COMMUNICATIONS TO THE EDITOR
1400
C-H
YIELD
OF
HT
\
40
20
n l
- 80
I
I
I
1
100 BOND DISSOCIATION ENERGY kcal /mole
tain residual water.'-5 When alkaline earth X and Y zeolites were heated from 500 to lOOO", Pickert, Rabo, Dempsey, and Schomaker found weight losses ranging from 1.040/, for R4gX to 0.25% for BaY.l As they point out, this weight loss could represent the removal of residual water and/or the loss of water formed by the condensation of zeolitic hydroxyl groups. Recently, Ward presented infrared evidence for the formation of pyridinium ions in divalent cation Y zeolites.2 He attributes their formation to the reaction of pyridine with acidic hydrogen resulting from the hydrolysis of residual water molecules attached to cations
M(OHJ2+ +RT(OH)+
+ H+
I n addition, he found that pyridinium ion concentration increases with decreasing cation radius, which is conFigure 1. sistent with his hydrolysis model and the weight loss data of Pickert, et al.' We have recently obtained evidence concerning the Table I: Relative HT Yields from Different C-H and N-H coordination of the residual water oxygen from singleBonds in Recoil Tritium Reactions" crystal X-ray studies of vacuum-activated CaX, SrX16 Speoific yield Bond dissociation and nickel(I1)-exchanged natural faujasite, NiFj RH of H T ~ energy8 These zeolites were evacuated for 8-16 hr at 400" and 9.9 104 CH4 torr and sealed, without exposure, in capillaries; 98 CzHe 15.3 then counter X-ray data were collected at room tem93 c-CSHlO 22 perature. The structures were solved using conven9.5 103 rt: 2 a" tional Fourier, difference Fourier, and least-squares 92 i 3 CDaNI-Iz 18 rt 1 86 i 3 (CD3hNH 51 f 4 techniques. A CaX difference map6 indicated scattering matter in SII' (see footnote of Table I for a de'Sample composition: Nz, 60-70 cm; 02,3.5 cm; He*, 1 cm; scription of the site nomenclature) , which was assigned RH, 5-10 cm. Yield per C-H or N-H bond under equivalent experimental conditions. to calcium ion, Ca3, based upon the Ca3-02 (framework oxygen) distance of 2.28(7) A. This new cation site is 0.03 from the plane of the supercage six-ring experimental error. Certainly, one can conclude that and 0.74 8 from the position a calcium ion would take the mechanism of abstraction is quite similar from in SII. Additional scattering matter found in the SII' both C-H and N-H bonds, involving in each case those region was assigned to oxygen, Ow, and attributed to factors that are important in controlling the bond a residual water molecule. The Ca3-Ow and Ca2 (caldissociation energy. Further testing of the correlation cium ion in a SI' site)-Ow distances, 2.34 (12) and and improvement of its accuracy requires experiments 2.64 (8) A, respectively, are the main bases for this with many additional N-H bonds to determine both assignment. Since the Ow peak on the difference map standard H T yields from recoil tritium reactions and is small and less than twice the background level, the the values of the corresponding bond dissociation following facts are given to support this assignment. energies. 1. Including Ow with partial occupancy in the model drops the refined weighted R from 0.083 to 0.079. T.TOMINAGA DEPARTMENT O F CHEMISTRY F. S. ROWLAND Applying a linear hypothesis test,8 we find that the UNIVERSITY OF CALIFORNIA
.'
IRVINE, CALIFORNIA92664 RECEIVED JANUARY 2, 1968
X-Ray Evidence for Residual Water in Calcined Divalent Cation Faujasite-Type Zeolites
Sir: There have been several reports that directly or indirectly support the thesis that divalent cation faujasite-type zeolites activated at 500" or below conThe Journal of Physical Chemistry
(1) P. E. Pickert, J. A. Rabo, E. Dempsey, and V. Schomaker, Proc. Intern. Congr. Catalysis, Srd, Amsterdam, 1.964, 1, 714 (1965). (2) J. W.Ward, presented at the 41st National Colloid Symposium, Buffalo, N. Y.,June 1967. (3) C.L. Angel1 and P. C. Schaffer, J. Phys. Chem., 70, 1413 (1966). (4) J. L. Carter, P. J. Lucchesi, and D. J. C. Yates, ibid., 68, 1386 (1964). (5) E. Dempsey, Research Department, hlobil Research and D e velopment Corp., Princeton, N. J., unpublished research. (6) D. € Olson, I. unpublished research. (7) D. H. Olson, in preparation. (8) W. C. Hamilton, Acta Cryst., 18, 502 (1966).
