Showing papers on "Dehydrogenation published in 2008"
TL;DR: It is shown that carbon nanotubes with modified surface functionality efficiently catalyze the oxidative dehydrogenation of n-butane to butenes, especially butadiene, and a high selectivity to alkenes was achieved for periods as long as 100 hours.
Abstract: Butenes and butadiene, which are useful intermediates for the synthesis of polymers and other compounds, are synthesized traditionally by oxidative dehydrogenation (ODH) of n-butane over complex metal oxides. Such catalysts require high O2/butane ratios to maintain the activity, which leads to unwanted product oxidation. We show that carbon nanotubes with modified surface functionality efficiently catalyze the oxidative dehydrogenation of n-butane to butenes, especially butadiene. For low O2/butane ratios, a high selectivity to alkenes was achieved for periods as long as 100 hours. This process is mildly catalyzed by ketonic CO groups and occurs via a combination of parallel and sequential oxidation steps. A small amount of phosphorus greatly improved the selectivity by suppressing the combustion of hydrocarbons.
754 citations
TL;DR: In this paper, an enhanced kinetics for hydrogen exchange in LiBH4 incorporated within nanoporous carbon scaffolds are described. And the authors demonstrate that confinement within a porous scaffold host is a promising approach for enhancing hydrogen uptake and release in reversible light-metal complex hydrides.
Abstract: Enhanced kinetics for hydrogen exchange in LiBH4 incorporated within nanoporous carbon scaffolds are described. Dehydrogenation rates up to 50 times faster than those in the bulk material are measured at 300 °C in a nanostructured hydride formed by filling a porous carbon aerogel host with LiBH4. Furthermore, the activation energy for hydrogen desorption, measured using the approach developed by Ozawa, is reduced from 146 kJ/mol for bulk LiBH4 to 103 kJ/mol for nanostructured LiBH4, and the faster kinetics result in desorption temperatures that are reduced by up to 75 °C. In addition, nanostructured hydrides exhibit increased cycling capacity over multiple sorption cycles. This work demonstrates that confinement within a porous scaffold host is a promising approach for enhancing hydrogen uptake and release in reversible light-metal complex hydrides.
412 citations
TL;DR: The excellent catalytic activity of Fe nanoparticles with no protective shell for the hydrolytic dehydrogenation of aqueous AB under argon and even in air at room temperature is reported.
Abstract: Iron, the most ubiquitous of the transition metals and the fourth most plentiful element in the Earth s crust, has been studied intensively because of its very potent magnetic and catalytic properties. However, its reactivity with respect to water and oxygen, especially on a nanoscale, generally limits its applications to a non-oxidizing environment where water and oxygen are not present. Recent studies involving coating Fe nanoparticles with an outer shell have succeeded in minimizing their oxidation and agglomeration. However, the presence of protective shell around the Fe particles is unfavorable for catalytic applications, where surface Fe active sites are needed. It is therefore understandable that, to date, there has been no report on the catalytic application of Fe nanoparticles without any protective shell other than the solvent components in aqueous solution in air. Fe nanoparticles that exert their powerful catalytic ability in aqueous solution or even in air will therefore significantly benefit both academic research and practical applications of iron-based materials. The search for effective hydrogen-storage materials is one of the most difficult challenges as we move towards a hydrogen-powered society as a long-term solution to current energy problems. Ammonia borane (AB; NH3BH3) has a hydrogen content of 19.6 wt%, which exceeds that of gasoline and therefore makes it an attractive candidate for chemical hydrogen-storage applications. The development of efficient and economical catalysts to further improve the kinetic properties under moderate conditions is therefore important for the practical application of this system. Herein we report the excellent catalytic activity of Fe nanoparticles with no protective shell for the hydrolytic dehydrogenation of aqueous AB under argon and even in air at room temperature. The Fe nanoparticles were pre-synthesized by reduction of FeSO4 with NaBH4 and then AB was immediately added to the solution to be catalytically hydrolyzed (AB/FeSO4/NaBH4 1.0:0.12:0.16). The gas generated was identified by mass spectrometry and its amount was measured volumetrically. Although black Fe nanoparticles were obtained rapidly, the evolution of 134 mL of hydrogen took more than 160 min (Figure 1a). The molar ratio of hydrolytically generated H2 to
407 citations
TL;DR: The stability of LiBH4 was investigated by pcT (pressure, concentration, and temperature) measurements under constant hydrogen flows and extrapolated to equilibrium by XRD (X-ray diffraction), and it was shown that the reaction can be reversed at a temperature of 600 degrees C and at a pressure of 155 bar.
