Designing selective sorbent materials to sequestrate toxic oxoanions with weaker binding tendency, especially oxoanions of selenium and arsenic (Se(IV)/Se(VI)/As(V)) is an arduous challenge. Herein, we demonstrate molecular shape recognition aided...
We
show how expanded Wang–Landau simulations can be used
to study the adsorption of methane–ethane mixtures in COF-102,
COF-105, and COF-108. This approach has several advantages. First,
a single simulation run is performed to determine key thermodynamic
properties such as the adsorption isotherms and selectivity. Second,
the combination of the expanded method with the Wang–Landau
sampling in the grand-canonical ensemble provides direct access to
the grand potential Ω = −k
B
T ln[Θ(μ1,μ2,V,T)] via a numerical evaluation
of the grand-canonical partition function. From there, we calculate
several thermodynamic quantities of adsorption, including the Gibbs
free energy, enthalpy, and entropy, which give important insights
into the mechanism of adsorption for the methane–ethane mixtures
in covalent organic frameworks (COFs). In particular, using a solution
thermodynamics approach, we identify a direct correlation between
the separation efficiency (selectivity) of a given COF and its energetic
efficiency (through desorption free energy calculations) for the methane/ethane
mixture, which in turn, allows us to rank the different COFs on the
basis of their methane/ethane separation performance.
The hydrogen-storage
(H-storage) capacities of different polyhydroxy adamantanes or adamantanols
coordinated (dressed or doped) with alkaline earth metal cations (Mg2+ and Ca2+) have been studied using the density
functional theory (DFT) method. The complex coordinated with one Mg2+ cation adsorbs a four H2 molecules with a binding
energy (BE) of 19.98 kcal/mol, whereas the complex coordinated with
one Ca2+ cation adsorbs a maximum of six H2 molecules
with a BE of 21.59 kcal/mol. The maximum number of metal cations that
can be anchored to polyhydroxy adamantane is four. The complexes dressed
with four Mg2+ and Ca2+ cations adsorb maxima
of 16 and 24 H2 molecules, respectively. The gravimetric
densities of the corresponding complexes are 8.4 and 10.4 wt %, respectively.
The results reinforce the idea that the creation of a number of open
metal sites on adamantanols enhances their hydrogen-storage capacities
and also their binding energies.
Heterometallic
metal organic frameworks (MOF) have attracted huge
interest for a wide range of applications including gas storage, separation,
and catalysis owing to their tunable electronic and magnetic properties.
Among several heterometallic MOF structures reported, iron containing
MOF structure, namely PCN-250, exhibits excellent thermal and chemical
stability. PCN-250 MOF consists of the trimetallic cluster node Fe2M linked with (H4ABTC)6 (H4ABTC = 3,3′,5,5′-azobenzenetetracarboxylic acid and
M = Cr(II), Mn(II), Fe(II), Co(II), Ni(II), or Zn(II)) to form a three-dimensional
porous network. In this work, we employed Density Functional Theory
(DFT) to investigate the strength of the interaction of O2 and N2 gas molecules with both linkers and coordinatively
unsaturated metal sites. In addition, grand canonical Monte Carlo
simulation is used to predict the adsorption isotherm at two different
temperatures, 273 and 298 K, in both homometallic and heterometallic
PCN-250. On the basis of the cluster model DFT calculations, we observe
almost a factor of 5 selectivity (O2/N2) in
Fe2Cr- and Fe2Mn-based PCN-250 MOF structures.
Incorporation of first-row transition metals with +2 oxidation state
showed enhanced binding of O2 over N2, correlating
well with charge transfer from the metal atom to the adsorbed O2 molecule. Agreeing qualitatively with DFT calculations, GCMC
simulations at 273 K showed higher uptake of O2 over N2 following the order Fe2Cr > Fe2Mn
>
Fe2Ni > Fe2Co > Fe2Zn, respectively.
Also, a selectivity of greater than one is predicted for O2 over N2 in all heterometallic PCN-250 structures based
on a single component adsorption isotherm at 1 bar.
Metal–organic frameworks (MOFs)
have attracted
much attention
for the effective capture of contaminants from air. Herein, density
functional theory (DFT) calculations and grand canonical Monte Carlo
(GCMC) simulations were combined to systematically assess the adsorption
performance of the cagelike UiO-66 nanoporous MOF functionalized by
metal(II) catecholate [CatM(II), where M(II) = Mg(II), Mn(II), Fe(II),
Co(II), Ni(II), Cu(II), Zn(II), Pd(II), and Pt(II)] with respect to
NO
x
potentially present at very low concentration
(from the ppm to ppb levels). The adsorption modes and energetics
of NO
x
toward metal(II) catecholate functions
were first examined systematically using cluster DFT calculations
in order to determine the optimum metal(II) for effective NO
x
capture. The best CatFe(II) was further incorporated
in the crystal structure of UiO-66 and force-field parameters to accurately
describe the specific interactions between Fe(II), and both NO
x
were derived from periodic DFT calculations
and further implemented in a GCMC scheme to predict the adsorption
isotherms in a whole range of gas pressure. These calculations revealed
that UiO-66-CatFe(II) exhibits steep-adsorption isotherms for both
NO
x
, leading to excellent adsorption uptake
at very low gas pressure (from 10–9–10–4 bar). This finding complements the portfolio of nanoporous
materials that has so far been almost exclusively tested in operation
conditions at much higher NO
x
concentration
(>1000 ppm).
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