Investigation of the NaA zeolite crystallization mechanism by i.r. spectroscopy

Investigation crystallizatipn spectroscopy

of the NaA zeolite mechanism by i.r.

S.R. StojkoviC Institute

of General and Physical

Chemixtry,

P.O. Box 551, Belgrade,

Yugoslavia

and B. AdnadjeviC Faculty of Science, P.O. Box 550, Belgrade, Yugoslavia (Received 11 May 1987; accepted 23 February 1988)

The crystallization mechanism during the synthesis of a type A zeolite was investigated by i.r. spectroscopy. The investigation has shown the presence of primary structural units (D4R) in sodium aluminosilica+a gel (S&3) and an increase in their concentration during crystallization. The ratio between band intensitiek of the primary structural units (D4R), at 560 cm-‘, and of the Si-0 bending vibration in the Si04 tetrahedron, at 470 cm-‘, increases during crystallization. Keywords:

Ix.; NaA

zeolite;

crystallization;

aluminosilicate

INTRODUCTION In their papers, Zhdanov’ and Kerr’ presuppose a transformation of amorphous sodium-silicate gel into zeolite crystals via liquid-phase components participating in the formation and further growth of crystallization seeds. Another type of crystallization mechanism, first proposed b Breck and Flanigen3 and then also Mirskii et al. 1 and McNicol et al.,’ assumes a direct transformation (in the solid phase) of amorphous SASG into zeolite, without dissolution of the solid phase and participation of the liquidphase components in the formation and growth of zeolite crystals. Dondur et aL6 formed a hypothesis on the formation of._zeolite seeds on the SASG boundary surface followed by interaction between the thusformed active complex and amorphous SASG during crystallization. On the basis of this hypothesis, possible characteristic vibrational frequencies of the zeolite crystallization precursor (aluminosilicate complex) in the region from 1080-980 cm-’ to 700-650 cm-’ were thoroughly investigated by i.r.7 From the results of the band analysis of the crystallized zeolite aluminosilicate gel, it has been concluded that zeolite crystallinity can be followed from the surface ratios of the bands at 560 cm-’ and 470 cm-‘, which are characteristic for D4R vibrations and Si-0 bending vibrations.

EXPERIMENTAL The NaA zeolite was obtained by the procedure described in Yugoslav patent 1438/81.* The synthesis

@ 1988

Butterworth

Publishers

was performed by using sodium silicate and sodium aluminate, produced by the Alumina Factory “BiraE,” Yugoslavia, having, respectively, the following par+meters: module 2.4, density 1.430 g/cm”, and temperature 333 K; and module 1.9, density 0.250 g/cm”, and temperature 363 K. Both solutions were added to water simultaneously for 30 min, at 365 K. Mixing was performed at 500 rpm. The reaction ratio was 2.73 Na20/A1203/1 .99 Si02/86 H20. Reaction mixture aliquots taken during zeolite crystallization were filtrated. The NapO, A1?03, and SiOp contents in the liquid phase were determined by standard chemical analysis. SASG was rinsed and dried at 383 K and then investigated by X-ray and i.r. spectroscopy. 1.r. spectra were recorded on a PerkinElmer 457 spectrophotometer; 1 mg/150 mg KBr pellets of the sample were prepared in vacuum and under pressure of 200 MPa. Complex bands in the region from 1100 to 900 graphically according to the cm-’ were deconvoluted analytical equation:’

Iv

IO

= 1 + o.,l?$JLj’+

0.1 (J+L(4

where Z, is the band intensity with the wavenumber v, I, is the maximum intensivity at vO, y is one-half of the band half-width, and 0.9 and 0.1 are empirical coefficients providing the best band shape under experimental conditions.

