# High-pressure adsorption phenomena in natural and synthetic zeolites with EAB topology

Tommaso Battiston, Davide Comboni, Paolo Lotti, Giorgia Ferrarelli, Massimo Migliori, Girolamo Giordano, Michael Hanfland, Gaston Garbarino, Hanns-Peter Liermann, G. Diego Gatta

► To cite this version:

Tommaso Battiston, Davide Comboni, Paolo Lotti, Giorgia Ferrarelli, Massimo Migliori, et al., High-pressure adsorption phenomena in natural and synthetic zeolites with EAB topology. Microporous and Mesoporous Materials, 2024, 365, pp.112873. 10.1016/j.micromeso.2023.112873. hal-05095802

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# High-pressure adsorption phenomena in natural and synthetic zeolites with EAB topology

Tommaso Battiston<sup>a, *</sup>, Davide Comboni<sup>a, b</sup>, Paolo Lotti<sup>a</sup>, Giorgia Ferrarelli<sup>c</sup>, Massimo Migliori<sup>d</sup>, Girolamo Giordano<sup>d</sup>, Michael Hanfland<sup>b</sup>, Gaston Garbarino<sup>b</sup>, Hanns-Peter Liermann<sup>e</sup>, G. Diego Gatta<sup>a</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

### Keywords:

EAB-Type zeolite

Synchrotron X-ray diffraction

Structure refinement

High-pressure

Compressibility

Crystal-fluid interaction

## ABSTRACT

The high-pressure behaviour and the crystal-fluid interaction of zeolites with EAB topology were investigated by *in-situ* single-crystal and powder synchrotron X-ray diffraction, both on synthetic and natural samples. The experiments were conducted using a diamond anvil cell and different pressure transmitting fluids, including: the non-penetrating *silicone oil* and the potentially penetrating *methanol:ethanol:H₂O* = 16:3:1 mixture, *methanol* and *distilled H₂O*. The zeolites intrinsic compressional behaviour investigated with a non-penetrating fluid showed the significant role of the extra framework population on the bulk compressibility. Notably, within the first ~0.5 GPa of the *silicone oil* ramp, the bulk modulus (*K*<sub>V0</sub> = *β*<sub>V0</sub><sup>1</sup>) resulted to be *K*<sub>V0</sub> = 62(1) GPa for the natural bellbergite and *K*<sub>V0</sub> = 16(4) GPa for the synthetic K-analogue. The synthetic EAB zeolites demonstrated a high host capacity for methanol molecules, which were not able to penetrate the natural bellbergite cavities. A comparative analysis between the behaviour of zeolites with EAB topology and structurally similar zeolites, such as erionite (ERI topology) and offretite (OFF), is provided.

## 1. Introduction

During the last decades, the research on microporous materials (such as synthetic or natural zeolites) at high pressure has been expanded to characterize the variety of behaviour of these structures under extreme conditions [1,2]. In addition, these investigations provided new insights into the potential industrial exploitability of zeolites at high-pressure: non-ambient conditions can enhance mass transfer phenomena due to the selective incorporation of molecules or atoms [3]. In this regard, zeolites with cavities in the form of channels (e.g.: MFI, FAU, etc.) have been more frequently investigated in comparison to those with cages. The EAB framework (i.e., Erionite of Aiello and Barrer) was firstly observed by Aiello and Barrer in 1970, as unexpected product during the synthesis of a TMA-E type zeolite [4]. The framework is characterized by two types of columns: the first is composed of an alternation of *eab*-type cages intercalated with double six rings (cages composed by two

subsequent rings, each one made by six tetrahedra), while the second is made by a superposition of gmelinite-type cages (Fig. 1a). Both columns extend along the *c* crystallographic axis direction (Fig. 1). The channels are connected in the (001) plane through 8-membered rings (hereafter 8 mRs), with free diameters of ~3.5 and ~4.5 Å. Ideally, the EAB framework is characterized by an accessible volume, which represents the portion of the occupiable volume that has continuity between all the unit cells, of about 14.3 % (https://asia.iza-structure.org/IZA-SC/frame work.php?STC=EAB). According to the classification of Gottardi and Galli, the EAB topology belongs to the ABC-6 family [5]. Therefore, the framework can be described as a repetition along the *c* axis of six-membered rings (hereafter 6 mRs) with ABBACC stacking sequence. This sequence is very similar to that of the ERI-framework (i.e., AABAAC), with which it was originally misidentified [4], LEV-framework (i.e., AABCCABBC) and OFF-framework (i.e., AAB). EAB zeolites, which crystal structure was firstly refined by Meier and Groner

<footnote><sup>a</sup> Dipartimento di Scienze della Terra, Università degli Studi di Milano, Via Botticelli 23, 20133, Milano, Italy</footnote>

<footnote><sup>b</sup> European Synchrotron Radiation Facility, 71 Avenue des Martyrs, CS40220, 38043, Grenoble, Cedex 9, France</footnote>

<footnote><sup>c</sup> Department of Chemical, Biological, Pharmaceutical and Environmental Sciences, University of Messina, Italy</footnote>

<footnote><sup>d</sup> C.E.Ca.S.P. Lab (Chemical Engineering Catalysis and Sustainable Processes Laboratory), University of Calabria, Rende, Italy</footnote>

<footnote><sup>e</sup> Deutsches Elektronen Synchrotron DESY, Notkestr. 85, 22607, Hamburg, Germany</footnote>

<footnote>* Corresponding author.
E-mail address: tommaso.battiston@unimi.it (T. Battiston).</footnote>
in 1981 [6], crystallize in the hexagonal system with space group P63/mmc. The natural isotypic framework mineral is a rare Sr-bearing zeolite named bellbergite, after its type locality (Bellberg volcano, Eifel, Germany) [7]. Natural bellbergite is a low-silica zeolite, characterized by a Si/Al ratio ~1, as its general formula is (K,Ba,Sr)2Sr2Ca2(Ca, Na)4[Al3Si3O12]6·30H2O.

Zeolites with EAB topology are still poorly investigated, likely because of their low thermal stability: at T ~400 °C, they undergo an irreversible first-order phase transition to a [SOD]-type framework [6, 8]. Recently, the work of Tong et al. [8] investigated the ion exchange properties of an EAB synthetic zeolite, revealing potential applications in alkali ions separations (i.e., K+, Na+ and Li+).

