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Porous texture modifications of a series of silica and silica-alumina hydrogels and xerogels: a thermoporometry study.
Studies in Surface Science and Catalysis 128 K.K. Ungeret al. (Editors) o 2000 Elsevier Science B.V. All rightsreserved.
623
Porous texture modifications of a series of silica and silica-alumina hydrogeis and xerogels : a thermoporometry study. J.P. REYMOND* and J.F. QUINSON** *LGPC, Ecole CPE, 43 Bd du 11 Novembre 1918, 69616 Villeurbanne Cedex, France. **GEMPPM, INSA Lyon, 20 Av. Albert-Einstein, 69621 Villeurbanne Cedex, France.
Abstract The objective of the paper is to identify and evaluate the influence of the main preparation operating parameters which govern the porous texture formation of silica and silica-alumina usable as catalyst support or matrix. The chosen preparation process is a sol-gel transition leading to hydrogels which are spray dried. The texture modifications of hydrogels and xerogels are evaluated from water thermoporometry, analytical method well suited to measurements on wet or dry gels. It allows to point out the influence of operating parameters of the preparation process and demonstrate that texture modifications induced in hydrogels still exist in xerogels. From the knowledge of texture evolution it is possible to master the pore texture of the end material.
1. I N T R O D U C T I O N One of the most important characteristics of a catalyst is its porous texture (specific surface area, pore volume, pore size and size distribution) which must allow good reactant and product circulations in the catalyst bulk. According to its use, it is necessary to give to a catalyst a tailor-made texture. As they present many advantages, silica-aluminas are widely used as matrices (for cracking catalysts) or supports (supported metals) of catalytic phases. The texture of precipitated silica-alumina depends on the texture of the silica when silica results from a sol-gel transition. The condensation of silicic acids leads to the formation of primary spherical particles (sol) which aggregate in defined conditions, forming the tridimensional network of the gel [1]. In the gel framework each primary particle of silica is connected to two or three particles [1 ] and the gel pores are the cavities existing between these particles [2]. The size of the particles, conjugated to their connectivity, defines the surface area, the volume and diameter of the gel pores. Thus, the silica texture could be controlled by mastering the size and the packing of the silica particles [3], characteristics which depend on the conditions of preparation of silica sol and gel. A change in the porous texture of silica can be obtained by varying the operating parameters of the preparation process [4] : pH and temperature of the sol-gel transition, gel ageing, reactant mixing, addition of porogens (organic polymers), change of the interrnicellar solvent, use of hydrothermal treatments, etc. However, two steps of the preparation of silicaalumina can induce important modifications of the texture of the starting silica : i) the precipitation of alumina in the silica gel and ii) the drying of the silica-alumina gel. To evaluate the effects of each operating parameter of the preparation process on the texture of the end product, it is necessary to determine the textural characteristics at each step of the process. As it is described in this paper, thermoporometry is well suited to such a determination.
624 2. E X P E R I M E N T A L 2.1. Preparation of silica and silica-alumina gels 9 Typically, studies of the texture of silica gels concern highly pure gels obtained from an hydrolysis-condensation process of silicium alkoxides dissolved in alcohol. Such a process does not imply the use of a continuous stirring of the reactant mixture (except for initial mixing of reactants) and leads to a monolithic alcohogel. Conversely, the present work is related to hydrogels prepared in an aqueous medium following a two-step process in which the solutions are continously stirred and lead to slurries. First step :preparation of a silica hydrogel. Typically a sodium silicate solution (water glass; SiOJNa20 = 3.44; silica content : 6 wt%) is partly neutralized (pH 9.5) under vigorous stirring by a sulfuric acid solution (35 or 20 wt%). After ageing (30ran), the silica hydrogel slurry (solid content between 5 and 10 wt%) can be filtered, washed and spray dried to obtain silica xerogels. The silica hydrogel is also used to prepare silica-alumina hydrogels. Second step : preparation of silica-alumina gels. Aluminium sulfate solution (33 wt% A12(SO4)3-18H:O) is added to the silica gel slurry. A further addition of an ammonia solution (20 wt% NH3) leads to the precipitation of alumina at pH 6. The obtained slurries of silicaalumina hydrogels are successively filtered under vacuum and washed several times to remove impurities, and spray-dryed in well defined conditions. Spray drying leads to solid spherical particles with reproducible physical characteristics, in particular pore size distribution, pore texture and particle size distribution. Preparations took place in a stirred glass reactor equipped with sensors (pH electrode, thermocouples and torquemeter on stirring shaft) which allow to control silica gelation and alumina precipitation. 2.2. Thermoporometry experiments : Texture of gels has been evaluated by thermoporometry. This calorimetric method, which has been described elsewhere [5], applies to hydrogels (wet materials) as well on xerogels (dry materials). Thermoporometry is based on the analysis of solidification of pure water confined in the pores of a material. The hydrogel slurries prepared in this work contain large amounts (20 to 30 wt%) of impurities, ions (Na +, SO2-4, NH 4+) resulting from the neutralisation reactions. As these impurities act on the solidification temperature of water they must be removed. The following experimental procedure has been applied in order to obtain pure samples : - the gel slurry is vacuum filtered (drying of the cake filtration on the filtration media must be avoided) - the cake filtration is washed with pure water on the filter. Large amounts of water improve the gel purity but they have a detrimental effect on silica hydrogel texture. An optimal amount of water is found to be 5 g for 1 g of gel. Thus, the total impurity content is less than 0.5 wt%. Hydrogel samples used to perform thermoporometry measurements are small pieces of the washed filtration cake (solid content is between 10 and 15 wt%). - the cake filtration is repulped in water and the resulting suspension is spray dried leading to the xerogel. Sample for thermoporometry is took from this xerogel.
