387
LATTICEPARAMETER YARIATIOR'SIN SODIUM FAUJASITES Glossary of Symbols
Greek Symbols
B
11
C
D k k
(1) m"
M n
Q R (R
r (s )
t
T VB
X
z
Solubility parameter; eq 8 Mass fraction of solute Effective Fick diffusion coefficient Berthoud first-order rate constant Boltzmann constant Liquid Mass flux across solid-liquid interface RIolecular weight Number density (per unit volume) Heat of solution (per gram of solute) Particle radius R/&; normalized particle radius Radial coordinate Solid Time Absolute temperature Recession velocity of interface, = m " / p s Moles of water (of hydration) per mole of solute ( k R / D ) ( l - cBat); dimensionless rate constant; eq 10
p u T
Dimensionless local undersaturation; eq 9 Thermal conductivity of solution; eq 23 Density Surface free energy Dimensionless time; eq 7
Miscellaneous O( ) ,
Order of magnitude symbol
Subscripts chem Chemically controlled (interface kinetics) Diffusion controlled diff Pertaining to solute S Life (of dissolving particle) life At liquid-solid interface R' Saturated sat Evaluated at t = 0 0 m Far from particle surface
Variation of the Lattice Parameter with Aluminum Content in Synthetic Sodium Faujasites.
Evidence for Ordering of the Framework Ions
by E. Dempsey, G. H. Kuhl, and D. H. Olson Mobil Research and Development Corporation, Research Department, Central Research Divisioti, Princeton, N e w Jersey (Received August 8 , 1068)
OS5.$0
A plot of crystal lattice parameter against si1icon:aluminum ratio for a range of hydrated sodium X- and Y-zeolites shows two distiiict breaks a t silicoii: aluminum ratios near 1.4: 1 and 2 : 1. A similar plot for dehydrated calcium materials published earlier by Breck and Flanigeii shows similar breaks. It is suggested that the breaks are related to phase changes occurring in the materials as t h e rilicoii-aluminum ordering changes.
Introduction
Experimental Section
Although the framework structures of a t least 25 zeolites are known, information concerning the ordering of silicon and aluminum ions in the framework has been reported for only a handful of these structures. Such information is vital for the development of any detailed theory of crystallization or reaction mechanisms. At this time, no concrete experimental data have been presented pertaining to the ordering of silicon and aluminum ions in faujasite-type zeolites. During the course of an accurate determination of the variation of the cubic lattice parameter with aluminum content in hydrated sodium faujasites, we noted distinct discontinuities in what was expected t o be a continuous linear relationship. The experimental data and their possible implications concerning silicon-aluminum ordering is discussed.
All of the sodium synthetic faujasites used in this study were of high quality, as judged by their chemical analyses, X-ray scattering power, and sorption capacity for water and cyclohexane. The X-ray measurements were made on a Siemens X-ray diffractometer equipped with a scintillation counter, pulse-height analyzer, and strip-chart recorder. A scan speed of 0,25"/min was used with a 2-cm/min chart speed. Before the measurements were made, the samples were equilibrated for 16 hr in a 75% r.h. constant-humidity cabinet. The cubic lattice parameters, ao, were measured using the double scanning diff ractometry technique,l which minimizes 0 20 errors. I n most instances, four diffraction peaks in the 50-60" 20 (Cu) range were measured and the (1) H. W. King and L. F. (1962).
S'assamillet, Advan. X - R a y Arial., 5 , 78 Volume 73, h'umbsr 2 Februaru 1960
388
E. DEMPSEY, G. H. KUHL,AND D. H. OLSON
resulting values of a0 were averaged. The precision of the uo values is estimated to be =k0.005 A. Because extrapolation techniques were not employed, this value represents the relative accuracy of the a. values.
1.00
1.09 1.18
Re sult s
1.28
The lattice parameters found are summarized in Table I, along with the corresponding SiOa:A1203molar ratios and the number of A1 atoms per unit cell. Three lattice parameters larger than any included in the table were also found, but since there was a proportion of A zeolite admixed with the faujasite material in these samples, it was impossible to determine the number of A1 atoms per unit cell for the faujasite alone. The largest of the three lattice parameters was 25.132 8.
