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AEROGELS, A SOURCE OF ADSORBENTS, INSULATORS, CATALYSTS AND CERAMICS
S. Teichner
To cite this version:
S. Teichner. AEROGELS, A SOURCE OF ADSORBENTS, INSULATORS, CATALYSTS AND CE- RAMICS. Journal de Physique Colloques, 1989, 50 (C4), pp.C4-1-C4-6. �10.1051/jphyscol:1989401�.
�jpa-00229476�
REVUE DE PHYSIQUE APPLIQUÉE
Colloque C4, Supplément au n°4, Tome 24, Avril 1989 C M
AEROGELS, A SOURCE OF ADSORBENTS, INSULATORS, CATALYSTS AND CERAMICS
S.J. TEICHNER
Laboratoire de Thermodynamique et Cynétique Chimiques, Université Claude Bernard Lyon I, 43 bd du 21 novembre 1918, F-69622 Villeurbanne Cedex, France
Ré sumé - La préparation et quelques p r o p r i é t é s remarquables des aérogels d'oxydes inorganiques, autres que l a s i l i c e , tant simples que b i n a i r e s ou t e r t i a i r e s , sont passées en revue.
Summary - The preparation and some remarquable properties of inorganic oxides, other than silica, aerogels, simple or binary and ternary, are reviewed.
During the First International Symposium on Aerogels, which was organized in Wiirzburg (G.F.R.)/1/, it was realized that the subject is becoming enough popular to deserve a periodical reassessement of the work done in this field. Also a Second Symposium is now organized in Montpellier (France). The first aerogels made by Kistler 111 were those of silica. It is still silica which is the main material produced in the form of aerogel. Its properties, thermal, acoustic, adsorbing, optical, were found so fascinating /3/ that their study is still far from being complete. New physical properties of silica aerogels are reported at the present Symposium. It is not therefore surprising that only little room is, for the moment, left for the study of other aerogels. Among these, organic aerogels /A/, zirconia aerogels /5/, cordierite aerogels 16/ and mixed aerogels of niobia and silica 111 and silico—aluminates /8/ are described. It seems reasonable to forecast that new properties and applications, difficult now to predict, will be found for the new non-silica aerogels and various multicomponent silica or non-silica aerogels.
SINGLE OXIDE AEROGELS
The purpose of this review is to bring the attention to this less studied class of non-silica aerogels. In the pioneering studies of Kistler /2/ who, besides silica, prepared also some other aerogels the main difficulty stemmed out from the tedious procedure of washing an initial aquagel from inorganic ions and of replacing water, which is the original solvent of the precursors, by converting the aquagel into alcogel. This procedure requires large volume equipment for washing, filtering and water replacement and impeded the availability of aerogels on a commercial scale, attempted by Monsanto in St Louis (Missouri). Indeed, the low density or large volume of silica aerogels mobilizes large volume of solvents in the various equipments.
The method developped by G. Nicolaon and the present author /9/ gave a new impetus to aerogels 110/ by shortening the time required for the preparation of aerogels, from a few weeks to a few hours. Simultaneously, the technology and the equipment are now restricted to the supercritical drying in an autoclave, without any requirement for extraction, filtering and water replacement. The method may be summarized by the example of the preparation of zirconia aerogel I 111.
Zirconium propylate is dissolved in propanol. The addition of the stoichiometric amount of water produces hydrolysis of the organic precursor with the precipitation of zirconia (more or less hydrated) :
Zr (OC,H,), + 4 H.O -»- Zr (OH), + 4 C„H,OH 3 7 4 2 4 3 7
This precipitation of zirconia (by hydrolysis) may be postponed by addition of an acid component like ethylacetoacetate and is only produced during the heating in the autoclave.
Indeed, by drying zirconia alcogel in the autoclave, in the supercritical conditions with respect to the propanol, its conversion to an aerogel is made. This method, which was initially developed for silica aerogel with silicon methoxide as the precursor dissolved in methanol, was generalized to any oxide aerogel /12/.