ZEOLITES,

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NaA

zeolite

crystallization:

S. R. StojkoviC

and

B. Adnadjevid

Si 02 (g/l 1 2

0

2

1

3

4 TIME,

Figure 1 on time

Change

of concentration

of A&O3

5 hours

and SiOz dependent

RESULTS AND DISCUSSION

Figure

Change of the liquid-phase chemical composition dependent on the NaA zeolite crystallization time is shown in Figures 1 and 2. The SASG crystallization diffractograms are given in Figure 3. The crystallinity of the zeolite in dependence on the crystallization time, determined from these diffractograms, is shown in Figure 4. The i.r. spectrum of the zeolite (Figure 5) during as well as after termination of crystallization contains a number of characteristic vibration bands in the regions 1100-900 cm -I, 700-650 cm-‘, 560 cm-‘, 470 cm-’ and 380 cm-‘. A band at about 1000 cm-‘, which corresponds either to the stretching vibrations of the Si-0 bond in Si04 or to the skeleton of bonded SiO,t tetrahedra, shows the greatest change during crystallization. Taking into account the increase of Nap0 and decrease of AlzOs in the starting solution, these phenomena can be ascribed to the change in the degree of polymerization of Si04 tetrahedra, that is, to the formation of the zeolite crystallization seed due to presence of the sodium-aluminiosilicate cluster, the

76 1

0

2

1

524

2

Change

ZEOLITES,

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difractogram

of SASG

precursor of phase transformation into the zeolite seed. A band at 700 cm-’ remains in an unchanged position up to 44% crystallinity. At a crystallinity higher than 44%, it shifts to 670 cm- ‘. These changes indicate that the beginning of seed interaction at the SASG surface Si is liberated and substituted by additional Al and Na. X-ray investigation shows the presence in the spectrum of a band at 560 cm-’ that corresponds to the D4R vibration existing even before the formation of zeolite crystals. The increasing intensity and half-width of this band are ascribed to the crystallization induced in SASG. to the bending The band at 470 cm-’ corresponds vibrations of the Si-0 bond in the Si04 tetrahedra. The unchanged position of this band and its halfwidth, which decreases with time, point to the rearrangement of SASG during the crystallization into zeolite. The band at 380 cm-’ that appears at 22% crystallinity corresponds to inlet aperture vibrations. The degree of crystallization of NaA zeolite is therefore determined by calculating the surface area ration of the bands at 560 cm-’ and 470 cm-‘. The

3 T I ME:

Figure

3

2

3

hours on time

Figure

4

Zeolite

crystallinity

as a function

L T I ME,

5 hours

of crystallization

NaA zeolite

crystallization:

S. I?. Stojkovid

and

Crystallinity,

X-ray

B. Adnadjevic

4 hours

lb00

Figure 5 tion time

I

I

I

I

I

1

1200

1000

800

600

LOO

200

1.r. spectrum

of NaA zeolite

dependent

on crystalliza-

reliability of the established method is proved by expressing the obtained ratio as a function of the degree of crystallinity determined by the X-ray method, as shown in Figure 6.

CONCLUSIONS On the basisof the obtained results, the crystallization mechanism of type A zeolite can be explained as a phase transformation of amorphous aluminosilicate gel into a zeolite structure. The precursor of such phase transformation is a cluster formed by the rearrangement of primary structural units as affected by sodium ions on the aluminosilicate gel surface.

Figure 6 Correlation i.r.-X-ray spectra

of SASG

and crystallinites

obtained

from

The crystallization process is explained by induction effects of the formed zeolite seed.

REFERENCES Zhdanov, S.P. Adv. Chem. Ser. 1971, 101,22 Kerr, G.T. J. Phys. Chem. 1960,70, 1047 Breck, D.W. and Flanigen E.M., presented at the 137th National Meeting of the American Chemical Society, Division of Inorganic Chemistry, Cleveland, Ohio, April 1960, Abstract, p. 33-M Mirskii, Ya. V., Mitrofopov, M.T. and Popkov, B.M., Tseoliiy, ikh sintez, vopstvo i primenenie, Moscow & Leningrad, Nauka, 1965, p. 192 McNicol, B.D., Pott, G.T. and Loss, K.R., J. Phys. Chem. 1972, 76,338a Dondur, V. et al. in VII Jugoslovenski kongres za hemiju i hemijsku technologiju, Vol. 3, Novi Sad, Yugoslavia, 1983 Burkov, K.A. et a/. Z.P.H. 1979, LII 1, 53 AdnadjeviC, 8. et al. Yugoslav Pat. 1438181 Ribnikar, S.V. Glasnik /-/em. DruStva Beograd 1979,44, 591

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