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J. Phys. Chem. B 2003, 107, 5212-5220
FT-IR Study of Carbon Monoxide Adsorption on Li-Exchanged Zeolite X Aida V. Rudakova,† Raul F. Lobo,† and Kirill M. Bulanin*,‡ Department of Chemical Engineering and Department of Materials Science and Engineering, Center of Catalytic Science and Technology, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: NoVember 13, 2002; In Final Form: March 24, 2003
Carbon monoxide adsorption on LiX-1.0 zeolite ([Li96‚(Si96Al96O384)]-FAU, Si/Al ) 1.0) has been studied by means of FT-IR spectroscopy at a wide temperature range (77-298 K) to investigate the nature and properties of the adsorption sites. We show the existence of two slightly different adsorption centers (Li+ cations at sites SIII and SIII′) in LiX-1.0 zeolite at room and low temperatures. Variable-temperature experiments confirmed that a phase transition from a cubic phase at ambient temperature to an orthorhombic phase at low temperature takes place between ∼220 and ∼235 K. Our studies indicate that cation redistribution takes place in this temperature range. The number of the Li+ cations accessible for CO adsorption (most probably at site SIII′) is increased significantly. We argue that this change in population of these sites is triggered by the presence of admolecules, as it is not observed in an unloaded adsorbent. The effective electric fields created by Li+ cations at different sites in the cavity of LiX-1.0 zeolite are reported.
1. Introduction Alkali-exchanged zeolites are used in important industrial processes such as gas separation and drying.1-4 Dehydrated zeolites are potentially useful as solid ionic conductors.5-7 The nature and location of the cations compensating the negative charge of the zeolite framework are the most important factors determining the material properties for these and other applications. Several probe molecules can be used to characterize the zeolites; among them is carbon monoxide. Carbon monoxide molecules interact with Lewis acid centers (cations) of metalexchanged zeolites. Savitz et al. examined adsorption of CO, N2, and O2 at 195 K in siliceous MFI and Li-, Na-, and K-exchanged MFI using microcalorimetry; they found that zerocoverage heats of adsorption for CO and N2 decrease with an increase of the metal cation size.8 IR spectroscopic investigations of CO adsorption have been reported on many cation-exchanged zeolites: MFI,9-23 MOR,10,19,24 silicalite,25 FER,26 FAU-type zeolites,10,18,19,23,27-31 EMT,32 A,10,33 L,34 and the titanosilicate ETS-10.35 The CO absorption band for different types of alkali-exchanged zeolites appears in the 2145-2188 cm-1 spectral region, and the frequency shift ∆ν(CO) ) νads - νgas increases with the decrease of the cation radius. These experimental results were supported by theoretical studies concerning CO interactions with naked alkali-metal cations36,37 as well as with alkali-metal cations in zeolites.9,38,39 Coordination of a CO molecule via either the oxygen or the carbon end in the interaction with a naked alkali cation, or with an alkali-metal cation in a zeolite, was considered elsewhere.9,23,36,37,39 The results obtained in ref 36 indicate that coordination via the carbon end is always more favorable and that the difference in the interaction energy decreases significantly on going from Li+ to Cs+. * Corresponding author. Fax: +1-302-831-3009. E-mail: kbulanin@ udel.edu. † Department of Chemical Engineering. ‡ Department of Materials Science and Engineering.
