J. CHEM. SOC. FARADAY TRANS., 1991, 87(16), 2675-2678

Infrared Spectroscopic Study of the Interaction with Simple Molecular Probes Part 1 .-Adsorption of Molecular Hydrogen on Alkaline Forms Loca I izat ion Sites

Leonid M. Kustov and Vladimir B. Kazansky N. D. Zelinsky Institute of Organic Chemistry, Moscow, USSR

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of Cations in Zeolites

of Zeolites as a Test for

The localization sites for univalent cations in cationic forms of zeolites (A, X, Y, ZSM-5 and mordenite) have been investigated using the low-temperature adsorption of molecular hydrogen as a spectroscopic test. The extent of H-H bond polarization, estimated from the low-frequency shift of the fundamental stretching vibration of perturbed H, molecules, was found to be strongly dependent on the basicity of the neighbouring oxygen atoms of the framework.

A number of papers and monographs have been devoted to the study of cation distribution in zeolites.'*2 X-Ray diffrac- tion analysis still remains one of the most effective methods for this purpose. Its application, however, is rather time con- suming. Various other spectroscopic methods can be also used for the study of localization of cations in zeolites, e.g. EPR has been applied to study cationic forms of zeolites con- taining paramagnetic Mn2+ ions as a probe.3 Far-IR spec- troscopy is also an effective and promising tool for this p u r p o ~ e . ~

P r e v i o u ~ l y ~ . ~ we have reported the use of molecular hydro- gen adsorbed at low temperature as a spectroscopic probe for the study of surface sites of adsorbents and catalysts. Polar- ization of H, molecules by centres of different nature resulted in the appearance of a set of bands in the IR spectra belong- ing to the stretching vibrations of perturbed H, molecules. It was shown in these ~ t u d i e s ~ - ~ that the use of hydrogen as a probe molecule can give more information about the nature of adsorption sites than other commonly used molecular probes. For instance, the analysis of IR spectra of hydrogen adsorbed at low temperatures allowed us to investigate aprotic sites in H-forms of different zeolites.

In the earlier studies of Forster et al.'O~" adsorption of dihydrogen on cationic forms of zeolite A has been also studied by means of IR spectroscopy. However, attention was focussed on the investigation of the influence of electric field symmetry at the adsorption site on the IR spectra of adsorbed hydrogen and to the interpretation of so-called 'side-bands' (v > 4160 cm- ') arising in the spectra.

In the present paper the low-temperature adsorption of H, is applied to the spectroscopic study of localization of alkali- metal cations in zeolites with different Si/Al ratios.

Experimental Sodium forms of A (Si/Al = l.O), X (Si/AI = 1.14 and 1.25), Y (Si/Al = 1.71, 2.35 and 2.9), mordenite (Si/A1 = 5.0) and ZSM-5 (Si/Al = 30) zeolites were used. The Si/A1 ratios were obtained from the chemical analysis. X zeolites containing Li', K' and Cs'cations were prepared from the Na-form by repeated ion exchange at 330 K according to the standard procedure. The extents of ion exchange of Na' for corre- sponding cations in these samples were 100, 100 and 70%, respectively.

The samples were placed into quartz IR cells and pretreat- ed at 670 K for 8 h in vacuum (0.1 Pa). The heating rate was ca. 5-10 K min-'.

Diffuse-reflectance IR spectra were measured at 77 K in the presence of hydrogen ( P = 0.07-35 kPa) using a Beckman Acta M-VII spectrophotometer supplied with a home-made diffuse reflectance a t t a ~ h m e n t . ~ ? ~

Results IR spectra of molecular hydrogen adsorbed at a pressure of 0.3 kPa on Na forms of mordenite [(a), (b ) ] , ZSM-5 (c), A [(d), (e)], X (f) and Y [(g), (h)] zeolites are shown in Fig. 1. Adsorption of H, at 77 K results in the appearance of bands

t- 4108 A

0.02AI /I 4110

( f )

( 9)

( h ) v/cm - '

Fig. 1 IR spectra of molecular hydrogen adsorbed at 77 K on NaM

zeolites PI,2 = 0.3 kPa [ (b) , (c), (eHg)] or 13 kPa [(a), (43. (h) was measured after evacuation (77 K, 1 min) of NaY zeolite with pre- adsorbed hydrogen

