I N F L U E N C E O F S I L I C I C ACID H Y D R O G E L D E H Y D R A T I O N

C O N D I T I O N S ON F O R M A T I O N O F S I L I C A G E L S T R U C T U R E ,

W I T H M O L E C U L A R SIEVE P R O P E R T I E S

E. I. Kosenko and V. L. Struzhko UDC 546.33.284:541.182.644

In a study of the influence of the temperature and rate of dehydration on the pore structure of silica gel, it has been shown that, in the drying stage, the primary condition for the formation of silica gels with molecular sieve properties is a constant rate of dehydration, ensuring uniform shrinkage of the gel framework.

In the process of silica gel synthesis, the dehydration of the hydrogel is an important stage, since the parameters of the silica gel pore structure can be varied by adjusting the conditions of water removal. Contradictory data have been reported

on the influence of dehydration conditions on the formation of silica gel structure [1-4]. Also, no information whatever is available on the process of dehydration in the synthesis of ultramicroporous silica gels, which have pore diameters smaller than

0.7 nm [5] and which have molecular sieve properties. Although such silica gels can be used in many industrial processes, they have not been the subject of any significant amount of research. In the interest of controlled synthesis of these materials,

research is needed on the influence of dehydration conditions on the formation of a structure with molecular-size pores. The present work has been aimed at obtaining such information. We have investigated the influence of the temperature and the dehydration kinetics on the dehydration process, considering that these are the two most important factors determining the final

pore structure [1, 6]. The hydrogel used in this study was obtained by the interaction of solutions of sodium silicate and sulfuric acid (sol

pH 2), freed of sodium sulfate by washing with distilled water acidified with HCI (pH = 2). The hydrogel was dried under

static conditions at various temperatures in the 293-463 K interval; in other experiments, the hydrogel was dried at 293 or 303 K in a flow of air. Experimentally determined weight losses were used to plot kinetic curves (Fig. 1) and dehydration rate curves (Fig. 2). From the isotherms obtained for the adsorption of CH3OH and CCI 4 vapor (respective molecular diameters

0.44 and 0.69 nm [7]), the adsorption-structural characteristics of the xerogels were calculated (Table 1). In our experiments on hydrogel dehydration under normal static conditions over a broad range of temperatures (293-463

K), as indicated in Table 1, we found that silica gel with a molecular sieve structure was not formed at any of the dehydration temperatures. Regardless of the temperature, supermicroporous silica gels were obtained, with pores that were accessible for

CC14 molecules. This is evidenced not only by the significant sorption of CC14 vapor by the xerogels, but also by the small value of the characteristic energy of adsorption E0; further evidence is provided by a comparison of the volume of micropores

W 0 and the limiting sorption volumes of the pores V s as calculated from the data for CH3OH and CCI 4. Only by carrying out the dehydration of low temperatures in a flow of air was it possible to form silica gels that gave essentially no sorption of CCI 4,

i.e., silica gels with molecular sieve properties. Such structural differences can evidently be attributed to the influence of

kinetic features of the dehydration (Fig. 2). At all temperatures, the dehydration process can be divided into two periods on the basis of the rate of water removal. The first period is characterized by a constant rate of dehydration, the second period by a decreasing rate; these period s correspond to the removal of interglobular and bound water, respectively [6, 8]. Dehydration in an air flow is an exception to this generalization, since it proceeds at a constant rate -- the specific condition

required for the formation of ultramicroporous silica gels with molecular sieve properties.

L. V. Pisarzhevskii Institute of Physical Chemistry, Academy of Sciences of Ukraine, Kiev. Translated from Teoreticheskaya i t~ksperimental'naya Khimiya, Vol. 29, No. 4, pp. 356-360, July-August, 1993. Original article submitted

September 21, 1993.

0040-5760/93/2904-0237512.50 �9 Plenum Publishing Corporation 237

AG, wt. %

619 ?

