2642
J. Phys. Chem. 1992,96, 2642-2645
Formation of Alkali Metal Partides h Alkali Metal Cation Exchanged X Zeollte Exposed to A t k a Metat Vapor: Control of Metat Particle Identity
Bo Xu and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: August 5, 1991)
The series of five alkali metal (M,) vapors formed by alkali metal azide decomposition were reacted with the series of alkali metal (MI) cation exchanged X zeolites to form alkali metal particles. For Rb+ and Cs+ exchanged X zeolites, Rb, and Cs, metal particles are formed regardless of the choice of M2vapor, so the cluster atoms must originate from the exchanged cations rather than the alkali metal vapor atom. Rb and Cs vapors exposed to any other alkali metal cation exchanged X zeolite (Li, Na, K) give Rb, and Cs, metal clusters, respectively, except for Cs/K-X. For the other M2/MI-X systems the metal particle seems to be formed from the M2metal vapor atoms, except for Na/Li-X. These results are discussed in terms of electron-transfer energetics, electron transfer via the zeolite framework, ion and atom sizes relative to ring sizes in the zeolite framework, and ion/atom migration.
Introduction The formation of dispersed alkali metal species in zeolites is of interest for catalytic applications.'-' When alkali cation exchanged X and Y zeolites (M-X and M-Y) react with an alkali metal vapor, two kinds of metal clusters are formed: ionic clusters l i e Nq3+,Naas+, K?+,and IC,'+ and alkali metal particles M," The Nan" and K,"+ ionic clusters are formed in Na-X/Y and K-X/Y zeolites, respectively, when reacted with Na, K, or Rb metal vapors.5" Thus the atoms constituting the ionic cluster can be derived from the original ions in the zeolite. All of the alkali metal particles, Li, Na,,, K,,, Rb,,, and Cs, have been formed in the corresponding M/M-X/Y zeolites and are characterized by different electron spin resonance (ESR) g value^.^^^^^ In this study a complete matrix of experiments involving the reaction of all five alkali metal vapors with all five alkali cation exchanged X zeolites is reported. The symbol M2/M1-X is used to denote a sample in which the M2 alkali metal vapor reacts with the MI alkali cation exchanged X zeolite. Particular attention is focused on the identity of the different metal particles formed in the M2/M1-X systems and on their probable locations.
Experimental Section
TABLE I: Room Temperature ESR g Values of Alkali Metal Puticks in Alkali McW Cation Exckmged X zeolltar Reacted with Alkali Metal Vapor and Assignment of Metal Partick Formed metal Li-X Na-X K-X Rb-X Cs-X vapor Li
Na
K Rb
Cs
(1) Martens, L. R. M.; Grobet, P. J.; Jacobs, P. A. Nature 1985,315,568. (2)Martens, L. R. M.; Grobet. P. J.; Verrneiren, W. J. M.; Jacobs, P. A. In New Lkvelopmenrs in Zeolite Science and Technology; Murakami, Y ., Iijima, A., Ward, J. W., Eds.; Kodansha: Tokyo, 1986;#p935. (3) Martens, L. R.M.; Venneiren, W. J. M.; Grobet. P. J.; Jacobs, P. A. Stud. Surf. Sci. Catal. 1987,31,531. (4)Rabo, J. A.;Angell, C. L.; Kasi, P. H.; Schomaker, V. Disc Faruduy Soc. 1966,41, 328. (5) Hamson, M. R.; Edwards, P. P.; Klinowski, J.; Thomas, J. M.; Johnson, D. C.; Page, C. J. J . Solid Stare Chem. 1984,54, 330. (6)Kasai, P. H.; Bishop, R. J., Jr. ACS Monograph 1976, 171, 350. (7) Harrison, M. R.; Klinowski, J.; Ramadas, S.;Thomas, J. M.; Johnson, D. C.; Page, C. J. J . Chem. SOC.,Chem. Commun. 1984,982. ( 8 ) Xu, 8.; Kevan, L. J . Chem. SOC.,Faraday Trans. 1991, 87, 2843. (9) Blazcy, K.W.;Mtiller, K. A.; Blatter, F.; Schumacher. E. Europhys. Letr. 1981. 4, 857.