[(a), @)I, NaZSM-5 (4, NaA C(4, ( e ) ] , NaX Cf) and Nay C(g), (WI

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2676 J. CHEM. SOC. FARADAY TRANS., 1991, VOL. 87

1 O I . I

3740 +

v/cm - ' Fig. 2 and NaZSM-5 (4 zeolites

IR spectra of hydroxyl groups in NaX (a), NaY (b), NaM (c)

in the region of the stretching vibrations of the H-H bond of molecular hydrogen (4040-4240 cm- '). Although the same range of frequencies is characteristic of interaction between adsorbed H, and OH groups of zeolites (vH-H = 410&4130 cm-'),6 the bands observed in the spectra of univalent cationic forms of zeolites should definitely be assigned to the molecules perturbed by metal cations. Indeed, as seen in Fig. 2, acidic hydroxyl groups (v = 36W3660 cm- ') in sodium or other alkaline forms of zeolites are either practically absent or their concentration is much lower than in the H-forms. Furthermore, even at low pressures of hydrogen (< 130 Pa) there are intense absorption bands of hydrogen adsorbed on cationic forms, whereas for the H-forms they usually appear only at pressures > 1.3 kPa. Finally, increasing the tem- perature of vacuum pretreatment of the samples (up to 820 K), which would result in dehydroxylation of cationic forms of zeolites, does not create any changes in the spectra of adsorbed hydrogen.

When the spectra of H, contain more than one line a decrease of H, pressure or evacuation of the samples at 77 K leads to a change in the relative intensities of the bands: the high-frequency lines (v > 4120 cm- ') start to disappear, whereas the low-frequency line (v = 4080-4100 cm- ') remains unchanged. This observation indicates that the set of bands in the IR spectra corresponds to H, molecules inter- acting with different types of sites which seem to be sodium ions in different positions in the zeolite channels.

High-silica Zeolites

Let us start with the spectra of Na-forms of zeolites with high Si/AI ratios because they are the simplest. The spectrum of H, adsorbed at 0.7 kPa on the Na-form of mordenite [Fig. l(h)] consists of a single band (vHPH = 4108 cm-') shifted by 56 cm-' towards lower frequencies compared with the fre-

quency of gas-phase H, (v = 4163 cm--'). This is evidence of the homogeneity of the structural environment of Na' cations in the mordenite framework. Indeed, in accordance with the literature data,' a substantial fraction of the cations in Na-mordenite are preferentially localized in one main type of site which is placed in the vicinity of the centre of the eight-membered rings.

An increase of the H, pressure to 13 kPa [Fig. l(a)] leads to the appearance of an additional low-intensity shoulder near 4120 cm-'. This may be connected with complexes of hydrogen and Na+ cations distributed among other available sites of localization.

For the NaZSM-5 zeolite there are no published data on the cation distribution. Our results on the study of the IR spectra of adsorbed hydrogen suggesf that in this case also one type of site for univalent cations predominates. The sites are likely to be similar to the localization sites of Na- mordenite since the maxima of the bands are very close.

NaA Zeolite

For NaA zeolite, only one absorption band at 4076 cm-' could be observed in the IR spectrum at low H, pressures [Fig. l(e)]. It was established" that in the sodium form of A-type zeolite 8 Na+ ions of the overall number of 12 are localized near the centres of the six-membered rings forming the sodalite cages. They are in fact slightly shifted towards the centre of the supercage. Thus, the line at 4076 cm-' should be assigned to the complexes with Na' cations of this type. At increasing H, pressure an additional high-frequency band at 4105 cm-' appears [Fig. l(d')] which could be con- nected with the remaining four Na' cations situated near the four-membered rings of D4R units.'

X-type Zeolites

The spectra of H, adsorbed on zeolites with moderate Si/Al ratios are more complicated than those observed for high- silica zeolites or A-type zeolite.

X-type zeolites are very close in their chemical composition to A-type zeolite and also contain six-membered rings in their framework. Hence, similar features could be expected in the spectra of H, adsorbed on these zeolites. However, for NaX, unlike A-type zeolite, two bands at 4095 and 4120 cm-' are observed even at low H, pressure. This could be explained by differences in cation distribution. Indeed, according to X-ray diffraction analysis, in dehydrated NaX zeolite there are two main localization sites for Na+ cations which are situated near the centres of the six-membered rings: inside (S,,) and outside (S,,) the sodalite cages.

