Sensitive monitoring of side-products formed in heavy metal ion exchanged zeolites

J. Weitkamp, H.G. Karge, H. Pfeifer and W. HBlderich (Eds.) Zeoliies and Related Microporous Marerials: &are of ihe A n 1994 Studies in Surface Science and Catalysis, Val. 84 0 1994 Elsevier Science B.V. All rights reserved.

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Sensitive monitoring of side-products formed in heavy metal ion exchanged zeolites M. Warkl, W. Lutz2, E. Loffler3, H. Kesslerl and G. Schulz-Eklo@

* ENSC Mulhouse, Laboratoire de Matenaux Mindraux, 3 rue Alfred Werner, F-68093 Mulhouse, France

Institut fir Angewandte Chemie Berlin-Adlershof e. V., Rudower Chaussee 5, D-12484 Berlin, Germany Adlershofer Umweltschutztechnik- und ForschungsgeseIlschafl mbH, Rudower Chaussee 5, D- 12484 Berlin, Germany 4 Institut fur Angewandte und Physikalische Chemie der Universitat Bremen,

D-28334 Bremen, Germany

ABSTRACT

During the high loading of zeolites with metal ions the possibility of a precipitation of hydroxidic or oxidic side-products must be taken into account in dependence on the solubility products of the corresponding metal hydroxides. The side-products generated can be sensitively monitored, in spite of their small amounts and their amorphous states, by pH measurements, XRD, W-VIS and KR spectroscopy, due to interactions with the zeolitic framework. This is demonstrated exemplary by the exchange of zinc ions into zeolite A. 1. INTRODUCTION

The exchange of sodium cations from aluminosilicate zeolites by transition-metal ions has been used for several decades to obtain effective sorbents and catalysts [ 13. Nowadays zeolites are modified for hrther applications, e.g., as composites with new optoelectronic and photocatalytic properties [2-41. The encapsulation of quantum-sized semiconductor particles inside the zeolite pore system, e.g., the sulfides of Cd, Pb, Zn or the oxides of Zn and Cd, requires a pure starting material. The ion exchange of zeolites is a complex process which is, for example, accompanied by an additional hydronium ion incorporation as well as a precipitation of metal hydroxides at the outer surface of the zeolite crystals. Although the detected amounts of precipitated hydroxide are very small (less than 1 weight % of the mass of the zeolite), their presence led, in a non negligible extent, to a formation of bulk-like sulfide particles in a subsequent sulfidation step observable by a strong tailing at the absorption edges in the UV-VIS spectra of these samples [5,6].

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2. EXPERIMENTAL

Zeolites of type A, X or Y with crystal sizes between 2 and 8 p n were ion exchanged at room temperature in 0.01 - 0.1 M chloride or nitrate solutions of several cations (Kf,Ca2+, Zn2+ and Pb2+). The stirred batches had a liquidsolid mass ratio of 100. In order to remove freshly precipitated hydroxide, some highly zinc loaded samples were treated for five minutes in 10 weight-% ammonia solutions directly after the exchange procedure. The exchange process was followed by pH-measurements under inert gas with a glass electrode (Methrom ES 605). The degrees of ion exchange were determined from the residual solutions by AAS (Philips Pye Unicam 9200). The washed, dried (353 K) and calcined (423 K) samples were characterized by means of X-ray diffraction and IR-spectroscopy. For the X-ray diffraction measurements a Bragg-Brentano-arrangement with fixed slits (Seifert, Iso-Debyeflex 1001) was used. The IR framework vibrations were recorded on a FTR spectrometer FTS 15 (Digilab) with a resolution of 4 cm-1 using the standard KBr wafer technique. The optical absorption behaviour of the samples was determined with a W-VIS spectrometer (PE, Lambda 9) equipped with a diffise reflectance attachment using pure NaA as a reference. 3. RESULTS AND DISCUSSION 3.1. pH measurements

The addition of as-synthesized zeolites NaA, NaX or NaY to stirred, carbon dioxide-free water results in an immediate increase of the pH value due to a hydrolysis reaction, e.g.,: NaA + (H20), c,HA + (H20),.1 + Naf + OHSodium cations are partially exchanged by hydronium ions, and the formed free hydroxyl ions are responsible for the alkaline pH of the slumes (Fig. 1). The sequence of alkalinity of the zeolites NaA > NaX NaY corresponds to the decreasing number of sodium ions present in the zeolite. In aqueous salt solutions, the alkaline reaction of the zeolite should be dominant since the acidity of usual salts in water is negligibly small (pH = 5-6). This is true for monovalent cations (Fig. 2). In the presence of bivalent cations, however, several effects must be considered: (i) the value of the solubility product of the hydroxides, (ii) the hydrolysis of the dissolved metal ions according to (Me: Ca, Zn or Pb) Me2+ + H,O c, [MeOH]+ + H+ and (iii) the hydronium ion re-exchange from the zeolite. Figure 2 demonstrates that even for calcium, a cation of relative high hydroxide solubility, the pH value of the exchange batch is drastically decreased compared to the value of NaA in water. Due to the large solubility product of 3. 1~10-~mo131-3, precipitation of Ca(OH)2 may be excluded. This means that any pH-decrease results from the formation of [CaOH]+ ions inside the zeolite matrix, a species which was found indeed in zeolites by IR measurements [7]. The amount of introduced [CaOH]+ can be calculated to be about 1-2 % of the supplied calcium ions.

