# Synthesis of OSDA-free offretite in the presence of cesium

Risheng Wang, Qiudi Yue, Sajjad Ghojavand, Edwin B Clatworthy, Diógenes Honorato Piva, Marie Desmurs, Valérie Ruaux, Svetlana Mintova

► To cite this version:

Risheng Wang, Qiudi Yue, Sajjad Ghojavand, Edwin B Clatworthy, Diógenes Honorato Piva, et al. Synthesis of OSDA-free offretite in the presence of cesium. Microporous and Mesoporous Materials, 2025, 398, pp.113795. 10.1016/j.micromeso.2025.113795. hal-05343433

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# Synthesis of OSDA-free offretite in the presence of cesium

Risheng Wang ID, Qiudi Yue ID, Sajjad Ghojavand ID, Edwin B. Clatworthy ID,
Diógenes Honorato Piva ID, Marie Desmurs, Valérie Ruaux, Svetlana Mintova * ID

Université de Caen Normandie, ENSICAEN, CNRS, LCS, Laboratoire Catalyse et Spectrochimie, 14000, Caen, France

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

## ARTICLE INFO

**Keywords:**

Offretite

OSDA-Free synthesis

Alkali metal cations

Synthesis optimization

CO₂ capture

## ABSTRACT

Zeolites containing 8-membered rings (8MRs) such as offretite have shown promising properties in selective adsorption of CO₂. However, the necessity of organic structure-directing agents (OSDAs) in the conventional synthesis of offretite leads to additional costs and energy consumption which critically restricts its industrial application. Herein, we report a novel OSDA-free synthetic strategy of offretite by direct synthesis in a colloidal precursor suspension containing inorganic cations as the structure-directing agents (SDAs). For the first time Cs⁺ was used as an SDA in a synthesis mixture to obtain offretite. Based on empirical evidence presented here and from the literature, we hypothesise that Cs⁺, in combination with K⁺ and Na⁺ cations, facilitates the formation of gme cages and avoids the formation of other crystalline phases such as LTL and PHI. By careful optimization, the effect of various parameters related to the purity and crystal sizes (from 15 µm to 2 µm) have been investigated, including synthesis temperature, composition of mixed cations, aging time, silica-to-aluminium ratio, aluminium source and water content. Offretite as pure phase, with a Si/Al ratio of 3.5, and tunable crystal sizes have been successfully prepared solely by using inorganic SDAs. The offretite exhibited a high sorption capacity for CO₂ (4.1 mmol/g at 0.95 bar and 0 °C), showing potential for selective CO₂ capture.

## 1. Introduction

Zeolites are a class of crystalline microporous materials whose ordered three-dimensional frameworks form channels and cages that can accommodate different guest molecules. Due to their high adsorption capacity and selectivity towards CO₂, as well as their high thermal and chemical stability, they are considered promising solid adsorbents for CO₂ separation [1]. FAU-type zeolite, with the pore size of 7.35 Å, has been the subject of a significant amount of research for CO₂ separation because of its large micropore volume and high amount of adsorption sites [2–4]. Its CO₂ separation performance is governed by the different thermodynamic affinities of guest molecules with the adsorption sites, since the large pore size allows facile access of many different small molecules to the microporosity [5]. In comparison, small-pore zeolites with 8-membered ring (8MR) pore apertures (typically 3.0–4.5 Å) have also been widely investigated for CO₂ capture due to the comparable size of the pore aperture with the kinetic diameter of CO₂ (~3.3 Å), which enables the molecular sieving effect [6–9]. As a consequence, 8MR zeolites such as CHA, GIS, MER and RHO, demonstrated significantly high selectivity towards CO₂ [10–15]. In addition, it has been shown recently that reducing the crystal sizes of these small-pore zeolites can

substantially enhance the CO₂ diffusion properties within intracrystal-line channels for better CO₂ separation performances [16–19].

Offretite zeolite, with its OFF-type topology comprising of can and gme cages, is characterized by a three-dimensional mixed-size pore system with 12-membered ring (12MR) channels propagating along the c-direction perpendicularly intersected by three 8MRs of the gme cages, creating a secondary sinusoidal channel system [20,21]. This feature suggests that offretite may exhibit desirable CO₂ adsorption properties, especially its potential in diffusion kinetics enhancement owing to the larger pore diameter (6.7 Å along the c-axis and 3.6 Å normal to c-axis) [22]. Offretite occurs as both a natural and synthetic zeolite, however, it frequently crystallizes with stacking faults and produces intergrowths with erionite (zeolite T, ZSM-34) [20,23–25]. By introducing organic molecules into the synthesis precursor mixture the ratio of these phases can be tuned to produce pure offretite. Early work by Aiello and Barrer demonstrated the synthesis of OFF-type zeolite with the assistance of tetramethylammonium (TMA⁺) cations as the organic structure-directing agent (OSDA) in the presence of Na⁺ and K⁺ cations; both TMA⁺ and K⁺ had to be present to obtain the OFF phase [26]. Further by carefully increasing the TMA⁺ amount in the parent gel, erionite could be avoided, resulting in pure offretite hexagonal crystals

<footnote>* Corresponding author.
E-mail address: svetlana.mintova@ensicaen.fr (S. Mintova).</footnote>
[25]. Subsequent investigations showed that pure offretite can be synthesized in the presence of different organic molecules including choline [27], 1,4-diazabicyclo[2.2.2]octane (DABCO) [28], tetraethylammonium (TEA⁺) [29], and cetyltrimethylammonium (CTA⁺) [30]. Interestingly, Yang et al. observed that the introduction of KCl to a glycerol-based precursor mixture directed the zeolite structure formation from zeolite omega (MAZ) to offretite, highlighting the role of K⁺ in directing the formation of the OFF structure [23]. However, the use of organic molecules significantly increases zeolite manufacturing costs and poses environmental concerns due to the additional energy requirements for calcination, and the associated emissions.

