J. CHEM. SOC. FARADAY TRANS., 1994, 90(21), 3367-3372 Comparative IR-Spectroscopic Study of Low-temperature H, and CO Adsorption on Na Zeolites Silvia Bordiga, Edoardo Garrone, Carlo Lamberti? and Adriano Zecchina" Dipartimento di Chimica Fisica, Chimica lnorganica e Chimica dei Materiali, via Pietro Giuria 7, 10125 Turin, Italy Carlos Otero Arean Departamento de Qdmica, Universidad de /as lslas Baleares, 07071 Palma de Mallorca, Spain Vladimir B. Kazansky and Leonid M. Kustov N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Moskow, Russia Extraframework cation sites in the sodium forms of the zeolites ZSM-5, mordenite, Linde-4A and faujasite-type X and Y have been investigated by using low-temperature adsorption of dihydrogen and carbon monoxide as IR spectroscopic probes.The extent of H-H and C-0 bond polarization was found to be dependent not only on the cation electrostatic field, but also on the neighbouring oxygen atoms of the zeolite framework. The influence of these oxygen atoms is most keenly felt by adsorbed molecular hydrogen, but they also affect the IR frequency shift of the stretching vibration of adsorbed carbon monoxide. The Si : Al ratio of the zeolite framework modu- lates the basic strength of the oxygen atoms, and this was found to be reflected in the IR stretching frequency of both adsorbed molecules, H, and CO. The possible role of extraframework cations as catalytically active sites has been postulated and discussed in the first papers'-4 on catalytic applications of zeolites.According to these ideas, such ions can activate adsorbed molecules due to their high polarization power. The extraframework cations protrude into the void internal space of the zeolite and expose adsorbed guest molecules to electric fieldssp7 in the order of 107-108 V cm-'. These intense fields are thought to be responsible for the activation and reactivity of adsorbed guests. Furthermore, the zeolite framework has a negative charge arising from A10,-units which replace neutral SiO, units. While the total negative charge (per unit cell) depends on the Si :A1 ratio, the local charge is a function of the ion- icity of the lattice. Negatively charged oxygen atoms sur-rounding an extraframework cation provide dual acid-base sites (of the Lewis type) which play an important role498.9 in the proposed mechanisms for many catalytic processes medi- ated by zeolites.The usual approach to an experimental estimation of polarization effects in zeolites is the use of adsorbed mol- ecules which act as spectroscopic probes. IR spectroscopy can then monitor the perturbation of the probe molecules in the adsorbed state. However, a suitable choice of different adsorbed molecules is needed for a better understanding of the combined effect of extraframework cations and their neighbouring (framework) oxide anions. This should ultim- ately lead to a better understanding of the catalytically active sites in zeolites. We present here a combined IR study of low- temperature adsorption of molecular hydrogen and carbon monoxide on several sodium zeolites, which have different Si : A1 ratios.The probe molecules were chosen on the basis of their specific (or distinct) interactions with the adsorption sites. Dihydrogen, being a symmetric molecule, has an H-H bond vibration which is IR-inactive. Therefore, molecular hydrogen does not show any absorption bands in the IR unless polarized, inside the zeolite cavities, by the electric field created by cations and surrounding anions. Thus, molec- ular hydrogen has the advantage of the absence of back- ground absorption from the gas phase. In addition, the H-H t Also at: INFN Sezione di Torino, Italy. stretching frequency exhibits, in the adsorbed (polarized) state, very large bathochromic shifts,"." up to 200 cm-'.Finally, this molecular probe has a diameter', of only 2.3 8, and can penetrate even inside the sodalite cages, which have six-membered-ring windows of cu. 2.6 8, in diameter. As a second molecular probe we used carbon monoxide. This molecule has a small dipole moment (0.1 DS) and