2128
J . Phys. Chem. 1984, 88, 2128-2131
Role of Soluble Species in the Crystallization of Mordenites Satoru Ueda,* Nobuo Kageyama, Mitsue Koizumi, The Institute of Scientific and Industrial Research, Osaka University, 8-1, Mihogaoka, Ibaraki, Osaka 567, Japan
Shuji Kobayashi, Yoshimichi Fujiwara, and Yoshimasa Kyogoku The Institute of Protein Research, Osaka University, Yamadaoka, Suita, Osaka 565, Japan (Received: April 14, 1983)
Mordenite, analcime, and zeolite P1 (cubic type) were formed from aqueous clear solutions with compositions in the range 10Na20~(0.075-0.4)A120,~(28-40)Si02~360H20, and their formation fields were investigated. Mordenite was obtained as a single phase, and the Si02/A1203mole ratios ( k values) of the framework were in the range of 11.4-16.3 and dependent on only the original composition of starting material. However, the k value of forming mordenite in a clear solution was kept constant during crystallization despite the large changes of the Si02/A1203ratios in the mother liquor with reaction time. As a result, the reduced content of alumina and silica in the solutions could be calculated and was found to be approximately linear as a function of the amount of crystals formed. The compositional variations of the mother liquor were related to the changes of the boundary curves between fields of formation. In the crystallization of mordenites, aluminosilicate-soluble species played a part as nutrients for the formation of crystal nuclei and for the growth of crystals.
Introduction Most zeolites crystallize from reactive aluminosilicate hydrogels consisting of amorphous solid and aqueous solution phases. In such heterogeneous systems, complicated phenomena such as precipitation of a gel phase, nucleation of zeolite(s), continued crystallization, and crystal growth of zeolite(s) can simultaneously occur, and a metastable phase usually is formed during the reaction. Finally, the metastable phases are transformed into more stable phases in the reaction system.’ Accordingly, both the compositions and the quantities of the solid phases as well as the solution phases vary continuously until crystallization is completed. This indicates that the nature of both the nutrient solid gel and the aqueous solution contributes directly to the zeolite formation. It is difficult to determine accurate compositions of these phases. So far, two theories have been proposed for the crystallization mechanism of zeolites: one is the theory of solid-solid conversion of nutrient gel to crystalline phases,2” and the other is the theory of nucleation and crystallization from Both theories B. Sand in “Proceedings of the Fifth International Conference on , L. V. Rees, Ed., Heyden, London, 1980, p 1. W. Breck and E. M. Flanigen in “Molecular Sieves”, R. M. Barrer, Ed.; Society of Chemical Industry, London, 1968 p 47. (3) R. Aiello, R. M. Barrer, and I. S.Kerr, Adu. Chem. Ser., No. 101, 44 ( 1971). (4) R. Aiello, C. Collella, and R. Sersale, Adu. Chem. Ser., No. 101, 51 (,-1 97 1 ). ( 5 ) B. D. McNicol, G . T. Pott, and K. R. Loos, J. Phys. Chem., 76,3388 (1972). (6) B. D. McNicol, G. T. Pott, K. R. Loos, and N. Mulder, Adu. Chem. Ser., No. 121, 152 (1973). (7) G.T. Kerr, J . Phys. Chem., 70, 1047 (1966). (8) G.T. Kerr, J . Phys. Chem., 72, 1385 (1968). (9) J. Ciric, J . Colloid Interface Sci., 28, 315 (1968). (10) A. Culfaz and L. B. Sand, Adu. Chem. Ser., 121, 140 (1973). (1 1) R. A. Cournoyer, W. L. Kranich, and L. B. Sand, J . Phys. Chem., 79, 1578 (1975). (12) A. K. Pate1 and L. B. Sand, ACS Symp. Ser., No. 40, 207 (1977). (13) H. Kacirek and H. Lechert, J . Phys. Chem., 80, 1291 (1976). (14) S. Ueda and M. Koizumi, Am. Mineral., 64, 172 (1979). (15) S. Ueda, H. Murata, M. Koizumi, and H. Nishimura, Am. Mineral., 65, 1012 (1980). (16) B. M. Lowe, N . A. MacGilp, and T. V. Whittam in “Proceedings of the Fifth International Conference on Zeolites”, L. V. Rees, Ed., Heyden, London, 1980, p 85. (17) J.-L. Guth, P. Canllet, and R. Wey in “Proceedings of the Fifth International Conference on Zeolites”, L. V. R e a , Ed., Heyden, London, 1980, p 30. (18) S . P. Zhdanov and N. N. Samulevitch in “Proceedings of the Fifth International Conference on Zeolites”, L. V. Rees, Ed., Heyden, London, 1980, p 75. - I .
