GEORGE T. KERR
2594
Chemistry of Crystalline Aluminosilicates.
V.
Preparation of
Aluminum-Deficient Faujasited
by George T. Kerr Mobil Research and Development Corporation, Central Research Division Laboratory, Princeton, New Jersey 08640 (Received January 24, 1968)
The slow addition of a dilute solution of ethylenediaminetetraacetic acid to sodium zeolite Y effects the removal of aluminum from the zeolite. A mechanism for the reaction is presented. The removal of up to about 50% of the aluminum yields highly crystalline products of improved thermal stability and increased sorptive capacity. As aluminum removal proceeds from about 60 to loo%, the crystallinity of the products decreases; aluminum-free products are completely amorphous.
Introduction Removal of aluminum from clinoptiolite2 and a synthetic erionite3 by leaching with hydrochloric acid has been reported recently. Faujasite, having lower ratios of silicon to aluminum than either clinoptiolite or erionite, is prone to crystal lattice attack by strong mineral acidse4 For synthetic faujasite, aluminum was removed by special thermal treatment of the hydrogen (acid) form of the zeolite, followed by cation exchange of aluminum by sodium using sodium hydroxide solution.6 I n the work reported here, aluminum is removed directly from sodium zeolite Y, a synthetic faujasite, by a special technique using ethylenediaminetetraacetic acid (H4EDTA), Experimental Section The Siemens Crystalloflex I1 X-ray diff ractometer was used, with a l-radian camera, for the diffraction analyses. A Du Pont 900 differential thermal analyzer was used a t a heating rate of 30"/min to determine the temperature at which crystal lattice collapse occurred.6 Reagents. Reagent grade chemicals were used throughout this study. The sodium zeolite Y had the following composition: ash, 75.7% (ignition at 1100") ; NazO, 13.5%; AL.03, 21.1%; and SiOz, 65.2%. Reaction Conditions. All reactions involving the zeolite and HIEDTA were conducted at reflux using 20.0 g of the zeolite. This quantity of zeolite contains the following quantities of oxides, expressed as gram formula weight: 0.0331, NazO; 0.0314, A1203; and 0.1595, Si02. For rapid addition of HlEDTA to the zeolite, 9.6 g (0.033 mol) of the acid was added directly to a slurry of zeolite in 300 ml of water. The mixture was stirred for 6 hr. For slow addition of the acid to the zeolite, a Soxhlet extractor was used. The 20.0 g of zeolite and the 300 ml of water were contained in the boiling flask; the appropriate quantity of acid was contained in a Soxhlet thimble within the extractor. At least The Journal of Physical Chemistry
18 hr was required for complete addition of the acid to the boiling flask. After filtration the samples were calcined with air or with nitrogen purge at 550-600" prior to sorption measurements.
Results The Effect of Rate of Addition of Acid to Sodium Zeolite Y . The same quantities of reactants were used for both slow and rapid addition. The properties of the products and of the initial zeolite are summarized in Table I. Cyclohexane sorptive capacities and the quality of the powder photographs indicate that the product of slow addition is the more crystalline. Both aluminum-deficient products, however, are significantly more thermally stable than the normal sodium zeolite. Although about 50% of the tetrahedrally coordinated aluminum and the accompanying sodium ions were removed from the slow-addition product, the sorptive capacity and the powder photograph indicate that little, if any, loss of crystallinity occurred. Composition of Aluminum-Deficient Products. A series of reactions were conducted using various quantities of H4EDTA and the slow-addition technique. The results of this study are depicted in Figure 1. The straight line conforms to the stoichiometry of the general equation
+ NaAIOz(SiOz)v+ xXaA1EDTA. HzO + (NaA102)1-,(Si02)y+ xH2O
xH4EDTA
where x 5 1 and y 1 2.5- The close fit of the experimentally derived points to the line in Figure 1 shows that the above stoichiometry is very closely followed (1) Part I V : G. T. Kerr, J . Phys. Chem., 72, 1385 (1968). (2) R. M. Barrer and M. B. Makki, Can. J . Chem., 42, 1481 (1964). (3) S. P. Zhdanov and B. G. Novikov, Dokl. Akad. Nauk SSSR, 166, 1107 (1966).
