Consecutive Hydrogenation Reactions Research Articles

Scalable synthesis of Cu clusters for remarkable selectivity control of intermediates in consecutive hydrogenation

Subnanometric Cu clusters that contain only a small number of atoms exhibit unique and, often, unexpected catalytic behaviors compared with Cu nanoparticles and single atoms. However, due to the high mobility of Cu species, scalable synthesis of stable Cu clusters is still a major challenge. Herein, we report a facile and practical approach for scalable synthesis of stable supported Cu cluster catalysts. This method involves the atomic diffusion of Cu from the supported Cu nanoparticles to CeO2 at a low temperature of 200 °C to form stable Cu clusters with tailored sizes. Strikingly, these Cu clusters exhibit high yield of intermediate product (95%) in consecutive hydrogenation reactions due to their balanced adsorption of the intermediate product and dissociation of H2. The scalable synthesis strategy reported here makes the stable Cu cluster catalysts one step closer to practical semi-hydrogenation applications.

A DFT approach for methanol synthesis via hydrogenation of CO on gallia, ceria and ZnO surfaces

A systematic theoretical study of the consecutive hydrogenation reactions of the CO molecule for the methanol synthesis catalyzed by different oxides of Zn, Ce and Ga is reported in this work. First, the CO hydrogenation with the formation of formyl species (HCO) was analyzed, followed by the successive hydrogenations that lead to formaldehyde (H2CO), methoxy (H3CO) and, finally, methanol (H3COH). The co-adsorption with H, in almost all the intermediate species, allows the corresponding hydrogenation reaction. Oxygen vacancies promote the reactivity in the generation of both formaldehyde and methoxy species. The formation of these species involves an important geometric difference between the initial and the final states, leading to high activation barriers. Comparing the surfaces studied in this work, we found that ZnO (0001)vacO has shown to be of a greater interest for methanol synthesis. However, the foregoing is not the most relevant of our results, but, instead, that the Brönsted Evans Polanyi (BEP) relationships between the initial or the final states and the transition states (TS) allowed to find a very good correlation between surface structure and reactivity.

Quantitative Differences in Sulfur Poisoning Phenomena over Ruthenium and Palladium: An Attempt To Deconvolute Geometric and Electronic Poisoning Effects Using Model Catalysts

Sulfur poisoning over noble-metal catalysts has traditionally been regarded as very complex and precluding from easy rational understanding, because of the problems of interference from using different supports, inability of controlling coverage due to nonuniform metal particle size, intrinsic size/shape effect of metal component, etc. Here, high-quality polyvinylpyrrolidone (PVP) polymer-supported ruthenium and palladium model nanocatalysts using without solid support are equivalently modified with preadsorbed mercaptoethanol over a range of surface concentrations in order to compare sulfur poisoning effects on the two important noble metals commonly used in industry. A typical consecutive hydrogenation reactions of alkyne to alkene and then to alkane is studied under mild reaction conditions in the liquid phase. The first stage alkyne hydrogenation is well-known to be surface insensitive, because of strong adsorption of alkyne on both metals. However, the second stage, surface-sensitive hydrogenation/isomerization of weakly adsorbed alkenes, is highly influenced by perturbations in metal surface electronic states induced by sulfur adsorbates. Using a combination of 13C NMR, Fourier transform infrared (FTIR) measurements of chemisorbed CO, kinetic products analysis and density functional theory (DFT) calculations, the electronic and geometric components of sulfur poisoning can be assigned in an almost-quantitative manner for the first time, over these two metal nanocatalysts. It is found that this sulfur adsorbate dwells preferentially on terrace sites for both metals at high coverage, causing deactivation by surface site blockage for the alkyne hydrogenation. The adsorbate can also deplete electron density from the metal surface (mixing with higher vacant band states of sulfur). As a result, reduction in adsorption strength for alkenes in the second-stage hydrogenation, leading to deactivation by electronic effects, is observed. This component is shown to contribute more significantly to the total deactivation for palladium (electron-rich metal) than ruthenium (electron-poor metal). At 60% sulfur coverage on Pd, the electronic contribution to surface adsorption can be totally cancelled out. This work clearly shows that the differing nature of metals can result in very different degrees of geometric and electronic deactivation upon sulfur adsorption over a size range of 2–3 nm without any interference from solid support, particle size/shape variations, giving important insights to developing more sulfur-tolerant catalysts in the future.

Iridium@graphene composite nanomaterials synthesized in ionic liquid as re-usable catalysts for solvent-free hydrogenation of benzene and cyclohexene

Synthesis of stable hybrid iridium@graphene nanomaterials in the ionic liquid (IL) 1-butyl-3-methyl-imidazolium tetrafluoroborate ([BMIm][BF4] through microwave irradiation (MWI) or electron-beam (e-beam) irradiation (IBA Rhodotron accelerator) induced decomposition of Ir4(CO)12 in the presence of graphene provides an easy method for the generation of small iridium nanoparticles with size distributions of 1.0 ± 0.4 and 2.7 ± 0.7 nm by MWI reactions with 90 and 60 min decomposition time, respectively, and of 3.6 ± 1.0 nm for e-beam irradiation synthesis. Graphene was derived by thermal reduction of graphite oxide (TRGO). Powder X-ray diffraction (PXRD), transmission electron microscopy (TEM) and energy-dispersive X-ray spectrometry (EDX) showed the formation of Ir nanoparticles which are evenly distributed on the TRGO sheets. Ir@TRGO proved to be a highly active (∼10000 mol cyclohexane x(mol Ir)−1x h−1 for benzene hydrogenation) and selective heterogeneous catalyst for the industrially relevant hydrogenation of benzene or cyclohexene to cyclohexane under mild conditions (100 °C, 10 bar H2) with quantitative conversion. The catalyst could be re-used over 10 consecutive hydrogenation reactions with similar activities. A brief correlation between activity and particle size points to an optimal diameter or surface regime for the catalytic activity with iridium particles of 3.6 ± 1.0 nm on TRGO giving here the highest activity in benzene hydrogenation.

