Synthesis and characterization of titanium silicate molecular sieves with zorite-type structure

MICROPOROUS MATERIALS Microporous Materials7 (1996) 73-80

ELSEVIER

Synthesis and characterization of titanium silicate molecular sieves with zorite-type structure Hongbin Du a, Fengqi Zhou ", Wenqin Pang a'*, Yong Yue b a Key Laboratory of Inorganic Hydrothermal Synthesis, Department of Chemistry, Jilin University, Changchun 130023, China b Wuhan Institute of Physics, The Chinese Academy of Science, Wuhan 430071, China Received 23 June 1995; accepted 21 February 1996

Abstract

The titanium silicate having zorite structure has been synthesized in the presence of organic amines. The influence of various synthesis parameters such as TiO2/SiO2 and OH-/SiO2 on the synthesis of zorite is discussed. The zorite samples have been characterized by powder X-ray diffraction (XRD), IR, TGA, SEM, NMR and adsorption tests. Keywords: Synthesis; Molecular sieves; Titanium; Silicate; Zorite; NMR studies

1. Introduction

After the discovery of a large family of microporous aluminophosphates [1], members of new families of microporous materials have recently been synthesized. Among these are the M(III)X(V)O4 family (GaPO4's, InPO4's, A1AsO4's, GaAsO4's), the GeO2 [2] family, metal sulfides [3] etc. Recently, an important novel class of materials containing both octahedral and tetrahedral framework atoms has been discovered [4, 5]. It is of interest to note that such materials will most likely have new topologies, novel compositions and potentials in catalysis and in separation science, e.g., in adsorption. Among these classes of novel materials are titanium silicates. There are several natural titanium silicates with an open framework, such as verplanckite [6], * Correspondingauthor. 0927-6513/96/$15.00 © 1996ElsevierScienceB.V. All rights reserved PII S0927-6513 (96) 00014-4

muirite [7] and zorite [8]. Only a few of these could be synthesized. For example, the titanium silicate ETS-10 with three-dimensional 12-ring pores, the structure of which comprises cornersharing SiO 4 tetrahedra and TiO 6 octahedra linked through bridging oxygen atoms and displays a considerable degree of disorder, has been synthesized in the N a 2 0 - K 2 0 - T i O 2 - S i O 2 - H 2 0 TMAOH system [9,10]. Another synthetic titanium silicate is ETS-4 [11], which possesses a framework structure similar to that of the mineral zorite. It contains 12- and 8-membered ring channels and TiO 6 octahedral chains. More recently, Chapman and Roe [12] reported on the synthesis of some titanium silicate materials. They prepared the synthetic analog of the zeolitic mineral zorite using strongly alkaline titanium silicate gels as reactants, for which the chemical formula of the unit cell was Nas.o(Tis.oSi~.4012.o)Ox, and suggested that the synthesized material contained XRD amorphous sodium titanate as an impurity.

74

Hongbin Duet al./Microporous Materials 7 (1996) 73-80

In the present paper, we report on the synthesis of the zorite-type titanium silicate. The synthetic products were characterized by XRD, IR, SEM, NMR, thermal analysis, and water sorption measurements.

2. Experimental

2.1. Synthesis of titanium silicates Tetrabutyltitanate (98%), fumed silica (99.5%), sodium hydroxide (96%), aqueous hydrogen peroxide (30%) and tetrapropylammonium bromide (TPABr, Fluka) were used as reagents. Gels of the following molar compositions were prepared for the synthesis: xNa20:yTiO2: 1.0SiO2:0.15TPABr:25H20:l.8H202, where x = 0.72-3.36 and y=0.07-0.43. In a typical preparation, the following procedure was followed: Sodium hydroxide (3.2 g) was first dissolved in distilled water ( 15 ml), and then tetrabutyltitanate (4.0 g) was added slowly to the solution under vigorous stirring, to which aqueous hydrogen peroxide (8 ml) was added, forming a clear, yellow solution after stirring. Then, TPABr (1.6 g) was added. Finally, fumed silica (2.4 g) was added to the above solution. The mixture was stirred for about 1 h at room temperature, and the crystallization was carried out in an autoclave under autogenous pressure at 453 K for 3-5 days. The crystalline products were filtered, washed with distilled water and dried at ambient temperature.

