Microwave-Assisted Interzeolite Transformations

1. Introduction

Zeolites are hydrated aluminosilicates whose active synthesis began in the 1950s, driven by their commercialization in separation and purification processes [1]. Initially, the zeolite production has been based on the hydrothermal conversion of aluminosilicate gels. Over time, this method proved effective not only for zeolites but also for a variety of chemically different zeolite-like materials [2,3] and for many years, gel-based synthesis remained the dominant method. However, in the pursuit of faster, more sustainable, and better-controlled synthesis, an alternative technique - interzeolite transformation (IZT) has reemerged, building on the early works of Barrer [4]. The IZT method involves the hydrothermal [5] or room temperature [6] transformation of a pre-existing (“parent”) zeolite into a structurally different "daughter" zeolite. This approach has gained significant attention due to its simplicity and potential advantages, including faster crystallization, the ability to produce frameworks with a high Si/Al ratio and advanced pore chemistry, the creation of hierarchical structures, and the ability to achieve low synthesis temperatures that are often difficult to reach using conventional gel-based zeolite synthesis [6,7,8,9,10,11]. It is important to note that IZT should not be confused with the polymorphic transformations that occur during zeolite crystallization from a gel medium, where no parent phase is involved. Among the parent phases, FAU-type zeolite is one of the most studied due to its wide availability, cost-effective synthesis, and low-density framework, which allows transformations into a range of lower-density zeolites [12]. One such transformation is the conversion of FAU to MER-type zeolite, reported by Kirschhock and colleagues in 2013 [13]. They used an Rb- and Na-containing hydroxide solution and FAU zeolite with a Si/Al ratio of 2.6, obtaining MER zeolite after 96 hours at 95 ^oC. Similarly, Chengyu et al. later reported the synthesis of MER zeolite by transforming NaY and HY-FAU (Si/Al = 2.4-2.6) zeolites in a K-containing solution at 100–150 ^oC for 96 hours [14]. MER zeolite was also obtained under dry conditions by the mechanochemical treatment of FAU zeolite (Si/Al = 2.4) with KOH at 110 ^oC for 120 min [15]. Additionally, the conversion of clinoptilolite-rich natural zeolite to MER has been reported [16].

EDI zeolite, another potassium-containing zeolite, was recently synthesized by IZT of FAU in concentrated KOH solutions, either at room temperature for 11–35 days or at 60 ^oC for 6–27 hours [6]. The IZT resulting in ANA-type structures was reported in 1999 by Chiyoda and Davis, who transformed NaY (Si/Al = 2.0–3.0) into ANA zeolite using sodium-containing solutions [17]. In 2010, Wang et al. synthesized ANA zeolite crystals with a regular icositetrahedron morphology by transforming ultrastable zeolite Y (Si/Al = 6.7) in NaOH solution at 100 ^oC for 42–288 hours [18]. In 2013, Kirschhock and colleagues also synthesized synthetic pollucite (Cs-ANA) by IZT of FAU zeolite (Si/Al = 2.6) in CsOH solution at 95 ^oC for 48 hours [13] More recently, ANA zeolite was synthesized through mechanochemically-assisted transformation of commercial FAU (Si/Al = 2.4) using CsOH or NaOH at 110°C for 120 min [15]. CAN zeolite was also synthesized under similar conditions using KOH [15]. Notably, the mechanochemically-assisted transformations of FAU are faster than the conventional synthesis of MER and ANA, which typically take 6 hours [19] and 5–10 hours [20], respectively.

While various types of radiation, such as UV light [21,22], gamma rays [23], and microwaves [24], have been used to control and accelerate zeolite crystallization from gel media, similar methods have not yet been applied to IZT-based synthesis. Microwave-assisted synthesis has been effective in accelerating conventional zeolite synthesis, including MER (180 ^oC for 12 min) [25], ANA (120 ^oC for 5h) [26], and Li-EDI (90 ^oC for 60 min) [27], among many others. However, this approach remains unexplored for IZT-based zeolite synthesis.

This work aims to introduce the first examples of microwave-assisted interzeolite transformation and show how this method can significantly accelerate crystallization. It also explores its potential for controlling zeolite crystal morphology, particle size, and crystal structure.

2. Results and Discussion

Four single-phase zeolite-type structures (EDI, MER, CAN, and ANA) were obtained by the transformation of FAU zeolite through hydrothermal treatment under microwave radiation. Table 1 compares the fastest reported IZTs leading to EDI, MER, CAN, and ANA zeolites and the same phases obtained in this work. The microwave-assisted synthesis at higher temperatures assures at least 20 times faster crystallization than the mechanochemically-assisted and at least 70 times faster than the conventional heating approach. Additionally, a non-zeolitic aluminosilicate with a Kalsilite (KAlSiO₄) [28] structure was obtained.

