J. Phys. Chem. 1995, 99, 10982-10987
10982
Ordered Distributions of A1 Atoms in the Framework of Faujasite Type and a Chiral Yt T. Takaishis Toyohashi University of Technology, Toyohashi, 441 Japan Received: March 3, 1995; In Final Form: May 11, 1 9 9 9 Ordered distributions of AI atoms in frameworks of faujasite family are deduced for various A1 contents. The proposed model explains observations that Y of a high crystallinity has the shape of a trigonal plate and that discontinuities appear in plots of Al contents vs lattice constants. For L a x and Lay, calculated populations of La on three kinds of sites are in good agreement with observed populations. It is predicted that Y containing 72 f 6 A1 atoms per unit cell is chiral. The model is further extended to a region of much greater silica content.
Introduction It has been accepted without doubt that X and Y zeolites have a symmetry of Fd3 or Fd3m irrespective of their Al contents.’ In 1979, however, Edwards et al. patented a method to produce a tabular Y with a trigonal morphology, as shown in Figure 1,2 but its theoretical importance has been overlooked by other scientists. Frequently, a crystal belonging to a space group of lower symmetry has a cubic morphology by virtue of twinning. But the reverse is not the case. For instance, a morphology of trigonal symmetry is never observed with a crystal belonging to a cubic space group. In the X-ray structural analysis of a zeolite, the difference between Si and AI atoms is usually ignored, and a higher symmetry than the actual one is provisionally assigned to the framework. The above result shows that the actual symmetry of their Y is trigonal, that is, A1 atoms are regularly distributed in the framework with a trigonal symmetry. The following Barrer’s statement, on the crystal g r ~ w t h explains ,~ why the crystal morphology is sensitively controlled by a geometry of A1 distribution. “The chemical process referred to the above must involve the formation of T-0-Si bonds (T = A1 or Si): +T-O-Naf Z-T-OH
+ HO-Si4 - ST-O-Si4 + NaOH + HO-Sit - Z-T-O-Si4 + H 2 0
Figure 1. Morphology of trigonal Y.’
Figure 2. Frameworks of chabazite and location of A1 atoms and Ca ions. 0, AI atom; 0, Ca2+.
family and show that the proposed model reasonably explains various observations not understood so far.
Secondary and Tertiary Building Units and so it may be inferred that the elimination of water or NaOH occurs with a lower E (activation energy) when T = Al than when T = Si. The above reactions between precursor molecules and surface require a steric fit, for example, the surface hydroxyls of the lattice at the crystal-solution interface and corresponding hydroxyl of the impinging precursor molecule which successfully condense onto the growing surfaces ...” Growth rates of each crystal face are directly related to the geometry of an A1 configuration in the framework of the respective face, and hence the crystal morphology also depend on the A1 configuration. One can deduce from a crystal shape a point group of the framework and may be able, in a fortunate case, to determine its space group by aid of some other knowledge. In the present paper, we deduce A1 configurations in members of the faujasite This paper was read before the Annual Meeting of Japan Ass. Zeolite, Nov 15, 1994. Retired, the permanent address is Hayama-cho, Nagae 1461-251, Kanagawa Prefecture, 240-01 Japan. Abstract published in Advance ACS Abstracts, June 15, 1995. +
*
@
The secondary building unit (SBU) of faujasite and chabazite is a double six-membered ring (D6R)’ that is connected in different ways in both. The Al configuration in mined chabazite, CW12Si24072, was determined in a preceding paper: as shown in Figure 2. D6Rs contain six or three A1 atoms (abbreviated D6R-6 and D6R-3, respectively), and Ca ions are located on the surface of D6R-6 and at the center of D6R-3. These cation sites correspond to site-I’ and site-I in faujasite, respectively. The unit cell contains one D6R-6 and two D6R-3s and two residual Ca ions on four-membered rings connecting D6Rs. These D6Rs have a 3-fold symmetry axis and may be much more stable than asymmetric ones containing one, two, four, or five A1 atoms. It is conceivable and our adopted working hypothesis that stable D6Rs have the same geometry in faujasite and chabazite. This is quite different than the working hypothesis of Dempsey et aL5 Connecting four D6Rs as shown in Figure 3, one can construct five kinds of tertiary building unit (TBU), given in Table 1. The unit cell of faujasite is constructed by four TBUs, containing 16 D6Rs in total. Connecting TBUs of the same
0022-365419512099-10982$09.00/0 0 1995 American Chemical Society
Framework of Faujasite Type and a Chiral Y
J. Phys. Chem., Vol. 99, No! 27, 1995 10983
L)
RI + v)
C
I
TABLE 1. Five Kinds of Tertiary Building Unit (TBU) and Unit Cells Generated from Them unit cell generated from 4 TBUs of the same kind
1 2 3 4 5
D6R-6
D6R-3
symbol"
4 3 2 1 0
0 1 2 3 4
TBU(24A1) TBU(21Al) TBU(18AI) TBU(15Al) TBU(12AI)
common name [All X X Y Y high-silica Y
96 84 72 60 48
I
A
symmetry
24.8
cubicFd3trigonal R3b pseudo-P4_lb trigonal R3b cubic Fd3
The number in the parentheses denotes that of A1 atoms contained in TBU. See Appendix.
