Impact of organic-ligand shell on catalytic performance of colloidal Pd nanoparticles for alkyne gas-phase hydrogenation

Monodispersed Pd nanoparticles (NPs) have been prepared by colloidal technique and deposited on a structured support consisting of carbon nanofibers (CNF) grown on sintered metal fibres (SMF). The surface properties of Pd NPs have been fine-tuned by (i) changing the nature of stabilizing agent (electrostatic vs. steric), (ii) controlling Pd NPs size (2-10 nm) and (iii) grafting N-containing ligands onto the CNF/SMF surface. In the semi-hydrogenation of acetylene (T= 393 K; 13= 1 bar) catalytic response was insensitive to the nature of the reducing agent where equivalent activity/selectivity were obtained over Pd NPs with similar dispersion, prepared with the same stabilizer. A similar product distribution was recorded over Pd NPs with similar crystal size irrespective of the colloidal stabilizer (electrostatic vs. steric). In contrast, a stronger inhibiting effect on hydrogenation rate has been found with electrostatic stabilizer (sodium di-2-ethylhexylsulfosuccinate) as compared to the steric ones (polyvinylpyrrolidone or polyvinylalcohol) and assigned to geometric and electronic effects. Decrease (from 8 2 nm) in Pd NPs size results in a concomitant decrease in activity (antipathetic size-sensitivity), but higher selectivity to target ethylene product. Grafting of nitrogen-containing modifiers (polyvinylpyridine or polyethylenimine) on the CNF/SMF support results in a significant increase in olefin selectivity (up to 93%) where the catalyst shows remarkable stability during 120 h on-stream. This is explained by the electronic modifications promoted by interactions between the Pd NPs and the grafted ligands as confirmed by XPS analysis. In comparison, stabilizer-free Pd/CNF/SMF has low selectivity to ethylene (65%). In summary, controlled size Pd (core) nanoparticles with organic ligands ( shell) demonstrated increased selectivity and remarkable stability in catalytic gas-phase alkyne semi-hydrogenation opening new tools for rational catalyst design. (C) 2014 Elsevier B.V. All rights reserved.

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

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

Rational Design of Pd-Based Catalysts for Selective Alkyne Hydrogenations

Rocio Micaela Crespo Quesada

Traditionally, the search for active and selective catalysts involved a rather tedious "hit-and-miss" approach in which hundreds, if not thousands catalysts were tested until the optimal substance was identified. With the advancing theoretical understanding of catalysis and the development of computational power, a new era of rational catalyst design is dawning. This approach, grounded on first principles, is based on new advances in synthesis, characterization and modeling with the ultimate aim of predicting the expected behavior of a catalyst based on chemical composition, molecular structure and morphology. The rational design of a catalyst is a complex process which spans across several levels of scale. In this thesis, the pursuit of a rational design for Pd-based catalysts effective in alkyne hydrogenations is presented. A multi-level integrated approach was thus applied ranging from the nano-scale design of the active sites for a specific reaction, taking into account its structure sensitivity, to the micro-scale design of the supported Pd nanoparticles including metal-additive and metal-support interactions as well as mass and heat transfer phenomena. In order to rationally design a catalyst at a nano-scale, i.e. a catalyst's active site, several methodologies have been hitherto applied, such as the study on single crystals or model catalysts. Here we present the use of metal nanoparticles with tuned sizes (5-30 nm) and shapes (cubes, octahedra and cube-octahedra) prepared via colloidal techniques. These nanoparticles can be tested per se and represent a new generation of model catalysts, complementing single crystal studies, which inherently lack the complexity of industrial catalysis. However, metal nanoparticles, especially those prepared in a controlled manner in order to tailor their shape and size, require the use of stabilizing and/or capping agents capable of directing their growth. These substances can mask the true catalytic behavior of the nanoparticles. Thus, it is of great importance to study the interactions of the active phase with the substances that are in close contact with it, in the so-called meso-scaled level of rational catalyst design. Therefore, the effect of the nature of the stabilizing agent on the catalytic response was studied, as well as the promoting effect of some of these substances. Finally, a methodology was developed capable of eliminating organic stabilizing agents from the surface of nanoparticles without compromising their morphological stability. The knowledge gathered in the first two levels can be applied further to reach the microscale rational catalyst design. In this thesis, a final catalyst consisting on well-defined stabilizer-free supported Pd nanoparticles was used in the hydrogenation of acetylene. This deep study of the catalytic behavior of well-defined Pd catalysts throughout several levels of scale and complexity have given us the tools needed to perform a rational catalyst design for alkyne hydrogenations. Depending on the specific reaction, the active phase can be optimized in terms of the desired activity and selectivity and can be tuned even further with the use of specific additives. Finally, the appropriate support also exerts a promoting effect on the nanoparticles and ensures their anchoring in addition to avoiding heat and mass transfer artifacts.

EPFL2011