Recently, the OSDA-free synthesis of zeolites has received renewed attention in an effort to minimize the use of organic molecules in zeolite synthesis, as well as a means to control the physico-chemical properties of zeolites, such as nanoparticle morphology, to improve their performance [31]. To date, OSDA-free synthesis approaches for nanozeolites have been developed yielding properties that are competitive with or superior to those of their conventionally synthesized counterparts. Previous work has shown that proper use of inorganic alkali metal (or alkaline earth metal) cations as inorganic structure-directing agents (SDAs) to replace OSDAs has become a promising way to obtain green zeolites [32–35]. By carefully tuning synthesis parameters such as the type and composition of cations, Si and Al sources, and aging times, various zeolites with finely controlled physio-chemical properties can be synthesized. For instance, syntheses of nano-sized FAU and EMT zeolites could be carried out with only Na⁺ as SDA [33,35]. Nano-sized RHO zeolite was also synthesized solely using Na⁺ and Cs⁺ cations as SDAs [10,11]. In addition, nano-sized CHA could be successfully prepared from a precursor containing Na⁺, K⁺ and Cs⁺, exhibiting enhanced CO₂ diffusion properties compared to micron-sized CHA [17,32,36]. By further tuning the composition of the Na⁺/K⁺/Cs⁺ cationic mixture and aging time of the precursor colloidal suspension, crystallization of BPH zeolite nanosheets could be achieved at ambient and low temperature conditions [34,37].

In a typical zeolite synthesis, alkali and alkaline earth metal hydroxides play multiple roles including as a source of OH⁻ anions to facilitate the solubilisation of the Si and Al sources, and as a source of alkali and alkaline earth metal cations which facilitate crystal growth [38]. Furthermore, alkali and alkaline earth metal cations, with respect to the particular availability of Si, Al and solvent in the precursor mixture, act as inorganic structure directing agents for a zeolite framework topology [39]. With respect to the OFF framework structure, it can be deconstructed into three composite building units (CBUs), the double six ring (d6r), cancrinite (can), and gmelinite (gme) units [21]. K⁺ cations have been identified as a SDA associated with the formation of Linde Type L consisting of CAN cages in its OSDA-free synthesis with K⁺ cations [40]. Additionally, Yang et al. observed the preferred formation of offretite (instead of omega zeolite) by the addition of K⁺ cations to the zeolite precursor mixture in aqueous glycerol medium, which is responsible for the formation of cancrinite cages (not present in omega zeolite). This highlights the critical role of K⁺ in offretite synthesis, which was also described by Moudafi et al. [23,41] Additionally, Moudafi et al. showed that, with a partial replacement of K⁺ (up to 75 %) with Na⁺ in the precursor mixture, both the nucleation and crystal growth of offretite were accelerated, while the Si/Al ratio was lower (3.0) compared to the K⁺/TMA⁺ directed synthesis (3.5), attributed to the increased supersaturation of Na⁺ [42]. Aiello and Barrer focused on the directing effect of TMA⁺ in offretite synthesis and proposed a unique role of TMA⁺ cations in the formation of gme cages (0.9–1 TMA⁺ per gme cage, non-exchangeable) emphasising the essential role of TMA⁺ in the development of the offretite nuclei [26]. Recently, Tsunoji et al. reported a multiple template synthesis strategy of AXF-type zeolite using a bulky OSDA combined with either TMA⁺ or Na⁺, where the Na⁺ cations were located in the gme cages, raising the question if a similar strategy can be used in the synthesis of offretite [43].

In this study, we present an environmentally friendly synthesis

strategy for offretite directed by a mixture of alkali metal cations as SDAs. The synthesis conditions have been systematically varied by adjusting parameters such as the crystallization temperature, aging time, composition, Al source, and water content. The goal of this approach is to investigate the effects of the synthesis parameters on the OSDA-free preparation of offretite in terms of phase purity, particle size, and morphology, and to elucidate the relationship between the zeolite properties and their suitability for CO₂ adsorption.

## 2. Experimental section

### 2.1. Chemicals

All reagents were used as received unless explicitly specified otherwise. Colloidal silica (LUDOX HS-30); aluminum powder (Alfa Aesar, 99.5 %); aluminum hydroxide (Alfa Aesar, 76.5 %); sodium aluminate (Sigma Aldrich, 40–45 % Na₂O, 50–56 % Al₂O₃); aluminum isopropoxide (Sigma Aldrich, 98 %); sodium hydroxide (Sigma Aldrich, 99 %); potassium hydroxide (Sigma Aldrich, 90 %); cesium hydroxide hydrate (Alfa Aesar, 15–20 % H₂O); ultrapure Milli-Q water was used for all synthesis steps.

