Journal of Molecular Catalysis, 27 (1984) 71 - 80 71

FRAMEWORK CHEMISTRY AND STRUCTURE INVOLVING ELECTRON TRANSFER AND EUROPIUM IN ZEOLITES

GALEN D. STUCKY

Central Research & Development Department, Experimental Station, E. I. du Pont de Nemours and Company, Wilmington, DE 19898 (U.S.A.)

LENNOX ITON, TIM MORRISON, GOPAL SHENOY

Material Science & Technology Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439 (U.S.A.)

STEVE SUIB

Department of Chemistry, University of Connecticut, Storrs, CT 06268 (U.S.A.)

and RICHARD P. ZERGER

Physical Science Department, McPherson College, McPherson, KS 67460 (U.S.A.)

Summary

The relationship between the zeolite framework and the coordination number and electron-transfer properties of Eu*+ and Eu3+ have been inves- tigated by time-resolved luminescence (TRL) EXAFS, EPR and Mossbauer spectroscopy. TRL-determined lifetimes are sensitive to non-framework oxygen atoms and can be used to help distinguish between coordination involving OH groups and the framework oxygen atoms. The suitability and some of the present limitations of this application of TRL, which is new to the study of zeolites, are described. The average coordination of hydrated Eu3+ in the large-pore zeolite Y is identical to that in aqueous solution, while the coordination number in zeolites A and ZSM-5 is smaller, with framework oxygen atoms displacing water from the Eu3+ coordination sites. Europium metal dissolved in liquid ammonia can be used to generate Eu(NH~)~*+ and solvated electrons. With zeolite X, this solution results in the formation of Nae3+ and two kinds of defects. These are assigned to an electron trapped in an oxygen vacancy arising from a Lewis acid site and a V-type hole center in which the unpaired electron resides on a non-bridging oxygen atom bonded to silicon.

Introduction

It is becoming increasingly clear that under even mild reaction con- ditions, one must view the zeolite framework as a dynamic system which can readily undergo thermal and adsorbatedriven framework distortions [l - 31; oxygen atom displacement with H20, O2 and CO [ 4 - 71; topotactic

0304-5102/84/$3.00 0 Elsevier Sequoia/Printed in The Netherlands

72

transformations associated with the rearrangement of framework oxygen bonds [ 81; and the dissociation of aluminum atoms from the zeolite frame- work [9].

Supported metal atoms add another dimension to the zeolite frame- work chemistry, particularly when they are involved in electron-transfer processes or are in the higher (> +l) oxidation states, so that they promote hydrolysis reactions and can effectively compete for the bonding and chem- istry which involves the framework oxygen atoms.

Europium has a number of characteristics which makes it an useful probe for studying supported metal atom siting and the associated frame- work chemistry under redox or changing thermal conditions. Kasai and Bishop have made use of the readily available Eu3+/Eu2+ redox potential of -0.429 V for a thermally driven water-splitting cycle between room tem- perature and 300 “C [lo]. In addition, like alkali and alkaline earth metals, europium metal can be dissolved in liquid ammonia to give a strongly re- ducing lower valent system. The 4f half-filled shell of Eu*+ and Gd3+ has a Russell-Saunders ground term of 8S,,2. The second-degree terms of the spin Hamiltonian are defined by the fine structure of the EPR spectrum and reflect the symmetry of the crystal field of these ions within the zeolite lattice. The emission lifetimes from the Eu3+ ( 5D, + 7F2) and Tb3+ ( ‘D4 + ‘FS) transitions have been used by Horrocks and Sudnick [ll] to monitor water coordination to europium and terbium chelate complexes and large molecules of biochemical interest. As shown below, this approach is also useful in following changes in Eu 3+ hydrate coordination in zeolites. Finally, europium has unusually large chemical isomer shifts (IS) associated with the 21.6 keV resonance of 151Eu, which may be divided into three groups: ionic Eu*+ compounds (4f’) (-16.0 to -10.9 mm s-l), metallic band sys- tems (-11.4 to -7.6 mm s-l), and ionic Eu3+ compounds (4f6) (-0.2 to +0.9 mm s-l). In this paper, a brief summary is given of the use of these properties for the study of metal-framework interactions in zeolites A, X, Y and ZSM-5.

