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Vibrational spectroscopy of H2, N2, CO and NO adsorbed on H, Li, Na, K-exchanged ferrierite
Microporous and Mesoporous Materials 34 (2000) 67–80 www.elsevier.nl/locate/micromeso
Vibrational spectroscopy of H , N , CO and NO adsorbed on 2 2 H, Li, Na, K-exchanged ferrierite S. Bordiga, G. Turnes Palomino, C. Paze`, A. Zecchina * Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universita` di Torino, Via P. Giuria 7, I-10125, Torino, Italy Received 15 April 1999; received in revised form 28 July 1999; accepted for publication 9 August 1999
Abstract The vibrational spectroscopy of H , N , CO and NO adsorbed on the zeolite ferrierite is described. The spectral 2 2 modifications induced by the adsorption process on the protonic form are compared with those observed on the alkaline-exchanged forms. Different environments of Brønsted sites and of alkaline counterions in ferrierite are evidenced, which differ in the local electric field associated with the cationic species. These different environments are associated with the cation locations in the channels and cavities. In particular, ions located in the 10-membered ring channels are more available for the interaction with CO, N and H , while ions in the cages form weaker adducts. 2 2 On passing from lithium-, to sodium- and to potassium-exchanged samples, the local fields probed by H , N and 2 2 CO are increasingly dependent upon the distribution of the ions in the framework. In K-ferrierite the interaction cannot be solely described in terms of 1:1 K+/B (B=N , CO) adducts. As far as H-ferrierite is concerned, it is 2 inferred from the observed shift of the n(OH ) mode upon interaction with H , N and CO that the bridged strong 2 2 acid groups show an acid strength very similar to that observed for H-ZSM-5, H-mordenite and H-Beta. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Acidity; Alkali-exchanged zeolite; CO adsorption; Ferrierite; Hydrogen adsorption
1. Introduction Ferrierite is a zeolite which contains 10-, 8-, 6and 5-membered rings in its structure [1,2] and shows interesting catalytic properties for the isomerization of linear butenes [3–7]. The zeolitic channel system of the ferrierite framework is characterized by 10-membered ring channels running parallel to the crystallographic [001] direction and by 8-membered ring channels running parallel to the [010] direction. * Corresponding author. Tel.: +39-11-6707537; fax: +39-11-6707855. E-mail address: [email protected] (A. Zecchina)
Recently, Zholobenko et al. [8] reported a detailed spectroscopic characterization of the acid sites using ammonia and isobutane as probes. They concluded that the strong Brønsted acid groups are mainly located in extended rings at the intersection of the 8- and 6- membered ring channels. In this paper the characterization of Brønsted acidity is continued by using H , N , CO and NO 2 2 as probes, taking advantage of the well-known fact that the n(HH ), n(NN ) and n(CO) stretching modes are known to be selectively perturbed by the interaction with the Brønsted groups and that a parallel modification of the vibrational properties of the n(OH ) of the Brønsted centers occurs [9–11].
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Furthermore, H , N and CO interact with any 2 2 exposed cation present in the channels: owing to this interaction, the n(HMH ), n(NN ) and n(CO) stretching frequencies shift upwards (N and CO) 2 and downwards (H )1 [9–11], the shift being pro2 portional to the value of the positive electric field centered at the cationic or at the Lewis sites [10,12,13]. On this basis it is evident that these very weak bases can be used to probe the strength of electric fields present in the channels associated with positively charged cations or any other positively charged center. In the case of H and N , 2 2 the perturbation is accompanied not only by a shift of the stretching frequency but also by an impressive increment of the extinction coefficient, which is especially evident for dinitrogen interacting with extra-framework Al3+ [10,14]: this property enhances the usefulness of dinitrogen (and hydrogen) as surface probe. As far as NO is concerned, much less is known in the literature: however, owing to the similarity of the electronic structures of N , CO and NO, it is expected that 2 NO also can respond to the electric field perturbation and consequently can be used as a probe of the environment of the cationic sites. Based on these considerations, H , N and CO 2 2 were used as probes to explore the acid strength of the Brønsted sites and the distribution of the electric fields in alkali-exchanged ferrierite, in an attempt to gain further information on the location of the cations in the 8- and 10-membered ring channels and in the cavities connecting them. NO was used as a probe for the electric field at the cationic sites as well.
Elementary chemical analysis gave an Si/Al ratio of 8, while 27Al MAS NMR measurements showed that all aluminum atoms were tetrahedrally coordinated. The ammonium and alkaline forms (Li, Na, K ) of ferrierite were obtained by conventional ion exchange procedures of the commercial sample with nitrate solutions. H-ferrierite samples were obtained by thermal decomposition of the ammonium forms. Self-supporting wafers were made from the samples and outgassed in dynamic vacuum (residual pressure<10−2 Pa) at 673 K for 2 h inside an IR cell which allowed in situ high-temperature treatments, gas dosage, and low-temperature measurements to be made. The IR spectra were recorded, at 2 cm−1 resolution on a Bruker IFS66 FTIR spectrometer equipped with a MCT cryogenic detector. Nitrogen and CO adsorption were measured at a temperature of about 100 K. For H adsorption, a specifically designed cell was used 2 which allowed spectra to be taken at a temperature as low as 80 K. The attainment of such a low temperature is essential for studying the H –cation interaction, because the low interaction 2 energy between H and H+ or Na+ keeps the 2 concentration of H+,H or Na+,H adducts at 2 2 100–110 K at critically low levels. H , CO, NO and N were high purity grade 2 2 from Matheson; NO was further purified before adsorption by repeated freeze–pump–thaw cycles.
3. Results and discussion 3.1. The structure of ferrierite and the siting of extra-framework ions
2. Experimental The original ferrierite sample used in this study was a commercial sample supplied by Engelhard Corporation (Iselin, NJ, USA) in an Na, K form.
1 The shifts Dn: caused by interaction with cationic centers are calculated by using as reference the n(NMN ) and n(CMO) frequencies of N and CO physically adsorbed in zeolite channels 2 respectively of 2327 and 2139 cm−1. For H the reference fre2 quency was that of n(HMH ) gas (because the frequency of physically adsorbed H is not available). 2
The ferrierite structure (Fig. 1) is based on 5-ring secondary building units; from their connection 10-ring channels (having a diameter of 0.43–0.55 nm) are formed. These parallel 10-ring channels are periodically connected along the [010] direction by cages having as pore access 8-membered rings (0.34–0.48 nm diameter); the cages extend for 0.6–0.7 nm in the [100] crystallographic direction of the space (Fig. 1(b)). From a crystallographic point of view, the ferrierite structure can be constructed starting from asymmetric
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Fig. 1. Computer graphic representation of the ferrierite structure: (a) Connolly representation of the accessible cavities; (b) details of the cage shape; (c) labels indicating the four T sites located in 10- and 8-membered rings as described in Ref. [1].
units having either four or five atoms; in this paper we always refer to the four-atom asymmetric unit and to the Vaughan’s notation [1] to identify the different sites. 10-membered rings are composed of atoms in sites T2, T3 and T4, while 8-membered rings are composed of atoms in T2 and T4 sites. T1 sites are located only in the 6-membered rings, and are therefore located only in the cages (Fig. 1(c)). Concerning the aluminum location, Fripiat et al. [15] and Blanco et al. [16 ] believe that the most populated sites are T2 and T4. The presence of aluminum on T4 sites located in the cages is inferred from neutron powder diffraction studies on D-ferrierite; however, a family of protons in the 10-ring channels has also been observed [17]. Using multinuclear MQMAS NMR spectroscopy, Sarv et al. [18] found that aluminum pairs can be present preferentially in the 6-membered rings which connect the two 10-membered rings ( T2 and T1 sites). From the existing data it is evident that
a definite conclusion about the aluminum location is not yet possible. Nevertheless, all the results point towards a preferential population of T2 sites of the cages. As for the counterion location, it has been observed with several techniques that they are preferentially located in the cages, but they are also present in the 10-membered ring channels [8,17–19]. A plausible representation of the possible locations of protonic groups associated with the T2 sites is shown in Fig. 2(a). Two kinds of environments can be distinguished, according to the fact that the protonic groups can be located in the 10-membered ring channels (A species) or in the cages (B species). A peculiarity of the ferrierite structure is represented by the 6-membered rings present in the cages. In fact, unlike the 6-membered rings separating sodalite cages and supercages in Y zeolites, in ferrierite the ring is flat. This implies that the Brønsted groups associated with T2 sites of the cages (B sites) are forced to interact with the oxygens of the ring. This does not happen for
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Fig. 2. (a) Ferrierite structure, viewed along the [001] direction. 10- and 6-membered rings are represented with their van der Waals spheres. Silicon atoms are grey, oxygens are dark grey, aluminum at T2 sites and associated protons are black. (b), (c) View along the [010] direction of the 8-membered ring entrance to the cage. The 8-membered rings are represented with their van der Waals spheres. In (b) and (c) two Li+ and two K+ ions ( located in the cages), respectively, are represented following the cation location suggested in Ref. [18].
alkali cations: in fact, owing to their higher ionicity and larger radius, they are no longer located in the plane of the flat 6-membered rings and are protruding into the cage showing themselves at the 8-membered ring window connecting the main channels with the cages ( Fig. 2(b) and (c)).
