Synthese, Charakterisierung und Anwendung von Pd-basierten intermetallischen Verbindungen hergestellt aus Hydrotalcit-ähnlichen Verbindungen

Für die Herstellung von geträgerten, intermetallischen Pd2Ga und PdZn Nanopartikeln wird ein neuartiger, leicht anwendbarer Syntheseansatz aus Hydrotalcit-ähnlichen (HT) Verbindungen vorgestellt. Die ternären HT Verbindungen mit der nominellen Zusammensetzung (Pd2+,M2+)0.70(M3+)0.30(OH)2(CO3)0.15 ∙ m H2O werden durch pH-kontrollierte Co-Fällung hergestellt. Die Kombination der zwei- und dreiwertigen Kationen variiert zwischen Mg2+/Ga3+ bzw. Zn2+/Al3+, um die Bildung von nanokristallinen Pd2Ga und PdZn intermetallischen Verbindungen geträgert auf porösem MgO/MgGa2O4 bzw. ZnO/ZnAl2O4 Support zu ermöglichen. Zusätzlich wird für die Herstellung einer monometallischen Pd-Referenz auf einem MgO/MgAl2O4 Träger ein PdMgAl HT synthetisiert. Der erfolgreiche Einbau von Pd in die HT Struktur erfordert eine oktaedrische Koordination des Pd2+, wobei Pd2+ Ionen in wässriger Lösung bevorzugt quadratisch planar koordiniert vorliegen. Bei gleichem Substitutionsgrad von M2+ durch Pd2+ konnte vollständiger Pd Einbau für die PdZnAl HT Verbindung erreicht werden. Im Falle von PdMgGa und PdMgAl HT liegt ein geringer Anteil des Pd2+ in segregierter Form auf der Oberfläche der HT Plättchen vor. Das nicht eingebaute Pd weist eine ähnliche Koordinationsphäre wie quadratisch planar koordiniertes Pd2+ in PdO auf. Die Grenze für den vollständigen Einbau von Pd2+ liegt bei unter 1 mol% Pd. Die thermische Zersetzung in reduzierender Atmosphäre führt zur Bildung von intermetallischen und metallischen Nanopartikeln in der Größenordnung von unter 2 bis 6 nm und monomodaler Partikelgrößenverteilung. Durch das Legieren von Pd mit Ga bzw. Zn wird sowohl die Kristallstruktur als auch die elektronische Struktur verändert, was zur Ausbildung von isolierten Adsorptionszentren an der Oberfläche führte. Des Weiteren wurde bei längerer CO Exposition und erhöhten CO Drücken eine dynamische Veränderung der Pd2Ga Oberfläche beobachtet. Dies lässt sich durch die Zersetzung von Pd2Ga zu metallischem Pd und Ga2O3 erklären. Der nanostrukturierte Pd2Ga Katalysator erzielt im Vergleich zum Pd2Ga Bulkkatalysator ähnliche Selektivität und Stabilität in der selektiven Semihydrierung von Ethin. Die Selektivität zu Ethen verbesserte sich gegenüber metallischem Palladium deutlich durch die Bildung der intermetallischen Pd2Ga Phase. Interessanterweise wird eine langsame Aktivierung des Katalysators im Reaktionsgas beobachtet, die sich mittels oxidativer Vorbehandlung drastisch verkürzt. Diese Dynamik kann durch das Zusammenspiel von der Oberflächenzersetzung zu Pd0 und Ga2O3 und der Umkehrung der starken Metall-Träger-Wechselwirkung erklärt werden, was zu einer deutlichen Aktivitätserhöhung führt. Zusätzlich zeigten Pd2Ga und PdZn Nanopartikel im Gegensatz zum reinen Pd Katalysator erhöhte Aktivitäten und Selektivitäten in der Reformierung von Methanol und der Methanolsynthese. Diese strukturell modifizierten Pd Katalysatoren weisen im Vergleich zum monometallischen Pd Katalysator deutlich verringerte CO Selektivitäten und erhöhte Bildung von Methanol auf.A novel, feasible synthesis approach for supported intermetallic Pd2Ga and PdZn nanoparticles derived from Hydrotalcite-like compounds (HTlc) is introduced. Ternary HTlc with the nominal composition (Pd2+,M2+)0.70(M3+)0.30(OH)2(CO3)0.15 ∙ m H2O are synthesized by pH-controlled co-precipitation. Mg2+/Ga3+ and Zn2+/Al3+ are chosen as M2+/M3+ combinations to permit formation of the nanocrystalline Pd2Ga and PdZn intermetallic compounds on a porous MgO/MgGa2O4 and ZnO/ZnAl2O4 support, respectively. In addition, a PdMgAl HTlc is prepared as monometallic Pd reference compound on a MgO/MgAl2O4 support. Incorporation of Pd2+ into the HT structure requires octahedral coordination, while Pd2+ ions prefer square planar coordination in aqueous solution. At the same substitution degree of M2+ by Pd2+, complete insertion is achieved for PdZnAl HT. In case of PdMgGa and PdMgAl HT a minor fraction is present as segregated Pd2+ on the external surface of the platelet-like particles with a local environment similar to PdO, i.e. in a square planar coordination. A limit of incorporation into the HT lattice exists at < 1 mol% for the Pd2+ containing precursors. Upon thermal decomposition in reductive atmosphere, intermetallic and metallic nanoparticles ranging from below 2 nm to 6 nm in size and exhibiting monomodal particle size distributions are formed. Alloying of Pd with Ga and Zn changes the crystal structure as well as the electronic structure and leads to the increased formation of isolated adsorption sites at the surface. Furthermore, dynamic surface changes of intermetallic Pd2Ga nanoparticles were noticed at longer exposure time to CO and higher CO coverage. This is attributed to the decomposition into metallic Pd and Ga2O3. The nanostructured Pd2Ga catalyst shows excellent performance in the selective semi-hydrogenation of acetylene similar to a bulk Pd2Ga model catalyst. In comparison to the elemental Pd catalyst the selectivity to ethylene is drastically improved by formation of Pd2Ga. Interestingly, the nanostructered catalyst slowly activates in the feed gas. The activation is triggered faster by a treatment in oxidative atmosphere. These dynamics of the Pd2Ga nanoparticles can be explained by the interplay of surface decomposition into Pd0 and Ga2O3 in oxygen and reversal of the strong-metal support interaction state leading to an increased activity. Furthermore, increased activities and selectivities in methanol steam reforming and methanol synthesis from CO2 are observed for the Pd2Ga and PdZn nanoparticles in contrast to the unmodified Pd particles. These structurally modified Pd catalysts exhibit a considerably lower selectivity to CO and enhanced formation of methanol compared to the monometallic Pd catalyst

