Zeolites are useful inorganic crystalline materials that have been widely applied for shape selective catalysis, adsorption, separation, ion exchange, and adsorption processes due to their versatile ordered pore structure, large specific surface area, high thermal and hydrothermal stability, controllable acid center skeleton, and exchangeable cations [1-4]. Most zeolites are synthesized under hydrothermal conditions from conventional amorphous aluminosilicate gels [5-8]. Recently, interzeolite transformation between zeolites has attracted significant attention owing to its benefits including relatively short crystallization time and specific zeolite structures such as CHA, AEI, and RTH that can be formed [9-12]. Generally, FAU zeolites with high silica to alumina ratios (SARs) are chosen as a starting material, where a large amount of orderly connected double 6-membered ring (D6R) units in the FAU framework play an important role during the transformation process [13, 14]. In addition, many successful examples of interzeolite transformation from FAU zeolites to other zeolite structures without D6R units (such as RUT, MER, and MWW) have been reported [15-17]. Currently, it is still challenging to investigate the behavior of D6Rs during interzeolite transformation.

Over the past decades, various techniques have been developed for identification of possible intermediates and mechanisms of zeolite crystallization, including X-ray diffraction and scattering [18], solid-state/liquid-state NMR spectroscopy [19, 20], mass spectrometry [21, 22], atomic force microscopy [23, 24], and electron microscopy [25, 26]. Compared to these techniques, UV Raman spectroscopy has certain advantages in terms of avoiding fluorescence and increasing sensitivity [27] and has proven to be a powerful technique to characterize zeolite crystallization because of its unique sensitivity to framework vibrations, particularly the ring structure. Therefore, it can provide structural information based on vibrational motion of the framework, even during the well-known "induction period" in zeolite synthesis, a "black" stage in terms of XRD pattern data [28, 29]. Unfortunately, no characterization of interzeolite transformation by UV Raman spectroscopy has been published to date.

In this study, UV Raman spectroscopy was used to monitor the behavior of the D6R units during the interzeolite transformation from FAU to CHA and MFI, showing that the D6R units basically remained during the transformation from FAU into CHA while they were divided to S6R units then into MFI structure during the transformation from FAU into MFI.

Powder X-ray powder diffraction (PXRD) patterns were measured using a Rigaku Ultimate VI X-ray diffractometer (40 kV, 40 mA) with Cu K_α (λ = 1.5406 Ǻ) radiation. The UV Raman spectra were recorded using a home-assembled UV Raman spectrograph with a Jobin-Yvon T64000 triple-stage spectrograph with a spectral resolution of 2 cm^–1. The single-frequency UV laser line at 266 nm originated from an efficient external cavity frequency doubler (Wavetrain, Spectra-Physics) of a single-frequency laser at 532 nm laser (Verdi 2, Coherent). The excitation source with an output of 30 mW was used and the power of the laser at was approximately 3.0 mW.

The frequency of the D6R structure extracted from the FAU crystallographic data was calculated using the B3LYP hybrid exchange-correlation functional. Geometry optimizations were performed at the basis set of 6-311+G (d, p). Frequency analysis was then performed to confirm the stability of the optimized molecular structures, and the enthalpies were then calculated from harmonic frequencies. All calculations were performed using the Gaussian 09 package [30]. The terminal O atoms were saturated with H atoms, which were positioned on the vector from the O to Si atom that the H was replacing. During structure optimization, all framework atoms were relaxed while the terminal H atoms were fixed to preserve the integrity of the zeolite structure.

The topological structures of FAU and the targets (CHA and MFI) are shown in Fig. 1. FAU and CHA share the same basic structural building blocks (D6R), but no D6R units exist in the framework of MFI. Therefore, the discussion regarding the D6R behavior will be divided into two parts, i.e. the target structures with and without D6R.

Figure 2a shows the UV Raman spectra of the interzeolite transformation from FAU to CHA zeolite. Between 0 and 6 h, three main Raman bands were observed belonging to vibrations of the FAU framework (Fig. S1 in the Supporting Information), yielding a sharp Raman band at 500 cm^–1 that could be assigned to the breathing mode vibration of the 4-membered ring (4R) in the FAU crystalline framework [31, 32]. The band at 298 cm^–1 was attributed to the bending mode of D6R in the FAU framework [33, 34], and a new shoulder band was observed at 480 cm^–1, which has not previously been reported for FAU with low SAR. DFT calculations were performed which verified that this new band could be attributed to the frequency of the breathing vibration mode of the 4R in D6R with much higher Raman activity.

