Search

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, π–π 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.

Our 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 carbide-derived 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.

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 opening/closing pressure for the application of interest.

Zinc(ii) and zirconium(iv) metal-organic 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.

RRRP, MLC and PAW would like to thank the Engineering and Physical Sciences Research Council (EPSRC) and CRITICAT Centre for Doctoral Training for Financial Support [Ph.D. studentship to RRRP; EP/L016419/1]. MML and PAW would also like to thank EPSRC for the FLEXICCS grant (Versatile Adsorption Process for the Capture of Carbon Dioxide from Industrial Sources; EP/N024613/1). KKC, MYM and KIH are grateful to the Bulgarian National Science Fund for the financial support (Contract no: DFNI T02/20). We thank Vladislav Blatov, Davide Proserpio, and Alexandrov Eugeny for helpful discussions regarding the topological analysis. A porous scandium MOF denoted STA-27 (St Andrews Porous Material-27) has been synthesised solvothermally using the 2,3,5,6-tetrakis(4-carboxyphenyl)pyrazine anion (TCPP4-) as the tetratopic carboxylate linker. STA-27 possesses a unique scandium-based 1D rod secondary building unit (SBU) comprising corner-sharing scandium Sc2O11 dimers connected via carboxylate groups from the linker. After activation under mild conditions STA-27 is an active Lewis acidic catalyst, while heating at elevated temperatures results in rupturing of the Sc-O-Sc linkages and a phase transition to a different topological type. Under similar synthesis conditions the smaller Al3+ and Ga3+ cations give isostructural MOFs with a different, previously reported, topology type based on chains of corner-sharing MO4(OH)2 octahedra: the Al-form possesses attractive properties for CO2 adsorption. Postprint

Hydrogen oxidation and oxygen reduction are two crucial energy conversion reactions, which are shown to be both strongly affected by the presence of intrinsically microporous polymer coatings on electrodes. Polymers of intrinsic microporosity (PIMs) are known to possess extremely high internal surface area and ability to bind gases under dry conditions. It is shown here that both, hydrogen- and oxygen gas binding into PIMs, also occurs under wet or “triphasic” conditions in aqueous electrolyte environments (when immersed in 0.01 M phosphate buffer at pH 7). For two known PIM materials (PIM-1 and PIM-PY), nanoparticles are formed by an anti-solvent precipitation protocol and then cast as a film onto platinum or glassy carbon electrodes. Voltammetry experiments reveal evidence for hydrogen and oxygen binding. Both, PIM-1 and PIM-PY, locally store hydrogen or oxygen gas at the electrode surface and thereby significantly affect electrocatalytic reactivity. The onset of oxygen reduction on glassy carbon is shifted by 0.15 V in the positive direction.

Metal-organic frameworks (MOFs) are coordination networks with organic ligands containing potential voids. Some MOFs show pronounced structural flexibility that may result in closing and re-opening these pores. Here, we show that collective flexibility in a MOF-DUT-8(Ni)-is controlled by conformational isomerism. DUT-8(Ni), a pillared-layer MOF with Ni 2 paddle-wheels, dabco pillars and naphthalene dicarboxylate (ndc) linkers, can crystallize in many conformational isomers that depend on the orientation of the non-linear ndc linkers with respect to each other. While the open form is compatible with several of these conformations, only one of them, with alternating linker orientations, is stable as the closed form. We show, by means of first principles calculations, that in the stable closed form, the appreciable lattice strain is compensated by London-dispersion forces between the ndc linkers that arrange with maximum overlap in a stacking order similar to the stacking in graphite. We substantiate these results by well-tempered metadynamics calculations on the DFT-based Born-Oppenheimer potential energy surface, by refined X-ray diffraction data and by nitrogen adsorption data obtained by experiment and grand-canonical Monte-Carlo simulations based on the DFT-optimized and PXRD-derived geometries. While the reported origin of flexibility cannot be generalized to all flexible MOFs, it offers a rational design concept of folding mechanisms in switchable MOFs by exploitation of the stabilization effect of linker stacking in the closed form.