In our study, the high-pressure behaviour of EAB-framework zeolites is investigated both on natural and synthetic samples, with particular emphasis on the stimulation of the P-mediated crystal-fluid interaction phenomena. The results are compared with the compressional behaviours of structurally similar ABC-6 family members, e.g. erionite and offretite [9,10]. As the variation in pressure, temperature and chemical composition (both of the P-transmitting fluid and samples) can mitigate or promote the adsorption phenomena, the in-situ high pressure X-ray diffraction experiments were performed using different classes of non-penetrating and potentially penetrating P-transmitting fluids (PTFs), also using fluids of potential industrial interest, such as methanol. Methanol is recognised as promising intermediate in the CO2-hydrogenation processes to “green” fuels (key step of carbon footprint reduction strategy through carbon dioxide chemical recycling [11–13]). In this regard, it could be of paramount interest to understand the interaction of this compound with the zeolite framework under pressure, close to that used in industrial processes. Considering the effective role of zeolites catalysts in the Methanol-To-Olefins (MTO) process, their

performances could be enhanced by adsorption of methanol using a modest pressure gradient [14]. Thus, experiments using pure methanol as PTF have been performed also on the EAB-type zeolites. The investigated samples consisted of single crystals of natural bellbergite and powders of synthetic Na⁺- and K⁺-EAB.

This study is part of a long-term project that aims at the characterization of the P-induced crystal-fluid interaction on natural and synthetic zeolitic species [15–18], following the previous works on the structurally similar ABC-6 zeolites erionite and offretite [9,10].

## 2. Materials and methods

### 2.1. EAB synthesis and characterization

An EAB zeolite (Si/Al = 7 in the synthesis gel) was synthesized following a well-established procedure [4]. The synthesis gel was prepared according to the following molar ratios:

$$4 \text{Na}_2\text{O} - 8 \text{TMAOH} - 15 \text{SiO}_2 - 1 \text{Al}_2\text{O}_3 - 460\text{H}_2\text{O}$$

The chemicals used for the synthesis were: silica gel 60 (Aldrich), sodium aluminate (NaAlO2, Aldrich), sodium hydroxide (Aldrich), tetramethylammonium hydroxide (TMAOH, Aldrich) and distilled water. The hydrothermal synthesis was conducted at 80 °C for 7 days in a tumbling oven. Product obtained was carefully washed with distilled water and dried at 90 °C overnight.

One fraction of the as-made sample in Na⁺-form (labelled as Na-EAB) was calcined at 550 °C (ramp of 5 °C/min under static air straight after drying). After calcination, XRD-pw revealed a reduction/disappearing of all the Bragg peaks ascribable to the EAB zeolite, together with the

<figure><img src="image_4.png" /><figcaption>Fig. 1. The EAB-framework with eab-cages, gmelinite-cages and the double-six-rings, in a skeletal and tetrahedral representation: A) view along a direction perpendicular to the [001]-axis showing the ABC positions of the six-membered-rings; B) view down the [001]-axis; C) structural model described in section 3.1 with the extraframework population (yellow Na atoms, green Sr atoms, dark blue Ca atoms, purple K atoms) in a view down the [001]-axis; D) view down the [100]-axis.</figcaption></figure>
appearance of those characteristic of the sodalite structure (i.e., SOD topology, stacking sequence ABC). As discussed in the Introduction section, EAB and SOD type frameworks belong to the same ABC-6 zeolite family and the SOD phase can be obtained from EAB by cracking one-twelfth of the siloxane bridges present in the EAB framework [19]. Therefore, to preserve the crystallinity of the EAB phase in the Na⁺-form, the calcination was carried out with a modified procedure: 4 h under N₂ flow at 450 °C and 4 h under air flow at 450 °C (ramp of 5 °C/min). The XRD pattern of the obtained material is shown in Figure S1.

In order to furtherly reduce the effect of calcination over EAB crystallinity, Na⁺ was completely exchanged with K⁺ in the as-made sample, by using the following ion exchange procedure [20]: three ion exchange cycles with 1 mol/L KCl solution at 80 °C for 3 h. After the ion exchange, thermogravimetric analysis was performed to establish the calcination temperature profile needed to remove the organic templating agent from the pores of the K⁺-form of EAB (labelled as K-EAB). Other than confirming the presence of the H₂O endothermic peak at about 100 °C, differential thermal analysis results proved that the organic template leaves the K-EAB framework at about 435 °C in air flow (Fig. S2). As a consequence of the DTA results, K-EAB sample was calcined in two steps: 4 h at 430 °C under N₂ flow followed by 4 h under air flow. Thermogravimetric analysis was repeated after the calcination process and the absence of the exothermic peak at 435 °C confirmed the lack of template molecules inside the zeolite pores (Fig. S2).

SEM-EDX measurements (Phenom ProX) were conducted to evaluate the Si/Al ratio, the counter ion/Al ratio and the chemical formula of the samples, calculated on the basis of Si + Al = 36 atoms per formula unit (apfu) and 72 O apfu (belonging to the framework) (Table 1). The atomic Si/Al ratio obtained was in accordance with literature data [4]. The atomic ratio between the counter ion and aluminium remained the same after the ion exchange to obtain K-EAB, confirming the success of the procedure.

The H₂O content in the synthesized samples was obtained by thermogravimetric analysis (TGA). As indicated by the DTA endothermic peak of moisture loss, a variation in the weight of the sample was observed (Fig. S3). The weight fraction of H₂O was evaluated by means of the weight loss of the sample between room temperature and 250 °C temperature at which a weight fluctuation was no longer observed.