The IR-spectroscopic investigation35 of CO adsorbed on the titanosilicate molecular sieve ETS-10 showed that on Naexchanged samples the Na+‚‚‚CO species, formed at the lowest dosage, evolve into Na+‚‚‚(CO)i (i ) 2, 3) species upon increasing the gas pressure. The formation of the weakly held (CO)i molecular clusters occluded in the supercages of Naexchanged zeolites X, Y, and EMT was also reported in refs 20, 30, and40. Hadjiivanov et al.32 reported similar results on the low-temperature adsorption and coadsorption of CO and 15N2 on Me-EMT samples (Me ) Li, Na, K, Rb, Cs). It was found that Na+, K+, Rb+, and Cs+ cations in EMT are able to form Me+(CO)2 species, as well as Me+(CO)(N2) mixed complexes. The bands in the 2155-2185 cm-1 region were assigned to CO polarized by the metal cations, while the bands located at 21442151 cm-1 were a result of mixed Me+(OC)(CO) species. On Li-EMT only Li+ mono-carbonyls were found after CO adsorption. A calorimetric study of CO in alkali-exchanged zeolite ZSM-58 also showed that in the case of the Li+ cation it is difficult for a second molecule to approach the site; however, for Na+ and K+ cations, two and more molecules can easily interact with a single cation simultaneously. LiX zeolite studied in the present work with the Si/Al ratio of nearly one (LiX-1.0) and with an ideal unit cell composition of [Li96‚(Si96Al96O384)] is widely used in industry for the production of oxygen from air.41 It is formed of alternating SiO4 and AlO4 tetrahedra and has the highest number of chargecompensating cations among all faujasites.5,42-44 Cations are distributed over six possible sites within the faujasite framework (Figure 1a). At room temperature, sites I′ and II are generally fully occupied in dehydrated zeolites X exchanged with alkali cations, leaving sites I and II′ either poorly occupied or empty. The occupation of a site I′ prevents the location of a cation in the nearby sites I, as is the case for II and II′. In the case of zeolite X-1.0 in the alkali form, 6-rings provide sites for twothird of the total number of cations. The remaining cations can be located in several alternative sites in the supercages. These sites can be classified according to their position relative to the 4-rings. The exact location of Li cations in the supercages of
10.1021/jp022428l CCC: $25.00 © 2003 American Chemical Society Published on Web 05/13/2003
CO Adsorption on Li-Exchanged Zeolite X
J. Phys. Chem. B, Vol. 107, No. 22, 2003 5213 X-1.0 at room and low temperatures. The estimated strengths of local electric fields are reported. 2. Experimental Section
Figure 1. (a) Cation positions in zeolite faujasite. (b) Illustration of the coordination of Li cations in the supercages of zeolite LiX-1.0.
zeolite X is still an issue that has not been completely established. At room temperature, the neutron refinements indicate that the Li cations are on site SIII (Figure 1b), where the Li+ is coordinated to four oxygen atoms. Below the phase transition (∼220 K), the refinements indicate that the cations move to site SIII′, but different reports disagree on the exact number of Li cations present at this location. It has also been shown by NMR spectroscopy that at room temperature the Li cations on SIII are moving rapidly.5 It is argued in this paper that this position may be an artifact of the Rietveld method, and is the average of four closely related locations surrounding SIII, namely two SIII′ and two nearly identical sites SIII′′ (our notation) in front of SIII′ (Figure 1b). The difference between SIII′ and SIII′′ is that the oxygen atoms are coordinated to a silicon atom and not to an aluminum atom. The geometry is slightly different as the Si-O bond length is 1.6 Å and the Al-O bond length is 1.73 Å, but the cation is coordinated to the same oxygen atoms in both SIII′ and SIII′′ sites. Energetically, these two locations are probably similar. To the best of our knowledge, there are only two IRspectroscopic studies of LiX-1.0 zeolite using nitrogen, oxygen, hydrogen, and deuterium adsorption on this zeolite.45,46 This paper describes the results of our experimental FT-IR study of vibrational spectra for carbon monoxide on Li-exchanged zeolite
