J. Phys. Chem. 1987, 91, 5800-5805
5800
chain except in the vicinity of the defects, and this seems a more likely possibility in polyazine. This leads to two midgap states, and in the case of oxidation, if the lower midgap state is singly occupied, a polaron is formed while if the lower midgap state is empty, a bipolaron is created. When two defects form on a polyazine chain, several possible geometries exist: the defects can both be on carbon atoms, they can both be on nitrogen atoms, or one defect can be on a carbon atom while the other is on a nitrogen atom. Each of these possibilities was considered, and the results were similar in all cases. Two midgap states were created from the valence and conduction band edges, leading to an increase in the band gap. In each case, the wave functions describing the midgap states were quite delocalized, generally encompassing about four monomer units.
Conductivity in these materials would m a t likely occur by some sort of hopping mechanism. Motion of a charge for any of the kinds of defect structures studied here would require substantial geometric changes-implying moderately large activation energies. Whether the defect is a domain wall, a polaron, or a bipolaron, the motion of the charge can be envisioned in terms of simple, a-bond shifts along the conjugated chain. This leads to a number of possible transition states for charge transport and thus could be expected to give a complicated conductivity mechanism.
Acknowledgment. Acknowledgment is made to Research Corporation for partial support of this work. Registry No. Polyazine, 85772-00-5.
Influence of Temperature on the Cation Distribution in Calcium Mordenite J. Elsen, G. S. D. King,* Laboratorium voor Kristallografie, Katholieke Universiteit Leuven, B-3030 Heverlee, Belgium
and W. J. Mortier Laboratorium voor Oppervlakte-scheikunde, Katholieke Universiteit Leuuen. B-3030 Heverlee, Belgium (Received: March 24, 1987)
The crystal structure of calcium mordenite has been studied at 150, 300,450, and 20 OC by single-crystal X-ray diffraction methods. The cation distribution changes with temperature although the framework is not affected. One calcium ion, in the small channel, is bonded to six framework oxygens and to two water molecules. This ion does not move on heating, but the water occupancy is reduced to that of the calcium ion. The second calcium ion, in the large channel, is bonded at room temperature to seven water molecules. As the temperature rises, water is lost and the cation becomes distributed over two pairs of sites, in both of which it is bonded to framework oxygens of the wall of the large channel and up to 150 OC also to water molecules. The site in the small channel has the best coordination for calcium, and its occupancy increases as the temperature rises.
Introduction Cations in zeolites act as Lewis acids. The strength of their interaction with adsorbed molecules depends on their nature and on their coordination. For instance, damage caused by removal of tetrahedral aluminium from the framework gives rise to the formation of strong Lewis acid sites.’ Previous studies2” have shown that the cation location depends principally on the temperature but also on the presence of adsorbed molecules. The present study is part of an attempt to relate the change in the cation location to temperature. Such information may then also apply to other catalytically more interesting systems for which no crystals can be obtained. A general rule proposed for Ca-Y zeolitesZ states that the occupancy of the “better coordinated” sites increases with temperature. These are site I for dehydrated zeolites, site I’ in the presence of water or ammonia, and site I1 in the presence of molecules that cannot penetrate the small cavities. These effects have also been observed to a lesser extent for hydrated Na-Y zeolites3 A comparative study of benzene adsorption on K-Y, Ca-Y, and Sr-Y has shown that these effects are strongly in(1) Freude, D.; Fr6hlich, T.; Hunger, M.; Pfeifer, H.; Scheler, G . Chem. Phys. Left.1983, 98, 263. Freude, D.; Frahlich, T.; Pfeifer, H.: Scheler, G. Zeolites 1983, 3, 171. (2) Dendooven, E.; Mortier, W. J.; Uytterhoeven, J. B. J . Phys. Chem. 1984,88, 1916. (3) Mortier, W. J.; Van Den Bossche, E.; Uytterhoeven, J. B. Zeolites 1984, 4, 41. (4) Van Dun, J. J. I.; Mortier, W. J.; Uytterhoeven, J. B. Zeolites 1985, 5, 257. (5) Mortier, W. J. J . Phys. Chem. 1977, 81, 1334. (6) Mortier, W. J.; Pluth, J. J.: Smith, J. V. Mater. Res. Bull. 1975, 10, 1037.
