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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.

MOFs and COFs are porous materials with a large variety of applications including gasstorage and separation. Synthesised in a modular fashion from distinct building blocks, anear in?nite number of structures can be constructed and the properties of the material canbe tailored for a speci?c application. While this modularity is a very attractive feature it alsoposes a challenge. Attempting to identify the best performing material(s) for a given appli-cation is experimentally intractable. Current research e?orts combine molecular simulationsand machine learning techniques to evaluate the simulated performance of hundreds of thou-sands of materials to identify top performing MOFs and COFs for a given application. Theseapproaches typically rely on moderated brute-force screening which is still resource-intensiveas typically between 70 - 100 % of the hundreds of thousands of materials must be simulatedto create a training set for the machine learning models used, restricting screening to rela-tively simple molecules. In this work we demonstrate our novel Bayesian mining approachto materials screening which allows 62 - 92 % of the top 100 porous materials for a range ofapplications to be readily identi?ed from large materials databases after only assessing lessthan one percent of all materials. This is a stark contrast to the 0 - 1 % achieved by conven-tional brute-force screening where porous materials are just chosen at random during a highthroughput screening. Through this accelerated virtual screening process, the identi?cation ofhigh performing materials can be used to more rapidly inform experimental e?orts and hencelead to an acceleration of the entire research and development pipeline of porous materials.

MOFs and COFs are porous materials with a large variety of applications including gasstorage and separation. Synthesised in a modular fashion from distinct building blocks, anear in?nite number of structures can be constructed and the properties of the material canbe tailored for a speci?c application. While this modularity is a very attractive feature it alsoposes a challenge. Attempting to identify the best performing material(s) for a given appli-cation is experimentally intractable. Current research e?orts combine molecular simulationsand machine learning techniques to evaluate the simulated performance of hundreds of thou-sands of materials to identify top performing MOFs and COFs for a given application. Theseapproaches typically rely on moderated brute-force screening which is still resource-intensiveas typically between 70 - 100 % of the hundreds of thousands of materials must be simulatedto create a training set for the machine learning models used, restricting screening to rela-tively simple molecules. In this work we demonstrate our novel Bayesian mining approachto materials screening which allows 62 - 92 % of the top 100 porous materials for a range ofapplications to be readily identi?ed from large materials databases after only assessing lessthan one percent of all materials. This is a stark contrast to the 0 - 1 % achieved by conven-tional brute-force screening where porous materials are just chosen at random during a highthroughput screening. Through this accelerated virtual screening process, the identi?cation ofhigh performing materials can be used to more rapidly inform experimental e?orts and hencelead to an acceleration of the entire research and development pipeline of porous materials.

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

Cancer remains hard to treat, partially due to the non-specificity of chemotherapeutics. Metal-organic frameworks (MOFs) are promising carriers for targeted chemotherapy, yet, to date, there have been few detailed studies to systematically enhance drug loading while maintaining controlled release. In this work, we investigate which molecular simulation methods best capture the experimental uptake and release of cisplatin from UiO-66 and UiO-66(NH2). We then screen a series of biocompatible, pH-sensitive zeolitic imidazolate frameworks (ZIFs) for their ability to retain cisplatin in healthy parts of the patient and release it in the vicinity of a tumor. Pure-component GCMC simulations show that the maximum cisplatin loading depends on the pore volume. To achieve this maximum loading in the presence of water, either the pore size needs to be large enough to occupy both cisplatin and its solvation shell or the MOF-cisplatin interaction must be more favorable than the cisplatin-shell interaction. Both solvated and non-solvated simulations show that cisplatin release rates can be controlled by either decreasing the pore limiting diameters or by manipulating framework-cisplatin interaction energies to create strong, dispersed adsorption sites. The latter method is preferable if cisplatin loading is performed from solution into a pre-synthesized framework as weak interaction energies and small pore window diameters will hinder cisplatin uptake. Here, ZIF-82 is most promising. If it is possible to load cisplatin during crystallization, ZIF-11 would outcompete the other MOFs screened as cisplatin cannot pass through its pore windows; therefore, release rates would be purely driven by the pH triggered framework degradation.