Experimental

Eu ion-exchanged zeolites were prepared from Linde Na-4A, Na-Y and Na-13X with compositions Na12(A102)12(Si02)12*27H20, Nas6(AlG2)s6-

(si”2h6- 256H,Q Na86(A102)86(SiO,),,,’ 26H,O. ZSM-5 was prepared according to example No. 27 of U.S. Pat. 3 702 886 [ 121. The X-ray powder diffraction pattern confirmed the presence of ZSM-5, and chemical analyses

gave (Na2G)7.3(A12Gs)s.s(SiG2)s7.4. Chemical analyses were carried out by neutron activation, wet chemical, and wavelength and energydispersive X-ray spectroscopic techniques.

Aqueous ion-exchange was undertaken using EuCl,. 6Hz0 at a buffered pH of 5.0 by taking 10 mmol 1-l of 0.1 N EuCl, and stirring for 12 - 24 h. The samples were repeatedly washed with deoxygenated and deionized

73

distilled water until no AgCl was formed upon addition of AgN03 to the wash water. Europium, sodium, silicon and aluminum analyses of the zeolite by neutron activation gave the europium exchange as (Eu,.,Na,.,),Al,,-

Si12048 (A), (Eu,,Naz),Al,,Si,360384 (Y) and ((Eu,.,,3+Nao.14)x0),.3-

W2W8.3WW87.4 (ZSM-5). The value of 3t, which is determined by the pro- ton exchange at pH = 5, was not established. For the luminescence lifetime samples, approximately 0.15 g of exchanged zeolite were loaded into 5 mm X 5 cm quartz tubes. In order to prepare fully hydrated H,O-D,O samples, various D,O/H*O mixtures ranging from 0% D20 to 100% D,O were added to the sample tubes and the zeolite-water mixture stirred to create a slurry. The sample tubes were tightly stoppered, centrifuged and the water mixture left overnight in contact with the zeolite. This process was repeated as necessary in order to obtain the desired D,O/H,O ratio. The samples were loaded directly, without air drying, into an air-tight sample holder for fluorescence measurements. Europium exchange with the zeolites using europium dissolved in liquid ammonia was carried out as described previously [ 131.

Luminescence lifetime measurements were made with a Molectron UV-24 nitrogen laser at a wavelength of 337 nm and with a 5 mJ per 12 ns pulse output. Emission from the Eu 3+ [5D, --f ‘F,] transition at 618 nm was monitored at an angle of 30” from the incident beam using a PRA B204 monochromator followed by a Hammamatsu R-928 photomultiplier tube operated at 1000 V. Decay constants were obtained using an exponential curve-fitting program (HP STAT l-23) which yielded correlation coefficients greater than 0.985 and a reproducibility from sample to sample of about 5%.

EPR spectra were recorded using a Varian E-Line or E-Line Century spectrometer operated at X-band (- 9.1 GHz) or Q-band (- 35 GHz) with 100 kHz field modulation. A strong pitch sample (g = 2.0028) was used as a g-value reference.

EXAFS measurements in the transmission mode were carried out at the Stanford Synchrotron Radiation Laboratory on beamline I-5. In all cases, at least three consecutive runs on each sample were summed to im- prove statistics as described previously [ 141.

Liquid nitrogen and room-temperature 15iEu Mossbauer spectra were obtained on a constant acceleration spectrometer. The lslEu data were computer fit to Lorentzian lines. All Mijssbauer samples were loaded in polyethylene cylinders and sealed in a dry box with a mixture of beeswax and paraffin.

Results and discussion

Framework and hydration coordination The degree of hydration of a polyvalent ion within a zeolite is deter-

mined by the cation charge/radius ratio; the framework Si/Al ratio, which determines the framework oxygen atom basicity; and by the volume

74

available for the complexed cation. Because of disorder and migration, the water coordination number is difficult to determine by diffraction tech- niques. Bennett and Smith [ 151, discussing the location of cations of hy- drated Ca faujasite, proposed that the water molecules switch places continuously with the cations inside the sodalite units so that sites I’ and II’ are partially occupied by both water molecules and cations. Olson et al

[16] have shown by single-crystal diffraction studies that in the hydrated rare-earth Ce faujasite zeolite, the rare earth ions are contained within the 13 A large cavity and coordinated to an undetermined number of water molecules; but upon dehydration at 350 “C, the metal ion is coordinated to three water molecules and the three framework atoms of the six ring at site I’. In contrast, hydrated zeolite X has partially dehydrated rare earth ions with framework coordination. Costenoble et al. [17] have discussed in detail the complexities of water, hydroxyl and cation distributions within zeolites La- and Ca-X and La-Y by TGA, IR and powder X-ray methods. They suggest that residual water is not present above 495 K in zeolite Y and 623 K in zeolite X.