## 2.2. Bellbergite characterization at ambient conditions

Natural bellbergite crystals, kindly provided by Bernd Ternes, from its type locality at the Bellberg volcano, near Mayen (Eifel, Germany), were selected for the investigations. The sample was characterized by the electron microprobe in wavelength dispersive mode (EMPA-WDS), using a JEOL JXA-8200 instrument at the Earth Science Department of the University of Milan (ESD-MI). Data collection was conducted with an accelerating voltage of 15 kV, a beam current of 5 nA, defocusing the beam width at 20 µm; the counting time was respectively 30 and 10 s for the peaks and the background. Matrix effect on raw data was corrected with the PhiRhoZ method, implemented in the JEOL suit of programs. Natural samples have been adopted for standardization (Ca, Al, Si – grossular; K – K-feldspar; Na – omphacite; Mg – forsterite; Sr – celestine; Ba – sanbornite). The analysis indicated that the sample was chemically

<table><caption>**Table 1**

Chemical formulae by EMPA-WDS analysis, calculated on the basis of 72 O apfu, of the investigated samples. The charge imbalance is likely compensated by silanol and H⁺.</caption><thead><tr><th>Sample</th><th>Chemical formula</th></tr></thead><tbody><tr><td>Bellbergite</td><td>(Ba<sub>0.05</sub>Na<sub>0.76</sub>K<sub>1.73</sub>Sr<sub>2.28</sub>Ca<sub>4.93</sub>Mg<sub>0.01</sub>) [Al<sub>17.46</sub>Si<sub>18.63</sub>] O<sub>72</sub> • 28.03H<sub>2</sub>O</td></tr><tr><td>Na-EAB</td><td>Na<sub>9.9</sub>Al<sub>12.4</sub>Si<sub>23.6</sub>O<sub>72</sub> • 17.3H<sub>2</sub>O</td></tr><tr><td>K-EAB</td><td>K<sub>11.2</sub>Al<sub>12.4</sub>Si<sub>23.6</sub>O<sub>72</sub> • 16.4H<sub>2</sub>O</td></tr></tbody></table>

homogenous. The chemical formula of bellbergite, calculated on the basis of 72 O apfu and assuming H₂O by difference to 100 wt%, is reported in Table 1.

The full structural characterization, based on single crystal (~100 × 40 × 40 µm³ in size) X-ray diffraction data collected at room conditions, was performed at the ESD-MI with a Rigaku XtaLAB Synergy-S diffractometer, equipped with a HyPix-6000HE detector and a PhotonJet-S Mo-Kα (λ = 0.71073 Å) microsource, with operating conditions at 50 kV and 1 mA. The data collection was performed with an omega step size of 0.5° and 50 s per step of exposure time. Indexing of the diffraction pattern, along with intensity integration and unit-cell parameters refinement, were performed using the CrysAlisPro software [21].

## 2.3. High-pressure experiments

In-situ high-pressure X-ray diffraction experiments, conducted on powder and single crystals, were performed at the ID15B beamline of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, and at the P02.2 beamline at PETRA-III, Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany. Samples of Na- and K-EAB were grinded by means of an agate mortar for about 15 min to obtain a fine grained powder (Ø grains <2 µm), while transparent prismatic crystals of bellbergite (~50 x 50 × 20 µm) were selected for the high-pressure experiments. Zeolite samples, along with few ruby spheres as P-calibrant (pressure uncertainties ± 0.05 GPa) [22], were loaded in membrane-driven diamond anvil cells (DAC) with Boehler-Almax design (diamonds culet diameter of 600 µm at ESRF and 400 µm at PETRA-III). Stainless-steel gaskets (original thickness ~ 250 µm) were pre-indented to ~ 70 µm and drilled by spark erosion to create a P-chamber with hole diameter of ~200 µm.

Powder of synthetic K- and Na-EAB have been investigated under in-situ high-pressure conditions at ID15B (ESRF), using two different classes of PTFs: 1) silicone oil (hereafter s.o.), up to 0.74(5) and 0.86(5) GPa, respectively for K- and Na-EAB; 2) methanol, up to 4.08(5) GPa. All the experiments have been performed below the hydrostatic limit for each PTF [23]. The choice of the fluids was based on the strategy previously applied in other crystal-fluid interaction experiments on microporous framework materials [9,10,14,16]. The use of nominally anhydrous PTF (i.e., silicone oil), which, due to its polymeric nature, is unable to penetrate the structural cavities, was made to set a benchmark to highlight the occurrence of mass transfer between the fluid and the sample; adsorption ensues a decrease of compressibility due to the so-called “pillar effect” played by guest species within the framework (see Ref. [1] for a review). The comparison with potentially penetrating PTFs data, therefore, provides insights on the occurrence of crystal-fluid interaction. On the other hand, the use of mixtures with different wt% of potentially penetrating molecules (i.e., methanol and distilled H₂O) is aimed to enhance differences in magnitude of the adsorption process, as already observed in previous experiments on other natural zeolites [9, 10].

Analogously, in-situ high pressure experiments on natural bellbergite were performed using different PTFs: 1) s.o., up to 1.59(5) GPa; 2) methanol:ethanol:H₂O = 16:3:1 mixture, up to 8.14(5) GPa (m.e.w., molecular diameters: H₂O ~ 2.65 Å, methanol ~ 3.76 Å, ethanol ~ 4.46 Å); 3) pure methanol, up to 2.96(5) GPa; 4) distilled H₂O, up to 0.99(5) GPa.

Investigations on natural bellbergite have been performed both at the beamline P02.2 at PETRA-III (s.o. and m.e.w.) and ID15B at ESRF (methanol and distilled H₂O). The data collection strategy on single crystals consisted of a ±32° stepwise ω-rotation, with 0.5° step width and 1 s of exposure time, using an incident monochromatic beam (λ ~ 0.29 Å and E ~ 42.7 KeV at P02.2; λ ~ 0.41 Å and E ~ 30 KeV at ID15B). The diffraction patterns were collected with a PerkinElmer 1621 XRD detector set at a distance of 395.38 mm from the sample at P02.2, while an Eiger2 X 9 M CdTe (340 × 370 mm) detector positioned at 180.68 mm from the sample was used at ID15B. Indexing of diffraction patterns,
unit-cell parameters refinement and intensity integration were always performed using the *CrysAlisPro* suite, which also provides the corrections for the absorption (due to DAC components) and the Lp-effects, through the use of the semiempirical *ABSPACK* routine implemented in the software.

