Equation of State of Fluid Methane from First Principles with Machine Learning Potentials.

The predictive simulation of molecular liquids requires potential energy surface (PES) models that are not only accurate but also computationally efficient enough to handle the large systems and long time scales required for reliable prediction of macroscopic properties. We present a new approach to the systematic approximation of the first-principles PES of molecular liquids using the GAP (Gaussian Approximation Potential) framework. The approach allows us to create potentials at several different levels of accuracy in reproducing the true PES and thus to determine the level of quantum chemistry that is necessary to accurately predict macroscopic properties. We test the approach by building a series of many-body potentials for liquid methane (CH4), which is difficult to model from first principles because its behavior is dominated by weak dispersion interactions with a significant many-body component. The increasing accuracy of the potentials in predicting the bulk density correlates with their fidelity to the true PES, whereas the trend with the empirical potentials tested is surprisingly the opposite. We conclude that an accurate, consistent prediction of its bulk density across wide ranges of temperature and pressure requires not only many-body dispersion but also quantum nuclear effects to be modeled accurately

Styrene Purification by Guest-Induced Restructuring of Pillar[6]arene

The separation of styrene (St) and ethylbenzene (EB) mixtures is important in the chemical industry. Traditionally, this is done using energy-intensive vacuum distillation columns. Adsorptive separation is an alternative approach. Here, we explore the St and EB adsorption selectivity of two pillar-shaped macrocyclic pillar[n]arenes (EtP5 and EtP6; n 5 and 6). Both crystalline and amorphous EtP6 can capture St from a St-EB mixture with remarkably high selectivity. We show that EtP6 can be used to separate St from a 50:50 v/v St:EB mixture, yielding in a single adsorption cycle St with a purity of > 99 %. Single crystal structures, powder X-ray diffraction patterns and molecular simulations all suggest that this selectivity is due to a guest-induced structural change in EtP6 rather than a simple cavity/pore size effect. This restructuring means that the material ‘self-heals’ upon each recrystallization, and St separation can be carried out over multiple cycles with no loss of performance

Structure–Property Relationships in Amorphous Microporous Polymers

Structural models and physical properties of several amorphous microporous polymers (AMPs) have been investigated using molecular dynamics simulations in an all-atom framework. The modeled structures of AMPs are quantitatively consistent with experimental observations. A linear relationship between the accessible surface area (ASA) and mass density of AMPs has been established. In the AMP network constituted by planar nodes, near-neighbor nodes are oriented parallel to each other. The microporous structural models are further validated by the calculation of CO₂ and N₂ adsorption isotherms at 298 and 77 K, respectively, obtained through Grand Canonical Monte Carlo (GCMC) simulations. The isotherms and isosteric heat of adsorption computed within a force field approach are able to well reproduce the experimental results. The nature of interactions between the functional groups of the AMPs framework and CO₂ have been identified. An excellent CO₂ uptake with high heat of adsorption has been observed in AMPs containing nitrogen-rich building blocks

Modelling adsorption in fluorinated TKL MOFs

<p>Ligand functionalisation resulting in unprecedented enhanced adsorption in a series of structurally similar, fluorinated metal-organic frameworks is studied using molecular computations and simulations. Strikingly anomalous experimental trends in the adsorption characteristics of the TKL FMOFs are investigated and understood here using classical and quantum chemical methods. Almost identical adsorption sites and energies for all the MOFs considered herein were observed. However, experimental isosteric heats and uptake amounts between these solids have been reported to be significantly different. Therefore, <i>ab initio</i> molecular dynamics simulations are performed to account for the drastic effects that flexible linkers can have on gas adsorption energetics, something that low-temperature crystallographic measurements cannot completely capture.</p

Equation of State of Fluid Methane from First Principles with Machine Learning Potentials.