Abstract: LiBH4 is a complex hydride and exhibits a high gravimetric hydrogen density of 18.5 wt %. Therefore it is a promising hydrogen storage material for mobile applications. The stability of LiBH4 was investigated by pcT (pressure, concentration, and temperature) measurements under constant hydrogen flows and extrapolated to equilibrium. According to the van 't Hoff equation the following thermodynamic parameters are determined for the desorption: enthalpy of reaction DeltarH = 74 kJ mol-1 H2 and entropy of reaction DeltarS = 115 J K-1 mol-1 H2. LiBH4 decomposes to LiH + B + 3/2H2 and can theoretically release 13.9 wt % hydrogen for this reaction. It is shown that the reaction can be reversed at a temperature of 600 degrees C and at a pressure of 155 bar. The formation of LiBH4 was confirmed by XRD (X-ray diffraction). In the rehydrided material 8.3 wt % hydrogen was desorbed in a TPD (temperature-programmed desorption) measurement compared to 10.9 wt % desorbed in the first dehydrogenation.
335 citations
TL;DR: In this paper, the authors review the dehydrogenation of cycloalkanes as useful reaction for storage of hydrogen in chemical hydrides and propose a novel approach for the supply of hydrogen, through liquid organic hydride (LOH) using catalytic reaction pair of dehydrogenization of cycloencane and hydrogenation of corresponding aromatics.
324 citations
TL;DR: It was shown that alkylamine-boranes can serve as a source of H2 in the Ru-catalyzed reduction of ketones and imines in AB and MeAB.
Abstract: The dehydrogenation of ammonia borane (AB) and methylammonia borane (MeAB) is shown to be catalyzed by several Ru-amido complexes. Up to 1 equiv of H2 (1.0 system wt %) is released from AB by as little as 0.03 mol % Ru within 5 min, and up to 2 equiv of H2 (3.0 system wt %) are released from MeAB with 0.5 mol % Ru in under 10 min at room temperature, the first equivalent emerging within 10 s. Also, a mixture of AB/MeAB yields up to 3.6 system wt % H2 within 1 h with 0.1 mol % Ru. Computational studies were performed to elucidate the mechanism of dehydrogenation of AB. Finally, it was shown that alkylamine-boranes can serve as a source of H2 in the Ru-catalyzed reduction of ketones and imines.
276 citations
265 citations
TL;DR: Improved dehydrogenation properties are attributed to the different bonding nature and reactivity of the metal amidoboranes compared to NH3BH3.
Abstract: Alkali- and alkaline-earth metal amidoboranes are a new class of compounds with rarely observed [NH2BH3]− units. LiNH2BH3 and solvent-containing Ca(NH2BH3)2·THF have been recently reported to significantly improve the dehydrogenation properties of ammonia borane. Therefore, metal amidoboranes, with accelerated desorption kinetics and suppressed toxic borazine, are of great interest for their potential applications for hydrogen storage. In this work, we successfully determined the structures of LiNH2BH3 and Ca(NH2BH3)2 using a combined X-ray diffraction and first-principles molecular dynamics simulated annealing method. Through detailed structural analysis and first-principles electronic structure calculations the improved dehydrogenation properties are attributed to the different bonding nature and reactivity of the metal amidoboranes compared to NH3BH3.
246 citations
TL;DR: It is demonstrated for the first time that mesostructured Ga–Al mixed-oxide solid solutions are highly promising supports for the fabrication of exceptionally effective gold catalysts for aerobic alcohol oxidation under mild conditions.