1401
COMMUNICATIONS TO THE EDITOR
Figure 1. Stereoscopic drawing of a sodalite cage in a faujasite-type structure showing the coordination of a residual water oxygen. Oxygens 0 2 and 0 4 are members of the supercage six-ring and oxygens 0 2 and 03 are members of the hexagonal prism six-ring.
Ow scattering matter can be included in the model at the 0.005 significance level. 2. The refined population parameter for Ow is 5.5 times its standard deviation. 3. The population parameter for the calcium ion in SII' is roughly equal to that for Ow, implying simultaneous occupancy of the SII' site and the formation of a Ca3-Ow bond (this agreement of occupancy factors for Ow and metal cation in SII' is also observed in SrX6 and I\TiFj7). In fact, the presence of residual water oxygen in site SII' is the most logical explanation for the -0.7-A movement of metal ions from SI1 to SII'. Table I: Summary of Nonframework Atom Sites" CaX
SIX
Site
No./u.~.~
No./u.o.
Cation (MI) Cation (M2)
SI SI'
7.5 (5) 17.3 (6)
11.2 (3) 7 . 0 (6)
Cation (M3) Cation (M4) ow
SII' SI1 SII'
9 . 0 (IO) 17.3 (6) 10.5 (19)
4.2 (8) 19.5 (6) 5 . 4 (15)
Atom type
NiFj No./u.o.
10.6 (1) 3 . 2 (3) 1.9 (6)b 1 . 9 (6) 6 . 4 (2) 1 . 9 (15)
+
The site designations used here are those introduced by Pickert, et uL1 SI is in the center of the hexagonal prism; SI' is adjacent to SI, and in the sodalite cage, SII' is in the sodalite cage adjacent to the supercage six-ring and SI1 is in the supercage adjacent to SII'. Except for site SI, the sites define a region in crystal space and not a definite point. 'There are two site SI' peaks assigned to nickel ions. The smaller is 2.0 from Ow scattering matter.' ' U.C. = unit cell.
Evidence for residual water, Ow in SII', was also found in SrX and NiFj, although in these cases the occupancy factors were lower (Table I). In these two structures, the population parameters for Ow and M3 were adjusted by computing a succession of difference maps.
The position and coordination of the residual water oxygen, Ow, is shown in Figure l a 9In addition to its coordination to M3 cations, Ow is coordinated to cations in SI' with M-0 distances of 2.64 (8), 2.67 (8), and 2.0 (3) in CaX, SrX, and NiFj, respectively. The Ow and M2 population parameters (Table I) indicate that an average of 1.6, 1.3, and 1.0 such bonds per Ow occur in these three zeolites when evacuated as described earlier. The multiple coordination of Ow explains its retention following the relatively severe dehydrating conditions. The multiple coordination would also promote the hydrolysis reaction which Ward2 has shown to occur. The M3 cation is tetrahedrally coordinated by Ow and three framework oxygens of the supercage six-ring. Further support for this coordination is afforded by the uv diffuse reflectance work of Williams, who found evidence for tetrahedrally coordinated cobalt in a cobalt-exchanged zeolite X heated at 400" in flowing Nz.'O Thus divalent cation faujasite-type zeolites, as well as the trivalent rare earth forms," retain water oxygens at 400" or above. This residual water no doubt influences the cation distributi~n.~The monovalent analogs of these zeolites dehydrate much more readily,1*2p6 as would be predicted from the lower cation charge density. Acknowledgment. Several helpful discussions with E. Dempsey of this laboratory are gratefully acknowledged. (9) Computer drawn using ORTEP. C. K. Johnson, ORTEP, Oak Ridge National Laboratory, Oak Ridge, Tenn., 1964. (10) C. J. Williams, unpublished research carried out a t Mobil Research and Development Corp., Princeton, N. J. (11) D. H. Olson, G. T. Kokotailo, and J. F. Charnell, presented a t the 41st National Colloid Symposium, Buffalo, N. Y., June 1967.
MOBILRESEARCH AND DEVELOPMENT CORPORATION PRINCETON, NEW JERSEY 08540 RECEIVED JANUARY 12, 1968
D. H. OLSON
Volume 76, Number 4
April 1968
