- Email: [email protected]
Hydrothermal modification of silica gels (xerogels) 1. Effect of treatment temperature on their porous structure
ELSEVIER
.
Colloids and Surfaces A: Physicochemicaland Engineering Aspects 105 (1995) 181- 189
COLLOIDS AND SURFACES
A
Hydrothermal modification of silica gels (xerogels) Effect of treatment temperature on their porous structure R. Leboda a,,, E. Mendyk a, A. Gierak a, V.A. Tertykh b a Faculty of Chemistry, Maria Curie-Sklodowska University, 20031 Lublin, Poland b Institute of Surface Chemistry, National Academy of Science of Ukraine, 252022 Kiev, Ukraine
Received 15 October 1994; accepted 16 May 1995
Abstract
The effect of hydrothermal treatment temperature (in the range 150-300°C) on the structural parameters of silica gels was investigated. Silica gels with different starting porous structures were used for modification. The specific surface areas of the modified sorbents varied from 830 to 12 m 2 g-1. Simple mathematical equations describing the changes in specific surface area and pore diameter with treatment temperature are proposed. From the mathematical and/or graphical dependences it is possible to program the hydrothermal treatment process to obtain silica gels with desired porous structures. Keywords: Effect of temperature; Hydrothermal modification; Silica gels
1. Introduction
Silica gel is one of the most often utilized adsorbents for adsorption, catalytic, biochemical and chromatographic purposes [ 1 - 3 ] . The wide range of applications of silica gels requires the preparation of adsorbents of different porosity, i.e. microporous, macroporous and various intermediate structures. Such materials may be obtained by appropriate preparation or additional modification of the porous structure of silica gels. The former is often difficult to carry out in practice, since the final structure of a silica gel depends on many factors [3-63. Therefore, silica gels prepared on an industrial scale are very often heterogeneous and their structures, non-reproducible. Paper presented at the XXIII European Chemistry at Interfaces Conference held in Kiev, Ukraine, 11 16 September 1994. * Corresponding author. 0927-7757/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-7757(95)03273-8
Hydrothermal treatment is one of the most effective methods used in silica gel structure modification [ 3 - 8 ] . A review of data in the literature indicates the fundamental work being done to describe the mechanism of hydrothermal treatment of hydrogels. The papers on xerogel hydrothermal modification present results for various silica gels, often of unknown origin. Moreover, these investigations have been carried out in different, often non-comparable conditions, so the data presented so far do not allow the determination of adequate procedures for the preparation of sorbents with desired structural parameters. These parameters depend on many factors such as the amount of sorbent, the mode of modification (liquid or gas phase), medium pH, initial structure of the sorbent, the temperature and the modification time. The first four factors were reported earlier [ 9 - 1 2 ] . The changes in the porous structure parameters (specific surface area, pore and globule diameter,
182
R. Leboda et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 181-189
etc.) with temperature by means of simple empirical equations are presented in this paper. Special attention has been paid to practical aspects of the relations that could be used in planning experimental conditions to obtain sorbents with desired porous structures. Another factor, the effect of autoclaving time, is discussed in Part 2 [ 12b].
wide-pore materials was also examined by the mercury porosimetry method employing a Carlo-Erba type 1500 porosimeter. The transmission electron micrographs (TEM) were obtained using a Tesla BS 613 electron microscope (Czechoslovakia). The overall accuracy of the data obtained from the sorptomatic and porosimetric methods was found to be less than 5%, and the reproducibility 1 3% according to the sample characteristics.
2. Experimental
2.1. Materials
3. Results
Four types of commercial silica gels were used: (a) narrow-pore Si-40 and medium-pore Si-100, produced by Merck (Darmstadt, Germany); (b) medium-pore "Kieselgel 60" (MN) from Macherey Nagel Co. (Dfiren, Germany); and (c) mediumpore silica gel (SRT), produced by Schuchardt (M0nchen, Germany). These materials were dried at 200°C for 6 h before hydrothermal treatment. The abbreviations of the adsorbents given in parentheses are used in the text, and the numbers in Si-40 and Si-100 refer to mean pore diameters given by the manufacturer.
Table 1 presents the results of hydrothermal treatment of silica gels in the temperature range 110-300°C. As can be seen, the hydrothermal treatment of silica gels causes significant changes in the internal structure of the sorbents. The higher the temperature of autoclaving, the greater are the observed changes in the structural parameters. As a consequence, the high-temperature treatment leads to gradual transformation of silica gel to macroporous materials with extremely low specific surface areas. Fig. 1 shows a set of adsorption-desorption isotherms of nitrogen as well as the pore size distribution curves for a silica gel of the Si-40 series. The gradual shift in the hysteresis loop towards higher relative vapour pressures, UPs, as well as a drop in the nitrogen adsorption was caused by the enlargement of pores with increasing temperature. Such changes in the adsorption isotherm and hysteresis loop are typical of hydrothermally treated silica gels [ 13-15]. The significant diminution and broadening of the dV/d log R curves (Fig. 1) are due to the reduced pore size homogeneity in the newly formed structures. This means that a very wide range of pore dimensions is formed during hydrothermal treatment. Similar effects were observed earlier by Kiselev et al. [15]. As a consequence, a significant drop in the sorption capacity V~ can be observed (Fig. 1, Table 1), a phenomenon that may be explained by the formation of wide pores which cannot be determined by the sorptomatic method (dp>500,~). In such pores the capillary condensation of nitrogen does not occur, which results in an apparent drop of the total pore volume Vp. On the other hand, the
2.2. Modification of samples Hydrothermal modification was carried out according to the following procedure: 2 g samples of silica gel were placed in quartz vials and introduced into a stainless steel autoclave of 0.3 1 volume. 20 ml of water was added to the bottom of the autoclave. After closing, the autoclave was heated to 110, 150, 200, 250 and 300°C for 6h. These conditions ensured saturated water vapour pressure [9]. After modification, the samples were dried at 200°C for 6 h.
