Location of cupric ions in Y zeolites by electron spin echo

J. Phys. Chem. 1903, 87, 4433-4437

interpretation of the experimental observations is as yet incomplete. It would be of interest to investigate the effect of temperature on emission spectra and decay times in order to elucidate the remaining anomalies and substantiate the mechanisms proposed in the present paper. Acknowledgment. We are most grateful to Dr. R. Humphreys and Professor D. Arnold for supplying the

4433

diazo compounds and the sample of DAC carbonium ions. We also thank them and D. Bethell for helpful discussions. Financial support from the Natural Science and Engineering Research Council is also acknowledged. Registry No. DPC, 3129-17-7;MCC, 64568-30-5;DCC, 64568-31-6; MMC, 14845-81-9; DMC, 14845-84-2;DBC, 6124260-2;DAC, 67155-34-4; DNC, 67155-33-3.

Location of Cupric Ions in Y Zeolites by Electron Spin Echo Spectrometry: Contrast between Sodium- and Potassium-Y Zeolites and Comparison with X Zeolites Tsunekl Ichikawa Facuity of EWin&ng,

HokkaMo University, Sapporo. 060, Japan

and Larry Kevan' Department of Chemkw, University of Houston, Houston, Texas 77004 (Received: December 8, 1982; In Final Form: February 16, 1983)

Electron spin echo spectra have been measured for Y zeolite containing a small amount of Cu2+. Locations of the Cu2+and the coordination with adsorbed water molecules have been determined by analyzing the observed nuclear modulation effecta of 27Aland D on the electron spin echo spectra. In initially hydrated sodium- and potassium-Y zeolites Cu2+is located on a line perpendicular to the hexagonal windows between the sodalite and supercages of the zeolite structure and displaced into the supercage (site SII*). Each Cu2+in SII* is coordinated to three lattice oxygens in the hexagonal window and three water molecules in the supercage. Upon dehydration at 673 K, the Cu2+environment changes significantly as shown by resolved hyperfine splittings in the g, region of Cu2+and an apparently shorter Cu2+-aluminumdistance. Cu2+moves to the region of the hexagonal prisms connecting two sodalite units at or near site I where site I is the center of the prism. Cu2+ appears to occupy two different locations in the region of site I possibly due to the unequal numbers of Si and A1 per hexagonal prism in Y zeolites or displaced into site I' in the sodalite cage. Upon rehydration Cu2+in K-Y zeolite moves back to ita original site SII*, but the Cu2+in Na-Y zeolite remains in the hexagonal prism. The difference between the rehydrated sodium- and potassium-Y zeolites is explained in terms of the different hydration energies of alkali cations. Comparison is made with Cu2+locations in X zeolites which contain a lower Si/Al ratio.

Introduction The cation locations in zeolite catalysts have been extensively studied because of their influence on catalytic properties. Electron spin resonance (ESR) spectroscopy gives information about the location and coordination environment of paramagnetic cations.' The cupric ion has been widely studied because this d9 ion has an easily observed ESR spectrum which is rather sensitive to the Cu2+ environment.23 However, the ESR spectrum does not give direct information about the local structure of the cations, such as the distance and number of first solvation shell nuclei. When the cations are surrounded by magnetic nuclei, this information is directly included in the superhyperfine interaction between the electron spins and the nuclear spins, but this is generally too weak for detection by normal ESR. We have recently demonstrated that electron spin echo (ESE) spectroscopy can detect the weak superhyperfine interaction between surrounding magnetic nuclei and paramagnetic species on catalytic oxide^.^-'^ This su(1) Lunsford, J. H. Adu. Catal. 1976,26, 137. (2) Conesa, J. C.; Soria, J. J. Chem. SOC.,Faraday Trans. 1 1979, 75, 406. (3) Herman, R. G.; Flentge, D. R. J. Phys. Chem. 1978,82, 720. (4) Ichikawa, T.;kevan, L. J.Chem. SOC., Faraday Trans. I , 1981,77, 2567. 0022-3654l03l2~07-4433$01.5010

perhyperfine interaction shows up as a modulation of the decay of the time domain ESE spectrum. By proper analysis of this modulation one can determine types, number, and distances of magnetic nuclei surrounding the paramagnetic specie^.'^ ESE spectroscopy is used here to elucidate the local structure of trace Cu2+in hydrated, dehydrated, and rehydrated zeolite Y with sodium and potassium as ex(5) Ichikawa, T.; Kevan, L. J. Am. Chem. SOC. 1981,103,5355.