Table I gives a few examples of single oxide aerogels prepared from various organic precursors like alcoholates, their pore volumes, surface areas and the state of crystallization.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989401
T a b l e I
A few examples of s i n g l e o x i d e a e r o g e l s
A e r o g e l p o r e yolume S u r f a e a r e a
5
b) C r y s t a l l i n e s t a t e(cm I g ) (m 18)
Amorphous Amorphous Amorphous Amorphous Amorphous Amorphous Amorphous
a ) Determined w i t h mercury p o r o s i m e t e r b ) Determined by B.E.T. n i t r o g e n a d s o r p t i o n
The l a r g e s u r f a c e a r e a s observed show t h a t s u r f a c e p r o p e r t i e s of a e r o g e l s may predominate o v e r t h e i r b u l k p r o p e r t i e s . I n c o n v e n t i o n a l , non d i v i d e d s o l i d s , t h e s u r f a c e p r o p e r t i e s a r e minimized. A d s o r p t i o n and c a t a l y s i s a r e l i n k e d w i t h s u r f a c e p r o p e r t i e s . I t i s t h e r e f o r e l o g i c a l t o experiment t h e s e p r o p e r t i e s f o r t h i s new c l a s s of p o t e n t i a l a d s o r b e n t s and c a t a l y s t s 1121. On t h e o t h e r hand, l a r g e pore volume o r low a p p a r e n t d e n s i t y of a e r o g e l s c a l l t h e a t t e n t i o n t o i n s u l a t i n g p r o p e r t i e s , t h e r m a l and a c o u s t i c 1 3 , l o / . The f a c t t h a t i n some c o n d i t i o n s of p r e p a r a t i o n a e r o g e l s may b e o b t a i n e d a s t r a n s p a r e n t m o n o l i t h s 1131 f o c u s t h e i r u s e f o r Cerenkov r a d i a t o r s ( 1 4 ) and g l a s s and c e r a m i c p r e c u r s o r s 115, 161. F i n a l l y , a e r o g e l s a r e p r e p a r e d i n o r g a n i c s o l v e n t s ( l i k e a l c o h o l s ) and i f w a t e r r e q u i r e d f o r h y d r o l y s i s of t h e o r g a n i c p r e c u r s o r i s n o t i n e x c e s s 1131 t h e r e s u l t i n g a e r o g e l i s u s u a l l y i n t h e amorphous s t a t e . F o r g l a s s e s , t h i s s t a t e i s p r e r e q u i s i t e i f a low-temperature p r o c e s s i s a p p l i e d , g i v i n g a n a d v a n t a g e o v e r c o n v e n t i o n a l h i g h - t e m p e r a t u r e p r o c e s s f o r p r o d u c t i o n of g l a s s e s . F o r c o m p o s i t e m a t e r i a l s l i k e c a t a l y s t s , c e r a m i c s , t h e amorphous s t a t e of components i n c r e a s e s t h e i r r e a c t i v i t y i n t h e s o l i d s t a t e . F o r multicomponent g l a s s e s and c e r a m i c s lower f i r i n g t e m p e r a t u r e s a r e r e q u i r e d . F o r c a t a l y s t s where t h e a c t i v e phase i s a p r o d u c t of i n t e r a c t i o n between two o r more s o l i d o x i d e s of t h e a e r o g e l i t i s e s s e n t i a l t o produce t h i s i n t e r a c t i o n a t low t e m p e r a t u r e s , i n o r d e r t o p r e s e r v e t h e t e x t u r e of t h e c a t a l y s t ( s u r f a c e a r e a and p o r e volume). I n d e e d , c a t a l y s t s s i n t e r a t h i g h t e m p e r a t u r e s , l o o s i n g t h e i r s p e c i f i c s u r f a c e a r e a and t h e i r c a t a l y t i c a l l y a c t i v e s u r f a c e . I f t h e o x i d e components of t h e c a t a l y s t a r e amorphous t h e i r i n t e r a c t i o n a t low t e m p e r a t u r e s ( s e e below) p r e s e r v e s h i g h s u r f a c e a r e a and t h e r e f o r e p r o v i d e s h i g h c a t a l y t i c a c t i v i t y p e r u n i t mass of c a t a l y s t .
