Showing papers on "Dehydrogenation published in 2014"

Catalytic dehydrogenation of light alkanes on metals and metal oxides.

Abstract: A study is conducted to demonstrate catalytic dehydrogenation of light alkanes on metals and metal oxides. The study provides a complete overview of the materials used to catalyze this reaction, as dehydrogenation for the production of light olefins has become extremely relevant. Relevant factors, such as the specific nature of the active sites, as well as the effect of support, promoters, and reaction feed on catalyst performance and lifetime, are discussed for each catalytic Material. The study compares different catalysts in terms of the reaction mechanism and deactivation pathways and catalytic performance. The duration of the dehydrogenation step depends on the heat content of the catalyst bed, which decreases rapidly due to the endothermic nature of the reaction. Part of the heat required for the reaction is introduced to the reactors by preheating the reaction feed, additional heat being provided by adjacent reactors that are regenerating the coked catalysts.

Direct, Nonoxidative Conversion of Methane to Ethylene, Aromatics, and Hydrogen

Abstract: The efficient use of natural gas will require catalysts that can activate the first C-H bond of methane while suppressing complete dehydrogenation and avoiding overoxidation. We report that single iron sites embedded in a silica matrix enable direct, nonoxidative conversion of methane, exclusively to ethylene and aromatics. The reaction is initiated by catalytic generation of methyl radicals, followed by a series of gas-phase reactions. The absence of adjacent iron sites prevents catalytic C-C coupling, further oligomerization, and hence, coke deposition. At 1363 kelvin, methane conversion reached a maximum at 48.1% and ethylene selectivity peaked at 48.4%, whereas the total hydrocarbon selectivity exceeded 99%, representing an atom-economical transformation process of methane. The lattice-confined single iron sites delivered stable performance, with no deactivation observed during a 60-hour test.

Critical Literature Review of the Kinetics for the Oxidative Dehydrogenation of Propane over Well-Defined Supported Vanadium Oxide Catalysts

Abstract: Producing propene by the oxidative dehydrogenation of propane (ODH) has become an attractive and feasible route for bridging the propene production-demand gap, either as a complementary route of the existing oil-based processes or as a new alternative from propane separated from natural gas. The industrial application of propane ODH has not succeeded so far due to low propene yields. Therefore, propane ODH has been extensively investigated in recent decades using different catalysts and reaction conditions. Although several important aspects have been discussed in previous reviews (e.g., supported vanadium oxide catalysts, bulk catalysts, productivity toward propene, etc.), other relevant aspects have not been addressed (e.g., support effects, loading effects, vanadia precursor or catalyst synthesis methods, surface impurities, structure–reactivity relationships, etc.). In this review, we endeavor to cover the majority of the publications with an emphasis on the following: (1) catalyst synthesis: to focus...

A Molecular Iron Catalyst for the Acceptorless Dehydrogenation and Hydrogenation of N‑Heterocycles

Abstract: A well-defined iron complex (3) supported by a bis(phosphino)amine pincer ligand efficiently catalyzes both acceptorless dehydrogenation and hydrogenation of N-heterocycles. The products from these reactions are isolated in good yields. Complex 3, the active catalytic species in the dehydrogenation reaction, is independently synthesized and characterized, and its structure is confirmed by X-ray crystallography. A trans-dihydride intermediate (4) is proposed to be involved in the hydrogenation reaction, and its existence is verified by NMR and trapping experiments.

Enhanced Hydrogen Storage Kinetics and Stability by Synergistic Effects of in Situ Formed CeH2.73 and Ni in CeH2.73-MgH2‑Ni Nanocomposites

Abstract: Mg-based materials are promising candidates for high capacity hydrogen storage. However, their poor hydrogenation/dehydrogenation kinetics and high desorption temperature are the main obstacles to their applications. This paper reports a method for in situ formation of cycle stable CeH2.73-MgH2-Ni nanocomposites, from the hydrogenation of as-melt Mg80Ce18Ni2 alloy, with excellent hydrogen storage performance. The nanocomposites demonstrate reversible hydrogen storage capacity of more than 4.0 wt %, at a low desorption temperature with fast kinetics and long cycle life. The temperature for the full hydrogenation/dehydrogenation cycle of the composites is significantly decreased to 505 K, which is about 100 K lower than that for pure Mg. The hydrogen desorption activation energy is 63 ± 3 kJ/mol H2 for the composites, which is significantly lower than those of Mg3Ce alloy and pure Mg (104 ± 7 and 158 ± 2 kJ/mol H2, respectively). X-ray diffraction and transmission electron microscopy have been used to revea...