Na
1.9999 K 1.9929
2.0004 Li or K 1.9998 Na or K 1.9997 K 1.9930
Rb
Rb
Rb
1.9640
cs
2.0053 Li 2.0016
1.9686
cs
1.9939 Rb
1.9936 Rb
1.9939 Rb
1.9929 Rb
1.9998
1.9931
K
Rb
1.9712
cs 1.9776 cs 1.9722 cs 1.9736 cs 1.9686 cs
TABLE II: Colldaetion ESR Line Widtb, (C) at Room T e m p e " for AIw3I Metal Cation Exelmaged X Zeolites h t e d i k b Alkali Metal Vapor metal vapor Li
Na
Na-X zeolite was obtained from Union Carbide Co. Other alkali metal cation exchanged zeolites were obtained by ion exchange as described previously.* Ion exchange is complete for Li-X and K-X and greater than 70% complete for Rb-X and Cs-X. Sodium azide was obtained from Aldrich Chemical Co. Other alkali metal azidea were synthesized as dewxibed previously.8 Alkali metal particles were prepared by an adapkions of the method developed by Martens et a1.l Each alkali metal cation exchanged zeolite was reacted with each alkali metal azide. Typically 0.2 g of zeolite was reacted with 0.02 g of alkali metal azide. In this way, the ratio of the number of atoms of alkali metal from the azide to the number of alkali cations in the zeolite was between 1/6 to 1/3. Due to the air sensitivity of the samples,
2.0034 Li 2.0032 Li 2.0008 K 1.9932
K Rb
Cs
Li-X Na-X K-X a a a 0.83 f 0.1 4.3 f 0.2 a 2.0 f 0.2 5.6 f 0.2 5.8 f 0.2 8.5 f 0.2 8.1 f 0.2 10.5 f 0.5 35.6 f 1 33.0 f 1 5.4 f 0.2
Rb-X 8.5 i 0.2 7.5 f 0.2 7.6 f 0.2 7.6 f 0.2 7.9 f 0.2
CS-X 44 f 1 47 f 1 23.2 f 0.5 23.2 f 0.5 21.3 0.5
Line width cannot be measured precisely because it is overlapped with another signal.
direct analysis of the amount of metal was not possible. ESR measurements were armed out on a Bruker ESP 300 ESR spectrometer at room temperature and 77 K near 9.3 GHz with 100-kHz magnetic field modulation.
Regults Room temperature ESR spectra of M-X samples reacted with alkali metal azides are given in Figures 1-5. Except for the Li/M-X samples which have a purple color, the rest of the samples are blue or dark blue in color depending on the concentration of the alkali metal. All samples give a single line at about g = 2.0 which is assigned to an alkali metal particle formed within the zeolite. Also hyperfine patterns are o b r v e d in Na/Li-X, M/Na-X, and M/K-X samples which are assigned to alkali metal ionic clusters formed in the @-cage, which have been analyzed previously.6q8 The g values of the central lines in these samples are listed in Table I. The line widths of the central lines are shown in Table 11. They do not change from room temperature to 77 K. All the central lines have a Lorentzian line shape and the microwave power saturation results are consistent with homogeneous broadening. In some samples, as shown in Table 11, the overlap
0022-3654/92/2096-2642$03.00/0Q 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, NO. 6, 1992 2643
Formation of Alkali Metal Particles
I I
IJ
WLi-X
-P
Na/Li-X
40G
1
Figure 1. ESR spectra at 300 K of Li-X zeolites reacted with alkali
NdRb-X
Figure 4. ESR spectra at 300 K of Rb-X zeolites reacted with alkali metal vapor from the corresponding azides.
metal vapor from the corresponding azides.
wL Li/Na-X
c100 --G t---,
Figure 2. ESR spectra at 300 K of Na-X zeolites reacted with alkali
metal vapor from the corresponding azides.
100G
Figure 5. ESR spectra at 300 K of Cs-X zeolites reacted with alkali
metal vapor from the corresponding azides. of all the ESR signals in these samples decrease with increasing temperature.I0
A\/"L
-
-
LYK-X
40G
Figure 3. ESR spectra at 300 K of K-X zeolites reacted with alkali metal vapor from the corresponding azides.
of the central line and the hyperfine lines precludes a precise measurement of the line width of the central line. The intensities
Discussion A. Ionic Clusters. The hyperfine patterns found in M/Na-X, M/K-X, and Na/Li-X samples originate from the sodium ionic clusters and potassium ionic clusters in the 8-cage. The identification of these ionic clusters in zeolite X and Y was discussed in previous s t ~ d i e s . ~The ~ ~ atoms J in the ionic clusters are composed of the exchanged metal cations in the zeolite except for Na/Li-X. Na/Li-X is the only one in the M/Li-X series in which a hyperfine pattern is observed. This hyperfine pattern has about the same coupling constant (32 G) as that found in Na/ Na-Y in which the Na43+ionic cluster in b-cage is formedSand thus is similarly assigned in Na/Li-X. This suggests that, in Li-X, a sodium ionic cluster can be formed which implies that, during sodium metal vapor exposure, sodium cations are generated by electron transfer to Li+ which then migrate into the /%cage. B. Identification of Metal Particles. Alkali metal particles formed in M/M-X systems were characterized by conduction ESR lines in previous work6 and have distinctly different g values. Table I shows these g values along the diagonal for M/M-X systems. This allows one to make the identifications indicated in Table I. For example, the g value of the central ESR signal (10) Kittel, C. Introduction to Solid State Physics; Wiley: New York, 1953; Chapters 12 and 13, p 223.
