Recent advances in the liquid‐phase synthesis of metal nanostructures of different sizes and shapes are reviewed regarding their catalytic properties. The controlled synthesis of nanostructures is based on the colloid chemistry techniques in the solution, which use organic nanoreactors and a variety of stabilizers. Their catalytic activity and selectivity depend on the particle’s shape and size, as shown for Suzuki and Heck coupling, hydrogenations, hydrogenolysis, oxidations, and electron‐transfer reactions. The knowledge of a reaction’s structure‐sensitivity relationship is important for the rational catalyst design in view of process intensification. Nanostructures can be used per se and in supported form to meet the requirements of an eventual process. © 2009 Taylor & Francis.
Palladium nanohexagons were prepared using a seed-mediated method. Their catalytic performance in 2-methyl-3-butyn-2-ol hydrogenation was compared to the one of monodispersed Pd nanospheres. Quantitative correlations between initial turnover frequencies (TOFs) and nanoparticle surface compositions showed independence of TOFs calculated per atoms on Pd(111) facets on particle size and shape. © 2009 Springer, reproduced with permission.
The solvent-free selective hydrogenation of 2-methyl-3-butyn-2-ol (MBY) to 2-methyl-3-buten-2-ol (MBE) was studied over a Pd/ZnO structured catalyst and compared to its behavior in water-assisted conditions. The catalytic behavior was correlated with the surface properties of the catalysts which were characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The catalyst showed high selectivity and stability with the performance being superior to that of the industrial Lindlar catalyst (50%). The addition of a sulphur-containing modifier in the reaction mixture was found to affect the activity and to hinder the over-hydrogenation reaction. The MBE yield of ∼97% was attained at MBY conversion >99%. The reuse of the catalyst showed that it deactivated by a 38% and that its selectivity slightly increased (∼0.5%) over 10 runs. The reaction kinetics was modeled using a Langmuir–Hinshelwood mechanism considering competitive adsorption for the organic species and dissociative adsorption for hydrogen. The kinetic experiments were planned and the results analyzed following a design of experiments (DOE) methodology. This approach led not only to a robust model that predicts the reaction rate in a wide range of reaction conditions but also to the determination of its kinetic parameters. © 2009 Elsevier, reprinted with permission.
The present invention relates to novel Pt metal group catalysts and a process for the preparation thereof wherein a reverse microemulsion of a Pt metal in a water-in- hydrocarbon system ids prepared, some solvent is evaporated, a C1C4-alcohol is added to form a precipitate of Pt metal nanoparticles, which can be used as structured catalysts f.e. in woven fabrics.
The invention relates to a structured catalyst based on sintered metal fibers (SMF) coated by a ZnO layer impregnated with Pd-nanoparticles, reactions of organic starting material with hydrogen in the presence of said catalyst and vitamins, carotinoids, perfume ingredients, and/or food or feed ingredients prepared by using this reaction.
Systematic studies of mass transfer interactions with intrinsic reaction kinetics were performed for the three-phase selective hydrogenation of 2-methyl-3-butyn-2-ol (MBY) to 2-methyl-3-buten-2-ol (MBE) over a modified Pd/CaCO3 catalyst under solvent free conditions. Hydrogen concentration in the liquid phase (CH2,b) was monitored in situ during the catalytic reaction by means of the “Fugatron” analyzer. Reactions were carried out in an autoclave at different stirring rates at two concentrations of hydrogen (5 and 13 mol·m−3). For stirring speeds higher than 1500 rpm no influence of gas−liquid mass transfer was observed. Hydrogen liquid−solid (L-S) mass transfer was found to be negligible, whereas the MBY mass L-S transfer becomes important at high MBY conversions at high hydrogen concentration. Low stirrer speed caused the reaction rate and MBE selectivity to decrease. No internal mass transfer limitations were observed, and conditions for the kinetic regime were found. The kinetics modeled followed the Langmuir−Hinshelwood mechanism and was consistent with the experimental data. © 2008 American Chemical Society, reprinted with permission.