Lewis Acid-Assisted Formic Acid Dehydrogenation Using a Pincer-Supported Iron Catalyst

Abstract: Formic acid (FA) is an attractive compound for H2 storage. Currently, the most active catalysts for FA dehydrogenation use precious metals. Here, we report a homogeneous iron catalyst that, when used with a Lewis acid (LA) co-catalyst, gives approximately 1,000,000 turnovers for FA dehydrogenation. To date, this is the highest turnover number reported for a first-row transition metal catalyst. Preliminary studies suggest that the LA assists in the decarboxylation of a key iron formate intermediate and can also be used to enhance the reverse process of CO2 hydrogenation.

Well-Defined Iron Catalysts for the Acceptorless Reversible Dehydrogenation-Hydrogenation of Alcohols and Ketones

Abstract: Acceptorless dehydrogenation of alcohols, an important organic transformation, was accomplished with well- defined and inexpensive iron-based catalysts supported by a cooperating PNP pincer ligand. Benzylic and aliphatic secondary alcohols were dehydrogenated to the corresponding ketones in good isolated yields upon release of dihydrogen. Primary alcohols were dehydrogenated to esters and lactones, respectively. Mixed primary/secondary diols were oxidized at the secondary alcohol moiety with good chemoselectivity. The mechanism of the reaction was investigated using both experiment and DFT calculations, and the crucial role of metal−ligand cooperativity in the reaction was elucidated. The iron complexes are also excellent catalysts for the hydrogenation of challenging ketone substrates at ambient temperature under mild H2 pressure, the reverse of secondary alcohol dehydrogenation.

Evaluation of Industrially Applied Heat‐Transfer Fluids as Liquid Organic Hydrogen Carrier Systems

Abstract: Liquid organic hydrogen carrier (LOHC) systems offer a very attractive method for the decentralized storage of renewable excess energy. In this contribution, industrially well-established heat-transfer oils (typically sold under trade names, e.g., Marlotherm) are proposed as a new class of LOHC systems. It is demonstrated that the liquid mixture of isomeric dibenzyltoluenes (m.p. −39 to −34 °C, b.p. 390 °C) can be readily hydrogenated to the corresponding mixture of perhydrogenated analogues by binding 6.2 wt % of H2. The liquid H2-rich form can be stored and transported similarly to diesel fuel. It readily undergoes catalytic dehydrogenation at temperatures above 260 °C, which proves its applicability as a reversible H2 carrier. The presented LOHC systems are further characterized by their excellent technical availability at comparably low prices, full registration of the H2-lean forms, and excellent thermal stabilities.

Mechanistic insight into size-dependent activity and durability in Pt/CNT catalyzed hydrolytic dehydrogenation of ammonia borane.

Abstract: We report a size-dependent activity in Pt/CNT catalyzed hydrolytic dehydrogenation of ammonia borane. Kinetic study and model calculations revealed that Pt(111) facet is the dominating catalytically active surface. There is an optimized Pt particle size of ca. 1.8 nm. Meanwhile, the catalyst durability was found to be highly sensitive to the Pt particle size. The smaller Pt particles appear to have lower durability, which could be related to more significant adsorption of B-containing species on Pt surfaces as well as easier changes in Pt particle size and shape. The insights reported here may pave the way for the rational design of highly active and durable Pt catalysts for hydrogen generation.