3. RESULTS AND DISCUSSION Although a great number of operating parameters of the preparation process has an effect on the texture of silica-alumina, only the effects of the main parameters are reported below.
625 In thermoporometry experiments the pore radius is deduced from the measurement of the solidification temperature and the volume of these pores is calculated from the energy involved during the phase transition. The pore radius distribution and the pore surface are then calculated. The pore texture can be described from numerical values (mean pore radius, total pore volume or surface, etc...) or by curves. For example, curves of figure 1 are the cumulative pore volume vs pore radius while curves of figure 2 are the pore radius distributions. Texture modifications are conveniently depicted by the pore size distribution curves.
Silica hydrogel Silica xerogel Silica-alumina hydrogel , Silica-alumina xerogel ;." ""'"
I ......
1200 ._.1000
"~S"ica-h-ydrogem 1000
. . . . . . . . . . . . . . . . . . . . . . . . . . .
9""
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o
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E 600 E
i m ( I ee
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-
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"6 > 400 ~ ix. ~) 2 0 0 0"-
.....
9.--~--- Silica xerogel --0.-- Silica-alumina hydrogel --0~- Silica-alumina xerogel
f L
_j~/i
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:
-
"0 200
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-
. . . . .
0
5 10 Pore radius (nm)
15
Figure 1 9 C o m p a r i s o n o f the texture o f silica and
silica-alumina (hydrogels and xerogels) from cumulative volume curves
0
5 10 Pore radius (nm)
15
Figure 2 9Comparison of the texture of silica and silica-alumina (hydrogels and xerogels) from pore size distribution curves.
It can already be noted that the texture of silica gels (hydrogel and xerogel) is characterized by a very narrow pore size distribution while silica-alumina gels have a wider pore size distribution.
3.1.Effect of stirring ." The gels are maintained under stirring throughout the preparation process. The influence of stirring on the gel texture has been evaluated from the comparison of the texture of gels obtained with stirring or without stirring (except for the initial mixing of reactants), all others operating parameters remaining constant. The results summarized in table 1 are related to fresh hydrogels (one day) or to aged hydrogels (7 days). The continuous stirring during silica gelation has a weak influence on the texture of silica hydrogel, a slight broadening of the pore size distribution is only observed. So, the silica hydrogel obtained with continous stirring can be assimilated to a suspension of small pieces of the monolith which would be obtained without stirring, and each piece has a texture quite similar to that of the monolith.
626 Table 1 9Effect of stirring on the texture of fresh and aged silica hydrogels ' Stinted hydrogel Hydrogel ageing (days) Pore volume (mm3/g) Mean pore radius (nm) Pore surface(m2/g) Pore range(nm)
1 460 3 388 2.3-3.7
7 653 3.1 517 2.2-4.2
Non stirred hydrogel 1 496 2.7 491 2.1-3.1
7 537 3 459 2.2-3.5
..
,.
It is noteworthy that ageing has the same effect on the two kinds of gel (stirred or non stirred) : increasing of pore volume, surface and mean diameter, and broadening of the pore range. As a consequence, to avoid these undesirable texture modifications, thermoporometry measurements must be carried out as soon as possible after the preparation of hydrogel samples (within 24 hours).
3.2.Effect of silica gelation pH: The gelation pH of silica is well known to be effective on gel texture [ 1]. Table 2 allows to compare the texture of monolithic silica gels prepared in acidic (pH=5.4) or basic (pH=l 0.5) gelation conditions. Table 2 9Effect of gelation pH on the texture of monolithic silica hydrogels. ,,.
.
pH
10.5
5.4
Pore volume (mm3/g)
i339
1936
mean pore radius (nm)
4.5
6.6
Pore surface (mZ/g)
614
,
..,
602
1
,,
The effect of gelation pH on silica texture is complex. A low pH value favors the formation of small elementary particles of silica, which would lead to the formation of small pores in the resulting gel. But, when the pH of the sol-gel transition has the neutral pH value the silica gelation is very fast, the silica particles exhibit a broad size distribution and the resulting gel network has a wide open porosity (large pore volume and mean diameter). On the other hand, textural modifications observed in table 2 are not only attributable to the pH change. To obtain silica gelation at pH 5.4 the preparation procedure should be changed by inversing the adding order of reactants : while basic pH is obtained in pouring sulfuric acid in the silicate solution (initial pH 12.5), acid pH is obtained in pouring silicate solution in acid solution. In this way instantaneous gelation at neutral pH is avoided. Intrinsic effects of pH change on gel texture are described by results of table 3 which gives the main textural characteristics of silica and silica-alumina hydrogels prepared at two basic pH according to the same preparation procedure 9 acid is poured in silicate under continuous stirring. The main effect due to a pH decrease is an increase of pore volume and radius. Another interesting fact appears in table 3 : textural modifications induced in silica hydrogel texture still exist in silica-alumina hydrogels.
627 Table 3 9Effect of pH gelation on the texture of stirred silica and silica-alumina hydrogels. Silica
Silica-alumina
pH
10.5
9.5
10.5
9.5
Pore volume (mm3/g)
516
687
1380
1450
mean pore radius (nm)
2.9
3.5
5.5
6.6
Pore surface (m2/g)
452
451
421
389
, - .
3.3.Effect of gel ageing: The slurry obtained after gelation of silica is maintained under moderate stirring during an ageing step. R.K. Iler [ 1] described several modifications of the silica gel network occuring during this period. Table 1 and curves of figure 3 show the texture evolution of a silica hydrogel aged in presence of its mother-liquor. It appears that pore radius, pore surface and pore volume increase with ageing time. This behaviour has been attributed to the phenomenon of dissolution and precipitation of silica. These phenomena, due to the presence of convex and concave curvatures in the silica gel network, strongly modify the gel texture. In presence of water (or aqueous salt solution) the silica phase is not stable.
