Anw. ctwn. SQiz, ,
(21) Y
08,1148 (1871).
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The crystal structure of hydrated TI+-exchanged zeolite X has been determined by single-crystal X-ray diffraction ,techniques. The cohpound crystallizes in space group Fd3m (a 25.089 (5) A). Reflection data were colleC~tedwith CU Ka (A 1,54182 A) radiation using a synthetic crystal with a maximum dimension of approximately 0.1 mm. A'total of 281 observed reflections'was obtained by counter methods and the structure was refined by leaat-squares techniques to a final conventional R factor of 0.13. TI+ ions are located inside the sodalite cages in front of the hexagonal prisms (site 1') and in fiont of the six-membered ring face of the sodalite cage on the supercage side (site 11). No water molecules and only 5296 of the cations could be located, Unlocated cations and water 'molecules were included as uniformly distributed spheres. The shortest TI+-0 distances aie 2.64 (5) and 2.79 (5) A.
I, Introduction In recent y e m many X-ray diffraction studies ha& been made of the siting of cations in zeolites with 'the fayjaeite framework structure.' However, particularly in monovalent cation-exchanged forms, there is always a proportion of cations that are not located by X-ray analysis (so-called sib I11 cations). This proportion is largest in hydrated Xtype zeolites (e.&, Na-X, 53% unlocated;2 K,X, 64% unlo: cated3). Possible sites for these cations have been indicated by several authors.'-! The most favored sites are those in the vicinity of the four-mem gs of oxygen atoms in the supercage (see Figure 1). s indeeil any ordering of these.cations then they ale on smmetry sites with low occupancy facto? or they,are spread over general (nonsym: metry) crystallographic positions. However, they may be obserGtjd in cases where the monovalent cation has a high a number, such as TI+.Furthermore, the ion-exC isotherm for the s (T1,Na)-X shows that the eelectidty of the wli6 + is high over the entire range of the exchange re This suggests'that TI+ is strofigly interacting with the zeolite surface even in the hydrated form.' The ion-erchiurge'system (T1,Na)-A shows a similar effect and indeed 9% of the TI+ ions were found by '
X-ray analysis to be,strongly bound to the zeolite surface in hydrated TI+-exchangedzeolite A.6 We have now studied the cry8taI structure o TI+-ekchanged zeolite X in the hope of locating the sitel11
Crystals of zeolite X suitable for single-crystal Xanalysis were prepared by the method of Charnell.7 Si some zeolite A cocrystallized, the Si(A1 ratio for ze0lite.X could' not be accurately determined by chemical analysis. We have therefore assumed the Si/Af ratio to be 1.18, the value found by Olson* for similarly brepared synthetic zeol i b X crys'lals, For X-ray .examinatioh zeolite X and A' cryatah we* eaaily separated owing to the di crystal product morphology. The cryatalline product ww contacted with 20 ml of a 0.1 M solution of TIN03 at room bsequent chemical analyde indicated that excomplete, A roughly octahedrally shaped crys= tal of zeolite X, approximately 0.1 mm on edge, wan selected' for %ray examination, Preliminary pmasion photugraphs &firmed the space -up to k Fd3m and no spu-
rho J o u r n ~ ~ o f ~ VOI.~ 70, ~ aNO.~23,Cto74 ~ ~ ~ ~ ~ ~ ,
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7
7 .
. At the dmpletion. of the
data COlIection'and st&ure enalysis,.the.crS;etal was mounted; for,demonktration' pur-8,
on a.PhiIip8PWilOO foiu-circleeutomatfc diffracb
meter. By means ofthe computer-controlled random'r&ip rocd lattice search mutine4t wai dhovered' tal contained a small .twin intergrowth; The. were shoi, to be of the t ~ ' ~ l l i ) , ( Oand l l )(1 the main and intergrowth c r y s w , respectively. nt reflections likely to be to be g d . Fdhermore, ons from the intergrowth effects of the twin on the results of the'structural analysis are believed to be
w criminator. Least-squares refinement based on 8, -8 (8 ,< 1 5 O ) values measured for ' Ka radiation (A 1.54182 A) g oximately eight equivalent daw s Ni. filtered Cu Kh.radiation (8 S SOo). The 8-28 scan method was employed with a scanning speed of O.G0/min (in e) and a scan range of 0.7O. Backgrounds were measured for half of the scan time on each side of the scan. As a check on electronic and crystal stability, a 'control reflection was monitored a t regular inNo significant variation in intensity of this reflecobserved during data collection. The initial stan'
,
.