I .40
a I-
Table I: Lattice Parametera, Sorption Capacity, and Aluminum Content of Sodium Synthetic Faujasites [SiOrl/
c--Sorption,
g/100 g--
Ha0
cy010
1~1~0~1
2.45 2.47 2.56 2.73 2.81 3.02 3.18 3.35 3.55 3.74 3.85 3.95 4.34 4.52 4.86 5.32 5.45 5.54 5.59 5.83
29.9 30.1 29.6 31.0 28.6 31.6 29.3 30.7 32.4 31.5 31.9 30.4 30.3 33.0 29.4 29.5 27.8 29.0 29.4 28.9
c
I
/
/
- 2.69 -
No. of AI atoms/unit
cs
17.1 18.0 19.2 19.8 17.4 18.3 18.5 18.9 19.2 19.4 19.0 18.8 18.3 20.7 19.7 19.2 17.8 20.5 20.1 18.6
cell
ao
86.2 85.9 84.2 81.2 79.8 76.5 74.1 71.8 69.2 66.9 65.6 64.5 60.6 58.9 56.0 52.5 51.5 50.9 50.6 49.0
24.996 24.983 24.968 24.931 24.943 24.913 24.883 21.845 24.834 24.798 24.823 24.810 24,763 24.782 24.709 24.655 24,674 24 675 24.673 24.669 I
3.00
LATTICE PARAMETER, a,
8
Figure 1. The number of A1 atoms per unit cell and the Si:A1 ratio us. a0 for hydrated synthetic sodium faujasites (bars on the left-hand side denote theoretical model compositions).
unit cell, are in good agreement with the points of discontinuity in Figure 1. I n Figure 1, a t 52 AI atoms/unit cell, another discontinuity is observed. Since the probable lower limit of 48 A1 atoms/unit cell is being approached, it is conceivable that the samples below 52 A1 atoms/unit cell in Figure P contain amorphous silica; the small variation in the lattice parameter shown for the last four points of Figure 1suggests that an increasing amount of amorphous silica may occur in these samples as we go to lower Si: A1 ratios. A corresponding discontinuity does not occur in Figure 2.
Discussion I n Figure 1, the values for a. are plotted as a function of the number of AI atom! per unit cell. Since the lattice parameter of 25.132 A fits on this plot a t a S i G : A1203 molar ratio of very close to 2.0, it is included. Two, if not three, discontinuities are apparent in Figure 1. Two approximately parallel straight lines may be drawn in the ranges 96-80 and 66-52 A1 atoms/unit cell. At about 80 and 66 A1 atoms/unit cell, distinct breaks occur. The data points between these two compositions also form a straight line but of a somewhat different slope from the lines in the other regions. Breck and Flanigen2 found two discontinuities in a similar plot using dehydrated calcium zeolites having a faujasite structure. Their data are shown in Figure 2. The breaks, in the regions 79-76 and 64-60 A1 atoms/ The Journal of Physical Chemistry
The existence of a t least two distinct breaks in the plot of aluminum atoms per unit cell (equivalent to the number of sodium cations per unit cell) us. lattice parameter (Figure l) implies that a t least three different faujasite phases may be occurring across the range of Si :A1 variation, The approximate agreement between the results described above (relating to water-loaded sodium faujasites) and those shown in Figure 2 (relating to dehydrated calcium faujasites) suggests that the different phase regions do not result from variation in the cation distribution as the cationic content of the zeolites varies with the %:AI ratio. Rather, the breaks (2) D. W. Breclc arid E. M.Flanigen in “Molecular Sieves,” Society of Chemical Industry, London, p 47 1968.
389
LATTICE PARAMETER VARIATIONS IN SODIUM FAUJASITES 90-
AI u c.