3.2. IR spectra of H , N and CO adsorbed on 2 2 H-ferrierite The IR spectra of increasing doses of H , N 2 2 and CO on H-ferrierite are shown in Figs. 3–5, respectively.
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Fig. 3. FTIR spectra of increasing doses of H adsorbed on H-ferrierite at about 80 K. Maximum equilibrium pressure=4 kPa. 2
Fig. 4. FTIR spectra of increasing doses of N adsorbed on H-ferrierite at about 100 K. Dotted lines in (b) correspond to the highest 2 pressures. Maximum equilibrium pressure=6.6 kPa.
The most important facts deserving a comment are as follows. (i) The peak with its maximum at 3605 cm−1, due to the unperturbed n(OH ) mode of the bridged strong Brønsted groups, is gradually eroded upon dosage of H , N and CO ( Figs. 3(a), 4(a) and 2 2 5(a)). It should be noted that this peak is highly
asymmetric on the low frequency side indicating that the Brønsted groups are heterogeneous. This means that the peak at 3605 cm−1 is constituted by two or more components. Zholobenko et al. [8] have deconvoluted this peak into four components at 3609, 3601, 3587 and 3565 cm−1. From Figs. 3(a), 4(a) and 5(a), it is inferred
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Fig. 5. FTIR spectra of increasing doses of CO adsorbed on H-ferrierite at about 100 K. Maximum equilibrium pressure=6.6 kPa.
that, upon interaction with diatomic molecules, the high frequency part of the n(OH ) band is preferentially eroded, and that the low frequency components (contributing to the tail ) are perturbed only at the highest equilibrium pressures. This implies that the OH groups responsible for the low frequency tail of the 3605 cm−1 peak are less prone to an interaction with basic probes. The most plausible explanation of these facts is that the band of Brønsted sites with a maximum at 3605 cm−1 is composite and that at least two different families are contributing. In particular, these are: (a) a family of OH groups responsible for the 3605 cm−1 maximum; we think that they are the most exposed ones, like those located in the largest rings (sites A); (b) a family of OH groups responsible for the low frequency tail; these OH groups could be located in sites B. Their lowered frequency could be because they are already engaged in very weak hydrogen bonding interaction. In fact, because of the planar nature of the 6-membered rings present in the cages (Fig. 2(a)), the Brønsted sites (B) associated with them can form bifurcated H-bonds with the neighboring oxygens, which explains both the lowered frequency and the smaller propensity to interact with external probes. (ii) New broader and composite bands at 3575
( H ), 3502 (N ) and at 3314 cm−1 (CO) are formed 2 2 upon gas dosage. These bands are asymmetric on the high frequency side where their shape is specular to that of the original n(OH ) absorption. Their intensity grows proportionally to the decrement of the intensity of the n(OH ) band of free Brønsted groups (isosbestic points are, in fact, observed at 3590, 3575 and 3525 cm−1, respectively). These broad bands ( FWHM #52; FWHM #60; HH NN FWHM #114)2 are the n(OH ) modes of the CO hydrogen bonded adducts shown in Scheme 1, which are downward shifted and broadened because of the hydrogen bonding interaction (the shifts, calculated on the maxima, are Dn: =30; HH Dn: =102; Dn: =290 cm−1). As illustrated in NN CO several contributions [20–25], the shifts (in particular those associated with the CO interaction) can be used to evaluate the acid strength of the Brønsted groups involved. The comparison of these data with those obtained on various zeolites show that these shifts (calculated on the maximum of the band and hence associated with the most exposed groups) are very similar to those observed for H-ZSM-5 [20], H-mordenite [10,11], and 2 The full width at half-maximum is calculated by considering the low frequency half of the band, because it is not influenced by sites heterogeneity.
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Scheme 1.
H-Beta [25]. An important observation regarding the shape of the peaks of the n(OH,B) (B=H , 2 N , CO) adducts is that, unlike that found for the 2 n(OH ) band before gas dosage, the tail responsible for the asymmetry is located on the high frequency side of the peaks (which originates from the mirror effect previously mentioned ). Moreover, in the CO case, the tail shows distinct signs of a composite structure, a shoulder at ~3400 cm−1 is clearly observable. We think that Brønsted species located in the smaller rings cannot form adducts with hydrogen, nitrogen and CO characterized by maximum interaction energy, because they are already engaged in hydrogen bonding interactions with skeletal oxygens (see species B in Fig. 2(a)). All these results shows distinct similarities with those found for N and CO/mordenite systems [10,11], 2 which is not unexpected as ferrierite sample studied here and mordenite have a similar aluminum content, and the access to the secondary cavities ( limited by 8-membered rings) is of similar diameter. As far as the interaction with H is concerned, 2 it should be noted that the shift caused on the n(OH ) stretching frequency is one order of magnitude smaller than that caused by the interaction with CO, in agreement with the lower proton affinity of the dihydrogen molecule. (iii) The n( HH ), n(NN ) and n(CO) modes are also perturbed by the interaction with the Brønsted groups. These bands are complex and asymmetric (Figs. 3(b), 4(b) and 5(b)), as shoulders are distinctly observable on the high (H ) and low fre2 quency sides (N , CO) of the main peaks at 4103, 2 2331 and at 2171 cm−1 assigned to H , N and 2 2 CO adducts with Brønsted acid groups, respectively. The shoulder at 4120 cm−1, which appears at the highest pressures, could be assigned (following Beck et al. [26 ]) to a pressure-induced absorption, due to the interactions between the
hydrogen molecules. As to the component at 4108 cm−1, it could be due either to hydrogen adsorbed on further cation sites or (together with the band at 4103 cm−1) to the ortho–para H split. 2 These spectra are very similar to those already observed for the interaction of H with 2 H-mordenite [9]. Note that the n(HH ) maximum is downward shifted (Dn: =−58 cm−1) with respect to the gas phase (Raman active vibration of para H at 4161 cm−1 [27]), while the n(NN ) and 2 n(CO) are upward shifted (Dn: (NN )=+4; Dn: (CO)=+32 cm−1, respectively1). The positive shifts of the n(CO) (carbon down complexes) and of the n(N ) are always observed when these 2 molecules are axially perturbed by an electric field generated by a hydrogen site with positive character. Negative shifts of the n( HMH ) are consistent with the formation of both end-on and side-on structures (Scheme 2). Following Senchenya and Kazansky [28], the hydrogen molecule is interacting simultaneously with the bridging hydroxyls and with the neighboring negatively charged oxygen centers. In other words, hydrogen is a probe of dual sites. The data obtained on H-ferrierite are not conclusive on this point. For the time being, it is useful to underline that the n(HMH ) stretching mode is more influenced by external perturbations than the n(CO) and n(NN ) (greater Dn: shift). This implies that H (with its simple HMH bond) is a better probe 2
Scheme 2.