A plot of the lifetime decay constant for Eu3+-exchanged Y-zeolite versus X(H20) = [H,O]/[D,O] is shown in Fig. 1. The linearity of the plot shows that the luminescence lifetime measurements can provide a reliable measure of the number of water molecules in the primary coordination sphere of the europium(II1) ion in zeolite Y. Using the same scales as those determined by Horrocks et al. [ 11, 19, 20 3, these data define the coordina- tion number (CN) of fully hydrated Eu3+ to be 8.7 (9.7 ms-‘). The water of hydration is easily displaced and if the zeolite is equilibrated overnight in a dessicator with a saturated solution of NH&l, a hydration CN of 6.9 (7.7 ms-‘) is found. Dehydration at 300 “C for 4 h results in a CN of 3.2 (4.1 ms-l). The smaller pore ZSM-5 gave a CN of 4.8 (5.7 ms-‘) in the

10.0 -

I.0 - -0

0.0 - I I I I I

0.0 0.2 0.4 0.6 0.8 I

XHZO

Fig. 1. Plot of decay constant, (h) in rns-’ versus the mole fraction of water for fully hydrated europium(III)-exchanged Y-zeolite.

75

fully hydrated form, while the CN in A, fully hydrated, is 5.0 (5.9 ms-‘) and decreases to 1.4 for Eu-A heated to 300 “C for 2 h under a vacuum of 10” torr. It is of interest to note that the values obtained for the CN of hydrated A (4.0 - 5.0) are consistent with the singlecrystal X-ray diffraction results of 4.50 reported by Firor and Seff [18] for hydrated europium- exchanged A. As for zeolite Y, the storage conditions are critical with respect to the local hydration CN of Eu3+.

There are aspects of Eu3+ coordination and hydration in the solid state not encountered in studies of large biomolecules [19, 201 which require considerably more study in order to extend the usefulness of the technique to zeolite systems. Preliminary studies by Richardson [Zl] suggest that in europium complexes, the quenching rate for an -OH group in a hydroxyl group is cu. l/2 that for an -OH group in a water molecule. We find, how- ever, a decay rate for Eu(OH)~.~H,O of 48.0 ms-’ which decreases upon dehydration. The luminescence decay constants for samples calcined under vacuum above 400 “C also are much larger with k = 18.5 ms-’ for Eu-Y calcined at 750 “C. It is not clear at this time to what extent the increase is related to the formation of Eu2+ and quenching by a charge-transfer transi- tion Eu3+ +- Eu2+; or, to quenching by hydrolysis species. In any event, once understood, the large change in the decay constant could provide an additional, useful means of studying europium ion chemistry within zeolites.

Time-resolved luminescence (TRL) gives information about the hydra- tion coordination. EXAFS describes the total local coordination environ- ment about the europium atom. Analysis of the L(II1) EXAFS spectra gives coordination numbers of S.?(5) for Eu-Y and 6.7(5) for Eu-A. The former agrees with that found by TRL but the latter is 2.7-times larger than the value obtained by TRL for a sample of hydrated Eu-A prepared under the same atmospheric conditions. The difference can be assigned to framework atom coordination sites.

Framework and metal ion redox &ehavior Aqueous exchange We have shown previously that the ion-exchange of Eu2+ with Eu(OH),-

(H,O), into zeolite A, reported to give EuSNa2Si,2A112048~ 12H20 [ 181, involves instead Eu 3+ almost exclusively [ 131. Eu(OH)z*HzO itself trans- forms via the reaction:

~Eu(OH)~*H,O - ZEUS + H,t

with a half-life which rapidly decreases if the isolated solid is exposed (from ca. 15 d to < 1 min) to deoxygenated water [13]. The electronic properties of aqueousexchanged EuJf, which has been thermally and/or hydrogen reduced to Eu’+, and Gd3+ in zeolites A, X and Y have been described [ 13,25, 271 and will only be discussed briefly.