### 2.2. OSDA-free synthesis of offretite (OFF) zeolite using alkali metal cations

OSDA-free synthesis of offretite was carried out under hydrothermal conditions with various colloidal composition (Table 1). In contrast to using typical TMA⁺ as the OSDA reported previously [26], in our presented system Na⁺, K⁺, and Cs⁺ (if required) cations were employed as the inorganic SDA for offretite synthesis. Typically, the aluminum source was dissolved in the potassium hydroxide solution denoted as mixture A. Depending on the Al source employed, the mixture could be transparent solution (aluminum powder, aluminum isopropoxide or sodium aluminate) or milky suspension (Al(OH)₃). Then the colloidal silica (HS-30) was added dropwise into another alkaline solution mixed with sodium hydroxide and cesium hydroxide (if required) to get a milky gel followed by heating at 90 °C for 10 min till a clear colloid suspension B was achieved. When the suspension B cooled to room temperature, mixture A was slowly added dropwise under a vigorous stirring in an ice bath condition. Afterwards, the resulting suspension was aged under stirring for certain days at room temperature (details in Table 1). The viscosity of the suspension would increase gradually during the first day of aging, and then keep stable (Fig. S1). The fully aged precursor was then subjected to hydrothermal (HT) treatment at 90–130 °C for certain amount of time to allow the growth of offretite crystals. The solid products were recovered by a high-speed centrifugation at 20000 rpm for 10 min and washed with the double distilled water until the pH of the supernatant was neutral. Finally, the obtained samples were dried overnight at 90 °C.

### 2.3. Characterization

X-ray powder diffraction (XRPD) patterns were measured with a PANalytical X'Pert Pro diffractometer using Cu Kα₁ radiation (λ = 1.5406 Å, 45 kV, 40 mA). The patterns were collected between 5 and 50° 2θ with a step size of ~0.0167° and time per step of 200 s. Scanning electron microscopy (SEM) was performed using a Tescan Mira I LMH operating at 30 kV and JEOL JSM-IT800 Schottky field emission scanning electron at 0.8 kV and 10 nA. The magic-angle spinning nuclear magnetic resonance (MAS NMR) spectra for ²⁹Si and ²⁷Al nuclei were recorded at 99.3 and 130.3 MHz, respectively, on a Bruker Avance 500 MHz (11.7 T) spectrometer using 4 mm OD zirconia rotor with a spinning frequency of 12 kHz. The chemical shifts are referenced to tetramethylsilane (TMS) and AlCl₃. Radiofrequency (rf) field strength of 36 and 50 kHz and recycle delays of 20 and 1 s were used, respectively. 10240 and 725 scans were collected for ²⁹Si and ²⁷Al nuclei,
<table><caption>**Table 1**

Conditions for the synthesis of zeolite samples.</caption><thead><tr><th>Sample</th><th>Aging time(d)</th><th>Temperature(°C)</th><th>Time(d)</th><th>n(K<sub>2</sub>O)</th><th>n(Na<sub>2</sub>O)</th><th>n(Cs<sub>2</sub>O)</th><th>SiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>Aluminium Source</th><th>n(H<sub>2</sub>O)</th><th>Product Phase<sup>a</sup></th></tr></thead><tbody><tr><td>S1</td><td>1</td><td>130</td><td>7</td><td>1.7</td><td>4</td><td>-</td><td>18</td><td>1</td><td>Al(OH)<sub>3</sub></td><td>280</td><td>OFF + PHI</td></tr><tr><td>S2</td><td>1</td><td>100</td><td>28</td><td>1.7</td><td>4</td><td>-</td><td>18</td><td>1</td><td>Al(OH)<sub>3</sub></td><td>280</td><td>OFF + PHI</td></tr><tr><td>S3</td><td>1</td><td>90</td><td>42</td><td>1.7</td><td>4</td><td>-</td><td>18</td><td>1</td><td>Al(OH)<sub>3</sub></td><td>280</td><td>OFF + PHI</td></tr><tr><td>S4</td><td>1</td><td>130</td><td>5</td><td>1.9</td><td>4</td><td>-</td><td>18</td><td>1</td><td>Al(OH)<sub>3</sub></td><td>280</td><td>OFF + LTL + PHI</td></tr><tr><td>S5</td><td>1</td><td>130</td><td>5</td><td>2.1</td><td>4</td><td>-</td><td>18</td><td>1</td><td>Al(OH)<sub>3</sub></td><td>280</td><td>OFF + LTL + PHI</td></tr><tr><td>S6</td><td>1</td><td>130</td><td>7</td><td>1.9</td><td>4</td><td>0.25</td><td>18</td><td>1</td><td>Al(OH)<sub>3</sub></td><td>280</td><td>OFF</td></tr><tr><td>S7</td><td>1</td><td>130</td><td>5</td><td>1.9</td><td>4</td><td>0.375</td><td>18</td><td>1</td><td>Al(OH)<sub>3</sub></td><td>280</td><td>OFF + ANA</td></tr><tr><td>S8</td><td>4</td><td>130</td><td>4</td><td>1.9</td><td>4</td><td>0.25</td><td>18</td><td>1</td><td>Al(OH)<sub>3</sub></td><td>280</td><td>OFF</td></tr><tr><td>S9</td><td>7</td><td>130</td><td>3</td><td>1.9</td><td>4</td><td>0.25</td><td>18</td><td>1</td><td>Al(OH)<sub>3</sub></td><td>280</td><td>OFF</td></tr><tr><td>S10</td><td>7</td><td>130</td><td>3</td><td>1.9</td><td>4</td><td>0.25</td><td>18</td><td>0.82</td><td>Al(OH)<sub>3</sub></td><td>280</td><td>OFF</td></tr><tr><td>S11</td><td>7</td><td>90</td><td>8</td><td>1.9</td><td>4</td><td>0.25</td><td>18</td><td>0.82</td><td>Al(OH)<sub>3</sub></td><td>280</td><td>Am.+OFF</td></tr><tr><td>S12</td><td>21</td><td>90</td><td>8</td><td>1.9</td><td>4</td><td>0.25</td><td>18</td><td>0.82</td><td>Al(OH)<sub>3</sub></td><td>280</td><td>OFF + Am.</td></tr><tr><td>S13</td><td>51</td><td>90</td><td>8</td><td>1.9</td><td>4</td><td>0.25</td><td>18</td><td>0.82</td><td>Al(OH)<sub>3</sub></td><td>280</td><td>OFF + Am.</td></tr><tr><td>S14</td><td>21</td><td>90</td><td>8</td><td>1.9</td><td>4</td><td>0.25</td><td>18</td><td>0.82</td><td>Al(-CH(CH<sub>3</sub>)<sub>2</sub>)<sub>3</sub></td><td>280</td><td>OFF + unknown</td></tr><tr><td>S15</td><td>21</td><td>90</td><td>8</td><td>1.9</td><td>4</td><td>0.25</td><td>18</td><td>0.82</td><td>NaAlO<sub>2</sub></td><td>280</td><td>CHA + LTL</td></tr><tr><td>S16</td><td>21</td><td>90</td><td>6</td><td>1.9</td><td>4</td><td>0.25</td><td>18</td><td>0.82</td><td>Al powder</td><td>280</td><td>OFF</td></tr><tr><td>S17</td><td>21</td><td>90</td><td>6</td><td>1.9</td><td>4</td><td>0.25</td><td>18</td><td>0.82</td><td>Al powder</td><td>210</td><td>OFF + LTL</td></tr><tr><td>S18</td><td>21</td><td>90</td><td>6</td><td>1.9</td><td>4</td><td>0.25</td><td>18</td><td>0.82</td><td>Al powder</td><td>140</td><td>LTL + OFF</td></tr></tbody></table>