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of surface site heterogeneity than CO and N . In 2 case of the dinitrogen only, we also observe the formation, at the highest filling conditions (i.e., when the channels become filled with a liquid-like phase) of two satellite bands at 2334 and 2324 cm−1. A small but nevertheless distinct shift of the main peak to lower frequency (2330 cm−1), accompanied by broadening and intensity decrement, is also observed at the highest equilibrium pressures. A reasonable explanation of the shift to lower frequencies could be that the hydrogen-bonded species (b) (Scheme 1) become gradually surrounded by physically adsorbed (IR-inactive) nitrogen causing a gradual modification of the dielectric constant of the medium and of the adsorbate–adsorbate interactions. The origin of the shoulders at 2334 and 2324 cm−1 remains, however, obscure, and other effects, such as the presence of rotovibrational components or the coupling with a frustrated translational mode at 10–15 cm−1 or (finally and more plausibly) the coordination of two nitrogen molecules, must be invoked. This problem remains open. (iv) In the CO/ H-ferrierite system, an intense and complex band at 2139 cm−1 (with a shoulder at 2133 cm−1), associated with liquid-like CO species, is also observed at the highest pressures. An analogous absorption is not found for adsorbed nitrogen and hydrogen, because the n(NN ) and the n(H–H ) of liquid nitrogen and hydrogen are practically not IR active (and because liquid-like H is not present at 80 K ). 2 In conclusion, the Brønsted groups absorbing at 3605 cm−1 show the strongest interaction with H , N and CO, while Brønsted groups absorbing 2 2 at lower frequencies (and responsible for the low frequency tail ) form weaker hydrogen bonds. The 3605 cm−1 peak is associated with unperturbed strong OH groups pointing outwards in the open spaces of the main channels (10-membered ring channels), while the low frequency tail of the 3605 cm−1 peak is due to OH groups located in narrower rings (i.e., planar 6-rings) of the cages. Finally, from the observed shift of the n(OH ), it is inferred that the acid groups have an acid strength very similar to that observed for H-ZSM-5, H-mordenite and H-Beta [24,25].
3.3. IR spectra of H , N and CO on Li-, Na- and 2 2 K-exchanged ferrierite The IR spectra of H adsorbed on 2 Na-exchanged ferrierite, N on Li-exchanged fer2 rierite and CO on Li-, Na- and K-exchanged ferrierite are illustrated in Fig. 6 ( H ), Fig. 7 (N ) 2 2 and Fig. 8 (CO). 3.3.1. H 2 The IR spectra in the n(HMH ) stretching region of dihydrogen adsorbed on Na-ferrierite are characterized by a main peak centered at 4098–4096 cm−1 (distinctly asymmetric on the high frequency side) which is accompanied by two weak and broad tails in the 4120–4160 and 4080–4040 cm−1 ranges. These tails on both sides of the central maximum have certainly rotational origin: this observation finds a precedent in the spectrum of dihydrogen adsorbed on Na-ETS-10 [29]. Their presence demonstrates that the hydrogen molecule adsorbed on Na+ has residual
Fig. 6. FTIR spectra of increasing doses of H adsorbed on 2 Na-ferrierite at about 80 K. Maximum equilibrium pressure= 4 kPa.
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Fig. 7. FTIR spectra of increasing doses of N adsorbed at 2 about 100 K on Li-ferrierite. Maximum equilibrium pressure=6.6 kPa.
rotational freedom. The asymmetric character of the main band reveals that Na+ sites are heterogeneous. The frequency of the main peak is downward shifted with respect to the gas phase (Dn: =−63 cm−1), which demonstrates that the polarizing power of Na+ (or of dual sites) is higher than that of the Brønsted groups. The higher polarizing power of Na+ (or of dual sites) is also demonstrated by the intensity of the peak of the dihydrogen adduct, which is one order of magnitude higher than that of the analogous species formed upon interaction with Brønsted sites. Also in this case we cannot decide about the end-on, side-on or dual-site structures of the dihydrogen species. 3.3.2. N 2 The most important observations concerning the spectra of adsorbed nitrogen are as follows. 3.3.2.1. Li-ferrierite. On Li-ferrierite ( Fig. 7) the main peak due to the n(NMN ) mode of adsorbed N is in the 2339–2338 cm−1 range. The frequency 2
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is nearly constant for spectra 1–6 (corresponding to equilibrium pressures≤61 Pa): in this coverage range also the shape of the band remains constant. The peak profile is neither Lorentzian nor Gaussian because the main band is constantly accompanied by weak, nearly symmetrically located shoulders at 2341 and 2336 cm−1. Under high filling conditions (spectra 7 and 8), the second component (2336 cm−1) appears more visible (shoulder). Finally, at the highest pressure (spectrum 9), the two bands form a nearly symmetric envelope. The band at 2338–2339 cm−1 is undoubtedly the n(NN ) of nitrogen polarized by the strong electric field of Li+ in A sites (Dn: =+11 cm−1) [10]. The shoulder at 2336 cm−1, which appears at higher pressure, is consequently due to N interacting with B sites. 2 It should be noted that a shoulder at 2327 cm−1, assigned to dinitrogen interacting with silanols, also develops at the highest equilibrium pressures. This assignment is based on previous results obtained on other N /zeolite systems [10] 2 and on the observation that its growth is accompanied by a perturbation of the n silanols band OH (shifting the peak to 3710 cm−1; spectra not shown for brevity). 3.3.2.2. Na- and K-exchanged zeolites. The spectra of N on Na- and K-exchanged ferrierite (spectra 2 not reported for the sake of brevity) show maxima at 2333 and 2329 cm−1, respectively. The halfwidth of the absorptions (taken at the highest pressure) is smaller than that observed for N /H-ferrierite and decreases on passing from Li2 to K-exchanged zeolites. With respect to Li-exchanged ferrierite, the n(NN ) peaks are less upward shifted (Dn: =+6; Dn: =+2 cm−1). Na+ K+ This is certainly associated with the smaller polarizing power of Na+ and K+ with respect to Li+ (as found on other alkali-exchanged zeolites) [10]. In contrast with Li-ferrierite, no distinct signs of heterogeneity are observable. In both cases the formation of a shoulder at 2327 cm−1 associated with N interacting with silanols is observed at the 2 highest equilibrium pressures. On the basis of the data described, it is evident that nitrogen as a probe seems unable to distinguish between different cation families.
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Fig. 8. FTIR spectra of increasing doses of CO adsorbed on (a) Li-, (b) Na-, and (c) K-ferrierite respectively, at about 100 K. Maximum equilibrium pressure=6.6 kPa.
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3.3.3. CO The following comments can be briefly made. 3.3.3.1. Li-ferrierite (Fig. 8(a)). The IR spectra for increasing doses of CO on Li-ferrierite (corresponding to equilibrium pressures in the 0–6.6 kPa range) are characterized by the initial formation of a band at 2190 cm−1 (spectra 1–7), clearly due to the n(CO) of Li+,CO adducts. The main band is distinctly tailed on the low frequency side. The presence of the above-mentioned tail represents a clear indication that the Li+ centers are heterogeneous. By analogy with the H-ferrierite we believe that this heterogeneity is associated with the presence of rings of different radii in the structure. In particular, a reasonable hypothesis is that the peak at 2190 cm−1 is due to Li+,CO adducts located in the 10-membered ring channels (A sites) and that the tail at lower frequency is due to CO adsorbed on Li+ sites on 8- and 6-membered rings of the cages (B sites). The species absorbing at 2190 cm−1 are quite stable and can be observed, although with reduced intensity, already at room temperature. When the CO pressure is increased from 10−2 to 6.6 kPa (spectra 9–16), the peak at 2190 cm−1 disappears and a new intense and broad and probably composite absorption at 2179 cm−1 develops, which overshadows the tail due to B sites. This behavior is very commonly observed when CO interacts with small extra-framework cations [11,13] and is due to the stepwise solvation of the cations following the process shown in Scheme 3. At high pressures also the characteristic absorption at 2138 cm−1, typical of liquid-like CO, emerges.
Scheme 3.