(6) Narayana, M.; Li, A. S. W.; Kevan, L. J. Am. Phys. Chem. 1981,

85, 132. (7) Ichikawa, T.;Yoshida, H.; Kevan, L. J.Chem. Phys. 1981, 75,2485. (8) Ichikawa, T.;Yoshida, H.; Kevan, L. J. Phys. Chem. 1982,86,881. (9) Ichikawa, T.; Kevan, L. J. Am. Chem. SOC.1980,102,2650. (10) Narayana, M.; Kevan, L. J. Am. Chem. SOC.1981, 103, 5729. (11) Narayana, M.; Kevan, L. J. Chem. Phys. 1981, 75,3269. (12) Narayana, M.; Kevan, L. J. Chem. Phys. 1982, 76,3999. (13) Narayana, M.; Janakiraman, R.; Kevan, L. Chem. Phys. Lett. 1982, 90,235. (14) Ichikawa, T.; Kevan, L. J. Am. Chem. SOC.1983,105, 402. (15) Narayana, M.; Kevan, L. J. Phys. C 1982,16, 361. (16) Narayana, M.; Narasimhan, C. S.; Kevan, L. J. Catal. 1983, 79, 237. (17) Narayana, M.; Kevan, L. J . Chem. Phys. 1983, 78, 3573. (18) Narasimhan,C. S.; Narayana, M.; Kevan, L. J.Phys. Chem. 1983, 87, 984. (19) Kevan, L. In "Time Domain Electron Spin Reeonance", Kevan, L.; Schwartz, R. N., Eds.; Wiley-Interscience: New York, 1979; Chapter 8.

0 1903 American Chemical Society

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The Journal of Physical Chemistty, Vol. 87, No. 22, 1983

changeable cations. It is shown that the site location in rehydrated zeolite Y is critically dependent on the nature of the major cation. Comparison with zeolite X, which has a lower Si/Al ratio, points up the dependence of this ratio on the Cu2+locations.

Experimental Section Binder-free Linde Na-Y zeolite proved to contain significant paramagnetic impurities (probably Fe3+)which decreased the electron spin echo decay time so that the nuclear modulation could not be quantitatively analyzed. So that these paramagnetic impurities would be reduced, Na-Y was stirred in 0.1 mol/dm3 of A1-EDTA (ethylenediaminetetraacetic acid) solution at 350 K and pH 4.5 for 2 h, and filtered three times. It was then stirred in 1 mol/dm3 of Na2C03solution at 350 K for 2 h to replace any removed Na+. Finally the zeolite was washed with distilled water. This procedure did not appear to affect the local crystalline environment around subsequently exchanged Cu2+ since the ESR and ESE spectra were identical with those of Cu2+ in untreated K-X zeolite. Potassium-Y zeolite (K-Y) was then prepared by conventional exchange of Na+ in purified Na-Y zeolite with K&O3 solution. Partially Cu2+exchanged Na-Y and K-Y (CuNa-Y and CuK-Y, respectively) were prepared by ion exchange as described previously.4 The extent of exchange of Na+ or K+ by Cu2+was approximately 0.6%. After filtration the excess water was removed by pressing the wet samples between filter paper. The resulting samples are designated as hydrated. Deuterated samples were prepared by exchanging all the H20 with D20 (Stohler Isotope Chemicals) at ambient temperature. Completely dehydrated samples were prepared by evacuating the hydrated samples at ambient temperature for 1 h, heating to 673 K at increasing temperature for intervals of 1h in vacuo, oxidizing at 673 K for 2 h under an oxygen pressure of 760 torr, and evacuating for 15 h at 673 K. Rehydrated samples were prepared by exposing the dehydrated samples to H20 and D20 vapor a t room temperature for 2 h. ESR spectra were obtained at 77 K on a Varian E-4 spectrometer. ESE spectra were obtained at 4.2 K on a home-built spectrometer with typical pulse powers of 100 W and typical pulse widths of 60 m 2 0 The field position for maximum ESE intensity was the g , component at 315 mT for a microwave frequency of 9.15 GHz. The nuclear modulations from Al in the zeolite framework were detected by measuring the two-pulse ESE spectra of the zeolite containing no deuterated compounds. The nuclear modulations from deuterium in the deuterated adsorbates were detected by measuring three-pulse ESE spectra with 0.26 ps selected between the first and second microwave pulses to detect the deuterium modulation while suppressing the aluminum mod~1ation.l~ Analysis The quantitative analysis of deuteron modulation has been described in detail.1g9213Two-pulse and three-pulse simulations are made for n equivalent interacting nuclei at distance r from the paramagnetic ion with an isotropic coupling constant a. The simulated modulation is multiplied by a generalized decay function given by eq l to D(T)= exp(ao + a l T + a 2 P ) (1) compare directly with experiment. T is the interpulse time (20) Ichikawa, T.;Kevan, L.; Narayana, P. A. J. Phys. Chem. 1980,83, 3378. (21) Mims, W. B.; Peiaach, J.; Davis, J. J. Chem. Phys. 1977,66,5536. (22) Ichikawa, T.; Kevan, L.; Bowman, M. K.; Dikanov, S. A.; Tsvetkov, D. J . Chem. Phys. 1979, 71,1167.

