A NEW METHOD OF PREPARATION OF AEROGEL-LIKE MATERIALS USING A FREEZE-DRYING PROCESS

HAL Id: jpa-00229480

https://hal.archives-ouvertes.fr/jpa-00229480

Submitted on 1 Jan 1989

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

A NEW METHOD OF PREPARATION OF AEROGEL-LIKE MATERIALS USING A

FREEZE-DRYING PROCESS

D. Klvana, J. Chaouki, M. Repellin-Lacroix, G. Pajonk

To cite this version:

D. Klvana, J. Chaouki, M. Repellin-Lacroix, G. Pajonk. A NEW METHOD OF PREPARATION

OF AEROGEL-LIKE MATERIALS USING A FREEZE-DRYING PROCESS. Journal de Physique

Colloques, 1989, 50 (C4), pp.C4-29-C4-32. �10.1051/jphyscol:1989405�. �jpa-00229480�

REVUE DE PHYSIQUE APPLIQUÉE

Colloque C4, supplément au n°4. Tome 24, avril 1989 Cf-29

A NEW M E T H O D OF PREPARATION OF AEROGEL-LIKE MATERIALS U S I N G A FREEZE-DRYING PROCESS

D. K L V A N A , J. C H A O U K I , M. REPELLIN-LACROIX* and G.M. PAJONK*

Ecole Polytechnique de Montreal, GGnie de Physique, Montreal, Canada 'University Claude Bernard, Lyon I, CNRS-ISM UA-231, F-69622

Villeurbanne Cedex, France

Résumé - Pour conserver les propriétés texturales des gels encore imbibés de solvant au moment de leur séchage il existe la possibilité d'évacuer le solvant dans les conditions hypercritiques (procédé à haute température conduisant aux aérogels) et aussi celle de sublimer ledit solvant qui conduit alors à l'obtention de cryogels (procédé à basse température). Une série de cryogels d'oxydes mixtes de nickel et d'aluminium ont été ainsi élaborés. Leurs propriétés texturales ont été déterminées par adsorption-désorption de N , par la méthode BET, la poroslmétrie au mercure tandis que leur structure a été étudiée par les rayons X.

Ces cryogels sont comparés aux aérogels de formulation chimique identique.

Finalement un échantillon de cryogel a été testé dans la réaction catalytlque d'hydrogénation du toluène en méthylcyclohexane qui constitue un procédé potentiel de stockage de l'hydrogène pour les systèmes de transport.

Abstract - In order to preserve the textural properties of a wet gel one possibility is to evacuate the solvent under hypercritical temperature conditions (high temperature process leading to the well known aerogel materials) and another one is to sublimate the solvent frozen at low temperature conditions (low temperature process leading to cryogels).

The preparation of a series sfeeveral nickel oxide on alumina cryogels is described in details. Their surface and textural properties are measured by physical adsorption-desorption of N , by the BET method and also by mercury porometry. The corresponding data are compared to those belonging to aerogels of same chemical compositions. Cryogels (as aerogels) appeared to be amorphous from their XRD patterns.

Finally one sample of a nickel on alumina cryogel is tested as a potential catalyst in a flow reactor for the reaction of hydrogenation of toluene into methylcyclohexane which is one possible means to store hydrogen in transport systems.

1 - INTRODUCTION

It is well known that during the drying-step of a wet gel the formation of a liquid-vapour meniscus in the porous network is responsible for its collapse.

The aerogel method (see ref. /I/ and 111 for instance) is a good means to preserve the textural properties of the gel but it suffers from some drawbacks such as (in general) high pressure, high temperature and impossibility to use water as a solvant which must be replaced (directly or not) by flammable organic compounds such as alcohols.

In principle the same purpose can be reached through the means of lyophylisatlon or freeze drying which is based on the sublimation of the solvant once it has been frozen /3/. To obtain cryogels necessitates two steps : first freeze the gel and second sublimate the solvant. It is important in this last stage to avoid carefully any melting of the solvant especially when a heat source is used to accelerate the sublimation step (see ref. /2, 4/).