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3.3.3.2. Na-ferrierite (Fig. 8(b)). The IR spectra of the first doses of CO on Na-ferrierite are characterized by two main bands, one narrow and strong at 2175 cm−1, and the second broader at 2159 cm−1. A very weak component at 2111 cm−1 is also observable (spectra 1–4). Upon increasing the equilibrium pressure, the first peak becomes progressively more intense and distinctly moves downwards (Dn: =−5 cm−1). At the highest equilibrium pressures this peak dominates the spectra of the CO/Na-ferrierite system. A similar but nearly opposite shift (Dn: =+3–4 cm−1) is simultaneously shown by the weak component at 2111 cm−1. On the basis of the considerations developed so far and of the experience accumulated on the IR spectroscopy of CO adsorbed on alkali-exchanged zeolites [11,13], the peaks at 2175 and at 2111 cm−1 are assigned to the n(CO) of Na+,CO and Na+,OC [30] complexes located in the main channels (A sites). Their simultaneous and opposite shifts upon increasing the CO dosage are simply the consequence of the solvation process which leads to the formation of Na+(CO) (n≥2) and n Na+(OC ) (CO) (m≥1) complexes. Multiple solm vation (responsible for the frequency shifts) is expected to occur only on cations located in the more open spaces. Of course the stretching frequency of Na+,CO complexes is lower than that of the corresponding Li+,CO, because of the lower polarizing power of Na+. Following the considerations developed before, the broad band at 2159 cm−1 could be due to CO adsorbed on Na+ in B sites. However, this hypothesis seems unable to account for why this band is so intense and stable. We think that an alternative explanation of this band based on the interaction of CO with more than one Na+ cation is possible. This point will be discussed in greater detail when the CO interaction with the K-ferrierite zeolite will be considered. Finally the weak absorption centered at 2138 cm−1 is the usual manifestation of liquid-like CO: its low intensity is the result of the space limitations imposed by the larger dimension of the ions populating the channels and cavities. 3.3.3.3. K-ferrierite (Fig. 8(c)). The spectra of CO on K-ferrierite are characterized by the pres-
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ence of a doublet at 2162 (medium) and at 2150 cm−1 (strong). Very weak bands are also observed at 2175 and 2118 cm−1 ( low dosage). The weak band at 2175 cm−1 is undoubtedly due to CO interacting with residual Na+ ions which have resisted the exchange procedure. Practically no signs of physically adsorbed CO can be observed. Following the considerations developed so far (and in agreement with the results obtained on other alkali-exchanged zeolites) [11,13], the 2162 cm−1 band is assigned to K+,CO adducts formed in the more open spaces of 10-membered ring channels (A sites), while the 2150 cm−1 band should be ascribed to adducts formed on B sites. However, the intensity and stability of the 2150 cm−1 band are now so high that this explanation does not seem appropriate. We think that in order to overcome these difficulties we have to reconsider the assumption that, in all cases, the different IR bands correspond to the n(CO) of different M+,CO complexes, i.e., complexes where the CO molecules are perturbed by the electrostatic field generated by a single cation [31]. To illustrate this point let us consider the effect of increasing the cation radius of two B sites located in the cage. The situation for Li-ferrierite and K-ferrierite viewed through the 8-membered rings representing the entrance to the cage is illustrated in Fig. 2(b) (Li) and Fig. 2(c) ( K ). We notice that in Li-ferrierite, the two Li+ sites in the cage are 0.5 nm distant and consequently they effectively interact with CO in an independent way. For Li-ferrierite the basic assumption that the interaction with CO is dominated by the electric field of a single cation is consequently valid. On K-ferrierite, the situation is completely different, as the K+ sites are now at a distance of only 0.38 nm apart. Any CO molecule penetrating the cage through the 8-membered ring channel necessarily interacts with the electrostatic field generated by two ions. In agreement with this hypothesis, it is worth underlining that, on passing from Na- to K-exchanged ferrierite, the second band is acquiring intensity (in relative terms) and becomes predominant in K-ferrierite. This in turns implies that the 2159 cm−1 band observed on Na-ferrierite can be more plausibly assigned to CO interacting with more than one cation than to a CO interacting
with a single Na+. The weak feature at 2118 cm−1 is likely due to K+,OC species on the ions located in the more open spaces. Finally, the very weak component at 2104 cm−1 is the 13CO satellite of the main band. 3.4. IR spectra of NO adsorbed on Li-, Na- and K-ferrierite The IR spectroscopy of NO adsorbed at low temperature on zeolites is a complicated subject because of the tendency of NO to form dimers (trans and cis) both in the gas and in the adsorbed phase. For this reason the use of NO as a probe of the local fields associated with the cationic sites is more difficult, and it is strictly limited to the lowest dosages where the probability of adsorbate– adsorbate interactions (and hence the formation of dimers) is minimized or suppressed. The IR spectra of NO adsorbed on K-ferrierite are shown in Fig. 9(a). As expected the spectra are very complex and become simpler only for the lowest dosages. We shall consequently concentrate only on the bands whose relative intensity grows when the adsorbate concentration in the channels is going to zero. This means that we will consider only the main peaks of the spectra obtained at the lower pressures (see Fig. 9(b)), the others being dominated by IR manifestations of the adsorbed dimers. We notice that the highest frequency bands are at 1906 (Li), 1894 (Na) and at 1882 cm−1 ( K ). These bands are shifted upward with respect to the gas phase (1876 cm−1): Dn: =+30; Li+ Dn: =+18 and Dn: =+14. If the frequency of Na+ K+ these bands is plotted against the frequency of the highest frequency bands observed for N and CO 2 adsorbed on the same zeolites ( Fig. 10), we obtain a straight correlation. This observation has two implications: (i) the NO molecule responds to the polarizing field of the cations in the same way as CO and N ; (ii) the cation–NO adducts charac2 terized by the highest frequencies have nitrogendown structure and are located in the main channels. Other considerations concerning the possible presence of NO species adsorbed on cationic sites located in the 8-membered rings cannot be made with confidence, because of the confusing effect
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Fig. 9. (a) FTIR spectra of increasing doses of NO adsorbed on K-ferrierite. (b) Low pressure interaction of NO on Li- (curve 1), Na- (curve 2) and K-ferrierite (curve 3) at about 120 K.
Fig. 10. (a) n: (N ) versus n: (NO) and (b) n: (CO) versus n: (NO) plots for 1:1 adducts in Li-, Na- and K-ferrierite. 2
of the dimers bands (which cover the 1850– 1700 cm−1 interval ).
4. Conclusions The modifications induced in the IR spectrum of H , N , CO and NO by the interaction with 2 2 H-, Li-, Na- and K-exchanged ferrierite are used
to evaluate the local electric fields associated with the different cationic sites locations. It is concluded that cationic sites in 10-membered ring channels show the highest polarizing power. On passing from Li- to Na- to K-exchanged ferrierite the local electric fields probed by diatomic probes are increasingly influenced by the distribution of the ions in the framework, the explanation of IR spectra of adsorbed
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probes in terms of 1:1 interactions consequently becoming less and less realistic.
Acknowledgements The present work is a part of a project coordinated by A. Zecchina and cofinancied by the Italian MURST (Cofin 98, Area 03). CNR (project 98.01987.CT03) was also acknowledged.
References [1] P.A. Vaughan, Acta Crystallogr. 21 (1966) 983. [2] R.E. Morris, S.J. Weigel, N.J. Henson, L.M. Bull, M.T. Janicke, B.F. Chmelka, A.K. Cheetham, J. Am. Chem. Soc. 116 (1994) 11 849. [3] P. Grandvallet, K.P. de Jong, H.H. Mooiweer, A.G.T.G. Kortbeek, B. Kraushaar-Czarnetzki, EP Patent No. 501 577, 1992, assigned to Shell Internationale Research Maatschappij B.V. [4] H.H. Mooiweer, K.P. de Jong, B. Kraushaar-Czarnetzki, W.H.J. Stork, B.C.H. Krutzen, Stud. Surf. Sci. Catal. 84 (1994) 2327. [5] M. Guisnet, P. Andy, Y. Boucheffa, N.S. Gnep, C. Travers, E. Benazzi, Catal. Lett. 50 (1998) 159. [6 ] P. Me´riaudeau, C. Naccache, H.N. Le, T.A. Vu, G. Szabo, J. Molec. Catal. A 123 (1997) L1. [7] A.C. Butler, C.P. Nicolaides, Catal. Today 18 (1993) 443. [8] V.L. Zholobenko, D.B. Lukyanov, J. Dwyer, W.J. Smith, J. Phys. Chem. B 102 (1998) 2715. [9] M.A. Makarova, V.L. Zholobenko, K.M. Al-Ghefalli, N.E. Thompson, J. Dewing, J. Dwyer, J. Chem. Soc. Faraday Trans. 90 (1994) 1047. [10] F. Geobaldo, C. Lamberti, G. Ricchiardi, S. Bordiga, A. Zecchina, G. Turnes Palomino, C. Otero Area´n, J. Phys. Chem 99 (1995) 11 167. [11] S. Bordiga, C. Lamberti, F. Geobaldo, A. Zecchina, G. Turnes Palomino, C. Otero Area´n, Langmuir 11 (1995) 527.