2.3. Sample testing The low-temperature (77 K) adsorption~desorption isotherms of nitrogen were measured using a Sorptomat 1800 apparatus (Carlo-Erba, Milano, Italy). The specific surface areas SBET of the sorbents by the BET method as well as pore volume and differential pore size distribution were calculated from these data. The structure of the
R. Leboda et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 181 189
183
Table 1 Structural characteristics of silica gels before and after hydrothermal treatment (treatment time 6 h) Adsorbent
Temperature
Surface area,
Pore volume,
Pore diameter,
(°C)
S (m 2 g 1)
Vp (cm 3 g - l )
dp (A)
N2,BET
Hg
Vs
VHg
ddom
110 150
830 647 208
-
0.63 0.62 0.54
-
40 54 104
200
108
-
0.42
-
226
250 300
49 20
38 26
0.34 -
0.58 0.56
488 -
110 150
350 264 177
95 95
0.95 0.93 0.79
0.51 0.58 0.69
108 140 232
200
74
45
0.42
0.75
-
760
513
250 300
27 12
44 16
-
-
0.89 0.79
1810 4166
1407 5267
103 154 369 1011 2275
73 112 286
62 104 244
842 2133
718 1820
82 101 195
68 84 162
400
333 758
Si-40
Si-100
MN
110 150
440 285 112
200 250
38 15
SRT
-
-
131 273
542 1054
490 1260
557 1365
184 184 292
108 141 215
-
60 106 264
-
48 23
0.41 0.12
0.76 0.74
-
780 2480
-
80 112 186
-
412
-
-
0.82 0.81 0.72
200
82
-
0.48
250
36
44
0.36
0.98
-
33 42
240
-
398 325 168
def. 30 38 117
0.80 0.80 0.62
110 150
V~, maximal adsorption capacity at
dHg
Globule diameter, D (A)
812
911
78
UPs= 1.
D=27300/S [-4,23]. ddom, the mean pore diameter at the maximum of the
dV/d log
porosimetric data (VHg, Table 1) indicate that for all hydrothermally modified materials the pore volume is practically constant, i.e. for wide-pore materials, Vng is close to V~ of the initial materials. Only at a temperature of 300°C can a slight decrease in the pore volume Vp be observed [16-18]. Fig. 2 shows the pore size distribution curves for the series Si-100 samples by the mercury porosimetry method. The trend of these curves confirms the changes discussed above and observed in the structure of hydrothermally modified materials. The gradual increase in Vng for this series of adsorbents (Table 1) is connected with the increasing number of sufficiently large pores for mercury penetration (unfortunately, the type of the porosimeter only permitted the measurement of pores
R curve.
larger than 100 A). This means that if, for an initial sample of Si-100 silica gel, about 50% of the total pore volume was not penetrated by mercury (for Vp = V~=0.95; Vng=0.51), then in the case of the material modified at 250°C this value is lower than 7%.
3.1. The mechanism ofhydrothermal treatment of silica gels Fig. 3 shows a selection of TEM photos of platinum-carbon replicas [19] of the samples of Si-100 silica gels. The initial silica gel is characterized by a highly uniform globular structure, with particle diameters of 80 A (Fig. 3a, Table 1). Significant changes in the structure of the modified
184
R. Leboda et aL/Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 181 189
2 3 E u
31
#
"t3
03
0
0.6 0.5 1
O.k 0.3 0.2 0.1 .. 0.2 O.k 0.6 reanve pressure-- P/Ps
08
1.0
Fig. 1. Adsorption-desorption isotherms of nitrogen and differential pore size distribution as a function of pore radius (inset) for initial silica gel Si-40 (1) and the same adsorbent hydrothermallymodifiedat 150 (2), 200 (3) and 250°C (4).
0-51
3
ol
°f 1.5
I
1
2
' s'o ~do
;',1[
2.5
it 3
's66"~'doo
3.5
4 ogl~
material are observed already at 150°C. At this stage, the process of hydrothermal treatment consists mainly in the mutual aggregation of neighbouring globules leading to the formation of spherical aggregates of larger dimensions (Fig. 3b). An increase in temperature to 200°C causes a further increase in the particle dimensions (Fig. 3c), as well as the pores between them (dark regions in Fig. 3). However, in this case the newly formed particles are so compact that the edges of primary globules are no longer visible. It should be pointed out that despite a fivefold increase in globule diameter, the silica gel still retains its globular structure. In the temperature range 150-200°C, owing to the spontaneous aggregation of primary globules into larger spheres, the decrease in the specific surface area (S) of the modified silica should be expected. This effect is confirmed in Fig. 4(b). The maximum change in S is localized near 150°C for all modified sorbents. At higher temperatures (250°C), deformation of the spheres due to the formation of "spindle-shaped" aggregates of different dimensions (Fig. 3d) is observed. Moreover, in the skeleton, apart from the regions formed by regular particles, regions of nonporous glassy mass become visible. These phenomena are undoubtedly the reason for the heterogeneity of the modified silica pore structure. Due to such changes, the globular structure of silica gels transforms into a spongy one that is typical of porous glasses [20,21]. The disappearance of the globular structure is also typical of the samples modified at 300°C (Fig. 3e). In this case, however, extremely large increases in the skeleton size as well as the pore diameter are found. It should be emphasized that the pore diameters evaluated from the photos show good agreement with the experimental data listed in Table 1. Comparing the experimental dp values with the calculated ones (assuming cylindrical pores), it can be stated that the effective pore diameter doff of modified silica gels can be satisfactorily expressed by means of the effective values calculated from
' 'sddd'"~,;~
Fig. 2. Differentialpore size distribution as a function of pore radius calculated from the mercury porosimetry data. 1, initial silica gel Si-100; 2, 3, 4 and 5, the same adsorbent hydrothermally modifiedat 150, 200, 250 and 300°C, respectively.
do~=4v/s
(1)
on the basis of measured Vp and S values [4]. The problem of evaluating the mean globule diameter
R. Leboda et al./Colloids Surfaces A." Physicochem. Eng. Aspects 105 (1995) 181-189
185
Fig. 3. Surface of initial silica gel Si-100 (a) examined by TEM, and the same adsorbent hydrothermally modified at temperatures of 150 (b), 200 (c), 250 (d) and 300°C (e).
D may be solved in a slightly different way (see Eq. (5)). Table 1 presents the hypothetical values of
globule diameter D for the modified silica gels calculated with the assumption that the modified materials retain their globular structure. Because
R. Leboda et aL/Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 181 189
186
b
o
b
800 ~
1(
700
~ 8
E ~- 50G
2
,,oo I .