The incorporation of defects into crystalline materials provides an important tool to fine-tune properties throughout various fields of materials science. We performed high-pressure powder X-ray diffraction experiments, varying pressures from ambient to 0.4 GPa in 0.025 GPa increments to probe the response of defective UiO-66 to hydrostatic pressure for the first time. We observe an onset of amorphization in defective UiO-66 samples around 0.2 GPa and decreasing bulk modulus as a function of defects. Intriguingly, the observed bulk moduli of defective UiO-66(Zr) samples do not correlate with defect concentration, highlighting the complexity of how defects are spatially incorporated into the framework. Our results demonstrate the large impact of point defects on the structural stability of metal–organic frameworks (MOFs) and pave the way for experiment-guided computational studies on defect engineered MOFs.

International audience; Whilst many metal–organic frameworks possess the chemical stability needed to be used as functional materials, they often lack the physical strength required for industrial applications. Herein, we have investigated the mechanical properties of two UiO‐topology Zr‐MOFs, the planar UiO‐67 ([Zr6O4(OH)4(bpdc)6], bpdc: 4,4′‐biphenyl dicarboxylate) and UiO‐abdc ([Zr6O4(OH)4(abdc)6], abdc: 4,4′‐azobenzene dicarboxylate) by single‐crystal nanoindentation, high‐pressure X‐ray diffraction, density functional theory calculations, and first‐principles molecular dynamics. On increasing pressure, both UiO‐67 and UiO‐abdc were found to be incompressible when filled with methanol molecules within a diamond anvil cell. Stabilization in both cases is attributed to dynamical linker disorder. The diazo‐linker of UiO‐abdc possesses local site disorder, which, in conjunction with its longer nature, also decreases the capacity of the framework to compress and stabilizes it against direct compression, compared to UiO‐67, characterized by a large elastic modulus. The use of non‐linear linkers in the synthesis of UiO‐MOFs therefore creates MOFs that have more rigid mechanical properties over a larger pressure range.

Pioneered by Lehn, Cram, Peterson and Breslow, supramolecular chemistry concepts have evolved providing fundamental knowledge of the relationships between the structures and reactivities of organized molecules. A particular fascinating class of metallo-supramolecular molecules are hollow coordination cages that provide cavities of molecular dimensions promoting applications in diverse areas including catalysis, enzyme mimetics and material science. Here we report the synthesis of coordination cages with exceptional cross-sectional diameters that are composed of multiple sub-cages providing numerous distinctive binding sites through labile coordination solvent molecules. The building principles, involving Archimedean and Platonic bodies, renders these supramolecular keplerates as a class of cages whose composition and topological aspects compare to characteristics of edge-transitive {Cu2} MOFs with A3X4 stoichiometry. The nature of the cavities in these double-shell metal-organic polyhedra and their inner/outer binding sites provide perspectives for post-synthetic functionalizations, separations and catalysis. Transmission electron microscopy studies demonstrate that single molecules are experimentally accessible., Host–guest chemistry in hollow coordination cages can be exploited for a range of applications, but is often limited by inner cavity dimensions. Here, Schmitt and co-workers fabricate supramolecular keplerates that possess ultra-large cross-sectional diameters and are composed of multiple sub-cages.

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.

International audience; We are pleased to introduce this special issue of the journal Molecular Simulation on the topic of molecular simulation of framework materials. This broad category of materials includes well-established inorganic materials, such as zeolites, the very topical metal organic frameworks (MOFs), as well as covalent organic frameworks and other molecular framework materials. These materials are all built following the same pattern, with an underlying net formed by nodes linked by atomic or molecular bridging units, although the nature of the interactions involved can vary widely from strong covalent bonds, coordinative bonds, π–π stacking, to hydrogen bonds, etc. Framework materials have received great attention and have been the focus of an ever-growing research effort in the past decade as their properties can be tuned for specific applications, such as gas storage and separation, catalysis, and drug delivery, among others.