EDI-type zeolite was obtained for 5 min (160 ^oC), 10 min (160 ^oC), 1h (80 ^oC), 6h (60 and 80 ^oC), and 22h (60 ^oC) at two different KOH/FAU and H₂O/KOH ratios (Figure 1a). In the system where KOH/FAU= 8.94, H₂O/KOH = 0.87 at 80 ^oC and within 22h occurs the transformation FAU-EDI-Kalsilite (Figure 1b) that follows a path from a more porous to less porous structure (FAU 12MR > EDI 8MR > Kalsilite 6MR) (Figure 1c). The framework density (FD) change also follows the general rule of transformation from less to more dense structure (FD_Si: FAU 13.3 T/1000 > EDI 16.3 T/1000 > Kalsilite 19.9 T/1000 ų) indicating that the thermodynamically more stable phase has more framework atoms per unit volume. From a symmetry point of view, FAU (cubic) -EDI (tetragonal) -Kalsilite (hexagonal) transformations represent a sequence of change from higher to lower symmetry. At 60 °C, FAU-EDI transformation takes 6 h and after its formation EDI structure remains stable up to 22 hours, with the main difference being the relatively higher peak intensity when the synthesis time is longer (Figure 1d). At the same (6h) or shorter (10 min) time for synthesis but at higher temperatures (120 ^oC and 160 ^oC) the run product of FAU transformation is Kalsilite (Figure 1e). These results highlight the importance of temperature for control of FAU-EDI zeolite. The agitation speed during crystallization was found also to influence the IZT. While at 10 rpm and 60 ^oC FAU transforms to EDI, when the agitation speed is increased to 600 rpm the run product is an amorphous phase (Figure 1f). In the studied systems it was observed that at low temperatures (60-80 ^oC), a low homogenization speed (10 rpm) favors the formation of a single daughter phase while a high homogenization speed (1200 rpm) at higher temperatures (150-170 ^oC) favors the formation of a single phase, which determined the choices of homogenization speed listed in Table 2.

Figure 2a shows the time-temperature dependence of transformations of FAU zeolite in a system where KOH/FAU = 8.94 and H₂O/KOH = 0.87. In general, the higher temperature accelerates the FAU-EDI transformation and at 80 ^oC it occurs just in 1h while at 60 ^oC the same process takes 6h. At more prolongated times for crystallization (22h) the low temperature (60 ^oC) results in EDI zeolite while for the same time of crystallization at 80 ^oC is formed Kalsilite. At much high temperatures (120 ^oC, 160 ^oC) the transformation FAU-Kalsilite occurs rapidly (10-60 min), and no EDI zeolite was captured. Figure 2b shows a field of crystallization with compositions that yield a single phase EDI, MER zeolite, and Kalsilite at 160 ^oC for 10 min of synthesis. The powder XRD patterns of phases obtained after synthesis at different H₂O/KOH ratios are shown in Figure 2c. It appears that at short times for crystallization and higher temperature, the capturing of EDI zeolite requires lower KOH/FAU (2) and higher H₂O/KOH (2) ratios compared to those at the low-temperature conditions in Figure 2a. The crystallization of phase pure MER zeolite is localized at KOH/FAU = 0.7-2.0 and H₂O/KOH = 4.5. Compared to EDI zeolite it is clear that the synthesis of MER zeolite requires a higher H₂O/KOH ratio. Independent of the KOH/FAU ratio (2-9) the high temperature promotes the crystallization of Kalsilite. Figure 2b demonstrates how adjusting the H₂O/KOH ratio, while maintaining a constant KOH/FAU ratio of 2, influences the crystallization process. As the amount of KOH in the synthesis mixture decreases, the sequence of zeolite formation shifts. In particular, the sequence of formation of Kalsilite, EDI and MER follow the progressive reduction of KOH concentration, with a low amount of KOH favoring the crystallization of MER, while a high one favors the transformation into EDI zeolite and Kalsilite, respectively. When the H₂O/KOH ratio deviates from the optimal value needed for the crystallization of a single-phase zeolite, secondary phases like GIS and PHI (HAR) begin to form. This is shown in Figure 2c,d, where varying the H₂O/KOH ratio from the optimal value results in secondary phases. The appearance of these phases reveals the importance of alkalinity for selection of the daughter phase.