1
(
100
g
200 300°C
o -Silo
30o
\
• si oo
""
n-MN
200 u
•~
lOC 0
0
,
L_
L
.L
50
100
150
200
250
300°C
Fig. 4. Dependence of the surface area of silica gels listed in Table 1 on hydrothermal treatment temperature (a). Inset (b) differential curves of surface area distribution as a function of mean hydrothermal treatment temperature.
of the deformation of the spheres into irregular aggregates it is very difficult to determine the dimensions of these globules from the photos. However, the structural elements that retain their spherical shape are of dimensions close to the D values listed in Table 1 (Fig. 3c-e). It should be emphasized that similar changes in the structure of hydrothermally modified silica gels were suggested earlier [ 15,22-24]. The changes in the structural parameters of the tested adsorbents presented in Table 1 as well as the microphotographs (Fig. 3) enable us to establish the general mechanism of hydrothermal treat: ment of silica gels. Each porous system tends towards a diminution of its surface energy by decreasing its specific surface area. In the case of silica gels, the surface decrease during the hydrothermal treatment occurs due to the increase in mean globule sizes. Silica solubility at a constant temperature is determined by the dispersion of particles, i.e. smaller globules forming the silica gel skeleton are of a higher solubility [3,25]. At moderate hydrothermal treatment temperatures (150-200°C) only the smallest particles undergo
total dissolution. Dissolved silica existing mainly in the form of Si(OH)4 and lower polysilicic acids deposit and condense on the surfaces of other particles, first of all at their points of contact, where the solubility of silica is the lowest because of the negative curvature radius E3]. As a result of these processes, the primary globules aggregate into spherical particles of various dimensions. So, a new geometrical structure of silica gel, formed by a spatial network of large globules, is produced. The increase in the dimensions of these globules, connected with the disappearance of small particles, causes a dramatic decrease in the specific surface area and an increase in the pore diameter in the modified material. Analogous processes, i.e. the dissolution of small and aggregation of larger globules, are also observed at higher temperatures (200-300°C). The increased solubility of silica observed at these temperatures also promotes the formation of local regions of nonporous glassy mass as well as the formation of a spongy structure with extraordinarily large pores. As a consequence, at temperatures above 200°C, adsorbents with specific surface areas of the order of a few m 2 g - ~ can be obtained. In the temperature range 250-300°C, apart from amorphous silica, very small quantities of crystalline quartz were also formed in the modified silica gel [15,22]. The data presented in this paper suggest that the proposed mechanism is universal and is independent of the type of the tested material, although the individual stages of this process can occur at different temperatures. The mechanism presented here also permits us to describe some additional details connected with the hydrothermal treatment of silica gels. An important feature of globular structure of porous solids is the fact that in such systems, the pore volume is a function of the globule packing density [4,26]. The constant pore volume Vp during the hydrothermal treatment of silica gels suggests that despite the globule diameter increase, their packing density in the skeleton of the modified materials does not change. Owing to the condensation of dissolved silica at the points of globule contact, the structure of silica gels hardens. Skeleton hardening makes the mutual dislocationof individual globules impossible owing to the limited bulk diffusion. Therefore, the pore
R. Leboda et al./Colloids Surfaces A." Physicochem. Eng. Aspects 105 (1995) 181-189
volume may remain practically unchanged despite the drastic reduction in specific surface area. From the above facts a very important conclusion can be drawn that the rebuilding of the internal structure of silica gels occurs mainly through a surface diffusion of dissolution products. The pore volume decrease at 300°C and above can be explained by the transition of the globular structure to a spongy one.
4. Discussion
4.1. Specific surface area From the practical viewpoint a very important question arises as to whether any simple relationships exist between the hydrothermal treatment temperature and changes in the structural parameters, i.e. S, dp and D. It should be emphasized here that this problem has not been addressed so far in the literature. Fig. 4(a) shows the conventional way of presenting the changes in the specific surface area S with temperature. Such a presentation is however, not very practical because of the complex character of the changes in S. The shapes of the curves in Fig. 4(a) suggest an exponential relationship between the parameters. Iler [3] summarized the results of silica solubility in water and showed the linear dependence of the solubility on the reciprocal of temperature (l/T) for various forms of silica (amorphous or crystalline). These results suggest that the rate of depolymerization or dissolution should be proportional to the specific surface area of silica and to the particle size. Goto [27] showed that the rate of dissolution is proportional to the surface area (i.e. inversely proportional to the value of D) but did not refer to particles smaller than 50 A because such particles show abnormally high solubility in water. Fournier and Rowe [28] stated that the solubility of amorphous silica in hydrothermal conditions (under saturated water vapour pressure) is given by the following logarithmic relationship: 731 log C=4.52 - - ~ - ,
(2)
where C is the concentration of SiO/(ppm) and T
187
is the hydrothermal treatment temperature. According to Eq. (2) the solubility of silica increases with temperature. For example, at 25°C the solubility is l l 7 p p m , and at 100°C it is 321 ppm. Iler [3] showed that similar relationships (2) exist for the other forms of silica (i.e. quartz, crystobalite). In the process of hydrothermal treatment of silica gel, there is an equilibrium between the dissolution and recondensation of silica. The parameters governing this equilibrium would change in a logarithmic manner with temperature as C in Eq. (2). From this viewpoint, the way of presenting changes in S and dp as a function of the reciprocal absolute temperature 1/T appears to be reasonable. Fig. 5 illustrates the dependence of log S on lIT. The proposed graphical presentation of the results shows that there exists a linear relationship between the specific surface area S and the hydrothermal treatment temperature. The experimental points lie relatively well on straight lines in the temperature range 110-300°C, the range most often used in practice. For comparison, the graphs of the relations obtained by Kiselev et al. [ 15 ] are also presented in Fig. 5. For temperatures below 100°C no significant changes in the pore structure are observed. Above this temperature a linear decrease in the specific surface area
1000
I
Si40
I
5OO
M N
!
E 7.£./3
110C
oJ
--~-~SRT
SilO0 Kiselev 1153
"
50 40
~ 3o .u 20
ITo=363K
mlO
I
i
I 2
i
I
46o 36o2~,o2bo l&o 16o
I
J
3
(0 2~
4K-1.103
6 °c '
Fig. 5. Surface area of silica gel as a function of the reciprocal temperature of hydrothermal treatment.
R_ Leboda et al./Colloids Surfaces A. Physicochem. Eng. Aspects 105 (1995) 181-189
188
is observed. Therefore appreciable changes in the structure of silica gels will only occur at temperatures above the boiling point of water. The lines parallel to the horizontal axis (Fig. 5) correspond to the specific surface areas of unmodified adsorbents. The intersections of these lines with the individual diagrams may be considered, in turn, as the points corresponding to the beginning of effective changes in the structure (T0=363 K). From Fig. 5, we can deduce the empirical linear equation
5000 400( 3001 200q I 001 ~ITo:363K
0~_ 501
~4oo I 300 -~ 200 ..6 o)
100
\ \,~___ _~nj_ -o .
where S~ is the specific surface area at a given treatment temperature, b is a constant describing the slope of the straight line, A is a constant corresponding to the specific surface area at the point of intersection with the asymptote, T is the hydrothermal treatment temperature, and To= 363 K. The constant A is approximately equal to the specific surface area So of the initial material. Therefore, Eq. (3) can be written in the form log ST=log So + b l
(,1)
-~--~o ) •
(4)
The globule diameter D (in A) and specific surface area S are connected by the simple mathematical relationship E26] 6
D-
x 104
dS'
(5)
where d is the real density of the silica gel. Therefore, analogous dependences can also be used to describe the changes in globule diameters during the hydrothermal treatment of silica gels.