1400
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' 3 days
1200 -I/
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-------
,, ',
--
--
i --
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day-i days
- - - - - - 7 days,
I
. . . . . . . ~"
0
2
4 6 Pore radius (nm)
8
10
Figure 3 : Effect of ageing time on texture of a silica hydrogel.
0
30 10 20 . Pore radius (nm)
40
Figure 4 : Effect of ageing time on the texture of a silica-alumina hydrogel.
As shown by curves of figure 4, the texture of a silica-alumina hydrogel, kept in suspension in its mother-liquor, does not change during ageing. This stabilising effect of alumina would be due to the blocking of intermicellar junctions of the silica gel network by incorporation of A10 4" tetrahedrons between SiO 4 tetrahedrons. This blocking phenomenon suggests that the textural evolution of silica hydrogels, observed during ageing, would be due to mass transferts through the grain boundaries. These transfers lead to an increasing of mean particle radius and mean pore radius. These results point out that ageing step offers a simple way to strongly modify the texture of silica hydrogels. A further precipitation of alumina in the aged silica gel stops the
628
modifications and leads to a mixed gel whose structure, and as a consequence the texture, is much more stable than the silica one.
3.4.Effect of alumina precipitation in silica hydrogel : A qualitative effect of alumina incorporation in silica has been described : alumina is a stabilizing agent for silica hydrogel texture. Thus, it is also interesting to study the effect of alumina content on the texture of silica-alumina gel. This alumina content is modified by adjustement of the amount of aluminium sulfate and, therefore, of the ammonia amount needed to precipitate the aluminium sulfate at constant pH (-~ 6). Table 4 and figure 5 depict the results obtained when alumina content increases up to 60 wt%. Table 4 : Effect of the alumina content on the texture of silica-alumina hydrogels Alumina content (wt%)
Pore v0'iume mm3/g
Mean pore radius nm
Pore surface m2:g
0 (pure silica) 5 9 25 36 60
516 768 1018 1380 1700 1007
2.9 3.4 3.2 5.5 4.8 6.1
452 523 533 420 367 284
H,..
,,,..
1000
8oo
E
- ~ 0 ~ Pure silica - . . . . . 5 wt% alumina -- .13-- lOwt% alumina 25 wt% alumina I" 36 wt% alumina - - . - X m 6 0 wt% alumina
I | ,, I I '; ! ~1; '~ I1~ ', t~,,
600
B
~~'400
I[
'
-
-
-
200
0 0
2
4
6
8 10 12 Pore radius (nm)
14
16
18
Figure 5 : Effect of the alumina content on the pore radius distributions of. silica-alumina hydrogels Precipitation of increasing amounts of alumina in the silica hydrogel leads to drastic modifications of the gel texture. Up to 10 wt% of alumina, silica-alumina texture is related to that of silica, although pore volume, pore radius and pore surface are more or less increased.
629 Above 10 wt% of alumina the pore size distribution broadens, pore surface decreases and pore volume reaches a maximum value for 36 wt% of alumina. Whatever the alumina content silica-alumina texture differs from that of silica gel. A NMR study of the incorporation of alumina in silica (not detailed in this paper) points out the existence of two aluminium species, A1TM and A1v~, and leads to the following conclusions [8]: for low alumina contents (< 10 %) alumina precipitation results in an isomorphic incorporation of A 1 0 4 tetrahedrons between the SiO4 tetrahedrons forming the elementary silica particles which constitute the gel network. That isomorphic substitution (A1TMspecies) does not induces important textural changes but creates surface acid sites of silica-alumina. for alumina content between 10 % and 40 %, part of alumina is not incorporated in the silica network but precipitates on the silica skeleton. The upper limit of incorporated alumina (A1TM) is reached for a total alumina content (A1~v and A1v~) near to 40 % and is found egal to 25 %. - for alumina content greater than 40 % precipitation of free alumina (A1w species) occurs in the cavities of the gel network. The silica-alumina gel is not only made up of a silica gel network in which A 1 0 4 tetrahedrons are incorporated. Indeed it results on the mixture of a silica-alumina framework partly covered with alumina bonded to silica and free alumina clusters. The silica hydrogel is a compliant material whose network building-up is not completed when alumina is added. So, alumina precipitation can easily modify the silica gel structure and induce drastic texture modifications. -
-
3.5.Effect of gelation temperature . The formation of silica from a sol-gel transition is a low activation energy reaction. Thus, it was only expected a slight effect of the preparation temperature on the gel texture (silica-alumina as well as silica). However, as shown in table 5, the effect of preparation temperature on the texture of silica and silica-alumina hydrogels is important. Table 5 9 Effect of gelation temperature on hydrogel texture.
Pore volume (mm3/g) mean pore radius (nm) Pore surface (m2/g) ,,,,,,
20 ~
Silica 40 ~
353 2.7 342
516 2.9 452
60 ~
20 ~
691 4.4 358
1073 4.8 387
,,,
S il ica-alumina 40 ~ 60 ~ 1380 5.5 421
1476 5.6 387 ,,,
As temperature is increased from 20 ~ to 60 ~ pore volume and mean radius of silica and silica-alumina gels are increased, while pore surface remains quite constant. This can be explained from the increase of the diameter of elementary particles of silica (silica sol) induced by the temperature increase [1]. As a consequence, the mean pore radius increases (pores are cavities in the gel network and between the particles), the number of elementary particles decreases (solid content is constant) resulting to the increasing of the pore volume. The weak variations of pore surface could be due to compensating effects.