The conversion of Eu3* to Eu2+ upon thermal heating was monitored by EPR and the 4f65d-4f7 transition at 440 - 480 nm. The 455 nm band for Eu2+-Y was observed initially at 325 “C upon degassing zeolite Y and was

76

completely removed by rehydration at room temperature. A broad band at 440 nm for Eu2’-A did not appear until heating at 400 “C and was then reduced to one-third of its integrated intensity upon room-temperature hydration. Arakawa et al. [28] have reported that oxygen is not given off until 475 “C for Eu3+- M, Eu3”-X or Eu3+-Y; a temperature somewhat higher than the temperature (373 “C) required for 0, evolution in the autoreduc- tion of cupric ions in Cu-Y [29]. Of considerable interest is the observation of an emission band at 380 nm in Eu-hectorite which can be attributed to the reduction of Eu3+ to Eu2+ upon heating the clay to 600 K [ 23 J.

In order to increase the amount of Eu2+ that can be exchanged into a zeolite and to explore electron transfer and zeolitic ammonia chemistry, ion-exchange of Eu(NH~)~~+ into zeolites X and Y was carried out by dis- solving metallic europium in liquid ammonia [13].

The EPR spectrum of ammonia~xch~ged Eu-Y is that expected for Eu2+ in an ammoniacal solution [30, 313 with bzo = 0.02 cm-’ and no evidence of framework interactions. As for hydrated Eu-X and Eu-Y, framework interactions are more important in ammonia-exchanged Eu-X. The axial components of the CF for zeolite X are distributed around b,* - 0.065 cm-’ and the rhombic components around b,* - 0.059 cm-‘.

The X-zeolite also showed trapped Na4 3+ electron centers at the initial stages of the exchange with 13-line nuclear hyperfine structure and lattice defects in substantial concentrations. Nag3+ has been observed previously in zeolites [32, 331 upon treatment of these zeolites with vapor-phase sodium.

Lattice defects can be generated by treatment of zeolite X with Na/ liquid NH,, as well as Eu/liquid NH3 (Fig. 2). Both defect resonances display axial g-factors. The more intense hole center (II), gll = 2.019 and g, = 2.005, is characterized by very well-resolved axial anisotropy with no 27A1 super- hyperfine or 14N hyperfine splitting. y-Irradiation of zeolites produces V, and V, trapped hole centers [34,35] of the form

(T = Al for V, ; T = Si for V,)

in which the unpaired electron occupies the p-orbital of an oxygen atom bridging two tetrahedral atoms of the framework. The rather large positive g-shift suggests that the hole center may be on a non-bridging oxygen atom (I) and stabilized by a neighboring, non-framework cation 136, 3’71.

‘Al O\ /

/ \ /

sl\

(I) There is no evidence of motional averaging to 295 K, and the trapped hole is tightly bound to the lattice.

I I

3.2 3.3 3.4

’ )

x 1.t 5 :c :

3.3 3.4 :

ci

3.2

Fig. 2. X-band EPR spectra of trapped hole and trapped electron centers in zeolites exchanged with metal ammonia solutions: (a) 95 K spectrum of Na-X-zeolite treated with Na/liquid NH3; (b) 295 K spectrum of Na-X-zeolite treated with Na/liquid NHs; (c) 95 K spectrum of Na-X-zeolite treated with Eu/liquid NH3; (d) 95 K spectrum of Ca-A-zeolite treated with Eu/liquid NHs.

The relatively large negative g-shift (Ag, - -0.023) of the center with gll = 1.968 and g, = 1.979 is consistent with a trapped electron center which has substantial delocalization of the unpaired electron into an unoccupied 4p orbital of the Si atom [ 38 - 401. The anisotropy of this resonance was resolved at 95 “C but not at 295 K due to the effects of thermal averaging evident in the spectrum of this center at 295 K. This would be expected for a defect such as II in which the unpaired electron is not tightly bound to the silicon cation. y-Irradiation of zeolites does not lead to significant amounts of stable trapped electron centers located on the framework lattice.