Highly Efficient Reversible Hydrogenation of Carbon Dioxide to Formates Using a Ruthenium PNP‐Pincer Catalyst

Abstract: The use of hydrogen as a fuel requires both safe and robust technologies for its storage and transportation. Formic acid (FA) produced by the catalytic hydrogenation of CO2 is recognized as a potential intermediate H2 carrier. Herein, we present the development of a formate-based H2 storage system that employs a Ru PNP-pincer catalyst. The high stability of this system allows cyclic operation with an exceptionally fast loading and liberation of H2. Kinetic studies highlight the crucial role of the base promoter, which controls the rate-determining step in FA dehydrogenation and defines the total H2 capacity attainable from the hydrogenation of CO2. The reported findings show promise for the development of practical technologies that use formic acid as a hydrogen carrier.

A Robust Binary Supramolecular Organic Framework (SOF) with High CO2 Adsorption and Selectivity

Abstract: A robust binary hydrogen-bonded supramolecular organic framework (SOF-7) has been synthesized by solvothermal reaction of 1,4-bis-(4-(3,5-dicyano-2,6-dipyridyl)dihydropyridyl)benzene (1) and 5,5′-bis-(azanediyl)-oxalyl-diisophthalic acid (2). Single crystal X-ray diffraction analysis shows that SOF-7 comprises 2 and 1,4-bis-(4-(3,5-dicyano-2,6-dipyridyl)pyridyl)benzene (3); the latter formed in situ from the oxidative dehydrogenation of 1. SOF-7 shows a three-dimensional four-fold interpenetrated structure with complementary O–H···N hydrogen bonds to form channels that are decorated with cyano and amide groups. SOF-7 exhibits excellent thermal stability and solvent and moisture durability as well as permanent porosity. The activated desolvated material SOF-7a shows high CO2 adsorption capacity and selectivity compared with other porous organic materials assembled solely through hydrogen bonding.

Selective conversion of m-cresol to toluene over bimetallic Ni–Fe catalysts

Abstract: The catalytic conversion of m-cresol in the presence of H2 has been investigated on SiO2-supported Ni, Fe, and bimetallic Ni–Fe catalysts at 300 °C and atmospheric pressure. Over the monometallic Ni catalyst, the dominant product is 3-methylcyclohexanone while 3-methylcyclohexanol and toluene appear in smaller amounts, even at high conversions. By contrast, on Fe and Ni–Fe bimetallic catalysts, the dominant product is toluene while the hydrogenation products (3-methylcyclohexanone and 3-methylcyclohexanol) are practically negligible in the entire range of conversions. To explain these differences, we have proposed a deoxygenation path that starts with the tautomerization of m-cresol to an unstable ketone intermediate (3-methyl-3,5-cyclohexadienone). The fate of this intermediate is determined by the ability of the catalyst to either hydrogenate the carbonyl group or the ring. The former would mostly occur on Fe and Ni–Fe catalysts that contain an oxophilic metal (Fe), while the latter would occur on Ni, which has a higher affinity for the aromatic ring. Hydrogenation of the carbonyl group produces a very reactive unsaturated alcohol (3-methyl-3,5-cyclohexadienol), which can be easily dehydrated to toluene. This would explain the high selectivity of Fe and Ni–Fe to toluene. By contrast, hydrogenation of the ring would result in 3-methylcyclohexanone, which can be further hydrogenated to 3-methylcyclohexanol. On supports that contain acid sites, which are active for dehydration, the formation of toluene would occur via dehydration of the alcohol and subsequent dehydrogenation. On the catalysts investigated in this work, dehydration of the corresponding alcohol does not occur, so the only path to toluene is via hydrogenation of the carbonyl of the unstable ketone intermediate. In addition, to the products mentioned above, xylenol is also observed in significant yields, which indicate that transalkylation of m-cresol is another reaction path occurring on these catalysts.

Platinum-promoted Ga/Al₂O₃ as highly active, selective, and stable catalyst for the dehydrogenation of propane.

Abstract: A novel catalyst material for the selective dehydrogenation of propane is presented. The catalyst consists of 1000 ppm Pt, 3 wt % Ga, and 0.25 wt % K supported on alumina. We observed a synergy between Ga and Pt, resulting in a highly active and stable catalyst. Additionally, we propose a bifunctional active phase, in which coordinately unsaturated Ga3+ species are the active species and where Pt functions as a promoter.

