Sustainable Route for Synthesis of All-Silica SOD Zeolite

: The development of the sustainable synthesis of zeolites has become a very hot topic in recent years. Herein, we report a sustainable route for synthesizing all-silica SOD zeolite under solvent-free conditions. The method of solvent-free synthesis includes mixing, grinding, and heating raw solids. The all-silica SOD zeolite obtained was well characterized by multiple measurement techniques (XRD, SEM, IR, thermogravimetric-di ﬀ erential thermal analysis (TG-DTA), and magic angel spinning nuclear magnetic resonance (MAS NMR)). The crystallization process of all-silica SOD zeolite was also investigated in detail by XRD, SEM, UV-Raman, and MAS NMR techniques. In addition, the e ﬀ ects of the crystallization compositions, including the molar ratios of Na 2 O / SiO 2 and ethylene glycol / SiO 2 , on the synthesis of the pure all-silica SOD zeolite were investigated at di ﬀ erent temperatures.

The all-silica SOD zeolite, firstly synthesized under solvothermal conditions by Bibby in 1985, is a promising candidate for the above applications. It is worth noting that a large amount of ethylene solvents were necessary in the synthesis, which acted as both an organic template and solvent [14][15][16]. Later, Gies et al. also reported that all-silica SOD zeolite was synthesized in the presence of pyrrolidine and ethylamine under solvothermal conditions [17]. Notably, a large amount of organic solvent was necessary in the process of all-silica SOD zeolite synthesis, which not only increased the production cost, but also produced polluted waste and thus led to the issue of environmental hazard. Therefore, it is highly desirable to develop a sustainable route for all-silica SOD zeolite synthesis.
Recently, a solvent-free method has been reported for synthesizing zeolites by mixing, grinding, and heating raw solids, which is regarded as a sustainable route. To date, various zeolites, 2 of 10 such as MFI, Beta, FAU, and MOR, have been successfully prepared under this route [18][19][20][21][22]. However, the synthesis of all-silica SOD zeolite has not yet been successful using this sustainable route. In this work, for the first time, we show a sustainable route for synthesizing all-silica SOD zeolite. The synthesis is well characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric-differential thermal analysis (TG-DTA), infrared spectroscopy (IR), UV-Raman, and magic angel spinning nuclear magnetic resonance (MAS NMR).

Synthesis
In a typical example of synthesizing the all-silica SOD zeolite, 0.5 g of Na 2 SiO 3 ·9H 2 O, 1.57 g of SiO 2 , and 2.6 g of ethylene glycol were added into a mortar one by one and mixed together uniformly. After grinding for 15 min at room temperature, the powder was transferred to a 15 mL autoclave and sealed. After crystallization at 200 • C for 24 h, the product was fully crystalline. After filtering, washing with water at room temperature, and drying at 100 • C for 12 h, the final product was obtained, yielding a 1.75 g sample. This sample is denoted as S-Si-SOD.