E a s i l y r e d u c i b l e o x i d e s c a n n o t be o b t a i n e d i n a s i n g l e s t a t e ( w i t h o u t a s u p p o r t ) i n t h e a e r o g e l form, u n l e s s t h e y a r e a s s o c i a t e d w i t h a r e f r a c t o r y o x i d e a e r o g e l c a r r i e r . F o r i n s t a n c e , o r g a n i c d e r i v a t i v e s of n i c k e l ( n i c k e l a c e t a t e ) , l e a d ( l e a d a c e t y l a c e t o n a t e ) o r c o p p e r ( c o p p e r a c e t a t e ) d i s s o l v e d i n a n a l c o h o l , h y d r o l y z e d and e v a c u a t e d i n s u p e r c r i t i c a l c o n d i t i o n s g i v e m e t a l l i c and o x i d i z e d p a r t i c l e s b e c a u s e of t h e r e d u c i n g c o n d i t i o n s i n t h e a u t o c l a v e . The m e t a l l i c p a r t i c l e s a r e o f t e n p y r o p h o r i c and by h a n d l i n g i n a i r t h e y a r e c o n v e r t e d i n t o o x i d e s i n a non-divided s t a t e . The c o r r e s p o n d i n g m e t a l s o r o x i d e s c a n be however o b t a i n e d i n t h e a e r o g e l form by a s s o c i a t i o n w i t h a r e f r a c t o r y s u p p o r t o r c a r r i e r l i k e s i l i c a o r alumina a e r o g e l 1131 which p r e s e r v e s t h e s t a t e of d i v i s i o n of t h e m e t a l : (Cu/Al 0 ) / 1 7 / , o r of t h e o x i d e : (NiO/A1203)/18/, (CuO/A1203)/19/. These s y s t e m s a r e mentiogea below.
A t t e m p t s were made however a t t h e p r e p a r a t i o n of p u r e m e t a l a e r o g e l s l i k e Cu, Pd o r Au 1201.
The c o r r e s p o n d i n g s a l t s a r e d i s p e r s e d i n a n o r g a n i c s o l v e n t which i s e v a c u a t e d 2 i n s u p e r c r i t i c a l c o n d i t i o n s . The r e s u l t i n g Cu powder e x h i b i t s a s u r f c e a r e a of 0 , 2 3 m / g whereas s u p p o r t e d on A1203 copper 1171 p r e s e n t s a s u r f a c e a r e a of 30 m
Z
/ g f o r a t o t a l s u r f a c e of t h e s o l i d of 660 m /g.2 I t was a l s o claimed t h a t p u r e chromia a e r o g e l s (Cr 0 ) w i t h a2 3
s u r f a c e a r e a of 516-785 m / g were p r e p a r e d i n methanol (21) w i t h some s a f e t y p r e c a u t i o n s because of a p o s s i b l e r e a c t i o n between CrO p r e c u r s o r and methanol.
3
C o n s i d e r a t i o n s c o n c e r n i n g t h e v a r i a b l e s d e t e r m i n i n g h i g h s u r f a c e a r e a and p o r e volume of o x i d e a e r o g e l s , l i k e t h e n a t u r e of t h e o r g a n i c p r e c u r s o r , i t s c o n c e n t r a t i o n i n t h e a l c o h o l and t h e amount of w a t e r used f o r h y d r o l y s i s , a r e g i v e n i n r e f e r e n c e s 112, 13, 221. B e f o r e c l o s i n g t h i s c h a p t e r i t s h o u l d b e mentioned t h a t v a r i o u s r e v u e a r t i c l e s were p u b l i s h e d on
a e r o g e l s 1 2 3 , 241 and a g e n e r a l p a t e n t on a e r o g e l s , whose v a l i d i t y i s d o u b t f u l , was r e c e n t l y d e p o s i t e d 1251. It i s e s s e n t i a l l y based o n t h e method d e s c r i b e d by t h e p r e s e n t a u t h o r 1131.