[12] C. Lamberti, S. Bordiga, F. Geobaldo, A. Zecchina, C. Otero Area´n, J. Chem. Phys. 103 (1995) 3158. [13] A. Zecchina, S. Bordiga, C. Lamberti, G. Spoto, L. Carnelli, C. Otero Area´n, J. Phys. Chem. 98 (1994) 9577. [14] A. Zecchina, C. Otero Area´n, Chem. Soc. Rev. (1996) 187. [15] J.G. Fripiat, P. Galet, J. Delhalle, J.M. Andre´, J.B. Nagy, E.G. Derouane, J. Phys. Chem. 89 (1985) 1932. [16 ] F. Blanco, G. Urbina-Villalba, M.M. Ramirez de Aguadelo, Molec. Simul. 14 (1995) 165. [17] A. Martucci, A. Alberti, G. Cruciani, P. Radaelli, P. Ciambelli, M. Rappacciuolo, Micropor. Mesopor. Mater. 30 (1999) 95. ˇ ejka, J. Phys. Chem. B 102 [18] P. Sarv, B. Wichterlowa´, J. C (1998) 1372. [19] I.J. Pickering, P.J. Maddox, J.M. Thomas, A.K. Cheetham, J. Catal. 119 (1989) 261. [20] L. Kubelkova´, S. Beran, J.A. Lercher, Zeolites 9 (1989) 539. [21] E.A. Paukshtis, R.I. Soltanov, E.N. Yurchenko, React. Kinet. Catal. Lett. 19 (1982) 105. [22] M.I. Izaki, H. Knozinger, Mater. Chem. Phys. 17 (1987) 201. [23] T.P. Beebe, P. Gelin, G.T. Yates, Surf. Sci. 148 (1984) 526. [24] A. Zecchina, S. Bordiga, G. Spoto, D. Scarano, G. Spano`, F. Geobaldo, J. Chem. Soc., Faraday Trans. 92 (1996) 4863. [25] C. Paze`, S. Bordiga, C. Lamberti, M. Salvalaggio, A. Zecchina, G. Bellussi, J. Phys. Chem. B 101 (1997) 4740. [26 ] K. Beck, H. Pfeifer, B. Staudte, J. Chem. Soc., Faraday Trans. 89 (1993) 3995. [27] G. Herzberg, Molecular Spectra and Molecular Structure I – Spectra of Diatomic Molecules, 2nd Edition, Van Nostrand, Princeton, NJ, 1950. [28] I.N. Senchenya, V.B. Kazansky, Kinet. Catal. 35 (1994) 61. [29] A. Zecchina, C. Otero Area´n, G. Turnes Palomino, F. Geobaldo, C. Lamberti, G. Spoto, S. Bordiga, Phys. Chem. Chem. Phys. 1 (1999) 1649. [30] C. Otero Are´an, A.A. Tsyganenko, E. Escalona Platero, E. Garrone, A. Zecchina, Agnew. Chem. Int. Ed. 37 (1998) 3161. [31] S. Bordiga, E. Garrone, C. Lamberti, A. Zecchina, C. Otero Area´n, V.B. Kazansky, L.M. Kustov, J. Chem. Soc., Faraday Trans. 90 (1994) 3367.
Vibrational spectroscopy of H , N , CO and NO adsorbed on 2 2 H, Li, Na, K-exchanged ferrierite S. Bordiga, G. Turnes Palomino, C. Paze`, A. Zecchina * Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universita` di Torino, Via P. Giuria 7, I-10125, Torino, Italy Received 15 April 1999; received in revised form 28 July 1999; accepted for publication 9 August 1999
Abstract The vibrational spectroscopy of H , N , CO and NO adsorbed on the zeolite ferrierite is described. The spectral 2 2 modifications induced by the adsorption process on the protonic form are compared with those observed on the alkaline-exchanged forms. Different environments of Brønsted sites and of alkaline counterions in ferrierite are evidenced, which differ in the local electric field associated with the cationic species. These different environments are associated with the cation locations in the channels and cavities. In particular, ions located in the 10-membered ring channels are more available for the interaction with CO, N and H , while ions in the cages form weaker adducts. 2 2 On passing from lithium-, to sodium- and to potassium-exchanged samples, the local fields probed by H , N and 2 2 CO are increasingly dependent upon the distribution of the ions in the framework. In K-ferrierite the interaction cannot be solely described in terms of 1:1 K+/B (B=N , CO) adducts. As far as H-ferrierite is concerned, it is 2 inferred from the observed shift of the n(OH ) mode upon interaction with H , N and CO that the bridged strong 2 2 acid groups show an acid strength very similar to that observed for H-ZSM-5, H-mordenite and H-Beta. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Acidity; Alkali-exchanged zeolite; CO adsorption; Ferrierite; Hydrogen adsorption
1. Introduction Ferrierite is a zeolite which contains 10-, 8-, 6and 5-membered rings in its structure [1,2] and shows interesting catalytic properties for the isomerization of linear butenes [3–7]. The zeolitic channel system of the ferrierite framework is characterized by 10-membered ring channels running parallel to the crystallographic [001] direction and by 8-membered ring channels running parallel to the [010] direction. * Corresponding author. Tel.: +39-11-6707537; fax: +39-11-6707855. E-mail address: [email protected] (A. Zecchina)
Recently, Zholobenko et al. [8] reported a detailed spectroscopic characterization of the acid sites using ammonia and isobutane as probes. They concluded that the strong Brønsted acid groups are mainly located in extended rings at the intersection of the 8- and 6- membered ring channels. In this paper the characterization of Brønsted acidity is continued by using H , N , CO and NO 2 2 as probes, taking advantage of the well-known fact that the n(HH ), n(NN ) and n(CO) stretching modes are known to be selectively perturbed by the interaction with the Brønsted groups and that a parallel modification of the vibrational properties of the n(OH ) of the Brønsted centers occurs [9–11].
1387-1811/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S1 3 8 7 -1 8 1 1 ( 9 9 ) 0 0 16 0 - 2
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Furthermore, H , N and CO interact with any 2 2 exposed cation present in the channels: owing to this interaction, the n(HMH ), n(NN ) and n(CO) stretching frequencies shift upwards (N and CO) 2 and downwards (H )1 [9–11], the shift being pro2 portional to the value of the positive electric field centered at the cationic or at the Lewis sites [10,12,13]. On this basis it is evident that these very weak bases can be used to probe the strength of electric fields present in the channels associated with positively charged cations or any other positively charged center. In the case of H and N , 2 2 the perturbation is accompanied not only by a shift of the stretching frequency but also by an impressive increment of the extinction coefficient, which is especially evident for dinitrogen interacting with extra-framework Al3+ [10,14]: this property enhances the usefulness of dinitrogen (and hydrogen) as surface probe. As far as NO is concerned, much less is known in the literature: however, owing to the similarity of the electronic structures of N , CO and NO, it is expected that 2 NO also can respond to the electric field perturbation and consequently can be used as a probe of the environment of the cationic sites. Based on these considerations, H , N and CO 2 2 were used as probes to explore the acid strength of the Brønsted sites and the distribution of the electric fields in alkali-exchanged ferrierite, in an attempt to gain further information on the location of the cations in the 8- and 10-membered ring channels and in the cavities connecting them. NO was used as a probe for the electric field at the cationic sites as well.
Elementary chemical analysis gave an Si/Al ratio of 8, while 27Al MAS NMR measurements showed that all aluminum atoms were tetrahedrally coordinated. The ammonium and alkaline forms (Li, Na, K ) of ferrierite were obtained by conventional ion exchange procedures of the commercial sample with nitrate solutions. H-ferrierite samples were obtained by thermal decomposition of the ammonium forms. Self-supporting wafers were made from the samples and outgassed in dynamic vacuum (residual pressure<10−2 Pa) at 673 K for 2 h inside an IR cell which allowed in situ high-temperature treatments, gas dosage, and low-temperature measurements to be made. The IR spectra were recorded, at 2 cm−1 resolution on a Bruker IFS66 FTIR spectrometer equipped with a MCT cryogenic detector. Nitrogen and CO adsorption were measured at a temperature of about 100 K. For H adsorption, a specifically designed cell was used 2 which allowed spectra to be taken at a temperature as low as 80 K. The attainment of such a low temperature is essential for studying the H –cation interaction, because the low interaction 2 energy between H and H+ or Na+ keeps the 2 concentration of H+,H or Na+,H adducts at 2 2 100–110 K at critically low levels. H , CO, NO and N were high purity grade 2 2 from Matheson; NO was further purified before adsorption by repeated freeze–pump–thaw cycles.
3. Results and discussion 3.1. The structure of ferrierite and the siting of extra-framework ions
2. Experimental The original ferrierite sample used in this study was a commercial sample supplied by Engelhard Corporation (Iselin, NJ, USA) in an Na, K form.
