A reflection on ‘Flexibility versus rigidity: what determines the stability of zeolite frameworks? A case study’

Abstract

Since their discovery, the quest for novel zeolite materials has led to an inflation of discovered topologies. But what determines the stability and synthetic feasibility of these porous materials? In 2014, Verheyen et al. published a manuscript in Materials Horizons discussing the stability of a new zeolite framework, OKO, from the viewpoint of framework flexibility (Mater. Horiz., https://doi.org/10.1039/C4MH00127C). The present commentary reflects on this work in a broader context, outlining two important criteria contributing to the stability of zeolite crystals, minimization of: (i) internal surface, and (ii) strain. Using examples selected from over a decade of zeolite research, the impact of these aspects is discussed in terms of enthalpic and entropic contributions to the minimization of crystal energy and overall Gibbs free energy of zeolite synthesis systems.

Introduction

Today, one can confidently state that the successful synthesis of zeolites – and porous solids in general – is primarily affected by two criteria:

(i) Minimization of internal surface: porous frameworks form as dense materials, as their internal voids are occupied by species that interact energetically favourably with the framework. These interactions stabilize the pore walls, lower the overall crystal energy, and provide a free-energy gain to the overall synthesis mixture.^1–3

(ii) Minimization of strain: frameworks that exhibit minimal local strain are energetically preferred.

In 2014, Verheyen et al. published a manuscript in Materials Horizons discussing the stability of an, at the time, new zeolite framework, OKO, from the viewpoint of framework flexibility.⁴ The all-silica extra-large-pore zeolite derived from UTL was the first case of a zeolite that reversibly transforms from a systematically interrupted framework (-COK-14) to the fully connected COK-14 with the new OKO topology.⁵ The stability of -COK-14 with respect to COK-14 could be explained by framework flexibility, a concept that builds on the stability criterion ‘Minimization of strain’.

In 2010, Treacy et al. had observed on a theoretical basis that zeolite topologies exhibit some degree of flexibility, i.e., a framework can accommodate small positional shifts of its atom-positions without disrupting typical bond lengths and angles.^6–9 For all known topologies in (hypothetical) siliceous form, a range of framework densities exists within which an ideal, fully connected framework remains strain-free.^6,10 Flexibility increases thermodynamic stabilization by allowing low-energy framework distortions, thereby contributing to the entropy of the material.⁹ The interplay of these two criteria, minimization of interfaces, i.e., maximal stabilization of pore walls and minimal strain, with associated flexibility, manifests in various facets of zeolite formation. As these stability criteria sometimes remained under the surface, we start by highlighting them for zeolite synthesis in general. Thereafter, we reflect on the impact of the work of Verheyen et al.⁴ Exploring the synthesis of novel large-pore structures, we find that, more often than not, the synthesis involves a (partially) unconnected stage as well, giving -COK-14 a remarkable place in modern zeolite synthesis strategies.

Impact of strain and surface minimization on zeolite stability

One of the most basic ways to stabilize a zeolite framework is to introduce heteroatoms during synthesis. Isomorphic substitution of Si by Al is most common, and addresses both criterion (i) and (ii).⁷ Incorporation of heteroatoms with a reduced charge, e.g., Al³⁺, introduces a negative framework charge. The charge is compensated by inclusion of (hydrated) cations, which fill the pores and minimize the free volume whilst stabilizing the internal surfaces (criterion (i)).^1,3,11–13 At the same time, Coulombic interactions between framework and cations contribute to the reduction of crystal energy. Compared to Si, heteroatoms like Al³⁺ exhibit a broader range of bond lengths, and angles. In all-silica zeolites, typical Si–O–Si angles between corner-sharing tetrahedra fall in the range between 130° and 160°, while retaining the tetrahedral geometry around silicon.^6,7,14 Including aluminium contributes to the reduction of the strain (criterion (ii)) by expanding the flexibility window derived for pure silica frameworks.^7,15 The industrially important LTA and FAU zeolite topologies illustrate the dual effect of isomorphic substitution of Si by Al: enhanced flexibility and pore wall stabilization by charge-compensating cations. Both LTA and FAU are built from relatively rigid building units—double four- or six-rings—and exhibit limited intrinsic flexibility (Fig. 1b, c, f and g).⁷

Fig. 1 Examples of zeolite topologies containing rigid double-ring structures. (a) EMT; (b) FAU; (c) LTA; (d) UTL; (e) -IRY; (f) double 6-ring (D6R); (g) double 4-ring (D4R); (h) double 3-ring (D3R).

