Electrophilic Organoiridiunn(III) Pincer Complexes on Sulfated Zirconia for Hydrocarbon Activation and Functionalization

Single-site supported organometallic catalysts bring together the favorable aspects of homogeneous and heterogeneous catalysis while offering opportunities to investigate the impact of metal–support interactions on reactivity. We report a (dmPhebox)Ir(III) (dmPhebox = 2,6-bis(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl) complex chemisorbed on sulfated zirconia, the molecular precursor for which was previously applied to hydrocarbon functionalization. Spectroscopic methods such as diffuse reflectance infrared Fourier transformation spectroscopy (DRIFTS), dynamic nuclear polarization (DNP)-enhanced solid-state nuclear magnetic resonance (SSNMR) spectroscopy, and X-ray absorption spectroscopy (XAS) were used to characterize the supported species. Tetrabutylammonium acetate was found to remove the organometallic species from the surface, enabling solution-phase analytical techniques in conjunction with traditional surface methods. Cationic character was imparted to the iridium center by its grafting onto sulfated zirconia, imbuing high levels of activity in electrophilic C–H bond functionalization reactions such as the stoichiometric dehydrogenation of alkanes, with density functional theory (DFT) calculations showing a lower barrier for β-H elimination. Catalytic hydrogenation of olefins was also facilitated by the sulfated zirconia-supported (dmPhebox)Ir(III) complex, while the homologous complex on silica was inactive under comparable conditions

The lightest organic radical cation for charge storage in redox flow batteries

In advanced electrical grids of the future, electrochemically rechargeable fluids of high energy density will capture the power generated from intermittent sources like solar and wind. To meet this outstanding technological demand there is a need to understand the fundamental limits and interplay of electrochemical potential, stability, and solubility in low-weight redox-active molecules. By generating a combinatorial set of 1,4-dimethoxybenzene derivatives with different arrangements of substituents, we discovered a minimalistic structure that combines exceptional long-term stability in its oxidized form and a record-breaking intrinsic capacity of 161 mAh/g. The nonaqueous redox flow battery has been demonstrated that uses this molecule as a catholyte material and operated stably for 100 charge/discharge cycles. The observed stability trends are rationalized by mechanistic considerations of the reaction pathways.United States. Dept. of Energy. Office of Basic Energy Sciences. Chemical Sciences, Geosciences, & Biosciences Division (Contract DE-AC02-06CH11357

Surface Basic Site Effect on Boron-Promoted Platinum Catalysts for Dry Reforming of Methane

Platinum has been shown to be an active catalyst for the dry reforming of methane (DRM), which converts CO2 and CH4 into 2CO and 2H2 (synthesis gases) that can further be processed to produce valuable chemical feedstocks. Catalytic activity is often improved by the addition of promoter atoms, which are typically transition metals or noble metals, such as PtNi and PtSn. Recently, boron has shown to be an effective and low-cost catalyst promoter. Pt/B/SiO2 catalysts were prepared for DRM catalysis and compared with Pt/SiO2 catalysts without boron promotion. Both catalysts had similar surface concentrations of platinum, but the catalytic activity at 750 °C after 14 h for boron-containing catalyst was very high, resulting in nearly 100% CO2 conversion and a H2/CO ratio close to unity, compared to 12% CO2 conversion and H2/CO of 0.35 for boron-free Pt/SiO2. The catalysts were investigated with X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), and CO2 temperature-programmed desorption (CO2-TPD) to identify the deactivating factors. It was determined that neither platinum nanoparticle sintering nor coking was a significant factor in catalyst deactivation; instead, boron had an effect on the reactive surface groups on the SiO2 support. These surface groups, such as hydroxyls and surface basic sites, enhance the adsorption of CO2 and potentially stabilize intermediate carbonate species, resulting in a high CO2 conversion for boron-promoted platinum catalysts

Grafted nickel-promoter catalysts for dry reforming of methane identified through high-throughput experimentation