(4) R. M. Barrer, Proc. Chem. SOC.,99 (1958). (5) G. T. Kerr, J . Phys. Chem., 71, 4155 (1967). (6) A. S. Berger and L. K. Yakovlev, Zh. Prikl. Khim., 38, 1240 (1965).
CHEMISTRY OF CRYSTALLINE ALUM~NOSILICATES
Table I: Rapid and Slow Addition of Ethylenediaminetetraacetic Acid to Sodium Zeolite Y Product of rapid addition
Formula weight ratio NazO: A1203 s i o 2:A&03 Cyclohexane sorption Grams/100 g of activated sample Grams/l g of Si02 in activated sample Fraction removedb Na A1 Temperature of crystal collapse, "C a
Product of slow addition
0.89 (0.96)" 0.87 (1.16)' 13.2 (0.167)" 1 0 . 3 (0.164)'
Initial zeolite
1.05 5.26
11.2
24.0
19.1
0.135
0.298
0.292
0.66 0.60 1040
0.575 0.515 1047
0 0 974
Composition of aqueous-phase product given in parentheses. Si from zeolite.
* Assuming no loss of
I n Table I1 is a resume of the compositions and properties of the samples shown in Figure 1. Composition of Aqueous Product from AluminumRemoval Reaction Mixtures. The aqueous phase from a slow-addition reaction in which 50% of the aluminum and sodium were removed from the zeolite was evaporated to a low volume. On cooling, the liquid yielded a crystalline solid which was filtered, was dried a t loo", and was analyzed. Anal. Calcd for NaAIEDTA.HzO: Na, 6.46; Al, 7.55; C, 33.8; H, 3.43; N, 7.86. Found: Na, 7.2; AI, 7.57; C, 34.4; H, 3.82; N, 8.03. The X-ray diffraction powder photograph of this material was identical with that of an authentic sample of NaAIEDTA.HzO prepared by treating A1 with NaHsEDTA in the ratio of 1 g-atom/l mol. A trace of mercuric chloride was added to the aqueous phase to enhance the reactivity of the metallic AI. These findings also confirm the validity of the general equation proposed for the removal of aluminum from sodium zeolite Y.
Table I1 : The Stoichiometry and Composition of Aluminum-Deficient Zeolite Y Moles of HdEDTA used/ formula weight of NaY
----Fraction Na
removed----A1
Temperature of crystal collapse,
-Grams of cyclohexane sorbed per-100 g of 1 g of SiOa ieolite in zeolite
OC
0 0.25 0.50 0.50 0.675 0.750
0 0.393 0.575 0.59 0.74 0.84
0 0.263 0.515 0.51 0.70 0.80
19.1 21.4 24.0 23.6 24.8 24.5
0.292 0,293 0.298 0.295 0.284 0.270
974 1033 1047 1050 993 993
1.00
1.o
1.0
21.8
0.224
Amorphous
Crystallinity by X-ray
Good Good Good Good Good Nearly amorphous Completely amorphous
Discussion The Mechanism of Aluminum Removal. In an earlier n 0.80
5a 0.40
article, a mechanism was proposed to explain the formation of cationic aluminum during conversion of hydrogen zeolite Y to a more stable substance.6 Reaction 1 shows how the sodium zeolite is partially con-
0
I
Na+ 20-Al-0-Si-0 MOLES H4EDTA USED PER EW, NaY
Figure 1. The stoichiometry of aluminum removal.
in practice. Therefore, any degree of aluminum removal can be easily achieved by use of the proper quantity of acid relative to the quantity of zeolite used.