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4 4 I I

7

DH - value

NaA

1

11 pH value

1

-

H20

1

2 3 4 5 0 1 2 3 4 time I hours trme I hours Figures 1/21 pH values of different types of zeolites in water (Fig. I ) and of NaA zeolite in several aqueous salt solutions (Fig. 2) versus the time of treatment. (The figures base on the figures 1 and 2 of ref. [5].) 0

1

6

In the case of Zn and Pb the solubility products of the corresponding hydroxides, which are

5.10-17and 2.10-16 mo13 P3, respectively, are several orders of magnitude lower.

Consequently, the pH values of zinc and lead exchange batches decrease drastically and fall below the values obtained for the pure salt solutions. This strong decrease results fiom the formation of hydroxides which precipitate at the crystal surface and which are clearly detectable by scanning electron microscopy [ 5 , 8 ] . The complexity of the exchange system can be remarkably demonstrated by a treatment of NaA in lower concentrated solutions, e.g., 0.01 M or 0.025 M. The addition of 5.104 mol ZnC12 (equivalent to 50 ml of 0.01 M solution) to 50 ml of a NaOH solution of pH = 10.2, accordingly to the alkalinity of NaA in water, leads to the final pH = 7.7 without observable precipitates. The drop of the pH value is assumed to originate fiom the formation of [ZnOH]+ ions by analogy to [CaOH]+ and/or the precipitation of very fine colloidal Zn(OH), particles not observable to the naked eye. After the addition of 500 mg zeolite NaA to the slurry the alkalinity rises, within 30 minutes, by one order of magnitude and results in a final pH = 8.8. The zeolite acts as a complexing agent for the Zn2+ ions and changes thereby the equilibrium states in the slurry. The incorporation of up to 5 % of the zinc cations as [ZnOH]+ can be also deduced from differences between the zinc and the sodium contents in the exchange solutions determined by AAS . Comparable effects were obtained by use of a ZnCl2 solution of intermediate concentration (0.025 M) (Fig. 3). In this solution, the number of Zn2+ cations present corresponds to only 80% of the total number of available cation sites in the zeolite. Small amounts of zinc hydroxide precipitated at the beginning of the process are re-dissolved later because of a progressive entering of zinc ions into the matrix. Thus the change of the pH value reflects the kinetics of the incorporation of zinc ions into the zeolite. Figure 4 shows that the differences in the alkalinity of the zeolites NaX and NaY is of little importance to the appearance of hydroxide precipitates, but that less hydroxide is formed in faujasites,which have a more open structure than zeolite A, indicated by the higher pH values.

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pH value

pH value 1

1

1

9-

0.01 M

7-

0.025M

5-

0.1 M

4

,

,

,

*

t

a

,

6,

,

NaA

'

2 3 4 5 0 1 2 3 4 time 1 hours time I hours Figures 3/4: pH values of zinc/zeolite batches with time for different zinc concentrations (zeolite A) (Fig. 3) and different types of zeolites (zinc concentration: 0.1 M) (Fig. 4). ( The curves in figure 3 are taken from ref. [5].)