kind in a regular way, one obtains five kinds of faujasite, with symmetries given in Table 1. (Their details are given in the Appendix.) The faujasites thus constructed are named as milestone faujasites and labeled as in Table 1. As for nonmilestone faujasites of high crystallinities, it is assumed that only populations of D6R-6 and D6R-3 differ from those in milestone faujasites, and no D6R of unstable form is contained. If unstable D6Rs are contained, they are understood as structural defects. With decreasing or increasing Al content from a milestone value, a crystal becomes less stable and finally its symmetry changes to that of a neighbor milestone, with a boundary located midway between two milestones. The symmetries given in Table 1 manifest themselves only when TBUs are regularly connected, and they are realized exclusively in high-quality crystals carefully grown. The third milestone phase with pseudo-P41 (or pseudo-P43) chiral symmetry, especially, may require very careful work. In a usual synthesis, crystals with pseudo-P4, or P43 symmetries may appear with equal probability, or they might be twinned. If some chiral molecule is properly used as a template, a pure chiral crystal with a high crystallinity may be synthesized.
I I
I
Figure 3. Tertiary building unit (TE3U) of faujasite framework.
TBU
:I!! 1, I , I
0 0
no. of D6Rs contained
l l -
25.0t
t
48
I I
I
[,A,,
! ,
I
I
54 60
-Gaatoms Figure 4. Lattice constant
a0
, A
~
*
I
72
78
-
l l l
-
l l
-
I , ! ,
I
66
l
I
84
90
96
per unit cell
vs Ga content per unit cell, [Gal, in
gallosilicate faujasites, and phase boundaries theoretically predicted. Correlation factor of points are 0.994 and 0.995 for lines, in the descending order of [Gal.
25.0
t c, C
RI
24.9
I I I I
C
I I I I
8
I
I
!-
I
I I
I
11
I I
1
II
Lattice Constants and Al- or Ga-Contents of Faujasite Families Lattice constants, ao, of gallosilicate faujasites depend upon their Ga contents [Gal, as shown in Figure 4,6 which also contains marks of milestone and phase boundaries. The gaps in the observed curve and theoretical phase boundaries have a good correspondence. Phase boundaries evidently exist at [Gal = 66 and, with a high probability, at [Gal = 78. Samples in a region of [Gal < 54 contained amorphous silica. One interpretation is that crystals with [Gal < 54 belong to a different phase and grow at a very low rate, which indirectly suggests the existence of a phase boundary at [Gal = 54. It is generally accepted that the same gaps, though less pronounced, exist in aluminosilicate f a ~ j a s i t e s . ~ ~ Dempsey ~-* et al.'s data5 are reanalyzed and shown in Figure 5 . Points B and C in the figure contradict their theory, which locates a phase boundary at [All = 64 but fit the present one. The reverse is the case with point A in the figure. Breck and Flanigen'
-AI atoms per unit cell Figure 5. Lattice constant a0 vs aluminum content per unit cell, [All, in faujasites, and phase boundaries theoretically predicted. Correlation coefficients of points are 0.991, 0.988, and 0.997 for lines, in the descending order of [All.
concluded, however, that there are discontinuities at [All = 77 in curves of a0 and adsorbed amounts of triethylamine vs [All of CaX. It is concluded that the phase boundary is located at [All = 78 in a limit of experimental errors.