a rather high polarizability. Therefore, the stretching frequency of CO adsorbed on coordinatively unsaturated cations (via the carbon atom) exhibits hypsochromic shifts' 3-1 resulting from polarization, which can be used to probe the corre- sponding electrostatic field. Reported shifts' 6-1 for CO adsorbed on metal oxides and halides are in the range of 10-100 cm- '. With transition-metal ions, possible charge- transfer effects can also result in shifts of the C-0 stretching frequency.However, for Na' ions this latter effect is not present. Experimental Commercially available sodium forms of ZSM-5 (Si : A1 = 39, mordenite (Si : A1 = 5.0), Y (Si :A1 = 2.35) and X (Si :A1 = 1.25) faujasites, and Linde-4A (Si : A1 = 1.0) were used. Diffuse reflectance IR spectra of adsorbed hydrogen were recorded for zeolite powders at liquid-nitrogen tem-perature, using a Beckman Acta N-7 spectrophotometer equipped with a home-made diffuse reflectance attachment. Further details were given elsewhere.20*2 ' FTIR transmittance spectra of adsorbed carbon monoxide were also recorded at liquid-nitrogen temperature, using a Bruker 88 spectrometer equipped with a MCT cryodetector and a silica cell with NaCl windows.The background created by the zeolite framework was subtracted from all the spectra recorded. Both techniques allowed in situ activation of the zeolites and IR measurements to be made at different equilibrium pressures of adsorbed gases. For activation, the zeolite powders (diffuse reflectance study) or self-supporting wafers (FTIR transmission spectra) were heated under vacuum at 673 K for 2 h. $ 1 D x 3.33564 x C m. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Results and Discussion Low-temperature Hydrogen Adsorption Diffuse reflectance IR spectra of molecular hydrogen adsorbed at 77 K on sodium forms of the different zeolite samples are depicted in Fig.1. Several bands are evident in the H-H bond vibration region. They originate from H, molecules perturbed by Na ions (and surrounding anions) + in different positions inside the zeolite micropores. The spectrum of Na-ZSM-5 [Fig. l(a)] is the simplest since only one IR absorption band, at 4110 cm-', was observed. This shows that there is only one major site of Na' location in Na-ZSM-5. No crystallographic data are avail- able on cation distribution in this zeolite, but indirect strongly suggest that channel intersections are the preferential location of extraframework cations. The spectrum of Na-mordenite at low hydrogen pressure [Fig. l(c)] is quite similar to that of Na-ZSM-5, with a slightly different position of the absorption maximum; 4108 cm-'.At higher pressure [Fig. l(b)] a shoulder develops at 4125 cm-', indicating the existence of at least two different sites of sodium location. This is consistent with current struc- tural Sodium ions in Na-mordenite are prefer-entially located at the centre of highly distorted eight-membered rings, and H, adsorption on these ions must be responsible for the 4108 cm-'band. The shoulder at 4125 cm-' may then be assigned to more loosely bonded Na' ions distributed among other available sites., 5-27 The IR spectra of H, on Na-A [Fig. l(d) and (e)] show only a major band at 4075 cm- ', which develops a shoulder (4110 cm-') at high H, equilibrium pressure. Compared to Na-mordenite, both of these IR absorptions are shifted to lower frequency and are more separated from each other.This is also consistent with structural data25*28 according to which the preferential location of the Na+ ions is near the 4110 I centre of the six-membered rings forming the sodalite cages. The IR absorption band at 4075 cm-' must be assigned to these cations. The shoulder at 4110 cm-' would thus corre- spond to Na' ions located in the remaining (less populated) positions. The IR reflectance spectra of hydrogen adsorbed on sodium forms of faujasites are more complex. For Na-Y [Fig. l(g) and (h)] they show three bands, the relative intensity of which changes with hydrogen pressure. The distribution of sodium ions in dehydrated Na-Y has been