Characterization
The X-ray diffraction (XRD) patterns were measured with a Rigaku Ultimate VI X-ray diffractometer (40 kV, 40 mA, Rigaku, Tokyo, Japan) using CuKα (λ = 1.5406 Å) radiation. Scanning electron microscopy (SEM) experiments were performed on Hitachi SU-1510 electron microscopes (Hitachi, Tokyo, Japan). The Fourier transform infrared spectroscopy (FTIR) spectra were recorded using a Bruker 66 V FTIR spectrometer (Bruker, Karlsruhe, Germany). UV-Raman spectra were measured with a Jobin-Yvon T64000 triple-stage spectrometer (Jobin-Yvon, Paris, France) with a spectral resolution of 2 cm −1 . The laser line at 325 nm of a He/Cd laser was used as an excitation source with an output of 50 mW. The power of the laser at the sample was about 3.0 mW. 1 H-13 C CP/MAS NMR experiments were performed on a Bruker AVANCE-III 500 spectrometer (Bruker, Karlsruhe, Germany) using a 1.9 mm MAS probe with a sample spinning rate of 30 kHz; a total of 30,000 scans were accumulated with a recycle time of 5 s. Single-pulse 29 Si MAS NMR experiments with 1 H decoupling were performed on a Bruker AVANCE-III 500WB spectrometer (Bruker, Karlsruhe, Germany) using a 7 mm probe with a π/4 pulse width of 4.0 µs, a 60 s pulse delay, and a spinning rate of 4 kHz. TG-DTA analysis was carried out with a NETZSCH STA 449C (Netstal, Bavaria, Germany) in air at a heating rate of 10 • C/min from room temperature to 800 • C. Figure 1 displays the XRD pattern of the as-made S-Si-SOD zeolite crystallized for 24 h at 200 • C under solvent-free conditions, giving a series of typical peaks associated with the SOD structure, in good agreement with the reported literature [14][15][16]. Figure 2 shows the different magnification SEM images of the as-made S-Si-SOD zeolite, which exhibits a blocky crystal morphology with a size of about 10 µm. Figure 3 shows the 29 Si MAS NMR spectrum of the as-made S-Si-SOD zeolite, giving three peaks with chemical shifts at around −117.  Figure 4a shows the 13 C MAS NMR spectrum of the as-made S-Si-SOD zeolite, giving the only one peak centered at about 64.0 ppm, which is assigned to the presence of ethylene glycol in the product. Figure 4b displays the IR spectra of the as-made S-Si-SOD zeolite. From Figure 4b, two bands at 2800-3000 cm −1 and one band centered at about 3440 cm −1 can be seen, which are assigned to the ethyl group and hydroxyl group, respectively. Figure 4c shows the TG-DTA of the as-made S-Si-SOD zeolite, which gives the major exothermic peak at about 300-500 °C accompanied by a weight loss at about 5.8%, which is attributed to the decomposition of ethylene glycol in the S-Si-SOD zeolite framework. The above results support that the ethylene glycol is indeed used as an organic template to direct the formation of the S-Si-SOD zeolite.   Figure 4a shows the 13 C MAS NMR spectrum of the as-made S-Si-SOD zeolite, giving the only one peak centered at about 64.0 ppm, which is assigned to the presence of ethylene glycol in the product. Figure 4b displays the IR spectra of the as-made S-Si-SOD zeolite. From Figure 4b, two bands at 2800-3000 cm −1 and one band centered at about 3440 cm −1 can be seen, which are assigned to the ethyl group and hydroxyl group, respectively. Figure 4c shows the TG-DTA of the as-made S-Si-SOD zeolite, which gives the major exothermic peak at about 300-500 °C accompanied by a weight loss at about 5.8%, which is attributed to the decomposition of ethylene glycol in the S-Si-SOD zeolite framework. The above results support that the ethylene glycol is indeed used as an organic template to direct the formation of the S-Si-SOD zeolite.  one peak centered at about 64.0 ppm, which is assigned to the presence of ethylene glycol in the product. Figure 4b displays the IR spectra of the as-made S-Si-SOD zeolite. From Figure 4b, two bands at 2800-3000 cm −1 and one band centered at about 3440 cm −1 can be seen, which are assigned to the ethyl group and hydroxyl group, respectively. Figure 4c shows the TG-DTA of the as-made S-Si-SOD zeolite, which gives the major exothermic peak at about 300-500 °C accompanied by a weight loss at about 5.8%, which is attributed to the decomposition of ethylene glycol in the S-Si-SOD zeolite framework. The above results support that the ethylene glycol is indeed used as an organic template to direct the formation of the S-Si-SOD zeolite.   Figure 4a shows the 13 C MAS NMR spectrum of the as-made S-Si-SOD zeolite, giving the only one peak centered at about 64.0 ppm, which is assigned to the presence of ethylene glycol in the product. Figure 4b displays the IR spectra of the as-made S-Si-SOD zeolite. From Figure 4b, two bands at 2800-3000 cm −1 and one band centered at about 3440 cm −1 can be seen, which are assigned to the  Figure 4c shows the TG-DTA of the as-made S-Si-SOD zeolite, which gives the major exothermic peak at about 300-500 • C accompanied by a weight loss at about 5.8%, which is attributed to the decomposition of ethylene glycol in the S-Si-SOD zeolite framework. The above results support that the ethylene glycol is indeed used as an organic template to direct the formation of the S-Si-SOD zeolite.