fluenced by the cation-benzene interaction energy; the effect could be clearly demonstrated only for Ca-Y zeolites4 The effect of temperature on the cation distribution in zeolites is the opposite to what would be expected for a Boltzmann distribution.’ The cations should become more evenly distributed as the temperature increases. Although this is true for the distribution of ferrous iron and magnesium in orthopyroxenes, it is not so for zeolites, since any redistribution of the cations among sites causes a change in the energy levels of these sites. In orthopyroxenes, with all sites occupied, exchange can take place only among occupied sites. The Boltzmann formalism is therefore not valid for zeolites. The effect of temperature on the mordenite structure has previously been investigated for dehydrated calcium m ~ r d e n i t e , ~ where the occupancy of the better coordinated site was shown to increase with temperature. In the present study, the influence of temperature on the cation location in hydrated calcium mordenite has been investigated by single-crystal X-ray diffraction techniques. Structure Determinations Single crystals of mordenite from Challis Valley, ID, were used. Because of the similar scattering factors of sodium and water, crystals were immersed for 2 months in a 1 M CaC12solution to exchange all sodium. An electron microprobe analysis of a crystal of the same preparation set6 gave the composition Cao42Al0 98SiS,03012.xH20. A single crystal of dimensions 0.35 X 0.12 X 0.10 mm was sealed in a silica capillary containing a drop of water. This crystal (7) Mortier, W . J. J . Phys. Chem. 1975, 79, 1447.
0022-3654/87/2091-5800$01.50/0
0 1987 American Chemical Society
Cation Distribution in Calcium Mordenite
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5801
TABLE I: Cell Parameters a, 8,
b, A c, 8,
20 "C (B)
150 OC (A)
18.091 (4) 20.418 (4) 7.508 ( I )
17.964 (5) 20.273 (7) 7.467 (3)
TABLE II: Experimental and Refinement Data' 300 "C (A) 17.955 (7) 20.295 (9) 7.468 (4)
450 "C (A) 17.979 (7) 20.229 (9) 7.456 (4)
reflectns measured reflectns obsd [I > 2a(I)J R (all reflectns) R (obsd only)
20 "C
150 O C
300 "C
450 "C
1373 913 0.101 0.069
1346 712 0.132 0.070
1346 636 0.173 0.089
1342 610 0.176 0.089
Tb
a Figure 1. c-axis projection of calcium mordenite at room temperature.
was used for all measurements at elevated temperature. A second crystal was used for the data collection at room temperture. Intensity data were collected a t 150, 300, and 450 "C (crystal A) and at 20 O C (crystal B), on a Syntex P21diffractometer using Nb-filtered Mo K a radiation (A = 0.71073 A). At each temperature, a half-sphere of reciprocal space out to (sin O)/A = 0.6 was measured by the w-scan technique with a scan width of l.Oo. The crystal was shown to be orthorhombic by means of Weissenberg photographs. Systematic absences for hkl with h k = 2n + 1 and for hOl with I = 2n + 1 agree with the space group Cmcm as assumed by Meier for mordenite.* For data collection at higher temperatures, a furnace was fixed to the X-circle of the diffractometer. The furnace consists of a U-shaped platinum heating element with a built-in ChromelAlumel thermocouple and is surrounded by a KAPTON film.g Calibration against the melting points of A g N 0 3 at 212 OC, KC103 at 350 OC, B a N 0 3 at 592 OC, and KI at 680 OC showed that the temperature can be controlled to within about 20 OC, regardless of x, and about 10 OC for a constant x value. Unit cell parameters, obtained by the refinement of 29 values of 24 reflections, are listed in Table I. After application of an empirical absorption correction,1° intensities of symmetry-equivalent reflections were averaged to give a unique set of independent reflections to which Lorentz and polarization factors were applied. Reflections with intensities greater than twice their standard deviation (based on counting statistics) were considered as observed (Table 11). Atomic scattering factors for Ca2+, for 0-, and for the T atoms were calculated according to the analytical approximation of Cromer and Mann.l' A weighted scattering factor, 0.16fA13+ 0.84fsi4+ was used for the T atoms. The parameters of the framework atoms of dehydrated calcium mordenite6 were used as starting values for the structure determination, which was carried out by alternate
a
___j
Figure 2. c-axis projection of calcium mordenite at 150 "C.
