Computer Simulation of Porous Materials

Current Approaches and Future Opportunities

Kim Jelfs, Kim Jelfs

• Publisher: Royal Society of Chemistry
• Year: 2021
• ISBN:
  9781839163326
• Edition: 1
• Topic: Tecnología e ingeniería
• Subtopic: Ciencias de los materiales

Computer Simulation of Porous Materials covers the key approaches in the modelling of porous materials, with a focus on how these can be used for structure prediction and to either rationalise or predict a range of properties including sorption, diffusion, mechanical, spectroscopic and catalytic. The book covers the full breadth of (micro)porous materials, from inorganic (zeolites), to organic including porous polymers and porous molecular materials, and hybrid materials (metal-organic frameworks). Through chapters focusing on techniques for specific types of applications and properties, the book outlines the challenges and opportunities in applying approaches and methods to different classes of systems, including a discussion of high-throughput screening. There is a strong forward-looking focus, to identify where increased computer power or artificial intelligence techniques such as machine learning have the potential to open up new avenues of research. Edited by a world leader in the field, this title provides a valuable resource for not only computational researchers, but also gives an overview for experimental researchers. It is presented at a level accessible to advanced undergraduates, postgraduates and researchers wishing to learn more about the topic.

Chapter 1

Introduction to Computational Modelling of Microporous Materials

Kim E. Jelfs^a

^a Department of Chemistry, Molecular Sciences Research Hub, Imperial College London White City Campus, Wood Lane W12 0BZ United Kingdom Email: [email protected]

1.1 Introducing Porous Materials Modelling

Porous materials are materials that have void spaces within them that can be occupied by guests to produce a range of different properties and functionalities. Depending upon the size of their pores, porous materials are classified as microporous (pores smaller than 2 nm), mesoporous (pores between 2 and 50 nm) or macroporous (pores larger than 50 nm). Here, we will focus mostly upon microporous materials, as these pores can host small molecules and atoms and therefore provide functionality across a range of applications from encapsulation, separation, catalysis, sensing and more. As a note on terminology, the term sorption is generally used in the discussion of porous materials as these materials will both adsorb guests onto surfaces as well as absorb guests into the material's bulk.

The computational modelling of materials is increasingly widespread, and porous materials modelling is no exception to this. Computational modelling in porous materials has already had decades of application, especially in the field of zeolites, with significant impact in rationalising the structure and property of the materials, frequently providing additional atomic-level insight into the materials that could not be uncovered through experimental characterisation alone. This a posteriori rationalisation of experimental observations built a foundation upon which many predictive studies have been built, for example, screening materials in advance for useful properties prior to experimental realisation. A commonly used term is computational materials design, but we urge caution in not overusing the term ‘design’ when a more appropriate term is ‘screening’ or ‘discovery’. We should leave the term ‘design’ for those occasions when our insight, knowledge and software truly have enabled design. Computational screening, with consideration of both whether a hypothetical material can be experimentally realised and have target properties, is powerful in its own right.

Ever increasing computing power, increasing computational literacy and programming ability, combined with open-source data and algorithms hold great potential for the further impact of modelling in the field of porous materials. As in all computational fields, there is significant emphasis on the potential for advances through the use of artificial intelligence, notably machine learning. There is enormous value in all porous material scientists understanding, even if not directly applying, the potential of computational modelling for the advancement of their own research. Beyond technological advancements, we can also expect an increasing focus on understanding and predicting more complex behaviour, for example, consideration of multifunctionality in a material, such as the interplay of porosity with optoelectronic properties, and the consideration of the importance and utilisation of defects as an opportunity for property tuning.

