Syntheses of mesoporous aluminosilicates from layered silicates containing aluminum

H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials

109

Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.

Syntheses of mesoporous aluminosilicates from layered silicates containing aluminum. S. Inagaki, Y. Yamada and Y. Fukushima Toyota Central R&D Labs., Inc., 41-1, Nagakute, Aichi, 480-11, JAPAN Addition of Al(NO3)3~

or NaAIO 2 as an AI source formed crystalline layered sodium

silicates containing AI. Aluminosilicate mesoporous molecular sieves were synthesized by heating

the

layered

sodium

silicates

with

various

amounts

of

AI

in

hexadecyltrimethylammonium chloride solution. The mesoporous aluminosilicates with hexagonal regularity and uniform pore-size distribution had only tetrahedral Al in the framework and large amount of acidity. Although the addition of Al(NO3)3~

formed not only the

layered sodium silicates but also cristobalite (SiO2) crystal, the addition of NaA10 2 formed no crystobalite. It suggested isomorphous substitution of Al for Si in the SiO 2 framework of the layered sodium silicates and the mesoporous materials. 1. INTRODUCTION Mesoporous molecular sieves such as MCM-411) and FSM-162) have attracted much attention, because of their applicability to catalysts and adsorbents for large size molecules. FSM-16 is synthesized from a layered sodium silicate by using surfactants 3). A folded sheet mechanism has been proposed for the formation of FSM-16 3'4).The topochemical synthetic method of FSM16 is expected for a new strategy for synthesizing various inorganic materials. Several attempts 5-9)have been made to incorporate AI in siliceous framework of the mesoporous materials and apply the aluminosilicate mesoporous materials to various catalytic reactions such as a synthesis of porphyrin 1~ Although aluminosilicate MCM-41 materials containing only tetrahedral Al in the framework have been synthesized, their structural regularity were lower than those of silicious MCM-41 materials 6'7). Only one or two peaks at a low angle region in X-ray diffraction patterns were observed for the aluminosilicate MCM-41 materials with a relatively low Si/Al ratio under 50. The structural regularity decreased with increasing Al contents and their diffraction peaks due to the hexagonal regularity disappeared for the samples with the Si/

110 AI ratios of 26) or 57). We also reported that an impregnation of FSM-16 with aluminum chloride aqueous solution produced a mesoporous aluminosilicate 5), although, the mesoporous materials contained AI in not only a tetrahedral site but also an octahedral site. The amount of acid was about an half of that on ZSM-5 with the same AI203 contents. Here we have tried to synthesize aluminosilicate mesoporous molecular sieves with high regularity and only tetrahedral AI from a layered sodium silicates containing AI. 2. EXPERIMENTAL

2.1. Preparation of Layered Sodium Silicates Containing Aluminum. Aqueous solutions of Al(NO3)309H20 or NaAIO 2 were added in sodium silicate aqueous solution (SiO2/Na20=2), and the mixtures were stirred at 50* C for 3h. The solutions were dried at 100 *C for 12h and vacuum-dried at 70 *C for 12h to yield sodium aluminosilicate glasses. The glasses were calcined at 700 *C for 6h to crystallize to layered sodium silicates containing AI. The Si/AI molar ratio in the initial mixtures ranged from 2.5 to 100 as listed in Table 1.

2.2. Conversion to Mesoporous Aluminosilicates. The layered sodium silicates were dispersed in water and stirred at room temperature for 3h. The filtered samples (50 g) were dispersed in 0.1 mol dm3 hexadecyltrimethylammonium chloride aqueous solution (1000 cm 3) and heated at 70 *C for 3h. 2N HC1 aqueous solution was added to the dispersion to adjust the pH value to 8.5, and they were maintained at 70 ~ for further 3 h. The filtered samples were dried at 60 ~ and calcined at 550 ~ for 6 h to remove organic fractions. Consequently, we got Al-containing FSM-16 samples with the bulk Si/AI molar ratio between 7.2 and 188 as listed in Table 1. The Si/AI ratios in the Al-containing FSM-16 materials were determined by inductively coupled plasma-atomic emission spectrometry. The higher Si/A1 ratios of the final products than the initial solutions were due to a dissolution of A1 during the dispersion of the layered sodium silicates in water.

