Volume 234, Issue 7-8 - Special Issue: Modelling Crystalline Microporous Materials / Guest Editors: Michael Fischer, Rob Bell

We review two concepts in description of free space in the zeolite-like frameworks: topological (natural tiling) and geometrical-topological (Voronoi net). Advantages and disadvantages as well as similarities and differences of both concepts are discussed. We also consider the packing model of zeolite framework assemblage, which is derived from the natural tiling approach. New statistical data are given for natural tilings and Voronoi models of all known 239 zeolite frameworks. A route to modeling and prediction of new zeolite frameworks is outlined.

Zeolites are important microporous framework materials, where 200+ structures are known to exist and many millions so-called hypothetical materials can be computationally created. Here, we screen the “Deem” database of hypothetical zeolite structures to find experimentally feasible and industrially relevant materials. We use established and existing criteria and structure descriptors (lattice energy, local interatomic distances, TTT angles), and we develop new criteria which are based on 5-th neighbor distances to T-atoms, tetrahedral order parameters (or, tetrahedrality), and porosity and channel dimensionality. Our filter funnel for screening the most attractive zeolite materials that we construct consists of nine different types of criteria and a total of 53 subcriteria. The funnel reduces the pool of candidate materials from initially >300,000 to 70 and 33, respectively, depending on the channel dimensionality constraint applied (2- and 3-dimensional vs. only 3-dimensional channels). We find that it is critically important to define longer range and more stringent criteria such as the new 5-th neighbor distances to T-atoms and the tetrahedrality descriptor in order to succeed in reducing the huge pool of candidates to a manageable number. Apart from four experimentally achieved structures (BEC, BOG, ISV, SSF), all other candidates are hypothetical frameworks, thus, representing most valuable targets for synthesis and application. Detailed analysis of the screening data allowed us to also propose an exciting future direction how such screening studies as ours could be improved and how framework generating algorithms could be competitively optimized.

Zeolite micropores become more energetically stable by the occlusion of organic structure directing agents (templates). This energetic stabilisation, if approximated by van der Waals zeo-template interactions, can be calculated in a fast way by using modern computing techniques incorporating big data handling algorithms for massive screening. A software suite is presented which calculates an arbitrarily large 2-D matrix (template×zeolite) giving the zeo-template van der Waals interaction energy corresponding to the minimum energy conformation assuming one template molecule in a pure silica zeolite unit cell. With the goal of simplicity, the software only needs two coordinate input files of template and zeolite unit cell. Though a number of approximations have been considered, the software allows to compare, for a given template, which competing zeolite phases may become more stabilised. Applied to zeolite hypothetical databases, it may be of help to suggest templates for their synthesis.

The theoretical extent of framework flexibility of Zeolite A ( LTA ) in response to the steric and geometric effects of different Si/Al compositions and extra-framework cation content has been explored using GASP software. Flexibility windows and compression mechanisms for siliceous LTA and aluminosilicate Na- LTA , Ca- LTA and K- LTA have been modelled. As expected, relatively small cations in the zeolite pores have little effect on the range of flexibility observed. Aluminosilicate LTA , Na- LTA and Ca- LTA frameworks exhibit identical flexibility windows and these frameworks also follow the same compression mechanisms. The introduction of larger K + ions, however, results in greater steric hindrance. This restricts the flexibility of the framework and alters the compression mechanism to accommodate these larger cations. It is shown that the limits of the flexibility window of Zeolite A are dependent on framework aluminium content and extra-framework cation size.

Removal of volatile organic compounds (VOCs) from indoor or outdoor environments is an urgent challenge for the protection of human populations. Inorganic sorbents such as zeolites are a promising solution to tackle this issue. Using dispersion corrected periodic DFT calculations, we have studied the interaction between sodium-exchanged faujasite zeolite and a large set of VOCs including aromatics, oxygenates and chlorinated compounds. The computed interaction energies range from about −25 (methane) to −130 kJ/mol (styrene). Methane is by far the less interacting specie with the NaY zeolite. All other VOCs present interaction energies higher in absolute value than 69 kJ/mol. Most of them show a similar adsorption strength, between −70 and −100 kJ/mol. While the electrostatic interactions are important in the case of oxygenates and acrylonitrile, van der Waals interactions predominate in hydrocarbons and chlorides. By monitoring the variation of molecular bond lengths of the different VOCs before and after adsorption, we have then evaluated the tendency of adsorbate to react and form by-products, since a significant stretching would evidently lead to the activation of the bond. While hydrocarbons, tetrachloroethylene and acrylonitrile seem to be not activated upon adsorption, all oxygenates and 1,1,2-trichloroethane could possibly react once adsorbed.

In this paper, we present a systematic quantum-mechanical density functional theory (DFT) study of adsorption of small gas molecules in cation-exchanged Engelhard titanosilicate ETS-10 crystalline materials. Adsorbates with a range of polarities were considered, ranging from polar (H 2 O), quadrupolar (CO 2 and N 2 ), to apolar (CH 4 ) atmospheric gases. Starting from the base-case of Na-ETS-10, other extra framework cations such as Li + , K + , Rb + and Cs + were considered. The DFT calculations were performed with the M06-L functional and were corrected for basis set superposition error with the counterpoise method in order to provide accurate and robust geometries and adsorption energies. For all adsorbates, the adsorption enthalpies decrease in the order Li + >Na + >K + >Rb + >Cs + , while adsorbate – cation interaction distances increase along the same order. For the two extreme cases, the enthalpies calculated at the M06-L/6-31++G** level of theory for CH 4 , N 2 , CO 2 , and H 2 O interaction with Li + (Cs + ) exchanged materials are −21.8 (−1.7) kJ·mol −1 , −19.0 (−10.7) kJ·mol −1 , −34.4 (−21.3) kJ·mol −1 , and −70.5 (−36.1) kJ·mol −1 , respectively. Additionally, the calculated vibrational frequencies are found to be in quite good agreement with the characteristic vibrational modes of alkali metal cation-exchanged ETS-10 and also with the available experimental frequencies for CH 4 , N 2 , CO 2 , and H 2 O interactions with alkali metal cations in the 12-membered channel of ETS-10.