Two Co2+ metal-organic framework materials, constructed from a combination of 1,4-benzenedicarboxylate (BDC) and either 2,2′-dipyridyl-N-oxide (DPNO) or 2,2′-dipyridyl-N,N′-dioxide (DPNDO), are synthesized under solvothermal reaction conditions, and their structures solved by single crystal X-ray diffraction. Both have three-dimensional structures that contain octahedral Co2+ centers with μ2-(η2)-BDC, and bidentate DPNO or DPNDO coligands that bridge pairs of metal centers but do not contribute toward the overall connectivity of the framework. Co3(BDC)3(DPNO)2 contains trimers of trans corner-shared Co-centered octahedra with one type of bridging BDC ligand forming terminal edges of the trimers, bridging to neighboring trimer units, and a second type, bridging pairs of metals and also connecting neighboring trimers. Co2(BDC)2(DPNDO) is constructed from one-dimensional inorganic chains consisting of cis- and trans-corner shared Co2+-centered octahedra. The DPNDO ligand is bis-bidentate, forming the edges of one type of octahedron and the trans corners of the second type, with the coordination for both octahedra completed by bridging BDC linkers, which in turn connect the inorganic chains to yield a three-dimensional structure. Thermogravimetric analysis shows both materials contain trapped solvent, and while Co3(BDC)3(DPNO)2 is unstable with respect to solvent loss, Co2(BDC)2(DPNDO), and its magnesium analogue, can be desolvated to yield permanently porous materials that show thermal stability up to 300 °C. For Co2(BDC)2(DPNDO), gas adsorption studies show permanent microporosity with moderate uptake of small gas molecules (N2, CO2, CH4, and C2H6), supported by Grand Canonical Monte Carlo calculations based on the assumption of rigid crystal structures, while gravimetric analysis shows rapid and reversible methanol adsorption at ambient pressure for both the Co and Mg analogues of the framework.

We have studied the adsorption of n-butane and isobutane in the mesoporous silica SBA-2. Our work has two purposes: (i) to better understand the structure of the material, and in particular the impact of calcination on the evolution of the pore network, and (ii) to investigate our ability to tune the structure of SBA-2 to separate normal and isoalkanes by molecular sieving. By a combination of experimental adsorption measurements, molecular simulation, and percolation analysis, we determined the evolution of the sizes of the pores and the connectivity of the pore network as the calcination temperature increases. For a certain range of calcination temperatures, the pore network drops below its percolation threshold for isobutane, while allowing the percolation of n-butane, giving an extremely high selectivity for n-butane over isobutane. This suggests that tuning the window size of SBA-2 and other structured mesoporous materials of this general type has the potential to generate optimized adsorbents for particular applications.

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).

In this multiscale study, four robust zirconium oxide based metal–organic frameworks (MOFs) were examined using powerful molecular simulation tools as well as indispensable full-scale PSA system modeling to determine their potential for H2 purification. Grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations were employed to evaluate the MOF working capacities, binary-mixture selectivities, and micropore transport diffusivities for each of the components of a steam methane reformer offgas (SMROG) stream: H2, CO, CH4, N2, and CO2. The small, functionalized pores of UiO-66(Zr)-Br were found to result in high N2 and CO selectivities and working capacities, whereas the slightly larger pore volume of UiO-66(Zr) favored higher CO2 and CH4 working capacities. The collective impact of impurity uptakes and selectivities on the purification of H2 from five-component steam methane reformer offgas mixtures was investigated through PSA column modeling. The breakthrough behavior of SMROG mixtures in columns containing MOF crystallites was determined using the simulated adsorption and diffusivity data as input. MOF breakthrough curves for single and two-layered beds were compared to those of commercial adsorbents including Zeolite 5A and Calgon PCB. Two of the MOFs, namely, UiO-66(Zr) and UiO-66(Zr)-Br, were found to have longer dimensionless breakthrough times than the commercial zeolite materials and are therefore expected to result in larger yields of high-purity H2 per PSA cycle. UiO-66(Zr)-Br was found to be the most promising of the four MOFs, having the longest dimensionless breakthrough times in both single and two-layered bed setups.