F i n a l l y , i t i s of i n t e r e s t t o q u o t e a p e c u l i a r r e a c t i v i t y of r e f r a c t o r y o x i d e s l i k e s i l i c a o r alumina u n d e r t h e form of a e r o g e l . I t i s w e l l known t h a t t h e s e o x i d e s i n a c o n v e n t i o n a l form a r e n o t a c t i v e a s c a t a l y s t s . They a r e r a t h e r used a s s u p p o r t s f o r t h e a c t i v e p a r t n e r of t h e c a t a l y s t , l i k e a t r a n s i t i o n m e t a l o r i t s o x i d e . Now, i t h a s been shown t h a t a e r o g e l s of s i l i c a and alumina c a n be a c t i v a t e d by hydrogen s p i l l o v e r f o r t h e c a t a l y t i c r e a c t i o n s , a t f a i r l y l o w t e m p e r a t u r e s , of h y d r o g e n a t i o n , h y d r o g e n o l y s i s , h y d r o c r a c k i n g a n d d e h y d r o c y c l i z a t i o n 126
-
291. T h i s a c t i v a t i o n by hydrogen s p i l l o v e r c o n s i s t s i n h e a t i n g t h e a e r o g e l a t 400-450°C under a t m o s p h e r i c p r e s s u r e of hydrogen i n t h e p r e s e n c e of a v e r y s m a l l amount of P t s u p p o r t e d on a l u m i n a ( t y p i c a l l y 0.5 mg of P t f o r 1 g of a e r o g e l ) . Hydrogen i s a d s o r b e d and d i s s o c i a t e d o n P t and m i g r a t e s t h e n ( s p i l l s o v e r ) o n t o t h e a e r o g e l . A f t e r t h i s a c t i v a t i o n , which by p a r t i a l s u r f a c e r e d u c t i o n of t h e o x i d e , c r e a t e s d e f e c t s which a r e c a t a l y t i c s i t e s of v a r i o u s t y p e s ( l i k e h y d r o g e n a t i o n and Lewis a c i d s i t e s ) , P t i s removed by a m e c h a n i c a l d e v i c e 1291. The o x i d e a e r o g e l e x h i b i t s t h e n v a r i o u s c a t a l y t i c p r o p e r t i e s l i k e h y d r o g e n a t i n g e t h y l e n e o r a c e t y l e n e a t 170°C i n t o e t h a n e , which a r e n o t found f o r t h e same a e r o g e l n o t a c t i v a t e d by s p i l l o v e r . The phenomenon of s p i l l o v e r of hydrogen and o t h e r a d s o r b e d s p e c i e s i s f u l l y d e s c r i b e d i n r e f . 1291. The m i g r a t i o n of s p i l l e d o v e r hydrogen from P t t o t h e non m e t a l l i c a c c e p t o r was a l s o used t o c o n v e r t a s i l i c a a e r o g e l , f u l l y covered by methoxy g r o u p s , i n t o s i l i c a a e r o g e l c o n t a i n i n g o n l y OH g r o u p s , w i t h a s i m u l t a n e o u s r e l e a s e of methane/
301.Si-OCH 3
+
H s p i l l . -t Si-OH+
CH4 ( g ) MIXED OXIDES AND SUPPORTED AEROGELS
It h a s been mentioned p r e v i o u s l y t h a t f o r some o x i d e s and m e t a l s i n a e r o g e l form a s u p p o r t o r c a r r i e r (A1 0 SiO ) i s r e q u i r e d i n o r d e r t o p r e s e r v e t h e d i v i d e d s t a t e of t h e a c t i v e p a r t n e r . ~ h& r s t m e t a l on a e r o g e l m a t e r i a l was n i c k e l on alumina. 2 2 I t was p r e p a r e d 113, 311 by h y d r o l y s i s of a m i x t u r e of n i c k e l a c e t a t e and aluminium s e e - b u t y l a t e d i s s o l v e d i n s e e - b u t a n o l . The mixed a l c o g e l A1 0 - N i O was t h e n e v a c u a t e d from t h e a l c o h o l , i n t h e
2 3 .
a u t o c l a v e , i n s u p e r c r i t i c a l c o n d i t i o n s w l t h r e s p e c t t o sec-butanol. T h e s e r e d u c i n g c o n d i t i o n s were m a i n t a i n e d by c o o l i n g t h e a u t o c l a v e i n a f l o w of hydrogen. T a b l e I1 g i v e s t h e s u r f a c e a r e a s , p o r e volumes, m e t a l l i c s u r f a c e a r e a s and mean N i p a r t i c l e s d i a m e t e r f o r Ni/A1203 a e r o g e l s w i t h v a r i o u s Ni/A1 r a t i o s . F o r a l l a e r o g e l s a s t o i c h i o m e t r i c amount of w a t e r was used f o r h y d r o l y s i s of t h e p r e c u r s o r s , a c c o r d i n g t o t h e e q u a t i o n s :
(CH3
-
CH2-
CH-
0 ) 3 A 1+
3 H20 +A1(OHI3+
3 CH3-
CH2-
CHOH-
CH3I
(CH3
-
COO)2 N i+
2 H20 + Ni(OH)2+
2 CH3 COOHT a b l e 11
N i c k e l on alumina a e r o g e l s
N i / A 1 S u r f a e a r e a P o r e yolume ~ e t i 1 l . c
5 9
s u r f a c e ,yrea N i c k e l p a r t i c l(cm / g ) (m / g Ni) (nm)
gy
d i a m e t e r (m 1 9 )a ) Determined by hydrogen c h e m i s o r p t i o n b) Determined by m a g n e t i c measurements
The h i g h e s t s u r f a c e a r e a s a r e observed f o r t h e f i r s t two samples. The good d i s p e r s i o n of N i , observed f o r a l l samples, i s n o t maintained i f an excess of water i s used f o r h y d r o l y s i s ( i n s t e a d of t h e s t o i c h i o m e t r i c amount). Simultaneously alumina i s no l o n g e r amorphous but c r y s t a l l i z e s under t h e form of boehmite. T h i s example t e n d s t o show t h a t m e t a l l i c d i s p e r s i o n i s c o r r e l a t e d w i t h t h e amorphous s t a t e of t h e c a r r i e r .