+
+
(8) Meier, W. M. Z . Kristallogr. 1961,115, 439. (9)Brown, G.E.;Sueno, S.; Prewitt, C. T.Am. Mineral. 1973,58,698. (1 0 ) SYNTEX XTL Operations Manual; Syntex Analytical Instruments: Cupertino, CA, 1976. (1 1) Cromer, D.T.;Mann, J. B. Acta Crystallogr., Sect. A: Cryst. Phys., Dvfr ., Theor. Gen. Crystallogr. 1968,A24, 321.
T' l
a
Figure 3. c-axis projection of calcium mordenite at 300 OC.
structure-factor and electron-density calculations (X-RAY 76). l 2 After the cations and the water molecules had been located, several anisotropic full-matrix least-squares refinement cycles gave the final R values listed in Table 11. The sites are alphabetically named from the center of the small channel to the large channel in the [OOl] direction (Mortier).I3 The positional, population, (12)Stewart, J. M.; Machin, P. A.; Dickinson, C. W.; Ammon, H. L.; Heck, H.; Flack, H. "The X-ray System"; Technical Report TR-446;Computer Science Center, University of Maryland: College Park, MD, 1976.
5802
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
Elsen et al.
TABLE III: Atomic Coordinates (XlO') with esd's atom
X
1984 (1) 3035 (2) 869 (2) 4139 (2) 1241 (3) 3778 (3) 2628 (3) 936 (5) 3319 (5) 1780 (5) 2309 (4) 2500
0 5000 0 5000 0 5000 3769 (8) 4651 (28) 1988 (2) 3040 (2) 866 (2) 862 (2) 1241 (4) 3765 (3) 2628 (4) 939 (5) 3306 (6) 1780 (6) 2301 (6) 2500 0 5000 0 5000 3331 (28)
0 5000 4171 (26)
Y
Z
At 20 'C 4272 (1) 3090 (1) 3831 (2) 2736 (2) 4149 (3) 3056 (3) 3774 (3) 3048 (4) 3035 (5) 4195 (4) 5000 2500 4053 (7) 2945 (5) 5000 4784 (5) 4325 (10) 3490 (17) 5308 (9) 4044 (10)
5421 (3) 4546 (3) 2500 7500 4312 (9) 5799 (8) 4867 (10) 2500 2500 7500 5000 5000 2500 7500 5000 2500 7500 2500 7500 9318 (36)
At 150 OC 4275 (1) 3089 (2) 3840 (2) 2263 (2) 4185 (4) 3032 (4) 3770 (3) 3022 (5) 3044 (6) 4212 (6) 5000 2500 4084 (9) 2955 (6) 5000 3779 (15) 4570 (23) 4323 (10) 3493 (43) 4548 (16)
5418 (4) 4544 (3) 2500 2500 4305 (10) 5752 (9) 4882 (12) 2500 2500 7500 5000 5000 2500 7500 5000 4949 (45) 2500 7500 2500 5066 (99)
populn parameter
0.38 (1) 0.42 (2) 0.74 (3) 1.42 (7) 0.95 (4) 0.77 (3)
0.44 (1) 0.12 (2) 0.06 (1) 0.82 (4) 0.89 (4) 0.52 (3)
atom
X
T(l) T(2) T(3) T(4) O(1) O(2) O(3) O(4) O(5) O(6) O(7) 0(8) O(9) O(10) Ca(N Ca(D') Ca(E) Wa(B)
1984 (2) 3036 (2) 871 (3) 4145 (3) 1228 (5) 3766 (4) 2625 (5) 929 (6) 3283 (7) 1743 (8) 2299 (6) 2500 0 5000 0 5000 3321 (34) 0
T(l) T(2) T(3) T(4) 0(1) O(2) O(3) o(4) O(5) O(6) O(7) O(8) O(9) O(10) Caw) Ca(D') Ca(E) Wa(B)
1988 (2) 3034 (2) 865 (3) 4149 (4) 1234 (5) 3775 (4) 2629 (5) 922 (6) 3291 (7) 1762 (7) 2297 (6) 2500 0 5000 0 5000 3214 (32) 0
Z
populn Darameter
At 300 "C 4270 (1) 3094 (2) 3830 (2) 2735 (2) 4167 (4) 3065 (4) 3774 (4) 3067 (6) 3061 (9) 4200 (6) 5000 2500 4103 (8) 2935 (10) 5000 3500 (23) 4560 (27) 4314 (12)
5421 (5) 4544 (4) 2500 7500 4282 (12) 5749 (10) 4872 (16) 2500 2500 7500 5000 5000 2500 7500 5000 4162 (1 10) 2500 7500
0.48 (2) 0.13 (2) 0.04 (1) 0.93 (4)
At 450 OC 4268 (1) 3096 (2) 3830 (2) 2743 (2) 4155 (4) 3056 (5) 3777 (4) 3046 (6) 3078 (8) 4188 (6) 5000 2500 4114 (8) 2978 (10) 5000 3319 (30) 4531 (25) 4242 (16)
5421 (4) 4555 (4) 2500 7500 4295 (12) 5741 (11) 4870 (14) 2500 2500 7500 5000 5000 2500 7500 5000 3570 (100) 2500 7500
0.50 (4) 0.12 (4) 0.06 (2) 0.42 (3)
V
.,
h
and thermal parameters are given in Tables I11 and IV and the interatomic distances and bond angles in Tables V and VI.