I
’
70 -
000
J:
(0)
(b)
(C)
Figure 3. The arrangement of A1 and Si ions in faujasite six rings for: (a) Si:Al = 1:1 (X-zeolite, mela); (b) Si:Al = 2: 1 (Y-zeolite, meta form); ( e ) Si: A1 = 2: 1 (Y-zeolite, para form). 50
bp‘
24.7
248
24 9
250
LATTICE PARAMETER,a,
25 I
A
Figure 2. The relation between the cell constant, a ~and , the Si: A1 ratio of dehydrated forms of calcium-exchanged X- and Y-zeolites. The degree of calcium exchange is greater than 85 mol % for all zeolite samples. Dehydration carried out by heating in air at 400’ for 16 hra2 (From Breck and Flanigen, ref 2.)
in the plot must be connected with changes that occur in the Si02-A10z framework structure of the zeolites. Clearly if silicon and aluminum ions occur at random over the zeolite framework, the ideal space group Fd3m should apply to all faujasite materials regardless of the Si:Al ratio, and the plot of Figure 1 should show a single straight line having no breaks. The breaks must then correspond to transilions from one type of siliconaluminum ordering to another. In the past few years, calculations have been made of the electrostatic properties of ionic models of faujasite materials.* At the time the models were set up, the generally accepted view of faujasite structures held that the silicon and aluminum ions were disordered relative to each other. In order to make reliable calculations, however, it was necessary to set up odered models of the structures. This was done in a way that was consistent with the assumed ionic nat8ureof the materiaIs, and is described in ref 3. It turns out that the description of the zeolite models in ref 3 and the Suininary and Discussion of that paper contain the substance of an explanation of Figure 1; for the moment, this explanation mpst be considered tentative. Starting out from the idealized 1: 1 material in which the silicon and aluminum ions alternate regularly t,hroughout the structure (according to Lowenstein’s rule) we see that (with suitable and plausible positioning of the exchangeable cations) the structure can be divided into identical units having zero net charge and dipole moment. The smallest unit obeying these criteria is the double sodalite unit having a center of symmetry a t the center of the hexagonal prism linking the two sodalite units. We thus consider all fauiasitetype materials as being formally constructed of identical
centrosymmetric double sodalite units placed a t the points of a fcc lattice. If we now go from the 1 : l material to higher silicon :aluminum ratios, preserving our zero net charge and dipole moment criteria, we can set up faujasite models having the following Si:Al ratios: 1:1, 1,18:1, 1.4:1, 1.67:1, 2:1, 2.43:1, 3 : l . These ratios are marked on the left side of Figure 1. The 1: 1 material has a very high symmetry: the space group Fd3 is strictly applicable and all silicon and aluminum ion sites are equivalent to each other. All six rings have the a-meta form (see Figure 3a). (Hexagons in the sodalite units (formed of six -XO- pairs, where 11is A1 or Si) are called site-I or site-I1 six rings, depending upon whether they form part of a hexagonal prism or part of the supercage structure.) Going to the 1.18:l material involves replacing one A1 ion per sodalite cage by a Si ion, ensuring that the two changes made in a typical double sodalite cage are at positions that are inverse with respect to each other. Since all A1 ions are initially equivalent, it is immaterial which centrosymmetric pair of ions is exchanged. The important points are that this process generates two b-meta six rings per sodalite cage (Figure 3b) and that no subsequent rearranyenaent of the A1 and Si ions is possible. Taking the next step, exchanging two more A1 ions for Si ions at inverse positions with respect to the center of symmetry of the double sodalite cage to produce the 1.4:1 ratio, creates two more b-meta rings per sodalite cage. Sow we have to choose the position of the A1 ions more carefully, since none of the ions in a 1.18: 1 sodalite cage is equivalent to any other one. Clearly the second change, Al-.Si, has to be made as far as possible in the sodalite cage from the first one, and again it turns out that no subsequent rearrangement of the Si and A1 ions will lower the energy. Since all we have done in going from the 1: 1 through the 1.18: 1 to the 1.4: 1material is to change A1 ions to Si ions without further rearrangement, we can say that up to and illcluding the 1.4: 1 material the order is typical of the 1:1 structure. It may be appropriate to reserve the designation X-zeolite for materials obeying this criterion. (3) E. Deinpsey in “Molecular Industry, London, 1968, p 293.
