Selective three-phase hydrogenation of aromatic nitro-compounds over β-molybdenum nitride

A tetragonal molybdenum nitride (β-Mo2N) has been prepared by temperature programmed treatment of MoO3 in flowing N2 + H2 and for the first time shown to catalyze the liquid phase selective hydrogenation (T = 423 K; PH2=11bar) of a series of para-substituted (-H, -OH, -O-CH3, -CH3, -Cl, -I and -NO2) nitrobenzenes to give the corresponding aromatic amine. Reaction over Pd/Al2O3, as a benchmark catalyst (Pd particle size ca. 18 nm), resulted in a composite hydrodechlorination/ hydrogenation of p-chloronitrobenzene (as a representative nitroarene) to generate nitrobenzene and aniline. β-Mo2N has been characterized in terms of temperature-programmed reduction (TPR), H2 chemisorption/temperature programmed desorption (TPD), BET surface area/pore volume, elemental analysis, powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning (SEM) and transmission (TEM) electronic microscopy. Elemental analysis, XRD, SEM and TEM have confirmed the formation of tetragonal β-Mo2N, characterized by an agglomeration of flake-like crystallites. Post-synthesis, the nitride was passivated by contact with 1% (v/v) O2/He at ambient temperature and XPS analysis has demonstrated the formation of a superficial passivating oxide overlayer without bulk oxidation. Pre-reaction, activation by TPR to 673 K was necessary to remove the passivating film. Hydrogen TPD has revealed significant hydrogen uptake (0.7 μmol m-2) associated with β-Mo2N. Nitro group reduction kinetics have been subjected to a Hammett treatment where the reaction constant (p = 0 4) is diagnostic of an increase in rate due to the presence of electron-withdrawing substituents on the aromatic ring, consistent with a nucleophilic mechanism. The results presented in this study establish the viability of β-Mo2N to promote selective nitroarene hydrogenation. © 2011 Elsevier B.V. All rights reserved.

Selective three-phase hydrogenation of aromatic nitro-compounds over beta-molybdenum nitride

Fernando Cardenas Lizana, Lioubov Kiwi, Daniel André Samuel Lamey

A tetragonal molybdenum nitride (beta-Mo2N) has been prepared by temperature programmed treatment of MoO3 in flowing N-2 + H-2 and for the first time shown to catalyze the liquid phase selective hydrogenation (T = 423 K; P-H2 = 11 bar) of a series of para-substituted (-H, -OH, -O-CH3, -CH3, -Cl, -I and -NO2) nitrobenzenes to give the corresponding aromatic amine. Reaction over Pd/Al2O3, as a benchmark catalyst (Pd particle size ca. 18 nm), resulted in a composite hydrodechlorination/hydrogenation of p-chloronitrobenzene (as a representative nitroarene) to generate nitrobenzene and aniline. beta-Mo2N has been characterized in terms of temperature-programmed reduction (TPR), H-2 chemisorption/temperature programmed desorption (TPD), BET surface area/pore volume, elemental analysis, powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning (SEM) and transmission (TEM) electronic microscopy. Elemental analysis, XRD, SEM and TEM have confirmed the formation of tetragonal beta-Mo2N, characterized by an agglomeration of flake-like crystallites. Post-synthesis, the nitride was passivated by contact with 1% (v/v) O-2/He at ambient temperature and XPS analysis has demonstrated the formation of a superficial passivating oxide overlayer without bulk oxidation. Pre-reaction, activation by TPR to 673 K was necessary to remove the passivating film. Hydrogen TPD has revealed significant hydrogen uptake (0.7 mu mol m(-2)) associated with beta-Mo2N. Nitro group reduction kinetics have been subjected to a Hammett treatment where the reaction constant (p = 0 4) is diagnostic of an increase in rate due to the presence of electron-withdrawing substituents on the aromatic ring, consistent with a nucleophilic mechanism. The results presented in this study establish the viability of beta-Mo2N to promote selective nitroarene hydrogenation. (C) 2011 Elsevier B.V. All rights reserved.