Abstract: The selective oxidation of alcohols is one of the most challenging reactions in green chemistry. Although a number of methods have been developed, the search for new, facile, cost-effective, and environmentally benign procedures that avoid the use of a large excess of toxic and expensive stoichiometric metal oxidants has attracted substantial interest. An attractive method is the direct oxidation of alcohols—promoted by reusable heterogeneous catalysts—using air or molecular oxygen (O2) under solventfree conditions or (in the case of solid alcohols) in green organic solvents. Ideally, the reaction should also be performed under mild conditions (preferably at room temperature) for the synthesis of complex, thermolabile compounds, which are typical in fine chemistry. Satisfactory results were attained in only very few cases, in which a large excess of base additives was required, and this was usually achieved at the expense of selectivity. Therefore, the development of excellent reusable catalysts for liquid-phase aerobic oxidation of alcohols under mild conditions would constitute a breakthrough in both green chemistry and organic synthesis. Recently, supported gold nanoparticles have attracted considerable attention because of their extraordinarily high activity and selectivity. The outstanding catalytic ability of gold is related to the size and shape of the nanoparticles, the degree of coordinative unsaturation of the gold atoms, and the interactions between gold and the oxide support. Although several gold systems have been reported for the catalysis of alcohol oxidation reactions, in most cases they have been applied at temperatures above 100 8C. Dehydrogenation is known to be the rate-limiting step in the oxidation of alcohols on various noble metals. Therefore, the combination of gold nanoparticles with a suitable support (characterized by an exceptional alcohol-dehydrogenation activity) may allow the fabrication of new, versatile gold catalysts that could be used for liquid-phase organic synthesis under mild conditions. Herein, we demonstrate for the first time that mesostructured Ga–Al mixed-oxide solid solutions are highly promising supports for the fabrication of exceptionally effective gold catalysts for aerobic alcohol oxidation under mild conditions. A series of binary mesostructured Ga–Al mixed-oxide supports (denoted as GaxAl6 xO9; x= 2, 3, 4), along with unitary oxides of g-Ga2O3 and g-Al2O3, was prepared through an alcoholic sol–gel pathway. The X-ray diffraction (XRD) patterns of all as-synthesized binary substrates are characteristic of g-Ga2O3/Al2O3 solid solutions with a spinel-type structure. When gold nanoparticles were deposited onto these high-surface-area materials, no gold diffraction line was detected, and the pattern showed no significant differences relative to that of the support, thus indicating that the structure of the catalyst was maintained. A representative transmission electron microscopy (TEM) image of the Au/ GaxAl6 xO9 sample confirms that the gold particles were evenly deposited on the Ga–Al mixed-oxide support, with most particles being smaller than about 6 nm (see the Supporting Information for TEM and XRD data). To check the possible alcohol-dehydrogenation capability of the Au/GaxAl6 xO9 materials, we adsorbed 2-propanol on their surface and performed temperature-programmed surface reaction (TPSR) measurements of the desorbed H2 molecules (see the Supporting Information). Ga-containing mixed-oxide supports were found to be indispensable for attaining highly active alcohol-dehydrogenation materials (Figure 1A). Furthermore, the dehydrogenation activity of the catalysts was observed to be strongly dependent on the composition of these supports. A strongly enhanced hydrogen signal was identified in the case of a Ga3Al3O9 solid solution containing a Ga/Al molar ratio of 1:1—in sharp contrast to what was observed for the reference gold catalysts Au/TiO2 and Au/Fe2O3 (provided by the World Gold Council), where no H2 species were detected. These results can be rationalized by assuming that the formation of Ga–Al mixed-oxide solid solutions may favor the creation of specific dehydrogenation sites as a consequence of the presence of Ga atoms at the surface atomic sites (Td and Oh) of Al2O3 and highly dispersed GaO4 tetrahedra in the surface spinels. [16] These sites are responsible for the considerably enhanced dehydrogenation activity observed for the Ga–Al mixed-oxide-supported Au catalysts. Our initial aerobic-oxidation studies focused on the case of benzyl alcohol (Figure 1B), with the aim to understand the effect of the composition of the support on the catalytic performance of the gold catalysts. The reactions were performed in a magnetically stirred glass batch reactor in the presence of a solvent (at 90 8C) under O2 and at [*] F. Z. Su, Dr. Y. M. Liu, L. C. Wang, Prof. Y. Cao, Prof. H. Y. He, Prof. K. N. Fan Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials Fudan University, Shanghai 200433 (P. R. China) Fax: (+86)21-6564-2978 E-mail: yongcao@fudan.edu.cn
223 citations
TL;DR: In this article, the performance of a Pt/CeZrO 2 catalyst was tested for ethanol decomposition, steam reforming, partial oxidation, and oxidative steam reforming and it was shown that increasing the reaction temperature greatly improved the stability of the catalyst.