3.6.Effect of drying 9 Thermoporometry measurements allow to evidence the effects of operating parameters of the preparation process on the texture of hydrogels of silica and silica-alumina. As the aim
630 of our work is to prepare xerogels usable as supports of heterogeneous catalysts the hydrogels must be dried. Thus, two questions arise" 1. what is the effect of drying on the gel texture ..9 2. do the texture changes obtained on hydrogels still exist after the drying step ? Thermoporometry applying also on xerogels can give the answers. The effects of three drying modes on gel texture have been studied and compared : tray drying : 15 hours, in air at 125 ~ after drying xerogel water content is 21.3 wt%. This drying mode leads to very hard pieces which must be ground. freeze drying : 24 hours, under vacuum (pressure : 10 Pa) at -48 ~ xerogel water content is 20.4 wt%. A powder, constituted of polyhedric particles, is obtained. spray drying : 20 seconds, air temperature : 125 ~ ; xerogel water content is 27.1 wt %. The spray drying provides powders constituted of well shaped particles. -
-
-
Results concerning drying of silica gels are summarized in table 6, while results concerning silica-aluminas are in table 7. Comparison of curves of figure 1 and 2 also illustrates the effect of the drying on gel texture.
Texture
Table 96 .Effect of drying on the texture of silica gels. Hydrogel Spray drying Tray drying Freeze drying
Vp(mm3/g) Rp (nm) S (m2/g)
Texture
516 2.9 452
176 2.3 230
141 2.3 179
215 2.2 291
Table 7 9Effect of drying on the texture of silica-alumina gels. Hydrogel Spray drying Tray drying Freeze drying
Vp (mm3/g) 1380 Rp (nm) 5.5 S (m2/g) 420 Radius range (nm) 4.1 - 11
425 2.9 359 2.3 - 4.1
236 2.3 161 2.4 - 5.5
793 3.1 414 2.8 - 11.2
Whatever the drying mode and the hydrogel type (silica or silica-alumina) the drying induces a large shrinkage of the gel 9 mean pore radius, total pore volume and pore surface are strongly reduced. However, the extent of textural modifications depends on the drying mode 9 the freeze drying is the less altering technique while the tray drying is the worst one. Texture modifications occur mainly during the first step of drying (the constant rate period) and are related to the visco-elastic properties of the gel network [6]. During the second step of drying (the falling rate period), liquid water leaves the capillaries and the pore walls can be damaged by forces linked to the existence of liquid-gas meniscus [6, 7]. In the absence of a liquid-gas meniscus, as it is the case for freeze drying (sublimation of ice), the solid-gas interface tension is weak and the porous volume decreasing due to the drying is smaller. When a meniscus exists at the liquid-gas interface in the pores, the observed textural evolutions agree with a shrinkage produced by capillary forces [7]. This case is well illustrated by the tray drying which conjugates a high drying duration, and a large and thick sample in which water concentration is heterogeneous during the drying. In the case of spray drying, liquid-vapor menisci also exist in the pores, but the material is divided in very small particles (d ~ 150 gm) leading to a very short drying duration (20 to 30 seconds), which, combined to a low drying temperature (solid temperature-90 ~ results into less detrimental shrinkage effects.
631 The drying mode and drying operating parameters must be carefully chosen to minimize the gel texture modifications. Table 8 depicts the effects of the preparation temperature on the texture of hydrogels and xerogels of silica and silica-alumina. As the preparation temperature is increased the pore volume and the mean pore radius of hydrogels as well as xerogels, are increased. Table 8 9Effect of gelation temperature on the texture of hydrogels and xerogels of silica and silica-alumina. Temperature Silica hydrogel Silica xerogel Silica-alumina Silica-alumina ~ hydrogel xerogel Vp R V~ R ... Vp R Vp R 60 809 4.1 487 2.2 2170 7.8 897 3.8 ,,,,
20
568
3
141
1.7
1751
5.6
554
2
From these results it can be concluded that the modifications generated in the silica hydrogels are altered by the drying step, but they still exist in the silica xerogels. They are also observed on the silica-alumina hydrogels and, finally, on the silica-alumina xerogels. Although undesirable changes can occur at each step of the preparation procedure of silica-alumina, textural modifications deliberately generated in the silica hydrogels can be preserved. 4. CONCLUSION As thermoporometry is suitable for hydrogel as well as for xerogel materials, it has been possible to study the effects of the operating parameters of the preparation process on the texture of silica and silica-alumina and to follow the changes carried out by each step of the process. Large textural modifications occur during the precipitation of alumina in silica hydrogels and during the drying step. It has been demonstrated that the texture modifications generated at the beginning of the process (silica hydrogel formation) still exist in the dried end-product (silica-alumina xerogel). The knowledge of the influence of each preparation step allows a better mastering of the texture of the end-product. The silica hydrogel network constitutes the framework of the silica-alumina gel. From a textural point of wiev, a silica-alumina xerogel seems to be an "image" of the starting silica hydrogel. Depending on the alumina content the silica-alumina gels exhibit predominantly a silica-like texture or an alumina-like texture. REFERENCES 1. R.K. Iler in "The Chemistry of Silica" (John Wiley and sons, New-York 1979). 2. C.J. Planck and C.L. Drake, J. Colloid. Sci. 2 (1947), 399. 3. S.A. Mitchell, Chem. Ind. (1966), 924. 4. A.G. Forster and J.M. Thorp in "The Structure and Properties of Porous Materials" (D.H. Everett and F.S. Stone Eds., Butterworths, London 1958), 227. 5. J. Dumas, J.F. Quinson and J. Serughetti, J. Non Crystal. Solids, 125 (1990), 244. 6. C.J. Brinker and G.W. Scherer, Sol Gel Science (Academic Press, San Diego 1990), p.454. 7. G.W. Scherer, J. Amer.Ceram. Soc. In Sol-Gel Science (Academic Press Inc; Boston 1990), pp 453-513. 8. I. Bia7, Thesis, Paris VI l Jniversitv, 1999.