By t h e same method Cu/A1203 a e r o g e l s , c a t a l y s t s of t h e p a r t i a l hydrogenation of cyclopentadiene, were o b t a i n e d , a s mentioned p r e v i o u s l y /17/. More r e c e n t l y Pd on alumina a e r o g e l s were prepared by a s i m i l a r process /32/. They a r e a c t i v e a s c a t a l y s t s of t h e hydrogenation of nitrobenzene t o a n i l i n e .
A s p e c i a l mention should be made of N i on Moo2 a e r o g e l ( S = 476 m / G ) 2 which show?
semiconducting behaviour and e x h i b i t s a c o n d u c t i v i t y a t 25-C i n t h e range of 0.1 (ohm-cm) and t h e a c t i v a t i o n energy of c o n d u c t i v i t y of t h e o r d e r of 0.04 eV.
Non-refractory o x i d e s i n a e r o g e l form, l i k e p r e v i o u s l y m e t a l s , deserve a l s o t o be supported on a c a r r i e r , i n o r d e r t o e x h i b i t high d i s p e r s i o n . For example, pure i r o n oxide a e r o g e l (a-Fe203
+
Fe 0 ) e x h i b i t s a s u r f a c e a r e a of 6.8 m 2 / g w i t h a mean diameter of p a r t i c l e s (determined b y 3 x h ) i n t h e range of 32.0 ( f o r Fe 0 ) t o 60.0 nm f o r 9-Fe2O3)/33/. For a3 4
mixed a e r o g e l Fe 0 /SiO ( w i t h 10 % Fe) t h e t o t a l s u r f a c e a r e a i s 760 m / g and t h e diameter of p a r t i c l e s of de$4 (no "e 2 0 ) i s i n t h e range of 9.0 nm.
2 3
I n t h e c a s e of chromia (Cr 0 ) a e r o g e l i t was proposed t o p r e p a r e t h i s s o l i d from chromium 2 3 .
a c e t a t e d i s s o l v e d i n methanol i n t h e presence of A 1 0 -Raschig r i n g s a s c o l l e c t i o n u n i t s f o r t h e a e r o g e l p a r t i c l e s /34/. Another p o s s i b i l i t y is2 $0 p r e p a r e a e r o g e l p e l l e t s (and n o t a powder) by a d d i t i o n of open-form s u p p o r t s t o t h e p r e c u r s o r a l c o g e l s . The r e s u l t i n g a e r o g e l i s t h u s r e t a i n e d w i t h i n t h e openings of a s u p p o r t i n g s t r u c t u r e /35/. Conversely, A 1 0 a e r o g e l s
2 3
s u p p o r t s having a high s p e c i f i c s u r f a c e and pore volume, l o w bulk d e n s i t y and a morphology c o n s i s t i n g of extremely t h i n f o l d e d ribbons o r p l a t e s can be impregnated w i t h s o l u t i o n of T i c 1 i n n-heptane and t h e n w i t h Al-sec-butylate i n i s o b u t a n e i n o r d e r t o o b t a i n a
~ i e ~ $ e r - ~ a t t a e t h y l e n e polymerization c a t a l y s t /36/ i n t h e adsorbed s t a t e and not a s a l i q u i d , s o l u b l e i n t h e r e a c t i o n medium. T h i s s o l u b i l i t y i s a s e r i o u s drawback f o r i n d u s t r i a l a p p l i c a t i o n s of Ziegler-Natta c a t a l y s t s .