The predictive simulation of molecular liquids requires potential energy surface (PES) models that are not only accurate, but computationally efficient enough to handle the large systems and long time scales required for reliable prediction of macroscopic properties. We present a new approach to the systematic approximation of the first-principles PES of molecular liquids using the GAP (Gaussian Approximation Potential) framework. The approach allows us to create potentials at several different levels of accuracy in reproducing the true PES, and thus to determine the level of quantum chemistry that is necessary to accurately predict macroscopic properties. We test the approach by building a series of many-body potentials for liquid methane (CH4), which is difficult to model from first principles because its behaviour is dominated by weak dispersion interactions with a significant many-body component. The increasing accuracy of the potentials in predicting the bulk density correlates with their fidelity to the true PES, whereas the trend with the empirical potentials tested is surprisingly the opposite. We conclude that an accurate, consistent prediction of its bulk density across wide ranges of temperature and pressure requires not only many-body dispersion, but also quantum nuclear effects to be modelled accurately

Two 3D metal–organic frameworks of Cd(II): modulation of structures and porous properties based on linker functionalities

Two new Cd(II) based metal–organic frameworks (MOFs), {[Cd(NH<SUB>2</SUB>-bdc)(bpe)]·0.5EtOH}<SUB>n</SUB> (1) and {[Cd(NO<SUB>2</SUB>-bdc)(azbpy)]·4H<SUB>2</SUB>O}<SUB>n</SUB> (2) (NH<SUB>2</SUB>-bdc = 2-amino terephthalic acid, bpe = 1,2-bis(4-pyridyl)ethane, NO<SUB>2</SUB>-bdc = 2-nitro terephthalic acid, azbpy = 4,4′-azobipyridine), have been synthesized by a solvent diffusion technique and structurally characterized. Both the frameworks are constructed based on exo-bidentate pyridyl type linkers of similar length but different functionalities. Compound 1 has a 3D structure in which the –NH<SUB>2</SUB> functional group of NH<SUB>2</SUB>-bdc is coordinated to Cd(II) and a 1D ultra-micropore accommodates ethanol guest molecules. The desolvated framework of 1 (1′) is rigid as realized from the PXRD patterns and shows a type-I CO<SUB>2</SUB> uptake profile with a reasonably high isosteric heat of adsorption value. Density functional theory (DFT) calculation shows that aromatic π electrons interact strongly with CO<SUB>2</SUB> and the binding energy is 33.4 kJ mol<SUP>-1</SUP>. Compound 2 has a two-fold interpenetrated 3D porous framework structure where pendent –NO<SUB>2</SUB> groups of NO<SUB>2</SUB>-bdc are aligned on the pore surface. The desolvated framework (2′) exhibits structural transformation and is nonporous to N<SUB>2</SUB>. Smaller and gradual CO<SUB>2</SUB> uptake in 2′ can be attributed to the structural contraction. The solvent (H<SUB>2</SUB>O, MeOH and EtOH) vapour adsorption studies suggest that the pore surface of 2′ is hydrophobic in nature

Dynamic entangled porous framework for hydrocarbon (C2-C3) storage, CO<SUB>2</SUB> capture, and separation

Storage and separation of small (C1–C3) hydrocarbons are of great significance as these are alternative energy resources and also can be used as raw materials for many industrially important materials. Selective capture of greenhouse gas, CO<SUB>2</SUB> from CH<SUB>4</SUB> is important to improve the quality of natural gas. Among the available porous materials, MOFs with permanent porosity are the most suitable to serve these purposes. Herein, a two-fold entangled dynamic framework {[Zn<SUB>2</SUB>(bdc)<SUB>2</SUB>(bpNDI)]⋅4DMF}<SUB>n</SUB> with pore surface carved with polar functional groups and aromatic &#x03C0; clouds is exploited for selective capture of CO<SUB>2</SUB>, C2, and C3 hydrocarbons at ambient condition. The framework shows stepwise CO<SUB>2</SUB> and C<SUB>2</SUB>H<SUB>2</SUB> uptake at 195 K but type I profiles are observed at 298 K. The IAST selectivity of CO<SUB>2</SUB> over CH<SUB>4</SUB> is the highest (598 at 298 K) among the MOFs without open metal sites reported till date. It also shows high selectivity for C<SUB>2</SUB>H<SUB>2</SUB>, C<SUB>2</SUB>H<SUB>4</SUB>, C<SUB>2</SUB>H<SUB>6</SUB>, and C<SUB>3</SUB>H<SUB>8</SUB> over CH<SUB>4</SUB> at 298 K. DFT calculations reveal that aromatic π surface and the polar imide (RNC=O) functional groups are the primary adsorption sites for adsorption. Furthermore, breakthrough column experiments showed CO<SUB>2</SUB>/CH<SUB>4</SUB> C<SUB>2</SUB>H<SUB>6</SUB>/CH<SUB>4</SUB> and CO<SUB>2</SUB>/N<SUB>2</SUB> separation capability at ambient condition