Porous texture characteristics of a homologous series of base-catalyzed silica aerogels

]OURblAL OF

ELSEVIER

Journal of Non-Crystalline Solids 190 (1995) 198-205

Porous texture characteristics of a homologous series of base-catalyzed silica aerogels Andrzej B. Jarz~bski a,*, Jarostaw Lorenc a, Yuri I. Aristov b, Natalia Lisitza b a Polish Academy of Sciences, Institute of Chemical Engineering, 44-100 Gliwice, Poland b Russian Academy of Sciences, Institute of Catalysis, Novosibirsk 630090, Russian Federation

Received 9 December 1994; revised manuscript received 16 March 1995

Abstract

Eighteen samples of base-catalyzed silica aerogels obtained using various tetraethylorthosilicate (TEOS) and water contents in the reaction system were characterized using N 2 adsorption. From the observed pore size distribution and adsorption isotherms the volumetric and surface fractal dimensions were determined and also the range of power-law behaviour of the porous network. The porous texture of both the bulk aerogel and surface of clusters appeared to be dependent on the concentration of reactants. The water content, however, emerges as a factor controlling surface fractal structure, its principal effect being exerted on the pore volume in the mesopore size range.

1. Introduction

Numerous examinations of textural properties of silica aerogels, performed using small-angle scattering methods and electron microscopy have already established their main properties, i.e., polymeric or colloidal structure, quite often exhibiting a fractal character over a limited length scale [1-3]. These structures appeared to be affected by processing conditions such as the type of catalyst, water/silicon ratio, W, polymerization protocol (single-stage or two-stage). Polymeric aerogels proved to be produced under acid catalysis, two-stage polymerization and small W, while colloidal structures were obtained from base catalysis when W was about 4 [4]. The size of the fractal domain was found to be

* Corresponding author. Tel: +48-32 310 811. Telefax: +48-32 310 318. E-mail: [email protected].

dependent on the pH of the alcogel synthesis reaction as well as on the apparent density of samples; lighter aerogels exhibit a fractal structure over a larger domain which for neutrally prepared or acidcatalyzed silicates can cover approximately two decades in length scales [5,6]. However, despite the growing interest in basecatalyzed aerogels from chemical engineers, and the apparent importance of micropore, and especially mesopore structure for adsorption and catalysis, the porous texture of aerogels has as yet proved rather elusive. It began to attract attention only recently [7,8], apart from the most general information on pore size distribution (PSD) reported earlier [9]. The aim of this paper is to shed light on the effect of reactant concentrations in the alcogel synthesis system on the porous texture of silica aerogels obtained from the single-step method with basic catalyst in the presence of fluorine anions.

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A.B. Jarzqbsla"et al. /Journal of Non-Crystalline Solids 190 (1995) 198-205

2. Experimental The alcogel samples were synthesized b y hydrolysis and condensation reaction of tetraethylorthosilicate ( T E O S ) - e t h a n o l mixture with the addition of ammonium hydroxide (15 × 10 -3 m o l / l ) and ammonium fluoride (1.6 × 10 -3 t o o l / l ) as catalyst system. Fluorine anions are known for their ability to catalyze hydrolysis and condensation reactions, the former in a basic process proceeding rather slowly, and thus contribute to a remarkable acceleration of the gelation process. Eighteen alcogel samples, in three sets, were prepared as given in the general code W B S in Table 1. W is the w a t e r / T E O S molar ratio, B stands for base catalyzed process and S is the volumetric concentration of TEOS in the E t O H - T E O S system. All the alcogels were aged for 6 days at 30°C prior to supercritical drying at 25 MPa and 300°C. Conventional analysis of the pore network structure most often focused only on the pore size distribution (PSD). The fractal geometry approach is recognized to be helpful to understand and quantify disordered material structures, both of solid phase

199

and of voids. Nitrogen adsorption at 77 K is usually applied by chemists to obtain information regarding porous texture in the mesopore size range ( 2 - 5 0 nm) suitable for calculation of the specific surface, by means of the Brunauer, Emmett and Teller (BET) method, and the PSD and hence also the pore volume in the mesopore range, e.g. from the Barrett, Joyner and Halenda (BJH) algorithm. These characteristics can be used directly to determine the surface fractal dimension, D~, and the volumetric fractal dimension of the pore network structure, D v. D s can be calculated by fitting the experimental adsorption isotherm to the fractal F r e n k e l - H a l s e y Hill (FHH) isotherm [10]:

N / N m cx [ 1 / l n ( P o / P ) ]

l/m,

(1)

where

m=3/(3-Ds),

m>3

m--1/(3-Ds),

m<3.