The Journal01 PhplCOl Chemlshy, VQl. 78, No. 23, 1074
111, Structure D;Bterrnhtion e& Wfi8 carried O u t with the 'I framework parameters of hydrated Na-X.2 Difference Fourier byntheees revealed the position of TI+ cations a t sites I' and II,lo There w a no significant electron density inside
TABLE XI: Cation 8iting in Hydrated Na+-,K+:, and TI+-EmchangedFornieof Zeolite X.
All occupancy factare are given 88 the number ofcations per unit cell. 6 Standard deviations, in parentheses, are given in units of the 1-t significantdigite of the corresponding parameter. 8 Reference 2. Reference 3.
'
TABm In:ht6ratomicDistanccw and Anglev for Hydrated Thallium-Exchnnged +lite
X
D+tancea*
(Si,Al)-O(l). (Si,Al)-O(2) (Si,Al)-O(3) (Si$1)-O(4). ' 0(1)-0(2)
1.67 (3) 1.64 (2) 1.70 (2) 1.69 (3) 2.73 (8) 2.76 (7) 2.67 (3)
0(1)-O(3)
0(1)4(4)
. 0(1)- (Si,Al)-0 (2) 0 (l)-(Si,Al)-O (3)
111(4) 110 (3) 105 (4) 110 (3)
O(l)-(Si,Al)-O(4) 0(2)-(Si,A1)-0(3) 0 (2)-(Si,Al)-O(4) 0(3)-(Si,A1)4(4)
111(4) 108 (6)
(Si,Al)-O(l)-(Si,Al) 139 (6) (Si,AI)-O(2)-(Si,Al) 146 (4) a'Standard deviations, in parenth&s, are given in b DiatancC are in Hngetriims.' e Angles are in degrees. 0.13 md R2 = 0,15 (where
0(2)-O(3) O(2I-W) 0(3)-0(4) TI(I')-O (3) Tl(l')-O@) (II)-0(2) WII)-o(4)
2.74 (6) 2 -76 (6) 2.74 (11) 2 -64 (6) 3.26 (8) 2.79 (6) 3.26 (11)
Anglead
(Si,AI)-0(3)-(Si,Al) 144 (3) (Si,Al)-0(4)-(Si,Al) 1%(6) 0(3)-Tl(1')-0(3) 95 (3) 0 (3)-Tl(I')-O(2) 128 (3) 0(2)-TI(I')-O (2) 108 (1) 0(2)-Tl(11)-0(2) 89 (3) 0(2)-"l(11)-0(4) 53 (1) 0(2)-TI(II)-0(4) 121 (3) 0(4)-Tl(II)-0(4) 106 (2) unite of the leaat significant digits of the corresponding parameter.
