The addition of carbon dioxide to four superbase ionic liquids, [P3333][Benzim], [P3333][124Triz], [P3333][123Triz] and [P3333][Bentriz] was studied using a molecular DFT approach involving anions alone and individual ion pairs. Intermolecular bonding within the individual ion pairs is characterised by a number of weak hydrogen bonds, with the superbase anion geometrically arranged so as to maximize interactions between the heterocyclic N atoms and the cation. The pairing energies show no correlation to the observed CO2 adsorption capacity. Addition of CO2 to the anion alone clearly resulted in the formation of a covalently-bound carbamate function with the strength of binding correlated to experimental capacity. In the ion pair however the cation significantly alters the nature of the bonding such that the overall cohesive energy is reduced. Formation of a strong carbamate function occurs at the expense of weakening the interaction between anion and cation. In the more weakly absorbing ion pairs which contain [123Triz]− and [Bentriz]−, the carbamate-functionalised systems are very close in energy to adducts in which CO2 is more weakly bound, suggesting an equilibrium between the chemi- and physisorbed CO2.

The influence of the nature of the cation on the interaction of the silicoaluminophosphate SAPO-34 with small hydrocarbons (ethane, ethylene, acetylene, propane, propylene) is investigated using periodic density-functional theory calculations including a semi-empirical dispersion correction (DFT-D). Initial calculations are used to evaluate which of the guest-accessible cation sites in the chabazite-type structure is energetically preferred for a set of ten cations, which comprises four alkali metals (Li+, Na+, K+, Rb+), three alkaline earth metals (Mg2+, Ca2+, Sr2+), and three transition metals (Cu+, Ag+, Fe2+). All eight cations that are likely to be found at the SII site (centre of a six-ring) are then included in the following investigation, which studies the interaction with the hydrocarbon guest molecules. In addition to the interaction energies, some trends and peculiarities regarding the adsorption geometries are analysed, and electron density difference plots obtained from the calculations are used to gain insights into the dominant interaction types. In addition to dispersion interactions, electrostatic and polarisation effects dominate for the main group cations, whereas significant orbital interactions are observed for unsaturated hydrocarbons interacting with transition metal (TM) cations. The differences between the interaction energies obtained for pairs of hydrocarbons of interest (such as ethylene–ethane and propylene–propane) deliver some qualitative insights: if this energy difference is large, it can be expected that the material will exhibit a high selectivity in the adsorption-based separation of alkene–alkane mixtures, which constitutes a problem of considerable industrial relevance. While the calculations show that TM-exchanged SAPO-34 materials are likely to exhibit a very high preference for alkenes over alkanes, the strong interaction may render an application in industrial processes impractical due to the large amount of energy required for regeneration. In this respect, SAPOs exchanged with alkaline earth cations could provide a better balance between selectivity and energy cost of regeneration.

Dispersion-corrected density-functional theory (DFT-D) calculations are used to study the interaction of hydrogen and carbon dioxide with ZIF-8, a prototypical zeolitic imidazolate framework (ZIF) with sodalite topology. Four distinct adsorption sites are identified for each of the two guest species. Two of the sites are associated with the six-ring windows, a third site is located close to one imidazolate moiety, and the fourth site is situated in the close proximity of two methyl substituents of the methylimidazolate linkers. For the case of hydrogen, where experimental data are available, the positions and the energetic ordering obtained in the DFT-D calculations agree well with these data. The investigation is then extended to two groups of isostructural systems. The first group consists of two boron imidazolate frameworks (BIFs), in which the tetrahedrally coordinated atoms (T atoms) differ from those in ZIF-8, while the methylimidazolate linker remains the same. The calculations show that the nature of the T atoms has only a very limited effect on the interaction with the guest molecules. The second group of derivatives comprises four systems that incorporate the same T atom as ZIF-8 (zinc), but linkers with different substituents X, with X = –H, –NO2, –NH2, –CHO. In these cases, the interaction with CO2 and, to a much lesser extent, hydrogen is increased at the adsorption site that is associated with the substituents, most prominently in the nitro- and aldehyde-functionalised systems. A detailed analysis of the adsorption geometries is used to explain the favourable effect of the substituents. Furthermore, it is shown how a reasonable estimate of the average interaction energy can be obtained from a weighted average over the different adsorption sites, accounting for their possible occupancy. In the case of ZIF-8, this averaged value is compared to experimental heats of adsorption, and the deviations are discussed. Finally, possible applications in hydrogen storage and CO2/H2 separation are discussed. All materials show similar affinities for hydrogen, indicating that their performance in H2 storage applications is largely independent of the structural modifications considered. Because the CO2/H2 selectivity is related to the difference in affinity towards the two species, it can be expected from the DFT-D results that the nitro- and aldehyde-functionalised systems will perform considerably better than ZIF-8, especially for the removal of relatively small amounts of carbon dioxide from a hydrogen feed. This finding is particularly encouraging as both systems are synthetically accessible.

