81 research outputs found
Understanding the kinetic and thermodynamic origins of xylene separation in UiO-66(Zr) via molecular simulation
Xylene isomers are precursors in many important chemical processes, yet their separation via crystallization or distillation is energy intensive. Adsorption presents an attractive, lower-energy alternative and the discovery of adsorbents which outperform the current state-of-the-art zeolitic materials represents one of the key challenges in materials design, with metal-organic frameworks receiving particular attention. One of the most well-studied systems in this context is UiO-66(Zr), which selectively adsorbs ortho-xylene over the other C8 alkylaromatics. The mechanism behind this separation has remained unclear, however. In this work, we employ a wide range of computational techniques to explore both the equilibrium and dynamic behavior of the xylene isomers in UiO-66(Zr). In addition to correctly predicting the experimentally-observed ortho-selectivity, we demonstrate that the equilibrium selectivity is based upon the complete encapsulation of ortho-xylene within the pores of the framework. Furthermore the flexible nature of the adsorbent is crucial in facilitating xylene diffusion and our simulations reveal for the first time significant differences between the intracrystalline diffusion mechanisms of the three isomers resulting in a kinetic contribution to the selectivity. Consequently it is important to include both equilibrium and kinetic effects when screening MOFs for xylene separations
Early stages of phase selection in MOF formation observed in molecular Monte Carlo simulations
Metal-organic frameworks (MOF) comprising metal nodes bridged by organic linkers show great promise because of their guest-specific gas sorption, separation, drug-delivery, and catalytic properties. The selection of metal node, organic linker, and synthesis conditions in principle offers engineered control over both structure and function. For MOFs to realise their potential and to become more than just promising materials, a degree of predictability in the synthesis and a better understanding of the self-assembly or initial growth processes is of paramount importance. Using cobalt succinate, a MOF that exhibits a variety of phases depending on synthesis temperature and ligand to metal ratio, as proof of concept, we present a molecular Monte Carlo approach that allows us to simulate the early stage of MOF assembly. We introduce a new Contact Cluster Monte Carlo (CCMC) algorithm which uses a system of overlapping "virtual sites" to represent the coordination environment of the cobalt and both metal-metal and metal-ligand associations. Our simulations capture the experimentally observed synthesis phase distinction in cobalt succinate at 348 K. To the best of our knowledge this is the first case in which the formation of different MOF phases as a function of composition is captured by unbiased molecular simulations. The CCMC algorithm is equally applicable to any system in which short-range attractive interactions are a dominant feature, including hydrogen-bonding networks, metal-ligand coordination networks, or the assembly of particles with "sticky" patches, such as colloidal systems or the formation of protein complexes.</p
Grand-canonical Monte Carlo adsorption studies on SBA-2 periodic mesoporous silicas
SBA-2 and STAC-1 are periodic mesoporous silicas with slightly different structures whose pore networks consist of spherical cavities interconnected by windows. This feature makes them attractive for adsorptive separation processes where the selectivity originates from molecular sieving. Recently, we were able to obtain realistic atomistic models for these materials by means of a kinetic Monte Carlo (kMC) method. In this paper, we evaluate the ability of the model to predict adsorption of both nonpolar (methane and ethane) and polar (carbon dioxide) adsorptives. Predictions are in good agreement with experimental data, demonstrating the potential of these kMC-based models for use in the design of adsorption processes and the materials used in them. In particular, we show that surface roughness is a key feature for predicting adsorption in SBA-2 materials at low pressures; this is especially relevant in prospective applications such as carbon dioxide capture. (Chemical Equation Presented)
Role of particle size and surface functionalisation on the flexibility behaviour of switchable metal-organic framework DUT-8(Ni)