2.3. Characterizations. X-ray powder diffraction pattems were obtained

by

using

a

Rigaku

RAD-B

Table 1 AI sources and Si/A1 molar ratios. Si/A1 ratio A1 sources

Initial solution 100 AI(NO3)3"9H20 50 20 5 4 NaAIO2

50 5 2.5

Final products 188 67 38 9.9 7.7 132 12 7.2

111 diffractometer with Cu-Kct radiation9 Solid state MAS NMR spectra were measured on a Bruker MSL-300WB spectrometer9 295i MAS NMR spectra were recorded at a frequency of 59.620 MHz spinning 4 kHz using pulses at 90-s intervals and 360 scans9 27A1MAS NMR spectra were recorded at a frequency of 78.205 MHz spinning 4 kHz using pulses at 2-s intervals and 1000 scans. Ammonia-TPD spectra were measured with a conventional TPD apparatus, in which the desorbed materials were detected by a thermal conductivity detector. About 0.1 g of the samples were vacuum-dried at 500* C for lh. The dried samples were then exposed to 13.3 kPa of an ammonia gas at 100 ~C for 45 min, followed by evacuating at 100 ~C for lh. The TPD spectra were measured from 100 ~ to 1000 *C at a heating rate of 10 ~C/min. in a helium flow as carrier gas controlled at 4.0x10 3 of W/F.

3. RESULTS AND DISCUSSION Figs. 1 and 2 show X-ray diffraction pattems of Al-containing FSM-16 prepared by using Al(NO3)39

and NaAIO z as AI sources9 The FSM-16 samples with Si/A1 ratios of 188, 132,

9 cristobalite

/ 8 r

1..8i~ ' /] -t / :

~

~,,~,~,,.~_~__'

10 20 20 (CuKa)

30

67 / .."

/

-

/ /

38 ~

....9..,.9.....

....7..,2... 10

40

Figure 1. X-ray diffraction patters of aluminosilicate FSM-16 materials various Si/A1 ratios prepared by using AI(NO3)a.9H20 as AI source.

112

,

0

5

10

.

.

10

1

3

2

"

20

' ~

30

40

20 (CuKet) Figure 2. X-ray diffraction patters of aluminosilicate FSM-16 materials various Si/AI ratios prepared by using NaAIO 2 as A1 source.

67, 38 and 12 showed three Or four obvious peaks assignable to hexagonal symmetry at an angle smaller than 10 ~ The diffraction patterns indicate that their structural regularities were higher than those of MCM-41 materials with the same Si/AI ratio reported previously6-9). For the samples with the Si/A1 ratio of 9.9, though the peak intensity was reduced, two or three peaks were still observed in a low angle region, which suggested preservation of hexagonal regularity. The unit cell dimensions of the samples prepared by using AI(NO3)3-9H20 decreased from 4.3 to 4.0 nm with decreasing the Si/A1 ratio from 188 to 9.9. Those of the samples with the Si/A1 ratios of 132 and 12 prepared from NaAIO 2as an AI source were 4.4 nm. While, the products with the low Si/AI ratios of 7.7 and 7.2 showed only one broad peak in a low angle region. On the othe hand, the product (Si/AI=7.7) prepared from AI(NO3)3-9H20 showed several peaks due to the other crystal in a high angle region. Those peaks were assigned to sodium aluminum silicate hydroxide hydrate crystal. FSM-16 materials were not formed in such a high AI content, which was attributable to using non-layered sodium silicates as starting materials formed from the sodium aluminosilicate glasses mentioned later. Diffraction peaks due to cristobalite crystal were also observed in a higher angle region for all the samples prepared by using AI(NO3)3"9H20, and their intensity was increased with increasing the added amount of AI(NO3)3.9H20. The samples prepared by using NaA102 had almost no formation of cristobalite as shown in Figure 2. These results suggested that the introduction of A1 ions into

113 the sodium silicates without any supply of Na ions expelled Si from the framework of the layered sodium silicate and FSM-16.

The AI introduction incorporating with Na formed sheet

silicates including A1 and FSM-16 without ejecting Si to form crystobalite crystals, which is represented by the following chemical equations, (1) and (2). Na2Si205 + xAI(NO3)3 + (3x/2)H20 ---> Na2Si2.xAl H30s +xSiO 2 +3xNO 2 +302

(1)

(1-x)Na2Si20 s + (2x)Na~O 2 + xH20 -> Na2Si2zAl2xH2xOs

(2)

4-AI

03 04 9

i

/AI= 12

/A,J= 38 ,

,

200 100

0 -100 -200 ppm Si / AI m

-

m

-

u

-

-80

|

-

u

-

-100 ppm

u

-120

Figure 3. ZrAl and 29Si MAS NMR spectra of as-synthesized FSM-16 materials. The FSM-16 samples with Si/AI ratios of 12 and 38 were prepared by using NaAIO 2 and AI2(NO3)a'9H20, respectively. At

o' /Al = 12 I

~

i

/

A

l

= 38

200 100 0 -100-200 ppm Si / Al 9

.