n
Discussion Variations in the framework dimensions are negligible. Figures 1, 2, 3, and 4 are STRUPL084 drawings14 of the structures at 20, 150, 300, and 450 OC, respectively. The total cation occupancy agrees well with the chemical analysis. Site A in the center of a puckered ring of eight TO, tetrahedra has the highest occupancy in both the hydrated and the dehydrated states. At 20 O C , the occupancy of site A is higher than in the dehydrated state, due to the extra stabilization by water molecules. This occupancy increases steadily from 0.38 at room temperature to 0.50 at 450 "C (Table 111). The maximum occupancy of 0.5 is consistent with alternate occupied and empty sites in the c direction. A full occupancy would mean an unreasonably strong interaction between cations 1.87 8, apart. This site A is coordinated to six framework oxygens and to two water molecules at site B. Up to 300 OC the water/calcium ratio is 2/1 but decreases to less than unity at 450 O C . From 300 OC, B is the only site with a water molecule. The cation and water distributions in the large cavity are more complicated. In the hydrated state, as already found,I5 half the calcium ions are located in the center of the large channel where ( 1 3) Mortier, W. J. Compilation of Extra Framework Sites in Zeolites; Butterworth: Guildford, Surrey, U.K., 1982; p 54. (14) Fischer, R. X . J . Appl. Crystallogr. 1985, 18, 258. (15) Mortier, W. J.; Pluth, J. J.; Smith, J. V.Mater. Res. Bull. 1976, 1 1 , 15.
I' a
Figure 4. c-axis projection of calcium mordenite at 450 O C .
they are coordinated only to water (Figure 1). The water molecules are located at the corners of a distorted pentagonal bipyramid. The occupancies of Wa(E), Wa(D), Wa(G), and Ca(E)
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5803
Cation Distribution in Calcium Mordenite TABLE Iv: Temperature Factors (Xlo'), with d ' s " atom
u22
4 3
u12
At 20 OC 15 (1) 21 (1) 18 (2) 23 (2) 29 (4) 35 (3) 64 (6) 103 (8) 29 (5) 21 ( 5 ) 44 (6) 94 (10) 61 (11)
u23
u13
-1 (1) -1 (1)
-3 (1) 3 (1)
0
0
0
0
-14 (4) -10 (3) 19 ( 5 )
-7 (4) 3 (4) -10 (4) 0 0
0 0 0 0
0
6 (4) 44 (7)
-15 (8) 0
0 0
0
50 (8) 21 ( 5 )
0 0
126 (14) 35 (11) 1692 (181) 654 (81) 347 (31)
-9 ( 5 ) 0 0 0 0
0 0 0
244 ( 5 8 )
At 150 OC 21 (1) 22 (1) 25 (2) 30 (3) 37 ( 5 ) 46 ( 5 ) 72 (7) 113 (9) 37 (6) 26 (6)
-30 (22) 0
-2 (2) -3 (1)
2 (2)
0
0
0
0
-10 ( 5 ) -16 (4) -1 (6)
-1 (6) 13 ( 5 ) -2 ( 5 )
0
0 0
0 0 0
50 (8)
95 (12) 60 (12) 73 (11) 40 (11)
0
11 (6) 29 (9)
-7 (9) 0 0 0
91 (22) 1685 (202) 534 (223)
0 0
-20 ( 5 ) 0
0