Flexible MOF nanoparticles, i.e. MOF nanoparticles that change their structure upon external stimuli such as guest uptake, are promising for numerous applications including advanced gas adsorption, drug delivery and sensory devices. However, the properties of MOFs are typically characterised based on the bulk material with no consideration of how the particle size and external surface influences their performance. This combined computational and experimental contribution investigates the influence of the particle size and surface functionalisation on the flexibility of DUT-8(Ni) (Ni2 (2,6-ndc)2 dabco, ndc = naphthalene dicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane, DUT=Dresden University of Technology). DUT-8 nanoparticles remain rigid in their open pore form while microparticles, synthesised under slightly different conditions, undergo gate opening upon nitrogen adsorption suggesting that the particle size has an important role to play in the flexibility of DUT-8. While the adsorption environment at the surface capped with modulators smaller than the 2,6-ndc ligand is very different compared to the bulk of the crystal with considerably weaker guest-framework interaction, simulations reveal that the nanoparticles should close. We conclude that the size of the nanoparticles is not the major contributor for keeping DUT-8 nanoparticles open but that it is more likely that defects or nucleation barriers dominate. Moreover, our work reveals for the first time that functionalising the external surface of nanoparticles with different modulators or capping groups offers the opportunity to manipulate the gate opening / closing pressure. This principle is generally applicable and could be exploited to tune the gate openig / closing pressure for the application of interest
Framework-isomerism:highly augmented copper(II) paddlewheel-based MOF with unusual (3,4)-net topology
Conformational isomerism controls collective flexibility in metal-organic framework DUT-8(Ni)
Combined Experimental and Computational Study of Polycyclic Aromatic Compound Aggregation: The Impact of Solvent Composition
The aggregation of polycyclic aromatic compound (PAC) molecules is sensitive to the solvent they are dissolved or suspended in. By using both dynamic light scattering and diffusion-ordered nuclear magnetic resonance spectroscopy, in combination with molecular dynamics simulations, the effect of chemical structure on the aggregation of PACs in both aromatic and alkane solvents were systematically investigated. A suite of triphenylene-based PACs offers a robust platform to understand the driving forces of aggregation mechanism across both nanometer and micrometer scales. Both the configuration, either parallel or otherwise, and the arrangement, whether compact or loose, of molecules in their aggregates are determined by a fine balance of different interactions such as those between the polar groups, ÏâÏ interactions between the aromatic cores, steric hindrance induced by the side chains, and the degree of solvation. These results suggest that molecular architecture is the major factor in determining how the model compounds aggregate. The shift from aromatic to aliphatic solvent only slightly increases the likelihood of aggregation for the model compounds studied while subtle differences in molecular architecture can have a significant impact on the aggregation characteristics
Molecular simulations studies of gas adsorption in metalâorganic frameworks
Using computational tools ranging from molecular simulations â including
both Monte Carlo and molecular dynamics methods â to quantum
mechanical (QM) calculations (primarily at density functional theory (DFT)
level), this work focuses on addressing some of the challenges faced in
molecular simulations of gas adsorption in metalâorganic frameworks
(MOFs). This work consists of two themes: one concerns gas adsorption in
MOFs with coordinatively unsaturated metal sites (cusâs), and the other one
deals with predicting and understanding the breathing behaviour of the
flexible MOF MIL-53(Sc).
It has been shown experimentally that incorporation of cusâs â also known as
âopenâ metal sites or unsaturated metal centres â into MOFs significantly
enhances the uptake of certain gases such as CO2 and CH4. As a result of the
considerably enhanced, localized guest-molecule interactions with the cusâs,
it, however, remains a challenge to predict correctly adsorption isotherms
and/or mechanisms in MOFs with cusâs using grand-canonical Monte Carlo
(GCMC) simulations based on generic classical force fields. To address this
problem, two multi-scale modelling approaches â which combine GCMC
simulations with QM calculations â have been proposed in this work. The
first approach is based on the direct implementation of a fluidâframework
potential energy surface, calculated by a hybrid DFT/ab initio method, in the
GCMC simulations. The second approach involves parameterization of ab
initio force fields for GCMC simulations of gas adsorption in MOFs with
cusâs. This approach focuses on the generation of accurate ab initio reference
data, selection of semiempirical model potentials, and force-field fitting
through a multi-objective genetic algorithm approach. The multi-scale
simulation strategy not only yields adsorption isotherms in very good
agreement with experimental data but also correctly captures adsorption
mechanisms, including the adsorption on the cusâs, observed experimentally
but absent from GCMC simulations based on generic force fields.