Propylene hydrogenation and propane dehydrogenation by a single-site Zn2+ on silica catalyst

Abstract: This study reports the highly selective (more than 95%) dehydrogenation of propane to propylene as well as the reverse hydrogenation reaction by silica-supported single-site Zn(II) catalyst. The ca...

Mg–TM (TM: Ti, Nb, V, Co, Mo or Ni) core–shell like nanostructures: synthesis, hydrogen storage performance and catalytic mechanism

Abstract: Magnesium (Mg) was coated by different transition metals (TM: Ti, Nb, V, Co, Mo, or Ni) with a grain size in the nano-scale to form a core (Mg)–shell (TM) like structure by reaction of Mg powder in THF solution with TMClx. The thickness of the TM shell is less than 10 nm. TPD-MS results show the Mg–Ti sample can release hydrogen even under 200 °C. It is experimentally confirmed that the significance of the catalytic effect on dehydrogenation is in the sequence Mg–Ti, Mg–Nb, Mg–Ni, Mg–V, Mg–Co and Mg–Mo. This may be due to the decrease in electro-negativity (χ) from Ti to Mo. However, Ni is a special case with a high catalytic effect in spite of the electro-negativity. It is supposed that the formation of the Mg2Ni compound may play an important role in enhancing the hydrogen de/hydrogenation of the Mg–Ni system. It is also found that the larger the formation enthalpy, the worse the dehydrogenation kinetics.

Tandem Dehydrogenation of Ammonia Borane and Hydrogenation of Nitro/Nitrile Compounds Catalyzed by Graphene-Supported NiPd Alloy Nanoparticles

Abstract: We report a facile synthesis of monodisperse NiPd alloy nanoparticles (NPs) and their assembly on graphene (G) to catalyze the tandem dehydrogenation of ammonia borane (AB) and hydrogenation of R-NO2 and/or R-CN to R-NH2 in aqueous methanol solutions at room temperature. The 3.4 nm NiPd alloy NPs were prepared by coreduction of nickel(II) acetate and palladium(II) acetlyacetonate by borane-tert-butylamine in oleylamine and deposition on G via a solution phase self-assembly process. G-NiPd showed composition-dependent catalysis on the tandem reaction with G-Ni30Pd70 being the most active. A variety of R-NO2 and/or R-CN derivatives were reduced selectively into R-NH2 via G-Ni30Pd70 catalyzed tandem reaction in 5–30 min reaction time with the conversion yields reaching up to 100%. Our study demonstrates a new approach to G-NiPd-catalyzed dehydrogenation of AB and hydrogenation of R-NO2 and R-CN. The G-NiPd NP catalyst is efficient and reusable, and the reaction can be performed in an environment-friendly pro...

Sodium hydroxide-assisted growth of uniform Pd nanoparticles on nanoporous carbon MSC-30 for efficient and complete dehydrogenation of formic acid under ambient conditions

Abstract: Highly dispersed Pd nanoparticles (NPs) deposited on nanoporous carbon MSC-30 have been successfully prepared with a sodium hydroxide-assisted reduction approach. The modification by NaOH during the formation and growth of particles results in the well-dispersed ultrafine Pd NPs on carbon. The combination of distinct interaction between metal and support and high dispersion of NPs drastically enhances the catalytic performance of the resulted catalyst, over which the turnover frequency (TOF) for heterogeneously catalyzed decomposition of formic acid (FA) reaches 2623 h−1 at 50 °C with 100% H2 selectivity, the highest value ever reported under ambient conditions, comparable to those acquired from the most active homogeneous catalysts. Even at 25 °C, the complete dehydrogenation of FA with a TOF as high as 750 h−1 can be achieved.

Carbon dioxide mediated, reversible chemical hydrogen storage using a Pd nanocatalyst supported on mesoporous graphitic carbon nitride

Abstract: Reversible, carbon dioxide mediated chemical hydrogen storage was first demonstrated using a heterogeneous Pd catalyst supported on mesoporous graphitic carbon nitride (Pd/mpg-C3N4). The Pd nanoparticles were found to be uniformly dispersed onto mpg-C3N4 with an average size of 1.7 nm without any agglomeration and further exhibit superior activity for the dehydrogenation of formic acid with a turnover frequency of 144 h−1 even in the absence of external bases at room temperature. Initial DFT studies suggest that basic sites located at the mpg-C3N4 support play synergetic roles in stabilizing reduced Pd nanoparticles without any surfactant as well as in initiating H2-release by deprotonation of formic acid, and these potential interactions were further confirmed by X-ray absorption near edge structure (XANES). Along with dehydrogenation, Pd/mpg-C3N4 also proves to catalyze the regeneration of formic acid via CO2 hydrogenation. The governing factors of CO2 hydrogenation are further elucidated to increase the quantity of the desired formic acid with high selectivity.