0022-3654/84/2088-2128$01.50/0
are derived from experimental data which were obtained through analysis of compositional changes of solid gel and solution and, in some cases, by using the data for crystallization rates and seeding effects. In order to avoid the complications of crystallization of zeolites from heterogeneous systems, we design the reaction system to eliminate the formation of solid gel by converting the reaction mixture to a homogeneous clear solution before crystallites are ~ b s e r v e d . ’ ~The J ~ solubility of aluminosilicate gel mainly is inversely proportional to the alumina content of the initial material. In the system Na20-A1203-Si02-H20, extremely low-alumina gels were completely dissolved even in solutions of several molar concentrations of silica, and analcime, zeolite B, hydroxysodalite, and mordenite crystallized from clear solution^.'^^^^ Further, gmelinite and zeolite Y were synthesized in a similar manner.Ig As a result, we have proposed that soluble aluminosilicate species might possess both composition and structure similar to those of ze~lites.’~J~J~ The concept of soluble species in zeolite crystallization was introduced first by Kerr7,8and thereafter by many other investigators who have supported the theory of crystallization from the solution phase. From those results, the soluble species can be regarded as amorphous silicate particles, or silicate and aluminosilicate polyanions, but their nature has not yet been established. The present studies focused on the crystallization of mordenite from homogeneous solutions as a function of the following three parameters: (1) the relationship between Si02/A1203ratios of the solution phase and the k values (Si02/A1,03 mole ratios) of synthesized mordenites, (2) the role of soluble aluminosilicate species in crystallization, and (3) the relative stability of soluble species against discontinuous changes of reaction temperatures. Experimental Section Initial hydrogels with compositions in the range 10Na20. (0.075-0.4)A1203~(28-40)Si02~360H20 were prepared. Calculated amounts of sodium hydroxide (99%), aluminum metal foil (99.99%), and distilled water were mixed in a 500 mL capacity polypropylene flask which was used for a reaction vessel. The mixture was heated at 100 OC in a water bath until the aluminum metal was completely dissolved, and finally a fixed volume of aqueous colloidal silica sol (0.3675 g/(mL of Si02) and 0.8350 (19) N. Kageyama, S . Ueda, and M. Koizumi, J . Chem. SOC.Jpn., Chem. Ind. Chem., No. 9, 1510 (1981). (20) S . Ueda, T. Fukushima, and M. Koizumi, J . Clay Sci. SOC.Jpn., 22, 18 (1982).
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984 2129 0
0.41
TABLE I: Representative Compositions of Reaction Mixtures
--_--.
1
\
I
I
Figure 1. Formation field of zeolites as a function of alumina and silica contents of initial gels. m and n: alumina and silica contents, respectively. Mo, An, and ZP: mordenite, analcime, and zeolite P (cubic type), respectively. Curves D1, D1,and D3: predictable directions in the reduction of alumina and silica contents of original solution MX (cf. Figure 3 ) . M10-M40: cf. Table I.