623
Porous texture modifications of a series of silica and silica-alumina hydrogeis and xerogels : a thermoporometry study. J.P. REYMOND* and J.F. QUINSON** *LGPC, Ecole CPE, 43 Bd du 11 Novembre 1918, 69616 Villeurbanne Cedex, France. **GEMPPM, INSA Lyon, 20 Av. Albert-Einstein, 69621 Villeurbanne Cedex, France.
Abstract The objective of the paper is to identify and evaluate the influence of the main preparation operating parameters which govern the porous texture formation of silica and silica-alumina usable as catalyst support or matrix. The chosen preparation process is a sol-gel transition leading to hydrogels which are spray dried. The texture modifications of hydrogels and xerogels are evaluated from water thermoporometry, analytical method well suited to measurements on wet or dry gels. It allows to point out the influence of operating parameters of the preparation process and demonstrate that texture modifications induced in hydrogels still exist in xerogels. From the knowledge of texture evolution it is possible to master the pore texture of the end material.
1. I N T R O D U C T I O N One of the most important characteristics of a catalyst is its porous texture (specific surface area, pore volume, pore size and size distribution) which must allow good reactant and product circulations in the catalyst bulk. According to its use, it is necessary to give to a catalyst a tailor-made texture. As they present many advantages, silica-aluminas are widely used as matrices (for cracking catalysts) or supports (supported metals) of catalytic phases. The texture of precipitated silica-alumina depends on the texture of the silica when silica results from a sol-gel transition. The condensation of silicic acids leads to the formation of primary spherical particles (sol) which aggregate in defined conditions, forming the tridimensional network of the gel [1]. In the gel framework each primary particle of silica is connected to two or three particles [1 ] and the gel pores are the cavities existing between these particles [2]. The size of the particles, conjugated to their connectivity, defines the surface area, the volume and diameter of the gel pores. Thus, the silica texture could be controlled by mastering the size and the packing of the silica particles [3], characteristics which depend on the conditions of preparation of silica sol and gel. A change in the porous texture of silica can be obtained by varying the operating parameters of the preparation process [4] : pH and temperature of the sol-gel transition, gel ageing, reactant mixing, addition of porogens (organic polymers), change of the interrnicellar solvent, use of hydrothermal treatments, etc. However, two steps of the preparation of silicaalumina can induce important modifications of the texture of the starting silica : i) the precipitation of alumina in the silica gel and ii) the drying of the silica-alumina gel. To evaluate the effects of each operating parameter of the preparation process on the texture of the end product, it is necessary to determine the textural characteristics at each step of the process. As it is described in this paper, thermoporometry is well suited to such a determination.
624 2. E X P E R I M E N T A L 2.1. Preparation of silica and silica-alumina gels 9 Typically, studies of the texture of silica gels concern highly pure gels obtained from an hydrolysis-condensation process of silicium alkoxides dissolved in alcohol. Such a process does not imply the use of a continuous stirring of the reactant mixture (except for initial mixing of reactants) and leads to a monolithic alcohogel. Conversely, the present work is related to hydrogels prepared in an aqueous medium following a two-step process in which the solutions are continously stirred and lead to slurries. First step :preparation of a silica hydrogel. Typically a sodium silicate solution (water glass; SiOJNa20 = 3.44; silica content : 6 wt%) is partly neutralized (pH 9.5) under vigorous stirring by a sulfuric acid solution (35 or 20 wt%). After ageing (30ran), the silica hydrogel slurry (solid content between 5 and 10 wt%) can be filtered, washed and spray dried to obtain silica xerogels. The silica hydrogel is also used to prepare silica-alumina hydrogels. Second step : preparation of silica-alumina gels. Aluminium sulfate solution (33 wt% A12(SO4)3-18H:O) is added to the silica gel slurry. A further addition of an ammonia solution (20 wt% NH3) leads to the precipitation of alumina at pH 6. The obtained slurries of silicaalumina hydrogels are successively filtered under vacuum and washed several times to remove impurities, and spray-dryed in well defined conditions. Spray drying leads to solid spherical particles with reproducible physical characteristics, in particular pore size distribution, pore texture and particle size distribution. Preparations took place in a stirred glass reactor equipped with sensors (pH electrode, thermocouples and torquemeter on stirring shaft) which allow to control silica gelation and alumina precipitation. 2.2. Thermoporometry experiments : Texture of gels has been evaluated by thermoporometry. This calorimetric method, which has been described elsewhere [5], applies to hydrogels (wet materials) as well on xerogels (dry materials). Thermoporometry is based on the analysis of solidification of pure water confined in the pores of a material. The hydrogel slurries prepared in this work contain large amounts (20 to 30 wt%) of impurities, ions (Na +, SO2-4, NH 4+) resulting from the neutralisation reactions. As these impurities act on the solidification temperature of water they must be removed. The following experimental procedure has been applied in order to obtain pure samples : - the gel slurry is vacuum filtered (drying of the cake filtration on the filtration media must be avoided) - the cake filtration is washed with pure water on the filter. Large amounts of water improve the gel purity but they have a detrimental effect on silica hydrogel texture. An optimal amount of water is found to be 5 g for 1 g of gel. Thus, the total impurity content is less than 0.5 wt%. Hydrogel samples used to perform thermoporometry measurements are small pieces of the washed filtration cake (solid content is between 10 and 15 wt%). - the cake filtration is repulped in water and the resulting suspension is spray dried leading to the xerogel. Sample for thermoporometry is took from this xerogel.
3. RESULTS AND DISCUSSION Although a great number of operating parameters of the preparation process has an effect on the texture of silica-alumina, only the effects of the main parameters are reported below.