Ab initio MD and potential energy surface sampling has been used to study the rearrangement processes in carboranes and their derivatives. A new mechanism is found, in addition to those previously proposed. The fact that theoretical activation energies are lower than those observed experimentally, and the differing activity of technetium and rhenium complexes, are rationalised by orbital symmetry constraints.

Density Functional Theory (DFT) calculations and Grand Canonical Monte Carlo (GCMC) simulation techniques are used to study CO2 adsorption in (CH3)2-, (OH)2-, NH2- and COOH-functionalized MIL-53(Al3+)lp. The study provides a comparative analysis of CO2 adsorption in the parent and functionalized structures, in terms of heats of adsorption, isotherms and CO2 binding sites. For each functionalized MIL-53 material, DFT calculations of CO2⋯framework binding energies at different CO2 locations within the porous framework provide detailed information about the nature and strength of the CO2⋯site interactions. The multi-technique computational approach allows a direct comparison between adsorption sites from DFT calculations (at 0 K) and those observed in GCMC at 303 K and in the 0.01–25 bar pressure range. The results demonstrate the influence of confinement on the occurrence of synergic CO2–ligand interactions, and also highlight the strong link between the nature of CO2 adsorption sites and adsorption properties at low pressure, in particular the beneficial impact of polar functional groups on CO2 uptake. At high pressures, radial distribution functions of CO2 distances from the pore centres, and snapshots of the density and energy distributions of CO2 in the MOF materials provide important insights into the dependence of the adsorption process upon structural parameters, including surface area and free volume. At very high pressures, physical properties of the functional groups govern the adsorption of CO2. These counterbalancing influences play an essential role in CO2 capture and are key factors in designing new MOFs for selective capture of CO2.Graphical abstractHighlights► The adsorption of CO2 on MIL-53 metal–organic frameworks was modelled by DFT and GCMC. ► The enhancement of CO2 capture by –CH3, –NH2, –OH and –COOH functionalised ligands is predicted and quantified. ► The enhanced affinity of CO2 by polar ligands, and the reduced volume available for adsorption are counterbalancing factors. ► The effect of confinement, together with synergistic effects from neighbouring functional groups are found to be important.

For most guest molecules, electrostatic interactions have a non-negligible impact on the adsorption properties of microporous materials, such as zeolites or metal–organic frameworks. In force-field-based simulations of adsorption, partial charges located at atomic sites are most commonly used to account for electrostatics. These charges are either derived empirically or obtained from electronic structure calculations. In previous work addressing the adsorption of CO2 in all-silica zeolites, we have used a first-principles approach to derive system-specific charges from density functional theory (DFT) calculations. While this approach has been shown to perform very well, it has the drawback that it requires a separate DFT calculation for every system. In this work, we develop a set of generic charges that reproduces interaction energies from dispersion-corrected DFT calculations equally well as the initial, system-specific set. The performance of this set of charges is then assessed using grand-canonical Monte Carlo simulations of CO2 adsorption and CO2/N2 mixture adsorption for a total of 24 zeolite frameworks. The results are compared to analogous simulations using the system-specific charges. While the qualitative features are reproduced very well, the quantitative deviations are often non-negligible, amounting to more than 20% in a number of cases. Therefore, the generic charge set can be recommended for screening studies, but system-specific charges should be employed for more detailed investigations.

The concept of natural tilings provides a unique definition of the building blocks that constitute zeolite frameworks. Knowledge of the natural tiling permits the identification of different frameworks with similar structural properties. On the basis of results from earlier work, which showed a close association between energetically preferred carbon dioxide adsorption sites and certain structural building units, we propose a list of criteria to identify natural tiles that could lead to a high affinity of a zeolite framework toward CO2. After identifying all recognized framework types that incorporate tiles with these features, we perform grand-canonical Monte Carlo simulations of CO2 adsorption and CO2/N2 mixture adsorption in these structures. Out of a set of 37 frameworks, we identify eight systems that exhibit relatively high CO2/N2 adsorption selectivities and CO2 working capacities. An inspection of the natural tilings of these systems reveals that they contain a relatively limited number of natural tiles, most of which occur in several of the eight systems. A subsequent analysis of the CO2 adsorption sites shows that identical tiles usually afford adsorption sites with very similar geometries and adsorption energies, highlighting the connection between the presence of certain tiles and the material’s affinity for carbon dioxide. While the results are not directly transferable to real-world applications, this study demonstrates that an analysis of the natural tiling permits the judicious choice of candidate topologies that are particularly promising for a given task, provided that some initial information on the relationship between the structural features and the property in question is available. Similar approaches can be imagined for various applications in adsorption and catalysis.

Force-field-based grand-canonical Monte Carlo (GCMC) simulations of carbon dioxide and nitrogen adsorption have been carried out for 18 different zeolite topologies, assuming a purely siliceous composition. The electrostatic potential in the pores of the zeolite framework is accurately modeled using point charges derived from density-functional theory calculations. These parameters are complemented by newly derived, empirical Lennard-Jones parameters, permitting an unambiguous decomposition of the total interaction energy into an electrostatic and a dispersive contribution. For each of the systems, such a decomposition is carried out, based on GCMC simulations of single-component adsorption. In the next step, mixture adsorption isotherms are computed, permitting the prediction of working capacities and adsorption selectivities, important quantities that determine the suitability of an adsorbent. In the final part, preferred carbon dioxide adsorption sites are calculated and analyzed for a subset of seven zeolite topologies. It is found that particular structural features, such as double-crankshaft chains and double eight-ring units, are common to those systems that exhibit the highest selectivities. While the majority of the systems have not been synthesized in their purely siliceous form, the conclusions can easily be generalized to other materials with zeolite topologies, such as (silico)aluminophosphates.