Powder XRD data of the synthetic EAB zeolites were collected using the same *Eiger2 X 9 M CdTe* detector set at a distance of 250 mm from the sample at ID15B. A first data collection was performed with the sample in air, in a quartz vial (Ø ~ 0.1 mm), before loading it into the DACs. The high-pressure data collection strategy consisted in a single-step ω-rotation (−5° ≤ ω ≤ +5°) with 10 s of exposure time. Additional details pertaining to the experimental setups of P02.2 and ID15b beamlines are given respectively in Refs. [24,25].

## 2.4. Elastic behaviour and equation of state

In order to model the compressional behaviour of our samples, an isothermal Birch-Murnaghan equation of state was used to fit the experimental P–V data. This equation assumes that the strain energy of a solid undergoing compression can be expressed as a Taylor series in the finite Eulerian strain, $f_e = [(V_0 / V)^{2/3} - 1] / 2$. The expansion in the Eulerian strain polynomial has the following form:

$$P(f_e) = 3K_{P0,T0}f_e(1 + 2f_e)^{5/2}\{1 + 3/2(K' - 4)f_e + 3/2[K_{P0,T0}K'' + (K' - 4)(K' - 3) + 35/9]f_e^2 + \dots \}$$

where $K_{P0,T0}$ (or $K_{V0}$) stands for the isothermal bulk modulus expressed as $K_{P0,T0} = K_{V0} = -V(\partial P / \partial V)_{T0} = \beta_{P0,T0}^2(\beta_{P0,T0}^2)$ represents the volume compressibility coefficient at room conditions), $K' = \partial K_{P0,T0} / \partial P$ and $K'' = \partial^2 K_{P0,T0} / \partial P^2$. The fits have been performed through the *EoSFit 7.0 GUI* software developed by Angel et al. [26].

# 3. Results

## 3.1. Crystal structure re-investigation of bellbergite

The single crystal X-ray diffraction pattern of the natural bellbergite collected at ambient conditions was characterized by the presence of diffuse scattering along the [001] direction (Fig. S4), suggesting a structural disorder of the tetrahedral framework sequence, as already described [7]. The structural model, based on the single crystal X-ray diffraction data and shown in Fig. 1c and d, was refined starting from the framework atomic sites positions obtained by *ab-initio* calculations made with the software *SUPERFLIP*, which adopts the charge-flipping algorithm, implemented in *Jana2006* [27,28]. The structure was solved in the space group *P6₃/mmc*. Positive residual peaks in the difference-Fourier maps of the electron density with a 6 mR geometry were ascribed to the presence of stacking faults between the B and the C positions (Fig. 1) of the sequence. T2', O3', O4' and O5' atomic sites were than included to the model, mutually exclusive with, respectively, T2', O3, O4 and O5 pertaining to the pristine bellbergite framework, restraining the overall site occupancy factor (s.o.f.) to 1.0. In addition, these sites were fixed to share the same displacement parameter ($U_{eq}$), which, along with those of the other framework sites, were refined as anisotropic. Overall, the disorder, in response to the stacking faults between B and C positions, affect ca. 10% of the framework.

A careful analysis of the difference-Fourier map was performed to locate the extraframework cations and H₂O molecules, providing similar results to the first bellbergite structural model proposed by Rudinger et al. [7]. A K site, labelled as K1, was located within the 8 mRs bordering the gmelinite cages (Table S1), bonded to framework oxygens (O4) and H₂O molecules (Ow1, Ow3, Ow6), with a refined s.o.f. equal to 0.51(2) (CN = 8). Two Ca sites were included into the model: Ca1, with a full s.o.f., located at the centre of the 6 mRs of the gmelinite cages (corresponding to the A position, Fig. 1), coordinating O1 and Ow1 (CN = 8), and Ca2, on the trigonal axis within the eab-type cage, with s.o.f.

equal to 0.29(5), which is entirely coordinated by H₂O molecules (CN = 9) (Table S1). The Na1 atomic site lies almost in the same position of Ca2, with which it is mutually exclusive, but slightly shifted along the trigonal axis (CN = 9). Even the Sr site, labelled as Sr1, is located within the eab-type cage (Table S1), sharing a mutual exclusivity relationship with Ca2 and Na1. Sr1, which has a s.o.f. equal to 0.45(1), is coordinated by both framework oxygens and H₂O molecules (CN = 10). The Ca2' atomic site, which is mutually exclusive with Ca2 and Na1, was included to the model the structural disorder. Likewise, Ow6', which is mutually exclusive with Ow6 and K1, was included to the model the disordered location of the H₂O sites. Each extraframework H₂O-oxygen site, except for Ow1, which is inside the gmelinite cage, was located within the eab-type cage. Atomic fractional coordinates, along with displacement parameters, s.o.f. and selected statistical parameters of the ambient-conditions structural refinement are reported in Table S1. The CIF file is deposited as supplementary material.

## 3.2. Bellbergite at high pressure

Fig. 2 and Table S2 show the evolution of the unit-cell parameters at high pressure of natural bellbergite, compressed in the aforementioned PTFs. The P–V plot highlights an overall similar compressibility from the experiments performed in *silicone oil*, *methanol* and *m.e.w.*, while a slightly lower compressibility is demonstrated by the crystal compressed in *distilled H₂O* (Fig. 2a). The intrinsic compressibility of bellbergite is provided by the refined isothermal bulk modulus ($K_{V0} = \beta_{V0}^{-1}$) based on *silicone oil* data, calculated via II-order Birch-Murnaghan equation of state fit in the pressure range $P_{amb} - 1.58(5)$ GPa, yielding to the following average result: $K_{V0} = 62(1)$ GPa ($V_0 = 2430.9(2)$ Å³). The elastic parameters obtained from the experiments in *methanol*, *m.e.w.* and *distilled H₂O*, along with the linear (axial) bulk moduli ($K_{I0}$) for *silicone oil* data, are reported in Table 2. The P–V trend of the data collected in *silicone oil* suggests that the compression is almost monotonic, with a slight increase of the slope at ~0.45(5) GPa, suggesting the occurrence of a modest softening (Fig. 2a, S5). Additionally, the compression in *methanol* appears to be monotonic up to 2.96(5) GPa, although a minor decrease in compressibility, detectable also from the *m.e.w.* experiment, is observable in the range 0.75(5)-1.50(5) GPa. In addition, bellbergite in *m.e.w.* shows a more pronounced decrease in bulk compressibility above ~2.5 GPa (Fig. 2a).