1 The shifts Dn: caused by interaction with cationic centers are calculated by using as reference the n(NMN ) and n(CMO) frequencies of N and CO physically adsorbed in zeolite channels 2 respectively of 2327 and 2139 cm−1. For H the reference fre2 quency was that of n(HMH ) gas (because the frequency of physically adsorbed H is not available). 2
The ferrierite structure (Fig. 1) is based on 5-ring secondary building units; from their connection 10-ring channels (having a diameter of 0.43–0.55 nm) are formed. These parallel 10-ring channels are periodically connected along the [010] direction by cages having as pore access 8-membered rings (0.34–0.48 nm diameter); the cages extend for 0.6–0.7 nm in the [100] crystallographic direction of the space (Fig. 1(b)). From a crystallographic point of view, the ferrierite structure can be constructed starting from asymmetric
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69
Fig. 1. Computer graphic representation of the ferrierite structure: (a) Connolly representation of the accessible cavities; (b) details of the cage shape; (c) labels indicating the four T sites located in 10- and 8-membered rings as described in Ref. [1].
units having either four or five atoms; in this paper we always refer to the four-atom asymmetric unit and to the Vaughan’s notation [1] to identify the different sites. 10-membered rings are composed of atoms in sites T2, T3 and T4, while 8-membered rings are composed of atoms in T2 and T4 sites. T1 sites are located only in the 6-membered rings, and are therefore located only in the cages (Fig. 1(c)). Concerning the aluminum location, Fripiat et al. [15] and Blanco et al. [16 ] believe that the most populated sites are T2 and T4. The presence of aluminum on T4 sites located in the cages is inferred from neutron powder diffraction studies on D-ferrierite; however, a family of protons in the 10-ring channels has also been observed [17]. Using multinuclear MQMAS NMR spectroscopy, Sarv et al. [18] found that aluminum pairs can be present preferentially in the 6-membered rings which connect the two 10-membered rings ( T2 and T1 sites). From the existing data it is evident that
a definite conclusion about the aluminum location is not yet possible. Nevertheless, all the results point towards a preferential population of T2 sites of the cages. As for the counterion location, it has been observed with several techniques that they are preferentially located in the cages, but they are also present in the 10-membered ring channels [8,17–19]. A plausible representation of the possible locations of protonic groups associated with the T2 sites is shown in Fig. 2(a). Two kinds of environments can be distinguished, according to the fact that the protonic groups can be located in the 10-membered ring channels (A species) or in the cages (B species). A peculiarity of the ferrierite structure is represented by the 6-membered rings present in the cages. In fact, unlike the 6-membered rings separating sodalite cages and supercages in Y zeolites, in ferrierite the ring is flat. This implies that the Brønsted groups associated with T2 sites of the cages (B sites) are forced to interact with the oxygens of the ring. This does not happen for
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Fig. 2. (a) Ferrierite structure, viewed along the [001] direction. 10- and 6-membered rings are represented with their van der Waals spheres. Silicon atoms are grey, oxygens are dark grey, aluminum at T2 sites and associated protons are black. (b), (c) View along the [010] direction of the 8-membered ring entrance to the cage. The 8-membered rings are represented with their van der Waals spheres. In (b) and (c) two Li+ and two K+ ions ( located in the cages), respectively, are represented following the cation location suggested in Ref. [18].
alkali cations: in fact, owing to their higher ionicity and larger radius, they are no longer located in the plane of the flat 6-membered rings and are protruding into the cage showing themselves at the 8-membered ring window connecting the main channels with the cages ( Fig. 2(b) and (c)).
3.2. IR spectra of H , N and CO adsorbed on 2 2 H-ferrierite The IR spectra of increasing doses of H , N 2 2 and CO on H-ferrierite are shown in Figs. 3–5, respectively.
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Fig. 3. FTIR spectra of increasing doses of H adsorbed on H-ferrierite at about 80 K. Maximum equilibrium pressure=4 kPa. 2
Fig. 4. FTIR spectra of increasing doses of N adsorbed on H-ferrierite at about 100 K. Dotted lines in (b) correspond to the highest 2 pressures. Maximum equilibrium pressure=6.6 kPa.
The most important facts deserving a comment are as follows. (i) The peak with its maximum at 3605 cm−1, due to the unperturbed n(OH ) mode of the bridged strong Brønsted groups, is gradually eroded upon dosage of H , N and CO ( Figs. 3(a), 4(a) and 2 2 5(a)). It should be noted that this peak is highly
asymmetric on the low frequency side indicating that the Brønsted groups are heterogeneous. This means that the peak at 3605 cm−1 is constituted by two or more components. Zholobenko et al. [8] have deconvoluted this peak into four components at 3609, 3601, 3587 and 3565 cm−1. From Figs. 3(a), 4(a) and 5(a), it is inferred
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Fig. 5. FTIR spectra of increasing doses of CO adsorbed on H-ferrierite at about 100 K. Maximum equilibrium pressure=6.6 kPa.
that, upon interaction with diatomic molecules, the high frequency part of the n(OH ) band is preferentially eroded, and that the low frequency components (contributing to the tail ) are perturbed only at the highest equilibrium pressures. This implies that the OH groups responsible for the low frequency tail of the 3605 cm−1 peak are less prone to an interaction with basic probes. The most plausible explanation of these facts is that the band of Brønsted sites with a maximum at 3605 cm−1 is composite and that at least two different families are contributing. In particular, these are: (a) a family of OH groups responsible for the 3605 cm−1 maximum; we think that they are the most exposed ones, like those located in the largest rings (sites A); (b) a family of OH groups responsible for the low frequency tail; these OH groups could be located in sites B. Their lowered frequency could be because they are already engaged in very weak hydrogen bonding interaction. In fact, because of the planar nature of the 6-membered rings present in the cages (Fig. 2(a)), the Brønsted sites (B) associated with them can form bifurcated H-bonds with the neighboring oxygens, which explains both the lowered frequency and the smaller propensity to interact with external probes. (ii) New broader and composite bands at 3575
( H ), 3502 (N ) and at 3314 cm−1 (CO) are formed 2 2 upon gas dosage. These bands are asymmetric on the high frequency side where their shape is specular to that of the original n(OH ) absorption. Their intensity grows proportionally to the decrement of the intensity of the n(OH ) band of free Brønsted groups (isosbestic points are, in fact, observed at 3590, 3575 and 3525 cm−1, respectively). These broad bands ( FWHM #52; FWHM #60; HH NN FWHM #114)2 are the n(OH ) modes of the CO hydrogen bonded adducts shown in Scheme 1, which are downward shifted and broadened because of the hydrogen bonding interaction (the shifts, calculated on the maxima, are Dn: =30; HH Dn: =102; Dn: =290 cm−1). As illustrated in NN CO several contributions [20–25], the shifts (in particular those associated with the CO interaction) can be used to evaluate the acid strength of the Brønsted groups involved. The comparison of these data with those obtained on various zeolites show that these shifts (calculated on the maximum of the band and hence associated with the most exposed groups) are very similar to those observed for H-ZSM-5 [20], H-mordenite [10,11], and 2 The full width at half-maximum is calculated by considering the low frequency half of the band, because it is not influenced by sites heterogeneity.
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Scheme 1.