The results presented in this paper are to be considered as preliminary ones at the time of this Symposium.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989405

2

-

EXPERIMENTAL

P r e p a r a t i o n o f NiO-A1 0 c r y o g e l s 2 3

The method u s e d i n t h e s o l - g e l p r o c e s s was d e r i v e d from t h e one a l r e a d y a p p l i e d f o r t h e p r e p a r a t i o n of a e r o g e l s 151. N i c k e l a c e t a t e was d i s s o l v e d i n e t h y l e n e g l y c o l i n a p r o p o r t i o n comprised between 10 t o 15 % i n w e i g h t w i t h r e s p e c t t o alumina. The r e q u i r e d amount of w a t e r f o x t h e h y d r o l y s i s was added t o t h i s f i r s t s o l u t i o n . A second one c o n s i s t i n g of s e c o n d a r y b u t y l a t e of aluminium i n 2-methyl-2 p r o p a n o l was added t o t h e f i r s t s o l u t i o n under v i g o u r o u s mixing. The so-obtained mixed a l c o g e l was t h e n f r o z e n i n a a c e t o n e b a t h c o o l e d w i t h l i q u i d n i t r o g e n a t

-

96'C i n t h i n l a y e r s and t h e n t r a n s f e r r e d i n a Labconco u n i t t o s u b l i m a t e t h e s o l v e n t s . The s o l v e n t s were e v a c u a t e d i n t h i s way w i t h i n 24 hours. The r e s u l t i n g c r y o g e l a p p e a r e d t o be a v e r y f i n e g r e e n powder.

T h r e e c r y o g e l s were p r e p a r e d w i t h t h e same p r o p o r t i o n of N i ( i n w e i g h t ) b u t w i t h t h r e e d i f f e r e n t r e a c t a n t t o s o l v e n t r a t i o s a s i n d i c a t e d i n t a b l e 1. They a r e l a b e l l e d NAC.

Cryogel R e a c t a n t t o s o l v e n t r a t i o s i n % Apparent d e n s i t i e s i n g/cm 3

NAC 4 NAC 5 NAC 6

T a b l e 1 - Composition and a p p a r e n t d e n s i t i e s of t h e NAC c r y o g e l s .

T e x t u r a l a n d s t r u c t u r a l p r o p e r t i e s

T a b l e 1 shows t h a t t h e h i g h e r t h e p r o p o r t i o n of t h e r e a c t a n t s used w i t h r e s p e c t t o t h e s o l v e n t s t h e g r e a t e r t h e a p p a r e n t d e n s i t i e s . However t h e BET a r e a s of t h e s e c r y o g e l s were n o t s i g n i f i c a n t l y d i f f e r e n t a s i t i s shown i n t a b l e 2.

Cryogel D e g a s s i n g t e m p e r a t u r e i n "C Weight l o s s i n % S (BET) i n m 2 / g

NAC 4 NAC 5 NAC 6

T a b l e . 2

-

BET s p e c i f i c a r e a s of t h e NAC c r y o g e l s .

The a d s o r p t i o n - d e s o r p t i o n N 2 i s o t h e r m s f o r NAC 4 and f o r a comparable a e r o g e l (NA) a r e show?

on f i g u r e 1 and t h e i r most i m p o r t a n t f e a c t u r e s a r e g i v e n i n t a b l e s 3 and 4 where VpN i s t h e micro- and meso-pore volumes measured by p h y s i c a l a d s o r p t i o n of N r i s t h e mean p o r e r a d i u s and S t i s t h e d e r i v e d s p e c i f i c a r e a g i v e n by t h e c o r r e s p o n d i n g t s o t s of f i g u r e 2.

From t h e s e d a t a i t c a n be concluded t h a t t h e NAC 4 c r y o g e l i s , from t h e p o i n t of view of i t s t e x t u r e , a l i t t l e b i t l e s s developed t h a n t h e a e r o g e l . Both t y p e s of powder a r e n e v e r t h e l e s s e s s e n t i a l l y macroporous (and amorphous a s d e m o n s t r a t e d by t h e i r XRD p a t t e r n s ) . T h 5 i r macropore volumes Vp were m a s u r e d w i t h a mercury p o r o s i m e t e r and t h e i r VpH were 0.8 cm / g

5

f o r t h e c r y o g e l a n t g 5.4 cm / g f o r t h e a e r o g e l . I t i s p r o b a b l y t h e grea%est d i f f e r e n c e r e c o r d e d between t h e s e two k i n d s of d r i e d g e l powders. E x p e r i m e n t s a r e i n p r o g r e s s t o improve t h e p o r o s i t y of t h e c r y o g e l s .

F i g . 1

-

A d s o r p t i o n d e s o r p t i o n N2 i s o t h e r m Fig. 2

-

"t" p l o t s

2 3

-

Sample S (BET) i n m / g

.

VpN i n cm / g r = 2 VpN/S i n (A)

Cryogel 348

Aerogel 579

T a b l e 3

-

Comparison of t h e t e x t u r a l p r o p e r t i e s between t h e c r y o g e l and t h e a e r o g e l

-- - - - - -- ---

Sample Type of i s o t h e r m C a p i l l a r y c o n d e n s a t i o n H y s t e r e s i s S t i n m 2 / g

Cryogel I1

Aerogel I1

T a b l e 4

-

T a b l e 3 continued.