118 (111)
-117 (74)
0
At 300 OC 24 (2) 26 (2) 30 (3) 28 (3) 53 (7) 46 (6)
0
-5 (2)
-1 (2) 0 (2)
-4 (2) 0
80 ( 9 )
123 (11) 37 (8) 36 (8) 66 (10) 117 (15) 57 (13) 70 (13) 68 (13) 500 (13) 119 (26)
0
0 0
-25 (6) -14 (6) 1 (7)
-4 (6) 18 (7) -17 (6)
0 0 0 0 8 (13) 0 0 0 0 0
At 450 'C 26 (2) 30 (2) 33 (3) 31 (3) 58 (7) 56 (6) 78 (8) 149 (13) 37 (7) 35 (8) 84 (1 1) 140 (15) 82 (16) 75 (14) 90 (14)
0 0 0
4 (6) 33 (9) 0 0
-25 (7) 250 (7) 0
-7 (2) -4 (2)
-1 (2)
2 (2)
0
0 0
-34 (6) -16 ( 5 ) 3 (7)
-2 (6) 24 (7) -21 (6)
0
0
0 0 0 0
0 0 5 (7)
-3 (13)
32 (10) 0
0
0 0
76 (36)
0 -33 (7)
0
0
+
"The temperature factor is e~p[-2n~(h*a*~u,, + k2b*2u22+ Pc*~u,,+ 2hka*b*uI2+ 2hla*c*u13 2kWc*u2,)]. *Isotropic temperature factor
are such that some water molecules cannot be bound to calcium ions. None of the water molecules of the hydrate complex is closer
than 3.1 A to the framework. This hydrate complex no longer exists at 150 O C (Figure 2). The calcium ions migrate partly
5804 The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 TABLE V Interatomic Distances (A) 20 o c 150 OC 1.601 (6) 1.589 (8) T(1)-0(1) 1.590 (8) 1.602 (6) T(1 )-0(3) 1.604 (4) 1.612 (3) T(1)-0(6) 1.606 (4) 1.629 (3) ~(1)-0(7) 1.589 (7) 1.642 (6) T(2)-0(2) 1.586 (7) 1.596 (6) ~(2)-0(3) 1.622 (4) 1.602 (4) T(2)-0(5) 1.576 (3) 1.583 (2) T(2)-0(8) 1.661 (8) 1.651 (7) ~(3)-0(1) 1.662 (10) 1.604 (8) T(3)-0(4) 1.633 (7) 1.637 (5) T(3)-0(9) 1.610 (5) T(4)-0( 10) 1.615 (4) 1.546 (10) 1.605 (9) T(4)-0(4) 1.584 (7) 1.577 (6) T(4)-0(2) Ca(A)-O( 1) 2.887 (6) 2.823 (8) Ca(A)-O(9) 2.695 (10) 2.633 (12) Ca(A)-Wa(B) 2.328 (12) 2.317 (12) Ca(A)-Ca(A) 3.750 (2) 3.733 (2) Ca(E)-O(3) 2.718 (37) Ca(E)-O(5) 3.093 (48) Ca(E)-O(7) 2.769 (37) 2.440 (68) Ca(E)-Wa(E') 2.753 (19) Ca(D')-O(2) 2.535 (33) Ca(D')-O( 10) 2.157 (30) Ca(D')-Wa(E') 1.918 (43) Ca(D')-Wa(D) 3.657 (47) Ca(D')-Ca( D') Ca(F)-Wa(D) 2.641 (36) Ca( F)-Wa (G) 2.590 (28) Ca(F)-Wa(E) 2.235 (15) 1.835 (12) Wa(E')-Wa(E')
300 OC 1.616 (9) 1.583 (9) 1.618 (5) 1.617 (5) 1.592 (9) 1.584 (9) 1.591 (5) 1.580 (4) 1.553 (12) 1.659 (8) 1.628 (9) 1.588 (8) 1.632 (12) 1.619 (9) 2.829 (8) 2.608 (12) 2.329 (14) 3.734 (2) 2.691 (44) 3.042 (58) 2.766 (45)
450 OC 1.610 (10) 1.577 (9) 1.610 (5) 1.612 (5) 1.601 (9) 1.575 (9) 1.602 (5) 1.576 (4) 1.632 (9) 1.591 (13) 1.657 (8) 1.601 (9) 1.601 (13) 1.605 (9) 2.850 (9) 2.586 (12) 2.413 (21) 3.728 ( 2 ) 2.560 (39) 2.943 (52) 2.663 (40)
2.663 (42) 2.745 (80)
2.785 (46) 3.010 (74)
2.482 (120) 1.596 (105)
WalDl
Figure 5. (a) Coordination of Ca(E) at 150 "C. (b) Coordination of Ca(D') at 150 OC.
