Heterolytic Dissociation Of Hydrogen Research Articles

Ru(III) single site solid micellar catalyst for selective aqueous phase hydrogenation of carbonyl groups in biomass-derived compounds

Catalytic processes in water have a lower environmental impact, cost, and toxicity than in organic solvents. Considering the high content of water in biomass, it would be natural to use aqueous phase catalytic technology for the production of valuable products. However, in the aqueous phase, most metal-based catalysts suffer from low activity, low selectivity and deactivation due to metal oxidation and leaching. In this paper, we propose a solid micellar Ru catalyst (Ru(III)@MCM) based on single-site Ru(III) species stabilized by cetyltrimethylammonium (CTA+) surfactant and immobilized in the walls of MCM-41 for the selective aqueous phase hydrogenation of carbonyl groups. This catalyst demonstrates exceptional selectivity, activity, and stability in comparison with conventional metallic catalysts. DFT modeling suggests that the reaction proceeds via heterolytic dissociation of hydrogen, forming a Ru-Hydride species, and subsequent hydride transfer to the carbonyl group. Water plays a key role in avoiding product inhibition.

In Situ Generation of Brønsted Acidity in the Pd-I Bifunctional Catalysts for Selective Reductive Etherification of Carbonyl Compounds under Mild Conditions

Selective synthesis of ethers from biomass derived carbonyl compounds is an important academic and industrial challenge. The existing processes based on strong acid or metallic catalysts cannot provide high selectivity to ethers due to the occurrence of side reactions. Hereby we propose a Pd-I bifunctional heterogeneous catalyst for the selective reductive etherification of aldehydes with alcohols. Extensive catalyst characterizations uncovered the presence of iodine species on the surface of Pd nanoparticles. Heterolytic dissociation of hydrogen on the I-Pd surface sites leads to the in situ generation of a Bronsted acid, which promotes the reaction toward the corresponding ethers with extremely high selectivity under very mild reaction conditions.

Low-Dimensional Indium Oxo-Species on the Surface of In2O3/Al2O3 Catalytic Material: The Sites of Dissociative Adsorption of Hydrogen

The formation and nature of surface indium species in the alumina-supported indium oxide (In2O3/Al2O3) catalytic material has been investigated by means of UV–vis spectroscopy and IR spectroscopy of adsorbed probe molecules CO and H2. It has been found for the first time that high-temperature reduction of the material in hydrogen results in substitution of protons of surface hydroxyl groups of the alumina support by indium (In+) cations that form low-dimensional surface cationic clusters/associates. Oxidation of the surface In+ cations leads to the formation of low-dimensional surface indium oxo-species. These oxo-species have been found to be active in heterolytic dissociation of hydrogen, which occurs with the formation of surface indium hydrides and hydroxyl groups. It has been suggested that the property of In2O3/Al2O3 material to catalyze oxidative dehydrogenation of hydrocarbons reported in the literature is due to the low-dimensional indium oxo-species in which indium cations are bonded to alumina ...

Kinetic Modeling of the Effect of H2S and of NH3 on Toluene Hydrogenation in the Presence of a NiMo/Al2O3 Hydrotreating Catalyst. Discrimination between Homolytic and Heterolytic Models

Toluene hydrogenation was studied within a wide range of hydrogen sulfide, hydrogen, toluene, and ammonia partial pressures on a standard sulfided NiMo/Al2O3 hydrotreating catalyst at 350 °C under a total pressure of 4.9−9.8 MPa. The results showed a complex inhibiting effect of H2S on the hydrogenation activity with an order of reaction relative to H2S varying between −0.05 and −0.5. Unexpectedly, this inhibiting effect was enhanced by the presence of ammonia. Several kinetic models based on the homolytic or heterolytic dissociation of hydrogen and hydrogen sulfide were investigated and discriminated by using the CHEMKIN/SURFACE CHEMKIN II tool. The heterolytic dissociation was supposed to occur on centers composed of an unsaturated Mo ion (on which hydride ions and organic molecules can adsorb) and of a sulfur anion, host of the proton generated by the heterolytic dissociation of hydrogen or hydrogen sulfide. It was concluded that toluene hydrogenation was most likely to occur through a heterolytic mech...

Theoretical study of hydrogen dissociation on dialuminum oxide clusters and aluminum–silicon–oxygen clusters, as models of extraframework aluminum species and zeolite lattice

The reactivity of coordinatively unsaturated centers of aluminum oxide clusters, found for example in extraframework aluminum species of steamed zeolites, was examined theoretically on the example of the reaction of a hydrogen molecule with dialuminum hydroxide clusters (HO) 2(H 2O) n AlOAl(OH) 2(H 2O) ( 1). In the cluster, one aluminum atom is tricoordinated and the other is tetracoordinated ( 2, n=0), or both aluminum atoms are tetracoordinated ( 3, n=1). The system studied is also a model for transitional aluminas, in which two or three tetracoordinated aluminum centers can occur next to each other. Density functional theory calculations with electron correlation at the B3LYP/6–31G ∗∗ level have identified a complex with physisorbed hydrogen and a complex with chemisorbed hydrogen in each case. Each of them was more stable than the corresponding complex formed by the corresponding one-aluminum cluster. The transition structures for chemisorption were identified. The reaction coordinate for chemisorption revealed that the reaction is a case of metal ion catalysis, rather than an acid–base reaction with heterolytic dissociation of hydrogen. The potential energy barrier (PEB) for hydrogen chemisorption was lower for the two-Al clusters than for the one-Al clusters. The chemisorption on a silicon–aluminum cluster, (HO) 2(H 2O)AlOSi(OH) 3 was also found to occur, but it had a higher PEB than for the corresponding two-aluminum cluster. Thus, zeolites can also exchange hydrogen, albeit less effectively than alumina, whereas the extraframework aluminum species in steamed zeolites should exchange hydrogen easier than the intact zeolite.

