Detection of solar-like oscillations in relics of the Milky Way: Asteroseismology of K giants in M4 using data from the NASA K2 mission

Asteroseismic constraints on K giants make it possible to infer radii, masses and ages of tens of thousands of field stars. Tests against independent estimates of these properties are however scarce, especially in the metal-poor regime. Here, we report the detection of solar-like oscillations in eight stars belonging to the red-giant branch (RGB) and red-horizontal branch (RHB) of the globular cluster M4. The detections were made in photometric observations from the K2 Mission during its Campaign 2. Making use of independent constraints on the distance, we estimate masses of the eight stars by utilizing different combinations of seismic and non-seismic inputs. When introducing a correction to the ν scaling relation as suggested by stellar models, for RGB stars we find excellent agreement with the expected masses from isochrone fitting, and with a distance modulus derived using independent methods. The offset with respect to independent masses is lower, or comparable with, the uncertainties on the average RGB mass (4–10 per cent, depending on the combination of constraints used). Our results lend confidence to asteroseismic masses in the metal-poor regime. We note that a larger sample will be needed to allow more stringent tests to be made of systematic uncertainties in all the observables (both seismic and non-seismic), and to explore the properties of RHB stars, and of different populations in the cluster

Ethene Oligomerization in Ni-Containing Zeolites: Theoretical Discrimination of Reaction Mechanisms

Ni-containing porous aluminosilicates are promising heterogeneous catalysts for oligomerization of ethene, but little is known about the catalytic cycle. In addition, it remains unclear why the aluminosilicates work without the alkyl aluminum cocatalyst needed in homogeneous catalysis. As the first of its kind, this work uses density functional theory (DFT) to identify the most probable mechanism of oligomerization and active site formation. The periodic DFT calculations employed the BEEF-vdW functional to consider both short-range interactions involved in bond formation and long-range interactions with the zeolite framework. The calculations targeted Ni-containing SSZ-24 zeolite as a representative catalyst and considered Ni⁺, Ni²⁺ ions, and neutral nickel atoms as active sites. We investigated the catalytic cycles of the metallacycle and Cossee–Arlman mechanisms that have been proposed in the literature, in addition to a new proton-transfer mechanism. Free energy profiles were derived at a typical experimental reaction temperature of 393 K and used to kinetically discriminate the mechanisms with the energetic span model. On the basis of the results, we predict the Cossee–Arlman mechanism known from homogeneous catalysts to prevail also in the zeolite catalyst. The calculated intrinsic enthalpy of activation of 77 kJ/mol for ethene dimerization agrees well with available experimental data. We further propose a mechanism for formation of the active nickel–alkyl species by reaction between ethene and isolated Ni²⁺ ions. The results hence provide a solid starting point for experimental investigations of the catalytic cycle, to validate our predictions and ultimately determine the atom-scale properties that control catalytic activity

Ethene Oligomerization in Ni-Containing Zeolites: Theoretical Discrimination of Reaction Mechanisms

Ni-containing porous aluminosilicates are promising heterogeneous catalysts for oligomerization of ethene, but little is known about the catalytic cycle. In addition, it remains unclear why the aluminosilicates work without the alkyl aluminum cocatalyst needed in homogeneous catalysis. As the first of its kind, this work uses density functional theory (DFT) to identify the most probable mechanism of oligomerization and active site formation. The periodic DFT calculations employed the BEEF-vdW functional to consider both short-range interactions involved in bond formation and long-range interactions with the zeolite framework. The calculations targeted Ni-containing SSZ-24 zeolite as a representative catalyst and considered Ni⁺, Ni²⁺ ions, and neutral nickel atoms as active sites. We investigated the catalytic cycles of the metallacycle and Cossee–Arlman mechanisms that have been proposed in the literature, in addition to a new proton-transfer mechanism. Free energy profiles were derived at a typical experimental reaction temperature of 393 K and used to kinetically discriminate the mechanisms with the energetic span model. On the basis of the results, we predict the Cossee–Arlman mechanism known from homogeneous catalysts to prevail also in the zeolite catalyst. The calculated intrinsic enthalpy of activation of 77 kJ/mol for ethene dimerization agrees well with available experimental data. We further propose a mechanism for formation of the active nickel–alkyl species by reaction between ethene and isolated Ni²⁺ ions. The results hence provide a solid starting point for experimental investigations of the catalytic cycle, to validate our predictions and ultimately determine the atom-scale properties that control catalytic activity

Reversible and Site-Dependent Proton-Transfer in Zeolites Uncovered at the Single-Molecule Level

Zeolite activity and selectivity is often determined by the underlying proton and hydrogen-transfer reaction pathways. For the first time, we use single-molecule fluorescence microscopy to directly follow the real-time behavior of individual styrene-derived carbocationic species formed within zeolite ZSM-5. We find that intermittent fluorescence and remarkable photostability of carbocationic intermediates strongly depend on the local chemical environment imposed by zeolite framework and guest solvent molecules. The carbocationic stability can be additionally altered by changing para-substituent on the styrene moiety, as suggested by DFT calculations. Thermodynamically unstable carbocations are more likely to switch between fluorescent (carbocationic) and dark (neutral) states. However, the rate constants of this reversible change can significantly differ among individual carbocations, depending on their exact location in the zeolite framework. The lifetimes of fluorescent states and reversibility of the process can be additionally altered by changing the interaction between dimeric carbocations and solvated Brønsted acid sites in the MFI framework. Advanced multidimensional magic angle spinning solid-state NMR spectroscopy has been employed for the accurate structural elucidation of the reaction products during the zeolite-catalyzed dimerization of styrene in order to corroborate the single-molecule fluorescence microscopy data. This complementary approach of single-molecule fluorescence microscopy, NMR, and DFT collectively indicates that the relative stability of the carbocationic and the neutral states largely depends on the substituent and the local position of the Brønsted acid site within the zeolite framework. As a consequence, new insights into the host-guest chemistry between the zeolite and aromatics, in terms of their surface mobility and reactivity, have been obtained.status: publishe

Reactivity Descriptor in Solid Acid Catalysis: Predicting Turnover Frequencies for Propene Methylation in Zeotypes

Recent work has reported the discovery of metal surface catalysts by employing a descriptor-based approach, establishing a correlation between a few well-defined properties of a material and its catalytic activity. This theoretical work aims for a similar approach in solid acid catalysis, focusing on the reaction between propene and methanol catalyzed by Brønsted acidic zeotype catalysts. Experimentally, the ammonia heat of adsorption is often used as a measure of the strength of acid sites. Using periodic DFT calculations, we show that this measure can be used to establish scaling relations for the energy of intermediates and transition states, effectively describing the reactivity of the acid site. This allows us to use microkinetic modeling to predict a quantitative relation between the ammonia heat of adsorption and the rate of propene methylation from first principles. We propose that this is the first step toward descriptor-based design of solid acid catalysts