1 - 1
0
0
I
+ H4EDTA-+
0 0 20-AI
I I
0
H O
I 1 I
0-Si-0
+ Na2H2EDTA
(1)
0 Volume 78, Number 7 July 1968
GEORGE T. KERR
2596
0 20-A1
H O
1 1
I
0-Si-0
I
I
0
0
-OH
+ 3Hz0 +
0
0 H
I HO-Si-0 H I 0 0
0
0
I I + O-Al-0-Si-0
+ HzO
(2)
+ NaAIEDTA.HzO + HzO
(3)
I - I0
0
A1(0H)z+
0
0
I I I - I0 0
+ NazHzEDTA+
O-Al-0-Si-0
A1(OH)2 +
0
0
I I 1 - 1
O-A1-0-Si-0
0
0
Na + verted to the hydrogen form by HIEDTA. The zeolite must be a t least partially in the hydrogen form for removal of aluminum; no reaction occurs between the sodium zeolite and NazH2EDTA. Reaction 2 is a composite of the two reactions presented in the earlier report.6 This reaction involves hydrolysis of a tetrahedral aluminum adjacent to a Brgnsted acid site, followed by neutralization of the resultant basic aluminum hydroxide by a Brgnsted acid site to yield the cationic aluminum species and water. I n reaction 3 the aluminum cation is exchanged by sodium ion and then chelated by the EDTA anion. By the mass action principle, the chelation effects essentially complete exchange of aluminum ions by sodium ions. In the earlier work the formation of the hydroxyaluminate ion, using sodium hydroxide solution, served this purpose.6 The composite of the three proposed mechanistic reactions is in agreement with the general equation presented in the Results section. Thermal Stability. Removal of a portion of the tetrahedrally coordinated aluminum from the zeolite framework has been showns (and Table 11) to increase its thermal stability. The reason for this increase in stability is not obvious, but it might be attributable to the formation of new Si-0-Si bonds.6 The four hydroxyls in the product of reaction 2 are each bonded to silicon; on heating, these four groups would be expected to condense to yield water and Si-0-Si bonds. The
The Journal of Physical Chemistry
observed contraction of the unit-cell dimension on aluminum removal is in line with this proposal.6 The temperatures reported here a t which lattice collapse occurs should not be considered as true transition points, such as melting. Berger and Yakovlev pointed out that a dehydrated zeolite, “having a large internal surface and excess of free energy, is in a state of unstable equilibrium and when the temperature is raised to a certain limit it undergoes sudden breakdown which leads to formation of an amorphous product and liberation of the excess energy in the form of heat.”6 Differential thermal analysis is a dynamic method and the heating rate affects the temperature of crystal loss in zeolites. As long as the rate is constant, dta can serve to compare the relative stabilities of zeolites. For the particular sodium zeolite Y used in this study, the optimum removal of aluminum for thermal stability appears to be in the range of 25-50%. Above 50%, not only does the thermal stability decrease but also serious lattice collapse begins to occur; on complete removal of aluminum, the product is X-ray amorphous. Sorptive Capacity. The sorptive data in Table I1 show that removal of aluminum generally increases the capacity for cyclohexane sorption on the basis of sample weight. However, the sorptive capacity per gram of SiOz in the zeolite remains nearly constant up to about 60-70% aluminum removal. If it is assumed that silicon remains intact in the framework on removal of aluminum, then the number of grams of silicon in a sample is proportional to the number of unit cells in the sample. If the cyclohexane capacity per unit cell is unchanged on removal of aluminum, then the number of grams of cyclohexane sorbed per gram of SiOzshould be unchanged. The assumption that framework silicon is unaffected by aluminum removal is supported by the SiOz:AlZO3ratios in the aqueous phases shown in Table I. Our observations on the sorptive-capacity increase are in agreement with those of Zhdanov and Koviltov who studied aluminum-deficient erionite. The decrease in sorptive capacity (based on the number of grams of SiOz in the samples), from which more than about 70% of the aluminum was removed, is in line with the decrease in crystallinity as determined by X-ray diffraction. Usually faujasites sorb only traces of cyclohexane on conversion from the crystalline to the amorphous state. The relatively high sorptive capacities of these amorphous products, containing almost no aluminum, behave more like silica gel than amorphous zeolites. This is not surprising, since the amorphous material obtained on complete aluminum removal is simply silica. The usual amorphous faujasite contains about 20% of A1203 and would not be expected to yield a silica-type product.