N / N m is the number of layers adsorbed on the surface at equilibrium pressure, P , and Po is the saturated vapour pressure of the adsorbate.

Table 1 Porous texture characteristics of the silica aerogels Aerogel

SBET (rn2 g- 1)

VpNz(cm3 g- 1)

Pa (g cm- 3)

Dv t

dI (nm)

d u (nm)

Dv

Ds

2B05 2B10 2B15 2B20 2B25 2B30 4B05 4B10 4B15 4B20 4B25 4B30 6B05 6B10 6B15 6B20 6B25 6B30

783 862 743 721 684 624 492 483 445 378 307 276 442 463 382 135 310 254

3.18 2.99 3.52 3.91 3.85 3.32 1.61 2.38 1.70 0.99 0.64 0.54 1.14 1.64 0.94 0.32 0.77 0.43

0.039 0.038 0.079 0.088 0.085 0.101 0.034 0.041 0.054 0.062 0.072 0.063 0.035 0.042 0.051 0.064 0.061 0.084

1.70 1.87 1.76 1.48 1.40 1.35 1.94 1.92 2.05 2.10 2.08 2.01 2.08 2.03 2.17 2.09 2.19 2.17

3.5 4.5 4.5 3.5 3.5 4.5 6.5 6.5 4.5 5.5 4.5 5.5 4.5 7.5 3.5 5.5 4.5 2.5

42.5 42.5 42.5 37.5 37.5 37.5 27.5 27.5 27.5 27.5 42.5 57.5 27.5 32.5 37.5 47.5 42.5 52.5

1.87 1.72 1.88 2.18 2.33 2.39 2.12 1.80 2.20 2.3l 2.34 2.47

2.57 2.59 2.59 2.60 2.61 2.62 a a

aD=2.

2.02 2.07 2.09 2.01 2.01 2.05 2.12 2A7 2.24

200

A.B. Jarz~bski et al. / Journal of Non-Crystalline Solids 190 (1995) 198-205

0.2 dV/dD

/

[cm 3/g nm]

i i~

\ \

I/

/

0.1

i l\

\

! i

;~ 0

Pore diameter I

t

I

L

10

20

30

40

[nm] 50

Fig. 1. Pore size distributions for aerogels: 2B05 ( X ), 2B15 (O), 2B20 ( • ), 2B30 ( • ).

D v can be estimated from the PSD plot using the equation [11] dV

--

(2)

ct R 2 - ° ~

dR

or its equivalent form (3)

V ot R 3 - ° v ,

where V ( R ) is the total volume of pores of size larger than R.

0.1 dV/dD

[em3/g nm]

0.05 /

--~-~----~0

Pore diameter [ ~ 1

I

I

t

i

10

20

30

40

Fig. 2. Pore size distribution for aerogels: 4B05 ( × ), 4B15 (O), 4B20 ( • ), 4B30 ( • ) .

50

201

A.B. Jarzqbski et al. /Journal of Non-Crystalline Solids 190 (1995) 198-205

In this work nitrogen adsorption isotherms were obtained using an appropriate instrument (®Micromeritics ASAP 2000) and the PSD were computed using the BJH method (desorption data) assuming cylindrical shape of pores. The apparent density of aerogels, Pa, was measured by mercury porosimetry (®Micromeritics AutoPore II 9220).

3. Results Examination of the PSDs obtained (see Figs. 1-3) showed that all aerogels had pores in the micro- and mesopore size range (in addition to macropores which generally are not detected by the nitrogen adsorption method), and that reactant concentrations had a very pronounced effect on the cumulative pore volume and the PSDs. All the PSDs of aerogels synthesized at W = 2 (i.e., 2B05-2B30) appeared to have a pore volume in the range detected 1-80 nm, and especially an abundance (peak) of pores in the range of diameters 3 - 4 0 nm, accounting for approximately 90% of total detected pore volume, Vp,2, as may be seen from Fig. 1. This maximum in mesopore diameters, as well as the cumulative pore volume, dimin-

ished gradually with increase in TEOS content and much more rapidly with increase in W. At W = 4 this maximum of mesopore diameters was still observed in the range 6-35 nm but only on the PSD of the less dense samples, i.e., 4B05-4B15 (see Fig. 2) and was not noted in samples obtained at greater water content. The water content, while affecting the total pore volume in the range 1-80 nm, appeared to have little bearing on the apparent density of aerogels, as can be seen from Table 1. The latter depends primarily on the TEOS content in the reaction system. The PSDs obtained provided grounds for estimation of the volumetric fractal dimension of the pore network. Owing to the effects of the lower and upper cut-off of the fractal range, Eq. (2) is not quite convenient to determine D v, especially when the range of fractal behaviour is not broad. To obtain a more accurate prediction Ehrburger-Dolle et al. [12] proposed applying a hyperbolic transformation of variables, which attenuates these cut-off effects. The results of computations of volumetric fractal dimension obtained using this modification (denoted Dvt) are listed in Table 1, together with the lower and upper limit of the power-law region, expressed in terms of pore diameters, d I and d u. For the selected samples