2011

Rational Catalyst Design for Selective Hydrogenations

Daniel André Samuel Lamey

Catalyst design for selective hydrogenations is of major importance for the manufacturing of fine chemicals. Catalytic procedure which uses scarce and expensive noble metals is very challenging in terms of exclusive attack of a single functionality or substituent. The approach taken in this thesis is based on rational catalyst design that calls on a combination of catalyst synthesis and characterization. This has been applied for the design of catalysts suitable for industrially relevant reactions, i.e. the partial catalytic reduction of substituted nitroaromatic compounds and alkynes. Molybdenum nitrides were selected as they represent a more sustainable alternative to noble metals, less expensive and easy to prepare. First, beta-Mo2N was used to catalyze the liquid phase selective hydrogenation of a series of para-substituted nitroarenes to give the corresponding aromatic amine. Incorporation of low amounts (0.25% wt.) of Au nanoparticles on beta-Mo2N enhanced hydrogen uptake and catalytic activity while delivering an ultraselective response. In a second step, the influence on the catalytic response of nitride crystallographic phase (beta- vs. gamma-Mo2N) and surface area (7-66 m2 g-1) was examined. Both phases promoted the exclusive hydrogenation of p-chloronitrobenzene to p-chloroaniline where the beta-form delivered a higher specific (per m2) rate; the one for gamma-Mo2N was independent of surface area. The inclusion of Au on both nitrides served to enhance p-chloroaniline production. Finally, it was shown that incorporation of N in Mo structure can increase nitrobenzene hydrogenation rates on Mo2N samples with higher nitrogen content. In contrast, -C=O hydrogenolysis was favored with benzaldehyde as a result of lower N content. As a powerful tool for tailoring metal nanoparticle morphology, colloidal methods were used to design structured catalysts effective for the selective alkyne and âNO2 group reduction. The development of a catalyst based on polyvinylpyridine modified structured carbon nanofibers on sintered metal fibers supported (4 nm) polyvinylpyrrolidone (PVP) stabilized Pd was shown to be an optimum formulation for acetylene semi-hydrogenation where the catalyst shows high selectivity (93%) and stability with time-on-stream. We have established that reducing agent does not play a critical role in catalytic response while steric (vs. electronic) stabilizers and larger colloidal metal crystallites are more efficient. PVP-stabilized Ni nanocrystals were also prepared via colloidal method and tested in the m-dinitrobenzene hydrogenation were an antipathetic structure sensitivity was recorded. TOF and selectivity to m-nitroaniline exhibited a marked dependency on the presence of the stabilizer. It was attributed to preferential adsorption of PVP on selective edge and vertex atoms, leaving the plane atoms free. Supporting the Ni nanoparticles on activated carbon fibers and removing traces of PVP by a UV-ozone treatment resulted in a dramatic increase in selectivity to target m-NAN even at high conversions. Two-sites Langmuir-Hinshelwood kinetic model has been applied to rationalize the results. In summary, this thesis demonstrates that the product distribution can be controlled on a nano-level by tuning the properties of the active phase through modifications of the structural parameters (allotropic form, composition), modifications on the nanoparticle microenvironment (organic ligand shell) and optimization of particle size.

EPFL2015

Transition metal clusters on h-BN/Rh(111)

Hamed Achour

The main aim of this dissertation is to study the catalytic aspects of very small transition metal clusters supported on h-BN/Rh(111). The catalytic reactions of interest were ammonia synthesis and CO oxidation on mass selected and soft landed iron and platinum clusters, respectively. A special focus was devoted to the investigation of the stability of these very small clusters on h-BN before and after the reaction. It was found that Pt7 clusters soft landed at room temperature with an energy of 1.2 eV per atom and annealed to 700 K exhibit Smoluchowski ripening. Above this temperature, these clusters undergo partial intercalation between the h-BN monolayer and the Rh(111). The intercalation initiates at 900 K and becomes more pronounced when the clusters are annealed under gas reaction conditions. A high catalytic activity was observed when the Pt catalyst remains supported on h-BN. In this case, the reaction starts at 480 K and follows Langmuir-Hinshelwood mechanism. However, the catalytic activity strongly reduces when the Pt clusters undergo partial intercalation. Nevertheless, in this case the reaction starts at only 380 K, revealing a reduction by 100 K in the Pt poisoning as a result of substrate effect and charge redistribution. In a second trail, the cluster-h-BN interaction was studied through h-BN irradiation with Pt clusters, at room temperature, within an energy window of 30-416 eV/atom. The results show that even irradiation at 30 eV/atom can lead to entire non thermal intercalation of the clusters. The energetic Pt clusters were found to be site selective as they settle only under the h-BN wires, in contrast to soft landed Pt7 clusters that were found to settle at the side edge of the h-BN depressions. After being exposed to irradiated Pt clusters, at an energy above 100 eV/atom, the h-BN layer starts to display visual cavity-type defects. These defects, induced by collision, disappeared after annealing to 600 K under gas reaction leaving behind an h-BN with no visual defects. CO temperature desorption spectroscopy (TDS) indicates that the intercalated Pt is inactive towards CO oxidation due to h-BN screening. Subsequently, we were able to produce a catalyst system made of soft-landed Pt/h-BN/intercalated Pt/Rh(111) using a combination between soft and energetic Pt deposition with which a reduction by 100 K in CO poisoning was obtained. Finally, ammonia synthesis was conducted on soft landed Fe clusters, supported on h-BN/Rh(111) and deposited at 100 K under Ultra High Vacuum (UHV). Scanning tunneling microscopy (STM) displays two imaging states of the as deposited clusters; a ring state surrounding h-BN depression which disappears above 300 K and a dot like structure. The Fe clusters are found to grow by Ostwald ripening after annealing to 600 K and above this temperature, they endure partial intercalation. It was found that, in the presence of iron, the nitrogen reaction gas experienced an exchange with the nitrogen species forming the h-BN layer. TPR (Temperature Programmed Reduction) of N

_2

with hydrogen revealed that nitrogen reduction occurs at 620 K under high vacuum following the Haber-Bosch method. The detection of reaction intermediates NH and NH

_2

together with desorbed atomic nitrogen, during ammonia synthesis, confirms that NH

_3

formation involves the stepwise hydrogenation of adsorbed nitrogen.

EPFL2017