Molecular aspects of the H 2 activation on MoS 2 based catalysts — the role of dynamic surface arrangements

In order to model one of the key steps in hydrosulfurization (HDS), density functional calculations of the dissociation of H 2 on the edges of a Co promoted MoS 2 (Co-Mo-S) slab have been performed. The results show that the heterolytic dissociation of hydrogen is not favored, but there appears to exist different homolytic pathways with similar activation barriers. It is shown that the dissociation process couples strongly to flexible sulfur atoms such that dynamic surface reconstructions occur during the adsorption process. By calculating the minimum energy reaction path we find that S atoms at the edge of the MoS 2 slabs can move up to extract one of the H atoms of the dissociating H 2 molecule and transport it to the other side of the slab where it diffuses on. Transition metal sulfides constitute an important class of heterogeneous catalysts and the active part of many enzymes, and we suggest that the catalytic activity of these systems may be closely coupled to the flexibility of the sulfur ligands.

CO hydrogenation on alumina pre-treated with sodium acetate aqueous solution

CO hydrogenation was studied over various alumina catalysts, untreated, pre-treated with sodium acetate (NaOAC) and calcined at 773 K after treatment. The catalysts were characterized by means of XRD and IR spectroscopy after pyridine adsorption. An apparent increase of ethylene selectivity over NaOAC pre-treated catalyst was observed. Both CO conversion and ethylene selectivity depended on the temperature of CO hydrogenation and a new γ-Al2O3 layer formed by surface boehmite transformation at elevated temperature on the original alumina surface. Several probe molecules, including HC1, NH3 and pyridine, were used to detect the active sites for CO hydrogenation. There are two kinds of active sites (e.g. Lewis acidic and basic sites) which seem to be involved in the heterolytic dissociation of hydrogen to form a hydroxyl. Thus, the hydroxyl possibly plays a very important role in the formation of formyl, which may be the intermediate of the methoxide, and the methoxide seems to be the intermediate for hydrocarbons.

On the nature of surface sites for heterolytic dissociation of hydrogen on Al2O3

A quantum-chemical analysis of the models for geminal OH groups of Al2O3 and of the processes of their dehydroxylation with further dissociative chemisorption of hydrogen has been carried out. Calculations were performed by the SCF MO LCAO method using STO-3G basis set in terms of the cluster approach.

Heterolytic dissociation of hydrogen on high-temperature methanol synthesis catalysts

The heterolytic dissociation of hydrogen is thought to constitute the hydrogen-activation step in the mechanism of methanol synthesis. No data have been reported until now for the adsorption of hydrogen on the high-temperature methanol synthesis catalysts based on Zn/Cr mixed oxides, with a Zn/Cr atomic ratio higher than 0.5. In the present communication results of an IR study of the adsorption of hydrogen and deuterium on stoichiometric (Zn/Cr = 0.5) and nonstoichiometric zinc-rich spinels (Zn/Cr = 0.6 and 1.0) are reported. X-ray diffraction (XRD) analysis of such samples indicates that all samples are monophasic, showing only the presence of the spinel-type phases. Furthermore, these samples were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), with an energy dispersive X-ray analyzer (EDX), and the presence of particles containing only zinc was not found. 30 references.

Isotopic study of hydrogen adsorption on magnesium oxide powders

By measuring temperature-programmed desorption (t.p.d.) spectra of hydrogen adsorbed on MgO powders outgassed at 1123 K, eight different states of adsorbed species have been found. Species W1 is reversibly adsorbed at 77 K and other irreversible species desorb with maximum rates at 190 (W2), 229 (W3), 293 (W4), 327 (W5), 460 (W6), ca. 500 (W7) and 608 K (W8). All the species are hydrogen chemisorbed on different active sites. The numbers of active sites are > 1.7 (W1), 32±3 (W2+W3), 23±3 (W4+W5), 1.9±0.5 (W6+W7) and 2.3±0.5 (W8)× 1015 site m–2 and the total number of active sites except for type W1 amounts to ca. 60 × 1015 site m–2.A dominant species W5 has been studied in detail. T.p.d. spectra, observed by using a mass-filter, of HD molecules have shown that molecular identity is conserved in the desorption process. This implies that the adsorbed species is immobile and localized on the surface and that pseudo-first-order kinetics controls the desorption process. The activation energy for H2 desorption at the peak maximum is 99.4±0.6 kJ mol–1, less than that for D2 desorption by only 1.5 kJ mol–1. The heat of adsorption is roughly estimated to be 90 kJ mol–1. The values of isotope effects (H/D) observed for species W5 on the sample having nearly half-coverage at 308 K are 2.2±0.2 and 0.71±0.02 for the adsorption rate and the equilibrium, respectively. It is revealed, on the basis of these isotope effects and available i.r. spectra on adsorption, that the formation of species W5 is consistent with a heterolytic dissociation of hydrogen on a surface ion pair O2–LC—Mg2+LC with very low coordination numbers. O—H and Mg—H bonds formed have no hydrogen bonding with neighbouring ions and behave as ‘free’ groups. Their frequencies, including bending modes, have been assigned.Species W2(and/or W3), dominant in the lower-temperature region, also seems to be formed via the heterolytic dissociation of hydrogen. Its adsorption process requires a higher activation energy than that of species W5, while the desorption process, controlled by first-order kinetics, requires a lower activation energy than that of species W5.