Rates of the Crystal Growth and AI Contents The rate of formation of T-0-Si bonds between a zeolite surface and visiting molecule is sensitively affected by a concentration and configuration of Al atoms in the surface framework, as mentioned before. Kacirek and Lechert measured
Takaishi
10984 J. Phys. Chem., Vol. 99, No. 27, 1995
I
phase
I
I
1
I
1
p41
R3
phase
phase
I
m
I I
.
(0
::
\
21
F
f
T
Figure 7. TBU(15AI) and site for La3+,and virtual TBU. (a, top) Names of La-site: filled circles, AI atom; hashed circles, La on site-1'. (b, bottom) Newly formed sodalite cage (shown by dotted lines) and 4 D6Rs (numbered as 1-4), constituting a virtual TJ3U.
I
II-I, -AI
I
,
I
1
I
I I
I
,
I
atoms per unit cell
Figure 6. Linear growth rates, k, of faujasite crystals vs [All: (a, top) log{W[SiO2],,1} vs [All; (b, bottom) activation energy in (W[si0&,1}
vs [All. linear growth rates, k, of zeolites X and Y with various Al contenkg Let [SiO2ls0l be the concentration of silica in a solution phase, and then log{kl[Si02],,1} is plotted against [All as shown in Figure 6a. Scatters of points in the figure are unavoidable owing to experimental difficulties, but points in the Fd3 and R? phases lie on lines with correlation coefficients 0.978 and 0.975, respectively. It is difficult to determine, with certainty, break points in the broken line, but one can say that Figure 6a is in good harmony with the present theory predicting break points at [All = 54 and 66. Kacirek and Lechert also determined the activation energy of the apparent growth rate, W[SiO2Is0~, which is plotted against [All in Figure 6b. The experimental points lie on a broken line with a break point at [All = 66, a phase boundary. This means that a change in the reaction mechanism occurs at this phase boundary, but only four experimental points are available, and further accumulation of experimental data is desirable. In Figure 6b, the activation energy evidently decreases with increasing [All, in accordance with the Barer's inference mentioned before but not in a simple manner. Measuring growth rates of each crystal face, as Fajula et al. did with zeolite-o,I0 and correlating them with respective surface Al configurations are tasks for the future.
Locations of La Ions in Fully Dehydrated Lax and LaY La3+ ion preferentially occupies a site surrounded by three Al atoms. If the present model is correct and all Al atoms form triads in six-rings, all monovalent cations in NaX or NaY can be replaced by La ions with a ratio of 3/1. Karge et al. carried out a solid-state ion exchange between (NH4)53.5Na1.3Al54.9Si137.70384 and Lac13 and attained almost 100% La exchange, i.e., nearly one La3+/three A1 atoms." Y54.9 is far from the milestone Ym and very near the phase boundary Y54;nevertheless, almost all Al atoms form triads in six-rings. This is strong support to the present model. There are three kinds of La-site surrounded by three Al atoms, as shown for TBU( 15A1) in Figure 7a. In the first place, let us calculate populations of La ions on each kind of site in Ya. An Al triad can occupy either the inner or outer six-rings of a D6R3, these states being denoted as D6R-34 and D6R-3t, respectively. The state of TBU( 15A1) in the figure is represented as i44. To realize a homogeneous charge distribution, D6R-34 and D6R-3t must appear randomly with a probability of '/*, while %fold axes o t D 6 R - 6 ~are always parallel to the c axis in Y w to maintain R3 symmetry. In Figure 7a, two La configurations are possible, i.e., 3.La/I La/I' and 4La/II (the site-I' on the outer surface of D6R-6 is not changed and is removed from consideration at the present stage), where La/I means La on site-I and other symbols have similar meanings. The former configuration is electrostatically less stable than the latter, as three dipoles, constructed from La3+ and Al-triad, direct outward as iii. The state &&i occurs with a probability of I/s. To calculate numbers of La site/unit cell, connections between TBUs must be taken into consideration. Tetrahedrally connected four TBUs form a sodalite cage in their center, as shown in Figure 7b. This newly formed cage and four D6Rs surrounding it constitute a virtual TBU as seen in Figure 7b. The name "virtual" TBU is given because all T-sites in it belong to
+
Framework of Faujasite Type and a Chiral Y
t
J. Phys. Chem., Vol. 99, No. 27, 1995 10985 7/8
x 2/8*La/II
C-axis
+ (1 - 7/8 x 2/8).La/I
while Al-triads on 0 sites does 2 x 7/8{‘/&&I 0
AI
+ (1 - ‘/,>.Ld’}
Then the La populations in La19Y57become [La/I] = 4 x 3 x [La’] =4 x
49/32
[Lam] = 4 x Figure 8. TBU in
Y57 which containing TBU(15AI) and TBU(12AI) in a ratio 3:l. 0, site occupied by A1 atom with a probability of ’/8.
