mechanism.

Core--strategy leading to high reversible hydrogen storage capacity for NaBH4.

Abstract: Owing to its high storage capacity (10.8 mass %), sodium borohydride (NaBH4) is a promising hydrogen storage material. However, the temperature for hydrogen release is high (>500 °C), and reversibility of the release is unachievable under reasonable conditions. Herein, we demonstrate the potential of a novel strategy leading to high and stable hydrogen absorption/desorption cycling for NaBH4 under mild pressure conditions (4 MPa). By an antisolvent precipitation method, the size of NaBH4 particles was restricted to a few nanometers (<30 nm), resulting in a decrease of the melting point and an initial release of hydrogen at 400 °C. Further encapsulation of these nanoparticles upon reaction of nickel chloride at their surface allowed the synthesis of a core–shell nanostructure, NaBH4@Ni, and this provided a route for (a) the effective nanoconfinement of the melted NaBH4 core and its dehydrogenation products, and (b) reversibility and fast kinetics owing to short diffusion lengths, the unstable nature of nic...

Simple copper/TEMPO catalyzed aerobic dehydrogenation of benzylic amines and anilines

Abstract: CuBr2 with 2,2,6,6-tetramethylpiperidyl-1-oxy (TEMPO) has been successfully employed for the aerobic oxidation of primary and secondary benzyl amines in aqueous acetonitrile. Such catalytic systems have previously been used extensively in alcohol oxidation reactions. The same catalyst system was also used for oxidative cross-couplings of benzylamines with anilines. The electronic and steric properties of the aniline partner were found to be of crucial importance in determining reactivity or lack thereof. A mechanism for these reactions is proposed based on the data obtained to date. In the absence of benzyl amines, electron-rich anilines were found to undergo dehydrogenative coupling and yields of the resulting azo products could be increased by replacing CuBr2 with CuBr. No ligand (e.g.pyridine) is required for either reaction to proceed and presumably water and acetonitrile solvate the copper-containing intermediates.

Rapid and energy-efficient synthesis of a graphene–CuCo hybrid as a high performance catalyst

Abstract: A general and energy-efficient strategy has been successfully applied for synthesis of a graphene–CuCo nanohybrid, which leads to the highest catalytic activity of Cu-based catalysts up to now toward the dehydrogenation of ammonia borane, and also the excellent activity for electrochemical hydrogen evolution reaction. Moreover, this general method can be easily extended to facile preparation of other graphene–metal systems.

A facile synthesis of nearly monodisperse ruthenium nanoparticles and their catalysis in the hydrolytic dehydrogenation of ammonia borane for chemical hydrogen storage

Abstract: A facile method for the preparation of nearly monodisperse Ru nanoparticles (NPs) and their record catalytic activity in the hydrolytic dehydrogenation of ammonia borane (AB) for chemical hydrogen storage are reported herein. Ru NPs were prepared by a novel protocol comprising the thermal decomposition and concomitant reduction of ruthenium(III) acetylacetonate (Ru(acac)3) in the presence of oleylamine (OAm) and benzylether (BE). In the protocol, OAm served as both a stabilizer and reducing agent and BE used as a solvent. Ru NPs were supported on aluminum oxide nanopowder (Ru@Al2O3) via a simple liquid impregnation method before their use as catalyst for the dehydrogenation of AB in water. Without any special treatment to remove the surfactants, Ru@Al2O3 showed high catalytic activity in the hydrolytic dehydrogenation of AB providing an initial turnover frequency (TOF) of 39.6 mol H2 (mol Ru min)−1. Their catalytic performance was further enhanced dramatically by the acetic acid treatment and the initial TOF value is increased to 83.3 mol H2 (mol Ru min)−1 that is the highest among the reusable Ru catalysts and even higher than Pt-based catalysts tested in the hydrolysis of AB. Additionally, the work reported here includes a wealth of kinetic data to determine the rate law and apparent activation parameters for the catalytic dehydrogenation of AB in water.

Aldehyde binding through reversible C-C coupling with the pincer ligand upon alcohol dehydrogenation by a PNP-ruthenium catalyst.

Abstract: Primary alcohol dehydrogenation by a PNP–Ru(II) catalyst was probed by low-temperature NMR experiments. Facile dehydrogenation occurred at −30 °C, but the resulting aldehydes were not found in solution, as they were trapped by the catalyst through a new mode of metal–ligand cooperation involving Ru–O coordination and an unusual, highly reversible C–C coupling with the PNP pincer ligand.

Formic Acid Dehydrogenation on Ni(111) and Comparison with Pd(111) and Pt(111)

Abstract: Spin-polarized density functional theory calculations have been performed to investigate formic acid dehydrogenation into carbon dioxide and hydrogen (HCO2H → CO2 + H2) on Ni(111). It is found that formic acid prefers the O (O═C) atop adsorption on nickel surface and the H (H–O) atom bridging two neighboring nickel atoms, and formate prefers the bidentate adsorption with O atop on nickel surface. The computed stretching frequencies for deuterated formic acid (DCO2H) and deuterated formate (DCO2) on Ni(111) agree well with the experimentally observed IR spectra. Formic acid dehydrogenation into surface formate and hydrogen atom (HCO2H → HCO2 + H) has barrier of 0.41 eV and is exothermic by 0.35 eV. Formate dehydrogenation into carbon dioxide and hydrogen atom (HCO2 → CO2 + H) has an effective barrier of about 1.0 eV and is the rate-determining step. Our computed adsorption configurations and energetic data for formic acid dehydrogenation on Ni(111) are very close to the reported results for Pt(111), but in...

Acceptorless and Base‐Free Dehydrogenation of Alcohols and Amines using Ruthenium‐Hydride Complexes

Abstract: An efficient, operatively simple, acceptorless, and base-free dehydrogenation of secondary alcohols and nitrogen-containing heterocyclic compounds was achieved by using readily available ruthenium hydride complexes as precatalysts. The complex RuH2(CO)(PPh3)(3) (1) and Shvos complex (2) showed excellent activities for the dehydrogenation of secondary alcohols and nitrogen containing heterocycles. In addition to complexes 1 and 2, the complex RuH2(PPh3)(4) (3) also showed moderate to excellent activity for the acceptorless dehydrogenation of nitrogen-containing heterocyclic compounds. Kinetic studies on the oxidation reaction of 1-phenylethanol using complex 1 were carried out in the presence and the absence of external triphenylphosphine (PPh3). External addition of PPh3 had a negative influence on the rate of the reaction, which suggested that dissociation of PPh3 occurred during the course of the reaction. Hydrogen was evolved from the oxidation reaction of 1-phenylethanol by using 1 mol% of 1 (88%) and 2 (92%), which demonstrated the possible usage of the catalytic systems in hydrogen generation.