From thermodynamic perspective, the alkalinity of the solution directly impacts the Gibbs free energy (ΔG) of dissolution and hence the availability of silica and alumina species for zeolite formation. Therefore, the fine balance of solubility of silica and alumina controlled by the KOH concentration determines the availability of these species for formation of certain zeolite structure. The relationship between the H₂O/KOH ratio in the growth solution and the K/(Al+Si) ratio in the parent phase shows that higher KOH concentrations reduce the free energy of formation for K-rich phases, making their formation thermodynamically favorable (Figure 2e). Considering these results one can suggest that the alkalinity not only lowers the nucleation barrier but also influences which crystalline phase forms by affecting the supersaturation levels for different phases (different polymorphs or phases have distinct solubility products and free energy profiles), interfacial energy surfaces (high alkalinity can reduce the surface energies for specific phase, thus lowering the nucleation barrier compared to other phases) and finally promote kinetically favorable phases due to rapid nucleation under high supersaturation.

Compared to the same IZT using conventional heating [6], the transformation of FAU to EDI via microwaves shows different results. At low temperature (60 ^oC), the transformation time (6 hours) is similar for both microwave and conventional heating methods. However, when the temperature is increased to 80 ^oC, microwave heating drastically reduces the crystallization time to just 1 hour, whereas with conventional heating, the product remains amorphous after the same duration (Figure S2). These results suggest that above a certain temperature, microwaves play a crucial role in accelerating crystallization.

Figure 3a shows the powder XRD patterns of samples obtained from kinetic studies conducted at KOH/FAU = 2.0 and H₂O/KOH = 4.5 at 160 ^oC. The changes in the diffractograms reveal that the IZTs are highly sensitive to the crystallization time. After 1 min of synthesis, the parent FAU structure is mostly preserved, but a peak shift towards smaller 2-theta angles is observed. The refinement of the unit cell shows a lattice parameter a = 24.93 Å, which corresponds to Si/Al = 1.3 and indicates a transformation of the parent zeolite Y into zeolite X. After 5 min of synthesis, the FAU structure transforms into a mixture of MER, CHA and GME zeolites. After 10 min of heating crystallize phase pure MER zeolite, which, after 60 min, transforms into a mixture of MER and NAT zeolites (Figure 3b). These results demonstrate that the IZTs are effectively controlled by the crystallization time.

MER zeolite was also obtained after 5 min at 150 ^oC and 1h at 160°C (Figure S1). The synthesis of MER zeolite is temperature-sensitive: at 150 ^oC, 160°C, or 170 ^oC, MER crystallizes in 10 min, but when the temperature is reduced to 140 ^oC, the parent FAU zeolite is preserved but with larger unit cell.

Figure 3c shows powder XRD patterns of the phases obtained after 10 min of synthesis at different temperatures in the system KOH/FAU = 2 and H₂O/KOH = 4.5. At 130 ^oC and 140 ^oC, the parent FAU structure is maintained, but there is a shift of XRD reflections (in the 20-35° 2θ range) towards smaller angles. The refined powder XRD pattern revealed a FAU unit cell (a = 24.91 Å) that corresponds to a Si/Al ratio of 1.3. This value is slightly lower than the value determined by EDS analysis (1.7), which indicates the presence of an amorphous silica. The lowest temperature at which MER zeolite crystallizes is 150 ^oC. At 160 ^oC, MER zeolite also forms but with lower crystallinity compared to the phase obtained at 170 ^oC.

Figure 3d shows the powder XRD patterns for the transformation of FAU to ANA and FAU to CAN zeolites. The FAU-ANA IZT was achieved at CsOH/FAU = 2.7 and H₂O/CsOH = 2.59 (at 170 ^oC for 1 hour), resulting in Cs-ANA with a Si/Al ratio of 3.3. A similar FAU-ANA transformation occurred at CsOH/FAU = 1.42 (at 160 ^oC for 10 min) with H₂O/CsOH = 4.9, producing Cs-ANA with a Si/Al ratio of 2.6. The shorter synthesis time and lower temperature led to more intense XRD reflections, indicating improved crystallinity. These results demonstrate that both the framework composition and crystallinity can be effectively controlled by adjusting the synthesis conditions. CAN zeolite was synthesized at NaOH/FAU = 2 and NaOH/CsOH = 4.5 at 160 ^oC for 10 min, resulting in framework with a Si/Al ratio of 1.2.

Figure 4a shows an SEM image of the parent FAU zeolite transformed into hierarchical FAU (Figure 4a-c) and MER zeolites (Figure 4d-f). The FAU samples obtained after synthesis at 130 ^oC (Figure 4b) and 140 ^oC (Figure 4c) (with KOH/FAU = 2 and H₂O/KOH = 4.5) exhibit hierarchical structures formed by the dissolution of the parent FAU phase. This dissolution occurs in such a way that the initial shape of the FAU particles is retained, but numerous nanofins are carved into the psrent particles. Compared to previous examples of post-synthetic treatment of FAU zeolites that result in hierarchal structures [10,30,31], the microwave-assisted approach show potential for more simple (organic-free) and faster crystal hierarchization. By keeping the initial batch composition, the same but increasing the synthesis temperature to 150 ^oC (Figure 4d), 160 ^oC (Figure 4e), or 170 ^oC (Figure 4f), the FAU zeolite transforms into MER zeolite with Si/Al ratio = 1.6–1.7. At the higher synthesis temperature (170 ^oC), the MER zeolite crystals are better faceted and more abundant compared to those obtained at the lower temperatures.