respectively. Inductively-coupled plasma mass spectrometry (ICP-MS) measurements were performed on a 7900 ICP-MS from Agilent Technologies. N₂ adsorption/desorption isotherms were measured at –196 °C using a Micromeritics 3Flex Surface characterization unit. CO₂ adsorption isotherm was measured at 25 °C using a Micromeritics ASAP 2020 instrument. The isosteric heat of adsorption was calculated using

ASAP 2020 plus software from Micromeritics. Samples for all adsorption measurements were pretreated with degassing at 350 °C under high vacuum for 12 h. Thermogravimetric analysis was performed on a NETZSCH STA 449F3 analyzer. Samples were heated from 35 to 800 °C with a heating ramp of 5 °C/min under a flow of air of 40 mL/min.

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** XRPD patterns of samples prepared from precursor mixtures with a molar oxide ratio of 18 SiO₂:1 Al₂O₃:1.7 K₂O: 4 Na₂O: 280 H₂O, with hydrothermal treatment carried out at different temperatures: 130 °C (S1); 100 °C (S2); and 90 °C (S3). P identifies peaks associated with the PHI structure.</figcaption></figure>

<footnote>a Framework Type Code and sample abbreviations: OFF = Offretite; PHI = Phillipsite; LTL = Linde Type L; ANA = Analcime; CHA = Chabazite; Am. = amorphous. In samples containing more than one identifiable phase, the major component is listed first.</footnote>
### 3. Results and discussion

Pure offretite (OFF) zeolite was synthesized using only alkali metal cations (Na⁺, K⁺, and Cs⁺) as SDAs by screening several synthesis parameters. Table 1 presents all the samples prepared during the optimization of the synthesis. The varied parameters were: aging time of the precursor mixture at room temperature (1–51 days), hydrothermal treatment time (3–42 days) and temperature (90–130 °C), Si/Al ratio in the initial precursor mixture (9–11), aluminum source (Al(OH)₃, Al (OC₃H₇)₃, NaAlO₂, or Al powder), the concentration and type of alkali metal cations, and the amount of water in the initial precursor mixture (Table 1).

#### 3.1. *The effect of synthesis temperature and time*

According to the OSDA-free synthesis protocol of OFF/ERI from Cichoki et al., an initial precursor mixture with a chemical composition of 18 SiO₂:1 Al₂O₃:1.7 K₂O: 4 Na₂O: 280 H₂O was chosen as the starting point to explore the OSDA-free synthesis of offretite [44]. Initially, the synthesis temperature (90–130 °C) was varied while simultaneously varying the HT time (7–42 days) to ensure the formation of crystalline material (samples S1–S3); Fig. 1, presents the XRPD patterns of the obtained products. In Fig. 1 the diffraction pattern of the OFF framework structure is observed for all samples (major diffraction peaks at 2θ = 7.68°, 13.34°, 20.47°, and 23.65°) [30]. However, all samples also possessed the PHI-type zeolite as an impurity. By decreasing the synthesis temperature from 130 °C (sample S1) to 100 °C and 90 °C

(samples S2 and S3, respectively), the amount of the PHI phase increased. Moreover, the reduction of the synthesis temperature gave rise to a significant increase in the HT time up to 42 days at 90 °C, which is undesirable at an industrial scale. As a result, the synthetic protocol at 130 °C (sample S1, Table 1) was selected for further optimization.