Vanadium-Node-Functionalized UiO-66: A Thermally Stable MOF- Supported Catalyst for the Gas-Phase Oxidative Dehydrogenation of Cyclohexene

Abstract: The OH groups on the Zr-based nodes of ultrastable UiO-66 can be metallated with VV ions in a facile fashion to give the derivative VUiO-66. This metallated MOF exhibits high stability over a broad temperature range and displays high selectivity for benzene under low-conversion conditions in the vapor-phase oxidative dehydrogenation of cyclohexene (activation energy ∼110 kJ/mol). The integrity of the MOF is maintained after catalysis as determined by PXRD, ICP-AES, and SEM.

Trends in Formic Acid Decomposition on Model Transition Metal Surfaces: A Density Functional Theory study

Abstract: We present a first-principles, self-consistent periodic density functional theory (PW91-GGA) study of formic acid (HCOOH) decomposition on model (111) and (100) facets of eight fcc metals (Au, Ag, Cu, Pt, Pd, Ni, Ir, and Rh) and (0001) facets of four hcp (Co, Os, Ru, and Re) metals. The calculated binding energies of key formic acid decomposition intermediates including formate (HCOO), carboxyl (COOH), carbon monoxide (CO), water (H2O), carbon dioxide (CO2), hydroxyl (OH), carbon (C), oxygen (O), and hydrogen (H; H2) are presented. Using these energetics, we develop thermochemical potential energy diagrams for both the carboxyl-mediated and the formate-mediated dehydrogenation mechanisms on each surface. We evaluate the relative stability of COOH, HCOO, and other isomeric intermediates (i.e., CO + OH, CO2 + H, CO + O + H) on these surfaces. These results provide insights into formic acid decomposition selectivity (dehydrogenation versus dehydration), and in conjunction with calculated vibrational frequenc...

Molecular catalysts for hydrogen production from alcohols

Abstract: An industrially applicable catalytic methodology for dihydrogen formation from a proton source remains at the forefront of all efforts to replace the present fossil fuel economy by a hydrogen economy. This review tries to summarize the achievements which have been made with molecular organometallic complexes as catalysts for the dehydrogenation of alcohols. Biology uses NAD+ as a metal-free hydrogen acceptor which converts with the help of enzymes (alcohol dehydrogenase, aldehyde dehydrogenase) alcohols in carbonyl compounds, NADH, and protons. In the regeneration of NADH to NAD+, electrons are stored in electron transfer enzymes (ferredoxines) which are subsequently used to reduce protons to hydrogen with the help of hydrogenases or nitrogenases which ensures a very low overpotential for the reduction. Man-made organometallic complexes are rather primitive with respect to this complex machinery but use some principles from biology as guide lines. Classical complexes like rhodium or ruthenium phosphane complexes achieve at best a few thousands of turn over frequencies (TOFs). Established reactions like oxidative addition of the hydroxyl group of the substrate to the metal centre, β-hydrogen elimination from the α-CH group of the coordinated alcohol, product dissociation, and reductive elimination of hydrogen are involved in the proposed catalytic cycles. Complexes which show metal–ligand cooperativity show a significantly better performance with respect to turn over frequencies (conversion rate = activity) and turn over numbers (number of product molecules per catalyst molecule = efficiency). In these catalytic systems, the alcohol substrate is converted with the help of active centres in the ligand backbone which participate directly and reversibly in the transformation of the substrate. Present results indicate that dehydrogenative coupling reactions of the type, R–CH2–OH + XH → RCOX + 2H2, proceed especially well and can be applied to a wide range of substrates including multiple dehydrogenative couplings leading to polyesters or polyamides. In photocatalytic conversions, alcohols can be deoxygenated to hydrocarbons, CO, and H2 which should be further explored in the future. New developments consist of the construction of organometallic fuel cells (OMFCs) where the anode is composed of molecular catalysts embedded into a conducting support material. Here no free hydrogen is evolved but it is directly converted to electric current and protons according to H2 → 2H+ + 2e. The review focuses on the catalysis with organometallic complexes but lists some selected results obtained with heterogeneous catalytic systems for comparison.