I t has been a l r e a d y mentioned t h a t t h e amorphous s t a t e and a high d i s p e r s i o n favour t h e i n t e r a c t i o n between t h e o x i d e s of a multicomponent aerogel. One of t h e b e s t examples i s t h e formation of a s p i n e l A 1 0 N i . I n t h e c a s e of a n a e r o g e l NiO/A1203 t h e s p i n e l phase i s d e t e c t e d a l r e a d y a f t e r hea$iig t o 400°C /37/ whereas a mechanical mixture of c r y s t a l l i n e N i O and A1203 r e q u i r e s f i r i n g temperatures i n t h e range of 800°C. A second p o i n t of i n t e r e s t i n t h i s i n t e r a c t i o n stems o u t from t h e s u r f a c e p r o p e r t i e s of t h e s p i n e l phase. A c a t a l y s t i s a c t i v e on i t s s u r f a c e and not i n t h e bulk. The a c t i v e phase may be t h e r e f o r e r e s t r i c t e d t o t h e s u r f a c e of t h e c a t a l y s t only. The s p i n e l A1204 N i has been shown t o behave a s a v e r y e f f i c i e n t c a t a l y s t of t h e conversion by NO of i s o b u t e n e i n t o m e t h a c r y l o n i t r i l e /37/. I n t h e c a s e of a b i n a r y a e r o g e l NiO/A1203, w i t h a r a t i o ~ i / ~ l = 0.5 (corresponding t o t h e composition of t h e s p i n e l ) , t h e s p i n e l phase i s p r e s e n t a l r e a d y a f t e r p r e h e a t i n g t o 400°C, t o g e t h e r w i t h t h e uncombined N i O and amorphous alumina.
Of course, a l l N i O can be i n t e r a c t e d w i t h A 1 0 a t some h i g h e r temperature t o g i v e bulk s p i n e l but simultaneously t h e s o l i d i s sintezec? w i t h t h e l o s s of c a t a l y t i c a c t i v i t y per gramme of c a t a l y s t o r per gramme of n i c k e l . However w i t h a r a t i o N i / A 1 = 0.05 i n t h e a e r o g e l , preheated a g a i n a t 40O0C, t h e formation of t h e s p i n e l a t t h e s u r f a c e of alumina c a r r i e r i s complete. I t s c a t a l y t i c a c t i v i t y p e r gramme of c a t a l y s t i s p r a c t i c a l l y t h e same a s t h e a c t i v i t y of t h e f i r s t a e r o g e l c a t a l y s t w i t h N i / ~ 1 = 0.5 but i t s a c t i v i t y p e r gramme of n i c k e l i s 7 times h i g h e r t h a n t h a t of t h e f i r s t c a t a l y s t . I n o t h e r words, N i O i s much b e t t e r used ( o r n o t wasted) under t h e form of A 1 0 N i when the' formation of t h e s p i n e l i s r e s t r i c t e d t o t h e i n t e r f a c e between two oxides. 2 4
Another example of t h e i n t e r a c t i o n between two o r more components of an a e r o g e l and of a b e t t e r u s e of t h e . a c t i v e p a r t n e r i s given by CuO/ZnO/ZrO t e r n a r y a e r o g e l c a t a l y s t of t h e s y n t h e s i s of methanol from (CO
+
H ) o r (C02+ HZ)react an?'^
/38/. Binary a e r o g e l s ZnO/A1203 and CuO/A1203, where t h e s u p p o r t i s 2 alumina a e r o g e l c o n t a i n i n g 5 % of ZnO o r CuO, a r e poor c a t a l y s t s of t h i s s y n t h e s i s . The same poor behaviour i s observed w i t h t h e a e r o g e l CuO/ZnO (50:50) without t h e s u p p o r t o r a t e r n a r y a e r o g e l CuO/ZnO/Al 0 (5:5:90). Now, s u p e r i o r r e s u l t s concerning t h e c a t a l y t i c a c t i v i t y a r e observed i f t h e supp%r? i s ZrO a e r o g e l f o r a t e r n a r y cuO/~nO/zrO (5:5:90) o r a b i n a r y CuO/ZrCJ2 (5:95) composition. Z i r c o i i a by i t s e l f i s b a r e l y a c t i v e i n t i e s y n t h e s i s of methanol. An ~ n t e r a c t i o n between z i r c o n i a and CuO i s observed a t 400°C ( t e m p e r a t u r e of t h e p r e t r e a t m e n t g i v i n g t h e h i g h e s t c a t a l y t i c a c t i v i t y ) . When t h eamount of CuO ( o r ZnO f o r t h e t e r n a r y c a t a l y s t ) i s i n c r e a s e d w i t h r e s p e c t t o Z r O ( u p t o 40
%) no improvement of t h e a c t i v i t y i s found. Again, a n i n t e r a c t i o n r e s t r i c t e d t o t h e i n t e r f a c e 2 between two ( o r t h r e e ) s o l i d p h a s e s i s s u f f i c i e n t i n t h e development of s u r f a c e p r o p e r t i e s , h e r e i n v o l v e d i n c a t a l y s i s .
