Multinuclear
solid-state nuclear magnetic resonance, mass spectrometry,
first-principles molecular dynamics simulations, and other complementary
evidence reveal that the coordination environment around the Zn2+ ions in MOF-5, one of the most iconic materials among metal–organic
frameworks (MOFs), is not rigid. The Zn2+ ions bind solvent
molecules, thereby increasing their coordination number, and dynamically
dissociate from the framework itself. On average, one ion in each
cluster has at least one coordinated N,N-dimethylformamide (DMF) molecule, such that the formula of as-synthesized
MOF-5 is defined as Zn4O(BDC)3(DMF)x (x = 1–2). Understanding
the dynamic behavior of MOF-5 leads to a rational low-temperature
cation exchange approach for the synthesis of metastable Zn4–xCoxO(terephthalate)3 (x > 1) materials, which have not been
accessible
through typical high-temperature solvothermal routes thus far.
The structure and properties of water
films in contact with metal surfaces are crucial to understand the
chemical and electrochemical processes involved in energy-related
technologies. The nature of thin water films on Pd, Pt, and Ru has
been investigated by first-principles molecular dynamics to assess
how the chemistry at the water–metal surface is responsible
for the diversity in the behavior of the water layers closer to the
metal. The characteristics of liquid water: the radial distribution
functions, coordination, and fragment speciation appear only for unconfined
water layers of a minimum of 1.4 nm thick. In addition, the water
layer is denser in the region closest to the metal for Pd and Pt,
where seven- and five-membered ring motifs appear. These patterns
are identical to those identified by scanning tunneling microscopy
for isolated water bilayers. On Ru densification at the interface
is not observed, water dissociates, and protons and hydroxyl groups
are locked at the surface. Therefore, the acid–base properties
in the area close to the metal are not perturbed, in agreement with
experiments, and the bulk water resembles an electric double layer.
Confinement affects water making it closer to ice for both structural
and dynamic properties, thus being responsible for the higher viscosity
experimentally found at the nanoscale. All these contributions modify
the solvation of reactants and products at the water–metal
interface and will affect the catalytic and electrocatalytic properties
of the surface.
Increasing the resistance to humid environments is mandatory for the implementation of isoreticular metal-organic frameworks (IRMOFs) in industry. To date, the causes behind the sensitivity of [Zn(4)(μ(4)-O)(μ-bdc)(3)](8)(IRMOF-1; bdc=1,4-benzenedicarboxylate) to water remain still open. A multiscale scheme that combines Monte Carlo simulations, density functional theory and first-principles Born-Oppenheimer molecular dynamics on IRMOF-1 was employed to unravel the underlying atomistic mechanism responsible for lattice disruption. At very low water contents, H(2)O molecules are isolated in the lattice but provoke a dynamic opening of the terephthalic acid, and the lattice collapse occurs at about 6% water weight at room temperature. The ability of Zn to form fivefold coordination spheres and the increasing basicity of water when forming clusters are responsible for the displacement of the organic linker. The present results pave the way for synthetic challenges with new target linkers that might provide more robust IRMOF structures.
Despite its ubiquity in homogeneous and enzymatic catalysis, concerted mechanisms have been overlooked for heterogeneously catalyzed reactions. The elusive nature of transition states leaves Density Functional Theory, DFT, as the only robust tool for their identification and characterization. By means of this method, we show that a concerted path takes part in the recently discovered semihydrogenation of propyne on CeO 2 , for which an excellent activity and selectivity have been reported. The high surface H coverage imposed by the experimental hydrogenation conditions induces site isolation and drives the reaction through a six-membered ring transition state. This unprecedented pathway accounts for many of the experimental observations, such as the unique syn-stereoselectivity, the excellent alkene selectivities, or the high temperature and large H 2 /alkyne ratios required.
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