Zeolite LiX-1.0, synthesized and analyzed for content for all containing elements as described in ref 45 was pressed into ∼10 mg cm-2 self-supporting pellets. Analysis data for this sample (Galbraith Laboratories, Inc.) presented in ref 45 are in agreement with our analysis using X-ray photoelectron spectroscopy (not reported in this paper). According to these data, some negligible traces of sodium (∼3% Na+ cations per unit cell) are present in our sample. The stainless steel cell used in the present work was described elsewhere.47 The gas-handling manifold was equipped with two Edwards-600 capacitance manometer gauges of 10-3 - 10.000 Torr and 10-1 - 1000.0 Torr pressure ranges. To remove water and any organic impurities, the pellets were heated in a vacuum (10-6 Torr) at a rate of 2 K/min up to 823 K, kept at these conditions for 3 h, and exposed to a few Torr of oxygen for 1 h. In low-temperature adsorption experiments, carbon monoxide gas was introduced into the cell with a sample cooled by filling the cell with liquid nitrogen. A common practice is to admit helium gas into the cell to improve thermal contact between the sample and the cold environment. However, we found that presence of helium influences the IR spectrum of adsorbed CO. To eliminate this effect we did not admit helium into the sample compartment of the cell during our low-temperature experiments (i.e., the effective sample temperature was slightly higher than 77 K). All CO gas dosed was absorbed by the sample at low temperature. The amount of adsorbed CO molecules was estimated from the known gas pressure in the known volume at the known temperature of the gas manifold. Adsorption of a new carbon monoxide portion was studied after complete removal of the previous portion of adsorbed CO. The sample coverages are given in terms of the number of adsorbed molecules per supercage (hereafter denoted n). The spectra were obtained with a Nicolet-510 FTIR spectrometer, at 1-2 cm-1 spectral resolution by collecting 256 scans. All the spectra shown in this work are background subtracted. The band positions were determined using OMNIC 4.0 software. The individual peak areas were obtained with the GRAMS/32 4.14 software. 12CO (Matheson, 99.995% purity) and 13CO (Isotec Inc., 99 atom % 13C, 10 atom % 18O) gases were used as supplied. 3. Experimental Results The spectra of 12CO and 13CO adsorbed at room temperature and at different equilibrium pressures are presented in Figures 2a and 2b, respectively. The spectrum of gaseous carbon monoxide was subtracted from those recorded after CO adsorption on the zeolite. At the smallest equilibrium pressure of 12CO (1.2 Torr), a weak broad absorption is observed with the maxima at 2183 and 2187 cm-1. As the equilibrium pressure is increased, these two features become more pronounced. The intensities of both bands are slightly different (the low-frequency component is stronger), but they grow together with the amount of gas dosed. 13CO/LiX spectra show identical behavior with the peak positions at 2139 and 2134 cm-1 (Figure 2b). A distinct feature of the IR spectra in Figure 2 is the presence of weak absorption near 2115 cm-1 for 12CO (Figure 2a) and 2070 cm-1 for 13CO (Figure 2b). Figure 3 shows the IR spectra of gradually increasing doses of carbon monoxide adsorbed at a nominal temperature of 77
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Figure 2. IR spectra of CO adsorbed at room temperature on LiX zeolite. Equilibrium pressure (from bottom to top) for 12CO adsorption (a) 1.2, 7.7, 20.2, 47.0, 74.8, and 248.1 Torr; for 13CO adsorption (b) 3.5, 7.5, 14.2, 48.4, and 87.5 Torr.
Figure 3. IR spectra of CO adsorbed on LiX-1.0 zeolite at 77 K (without helium). The coverage (n) (from bottom to top): for 12CO (a) 0.02, 0.05, 0.11, 0.24, 0.35, 0.48, 0.60, 1.01, and 1.75; for 13CO (b) 0.02, 0.05, 0.12, 0.23, 0.33, 0.45, 0.56, 0.84, and 1.58.