212 citations
TL;DR: In this paper, the 5% Au/Al2O3 catalyst was used for the aerobic dehydrogenation of amines to imines under the mild conditions of 1 atm O2 and 100 °C.
TL;DR: A highly efficient heterogeneous catalytic system using hydrotalcite-supported Cu nanoparticles (Cu/HT) that can successfully promote the oxidant-free dehydrogenation of various alcohols under liquid-phase conditions is developed.
TL;DR: In situ(11)B NMR monitoring, computational modeling, and external trapping studies show that selectivity and extent of H(2) release in metal-catalysed dehydrogenation of ammonia borane are determined by coordination of reactive aminoborane, NH( 2)BH(2), to the metal center.
TL;DR: In this paper, the properties of Mg-Al alloys in relation to hydrogen storage are reviewed and it is observed that the enthalpy of hydride formation of magnesium is lowered upon alloying with Al due to a slightly endothermic disproportionation reaction.
TL;DR: In this paper, it was shown that the Si/Al ratio of the zeolite, and consequently the density of available Bronsted acid sites, plays a dramatic role on the anchoring mode of the molybdenum and on its catalytic activity.
Abstract: Whereas the hegemony of the Fischer–Tropsch synthesis is well established for the valorization of methane in the case of important gas fields, the methane dehydro-aromatization reaction remains an interesting solution to convert methane obtained as by-product of oil extraction and thus fight against flaring and global warming. Until now, it was generally accepted that the molybdenum is anchored on the Bronsted acid sites of the zeolite as (Mo2O5)2+ species. The molybdenum performs the dehydrogenation and coupling of CH4 to ethylene which is consecutively oligomerized to benzene over the Bronsted acid sites. In the present work, we bring evidence that this picture is actually more complicated than it seems. The Si/Al ratio of the zeolite, and consequently the density of available Bronsted acid sites, plays a dramatic role on the anchoring mode of the molybdenum and on its catalytic activity.
TL;DR: Delta H for the dehydrogenation of AB, MeAB, and AB/MeAB mixtures has been determined by calorimetry and the experimental heats of reaction are compared to results from computational studies.
Abstract: Dehydrogenation of amine boranes is catalyzed efficiently by the iridium pincer complex (kappa (3)-1,3-(OP ( t )Bu 2) 2C 6H 3)Ir(H) 2 ( 1). With CH 3NH 2BH 3 (MeAB) and with AB/MeAB mixtures (AB = NH 3BH 3), the rapid release of 1 equiv of H 2 is observed to yield soluble oligomeric products at rates similar to those previously reported for the dehydrogenation of AB catalyzed by 1. Delta H for the dehydrogenation of AB, MeAB, and AB/MeAB mixtures has been determined by calorimetry. The experimental heats of reaction are compared to results from computational studies.
TL;DR: The crystal structure of solvent-free Li2B12H12 has been determined by powder X-ray diffraction and confirmed by a combination of neutron vibrational spectroscopy and first-principles calculations.
Abstract: The crystal structure of solvent-free Li2B12H12 has been determined by powder X-ray diffraction and confirmed by a combination of neutron vibrational spectroscopy and first-principles calculations. This compound is a possible intermediate in the dehydrogenation of LiBH4, and its structural characterization is crucial for understanding the decomposition and regeneration of LiBH4. Our results reveal that the structure of Li2B12H12 differs from other known alkali-metal (K, Rb, and Cs) derivatives.
TL;DR: In this article, a three-step reaction pathway consisting of Ir-catalyzed alcohol dehydrogenation to aldehyde, amine-aldehyde condensation to imine, and then Ir-caralyzed imine hydrogenation to amine was studied.
TL;DR: In this article, LiBH4 was incorporated into an activated carbon (AC) scaffold using a chemical impregnation method to improve both the hydrogen sorption kinetics and thermodynamics, compared to the bulk hydride.