Monodispersed Pd nanoparticles (8, 11, and 13 nm in diameter) as confirmed by high resolution transmission electron microscopy were prepared via the reverse microemulsion method and deposited on structured supports consisting of carbon nanofibers (CNF) grown on sintered metal fibers (SMF). The CNF/SMF supports were subjected to oxidative treatments to introduce O-functional groups on the CNF surface. These groups were characterized by temperature-programmed decomposition (TPD) and X-ray photoelectron spectroscopy. The catalysts were used to study (a) the effect of Pd size and (b) the effect of the support nature on the selective acetylene hydrogenation. Antipathetic size dependence of TOF disappeared at particle size bigger than 11 nm. Initial selectivity to ethylene was found size-independent. The deactivation due to coke deposition was faster for smaller particles. The structure-sensitivity relations for the catalysts investigated are discussed in terms of “geometric” and “electronic nature” of the size effect and rationalized regarding Pd−Cx phase formation which is size-dependent. Supports with increased acidity diminished the formation of coke and changed the byproduct distribution toward ethane. © 2008 American Chemical Society, reprinted with permission.
A novel method for isolation of monodispersed Pd nanoparticles from a reverse microemulsion was developed using hydrocarbon evaporation and methanol-assisted particle purification from a surfactant. Fcc Pd nanoparticles of 6, 8, 11, and 13 nm in diameter were isolated from water/AOT/isooctane mixture and used to study a size effect during solvent-free hydrogenation of 2-methyl-3-butyn-2-ol to 2-methyl-3-buten-2-ol. The initial TOF calculated per mole of surface palladium atoms was duplicated when particle size was increased from 6 to 13 nm but remained constant when accounted per number of specific Pd atoms on Pd(111) facets. Selectivity to olefinic alcohol was not size-dependent, but an increase in particle size decreased the byproduct ratio of dimers to saturated alcohol. Acetylenic alcohol hydrogenation is shown to be a structure-sensitive but size-independent reaction for Pd particles with size of 6–13 nm. The work shows also that the Pd size controlled the reaction rate and the byproduct distribution. © 2007 Elsevier, reprinted with permission.
An innovative structured catalyst based on sintered metal fibers (SMFs) coated by a grain-structured ZnO layer was developed for the selective 3-phase hydrogenation of functionalized alkynes. The catalyst was synthesized by the deposition of Pd0-sol with 7-nm nanoparticles stabilized by molybdate anions on ZnO/SMF after reduction in hydrogen at 773 K. The crystallites of PdZn alloy (24 nm) were formed by high-temperature treatment in hydrogen and contained partly Pd(0) nanoparticles. Pd/ZnO/SMF (0.2 wt% Pd, 6 wt% ZnO) allowed >95% yield of 2-methyl-3-buten-2-ol (MBE) during 2-methyl-3-butyn-2-ol hydrogenation at 308 K and 5 bar pressure using water as the solvent and adding a small amount of quinoline. The consecutive reaction of MBE hydrogenation was suppressed due to the presence of PdZn in the active phase. The catalyst activity of the catalyst was an order of magnitude higher compared with that of commercial Lindlar catalyst (Pd, Pb/CaCO3), which quickly deactivated in aqueous media due to irreversible chemical transformations of the CaCO3 support. The structured catalyst developed in this study could be reused after ultrasonic regeneration, retaining the original selectivity and most of its initial activity. The catalyst showed high mechanical stability, precluding leaching of its components during the reaction. Due to easy shaping, high permeability, and low pressure drop during the fluid passage, the structured Pd/ZnO/SMF catalyst is suitable for a continuously operated staged bubble column reactor. © 2007 Elsevier, reprinted with permission.
The structure sensitivity of a liquid-phase 1-hexyne hydrogenation was studied using monodispersed nonsupported Pd nanoparticles of 6, 8, 11, 13, and 14 nm diameter. The particles were prepared via a reverse microemulsion water/AOT/isooctane. The size was varied by the water-to-surfactant ratio. A 15-fold TOF increase was observed with increase of the particle diameter from 11 to 14 nm. For particles above 14 nm, TOF approached the value of the Pd black catalyst. It was observed that the selectivity toward 1-hexene did not vary with the particle size and was 96.5% at the conversion of 1-hexyne of 85%. The byproduct distribution was characterized by the ratio of selectivities to 2-hexenes/n-hexane and decreased 5-fold with particle diameter from 6 to 11 nm but remained constant for bigger particles. The experimental results indicate a predominance of a “geometric” nature of the size effect observed during selective 1-hexyne hydrogenation, suggesting that an ensemble of neighboring Pd surface atoms constitutes the active center responsible for hydrogenation. © 2007 American Chemical Society, reprinted with permission.