Ga
1
= 6.125
7/8
= 3.5
Observed populations in La19Y57, on the other hand, areI2 [La/I] = 11.7,
[ L a ’ ] = 2.5,
[La/II] = 1.4
[Ld] =0 X84.
neighbor TBUs, and it is not a real but a conceptual TBU being introduced to describe connections between real TBUs. To realize a homogeneous charge distribution, the distribution of A1 atoms must be the same in both real and virtual TBUs. If an Al-triad in D6R-3 is in the state 4, it can participate in the formation of La/IIs in a real TBU with a probability of (1/2)3, as seen in Figure 7a, while, if t, it can do so in the virtual one. Thus the Al-triad in D6R-3 of Ym produces La-sites such as (1 - 2/8).La/I
+ 2/8*La/II
Each Al-triad in D6R-6, on the other hand, takes part in the formation of La/II, either in the real or virtual TBU, with a probability of ‘/8 and produces La sites such as
+
(1 - l/,).~a/~’ ‘/,.L~/II
[La/I] = 12 x (1 - 2/8) = 9
2/8
+4 x 2 x
V2) = 12
[La/II] = 4 x 4 x 2 x
= 16
Bennett and Smith determined the La distributions in La26.4Nc~dC82as13 [ L a ] = 5.2,
I/*
[La/I’] = 14.1,
[LaAI] = 6.3
with 25.6 La in total, which is slightly smaller than the analytical value 26.4 La. It is considered that 4.6 Na+ contained cannot be ion-exchanged owing to detects destroying Al-triads. There may be various kinds of detects, but here only one example is shown in Figure 9b. In the figure, one Al atom in a D6R-6 is misreplaced by Si atom, four La sites on I1 are destroyed, and the following change is introduced:
-
La/I
+ 2*La/I’ + 2*Na/other site
By introducing 2.3 (=4.6/2) defects of this kind into Xed, one has a crystal La25.7Na4.6X81.7and [La/I] = 2.3,
[La/I’] = 12
+ 2 x 2.3 = 16.6
[La11 = 16 - 4 x 2.3 = 6.8
[La/I’]=4x2x(1-’/8)=7
[Lam]= 12 x
[Lax] = 12 x 2 x (1 -
4*La/II
The unit cell of Ym contains 4 TBU(lSAl), Le., 12 D6R-3s and 4 D6R-6s, and La populations in it become
=4
where [ 3 denotes a number per unit cell. For comparisons with experimental results, we calculate numbers of La sites in Lal9Y57, which contains TBU(15Al) and TBU(12Al) in a ratio of 3:l. In Figure 8, we consider the A1 occupancy factor of sites marked by 0. The site are always occupied by Al-triads in TBU( 15A1), while with a probability of in TBU(12A1), and the averaged occupancy factor of A1 becomes 7/8 (=3/4 l/2 x An Al-triad in D6R-3 produces La sites:
+
= 9.375
with 15.6 La in total, which is considerably smaller than the analytical value 19 La, and the amount of structural defects is unknown. With these situations in view, it may be said that the theory well explains the experimental results. Next, another trigonal phase, around the second milestone x84, is investigated. The unit cell Of X84 contains 4 TBU(21Al)s, or 12 D6R-6s and 4 D6R-3s. The 3-fold axes of D6R-3s are always parallel to the c axis. An Al-triad in D6R-3 takes part in the formation of La/II either in a real or virtual TBUs, as seen in Figure 9; that is, La/I does not appear in this D6R-3. Then, the La populations in the unit cell of x84 become
C-axis
(a) Figure 9. TBU(21AI) in
25/32
Further sophisticated improvements are possible but may mean less at the present stage, and one must be contented with the agreement in the general tendency.