studied by X-ray diffraction It was concluded that Na+ prefer- entially occupies s,, sites inside the large cavities and, to a lesser extent, S,.sites inside sodalite cages and S, sites inside hexagonal prisms. This correlates with the IR spectrum of Fig. l(g) if the 4102 cm-' band (which shows the largest shift from free H,) is ascribed to dihydrogen perturbed by Na+ ions in S,, positions, while the bands with maxima at 4125 and 4150 cm-' are assigned to H, molecules under the influ- ence of Na' ions located at SItand S, sites, respectively. Such an interpretation is further confirmed by the spectra of hydrogen adsorbed on Na-X [Fig. l(f)] where similar bands of H, molecules perturbed by Na' at S,, and SIPsites are observed, while the band corresponding to S, sites is absent.This suggests, in agreement with X-ray diffraction that in dehydrated faujasite-type zeolites site S, is less populated in Na-X than in Na-Y. Further details on the assignment of IR bands for molecular hydrogen adsorbed on these zeolites are given elsewhere.,' Low-temperature Carbon Monoxide Adsorption Transmittance IR spectra of carbon monoxide adsorbed at 77 K on Na-ZMS-5, Na-mordenite, Na-Y, Na-X and Na-A zeo- lites at increasing micropore filling are presented in Fig. 2-6. The most prominent feature of these spectra is the presence of two major absorptions, one of them shifted to higher fre- quency (as compared to free CO)and the other one appear- ing at about 2138 cm-'. This latter band is a~signed~,-~~ to non-specifically adsorbed CO which, at relatively high equi- librium pressure, behaves as a liquid-like phase inside the zeolite cavities or channels.This attribution has been con- firmed by an accurate band fitting analysis performed on H-ZSM-5 at different CO equilibrium pressures. The relative intensity of this band is highest for Na-ZSM-5 and lowest for Na-X and Na-A. This could be due to the fact that Na-X and Na-A have the highest extraframework cation population, leaving less space available for the intake of CO in a hindered rotational state. More quantitative consider- ations cannot be made because the amount of weakly Fig. 1 Diffuse reflectance IR spectra of H, adsorbed at 77 K on 2300 2200 2100 . 2000 Na-ZSM-5 (a),Na-M (b) and (c), Na-A (d) and (e), Na-X (f) and wavenumbers/cm -' Na-Y (9)and (h).P,, = 0.3 for (a), (b), (e), (f) and (9)or 13 kPa for (c) Fig. 2 FTIR spectra in the CO-stretching region of Na-ZSM-5 at and (6).Spectrum (h) was taken after outgassing (77 K, 1 min) the increasing CO equilibrium pressure, from ca. 3 Pa up to 3 kPa. The Na-Y zeolite samples with preadsorbed hydrogen. zeolite blank has been subtracted from each spectrum. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1 .o Q, t 2s: 0.5 9 0 1 2300 2200 2100 ’ 2000 wavenumber/cm-Fig. 3 FTIR spectra in the CO-stretching region of Na mordenite at increasing CO equilibrium pressure. Conditions as in Fig. 2. 2-2300 2200 2100 2000 ’ wavenum ber/cm - Fig. 4 FTIR spectra in the CO-stretching region of Na-Y at increasing CO equilibrium pressure.Conditions as in Fig. 2. adsorbed CO is critically dependent upon small fluctuations of the actual temperature of the zeolite wafer. The high- frequency band must correspond to CO polarized by the Na’ ions of the zeolites. In a recent of low-temperature CO adsorption on alkali-metal-exchanged ZSM-5 zeolites (Si :A1 = 14), the position of this band was found to shift gradually from 2188 cm-’ for Li-ZSM-5 down to 2157 cm-’ for Cs-ZSM-5 showing that this high-frequency band is cation specific. For Na-ZSM-5 the cation-specific band (at 2178 cm-’) is narrow and single (see Fig. 2), which is consistent with the fact that this zeolite has only one type of extraframework cation site, as already found in the study of dihydrogen adsorption.A detailed discussion on the minor absorptions present in Fig. 2 has already been performed else~here~~.~~?