4.2. Porous structure:pore diameter The second important parameter of the geometrical structure of silica gel is the pore diameter de . Fig. 6 presents the dependences of the pore diameter dp on the reciprocal temperature. Again, linear dependences are observed. The changes in dp with temperature may be described by the empirical linear equation
log dp=log do -b2 ( 1 - ~oo),
(6)
~ 50
.
.
.
-\~ I
l
I
M~ .
Zx
.
Sit~O i
2 t,O0 300250200 150
.
I
3
16o
_ i
/., K-1.103
sb is
6°c
Fig. 6. Pore diameter of silica gels modified with water vapour as a function of reciprocal temperature.
where do is the pore diameter of initial material, and b2 is the constant for a given sorbent, corresponding to the slope of the straight line.
4.3. Practicalaspects of the derivedequations The dependences presented in Figs. 5 and 6, as well as their mathematical descriptions (Eqs. (3)-(6)), may be of great practical importance. They permit to foresee the hydrothermal treatment temperatures for the transformation of narrowpore silica gels into wide-pore sorbents of controlled porosity. The only numerical values that need to be known are those of the constant b. These values are easily obtained from the slopes of the straight lines, especially from log(ST, dp)= f(I/T) coordinate systems (bl = 1434 m 2 g- x K or b2 = 1472 ,~ K; Table 2).
5. Conclusions
On the basis of the presented results, the following conclusions can be drawn: 1. The hydrothermal treatment of silica gels causes the rebuilding of their porous structure. 2. Owing to the modification with water vapour, such parameters as the pore diameter (dp), the
R. Leboda et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 181-189 Table 2 Values of k for empirical equations (4 and 6) (b=mean values of b for all investigated adsorbents) Adsorbent
bl (m2 g-1 K)
b2 (~, K)
Si-40 Si-100 MN SRT Kiselev[,15]
1530 1360 1500 1200 1480 1434
1350 1500 1800 1180 1530 1472
specific surface area (S) a n d globule diameter (D) are changed. The m a i n p a r a m e t e r that determines all other p a r a m e t e r s is the globule diameter (D). 3. D u r i n g h y d r o t h e r m a l t r e a t m e n t the skeleton of silica gels becomes more rigid, b u t does n o t u n d e r g o shrinking. As a consequence, the pore v o l u m e Vp of the modified materials does practically n o t change. 4. C h a n g e s in the structural parameters S, dp a n d D can a p p r o x i m a t e l y be presented by the empirical linear e q u a t i o n s log S = f ( 1 / T ) a n d log dp = / ( l / T ) . 5. At high t r e a t m e n t temperatures (above 250°C), a gradual t r a n s f o r m a t i o n of the globular structure of the silica gel to a spongy one takes place. 6. The high heterogeneity of the p o r o u s structure of h y d r o t h e r m a l l y modified silica gels in comp a r i s o n with the structure of initial materials is a characteristic feature of these adsorbents. 7. U s i n g the m e t h o d of h y d r o t h e r m a l treatment, wide-pore a d s o r b e n t s of highly differentiated specific surface areas S can be obtained.
Acknowledgement Financial support from KBN, Project No. 205119101, is gratefully acknowledged.
References [1] G.V. Lisichkin (Ed.), Modified Silicas In Adsorption, Catalysis and Chromatography, Khimiya, Moscow, 1975 (in Russian).
189
[-2] V.A. Tertykh and L.A. Belyakova, Chemical Reactions Involving Silica Surfaces, Naukova Dumka, Kiev, 1991 (in Russian). [-3] R.K. Iler, Chemistry of Silica, Wiley, New York, 1979. [-4] R. Leboda and E. Mendyk, Mater. Chem. Phys., 27 (1991) 189. [-5] C.J. Brinker and G.W. Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic Press, San Diego, 1990. [6] C. Okkerse, in B.G. Linsen (Ed.), Physical and Chemical Aspects of Adsorbents and Catalysts, Academic Press, London, 1970, p. 233. [7] V.M. Chertov, V.V. Cirina, V.M. Shamrikov and V.I. Malkiman, Izv. Akad. Nauk SSSR, Neorg. Mater., 27 (1991) 752. [8] R. Leboda, A. Gierak, R. Charmas and A. Lodyga, React. Cat. Lett., 50 (1993) 63. [9] E. Mendyk, R. Leboda and A. Gierak, Mater. Chem. Phys., 31 (1992) 355. [,10] R. Leboda, E. Mendyk and V.A. Tertykh, Mater. Chem. Phys., 42 (1996) 7. [-11] R. Leboda, E. Mendyk and V.A. Tertykh, Mater. Chem. Phys., in press [,12] (a) E. Mendyk, R. Leboda and A. Gierak, Mater. Chem. Phys., 20 (1988) 87. (b) R. Leboda, E. Mendyk, A. Gierak and V.A. Tertykh, Colloids Surfaces A: Physicochem. Eng. Aspects, 105 (1995) 191. [13] R. Leboda, E. Mendyk, V.V. Sidortchuk and V.A. Tertykh, Mater. Chem. Phys., 38 (1994) 146. [14] V.S. Komarov, Adsorbents and Their Properties, Ed. Nauka i Tiekhnika, Minsk, 1977 (in Russian). [15] A.V. Kiselev, V.M. Lukyanovich, Y.S. Nikitin, E.B. Oganesyan and A.I. Sarakhov, Kolloidn. Zh., 31 (1969) 388. [16] A.V. Kiselev,Y.S. Nikitin and E.B. Oganesyan, Kolloidn. Zh., 30 (1968) 842; 31 (1969) 525. [17] B. Bohlen, M. Buehlman and A. Guyer, Chimia (Aarau), 20 (1966) 32. [18] M.R. Buehlman, Promotionsarbeiten No. 3540 (1964) 112. [19] E. Tracz and R. Leboda, J. Chromatogr., 346 (1985) 346. [20] A.L. Dawidowicz and E. Mendyk, Mater. Chem. Phys., 21 (1989) 463. [21] E. Mendyk and A.L. Dawidowicz, Mater. Chem. Phys., 24 (1989) 13. [22] A.V. Kiselev,Y.S. Nikitin and E.B. Oganesyan, Kolloidn. Zh., 28 (1966) 662. [23] B.M. Miciuk, Teor. Eksp. Khim., 19 (1983) 600. [24] V.V. Strelko, Kolloidn. Zh., 32 (1970) 430. [25] I.E. Neimark, Synthetic Mineral Adsorbents and Supports of Catalysts, Naukova Dumka, Kiev, 1982 (in Russian). [26] A.P. Karnaukhov, Kinet. Kataliz, 12 (1971) 1025. [27] K. Goto, Bull. Chem. Soc. Jpn., 31 (1958) 900. [28] R.O. Fournier and J.J. Rowe, Am. Miner., 62 (1977) 1052.