In this book, the key foundations for porous materials modelling are laid out, along with the major areas where modelling is applied, successes so far, as well as the future prospects for the field. In this chapter (Chapter 1), the key classes of microporous materials are introduced, along with the applications of porous materials. This is followed by the introduction of the key types of modelling approaches that are applied in the area of porous materials. This chapter should therefore provide the foundation for accessing the following chapters that focus on specific application areas of porous materials modelling. Chapter 2 (Addicoat) outlines the approaches and successes in the structure prediction of porous materials, covering both bottom-up approaches from building blocks and top-down approaches from libraries of topological nets. Examples are discussed from zeolites, metal–organic frameworks, polymers and porous molecules. With a known structure of a material, either from a simulation or an experimental crystal structure, many further simulations analysing properties and function can be performed. Chapter 3 (Evans) discusses the analysis of the mechanical properties of porous materials. How to analyse the strength and flexibility of porous materials is covered, along with the potential to uncover unusual properties, such as negative thermal expansion. The modelling of both sorption and diffusion behaviour in porous materials is covered next (Glover and Besley, Chapter 4). This spans sorption simulations of guest uptake, pore analysis, and molecular dynamics and enhanced sampling techniques for guest diffusion. Modelling the spectroscopic and catalytic properties of porous materials is then covered (Morales-Vidal and Ortuño, Chapter 5), including a discussion of how to choose appropriate methods, followed by examples of mechanistic investigations, focused upon metal–organic frameworks and covalent organic frameworks. Finally, the future for the field, in particular through the application of artificial intelligence techniques, is covered (Jensen and Olivetti, Chapter 6).

1.2 An Overview of Microporous Material Classes

There are a large range of microporous materials and these can have a variety of chemical compositions, whether inorganic or organic in nature, as well as hybrid materials containing both inorganic and organic components. Microporous materials also vary by their degree of crystallinity, with some having both short- and long-range order, and amorphous porous materials lacking any long-range order at all. In this section, the key classes of porous material that are discussed in later chapters will be introduced.

1.2.1 Zeolites

Zeolites are naturally occurring crystalline aluminosilicate minerals that are typically found in regions where there has been volcanic activity. The name zeolite originates from how the rock appeared to ‘boil’ upon heating, with the ancient Greek zein for ‘to boil’ and lithus for ‘rock’, so literally ‘zeolite’ means ‘boiling rock’. It was not until the 1940s that Barrer found a method to synthesise zeolites in a laboratory. Barrer was able to synthesise zeolites via mimicking the conditions where they form naturally, in particular the high temperature and pressure, along with organic template molecules that would direct the formation to a targeted zeolite structure. There are now more than two hundred zeolite structures that are either naturally occurring or have been synthesised in the laboratory.¹

Zeolites are composed of [SiO₄]^{4 −} and [AlO₄]^{5 −} tetrahedra, and these primary building units, known as T-sites, are linked by the bridging oxygen atoms to produce corner-sharing tetrahedra. Through the multitude of ways that these tetrahedra can be linked and arranged to form crystalline arrays, there are millions of hypothetical zeolite topologies. There are a series of what are known as ‘secondary building units’ or SBUs that can be used to characterise a zeolite topology. Examples of SBUs include n-membered rings (where n = 3, 4, 5, 6…), and then the equivalent doubled rings, double 4-ring (D4R), double 5-ring (D5R), double 6-ring (D6R) and so on that have n bridging oxygens linking the two rings, as well as a small number of other SBUs linking different sized rings. Each zeolite has a three letter framework type code assigned by the International Zeolite Association,¹ which will be unique to a zeolite topology. An example of the zeolite with the framework type code MFI is shown in Figure 1.1.

Figure 1.1 An example of a zeolite structure. This is the zeolite MFI viewed down the b-axis, with the a- and c-axes of the unit cell (shown in black) labelled. Ten membered-ring channels run down the a- and b-axes.

The microporosity of zeolites originates either from the channel formed in an SBU or from the channels formed between SBUs when they are connected into a zeolite topology. The pore dimensions of zeolites vary across the full range of the definition of microporous materials. The first SBU where a small guest can feasibly pass through is the 6MR, with a diameter of ∼2 Å. Not only will the pore dimensions of different zeolite structures vary, but obviously also the shape of the channels, such as straight or curved channels, and some with larger cavities at points in a channel or side pockets connected to a main channel. Zeolites vary further by their pore dimensionality. There are zeolites with only 1-dimensional channels, those with 2-dimensional channels and those with 3-dimensional interconnected channels.