TL;DR: In this article, the authors present a review of the combined theoretical and experimental studies on hydrogen spillover mechanisms in solid-state materials where, for the first time, the complete mechanisms that dictate hydrogen spilloff processes in transition metal oxides and nanostructured graphitic carbon-based materials have been revealed.
Abstract: Hydrogen spillover has emerged as a possible technique for achieving high-density hydrogen storage at near-ambient conditions in lightweight, solid-state materials. We present a brief review of our combined theoretical and experimental studies on hydrogen spillover mechanisms in solid-state materials where, for the first time, the complete mechanisms that dictate hydrogen spillover processes in transition metal oxides and nanostructured graphitic carbon-based materials have been revealed. The spillover process is broken into three primary steps: (1) dissociative chemisorption of gaseous H2 on a transition metal catalyst; (2) migration of H atoms from the catalyst to the substrate and (3) diffusion of H atoms on substrate surfaces and/or in the bulk materials. In our theoretical studies, the platinum catalyst is modeled with a small Pt cluster and the catalytic activity of the cluster is examined at full H atom saturation to account for the essentially constant, high H2 pressures used in experimental studies of hydrogen spillover. Subsequently, the energetic profiles associated with H atom migrations from the catalyst to the substrates and H atom diffusion in the substrates are mapped out by calculating the minimum energy pathways. It is observed that the spillover mechanisms for the transition metal oxides and graphitic carbon-based materials are very different. Hydrogen spillover in the transition metal oxides is moderated by massive, nascent hydrogen bonding networks in the crystalline lattice, while H atom diffusion on the nanostructured graphitic carbon materials is governed mostly by physisorption of H atoms. The effects of carbon material surface curvature on the hydrogen spillover as well as on hydrogen desorption dynamics are also discussed. The proposed hydrogen spillover mechanism in carbon-based materials is consistent with our experimental observations of the solid-state catalytic hydrogenation/dehydrogenation of coronene.
TL;DR: In this article, N-heterocyclic carbene (NHC) nickel complexes catalyze the dehydrogenation of ammonia-borane, a candidate for chemical hydrogen storage, through proton transfer from nitrogen to the metal-bound carbene carbon, instead of the B−H or N−H bond activation.
Abstract: Density functional Tao−Perdew−Staroverov−Scuseria calculations with all-electron correlation-consistent polarized valence double-ζ basis set demonstrate that N-heterocyclic carbene (NHC) nickel complexes catalyze the dehydrogenation of ammonia-borane, a candidate for chemical hydrogen storage, through proton transfer from nitrogen to the metal-bound carbene carbon, instead of the B−H or N−H bond activation This new C−H bond is then activated by the metal, transferring the H to the metal, then forming the H2 by transferring a H from B to the metal, instead the β-H transfer This reaction pathway explains the importance of the NHC ligands in the dehydrogenation and points the way to finding new catalyst with higher efficiency, as partial unsaturation of the M-L bond may be essential for rapid H transfers
TL;DR: Rhodium amine-borane sigma-complexes of H3BNHMe2 have been isolated which are potential intermediates in the catalytic dehydrogenation of H1B and NHMe2.
Abstract: Rhodium amine-borane σ-complexes of H3BNHMe2 have been isolated which are potential intermediates in the catalytic dehydrogenation of H3B·NHMe2.
TL;DR: In this article, a series of mixed Ga x Al 10− x O 15 oxides (with x varying from 0 to 10) synthesized through an alcoholic coprecipitation pathway was used for dehydrogenation of propane to propylene in the presence or absence of carbon dioxide.
TL;DR: In this article, a series of supported V2O5/Al2O3 catalysts were synthesized by incipient wetness impregnation with vanadium isopropoxide and subsequent calcination.
TL;DR: Pd(OAc)2-mediated dehydrogenation of an alkyl group to a double bond or a η3-allylic complex via sp3 C−H bond activation and allylic oxidation is reported in this paper.
TL;DR: In this article, a complete kinetic model of propane dehydrogenation to produce propene over a Pt-Sn-K/Al2O3 catalyst was obtained, and a suitable mathematical model was developed for simulating the process behaviour in a two-zone fluidized bed reactor (TZFBR).