#### 3.2. *The effect of the K⁺ content in the precursor mixture*

As mentioned earlier, K⁺ cations have been identified as associated with the formation of CAN cages; specifically in the synthesis of LTL [23]. The effect of K⁺ cation content in the precursor mixture was varied (samples S4 and S5) and the XRD patterns of the solid products obtained after HT are presented in Fig. 2. The intensification of the offretite peaks in the XRPD pattern is attributed to the increased concentration of K⁺ cations in the starting precursor mixture, accelerating the growth of the crystalline domains, and reducing the HT time to 5 days. However, the appearance of Linde Type L (LTL framework structure), in addition to phillipsite, was observed when the molar ratio of K₂O was increased from 1.7 (sample S1) to 1.9 (sample S4). A further increase in the molar ratio of K₂O to 2.1 (sample S5) resulted in an increased amount of the denser phillipsite. Regarding the similar CBU composition of the LTL and OFF structures, it was speculated that Na⁺ is not efficient in supporting the large *gme* cages, therefore the LTL structure, without *gme* units, was formed in a minor amount.

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** XRPD patterns of samples prepared from precursor mixtures with a molar oxide ratio of 18 SiO₂:1 Al₂O₃: x K₂O: 4 Na₂O: 280 H₂O, where x is set to 1.7 (S1), 1.9 (S4) and 2.1 (S5). P and L identify peaks associated with the PHI and LTL structures, respectively.</figcaption></figure>
### 3.3. The effect of $Cs^+$ content in the precursor mixture

To leverage the structure directing effect of different inorganic cations, $Cs^+$ was introduced to the synthesis precursor mixture. The addition of $Cs^+$, a large alkali metal cation, is associated with the formation of zeolite topologies possessing 8MRs, such as ANA, RHO and CHA [10, 11, 23, 32, 36, 45]. In addition, using combined 3D electron diffraction and DFT modelling, $Cs^+$ cations were identified in the 8MR of the *afs* CBU which forms part of the BPH structure [37]. The $Cs^+$ cation possesses a hydrated ionic radius (~3.3 Å) and is frequently observed to occupy double (*d8r*) or single eight ring (*s8r*) structures in RHO depending on its hydration sphere. Recently, Yamada et al. employed atom-selective analyses to reveal how $Cs^+$ cations facilitate the formation of a crystalline-like coordination environment, attributed to the *d8r* structure, within the amorphous precursor mixture before the establishment of long-range crystalline order [46]. In comparison, due to its smaller size, $Na^+$ cations prefer to occupy positions near the *s6rs*, *s8rs* or within the *d8rs* depending on the hydration sphere, zeolite cation composition, and Si/Al ratio [11, 47, 48]. This is in line with DFT calculations of the cation sitings within offretite zeolite, whereby the most stable site for the $Na^+$ cations was found to be at the six-membered ring of the *gme* CBU [49]. From the considerations described above we may hypothesise that $Cs^+$ cations may play a role in facilitating the formation of the *s8r* structure which forms part of the *gme* cage, however, the precise mechanism was not elucidated. The XRPD patterns of the solid products obtained from the HT of the precursor mixtures containing $Cs^+$ cations are shown in Fig. 3. Continuing with the molar oxide ratio of

sample S4, two different amounts of $Cs^+$ were introduced into the precursor mixture; the HT time was increased from 5 to 7 days for the precursor mixture with a lower amount of $Cs^+$ cations present. Interestingly, with the introduction of a small amount of $Cs^+$, a phase-pure and fully crystalline offretite zeolite was achieved (sample S6). By adding more $Cs^+$ (sample S7), we observed that the HT time could be reduced to 5 days (Table 1); a minor amount of ANA-type zeolite appeared.

### 3.4. The effect of the aging time of the precursor mixture

The aging time of the precursor mixture, typically performed at room temperature, has a significant influence on the outcome of a zeolite synthesis, particularly with respect to particle size and morphology [36]. The desirable amount of time depends heavily upon the type of the synthesis reagents, their addition order, and physical treatment, such as rapid stirring. This affects the structure and availability of precursor species [50,51]. Depending on the particular composition and aging time of a precursor mixture, zeolite formation may progress entirely from nucleation to crystallization at room temperature [34]. Following the optimization of the $Cs^+$ content, the conditions used to synthesize sample S6 were employed for optimizing the aging time to explore the tunability of the offretite crystal morphology, and the HT time required to achieve a crystalline product. Two samples were prepared: samples S8 and S9 where the aging time was set to 4 and 7 days, respectively (Table 1). The XRPD patterns of the solid products obtained after HT treatment are presented in Fig. 4a. When the aging time was set to 4 days

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** XRPD patterns of samples prepared from precursor mixtures with a molar oxide ratio of 18 $SiO_2$:1 $Al_2O_3$: 1.9 $K_2O$: 4 $Na_2O$: y $Cs_2O$: 280 $H_2O$, where y is set to 0 (S4), 0.25 (S6) and 0.375 (S7). P, A and L identify peaks associated with the PHI, ANA and LTL structures, respectively.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** (a) XRPD patterns of samples prepared from precursor mixtures with different aging times of 1 day (S6), 4 days (S8), and 7 days (S9), followed by HT at 130 °C for 7 days (S6), 4 days (S8), and 3 days (S9). SEM images of samples S8 (b) and S9 (c).</figcaption></figure>

(sample S8), the HT time required to achieve a crystalline product decreased significantly from 7 to 4 days at 130 °C, compared to sample S6 (Table 1). Increasing the aging time to 7 days (sample S9) decreased the required HT time at 130 °C to 3 days (Table 1), improving the energy efficiency of the synthesis. The reduction in the required amount of HT time is consistent with the observations concerning the preparation of nanosized CHA, where increasing the aging time from 4 to 17 days increased the rate of crystallization and reducing the HT time from 7 to 3 h at 90 °C [36]. SEM analysis reveals that both samples S8 and S9 have hexagonal-prism shaped crystals of approximately 15 µm in length and 2 µm in width (Fig. 4b and c).