Since the S,, sites in faujasites are very similar to the most populated localization sites of A-type zeolite we may assign the low-frequency line (4095 cm-') to the complexes of H, with Na' cations in S,, sites. The other band at 4120 cm- is likely to be attributed to H, molecules coordinated to sodium ions in S,, sites.

This assignment is confirmed by a comparison of the spectra of H, adsorbed on X zeolites containing different alkali-metal cations (Fig. 2). When going from LiX to CsX faujasite a change in the ratio of intensities for the bands near 4095 and 4120 cm-' is observed. Thus, for LiX zeolite the low-frequency band predominates in the spectrum of adsorbed hydrogen, whereas for the Cs form both lines are approximately of the same intensity. It is known' that with increasing ionic radius from K to Cs cations, an increasing number of S,, sites become inaccessible as compared with the Li and Cs forms. In agreement with the assignment made above, the bands near 4095 and 4120 cm-' in the spectra of

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4 0 9 6

v/cm - ' Fig. 3 IR spectra of hydrogen adsorbed at 77 K on LiX (a) , KX (b) and CsX ( c ) zeolites; PH2 = 13 kPa

MeX zeolites should be ascribed to H, complexes with Me+ cations localized near S,. and S,, positions, respectively.

NaY Zeolite

The most complicated spectra are obtained for NaY zeolites. These spectra contain three well resolved bands: 4102, 4125 and 4150 cm-'. This is in agreement with the literature data' since according to X-ray analysis in dehydrated NaY zeolite there are three localization sites for Na+ cations: S,, S,. and SII. Two bands in the IR spectrum of hydrogen adsorbed on this sample (4102 and 4125 cm- ') have almost the same posi- tions as the lines observed for NaX zeolites (4095 and 4120 cm-'). Therefore we assign them to similar complexes with Na' cations localized near S,, and S,, sites.

The band at 4150 cm-', which is only slightly shifted com- pared with the gas-phase vHPH frequency (Av = 10-15 cm-'), was not observed in the spectra of X-type zeolites, hence it could be attributed to H, complexes with Na' cations in S, sites, which are more characteristic for Y zeolite than for X zeolite. These sites are localized at the centres of the hexago- nal prisms and seem to be hardly accessible to direct H, adsorption because of the large kinetic diameter of the hydro- gen molecule (ca. 2.9 A) compared to the entrance of a four- or six-membered ring (< 2.6 A). However, H, may interact with such ions, forming weak complexes with long H,. . .Na +

distances when the adsorbed molecules are placed in the supercages of the faujasite or inside the sodalite cages. This model is in agreement with the low value of the frequency shift and low stability of such complexes.

This assignment is also confirmed by a comparison of the relative intensities of the bands with the known data on the population of the localization sites of Na' cations in NaY zeolites. For the dehydrated NaY with Si/A1 = 2.35 the fol- lowing distribution of Na' ions occupying the S,, SI, and S,, sites was reported'V2: 7.5 : 19.5 ; 30.3. As could be seen from Fig. I , this relation is very close to the ratio of intensities for the corresponding bands at 4150,4102 and 4125 cm-'.

Discussion As we inferred earlier6 the extent of polarization of H, mol- ecules, and hence, the shift of the H-H vibration in a complex with an aprotic centre is dependent not only on the properties of the cation accepting electron density but also on the basicity of the neighbouring oxygen atom. This has been confirmed by quantum-chemical calculations on the H, mol- ecule coordinated to a Lewis-acid site modelled by the cluster Al(OH), .' This molecule prefers the two-point adsorption on the pair including a low-coordinated A1 ion and an

oxygen atom. Therefore, it is reasonable to assume that in the case of H, adsorption on the exchangeable cations in zeolites the extent of its polarization will also be dependent upon oxygen environment of these cations, in particular, upon the basicity of oxygen atoms and on the symmetry of the coordi- nation sphere.

Indeed, according to Mortier and Geerling~'~. ' the basic properties of oxygen atoms O(2) and O(4) which are prefer- entially located near the sites s,, and SII, respectively, strengthen from O(4) to 0(2), i.e. in the same manner as the extent of H, polarization.