Critical Factors for Synthesizing S-Si-SOD Zeolite
In the preparation process of S-Si-SOD zeolite, various factors influence the results of crystallization, such as the ratios of Na 2 O/SiO 2 , EG/SiO 2 and the synthesis temperature. Notably, the ratio of Na 2 O/SiO 2 was very crucial to the synthesis of the final product. When the Na 2 O/SiO 2 ratio was 0.01, the product was always amorphous (Table 1, Run 1, Figure 5a). Increasing the ratio of Na 2 O/SiO 2 to 0.04, the S-Si-SOD zeolite product was dominant together with some amorphous phase (Table 1, Run 2, Figure 5b). Further increasing the ratio of Na 2 O/SiO 2 to 0.06, the S-Si-SOD zeolite product with high crystallinity could be successfully obtained (Table 1, Run 3, Figure 1). However, a higher Na 2 O/SiO 2 ratio would lead to the production of some amorphous phase again (Table 1, Run 4, Figure 5c). In addition, the ratio of EG/SiO 2 was also a very important factor for crystallization. When EG was absent in the starting mixture, the quartz and amorphous phase was always obtained (Table 1, Run 5, Figure 5d). When the EG/SiO 2 ratio was 1.0, the S-Si-SOD would be obtained accompanied by some amorphous phase (Table 1, Run 6, Figure 5e). Moreover, the influence of temperature on crystallization was also investigated. When the temperature was 160 • C, the product obtained was the amorphous phase (Table 1, Run 7, Figure 5f). When the temperature was 230 • C, the S-Si-SOD zeolite product could be obtained and the crystallization time was significantly decreased ( Table 1, Run 8, Figure 5g). Furthermore, it was found that the addition of seed in the initial mixture also influenced the crystallization. When seed added it, the crystallization time greatly decreased and it only required 3 h (Table 1, Run 9, Figure 6a). More interestingly, the size of the S-Si-SOD zeolite was Crystals 2019, 9, 338 5 of 10 about 1-2 µm, which was significantly smaller than that of the product without the addition of seed (Figure 6b).       Based on the aforementioned experimental results, it could be concluded that the synthesis of S-Si-SOD zeolite was greatly influenced by many factors and S-Si-SOD zeolite with high crystallinity could only be obtained in a narrow range of proportions of Na 2 O-EG-SiO 2 at an appropriate temperature.

Crystallization Process of S-Si-SOD Zeolite
In order to understand the synthesis, the crystallization process at 200 • C was characterized by XRD, SEM, UV-Raman, and MAS NMR techniques in detail ( Figures 7-10). The photographs of the products before and after crystallization are shown in Figure 11, revealing that the product was always in a solid phase, indicating solvent-free synthesis rather than solvothermal synthesis. The XRD patterns showed that the weak peaks associated with the SOD zeolite structure appeared after crystallization for 12 h (Figure 7c). At the same time, some crystals could be observed via the SEM image (Figure 8b). Increasing the time from 14 to 20 h, the intensity of the peaks gradually increased (Figure 7d-f) and more crystal could be produced (Figure 8c,d). With further increasing the crystallization time to 24 h (Figure 7g), the intensity of the XRD peaks was similar to that of the product crystallized at 20 h, which means that the crystallization was completely finished at 20 h.  zeolite structure appeared after crystallization for 12 h (Figure 7c). At the same time, some crystals could be observed via the SEM image (Figure 8b). Increasing the time from 14 to 20 h, the intensity of the peaks gradually increased (Figure 7d-f) and more crystal could be produced (Figure 8c,d).