When the crystallization time reached 6 h, two new Raman bands appeared at approximately 465 and 330 cm^–1 in accordance with the weak XRD peaks at 9.6°, 16.2° and 20.8° associated with the CHA framework (Fig. S1). The band at 465 cm^–1 was attributed to 4R in the CHA structure, which became stronger with increasing crystallization time [35]. Meanwhile, the intensity of the 500 cm^–1 band gradually decreased and eventually vanished, indicating that the 4Rs in the FAU structure were gradually transferred into 4Rs in the CHA structure. Interestingly, the intensities of the characteristic D6R bands (298 and 480 cm^–1) were remained nearly identical because D6R units exist in both FAU and CHA frameworks. In addition, a novel band at 330 cm^–1 appeared, which was attributed to D6R in the CHA structure [36, 37]. The intensity of this band strengthened with increasing crystallization time.

Figure 2b shows the FAU and CHA crystallinity and intensity of the selected band (298 cm^–1) as a function of crystallization time. With increasing crystallization time, the crystallinity of the FAU zeolite significantly decreased, which was accompanied by a corresponding increase of the CHA zeolite crystallinity. Interestingly, although there was a conversion balance of the two crystalline phases during the transformation, the intensity of the D6R species largely remained the same within the experimental error range. This indicated that the D6R structure was stable and the D6R structural integrity was maintained rather than decomposed during the conversion from FAU to CHA. These results provide direct evidence for the transformation of D6R units from the FAU to CHA structure.

Figure 3a shows the UV Raman spectra of the interzeolite transformation from FAU to MFI zeolite. As mentioned previously, the Raman spectrum of the FAU zeolite only showed bands at 500, 480, and 298 cm^–1. After crystallization for 3 h during the transformation to MFI, the Raman bands associated with the characteristic vibrations of the MFI zeolite framework (433 and 475 cm^–1 for 4R, 289 cm^–1 for S6R, and 377 cm^–1 for the 5R units in the crystal phase) gradually appeared [38-41]. Simultaneously, the XRD patterns of the samples exhibited clear characteristic peaks of the zeolite MFI framework (Fig. S2 in the Supporting Information). Unlike the interzeolite transformation from FAU to CHA, the Raman spectra of the interzeolite transformation between FAU and MFI showed an apparent and parallel reduction in the band intensities at 500, 298, and 480 cm^–1. These bands disappeared when the MFI zeolite dominated crystalline phase in the product, indicating that the D6R structure was unstable during this process and was decomposed.

Figure 3b shows the FAU/MFI crystallinity and intensity of selected Raman bands (298, 289, and 377 cm^–1) as a function of crystallization time. With increasing crystallization time, the crystallinity of the FAU zeolite decreased and the intensity of the D6R structure significantly decreased and eventually completely vanished before the disappearance of FAU structure, suggesting that the D6R was unstable and likely decomposed into smaller species during the transformation. Interestingly, the emergence of the S6R units occurred earlier than the formation of MFI crystallites, where the concentration of S6R units increased sharply and reached a maximum in a very short time. Thus, it is reasonable to infer that the S6R units exist during the crystal nucleation period and participated in the nucleation. Moreover, the growth rate of the 5R units approximates the crystallinity of the MFI zeolite, indicating that the formation of 5R units occurs during MFI crystallization from the assembly of S6Rs. These results suggest that the D6R structure decomposed into S6R units instead of 4R units during the formation of MFI structure, while the 5R units were only observed after formation of MFI crystals.

It is well known that DFT calculations can be used to explore the reaction activities of zeolites [42, 43]. Herein DFT was used to investigate the D6R hydrolysis in FAU zeolite, as shown in Fig. 4. Two hydrolysis processes are proposed for the D6R units: (i) D6R hydrolysis to form two S6Rs and six H₂O molecules, and (ii) D6R hydrolysis to form three S4Rs and six H₂O molecules. Figure 4a and 4b provide the optimized geometries and corresponding reaction enthalpies of the proposed hydrolysis processes. The calculated reaction enthalpies for D6R hydrolysis into two S6Rs (–16.3 kcal/mol) was significantly lower than that form three 4Rs (–4.6 kcal/mol), indicating that D6R hydrolysis to form two S6Rs is preferable, in good agreement with the experimental UV Raman results (Fig. 2 and 3).

According to the results presented above, the possible pathways of interzeolite transformation from FAU to CHA and MFI zeolites are proposed in Fig. 5. Depending on the structural similarity between the staring and target zeolites, the D6R units of the FAU zeolite display two responses: a direct transformation to CHA, or decomposition into S6R and adoption of the MFI structure.

In summary, interzeolite transformation from FAU to CHA and MFI zeolites was carefully investigated by UV Raman spectroscopy. A nearly direct transformation of the D6R structure occurred when starting from FAU to the target CHA zeolite as they share the same basic composition unit (D6R). However, when the target zeolites do not contain the same basic structures, such as for the transformation of FAU to MFI, the D6R units are decomposed into two S6Rs as building blocks for further assembly and growth of the target product. The insights provided by this study may be important for further understanding of interzeolite transformation.