Small angle scattering study of silica gels and zeolites
T. Beelen, W. Dokter, H. Van Garderen, R. Van Santen, E. Pantos
To cite this version:
T. Beelen, W. Dokter, H. Van Garderen, R. Van Santen, E. Pantos. Small angle scattering study
of silica gels and zeolites. Journal de Physique IV Colloque, 1993, 03 (C8), pp.C8-393-C8-396.
<10.1051/jp4:1993882>. <jpa-00252313>
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Submitted on 1 Jan 1993
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JOURNAL DE PHYSIQUE IV
Colloque C8, supplkment au Journal de Physique I, Volume 3, dkcembre 1993
Small angle scatte~ingstudy of silica gels and zeolites
T.P.M. BEELEN, W.H. DOKTER, H.E VAN GARDEREN, R.A. VAN S A N T E N and E. PANTOS*
Eindhoven University of Technology, PO. Box 513,5600 MB Eindhoven, The Netherlands
* SRS Daresbuly Laboratory, Wammngton,
UK.
Abstract.
Silica hydrogels show fractal properties corresponding with DLCAA or RLCCA. During aging
silica is redistributed, resulting in reinforcement of the tenuous structure and only slightly
disturbing fractal geometry. In zeolitic precursors no mass-fractals are present, but SAXS spectra
show strong indications for the presence of surface-fractals. Computer simulations do confirm the
experimental results.
Introduction.
For most applications of silica gel, size, form and distribution of pores are the most important properties for
succesful use [I]. Also for zeolites, crystalline silica/alumina or silica compounds with a rich variety on
chamellcage structures, the microporous structure is determining high selectivity in catalysis, selective
adsorption of gases or possibilities for ion exchange [2].
Owing to difficult accessibiity of colloidal silica/alumina systems for "classical"analysis, knowledge concerning
precursors of zeolites or silicas is still very scarce. However, using recent progress in NMR, microscopy and
small angle scattering considerable progress has been achieved. Especially aggregation phenomena in the
recently developed sol-gel techniques can be described quite succesfullyusing SAXS, SANS or light scattering
in combination with fractal concepts [3,4,5].
To produce porous silicas or zeolites on industrial scale, in most applications water glass is used as silica
source. Acidifcation of diluted water glass solutions may result in gels composed of fractal aggregates with
the fractal region covering several decades at favourable conditions [6]. To prepare porous silicas aging of
the wet gels is necessary [1,1. In this process silica monomers are redistributed, reinforcing the ramified
silica skeleton and therefore avoiding collapse of the fragile network during or after drying. During gelation
and aging the fractal dimension D of the wet system is changing only slightly, corresponding with the
conservation of diffusion-limited cluster-cluster aggregation (DLCCA) or reaction-limited cluster-cluster
aggregation (RLCCA) structure [3] on sub-micron scale [7].
Contrary, in precursor phases of zeolites no DLCCA/RLCCA aggregates are present.
Experimental.
Silica gels (4 wt% SiO3 have been prepared by pH-controlled addition of water glass to diluted HCI with
catalytic quantities of NaF (F/Si = 0.01). Wet gels were aged at room temperature. After freeze-drying
porosity was determined by physical adsorption (BET).
Zeolite-Nay was prepared at standard conditions [2] with aging at 50 OC and crystallization at 100 "C.
Identification of zeolite-Nay and determination of yield was performed by X-ray diffraction methods.
SAXS spectra were measured at Daresbury Laboratory (UK) at NCD-station 8.2 using synchrotron radiation
at 0.154 nm and a point like source (no desmearing). The camera length was 2.3 m and a quadrant detector
(512 channels) was used.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1993882
JOURNAL DE PHYSIQUE IV
Results and discussion.
To prepare fractal aggregates water glass was added to diluted HCI (4 wt% SiO,, pH = 4). To optimize
reactivity of the elementary condensation/hydrolysis reactions catalytic quantities of NaF were added [I].