The f l u i d i z a t i o n of a e r o g e l c a t a l y s t s i n a dynamic r e a c t o r removes a l l d i f f i c u l t i e s i n t h e u s e o f t h e s e powdered c a t a l y s t s on a commercial s c a l e /39/.
However a t o t a l i n t e r a c t i o n ( b u l k ) i s a l s o f a v o u r e d between t h e components of a n a e r o g e l . I t may r e q u i r e h i g h e r t e m p e r a t u r e t h a n t h e s u r f a c e i n t e r a c t i o n but c o n v e n t i o n a l non-aerogel components r e q u i r e s t i l l much h i g h e r f i r i n g t e m p e r a t u r e s and a l o n g e r t i m e t o g i v e a b u l k i n t e r a c t i o n . T h i s . i s , i n p a r t i c u l a r , t h e c a s e of t h e s u p e r c o n d u c t o r c e r a m i c s Y Ba2 Cu 0 which i s e a s i l y s y n t h e t i z e d by h e a t i n g t h e p r e c u r s o r a e r o g e l , a s d e s c r i b e d a t t h i s S & P O ~ Z &
/ 4 0 / , a t lower t e m p e r a t u r e s and i n a s h o r t e r t i m e t h a n f o r t h e mechanical m i x t u r e of c r y s t a l l i z e d p r e c u r s u r s , o x i d e s o r c a r b o n a t e s .
Even i f t h e f i n a l c r y s t a l l i z a t i o n i s n o t d e s i r e d , which i s t h e c a s e of g l a s s e s , t h e a e r o g e l l e a d s t o a n e a s i e r d e n s i f i c a t i o n , ( i n a s h o r t e r t i m e ) , a t a lower f i r i n g t e m p e r a t u r e and a t a lower p r e s s u r e a p p l i e d . T h i s i s i n p a r t i c u l a r observed i n l o w - t e m p e r a t u r e c o r d i e r i t e g l a s s
(magnesium s i l i c o - a l u m i n a t e ) made by a e r o g e l p r o c e s s /41/.
Many b i n a r y o r t e r n a r y a e r o g e l s were p r e p a r e d a s c a t a l y s t s , but i t i s n o t t h e purpose of t h i s r e v u e t o examine t h e i r p r o p e r t i e s . The c a t a l y t i c a c t i v i t y was o n l y c o n s i d e r e d h e r e a s a t o o l i n l a b e l l i n g s u r f a c e p r o p e r t i e s of a e r o g e l s and d e r i v e d s o l i d s .
I n c o n c l u s i o n , a tremendous amount of r e s e a r c h work h a s s t i l l t o be done i n o r d e r t o e x p l o r e p r o p e r t i e s and a p p l i c a t i o n s of n o n - s i l i c a a e r o g e l s . E l e c t r i c a l c o n d u c t o r a e r o g e l s , s u p r a c o n d u c t o r a e r o g e l p r e c u r s o r s , new a d s o r b e n t s , a e r o g e l p r e c u r s o r s f o r new t y p e s of g l a s s e s and c e r a m i c s , e x h i b i t i n g s i m u l t a n e o u s l y p e c u l i a r i s o l a t i n g p r o p e r t i e s ( a c o u s t i c and t h e r m a l ) and new a e r o g e l c a t a l y s t s a r e t h e non-exhaustive examples of t h e f i e l d t o be developped a t t h e 3d I.S.A.
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