Synthesis and characterization of gallosilicate mesoporous molecular sieves SBA-15

Studies in Surface Science and Catalysis 156 M. Jaroniec and A. Sayari (Editors) 9 2005 Elsevier B.V. All rights reserved

133

Synthesis and characterization of gallosilicate mesoporous molecular sieves SBA-15 C.-F. Cheng* and H.-H. Cheng Department of Chemistry and Center of Nanotechnology, Chung Yuan Christian University, 22 Pu-Jen, Pu-chung Li, Chung-Li (32023), Taiwan. E-Mail: [email protected]

A series of gallosilicate nanoporous materials with the SBA-15 structure have been directly synthesized by using a novel method controlling the TMAOH/Ga mole ratio in gel and characterized by ICP elemental analysis, powder XRD, N2 adsorption/desorption, ~3C, 71Ga MAS NMR, SEM and TEM. When the TMAOH/Ga molar ratio in gel increases, ratio of gallium incorporated in SBA-15 structure increases, and furthermore pore size is enlarged. N2 adsorption/desorption measurement and TEM demonstrate high mesoporosity of gallosilicate SBA-15. More than 90 % of Ga is 4-coordinated and incorporated into the framework of SBA-15 as revealed by 71Ga MAS NMR spectra. 1. INTRODUCTION The introduction of gallium into silicates or aluminosilicates results in high selectivity to aromatics in the catalytic conversion of olefins and paraffins. The cyclar process, which transforms C3-C5 alkanes into aromatics and hydrogen, was developed jointly by UOP and BP and proceeds over gallium- and/or zinc- containing zeolites. H-forms of zeolites are active catalysts for aromatization of light alkanes and alkenes. Aromatization proceeds simultaneously with cracking and hydrogen-transfer reaction which lead to light alkanes, and the redistribution of hydrogen atoms severely restricts the maximum yield of aromatic products. Gallium sites also exhibit good catalytic activity for alkane dehydrogenation. Some of the hydrogen lost during aromatization appears in the products as molecular hydrogen, thus considerably enhancing the selectivity toward aromatics [1-3]. The synthesis and catalytic application of gallosilicate MCM-41 have been reported by various research groups [4-8]. Klinowski et al. [4-6] first reported synthesis and characterization of a range of gallosilicate mesoporous sieves MCM-41 prepared by using gallium nitrate as the source of gallium. In 2000, Chatterjee et al. [7] reported characterization and synthesis of highly ordered gallium MCM-41 with varied Si/Ga (10-100) ratio at room temperature. In 2001, Okumura et al. [8] illustrated the prominent catalytic activity of Ga-containing MCM-41 in the Friedel-Crafts alkylation and the highest yield of diphenylmethane was obtained on the Ga impregnated MCM-41. Recently, ~hao et al. [9] reported the synthesis of a novel hexagonal mesoporous silica denoted SBA-1 $ prepared with nonionic triblock copolymer of EO20POToEO20to organize the polymerizing silica precursor and form a hexagonal structure. Besides its large uniform pore size (up to 30 rim), this material has thicker walls than MCM-41, resulting in much higher stability. The hexagonal mesoporous silica molecular sieve SBA-15 has already been tested

134 for several applications in the fields of catalysis, [10] separations [ 11], and advanced optical materials [ 12]. Many efforts have been carried out to prepare SBA-15 containing heteroatoms in the framework such as titanium [ 13], vanadium [ 14], zirconium [ 15] and aluminum [ 16] to generate the catalytic activity of selective oxidation and cracking. Incorporation of gallium into the siliceous framework of SBA-15 by direct synthesis seems unlikely because SBA-15 is synthesized in strong acid media (pH value < 1) and most gallium species do not favor the condensation with silica source under this condition. In this paper, we first report a direct synthesis of high quality gallosilicate SBA-15 by using a novel method controlling the TMAOH/Ga mole ratios in gel. Most gallium is incorporated into the framework of SBA-15 silica in four-coordination. These gallosilicate SBA-15 has been characterized with XRD, N2 adsorption/desorption measurement, ~3C, 71Ga MAS NMR, ICP-AES, SEM and TEM. 2. E X P E R I M E N T A L S 2.1. Materials