~~and is not relevant in this context. For Na-mordenite, Fig. 3, the CO spectra are quite complex as two main bands are observed at 2177 and ca. 2160 cm-’. This is consistent with the structure of mordenite3* where two types of channels are present: the first similar to the linear channel present in ZSM-5; the second perpendicular to the first, obstructed by narrow windows located at ca. 8 A from the intersection with the main channel and forming elongated nanopockets. The 2177 cm- similar to the single 2178 cm-’ band observed on Na-ZSM-S/CO spectra, is so assigned to CO interacting with Na+ cations located in the main channel, while the band at ca.2160 cm-’ is attributed to CO interacting with Na+ ions in the lateral obstructed nanopockets. It is most noticeable that two bands are also observed in the H, adsorption on Na-mordenite I Q, C n 2300 2200 2100 2000 wavenumber/cm-’ Fig. 5 FTIR spectra in the CO-stretching region of Na-X at increasing CO equilibrium pressure. Conditions as in Fig. 2. 2300 2200 2100’ 2000 wavenumber/cm - Fig. 6 FTIR spectra in the CO-stretching region of Na-A at increasing CO equilibrium pressure. Conditions as in Fig. 2. [vide supra Fig. l(a)]. According to structural data,”-,’ CO penetrating even partially into these narrow obstructed cavi- ties is expected to undertake interactions not only with one cation but also with the walls.In presence of such multiple interactions, the relationship between the CO frequency shift and the electric field generated by an isolated cation” becomes too rough an approximation and the role played by the neighbour framework atoms cannot be neglected. On the contrary, the validity of this approximation has been proven in the case of a more diluted zeolite37 (ZSM-5 with Si :A1 = 14). In fact, the study of the interaction at 77 K of CO with a complete series of alkali-metal-exchanged ZSM-5 zeolites indicated that a quasi-linear relation exists between the CO frequency shift and the variable l/(Rx + R,,)’, where R,, represents the CO radiu~”*~~and R, the cation radius4’ (X = Li, Na, K, Rb, Cs).The similarity of the frequency of the first CO band with that of Na, ZSM-5, indicates that this relation holds also for alkali-cation-exchanged mordenites. A more detailed discussion of this problem has been performed el~ewhere.~’Note that the assignment of the 2177 cm- band to Na+...CO interactions involving cations in the main channel, and the 2160 cm-’ band to similar interaction with cations placed at the bottom of lateral pockets, is in agree- ment with, (i) the presence of a similar high-frequency band (at 2178 cm-’) for Na-ZSM-5 where only the main channel sites are available, and (ii) the fact that cations in side pockets are more shielded by negatively charged oxygens and there- fore exert a smaller polarization on adsorbed CO molecules.Only one cation-specific CO band (at 2172 cm-’) was found for Na-Y (Fig. 4) while H, gave two or three bands [Fig. l(g) and (h)]. This is due to the fact that the larger CO molecule, which has a diameter12v3' of ca. 3.5-4.2 A, cannot penetrate inside the sodalite cages and hexagonal prisms, thus only the S,, sites (in the supercage) are probed with CO. For Na-X the situation is more complex. Fig. 5 shows that the cation-specific band has a maximum at 2164 cm-and a shoulder at about 2170 cm-'. We assign the maximum to CO molecules interacting with Na+ ions at S,, sites, while the shoulder is probably due to a similar interaction with Na+ at (less populated) S,,, sites also within the supercage.Site SnIis located slightly over a four-membered ring, therefore the cor- responding cation is more coordinatively unsaturated than that at site S,, (which protrudes from a six-membered ring). This would explain why the Na+ ion at S,, polarizes the CO molecule more strongly than that at S,, thus giving a high- frequency shoulder in the IR absorption band. The foregoing assignment is also consistent with the single cation-specific band observed for CO or Na-Y, since in Y-type zeolites S, sites are usually vacant. The fact that dihydrogen shows no resolution of the band assigned to Na+ ions in the supercage [Fig. l(f)] seems to suggest that H, is a less sensitive spec- troscopic probe. A detailed