On the catalytic activity of cluster anions in styrene hydrogenation: considerable enhancements in ionic liquids compared to molecular solvents

Many low oxidation state transition metal (carbonyl) clusters are salts and are highly soluble in ionic liquids. Since many are also effective catalysts/precatalysts for hydrogenation reactions, we have conducted a catalytic study of these compounds in ionic liquids and found that the activity of certain clusters to hydrogenate alkene substrates, when immobilised in ionic liquids, is up to 3.6-fold faster than that observed in organic solvents. Using high-pressure NMR it has been possible to trace the improvements in activity to the increased stability of the cluster species in the ionic liquid. The ionic liquid also gives rise to higher regioselectivity in the hydrogenation of cyclic dienes to monoenes compared to that observed in organic solvents. (C) 2004 Elsevier B.V. All rights reserved.

Nanoparticle - Ionic Liquid Based Catalytic Systems

Alena Karakulina

In recent years ionic liquids have emerged as an important class of compounds for the synthesis of metal nanoparticles. The significant advantage of using ionic liquids is their dual role of reaction solvent and nanoparticle stabilizer. The variability of ionic liquids enables their chemical properties to be tuned by the rational introduction of functional moieties. Metal nanoparticles and ionic liquids appear to be ideal complementary components for the construction of multifunctional catalytic systems. The development of multifunctional systems based on the combination of catalytically active nanoparticles and functionalized ionic liquids allows the integration of complex multi-step catalytic reactions in a one-pot process. The design of efficient metal nanoparticle ¿ functionalized ionic liquid catalysts is currently attracting much attention. The thesis is centered on the design, synthesis, characterisation and the catalytic applications of novel metal nanoparticle ¿ ionic liquid based catalytic systems. In the first chapter a general introduction to the field of metal nanoparticles immobilized in ionic liquids is presented, and the recent evolution of metal nanoparticle ¿ acidic ionic liquid multifunctional catalysts is reviewed. The second chapter covers the studies on chlorometallate ionic liquids. Lewis acidity of various chlorometallate systems was evaluated with respect to the speciation present in these systems. The speciation in chlorozincate ionic liquids was determined by different techniques. The obtained scale of Lewis acidity of the chlorometallate ionic liquids and the understanding of the speciation in these systems allowed accurate selection of the appropriate system for catalytic applications. The third and the fourth chapters describe the studies concerning the ability of the chlorometallate ionic liquids to function in a cooperative manner with rhodium nanoparticles in hydrogenation reactions. A novel bifunctional catalyst, consisting of rhodium nanoparticles dispersed in a Lewis acidic ionic liquid medium, was developed. In the third chapter the rhodium nanoparticle component of the catalyst is characterized. Subsequently, the combined bifunctional catalytic system was used to catalyse the hydrogenation of aromatic compounds and was found to exhibit excellent activity under mild conditions. In the fourth chapter, the high activity of the rhodium nanoparticle ¿ Lewis acidic ionic liquid catalyst the direct hydrogenation of challenging heteroarene substrates is demonstrated. Chlorozincate(II), chloroaluminate(III) and chlorogallate(III) ionic liquids were found to be effective as a second active component of the catalytic system. The chemoselective reduction of different quinolines, pyridines, benzofurans was performed at mild conditions ¿ 30 bar and 80-120°C and the corresponding heterocycles were obtained in high yields. The high selectivity of the catalyst and its tolerance to different functional groups was demonstrated on a broad range of substrates. The recyclability of the rhodium nanoparticle ¿ Lewis acidic ionic liquid catalyst was evaluated. The final chapter presents two reduced graphene oxide supported rhodium nanoparticle catalysts obtained in ionic liquid media via microwave-assisted and conventional thermal heating methods. The rhodium nanoparticle-reduced graphene oxide composites were characterized by different techniques. The catalytic performance of the rhodium nanoparticle-reduced graphene oxide composites was evaluated in the hydrogenation of quinoline compounds under mild conditions, i.e. 10 bar and 80°C. The advantages of the microwave-assisted ionic liquid synthesis for the production of highly efficient catalysts were demonstrated. The long-term stability of the catalyst obtained by microwave-assisted method was studied and the catalyst could be recycled several times without significant loss in activity and selectivity.

