Synthesis of NON single crystal from HF-SiO2-N,N-dimethyl-piperidine chloride-Et3N system

Studies in Surface Science and Catalysis, volume 158 J. (~ejka,N. Zilkov~iand P. Nachtigall (Editors) 9 2005 ElsevierB.V. All rights reserved.

239

Synthesis of NON single crystal from HF-SiO2-N,N-dimethylpiperidine chloride-EtaN system H. Xu a'b'c, L. Liu c, J.-G. W a n g a and J.-X. Dong c*

aState Key Laboratory of Coal Conversion, Institute, Chinese Academy of Sciences, Taiyuan 030001, Shanxi, P. R. China bGraduate School of the Chinese Academy of Sciences, Beijing 100039, P. R. China CResearch Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan 030024, Shanxi, E R. China Nonasil single crystals (200•215 with perfect morphology have been successfully synthesized from HF-SiO2-N,N-dimethylpiperidine chloride-EtaN system. Compared with other methods, NON prepared in this system has larger size and more regular morphology. The NON single crystals were characterized by XRD, SEM, FT-IR, 29Si NMR spectroscopy. The factors that influence the morphology of NON crystals during the crystallization have been studied. The experiments confirmed that N,N-dimethylpiperidine Chloride was a template for preparing nonasil in a near non-aqueous system. 1. I N T R O D U C T I O N The synthesis of large zeolite single crystals is of extensive interest for a large number of requirements, including single-crystal structure analysis, studies of zeolite crystal growth mechanism, studies of adsorption and diffusion, the determination of anisotropic electrical, magnetic or optical properties [1-3]. Since zeolites are metastable phases formed in hydrothermal systems, small fine-powder and small aggregated crystallites are usually formed. Therefore, efforts have been made to develop synthesis routes for large single crystals. The Fsynthesis route has been widely used in the formation of large single crystals. Addition of F favors mineralization and induces crystallization in neutral systems instead of the traditional basic system. The hydrothermal synthesis of microporous silica phases using fluoride anions instead of hydroxide anions as mineralizers has been known since the pioneering work by Flanigen and Patton [4] and extensively used by others during the last two decades [5-6]. Zeolites are normally crystallized from aqueous system. Bibby and Dale [7] first reported the synthesis of silica-SOD using an organic solvent, which opened a new route for the synthesis of zeolites and zeolites-related materials, especially for the formation of large single crystals. Using alcohol as the solvent, some aluminosilicate and all-silicate zeolite large single crystals can be synthesized [8].

240 Nonasils (NON) belong to the clathrasilicates, a distinct class of porous tectosilicates. Clathrasilicates are clathrate compounds with three-dimensional four-connected silicate host frameworks. The framework consists of only four-member rings, five-membered rings and six-member rings of interconnected [TO4] tetrahedra, which form three different types of cages: the [5464]-, the [4158]-, and the [586~2]-cages. Clathrasilicates are closely related to zeolites but possess cage-like voids that enclathrate (mostly organic) guest species during the synthesis. The openings of the cages are too small to expel or exchange these guest molecules without their decomposition [9]. NON may be prepared by hydrothermal synthesis from aqueous siliceous solutions containing various organic molecules [10-13], only badly intergrown crystals or powder of NON have been available. The reported template of NON in hydrothermal synthesis systems was cobalticinium hexafluorophosphate, trimethylene bistrimethylammonium iodide, and dimethylpiperidinium iodide. However, in a near nonaqueous system, synthesis of NON single crystals with larger size and perfect morphology in fluoride has not been reported yet. In a near nonaqueous system, large crystals of zeolite MFI, FER may be synthesized by adding F-[ 14]. In the similar path, we synthesized large single crystals of NON in HF-SiO2Et3N system with the presence of N,N-dimethylpiperidine Chloride. XRD, FT-IR, 29Si NMR and SEM characterized typical synthesized samples. The structure-directing role of N,N-dimethylpiperidine chloride in the synthesis of large single crystals was proved. 2. EXPERIMENTAL The reactants were fumed silicon (A1203<10 ppm), hydrofluoric acid (HF>40%) and N,N-dimethylpiperidine Chloride (>98%). The solvents were triethylamine (Et3N, >98%), tert-butylamine (t-ButNH2), n-dipropylamine (Pr2NH, >98%) and dibutylamine (But2NH,>98%). Certain amounts of reactants and solvent were mixed and stirred homogeneously in a Teflon-lined stainless-steel autoclave. The autoclave was then sealed, put into an oven and heated at 150-220 ~ for 10-30 days under autogeneous pressure. The product was filtered, washed and dried at 90 ~ The powder XRD data were collected in air by a RigaKu D/max 2500 using CuKa radiation, electric current of 100 mA and voltage of 40 kV. SEM photos were taken by a JEOL JSM-35C. The samples were coated with gold. The IR spectra were recorded on a Perkin-Elmer 1730 FT-IR spectrometer. The samples were prepared using the standard KBr pellet method. Solid-state NMR experiments were carried out using a Varian Infinity-plus 400 spectrometer operating at a magnetic field strength of 9.4 T. Si chemical shifts were referenced to tetramethylsilane (TMS). 3. RESULTS AND DISCUSSION 3.1. XRD pattern X-ray powder diffraction method was used for checking the phase of synthesized samples. Fig. 1 showed the XRD pattern of typical synthesized NON sample. Comparing with the standard XRD pattern of NON[ 15], the XRD data of the sample were similar to reported data