Discussion Can Table 1 and Figure 2 be extended into a region of much lower A1 contents or not? This is an interesting and practically important problem. The extension is done by assuming that an Al-free D6R (abbreviated as D6R-0) is more stable than those containing one or two A1 atoms, owing to its preferable symmetry, and that a stable high-silica Y contains only D6R-3
10986 J. Phys. Chem., Vol. 99, No. 27, 1995
Takaishi
EMT
b
I
6. AI occupanc
factor 1/2. 6% center of symmetry, located a t the center of D6R-6 0,
At
Figure 11. High-resolutionelectron micrograph of an edge of Y zeolite, supplied by 0. Terasaki and T. Oksuna.
W
I
I
I
I
I
I
I
I
Figure 10. Trigonal Y ~ with o R? symmetry, projected on (OOO1). Center of symmetry is located on the center of D6R shown in the insert.
and D6R-0. Then, there occur milestone compositions at [All = 36,24, 12, and 0 and phase boundaries at 42,30, 18, and 6. Symmetries of these phases are R3, pseudo-P41, R3, and Fd3, in the order of descending A1 contents, quite analogous to Table 1. At present, we do not have enough data to prove this expectation, and its proof is a future issue. Ultrastable Y (USY), used as fluidized cracking catalysts, has a composition in the concerned range, but it is prepared by dealumination, and the above model cannot be directly applied to USY. In the first place, the proposed phase diagram must be ascertained, and then comparisons between as-synthesized and dealuminated Y's, with the same A1 content, may afford some valuable information for their thermal stability. In such studies, a useful means is a titration of Al-triads by La ion exchange, which determines the number of Al-triads and population of unexchangeable cations by avoiding incomplete exchanges due to hydration of La3+.lo Unexchangeable cations stem from asymmetric D6Rs that are unstable and may become sources of collapse of the framework. The spectrum of 29Si MAS NMR of a faujasite is seriously affected by Al-misplacing defects. Direct comparisons between the theory and observations with poorly defined samples are pointless. Descriptions of details of calculations of spectra are postponed to the future, when meaningful comparisons become available.
Acknowledgment. The author thanks Dr. T. Ohsuna of Iwaki Meisei University and Dr. 0. Terasaki of Tohoku University for supply of photograph. Appendix Figure 10 shows connections of TBU(15Al)s that produce Ym with R3 symmetry. All 3-fold axes of D6R-6s in TBUs are parallel to the c axis of the crystal. In x84 constructed from
Figure 12.
0
:
= I-
: D6R-6
-}
: D6R-3
Y72 with
sodalite cage
pseudo-P41 symmetry.