0

1

3.2. XRD investigations

5

With the introduction of zinc cations into zeolite A, typical changes in the X-ray diffraction patterns take place. Due to the higher charge and the smaller ionic radius of the zinc ions compared to sodium ions, the lattice constant ofthe zeolite decreases from a = 24.61 f 0.OlA WaA) to 24.21 k 0.01 A (ZnNaA - exchange degree. 92 %) [9]. Furthermore, the intensity of some reflections is changed. The reflection (400) of the space group Fm3c, labelled (1) in Figure 5, appears due to the introduction of additional cations into zeolite A [10,11] and occurs independently of the existence of hydroxidic or oxidic sideproducts even for small degrees of ion exchange (Fig. 5b). The existence of side-products is accompanied by the occurrence of further weak, but significant, reflections with d m values of 7.4, 5.6 and 3.2 A exhibiting a distinctly increased full width at half height. The reflections which occur after the conversion of zinc hydroxide into zinc oxide at 423 K are labelled (2) in Figure 5c. Contrary to that, in highly exchanged faujasites no additional reflections were observed, probably due to yet lower amounts of sideproducts present and to the fact, that the more open faujasite framework reacts more flexible to a distortion of some TO4 tetrahedra, induced by an interaction of nano-oxide particles with the lattice. In zeolite A larger areas of the framework are influenced by the distortion. The reflections cannot be assigned to any modification of zinc hydroxide or oxide according to the JC-PDS, as well as no reflections stemming from any known zinc hydroxide or oxide phase are observable in the patterns. By analogy to the discussion by Lutz et a1.[8] the additional reflections (2) can formally be identified as (3 1 l), (33 1) and (553) reflections of space group F m k with a formal lattice constant of a = 24.3 0.1 A. We assume a partial change of the framework symmetry of the zeolite under the influence of occluded species, probably amorphous zinc oxide particles or [ZnOH]+ ions, as a reason for the appearance of these reflections. Washing of the zeolites directly after the exchange procedure with ammonia leads to only a slight reduction of the degree of ion exchange of 5-10 %, due to the formation of water soluble zinc ammine complexes, but to a complete disappearance of the additional reflections (Fig. 5d).

*

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This can be attributed to a preferred removal of more loosely bound zinc hydroxide nanoparticles and [ZnOH]+ ions compared to Zn2+ ions. 4000

Cps

0 5.0

ol 5.0

18.5

32.0

5.0

18.5 2 theta / degrees

32.0

5.0

2 theta I degrees

,

18.5

2 &eta / d e m

32.0

,

18.5 2 theta 1 degrees

32.0

Figure 5 : XRD patterns of zinc loaded A zeolites: (a) NaA, (b) ZnNaA (30%), exchanged with 0.01 M ZnCl2, (c) ZnNaA (92%), exchanged with 0.1 M ZnC12 and (d) sample (c) treated with ammonia after the exchange procedure. 3.3. IR measurements The spectrum of the parent zeolite NaA shows the typical IR framework vibration bands at 465 cm-I (TO4 bending vibration), at 558 cm-1 (double-4-ring vibration), and at 1004 cm-I (T-0-T stretching vibration). In contrast to other types of bivalent cations (Ca, Mg, Co, Ni, and Cd, Fig. 6 ) , for which no precipitation of side-products occurs during the exchange procedure due to the relatively large solubility products of the hydroxides, the introduction of zinc ions causes even at low degrees of exchange (30%) slight shifts of the T-0-T stretching and the double-4-ring vibrations to higher wavenumbers. The fiamework vibration bands are broadened, which results, for high loaded ZnNaA samples (degrees of ion exchange higher than approximately 60%), in a splitting of the degenerated T-0-T stretching vibration band into three bands. Additionally, the TO4 bending vibration is shifted to lower wavenumbers and decreases in intensity with increasing degree of ion exchange (Fig. 7).

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Figure 6: IR framework vibrations of zeolite A partially exchanged with different metal cations.

I

1000 wovenumbers/cm-

500

1

'

The intensity of the splitted bands is drastically reduced when the side-products are almost removed by a treatment with ammonia prior to calcination (Fig. 8). Due to the similarities of the behaviour of the samples in the X-ray diffraction patterns and the IR spectra, we believe that the same effect is responsible for the splitted IR framework vibrations. That is the interaction of very small zinc oxide particles or [ZnOH]' ions with the zeolite framework reducing the "local" symmetry of the tetrahedra.

(c)

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0.21

673

' (b)

1004

1000

wovenumbers/crn-'

wavenurnbers/cm-'

Figure 7: JR spectra of (a) NaA and ZnNaA zeolites with different degrees of ion exchange: (b) 30%, (c) 80% and (d) 92%.

Figure 8:

500

1

IR framework vibrations of (a) NaA, (b) ZnNaA (92%) and (c) sample (b) treated with ammonia after the exchange procedure.

