Preparation of Aluminosilicate Ferrierite Zeolite Nanosheets with Controllable Thickness in the Presence of a Sole Organic Structure Directing Agent

Preparation of aluminosilicate ferrierite (FER) zeolite nanosheets with controllable thickness in the presence of a sole organic ammonium is attractive, but still challenging. In this report, with the employment of N,N-diethyl-cis-2,6-dimethylpiperidinium (DMP) as both a structure directing agent and crystal growth inhibitor, aluminosilicate FER zeolite nanosheets, with a variety of crystal thicknesses, ranging from 6 to 200 nm, are successfully synthesized under hydrothermal conditions. Very interestingly, the amount of DMP in the starting gel is the key factor for crystal thickness control of aluminosilicate FER zeolite nanosheets. The obtained FER products, with different thicknesses, are well characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), N2 sorption, thermogravimetric analysis (TG), inductively coupled plasma (ICP), and magic angle spinning nuclear magnetic resonance (MAS NMR) techniques. This simple strategy might provide a novel avenue for the synthesis of other zeolite nanosheets with controllable thickness.

Molecules 2020, 25, 771 2 of 10 FER zeolite is a medium-pore zeolite containing 10-Membered Ring (10-MR) channels (0.42 × 0.54 nm) in the [001] direction and 8-MR channels (0.35 × 0.48 nm) in the [010] direction [25][26][27][28][29][30][31]. Aluminosilicate FER zeolite, which is represented by ZSM-35, shows distinguished catalytic performances in many reactions, such as skeletal isomerization, high-olefin catalytic cracking, carbonylation of dimethyl ether, methanol to olefins, and NO x reduction [25,26,[32][33][34][35][36][37][38][39][40] To further enhance the catalytic performance, the preparation methods for aluminosilicate FER zeolites with controllable morphology and crystal sizes are particularly attractive. Corma et al. prepared FER nanocrystals with sizes of 10-20 nm using piperidine and surfactant as dual templates [25]. Meanwhile, FER nanoneedles with a diameter of 10 nm were developed by adding choline as an organic template [41]. Despite the fact that the diameter of the aluminosilicate FER zeolite crystals is very small in the above works, the crystal size could not be adjusted. Recently, Xu et al. reported on the preparation of sheet-like FER zeolite, with controllable thickness from 100 nm to 2 µm, using piperidine as a structure directing agent and cetyltrimethyl ammonium bromide (CTAB) as crystal growth inhibitors in one synthesis system [42]. However, the thinnest FER zeolite crystal in this work is still too thick and the dual template method is complicated. At present, it is still a great challenge to control the different nanosheet thicknesses of aluminosilicate FER zeolite within the thickness of 100 nm using the facile synthesis method.
More recently, we reported a simple method for synthesizing ultrathin nanosheets of aluminosilicate FER zeolite with the thickness of 6-8 nm by using a sole small organic ammonium (N,N-diethyl-cis-2,6-dimethylpiperidinium, DMP). Very interestingly, the DMP molecules not only direct the FER zeolite structure, but also inhibit the growth of FER zeolite on [100] direction, according to the theoretical calculation [43]. Therefore, it might give an opportunity for controlling the nanosheet thickness of aluminosilicate FER zeolite just by changing the amount of DMP molecules in the same synthesis system.
In this work, we report the synthesis of FER zeolite nanosheets with controllable thickness using a sole organic ammonium as both a structure directing agent and crystal growth inhibitor. Very interestingly, by adjusting the amount of DMP in the starting gel, the thickness of FER nanosheets can be adjusted, ranging from 6 to 200 nm. Figure 1 shows the XRD patterns of aluminosilicate FER zeolite nanosheets with the different amount of DMP in the starting gel. Each XRD pattern of FER sample shows typical peaks of FER zeolite structure. The peaks at 9.3 • associated with the [200] reflection widen with the increasing amount of DMP in the starting gel (Table 1), suggesting thinner thickness of FER zeolite along the [100] direction, which has been further confirmed by the results of the SEM images ( Figure 2). According to the results of the SEM images in Figure 2 and sample synthesis conditions in Table 1, the thickness of FER-0 nanosheets is approximately 100-200 nm, without the addition of DMP in the starting gel (Figure 2a). When a small amount of DMP was added (DMP/SiO 2 = 0.015) into the starting gel, the thickness of FER-0.015 nanosheets turned to around 50-100 nm (Figure 2b). Increasing the DMP/SiO 2 ratio to 0.030, the nanosheets of FER-0.030 become thinner (30-60 nm, Figure 2c). Further increasing the DMP/SiO 2 ratio to 0.060, the thickness of FER-0.06 nanosheets is around 10-20 nm (Figure 2d). Finally, when the DMP/SiO 2 ratio reaches 0.12, the thinnest aluminosilicate FER zeolite nanosheets (FER-0.12) with a thickness of 6-8 nm would be obtained (Figure 2e) [43]. In addition, the SiO 2 /Al 2 O 3 ratios of the FER zeolite samples with different thicknesses are all around 16.0-17.0 (Table 1), even if the amount of DMP molecules added in the synthesis gel is different. The above results show that the DMP molecules would inhibit the crystal growth of the aluminosilicate FER nanosheets, and thus obtain the different thickness of FER nanosheets from 6 to 200 nm.      Figure 3 shows nitrogen sorption isotherms of the aluminosilicate H-FER zeolite nanosheet samples. The micropore volumes of these samples are the same (0.14 cm 3 /g), while the mesopore volumes of these samples are increased with the decreasing of the crystal thickness, as shown in Table  2. Moreover, the external surface area and the Brunauer-Emmett-Teller (BET) urface area are higher when the crystal thickness of the aluminosilicate FER zeolite nanosheet samples decrease. The higher  Figure 3 shows nitrogen sorption isotherms of the aluminosilicate H-FER zeolite nanosheet samples. The micropore volumes of these samples are the same (0.14 cm 3 /g), while the mesopore volumes of these samples are increased with the decreasing of the crystal thickness, as shown in Table 2. Moreover, the external surface area and the Brunauer-Emmett-Teller (BET) urface area are higher when the crystal thickness of the aluminosilicate FER zeolite nanosheet samples decrease. The higher external surface area and BET surface area means a higher exposure degree, which is in good agreement with that of the zeolite nanosheets.

