17 research outputs found
Penilaian Karya Ilmiah C-11
Recent
experimental work has shown that variations in the confinement
of <i>n</i>-butane at Brønsted acid sites due to changes
in zeolite framework structure strongly affect the apparent and intrinsic
enthalpy and entropy of activation for cracking and dehydrogenation.
Quantum chemical calculations have provided good estimates of the
intrinsic enthalpies and entropies of activation extracted from experimental
rate data for MFI, but extending these calculations to less confining
zeolites has proven challenging, particularly for activation entropies.
Herein, we report our efforts to develop a theoretical model for the
cracking and dehydrogenation of <i>n</i>-butane occurring
in a series of zeolites containing 10-ring channels and differing
in cavity size (TON, FER, -SVR, MFI, MEL, STF, and MWW). We combine
a QM/MM approach to calculate intrinsic and apparent activation parameters,
with thermal corrections to the apparent barriers obtained from configurational-bias
Monte Carlo simulations, to account for configurational contributions
due to global motions of the transition state. We obtain good agreement
between theory and experiment for all activation parameters for central
cracking in all zeolites. For terminal cracking and dehydrogenation,
good agreement between theory and experiment is found only at the
highest confinements. Experimental activation parameters, especially
those for dehydrogenation, tend to increase with decreasing confinement.
This trend is not captured by the theoretical calculations, such that
deviations between theory and experiment increase as confinement decreases.
We propose that, because transition states for dehydrogenation are
later than those for cracking, relative movements between the fragments
produced in the reaction become increasingly important in the less
confining zeolites
Reliably Modeling the Mechanical Stability of Rigid and Flexible Metal–Organic Frameworks
ConspectusOver the
past two decades, metal–organic frameworks (MOFs)
have matured from interesting academic peculiarities toward a continuously
expanding class of hybrid, nanoporous materials tuned for targeted
technological applications such as gas storage and heterogeneous catalysis.
These oft-times crystalline materials, composed of inorganic moieties
interconnected by organic ligands, can be endowed with desired structural
and chemical features by judiciously functionalizing or substituting
these building blocks. As a result of this reticular synthesis, MOF
research is situated at the intriguing intersection between chemistry
and physics, and the building block approach could pave the way toward
the construction of an almost infinite number of possible crystalline
structures, provided that they exhibit stability under the desired
operational conditions. However, this enormous potential is largely
untapped to date, as MOFs have not yet found a major breakthrough
in technological applications. One of the remaining challenges for
this scale-up is the densification of MOF powders, which is generally
achieved by subjecting the material to a pressurization step. However,
application of an external pressure may substantially alter the chemical
and physical properties of the material. A reliable theoretical guidance
that can presynthetically identify the most stable materials could
help overcome this technological challenge.In this Account,
we describe the recent research the progress on
computational characterization of the mechanical stability of MOFs.
So far, three complementary approaches have been proposed, focusing
on different aspects of mechanical stability: (i) the Born stability
criteria, (ii) the anisotropy in mechanical moduli such as the Young
and shear moduli, and (iii) the pressure-versus-volume equations of
state. As these three methods are grounded in distinct computational
approaches, it is expected that their accuracy and efficiency will
vary. To date, however, it is unclear which set of properties are
suited and reliable for a given application, as a comprehensive comparison
for a broad variety of MOFs is absent, impeding the widespread use
of these theoretical frameworks.Herein, we fill this gap by
critically assessing the performance
of the three computational models on a broad set of MOFs that are
representative for current applications. These materials encompass
the mechanically rigid UiO-66Â(Zr) and MOF-5Â(Zn) as well as the flexible
MIL-47Â(V) and MIL-53Â(Al), which undergo pressure-induced phase transitions.
It is observed that the Born stability criteria and pressure-versus-volume
equations of state give complementary insight into the macroscopic
and microscopic origins of instability, respectively. However, interpretation
of the Born stability criteria becomes increasingly difficult when
less symmetric materials are considered. Moreover, pressure fluctuations
during the simulations hamper their accuracy for flexible materials.