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.

Many zeolitic imidazolate frameworks (ZIFs) are promising candidates for use in separation technologies. Comprising large cavities interconnected by small windows they can be used, at least in principle, as molecular sieves where molecules smaller than the window size are able to diffuse into the material while larger molecules are rejected. However, "swing effect" or "gate opening" phenomena resulting in an enlargement of the windows have proven to be detrimental. Here, we present the first systematic experimental and computational study of the effect of chemical functionalization of the imidazole linker on the framework dynamics. Using high-pressure (HP) single-crystal X-ray diffraction, density functional theory, and grand canonical Monte Carlo simulations, we show that in the isostructural ZIF-8, ZIF-90, and ZIF-65 functional groups of increasing polarity (-CH 3, -CHO, and -NO 2) on the imidazole linkers provide control over the degree of rotation and thus the critical window diameter. On application of pressure, the substituted imidazolate rings rotate, resulting in an increase in both pore volume and content. Our results show that the interplay between the guest molecules and the chemical function of the imidazole linker is essential for directing the swing effect in ZIF frameworks and therefore the adsorption performance.

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.

The authors acknowledge the support of the EPSRC/St Andrews Criticat CDT (RRRP, PAW) and the European Community Seventh Framework Program (FP7/2007-2013) number 608490 (project M4CO2) (KKC, MYM, KIH, PAW). SEA would like to thank the Royal Society and Wolfson Foundation for a merit award. This research made use of the Balena High Performance Computing (HPC) Service at the University of Bath. The research data (and/or materials) supporting this publication can be accessed at DOI: http://dx.doi.org/10.17630/6ffeed8a-e75f-4648-968f-3ed32a94e9a0. A fully interpenetrated 8,3-connected zirconium MOF with the the-i topology type, STA-26 (St Andrews porous material-26), has been prepared using the 4,4',4"-(2,4,6-trimethylbenzene-1,3,5-triyl)tribenzoate (TMTB) tritopic linker with formic acid as a modulating agent. In the as-prepared form STA-26 possesses Im-3m symmetry compared with the Pm-3m symmetry of the non-interpenetrated analogue, NU-1200, prepared using benzoic acid as a modulator. Upon removal of residual solvent there is a shift between the interpenetrating lattices and a resultant symmetry change to Cmcm which is fully reversible. This is observed by X-ray diffraction and 13C MAS NMR is also found to be remarkably sensitive to the structural transition. Furthermore, heating STA-26(Zr) in vacuum dehydroxylates the Zr6 nodes leaving coordinatively unsaturated Zr4+ sites, as shown by IR spectroscopy using CO and CD3CN as probe molecules. Nitrogen adsorption at 77 K together with grand canonical Monte Carlo simulations confirms a microporous, fully interpenetrated, structure with pore volume 0.53 cm3 g−1 while CO2 adsorption at 196 K reaches 300 cm3 STP g−1 at 1 bar. While the pore volume is smaller than that of its non-interpenetrated mesoporous analogue, interpenetration makes the structure more stable to moisture adsorption and introduces shape selectivity in adsorption. Postprint

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.

A solid-state approach that takes advantage of the ordered 3D arrangement of active secondary building units allows the preparation of new interlocked MOFs that grow hetero-epitaxially on the crystal faces of a precursor phase that acts as a "topological blueprint". The synthetic strategy is exemplified by using rigid acetylene-based ligands to produce highly augmented Cu(II) acetate-based MOFs.

The syntheses, calculated surface areas and N2uptakes of two highly augmented {Cu}2‘paddlewheel’-based MOFs provide a direct comparison oftboandptoframework polymorphs.

It has been 10 years since the original publication that presented Music, a multipurpose simulation code, written in Fortran 90. Since then, the code has been downloaded over 350 times and used in ...

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.