OY41 0

0’ ‘0 SiYo + e- -

0’ ‘0 O\Al 0’ ‘0

q S/C

A’0

(II)

Defect sites involving 3coordinate aluminum produced by ionizing radiation of aluminum-doped silica have been described by Brower [ 41, 421. In that case, the electron was trapped by the 3coordinate alumina. In our case, the electron is trapped by the positively charged silicon atom. Defects analogous to both of the types described above have been extensively studied in y-irradiated fused silica and alkali silicate glasses where they exhibit high thermal stability. Lewis acid sites are the electron acceptors in the formation of radical cations upon adsorption of organic molecules (for example, benzene) on dehydrated zeolites. The mechanism proposed by Shih [43] in which the V, center serves as the electron acceptor is not

78

supported by the ESR evidence. The spectrum of the Vi center [34] is quite different from the spectrum assigned by Shih. The production of paramag- netic defect centers in zeolites by chemical reaction is unusual.

Several observations can be made about the nature of the chemical species present in the zeolites from lslEu Mijssbauer spectroscopy. The sensitivity of detection for the europium Mossbauer spectra in this experi- ment was approximately 1% (weight). With this limitation, no europium metal was observed for ammonia-exchanged A, X, or Y. The observed Eu*+ Mossbauer chemical shifts varied from -13.5(2) to -14.71(8) mm s-l (relative to SmFs) which are at the lower end of the range of Eu” chemical shifts and well below that for europium metal (-7.8 mm s-i). Eu3+ shifts were also observed (0.63 to 1.24 mm s-l). The solid hexamine Eu(NH~)~*+ can be crystallized from solution and decomposes gradually at room tem- perature to metallic Eu, amides (Eu(NH*)*) or imides (EuNH) [44, 451. The Miissbauer chemical shifts for Eu(NH~)~~+ and Eu(NH,), are -12.6 and -11.9 mm s-l respectively [46, 471 and were not observed. The temperature dependence of the EPR of these compounds also differs substantially from that observed here [ 30, 311. Other known species considered include Eu,03 (diamagnetic, 1.0 mm s-l); EuO (11.31 mm s-i); and Eu304 (-12.4 mm s-l and 0.7 mm s-l). The isomer shifts for both the Eu3+ and Eu*+ ions in the ammonia-exchanged zeolites are not consistent with these values.

In the case of europium/liquid ammonia treated Ca-A, the EPR spec- trum revealed Eu2+/Eu3+ ferromagnetic exchange through the excited ‘Fi state of Eu3+ (375 cm-’ above ‘F,). At very low temperatures (8 K), the EPR spectra manifest the effect of Eu2+/Eu3+ exchange.

Additional XPS and other data are needed to establish the external versus internal zeolite distribution of europium in this sample of zeolite A.

Conclusions

The metal-framework atom chemistry in the presence of adsorbates, although not a new topic, emphasizes both the many dimensions of zeolite chemistry and the very non-passive role of the framework. The manner in which framework properties and chemistry determine coordination prop- erties and the electronic state of the metal atom as a function of temperature and adsorbate is a multi-dimensional field with much yet to be learned. The EPR, TRL and EXAFS studies confirm the effect of framework basicity and channel size on metal atom hydrate/framework coordination. In both ammonia- and water-exchanged Eu-Y, there is no observable interaction with the zeolite framework at room temperature in the fully solvated form. TRL is useful in examining hydrate coordination for samples with a low metal ion loading, or in situations where long-range order is not present. Additional study is needed to understand the effect of hydroxyl groups and mixed valency on luminescence decay times. The surprisingly easy delo- calization of electrons within the zeolite cages, first noted by Kasai and

79

coworkers [32, 331 and reaffirmed with the ammonia-exchanged europium and sodium systems, suggests that chemistry involving electron-transfer processes may be particularly efficient in zeolites.

Acknowledgements

The authors wish to thank Gary Holtom and the Regional Laser Labo- ratory of the University of Pennsylvania. The support of NSF through Grant CHE-8204417 is also appreciated. The work at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Basic Energy Sciences.

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