Kinetics of Liquid - Phase Hydrogenation of Straight Chain C10 to C13 Di-Olefins Over Ni/Al2O3 Catalyst

Straight chain C10 to C13 di-olefins were selectively hydrogenated to increase the mono-olefins content in the feed to alkylation reactor in the process of production of Linear Alkyl Benzene (LAB). The reaction was carried out in a liquid phase up-flow fixed bed reactor at temperature of 448-503 °K and pressure of 1.08-1.96 MPa, which keeps hydrogen dissolved in the hydrocarbon feed. Under the above process conditions the reactor will be virtually two phase (solid-liquid) instead of three phase (solid-liquid-vapour). Kinetics of hydrogenation of straight chain C10 to C13 di-olefins on nickel – alumina (Ni/Al2O3) catalyst was studied in the temperature range of 458-488 °K. The reaction scheme considered includes two consecutive hydrogenation reactions. Various rate models based on the modified Power Law, Horiuti-Polanyi mechanism & proposed by Somers. A.; were derived for the two consecutive hydrogenation reactions and subjected to model discrimination. Parameter estimation was done utilizing the Levenberg-Marquardt Algorithm for global convergence using MATLAB software. Out of the various models tested, rate model based on power law with modification fitted the data well. The estimated rate constants of the best model are thermodynamically sound and statistically consistent.

Reactions of gas-phase H atoms with atomically and molecularly adsorbed oxygen on Pt(111)

The interaction of gas-phase H atoms with ordered and disordered adlayers of atomic oxygen, hydroxyl, and molecular oxygen on Pt(111) surfaces was investigated by in situ mass spectrometry and post-reaction TPD (temperature programed desorption). Exposure of oxygen adlayers to gas-phase H atoms at 85 K leads to formation of H2O via two consecutive hydrogenation reactions: H(g)+O(a)→OH(a) followed by H(g)+OH(a)→H2O(g,a). Both reaction steps are highly exothermic, and nascent H2O molecules partially escape into the gas phase before being thermally accommodated on the surface. Empty surface sites and hydrogen bonding promote thermalization of H2O. Separate experiments performed with OH-covered Pt(111) surfaces reveal that the hydrogenation of hydroxyl is a slow reaction compared to the hydrogenation of atomic oxygen; additionally, the abstraction of H from OH by gas-phase D atoms, OH(a)+D(g)→O(a)+HD(g), was detected. Abstraction of H from adsorbed H2O was not observed. Admission of gas-phase H atoms to O2-covered Pt(111) surfaces at 85 K leads to the desorption of O2 and H2O. The thermodynamic stability of the HO2 radical suggests that the reaction is initiated by hydrogenation of molecular oxygen, O2(a)+H(g)→HO2. The intermediate HO2 either decomposes via dissociation of the HO–O bond, HO2→OH(a)+O(a), finally leading to the formation of H2O (∼85%), or via dissociation of the H–O2 bond thus leading to desorption of O2 (∼15%). The whole reaction sequence of formation and decomposition of HO2 is fast compared to the formation of H2O via hydrogenation of atomic oxygen and hydroxyl. The observed coverage dependence of the reaction kinetics indicates the dominance of hot-atom mediated reactions.

Reaction calorimetry as an in-situ kinetic tool for characterizing complex reactions

Abstract Due to its ability to provide directly reaction rate data and its in-situ nature, reaction calorimetry has become one of the most powerful probes of reaction pathways and mechanisms of chemical reactions by virtue of providing high-quality kinetic data. In this paper, examples of enantioselective hydrogenation and selective consecutive hydrogenation reactions are presented to demonstrate the high quality of kinetic data obtainable from the reaction calorimetry. They are also used to illustrate the use of reaction calorimetry for elucidating reaction pathways and mechanisms from detailed kinetic and thermodynamic information about individual step involved in multi-step reactions that is otherwise difficult to obtain without calorimetry.

Kinetic analysis of the liquid phase hydrogenation of 2‐ethyl‐hexenal in the presence of supported Ni, Pd and Ni‐S catalysts

AbstractThe kinetics of the consecutive hydrogenation reactions 2‐ethyl‐hexenal → 2‐ethyl‐hexanal → 2‐ethyl‐hexanol were studied in the liquid phase in presence of commercial Ni, Pd and Ni‐S catalysts. The Pd and Ni‐S catalysts were extremely selective with respect to the formation of 2‐ethyl‐hexanal, while the full reaction sequence was readily catalyzed by nickel.A Langmuir‐Hinshelwood model with dissociative hydrogen adsorption was found to be the most probable model, for all three catalysts. The proposal of dissociative hydrogen adsorption of the Ni‐S catalyst was supported by an independent gas‐phase experiment, using the H2/D2 exchange reaction as a model reaction for the hydrogen àdsorption process.

Influence of hydrogen pressure on selectivity in consecutive hydrogenation reactions

Etude par simulation mathematique, en vue de demontrer comment la vitesse d'adsorption influence l'ordre apparent d'une reaction et l'influence de l'hydrogene sur la selectivite de reactions d'hydrogenation consecutive