2644 The Journal of Physical Chemistry, Vol. 96, No. 6, 199I2 TABLE I11 AH Values for Gas-Phase Electron Transfer from Alkali Metal Atoms to Alkali Metal Cations' (kJ/mol) AH Li' Na+ K+ Rb+ Cs' Lio 0 244 1014 1172 1445 Nao -244 0 770 928 1201 KO -1014 -770 0 158 43 1 -928 -158 Rbo -1172 0 273 -1201 -431 -273 0 CSO -1445
'Values are calculated from the gas-phase ionization potentials of alkali metal atoms from ref 13a. in Na/Li-X is very close to the value of a lithium metal particle and is so assigned. Table I1 shows the line widths of the metal particles. In general, similar line widths are expected to a first approximation for the same particles. Also, the line widths are expected to increase with atomic size due to an increasing spin-orbit interaction."J2 The data in Table I1 are reasonably consistent with the metal particle identifications made in Table I. In the Na-X zeolites exposed to different alkali metal vapors the g values of the conduction ESR line indicate that the metal particles formed in each M/Na-X sample are composed of M atoms. This is also true in the M/K-X systems, except for Cs/K-X, although there are some uncertainties for Li/K-X and Na/K-X. It is evident that the spectra for Rb-X and Cs-X samples (Figures 4 and 5 ) and their paramagnetic parameters (Tables I and 11) are different from those in Li-X and Na-X samples. For M/Rb-X and M/Cs-X the g values are about the same (Table I), indicating formation of the same metal particle for all M. The line widths are also about the same for the M/Rb-X samples and fall into two groups in the M/Cs-X samples. These observations indicate that the metal species formed in these two kinds of zeolites are composed of the exchanged metal cations in the zeolites. C. Electron-Transfer Thermochemistry. In Table 111, the gas-phase enthalpies for electron transfer from alkali metal atoms to alkali metal cations are given.lga Positive values in the table indicate that electron transfer from the alkali atoms with smaller atomic number to the alkali cations with larger atomic number is endothermic in the gas phase. The gas-phase thermochemistry may be expected to apply if the metal vapor atom can closely approach the cation within the zeolite structure. This seems to apply to Na/Li-X, Cs/Rb-X, and Cs/K-X in which electron transfer from the metal vapor atom occurs. This may apply to Li/Na-X, Li/K-X, and Na/K-X, but in these cases electron transfer from the metal vapor atom does not occur as expected from the gas-phase thermochemistry. It may also be expected that the gas-phase thermochemistry is significantly modified by the highly ionic nature of the zeolite lattice. This can either result in no electron transfer when expected or reverse electron transfer from a smaller atomic number atom to a larger atomic number cation. Such effects should be more important when the metal atom to metal cation approach distance is greater. The following systems show no electron transfer although such would be exothermic in the gas phase: K/Li-X, Rb/Li-X, Cs/Li-X, K/Na-X, Rb/Na-X, Cs/Na-X, and Rb/K-X. I t is rather interesting that the Cs/K-X system does not also fall in this list. Apparently the gas-phase exothermicity is sufficiently greater for Cso K+ electron transfer compared to Rbo K+ electron transfer so that in X zeolite the former occurs and the latter does not. The third category of systems is where electron transfer occurs, but in the reverse direction from that predicted by the gas-phase thermochemistry. These examples all include electron transfer from a smaller atomic number metal vapor atom to the large
-
-
(1 1) Elliott, R. J. Phys. Rev. 1954, 96, 266. (12) Yafet, Y. Solid Sfate Phys. 1963, 1 4 , 1. (13) (a) Handbook of Chemistry and Physics, 63th ed.; Weast, R. C., Astle, M. J., Eds.; Chemical Rubber: Boca Raton, FL, 1982, E64. (b) Ibid. p F-179.
Xu and Kevan TABLE I V Comparison of the Diameters (nm) of Alkali Metal Atoms and Cations and of Rings and Cages in Zeolite X alkali ring alkali atom metal cation zeolite or cage atoms diameter' cations diametefl rings/cages diameterC 0.14 4-ring 0.16 Lio 0.30 Li+ Na+ 0.19 6-ring 0.28 Nao 0.37 KO 0.46 K+ 0.27 12-ring 0.80 Rb+ Rbo 0.49 0.29 @-cage 0.66 Cso 0.53 Cs+ 0.33 a-cage 1.30 "Data taken from ref 14. *Data taken from ref 13b.