### 3.5. The effect of the aluminium content in the precursor mixture

The effect of the aluminium content of the offretite precursor mixture was investigated; the Si/Al ratio of the precursor mixture was increased from 9 to 11, corresponding to samples S9 and S10, respectively (Table 1). The XRPD patterns of the solid products corresponding to samples S9 and S10 are presented in Fig. 5a. XRPD analysis reveals that both samples are pure offretite, while SEM analysis (Fig. 5b and c) shows that both samples exhibit similar particle lengths (15 µm vs. 10 µm) and width (2 µm vs. 1.5 µm). This indicates that the Al content of the precursor mixture does not significantly influence the particle size.

### 3.6. The effect of aging time and synthesis temperature on the offretite crystal size

Decreasing the discrete crystal size of zeolites has been found to be an effective strategy to improve their CO₂ separation performance [17]. Previous studies have shown that an effective approach to synthesize

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** (a) XRPD patterns of samples prepared from precursor mixtures with a molar oxide ratio of 18 SiO₂:x Al₂O₃:1.9 K₂O: 4 Na₂O:0.25 Cs₂O:280 H₂O, where x was set to 1 or 0.82 to give a Si/Al ratio of 9 (S9) or 11 (S10) of the precursor mixture. SEM images for samples S9 (b) and S10 (c).</figcaption></figure>

smaller zeolite crystals is to prolong the aging time while decreasing the synthesis temperature [36]. To integrate this concept into the current strategy, the synthesis temperature of 90 °C was selected to slow down the crystallization kinetics, and the aging time of the precursor mixture was set to 7, 21, and 51 days (samples S11, S12, and S13 respectively, Table 1). The XRPD patterns of the solid products obtained after HT for S11, S12, and S13 are presented in Fig. 6a. As shown in Fig. 6, the reduction in the synthesis temperature from 130 °C to 90 °C significantly impacts the rate of crystallization. According to sample S3, around six weeks was required to obtain fully crystalline offretite at 90 °C if only 1 day of aging was employed (Fig. 1). When prolonging the aging time to 7 days (sample S11), the synthesis yielded mostly amorphous material with the appearance of weak reflections corresponding to the OFF structure. Small quantities of crystalline zeolite were visible by SEM (Fig. 6b), which is consistent with the XRPD data (Fig. 6a). Increasing the aging time to 21 days (sample S12) resulted in the formation of offretite zeolite with crystals of approximately 2 µm in length and 400 nm in width (Fig. 6c). However, unreacted amorphous material was observed in the final product (Fig. 6c). By increasing the aging time further to 51 days, more crystalline offretite (sample S13, Fig. 6a) with small prism like crystals was obtained (Fig. 6d), however a minor amount of amorphous material was still present in the sample.

### 3.7. The effect of the aluminium source

Considering the energy and time efficiency of the synthesis, a long aging time is undesirable. Liu et al. proposed that different aluminium (Al) sources can significantly alter the zeolite crystallization kinetics; the synthesis of LTA using aluminium sulfate was observed to occur much faster than using aluminium chloride as the Al source [52,53]. Here, the
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** (a) XRPD patterns of samples obtained from precursor mixtures with different aging times of 7 days (S11), 21 days (S12), and 51 days (S13), followed by HT treatment at 90 °C for 8 days. SEM images of sample S11 (b), S12 (c) and S13 (d), orange circles highlight typical amorphous particles. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

effect of four different Al sources in the precursor mixture including Al (OH)₃, Al(OPri)₃, NaAlO₂, and Al powder corresponding to samples S12, S14, S15, and S16, respectively were compared (Table 1). The synthesis parameters of the samples with different Al sources were derived from sample S12. From XRPD analysis of the solid products, the use of certain Al sources in the precursor mixture can result in the formation of different zeolite structures. The replacement of Al(OH)₃ with Al(–CH(CH₃)₂)₃ yields the crystallization of offretite as well as the appearance of a diffraction peak ascribed to an unknown phase at 2θ = 18°. The use of NaAlO₂ as the Al source results in the crystallization of CHA and a minor amount of LTL, while the use of Al powder affords pure offretite with a shortened synthesis time of 6 days due to the faster crystallization kinetics (Fig. 7a). As a result, when Al powder was used, offretite zeolite (approximately 2 µm in length and up to 400 nm in width) was obtained (Fig. 7e).

## 3.8. The effect of water content

Generally, water functions as an ion and mass transporter in the crystallization system [54]. Changes to the water content alters the alkalinity level of the precursor mixture which can lead to the formation of different phases, such as LTL or CHA [30,44,55]. While a higher level of alkalinity in the synthetic mixture increases the probability of crystallizing multiple zeolite structural phases, it can also result in the formation of smaller discrete crystals because of the equilibrium shift

between condensation and dissolution of Si species forced by high alkalinity during the induction period [56,57]. Using the optimised synthesis conditions of sample S16, the water amount was carefully reduced to 210 and 140 after the initial precursor suspension gel have been rapidly stirred for 30 min, corresponding to samples S17 and S18 respectively (Fig. 8). XRPD analysis of the solid products obtained from S17 and S18 revealed that offretite was the only phase formed with a H₂O molar ratio from 210 to 280 (n(H₂O)/n(SiO₂) = 11.2–15.5). However, when decreasing the H₂O molar ratio to 140 (n(H₂O)/n(SiO₂) = 7.8), LTL zeolite became the dominant product, with a minor amount of offretite present. This likely resulted from the high alkalinity promoting the growth of LTL, which is in line with previous reports [30,58].