discussion on and relative assign- ments of the minor absorptions present in the Na-Y and Na-X spectra shown in Fig.4 and 5 has been presented else- where.34 For Na-A only a main band at 2163 cm-' is observed, which corresponds to the most intense peak at 4075 cm-'of dihydrogen, shown in Fig. l(d) and (e). The observed peak corresponds to CO on Na+ ions at the centre of six-member rings of the sodalite cages. Owing to the high cationic popu- lation (compared to ZSM-5 and mordenite) and to the small dimensions of the cavities (compared to faujasites), it is quite probable that CO adsorbed on the Naf sites is suffering from simultaneous perturbing effects from other neighbouring cations. Also, in this case a simple relation between the CO frequency shift and the electric field generated at a single cationic position could be an exceedingly rough approx-imation.In this context, we note that due to the stronger interaction of CO with Na+ ions (as compared to H,) the bands of spe- cifically adsorbed molecules are more evident even at lower dosages. Moreover, the band maxima shift slightly toward lower frequencies when going from low to higher CO cover-age. This is a consequence of a gradual decrease of the elec- trostatic charge surrounding the cations when more dipolar CO molecules are admitted into the internal space of the zeloite~.~~ Comparison of Adsorbed Dihydrogen and Carbon Monoxide Table 1 summarizes the observed absorption frequencies of specifically adsorbed dihydrogen and carbon monoxide on the different Na-zeolites.To facilitate discussion, the corre- sponding frequency shifts from the free molecules are also reported. In the case of molecular hydrogen, when more than one band was observed in the spectra the shifts refer to the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 lowest frequency line (highest bathochromic shift). For CO, when more than one cation-sensitive band was observed, the quoted shifts correspond to the highest frequency (highest hypsochromic shift). The data reported in Table 1 demonstrate a clear depen- dence of the observed frequency shifts upon the aluminium content of the zeolite framework. This applies to both molec- ular probes. However, the variation of the shifts for adsorbed H, follows a trend which is opposite to that observed for adsorbed CO.The bathochromic shift of the H-H stretching vibration is largest for the most basic Na-A zeolite and smal- lest for Na-ZSM-5, while for the hypsochromic shifts of the C-0 stretching vibration Na-ZSM-5 shows the largest effect and Na-A the smallest one. This is a consequence of different mechanisms of perturbation of the adsorbed molecules, which can be explained from the results of quantum chemical analysis of the interaction of CO and H, with A10 clusters mimicking A13 + ions at the surface of aluminium oxide. Semi-empirical calculations 14*42 concerning CO adsorp- tion on A13+ ions in trigonal or square pyramidal coordi- nation provide support for a linear interaction through the carbon end as shown in Scheme 1.No tendency was found for interactions involving the oxygen end of the CO molecule. Maps of the molecular elec- trostatic potential (MEP) of CO, calculated including corre- lation effects,I4 show that the oxygen atom has a second negative zone, albeit less pronounced than that at the carbon end of the molecule. Therefore, CO could, at the most, coor- dinate to two Lewis acid sites through both ends of the mol- ecule. No tendency to interact with Lewis basic sites is expected, because the positive lobes in the MEP are not sub- stantial, particularly in the neighbourhood of the oxygen atom.l4 In conclusion, carbon monoxide probes exclusively the Lewis acid sites which polarize the molecule (through the carbon end) increasing the C-0 For dihydrogen adsorption the situation is quite different. Ab initio SCF MO calculations are available43 for an adsorp- tion site modelled by the Al(OH), cluster with standard bond angles and bond lengths, as shown in Fig.7. These calcu- lations were performed using the GAUSSIAN 80 