Ichikawa and Kevan

and the parameters, ao, al, and a2are fit empirically. The values of n are constrained to be integral and are deter8; r is determined to the nearest integral n up to n mined to *0.01 nm, and a to &lo%. The A1 modulations can only be qualitatively analyzed because of the large quadrupole moment of A1 nuclei. However, quite useful qualitative information can be obtained from Al modulations, particularly on a comparative basis, from the following characteristics of the modulation: (1)The amplitude of the nuclear modulation increases with the number of nearest nuclei. (2) The decay of the amplitude of successive nuclear modulation cycles increases with decreasing distance to the nearest nuclei. (3) The amplitude of the frequency component at twice the nuclear Larmor frequency (double frequency component) in the two-pulse ESE spectrum increases with decreasing distance to the nearest nuclei. The Al modulations can also be usefully analyzed by a ratio analysis procedure.22 In this method two smooth curves joining all the maxima and minima of the A1 modulation are drawn. These curves, measured as a function of the time interval between the first and second pulse, 7 , are denoted by V-(T) and Vmin(7),respectively. The experimental modulation amplitude is defined by

-

When an electron spin is surrounded by n equivalent nuclear spins, the theoretical modulation amplitude is given by

RP(7) = (Rth(7,r,a))" (3) By equating Rex(7)to RF(7) and taking the log twice one has log (log Rex(7)}- log (log Rth(7,r,a)} = log n

(4)

Therefore if one compares two ESE modulations with different n and the same r and a, the difference of the double logarithmic modulation amplitudes gives a 7-independent constant which is equivalent to the logarithmic ratio of the number of surrounding nuclei

Results Figure 1shows the ESR, three-pulse ESE, and two-pulse ESE spectra for hydrated CuNa-Y. The magnetic field dependence of the ESE intensity is the same as the first integral of the ESR spectrum for all the samples. The ESR spectrum with A,, N 14 mT is assigned to that of Cu2+ under distorted octahedral symmetry.23 The best simulation of the D modulation in the three-pulse ESE spectrum indicates that one Cu2+is surrounded by six deuterons with a Cu2+-D(D20) distance of 0.28 nm. This indicates that the Cu2+ is coordinated to three water molecules with a Cu2+-O(D20)distance of 0.22 nm. The ESR and ESE spectra for hydrated and rehydrated CuK-Y are similar to those of hydrated CuNa-Y, indicating that the local structure of Cu2+in these CuK-Y zeolites is the same as that in hydrated CuNa-Y. Deuterium modulation is not observed after dehydrating CuNa-Y at 673 K consistent with the absence of coordinated waters. As shown in Figure 2 for CuNa-Y(I), dehydrated CuNa-Y gives an ESR spectrum with slightly resolved hyperfine splittings in the g, region. However, (23) Hathaway, B. J.; Billing, D. E. Coord. Chem. Reu. 1970, 5, 143.