## 3.9. Physiochemical properties of offretite samples

Based on the broad study of the different parameters affecting the synthesis of offretite (vide supra), two samples were selected for further study: S10 and S16 which correspond to offretite crystals of 15 µm length (renamed OFF-15000) and 2 µm length (renamed OFF-2000), respectively (Fig. S2). The reproducibility of the optimised synthesis of OSDA-free offretite was demonstrated for sample S16 (Fig. S3). The chemical composition of the samples OFF-15000 (S10) and OFF-2000 (S16) were determined by ICP-MS, and the results are shown in Table S1. Both samples present near identical chemical compositions, with slight differences in the Na⁺ and K⁺ extra-framework cation
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** (a) XRPD patterns of samples prepared from precursor mixtures with a molar oxide ratio of (18 SiO₂:0.82 Al₂O₃:1.9 K₂O: 4 Na₂O:0.25 Cs₂O:280 H₂O) with different aluminium sources: Al(OH)₃ (S12); Al(–CH(CH₃)₂)₃ (S14); NaAlO₂ (S15); Al powder (S16). SEM images for S12 (b), S14 (c), S15 (d) and S16 (e). U and L identify peaks associated with the unknown and LTL structures, respectively.</figcaption></figure>

content and a Si/Al ratio of 3.5. Notably, this is similar to, or higher than, the Si/Al ratio of offretite obtained when using OSDAs such as TMA⁺ [29,42]. The water content of the offretite zeolite samples was evaluated by thermogravimetric (TG) analysis (Fig. 9a). The majority of the mass loss occurs between 35 and 250 °C attributed to the release of water adsorbed on the external surface and within the zeolite microporosity. A total mass loss at 300 °C was 11.7 % and 12.2 % for OFF-15000 (S10) and OFF-2000 (S16), respectively. A slightly higher loss between 350 °C and 800 °C was found for OFF-2000 (0.6 %) than for OFF-15000 (0.1 %), attributable to the condensation of silanols [59]. The porosity of OFF-15000 (S10) and OFF-2000 (S16) was analysed by measuring the N₂ sorption isotherms at –196 °C. Both samples exhibit dramatically low uptake in the low-pressure region (P/P₀ < 0.02) indicative of restricted access of N₂ to the microporosity of the zeolite, and consequently a low micropore volume. The different uptake in the low-pressure region may be due to the trace amount of LTL phase present in the sample, or the effect of silanols on the cation motions. The isotherm of OFF-15000 plateaued immediately after P/P₀ = 0.02 (see Fig. 9b) ascribed to the low external surface area to volume ratio of the

large crystals (15 µm). In comparison, for OFF-2000 with 2 µm long crystals, the isotherm exhibits an increasing uptake from 0.02<P/P₀ < 0.85 due to multi-layer formation of nitrogen. Above P/P₀>0.85, the N₂ adsorption isotherm of OFF-2000 exhibits increased uptake due to interparticle mesoporosity.

Fig. 10 presents the ²⁹Si and ²⁷Al MAS NMR spectra of samples OFF-15000 (S10) and OFF-2000 (S16). The offretite structure contains two distinct T-sites, therefore, two peaks in ²⁹Si NMR spectrum depending on the surrounding Al atoms are present. As shown in Fig. 10a and c, six peaks are observed in the ²⁹Si MAS NMR spectra; the peaks at 90, 93, 98, 103, 108 and 113 ppm are assigned to (Si[AlO]₄), (Si[SiO][AlO]₃), (Si[SiO]₂[AlO]₂), (Si[SiO]₃[AlO]₁), (Si[SiO]₄, and (Si[SiO]₄ at different T sites) species, respectively, according to previous work [60]. The similar Si distribution observed for the two samples is consistent with their similar extra-framework cation and framework composition based on ICP-MS (Table S1). The Si/Al ratios were calculated as 3.7 (OFF-15000) and 3.6 (OFF-2000) respectively based on the deconvolution of the ²⁹Si spectra (Fig. 10a–c; Table S2), which is close to the ratio of 3.5 for both samples determined by ICP-MS analysis (Table S1). The ²⁷Al spectra display a single peak at approximately 59 ppm for both samples, indicating that all of the Al atoms are tetrahedrally coordinated in the zeolite framework and no octahedrally coordinated Al is present [36,61,62].

### 3.10. CO₂ adsorption on offretite samples

The two offretite samples, OFF-15000 and OFF-2000, with similar chemical composition but different crystal sizes, were investigated for CO₂ adsorption. The CO₂ adsorption isotherms measured at 0, 25 and 40 °C (Fig. 11) show that both offretite samples exhibit similar CO₂ adsorption profiles and capacity due to their near identical chemical compositions. The equilibrium CO₂ adsorption capacities at 95 kPa for OFF-2000 decreased with increasing the temperature: 0 °C (4.1 mmol g⁻¹), 25 °C (3.4 mmol g⁻¹), 40 °C (3.0 mmol g⁻¹), that is equivalent to 5.52, 4.58 and 4.04 CO₂ molecules per unit cell, respectively [32]. In comparison to other zeolites with similar Si/Al ratios or cation compositions considered for CO₂ adsorption, the OSDA-free offretite exhibits comparable capacity to Na,Cs-RHO (Si/Al = 4) and nano-sized mixed-cation (Na⁺, K⁺, Cs⁺) CHA [15,63,64], but lower than that of conventional Na-X and Na-A. Using the isotherms recorded at different temperatures, the CO₂ isosteric enthalpies of adsorption were calculated using the Clausius–Clapeyron equation (Fig. 11b). Both samples exhibit similar profiles with increasing enthalpies indicating the similar distributions of adsorption sites. The isosteric enthalpy of adsorption values vary from 45 kJ mol⁻¹ (at 0.5 mmol/g CO₂ loading) to 55 kJ mol⁻¹ (at 3.0 mmol/g CO₂ loading), indicative of a slightly stronger interaction between CO₂ molecules and the adsorption sites compared with nano-sized mixed-cation CHA and Na-X [15,65]. Boer et al. suggested that an enthalpy range of 25–50 kJ mol⁻¹ is favorable for physical adsorbents and higher enthalpy usually corresponds to higher selectivity towards a particular guest molecule in competitive adsorption [66], suggesting that the optimised offretite offers potential as a solid adsorbent for selective CO₂ adsorption.