241

on the position and relative intensity of diffraction peaks. This confirmed that the sample prepared was pure NON phase.

4000 ~.

3000

9~

2000 1000 0 5

10

15

20

25

30

2 Theta/degree Fig. 1. XRD pattern of NON

3.2. Synthesis conditions Table 1 Typical synthesis conditions and product morphology Sample

Molar reaction mixture composition HF SiO2 R Et3N A 0.0 1.5 2.0 13.2 B 2.0 1.5 2.0 13.2 C 4.0 1.5 2.0 13.2 D 6.0 1.5 2.0 13.2 E 4.0 1.5 0.0 13.2 F 4.0 1.5 0.5 13.2 G 4.0 1.5 1.0 13.2 H 4.0 1.5 4.0 13.2 I 4.0 1.5 6.0 13.2 Crystallization temperature 453 K, 10 days. R: N,N-dimethylpiperidine chloride; a: single crystal

Products amorphous NON a

NON a NON a amorphous amorphous NON+Amorphous NON a NON a

In the experiments, the synthesis conditions of zeolite NON were investigated through changing the chemical composition in reactive system. The typical results were listed in Table 1. First, there was amorphous solid in given product without adding hydrofluoric acid. The given sample was zeolite NON with suitable HF in SiO2- N,N-dimethylpiperidine

242 chloride-Et3N system. Zeolite NON may be obtained in F-/Si=1.33-4.00. This showed that Fanion was an important chemicals in synthesis zeolite NON. Next, N,N-dimethylpiperidine chloride was another important chemicals in synthesis zeolite NON. Zeolite NON may be obtained in N,N-dimethylpiperidine chloride/Si=0.67-4.0. But the product was amorphous solid without N,N-dimethylpiperidine chloride. According to experimental data and reference's work [ 1], the authors believed that N,N-dimethylpiperidine chloride played a role of structure-direction agent in synthesis zeolite NON. Triethylamine served only as a solvent. The best experimental temperature was 453 K. 3.3. The effect of solvents

A number of manageable factors and conditions may affect the formation of large-size crystals. In the experiments, the solvent effect was studied by choosing tert-butylamine (t-ButNH2), n-dipropylamine (Pr2NH), triethylamine (Et3N) and n-dibutylamine (But2NH) as solvents. The molar composition of the reaction mixture was: 4.0HF: 1.5SIO2: 2.0N,N-dimethylpiperidine chloride: 13.2amine. The results were shown in Table 2. Certainly, Table 2 The effect of the solvents Solvents Template Products Et3N Yes NON a Et3N No amorphous t-ButNH2 Yes NON a t-ButNH2 No amorphous ButzNH Yes NON a ButzNH No amorphous PrzNH Yes NON a PrzNH No amorphous Crystallization temperature 453 K, l0 days. R: N,N-dimethylpiperidine chloride; a: single crystal