Location of Cupric Ions in Y Zeolites

15 >.

t m

510 t-

z

I

5

n

The Journal of Physical Chemistry, Vol. 87, No. 22, 1983 4435

ESR 50mTI

I \ -DMOD.(--,EXF! --J-

I

I

n

ESR(\

li. I >

I-

***,SIMUL.

a I0.2MHz

.. 'Ik

I

I

0 I

-

TIME/Lrs Figure 1. ESR and ESE spectrum for hydrated CuNa-Y. The threepulse ESE spectrum showing deuterium modulation (D MOD.) is obtained from CuNa-Y hydrated with D,O. The two-pulse ESE spectrum showing aluminum modulation (Ai MOD.) is obtained from CuNa-Y hydrated with H,O.

'.

I

AI MOD. I

'--. .-.-- I 1

1 T IME/&s

I

2

3

Figure 3. ESR and ESE spectra for rehydrated CuNa-Y. The threepulse ESE spectrum showing deuterium modulation (D MOD.) is obtained from CuNa-Y hydrated with D,O. The two-pulse ESE spectrum showing aluminum modulation (AI MOD.) is obtained from CuNa-Y rehydrated with HzO.

E SR

1

31

I

I'I I

w k -1 z

Figure 4. Comparison of the amplitude of the aluminum modulation for hydrated CuNa-Y (0),dehydrated CuNa-Y(1) (A),and rehydrated CuNa-Y(0).

1 0

1

0.5

I

I

1.o Z/W

1.5

2.0

F w r e 2. ESR and two-pulse ESE spectra for dehydrated CuNa-Y(1) (-) and dehydrated CuNa-Y(I1) (---). The ESE modulatlon is due to alumlnum. Dehydration usually gave type I spectra, but occasbnaly type I1 spectra are observed.

the resolution of the hyperfine structure in the ESR spectrum is much lower than that for CuNa-X zeolite dehydrated identically which we previously studied.14 CuNa-Y (I) gives deeper A1 modulation than hydrated CuNa-Y. Although the type I ESR and ESE spectra are usually observed after dehydration, occassionally a sample showed much better resolved hyperfine structure in the g, region of the ESR spectrum after nominally the same dehydration treatment as shown in CuNa-Y(I1) in Figure 2. In experiments on other zeolite preparations it appears that better ESR hyperfine resolution is correlated with slower heating during dehydration. The ESE spectrum of CuNa-Y(I1) shows shallower A1 modulation with a stronger double frequency component. These results suggest that Cu2+mainly migrates into Al-rich sites (type I) upon dehydration, but occassionally Cu2+migrates into A1 poor sites (type 11) where, however, the distance between the Cu2+and one specific Al is short enough to give a stronger double frequency component. The type I Cu2+ sites show barely resolved hyperfine structure in the g, region while the type I1 Cu2+sites show well-resolved hyperfine structure in the g, region. The ESR and ESE

spectra for dehydrated CuK-Y are similar to those for dehydrated CuNa-Y, indicating no difference in the local structure of the Cu2+in Na-Y and K-Y zeolites. Figure 3 shows the ESR and ESE spectra for rehydrated CuNa-Y. Although the ESR spectrum is similar to that for CuNa-Y, the ESE spectra show much shallower D modulation than those for hydrated CuNa-Y. The best simulation of the D modulation indicates that the Cu2+ interads with only one water molecule with a Cu2+-D(D20) distance of 0.36 nm. Since this distance gives a Cu2+-0(D20)distance of 0.30 nm, it can be concluded that the Cu2+in the rehydrated CuNa-Y is not directly coordinated to a water molecule. Thus we consider it to be coordinated only to lattice oxygens. The relatively deep Al modulation indicates that Cu2+in rehydrated CuNa-Y is surrounded by more A1 than is Cu2+in hydrated CuNa-Y or CuK-Y. Figure 4 compares the amplitude of Al modulation more quantitatively by ratio plots. Note that the curves for initially hydrated and for rehydrated CuNa-Y are the same shape. This indicates that the difference in A1 structure around Cu2+in these samples differes only in the number of interacting aluminums. This difference indicates that the number of A1 surrounding Cu2+ in rehydrated CuNa-Y is 1.6 to 2.0 times larger than that in initially hydrated CuNa-Y. Note also that the ratio plot for dehydrated CuNa-Y(1) is not the same shape as those for hydrated and rehydrated CuNa-Y. This suggests that the r and possibly a parameters for Al are different as well as the n parameter for the hydrated and dehydrated

