JOURNAL OF MATERIALS SCIENCE LETTERS 13 (1994) 1077-1080

Preparation and properties of fibrous boehmite sol and its application for thin porous membrane S. FURUTA, H. KATSUKI, H. TAKAGI

Saga Ceramics Research Laboratory, 3037-7 Chubu-hei, Arita-machi, Saga Prefecture 844, Japan

Inorganic porous membranes (pore size <10 #m) are widely used in separation processes such as gas filtration and purification of products in chemical synthesis [1-5]. For separation processes, the porous membranes must be as thin as possible because of the permeability of the medium [6]. Fibrous materials are expected to be superior to spherical ones in preparing thin membranes for the following reasons: (i) crack-free membranes can be prepared easily using fibrous materials, and (ii) fibrous crystals cannot easily be sintered to each other at high temperatures. Some types of boehmite crystals, known to be fibrous, will be useful for preparing thin porous membranes. If the crystal size of fibrous boehmite can be controlled, porous membranes with fibrils may retain porous structure after transforming into ),-alumina at 500-1000°C. Futhermore, because fibrous boehmite has good chemical resistance in acid solutions, it is expected to be used for purification of the products in chemical synthesis, for example, filtration of polymer particles before separation by the high pressure liquid chromatography (HPLC) method. However, most commercial boehmites have very fine fibrils (less than 0.1 #m in length) or are not fibrous. They are produced by calcining gibbsite [7]. Recently, there have been several reports on synthesizing size-controlled fibrous boehmite by hydrothermal treatment [8-10]. In this study, we synthesized fibrous boehmite sol and prepared thin porous membranes on porous alumina ceramic supports. Aluminium iso-propoxide and aluminium chloride were used as raw materials to synthesize boehmite sol. HC1 was added to the solution in the system of aluminium iso-propoxide to synthesize the stabilized sol. The A1/C1 molar ratio was controlled by changing the fraction of HC1 and A1 powder in each process. Hydrothermal treatment was carried out at 150-200 °C for 10 or 20 h in an autoclave. The structure and morphology of boehmite crystals were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Crystallite size was calculated from peak broadening of XRD patterns by the Scherrer equation. Furthermore, calcined porous alumina sheets with pore size 2.0/~m, porosity 48% and thickness 1.0 mm were dipped into the sol for 5 s. After dipping, sheets were dried overnight at room temperature. This dipping-drying process was repeated again. The orientation of the boehmite crystals on the supports was investigated by X R D analysed from the surface. 0261-8028 ©1994 Chapman & Hall

Pore size distribution and porosity were measured by mercury porosimetry. Filtration test of starch powder suspension was carried out by a direct flow method using alumina sheet coated with boehmite sol and fired at 500 °C. Starch powder (below 0.1 #m) was prepared by precipitation in acetonitrile solution. If CI- ions are present in a mixture during hydrothermal reaction, they affect some of the properties of the resulting sol. Fig. 1 shows the species in products produced by changing the ratio A1/C1 in the raw materials and the temperature of hydrothermal treatment. In the case of aluminium chloride, there was no XRD peak of crystal before hydrothermal treatment. Boehmite crystals were confirmed after treatment over 150 °C, but aluminium chloride crystals were also formed in some cases. In the case of aluminium iso-propoxide, when the A1/CI ratio was low and the temperature of hydrothermal treatment was high, aluminium chloride crystals tended to precipitate. From the above results, the following reaction may be assumed: AlCl3(s) + 2H20(g) ~,~ A1OOH(s) + 3HCl(g) The changes of Gibbs free energy between the left and right sides of this reaction were calculated as shown in Table I. AG gave negative values at 130-193 °C, which indicates that the reaction will move towards the products side easily. A G decreases with rising temperature so more boehmite may be produced at high temperatures. However,

200

•

gG

20C

ID

ID••

• •

•

ooo

•

•

EP

a~

150

0

0

0

0

0

(a) 0.5 1.0 1.5 AI / CI (molar ratio)

150

•

(b)

1 2 3 4 5 AI / CI (molar ratio)

Figure I Effect of A1/C1 ratio and temperature of hydrothermal treatment on product species. (a) Aluminium chloride and (b) aluminium iso-propoxide. (O), No crystal phase; (tD), boehmite and Ale13 phases; (O), boehmite only.