.
Colloids and Surfaces A: Physicochemicaland Engineering Aspects 105 (1995) 181- 189
COLLOIDS AND SURFACES
A
Hydrothermal modification of silica gels (xerogels) Effect of treatment temperature on their porous structure R. Leboda a,,, E. Mendyk a, A. Gierak a, V.A. Tertykh b a Faculty of Chemistry, Maria Curie-Sklodowska University, 20031 Lublin, Poland b Institute of Surface Chemistry, National Academy of Science of Ukraine, 252022 Kiev, Ukraine
Received 15 October 1994; accepted 16 May 1995
Abstract
The effect of hydrothermal treatment temperature (in the range 150-300°C) on the structural parameters of silica gels was investigated. Silica gels with different starting porous structures were used for modification. The specific surface areas of the modified sorbents varied from 830 to 12 m 2 g-1. Simple mathematical equations describing the changes in specific surface area and pore diameter with treatment temperature are proposed. From the mathematical and/or graphical dependences it is possible to program the hydrothermal treatment process to obtain silica gels with desired porous structures. Keywords: Effect of temperature; Hydrothermal modification; Silica gels
1. Introduction
Silica gel is one of the most often utilized adsorbents for adsorption, catalytic, biochemical and chromatographic purposes [ 1 - 3 ] . The wide range of applications of silica gels requires the preparation of adsorbents of different porosity, i.e. microporous, macroporous and various intermediate structures. Such materials may be obtained by appropriate preparation or additional modification of the porous structure of silica gels. The former is often difficult to carry out in practice, since the final structure of a silica gel depends on many factors [3-63. Therefore, silica gels prepared on an industrial scale are very often heterogeneous and their structures, non-reproducible. Paper presented at the XXIII European Chemistry at Interfaces Conference held in Kiev, Ukraine, 11 16 September 1994. * Corresponding author. 0927-7757/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-7757(95)03273-8
Hydrothermal treatment is one of the most effective methods used in silica gel structure modification [ 3 - 8 ] . A review of data in the literature indicates the fundamental work being done to describe the mechanism of hydrothermal treatment of hydrogels. The papers on xerogel hydrothermal modification present results for various silica gels, often of unknown origin. Moreover, these investigations have been carried out in different, often non-comparable conditions, so the data presented so far do not allow the determination of adequate procedures for the preparation of sorbents with desired structural parameters. These parameters depend on many factors such as the amount of sorbent, the mode of modification (liquid or gas phase), medium pH, initial structure of the sorbent, the temperature and the modification time. The first four factors were reported earlier [ 9 - 1 2 ] . The changes in the porous structure parameters (specific surface area, pore and globule diameter,
182
R. Leboda et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 181-189
etc.) with temperature by means of simple empirical equations are presented in this paper. Special attention has been paid to practical aspects of the relations that could be used in planning experimental conditions to obtain sorbents with desired porous structures. Another factor, the effect of autoclaving time, is discussed in Part 2 [ 12b].
wide-pore materials was also examined by the mercury porosimetry method employing a Carlo-Erba type 1500 porosimeter. The transmission electron micrographs (TEM) were obtained using a Tesla BS 613 electron microscope (Czechoslovakia). The overall accuracy of the data obtained from the sorptomatic and porosimetric methods was found to be less than 5%, and the reproducibility 1 3% according to the sample characteristics.
2. Experimental
2.1. Materials
3. Results
Four types of commercial silica gels were used: (a) narrow-pore Si-40 and medium-pore Si-100, produced by Merck (Darmstadt, Germany); (b) medium-pore "Kieselgel 60" (MN) from Macherey Nagel Co. (Dfiren, Germany); and (c) mediumpore silica gel (SRT), produced by Schuchardt (M0nchen, Germany). These materials were dried at 200°C for 6 h before hydrothermal treatment. The abbreviations of the adsorbents given in parentheses are used in the text, and the numbers in Si-40 and Si-100 refer to mean pore diameters given by the manufacturer.
Table 1 presents the results of hydrothermal treatment of silica gels in the temperature range 110-300°C. As can be seen, the hydrothermal treatment of silica gels causes significant changes in the internal structure of the sorbents. The higher the temperature of autoclaving, the greater are the observed changes in the structural parameters. As a consequence, the high-temperature treatment leads to gradual transformation of silica gel to macroporous materials with extremely low specific surface areas. Fig. 1 shows a set of adsorption-desorption isotherms of nitrogen as well as the pore size distribution curves for a silica gel of the Si-40 series. The gradual shift in the hysteresis loop towards higher relative vapour pressures, UPs, as well as a drop in the nitrogen adsorption was caused by the enlargement of pores with increasing temperature. Such changes in the adsorption isotherm and hysteresis loop are typical of hydrothermally treated silica gels [ 13-15]. The significant diminution and broadening of the dV/d log R curves (Fig. 1) are due to the reduced pore size homogeneity in the newly formed structures. This means that a very wide range of pore dimensions is formed during hydrothermal treatment. Similar effects were observed earlier by Kiselev et al. [15]. As a consequence, a significant drop in the sorption capacity V~ can be observed (Fig. 1, Table 1), a phenomenon that may be explained by the formation of wide pores which cannot be determined by the sorptomatic method (dp>500,~). In such pores the capillary condensation of nitrogen does not occur, which results in an apparent drop of the total pore volume Vp. On the other hand, the
2.2. Modification of samples Hydrothermal modification was carried out according to the following procedure: 2 g samples of silica gel were placed in quartz vials and introduced into a stainless steel autoclave of 0.3 1 volume. 20 ml of water was added to the bottom of the autoclave. After closing, the autoclave was heated to 110, 150, 200, 250 and 300°C for 6h. These conditions ensured saturated water vapour pressure [9]. After modification, the samples were dried at 200°C for 6 h.
2.3. Sample testing The low-temperature (77 K) adsorption~desorption isotherms of nitrogen were measured using a Sorptomat 1800 apparatus (Carlo-Erba, Milano, Italy). The specific surface areas SBET of the sorbents by the BET method as well as pore volume and differential pore size distribution were calculated from these data. The structure of the
R. Leboda et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 181 189
183
Table 1 Structural characteristics of silica gels before and after hydrothermal treatment (treatment time 6 h) Adsorbent
Temperature
Surface area,
Pore volume,
Pore diameter,
(°C)
S (m 2 g 1)
Vp (cm 3 g - l )
dp (A)
N2,BET
Hg
Vs
VHg
ddom
110 150
830 647 208
-
0.63 0.62 0.54
-
40 54 104
200
108
-
0.42
-
226
250 300
49 20
38 26
0.34 -
0.58 0.56
488 -
110 150
350 264 177
95 95
0.95 0.93 0.79
0.51 0.58 0.69
108 140 232
200
74
45
0.42
0.75
-
760
513
250 300
27 12
44 16
-
-
0.89 0.79
1810 4166
1407 5267
103 154 369 1011 2275
73 112 286
62 104 244
842 2133
718 1820
82 101 195
68 84 162
400
333 758
Si-40
Si-100
MN
110 150
440 285 112
200 250
38 15
SRT
-
-
131 273
542 1054
490 1260
557 1365
184 184 292
108 141 215
-
60 106 264
-
48 23
0.41 0.12
0.76 0.74
-
780 2480
-
80 112 186
-
412
-
-
0.82 0.81 0.72
200
82
-
0.48
250
36
44
0.36
0.98
-
33 42
240
-
398 325 168
def. 30 38 117
0.80 0.80 0.62
110 150
V~, maximal adsorption capacity at
dHg
Globule diameter, D (A)
812
911
78
UPs= 1.