TABLE VI: Bond Angles (deg) O( 1)-T( 1)-0(3) O(l)-T(1)-0(6) O( I)-T( 1)-0(7) O(3)-T( 1)-0(6) O(3I-V 1)-0(7) O(6)-T( 1)-0(7) 0(2)-T(2)-0(3) 0(2)-~(2)-0(5) 0(2)-T(2)-0(8) 0(3)-T(2)-0(5) O( 3)-T( 2)-O( 8) 0(5)-T(2)-0(8) O( 1)-T(3)-0(4) O( 1)-T(3)-0( 1) 0(4)-T(3)-0(9) 0(9)-T(3)-0( 1) 0(2)-T(4)-0( 10) 0(2)-T(4)-0(4) O(2)-T(4)-0( 2) O( 10)-T(4)-0( 4) T( 1)-O( l)-T(3) T(2)-0(2)-T(4) T ( 1)-0(3)-T(2) T ( 3)-0(4)-T(4) T ( 2)-O( 5)-T( 2) T ( 1)-O(6)-T( 1) T( 1)-O(7)-T( 1) T(2)-0(8)-T( 2) T( 3)-0(9)-T(3) T(4)-0( 10)-T(4)
Elsen et al.
20 OC
150 OC
300 OC
450 OC
112.1 (3) 107.2 (4) 110.2 (3) 110.9 (4) 105.5 (3) 111.1 (3) 109.2 (3) 106.4 (4) 110.2 (2) 109.5 (5) 110.5 (2) 111.1 (4) 111.2 (3) 111.0 (3) 110.4 (6) 106.4 (3) 106.9 (3) 112.2 (3) 108.2 (3) 110.1 (5) 146.4 (4) 145.2 (4) 158.5 (4) 170.9 (6) 142.6 (7) 151.1 (6) 137.8 (5) 180.0 (14) 147.8 (9) 149.4 (7)
113.9 (4) 107.6 (5) 107.4 (4) 111.2 (5) 106.7 (4) 110.0 (4) 110.9 (4) 107.0 (4) 109.0 (3) 109.9 (6) 109.7 (3) 110.3 (5) 112.8 (4) 105.0 (4) 108.4 (5) 112.2 (7) 109.7 (4) 111.1 (6) 107.7 (3) 111.0 (4) 144.6 (6) 147.4 (5) 159.6 (5) 170.3 (7) 144.6 (7) 151.6 (8) 138.9 (7) 180.0 (8) 144.7 (12) 148.2 (8)
113.1 (5) 105.6 (6) 108.1 (4) 112.8 (6) 106.1 (5) 110.2 (5) 109.2 (5) 108.1 (6) 110.6 (4) 108.3 (8) 110.3 (4) 110.2 (6) 113.1 (4) 103.3 (5) 109.7 (5) 113.4 (7) 107.5 (5) 112.3 (4) 107.8 (5) 109.4 (9) 144.6 (6) 144.8 (6) 158.9 (6) 171.5 (8) 147.3 (9) 147.3 (9) 139.0 (7) 180.9 (11) 140.9 (11) 145.4 (5)
113.0 (5) 106.0 (6) 108.6 (5) 111.8 (6) 106.0 (5) 111.5 (4) 110.2 (5) 106.6 (5) 110.6 (4) 107.2 (7) 110.9 (4) 111.1 (5) 112.0 (4) 110.2 (5) 114.0 (8) 104.1 (5) 106.5 (5) 111.1 (4) 109.6 (5) 111.9 (9) 145.9 (6) 145.1 (6) 158.0 (6) 171.7 (9) 146.2 (9) 148.5 (9) 139.7 (7) 180.0 (17) 139.5 (11) 145.5 (14)
~
toward the sites D', located asymmetrically in the ring of eight TO4 tetrahedra adjacent to the large channel and bound to three oxygens (D is in the center of this ring), and partly toward site E by the wall of the large channel defined by the elongated ring of six TO4tetrahedra, where it is coordinated to four of the six oxygens. At 300 and 450 "C the Ca(E) ion is coordinated only to these framework oxygens. The calcium at position E is least affected but moves significantly closer to the framework oxygens as the temperature rises. This may be due to residual water molecules too few in number to show up in the electron-density
map, their amount decreasing continuously with rising temperature. The Ca(E) ions are still coordinated to four water molecules at 150 "C (Figure sa). These water molecules are close to the Wa(E) positions in the fully hydrated state but have moved away from the framework in the (101) directions in order to allow the incorporation of a calcium ion. At 150 OC, the Ca(D') coordination resembles a distorted octahedron formed by three water molecules [two Wa(E') and one Wa(D)] and three framework oxygens [two O(2) and one O(10); Figure 5b]. Because of the difficulty of estimating the water occupancies which are strongly correlated with the temperature factors in the least-squares refinement, it is not certain that all adsorbed molecules at this temperature are associated with cations. The large temperature-factor values may be due to disorder of the water molecule positions. Their occupancy, however, is such that not all the water molecules can be associated with cations. All these water molecules are removed by 300 "C when the Ca(D') has migrated closer to the framework oxygens. Unlike the