A new procedure for preparing aerogel catalyst

PREPARATION OF CATALYSTSVI Scientific Bases for the Preparationof HeterogeneousCatalysts G. Ponceletet al. (Editors) 1995 ElsevierScience B.V.

427

A n e w p r o c e d u r e f o r p r e p a r i n g aerogel catalyst C.-M. Zhang, S.-Y. Chen and S.-Y. Peng State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Science, Box 165 Tai Yuan Shanxi 030001 P.R. China. 1. INTRODUCTION Aerogel, a new kind of catalytic materials, exhibits several advantages [1], such as very high porosity, i.e. big pore volume, high specific surface area and very good textural stability during h e a t t r e a t m e n t at high t e m p e r a t u r e s . Aerogel can be p r e p a r e d by the removal of solvent from a wet gel at a t e m p e r a t u r e and pressure above the critical t e m p e r a t u r e and pressure of the solvent. Some aerogel may have unusual catalytic activities, selectivities and stabilities for several reactions. But the large number of variables involved in the synthesis of the gel and the process of supercritical drying makes it very difficult to understand clearly how to prepare special catalysts [2]. The process of supercritical fluid drying is not a suitable method for the p r e p a r a t i o n of some catalytic m a t e r i a l s , which are of low m e l t i n g points a n d easily crystallized, such as molybdenum oxide. In order to expand the use of this technique, a new procedure, i.e. gel-like method, is presently proposed for the preparation of aerogel. In the gel-like method, the gel is filled by solvent and a modifying agent(such as detergent). For the preparation of hydrogel with this procedure, a modifying agent should be used. The gel-like can be dried with supercritical fluid. The influence of SCF drying and calcination on aerogel are discussed. Two kinds of aerogels were p r e p a r e d by the gel-like method : alumina, which is easily prepared gel-like and molybdenum oxide, which is more difficult to prepare. 2. EXPERIMENTAL 2.1. Supercritical d r y i n g eqm'pment The scheme of the supercritical drying equipment is shown in Figure 1. 2.2. C h a r a c t e r i z a t i o n techniques Surface areas were calculated by the BET equation from the nitrogen adsorption isotherms at 77K. The N2 isotherm curves were obtained with a Carlo-Erba-1800 adsorption equipment. X-ray diffraction (XRD) patterns were recorded with a Rigaku Dmax-rA diffractometer operated at 40KV and 33mA with nickel filtered CuK radiation. Morphological study was carried out by T r a n s m i s s i o n Electron Microscopy (TEM) using a Philips EM400, a n d

428 S c a n n i n g E l e c t r o n Microscopy (SEM) u s i n g a H i t a c h i H-600. Thermogravimetric data were obtained with STA-780 Thermogravimetric Analyser. aldnooouJJaql

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aBno6 adNSSadd

Figure 1. Autoclave equipment for the evacuation of the solvent under hypercritical conditions.

3. RESULTS AND DISCUSSION 3.1. Alumina aerogel p r e p a r e d from gel-like The preparation scheme is shown in Figure 2. A1Cl3 water solution HC1 Polyethyl alcohol r acidic Change the pH ~lumina solution [gel-like

Mumina aerogel

SCFDT Wash with alcohol

I

Alumina alcogel

Figure 2 : Preparation scheme of alumina aerogel by gel-like process All the chemical reagents were chemically pure. The content of the modifying agent in the gel was much less than 10 percent. The alcogel was vigorously stirred for 15 minutes and then poured into a cylindrical stainless tube (30cm diameter and 250cm high) before hypercritical evolution. To evacuate the solvent under hypercritical condition, the tube containing the alumina gel was placed inside the autoclave with a capacity of 1 liter. The

429 autoclave was flushed with nitrogen several times : ethanol was used as supercritical solvent (for pure ethanol T=520K, P=6.7MPa) and was pumped into the autoclave, and the rise of t e m p e r a t u r e was programmed and the pressure was controlled automatically. The supercritical state was kept for 30 minutes. Thereafter, the pressure was decreased by venting off the solvent vapor. When the pressure reached atmospheric pressure, the heating was turned off, the autoclave was flushed overnight with dry nitrogen and the product was removed after cooling the autoclave to room temperature. Using the above described operation procedure, a series of samples was obtained. Their texture and properties are discussed below : a. Texture comparison of the aerogel prepared with this procedure and the conventional aerogel. The TEM and SEM observations indicate that there are large differences between the two kinds of aerogel. The alumina aerogel is laminar, whereas conventional alumina aerogel (TEM) is agglomerated.

a

b

Figure 3.: TEM (a) and SEM (b) micrographs of the alumina aerogel XRD results indicated t h a t A1203 aerogel was amorphous. After calcination at 773K for 4 hours, the alumina aerogel was transformed into a-A12 03. This is the same as the conventional alumina aerogel. The distribution of particle diameter before and after calcination are shown in Figure 4. b. Surface areas and pore volumes The surface areas and pore volumes of the products obtained at different t e m p e r a t u r e s and pressures (all the samples had the same content of modifying agent) are given in Table 1.