The compressional behaviour along the directions of the main crystallographic axes is reported in Fig. 2b and c. Along the a-axis, which reflects the behaviour on the (001) plane, the structure resulted markedly less compressible in *distilled H₂O* with respect to the other PTFs (Fig. 2b). In addition, within the same plane, a mild decrease in compressibility is also observable from the *methanol* and *m.e.w.* experiments, in the range 0.75(5)-1.50(5) GPa. Conversely, the crystal compressed in *silicone oil* shows a monotonic shortening along [100] within the whole investigated range (i.e., $P_{amb}-1.58(5)$ GPa). However, the compression in *silicone oil* induces an increase in linear compressibility along the c-axis above 0.48(5) GPa (Fig. 2c), which corresponds to the P-value at which the slight softening in the unit-cell volume evolution takes place (Fig. 2a). Similar softenings have also been detected from *distilled H₂O* and *methanol* experiments, above 0.31(5) GPa. Along the same direction, the structure exhibits a further increase in linear compressibility above 1.50(5) GPa, when compressed in *methanol*. Conversely, from the *m.e.w.* ramp, the structure along the c direction seems to show a monotonical softening up to 1.14(5) GPa (Fig. 2c). Above that pressure value, the structure resulted to be almost uncompressible up to 2.98(5) GPa. A further shortening is observable based on the last three data collected, up to of 4.13(5) GPa, whereas an elongation of about ~0.2% along the c-axis direction is observable at 2.16(5) GPa.

## 3.3. Synthetic EAB zeolites at high pressure

The evolution of the unit-cell volume and axial parameters of both
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Evolution with pressure of the unit-cell parameters, normalized to ambient *P*-values, of natural bellbergite single crystals compressed in *silicone oil*, *m.e.w.*, *methanol* and *distilled H₂O*; (A) unit-cell volume; (B) *a* unit-cell parameter; (C) *c* unit-cell parameter. The e.s.d. are smaller than the symbols.</figcaption></figure>

**Table 2**

Refined elastic parameters of natural bellbergite and synthetic Na- and K-EAB zeolites, compressed in different pressure-transmitting fluids (see text for further details). Results are based on II-order Birch-Murnaghan equation of state fits (*fixed).

<table><thead><tr><th></th><th><i>P</i> range (GPa)</th><th><i>V</i><sub>0</sub><i>l</i><sub>0</sub> (Å<sup>3</sup>, Å)</th><th><i>K</i><sub>V0,10</sub> (GPa)</th><th><i>K</i>'</th></tr></thead><tbody><tr><td colspan="5"><strong>bellbergite in s.o.</strong></td></tr><tr><td><i>V</i></td><td><i>P</i><sub>amb</sub>-1.59(5)</td><td>2430.9(2)</td><td>62(1)</td><td>4*</td></tr><tr><td><i>a</i></td><td><i>P</i><sub>amb</sub>-1.59(5)</td><td>13.2522(5)</td><td>56(5)</td><td>4*</td></tr><tr><td><i>c</i></td><td><i>P</i><sub>amb</sub>-0.48(5)</td><td>15.9824(9)</td><td>113(11)</td><td>4*</td></tr><tr><td><i>c</i></td><td>0.74(5)-1.59(5)</td><td>16.04(1)</td><td>42(3)</td><td>4*</td></tr><tr><td colspan="5"><strong>bellbergite in m.e.w.</strong></td></tr><tr><td><i>V</i></td><td><i>P</i><sub>amb</sub>-5.83(5)</td><td>2453(1)</td><td>66.5(8)</td><td>4*</td></tr><tr><td colspan="5"><strong>bellbergite in methanol</strong></td></tr><tr><td><i>V</i></td><td><i>P</i><sub>amb</sub>-2.96(5)</td><td>2443(1)</td><td>62(1)</td><td>4*</td></tr><tr><td colspan="5"><strong>bellbergite in distilled H<sub>2</sub>O</strong></td></tr><tr><td><i>V</i></td><td><i>P</i><sub>amb</sub>-0.99(5)</td><td>2452(1)</td><td>74(5)</td><td>4*</td></tr><tr><td colspan="5"><strong>K-EAB in s.o.</strong></td></tr><tr><td><i>V</i></td><td><i>P</i><sub>amb</sub>-0.45(5)</td><td>2308(10)</td><td>16(4)</td><td>4*</td></tr><tr><td><i>V</i></td><td>0.45(5)-0.74(5)</td><td>2478(90)</td><td>4(2)</td><td>4*</td></tr><tr><td colspan="5"><strong>K-EAB in methanol</strong></td></tr><tr><td><i>V</i></td><td>0.48(5)-1.00(5)</td><td>2319(2)</td><td>149(21)</td><td>4*</td></tr><tr><td><i>V</i></td><td>1.00(5)-4.08(5)</td><td>2331(3)</td><td>92(4)</td><td>4*</td></tr><tr><td colspan="5"><strong>Na-EAB in s.o.</strong></td></tr><tr><td><i>V</i></td><td><i>P</i><sub>amb</sub>-0.45(5)</td><td>2295(9)</td><td>19(5)</td><td>4*</td></tr><tr><td><i>V</i></td><td>0.45(5)-0.86(5)</td><td>2333(22)</td><td>11(2)</td><td>4*</td></tr><tr><td colspan="5"><strong>Na-EAB in methanol</strong></td></tr><tr><td><i>V</i></td><td>0.48(5)-1.00(5)</td><td>2311(1)</td><td>231(39)</td><td>4*</td></tr><tr><td><i>V</i></td><td>1.00(5)-4.08(5)</td><td>2326(3)</td><td>99(5)</td><td>4*</td></tr></tbody></table>