Crystal size 200x 170x25/xm 100x 100x 10/xm 100x 100x32#m 150x 150x27#m

the samples synthesized were also zeolite NON when tert-butylamine, n-dipropylamine, and n-dibutylamine replaced triethylamine. Then, the products were all amorphous silica without employing N,N-dimethylpiperidine chloride as template,. It proved that these organic amine solvents had not structure-directing roles. The role of them was only the solvent. SEM technique was used in observing synthesized samples. From the photos in Fig. 2, it was found that the type of solvent had obvious effect on the morphology of NON. Obviously, using other organic amine instead of Et3N, the crystal sizes of NON single crystals were different in HF-SiOz-N,N-dimethylpiperidine chloride-Et3N system. In these solvents, Et3N was the best solvent to form large and regular single crystals (200x 170x25/~m). The solvent of the sample (b) was tert-butylamine, the morphology was same with sample (a), but the crystal size was 100x 100x 10/xm, it was only nearly half of the sample (a). The solvent of the sample (c) was n-dibutylamine, the crystal size was 100x 100x32/xm, it was smaller than the

243 sample (a), but it was thicker. The solvent of the sample (d) was n-dipropylamine, the facade is not regular and glazed. From the SEM photographs we can also observe the morphology differences with different solvents, between amine and water. From the SEM photographs shown in Fig. 2, the sample (a) synthesized in the system using Et3N as solvent is perfect large single crystal. It is 25 #m thicknesses of the single crystal and the width is about 200#mx 170#m. The facade is much more regular and glazed than other NON single crystal [ 16].

(a)

(c)

(b)

(d)

Fig. 2. SEM photographs of NON crystals (a) solvent: Et3N; (b) solvent: t-ButNH2; (c) solvent: But2NH; (d) solvent: Pr2NH

244 3.4. Framework vibration

The framework vibration of NON was recorded on a Perkin-Elmer 1730 FT-IR spectrometer (see Fig. 3). According to Flanigen article's assigning method [17], there were the T-O band bending vibration, double ring stretching vibration, symmetric stretching and asymmetric stretching vibration of T-O-T in synthesized NON. The vibrational data may be assigned as: T-O band bending vibration at 455, 469 and 488cm-~; double ring stretching vibration at 586 and 633 cm-1; symmetric stretching vibration at 664, 701,720 and 783cm1; asymmetric stretching vibration at 1043, 1085, and 1107 cm -~.

?0a~ 6o o

5o

30 2O i0 1200

1100

1000

, 900

. 800

.

. 700

. 600

500

, 400

Wavenumber (era-1 ) Fig. 3. FT-IR pattern of typical samples 3.5.

29Si NMR

Fig. 4 shows the 29SiNMR spectra of the NON single crystal. The as-synthesized sample had six different chemical shifts,-105.6 ppm, 110.4 ppm, 112.5 ppm, 115.4 ppm, 116.6 ppm and 118.1 ppm. Pure silica materials synthesized in hydroxide medium in the presence of organic cations typically present a large concentration of Q3 species, i.e., Si(OSi)3OH groups, which can be detected by NMR. Typically, the concentration of Q3 species is four times larger than the concentration of positive charges in the channels [18]. By contrast, pure silica materials prepared in fluoride medium at near to neutral pH typically present a very low concentration of such Q3 species and in this sense are essentially defect-free. This is demonstrated by the lack of significant resonances assignable to Q3 species(-90 to -104 ppm chemical shift range) in the 29Si NMR spectra [19]. It can be seen that 29Si NMR of NON sample had no -90 to -104 ppm chemical shift range.

245

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Chemical shift (ppm) Fig. 4.298i NMR spectra of the NON single crystal 4. CONCLUSION NON single crystals with better morphology and large size can be synthesized in the HF-SiO2- Et3N system containing N,N-dimethylpiperidine Chloride, which acts as structure directing agent. F- and organic amine solvent provided a feasible medium together, without any one of them, the perfect crystals can not be formed. From our experiment results, the choice of organic amine solvent is also very important, it directly affects the morphology of single crystals. ACKNOWLEDGEMENTS The authors are grateful to the National Science Foundation of China (Grant No. 20373047) and the Science Foundation of Shanxi (Grant No. 20041024) for financial support. REFERENCES

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