625 In thermoporometry experiments the pore radius is deduced from the measurement of the solidification temperature and the volume of these pores is calculated from the energy involved during the phase transition. The pore radius distribution and the pore surface are then calculated. The pore texture can be described from numerical values (mean pore radius, total pore volume or surface, etc...) or by curves. For example, curves of figure 1 are the cumulative pore volume vs pore radius while curves of figure 2 are the pore radius distributions. Texture modifications are conveniently depicted by the pore size distribution curves.
Silica hydrogel Silica xerogel Silica-alumina hydrogel , Silica-alumina xerogel ;." ""'"
I ......
1200 ._.1000
"~S"ica-h-ydrogem 1000
. . . . . . . . . . . . . . . . . . . . . . . . . . .
9""
800 E
o
E800
__
c
.,
t~
E 600 E
i m ( I ee
600 -
-
-
IZ
"6 > 400 ~ ix. ~) 2 0 0 0"-
.....
9.--~--- Silica xerogel --0.-- Silica-alumina hydrogel --0~- Silica-alumina xerogel
f L
_j~/i
-o 4 0 0
:
-
"0 200
; II
-
. . . . .
0
5 10 Pore radius (nm)
15
Figure 1 9 C o m p a r i s o n o f the texture o f silica and
silica-alumina (hydrogels and xerogels) from cumulative volume curves
0
5 10 Pore radius (nm)
15
Figure 2 9Comparison of the texture of silica and silica-alumina (hydrogels and xerogels) from pore size distribution curves.
It can already be noted that the texture of silica gels (hydrogel and xerogel) is characterized by a very narrow pore size distribution while silica-alumina gels have a wider pore size distribution.
3.1.Effect of stirring ." The gels are maintained under stirring throughout the preparation process. The influence of stirring on the gel texture has been evaluated from the comparison of the texture of gels obtained with stirring or without stirring (except for the initial mixing of reactants), all others operating parameters remaining constant. The results summarized in table 1 are related to fresh hydrogels (one day) or to aged hydrogels (7 days). The continuous stirring during silica gelation has a weak influence on the texture of silica hydrogel, a slight broadening of the pore size distribution is only observed. So, the silica hydrogel obtained with continous stirring can be assimilated to a suspension of small pieces of the monolith which would be obtained without stirring, and each piece has a texture quite similar to that of the monolith.
626 Table 1 9Effect of stirring on the texture of fresh and aged silica hydrogels ' Stinted hydrogel Hydrogel ageing (days) Pore volume (mm3/g) Mean pore radius (nm) Pore surface(m2/g) Pore range(nm)
1 460 3 388 2.3-3.7
7 653 3.1 517 2.2-4.2
Non stirred hydrogel 1 496 2.7 491 2.1-3.1
7 537 3 459 2.2-3.5
..
,.
It is noteworthy that ageing has the same effect on the two kinds of gel (stirred or non stirred) : increasing of pore volume, surface and mean diameter, and broadening of the pore range. As a consequence, to avoid these undesirable texture modifications, thermoporometry measurements must be carried out as soon as possible after the preparation of hydrogel samples (within 24 hours).
3.2.Effect of silica gelation pH: The gelation pH of silica is well known to be effective on gel texture [ 1]. Table 2 allows to compare the texture of monolithic silica gels prepared in acidic (pH=5.4) or basic (pH=l 0.5) gelation conditions. Table 2 9Effect of gelation pH on the texture of monolithic silica hydrogels. ,,.
.
pH
10.5
5.4
Pore volume (mm3/g)
i339
1936
mean pore radius (nm)
4.5
6.6
Pore surface (mZ/g)
614
,
..,
602
1
,,
The effect of gelation pH on silica texture is complex. A low pH value favors the formation of small elementary particles of silica, which would lead to the formation of small pores in the resulting gel. But, when the pH of the sol-gel transition has the neutral pH value the silica gelation is very fast, the silica particles exhibit a broad size distribution and the resulting gel network has a wide open porosity (large pore volume and mean diameter). On the other hand, textural modifications observed in table 2 are not only attributable to the pH change. To obtain silica gelation at pH 5.4 the preparation procedure should be changed by inversing the adding order of reactants : while basic pH is obtained in pouring sulfuric acid in the silicate solution (initial pH 12.5), acid pH is obtained in pouring silicate solution in acid solution. In this way instantaneous gelation at neutral pH is avoided. Intrinsic effects of pH change on gel texture are described by results of table 3 which gives the main textural characteristics of silica and silica-alumina hydrogels prepared at two basic pH according to the same preparation procedure 9 acid is poured in silicate under continuous stirring. The main effect due to a pH decrease is an increase of pore volume and radius. Another interesting fact appears in table 3 : textural modifications induced in silica hydrogel texture still exist in silica-alumina hydrogels.
627 Table 3 9Effect of pH gelation on the texture of stirred silica and silica-alumina hydrogels. Silica
Silica-alumina
pH
10.5
9.5
10.5
9.5
Pore volume (mm3/g)
516
687
1380
1450
mean pore radius (nm)
2.9
3.5
5.5
6.6
Pore surface (m2/g)
452
451
421
389
, - .
3.3.Effect of gel ageing: The slurry obtained after gelation of silica is maintained under moderate stirring during an ageing step. R.K. Iler [ 1] described several modifications of the silica gel network occuring during this period. Table 1 and curves of figure 3 show the texture evolution of a silica hydrogel aged in presence of its mother-liquor. It appears that pore radius, pore surface and pore volume increase with ageing time. This behaviour has been attributed to the phenomenon of dissolution and precipitation of silica. These phenomena, due to the presence of convex and concave curvatures in the silica gel network, strongly modify the gel texture. In presence of water (or aqueous salt solution) the silica phase is not stable.
1400
T
....
1 day
/
-
' 3 days
1200 -I/
I
......
I! ~
....
. . . . .