Na- and K-EAB with pressure is reported in Fig. 3 and Table S2. The results from non-penetrating *silicone oil* data suggest that the structure undergoes a monotonic compression up to 0.45(5) GPa, followed by a slight increase in compressibility, both in Na- and K-EAB, up to 0.86(5) GPa and 0.74(5) GPa, respectively. The refined isothermal bulk moduli are reported in Table 2. Conversely, results from *methanol* experiments revealed a marked expansion of the unit-cell volume in the pressure range *P*<sub>amb</sub>-0.40(5) GPa, either in Na- and K-EAB, with a slightly higher magnitude observable in Na-EAB (Fig. 3a). The expansion is also observable from the shifts of the symmetry-independent Bragg peaks 220, 312, 400, 401 and 402, of the K-EAB powder diffraction patterns compressed in *methanol*, as shown in Fig. 4. This peculiar behaviour is also evident along the main crystallographic axes. In the *methanol* experiment, the expansion along the *a*-axis up to 0.55(5) and 1.00(5) GPa, for K- and Na-EAB respectively, is followed by a mild compression up to ~ 4 GPa (Fig. 3b). According to the observations on the bulk volume evolution, the magnitude of the expansion along the *a*-axis is still larger in Na-EAB. Along the same direction, non-penetrating *silicone oil* ramps showed an abrupt compression for both the Na- and K-EAB already from the first kbar of pressure, followed above 0.45(5) GPa in K-EAB by an abrupt increase in bulk compressibility, that is observable also in Na-EAB, with a lower magnitude (Fig. 3b). The results were different along the *c*-axis direction. The data collected in *methanol* showed an expansion only within the first 0.40(5) GPa for both the samples, followed by a sudden increase in compressibility, larger than that observed along the *a*-axis direction (Fig. 3c). Above 0.40(5) GPa, the compression is monotonic up to 4.04(5) GPa (Fig. 3c). In addition, Na-EAB data collected in *silicone oil* showed an unexpected elongation of the structure along the *c*-axis direction within the first 0.34(5) GPa. Above that pressure, the structure is compressed mainly along [001].
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Evolution with pressure of the unit-cell parameters, normalized to ambient *P*-values, of synthetic EAB zeolite powders compressed in silicone oil and methanol; (A) unit-cell volume; (B) *a* unit-cell parameter; (C) *c* unit-cell parameter. The e.s.d. are smaller than the symbols.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Evolution with pressure of the X-ray powder diffraction patterns of Na-EAB compressed in methanol. The Bragg reflections 220, 312, 400, 401 and 402 are marked.</figcaption></figure>

The anisotropy of the compressional behaviour is different in K-EAB, which does not show any expansion along the *c*-axis, before the increase in linear compressibility above 0.45(5) GPa. Data collected in decompression suggested the complete reversibility of the phenomena (Table S2).

# 4. Discussion

## 4.1. High-pressure elastic behaviour without crystal-fluid interaction

High-pressure investigation performed in non-penetrating PTFs allowed to model and describe the intrinsic elastic behaviour of the EAB framework with different types and configurations of extraframework components. This emphasized the markedly different compressional behaviour of natural and synthetic zeolites. The refined isothermal bulk modulus of the natural bellbergite (i.e., $K_{V0}$ = 62(1) GPa between $P_{amb}$-1.59(5) GPa) was found to be close to that reported for the structurally similar zeolites offretite ($K_{0.9}Ca_{1.1}Mg_{0.7}[Al_{4.7}Si_{13.3}]O_{36} \cdot 11.5H_2O$, 6 mRs stacking sequence AAB, $K_{V0}$ = 59(2) GPa [10]) and levyne ($Ca_{2.3}Na_{0.8}K_{0.3}[Al_{5.6}Si_{12.4}]O_{36} \cdot 17.5H_2O$, 6 mRs stacking sequence AABCCABBC, $K_{V0}$ = 59(2) GPa for $P < 1$ GPa [29]). Elastic parameters of the synthetic EAB samples showed a markedly larger intrinsic compressibility with respect to natural bellbergite, being $K_{V0}$ = 16(4) and 19(5) GPa, at $P < 0.45(5)$ GPa, and $K_{V0}$ = 4(2) and 11(2) GPa at $P > 0.45(5)$ GPa, in K- and Na-EAB, respectively. Such a different compressional behaviour can be explained by the dissimilar content of cationic and molecular extraframework population among the samples: the guest species, in terms of nature and abundance, act as “pillars” and are able to govern the bulk volume compression of a microporous sample [see Refs. [30,31]], for a review]. To determine the occupiable volume, calculations of the structural voids were performed using the tools implemented in the *Olex2* software [32], according to the refined structural models. In synthetic Na-EAB, the volume occupied by both framework and extraframework atoms, is calculated to be 1688.1 Å³, corresponding to ~74.7 % of the unit-cell volume at ambient conditions. Conversely, the volume occupied by atoms and molecules in natural bellbergite sums to
2236.6 Å³, which corresponds to the ~92.3 % of the entire unit-cell volume at room conditions. This difference can explain the overall higher bulk modulus (i.e., lower compressibility) observed for the natural sample.

In both natural and synthetic samples, a softening (i.e., which implies a change of the compressional regime) was observed at ~0.45 GPa. In the natural sample, as only a few data points were collected before the increase in compressibility, the calculation of elastic parameters in two separate pressure ranges (i.e., P < 0.48(5) GPa and P > 0.48(5) GPa) is not statistically robust. Nevertheless, it is worth to note that the softening occurred in all the samples compressed in *silicone oil* above ~0.45 GPa. Although its effect is modest, in terms of compressibility variation, its occurrence is realistic as it is observed far below the hydrostatic limit of the PTF (i.e., ~ 0.9 GPa according to Angel et al. [23]). A similar behaviour was observed in the structurally similar zeolite offretite [10], where it was consequent to a rearrangement in the extraframework population. However, as the investigated synthetic and natural EAB samples have a very different extraframework content, this peculiar increase in compressibility was interpreted mainly as a consequence of a structural rearrangement involving the EAB aluminosilicate framework. The structure response along the principal crystallographic axes suggests that the cause is likely attributable to a change in the mechanism of deformation along [001], rather than within the plane (001) (Fig. 3). This reveals an intrinsic difference between the deformation mechanisms of the EAB framework and the structurally similar OFF, in which the extraframework rearrangements generate a *P*-induced softening on the (001) plane [9,10]. In addition, a slight expansion along the *c*-cell edge was observed in synthetic Na-EAB compressed in *silicone oil* up to ~0.45(5) GPa (Fig. 3c). The low magnitude of the increase does not generate strong consequences on the crystalline edifice; nevertheless, a slight decrease of the linear compressibility along the *a*-axis is observable at P > 0.45(5) GPa. The absence of structural refinements, based on the high-pressure data, does not allow a full understanding of the phenomenon (i.e., to definitely exclude that it is an artifact induced by the Rietveld full-profile fit); however, a possible explanation is proposed. A similar behaviour was described by Gatta and Wells [33] for the structurally similar levyne (LEV topology). In that case, it resulted to be the effect of a deformation mechanism that stirred the double-six-ring along [001] in the low-*P* regime. Considering the similarity between EAB and LEV topologies, and the presence of a double-six-ring intercalated to the gmelinite cages, a similar phenomenon might occur also in EAB. Conversely, K-EAB did not shows any structural expansion along [001], at least at a significant level. The different compressional behaviour of the Na-end member might be explained by the nature and size of the extraframework cation that, as already observed by Gatta et al. [30], might trigger different deformation mechanisms.