4436

Ichikawa and Kevan

The Journal of Physical Chemktty, Vol. 87, No. 22, 1983 0.22nm !’W”O

HYDRATED RMVDRATED CUK-V

Pv Flgure 5. Cation sites and their designations in Y zeolite.

zeolites. However, the aluminum quadrupole coupling could change with dehydration to make the comparison with hydrated samples unclear.

Discussion This zeolite Y is composed of A102 and Si02 with an Al/(Al+ Si) ratio of 0.30 bonded together to form truncated octahedra with eight hexagonal windows and six square windows. These truncated octahedra are called sodalite cages and are tetrahedrally bonded together by hexagonal prisms to form larger supercages. Figure 5 shows various cation sites in Y zeolites identified by X-ray ~rystallography.~~ SU is near the center of the sodalite cage, SIV is near the center of the super cage, SI is the center of the hexagonal prism, SI1 is the center of a hexagonal window between the sodalite cage and the super cage, and SII’ and SII* correspond to displacement into the sodalite cage and into the supercage, respectively, along an axis perpendicular to a hexagonal window. Finally, SIII is used in a broad sense to cover sites in the supercage near the square windows. Cu2+in both Na-Y and K-Y hydrated zeolites is coordinated to three water molecules and has an ESR spectrum consistent with distorted octahedral coordination. Cu2+ has similar characteristics in hydrated K-X14 and Na-A= zeolites where it has been assigned to site SII* which projects into the supercage opposite a hexagonal window as shown in Figure 6. This assignment is consistent with other ESR work alone with indicates that Cu2+in hydrated Y zeolite is preferentially located in the supercage.mfl We have discussed elsewhere how the Cu2+ location is dependent on the nature of the major cation and on A and X zeolite structural types.25 Rehydrated CuK-Y gives ESR and ESE spectra identical with originally hydrated CuK-Y and hence Cu2+in this preparation is again assigned to site HI*. However, rehydrated CuNa-Y gives quite different ESE spectra illustrating a dramatic effect of the major cation. The weak deuterium and strong aluminum modulation suggest site I in the hexagonal prisms for this Cu2+as shown in Figure 6. Water molecules are too large to occupy the hexagonal prism so the water interaction observed probably involves a water molecule in the sodalite cage. In site I Cu2+is coordinated octahedrally to six oxygens. Thus the ESR spectrum is similar to that for Cu2+in site SII* but the ESE spectrum shows much deeper aluminum modulation. (24) Smith, J. V. In “Zeolite Chemistry and Catalysis”;&bo, J. A., Ed.; American Chemical Society: Washington, DC, 1976; Chapter 1. (25) Kevan, L.; Narayana, M. In ‘Intrezeolite Chemistry”;Stucky, G., Ed.; American Chemical Society: Washington, DC, 1983; Amer. Chem. Soc. Symp. Ser.; in press. (26) Herman, R. G.; Flentge, D. R. J. Phys. Chem. 1978, 82, 720. (27) Conesa, J. C.; Soria, J. J. Chem. SOC.,Faraday Trans. 1 1978, 74, 406.

SI REHYDRATED CuNa-Y

SIOFFSET

: :E

DMVDRATED

Flgwa 6. Models for Cu2+ site locations in Y zeolite. The coordination to zeolitic oxygens in &rings is shown: the actual location of these oxygens is not exactly in the center of the 6-ring sides as is drawn for convenience.