program and 3-21-G basis set, and optimizing the adsorption complex geometry with respect to the H-H distance and the position of H, relative to the adsorption site. The resulting structure of the molecular complex (Fig. 7) shows that dihydrogen acts as a probe for the surfaee acid-base pairs, where the contri- bution of both basic oxygen and low coordinated cations are equally important. The adsorbed molecule becomes slightly Scheme 1 Table 1 Stretching frequencies of adsorbed H, and CO, and corresponding frequency shifts from the free molecules ~~ ~~~ H-H stretching C-0 stretching frequency of Avu -: downward frequency of adsorbed Av,: upward shift zeolite Si : A1 ratio adsorbed H,/cm-' shift from H, gas4 co/cm - from CO gas* Na-A Na-X Na-Y Na-MOR Na-ZSM-5 1.o 1.25 2.35 5.0 35 4075, 41 10 (sh) 4096, 4125 (sh) 4102,4125,4150 4108,4125 (sh) 4110 -88 -67 -61 -55 -53 2163 2164 2172 2177 2178 +20+21+29+34+35 v~~ =-4163 m-'.vc0 = 2143 m-'. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 H I H Fig. 7 Model for H, adsorption polarized, which renders it IR active and the H-H stretching frequency is lowered. The calculated adsorption heat43 was 29 kJ mol-' which agrees with the fact that molecular hydro- gen is reversibly adsorbed, even at liquid-nitrogen tem-perature.The results shown in Table 1 can now be easily explained. Bathochromic shifts of the H-H stretching frequency should depend both on the polarizing power of the extraframework cation and on the basic strength of neighbouring oxygen atoms. The latter is the highest for Na-A, which results in the strongest perturbation of adsorbed dihydrogen. The weakest perturbation corresponds to Na-ZSM-5 where the basic strength of the oxygen atoms is lowest because the increased Si : A1 ratio increases the covalent character of the zeolite framework. We note that the different frequency shifts observed for adsorbed H, (Table 1) cannot be explained on the basis of different geometries of the corresponding adsorp- tion sites, since S,, sites in both Na-X and Na-Y possess the same geometry.For adsorbed carbon monoxide at an isolated cation site the perturbation depends only on the net positive charge at the extraframework site. However, this is influenced by the surrounding oxygens, which provide a compensating negative charge. Neighbouring basic oxygens in the more ionic zeolite frameworks, which correspond to lower Si : A1 ratio^,^^.^^ decrease the positive electric field in the proximity of the cation sites. This is why the smallest shift of the C-0 stretching frequency is observed for Na-A, while the largest effect corresponds to Na-ZSM-5.The C-0 stretching frequencies of adsorbed carbon mon- oxide (Table 1) can be compared with those of CO adsorbed at low temperature on alkali-metal halides. For CO on NaCl films, several author^'^.^^ have reported an IR absorption band at 2159 cm-' and assigned it to CO interacting with coordinately unsaturated Na+ ions. Hauge et ~l.,~'who studied the interaction of CO with some alkali-metal fluo- rides in an argon matrix, found a C-0 stretching frequency of 2172 an-' for the CO/NaF system. It thus appears that Na' ions in Na-X have about the same polarizing power as in halide films, while in Na-ZSM-5 this polarizing ability is greater, and more similar to that found in single alkali-metal fluoride molecules. This increased polarizing power for adsorbed CO must be correlated with the lower basicity of framework oxygen atoms in Na-ZSM-5, as compared to Na-A, a fact also revealed by the experiments on the adsorp- tion of molecular hydrogen. In summary, it was shown that by using H, and CO as IR spectroscopic probes, not only the different extraframework cation sites, but also their environments can be examined with confidence.The ionicity of the zeolite framework increases with decreasing Si : A1 ratio, and this affects the IR stretching frequencies of the adsorbed molecules. Note, however, that this effect tends to level out when the Si : A1 ratio is greater than five. 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