Ca(E) cation, Ca(D') is further displaced from the framework oxygens at 450 OC, while in the dehydrated state, its population is reduced by half (Table 111). This is also partly true for the site E occupancy, suggesting that the water atmosphere significantly influences the calcium ion location, despite the fact that no water molecules could be located. The number of water molecules located decreases gradually. The water molecule occupancy associated with the cations increases steadily until, at 450 OC, all water molecules are associated with cations at well-defined sites. The occupancy of the site D' and site E cations is such that the influence of the water atmosphere is still apparent. This situation is similar to that in hydrated Ca-Y zeolite,2 where the occupancy within the sodalite cage increases with temperature up to 250 O C . At higher temperatures the octahedral site I is preferred, although some water molecules remain in the cage. As no such framework site is available in mordenite, site A is the best coordinated. Its occupancy increases as the temperature rises. Galli et al. in a comparative study of potassium-exchanged heulandite at 20, 100, and 320 OC16 found that dehydration takes (16) Galli, E.; Gottardi, G.; Mayer, H.; Preisinger, A,; Passaglia, E. Acta Crystallogr., Sect. B: Srruct. Sci. 1983, 839, 189.
J . Phys. Chem. 1987, 91, 5805-5807
5805
on the framework. At higher temperatures the channels in
place in two steps and is accompanied by a deformation of the silicate framework. At 320 O C dehydration was complete, but the experimental conditions were different from those used here (their crystal was exposed to the atmosphere). In heulandite, there are three potassium positions at room temperature. Two of these potassium atoms, in the largest channel (A), remain in position as the temperature rises, while water is lost. The third migrates to the center of a smaller channel (B) as the temperature rises. An interpretation of the potassium occupancies is more difficult because of split positions, difficulties in the attribution of the extra-framework sites to cations or water molecules, and the framework deformation at higher temperatures. The main difference between the two structures is the influence of temperature
heulandite become longer and narrower, making possible (or as a consequence of) cation coordination to the framework oxygens. Potassium atoms migrate toward these favorable sites as the temperature is increased. The mordenite structure on the other hand is stable up to 800 O C so that the water molecules are necessary for the cation coordination.
Acknowledgment. W.J.M. is a Senior Research Associate (Onderzoeksleider) of the Belgian Nationaal Fonds voor Wetenschappelijk Onderzoek. The authors thank J. V. Smith for the gift of the sample. Registry No. Calcium mordenite, 12173-98-7.
Diffusion of Methyl Red and Its Photoexcited Isomer in Poly(ethy1ene glycol) J. L. Xia, S. S. Gong, and C. H. Wang* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (Received: April 8, 1987)
Laser-induced holographic grating relaxation of methyl red dissolved in poly(ethy1ene glycol) has been investigatedas a function temperature, the writing laser power intensity, and the crossing angle of two writing laser beams. The decay-growth-decay curve shape is due to simultaneousdiffusion of methyl red and its photoproduct. The temperature dependence of the diffusion coefficients of two isomers is well described by free volume theory developed by Vrentas et al.