Crystallization Process of S-Si-SOD Zeolite
With further increasing the crystallization time to 24 h (Figure 7g), the intensity of the XRD peaks was similar to that of the product crystallized at 20 h, which means that the crystallization was completely finished at 20 h.    Figure 9 showed the UV-Raman spectra of the as-made S-Si-SOD zeolite at various time. Before crystallization, the starting solid mixture exhibited a major broad band at about 494 cm −1 , which is assigned to the four-membered rings (4MRs) in the amorphous phase (Figure 9a). After crystallization for 12 h, the product showed the band at about 457 cm −1 , which can be reasonably assigned to the 4MRs associated with the SOD zeolite structure (Figure 9c). The intensity of this band increased with the crystallization time, which was in accordance with the SOD zeolite crystallinity (Figure 9d-f). The above result shows that the transformation was from the 4MRs in the amorphous phase to 4MRs in the SOD zeolite structure, suggesting that the 4MRs in the amorphous phase act as be the active building units for the crystallization of S-Si-SOD zeolite. Figure 10 shows the 29 Si MAS NMR spectra of the as-made S-Si-SOD zeolite at various times. Before crystallization, the 29 Si MAS NMR spectrum exhibited the main peaks with chemical shifts at about −111.0 and −101.3 ppm, assigned to the Q 4 and Q 3 silica species in the solid silica gel ( Figure  10a). As the crystallization time increased, the intensity of the signal peak with the chemical shift at around −117.2 ppm, associated with the Q 4 silica species in the SOD zeolite product, was observe to gradually increase (Figure 10b-f). This indicates the formation of more SOD zeolite crystals, which is a b Figure 11. Photographs of the samples (a) before and (b) after crystallization at 200 • C. Figure 9 showed the UV-Raman spectra of the as-made S-Si-SOD zeolite at various time. Before crystallization, the starting solid mixture exhibited a major broad band at about 494 cm −1 , which is assigned to the four-membered rings (4MRs) in the amorphous phase (Figure 9a). After crystallization for 12 h, the product showed the band at about 457 cm −1 , which can be reasonably assigned to the 4MRs associated with the SOD zeolite structure (Figure 9c). The intensity of this band increased with the crystallization time, which was in accordance with the SOD zeolite crystallinity (Figure 9d-f). The above result shows that the transformation was from the 4MRs in the amorphous phase to 4MRs in the SOD zeolite structure, suggesting that the 4MRs in the amorphous phase act as be the active building units for the crystallization of S-Si-SOD zeolite. Figure 10 shows the 29 Si MAS NMR spectra of the as-made S-Si-SOD zeolite at various times. Before crystallization, the 29 Si MAS NMR spectrum exhibited the main peaks with chemical shifts at about −111.0 and −101.3 ppm, assigned to the Q 4 and Q 3 silica species in the solid silica gel (Figure 10a). As the crystallization time increased, the intensity of the signal peak with the chemical shift at around −117.2 ppm, associated with the Q 4 silica species in the SOD zeolite product, was observe to gradually increase (Figure 10b-f). This indicates the formation of more SOD zeolite crystals, which is in good agreement with the results of the XRD patterns and SEM images. The above results reveal that the condensation of solid silica assisted with EG might be an indication of the formation of the SOD zeolite structure.

Conclusions
In summary, we developed a sustainable route to successfully synthesize all-silica S-Si-SOD zeolite under solvent-free conditions. For the first time, S-Si-SOD zeolite was successfully synthesized using this sustainable route. Compared with traditional solvothermal synthesis, the novel route in this work has obvious advantages from the sustainability point of view with respect to high product yield, environment-friendly production, and simple procedures. The above features should open an alternative platform for the efficient synthesis of all-silica SOD zeolite.