The second challenge that this work aims to address concerns the âbreathingâ
phenomenon of MOFs, in which the framework structure adapts its pore
opening to accommodate guest molecules, for example. The breathing effect
gives rise to some exceptional properties of these MOFs and hence promising
applications. However, framework flexibility often poses a challenge for
computational studies of such MOFs, because suitable flexible force fields for
frameworks are lacking and the effort involved in developing a new one is
no less a challenge. Here, an alternative to the force-field-based approach is
adopted. Ab initio molecular dynamics (AIMD) simulations â which combine
classical molecular dynamics simulations with electronic-structure
calculations âon the flyâ â have been deployed to study structural changes of
the breathing MOF MIL-53(Sc) in response to changes in temperature over
the range 100â623 K and adsorption of CO2 at 0â0.9 bar at 196 K. AIMD
simulations employing dispersion-corrected DFT accurately simulated the
experimentally observed closure of MIL-53(Sc) upon solvent removal and the
transition of the empty MOF from the closed-pore phase to the very-narrow-pore
phase with increasing temperature. AIMD simulations were also used to
mimic the CO2 adsorption of MIL-53(Sc) in silico by allowing the MIL-53(Sc)
framework to evolve freely in response to CO2 loadings corresponding to the
two steps in the experimental adsorption isotherm. The resulting structures
enabled the structure determination of the two CO2-containing intermediate
and large-pore phases observed by experimental synchrotron X-ray diffraction
studies with increasing CO2 pressure; this would not have been possible for
the intermediate structure via conventional methods because of diffraction
peak broadening. Furthermore, the strong and anisotropic peak broadening
observed for the intermediate structure could be explained in terms of
fluctuations of the framework predicted by the AIMD simulations.
Fundamental insights from the molecular-level interactions further revealed
the origin of the breathing of MIL-53(Sc) upon temperature variation and
CO2 adsorption.
Both the multi-scale simulation strategy for gas adsorption in MOFs with
cusâs and the AIMD study of the stimuli-responsive breathing behaviour of
MIL-53(Sc) illustrate the power and promise of combining molecular
simulations with quantum mechanical calculations for the prediction and
understanding of MOFs
Inclusion and release of ant alarm pheromones from metalâorganic frameworks
Zinc(II) and zirconium(IV) metalâorganic frameworks show uptake and slow release of the ant alarm pheromones 3-octanone and 4-methyl-3-heptanone. Inclusion of N-propyl groups on the MOFs allows for enhanced uptake and release over several months. In preliminary field trials, leaf cutting ants show normal behavioural responses to the released pheromones
Effect of Pore Geometry on Ultra-Densified Hydrogen in Microporous Carbons
This is the final version. Available on open access from Elsevier via the DOI in this recordOur investigations into molecular hydrogen (H2) confined in microporous carbons with
different pore geometries at 77 K have provided detailed information on effects of pore shape on
densification of confined H2 at pressures up to 15 MPa. We selected three materials: a disordered,
phenolic resin-based activated carbon, a graphitic carbon with slit-shaped pores (titanium carbidederived carbon), and single-walled carbon nanotubes, all with comparable pore sizes of < 1 nm.
We show via a combination of in situ inelastic neutron scattering studies, high-pressure H2
adsorption measurements, and molecular modelling that both slit-shaped and cylindrical pores
with a diameter of ~0.7 nm lead to significant H2 densification compared to bulk hydrogen under
the same conditions, with only subtle differences in hydrogen packing (and hence density) due to
geometric constraints. While pore geometry may play some part in influencing the diffusion
kinetics and packing arrangement of hydrogen molecules in pores, pore size remains the critical
factor determining hydrogen storage capacities. This confirmation of the effects of pore geometry
and pore size on the confinement of molecules is essential in understanding and guiding the
development and scale-up of porous adsorbents that are tailored for maximising H2 storage
capacities, in particular for sustainable energy applications.Engineering and Physical Sciences Research Council (EPSRC
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