Despite their high rigidity in highly siliceous form,^7,16 both LTA and FAU are readily obtained in the presence of aluminium. In fact, they were among the first aluminosilicate zeolites reported in the literature.^17–19 Inclusion of Al and inorganic cations assists in reducing strain by enhancing flexibility (criterion (ii)), while the extra framework species balancing the framework charge stabilise the pore walls (criterion (i)). It is so effective in stabilizing frameworks that it often leads to materials with a low Si/Al ratio (1–4).^20–22 Unfortunately, this can severely reduce the (hydro)thermal stability and, therefore, the application potential of materials. This hurdle can be overcome by (partial) replacement of inorganic cations with organic structure-directing agents (OSDAs).²³ The high-silica variants of LTA and FAU can, for example, only be synthesized by using OSDAs to stabilise the developing pores.^24–26 In addition to OSDAs, fluoride ions can also be introduced during synthesis. Fluoride is the only ligand allowing Si to adopt 5- or even 6-fold coordination,²⁷ thereby facilitating siloxane bond formation.^28–32 When incorporated into zeolite frameworks, F⁻ can bind to strained structural units, enhancing flexibility and enabling higher Si/Al ratios (criterion (ii)).^32–35

Another example illustrating the impact of framework flexibility on the stability of zeolites is the preference for FAU formation over EMT, despite their structural similarity. Both topologies consist of sodalite cages linked by double six-rings (Fig. 1a, b and f).³⁶ While aluminosilicate FAU is easily obtained in inorganic synthesis, EMT requires intergrowth with FAU, or the presence of OSDAs, to specifically stabilize its cages.^37–39 Assuming similar Si/Al ratios, the cation-mediated surface stabilization (criterion (i)) should be comparable. Hence, the preferential formation of FAU can be attributed, at least partly, to its higher flexibility compared to EMT (criterion (ii)).⁷

Optimal interaction between the forming zeolite material and structure directing agents (SDAs) is critical for the surface stabilization criterion.^3,35,40 During crystallization, framework elements and water compete in the liquid to coordinate with the SDAs, which are mostly inorganic and/or organic cations.^23,40–43 For zeolite formation to occur, the cation coordination by the zeolite framework and other extra-framework guest molecules must be more favourable than solvation of the cation by the synthesis medium.^3,40 This provides a thermodynamic basis for phase selection, dictated by the balance between cation solvation in the liquid phase and cation coordination inside the framework.^23,40–43 The following example illustrates the importance of this requirement while also bringing the role of entropy into perspective. As will be shown later, entropy changes play a significant role in the reversible opening and closing of COK-14.

From a single synthesis composition, both faujasite (FAU) and gismondine (GIS) zeolites can form, simply by increasing the synthesis temperature from 338 to 373 K. FAU is formed at the lower temperature, whereas GIS is favoured at 373 K.²¹ FAU features a relatively rigid topology, comprising small and large cages and extended cation–water networks.¹⁶ GIS, on the other hand, possesses uniformly sized intermediate cages, exhibits high framework flexibility, and contains significantly fewer hydrated cations in its pores. Higher synthesis temperatures amplify entropy: GIS's flexible framework allows greater atomic motion, and its lower water content increases the number of free, mobile water molecules in the synthesis medium.² This relative entropic contribution to the Gibbs free energy explains the preference for GIS at elevated temperatures. At even higher temperatures, the more rigid analcime (ANA) is favoured, offering optimal cation solvation by the framework and minimal water content.^21,44