Ab initio molecular dynamics (AIMD) simulations have been used to predict structural transitions of the breathing metal-organic framework (MOF) MIL-53(Sc) in response to changes in temperature over the range 100-623 K and adsorption of CO2 at 0-0.9 bar at 196 K. The method has for the first time been shown to predict successfully both temperature-dependent structural changes and the structural response to variable sorbate uptake of a flexible MOF. AIMD employing dispersion-corrected density functional theory 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 (symmetry change from P2(1)/c to C2/c) with increasing temperature, indicating that it can directly take into account entropic as well as enthalpic effects. We also used AIMD simulations 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. These simulations illustrate the power of the AIMD method for the prediction and understanding of the behavior of flexible microporous solids.

The reaction of zinc(II) nitrate and 4,4'-ethynylenedibenzoic acid (H(2)edb) in DMF at 80 degrees C gave the metal-organic framework material [Zn4O(edb)(3)(H2O)(2)]center dot 6DMF 1 in which edb ligands connect Zn4O centres into a doubly-interpenetrated cubic network with a similar topology to observed with other linear dicarboxylates in the IRMOF series. Analysis of the nitrogen isotherm revealed the material to have a BET surface area of 1088 m(2) g(-1), which is approximately one-third of the value calculated from GCMC simulations, suggesting incomplete activation or pore blocking in the activated material. The reaction of cobalt(II) nitrate and H(2)edb in DMF gave [Co-3(edb)(3)(DMF)(4)]center dot 2.6DMF 2. The structure of 2 is based on Co-3(O2CR)(6) linear secondary building units that are linked by the edb ligands into a two-dimensional network. When 2 was placed under vacuum, a colour change from pale pink to deep blue was observed, which is consistent with loss of the coordinated DMF molecules. When treated with [Co-2(CO)(8)], crystals of 1 turned dark red, and IR analysis is consistent with coordination of Co-2(CO)(6) fragments to the alkyne groups. However, the colour change was restricted to the external crystal surfaces. This is a likely consequence of partial framework collapse, which occurs following coordination of Co-2(CO)(6) to the alkyne groups. Coordination changes the preferred angle between carboxylate groups in the edb ligand, which in turn introduces strain into the network.

ZIF-8 is a zeolitic imidazole-based metal-organic framework with large cavities interconnected by narrow windows. Because the small size of the windows, it allows in principle for molecular sieving of gases such as H-2 and CH4. However, the unexpected adsorption of large molecules on ZIF-8 suggests the existence of structural flexibility. ZIF-8 flexibility is explored in this work combining different experimental techniques with molecular simulation. We show that the ZIF-8 structure is modified by gas adsorption uptake in the same way as it is at a very high pressure (i.e., 14 700 bar) due to a swing effect in the imidazolate linkers, giving access to the porosity. Tuning the flexibility, and so the opening of the small windows, has a further impact on the design of advanced molecular sieving membrane materials for gas separation, adjusting the access of fluids to the porous network.

The adsorption of deutero-methane (CD4) in Cu3(btc)2 (HKUST-1) was investigated at 77 K using high-resolution neutron powder diffraction. Rietveld refinement of the neutron data revealed a sequential filling of the rigid framework at distinct preferred adsorption sites, and showed the importance of open metal sites even for non-polar molecules such as methane. Four main adsorption sites were identified, located inside the small and two larger pores of the framework. The shorter distances between the CD4 center and the pore wall atoms are covering a range from 3.07 to 3.547 Å. The maximum occupation of 170 CD4 molecules per unit cell, estimated from the refined occupancy of the adsorption sites, is close to the value estimated from volumetric adsorption isotherms at 77 K (176 molecules per cell). Molecular simulation gave further insight into the adsorption mechanism.

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.