2.2. Characterization methods Powder X-ray diffraction (XRD) data were recorded using a Rigaku D/MAX-IlIA diffractometer with Cu K~ X-ray radiation. The unit cell parameters were determined by a least-squares fit to the interplanar spacings of 7 to 8 strong reflections, accurately measured in the 10-40 ° 20 angular region, using ~-A1203 as an internal standard. The percent crystallinity was calculated with reference to the strongest intensities of all the XRD peaks (5-40 ° 20). A Hitachi X-650B scanning electron microscope was used for the SEM experiments. Thermogravimetric analysis (TGA) was

done on a Perkin Elmer TGA-7 thermal analyzer under a flow of N 2 at a heating rate of 10 K min -a. Infrared (IR) spectroscopy was recorded by means of a Nicolet 5DX FT IR instrument using the KBr pellet technique. 13C and 298i magicangle spinning nuclear magnetic resonance spectra (MAS NMR) were recorded on a Bruker MSL-400 spectrometer with a magnetic field strength of 9.4 T. Magic-angle spinning speeds of 4 kHz were used for 29Si and 3 kHz for 13C. The cross-polarization technique was applied at 100.6 MHz for ~3C spectra. The contact time was 5.0 ms and the scan number 1000 with a recycle delay of 2 s. 29Si spectra were obtained at 79.5 MHz, and the single pulse excitation technique was used. The acquisition parameters adopted were: pulse width, 4 #s; recycle delay, 4 s. The chemical shifts were relative to external standards of tetramethylsilane. Adsorption was measured isothermally in a McBai-Bakr balance. The titanium silicate sample was activated by heating to the final activation temperature 453 K while maintaining a vacuum of < 10 -3 mbar for about 3 h, and the sample was cooled to room temperature under vacuum. Chemical analysis of the crystallized products was performed by the ICP-AES method (Leeman Labs., USA).

3. Results and discussion

3.1. Synthesis 3.1.1. Effect of the T i 0 2 / S i O 2 ratio The effect of TiO2/SiO 2 on the crystallization of zorite is shown in Fig. 1. Curves a-c represent the crystallization of reaction mixtures of TiO2/SiO2=0.10 , 0.24 and 0.43 respectively. Changing the titanium content in the reaction mixture shows that both the induction period and the crystal growth strongly depend on the TiO2/SiO 2 ratio. As the titanium content increased, the rates of nucleation and crystal growth increased, indicating the formation of more nuclei responsible for nucleation and subsequent crystallization. This is different to the synthesis of the titanium silicates TS-1 [13] and TS-2 [14]. However, the effect is not significant above a

Hongbin Duet al./Microporous Materials 7 (1996) 73-80

75

b

100

80

"E 60 (n

0

40

C tt'-

20

>

m ©

10

20

30

40

50

60

Time (h)

b

Fig. 1. Influence of the TiO2/SiO2 ratio on the crystallization of zorite (a) TiO2/SiO2=0.10, (b) TiO2/SiO~=0.24 and (c) TiOz/SiO 2 =0.43. T =453 K, OH /SiO 2 = 1.9, H20/SiO 2 = 25, TPABr/SiO 2 = 0.15.

TiO2/SiO2 ratio of 0.24. In our synthesis of zorite with TiO2/SiO2<0.07, zorite accompanied with other unknown phases was obtained (Fig. 2a), and with TiO2/SiO2>0.5, the crystallinity decreased and impurity products were synthesized (Figs. 2c and 2d). Table 1 lists the TiO2/SiO2 molar ratios in the reaction mixture and in the crystalline products with their corresponding yields (grams of zorite obtained per gram of gel). As the TiO2/SiO2 molar ratios in the mixture decreased, the yield of the crystalline products decreased rapidly. The TiO2/SiO2 ratio in the crystalline products, however, changed slowly. The results indicate that zorite has a narrow TiO2/SiO2 ratio in the framework, although it can be synthesized from gels with a broad range of TiO2/SiO2 molar ratios. This can be also proven by the following result: No Ti was detected in the mother liquor after filtration of the crystalline solids, but a large amount of Si in the mother liquor of low TiO2/SiO2 mixtures was found. With increasing Ti content in the reaction mixtures, the size of the zorite crystals decreased. At low Ti concentration, the rates of nucleation and crystallization were very low, and large crystallites