We used molecular simulation to examine the nature of enantioselectivity in the homochiral metal–organic framework [Ni2(l-asp)2(bipy)] for a number of chiral diols and compared the results to experimental enantiomeric excess (ee) data available in the literature. For all cases, our simulation results not only reproduced the general preference of the R-enantiomers in the framework derived from l-asp but also captured more subtle trends. Studying the adsorption process on the molecular level, we show that the R-enantioselectivity is strongly related to better packing effects at high loadings resulting in higher ee values for 1,3-butanediol in comparison with 1,2-butanediol and 1,2-propanediol. We demonstrate that the level of enantioselectivity is highly dependent on the chain length and the position of hydroxyl groups on the carbon chain. We show that whereas the chirality of the MOF framework assists the separation mechanism, the more dominant factor is the perfect match between guest–framework size and s...

SBA-2 and STAC-1 are two related periodic mesoporous silicas (PMSs) that have regular networks of spherical, interconnected pores; the pores are similar in the two materials but the networks differ in their symmetry. The nature of the interconnected network of pores in these materials gives rise to interesting properties related to their potential use in separation processes. In this work, we extend a kinetic Monte Carlo (kMC) technique, originally derived for MCM-41, a simpler PMS, and apply it to mimic the condensation, aggregation, deformation, and calcination stages of the synthesis of SBA-2 and STAC-1. Our simulated synthesis results suggest that the pores are connected through windows formed during micelle aggregation because of the close packing of the spherical micelles and the presence of water molecules at the silica–micelle interface. The simulated materials were validated by comparing properties such as unit cell size, pore size, pore shape, and wall density to results from experimental X-ray ...

The incorporation of coordinatively unsaturated metal sites (cus’s), also known as open metal sites, into metal–organic frameworks (MOFs), significantly enhances the uptake of certain gases, such as CO2 and CH4, especially at low loadings when fluid–framework interactions play the predominant role. However, due to the considerably enhanced, localized guest interactions with the cus’s, it remains a challenge to predict correctly adsorption isotherms and mechanisms in MOFs with cus’s using grand-canonical Monte Carlo (GCMC) simulations based on generic classical force fields. To address this problem, we carefully investigated several well-established semiempirical model potentials and used a multiobjective genetic algorithm to parametrize them using accurate ab initio data as reference. The Carra–Konowalow potential, a modified Buckingham potential, in combination with the MMSV potential for the cus’s gives not only adsorption isotherms in very good agreement with experiments but also correctly captures the...

As a class of porous materials, metal–organic frameworks (MOFs) show promise for the adsorption-based separation of mixtures of gases. The design of any process involving selective adsorption requires knowledge of mixture adsorption isotherms. Ideal adsorbed solution theory (IAST) predicts mixture adsorption equilibria using only single-component data, thereby minimizing the need for experimental adsorption data. In this work we perform a systematic study of the applicability of IAST to MOFs by using grand canonical Monte Carlo (GCMC) simulations to investigate the suitability of IAST for the prediction of the adsorption of mixtures of molecules of differing sizes, asphericities, and polarities in a range of structurally different MOFs. We show that IAST is generally accurate for MOFs. Where we find IAST is less accurate, deviations result from both mixture effects, in the form of nonidealities in the adsorbed phase, and characteristics of the adsorbent structures. In terms of the MOF structure, departure...

The scandium analogue of the flexible terephthalate MIL-53 yields a novel closed pore structure upon removal of guest molecules which has unusual thermal behaviour and stepwise opening during CO(2) adsorption. By contrast, the nitro-functionalised MIL-53(Sc) cannot fully close and the structure possesses permanent porosity for CO(2).