K. All cation-specific bands show a positive frequency shift with respect to the free CO molecule (2143 cm-1). As mentioned earlier, we observed changes in the spectra of adsorbed CO that occur if helium gas was pre-admitted into the sample cell. Transformation of the spectral band in the presence of He is shown in Figure 4. The kinetic diameter of helium is 0.26 nm,48 that is close to the open aperture of the small zeolite cavities. Dosed before CO adsorption, helium would be able to penetrate β-cages as well as hexagonal prisms through openings not blocked by the Li+ cations. It may cause slight zeolite structure changes, affect cations distribution in the framework, and affect local electric fields created by cations. Moreover, predosed helium slows diffusion of the CO molecules in zeolite channels. A more detailed study of this new phenomenon is required, which is beyond the scope of this paper. All the results of our low-temperature experiments described below were obtained without He being admitted into the cell. Even at the lowest 12CO coverage of n ∼ 0.02, two peaks are observed at 2192 and 2186 cm-1 (Figure 3a). Further increase of coverage results in a simultaneous intensity growth of both bands accompanied by a slight band shift, from 2192 cm-1 to 2191 cm-1. At coverages of ∼0.24 < n < ∼1, the
high-frequency band growth slows down, while the lowfrequency band intensity continues to increase with a higher rate. Only one saturated feature was observed in the spectrum recorded at the highest loadings (n ∼ 1.75). For the 13C16O and 13C18O isotopomers, the corresponding bands appear at 2143, 2138 cm-1 and 2089, 2085 cm-1, respectively (Figure 3b). At n ∼ 1.6, a single saturated feature due to adsorbed 13C16O and the bands due to adsorbed 13C18O at 2088 and 2082 cm-1 appear in the spectrum. A shoulder at 2172 cm-1 is observed at n > 0.24 for 12CO (Figure 3a) and a similar band at 2124 cm-1 appears in the spectra for 13CO (Figure 3b). In accordance with the published data,19,23,29,49 the band at 2172 cm-1 could be assigned to the C-O stretching mode of the Na+‚‚‚CO adducts. This is possible because our XPS data indicated the presence of a small amount of the Na+ cations in the studied zeolite sample. No further consideration of this band will be given here, since it was fully discussed in the cited literature. It should be noted that, at both room and low temperatures and at all CO coverages, the physisorbed species usually characterized by the IR band near 2139 cm-1 were not observed here. Figure 5 shows the coverage dependence of the band areas
CO Adsorption on Li-Exchanged Zeolite X
Figure 4. IR spectra of 12CO adsorbed on LiX-1.0 zeolite at 77 K in the presence of helium (He was introduced first). Equilibrium He pressure was 0.5 Torr. The coverage (n) (from bottom to top): 0.01, 0.04, 0.15, 0.3, 0.51, and 0.96.
(in cm-1) for the 12CO/LiX and 13CO/LiX systems studied. The values of the total band areas were obtained by adding the areas for the separated low-frequency and high-frequency bands. The coverage dependencies of the total band area for both isotopomers were approximated by the same linear function with the correlation coefficient of 0.98. Similar dependencies for the separated low- and high-frequency bands of both isotopomers significantly differ, though both are linear with the correlation coefficient of 0.92. We have recorded a series of spectra of adsorbed carbon monoxide decreasing and increasing the temperature for different CO loadings (Figures 6-8). Depicted in Figure 6a are the spectra of CO adsorbed on LiX-1.0 zeolite at room temperature and the initial equilibrium pressure of CO gas of 2.2 Torr and collected while decreasing the temperature from 296 to 77 K. At 77 K, all CO was adsorbed, and estimated n was 1.1 molecules per supercage. In the spectrum recorded at ambient temperature, two bands were observed at 2189 and 2183 cm-1. Lowering of the temperature to 240 K results in a simultaneous increase of the intensity for both bands. An important point is