20~~t~ . .__ 0 / 2 3 4 5 6'r

Fig. 1. Kinetic curves for removal of moisture from silica hydrogel at various temperatures:

1) 293 K, air flow; 2) 353; 3) 373; 4) 393; 5) 423; 6) 443; 7) 463 K. AG is the weight loss; r is the dehydration time.

AG/Ar wt. %/min

# 7

4

3 6

~ 4o ~ 4o z,h

Fig. 2. Curves for rate of hydrogel dehydration at following temperatures: 1) 293 K, air flow; 2) 353; 3) 373; 4) 393; 5) 423; 6) 443; 7) 463 K.

From the results of these experiments we can draw certain conclusions regarding the role of dehydration temperature

and kinetics in forming a silica gel structure with molecular sieve properties. According to the globular model of silica structure and the mechanisms of formation and rearrangement of its framework, temperature is an important factor determining

the formation of structure as the hydrogel is transformed into a xerogel, since the temperature influences the rate of silicic acid transcondensation processes that lead to the growth of globules and also influences the magnitude of capillary compressive

forces that cause shrinkage of the hydrogel skeleton. Higher temperatures should favor the formation of structures with higher sorption capacity. However, our experimental data have shown that in the dehydration of a hydrogel formed by minimum-size globules [5, 9], changes in the dehydration temperature have practically no effect on the pore structure of the xerogel: Within

the temperature interval 293-463 K, the supermicroporous silica gels that were obtained had identical parameters of pore structure (Ssp, V s, W 0, E0). This may indicate that hydrogel transcondensation processes proceed to the same degree regardless of the temperature (since as the temperature is increased, the duration of the hydrogel --, xerogel transition decreases). Another factor may be a predominant influence of the capillary radius on the magnitude of capillary forces, so that the hydrogel framework is drawn together under the influence of capillary forces to approximately the same degree, regardless of the

temperature. As demonstrated experimentally, the most important factor in forming the structure of silica gels with molecular sieve

properties is the dehydration kinetics. This obviously indicates that the formation of pore structure during dehydration may be influenced significantly by the presence of sections with different densities (different numbers of particle contacts), and also by differences in the temperature and moisture content of the surface and inner layers of the material, which can lead to differences in shrinkage at various points in the volume [6, 8]. These nonuniformities can probably be eliminated by main-

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TABLE 1. Influence of Dehydration Conditions on Pore Structure of Silica Gel

r ,K

Ssp, m2/g

CH3OH CCI 4

Vs, cm3/g WO, cm3/g EO, kJ/mole Vs, cm3/g

2 9 3 7 2 0 0 , 2 7 - - . - - 0,11

303 720 0,31 0,19 12,3 0,16

313 7OO 0,29 -- -- 0,15

333 750 0,32 0,21 12,8 0 ,18

3 5 3 7 4 0 0 , 3 2 - - - - 0 ,18

363 690 0,30 0,21 12,6 0,2.3

378 760 0,32 0,19 12'2 0,23

393 720 0,32 -- -- 0,19

423 700 0,29 0,18 13,0 0,19

443 730 0,31 -- -- 0,20

463 730 0,32 0,20 12,6 0,19

293* 670 0,26 0,19 14,5 0,01

303* 650 0,23 0,20 14,6 0,004

*In air flow.

taining a constant dehydration rate, presumably ensuring uniformity of compression of the gel framework as a consequence

of uniform withdrawal of moisture from the vaporization zone. This will tend to equalize the moisture content and the

concentration of particles, both in the near-surface zone and in the volume of the material, and will lead to the formation of

a structure with molecular sieve properties.

Thus, the kinetics of dehydration play an important role in forming the structure of microporous silica gels. The

dehydration of a silica hydrogel at a constant rate of water removal results in a uniform close packing of the polysilicic acid

globules and the formation of silica gels with a new structure - - ultramicroporous - - with molecular sieve properties.

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