0.1

dV/dD [cma/g nm]

0.05

~

0

~

10

~

20

30

Pore diameter [nm]

40

Fig. 3. Poresize distributionfor aerogels:6B05(×), 6B15(O), 6B20(•), 6B30(•).

50

202

A.B. Jarzfbski et al. /Journal of Non-Crystalline Solids 190 (1995) 198-205

tn{IV(d~)-V(d)l/lV(d)-V(d,)i}

3 2 1

0 -1 -2 -3 -4 -5 -6

I

b

I

I

-3

-2

-1

0

-7

-4

ln[(d-dl)/(du-d)] I

1

2

Fig. 4. Plot of Eq. (3) in the transformedvariables for aerogels: 2B05 (•), 2B15 ( • ), 2B30 ( + ), 2B25 ( *).

prepared at W equal to 2 or 4 this behaviour is portrayed in Figs. 4 and 5. The transformation method applied was previously found to be quite effective for D v < 2, by contrast with the larger Dv where it appeared to underpredict real values [12]. Hence additional computations performed used directly Eq. (3). These results, denoted as D v, are given in Table

1 while the power-law characteristics of the pore network of selected samples are shown in Fig. 6. While the volumetric fractals may be linked to the spatial dependence of the transport properties of porous solids [13], the surface fractal characteristics directly affect the process of adsorption [10]. All experimental adsorption isotherms proved to obey

In {[V(dO-V(d)]/[V(d)-V(d.)] } 3

÷

2 I

0 -I -2

-3 -4

In I

-4

-3

,

-2

-1

/(du-d)] d'dl

0

1

2

3

Fig. 5. Plot of Eq. (3) in the transformedvariables for aerogeis: 4B10 ( • ), 4B20 ( • ), 4B25 ( *), 4B30 ( + ).

203

A.B. Jarzgbski et al. /Journal of Non-Crystalline Solids 190 (1995) 198-205

InIVN2-VN2 (d)l

x ~" x ......... x

-0.5

""212;2 -1 -1.5 -2 -2.5 -3 In(d,v)

-3.5

•

0

i

I

I

I

1

2

3

4

Fig. 6. Plot of Eq. (3) for aerogels: 4B20 ( × ), 4B30 (0), 6B20 ( • ), 6B30 ( • ).

Eq. (1) over a range of P / P o between 0.5 and 0.9, as illustrated in Fig. 7 for five aerogels. Surprisingly enough, adsorption isotherms on aerogels synthesized with low water content (2B05-2B30) all appeared to exhibit FHH isotherm slope parameter, m, less than 3 (in fact between 2 and 3) by contrast with

those on aerogels prepared with W >_ 4, for which tn was always greater than or close to 3. Consequently D s was computed using different expressions, as given in Eq. (1). Calculated surface fractal dimensions are listed in Table 1.

10 N / N n k ~+ B°

,1~

°°° e $ • Sx o x

x

°°

+

0

x

1

. . . . . . . .

0.01

I

0.10

. . . . . . . .

I

1.00

10.00

Fig. 7. Fitting of Eq. (1) to experimental data for aerogels: 2B05 ( *), 2B25 ( + ), 6B10 ( • ), 4B30 (C)), 6B30 ( × ).