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(JFd IFd)2/ZIF#]1/2)! The refined positional; cy, and temperature factars h r ~given in Table'I, ana the observed and calculated , structure facbri are available elsewhere,*2Js, ,
IBr. Mrsowiana In h y d r d d thllium-exchhnged zeolite X,TI+ions are ' located at sites I' and I1 (see Figure 1); Surprisingly, there are no TI+catione (or residual Na+ cations) inside the hexagonal priime (site I). Partial occupancy of this site might be exfor an even thee balance distribution to b maintained over itk framework. Such is thd case for K+a.exchanged forms.of zebtite, d at aitae I, I', b d I1 (see Table 11). Distance calculations ahow that the hexagonal prism (centered at the origin) in in fact ,sufficiently large to accommodate a 'Fl+ (origin 4 0 ( 3 ) , .2,$6(7); origin 0(2), 3.62 6)A), Moreover, the calculated wg1e.e [0(3)-originO(V),.W3)O; O(a)-oaigin-O(B"), 94(3)O] indicata that .the
coordination geometry would b very close to octahedral. Howevei, T1+ ions must diffuse through the single askmembed.rinp forming the entrances to the d e l i t e W e in order to be located a t site 1'. Furthemore, the minimum dimetam of these two crystallographidly different sirmembered rings are very bimilar (hexagonal prism six-rh diameter, O(S) 0(2), 6,29.(9); rn+ res-‘ 5.6 A**! 0(4), 6,28 (11) A). lite cage eix=r %forthe abaence.of Tl+ There are ioria at rite 1. thermodynamicallyunfavorable; second; ithe six-membered ring entrance to the hexagonal prism inhibite diffusion. T h e .structurall.data alone do not allow ua to -dietingutoh betwe6,i theae two possibilities, However, in view of th coordination geometry at site I for a pancy.of thie site by other monovale and K + ' ( mTable 111, the thermod c argument seems leea likely. In this CBBB the double six-ring etructurd unit could conceivably be more rigid than a single six-mem-
+
0
e-
I
rhe, Journal of ~ h ~ s t c~hernrsrrn ir voi. 111, NO. 29, IWQ 1--1-1
TABLE*:
~rtionre~~6dinref~drm.f.11L) pop
!Qa Thd.Mum-Whingeilk l i G X
. AAti-$z(s)
'Atom
"-defining ;.i>tam
'0(3)
gg;)
q(2)
"o(4)
0 (3) 0 (2)
l1.37 '0.21 .1.66 0.41
mII)
All deviations.are positive, indicating that the atoms all lie on theoppoaite side of the p l h e tdthe origii. b e d ring and thereby increase the activation energy for diffusion of Tl+ions into the hexagonal prism. hithermore, there 'is good evidence from ion-exchange isotherm studies16 that for k g e ionic radii monovalent cap ions such &Rb+ and Cs+ diffusion through eix-memhred ring systems in h l i t e s X and Y cannot &cur at room temperature under equilibrium' conditions, whereas for small Li+ diffusion into the 0.4
and Rb+ (rRb+
en distance is lees
protrusion of y+ions above a sir-membered ring was obsewed in the hydrated TI-A structure.6 Also from Table IV it can be seen that ingle six-membered ring formed by en atoms is more buckled than the O(2) and O(4) double six-membered ring formed by O(2) and O(3) oxygen atoms. The distribution of cations in hydrated Ti+-exchanged zeolite X is compared with that in the Na+ and K+ forms in Table 11. Clearly, the absence of cations at site I results in a relative increase in the number of cations a t site I' to balance the iregative framework charge. However,'the total numbee of Tl+cations a t site I"(22.7 (3) per unit cell) is somewhat highei than the sum of the cations at sites I and I' in the +?a+- and K+-exchanged forms (Na+, 17.0; K+, 16,l.pP unit cell). Neverthelem, the number of cationsloc a t d at site I1 is very'similar for hree monovalent exchanged fopma, "he high &mpa f site I' in the T1+rimately three T1+ l m t TI(1') TIW) anpoach inside this cage is therefore 3.76 (4) A. . The ion-exchange iRbtherms for the systems (TI,Na)-X and (TI,Na)-YI ( 2 5 O and 0.1 total normality) measured Sherry5 are s h h in Figure 2, Asdpreviously discussed, m l i k X the Na+ ione can k completely exchanged for TI+ and a high wlectivity for TI+ is demonstrated throughout the exchange reaction. The selectivity was attributed to the high polarizability of TI+ and considered to be indicaThe Joumslot~yslcelChrmktry,Vol. 78, No,23, 1974
. 0.6
OB
.t.0 STI
2. TI+-?&+ kothtwm at 250,O.l total nmnaltty: tap cwe, e x momcurbi3,z w e Y '
tivk of strong binding on he zeolite surface. This is4n agreement with the rather high occupancy factor of site I' and the short framework-cation distance, T1(1