10

20

30

40

Two-Theta

Fig. 2. X-ray diffraction patterns of zorite showing the influence of the TiO2/SiO2 ratios on the crystallization. Curves a-d correspond to the TiO2/SiO2 ratios in the gel of 0.07, 0.24, 0.50 and 0.60, respectively. Table 1 Chemical composition and product yields of crystalline zorite a TiOz/SiO2 in the mixture

TiO2/SiOz in the product

Yields of products (%)

0.43 0.29 0.14 0.07

1.00 0.71 0.59 0.56

45 25 8 7

a 1.0SiO2:l.92NaOH:0.15TPABr:25H20:l.8H202 5 days.

' T=453 K,

could be produced. As seen in Fig. 3a, the samples of zorite synthesized at a TiO2/SiO2 ratio of 0.07 with TPABr as the template were composed of single, platelet crystallites, approximately 40 × 13 #m in length and width. The samples prepared at a TiO2/SiO2 ratio of 0.4 were composed

Hongbin Duet al./Microporous Materials 7 (1996) 73-80

76

%

'""

,,

Fig. 3. Scanning electron micrographs of zorite synthesized with TPABr as template at (a) TiO2/SiO 2 =0.07, (b) TiO2/SiO 2 = 0.40, and (c) with T M A O H as template (TiO2/SiO2 =0.07).

of small crystallites (Fig. 3b). With TMAOH as the template, t h e same results were observed, except that most of the zorite crystals were intergrown or twinned aggregations (Fig. 3c).

3.1.2. Effect of the OH-/SiO 2 ratio The effect of changing the OH-/SiO2 molar ratio in the initial reaction mixture on the crystallization of zorite is illustrated in Table 2. It is obvious that the OH-/SiO 2 molar ratio plays an important role in the formation of zorite. Compared with the synthesis of aluminosilicate zeolites, the crystallization of zorite needed relatively high alkalinity. At low sodium hydroxide concentration, some titanium silicate zeolites were favored by using special organic amines as templates, for example, TS-1 by TPABr [13], TS-2 by TBABr [14] and ETS-10 by TMAOH [9,10] etc. In our synthesis (OH-/SiO 2 = 0.72), a ZSM-5 type

zeolite accompanied by an amorphous phase was obtained. As the concentration of sodium hydroxide increased, an unknown phase with a small amount of zorite was crystallized (Fig. 4a). At OH-/SIO2=1.9-3.0, good crystallites of zorite were synthesized (Fig. 4b). On further increasing

c

r~

Table 2 Influence of the O H - / S i O 2 ratio on the crystallization of zorite a OH-/SiO 2

Time (days)

Crystal phase

0.72 1.44 1.92 2.88 3.36

20 l0 3 3 20

ZSM-5 Zorite + unknown Zorite Zorite Am b

a 1.0 SiOz:O.24TiOz:O.15 TPABr:25 H20:1.8 HzO2, T=453 K b Am = amorphous

a

10

20

30

40

Two-Theta Fig. 4. X-ray diffraction patterns of zorite showing the influence of the OH-/SiO2 ratios on the crystallization, a - d correspond to the O H - / S i O 2 ratios in the gel of 1.0, 1.9, 2.8 and 3.4, respectively.

Hongbin Duet al./Microporous Materials 7 (1996) 73 80

in sodium hydroxide concentration, the crystallinity of zorite became poor (Fig. 4c). Beyond the OH-/SiO 2 ratio of 3.4, no crystalline product was obtained. Perhaps, the crystalline material was redissolved at very high alkalinity. 3.2. Characterization 3.2.1. XRD analysis The X-ray diffraction pattern of one sample (Fig. 5a) clearly shows that the synthesized products have the zorite structure. The pattern is matched to that of the mineral zorite [15], which is shown in Fig. 5b for comparison. The pattern can be indexed, and the unit cell parameters are given in Table 3, along with their elemental compositions. Chapman and Roe [12] suggested that the unit cell composition ofzorite, based on the stoichiomeI

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Fig. 5. X-ray diffractionpatterns of (a) as-synthesizedzorite and (b) the mineralzorite.