_--looo
&
II/I
_
/
I1,1
800 +
"0
7 days 152 days
II , ';
E
E
3so T
600
'
~, 4oo t"
:-.~ !/~ '/
II/'1/,,, :
0
I
'
-------
,, ',
--
--
i --
3
day-i days
- - - - - - 7 days,
I
. . . . . . . ~"
0
2
4 6 Pore radius (nm)
8
10
Figure 3 : Effect of ageing time on texture of a silica hydrogel.
0
30 10 20 . Pore radius (nm)
40
Figure 4 : Effect of ageing time on the texture of a silica-alumina hydrogel.
As shown by curves of figure 4, the texture of a silica-alumina hydrogel, kept in suspension in its mother-liquor, does not change during ageing. This stabilising effect of alumina would be due to the blocking of intermicellar junctions of the silica gel network by incorporation of A10 4" tetrahedrons between SiO 4 tetrahedrons. This blocking phenomenon suggests that the textural evolution of silica hydrogels, observed during ageing, would be due to mass transferts through the grain boundaries. These transfers lead to an increasing of mean particle radius and mean pore radius. These results point out that ageing step offers a simple way to strongly modify the texture of silica hydrogels. A further precipitation of alumina in the aged silica gel stops the
628
modifications and leads to a mixed gel whose structure, and as a consequence the texture, is much more stable than the silica one.
3.4.Effect of alumina precipitation in silica hydrogel : A qualitative effect of alumina incorporation in silica has been described : alumina is a stabilizing agent for silica hydrogel texture. Thus, it is also interesting to study the effect of alumina content on the texture of silica-alumina gel. This alumina content is modified by adjustement of the amount of aluminium sulfate and, therefore, of the ammonia amount needed to precipitate the aluminium sulfate at constant pH (-~ 6). Table 4 and figure 5 depict the results obtained when alumina content increases up to 60 wt%. Table 4 : Effect of the alumina content on the texture of silica-alumina hydrogels Alumina content (wt%)
Pore v0'iume mm3/g
Mean pore radius nm
Pore surface m2:g
0 (pure silica) 5 9 25 36 60
516 768 1018 1380 1700 1007
2.9 3.4 3.2 5.5 4.8 6.1
452 523 533 420 367 284
H,..
,,,..
1000
8oo
E
- ~ 0 ~ Pure silica - . . . . . 5 wt% alumina -- .13-- lOwt% alumina 25 wt% alumina I" 36 wt% alumina - - . - X m 6 0 wt% alumina
I | ,, I I '; ! ~1; '~ I1~ ', t~,,
600
B
~~'400
I[
'
-
-
-
200
0 0
2
4
6
8 10 12 Pore radius (nm)
14
16
18
Figure 5 : Effect of the alumina content on the pore radius distributions of. silica-alumina hydrogels Precipitation of increasing amounts of alumina in the silica hydrogel leads to drastic modifications of the gel texture. Up to 10 wt% of alumina, silica-alumina texture is related to that of silica, although pore volume, pore radius and pore surface are more or less increased.
629 Above 10 wt% of alumina the pore size distribution broadens, pore surface decreases and pore volume reaches a maximum value for 36 wt% of alumina. Whatever the alumina content silica-alumina texture differs from that of silica gel. A NMR study of the incorporation of alumina in silica (not detailed in this paper) points out the existence of two aluminium species, A1TM and A1v~, and leads to the following conclusions [8]: for low alumina contents (< 10 %) alumina precipitation results in an isomorphic incorporation of A 1 0 4 tetrahedrons between the SiO4 tetrahedrons forming the elementary silica particles which constitute the gel network. That isomorphic substitution (A1TMspecies) does not induces important textural changes but creates surface acid sites of silica-alumina. for alumina content between 10 % and 40 %, part of alumina is not incorporated in the silica network but precipitates on the silica skeleton. The upper limit of incorporated alumina (A1TM) is reached for a total alumina content (A1~v and A1v~) near to 40 % and is found egal to 25 %. - for alumina content greater than 40 % precipitation of free alumina (A1w species) occurs in the cavities of the gel network. The silica-alumina gel is not only made up of a silica gel network in which A 1 0 4 tetrahedrons are incorporated. Indeed it results on the mixture of a silica-alumina framework partly covered with alumina bonded to silica and free alumina clusters. The silica hydrogel is a compliant material whose network building-up is not completed when alumina is added. So, alumina precipitation can easily modify the silica gel structure and induce drastic texture modifications. -
-
3.5.Effect of gelation temperature . The formation of silica from a sol-gel transition is a low activation energy reaction. Thus, it was only expected a slight effect of the preparation temperature on the gel texture (silica-alumina as well as silica). However, as shown in table 5, the effect of preparation temperature on the texture of silica and silica-alumina hydrogels is important. Table 5 9 Effect of gelation temperature on hydrogel texture.
Pore volume (mm3/g) mean pore radius (nm) Pore surface (m2/g) ,,,,,,
20 ~
Silica 40 ~
353 2.7 342
516 2.9 452
60 ~
20 ~
691 4.4 358
1073 4.8 387
,,,
S il ica-alumina 40 ~ 60 ~ 1380 5.5 421
1476 5.6 387 ,,,
As temperature is increased from 20 ~ to 60 ~ pore volume and mean radius of silica and silica-alumina gels are increased, while pore surface remains quite constant. This can be explained from the increase of the diameter of elementary particles of silica (silica sol) induced by the temperature increase [1]. As a consequence, the mean pore radius increases (pores are cavities in the gel network and between the particles), the number of elementary particles decreases (solid content is constant) resulting to the increasing of the pore volume. The weak variations of pore surface could be due to compensating effects.