The interplay between cation hydration and framework-interaction (criterion (i)), along with framework dynamics (criterion (ii)) offers exciting possibilities for tailoring zeolite materials towards specific applications. GIS, in particular, is among the most flexible known zeolite frameworks.^7,45 In its as-synthesized, hydrated state, GIS contains partially hydrated cations and a fully relaxed lattice. Upon dehydration, however, the loss of coordinating water forces the cations to interact strongly with pore walls, inducing substantial lattice deformation. This strain can be partially relieved through interactions with alternative ligands such as CO₂. As a result, dry, flexible zeolites like GIS are highly efficient for CO₂ adsorption.^45–49 Reintroduction of water reverses CO₂ adsorption—provided that the cation's affinity for water exceeds that for framework oxygen or the adsorbed molecule.^45,50–53

Strategies to increase pore size in zeolites

The previous examples show how the incorporation of aluminium (and fluoride) can reduce the strain in high-silica materials. Reflecting on the impact of the discovery of -COK14 on the field, this section discusses some more exotic strain–relaxation phenomena, such as interrupted frameworks and flexibility-inducing heteroatoms, and their contributions to the synthesis of extra-large-pore zeolites. The examples represent the important trends in zeolite synthesis research over the last decade.

Similar to the strain relaxation effects occurring upon isomorphic substitution of Si by Al, strain–relaxation effects also occur upon introduction of heteroatoms that do not introduce framework charge but solely enhance flexibility (criterion (ii)). A notable example is the substitution of Si by Ge. When combined with pore-filling structure-directing agents and optional fluoride ions,^54–56 Ge incorporation enables the synthesis of extra-large-pore zeolites. Interestingly, the presence of large pores is often associated with small ring structures, such as double 4-rings (D4Rs).^57,58 This suggests that even large-pore frameworks often feature dense, locally rigid regions, as recently demonstrated in aluminosilicate zeolites.¹ In high-silica, fully connected frameworks, these dense motifs impose significant strain.^7,14 Purely siliceous D4Rs are inherently unstable and difficult to form via conventional hydrothermal synthesis, as exemplified by LTA zeolites. In contrast, the substitution of Si by Ge overcomes this limitation due to the more flexible coordination geometry of Ge. As a result, extra-large-pore silicogermanate zeolites containing D4Rs (e.g., UTL; Fig. 1d and g)—and even double 3-ring (D3R) units (e.g., -IRY; Fig. 1e and h)—have been successfully synthesised.^56,59,60

Silicogermanates are particularly susceptible to hydrolytic degradation due to the instability of Ge–O bonds. While this limits their direct application,⁶¹ it offers a synthetic advantage: the selective removal of Ge enables access to topologies otherwise inaccessible by direct synthesis due to their low flexibility. In some cases, Ge can post-synthetically be replaced by Al, yielding stable aluminosilicate frameworks that cannot be directly synthesized.⁶¹

This top-down synthesis strategy enables the application of the principles of click chemistry, as known in organic synthesis. Pre-formed building units—such as zeolitic layers derived from hydrolysed Ge-containing precursors—can be reassembled into new structures. This approach underpins the ADOR (assembly–disassembly–organization–reassembly) concept,^62–65 and the related inverse sigma transformation of Ge-UTL into the all-silica COK-14 material with OKO topology.^5,63,64

Instead of harvesting building units from 3D silicogermanates (top-down), intermediate 2D layers can also form in situ during one-pot syntheses. This bottom-up approach has been observed in the synthesis of zeolites with FER and MWW topologies, and more recently in the formation of LEU-2 by Bae and coworkers.^65–68