Confinement of molecules in one dimensional arrays of channel-shaped cavities has led to technologically interesting materials. However, the interactions governing the supramolecular aggregates still remain obscure, even for the most common guest molecule: water. Herein, we use computational chemistry methods (#compchem) to study the water organization inside two different channel-type environments: zeolite L – a widely used matrix for inclusion of dye molecules, and ZLMOF – the closest metal-organic-framework mimic of zeolite L. In ZLMOF, the methyl groups of the ligands protrude inside the channels, creating nearly isolated nanocavities. These cavities host well-separated ring-shaped clusters of water molecules, dominated mainly by water-water hydrogen bonds. ZLMOF provides arrays of “isolated supramolecule” environments, which might be exploited for the individual confinement of small species with interesting optical or catalytic properties. In contrast, the one dimensional channels of zeolite L contain a continuous supramolecular structure, governed by the water interactions with potassium cations and by water-water hydrogen bonds. Water imparts a significant energetic stabilization to both materials, which increases with the water content in ZLMOF and follows the opposite trend in zeolite L. The water network in zeolite L contains an intriguing hypercoordinated structure, where a water molecule is surrounded by five strong hydrogen bonds. Such a structure, here described for the first time in zeolites, can be considered as a water pre-dissociation complex and might explain the experimentally detected high proton activity in zeolite L nanochannels.

Phonons are quantum elastic excitations of crystalline solids. Classically, they correspond to the collective vibrations of atoms in ordered periodic structures. They determine the thermodynamic properties of solids and their stability in the case of structural transformations. Here we review for the first time the existing examples of the phonon analysis of adsorption-induced transformations occurring in microporous crystalline materials. We discuss the role of phonons in determining the mechanism of the deformations. We point out that phonon-based methodology may be used as a predictive tool in characterization of flexible microporous structures; therefore, relevant numerical tools must be developed.

The mechanism inducing the breathing in flexible metal-organic frameworks, such as MIL-53(Al), is still not fully understood. Herein, the influence of lattice vibrations on the breathing transition in MIL-53(Al) is investigated to gain insight in this phenomenon. Through solid-state density-functional theory calculations, the volume-dependent IR spectrum is computed together with the volume-frequency relations of all vibrational modes. Furthermore, important thermodynamic properties such as the Helmholtz free energy, the specific heat capacity, the bulk modulus, and the volumetric thermal expansion coefficient are derived via these volume-frequency relations using the quasi-harmonic approximation. The simulations expose a general volume-dependency of the vibrations with wavenumbers above 300 cm −1 due to their localized nature. In contrast, a diverse set of volume-frequency relations are observed for vibrations in the terahertz region (<300 cm −1 ) containing the vibrations exhibiting collective behavior. Some terahertz vibrations display large frequency differences over the computed volume range, induced by either repulsion or strain effects, potentially triggering the phase transformation. Finally, the impact of the lattice vibrations on the thermodynamic properties is investigated. This reveals that the closed pore to large pore phase transformation in MIL-53(Al) is mainly facilitated by terahertz vibrations inducing rotations of the organic linker, while the large pore to closed pore phase transformation relies on two framework-specific soft modes.

Molecular simulations for gas adsorption in microporous materials with flexible host structures is challenging and, hence, relatively rare. To date, most gas adsorption simulations have been carried out using the grand-canonical Monte Carlo (GCMC) method, which fundamentally does not allow the structural flexibility of the host to be accounted for. As a result, GCMC simulations preclude investigation into the effect of host flexibility on gas adsorption. On the other hand, approaches such as molecular dynamics (MD) that simulate the dynamic evolution of a system almost always require a fixed number of particles in the simulation box. Here we use a hybrid GCMC/MD scheme to include host flexibility in gas adsorption simulations. We study the adsorption of three gases – CH 4 , CO 2 and SF 6 – in the crystal of a porous organic cage (POC) molecule, CC3-R, whose structural flexibility is known by experiment to play an important role in adsorption of large guest molecules [L. Chen, P. S. Reiss, S. Y. Chong, D. Holden, K. E. Jelfs, T. Hasell, M. A. Little, A. Kewley, M. E. Briggs, A. Stephenson, K. Mark Thomas, J. A. Armstrong, J. Bell, J. Busto, R. Noel, J. Liu, D. M. Strachan, P. K. Thallapally, A. I. Cooper, Separation of rare gases and chiral molecules by selective binding in porous organic cages. Nat. Mater. 2014 , 13 , 954, D. Holden, S. Y. Chong, L. Chen, K. E. Jelfs, T. Hasell, A. I. Cooper, Understanding static, dynamic and cooperative porosity in molecular materials. Chem. Sci. 2016 , 7 , 4875]. The results suggest that hybrid GCMC/MD simulations can reproduce experimental adsorption results, without the need to adjust the host–guest interactions in an ad hoc way. Negligible errors in adsorption capacity and isosteric heat are observed with the rigid-host assumption for small gas molecules such as CH 4 and CO 2 in CC3-R, but the adsorption capacity of the larger SF 6 molecule in CC3-R is hugely underestimated if flexibility is ignored. By contrast, hybrid GCMC/MD adsorption simulations of SF 6 in CC3-R can accurately reproduce experiment. This work also provides a molecular level understanding of the cooperative adsorption mechanism of SF 6 in the CC3-R molecular crystal.