Effect of hydrogen donor on liquid phase catalytic transfer hydrogenation of furfural over a Ru/RuO2/C catalyst

Abstract: The effect of alcohol hydrogen donor on methyl furan production through catalytic transfer hydrogenation of furfural in the liquid phase has been investigated over a mildly calcined Ru/C catalyst in the temperature range of 110–200 °C. It has been found that several parameters contribute to furfural hydrogenolysis, including alcohol dehydrogenation activity, solvent properties, as well as side reactions such as etherification between the intermediate, furfuryl alcohol, and the hydrogen donor. Methyl furan yield increases from 0 to 68% at 180 °C following the order of 2-methyl-2-butanol

Propane dehydrogenation over Pt–Cu bimetallic catalysts: the nature of coke deposition and the role of copper

Abstract: This paper describes an investigation of the promotional effect of Cu on the catalytic performance of Pt/Al2O3 catalysts for propane dehydrogenation. We have shown that Pt/Al2O3 catalysts possess higher propylene selectivity and lower deactivation rate as well as enhanced anti-coking ability upon Cu addition. The optimized loading content of Cu is 0.5 wt%, which increases the propylene selectivity to 90.8% with a propylene yield of 36.5%. The origin of the enhanced catalytic performance and anti-coking ability of the Pt–Cu/Al2O3 catalyst is ascribed to the intimate interaction between Pt and Cu, which is confirmed by the change of particle morphology and atomic electronic environment of the catalyst. The Pt–Cu interaction inhibits propylene adsorption and elevates the energy barrier of C–C bond rupture. The inhibited propylene adsorption diminishes the possibility of coke formation and suppresses the cracking reaction towards the formation of lighter hydrocarbons on Pt–Cu/Al2O3, while a higher energy barrier for C–C bond cleavage suppresses the methane formation.

Metal-Free Carbon Catalysts for Oxidative Dehydrogenation Reactions

Abstract: Catalysis over carbon, especially nanocarbon, is an attractive topic in material science and chemical engineering fields due to its significant advantages compared with conventional metal or metal oxide catalysts. This paper summarizes the recent developments, basic concepts, and commonly accepted understandings on the nature of carbon catalysis in oxidative dehydrogenation reactions, including: introduction and comparison of various reaction systems; identity and quantity of active sites on carbon catalysts; mechanism for the reactions; and structure− selectivity relations for modified carbon catalysts. These fruitful conclusive achievements are the basis for in-depth comprehension of carbon-catalyzed oxidative dehydrogenation process at the molecular level, and many other efforts, such as detailed kinetic study, precisely controllable synthetic technique for nanocarbon catalysts, are still needed to further push carbon catalysis fields to practical applications.

Design of a metal-promoted oxide catalyst for the selective synthesis of butadiene from ethanol.

Abstract: The synthesis of buta-1,3-diene from ethanol has been studied over metal-containing (M=Ag, Cu, Ni) oxide catalysts (MO(x)=MgO, ZrO2, Nb2O5, TiO2, Al2O3) supported on silica. Kinetic study of a wide range of ethanol conversions (2-90%) allowed the main reaction pathways leading to butadiene and byproducts to be determined. The key reaction steps of butadiene synthesis were found to involve ethanol dehydrogenation, acetaldehyde condensation, and the reduction of crotonaldehyde with ethanol into crotyl alcohol. Catalyst design included the selection of active components for each key reaction step and merging of these components into multifunctional catalysts and adjusting the catalyst functions to achieve the highest selectivity. The best catalytic performance was achieved over the Ag/ZrO2/SiO2 catalyst, which showed the highest selectivity towards butadiene (74 mol%).