Results and Discussion
Molecules 2020, 25, 771 5 of 10 external surface area and BET surface area means a higher exposure degree, which is in good agreement with that of the zeolite nanosheets.   Figure S1 shows the TGA curves of the as-synthesized aluminosilicate FER zeolite samples, exhibiting different weight loss associated with the decomposition of organic structure directing agent (OSDA) in the micropores of the FER zeolite samples. This result shows that the DMP molecules are the structure directing agent for directing the generation of aluminosilicate FER zeolite. Very interestingly, the weight loss of the samples is very consistent with the DMP amount in the synthesis process. Figure 4A shows the 29 Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectra of the aluminosilicate FER zeolite samples. These samples exhibit peaks at around −114, −111, −109, −106, and −103 ppm, which can be reasonably assigned to Si(4Si) (−114, −111, and −109 ppm), Si(3Si) (−106 ppm), and Si(2Si) (−103 ppm) species, respectively. According to the spectra and the structural information in Table S1, the Si(4Si) and Si(2Si) species of the samples decrease along with the increasing amount of DMP in the process of synthesis, while the Si(3Si) species of the samples increases. This phenomenon might be the result from both the structure directing and growth inhibition effect of the DMP molecules. Figure 4B shows the 27 Al MAS NMR spectra of the aluminosilicate FER zeolite samples. All of the samples give the one peak with the chemical shift at about 54 ppm associated with the tetrahedrally coordinated aluminum species in the FER zeolite framework. At the same time, there is no signal with the chemical shift at about zero ppm, suggesting the absence of extra-framework aluminum species in these samples. This result shows that the addition of different amounts of DMP molecules in the synthesis has no effect on the aluminum coordination of the samples.   Figure S1 shows the TGA curves of the as-synthesized aluminosilicate FER zeolite samples, exhibiting different weight loss associated with the decomposition of organic structure directing agent (OSDA) in the micropores of the FER zeolite samples. This result shows that the DMP molecules are the structure directing agent for directing the generation of aluminosilicate FER zeolite. Very interestingly, the weight loss of the samples is very consistent with the DMP amount in the synthesis process. Figure 4A shows the 29 Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectra of the aluminosilicate FER zeolite samples. These samples exhibit peaks at around −114, −111, −109, −106, and −103 ppm, which can be reasonably assigned to Si(4Si) (−114, −111, and −109 ppm), Si(3Si) (−106 ppm), and Si(2Si) (−103 ppm) species, respectively. According to the spectra and the structural information in Table S1, the Si(4Si) and Si(2Si) species of the samples decrease along with the increasing amount of DMP in the process of synthesis, while the Si(3Si) species of the samples increases. This phenomenon might be the result from both the structure directing and growth inhibition effect of the DMP molecules. Figure 4B shows the 27 Al MAS NMR spectra of the aluminosilicate FER zeolite samples. All of the samples give the one peak with the chemical shift at about 54 ppm associated with the tetrahedrally coordinated aluminum species in the FER zeolite framework. At the same time, there is no signal with the chemical shift at about zero ppm, suggesting the absence of extra-framework aluminum species in these samples. This result shows that the addition of different amounts of DMP molecules in the synthesis has no effect on the aluminum coordination of the samples.
The acidity of the aluminosilicate H-FER zeolite nanosheet samples is investigated by the NH3-TPD technique. The NH3-TPD curves in Figure S2 show that the samples with thinner crystal thickness have fewer acidic sites. This might be caused by the growth inhibition effect of the DMP molecules, which is in good agreement with the literature [42].