The gallosilicate SBA-15 materials were prepared using gallium(III) nitrate octahydrate, Ga(NOa)3"8H20, (Acros) and tetraethyl orthosilicate, TEOS, (Aldrich) as a gallium and silica sources, respectively. In our typical synthesis, amphiphilic triblock copolymer, poly(ethylene oxide)20-poly(propylene oxide)70-poly(ethylene oxide)20, (EO20POToEO20, average M.W. 5800, Aldrich) was dispersed in double distilled water. This solution was mixed with gallium nitrate hydrate and tetramethylammonium hydroxide (TMAOH) under stirring to obtain homogeneous solution. Finally, tetraethyl orthosilicate was added to this homogeneous solution with stirring. This mixture gel was stirred at room temperature for 3 days and then crystallized at 363 K for 20 h. After crystallization, the gallosilicate SBA-15 products were filtered off, washed with distilled water, dried in air at 70 ~ and calcined at 550 ~ for 6 h. The molar composition of final gel mixture was 1 TEOS : 220 H20 : 0.1 Ga(NO3)3 : 0-0.3 TMAOH : 0.017 EO20PO70EO20, and these gallosilicate SBA-15 samples are designated as GaSBA- 15-10, where l 0 stands for the Si/Ga molar ratio in initial gel mixture. 2.2. Characterization methods

X-ray Diffraction. The X-ray diffraction (XRD) patterns of calcined sample were recorded using beam line BL17A of the National Synchrotron Radiation Research Center (NSRRC), Taiwan, with a wavelength of 0.1326 nm. N2 Adsorption Measurement. The specific surface area, AaET, was determined from the linear part of the BET plot (P/P0 = 0.05 to 0.30) using Micromeritics ASAP 2010 system at 77 K. The pore size was obtained from maximum of pore size distribution using BJH model applied to adsorption data. ICP-AES. The content of Si and Ga were determined using a Jarrell-Ash ICAP 9000 inductively coupled plasma - atomic emission spectrometer (ICP-AES). Solid-State NMR. MAS NMR spectra were recorded at 9.4T using a Bruker Avance 400 spectrometer, zirconia rotors 4 mm in diameter spun at 7 kHz. 7~Ga spectra were measured at 122.0 MHz with 15 o pulse, 0.5 s recycle delay. External Ga(H20)63+ was used as a reference. 13C spectra were measured at 100.6 MHz using the CP technique and TMS as a reference. Scanning Electron Microscopy. Morphology and size of particles were taken with a Field Emission JEOL-JSM 6330F instrument operated at an accelerating voltage of 15 keV. Transmission Electron Microscopy. TEM microphotographs were taken with a JEOL-JEM 2010 instrument operated at 200 keV.

135

3. RESULTS AND DISCUSSION 3.1 X-ray Diffraction. In a typical synthesis of Ga-SBA-15 in 2 M HC1 aqueous solution at a fixed Si/Ga molar ratio of 10 in gel, the product has a good structure and crystallinity of hexagonal SBA-15 (not shown) but lessl than 0.02% of gallium was incorporated into SBA-15 product as listed in Table 1. Even though the acid concentration decreases to 0.1 M, 1.4 % gallium was merely incorporated into SBA-15 product. In strong acid environment, it is very difficult to incorporate gallium into SBA-15 framework due to the unfavorable condensation reaction between gallium hydrate cation and cationic silicate oligomer. It appears that adjusting the acid concentration of final gel to lower value results in increasing the ratio of gallium in SBA-15 product. Therefore, adding base such as TMAOH to gel can raise pH value of final gel, and thus lead to increase the ratio of gallium incorporated into SBA-15 products. It should be noticed that the ratio of gallium incorporated into SBA-15 is determined by pH value of final gel instead of initial gel. However, it is quite difficult to control the pH value of final gel simply by direct adjustment of pH value for initial gel because not only the amount of gallium but also gelling time and temperature affect the pH value of final gel. A novel method controlling the TMAOH/Ga mole ratios in gel was used to dominate the final acid concentration and gallium amount incorporated into SBA-15 structure.

(B)

(A)

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8 1I0 112 114 116 18

Pore diameter (nm)

Fig. 1. (A) XRD patterns and (B) pore size distributions of calcined GaSBA-15-10 synthesized at different TMAOH/Ga molar ratio. (a) 0, (b) 0.25, (c) 0.5, (d) 0.75, (e) 1.0, and (e) 1.5. Fig. I(A) shows the XRD pattems of the gallosilicate SBA-15-10 products obtained using different TMAOH/Ga molar ratios from 0 to 1.5 at initial gel without adding acid at a fixed Si/Ga molar ratio of 10 in initial gel, which compositions of products as determined by ICP-AES are also shown in Table 1. It was noticed that long reaction time for three days is required instead of one day for crystallizing SBA-15 in 2M HC1 due to deficient acid catalysis. Samples prepared with TMAOH/Ga molar ratio below 0.75 all exhibit a well-ordered hexagonal SBA-15 structure with three characteristic XRD peaks of (100), (110), (200).