After 30 minutes (before gelation) the solution contained mass-fractal aggregates with D = 1.7 (figure I),
corresponding with DLCCA [3]. After gelation and short aging the aggregates changed as shown in figure
1 (5 hours) with D increasing to 2.0. According to Iler [I] during aging silica particles (monomers)
preferentially dissolve from convex and recondensate at concave or less convex surfaces, resulting in filling
of crevices and necks between branches or particles and in growth of big particles at the expense of small
ones. However, during aging of ramified DLCCA aggregates, weakly (single) bonded particles or clusters
may dissolve also and after a Brownian trajectory be bonded again elsewhere. This assumption is confirmed
by NMR 181: during aging of fractal aggregates intensity is shifted from Q, and Q, to Q and Q4bands. Our
observation of the increase of D to 2.0 is in accordance with this picture: RLCCA systems show D-values
5
-
41.4 hrs.
4
-
/--.
?
0
V
3 -
5.0 hrs.
V
......
2 -
0,
0
-
1
-
0.5 h r s
0
-0.80
I
-0.52
1
I
1
-0.24
0.04
0.32
0.60
log ( Q (nm- 1))
Figure 1. SAXS spectra of silica hydrogel after 0 5 hrs (aggregation), 5.0 hrs (short aging) aod 41.4 hrs
(prolonged aging). To compare spectra curves are shifted vertically.
Figure 2. Aggregate prior to aging (left) and after aging (right).
at 2.0 - 2.2 and both RLCCA and aging in DLCCA are offering particles and clusters a "second chance" to
find more favourable sites (for example many-bonded sites in crevices, between branches deeper in the
"fjords" of the aggregates) resulting in higher D-values.
However, during prolonged aging most of the weakly and single bonded particles and clusters will be
dissolved and more strongly bonded elsewhere, or the single bond is reinforced by condensation of
monomers or small particles. Therefore the convex-concave aging mechanism will be prevailing, providing
local transport of silica monomers or particles towards crevices and necks and reinforcing the network.
Because the general structure of the aggregates is almost not changed by this process, D will be constant or
decrease very slightly because the mass gradient will increase due to the formation of bigger particles
preferentially in the kernel of the aggregate. Accordingly, after aging for 41.4 hours the SAXS spectrum of
the wet gel shows a fractal region with almost the same D (figure I), but with a crossover to the Porod
region [5] at much lower q, corresponding with bigger elementary particles. This crossover had to be
expected at q > 0.6 nm-' without or with short aging. In figure 2 a visualization of the aging process is given
[71.
In zeolites or zeolitic precursors no (mass)fractal aggregates could be detected. Although many zeolites are
prepared at conditions very different from the conditions for silicas, most parameters are not decisive. For
example, some zeolites may be prepared at low pH [9] or at lower temperatures (zeolite-A) and in silicas
the addition of templates or alumina does not prevent DLCCA or RLCCA.
Surprisingly, zeolitic precursors show strong evidence for the presence of surface fractals.
Surface fractals are reported for porous coal [lo] and colloids [ll] and are characterized in SAS-spectra by
straight lines in log(1)-log(q) plots with slopes between -3 and -4. The surface fractal dimension D, is
determined by: slope = -(6 - D,).
A typical example for zeolites is given in figure 3 (zeolite-Nay). During aging at 50 OC a surface fractal with
D, = 2.4 is observed. When the preparation is continued at 100 OC (crystallization stage), D, decreases to
2.0 (smooth surface) and during crystal growth (as observed by X-ray diffraction) again D, = 2.4 is observed.
Similar effects have been observed for zeolite-A [12] and silicalite [13]. However, to understand the relation
of surface fractals with aging precursors and growing crystals, more experimental information has to be obtained.