Monodispersed Pd-nanoparticles prepared via a modified microemulsion method were deposited on active carbon fibers (ACF) fabrics and studied in a semi-batch 1-hexyne liquid-phase hydrogenation. The catalyst Pd(0.45 wt%)/ACF demonstrated >96% selectivity to 1-hexene up to 90% of conversion. Initial activity at 303 K, 1.3 MPa pressure, in n-heptane as a solvent was . Assuming a Langmuir–Hinshelwood kinetics with a weak hydrogen adsorption a kinetic model was developed. It was shown to be consistent with the experimental data and allowed determining the main kinetic parameters. © 2006 Elsevier, reprinted with permission.
An innovative staged bubble column reactor (SBCR) with a structured catalyst based on sintered metal fibers (SMFs) coated with a thin ZnO layer has been designed for continuous three-phase hydrogenations. ZnO/SMF thin plates (thickness ≈ 0.3 mm) were found to be suitable stages for the SBCR because of their rigid open structure with high porosity (>90%) and good adhesion of ZnO. The hydrodynamic characteristics of the SBCR, including pressure drop and residence time distribution, were investigated at varying superficial liquid (uL0 ≤ 0.6 cm/s) and gas (uG0 ≤ 10 cm/s) velocities for different gas/liquid systems. A semiempirical model was developed for describing the influence of the superficial fluid velocities as well as the gas and liquid physical properties on the pressure drop during SBCR operation and was found to be consistent with experiments. © 2007 American Chemical Society, reprinted with permission.
Pd nanoparticles (2 nm) stabilized in the micelle core of poly(ethylene oxide)-block-poly-2-vinylpyridine were studied in 2-butyne-1,4-diol partial hydrogenation. Both unsupported micelles (0.6 kgPd/m3) and supported ones on γ-Al2O3 (0.042 wt.% Pd) showed nearly 100% selectivity to 2-butene-1,4-diol up to 94% conversion. The only side product observed was 2-butane-1,4-diol. The catalysis was ascribed to Pd nanoparticles’ surface modified by pyridine units of micelles and alkali reaction medium (pH of 13.4). TOFs over the unsupported and supported catalysts were found to be 0.56 and 0.91 s−1(at 323 K, 0.6 MPa H2 pressure, solvent 2-propanol/water = 7:3), respectively. Reaction kinetics fit the Langmuir–Hinshelwood model assuming weak hydrogen adsorption. The experiments on the catalyst reuse showed that Pd nanoparticles remain inside the micelle core, but the micelles slightly desorbed (less then 5%) during the catalytic run. © 2004 Elsevier, reprinted with permission.
Selective dehydrolinalool (3,7-dimethyloct-6-ene-1-yne-3-ol, DHL) hydrogenation to linalool (3,7-dimethylocta-1,6-diene-3-ol, LN, a fragrant substance) was studied with Pd nanoparticles formed in poly(ethylene oxide)-block-poly-2-vinylpyridine (PEO-b-P2VP) micelles with varying solvent composition (‘isopropanol (i-PrOH):water’ ratio) and the pH of the reaction medium. According to transmission electron microscopy (TEM) and atomic force microscopy (AFM), isopropanol fraction and KOH loading control the micellar characteristics governing catalytic properties. The larger and denser the micelles, the slower the reaction due to internal diffusion limitations within the micelles. Denser micelle cores provide better modification of the Pd nanoparticle surface with pyridine units and higher selectivity. The highest selectivity (99.4%) was obtained at pH of 9.4 and 95 vol.% of isopropanol. The highest observed TOF value was found to be 24.4 s−1 at pH of 13.0 and 70 vol.% of isopropanol. KOH and isopropanol were shown both to affect the micelle characteristics and act as modifiers of the catalyst surface. The hydrogenation kinetics was studied and zero order with respect to dehydrolinalool was found. © 2004 Elsevier, reprinted with permission.