H-Beta [25]. An important observation regarding the shape of the peaks of the n(OH,B) (B=H , 2 N , CO) adducts is that, unlike that found for the 2 n(OH ) band before gas dosage, the tail responsible for the asymmetry is located on the high frequency side of the peaks (which originates from the mirror effect previously mentioned ). Moreover, in the CO case, the tail shows distinct signs of a composite structure, a shoulder at ~3400 cm−1 is clearly observable. We think that Brønsted species located in the smaller rings cannot form adducts with hydrogen, nitrogen and CO characterized by maximum interaction energy, because they are already engaged in hydrogen bonding interactions with skeletal oxygens (see species B in Fig. 2(a)). All these results shows distinct similarities with those found for N and CO/mordenite systems [10,11], 2 which is not unexpected as ferrierite sample studied here and mordenite have a similar aluminum content, and the access to the secondary cavities ( limited by 8-membered rings) is of similar diameter. As far as the interaction with H is concerned, 2 it should be noted that the shift caused on the n(OH ) stretching frequency is one order of magnitude smaller than that caused by the interaction with CO, in agreement with the lower proton affinity of the dihydrogen molecule. (iii) The n( HH ), n(NN ) and n(CO) modes are also perturbed by the interaction with the Brønsted groups. These bands are complex and asymmetric (Figs. 3(b), 4(b) and 5(b)), as shoulders are distinctly observable on the high (H ) and low fre2 quency sides (N , CO) of the main peaks at 4103, 2 2331 and at 2171 cm−1 assigned to H , N and 2 2 CO adducts with Brønsted acid groups, respectively. The shoulder at 4120 cm−1, which appears at the highest pressures, could be assigned (following Beck et al. [26 ]) to a pressure-induced absorption, due to the interactions between the
hydrogen molecules. As to the component at 4108 cm−1, it could be due either to hydrogen adsorbed on further cation sites or (together with the band at 4103 cm−1) to the ortho–para H split. 2 These spectra are very similar to those already observed for the interaction of H with 2 H-mordenite [9]. Note that the n(HH ) maximum is downward shifted (Dn: =−58 cm−1) with respect to the gas phase (Raman active vibration of para H at 4161 cm−1 [27]), while the n(NN ) and 2 n(CO) are upward shifted (Dn: (NN )=+4; Dn: (CO)=+32 cm−1, respectively1). The positive shifts of the n(CO) (carbon down complexes) and of the n(N ) are always observed when these 2 molecules are axially perturbed by an electric field generated by a hydrogen site with positive character. Negative shifts of the n( HMH ) are consistent with the formation of both end-on and side-on structures (Scheme 2). Following Senchenya and Kazansky [28], the hydrogen molecule is interacting simultaneously with the bridging hydroxyls and with the neighboring negatively charged oxygen centers. In other words, hydrogen is a probe of dual sites. The data obtained on H-ferrierite are not conclusive on this point. For the time being, it is useful to underline that the n(HMH ) stretching mode is more influenced by external perturbations than the n(CO) and n(NN ) (greater Dn: shift). This implies that H (with its simple HMH bond) is a better probe 2
Scheme 2.
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of surface site heterogeneity than CO and N . In 2 case of the dinitrogen only, we also observe the formation, at the highest filling conditions (i.e., when the channels become filled with a liquid-like phase) of two satellite bands at 2334 and 2324 cm−1. A small but nevertheless distinct shift of the main peak to lower frequency (2330 cm−1), accompanied by broadening and intensity decrement, is also observed at the highest equilibrium pressures. A reasonable explanation of the shift to lower frequencies could be that the hydrogen-bonded species (b) (Scheme 1) become gradually surrounded by physically adsorbed (IR-inactive) nitrogen causing a gradual modification of the dielectric constant of the medium and of the adsorbate–adsorbate interactions. The origin of the shoulders at 2334 and 2324 cm−1 remains, however, obscure, and other effects, such as the presence of rotovibrational components or the coupling with a frustrated translational mode at 10–15 cm−1 or (finally and more plausibly) the coordination of two nitrogen molecules, must be invoked. This problem remains open. (iv) In the CO/ H-ferrierite system, an intense and complex band at 2139 cm−1 (with a shoulder at 2133 cm−1), associated with liquid-like CO species, is also observed at the highest pressures. An analogous absorption is not found for adsorbed nitrogen and hydrogen, because the n(NN ) and the n(H–H ) of liquid nitrogen and hydrogen are practically not IR active (and because liquid-like H is not present at 80 K ). 2 In conclusion, the Brønsted groups absorbing at 3605 cm−1 show the strongest interaction with H , N and CO, while Brønsted groups absorbing 2 2 at lower frequencies (and responsible for the low frequency tail ) form weaker hydrogen bonds. The 3605 cm−1 peak is associated with unperturbed strong OH groups pointing outwards in the open spaces of the main channels (10-membered ring channels), while the low frequency tail of the 3605 cm−1 peak is due to OH groups located in narrower rings (i.e., planar 6-rings) of the cages. Finally, from the observed shift of the n(OH ), it is inferred that the acid groups have an acid strength very similar to that observed for H-ZSM-5, H-mordenite and H-Beta [24,25].
3.3. IR spectra of H , N and CO on Li-, Na- and 2 2 K-exchanged ferrierite The IR spectra of H adsorbed on 2 Na-exchanged ferrierite, N on Li-exchanged fer2 rierite and CO on Li-, Na- and K-exchanged ferrierite are illustrated in Fig. 6 ( H ), Fig. 7 (N ) 2 2 and Fig. 8 (CO). 3.3.1. H 2 The IR spectra in the n(HMH ) stretching region of dihydrogen adsorbed on Na-ferrierite are characterized by a main peak centered at 4098–4096 cm−1 (distinctly asymmetric on the high frequency side) which is accompanied by two weak and broad tails in the 4120–4160 and 4080–4040 cm−1 ranges. These tails on both sides of the central maximum have certainly rotational origin: this observation finds a precedent in the spectrum of dihydrogen adsorbed on Na-ETS-10 [29]. Their presence demonstrates that the hydrogen molecule adsorbed on Na+ has residual
Fig. 6. FTIR spectra of increasing doses of H adsorbed on 2 Na-ferrierite at about 80 K. Maximum equilibrium pressure= 4 kPa.
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Fig. 7. FTIR spectra of increasing doses of N adsorbed at 2 about 100 K on Li-ferrierite. Maximum equilibrium pressure=6.6 kPa.
rotational freedom. The asymmetric character of the main band reveals that Na+ sites are heterogeneous. The frequency of the main peak is downward shifted with respect to the gas phase (Dn: =−63 cm−1), which demonstrates that the polarizing power of Na+ (or of dual sites) is higher than that of the Brønsted groups. The higher polarizing power of Na+ (or of dual sites) is also demonstrated by the intensity of the peak of the dihydrogen adduct, which is one order of magnitude higher than that of the analogous species formed upon interaction with Brønsted sites. Also in this case we cannot decide about the end-on, side-on or dual-site structures of the dihydrogen species. 3.3.2. N 2 The most important observations concerning the spectra of adsorbed nitrogen are as follows. 3.3.2.1. Li-ferrierite. On Li-ferrierite ( Fig. 7) the main peak due to the n(NMN ) mode of adsorbed N is in the 2339–2338 cm−1 range. The frequency 2
75
is nearly constant for spectra 1–6 (corresponding to equilibrium pressures≤61 Pa): in this coverage range also the shape of the band remains constant. The peak profile is neither Lorentzian nor Gaussian because the main band is constantly accompanied by weak, nearly symmetrically located shoulders at 2341 and 2336 cm−1. Under high filling conditions (spectra 7 and 8), the second component (2336 cm−1) appears more visible (shoulder). Finally, at the highest pressure (spectrum 9), the two bands form a nearly symmetric envelope. The band at 2338–2339 cm−1 is undoubtedly the n(NN ) of nitrogen polarized by the strong electric field of Li+ in A sites (Dn: =+11 cm−1) [10]. The shoulder at 2336 cm−1, which appears at higher pressure, is consequently due to N interacting with B sites. 2 It should be noted that a shoulder at 2327 cm−1, assigned to dinitrogen interacting with silanols, also develops at the highest equilibrium pressures. This assignment is based on previous results obtained on other N /zeolite systems [10] 2 and on the observation that its growth is accompanied by a perturbation of the n silanols band OH (shifting the peak to 3710 cm−1; spectra not shown for brevity). 3.3.2.2. Na- and K-exchanged zeolites. The spectra of N on Na- and K-exchanged ferrierite (spectra 2 not reported for the sake of brevity) show maxima at 2333 and 2329 cm−1, respectively. The halfwidth of the absorptions (taken at the highest pressure) is smaller than that observed for N /H-ferrierite and decreases on passing from Li2 to K-exchanged zeolites. With respect to Li-exchanged ferrierite, the n(NN ) peaks are less upward shifted (Dn: =+6; Dn: =+2 cm−1). Na+ K+ This is certainly associated with the smaller polarizing power of Na+ and K+ with respect to Li+ (as found on other alkali-exchanged zeolites) [10]. In contrast with Li-ferrierite, no distinct signs of heterogeneity are observable. In both cases the formation of a shoulder at 2327 cm−1 associated with N interacting with silanols is observed at the 2 highest equilibrium pressures. On the basis of the data described, it is evident that nitrogen as a probe seems unable to distinguish between different cation families.
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Fig. 8. FTIR spectra of increasing doses of CO adsorbed on (a) Li-, (b) Na-, and (c) K-ferrierite respectively, at about 100 K. Maximum equilibrium pressure=6.6 kPa.