Figure 5 shows a schematic model of the pathways of FAU transformation where the parent FAU can be selectively transformed to hierarchal FAU with a lower Si/Al ratio or to a MER zeolite. The transformation is controlled by simple temperature adjustment that has previously been shown to have a significant effect on the crystallization route [32].

Figure 6 shows SEM images of EDI zeolite obtained through IZT under different conditions. The FAU-EDI transformation (KOH/FAU = 8.94 and H₂O/KOH = 0.87) at 60 ^oC, regardless of the synthesis time (6 or 22 hours), results in nanoparticles (Figure 6a,b). At 80 ^oC, short synthesis times (1 hour) also lead to nanoparticles (Figure 6c), but when the synthesis time is increased to 6 hours, larger prismatic particles form (Figure 6d). Figure 6e,f shows EDI zeolite obtained at 160 ^oC (KOH/FAU = 2.0 and H₂O/KOH = 2.0), after 5 and 10 min of crystallization, respectively. Despite the relatively short crystallization times, the EDI zeolite forms well-shaped, intergrown prismatic particles that range from submicron to micrometric sizes. These results demonstrate that, although the higher temperature accelerates the crystallization process, faster crystallization does not result in smaller crystals.

Figure 7a,b shows SEM images of ANA zeolites synthesized by the transformation of FAU zeolite under two different conditions: (1) 170 ^oC for 1 hour (with CsOH/FAU = 2.7 and H₂O/CsOH = 2.59) (Figure 7a) and (2) 160 ^oC for 10 min (with CsOH/FAU = 1.42 and H₂O/CsOH = 4.9) (Figure 7b). In both cases, ANA zeolite crystallizes as aggregates of submicron, sphere-like particles. In the first system (170 ^oC for 1 hour), the submicron particles are decorated with second-generation nanoparticles (50-70 nm). This feature is absent in the second system (160 ^oC for 10 min), suggesting that the differences in supersaturation lead to secondary nucleation and growth in the first system, but not in the second. Additionally, in a system where NaOH/FAU = 2 and H₂O/NaOH = 4.5, FAU zeolite transforms into a CAN-type structure at 160 ^oC in 10 min. The resulting phase crystallizes as micrometric intergrowths of prismatic crystals, which is typical for CAN-type materials [33].

3. Experimental

The IZT transformations were performed using a parent FAU zeolite with Si/Al ratio close to 3.0 (±0.1) synthesized by a previously reported procedure [6]. The obtained parent phase was placed in alkaline solutions of NaOH (≥98%, Sigma-Aldrich), KOH (90%, Sigma-Aldrich), and CsOH∙H₂O (≥90%, Sigma-Aldrich) following the conditions described in Table 2. As a base for all synthesis was used 0.5 g of FAU zeolite. The crystallization was performed using a microwave reactor Monowave 400 (Anton Paar) and a SiC autoclave. The time for reaching the desired temperature was 5 min for all synthesis. After the synthesis, each sample was filtered and washed with distilled water.

The powder X-ray diffraction (XRD) patterns were obtained using a Bruker D8 Discover diffractometer working with copper radiation (λ₁ = 1.5406 Å, λ₂ = 1.5444 Å) in θ-2θ mode and using step size 0.04°, time per step 0.2 s and a LynxEye detector. Phase identification and quantification were carried out using the EVA software (Bruker AXS) with integrated database of the International Center for Diffraction Data (ICDD). Unit cell parameters were refined using the Le Bail method with TOPAS-3 (Bruker AXS) software. The Si/Al ratios in the FAU samples were calculated by the Breck-Flanigen correlation: Si/Al = ((192 × 0.00868)/(a₀ – 24.191)) – 1) [34]. Scanning electron microscopy (SEM) micrographs and energy dispersive spectroscopy (EDS) chemical analysis were performed on a NanoSEM-FEI Nova 200 equipped with an EDAX Pegasus X4M detector.

4. Conclusions

This work introduces microwave radiation in the synthesis of zeolites by interzeolite transformation. This approach provides control over the particle size and fast access to zeolite structures such as EDI, MER, ANA and CAN. An additional derivate of the microwave-assisted IZT is the introduction of an alternative pathway to hierarchical faujasites by fast desilication of a parent zeolite in a short time. In perspective, further development of the microwave-assisted IZT is expected to result in other zeolite framework types.