g/(mL of H20)) was added dropwise to the solution with constant stirring. The total volume of hydrogel was adjusted to 300 mL. The reaction mixture was heated at 100 "C under atmospheric pressure for 4-240 h. After the initial hydrogel was prepared, a large amount of amorphous solid was formed in solution but dissolved completely by heating within 1.5 h with stirring at 10-min intervals. Crystallites were observed forming in the clear solution after 40-45 h at reaction temperature. No detectable amorphous solid appeared again in the solution. After the run, the solid product was separated from solution by filtration, washed with hot water, and dried at 110 O C for 48 h in an electric oven. It was kept in a desiccator at constant humidity a t 25 OC until weighed with a chemical balance to determine yields of products. The identification of the product was made with an X-ray diffractometer (Cu K a radiation), an optical microscope, and a scanning electron microscope. Aluminum and silicon contents of the products were determined within an error of f0.4% by X-ray fluorometry (ED type) using Ti K a radiation on a finely powdered 0.5-g sample pressed under 200 kg/cm2 into a 13 mm X 3 mm disk. The effect of the fall of the temperature of mother solution on soluble species was examined by the following procedures: Reaction mixtures M10 and M30 indicated in Figure 1 were preheated at 100 O C for a period of 2-40 h and converted to clear solutions. They were quenched to rmm temperature and allowed to stand for 24 h. In all cases, no change could be observed in them during the period. The quenched solutions were heated soon again a t 100 OC for 192 h.
Results and Discussion Crystallization and Composition of Mordenite. Formation Field of Zeolites. Mordenite was formed over the whole range of initial gel composition, and, in a certain range, analcime and zeolite P coexisted with mordenite. The formation field for these zeolites is represented in Figure 1, in which 160 different compositional points in the system were examined and 4 fields determined. Sharp boundary curves could be drawn between the single-phase area (Mo) and the two-phase areas (Mo + An and Mo + ZP). These relations were reproducible. Representative compositions of the reaction mixtures for mordenite crystallization are listed in Table I. In the single-phase area, typical crystallization rates of mordenite are given in Figure 2. After the crystallites appeared in a clear solution, they grew gradually and were left suspended for 80-90 h. The grown crystallites settled to the bottom of the vessel, continued to grow slowly for 100 h or more, and attained the final dimension of 3-6 Km. This process obtained in all cases of mordenite crystallization.
symbolsa
Na,O
RM 1 M1 M2 M3 M4 M5 M6 M7 M10 M 20 M30 M40 M50
10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00
A1,0,
SiO,
H,O
32.00 360 38.00 360 34.00 360 0.15 38.00 360 0.15 34.00 360 0.1 5 31.00 360 0.10 33.00 360 0.10 31 .OO 360 0.35 38.00 360 0.35 34.00 360 0.25 36 .OO 360 0.25 34.00 360 0.40 40.00 360 M5 1 0.30 40.00 360 M52 0.20 40.00 360 M53 0.10 40.00 360 a The symbols in the first column are applied hereinafter. 0.1 0
0.25 0.25
Time IN Figure 2. Crystallization of mordenites from clear solutions as a function of time. Curves 1 and 3, and 2 and 4 correspond to the crystallizations from gels M10 and M30, respectively (cf. Table I). The plotted values on curves 3 and 4 were obtained as follows: after the original solutions were heated for 8 h, they were allowed to stand for 24 h at room temperature and then heated again for 192 h.
Relation between Composition of Mother Liquor and S i 0 2 / AI2O3Ratio ( k Value) of Mordenite. As crystallization proceeds, the contents of soda, alumina, silica, and water in the mother liquor decrease proportionally to the amounts of mordenite formed. However, the calculation for the compositional change of the solution actually cannot be made, unless the product consists of a single phase. Therefore, the effect of composition of mother liquor on the k value of formed mordenite was examined with the following procedure: (step 1) the initial hydrogel was heated for 168 h or more, and the product was separated from the mother solution; (step 2) the filtrate obtained in step 1 was heated again for 72 h or more, and the product was separated from the mother solution; (step 3) the filtrate obtained in step 2 was heated for 120 h, and the product was separated from the solution. In steps 2 and 3, powdered mordenite seeds were added to the filtrates in order to accelerate the crystallization. The temperature of the filtrates fell to about 75 O C during filtration, but no other phase than mordenite was formed from the filtrates. The initial hydrogels M10, M20, M30, and M40 plotted in Figure 1 were analyzed, and the results are listed in Table 11. As seen from the sixth and last columns in Table 11, the Si02/A1,0, ratios of the mother solutions greatly increase with subsequent steps, because the relative content of alumina greatly decreases compared with that of silica, whereas the k values of the mordenites formed are approximately constant from steps 1
2130
The Journal of Physical Chemistry, Vol. 88, No. 10, 1984
Ueda et al.