Another strategy to reduce framework strain (criterion (ii)), involves relaxing the requirement of full framework connectivity (i.e., the case of -COK14). With the continued search for large- and ultra-large-pore zeolites, relaxing this requirement opens the door to form novel materials with compelling properties, as well as new opportunities to functionalize them into hybrid structures. The OKO topology, which serves as the motivation for this reflection, exemplifies how systematically interrupted frameworks can form and become stabilized.^4,69 Through controlled acid hydrolysis of Ge from UTL-type zeolites at room temperature, crystalline but interrupted frameworks can be formed. The resulting material, designated ‘-COK-14’, contains systematically hydrolyzed siloxane bridges (Fig. 2). In this state, the framework is highly flexible, and adjacent silanol groups are stabilized by hydrogen bonding with water molecules within the pores.⁴ Upon thermal treatment in the absence of water, these neighboring silanols condense into strained 4-rings. This condensation is driven by entropy gains associated with the release of water—both physically adsorbed and chemically bonded. Remarkably, this transformation is fully reversible. Water addition reopens the rings, while heating reforms the closed, rigid 4-ring.^4,5 At present, COK-14 is the only zeolite known to exhibit reversible switching between a relaxed, open structure with adjacent silanol groups and a strained, closed-ring framework. Notably, the closed and rigid form cannot be obtained via direct hydrothermal synthesis. Only dry thermal activation—by removing water—yields this otherwise inaccessible phase. A similar phenomenon was recently described by Valtchev and coworkers in their work on the synthesis of the first inherently mesoporous zeolite.⁷⁰ The structure forms with unconnected hydroxyls that coordinate with the OSDA. Upon calcination, the silanol groups condense to form a fully connected material, transforming the 28-ring channel from a bi-lobal to an elliptical shape. The reversibility of this transformation has not yet been explored.

Fig. 2 Reversible, entropy driven formation of a single 4-ring, upon calcination of -COK-14 (right). Exposure of COK-14 (left) to a humid atmosphere reversibly reopens the ring structure.

A considerable number of high-silica zeolites with interrupted frameworks can be synthesized directly.⁶⁹ However, they can also be obtained through post-synthetic modification. One effective strategy involves the intentional destabilization of the zeolite framework via the removal of heteroatoms such as Al. This process induces framework interruption—typically through partial amorphization or mesopore formation⁷¹—and introduces structural flexibility that relaxes and thus stabilizes the material. A well-known example is the synthesis of the ultrastable Y zeolite with the FAU topology. Steaming-induced dealumination initially forms silanol nests. Subsequent treatments can trigger framework re-organization, leading to catalytically active materials with hierarchical pore systems.^71–73

Until recently, the application of click chemistry-inspired strategies in zeolite synthesis was largely limited to 2D-to-3D assembly. The recent discovery of ZEO-3 and ZEO-5, which exhibit JZT and HZF topologies, respectively, has provided striking examples of how novel porous zeolites can be constructed by condensing silanol groups under dry, water-free conditions.^74,75

Illustrating 1D-to-3D assembly, both ZEO-3 and ZEO-5 originate from pre-organized one-dimensional building units (1DBU), which are topotactically condensed into three-dimensional structures. In the case of ZEO-3, this condensation occurs directly; in ZEO-5, it follows the incorporation of additional silicate species through methylated silicon groups. The required 1DBUs are harvested from ZEO-2, a crystalline parent phase composed of siliceous chains built from interconnected 5-rings and terminated by unconnected 4-rings (Fig. 3). ZEO-2 is stabilized by hydrogen bonding between the chains and by OSDAs occupying the pore system.⁷⁴

Fig. 3 1D building unit of ZEO-2, showing the hydroxyl groups that allow consequent 1D-to-3D transformation into ZEO-3 and ZEO-5 in the direction of the green arrows.

As in the case of COK-14, the silanol groups in ZEO-2 are geometrically aligned for condensation but only react at elevated temperatures, releasing water and decomposed OSDA to form ZEO-3 with the JZT topology. Even more remarkably, ZEO-5—featuring a fully connected, siliceous HZF topology—is achieved by introducing methylated silane species between neighbouring 1DBUs in ZEO-2.⁷⁵ This synthesis route yields an interrupted framework where adjacent silicon atoms are terminated by methyl groups instead of silanols, as observed in ZEO-2 and COK-14. Upon calcination, these groups condense to form an additional 4-ring, giving rise to unique triple four-ring units (T4Rs) and setting a record for ring size with 20 tetrahedral atoms.⁴

This accomplishment encapsulates the key principles discussed in this commentary with respect to their contribution to zeolite stabilization: (i) minimization of internal surfaces through hydrogen bonding and/or OSDA stabilization in aqueous media; (ii) reduction of strain, which otherwise prevents the formation of purely siliceous double or even triple four-rings via conventional hydrothermal synthesis. These examples also illustrate how entropy gains at elevated temperatures, via release of small volatile species (water or organic species), can enable the formation of otherwise inaccessible structures.