Overall, it has been shown that Cs⁺ cations are capable of replacing OSDAs typically used in the synthesis of offretite. This was achieved after finely tuning the cationic composition in initial gel to afford fully crystalline offretite while avoiding the formation of other crystalline phases. With further optimization, the offretite crystal sizes were successfully controlled without any further additives. The samples with different crystal sizes show comparable properties and performance for CO₂ adsorption. This work demonstrates a new environmentally friendly synthesis and broadens the scope of synthesis strategies for preparing offretite, which is a promising material for gas separation.

## 4. Conclusions

A novel OSDA-free synthesis strategy for offretite has been
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** XRD patterns of samples prepared from precursor mixtures with a molar oxide ratio of (18 SiO₂:0.82 Al₂O₃:1.9 K₂O: 4 Na₂O:0.25 Cs₂O: x H₂O) with different water: 280 H₂O (S16); 210 H₂O (S17); 140 H₂O (S18).</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** (a) TG (solid line) and dTG (dash line) curves from 35 °C to 800 °C, and (b) N₂ adsorption/desorption isotherms of samples OFF-15000 (S10) in black and OFF-2000 (S16) in red measured at -196 °C (filled and empty squares represent the adsorption and desorption isotherms respectively). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

Instead of using organic molecules in the conventional synthesis of offretite, a mixture of inorganic alkali metal cations as structure-directing agents was successfully used for the first time to obtain OSDA-free offretite. A systematic investigation of the synthesis parameters (hydrothermal treatment temperature, composition of mixed cations, SiO₂/Al₂O₃ ratio, aging time, aluminum source and water content) has been carefully carried out to obtain the offretite phase in high purity. By employing different characterization techniques (XRPD, SEM, TG, NMR, ICP-MS and physical adsorption), the effects of changing the synthesis parameters on the synthesis of offretite have been revealed. The presence of a minor amount of different structural phases may compete with offretite during the crystallization stage, such

as PHI and LTL. We propose that the inclusion of Cs⁺ cations in the high-SiO₂ synthesis mixture, in combination with K⁺ and Na⁺ cations, facilitates the formation of gme cages, and helps to avoid the formation of the PHI and LTL phases. In combination with the factors of aging and HT treatment, fully crystalline offretite with sizes between 15 µm and 2 µm and a Si/Al ratio of 3.5 has been prepared. These optimizations offer a new perspective on the capability of specific mixtures of alkali metal cations, combined with different Al sources, on their directing effect for the formation of specific zeolite structures. The OSDA-free offretite with 2 µm long crystals exhibited a high CO₂ adsorption capacity at 95 kPa and moderate isosteric enthalpy of adsorption. These results show that offretite has great potential as a solid adsorbent for separation
<figure><img src="image_13.png" /><figcaption>**Fig. 10.** $^{29}$Si MAS NMR spectra of (a) OFF-15000 (S10) and (c) OFF-2000 (S16), and $^{27}$Al MAS NMR spectra of OFF-15000 (b) and OFF-2000 (d). Black and red spectra represent experimental and simulated spectra respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 11.** (a) CO₂ adsorption isotherms of sample OFF-15000 (S10) and OFF-2000 (S16) measured at 0 °C (black), 25 °C (blue) and 40 °C (magenta); (b) CO₂ isosteric enthalpy of adsorption of OFF-15000 and OFF-2000. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>
applications, and the OSDA-free synthesis strategy has been fully proved as an easy, economic, environmentally friendly and feasible way to prepare offretite zeolite.

### CRediT authorship contribution statement

**Risheng Wang:** Writing – original draft, Methodology, Investigation, Formal analysis, Data curation. **Qiudi Yue:** Writing – review & editing, Validation, Methodology. **Sajjad Ghojavand:** Writing – review & editing, Validation, Formal analysis. **Edwin B. Clatworthy:** Writing – review & editing, Methodology. **Diógenes Honorato Piva:** Writing – review & editing, Formal analysis. **Marie Desmurs:** Formal analysis, Data curation. **Valérie Ruaux:** Formal analysis, Data curation. **Svetlana Mintova:** Supervision, Resources, Project administration, Funding acquisition, Conceptualization.

### Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Svetlana Mintova reports financial support was provided by European Research Council. Svetlana Mintova reports a relationship with CNRS Normandy Delegation that includes: funding grants. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgement

The support of the Centre for Zeolites and Nanoporous Materials, Label of Excellence, Normandy Region (CLEAR), and the European Union (ERC, ZEOLiGhT, 101054004) is acknowledged. However, the views and opinions expressed are solely those of the author(s) and do not necessarily reflect those of the European Union or the European Research Council. Neither the European Union nor the granting authority can be held responsible for them.

### Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.micromeso.2025.113795.

### Data availability

No data was used for the research described in the article.

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