Synthesis of OSDA
The iodide form of the OSDA, N,N-diethyl-cis-2,6-dimethylpiperidine iodide was synthesized by mixing cis-2,6-dimethylpiperidine, iodoethane, and KHCO 3 in the methanol, followed by refluxing for 4 days. The excess of KHCO 3 was removed, and then the solvent and the excess iodoethane was evaporated. Next, the product was washed with diethyl ether and converted to the hydroxide form using an anion exchange resin.

Synthesis of Aluminosilicate FER Zeolite Nanosheets
In a typical run for synthesizing aluminosilicate FER zeolite nanosheets without the addition of DMP molecules, 0.165 g of NaAlO 2 and 0.254 g of NaOH was dissolved into deionized water. Then 2.95 g of colloidal silica (40 wt% SiO 2 in water) was added and the mixture was stirred for 2 h. Next, 0.024 g of FER zeolite seeds was added. Finally, the mixture was transferred into a Teflon-lined autoclave oven and crystallized at 140 • C for 48 h under rotation conditions (50 rpm). After filtering, washing, and drying, the product was gained.
In a typical run for synthesizing aluminosilicate FER zeolite nanosheets with the use of DMP, 0.165 g of NaAlO 2 and 0.254 g of NaOH was dissolved into deionized water. Next, a certain amount of DMP solution and 2.95 g of colloidal silica (40 wt% SiO 2 in water) were added. After stirring for 2 h, the mixture was transferred into a Teflon-lined autoclave oven and crystallized at 140 • C for 48 h under rotation conditions (50 rpm). After filtering, washing, and drying, the products were gained.
The as-made form of the products were denoted as FER-x (x represent the ratio of DMP/SiO 2 in the starting gel). The H-form of the samples (H-FER-x) were obtained by calcining and ion-exchanging the FER-x samples with the NH 4 NO 3 solution.

Methods
X-ray powder diffraction (XRD) patterns were measured with a Rigaku Ultimate VI X-ray diffractometer (40 kV, 40 mA) using Cu Kα (λ = 1.5406 Å) radiation. Scanning electron microscopy (SEM) experiments were performed on Hitachi SU-8010 electron microscopes. The N 2 sorption isotherms at the temperature of liquid nitrogen were measured using Micromeritics ASAP 2020M and Tristar system. The thermogravimetric analysis (TGA) experiments were carried out on a Perkin-Elmer TGA 7 unit in air, at a heating rate of 10 • C/min, in the temperature range from room temperature to 800 • C. 29 Si and 27 Al MAS NMR spectra were recorded on a Varian Infinity Plus 400 spectrometer. The sample composition was determined by inductively coupled plasma (ICP) with a Perkin-Elmer 3300DV emission spectrometer. The acidity of the samples was measured by the temperature-programmed-desorption of ammonia (NH 3 -TPD). The 50 mg of samples were placed in a quartz tube and pretreated in He flow at 600 • C for 30 min. Then the temperature was reduced to 150 • C. NH 3 passed through the samples until it reached equilibrium for 30 min. When the baseline was stable, the signal of NH 3 desorption was monitored by the thermal conductivity detector (TCD) in He flow at a heating rate of 20 • C/min from 150 to 650 • C.

Conclusions
In summary, aluminosilicate FER zeolite nanosheets with controllable thickness are successfully prepared with the use of a sole small organic ammonium (DMP). The addition of different amounts of DMP molecules in the starting gel is the key factor for successful control of the FER nanosheet crystal thickness. All of the nanosheets of the FER zeolite samples have good crystallinity, uniform morphology, large BET surface area, four-coordinated aluminum species, and abundant acidic sites.