D=27300/S [-4,23]. ddom, the mean pore diameter at the maximum of the
dV/d log
porosimetric data (VHg, Table 1) indicate that for all hydrothermally modified materials the pore volume is practically constant, i.e. for wide-pore materials, Vng is close to V~ of the initial materials. Only at a temperature of 300°C can a slight decrease in the pore volume Vp be observed [16-18]. Fig. 2 shows the pore size distribution curves for the series Si-100 samples by the mercury porosimetry method. The trend of these curves confirms the changes discussed above and observed in the structure of hydrothermally modified materials. The gradual increase in Vng for this series of adsorbents (Table 1) is connected with the increasing number of sufficiently large pores for mercury penetration (unfortunately, the type of the porosimeter only permitted the measurement of pores
R curve.
larger than 100 A). This means that if, for an initial sample of Si-100 silica gel, about 50% of the total pore volume was not penetrated by mercury (for Vp = V~=0.95; Vng=0.51), then in the case of the material modified at 250°C this value is lower than 7%.
3.1. The mechanism ofhydrothermal treatment of silica gels Fig. 3 shows a selection of TEM photos of platinum-carbon replicas [19] of the samples of Si-100 silica gels. The initial silica gel is characterized by a highly uniform globular structure, with particle diameters of 80 A (Fig. 3a, Table 1). Significant changes in the structure of the modified
184
R. Leboda et aL/Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 181 189
2 3 E u
31
#
"t3
03
0
0.6 0.5 1
O.k 0.3 0.2 0.1 .. 0.2 O.k 0.6 reanve pressure-- P/Ps
08
1.0
Fig. 1. Adsorption-desorption isotherms of nitrogen and differential pore size distribution as a function of pore radius (inset) for initial silica gel Si-40 (1) and the same adsorbent hydrothermallymodifiedat 150 (2), 200 (3) and 250°C (4).
0-51
3
ol
°f 1.5
I
1
2
' s'o ~do
;',1[
2.5
it 3
's66"~'doo
3.5
4 ogl~
material are observed already at 150°C. At this stage, the process of hydrothermal treatment consists mainly in the mutual aggregation of neighbouring globules leading to the formation of spherical aggregates of larger dimensions (Fig. 3b). An increase in temperature to 200°C causes a further increase in the particle dimensions (Fig. 3c), as well as the pores between them (dark regions in Fig. 3). However, in this case the newly formed particles are so compact that the edges of primary globules are no longer visible. It should be pointed out that despite a fivefold increase in globule diameter, the silica gel still retains its globular structure. In the temperature range 150-200°C, owing to the spontaneous aggregation of primary globules into larger spheres, the decrease in the specific surface area (S) of the modified silica should be expected. This effect is confirmed in Fig. 4(b). The maximum change in S is localized near 150°C for all modified sorbents. At higher temperatures (250°C), deformation of the spheres due to the formation of "spindle-shaped" aggregates of different dimensions (Fig. 3d) is observed. Moreover, in the skeleton, apart from the regions formed by regular particles, regions of nonporous glassy mass become visible. These phenomena are undoubtedly the reason for the heterogeneity of the modified silica pore structure. Due to such changes, the globular structure of silica gels transforms into a spongy one that is typical of porous glasses [20,21]. The disappearance of the globular structure is also typical of the samples modified at 300°C (Fig. 3e). In this case, however, extremely large increases in the skeleton size as well as the pore diameter are found. It should be emphasized that the pore diameters evaluated from the photos show good agreement with the experimental data listed in Table 1. Comparing the experimental dp values with the calculated ones (assuming cylindrical pores), it can be stated that the effective pore diameter doff of modified silica gels can be satisfactorily expressed by means of the effective values calculated from
' 'sddd'"~,;~
Fig. 2. Differentialpore size distribution as a function of pore radius calculated from the mercury porosimetry data. 1, initial silica gel Si-100; 2, 3, 4 and 5, the same adsorbent hydrothermally modifiedat 150, 200, 250 and 300°C, respectively.
do~=4v/s
(1)
on the basis of measured Vp and S values [4]. The problem of evaluating the mean globule diameter
R. Leboda et al./Colloids Surfaces A." Physicochem. Eng. Aspects 105 (1995) 181-189
185
Fig. 3. Surface of initial silica gel Si-100 (a) examined by TEM, and the same adsorbent hydrothermally modified at temperatures of 150 (b), 200 (c), 250 (d) and 300°C (e).
D may be solved in a slightly different way (see Eq. (5)). Table 1 presents the hypothetical values of
globule diameter D for the modified silica gels calculated with the assumption that the modified materials retain their globular structure. Because
R. Leboda et aL/Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 181 189
186
b
o
b
800 ~
1(
700
~ 8
E ~- 50G
2
,,oo I .
1
(
100
g
200 300°C
o -Silo
30o
\
• si oo
""
n-MN
200 u
•~
lOC 0
0
,
L_
L
.L
50
100
150
200
250
300°C
Fig. 4. Dependence of the surface area of silica gels listed in Table 1 on hydrothermal treatment temperature (a). Inset (b) differential curves of surface area distribution as a function of mean hydrothermal treatment temperature.