,

-80

.

,

-

,

-

-100 ppm

,

.

9

-120

Figure 4. 27A1and 29Si MAS NMR spectra of calcined FSM-16 materials. The FSM-16 samples with Si/AI ratios of 12 and 38 were prepared by using NaAIO 2 and AI2(NO3)3~ O, respectively.

114

6- Na2Si20 5

Cristobalite---

~

~

~ . . . .

_

=

1

8

8

=

.

9.9 . . . .

10

;0 20(CuKet)

Figure 5. X-ray diffraction patterns of calcined products at 700 ~C of aluminosilicate glasses with various Si/AI ratios prepared by using A12(NOa)a~

These results strongly suggest that AI is incorporated in the silicate framework of the layered sodium silicates and the FSM-16 materials. BET surface areas were 800-1300 mZ/g for the Al-containing FSM-16 samples and 30-300 m2/g for the sodium aluminum silicate hydroxide hydrate crystals. NMR spectra of the as-synthesized FSM-16 samples and the calcined samples are shown in Figures 3 and 4. 27A1MAS NMR spectra of the Al-containing FSM-16 samples with the Si/A1 ratios of 38 and 12, prepared by using Al(NOa)3*9H20 and NaAIO2 as A1 sources respectively, showed almost only one signal due to tetrahedral AI before and after calcination. Broadening of the signal during calcination suggesting somewhat dealumination from the framework was observed for the sample prepared by using AI(NO 3)3~ Obvious broadening was not observed for the sample prepared by using NaA102, which suggested its higher thermal stability than the sample prepared by using Al(NOa)a~ The incorporation of A1 in the silicious framework was also confirmed by 29SiMAS NMR spectra. Lower shifts at 3-7 ppm of signals assigned to Q3 and Q4environmental SiO4were observed for both as-synthesized and calcined samples after introduction of AI as shown in Figures 3 and 4. Such a peak shift was also observed for zeolites with Si (OSi)3(OA1). Figure 5 shows X-ray diffraction patterns of the calcined sodium aluminosilicate glasses prepared by using AI(NO3)3~

The diffraction patterns due to a layered sodium silicates, 6-

Na2Si205 were observed for the calcined products with Si/AI ratio over 9.9, while those were

115

|

[-

l~ "6~

|

l

ii

(This work)

//

IF

'

FSM-16

)llrSi/Al=9.9

0""""

FSM-16

I

(Impregnation)

o ........

..O ......

silica-alumina

== 0.5~[ / <

0.0 0

10

20

30

AI20 3 contents (wt%) Figure 6. Relationship between amount of acid and AI203 contents of various aluminosilicate materials determined by NHa-TPD. hardly observed for the samples with the low Si/AI ratio of 7.7. Cristobalite was contained in the layered sodium silicates, and its content increased with added amount of AI(NOa)a.9H20 as shown in Figure 5. Although 6-Na2Si205 crystal also formed in the calcined products prepared by using NaAIO 2with the Si/Al ratio higher than 12, cristobalite did not formed in all samples. These results support the expelling of Si from the framework of the layered sodium silicates mentioned above. The d-spacings of the diffraction peaks of those layered silicates containing Al were similar to those of •-Na2Si205. Acid amounts of the aluminosilicate FSM-16 materials were larger than those prepared by impregnating aluminum chloride solution and usual amorphous aluminosilicate with the same Si/Al ratio reported previously~3as shown in Figure 6. The acid amounts were 0.7 times of those of ZSM-5. It indicates highly level incorporation of Al in the framework of FSM-16 prepared from the layered silicate containing Al. NHa-TPD profiles of the aluminosilicate FSM-16 materials resembled in those observed in amorphous aluminosilicates, which suggested broad distribution of acid-strength. The successive formation of mesoporous molecular sieves with high regularity and high alumina would be attributable to using a layered silicate containing Al as starting material, although further studies are necessary to clarify the mechanism.

116 4. CONCLUSION The aluminosilicate mesoporous molecular sieves with high alumina and highly regular structure were prepared from layered sodium silicates containing A1. The expelling of Si from the framework of the layered silicates, the large amount of acidity and the NMR results indicated the high level incorporation of AI in the framework of the FSM-16 materials. REFERENCES

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