Introduction Besides being widely known as a technique for producing a three-dimensional picture, holography also serves as a powerful tool for the investigation of a variety of photochemical and photophysical processes.' In this communication, we are concerned with the application of the laser-induced holographic grating relaxation (LIHGR) technique to study the diffusion process of methyl red dissolved in poly(ethy1ene glycol) melt. Methyl red (MR) is known to exhibit trans-cis photoisomerism.* By irradiation of light in the green spectral region (e.g. the 514.5-nm line from an Ar' laser), the stable trans form is converted into the cis form which thermally reverts back to the trans form in the dark. The conversion to the trans form is accelerated by the interaction with the 632.8-nm He-Ne laser r a d i a t i ~ n . ~ . ~ It is, thus, important to realize that the apparent lifetime of M R is strongly affected by the reading power of the He-Ne laser. This appreciation is necessary in order to obtain the correct mass diffusion coefficient of M R in the polymer host. Experimental Section Poly(ethy1ene glycol) (PEG) having a molecukr weight of 1540 (Mw/MN = 1) was purchased from Polysciences, Inc. The sample used in the experiment is prepared by dissolving approximately 0.1% (by weight) of MR (mp 179 OC, purchased from the Aldrich Chemical Co.) in PEG at 60 OC. The PEG-MR sample is placed in a Pyrex cell ( 1 mm thick) for the LIHGR experiment. The holographic grating is induced by crossing two equal intensity beams derived from the main beam of an argon ion laser. The wavelength of the laser radiation is 514.5 nm and the laser is operated at a power level of 40 mW. It is attenuated by an appropriate factor before being incident on the sample. The writing time is about 100 ms, controlled by an electronically actuated shutter. The crossing angle 8 varies from 3 . 8 O to 15'. At 3 . 8 O it corresponds to a grid spacing of 7.72 pm. The optical (1) Brauchle, C.; Burland, D. M. Angew. Chem., Int. Ed. Engl. 1983, 22, 582. (2) Brown, G. M. Photochromism, Techniques of Chemistry, Vol. 111; Wiley-Interscience: New York, 1971. ( 3 ) Urbach, W.; Hervert, H.; Rondelez, F.J. Chem. Phys. 1985,83, 1877. (4) Gong, S . S.; Christensen, D.; Zhang, J.; Wang, C . H. J . Phys. Chem.,
submitted for publication.
0022-3654/87/2091-5805$01.50/0
setup employed in the present experiment is similar to that employed in the previous work,5 with the exception of using a He-Ne laser as the reading beam and a monochromator to separate the signal from the writing beams. A boxcar integrator interfaced to an IBM PC is used to collect the data for further analysis.
Results and Discussion A typically observed intensity Zvs. t (time) curve (obtained at 54 O C , 8 = 3.8') in the present LIHGR experiment is shown in Figure 1. This type of the decay-growth-decay curve shape has been interpreted in the camphorquinone (CQ)-poly(styrene) (PS) system as due to the relaxation of multiple gratings associated with the diffusion of C Q and its photoproduct.6 By fitting the curve shape to an analytical expression, we have demonstrated that the multiple grating effect can be used to extract the translational diffusion coefficient data of CQ and its photoproduct in poly(methy1 methacrylate) (PMMA) simultaneously. In irradiating the polymer system containing CQ, CQ is photochemically converted into a nonreversible photoproduct. Thus, it is straightfoward to extract the diffusion data associated with both species. In the LIHGR experiment involving the polymer system containing MR, the photoproduct is a photoexcited cis isomer, which can be converted back to the trans form either by irradiation with light or by a thermally activated process. In order to avoid the competing effect arising from photochemical kinetics, it is necessary to employ the reading He-Ne laser beam at a sufficiently low power level such that the apparent lifetime of the cis isomer is much longer than the time of diffusion. Not only is the LIHGR curve dependent on the power of the reading laser, but also is it dependent on the power of the writing beams. Shown in Figure 2 is a set of the LIHGR curves, obtained at T = 65 OC and 8 = 3.8' to show the effect of varying the power of the writing argon ion laser beams. As one notes, the growth increases with increasing writing beam power. We now show that the increase is due to the production of a greater amount of the photoproduct (Le. the cis form of MR) resulting from greater writing power used. However, employing too much writing beam power may cause photodecompo~ition.~ ( 5 ) Zhang, J.; Wang, C. H.; Chen, Z. C . J . Chem. Phys. 1986,85,5359. ( 6 ) Zhang, J.; Yu, B. K.; Wang, C. H. J . Phys. Chem. 1985, 90, 1299.
0 1987 American Chemical Society