TABLE 11: Crystallization of Mordenites from the Initial Solutions and Their Filtrates initial compnb sol#
Na,O
A1203
M10 F11 F12
10.00 10.00 10.00
M20 F21 F22
amt of formed
SiO,
H,O
0.350 0.304 0.147
38.00 37.48 35.69
360 (360) (360)
109 123 24 3
168 120 120
0.2 0.4
10.00 10.00 10.00
0.350 0.306 0.135
34.00 33.60 32.02
360 (360) (360)
97 110 23 7
192 72 120
0.2 0.4
M30 F3 1 F32
10.00 10.00 10.00
0.250 0.210 0.093
36.00 35.57 34.33
360 (360) (360)
144 169 369
168 120 120
0.2 0.4
M40 F4 1 F4 2
10.00 10.00 10.00
0.250 0.204 0.079
32.00 31.61 30.55
360 (360) (360)
128 155 387
192 72 120
0.2 0.4
SiO,/A1,0,
run, h
seed,c g/L mordenites, wt %
kd
13.40 14.57 13.73 total = 72.70 12.76 49.42 20.40 total = 82.58 16.50 46.62 12.24 total = 75.36 18.67 49.94 15.32 total = 83.93
14.8 14.9 14.8 12.2 12.3 12.3 14.0 14.1 14.0
11.5 11.6 11.5
a Solutions F11, F21, F31, and F41 were filtrates obtained from the initial solutions M10, M20, M30, and M40, respectively. Solutions F12, F22, F32, and F42 were filtrates obtained from the solutions F11, F21, F31, and F41, respectively. Values calculated by using formula 4 from the amount of formed mordenites (wt %) except for solutions M10, M20, M30, and M40. Grams per liter of solution. The k values (Si0,/A1,03 ratios of formed mordenites) were corrected on the basis of seed contents (k = 9.3 2 for seed)
*
to 3 in all cases. However, if the initial gel compositions differ, it is noteworthy that the products have different k values. These results indicate that the composition of the mordenite formed does not change with compositional changes in the mother liquor but is governed by the initial composition of the gel. This is in agreement with the results of our previous ~ o r k . ' ~The , ' ~ k value of the mordenites formed in step 2 tended to increase slightly compared with those in the other steps. As seen from the eighth and ninth columns in Table 11, probably it is related to rapid crystallization from seeding. From these results, the decreasing amounts of alumina and silica in the mother liquor as a function of the amount of mordenite can be calculated. The water content of the initial gel is far greater than the soda, alumina, and silica contents, and hence its reducing effect is assumed to be negligible. The anhydrous composition of the reaction mixture can be expressed as follows: 10Na20~mA1203~nSi02
(1)
where m and n are the number of moles of alumina and silica, respectively, and m < 10 < n. Assuming that n mol of alumina is consumed to form mordenite, the composition of the solution is given as follows: (10 - x)Na20.(m - x)A1203.(n - kx)Si02 (2)
0.41
I
j
\
s
I
I
I
I
M50
Figure 3. Relationship between formation field of mordenites and the
reduction of alumina and silica contents of solutions. The reducing amounts of alumina and silica were calculated with formula 4 and represented by values along dotted lines. RM1, Ml-M53: cf. Table I.
where the value of k is constant. The decrease in the amount of soda is equal to that of alumina because of required electroneutrality. In addition, the following approximation can be made in formula 2 referring to the composition of the solution: lONa,O-lO( m-")A1203.10( 10-x
"k")Si02 10 - x
(3)
where x