Next-Generation Cellulosic Filaments from Hemp Pulp via Dry-Jet Wet Spinning Using HighPerCell^® Technology

Fiber demand of cellulosic fibers is rapidly increasing; however, these fibers are mainly based on the use of wood pulp (WP), which often have long transport times and, consequently, a high CO2 footprint. So, alternative pulps based on non-wood, annual fast-growing plants are an option to cover the demand for raw materials and resources. Herein, we report on the use of a novel developed hemp pulp (HP) for man-made cellulosic fiber filament spinning. Commercial WP was used as a reference material. While HP could be used and directly spun as received without any further pretreatment, an additional step to adjust the degree of polymerization (DP) was needed to use the wood pulp. Continuous filaments were spun using a novel dry-jet wet spinning (HighPerCell® process) technique, which is based on the use of 1-ethyl-3-methylimidazolium octanoate ([C2C1im][Oc]) as a solvent. Via this approach, several thousand meters (12,000 m–15,000 m) of continuous multifilament filaments were spun. The HP pulps showed excellent spinning performance. The novel approach allows the preparation of cellulosic fibers for either technical—with high tensile strength—or textile—possessing a low fibrillation tendency—applications. Textile hemp-based filaments were used for first weaving trials, resulting in a flawless fabric

Poly(Ethylene Furanoate) along Its Life-Cycle from a Polycondensation Approach to High-Performance Yarn and Its Recyclate

