A Two-Step Catalytic Cycle for the Acceptorless Dehydrogenation of Ethane by Group 10 Metal Complexes: Role of the Metal in Reactivity and Selectivity

Doping Efects on Ethane/Ethylene Dehydrogenation Catalyzed by Pt2X Nanoclusters

The catalytic dehydrogenation of light alkanes is key to transform low-cost hydrocarbons to high value-added chemicals. Although Pt is extremely efficient at catalyzing this reaction, it suffers from coke formation that deactivates the catalyst. Dopants such as Sn are widely used to increase the stability and lifetime of Pt. In this work, the dehydrogenation reaction of ethane catalyzed by Pt3 and Pt2X (X=Si, Ge, Sn, P and Al) nanocatalysts has been studied computationally by means of density functional calculations. Our results show how the presence of dopants in the nanocluster structure affects its electronic properties and catalytic activity. Exploration of the potential energy surfaces show that non-doped catalyst Pt3 present low selectivity towards ethylene formation, where acetylene resulting from double dehydrogenation reaction will be obtained as a side product (in agreement with the experimental evidence). On the contrary, the inclusion of Si, Ge, Sn, P or Al as dopant agents implies a selectivity enhancement, where acetylene formation is not energetically favoured. These results demonstrate the effectiveness of such dopant elements for the design of Pt-based catalysts on ethane dehydrogenation.

Ion-molecule reactions of mass-selected ions

Gas-phase reactions of mass-selected ions with neutrals covers a very broad area of fundamental and applied mass spectrometry (MS). Oftentimes, ion-molecule reactions (IMR) can serve as a viable alternative to collision-induced dissociation and other ion dissociation techniques when using tandem MS. This review focuses on the literature pertaining applications of IMR since 2013. During the past decade considerable efforts have been made in analytical applications of IMR, including advances in one of the major techniques for characterization of unsaturated fatty acids and lipids, ozone-induced dissociation, and the development of a new technique for sequencing of large ions, hydrogen atom attachment/abstraction dissociation. Many advances have also been made in identifying gas-phase chemistry specific to a functional group in organic and biological compounds, which are useful in structure elucidation of analytes and differentiation of isomers/isobars. With "soft" ionization techniques like electrospray ionization having become mainstream for quite some time now, the efforts in the area of metal ion catalysis have firmly moved into exploring chemistry of ligated metal complexes in their "natural" oxidation states allowing to model individual steps of mechanisms in homogeneous catalysis, especially in combination with high-level DFT calculations. Finally, IMR continue to contribute to the body of knowledge in the area of chemistry of interstellar processes.

DFT characterisation of a PdII → IIII adduct, and a PdII complex formed after oxidative alkenylation of PdII by [Ph(alkenyl)IIII]+, in Pd-mediated synthesis of benzofurans involving PdIV, annulation and chain-walking

The synthesis of benzofurans by the reaction of the palladium(II) complex Pd{1-C₆H₄-2-OCH(CO₂Et)-C,C}(bipy) (bipy = 2,2'-bipyridine) with hypervalent iodine(III) reagents [Ph(CHCHR)I]⁺ has been examined by Density Functional Theory. Results highlight the role of oxidative alkenylation to form Pd^IV intermediates and the role of initial adduct formation in this process, an annulation process facilitated by Pd^II, and the role of 'chain-walking' at Pd^II centres to allow formation of the lowest energy product. Computation (R = Me) allows assignment of an initially formed adduct with a 'Pd^II → I^III' interaction at -50 °C, and, after oxidative alkenylation of Pd^II and reductive elimination from a Pd^IV centre via Ar⋯Alkenyl coupling, formation of a second intermediate with a structure consistent with NMR detection (R = n-hexyl) at -30 °C is obtained. This Pd^II complex, containing a coordinated alkene group in Pd{1-(RHC^γC^β)C₆H₄-2-OC^αH(CO₂Et)-η²-C^αC^β,C}(bipy), undergoes a 5-exo-trig annulation by forming a C^α-C^β bond to give a complex with a bicyclic carbon skeleton suitable for subsequent formation of benzofurans. A series of facile rearrangements including chain-walking results in formation of a lowest energy complex of three feasible hydrido(alkene)palladium(II) species, leading to decomposition and release of the observed benzofuran isomer isolated under synthesis conditions. The computational study allows reinterpretation of the NMR data reported previously, in particular the determination of barriers in the reaction pathway allowing assignment of structure for key intermediates.

Mechanism of Deoxygenation and Cracking of Fatty Acids by Gas-Phase Cationic Complexes of Ni, Pd, and Pt

Deoxygenation and subsequent cracking of fatty acids are key steps in production of biodiesel fuels from renewable plant sources. Despite the fact that multiple catalysts, including those containing group 10 metals (Ni, Pd, and Pt), are employed for these purposes, little is known about the mechanisms by which they operate. In this work, we utilized tandem mass spectrometry experiments (MSn) to show that multiple types of fatty acids (saturated, mono-, and poly-unsaturated) can be catalytically deoxygenated and converted to smaller hydrocarbons using the ternary metal complexes [(phen)M(O2CR)]+], where phen = 1,10-phenanthroline and M = Ni, Pd, and Pt. The mechanistic description of deoxygenation/cracking processes builds on our recent works describing simple model systems for deoxygenation and cracking, where the latter comes from the ability of group 10 metal ions to undergo chain-walking with very low activation barriers. This article extends our previous work to a number of fatty acids commonly found in renewable plant sources. We found that in many unsaturated acids cracking can occur prior to deoxygenation and show that mechanisms involving group 10 metals differ from long-known charge-remote fragmentation reactions.