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3.4. UV-VIS measurements

For highly loaded and calcined ZnNaA zeolites with extra reflections in the XRD patterns and an observable splitting of the T-0-T stretching vibration band, an absorption edge around 390 nm (point of idlexion: 378 nm) representing bulk ZnO was found. The weak absorption (a F(R) value of 0.1 corresponds to a loss of transmittance of about 10 %) originates from the ZnO precipitates on the surface of the zeolite crystals (Fig. 9a). ZnNaA zeolites showing no indications for the presence of side-products, absorb very little below 300 nm with a maximum around 250 nm and slightly more around 210 nm (Fig. 9b). The intensity of the relative sharp absorption around 210 nm depends on the degree of ion exchange and increases strongly after calcination of the zeolite samples at elevated temperatures (873 K). This absorption behaviour agrees well with results obtained for Zn40(acetate)6-complexes in ethanolic solutions [ 121. Preliminary force field calculations show, that the [Zn40l6' clusters are probably stabilized in the sodalite cages comparable to [Zn4SeI6+ clusters in sodalites [ 131. All zinc ions are tetrahedrally coordinated by the oxygen of the cluster and three oxygens of the zeolitic framework or adsorbed water molecules. The interpretation of the absorption around 250 nm is more difficult. Turk et al. [14] reported a much stronger absorption in zinc loaded and calcined A zeolites starting at 350 nm and assigned it to the presence of ZnO nano-particles with diameters close to the size of the zeolite cages, but our attempts to reproduce their results failed. Furthermore the existence of uncharged nano-oxide particles in zeolite cages is not very probable because of the known drastically enhanced solubility of metal oxides with decreasing particle size [ 151. Even sulfides exhibiting a much smaller solubility product were found to be non-stoichiometric in zeolitic pores and interact with the zeolitic host [16]. Due to the more pronounced blue-shift of the absorption in our samples we assume an interaction of [ZnOH]+ ions with the zeolitic framework to be responsible for the absorption. Support for our interpretation is given by (i) the low intensity of the absorption in our spectra, (ii) an observed independency on the degree of ion exchange and (iii) the behaviour of the pH value mentioned for low concentrated solutions.

0.10 n

E

W

b-

0.05

6

200

300

" " ' " ' ' * ~

400

wavelength (nm)

500

Figure 9: W - V I S reflectance spectra of zinc loaded A zeolite with (a) and without (b) sideproducts.

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4. CONCLUSIONS

During high loading of zeolites with heavy metal cations by ion exchange, hydroxides can precipitate idon the zeolite crystals. The precipitation of hydroxides as undesired sideproducts can be monitored by the pH of the exchange batch which becomes more acidic than the salt solution alone. To avoid the precipitation, the cation concentration in the exchange solutions must be limited, so that the solubility product of the corresponding hydroxide is not exceeded. But even from diluted solutions the exchange of a small amount of partially hydroxylated cations into the zeolite was observed. Zinc oxide idon zeolites ZnNaA formed from the hydroxides by calcination causes changes in the symmetry of the zeolite lattice which result in broadened individual reflections in the XRD patterns and a shifting and splitting of zeolitic IR framework vibrations. Weak optical absorptions of zinc loaded A zeolites between 200 and 390 nm result either from bulky ZnO precipitated at the crystal surface or from interactions of [ZnOH]' or [Zq0l6+ ions with the zeolitic framework. REFERENCES 1. J.A. Rabo,Ed., Zeolite Chemistry and Catalysis, ACS Monograph 171, h.Chem.Soc., Washington DC, 1976 2. G.A. Ozin, Adv. Mater. 4 (1992) 612 3. M. Wark, G. Schulz-Ekloff and N.I. Jaeger, Catal. Today 8 (1991) 467 4. P.V. Kamat, Chem. Rev. 93 (1993) 267 5. M. Wark, W. Lutz, G. Schulz-Ekloff and A. Dyer, Zeolites 13 (1993) 658 6. M. Wark, G. Schulz-Ekloff, N.I. Jaeger and W. Lutz, Mat. Res. SOC.Symp. 233 (1991) 133 7. P.A. Jacobs and J.B. Uytterhoeven, J. Chem. SOC.,Farad. Trans. I 6 9 (1973) 373 8. W. Lutz, H. Fichtner-Schmittler, M. Bulow, E. Schierhorn, N. van Phat, E. Sonntag, I. Kosche, S . Amin and A. Dyer, J. Chem. SOC.,Farad. Trans. I 8 6 (1990) 1899 9. U. Steinicke, W. Lutz, M. Wark and K. Jancke, Cyst. Res. Technol. 28 (1994) 149 10. T.B. Reed and D.W. Breck, J. h e r . Chem. SOC.78 (1956) 5972 1 1. I.J. Gal, 0.Jankovic, S. Malicic, P. Rodavanov and M.Tododrovic, J. Chem. SOC.,Farad Trans. 67 (197 1) 999 12. H. Kunkely and A. Vogler, J. Chem. SOC.,Chem. Commun. (1990) 1204 13. K.L. Moran, T.E. Gier, W.T.A. Harrison, G.D. Stucky, H. Eckert, K. Eichele and R.E. Wasylishen, J. Am. Chem. SOC.115 (1993) 10553 14. T. Turk, F. Sabin and A. Vogler, Mat. Res. Bull. 27 (1992) 1003 15. P. Schindler, H. Althaus, F. Hofer and W. Minder, Helv. Chim. Acta 48 (1965) 1204 16. O.P. Tkachenko, E.S. Shpiro, M. Wark, G. Schulz-Ekloff and N.I. Jaeger, J. Chem. SOC.,Farad. Trans. 89 (1993) 3987