430 For the supercritical p r e p a r a t i o n s , the surface a r e a of the products decreases w i t h the i n c r e a s e before calcinetion of p r e s s u r e a n d t e m p e r a t u r e . This could be e x p l a i n e d by t h e h i g h e r pressure and temperature which after c ~ I c i n a ti.on m a y p a r t i a l l y destroy the s t r u c u r e of 0 > gel before r e a c h i n g the s u p e r c r i t i c a l condition. The modifying a g e n t h a s a major influence on the p r e p a r a t i o n of 0 a l u m i n a aerogel. A suitable c o n t e n t of modifying a g e n t h a s a f a v o r a b l e i% o effect on the surface area. T h i s is V" -- d 'k shown by the difference of the surface a r e a before a n d a f t e r c a l c i n a t i o n of the s a m p l e s as c o m p a r e d w i t h t h e surface a r e a of the original aerogel powder. W h e n the alcogel is dried in U~ 9 SCF, the modifying a g e n t is p a r t i a l l y removed and the gel s t r u c t u r e is kept. is | I It_ - r If t h e s a m p l e is c a l c i n e d , t h e 2 4 6 8 m o d i f y i n g a g e n t is b u r n t off. PORE DIAMETER (nm) Therefore, the modifying agent p r e v e n t s t h e collapse of t h e gel structure and enlarges the surface Figure 4. 9 Distribution of pore particle a r e a a n d pore volume. F r o m S E M d i a m e t e r of a l u m i n a aerogel. photograph, it m a y be observed t h a t the modifying a g e n t leads to a change of the surface morphology a n d pore volume; the l a m i n a r p r o d u c t s a r e v e r y thin. As a c o m p a r i s o n , a few characteristics of the aerogels p r e p a r e d with and without modifying a g e n t are given in Table 2.

4

,,

Table 1. NO

1 2 3 4 5 6

T(K)

547 547 547 551 561 571

P(MPa)

6.7 7.8 9.2 8.2 8.2 8.2

original powder

S(m2/~) 525 300 263 217 177 156

V(ml/g) 1.19 -

calcined (673K) samples 4h. S(m2/g) V ( m l / g ) 438 256 236 598 1.67 223 218 -

8h S(m2/g) 360 -

The aerogel with the modifying a g e n t has h i g h e r pore volume. Although the a l u m i n a gel in acid solution cannot form the h y d r a t e d skeletal inorganic compound, addition of the modifying a g e n t r e s u l t s in bigger pore volume.

431 After b u r n i n g off the modifying agent, the pore volume reached 1.67 ml/g, which is larger t h a n t h a t of the common aerogel. TG results indicate t h a t the modifying agent is the gel could not be completely removed during the SCF drying operation, as shown in Figure 5.

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g

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I

J

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200 400 600 800 TEMPERATURE (*C) Figure 5. 9 Thermogravimetric curve of A1203 aerogel with modifying agent Table 2 Comparison of the aerogels prepared from the two processes

S(m2/g) Vp(ml/g) ,

common alumina aerogel aerogel from this procedure

, .....

600

598

, L

1.30 1.67

.

.

R(A) .

.

app.

state

den.

.

30-50 50

0.02-0.04 amorphous 0.05 amorphous

The IR spectra of pyridine adsorbed show bands at 1450 cm -1 a n d 1490 cm -1, indicating t h a t there exist only Lewis acid centers on the surface of the alumina aerogel. This is the same result as for the conventional alumina aerogel.

3.2. MoOx aerogel The m a t e r i a l s used were a m m o n i u m h e p t a m o l y b d a t e (abbr.A.H.M.) deionised w a t e r and agents A and B (A.R.). P r e p a r a t i o n : the p r e p a r a t i o n procedure is shown in Figure 6. The t e x t u r a l c h a r a c t e r i s t i c s of the conventional and the MoO3 aerogel are shown in Table 3.

432 Table 3 Textural characteristics of conventional and MoO3 surface area (m2/~) 1.6 19.13

C-MoO3 A-MoO3

Pore volume (ml/g) 0 0.12

~NH4)6Mo7024

apparent density (ml/g) 3.0 0.33

MoOx aerogel

1

+water

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A agent

!

.solution

SCFDT B agent

gel product transparent

] alcogel

]

methanol

Figure 6. : Preparation of MoOx hydrogel and aerogel The influence of SCF condition and calcination on the surface area and pore volume are shown in Table 4. a. Influence of supercritical conditions on MoO3 aerogel. Table 4 Influence of supercritical conditions on MoO3 aerogel N~ T(K) P(MPa) S(m2/g)

711

712

713

716

726

712

727

573 14 22.15

573 11.5 19.13

573 10 16.8

533 11.5 4.9

552 11.5 17.18

573 11.5 19.13

593 11.5 21.79

With the increase of pressure and t e m p e r a t u r e , the surface a r e a increases. The modifying agent is easily completely destroyed (decomposition temperature less t h a n 523K as shown in Figure 7) at supercritical condition. The modifying agent and MoO3 at high pressure and t e m p e r a t u r e have a s t r o n g interaction. The gel structure could be kept. At the supercritical condition, the modifying agent is completely destroyed. So, the modifying agent could be the main reason for the increase of the surface area. b. Influence of the calcination temperature on MoO3 The sample has been calcined at the different temperatures. The results are shown in Table 5.

433 Table 5 Influence of calcination temperature on MoO3 aerogel T(K)

non calcined

473

523

573

673

19

9.28

8.18

8.37

7.3

,|

S(m2/g)

From the above discussion, it is surface area of the aerogel prepared modifying agent. This experimental widely used in the preparation of other

possible that the large influence on the by the gel-like method is due to the procedure to prepare aerogel may be catalytic components.

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4(J0 6()0 8(J0 TEMPERATURE (*C)

Figure 7. 9 Thermogravimetric curve of MoO3 aerogel R~ERENCI~ 1. G.M. Pajonk, Appl. Catal. 72 (1991), 217 2. L.L. Hench, Chem. Rev. 90 (1990), 33