## 4.2. *P*-mediated crystal-fluid interaction

The phenomenon of *P*-induced adsorption within the cavities of a microporous material usually results in a stiffening of the structure, due to the “pillar” effect induced by the new guest species filling the structural voids (see Ref. [3] for a review). As a consequence, the choice of *silicone oil* as non-penetrating PTF allows, along with the characterization of the intrinsic compressibility of the samples, to set a benchmark for the evaluation of the magnitude of the crystal-fluid interaction phenomena when penetrating fluids are used. To enhance such a phenomenon, PTFs with different partial pressures of potentially penetrating molecules (i.e., *methanol*, *m.e.w.* and *distilled H₂O*) were used. This approach has proven to induce varying degrees of crystal-fluid interaction [9,10].

The *P*–*V* diagram in Fig. 2 indicates that the natural sample of bellbergite exhibits no significant interaction with fluids, as evidenced by its similar bulk compressibility among different PTFs. However, the ramp performed in *distilled H₂O* resulted in a slightly higher bulk modulus (i.e., $K_{V0}= 74(5)$ GPa) within the ~1 GPa, suggesting a modest adsorption of

guest H₂O molecules from the PTF. This is further supported by a markedly different compressional behaviour within the (001) plane (Fig. 2b) with respect to the other PTFs. In agreement to what was observed in erionite and offretite [10], changes in compressibility, in response to the crystal-fluid interaction, of the bellbergite structure are mainly manifested perpendicularly to the direction of cage-columns corresponding to [001]. Additionally, a modest decrease in compressibility on the (001) plane is detectable also from the *methanol* experiment between 0.51(5)-1.09(5) GPa, which may be ascribed to a minor H₂O intrusion, due to its hydrophilic nature (Fig. 2b) (assuming a low H₂O content even in nominally pure methanol). Nevertheless, its magnitude appears to be negligible, as no significant relevance is shown by the *P*–*V* trend (Fig. 2).

As shown in Fig. 2c, in the *m.e.w.* experiment, the structure of bellbergite exhibited almost no compression along [001] between 1.45(5)-2.99(5) GPa. Although it had a slight impact on the bulk volume compressibility, this result might be attributed to the selective adsorption of H₂O molecules from the mixture (which keeps its liquid state and hydrostaticity at those pressures [23]). Additionally, since no strong evidence of crystal-fluid interaction is observed in the *methanol* ramp and ethanol has a larger kinetic molecule diameter (~4.3 Å) with respect to methanol (~3.6 Å), the sorption of alcohols molecules might be considered as unlikely. However, it is interesting to note that in *m.e.w.* the crystal-fluid interaction affects the structure deformability along [001], instead on the plane (001) (Fig. 2), as it is shown by the *distilled H₂O* experiment. This difference might be explained by an adsorption occurring at higher pressure, for which it could be responsible the minor wt% of H₂O molecules within the mixture. Intuitively, if the same molecule is sorbed at significantly different pressure, it will interact with different structures of the same host zeolite. Unfortunately, the lack of structural refinements prevents any direct “observation” of adsorbed guest sites within the channels.

Conversely, CH₃OH resulted to be highly penetrating in synthetic EAB zeolites, either in K- and Na-endmembers. As a result, a spectacular unit-cell volume expansion was observed within the first 0.40(5) GPa (Fig. 3), and, above that pressure, the structures show a markedly lower compressibility if compared to the *silicone oil* experiments. As observed in the natural sample (except for the *m.e.w.* ramp), the crystal-fluid interaction has a larger impact on the (001) plane compressibility, rather than along [001] (Fig. 3). In *methanol* experiment, the softening, which in *silicone oil* occurs ~0.45 GPa, is observed above 1 GPa, as a consequence of the cavities stuffing due to the new guest molecules.

<figure><img src="image_8.png" /><figcaption>Fig. 5. *P*-induced evolution of the normalized (to ambient values) unit-cell volumes of natural bellbergite and synthetic Na-EAB zeolite, compressed in non-penetrating (*silicone oil*) and potentially-penetrating (*methanol* and *distilled H₂O*) *P*-transmitting fluids. The e.s.d. are smaller than the symbols.</figcaption></figure>
The markedly different adsorption capacity between bellbergite and the synthetic analogues (Fig. 5) reflects the different *occupiable volume* between the two structures, as already mentioned in section 4.1. An additional minor effect potentially able to hinder the sorption capacity of the natural bellbergite crystals is ascribable to the framework disorder, in response to the stacking faults between B and C positions (Fig. 1), which affect *ca.* 10 % of the structure (Table S1). However, the different magnitude of the crystal-fluid interaction phenomena may also be controlled by the different particle size of the natural and synthetic zeolite samples investigated in this study. As described by Lotti et al. [16], the sample size can somehow govern the magnitude of the P-induced intrusion process, being fine powdered zeolites facilitated in the adsorption of guest molecules for the higher surface/volume ratio [16].