Based on computational studies, we propose new metal−organic framework materials, in which the bridging ligands have been functionalized by different substituents, with the aim of improving the CO2 adsorption capacity of the material. The materials are based on the large-pore form of MIL-53(Al3+), with the following functional groups: OH-, COOH-, NH2-, and CH3-. For each form, adsorption heats and isotherms were simulated using the Grand Canonical Monte Carlo method which were found to be consistent with DFT calculations. The study illustrates the enormous impact of the functional groups in enhancing CO2 capture in the pressure range 0.01−0.5 bar and at room temperature. It also provides important insights into the structural factors which play a key role in the CO2 adsorption process in the functionalized MOFs. We propose the material (OH)2-MIL-53(Al3+) as an optimal candidate for improved CO2 capture at low pressures.

Density functional theory (DFT) and force-field-based calculations have been carried out on the breathing metal−organic framework MIL-53(Cr) in both its large- and narrow-pore forms. In its sorbate-free form, the large-pore structure appears to be the global minimum. We develop a hybrid force field combining ionic-model potentials, used for modeling inorganic solids, with molecular mechanics terms for the organic part. This gives an energy difference of close to 30 kJ mol−1 between the large- and narrow-pore forms. Calculations in which water molecules are introduced into the structures illustrate how the energetics of physisorption are able to drive the pore breathing process, with the water molecules, being more strongly stabilized in the narrow-pore form, favoring pore closure at loadings of more than one molecule per unit cell.

The influence of the nature of the cation on the interaction of the silicoaluminophosphate SAPO-34 with small hydrocarbons (ethane, ethylene, acetylene, propane, propylene) is investigated using periodic density-functional theory calculations including a semi-empirical dispersion correction (DFT-D). Initial calculations are used to evaluate which of the guest-accessible cation sites in the chabazite-type structure is energetically preferred for a set of ten cations, which comprises four alkali metals (Li+, Na+, K+, Rb+), three alkaline earth metals (Mg2+, Ca2+, Sr2+), and three transition metals (Cu+, Ag+, Fe2+). All eight cations that are likely to be found at the SII site (centre of a six-ring) are then included in the following investigation, which studies the interaction with the hydrocarbon guest molecules. In addition to the interaction energies, some trends and peculiarities regarding the adsorption geometries are analysed, and electron density difference plots obtained from the calculations are used to gain insights into the dominant interaction types. In addition to dispersion interactions, electrostatic and polarisation effects dominate for the main group cations, whereas significant orbital interactions are observed for unsaturated hydrocarbons interacting with transition metal (TM) cations. The differences between the interaction energies obtained for pairs of hydrocarbons of interest (such as ethylene–ethane and propylene–propane) deliver some qualitative insights: if this energy difference is large, it can be expected that the material will exhibit a high selectivity in the adsorption-based separation of alkene–alkane mixtures, which constitutes a problem of considerable industrial relevance. While the calculations show that TM-exchanged SAPO-34 materials are likely to exhibit a very high preference for alkenes over alkanes, the strong interaction may render an application in industrial processes impractical due to the large amount of energy required for regeneration. In this respect, SAPOs exchanged with alkaline earth cations could provide a better balance between selectivity and energy cost of regeneration.

Ab initio MD and potential energy surface sampling has been used to study the rearrangement processes in carboranes and their derivatives. A new mechanism is found, in addition to those previously proposed. The fact that theoretical activation energies are lower than those observed experimentally, and the differing activity of technetium and rhenium complexes, are rationalised by orbital symmetry constraints.

The addition of carbon dioxide to four superbase ionic liquids, [P3333][Benzim], [P3333][124Triz], [P3333][123Triz] and [P3333][Bentriz] was studied using a molecular DFT approach involving anions alone and individual ion pairs. Intermolecular bonding within the individual ion pairs is characterised by a number of weak hydrogen bonds, with the superbase anion geometrically arranged so as to maximize interactions between the heterocyclic N atoms and the cation. The pairing energies show no correlation to the observed CO2 adsorption capacity. Addition of CO2 to the anion alone clearly resulted in the formation of a covalently-bound carbamate function with the strength of binding correlated to experimental capacity. In the ion pair however the cation significantly alters the nature of the bonding such that the overall cohesive energy is reduced. Formation of a strong carbamate function occurs at the expense of weakening the interaction between anion and cation. In the more weakly absorbing ion pairs which contain [123Triz]− and [Bentriz]−, the carbamate-functionalised systems are very close in energy to adducts in which CO2 is more weakly bound, suggesting an equilibrium between the chemi- and physisorbed CO2.