of the deformation of the spheres into irregular aggregates it is very difficult to determine the dimensions of these globules from the photos. However, the structural elements that retain their spherical shape are of dimensions close to the D values listed in Table 1 (Fig. 3c-e). It should be emphasized that similar changes in the structure of hydrothermally modified silica gels were suggested earlier [ 15,22-24]. The changes in the structural parameters of the tested adsorbents presented in Table 1 as well as the microphotographs (Fig. 3) enable us to establish the general mechanism of hydrothermal treat: ment of silica gels. Each porous system tends towards a diminution of its surface energy by decreasing its specific surface area. In the case of silica gels, the surface decrease during the hydrothermal treatment occurs due to the increase in mean globule sizes. Silica solubility at a constant temperature is determined by the dispersion of particles, i.e. smaller globules forming the silica gel skeleton are of a higher solubility [3,25]. At moderate hydrothermal treatment temperatures (150-200°C) only the smallest particles undergo
total dissolution. Dissolved silica existing mainly in the form of Si(OH)4 and lower polysilicic acids deposit and condense on the surfaces of other particles, first of all at their points of contact, where the solubility of silica is the lowest because of the negative curvature radius E3]. As a result of these processes, the primary globules aggregate into spherical particles of various dimensions. So, a new geometrical structure of silica gel, formed by a spatial network of large globules, is produced. The increase in the dimensions of these globules, connected with the disappearance of small particles, causes a dramatic decrease in the specific surface area and an increase in the pore diameter in the modified material. Analogous processes, i.e. the dissolution of small and aggregation of larger globules, are also observed at higher temperatures (200-300°C). The increased solubility of silica observed at these temperatures also promotes the formation of local regions of nonporous glassy mass as well as the formation of a spongy structure with extraordinarily large pores. As a consequence, at temperatures above 200°C, adsorbents with specific surface areas of the order of a few m 2 g - ~ can be obtained. In the temperature range 250-300°C, apart from amorphous silica, very small quantities of crystalline quartz were also formed in the modified silica gel [15,22]. The data presented in this paper suggest that the proposed mechanism is universal and is independent of the type of the tested material, although the individual stages of this process can occur at different temperatures. The mechanism presented here also permits us to describe some additional details connected with the hydrothermal treatment of silica gels. An important feature of globular structure of porous solids is the fact that in such systems, the pore volume is a function of the globule packing density [4,26]. The constant pore volume Vp during the hydrothermal treatment of silica gels suggests that despite the globule diameter increase, their packing density in the skeleton of the modified materials does not change. Owing to the condensation of dissolved silica at the points of globule contact, the structure of silica gels hardens. Skeleton hardening makes the mutual dislocationof individual globules impossible owing to the limited bulk diffusion. Therefore, the pore
R. Leboda et al./Colloids Surfaces A." Physicochem. Eng. Aspects 105 (1995) 181-189
volume may remain practically unchanged despite the drastic reduction in specific surface area. From the above facts a very important conclusion can be drawn that the rebuilding of the internal structure of silica gels occurs mainly through a surface diffusion of dissolution products. The pore volume decrease at 300°C and above can be explained by the transition of the globular structure to a spongy one.
4. Discussion
4.1. Specific surface area From the practical viewpoint a very important question arises as to whether any simple relationships exist between the hydrothermal treatment temperature and changes in the structural parameters, i.e. S, dp and D. It should be emphasized here that this problem has not been addressed so far in the literature. Fig. 4(a) shows the conventional way of presenting the changes in the specific surface area S with temperature. Such a presentation is however, not very practical because of the complex character of the changes in S. The shapes of the curves in Fig. 4(a) suggest an exponential relationship between the parameters. Iler [3] summarized the results of silica solubility in water and showed the linear dependence of the solubility on the reciprocal of temperature (l/T) for various forms of silica (amorphous or crystalline). These results suggest that the rate of depolymerization or dissolution should be proportional to the specific surface area of silica and to the particle size. Goto [27] showed that the rate of dissolution is proportional to the surface area (i.e. inversely proportional to the value of D) but did not refer to particles smaller than 50 A because such particles show abnormally high solubility in water. Fournier and Rowe [28] stated that the solubility of amorphous silica in hydrothermal conditions (under saturated water vapour pressure) is given by the following logarithmic relationship: 731 log C=4.52 - - ~ - ,
(2)
where C is the concentration of SiO/(ppm) and T
187
is the hydrothermal treatment temperature. According to Eq. (2) the solubility of silica increases with temperature. For example, at 25°C the solubility is l l 7 p p m , and at 100°C it is 321 ppm. Iler [3] showed that similar relationships (2) exist for the other forms of silica (i.e. quartz, crystobalite). In the process of hydrothermal treatment of silica gel, there is an equilibrium between the dissolution and recondensation of silica. The parameters governing this equilibrium would change in a logarithmic manner with temperature as C in Eq. (2). From this viewpoint, the way of presenting changes in S and dp as a function of the reciprocal absolute temperature 1/T appears to be reasonable. Fig. 5 illustrates the dependence of log S on lIT. The proposed graphical presentation of the results shows that there exists a linear relationship between the specific surface area S and the hydrothermal treatment temperature. The experimental points lie relatively well on straight lines in the temperature range 110-300°C, the range most often used in practice. For comparison, the graphs of the relations obtained by Kiselev et al. [ 15 ] are also presented in Fig. 5. For temperatures below 100°C no significant changes in the pore structure are observed. Above this temperature a linear decrease in the specific surface area
1000
I
Si40
I
5OO
M N
!
E 7.£./3
110C
oJ
--~-~SRT
SilO0 Kiselev 1153
"
50 40
~ 3o .u 20
ITo=363K
mlO
I
i
I 2
i
I
46o 36o2~,o2bo l&o 16o
I
J
3
(0 2~
4K-1.103
6 °c '
Fig. 5. Surface area of silica gel as a function of the reciprocal temperature of hydrothermal treatment.
R_ Leboda et al./Colloids Surfaces A. Physicochem. Eng. Aspects 105 (1995) 181-189
188
is observed. Therefore appreciable changes in the structure of silica gels will only occur at temperatures above the boiling point of water. The lines parallel to the horizontal axis (Fig. 5) correspond to the specific surface areas of unmodified adsorbents. The intersections of these lines with the individual diagrams may be considered, in turn, as the points corresponding to the beginning of effective changes in the structure (T0=363 K). From Fig. 5, we can deduce the empirical linear equation
5000 400( 3001 200q I 001 ~ITo:363K
0~_ 501
~4oo I 300 -~ 200 ..6 o)
100
\ \,~___ _~nj_ -o .
where S~ is the specific surface area at a given treatment temperature, b is a constant describing the slope of the straight line, A is a constant corresponding to the specific surface area at the point of intersection with the asymptote, T is the hydrothermal treatment temperature, and To= 363 K. The constant A is approximately equal to the specific surface area So of the initial material. Therefore, Eq. (3) can be written in the form log ST=log So + b l
(,1)
-~--~o ) •
(4)
The globule diameter D (in A) and specific surface area S are connected by the simple mathematical relationship E26] 6
D-
x 104
dS'
(5)
where d is the real density of the silica gel. Therefore, analogous dependences can also be used to describe the changes in globule diameters during the hydrothermal treatment of silica gels.