Whereas grand-canonical Monte Carlo (GCMC) simulations based on generic force fields provide good predictions of adsorption isotherms in metal–organic frameworks (MOFs), especially at higher temperature, they fail to correctly describe the adsorption mechanism in MOFs with coordinatively unsaturated sites (cus's) at low temperatures, even for nonpolar fluids such as methane. To address this problem, we directly implemented the potential energy surface calculated by a hybrid DFT/ab inito method in the GCMC simulations using the adsorption of methane on CuBTC as an example. A comparison with previously published in situ experiments shows that our approach not only quantitatively predicts adsorption isotherms for a wide range of temperatures and pressures but also provides the correct description of the adsorption mechanism, including adsorption on the cus's. We also show that care must be taken when selecting the ab initio method to be coupled with GCMC simulations to obtain accurate predictions.

The crystal structure of the small pore scandium terephthalate Sc2(O2CC6H4CO2)3 (hereafter Sc2BDC3, BDC = 1,4-benzenedicarboxylate) has been investigated as a function of temperature and of functionalization, and its performance as an adsorbent for CO2 has been examined. The structure of Sc2BDC3 has been followed in vacuo over the temperature range 140 to 523 K by high resolution synchrotron X-ray powder diffraction, revealing a phase change at 225 K from monoclinic C2/c (low temperature) to Fddd (high temperature). The orthorhombic form shows negative thermal expansivity of 2.4 × 10–5 K–1: Rietveld analysis shows that this results largely from a decrease in the c axis, which is caused by carboxylate group rotation. 2H wide-line and MAS NMR of deuterated Sc2BDC3 indicates reorientation of phenyl groups via π flips at temperatures above 298 K. The same framework solid has also been prepared using monofunctionalized terephthalate linkers containing -NH2 and -NO2 groups. The structure of Sc2(NH2-BDC)3 has be...

A detailed analysis of the solvothermal synthesis in DMF of the polymeric magnesium formate {H[Mg(HCOO)(3)]⊃NHMe(2)}(∞) (1) from Mg(ClO(4))(2)·6H(2)O revealed that the final crystalline product is formed after an acid-catalyzed DMF hydrolysis, producing formic acid and dimethylamine. The former bridges magnesium(II) centers, creating the 3D scaffold, while the latter is trapped inside the cubic cavities of the polymer, engaging in strong hydrogen bonding with the formate ions of the cage. After thermal activation and guest removal, the material was tested for hydrogen uptake at T = 77 K over the 0-80 bar pressure range, and the existence of preferred H(2) adsorption sites was assessed through grand canonical Monte Carlo (GCMC) simulations. No specific low-energy site was found, and the H(2) molecules positions within the framework cavities are mainly dependent on packing effects. Thus, at low H(2) loadings the most populated site is the center of the cubic cavities, even though, at higher pressures, two more "localized" positions have been found by the simulation because of the reduced freedom of movement. The maximum experimental H(2) uptake corresponds to 8.8 mg/g or 13.5 mg/cm(3).

The diffusion of binary mixtures of CO2 and N2 in a series of functionalized MCM-41 materials was studied using molecular dynamics (MD) simulations. Transport diffusion coefficients were obtained in order to assess the effect of surface functionalization with organic groups. Unmodified MCM-41 was found to be transport selective for N2. The spatial distribution of the two coadsorbed species showed that N2 molecules permeated faster and occupied the central pore region to a greater extent than CO2 molecules. CO2 was found to have a much lower permeability than N2 due to stronger interactions with the surface of MCM-41 and a tendency to become trapped in nooks in the amorphous pore wall. This effect was not reproduced when using a more simplistic model pore with a smooth surface demonstrating the importance of using a realistic pore model. Larger aminophenyl surface groups were found to significantly reduce the N2 selectivity due to a decrease in the flux of N2 and an increase in the flux of CO2. We explain ...

The isosteric enthalpy of adsorption for neopentane at relative pressures down to 3 × 10(-8) in MCM-41 was predicted for the temperature range from -15 to 0 °C. At such low pressures and temperatures, experimental measurements become problematic for this system. We used an atomistic model for MCM-41 obtained by means of a kinetic Monte Carlo method mimicking the synthesis of the material. The model was parametrized to represent experimental nitrogen adsorption isotherms at 77 K using grand canonical Monte Carlo simulations. The simulated isosteric enthalpy of adsorption shows very good agreement with available experimental data, demonstrating that GCMC simulations can predict heats of adsorption for conditions that are challenging for experimental measurements. Additional insights into the adsorption mechanisms, derived from energetic analysis at the molecular level, are also presented.