Abstract: A complete kinetic model of propane dehydrogenation to produce propene over a Pt–Sn–K/Al2O3 catalyst was obtained. This has been investigated over the temperature range of 460–540 °C at atmospheric pressure. A Langmuir–Hinshelwood mechanism provides the best fit for propane dehydrogenation, while a monolayer–multilayer mechanism is proposed for modelling the coke formation. In addition, the reaction rate of coke formation and its influence on catalyst deactivation and subsequent regeneration have been studied. Finally, a suitable mathematical model is developed for simulating the process behaviour in a two-zone fluidized bed reactor (TZFBR).
TL;DR: In this article, the authors investigated the hydrogenation of the CaH2+MgB2 composite and the dehydration of the resulting products by in situ time-resolved synchrotron radiation powder X-ray diffraction, high-pressure differential scanning calorimetry, infrared, and thermovolumetric measurements.
Abstract: The hydrogenation of the CaH2+MgB2 Composite and the dehydrogenation of the resulting products are investigated in detail by in situ time-resolved synchrotron radiation powder X-ray diffraction, high-pressure differential scanning calorimetry, infrared, and thermovolumetric measurements. It is demonstrated that a Ca(BH4)(2)+MgH2 composite is formed by hydrogenating a CaH2+MgB2 composite, at 350 degrees C and 140 bar of hydrogen. Two phases of Ca(BH4)(2) were characterized: alpha- and beta-Ca(BH4)(2). alpha-Ca(BH4)(2) transforms to beta-Ca(BH4)(2) at about 130 degrees C. Under the conditions used in the present study, beta-Ca(BH4)(2) decomposes first to CaH2, Ca3Mg4H14, Mg, B (or MgB2 depending on experimental conditions), and hydrogen at 360 degrees C, before complete decomposition to CaH2, Mg, B (or MgB2), and hydrogen at 400 degrees C. During hydrogenation under 140 bar of hydrogen, beta-Ca(BH4)(2) is formed at 250 degrees C, and alpha-Ca(BH4)(2) is formed when the sample is cooled to less than 130 degrees C. Ti isopropoxide improves the kinetics of the reactions, during both hydrogenation and dehydrogenation. The dehydrogenation temperature decreases to 250 degrees C, with 1 wt % of this additive, and hydrogenation starts already at 200 degrees C. We propose that the improved kinetics of the above reactions with MgB2 (compared to pure boron) can be explained by the different boron bonding within the crystal structure of MgB2 and pure boron. (Less)
TL;DR: In this paper, the authors showed that the reaction is dependent on the size of the particle and showed that catalysts with smaller particles (∼3nm) are more active and less selective than those with larger particles.
TL;DR: In this paper, the effect of CO 2 on the yield of propene was found for all the studied catalysts in a flow apparatus at 773 and 923 K. This effect was discussed on the basis of temperature-programmed reaction of CO2 with H 2 as well as H 2 -TPR experiments after regeneration of the reduced catalyst with pure CO 2.
Abstract: Incipient wetness technique was used for a preparation of MCM-supported chromium oxide materials with a Cr loading ranging from 0.7 to 13.7 wt%. The obtained samples were characterized by XRD, UV–vis-DRS, H 2 -TPR and BET, and tested as catalysts in dehydrogenation of propane with CO 2 . The catalytic reaction was studied in a flow apparatus at 773–923 K. All the tested catalysts exhibited a good catalytic performance in the dehydrogenation of propane with CO 2 . The best results were achieved over the sample containing 6.8 wt% of Cr. In this case, the selectivity to propene was above 80%, while the conversion of propane increased from 21% (at 773 K) to 62% (at 923 K). The promoting effect of CO 2 on the yield of propene was found for all the studied catalysts. This effect was discussed on the basis of temperature-programmed reaction of CO 2 with H 2 as well as H 2 -TPR experiments after regeneration of the reduced catalyst with pure CO 2 . It was suggested that one of reasons of a higher yield of propene observed in the presence of CO 2 compared to that measured in the absence of CO 2 was coupling of the dehydrogenation of propane with the reverse water–gas shift reaction.