In contrast, the pressure-versus-volume equations of state are determined
in a thermodynamic ensemble specifically targeted to mitigate the
effects of these instantaneous fluctuations, yielding more accurate
results. The critical Account presented here paves the way toward
a solid computational framework for an extensive presynthetic screening
of MOFs to select those that are mechanically stable and can be postsynthetically
densified before their use in targeted applications
High-Throughput Screening of Extrinsic Point Defect Properties in Si and Ge: Database and Applications
Increased computational resources
now make it possible to generate
large data sets solely from first principles. Such “high-throughput”
screening is employed to create a database of embedding enthalpies
for extrinsic point defects and their vacancy complexes in Si and
Ge for 73 impurities from H to Rn. Calculations are performed both
at the PBE and HSE06 levels of theory. The data set is verified by
comparison of the predicted lowest-enthalpy positions with experimental
observations. The effect of temperature on the relative occupation
of defect sites is estimated through configurational entropy. Potential
applications are demonstrated by selecting optimal vacancy traps,
directly relevant to industrial processes such as Czochralski growth
as a means to suppress void formation
High-Throughput Screening of Extrinsic Point Defect Properties in Si and Ge: Database and Applications
Increased computational resources
now make it possible to generate
large data sets solely from first principles. Such “high-throughput”
screening is employed to create a database of embedding enthalpies
for extrinsic point defects and their vacancy complexes in Si and
Ge for 73 impurities from H to Rn. Calculations are performed both
at the PBE and HSE06 levels of theory. The data set is verified by
comparison of the predicted lowest-enthalpy positions with experimental
observations. The effect of temperature on the relative occupation
of defect sites is estimated through configurational entropy. Potential
applications are demonstrated by selecting optimal vacancy traps,
directly relevant to industrial processes such as Czochralski growth
as a means to suppress void formation
Mechanical Properties from Periodic Plane Wave Quantum Mechanical Codes: The Challenge of the Flexible Nanoporous MIL-47(V) Framework
Modeling the flexibility of metal–organic
frameworks (MOFs)
requires the computation of mechanical properties from first principles,
e.g., for screening of materials in a database, for gaining insight
into structural transformations, and for force field development.
However, this paper shows that computations with periodic density
functional theory are challenged by the flexibility of these materials:
guidelines from experience with standard solid-state calculations
cannot be simply transferred to flexible porous frameworks. Our test
case, the MIL-47Â(V) material, has a large-pore and a narrow-pore shape.
The effect of Pulay stress (cf. Pulay forces) leads to drastic errors
for a simple structure optimization of the flexible MIL-47Â(V) material.
Pulay stress is an artificial stress that tends to lower the volume
and is caused by the finite size of the plane wave basis set. We have
investigated the importance of this Pulay stress, of symmetry breaking,
and of <i>k</i>-point sampling on (a) the structure optimization
and (b) mechanical properties such as elastic constants and bulk modulus,
of both the large-pore and narrow-pore structure of MIL-47Â(V). We
found that, in the structure optimization, Pulay effects should be
avoided by using a fitting procedure, in which an equation of state <i>E</i>(<i>V</i>) (EOS) is fit to a series of energy
versus volume points. Manual symmetry breaking could successfully
lower the energy of MIL-47Â(V) by distorting the vanadium–oxide
distances in the vanadyl chains and by rotating the benzene linkers.
For the mechanical properties, the curvature of the EOS curve was
compared with the Reuss bulk modulus, derived from the elastic tensor
in the harmonic approximation. Errors induced by anharmonicity, the
eggbox effect, and Pulay effects propagate into the Reuss modulus.
The strong coupling of the unit cell axes when the unit cell deforms
expresses itself in numerical instability of the Reuss modulus. For
a flexible material, it is therefore advisible to resort to the EOS
fit procedure
Critical Analysis of the Accuracy of Models Predicting or Extracting Liquid Structure Information
This work aims at a critical assessment
of properties predicting
or extracting information on the density and structure of liquids.
State-of-the-art NVT and NpT molecular dynamics (MD) simulations have
been performed on five liquids: methanol, chloroform, acetonitrile,
tetrahydrofuran, and ethanol. These simulations allow the computation
of properties based on first principles, including the equilibrium
density and radial distribution functions (RDFs), characterizing the
liquid structure. Refinements have been incorporated in the MD simulations
by taking into account basis set superposition errors (BSSE). An extended
BSSE model for an instantaneous evaluation of the BSSE corrections
has been proposed, and their impact on the liquid properties has been
assessed. If available, the theoretical RDFs have been compared with
the experimentally derived RDFs. For some liquids, significant discrepancies
have been observed, and a profound but critical investigation is presented
to unravel the origin of these deficiencies. This discussion is focused
on tetrahydrofuran where the experiment reveals some prominent peaks
completely missing in any MD simulation. Experiments providing information
on liquid structure consist mainly of neutron diffraction measurements
offering total structure factors as the primary observables. The splitting
of these factors in reciprocal space into intra- and intermolecular