J. Phys. Chem. B, Vol. 107, No. 22, 2003 5215 that the relative intensities of these two peaks remain almost the same until this temperature. Further cooling of the sample results in a faster growth of the low-frequency band. A similar behavior is observed when the sample was heated (Figure 6b). The initial CO loading at 77 K was 1.1 molecules per supercage. At the lowest temperatures, two bands at 2190 and 2184 cm-1 appear, the low-frequency feature being more intense. With increasing temperature, both features decrease and shift slightly to 2189 and 2183 cm-1. Again, at temperatures from about 245 K and higher, the band intensities become practically equal. In both series of experimentsstemperature decrease and temperature increases spectra recorded at temperatures below 152 K have not been shown in Figures 6a and 6b because of the saturation of the low-frequency band. However, we should mention that the high-frequency band intensity in this temperature range remains constant. Figure 7 shows a plot of band areas vs inverse temperature in the case of variable-temperature CO adsorption presented in Figure 6a. This plot covers the temperature range from 182 up to 296 K. The data for temperatures lower than 182 K were not included because of the impossibility of estimating the intensity of saturated low-frequency band. The values of the total band areas in Figure 7a can be fitted in the temperature range from 182 to 235 K by a straight line with the correlation coefficient of 0.97. A break occurs in the range from 235 to 217 K, after which a linear dependence is restored. The reciprocal temperature dependence of the low-frequency band areas in Figure 7b shows the same behavior. The slope changes in the same temperature range, from 235 and 217 K. The high-frequency band area data fit a straight line with a less steep slope and the correlation coefficient of 0.95. Temperature variation of the CO band areas was also studied using different initial pressures of carbon monoxide upon cooldown of the sample. Figure 8a shows a plot of the total and separated band areas for initial CO pressure of 2.75 Torr. In this case, carbon monoxide remains in the gas phase during the whole variable-temperature experiment. This implies that at all temperatures of adsorption, there were enough adsorbate molecules to cover all the existing active sites. The dependencies presented in Figure 8a look similar to those depicted in Figure 7 for the initial CO pressure of 2.2 Torr, but the slope is steeper. We note that the area for the high-frequency band (Figure 8a, squares) remains constant at temperatures below 220 K. Figure
Figure 5. Coverage dependence of the band areas: (a) the total band area for 12CO absorption bands (triangles), for 13CO adsorption bands (open squares); (b) the band areas for the 12CO absorption bands at 2192 cm-1 (solid squares) and 2186 cm-1 (open circles), for the 13CO absorption bands at 2143 cm-1 (open triangles) and 2138 cm-1 ( solid triangles).
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Figure 6. IR spectra of CO adsorbed on LiX-1.0 zeolite (without helium) at variable temperatures. (a) Temperature decrease. CO was adsorbed at 296 K. Initial equilibrium pressure of CO was 2.2 Torr. (b) Temperature increase. CO was adsorbed at 77 K. Initial coverage of CO was 1.1 molecules/supercage.
Figure 7. Reciprocal temperature dependence of the band areas in the case of CO adsorption with decreasing temperature: (a) the total band area for 12CO absorption bands (solid triangles), (b) the band areas for the 12CO absorption bands at 2192 cm-1 (open squares) and 2186 cm-1 (circles). CO was adsorbed at 296 K. Initial equilibrium CO pressure was 2.2 Torr.
Figure 8. Reciprocal temperature dependence of the band areas in the case of temperature decrease. CO was adsorbed at 296 K. Initial equilibrium CO pressure was (a) 2.75 Torr, (b) 0.68 Torr. Triangles are for the total absorption band area, open circles are for the low-frequency (2186 cm-1) band area, squares are for the high-frequency (2192 cm-1) band area.
8b shows the results obtained with an initial CO pressure of 0.68 Torr. Carbon monoxide remains in the gas phase until 220 K. Band areas slowly increase with decreasing temperature as
long as there are molecules in the gas phase and cause to change below 220 K. The plots for the high- and low-frequency bands coincide.
CO Adsorption on Li-Exchanged Zeolite X
Figure 9. IR spectra of N2 adsorbed on LiX-1.0 zeolite at room temperature. Equilibrium pressure (from bottom to top): 31, 87, 200, 255, and 400 Torr.