Mesoporous adsorbents such as MCM-41 are promising materials for use in the separation stage of carbon capture and storage (CCS). Using molecular simulation, we investigated the effectiveness of functionalizing MCM-41 with organic surface groups for CO2/N2 separation from a typical power-plant flue-gas stream. The adsorption isotherms for pure CO2 and ternary flue-gas mixtures were determined in functionalized MCM-41 using grand-canonical Monte Carlo (GCMC) simulations. Both CO2 uptake capacities and CO2/N2 selectivities were studied in order to predict the performance of functionalized materials in the removal of CO2 from flue gases. The effects of pore size, surface group concentration, alkyl chain length, surface group rigidity, and concentration of polar amino moieties on adsorption were studied in detail. Surface functionalization of MCM-41 resulted in materials exhibiting a wide range of CO2/N2 selectivities. Insight from these simulations allowed for the design of a tailor-made surface group encomp...

Metal-organic frameworks (MOFs) have shown adsorption behavior that is not observed in other microporous materials such as zeolites or activated carbons. This study used grand canonical Monte Carlo simulation to evaluate a particular form of behavior, which corresponds to the presence of unusual type V adsorption isotherms. Study of a series of MOFs in the IRMOF family, containing chemically similar linkers of different length, showed that the presence of type V adsorption depends on a fine balance between the strength of the fluid-fluid and fluid-solid interactions, which in turn is a strong function of the length of the linker and therefore the pore size. A transition from type V behavior to the more common type I behavior is observed as the temperature increases. The temperature at which this transition occurs increases, and the transition becomes more diffuse, as the length of the linker increases. This type V behavior leads to an interesting possibility in the design of MOF adsorbents for use in gas separation and gas storage applications.

Metal−organic frameworks (MOFs) synthesized in a building-block approach from organic linkers and metal corner units offer the opportunity to design materials with high surface areas for adsorption applications by assembling the appropriate building blocks. In this paper, we show that the surface area calculated in a geometric fashion from the crystal structure is a useful tool for characterizing MOFs. We argue that the accessible surface area rather than the widely used Connolly surface area is the appropriate surface area to characterize crystalline solids for adsorption applications. The accessible surface area calculated with a probe diameter corresponding to the adsorbate of interest provides a simple way to screen and compare adsorbents. We investigate the effects of the probe molecule diameter on the accessible surface area and discuss the implications for increasing the surface area of metal−organic frameworks by the use of catenated structures. We also demonstrate that the accessible surface area...

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.

Grand canonical Monte Carlo simulations were performed to predict adsorption isotherms for hydrogen in a series of 10 isoreticular metal-organic frameworks (IRMOFs). The results show acceptable agreement with the limited experimental results from the literature. The effects of surface area, free volume, and heat of adsorption on hydrogen uptake were investigated by performing simulations over a wide range of pressures on this set of materials, which all have the same framework topology and surface chemistry but varying pore sizes. The results reveal the existence of three adsorption regimes: at low pressure (loading), hydrogen uptake correlates with the heat of adsorption; at intermediate pressure, uptake correlates with the surface area; and at the highest pressures, uptake correlates with the free volume. The accessible surface area and free volume, calculated from the crystal structures, were also used to estimate the potential of these materials to meet gravimetric and volumetric targets for hydrogen storage in IRMOFs.