A p p l i c a t i o n t o a c a t a l y t i c r e a c t i o n

The h y d r o g e n a t i o n of t o l u e n e i n t h e presence of NAC 4 was o p e r a t e d i n a small s t a i n l e s s s t e e l U t u b e r e a c t o r i n flow c o n d i t i o n s . The b i n a r y c r y o g e l was reduced a t 400°C i n f l o w i n g hydrogen f o r 16 h o u r s and t h e n cooled down t o t h e r e a c t i o n t e m p e r a t u r e of 130°C. The o n l y product d e t e c t e d by g a s chromatography was methylcyclohexane, t h e t o l u e n e c o n v e r s i o n being of 2 %. Experiment d e t a i l s a r e g i v e n i n r e f . / 6 / .

P r e p a r a t i o n o f SiO c r y o g e l s 2

Sodium m e t a s i l i c a t e i n w a t e r was a c i d i f i e d by H C 1 a t room t e m p e r a t u r e . A f t e r washing t h e a q u a g e l i t was f r o z e n i n t h i n l a y e r a t

-

40°C and evacuated by s u b l i m a t i o n i n t h e Labconco u n i t . P r e l i m i n a r y r e s u l t s show t h a t t h e SiO c r y o g e l s developed t e x t u r a l and s t r u c t u r a l p r o p e r t i e s a s h i g h a s t h o s e c o r r e s p o n d i n g t o t2he s i m i l a r a e r o g e l s of s i l i c a . A s i n t h e c a s e of t h e N i O - A 1 0 mixed o x i d e s t h e a e r o g e l s e x h i b i t e d a VpN a g a i n t w i c e h i g h e r t h a n t h a t of

2 3

t h e s i m i l a r c r y o g e l s 171. But c o n t r a r y o t h e Ni0-A1 0

5

c r y o g e l s t h e S i 0 2 c r y o g e l s e x h i b i t e d l a r g e BET s p e c i f i c a r e a s c l o s e t o 700 m / g and equi$a?ent t o t h o s e measured f o r t h e s i m i l a r s i l i c a a e r o g e l s ( p r e p a r e d from t h e sodium m e t a s i l i c a t e p r e c u r s o r ) / 7 / .

REVUE DE PHYSIQUE APPLIQUEE

CONCLUSIONS

A t t h e p r e s e n t time t h e c r y o g e l s can be prepared e i t h e r under t h e form of an a l c o g e l NiO-A1 0 o r a n aquagel S i 0 2 i n t h e sol-gel s t e p .

2 3

The t e x t u r a l and s t r u c t u r a l c h a r a c t e r i s t i c s of t h e s o o b t a i n e d c r y o g e l s a r e c l o s e t o t h o s e e x h i b i t e d by t h e a e r o g e l s of t h e same g r o s s chemical composition. But t h e p r e l i m i n a r y experiments showed t h a t i n t h e c a s e of t h e b i n a r y oxides Ni0-A1203 t h e c r y o g e l s have s m a l l e r t o t a l porous volumes a s w e l l a s s p e c i f i c a r e a s t h a n t h e a e r o g e l s .

For s i l i c a , t h e c r y o g e l s develop s u r f a c e a r e a s e q u a l t o t h o s e of t h e a e r o g e l s but t h e y have a micro and meso p o r o s i t y which i s h a l f a s l a r g e a s t h a t of t h e a e r o g e l s .

111 "Aerogels", J . F r i c k e Ed., S p r i n g e r Verlag, B e l i n 1986.

121 T e i c h n e r , S.J. i n op c i t r e f . 1 , p. 22

Pajonk, G.M. and T e i c h n e r , S.J., i n op c i t r e f . 1, p. 143.

131 S c h n e t t l e r , F . J . , Monforte, F.R. and Rhodes, W.W. i n "Science of Ceramics", Vol. 4. The B r i t i s h Ceramic S o c i e t y Stoke on T r e n t , 1968, p. 79.

141 Beyer, L.A., Kalnas, C.E., Roy, C.E. and Lloyd, I.K., Am. Ceram. Soc. Bull.,

66

⁽¹⁹⁸⁷⁾

1647.

151 Gardes, G.E.E., Pajonk, G. and T e i c h n e r , S.J., Bull. Soc. Chim. France, 1876, 1321.

161 Klvana, D . , Chaouki, J . , Kuschorsky, D., Chavarie, C. and Pajonk, G.M., Appl. Cat.,

62

(1988) 121.

171 Abouarnadasse, S., Chaouki, J . , Klvana, D., Chavarie, C. and Pajonk, G.M., i n press.

The a u t h o r s a r e g r a t e f u l t o NSERG (Canada) and t o Centre Jacques C a r t i e r (Lyon) f o r funding t h i s research.