contributions is extensively discussed, together with their sensitivity
in reproducing correct RDFs in coordinate space
On the Thermodynamics of Framework Breathing: A Free Energy Model for Gas Adsorption in MIL-53
When
adsorbing guest molecules, the porous metal–organic
framework MIL-53Â(Cr) may vary its cell parameters drastically while
retaining its crystallinity. A first approach to the thermodynamic
analysis of this “framework breathing” consists of comparing
the osmotic potential in two distinct shapes only (large-pore and
narrow-pore). In this paper, we propose a generic parametrized free
energy model including three contributions: host free energy, guest–guest
interactions, and host–guest interaction. Free energy landscapes
may now be constructed scanning all shapes and any adsorbed amount
of guest molecules. This allows us to determine which shapes are the
most stable states for arbitrary combinations of experimental control
parameters, such as the adsorbing gas chemical potential, the external
pressure, and the temperature. The new model correctly reproduces
the structural transitions along the CO<sub>2</sub> and CH<sub>4</sub> isotherms. Moreover, our model successfully explains the adsorption
versus desorption hysteresis as a consequence of the creation, stabilization,
destabilization, and disappearance of a second free energy minimum
under the assumptions of a first-order phase transition and collective
behavior. Our general thermodynamic description allows us to decouple
the gas chemical potential ÎĽ and mechanical pressure <i>P</i> as two independent thermodynamic variables and predict
the complete (ÎĽ, <i>P</i>) phase diagram for CO<sub>2</sub> adsorption in MIL-53Â(Cr). The free energy model proposed
here is an important step toward a general thermodynamics description
of flexible metal–organic frameworks
Molecular Dynamics Kinetic Study on the Zeolite-Catalyzed Benzene Methylation in ZSM‑5
The methylation of arenes is a key
step in the production of hydrocarbons
from methanol over acidic zeolites. We performed ab initio static
and molecular dynamics free energy simulations of benzene methylation
in H-ZSM-5 to determine the factors that influence the reaction kinetics.
Special emphasis is given to the effect of the surrounding methanol
molecules on the methylation kinetics. It is found that for higher
methanol loadings, methylation may also occur from a protonated methanol
cluster, indicating that the exact location of the Brønsted acid
site is not essential for the zeolite-catalyzed methylation reaction.
However, methylations from a protonated methanol cluster exhibit higher
free energy barriers than a methylation from a single methanol molecule.
Finally, comparison with a pure methanol solvent reaction environment
indicates that the main role of the zeolite during the methylation
of benzene is to provide the acidic proton and to create a polar environment
for the reaction. The metadynamics approach, which is specifically
designed to sample rare events, allows exploring new reaction pathways,
which take into account the flexibility of the framework and additional
guest molecules in the pores and channels of the zeolite framework.
This approach goes beyond the often applied static calculations to
determine reaction kinetics
Assessment of Atomic Charge Models for Gas-Phase Computations on Polypeptides
The concept of the atomic charge is extensively used
to model the
electrostatic properties of proteins. Atomic charges are not only
the basis for the electrostatic energy term in biomolecular force
fields but are also derived from quantum mechanical computations on
protein fragments to get more insight into their electronic structure.
Unfortunately there are many atomic charge schemes which lead to significantly
different results, and it is not trivial to determine which scheme
is most suitable for biomolecular studies. Therefore, we present an
extensive methodological benchmark using a selection of atomic charge
schemes [Mulliken, natural, restrained electrostatic potential, Hirshfeld-I,
electronegativity equalization method (EEM), and split-charge equilibration
(SQE)] applied to two sets of penta-alanine conformers. Our analysis
clearly shows that Hirshfeld-I charges offer the best compromise between
transferability (robustness with respect to conformational changes)
and the ability to reproduce electrostatic properties of the penta-alanine.
The benchmark also considers two charge equilibration models (EEM
and SQE), which both clearly fail to describe the locally charged
moieties in the zwitterionic form of penta-alanine. This issue is
analyzed in detail because charge equilibration models are computationally
much more attractive than the Hirshfeld-I scheme. Based on the latter
analysis, a straightforward extension of the SQE model is proposed,
SQE+Q<sup>0</sup>, that is suitable to describe biological systems
bearing many locally charged functional groups
Tandem Addition of Phosphite Nucleophiles Across Unsaturated Nitrogen-Containing Systems: Mechanistic Insights on Regioselectivity
The addition of phosphite nucleophiles
across linear unsaturated imines is a powerful and atom-economical
methodology for the synthesis of aminophosphonates. These products
are of interest from both a biological and a synthetic point of view:
they act as amino acid transition state analogs and Horner–Wadsworth–Emmons
reagents, respectively. In this work the reaction between dialkyl
trimethylsilyl phosphites and <i>α,β,γ,δ</i>-diunsaturated imines was evaluated as a continuation of our previous
efforts in the field. As such, the first conjugate 1,6-addition of
a phosphite nucleophile across a linear unsaturated <i>N</i>-containing system is reported herein. Theoretical calculations were
performed to rationalize the observed regioselectivites and to shed
light on the proposed mechanism