A wide variety of new nanoporous metal−organic materials are being synthesized by many research groups using supramolecular chemistry and directed assembly in a building block approach based on corner units and linker molecules. These materials may lead to revolutionary advances in adsorption separations because the properties of these materials may be tailored in a synthetically predictable manner. In this paper, we use molecular simulations to assess the suitability of one group of metal organic materials, namely, isoreticular metal−organic frameworks (IRMOFs), as adsorbents for mixture separations. By using grand canonical Monte Carlo simulations, the influence of the linker molecule on the adsorption of methane, n-butane, and their mixtures is determined. Detailed analysis of the energetics as well as the siting of molecules in the cavities allows us to resolve the impact of the linker molecules on the selectivity and to propose new, not yet synthesized materials, which show even higher selectivities....

One of the strategic goals of the modern automobile manufacturing industry is to replace gasoline and diesel with alternative fuels such as natural gas. In this report, we elucidate the desired characteristics of an optimal adsorbent for gas storage. The U.S. Department of Energy has outlined several requirements that adsorbents must fulfill for natural gas to become economically viable, with a key criterion being the amount adsorbed at 35 bar. We explore the adsorption characteristics of novel metal-organic materials (IRMOFs and molecular squares) and contrast them with the characteristics of two zeolites, MCM-41, and different carbon nanotubes. Using molecular simulations, we uncover the complex interplay of the factors influencing methane adsorption, especially the surface area, the capacity or free volume, the strength of the energetic interaction, and the pore size distribution. We also explain the extraordinary adsorption properties of IRMOF materials and propose new, not yet synthesized IRMOF structures with adsorption characteristics that are predicted to exceed the best experimental results to date by up to 36%.

Grand canonical Monte Carlo and molecular dynamics simulations have been performed for methane, n-alkanes, cyclohexane and benzene in two novel nanoporous metal–organic materials. The first material, bipyridine molecular squares, consists of discrete square molecules with corners formed by rhenium complexes and edges formed by bipyridine links, giving a small cavity within each square. The material is considered in both its crystalline form and an amorphous packing of squares. The second material is IRMOF-1, a periodic, crystalline structure also with metal corners and organic bridging units. Adsorption isotherms and self-diffusion coefficients are reported and provide insight into molecular interactions in these materials.

Most transport diffusion simulation studies are carried out in single pores with simple geometries such as slit-like pores or straight cylindrical pores. Such models are gross simplifications of the structure of real, geometrically heterogeneous adsorbents. We carried out grand canonical molecular dynamics simulations of binary, counter diffusing CH4/CF4 mixtures in different cylindrical model pores with pore radii ranging from 11 to 23 A. Each of these pore models represents a feature that can be found in real adsorbents such as pores with amorphous surfaces, pores with a change in the cross-sectional area, pores with kinks and simple pore networks. The results were compared to simulation results in straight pores with homogeneous surfaces to investigate the extent to which transport diffusion can be represented by these simplified model pores. Our results show that transport diffusion is hardly influenced by the different pore structures studied in this work. The simulation results can be reproduced by straight pores if the correct average radius, flow length and enclosing gradient are chosen.

Grand canonical Monte Carlo (GCMC) and dual-control-volume grand canonical molecular dynamics (DCV-GCMD) simulations were carried out with Lennard-Jones model fluids in carbon nanotubes, with the objective of investigating the effect of varying molecular properties on adsorption and diffusion. The influence of the molecular weight, and the Lennard Jones parameters a (a measure of the molecule size) and e (a measure of the interaction strength) on adsorption isotherms, fluxes, and transport diffusivities was studied. For these simulations, the properties of component 1 in the mixture were held constant and one of the properties of component 2 was changed systematically. Furthermore, the validity of Graham's law, which relates the fluxes of two counter diffusing species to their molecular weight, was investigated on a molecular level. Graham's law is fulfilled for the whole range of molecular weights and Lennard-Jones parameters a investigated. However, large deviations were observed for large values of e 2 . Here, the interaction of the two components in the mixture becomes so strong that component 1 is dragged along by component 2.