136 When the TMAOH/Ga molar ratio is exceeds 0.75, an increase in the TMAOH/Ga molar ratio reduces the (100) reflection intensity, suggesting a gradual lowering of the crystallinity of the product. Nevertheless, the proportion of gallium incorporated into the SBA-15 products rises from 20% to 43 %, as indicated in Table 1. The amount of gallium incorporated into the SBA-15 structure increases with the TMAOH/Ga molar ratio at the expense of the crystallinity of hexagonal SBA-15 because of insufficient hydrogen bonding and the electrostatic interaction between the surfactant and the gallosilicate oligomer, to construct the SBA-15 architecture. Hence, an adequate acid concentration in the final gel must be maintained for balance and to optimize the proportion of gallium incorporated into the SBA-15 products and the crystallinity of the hexagonal SBA-15.

3.2 Pore Size Distribution and Solid-state NMR Fig. I(B) shows pore size distribution of gallosilicate SBA-15 synthesized at different TMAOH/Ga molar ratios. As the TMAOH/Ga molar ratio in gel increases, the pore diameter increases from 6.9 to 8.4 nm and total pore volume increase from 0.9 to 1.38 cm3/g in GaSBA15-10 materials as listed in Table 1. ]3C MAS NMR spectra of gallosilicate SBA-15 materials prepared at different TMAOH/Ga molar ratios are shown in Fig. 2. A consistent increase in the intensity of the line at c a . 58 ppm attributed to methyl group in TMAOH with increasing TMAOH/Ga molar ratio indicates that it is proper to attribute partly the pore enlargement of GaSBA-15 to TMAOH probably situated between silicate and micelles and further expanding the micelle size. 7]Ga MAS NMR spectra of calcined GaSBA-15-10 prepared at different TMAOH/Ga ratios in Fig. 3. all exhibit one major resonance peaks at ca. 140 ppm from four-coordinated [17] gallium (framework) and another minor resonance peak at ca. 0 ppm from six-coordinated gallium (extraframework), showing that more than 90% of Ga is incorporated into SBA-15 structure for all samples except that prepared without adding TMAOH. Thus, it appears that TMAOH can assist in incorporating Ga into SBA-15 structure. The intensity of 140 ppm peak increases with increasing TMAOH/Ga ratios, in agreement with ICP results, again indicating that increasing of TMAOH/Ga molar ratio give rise to increase gallium ratio incorporated in SBA-15 structure owing to favor condensation of Ga species and silicate ion in lower acid solution. nTMAOH I nG, ~ :

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Chemical Shift (ppm) Fig. 2. ~3C MAS NMR spectra of as-made GaSBA- 15-10 prepared at different TMAOH/Ga molar ratios. TMAOH/Ga molar ratio = (a) 0, (b) 0.25, (c) 0.5, (d) 0.75, and (e) 1.5.

o' bo

Chemical Shift (ppm) Fig. 3. 71GaMAS NMR spectra of calcined GaSBA- 15- ! 0 prepared at different TMAOH/Ga molar ratios. TMAOH/Ga molar ratio = (a) 0, (b) 0.25, (c) 0.5, (d) 0.75, and (e) 1.5.

137 3.3 SEM and TEM images. SEM images in Fig. 4(A) reveals that the extracted GaSBA-15-10 product has rod-like morphology with the length around 2 ~tm. TEM image in Fig. 4(B) views with a slanted angle with the pore axis, and therefore both of pore and perpendicular channel can be viewed in the same microghaph. TEM image reveals not only a regular hexagonal array of uniform channels but also pore channel along the rod.

Fig. 4. (A) SEM and (B) TEM micrographs of gallosilicate SBA-15. Table 1 Physical properties of calcined gallosilicate SBA- 15-10 prepared by using different TMAOH/Ga molar ratios in gel. i

~,nthesis Condition

Si / Ga of pH of final gel

nTM~o./nc~products nsi/n~a in initial gel in initial gel (ICP) 10a 0 4871

.

.

SBET (cm2/g)

Do Vt (cm3/g) (rim)

< 1

754

0.93

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3.4

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.

hw

(nm)

.

l0 b

0

712

<1

676

0.78

6.4

4.1

10

0

50.1

1.78

617

0.90

6.9

5.6

10

0.25

41.7

1.83

743

1.00

7.4

5.1

10

0.50

32.5

1.92

765

1.24

7.7

4.8 5.0

10

0.75

29.2

1.96

848

1.27

7.9

10

1.0

26.0

2.06

634

1.28

8.1

4.8

10

1.5

23.4

2.37

642

1.38

8.4

5.3

a" Prepared by using 2.0 M HC1, b" Prepared by using 0.1 M HC1 SBET" BET Surface Area, Vt" Total Pore Volume calculated from P/Po = 0.99, Dp: Pore Diameter (a 0 = (2 / ~/-3)x dl00), hw: Wall Thickness (hw i

,

ao-

,,,

Dp). ,,

ACKNOWLEDGMENTS The supports of this work by the National Science Council, Taiwan (NSC92-2113M-001-049 & NSC93-2745-M-033-004) are gratefully acknowledged.

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