TJ3U(21Al)s, the roles of D6R-3 and D6R-6 are interchanged, and a crystal of the same symmetry is obtained with the same array of TBU(21Al)s. The TEM photograph in Figure 11 shows that a surface edge of Y consists of an array of D6Rs. By constructing a model of the framework of Y72 with the morphology of Figure 1, one can show the following: On the base plane, the (0001) plane, only D6R-6s appear, while on the side faces only D6R-3s do. (The side faces are { 1li. 1) planes in the hexagonal axes and correspond to (111) planes in the cubic axes of the usual morphology of octahedron.) The area of the base plane depends
Framework of Faujasite Type and a Chiral Y Y
l
Y
J. Phys. Chem., Vol. 99, No. 27, 1995 10987
I
n
m
In Figure 12 of Y72, D6R-6s, and D6R-3s constitute a pattem of P41 symmetry, as do extraframework cations also. Strictly speaking, however, the Al configuration does not have P41 symmetry, since P41 is not a subgroup of Fd3. As adsorptive and catalytic properties of Y72 are determined by the configuration of extraframework cations, let us use the term "pseudoP41 symmetry" to represent these situations. Y72 with pseudoP43 symmetry is similarly obtained. Figure 13 shows other conceivable connections of TBU( 18Al)s, which are discarded for the following reasons: The first is the claim, given in the preceding section, that the real and virtual TBUs must be equivalent to ensure a homogeneous charge distribution. (In the figure, virtual TBUs are located at levels zlc = '/4 or 3/4.) The second is a generally accepted view that a framework structure with a higher symmetry, if possible, is more stable. Many authors have theoretically predicted the existence of zeolites with pores of chiral On the other hand, the pores in Y72 is of achiral geometry, and its chirality stems from the Al configuration in the framework and is of a new type. References and Notes
Iv
V
ly
&
/ I
/ I
VI Figure 13.
Y72 with
other symmetries.
upon the ratio (stacking rate of the sheets of D6Rds)/(stacking rate of the sheets of D6R-3s), R,. With increasing R,, the area decreases, and in the limit of R, >> 1, the base plane disappears and the usual morphology of octahedron is realized. The trigonal plate form of Figure 1 appears only under the condition of R, 1, that is, that the stacking of the array of D6R-6s proceeds, in the given solution, with a lower rate than that of D6R-3s.
(1) In Atlas of Zeolite Structure Types; Meier, W. M., Olson, D. H., Eds.; Butterworth-Heineman: London, 1992. (2) Edwards, G. C.; Vaughan, D. E. W.; Albers, E. W. U.S. Patent, 4,175,059, Nov 20, 1979. (3) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982; p 152. (4) Takaishi, T.; Kato, M. Zeolites, submitted. (5) Dempsey, E.; Kiihl, G. H.; Olson, D. H. J . Phys. Chem. 1969, 73, 387. (6) Kiihl, G. H. J . Inorg. Nucl. Chem. 1971, 33, 3261 (7) Breck, D. W.; Flanigen, E. M. In Molecular Sieves; SOC.Chem. Industry: London, 1968; p 47. (8) Smith, J. V. In Molecular Sieve Zeolites-I; Adv. Chem. Ser. No. 101; American Chemical Society: Washington, DC, 1971; p 171. (9) Kacirek, H.; Lechert, H. J . Phys. Chem. 1975, 79, 1589; 1976, 80, 1291. (IO) Fajula, F.; Nicolas, S.;Di Renzo, F.; Gueguen, C.; Figueras, F. In Zeolite Synthesis; Occelli, M. L., Robson, H. E., Eds.; ACS Symp. Ser.; American Chemical Society: Washington, DC, 1989; Chapter 34. (11) Karge, H. G.: Mavrodinavo, V.; Zhem, Z.; Beyer, H. K. In Guidelines for Mastering the Properties of Molecular Sieves, NATO AS1 Series; Barthomeuf, O., Derouane, E. G., Holderich, W., Eds.; Plenum Press: New York, 1990; p 157. (12) Bennett, J. M.; Smith, J. V. Mater. Res. Bull. 1967, 4, 7. (13) Bennett, J. M.; Smith, J. V. Mater. Res. Bull. 1967, 4, 77. (14) Treacy, M. M. J.; Newsam, J. M. Nature 1988, 332, 249. (15) Higgins, J. B.; LaPierre, R. B.; Schlenker, J. L.; Rohrman, A. C.; Wood, J. D.; Kerr, G. T.; Rohrbaugh, W. J. Zeolites 1988, 8, 446. (16) Anderson, M. W.; Terasaki, 0.;Ohsuna, T.; Philippou, A,; MacKay, S. P.; Ferreira, A.; Rocha, J.; Lidin, S. Nature 1994, 367, 347. (17) Akporiaye, D. E. J . Chem. SOC., Chem. Commun. 1994, 1771. (18) Davis, M. E.; Lobo, R. F. Chem. Mater. 1992, 4 , 756. JF'950625X