The above two assignments seem fairly secure. But in the dehydrated samples the situation is less clear. Upon dehydration of both CuNa-Y and CuK-Y only aluminum modulation is seen as expected. It is significantly deeper than in the originally hydrated zeolites which suggests that Cu2+has moved into site SI. However, dehydration could also change the aluminum quadrupole coupling which would complicate interpretation of the aluminum modulation. Also, the modulation pattern is not the same as for rehydrated CuNa-Y in which Cu2+is assigned to site SI. This is shown by the ratio plot in Figure 4 in which the dehydrated CuNa-Y results do not parallel either the originally hydrated or the rehydrated CuNa-Y results. This latter difference probably reflects changes in the framework structure between hydrated and dehydrated samples. For example, the S i U A l angle typically changes on dehydration. The simplest interpretation seems to be that Cu2+in dehydrated Na-Y or K-Y zeolites is located in the hexagonal prisms. There may be two sites which could be related to a nonuniform electrostatic field in the hexagonal prisms since the Si/Al ratio in this Y zeolite is 2.5. This contrasts with a ratio of only 1.2 in the X zeolite we studied.14 To obtain a more quantitative geometrical picture one must be able to quantitatively analyze the aluminum modulation in terms of number and distances of interacting aluminums. An alternate interpretation is that type I Cu2+is mainly in site SI and type I1 Cu2+ is mainly in site SI’. This correlates with the weaker aluminum modulation in type I1 Cu2+ but it does not explain the apparently more prominent double modulation component in type I1 Cu2+. Site SI’ might be favored if Cu2+ coordinates to three framework oxygens in one hexagonal face of a hexagonal prism and to an extra framework oxygen formed during dehydration. The presence of extra framework oxygen in dehydrated zeolites has been indicated by X-ray crystallography and by temperature-programmed desorption.29 Upon rehydrating CuK-Y and Cu2+moves from the hexagonal prism to site SII* but in CuNa-Y the Cu2+stays in the hexagonal prism. The difference in the Cu2+location in the rehydrated Na-Y and K-Y zeolites can be qualitatively explained as follows. The preferential site locations for metal cations are roughly determined by the electrostatic energy of the site for the cations and the (28) Gallezot, P.;Ben Taarit, T.; Imelik, B. J . Catal. 1972,26, 295. (29) Iwamoto, M.;Nakamura, M.; Nagano, H.; Kagawa, S.; Selyama, T. J. Phys. Chem. 1982,86, 153.

J. Phys. Chem. iW3, 87, 4437-4441

adsorption (coordination or solvation) energy of the cation with adsorbed molecules. In zeolites AIOzhas an effective negative charge. Site SI has the largest s u m of electrostatic contributions from this. Therefore, if there are no adsorbed molecules, site SI is the most preferred site for the cations. However, since adsorbed molecules cannot occupy site SI, it has the lowest adsorption energy. In contrast, site SIV near the center of the supercage has the lowest electrostatic energy and the highest adsorption energy because the free space around site SIV can allow the cation to be fully coordinated by adsorbed molecules. Upon ion exchange with C U ~ + ( H ~ O Cu2+ ) ~ ,loses half of its coordinated water molecules and is trapped in site SII*. Through this process the Cu2+gains more electrostatic energy than it loses adsorption energy. When the adsorbed molecules are removed thermally, Cu2+goes into the hexagonal prism to maximize the electrostatic energy. Upon rehydration, most of the alkali cations are hydrated. The migration of Cu2+from the hexagonal prism to the site SII* is normally accompanied by the migration of an alkali cation to SI. If the alkali cation is hydrated it must lose its coordinated water before going into SI, so the total energy for transferring Cu2+out of the hexagonal prism increases with increasing adsorption (hydration) energy of the hydrated alkali cation. The hydration energy is 27% higher for Na+ with a smaller ionic radius than for K+. In the postulated model this difference is apparently sufficient so that, upon