Sieves,” Society of Clieinic:11
390 Making two more Al-tSi changes in our unit, at inwrse points, produces the 1.67: 1 material and two more b-nzeta rings per sodalite cage. Having made the change at the energetically most favorable position, however, we find that now a rearrangement of the Si and A1 ions will diminish the electrostatic energy. (For practical reasons changes in repulsive energy have not been calculated, but the changes in repulsive energy should operate with, rather than against, the electrostatic energy changes.) The result of the redistribution of Si and A1 ions is to create six rings of the para type (Figure 3c). Now we have moved away from the typical 1: 1 order. The limit for the pure 1 : 1 order type, characterized by having all site-I and -11 six rings of the meia forms and designated Im, IIm, is the 1.4: 1 ratio. It is just at this point that Figure 1 shows its first break, point A. The “mixed” quality of the 1.67:1 material may be illustrated by the designation Im, IImp, site-I1 six rings having both the meta and para forms. The next step, to the 2 : 1 material, permits a further rearrangement, after the two A 1 4 3 changes have been made, with the creation of two more pai*a rings. Now we have the situation (see the Disussion in ref 3 and Table I of that paper) in which all site-I six rings are meta and all site-I1 six rings are para, designated Im, IIp. Furthermore (see Table IV of ref 3), all site 11’s appear to be equivalent. This suggests that at the 2 : 1 composition a new symmetrical structure appears, which we have designated Y. The region between Si:AI > 1.4:l and Si:Al < 2 : l appears to have the character of a transition region between the X and Y phases. The second break in Figure 1 occurs close to the 2: 1 Si:Al point but riot quite a t it. It is difficult to speculate about this since the models used have only the discrete &:A1 molar ratios indicated above and in Figure 1. It may be that materials near the end of the transition range in Figure 1 anticipate the structure existing at and beyond Si :A1 = 2 : 1. In some support of this, it should be observed that the material of point
The Journal of Physical Chemistry
E, DEMPSEY, G. H. KUHL,AND D. H. OLSON B has a composition that may be represented as being 2: 1 with one in every five sodalite cages containing an extra A1 ion. A detailed analysis of the situation for the 2.43 :1 and 3: 1 materials has not been carried out. An examination of the models s h o ~ s however, , that for the 2 : 1 material two diametrically opposing square faces of every sodalite cage carry two -41 ions (the remainder carry only one each). Clearly, to achieve the optimum reduction in electrostatic energy on going to Si:Al = 2.43: 1, an A1 ion on one of these squares has to be replaced by a silicon ion, and then, to reach the 3 : 1 ratio, an A1 ion has to be replaced in the remaining square carrying two A1 ions. After making each of these changes, it appears that further reduction of energy cannot be achieved by rearranging the Si and A1 ions. This is equivalent to saying that the 2.43:1 arid 3: 1 materials properly belong bo the same phase region as the 2: 1 material and that neither defines a ncw phase. At this stage, it) seems pointless to speculate on the apparent discontinuity at D in Figure 1, beyond the comments made in the Results section; the model discussed has no explanation of the discontinuity. In summary, the form of Figures 1 and 2 implies that in zeolites having the faujasite structure the silicon and aluminum ions are ordered rather than disordered. The existence of the breaks in the figures is probably related to changes in the nature of the silicon-aluminum ordering as the &:A1 ratio is varied; the position of the breaks lends support to the particular ordered models described earlier.3 The fact that the section of Figure 1 between points A and B has a greater slope than the other two sections, which are approximately parallel, may be evidence for the proposed ‘%ransition”nature of zeolites in this range. Aclcnozcledgment. We wish to thank G. T. Kerr and G. R. Landolt for supplying us with some of the zeolite samples used in this study.