Time (min.)
l o g Q (nm-')
Figure 3. Surface fractal dimension of reaction mixtures during synthesis of zeolite-Nay (left). SAXS spectra
of samples after 20 min (a), 210 min @) and 730 min (c) (right). To compare spectra curves are shifted
vertically.
To explain the absence of (mass)fractal in zeolitic precursors the influence of concentration has to be
discussed. At high concentrations the number of collisions between particles or clusters will increase very
fast and the "wanderings" of the particles or clusters, representative in the formation of (mass)fractal
aggregates, might be too short to create the typical screening effects of DLCCA or RLCCA.
Because experiments with silicas at high concentrations could not be executed due to experimental
difficulties, computer simulations have been performed. With GRASP [14] off-lattice box DLCCA
calculations with 27000 particles have been carried out, concentration varying between 0.25 and 30 ~01%.T o
be able to compare simulation results with experimental SAXS curves, D A M [14] was used to calculate
SAXS curves from the coordinates of the particles in the aggregates after simulation. To avoid complications
caused by the form factor of the (spherical) particles, in figure 4 the structure factors S(q) are showed
JOURNAL DE PHYSIQUE IV
396
(instead of intensity I(q)) as function of concentration. In accordance with experimental evidence, at low
concentrations (below 5 %) a distinct fractal region is observed with D = 1.74. At higher concentrations the
straight line is contracting considerably together with increasing slope and fractal dimension. At 20% no
fractal region can be indicated anymore and even small diffraction peaks are appearing due to preferent
distances at the scale of the particles. However, at 20 and 30 % strong indications for a new fractal region
can be observed at the scale of the box (q = 0.1 1.0) with D = 3.0 - 3.6, a remarkable similarity with the
surface fractals of zeolitic precursors. However, the determination of D is rather problematic (no perfect
straight line) and although 27000 particles are sufficient to simulate mass-fractals, probably simulations with
100,000 particles will be necessary to confirm the formation of surface fractals at high concentrations.
-
2 1
-3
I
-2.5
-2
I
-1.5
I
I
1
-0.5
log (q)
-1
0
0.5
1
1.5
Figure 4. Calculated structure factors of simulated DLCCA (27000 particles) at varying concentrations. TO
compare spectra curves are shifted vertically.
Acknowledgement.
This work was initiated and supported by the Dutch Department of Economic Affairs in terms of IOPKatalyse. SAXS measurements were performed at the Synchrotron Radiation Source, Daresbury, UK, under
terms of the SERC/NWO agreement.
The assistance is appreciated of dr.W.Bras of the SRS.
References.
f l ] R.K.Iler, The Chemistry of Silica, Wiley & Sons Inc., New York, 1979.
[2] H. van Bekkum, E.M. Flanigan and J.C. Jansen (Eds), Introduction to Zeolite Science and Practice,
Elsevier, Amsterdam, 1991.
[3] R. Jullien and R. Botet, Aggregation and Fractal Aggregates, World Scientific, Singapore, 1987.
[4] T. Vicsek, Fractal Growth Phenomena (2' ed.), World Scientific, Singapore, 1992
[5] J.D.F. Ramsay, Chem.Soc.Rev., 15 (1986) 335
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Mat.ResSoc.Syrnp.Proc. 271 (1992) 263
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Santen, accepted for publication in Coll.& Surf.
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[lo] H.D. Bale and P.W. Schmidt, Phys.Rev.Letters 53 (1984) 596
[ l l ] K.D. Keefer and D.W. Schaefer, Phys.Rev.Letters 56 (1986) 2376
[12] To be published
[13] W.H. Dokter, T.P.M. Beelen, H.F. van Garderen, C.P.J. Rummens, J.W. de Haan, L.J.M. van de Ven,
R A . van Santen and J.D.F. Ramsay, submitted for publication in Zeolites.
[14] H.F. van Garderen, W.H. Dokter, T.P.M. Beelen, R A . van Santen and E. Pantos, submitted for
publication in Modelling and Simulation in Materials Science and Engineering.