Rational Catalyst Design for Selective Hydrogenations

Daniel André Samuel Lamey
EPFL, 2015
EPFL thesis

Abstract

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.

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https://infoscience.epfl.ch/record/206852?ln=en

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Daniel André Samuel Lamey
EPFL, 2015
EPFL thesis

Abstract

Official source

https://infoscience.epfl.ch/record/206852?ln=en

About this result

Related concepts (38)

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Related publications (108)

Related publications

Related publications (108)

Structured Non-Noble Metal Catalysts for the Liquid-Phase Reduction of Nitroaromatics

Oliver Anthony Beswick

The industrial production of aromatic amines, essential intermediates for the synthesis of several drugs, dyes, pigments and agrochemicals, requires the use of heterogeneous catalysis. These molecules were formerly produced by the Béchamp reduction, a non-catalytic batch process requiring costly downstream separation steps. Catalytic approaches afford the implementation of continuous processes producing less waste. While it is generally not the primary goal when improving processes to make them more environmentally friendly, it is often one of the consequences, thereby improving the image of the chemical industry. The development of new catalysts is one of the major axes for optimizing industrial production of chemicals. Often multidisciplinary, it involves a better understanding of existing systems, advances in characterization and modelling techniques for the discovery of new catalytic materials. This thesis aims at designing new catalysts based on non-noble metals (e.g. Fe, Co, Ni) for the chemoselective hydrogenation of nitroaromatics in the liquid phase. The use of activated carbon fibres (ACFs) as a structured support has been at the centre of the strategy applied during this work in order to develop new catalysts capable of surpassing the best systems based on Fe, Co or Ni. The high porosity of ACFs, composed of very thin pores (~ 2 nm) and a high specific surface area, has been exploited to generate and stabilize highly dispersed crystallites of active phase. The formation of ~2 nm metal oxides (MOx) nanoparticles (NPs) resulting from the impregnation/pyrolysis of Fe, Co or Ni precursors within the ACFs was performed. The reduction of nitroaromatics with different functional groups was carried out by catalytic hydrogenation with molecular hydrogen and by catalytic transfer hydrogenation (CTH) with hydrazine as reducing agent. The MOx/ACF catalysts were shown to be active and selective under mild conditions in the CTH of para-chloronitrobenzene (p-CNB) to para-chloroaniline (p-CAN). The FeOx/ACF was tolerating many substituents of nitroarenes during the -NO2 hydrogenation. The CoOx/ACFHNO3 catalyst was less active compared to the Fe-based catalyst but afforded a meta-vinylaniline (m-VA) yield above 99% during the reduction of the meta-nitrostyrene (m-NS), exceeding, to the best of our knowledge, the best performance reported to date. The very low activity of the NiOx/ACF catalyst in CTH has prompted us to test Ni-based catalyst in the hydrogenation of nitroarenes with gaseous hydrogen. Raney Nickel is a well-known catalyst used in industry for this reaction. First, the existence of particles size effects was evaluated using unsupported Ni PVP-stabilized NPs (2¿14 nm). An antipathetic structure sensitivity of the meta-dinitrobenzene (m-DNB) hydrogenation to meta-nitroaniline (m-NAN) was demonstrated, showing a 4.6-fold increase in the turnover frequency over the 14 nm NPs. Once supported on ACFs and cleaned from PVP by a UV-ozone treatment, the 2 nm Ni NPs afforded an increase of selectivity towards m-NAN from 34 to 96% at close to full conversion (Xm-DNB = 99%). The selective transformation favouring m-NAN has been attributed to reactions taking place at the edges and vertices of the NPs. The Langmuir-Hinshelwood two-site kinetic model was consistent with the experimental data. The incorporation of gold to form Ni-Au (1:1) bimetallic NPs led to a significant increase in the selectivity (99%), possibly by mimicking the electronic

EPFL2017

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

Catalyst design is of major importance for fine chemicals industry due to complexity of the synthesis and high product quality requirements. Moreover, the catalyst formulation has to comply with the standards of health, safety and environmental regulations. The conventional systems often do not satisfy the latter criteria and, therefore, have to be improved to become both efficient and eco-friendly. The advancing technologies of materials synthesis and characterization allow catalyst development from model systems (i.e. colloidal metal nanoparticles (NPs)) towards real heterogeneous catalysts for industrial implementation. The main approach taken in this thesis is based on surface and electronic structure design of the catalytically active NPs (Pd, Pt) by means of size control (2.1 – 9.8 nm) and modification by a second component (Ag, Cu, Zn) during the catalyst preparation. We analyzed the size effect of unsupported Pd NPs in the selective hydrogenation of alkynols with different alkyl chains, i.e. C16 in dehydroisophytol (to isophytol) and C1 in methylbutynol (to methylbutenol). Antipathetic structure sensitivity was established where larger Pd NPs exhibit intrinsically higher (up to ca. 2-fold) specific activity (per Pd surface atom). A higher Pd dispersion delivered greater yields to isophytol (89 -> 93%), while the product distribution in methylbutynol hydrogenation was insensitive to NP size. The distinct size effect on hydrogenation performance for both α-alkynols is associated with modifications in the adsorption strength linked to the hydrocarbon chain length. Furthermore, the incorporation of Ag or Cu in Pd NPs had a critical impact on yield to the target isophytol (up to 97%). This result is attributed to the dilution of the Pd surface sites by a second metal (Ag, Cu) and a modification of the Pd electronic properties. Pd-Ag NPs, having shown the highest selectivity, were further deposited on a structured support based on sintered metal fibers (SMF) coated with ZnO. The improved selectivity achieved over the unsupported Pd-Ag colloidal particles was retained over the structured catalytic system. The resulted Pd-Ag/ZnO/SMF (patent WO 2013/060821 A1) represents a viable alternative to the conventional Lindlar catalyst. Another bimetallic system was studied in selective 3-nitrostyrene hydrogenation, where Pt catalyst was modified by alloying with Zn through the reactive metal– support interactions. Pt/ZnO catalyst containing monodispersed Pt NPs (ca. 2.3 nm) favored 3-vinylaniline formation (97%) showing a remarkable activity (close to that of the benchmark Pt catalysts), i.e. the catalyst had superior combined selectivity/activity response as compared to the reported literature. Additionally, the catalytic tests were performed over bimetallic Pt-Zn NPs embeded within the hypercross-linked polystyrene (HPS) support. The controlled pore size of HPS (ca. 4 nm) allows immobilization and a NP size control by confining Pt within the nanocavities of the polymeric matrix. The catalytic results demonstrated boosted yields towards 3-vinylaniline (16 -> 97%) over Pt-Zn/HPS catalyst as compared to monometallic Pt/HPS. The findings presented here over monodispersed NPs establish the basis of catalyst development for the selective production of fine chemicals. The product distribution can be controlled on a nano-level by tuning the properties of the active phase through the particle size optimization and incorporation of a second metal.

EPFL2014