### 4.3. Comparison with structurally similar species

In Fig. 6, the comparative high-pressure behaviour and crystal-fluid interaction of bellbergite, erionite and offretite, three zeolitic species belonging to the ABC-6 group, is shown. The three structural models refined at ambient conditions show different fractions of the unit-cell volume occupied by the atoms (both framework and extraframework), namely ~ 83 % in offretite, ~ 89 % in erionite and ~ 92 % in bellbergite. Consequently, offretite has the largest occupiable volume, followed by erionite and then bellbergite. However, erionite was found to experience the highest adsorption of new guest species, followed by offretite and

bellbergite. While the different magnitude of the crystal-fluid interaction between erionite ($K_{2.4}Ca_{2.0}Mg_{1.0}[Al_{9.0}Si_{27.1}]O_{72.2}·26.8H_2O$) and offretite can be mainly explained by the different framework geometries and arrangement of the extraframework cations [10], in bellbergite the lower Si/Al ratio (~3.0, 2.8 and 1.1 in erionite, offretite and bellbergite, respectively; Table 1; [9,10]), and the consequent comparatively larger amount of extraframework cations within the structural cavities, play a key role in reducing drastically the available volume for hosting new guest species.

Pressure-induced selective adsorption of alcohol molecules at pressures of a few kbar has already been observed for MFI (methanol) and CHA (ethanol) zeolites [14,34]. Nevertheless, the volume expansion generated by the adsorbed methanol molecules in the studied EAB zeolites is remarkable in terms of magnitude. As shown in Fig. 7, the P-induced sorption of $CH_3OH$ molecules in EAB zeolites is larger than in MFI (i.e., one of the most widely used zeolites in Methanol-To-Hydrocarbons, hereafter *MTH*, processes and in petroleum industry, patented by Mobil Oil Corporation as ZSM-5). This behaviour might be explained by a different Si/Al ratio between the two species, as a higher Al content in the framework favours the affinity between zeolites and polar adsorbate molecules [35,36], and, in addition, even by the different channels configuration of the two frameworks. On the other hand, the EAB Si/Al might represent a major issue for the application to *MTH* processes, as it implicates a higher density of acid sites that favours a faster coking rate [36].

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Compressional evolution of the unit-cell volume (normalized to ambient values) of three natural zeolites sharing similar frameworks belonging to the ABC-6 group (OFF, ERI and EAB), compressed in non-penetrating (silicone oil) and potentially-penetrating (m.e.w. and distilled $H_2O$) fluids: (A) bellbergite; (B) erionite; (C) offretite. The same graphical scale was adopted. The e.s.d. are smaller than the symbols.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Compressional evolution of the unit-cell volumes (normalized to ambient values) of synthetic Na-EAB and synthetic Na-Al MFI zeolites (Comboni et al., 2020) [12], compressed in non-penetrating (silicone oil) and potentially-penetrating (methanol) P-transmitting fluids. The e.s.d. are smaller than the symbols.</figcaption></figure>

## 5. Conclusions

This manuscript reports on the high-pressure behaviour of both synthetic and natural [EAB]-type zeolites. The experiments were performed using both non-penetrating (i.e., silicone oil) and potentially penetrating (i.e., methanol:ethanol:H₂O = 16:3:1 mixture, methanol and distilled H₂O) pressure-transmitting fluids.

The data collected in silicone oil, which describe the intrinsic compressibility of the samples, highlight a slight increase in bulk compressibility not related to any phase-transition, which occurs in both synthetic and natural samples. The lack of structural refinements did not fully unveil the deformation mechanisms at the atomic scale but, similarly to what described for natural levyne [33], the increase in compressibility might be related to the behaviour of the double-six rings. Nevertheless, a markedly higher compressibility is shown by the synthetic samples, marking the role played by the extraframework population on the bulk compressibility. This is observable also from the experimental data based on potentially-penetrating fluids, in which the magnitude of crystal-fluid interaction is spectacularly higher in synthetic EAB, while it is minor in natural bellbergite and occurs only when compressed in distilled H₂O. Furthermore, both the synthetic EAB samples show a spectacular unit-cell volume increase within the first 0.4 GPa when compressed in methanol, proving a significant interaction with this molecule. The same fluid resulted to be non-penetrating, at least at a significant level, in natural bellbergite.

It can be concluded that synthetics EAB show a remarkable methanol adsorption capacity, even at pressure of a few kbar (i.e., values exploitable in industrial high-pressure processes), although the lack of structural refinements did not allow us to provide the position of the new guest sites within the cavities of the crystalline edifice. Nevertheless, the presented results can contribute to the development of new applications for EAB-type zeolites in the methanol chemistry. Based on the results of the study and on the previous considerations, a dealuminated EAB sample, in order to rise the Si/Al ratio, represents a possible candidate for future investigations of the P-mediated zeolite-methanol interactions.

## CRediT authorship contribution statement

**Tommaso Battiston:** Writing – original draft, Investigation, Formal analysis, Conceptualization.
**Davide Comboni:** Writing – review &

editing, Investigation.
**Paolo Lotti:** Writing – review & editing, Formal analysis, Conceptualization.
**Giorgia Ferrarelli:** Investigation.
**Massimo Migliori:** Writing – review & editing.
**Girolamo Giordano:** Writing – review & editing.
**Michael Hanfland:** Investigation.
**Gaston Garbarino:** Investigation.
**Hanns-Peter Liermann:** Writing – review & editing.
**G. Diego Gatta:** Writing – review & editing, Supervision, Funding acquisition, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

Data will be made available on request.

## Acknowledgements

We acknowledge ESRF (Grenoble, France) and DESY (Hamburg, Germany, a member of the Helmholtz Association HGF) for the allocation of the beamtimes. Parts of this research were carried out at PETRA III using beamline P02.2, during the beamtime allocated for proposal I-20211179 EC. Andrea Risplendente is thanked for the EMPA-WDS chemical analysis. Dr. Bernd Ternes is gratefully thanked for the samples of natural bellbergite. GDG, DC, PL and TB acknowledge the support of the Italian Ministry of University and Research through the projects "PRIN2017—Mineral reactivity, a key to understand large-scale processes" (2017L83S77) and "Dipartimenti di Eccellenza 2023–2027". GF, MM and GG acknowledge the support of Italian Ministry of University and Research, program PNR 2015–2020 (D.D. 1735 10.072017) "Industrial research in the 12 areas of specialization", specialization area "Energy", Project "WWGF - Gasification of Wet Organic Waste with Percritical Water for the Production of Biomethane and LNG", grant number ARS01_00868. Two anonymous reviewers and the editor H. Pastore are warmly thanked.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.micromeso.2023.112873.

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