4.2. Porous structure:pore diameter The second important parameter of the geometrical structure of silica gel is the pore diameter de . Fig. 6 presents the dependences of the pore diameter dp on the reciprocal temperature. Again, linear dependences are observed. The changes in dp with temperature may be described by the empirical linear equation
log dp=log do -b2 ( 1 - ~oo),
(6)
~ 50
.
.
.
-\~ I
l
I
M~ .
Zx
.
Sit~O i
2 t,O0 300250200 150
.
I
3
16o
_ i
/., K-1.103
sb is
6°c
Fig. 6. Pore diameter of silica gels modified with water vapour as a function of reciprocal temperature.
where do is the pore diameter of initial material, and b2 is the constant for a given sorbent, corresponding to the slope of the straight line.
4.3. Practicalaspects of the derivedequations The dependences presented in Figs. 5 and 6, as well as their mathematical descriptions (Eqs. (3)-(6)), may be of great practical importance. They permit to foresee the hydrothermal treatment temperatures for the transformation of narrowpore silica gels into wide-pore sorbents of controlled porosity. The only numerical values that need to be known are those of the constant b. These values are easily obtained from the slopes of the straight lines, especially from log(ST, dp)= f(I/T) coordinate systems (bl = 1434 m 2 g- x K or b2 = 1472 ,~ K; Table 2).
5. Conclusions
On the basis of the presented results, the following conclusions can be drawn: 1. The hydrothermal treatment of silica gels causes the rebuilding of their porous structure. 2. Owing to the modification with water vapour, such parameters as the pore diameter (dp), the
R. Leboda et al./Colloids Surfaces A: Physicochem. Eng. Aspects 105 (1995) 181-189 Table 2 Values of k for empirical equations (4 and 6) (b=mean values of b for all investigated adsorbents) Adsorbent
bl (m2 g-1 K)
b2 (~, K)
Si-40 Si-100 MN SRT Kiselev[,15]
1530 1360 1500 1200 1480 1434
1350 1500 1800 1180 1530 1472
specific surface area (S) a n d globule diameter (D) are changed. The m a i n p a r a m e t e r that determines all other p a r a m e t e r s is the globule diameter (D). 3. D u r i n g h y d r o t h e r m a l t r e a t m e n t the skeleton of silica gels becomes more rigid, b u t does n o t u n d e r g o shrinking. As a consequence, the pore v o l u m e Vp of the modified materials does practically n o t change. 4. C h a n g e s in the structural parameters S, dp a n d D can a p p r o x i m a t e l y be presented by the empirical linear e q u a t i o n s log S = f ( 1 / T ) a n d log dp = / ( l / T ) . 5. At high t r e a t m e n t temperatures (above 250°C), a gradual t r a n s f o r m a t i o n of the globular structure of the silica gel to a spongy one takes place. 6. The high heterogeneity of the p o r o u s structure of h y d r o t h e r m a l l y modified silica gels in comp a r i s o n with the structure of initial materials is a characteristic feature of these adsorbents. 7. U s i n g the m e t h o d of h y d r o t h e r m a l treatment, wide-pore a d s o r b e n t s of highly differentiated specific surface areas S can be obtained.
Acknowledgement Financial support from KBN, Project No. 205119101, is gratefully acknowledged.
References [1] G.V. Lisichkin (Ed.), Modified Silicas In Adsorption, Catalysis and Chromatography, Khimiya, Moscow, 1975 (in Russian).
189
[-2] V.A. Tertykh and L.A. Belyakova, Chemical Reactions Involving Silica Surfaces, Naukova Dumka, Kiev, 1991 (in Russian). [-3] R.K. Iler, Chemistry of Silica, Wiley, New York, 1979. [-4] R. Leboda and E. Mendyk, Mater. Chem. Phys., 27 (1991) 189. [-5] C.J. Brinker and G.W. Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic Press, San Diego, 1990. [6] C. Okkerse, in B.G. Linsen (Ed.), Physical and Chemical Aspects of Adsorbents and Catalysts, Academic Press, London, 1970, p. 233. [7] V.M. Chertov, V.V. Cirina, V.M. Shamrikov and V.I. Malkiman, Izv. Akad. Nauk SSSR, Neorg. Mater., 27 (1991) 752. [8] R. Leboda, A. Gierak, R. Charmas and A. Lodyga, React. Cat. Lett., 50 (1993) 63. [9] E. Mendyk, R. Leboda and A. Gierak, Mater. Chem. Phys., 31 (1992) 355. [,10] R. Leboda, E. Mendyk and V.A. Tertykh, Mater. Chem. Phys., 42 (1996) 7. [-11] R. Leboda, E. Mendyk and V.A. Tertykh, Mater. Chem. Phys., in press [,12] (a) E. Mendyk, R. Leboda and A. Gierak, Mater. Chem. Phys., 20 (1988) 87. (b) R. Leboda, E. Mendyk, A. Gierak and V.A. Tertykh, Colloids Surfaces A: Physicochem. Eng. Aspects, 105 (1995) 191. [13] R. Leboda, E. Mendyk, V.V. Sidortchuk and V.A. Tertykh, Mater. Chem. Phys., 38 (1994) 146. [14] V.S. Komarov, Adsorbents and Their Properties, Ed. Nauka i Tiekhnika, Minsk, 1977 (in Russian). [15] A.V. Kiselev, V.M. Lukyanovich, Y.S. Nikitin, E.B. Oganesyan and A.I. Sarakhov, Kolloidn. Zh., 31 (1969) 388. [16] A.V. Kiselev,Y.S. Nikitin and E.B. Oganesyan, Kolloidn. Zh., 30 (1968) 842; 31 (1969) 525. [17] B. Bohlen, M. Buehlman and A. Guyer, Chimia (Aarau), 20 (1966) 32. [18] M.R. Buehlman, Promotionsarbeiten No. 3540 (1964) 112. [19] E. Tracz and R. Leboda, J. Chromatogr., 346 (1985) 346. [20] A.L. Dawidowicz and E. Mendyk, Mater. Chem. Phys., 21 (1989) 463. [21] E. Mendyk and A.L. Dawidowicz, Mater. Chem. Phys., 24 (1989) 13. [22] A.V. Kiselev,Y.S. Nikitin and E.B. Oganesyan, Kolloidn. Zh., 28 (1966) 662. [23] B.M. Miciuk, Teor. Eksp. Khim., 19 (1983) 600. [24] V.V. Strelko, Kolloidn. Zh., 32 (1970) 430. [25] I.E. Neimark, Synthetic Mineral Adsorbents and Supports of Catalysts, Naukova Dumka, Kiev, 1982 (in Russian). [26] A.P. Karnaukhov, Kinet. Kataliz, 12 (1971) 1025. [27] K. Goto, Bull. Chem. Soc. Jpn., 31 (1958) 900. [28] R.O. Fournier and J.J. Rowe, Am. Miner., 62 (1977) 1052.