The sorption isotherms for binary mixtures of methane, ethane, propane and tetrafluoromethane have been determined in carbon nanotubes using configurational bias Monte Carlo simulation techniques. At high loadings, a curious maximum for equimolar gas-phase mixtures occurs with increasing pressure in the absolute adsorption isotherm of one or both adsorbing species. It was detected that there exist two fundamentally different reasons for this maximum. First, due to a higher packing efficiency, one component is able to displace the other component at high loadings. Here, it must be stressed that the displaced component is not necessarily the larger molecule. Second, non-ideality effects of the bulk gas phase can be made responsible for this maximum. The acceptance probability of a molecule insertion in a grand canonical Monte Carlo step is proportional to the component fugacity. If, owing to non-ideality effects of the gas phase, the fugacity of one component does not increase as steeply with pressure as the other component, a maximum can occur in the absolute adsorption isotherm of this component. These findings were demonstrated for various binary mixtures of CH 4 , CF 4 , C 2 H 6 and C 3 H 8 .

Understanding transport diffusion on a molecular level helps to develop improved adsorbents with tailored rate and equilibrium properties. Dual-control-volume grand canonical molecular dynamics (DCV-GCMD) simulations allow the direct simulation of transport diffusion on a molecular level. DCV-GCMD simulations of CH4/CF4 mixtures in carbon nanotubes were carried out. An approach to get composition dependent transport diffusivities directly from DCV-GCMD simulations is introduced. Composition dependent transport diffusivities and fluxes are calculated for varying driving forces in order to investigate the influence of the very large driving force in simulations which is about four orders of magnitude larger than in real experimental systems. Whereas, the flux depends on the driving force the transport diffusivity is independent of it so that DCV-GCMD simulation can be used to simulate transport under experimental conditions. Furthermore, the results of composition dependent diffusivities at four different temperatures are presented. A linear function describes the composition dependence and reproduces the simulated concentration profiles very well. The analysis of the temperature dependence indicates that the transport in the investigated system is due to liquid-like molecular diffusion and not to activated diffusion.

We report experimental measurements of the adsorption of methane and ethane, and binary mixtures of these components, on MCM-41. This adsorbent, which has a simple, regular pore structure and a chemically homogeneous surface, was used as a model system on which to evaluate methods for the prediction of adsorption equilibrium. Two methods were evaluated: ideal adsorbed solution theory and grand canonical Monte Carlo simulation. Both methods gave accurate predictions of binary adsorption. In addition, the grand canonical Monte Carlo method was able to accurately predict pure-component adsorption.

Previous high-pressure experiments have shown that pressure-transmitting fluids composed of small molecules can be forced inside the pores of metal organic framework materials, where they can cause phase transitions and amorphization and can even induce porosity in conventionally nonporous materials. Here we report a combined high-pressure diffraction and computational study of the structural response to methanol uptake at high pressure on a scandium terephthalate MOF (Sc2BDC3, BDC = 1,4-benzenedicarboxylate) and its nitro-functionalized derivative (Sc 2(NO2-BDC)3) and compare it to direct compression behavior in a nonpenetrative hydrostatic fluid, Fluorinert-77. In Fluorinert-77, Sc2BDC3 displays amorphization above 0.1 GPa, reversible upon pressure release, whereas Sc2(NO 2-BDC)3 undergoes a phase transition (C2/c to Fdd2) to a denser but topologically identical polymorph. In the presence of methanol, the reversible amorphization of Sc2BDC3 and the displacive phase transition of the nitro-form are completely inhibited (at least up to 3 GPa). Upon uptake of methanol on Sc2BDC3, the methanol molecules are found by diffraction to occupy two sites, with preferential relative filling of one site compared to the other: grand canonical Monte Carlo simulations support these experimental observations, and molecular dynamics simulations reveal the likely orientations of the methanol molecules, which are controlled at least in part by H-bonding interactions between guests. As well as revealing the atomistic origin of the stabilization of these MOFs against nonpenetrative hydrostatic fluids at high pressure, this study demonstrates a novel high-pressure approach to study adsorption within a porous framework as a function of increasing guest content, and so to determine the most energetically favorable adsorption sites.

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.