4457

rehydration, Cu2+in CuNa-Y stays in the hexagonal prism, while in CuK-Y, Cu2+migrates from the hexagonal prism to site SII* and K+ replaces Cu2+in the hexagonal prism. In addition to hydration energy the diffusion of the hydrated alkali cations through the 6-rings is probably relevant. The diffusion coefficients are difficult to predict but it is interesting that in water the mobility of K+is 50% greater than that of Na+. If relevant to zeolites, this is in the correct direction to contribute to the preferential Cu2+ locations found. Since Cu2+ in CuNa-Y does not migrate into site SI directly upon ion exchange as is the case in CuNa-X,’* it is concluded that some activation energy is necessary for Cu2+migration from the supercage to the hexagonal prism. The electrostatic energy of site SI in the hexagonal prism is greater in X zeolites than in Y zeolites because of the lower Si/Al ratio in X zeolites. Within this model this additional energy is sufficient to overcome the activation energy so that Cu2+in CuNa-Y can migrate directly into site SI during ion exchange.

Acknowledgment. This work was supported by the National Science Foundation and the Robert A. Welch Foundation. We thank M. Narayana for his critical comments. Equipment support from the University of Houston Energy Laboratory is greatly appreciated. Registry No. Cu, 7440-50-8; HzO, 7732-18-5.

Composltlon of Clathrate Gas Hydrates of H2S, Xe, SO,, CI,, CH,CI, CH,Br, CHCIF,, CCI2F2,and C3Hs George H. Cady Depertment of Cbmbm, UnherSny of Washlngfon, Siwttk, Washington 98195 (Received: December 9, 1982)

Compositions of the clathrate hydrates of gases have been determined at 0 “C by quantitative synthesis at various constant pressures. Within the limits of experimental accuracy, the hydration numbers of CC12F2and C3H8are 17.0. Hydration numbers of H2S,Xe, SOz, CH3Cl,and (probably) CH3Br and CHCIFzdecrease as the pressure increases in quantitative agreement with predictions of the statistical theory of van der Waals and others, as applied to structure I hydrates. For chlorine there is qualitative but not quantitative agreement. Hydrogen sulfide moleculeti appear to occupy equally the two sizes of cavities in structure I solid. With increasing molecular size in the order, Xe, SOz, Clz, CH3Cl,CH3Br,CHC1F2,the tendency to occupy the smaller type of cavity becomes less in comparison to the tendency to occupy the larger type. However, even the largest of these molecules apparently occupies some of the smaller sites.

In an earlier paper the author described a procedure for the quantitative synthesis of a clathrate gas hydrate by slow condensation of a hewn weight of water vapor upon a cold surface at constant temperature in the presence of the gas a t constant pressure.’ The influence of pressure and temperature upon the composition of several hydrates was reported and the experimental results were compared with those predicted by the statistical theory of van der Waals and others.2-5 In some cases agreement was good; in others, however, it appeared that agreement might not (1) G. H. Cady, J. Phys. Chem., 86, 3225 (1981). (2) J. H. van der Waals, Tram. Faraday Soc., 52,184 (1956). (3) R. M. Barrer and W. L. Stuart,R o c . R. Soc. London,Ser. A, 243, 172 (1957). (4) J. H. van der Waals and J. C. Plateeuw, Adu. Chem. Phys., 2, 1 (1959). (5) D. W. Davidaon in ‘Water: A Comprehensive Treatise”,Vol2, F. Franks Ed., Plenum Press, New York, 1976, Chapter 3. 0022-3654/83/2087-4437$01.50/0

be good. Additional experimental information of the type which was needed is ?ow presented.

Experimental Section Some of the new data were obtained with the procedure described previously.’ In this method a weighed sample of water (usually about 0.2 g) distilled from the bottom of a reactor containing the gas a t constant pressure and condensed upon the surface of a cold finger, usually at 0 OC. The hydrate was then weighed and the hydration number, n, was calculated from eq 1. A somewhat difg of water X mol wt of gas n= (1) g of combined gas X mol w t of water ferent procedure was used to obtain some of the new data. Weighings of the new reactor shown in Figure 1were made without the plastic cup in place. About 0.2-0.5 g of water 0 1983 American Chemical Society