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3.3.3. CO The following comments can be briefly made. 3.3.3.1. Li-ferrierite (Fig. 8(a)). The IR spectra for increasing doses of CO on Li-ferrierite (corresponding to equilibrium pressures in the 0–6.6 kPa range) are characterized by the initial formation of a band at 2190 cm−1 (spectra 1–7), clearly due to the n(CO) of Li+,CO adducts. The main band is distinctly tailed on the low frequency side. The presence of the above-mentioned tail represents a clear indication that the Li+ centers are heterogeneous. By analogy with the H-ferrierite we believe that this heterogeneity is associated with the presence of rings of different radii in the structure. In particular, a reasonable hypothesis is that the peak at 2190 cm−1 is due to Li+,CO adducts located in the 10-membered ring channels (A sites) and that the tail at lower frequency is due to CO adsorbed on Li+ sites on 8- and 6-membered rings of the cages (B sites). The species absorbing at 2190 cm−1 are quite stable and can be observed, although with reduced intensity, already at room temperature. When the CO pressure is increased from 10−2 to 6.6 kPa (spectra 9–16), the peak at 2190 cm−1 disappears and a new intense and broad and probably composite absorption at 2179 cm−1 develops, which overshadows the tail due to B sites. This behavior is very commonly observed when CO interacts with small extra-framework cations [11,13] and is due to the stepwise solvation of the cations following the process shown in Scheme 3. At high pressures also the characteristic absorption at 2138 cm−1, typical of liquid-like CO, emerges.
Scheme 3.
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3.3.3.2. Na-ferrierite (Fig. 8(b)). The IR spectra of the first doses of CO on Na-ferrierite are characterized by two main bands, one narrow and strong at 2175 cm−1, and the second broader at 2159 cm−1. A very weak component at 2111 cm−1 is also observable (spectra 1–4). Upon increasing the equilibrium pressure, the first peak becomes progressively more intense and distinctly moves downwards (Dn: =−5 cm−1). At the highest equilibrium pressures this peak dominates the spectra of the CO/Na-ferrierite system. A similar but nearly opposite shift (Dn: =+3–4 cm−1) is simultaneously shown by the weak component at 2111 cm−1. On the basis of the considerations developed so far and of the experience accumulated on the IR spectroscopy of CO adsorbed on alkali-exchanged zeolites [11,13], the peaks at 2175 and at 2111 cm−1 are assigned to the n(CO) of Na+,CO and Na+,OC [30] complexes located in the main channels (A sites). Their simultaneous and opposite shifts upon increasing the CO dosage are simply the consequence of the solvation process which leads to the formation of Na+(CO) (n≥2) and n Na+(OC ) (CO) (m≥1) complexes. Multiple solm vation (responsible for the frequency shifts) is expected to occur only on cations located in the more open spaces. Of course the stretching frequency of Na+,CO complexes is lower than that of the corresponding Li+,CO, because of the lower polarizing power of Na+. Following the considerations developed before, the broad band at 2159 cm−1 could be due to CO adsorbed on Na+ in B sites. However, this hypothesis seems unable to account for why this band is so intense and stable. We think that an alternative explanation of this band based on the interaction of CO with more than one Na+ cation is possible. This point will be discussed in greater detail when the CO interaction with the K-ferrierite zeolite will be considered. Finally the weak absorption centered at 2138 cm−1 is the usual manifestation of liquid-like CO: its low intensity is the result of the space limitations imposed by the larger dimension of the ions populating the channels and cavities. 3.3.3.3. K-ferrierite (Fig. 8(c)). The spectra of CO on K-ferrierite are characterized by the pres-
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ence of a doublet at 2162 (medium) and at 2150 cm−1 (strong). Very weak bands are also observed at 2175 and 2118 cm−1 ( low dosage). The weak band at 2175 cm−1 is undoubtedly due to CO interacting with residual Na+ ions which have resisted the exchange procedure. Practically no signs of physically adsorbed CO can be observed. Following the considerations developed so far (and in agreement with the results obtained on other alkali-exchanged zeolites) [11,13], the 2162 cm−1 band is assigned to K+,CO adducts formed in the more open spaces of 10-membered ring channels (A sites), while the 2150 cm−1 band should be ascribed to adducts formed on B sites. However, the intensity and stability of the 2150 cm−1 band are now so high that this explanation does not seem appropriate. We think that in order to overcome these difficulties we have to reconsider the assumption that, in all cases, the different IR bands correspond to the n(CO) of different M+,CO complexes, i.e., complexes where the CO molecules are perturbed by the electrostatic field generated by a single cation [31]. To illustrate this point let us consider the effect of increasing the cation radius of two B sites located in the cage. The situation for Li-ferrierite and K-ferrierite viewed through the 8-membered rings representing the entrance to the cage is illustrated in Fig. 2(b) (Li) and Fig. 2(c) ( K ). We notice that in Li-ferrierite, the two Li+ sites in the cage are 0.5 nm distant and consequently they effectively interact with CO in an independent way. For Li-ferrierite the basic assumption that the interaction with CO is dominated by the electric field of a single cation is consequently valid. On K-ferrierite, the situation is completely different, as the K+ sites are now at a distance of only 0.38 nm apart. Any CO molecule penetrating the cage through the 8-membered ring channel necessarily interacts with the electrostatic field generated by two ions. In agreement with this hypothesis, it is worth underlining that, on passing from Na- to K-exchanged ferrierite, the second band is acquiring intensity (in relative terms) and becomes predominant in K-ferrierite. This in turns implies that the 2159 cm−1 band observed on Na-ferrierite can be more plausibly assigned to CO interacting with more than one cation than to a CO interacting
with a single Na+. The weak feature at 2118 cm−1 is likely due to K+,OC species on the ions located in the more open spaces. Finally, the very weak component at 2104 cm−1 is the 13CO satellite of the main band. 3.4. IR spectra of NO adsorbed on Li-, Na- and K-ferrierite The IR spectroscopy of NO adsorbed at low temperature on zeolites is a complicated subject because of the tendency of NO to form dimers (trans and cis) both in the gas and in the adsorbed phase. For this reason the use of NO as a probe of the local fields associated with the cationic sites is more difficult, and it is strictly limited to the lowest dosages where the probability of adsorbate– adsorbate interactions (and hence the formation of dimers) is minimized or suppressed. The IR spectra of NO adsorbed on K-ferrierite are shown in Fig. 9(a). As expected the spectra are very complex and become simpler only for the lowest dosages. We shall consequently concentrate only on the bands whose relative intensity grows when the adsorbate concentration in the channels is going to zero. This means that we will consider only the main peaks of the spectra obtained at the lower pressures (see Fig. 9(b)), the others being dominated by IR manifestations of the adsorbed dimers. We notice that the highest frequency bands are at 1906 (Li), 1894 (Na) and at 1882 cm−1 ( K ). These bands are shifted upward with respect to the gas phase (1876 cm−1): Dn: =+30; Li+ Dn: =+18 and Dn: =+14. If the frequency of Na+ K+ these bands is plotted against the frequency of the highest frequency bands observed for N and CO 2 adsorbed on the same zeolites ( Fig. 10), we obtain a straight correlation. This observation has two implications: (i) the NO molecule responds to the polarizing field of the cations in the same way as CO and N ; (ii) the cation–NO adducts charac2 terized by the highest frequencies have nitrogendown structure and are located in the main channels. Other considerations concerning the possible presence of NO species adsorbed on cationic sites located in the 8-membered rings cannot be made with confidence, because of the confusing effect
S. Bordiga et al. / Microporous and Mesoporous Materials 34 (2000) 67–80
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Fig. 9. (a) FTIR spectra of increasing doses of NO adsorbed on K-ferrierite. (b) Low pressure interaction of NO on Li- (curve 1), Na- (curve 2) and K-ferrierite (curve 3) at about 120 K.
Fig. 10. (a) n: (N ) versus n: (NO) and (b) n: (CO) versus n: (NO) plots for 1:1 adducts in Li-, Na- and K-ferrierite. 2
of the dimers bands (which cover the 1850– 1700 cm−1 interval ).
4. Conclusions The modifications induced in the IR spectrum of H , N , CO and NO by the interaction with 2 2 H-, Li-, Na- and K-exchanged ferrierite are used
to evaluate the local electric fields associated with the different cationic sites locations. It is concluded that cationic sites in 10-membered ring channels show the highest polarizing power. On passing from Li- to Na- to K-exchanged ferrierite the local electric fields probed by diatomic probes are increasingly influenced by the distribution of the ions in the framework, the explanation of IR spectra of adsorbed
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probes in terms of 1:1 interactions consequently becoming less and less realistic.
Acknowledgements The present work is a part of a project coordinated by A. Zecchina and cofinancied by the Italian MURST (Cofin 98, Area 03). CNR (project 98.01987.CT03) was also acknowledged.
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