1077

TABLE I The changes of Gibbs free energy of a hydrothermal treatment Temperature (°C)

AG (kcalmol-I)

25 130 140 150 160 170 180 190 193

-55.4 -30.7 -31.0 -31.3 -31.6 -31.9 -32.1 -32.4 -32.5

8000

C

=

,~ .~.~

~o ¢--=

5

the reaction moves towards the left with rising temperature when aluminium iso-propoxide is used as a raw material. If C1- increases, the reaction will move towards the left. Some morphologies of boehmite crystals by T E M are shown in Fig. 2. The crystal growth of boehmite obtained from aluminium iso-propoxide (with A1/C1 ratio 4.3) by a hydrothermal treatment at

Figure 2 Morphologiesof boehmite crystalsprepared by a hydrothermal treatment at 180°C for 2Oh: (a) from aluminium iso-propoxideand (b) from aluminiumchloride. 1078

20

40 26 (Cu K~)

60

Figure 3 XRD patterns of boehmite obtained from aluminium iso-propoxide (A1/C1 ratio 4.3) (a) before hydrothermal treatment, and after treatment at 150 °C for (b) 10 h and (c) 20 h, and at 180 °C for (d) 10 h and (e) 20 h.

150-200 °C was examined by X R D (Fig. 3). Aluminium chloride crystals were not formed in this reaction condition. The crystallite size of boehmite measured by X R D using (020) and (120) peaks tended to increase with rising temperatures and increasing treatment time. Prior to treatment, the crystals were very fine (2.2 nm and 3.8 nm, calculated from the 0 2 0 and 1 2 0 lines, respectively), but increased to 8.2 nm and 14.3 nm after treatment at 200 °C for 20 h. This tendency agrees with the T E M observations. The aspect ratio of boehmite crystals from aluminium iso-propoxide was 2 - 5 and their lengths were 10-100 nm. However, in the case of aluminium chloride, crystals grew to fibrils 0 . 5 - 1 . 0 / t m in length and high aspect ratio. In this way, the crystal size and shape of boehmite can be changed by the conditions of hydrothermal treatment and the species of materials used. Large fibrils with high aspect ratio can be produced from aluminium chloride, which is a cheaper material than aluminium iso-propoxide. Fig. 4 shows microstructures of the surfaces and fractured sections of boehmite derived from aluminium chloride and coated on porous alumina sheets. Crack free thin porous membranes with overlapping fibrous boehmite up to several tens of micrometres thick may be observed. Furthermore, we can observe fibrous particles going into the porous structure of the support body (20-50/~m in depth), so there was no crack between the surface membrane and the support body. The pore size of the thin membrane composed of boehmite was about 0.1-0.2 pm by scanning electron microscopy (SEM) observation. Fibrous particles in the thin membrane condensed and changed its structure after firing at 500 °C and transformation to 7-alumina. The thickness of the membrane could be changed by altering the dipping time. X R D analysis revealed that most of the boehmite crystals were orientated on the support alumina sheet. (0 2 0) and (0 8 0) diffraction peaks were stronger than other peaks, compared with the peaks of the boehmite crystal only (Fig. 5), and the degrees of orientation of the crystals

Figure 4 Morphologiesof (a, b) the surfacesand (c, d) fracturedsectionsof thin boehmitemembranesderivedfrom aluminiumchloride.

calculated from the ratio of relative XRD intensity ratio were as follows: I12o/Io2o

_ 0.35/18

I12oflo20(JCPDS) 65/100 = 0.0194/0.65 = 0.03 I120/I020

(membrane)

_ 4.5/6.85

I120f1020(JCeDS)

65/100

= 0.657/0.65 = 1.01

(crystals only)