We report on the pilot scale synthesis and melt spinning of poly(ethylene furanoate) (PEF), a promising bio-based fiber polymer that can heave mechanical properties in the range of commercial poly(ethylene terephthalate) (PET) fibers. Catalyst optimization and solid state polycondensation (SSP) allowed for intrinsic viscosities of PEF of up to 0.85 dL·g−1. Melt-spun multifilament yarns reached a tensile strength of up to 65 cN·tex−1 with an elongation of 6% and a modulus of 1370 cN·tex−1. The crystallization behavior of PEF was investigated by differential scanning calorimetry (DSC) and XRD after each process step, i.e., after polymerization, SSP, melt spinning, drawing, and recycling. After SSP, the previously amorphous polymer showed a crystallinity of 47%, which was in accordance with literature. The corresponding XRD diffractograms showed signals attributable to α-PEF. Additional, clearly assignable signals at 2θ > 30° are discussed. A completely amorphous structure was observed by XRD for as-spun yarns, while a crystalline phase was detected on drawn yarns; however, it was less pronounced than for the granules and independent of the winding speed

Stereospecific Ring-Opening Metathesis Polymerization (ROMP) of endo-Dicyclopentadiene by Molybdenum and Tungsten Catalysts

United States. Department of Energy (DE-FG02-86ER13564

Investigation of the Tendency of Carbon Fibers to Disintegrate into Respirable Fiber-Shaped Fragments

Recent reports of the release of large numbers of respirable and critically long fiber-shaped fragments from mesophase pitch-based carbon fiber polymer composites during machining and tensile testing have raised inhalation toxicological concerns. As carbon fibers and their fragments are to be considered as inherently biodurable, the fiber pathogenicity paradigm motivated the development of a laboratory test method to assess the propensity of different types of carbon fibers to form such fragments. It uses spallation testing of carbon fibers by impact grinding in an oscillating ball mill. The resulting fragments were dispersed on track-etched membrane filters and morphologically analyzed by scanning electron microscopy. The method was applied to nine different carbon fiber types synthesized from polyacrylonitrile, mesophase or isotropic pitch, covering a broad range of material properties. Significant differences in the morphology of formed fragments were observed between the materials studied. These were statistically analyzed to relate disintegration characteristics to material properties and to rank the carbon fiber types according to their propensity to form respirable fiber fragments. This tendency was found to be lower for polyacrylonitrile-based and isotropic pitch-based carbon fibers than for mesophase pitch-based carbon fibers, but still significant. Although there are currently only few reports in the literature of increased respirable fiber dust concentrations during the machining of polyacrylonitrile-based carbon fiber composites, we conclude that such materials have the potential to form critical fiber morphologies of WHO dimensions. For safe-and-sustainable carbon fiber-reinforced composites, a better understanding of the material properties that control the carbon fiber fragmentation is imperative

Stereospecific Ring-Opening Metathesis Polymerization (ROMP) of <i>endo</i>-Dicyclopentadiene by Molybdenum and Tungsten Catalysts