EPFL2016

Novel catalytic applications of carbon nanofibers on sintered metal fibers filters as structured supports

Marina Ruta

Supported metal catalysts are important from both an industrial and a scientific point of view. They are used, amongst others, in large-scale processes such as catalytic reforming, hydrotreating, polymerization reactions and hydrogenations. Often, these catalysts consist of nanosized metal particles deposited on a suitable support, which acts as an anchor for the active phase and, in several cases, contributes to improve the overall catalyst performances. The growth of carbon nanofibers on sintered metal fibers filters (CNF/SMF) by hydrocarbons catalytic decomposition results in a structured composite material presenting all the suitable characteristics for its application as catalyst support. The main objective of this thesis was to develop novel catalytic systems based on CNF/SMF as catalytic support for continuous gas-phase hydrogenations. At first, the synthesis of CNF/SMF was optimized to tailor the properties of the material for further applications as catalytic support. The use of ethylene as carbon precursor and high synthesis temperature (973K) provided high yields of well-ordered CNF with increased specific surface area (SSA), up to 516 m2/g. On the other hand, the synthesis of CNF by ethane decomposition, which is less reactive than ethylene, resulted in a support with higher permeability, minimizing the pressure drop and being more suitable in a continuous-flow reactor. After the synthesis, the CNF surface was activated by treatment with O3 or H2O2. A novel technique, which consist of XPS analysis after step-wise TPD in UHV conditions, was developed and applied for the surface characterization. It allowed to assess the nature of the functional groups created on the CNF surface. The O3-activation was found the most effective for increasing the acidity of the CNF/SMF surface, yielding a relatively high amount of carboxylic functional groups. These groups are important because involved in the chemical anchoring and stabilization of the active metal deposited on the support. Industrially, many hydrogenations are performed over supported metal catalyst. The majority of these reactions are known to be structure sensitive reactions, so the control of the metal particle size is crucial in order to study the catalyst activity and selectivity. We prepared monodisperse-sized Pd nanoparticles (8, 11 and 13 nm) via the reverse microemulsion method and deposited on CNF/SMF and activated-CNF/SMF for a two-fold study: the effect of the nanoparticles size and the effect of the support nature on the selective hydrogenation of acetylene. The antipathetic size dependence of the TOF disappeared at particle size bigger than 11 nm. The initial selectivity to ethylene (∼60%) was found size-independent. The structure-sensitivity relations have been discussed in terms of "geometric" and "electronic" nature of the size-effect and rationalized assuming a Pd-Cx phase formation which is known to be size-dependent. CNF/SMF supports with increased acidity diminished the formation of coke and changed the by-products distribution, diminishing the catalyst deactivation. Subsequently, CNF/SMF was applied to develop a new catalytic system: the Structured Supported Ionic Liquid Phase (SSILP) catalyst, embedding an active Rh complex. The selective gas-phase hydrogenation of 1,3-cyclohexadiene to cyclohexene was used as model reaction. The SSILP based on CNF/SMF supports showed a high turnover frequency, up to 250 h-1 and selectivity > 96%. Multiple advantages of supporting a homogeneous active phase, dissolved in IL, on CNF/SMF have been demonstrated: the thin layer of IL was homogeneously distributed over the mesoporous support, so mass transfer limitations could be avoided, furthermore the chemical inertness of the CNF prevented any interaction between the active phase dissolved in IL ensuring an efficient use of the Rh complex. The SSILP concept can be applied not only for the "heterogenization" of a transition metal catalyst, but also to obtain metal nanoparticles with monodispersed size via the reduction of a metal precursor dissolved in IL. In this case, IL provides a suitable environment for the nucleation and growth of the nanoparticles. By applying the SSILP concept to the selective hydrogenation of acetylene, the influence of IL on the stability of the nanoparticles was investigated. Moreover, the different solubilities of reactants and products in the IL phase allowed to improve the selectivity toward ethylene. Monodispersed Pd nanoparticles of 5 and 10 nm in SILP on CNF/SMF showed excellent long-term stability. The lower solubility of ethylene than acetylene in the IL had the beneficial effect of allowing high selectivity to ethylene up to 85%, due to the inhibition of its consecutive hydrogenation to ethane. In conclusion, a novel catalytic system for continuous-flow, gas-phase hydrogenations was conceived, taking advantage of the suitable characteristics of CNF/SMF supports. The SSILP concept, involving both homogeneous and heterogeneous active phases has been demonstrated, indicating a great potential and flexibility for continuous-flow industrial applications.

EPFL2008

Switching the Mechanism of Catalyst Activation by Ionic Liquids

Corinne Daguenet, Paul Joseph Dyson

In ionic liqs. (ILs), the activation of a ruthenium(II) catalyst precursor occurs via an unexpected mechanism, which is different from the activation mechanism that takes place in water. The effectiveness of a biphasic catalytic hydrogenation reaction using the ruthenium(II) precatalyst is highly dependent on the nature of both anion and cation of the IL. Evidence is provided that chloride ions are required to generate the active catalyst. Moreover, the influence of the IL cation on the rate of hydrogenation is correlated with the differences in capability of the IL to solvate chloride. A kinetic study provides insights into the mechanism of the formation of the active catalytic species.