Thus, the crystals were considered to be parallel to the porous support body. The pore size distribution of alumina sheet with boehmite membrane was measured by mercury porosimetry (Fig. 6). The median diameter of the

support body was about 2.0/~m and that of the boehmite membrane was about 0.2 ~m. Furthermore, the nitrogen adsorption method revealed that there is no peak of pore distribution below 10 nm. The pore size of the membrane became smaller after firing at 500 °C. Under a filtration test of ,/-alumina membrane derived b y firing boehmite, the starch powder did not filter through the membrane and the filtrate was clear. The porous alumina before coating with y-alumina membrane could not obstruct the starch powder. Usually, filtration of polymer before H P L C analysis is carried out to remove insoluble polymer particles of 0.1-0.3/~m [11], so porous alumina sheets coated with y-alumina membrane may be useful in the filtration of fine polymer particles

1079

20000 (020) c~ v

m e-

e"

o o

o(120)

o

(051) °

(140)

(051)

0

(

)

3000

(020) el.

(120) •

to31)

•

m t-

t-

(b]

tl

20

~

~(200'

40 20 (Cu K(~)

(231,

60

before separating the synthesized products by HPLC. Based on our study, the following conclusions could be obtained. Firstly, fine boehmite fibrils could be synthesized from some aluminium compounds by a hydrothermal treatment. Their particle size could be controlled by A1/C1 ratio, conditions of hydrothermal treatment and species of raw materials. High aspect ratio of boehmite crystals could be produced from cheap materials, such as aluminium chloride. Secondly, crack free thin porous membrane composed of boehmite fibrils and 7-alumina on the porous alumina support were prepared by dipping-drying-firing processes. Thirdly, boehmite crystals of the thin membrane coated on porous alumina were orientated parallel to the support body. Finally, thin porous v-alumina membrane prevented fine starch particles under a filtration test.

References 1.

Figure5 XRD patterns of (a) boehmite coated on a porous alumina sheet and (b) as-grown boehmite by a hydrothermal treatment of aluminium iso-propoxide and aluminium chloride at 200 °C. (O), Boehmite; (O), o-alumina.

C. J. R. GONZALEZ-OLIVER, M. SCHNEIDER, K. NAWATA and H. KUSANO, J. Non-Cryst. Solids 100

(1988) 274. 2.

K . K . CHAN and A. M. BROWSTEIN, Ceram. Bull. 70

(1991) 703. 3. H. ISHIKAWA,Membrane 14 (1989) 186. 4.

S. KITAO, H. KAMEDA and M. ASAEDA, ibid. 15

(1990) 222. 5.

~1oo I

6.

Q

E

7.

O >

8.

P O

o.

50

9.

e-

10.

(D

a

!a!,^A , ...... l'0'--J 0.1

1.0

10 0.1 1.0 Pore diameter (gm)

10

Figure 6 Pore size distribution of porous alumina sheets (a) coated with boehmite membrane and (b) alumina sheet only.

1080

A . J . BURGRAAF, K. KEISER and B, A. VAN HASSEL (editors), "Surface and interfaces of ceramic materials" (Kluwer, Dordrecht, 1990) p. 705. H. SAITO and N. KATO (editors), "Application of fine ceramics", Vol. 1 (Taiga, Tokyo, 1986) p. 77. S. ONUMA (editor), "Yogyo kogaku handbook" (Gihodo, Tokyo, 1966) p. 156. T. OKUBO, M. WATANABE, K. KUSAKABE and S. MOROOKA, J. Mater. Sci. 25 (1990) 4822. P . A . BUINING, C. PATHMAMANOHARAN, M. BOBOOM, J. B. H. JANSEN and H. N. W. LEKKERKERKER, J. Amer. Ceram. Soc. 73 (1990) 2385. P . A . BUINING, C. PATHMAMANOHARAN, J. B. H. JANSEN and H. N. W. LEKKERKERKER, ibid. 74

(1991) 1303. 11. K. NAKANISHI and T. TANAKA, Chem. Engng. 6 (1991) 22.

Received 26 January and accepted 9 February 1994