77

try, corresponds to Na6Ti5Si12037, and concluded that synthetic zorite with a TiO2/SiO2 ratio of 0.62 contained some X-ray amorphous sodium titanate as an impurity. In our synthesis, the synthetic zorite has a TiO2/SiO2 ratio of 0.51-1.0. However, the unit cell parameters change very little. In addition, the zorite prepared with high TiO2/SiOz ratios in the gel is accompanied by other phases, possibly by titanates, according to the X-ray diffraction pattern (Fig. 2d). Thus, we presume that the synthetic zorite could also contain some amorphous sodium titanate. 3.2.2. Infrared spectroscopy The infrared spectrum of one sample is shown in Fig. 6. It contains bands at 1128, 980, 697, 671, and 451 cm -1, which are similar to those in aluminosilicates. According to Flanigen et al. [16], the bands at 1128, 980, 697 and 671 cm -1 can be attributed to the asymmetric and symmetric stretching vibrations of the framework TO4 polyhedra. The band appearing at 451 cm-t may be due to the vibrations of the T-O bending modes. One additional strong band appeared at 916 cm 1 and a shoulder at 815 cm 1. For the titanium silicate TS-1, an additional band at 960 cm -1 has been attributed to the vibrations of non-bridging Si-O bonded to a Ti 4+ (O3Si-O-Ti) ion in the SiO4 structure. This band has been claimed to indicate the incorporation of titanium into the framework of the silicalite structure [17,18]. Recently, Gabelica-Robert and Tarte [19] investigated the infrared and Raman spectra of fresnoite and fresnoite-like pyrosilicates on the basis of a factor group analysis and with the help of 28-298i and 4'~-5°Ti isotopic shifts. The bond at about 915 cm -1 in Ba2TiSi20 8 was assigned to the locally antisymmetric Vas(SiO3) mode. The bands in the 800-900 cm-1 region were assigned to the normal modes including the v(SiO3) and the y(Ti-O) vibrations. Thus in the as-synthesized zorite, the band at 916 cm-1 may be attributed to a stretching mode of a Si-O- group. The shoulder at 815 cm-1 may be due to the vibrations of the Ti-O and Si-O bond. 3.2.3. NMR results The absence of a 13C MAS NMR signal in the as-synthesized samples is a direct proof for the

Hongbin Duet al./Microporous Materials 7 (1996) 73-80

78

Table 3 The unit cell parameters and compositions for zorite Unit cell composition

Na6(TisSi12)Oas(OH)4 • 11H20 Nas.o(TiT.4Si12.o)Ox Na6.3(Ti6.1Sil2.0)Ox Na6.5(Ti6.sSi12.o)Ox Na~.2(Tis.sSi12.0)Ox

a

b

c

23.241 (7) 23.234(10) 23.743 23.744 23.740

7.238(4) 7.185(4) 7.307 7.308 7.307

6.955(4) 6.964(4) 7.144 7.144 7.139

0 t-.

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I-

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o

I

1400

Ref.

Unit cell parameters (,~)

1000

I

I

800

600

I

400

Wavenumber (cm"1 )

Fig. 6. Infrared spectra of as-synthesized zorite.

absence of occluded template. Element analysis shows that the as-synthesized samples do not contain organic molecules. Similar results have been found for the aluminophosphate VPI-5 [20], which was synthesized with n-dibutylamine (DBA) as organic template. In the channels of VPI-5 water molecules were occluded rather than organic amine molecules. 29Si MAS N M R shifts are in particular sensitive to the O-Si-O bond angle which is in turn very sensitive to the nature of the element in the coordination sphere surrounding the SiO4 tetrahedra: The larger the bond angle the lower the chemical shift [21]. Similarly, Anderson [22] et al. studied the 29Si N M R spectrum of the titanium silicate ETS-10 and attributed the three resonances at -94.1, -95.8 and -96.5 plum to Si(3Si, 1Ti) and the resonance at -103.3 ppm to Si(4Si, 0Ti). In