In terms of chemical composition, while an all-siliceous zeolite would only contain [SiO₄]^{4 −} tetrahedra, typically zeolites are aluminosilicates, with some [SiO₄]^{4 −} substituted for [AlO₄]^{5 −} tetrahedra. While the chemical composition may vary, all zeolites with the same framework structure (topology) will still have the same framework type code. For example, the code MFI refers to the structure that is common to both the all-siliceous silicialite-1 material (see Figure 1.1) and for ZSM-5 that contains both silicon and aluminium. In every instance where an Al^{3 +} ion substitutes a Si^{4 +} ion, there needs to be a ‘charge-compensating’ cation to maintain the zeolite structure's overall charge neutrality. These cations are generally not part of the framework itself but are more loosely bound in the zeolite's pores. The cations can be the organic templates used in the synthesis, or metal cations such as sodium or calcium, with the latter ions generally being solvated by water unless the zeolite has been dehydrated. Alternatively, the compensating cation can be a proton, and this then imparts a high acidity to the zeolite structure, that can be used in catalytic applications. Overall, the majority of zeolite applications stem from the presence of compensating cations and thus as a result of the aluminium substitution in the framework. The Si/Al ratio for a zeolite can range from 1 to infinity, with the lower limit of 1 a result of Lowenstein's rule that states that [AlO₄]^{5 −} tetrahedra will not form direct Al–O–Al bridges.

Beyond zeolites, there are a range of related materials known as ‘zeotypes’. Zeotypes can adopt the same framework structures as zeolites, but have different chemical compositions. For example, there is the family of aluminophosphates (ALPOs) consisting of Al^{3 +} and P^{5 +} ions (without the need for any compensating cation), SAPOs consisting of silicon, aluminium and phosphorus, zinc phosphates, and germanium sulfides. Through altering the chemical composition and in particular through the possibility to incorporate transition metal ions, this brings access to a further range of properties, particularly in terms of catalysis.

The applications for zeolites stem in particular from their chemical and physical stability, the presence of charge compensating cations, and the fact that zeolite pore dimensions are commensurate with small guest molecules and zeolite structures are available with a wide range of pore dimensions, shapes and topologies, allowing one to ‘pick’ an appropriate zeolite for a specific target application. Naturally occurring zeolites are typically very cheap and available on a large scale, and some synthetic zeolites are also available at a reasonably low cost. Zeolites have three major applications, several of which involve multimillion tonne productions. These applications are in ion exchange, catalysis and as molecular sieves (see Figure 1.2).

Figure 1.2 The main applications of zeolites.Reproduced from ref. ² with permission from the Royal Society of Chemistry.

In ion exchange, the charge compensating cations in the zeolite are able to be exchanged with other cations, as the cations remain loosely bound. The selectivity of a zeolite towards a cation is determined by the topology and chemical composition, as this will affect the ability of an ion to diffuse through the pores and the strength of the binding at the sorption site. Zeolite A has been used in ion exchange on a large scale in washing detergents, where it acts as a water softener by selectively removing Ca^{2 +} ions from hard water. Zeolites have also been used to remove radioactive ions from nuclear wastes or after nuclear disasters, or toxic ions from the waste water of heavy industry.

Zeolites are highly hydrophilic materials and will absorb significant quantities of water even at raised temperatures. This has led to their use as drying agents, for example, in hospitals for drying gases used for medical treatments or on a large scale in chemical industry, such as drying liquid propane. Zeolites can perform molecular separations, thus acting as ‘molecular sieves’, by several different mechanisms:

• (i) Size exclusion: excluding molecules with dimensions larger than the zeolite's pores, and thus separating the larger molecules from a mixture.
• (ii) Sorption differences: where two different components of a mixture have different enthalpies of sorption within a zeolite. For example, in pressure swing adsorption, a mixture would be forced into a zeolite under pressure, then the pressure will be lowered slightly and only the more weakly bound guest component will be released from the z...