3.6.Effect of drying 9 Thermoporometry measurements allow to evidence the effects of operating parameters of the preparation process on the texture of hydrogels of silica and silica-alumina. As the aim
630 of our work is to prepare xerogels usable as supports of heterogeneous catalysts the hydrogels must be dried. Thus, two questions arise" 1. what is the effect of drying on the gel texture ..9 2. do the texture changes obtained on hydrogels still exist after the drying step ? Thermoporometry applying also on xerogels can give the answers. The effects of three drying modes on gel texture have been studied and compared : tray drying : 15 hours, in air at 125 ~ after drying xerogel water content is 21.3 wt%. This drying mode leads to very hard pieces which must be ground. freeze drying : 24 hours, under vacuum (pressure : 10 Pa) at -48 ~ xerogel water content is 20.4 wt%. A powder, constituted of polyhedric particles, is obtained. spray drying : 20 seconds, air temperature : 125 ~ ; xerogel water content is 27.1 wt %. The spray drying provides powders constituted of well shaped particles. -
-
-
Results concerning drying of silica gels are summarized in table 6, while results concerning silica-aluminas are in table 7. Comparison of curves of figure 1 and 2 also illustrates the effect of the drying on gel texture.
Texture
Table 96 .Effect of drying on the texture of silica gels. Hydrogel Spray drying Tray drying Freeze drying
Vp(mm3/g) Rp (nm) S (m2/g)
Texture
516 2.9 452
176 2.3 230
141 2.3 179
215 2.2 291
Table 7 9Effect of drying on the texture of silica-alumina gels. Hydrogel Spray drying Tray drying Freeze drying
Vp (mm3/g) 1380 Rp (nm) 5.5 S (m2/g) 420 Radius range (nm) 4.1 - 11
425 2.9 359 2.3 - 4.1
236 2.3 161 2.4 - 5.5
793 3.1 414 2.8 - 11.2
Whatever the drying mode and the hydrogel type (silica or silica-alumina) the drying induces a large shrinkage of the gel 9 mean pore radius, total pore volume and pore surface are strongly reduced. However, the extent of textural modifications depends on the drying mode 9 the freeze drying is the less altering technique while the tray drying is the worst one. Texture modifications occur mainly during the first step of drying (the constant rate period) and are related to the visco-elastic properties of the gel network [6]. During the second step of drying (the falling rate period), liquid water leaves the capillaries and the pore walls can be damaged by forces linked to the existence of liquid-gas meniscus [6, 7]. In the absence of a liquid-gas meniscus, as it is the case for freeze drying (sublimation of ice), the solid-gas interface tension is weak and the porous volume decreasing due to the drying is smaller. When a meniscus exists at the liquid-gas interface in the pores, the observed textural evolutions agree with a shrinkage produced by capillary forces [7]. This case is well illustrated by the tray drying which conjugates a high drying duration, and a large and thick sample in which water concentration is heterogeneous during the drying. In the case of spray drying, liquid-vapor menisci also exist in the pores, but the material is divided in very small particles (d ~ 150 gm) leading to a very short drying duration (20 to 30 seconds), which, combined to a low drying temperature (solid temperature-90 ~ results into less detrimental shrinkage effects.
631 The drying mode and drying operating parameters must be carefully chosen to minimize the gel texture modifications. Table 8 depicts the effects of the preparation temperature on the texture of hydrogels and xerogels of silica and silica-alumina. As the preparation temperature is increased the pore volume and the mean pore radius of hydrogels as well as xerogels, are increased. Table 8 9Effect of gelation temperature on the texture of hydrogels and xerogels of silica and silica-alumina. Temperature Silica hydrogel Silica xerogel Silica-alumina Silica-alumina ~ hydrogel xerogel Vp R V~ R ... Vp R Vp R 60 809 4.1 487 2.2 2170 7.8 897 3.8 ,,,,
20
568
3
141
1.7
1751
5.6
554
2
From these results it can be concluded that the modifications generated in the silica hydrogels are altered by the drying step, but they still exist in the silica xerogels. They are also observed on the silica-alumina hydrogels and, finally, on the silica-alumina xerogels. Although undesirable changes can occur at each step of the preparation procedure of silica-alumina, textural modifications deliberately generated in the silica hydrogels can be preserved. 4. CONCLUSION As thermoporometry is suitable for hydrogel as well as for xerogel materials, it has been possible to study the effects of the operating parameters of the preparation process on the texture of silica and silica-alumina and to follow the changes carried out by each step of the process. Large textural modifications occur during the precipitation of alumina in silica hydrogels and during the drying step. It has been demonstrated that the texture modifications generated at the beginning of the process (silica hydrogel formation) still exist in the dried end-product (silica-alumina xerogel). The knowledge of the influence of each preparation step allows a better mastering of the texture of the end-product. The silica hydrogel network constitutes the framework of the silica-alumina gel. From a textural point of wiev, a silica-alumina xerogel seems to be an "image" of the starting silica hydrogel. Depending on the alumina content the silica-alumina gels exhibit predominantly a silica-like texture or an alumina-like texture. REFERENCES 1. R.K. Iler in "The Chemistry of Silica" (John Wiley and sons, New-York 1979). 2. C.J. Planck and C.L. Drake, J. Colloid. Sci. 2 (1947), 399. 3. S.A. Mitchell, Chem. Ind. (1966), 924. 4. A.G. Forster and J.M. Thorp in "The Structure and Properties of Porous Materials" (D.H. Everett and F.S. Stone Eds., Butterworths, London 1958), 227. 5. J. Dumas, J.F. Quinson and J. Serughetti, J. Non Crystal. Solids, 125 (1990), 244. 6. C.J. Brinker and G.W. Scherer, Sol Gel Science (Academic Press, San Diego 1990), p.454. 7. G.W. Scherer, J. Amer.Ceram. Soc. In Sol-Gel Science (Academic Press Inc; Boston 1990), pp 453-513. 8. I. Bia7, Thesis, Paris VI l Jniversitv, 1999.