204

A.B. Jarz¢bski et al. /Journal of Non-Crystalline Solids 190 (1995) 198-205

4. Discussion

Both Table 1 and Figs. 4-6 clearly demonstrate that the pore networks in the mesopore size range exhibit fractal character in the pore diameter scale. The volumetric fractal dimension of aerogel porous texture appears to be dependent on the water content in the reaction system, increasing with increase in this content. The TEOS concentration also affects the power-law behaviour, i.e., both D v and the range of the fractal domain, the latter notably for higher W. However, unlike the mass fractal domain of aerogel texture determined by scattering methods, the range of power-law behaviour of the pore network appears to wider and not narrow with increase in TEOS concentration, and hence the apparent density of aerogels. The increase in D v with increasing W and S clearly demonstrates a compaction of the pore network structure in the mesopore size range. This compaction is not unexpected. A slight increase of mass fractal dimension with increase in the H20 content was observed earlier [14] for acid-catalyzed alcogels. Also not surprising are low Dv values exhibited by the porous texture of aerogels prepared at W = 2. This water content, although stoichiometric from the overall hydrolitic polycondensation reaction point of view, is bound to produce incomplete hydrolysis of tetra-alkoxysilanes, thus leading to weakly branched particle structures [14,15]. Moreover, this trend is consistent with the effect of water and TEOS content on the pore network connectivity/mean coordination pore network number found recently for the same situation [8]. It should be noted that the N 2 adsorption-desorption method was recently reported [16] to be itself the cause of a marked decrease in the pore volume of aerogels and especially volume shrinkage in lightweight aerogels. This poses a crucial question regarding the reliability of results presented. In our opinion, even if the method itself affected the pore volume distribution in the range investigated, it might be expected to affect it on much the same scale for aerogels of similar densities (TEOS content). Since the effect of water on the PSDs in aerogels of similar densities is quite pronounced then the observed qualitative differences in the relevant PSDs should be ascribed rather to the water effect in the reaction system than to disturbances induced by the measure-

ment technique. The effect of TEOS is less evident and hence more likely to be misjudged. Despite possible shrinkage of aerogels caused by the nitrogen adsorption method the pore volume of aerogels in the mesopore size range is, on the whole, still significantly larger for lighter aerogels (with larger shrink capacity) than for denser aerogels (with smaller shrink capacity). As the increase in volume fractal dimension of the mesopore network, Dr, with increase in TEOS content results directly from this trend the possible shrinkage induced by nitrogen adsorption is not likely to upset the tendencies reported, although a small change cannot be precluded. The difference between mass fractal characteristics of aerogel texture obtained by scattering methods (SAXS, SANS) and those reported above might, in our opinion, be attributed to a physical difference in the objects 'seen' by the methods, i.e., overall structure 'seen' by scattering methods and the structure of voids (pore network) identified by the nitrogen adsorption-desorption method. Two classes of surface fractal dimension can be resolved in the aerogels investigated. All the samples prepared at low water content (2B05-2B30) had fractally rough surfaces (D s ranging from 2.5 to 2.6) while those of aerogels synthesized with W > 4 (4B s and 6B s) proved to be fractally smooth (D s between 2.0 and 2.2). This difference is in agreement with theoretical predictions of the surface structure of clusters grown according to the 'poisoned' Eden, and Eden rules [3]. It is also consistent with the results obtained from small-angle scattering experiments reporting a Porod slope of about - 4 for base-catalyzed silica aerogels synthesized with W = 4 [3,17]. These distinct surface characteristics of aerogels obtained under low and high water regimes, observed in adsorption isotherms, are responsible for various physical regimes of adsorption in the P/Po range 0.5 to 0.9, i.e., (i) capillary condensation on aerogels of the first group, as postulated by the Pfeifer and Cole theory for 2 < m < 3 [10]; (ii) FHH regime (associated with van der Waals forces) on aerogels synthesized with W > 4 [10]. Even superficial inspection of Table 1 shows clearly that the water content controls not only the fractal roughness but also the bulk pore volume in

A.B. Jarz¢bski et al. /Journal of Non-Crystalline Solids 190 (1995) 198-205

the mesopore size range. The mesopore network volume shown by aerogels obtained from the low water process, together with very high specific surface area seen in all 2B samples, provides additional incentives to further studies on base-catalyzed aerogels synthesized under sub-stoichiometric water regime. Extraordinary adsorption capacities detected most recently in these aerogels [18] may point to possible exciting directions to follow.

5. Conclusions Pore networks in base-catalyzed silica aerogels have a fractal structure over a major part or even the entire range of mesopores. Both surface fractal dimensions of silica aerogels and volume fractal dimension of the pore network appear to be markedly dependent on water content, and to a lesser degree on the TEOS concentration in the reaction system. Aerogels obtained using low water content have a porous structure in the mesopore size range which may indicate remarkable adsorption capacities. The N 2 adsorption method supplemented with the appropriate analytical tools proves to be quite an effective method to provide reliable surface structure characteristics which agree well with small-angle scattering results. However, volume fractal dimension characteristics of the aerogel pore networks obtained from the adsorption method appear to differ from mass fractal dimension characteristics of aerogels determined from SAXS or SANS. A.B.J. and J.L. gratefully acknowledge the financial support of the State Committee for Scientific

205

Research (KBN) for this work under Grant PB 3 1309 while Y.I.A. and N.L. gratefully acknowledge the support of the International Science Foundation under Grant N RA3 000.

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