High-throughput synthesis of a series of monometallic and bimetallic catalysts (45 bimetallic and 50 mono-metallic samples) consisting of nickel and one of nine different metal promoters (B, Co, Cu, Fe, Mg, Mn, Sn, V and Zn) supported on one of five different metal oxides (alumina, ceria, magnesia, silica and titania) is carried out via organometallic grafting using a robotic platform. The catalysts are evaluated for their activity and selectivity for the dry reforming of methane at a feed ratio of CH4:CO2 of 1 at 650-800 degrees C in a parallel flow reactor system. The type of oxide support prevails over the type of additive for both catalyst activity and stability. On Al2O3 and MgO, Fe was found to be the best promoter; on SiO2, Cu is the best promoter at 700 degrees C and higher, while on TiO2, Mn is found to enhance the conversion at 800 degrees C. On CeO2, all additives except Fe have beneficial effects. Twenty-five catalysts show > 90% methane conversion with ten catalysts showing > 95% conversion at 800 degrees C with the H-2:CO ratios ranging from 0.8 to 1.2. Amongst the ten highest performers, NiFe/Al2O3 and NiFe/MgO are more active than Ni/Al2O3 and Ni/MgO, respectively and were stable over a period of 25 h at 800 degrees C. Characterization on the as-prepared samples reveals highly dispersed phase, while after reduction in H-2, highly dispersed and reduced nickel particles up to 10 nm are formed. The particles do not increase in size under dry reforming reaction conditions at 800 degrees C. An increased hydrogen consumption observed during H-2-TPR of the nickel particles is positively correlated with methane conversion for Al2O3-based catalysts. The resistance to deactivation by coking and variation in coke structure are investigated by spectroscopic and microscopic methods to identify the relationship between metal promoters, alloy formation, and type of surface carbon deposits. Carbon whiskers were observed on the ten selected spent samples and are preferentially deposited on Ni rather than on the promoters. Carbon nanotube formation and metal particle removal from support were not observed to cause deactivation while amorphous carbon formation was clearly linked to catalyst deactivation, as amorphous carbon could encapsulate Ni, either on the support or at the end of the carbon nanotube. The organometallic grafting technique is an efficient and suitable technique for synthesizing highly dispersed and homogeneous phases which lead to high conversion and high durability for dry reforming of methane

Mechanistic Aspects of a Surface Organovanadium(III) Catalyst for Hydrocarbon Hydrogenation and Dehydrogenation

Understanding the mechanisms of action for base metal catalysis of transformations typically associated with precious metals is essential for the design of technologies for a sustainable energy economy. Isolated transition-metal and post-transition-metal catalysts on oxides such as silica are generally proposed to effect hydrogenation and dehydrogenation by a mechanism featuring either sigma-bond metathesis or heterolytic bond cleavage as the key bond activation step. In this work, an organovanadium(III) complex on silica, which is a precatalyst for both olefin hydrogenation and alkane dehydrogenation, is interrogated by a series of reaction kinetics and isotopic labeling studies in order to shed light on the operant mechanism for hydrogenation. The kinetic dependencies of the reaction components are potentially consistent with both the sigma-bond metathesis and the heterolytic bond activation mechanisms; however, a key deuterium incorporation experiment definitively excludes the simple sigma-bond metathesis mechanism. Alternatively, a two electron redox cycle, rarely invoked for homologous catalyst systems, is also consistent with experimental observations. Evidence supporting the formation of a persistent vanadium(III) hydride upon hydrogen treatment of the as-prepared material is also presented

Origin of Electrochemical, Structural, and Transport Properties in Nonaqueous Zinc Electrolytes.

Through coupled experimental analysis and computational techniques, we uncover the origin of anodic stability for a range of nonaqueous zinc electrolytes. By examination of electrochemical, structural, and transport properties of nonaqueous zinc electrolytes with varying concentrations, it is demonstrated that the acetonitrile-Zn(TFSI)2, acetonitrile-Zn(CF3SO3)2, and propylene carbonate-Zn(TFSI)2 electrolytes can not only support highly reversible Zn deposition behavior on a Zn metal anode (≥99% of Coulombic efficiency) but also provide high anodic stability (up to ∼3.8 V vs Zn/Zn(2+)). The predicted anodic stability from DFT calculations is well in accordance with experimental results, and elucidates that the solvents play an important role in anodic stability of most electrolytes. Molecular dynamics (MD) simulations were used to understand the solvation structure (e.g., ion solvation and ionic association) and its effect on dynamics and transport properties (e.g., diffusion coefficient and ionic conductivity) of the electrolytes. The combination of these techniques provides unprecedented insight into the origin of the electrochemical, structural, and transport properties in nonaqueous zinc electrolytes

Role of Boron in Enhancing the Catalytic Performance of Supported Platinum Catalysts for the Nonoxidative Dehydrogenation of n-Butane

Platinum-based supported catalysts for hydrocarbon conversion are among the most effective for selective dehydrogenation and isomerization processes. However, high process temperatures and the possibility of coke formation require catalyst modifications to mitigate such effects. One of the emerging approaches to prevent platinum catalyst deactivation is the use of boron additives that have been proposed to prevent coking. Despite such a valuable property of boron, the mechanisms for extending the catalyst lifetime and the decrease in coke formation based on this method are still poorly understood. The type and transformations of boron species on silica surface were investigated as a function of boron introduction, platinum addition, catalyst activation, and catalytic reactivity by a combination of X-ray photoelectron spectroscopy, electron microscopy, solid-state nuclear magnetic resonance spectroscopy, and density functional theory calculations to uncover the possible role of boron modification in improving the catalytic performance. Catalytic nonoxidative dehydrogenation of n-butane revealed that incorporation of boron improved the catalytic activity (similar to 3x) and stability of Pt/SiO2. The role of boron in enhancing catalytic performance was attributed to facilitating the migration of alkyl groups from platinum catalytic centers to tetrahedrally coordinated boron sites