[8] [ 12] this work this work this work

the as-synthesized zorite, two lines with chemical shifts of - 9 0 . 7 and - 9 5 . 8 p p m referenced to tetramethylsilane were observed (Fig. 7). The resonance at -95.8 ppm can be attributed to Si(3Si, 1Ti), and the other to Si(2Si, 2Ti). The ratios of these two environments in all samples are the same. These assignments are consistent with the framework connectivity of zorite shown in Fig. 8. In the a, b plane, infinite chains of TiO 6 cornersharing octahedra lined up in the b direction are surrounded by vertex-shared SiO4 tetrahedra such that each titanium is connected via oxygen to 2 silicon atoms and each silicon to 2 titanium atoms. These chains are connected by Si tetrahedra and Ti octahedra in such a way that 12-membered and 7-membered rings are formed. The silicon atoms located in a 12-membered or a 7-membered ring

J

i

-20

-40

-60

-80

-100

i

-120

i

-140

PPM

Fig. 7. 29Si MAS NMR spectrum of zorite (TiO2/SiO2 =0.60 in the products).

79

Hongbin Duet aL/Microporous Materials 7 (1996) 73-80

/,

them by powder X-ray diffraction. The results show that zorite began to lose its crystallinity at 523 K and became an amorphous phase when held at 823 K for 3 h (Fig. 10).

•

Ti

•

Si

lb a Fig. 8. Schematic diagram of the structure of zorite in the a, b plane.

are Si(2Ti, 2Si) and Si(1Ti, 3Si), respectively. These distinct types of silicon atoms can be resolved in the 29Si MAS N M R spectrum. 3.2.4. Thermogravimetry and thermal stability TGA results for as-synthesized zorite are shown in Fig. 9. Examination showed a total weight loss of 12.4%. This weight-loss, beginning at 373 K and completed at 573 K, is considered to be mainly due to the removal of water located on the external surface and in the pores. The thermal stability of as-synthesized zorite was examined by taking portions of samples calcined at various temperatures, and analyzing

3.2.5. Adsorption The water adsorption isotherm for the as-synthesized titanium silicate is shown in Fig. 11. It resembles that of NaX or AIPO4-17 and is a typical type I adsorption isotherm indicating that the zorite surface is hydrophilic. The adsorption capacity for water is 13 wt.-% at P/Po=0.2. The standard 'plug gauge' molecules such as n-hexane and cyclohexane [23] were used to determine the size of the pore openings to the channels of the microporous material. 1 wt.-% of n-hexane is adsorbed which reflects extracrystalline adsorption on the external zorite surface. The results show that the pore size of the as-synthesized zorite is

C

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I

t~ 100

a =O")

90

80

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30

150

I

I

270

I

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I

390

n

I

510

10

a

630

20

30

40

Two-Theta

Temperature (°C) Fig. 9. TG curve for zorite.

Fig. 10. X-ray diffraction patterns of zorite calcined at (a) 423 K, (b) 523 K, (c) 623 K, and (d) 823 K for 3 h, respectively.

Hongbin Duet al./Microporous Materials 7 (1996) 73-80

80

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12

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8

~° 6 E 4

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0

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0.2

0.5

0.4

0.5

0.6

0.7

0.8

P/Po

Fig. 11. H20 adsorption isotherm for the as-synthesized zorite at 293 K.

less t h a n 4.3 A. Similar results have been r e p o r t e d by K u z n i c k et al. [24].

4. Conclusions T i t a n i u m silicates with z o r i t e - t y p e structure have been synthesized in the presence o f a n o r g a n i c amine. T h e TiO2/SiO2 a n d O H - / S i O 2 ratios p l a y i m p o r t a n t roles in the f o r m a t i o n o f zorite, a n d the l a t t e r is a critical p a r a m e t e r in the synthesis. T h e as-synthesized samples c o n t a i n n o o r g a n i c a m i n e s a n d c a n a d s o r b a significant a m o u n t o f w a t e r after being a c t i v a t e d at 453 K u n d e r v a c u u m . T h e investigation o f t h e r m a l stability shows t h a t synthetic zorite is n o t stable a b o v e 573 K. N M R results allow a distinction between v a r i o u s types o f silicon a t o m s in zorite.

Acknowledgement W e are grateful to the N a t i o n a l N a t u r a l Science F o u n d a t i o n o f C h i n a a n d the K e y L a b o r a t o r y o f Inorganic Hydrothermal Synthesis o f Jilin U n i v e r s i t y for s u p p o r t s .

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