We report an examination of the ring-opening metathesis polymerization (ROMP) of <i>endo-</i>dicyclopentadiene (DCPD) by 10 well-defined molybdenum-based and 16 tungsten-based alkylidene initiators. Five tungsten-based MAP (monoaryloxide pyrrolide) initiators with the general formula W­(X)­(CHCMe₂Ph)­(Me₂Pyr)­(OAr) (X = arylimido, alkylimido, or oxo; Me₂Pyr =2,5-dimethylpyrrolide; OAr = an aryloxide) were found to yield >98% <i>cis</i>, >98% <i>syndiotactic</i> poly­(DCPD); they are W­(N-<i>t</i>-Bu)­(CHCMe₃)­(pyr)­(OHMT) (<b>2</b>, OHMT = O-2,6-(2,4,6-Me₃C₆H₂)₂C₆H₃, pyr = pyrrolide), W­(N-2,6-<i>i</i>-Pr₂C₆H₃)­(CHCMe₂Ph)­(pyr)­(OHMT) (<b>3</b>), W­(O)­(CHCMe₂Ph)­(Me₂Pyr)­(OHMT)­(PPh₂Me) (<b>7</b>, Me₂Pyr =2,5-dimethylpyrrolide), W­(O)­(CHCMe₂Ph)­(Me₂Pyr)­(ODFT)­(PPh₂Me) (<b>9</b>, ODFT = O-2,6-(C₆F₅)₂C₆H₃), and W­(O)­(CHCMe₂Ph)­(Me₂Pyr)­(OTPP)­(PMePh₂) (<b>10</b>, OTPP = O-2,3,5,6-Ph₄C₆H). Two biphenolate alkylidene complexes, Mo­(N-2,6-Me₂C₆H₃)­(CHCMe₂Ph)­(<i>rac</i>-biphen) (<b>17</b>) and W­(N-2,6-Me₂C₆H₃)­(CHCMe₂Ph)­(<i>rac</i>-biphen) (<b>22</b>, biphen =3,3′-(<i>t-</i>Bu)₂-5,5′-6,6′-(CH₃)₄-1,1′-biphenyl-2,2′-diolate), were found to yield >98% <i>cis</i>, >98% <i>isotactic</i> poly­(DCPD). <i>Cis</i>, <i>syndiotactic</i> or <i>cis</i>, <i>isotactic</i> poly­(DCPD)­s (made with 50–1000 equiv of DCPD) are accessible within seconds to minutes in dichloromethane at room temperature. No isomerization or cross-linking reactions are observed, and addition of a chain transfer reagent (1-hexene) or the use of THF as a solvent does not decrease the stereospecificity of the polymerizations. <i>Cis</i>, <i>syndiotactic</i> and <i>cis</i>, <i>isotactic</i> poly­(DCPD)­s can be distinguished readily from each other by ¹³C NMR spectroscopy. Hydrogenation of each stereoregular poly­(DCPD) produces H-poly­(DCPD)­s that have melting points near 270 °C (<i>syndiotactic</i>) or 290 °C (<i>isotactic</i>) and high crystallinities (<i>w</i>_c = 0.83 for <i>syndiotactic</i> and <i>w</i>_c = 0.74 for <i>isotactic</i>)

Dynamic Surface Processes of Nanostructured Pd₂Ga Catalysts Derived from Hydrotalcite-Like Precursors

The stability of the surface termination of intermetallic Pd₂Ga nanoparticles and its effect on the hydrogenation of acetylene was investigated. For this purpose, a precursor synthesis approach was applied to synthesize supported intermetallic Pd₂Ga nanoparticles. A series of Pd-substituted MgGa-hydrotalcite (HT)-like compounds with different Pd loading was prepared by coprecipitation and studied in terms of loading, phase formation, stability and catalytic performance in the selective hydrogenation of acetylene. Higher Pd loadings than 1 mol % revealed an incomplete incorporation of Pd into the HT lattice, as evidenced by XANES and TPR measurements. Upon thermal reduction in hydrogen, Pd₂Ga nanoparticles were obtained with particle sizes varying with the Pd loading, from 2 nm to 6 nm. The formation of intermetallic Pd₂Ga nanoparticles led to a change of the CO adsorption properties as was evidenced by IR spectroscopy. Dynamic changes of the surface were noticed at longer exposure times to CO and higher coverage at room temperature as a first indication of surface instability. These were ascribed to the decomposition into a Ga-depleted Pd phase and Ga₂O₃, which is a process that was suppressed at liquid nitrogen temperature. The reduction of the Pd precursor at 473 K is not sufficient to form the Pd₂Ga phase and yielded a poorly selective catalyst (26% selectivity to ethylene) in the semihydrogenation of acetylene. In accordance with the well-known selectivity-promoting effect of a second metal, the selectivity was increased to 80% after reduction at 773 K due to a change from the elemental to the intermetallic state of palladium in our catalysts. Interestingly, if air contact was avoided after reduction, the conversion slowly rose from initially 22% to 94% with time on stream. This effect is interpreted in the light of chemical response of Pd and Pd₂Ga to the chemical potential of the reactive atmosphere. Conversely to previous interpretations, we attribute the initial low active state to the clean intermetallic surface, while the increase in conversion is related to the surface decomposition of the Pd₂Ga particles