It follows from the data obtained that the extent of H, polarization on Na+ cations is considerably affected by the composition of the crystal lattice of zeolites, i.e. by the Si/AI ratio. Thus, the sodium forms of the zeolites under study could be arranged according to the increase of the frequency shift of the low-frequency band in IR spectra of adsorbed hydrogen corresponding to the strongest complexes with Na+ in six-ight-membered rings (vH- = 407W110 cm- ' ) in the following sequence:

NaZSM-5 < NaMOR < NaY < NaX < NaA

The range of the shifts for zeolites which differ in the Si/AI ratio is ca. 35 cm- '.

Note that the influence of the nature of alkali-metal ions on the extent of hydrogen polarization is less pronounced. Fig. 2 shows that the most intense line in the spectra of hydrogen adsorbed on X zeolites with different monovalent cations practically does not change its position: 4092 cm-' for CsX, 4094 cm-' for KX, 4095 cm-' for NaX and 4096 cm- ' for LiX.

In the spectra of hydrogen adsorbed on alkaline forms at high pressure the bands with maxima higher than the fre- quency of the fundamental gas-phase H-H vibrations could also be observed (see Fig. 3). This effect is more significant for zeolites with low Si/AI ratios. These bands could be assigned"-" to a combination of the stretching vibration of H-H bond, with some low-frequency mode representing a hindered rotation of the whole molecule relative to an adsorption site. Fig. 3 shows that unlike the low-frequency lines the positions of these high-frequency bands are much more sensitive to the nature of the alkali-metal cation. Thus, the maximum of the high-frequency band is shifted from 4239 to 4222 cm- ' with a decrease of the ionic radius of the alkali- metal cation, from Csf to Li', i.e. with increasing polarizing power of a cation.

Thus, the data discussed demonstrate some new pos- stbilities for IR spectroscopic study of hydrogen adsorption for the investigation of localization sites for cations in zeolites and of the interaction between exchangeable cations and adsorbed hydrogen. The extent of H, molecule polarization on alkali-metal cations is strongly dependent upon the com- position and type of the zeolite framework as well as upon the localization site of cations, i.e. upon its coordination by oxygen and basic properties of surrounding oxygen atoms.

References J. W. Smith, in Zeolite Chemistry and Catalysis, ed. J. Rabo, ACS Monograph 171, ACS, Washington D.C., 1976. W. J. Mortier, Compilation of Extraframework Sites in Zeolites, Butterworth, Guildford, 1982. I. D. Micheikin, G. M. Zhidomirov and V. B. Kazansky, Adu. Chem. (Russ.), 1972, 41,909. Ch. Peuker and D. Kiinath, J. Chem. Soc., Faraday Trans. I , 1981,77,2079. L. M. Kustov, A. A. Alexeev, V. Yu. Borovkov and V. B. Kazansky, Bull. Acad. Sci. USSR, 1981,261, 1374. V. B. Kazansky, V. Yu. Borovkov and L. M. Kustov, Proc. 8th Znt. Congr. Catal., Dechema, Berlin, 1984, vol. 3, p. 3.

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7 V. B. Kazansky, L. M. Kustov and V. Yu. Borovkov, Zeolites, 1983,3, 77.

8 L. M. Kustov, V. Yu. Borovko v and V. B. Kazansky, Kinet. Catal., 1984,25,47 1.

9 L. M. Kustov, I. V. Mishin, V. Yu. Borovkov and V. B. Kazansky, Kinet. Catal., 1984, 25,724.

10 H. Forster, M. Schuldt and W. Frede, J . Mol. Struct., 1982, 80, 195.

1 1 H. Forster and M. Schuldt, J . Mol. Struct., 1978,47,339.

J. CHEM. SOC. FARADAY TRANS., 1991, VOL. 87

12 R. Y. Tanagida, A. A. Amaro and K. Seff, J . Phys. Chem., 1973, 77, 805.

13 V. M. Zelenkovskii, G. M. Zhidomirov and V. B. Kazansky, Zh. Phys. Chem., 1984,5((, 1788.

14 W. J. Mortier and P. Geerlings, J . Phys. Chem., 1980,84, 1982. 15 W. J. Mortier, J. Catal., 1978,55, 138.

Paper 0/052771; Received 23rd November, 1990

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