Although the criteria discussed above were derived from experimental observations and basic geometrical modeling, we are certain that advanced theoretical modelling could greatly accelerate progress in zeolite science. While computational studies have already played an essential role in understanding many aspects of zeolite synthesis,¹² there is significant potential to develop dedicated modeling workflows to systematically evaluate how these stabilisation and relaxation criteria influence both known and hypothetical zeolite structures.

Such workflows should evaluate internal and external surface energies, crystal and local strains, and structural flexibility by simulating how materials respond to deformation.⁷⁶ In terms of strain (criterion (ii)), newly developed modeling tools now allow for establishing causal relationships between atomic structure and mechanical stress—an approach known as strain engineering.⁷⁷ These methods show strong promise for application in zeolite research.

For flexibility assessment, it is crucial to define suitable response functions for how a given material behaves under various thermodynamic conditions. Molecular dynamics simulations performed in appropriate thermodynamic ensembles can help assess the stability and metastability of frameworks.⁷⁸ However, a precise definition of flexibility remains essential.¹⁰ Flexibility can be influenced by temperature, pressure, framework composition, the nature of extra-framework species, and the presence of guest molecules. Consequently, the number of required simulations could grow exponentially.

To address this challenge, machine learning (ML) methodologies are emerging as powerful tools. ML models can dramatically accelerate the simulation of interatomic interactions, making large-scale screening feasible.⁷⁹ In synergy with experimental efforts, such approaches could provide valuable insights into the fundamental factors governing zeolite formation and guide the rational design of new zeolites.

Conclusion & outlook

Minimization of strain and internal surface are crucial factors in explaining zeolite formation. Following our work on -COK-14 and its interruption-induced stability, it is not surprising that an increasing number of interrupted large- and extra-large-pore zeolites are being reported with periodic or random interruptions that help enhance material flexibility without necessarily compromising structural stability. Such large and extra-large-pore materials typically contain dense structural elements surrounding and supporting their otherwise very open structure. In this respect, increased pore sizes will therefore emphasize the importance of click chemistry, generating or harvesting dense (interrupted) elements in the first steps of a synthesis, followed by the pre-organization of these structures around the channels and pores, and finally, a consolidation that reconnects the structural elements into frameworks with suitable flexibility.

Aside from synthesis-based research efforts, further advancements in this area will also require an in-depth understanding of the mechanisms and processes contributing to the stabilization of zeolite pores and crystal structure. Enthalpic as well as entropic contributions will have their place in the assessment of: (i) how feasible the synthesis of a specific topology is, and (ii) how to provide maximal stabilization to it.

Conflicts of interest

There are no conflicts to declare.

Data availability

This manuscript does not contain original datasets.

Acknowledgements

E. B. & C. K. acknowledge joint funding by the FWO Vlaanderen (FWO; G083318N and G0AC524N) and FWF (funder ID 10.13039/501100002428, grant IDs 10.55776/I3680 & 10.55776/I6800), as well as funding from KU Leuven (SIONA, C14/22/099). NMRCoRe is supported by the Hercules Foundation (AKUL/13/21), by the Flemish Government as international research infrastructure (I001321N), and by Department EWI via the Hermes Fund (AH.2016.134). W. Y. acknowledges the NSFC (22288101) and the ‘111 Center’ (B17020). B. S. acknowledges a C2 project from KU Leuven (Gorilla projectcode: 3E230473). VVS acknowledges the Research Board of Ghent University (BOF).

Notes and references

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