4. Discussion Adsorption of CO at Room and Low Temperatures. First we consider the accessibility of the sites in LiX-1.0 zeolite. It is well-known that Li cations in LiX-1.0 zeolite are found in three crystallographic positions, e.g., SI′, SII, and SIII (Figure 1a).5 Cations at the site SI′ are inaccessible to incoming molecules.38,43,50 The lithium cation in the SII site is embedded in the six-ring window as a result of its small size and short Li-O bond length. The framework oxygen atoms of the 6-ring appear to shield the Li+ cation, via steric repulsion, from direct contact with the adsorbate molecules.43,50,51 Thus, the SIII site cations are the most probable sites for initial adsorption. This suggestion was confirmed by previous study of nitrogen adsorption (Figure 9). Only a single band is observed in the IR spectrum of N2 adsorbed on LiX-1.0 zeolite at room temperature, independent of coverage. There is not, however, complete agreement about where exactly these SIII site Li cations reside in the supercages. Feuerstein et al.5 reported that the SIII cations are mobile at temperatures above T ) 273 K. The position determined by neutron diffraction at T ) 295 K may actually be the average position of the lithium cation as it jumps between sites at opposite corners of the four rings. They have proposed a model for the motion of the Li cations near the four-ring windows in which the cations are jumping between SIII and SIII′ sites (Figure 1b).5 Although recent Monte Carlo simulations50 have shown that there is a very little energetical difference between these two SIII and SIII′ sites, we should stress an important geometric difference between them. Site SIII (of which there are 48/unit cell) points toward the center of the supercage. In the case of LiX-1.0 zeolite, we assume 32 Li+(SIII), i.e., we have 4 Li+/ supercage on the average. Site SIII′ (of which there are 96/unit cell) is located in the plane of the 12-ring windows and thus these lithium cations are “shared” by two supercages. If there are 32 Li+ at this position, we would have only two Li cations per 12-ring window in this material. Having three Li cations in the window is highly unlikely since geometry would force the charged Li cations to be very close to each other. Adsorption of carbon monoxide on LiX-1.0 zeolite at room and low temperatures yields two main IR-absorption bands, even at low coverages (Figures 2 and 3). The fundamental vibrational frequencies of the 12CO molecules adsorbed at different tem-
J. Phys. Chem. B, Vol. 107, No. 22, 2003 5217 peratures on Li-containing systems are collected in Table 1. The observed band frequencies at 2183 and 2187 cm-1 for carbon monoxide adsorbed at room temperature (Figure 2a) are in a good agreement with the vibrational frequencies of CO adsorbed on different Li-containing zeolites reported in previous works, especially in an FT-IR study of CO adsorption at room temperature on LiNaX (Li83%) zeolite (Si/Al ) 1.25).40 The values for 12CO adsorbed on Li-exchanged zeolites at 77 K presented in Table 1 are also close to the values of 2192 and 2186 cm-1 obtained in our work. These bands have been previously assigned to the stretching mode in Li+‚‚‚CO species of carbon monoxide perturbed by the electrostatic field created by Li+ ions in different coordination environments in zeolites. It is generally recognized that the CO molecules can coordinate to alkali cations in two different waysseither via the carbon end or via the oxygen end.9,23,36,37,39 It is also firmly established that formation of O-bonded CO adducts in different systems leads to a bathochromic shift of the C-O fundamental stretching mode, i.e., a downward shift with respect to the free 12CO frequency.9,15,22,23,26,52-56 Our experimental data show the bands at 2119 and 2112 cm-1 (Figure 2a) and the bands at 2073 and 2065 cm-1 (Figure 2b), which can be assigned to the fundamental 12C-O and 13C-O stretching mode in Li+‚‚‚OC adducts, respectively. The band at 2080 cm-1 in Figure 2b belongs to the adsorbed 13C18O molecules. In the spectra of CO adsorbed at low temperature (Figure 3), the cation-specific bands show positive frequency shifts. This hypsochromic shift can readily be ascribed to interaction of CO via the carbon end with the electric field set by the Li+ cations.9,36,39,57 There are no bands corresponding to the Obonded species. There are two plausible reasons that may explain the existence of two main absorption features in the spectra of CO adsorbed on LiX-1.0. The first alternative is that there are two different types of cationic sites accessible for CO adsorption with the formation of similar mono-carbonyl species. Another one is that two CO molecules can be adsorbed on the same site, so formation of mono-carbonyl and di-carbonyl species is suggested. We do not have any evidence in support of the formation of geminate species on Li+ ions in LiX-1.0 zeolite. Also the observed behavior of bands does not agree with this suggestion. The bands start to grow and grow together, and neither one disappears at higher coverages. Many experimental data are available on the IR-spectra of CO adsorbed on transition-metal-, alkaline-earth-metal-, and alkali-metal-exchanged zeolites. Formation of the Li+‚‚‚(CO)2 adducts at low temperatures was reported in refs 15,26,56. Zecchina et al.15 studied interaction of CO with the Li-containing ZSM-5 zeolites, namely Li-Na-ZSM-5, with the Li+/Na+ ratio ranging from ∼1 to ∼0.25 (Si/Al ) 14). Among other features, they observed a weak absorption at 2195 cm-1 and assumed this band to be associated with a small fraction of highly coordinated Li+ ions located in a special (defective) position of the lattice. The disappearance of this peak at elevated pressures of CO was explained by coordination of a second CO molecule observed by the authors for NaCl, KCl, MgO, and ZnO crystallites. It is known that cations in a zeolite are much more protruding than those at the surfaces of metal oxides or halides, more resembling naked ions. Estimated values of adsorption energies on alkali-metal-exchanged zeolites are higher than those on the corresponding metal oxides and halides.9 Bordiga et al.26 studied the CO adsorption on a Liexchanged ferrierite (Si/Al ) 8, the level of the ion exchange conversion was not indicated). When the CO pressure was
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TABLE 1: Vibrational Frequencies of 12CO Adsorbed at Different Temperatures on Li-containing Systems
system
level of Li+ exchange conversion, %
Li+
CO/isolated cation CO/isolated Li+ cation CO/isolated Li+ cation CO/Li+AlH(OH)3CO/Li+AlSi4O4H12CO/Li-MOR CO/Li-FER CO/Li-EMT CO/Li-ZSM-5 CO/Li-ZSM-5 CO/Li-ZSM-5 CO/Li-Y CO/LiNa-X CO/Li-X CO/Li-X a
70 c 75 50 c c c 83 100 100
ν(12CO), cm-1 2317 2228 2176 2263 2183 2188 2190 2183 2188 2195 2185 2179 2184 2183, 2187 2186, 2192
temperature of spectrum record, K
ref b
100-110 100 85 77 77 226 90 298 298 77
9 ,36b 72b 61b 39b 72b 24a 26a 32a 15a 56a 21a, 52a 19a 40a this work this work
Experimental work. b Theoretical work. c The level of Li+ exchange conversion was not indicated.
increased to 49.5 Torr, the authors observed a new intense composite adsorption band at 2179 cm-1, which was attributed to the stepwise solvating of the cations forming Li+(CO)i (i > 1). Otero Arean et al.56 proposed that CO adsorbed on LiZSM-5 (the Si/Al ratio and the level of the ion exchange conversion was not indicated) can be coordinated to Li+ ions in several different modes, including formation of C-bonded mono- (2195 cm-1) and di-carbonyls (2188 cm-1) and also corresponding O-bonded Li+‚‚‚OC species (2110 and 2100 cm-1). It should be noted that there are no suggestions on the formation of Li+(CO)2 species in theoretical works on CO interaction with alkali metal cations in faujasite zeolites. The difference in the interaction energy depends on the zeolite framework structure, the distribution of cations inside it, and cation accessibility. We do not believe that a small lithium cation in S the III (SIII′) site of LiX-1.0 zeolite can coordinate two molecules and form di-carbonyls. We prefer the first suggestion considered above that the pair of the observed IR-absorption bands should be assigned to the CO stretching mode in Li+‚‚‚CO adducts, where lithium cations are situated at two slightly different sites inside the zeolite framework. According to the investigation of structure and cation location by NMR spectroscopy and neutron diffraction methods,5 these two slightly different sites responsible for the adsorption are lithium cations at SIII and SIII′ sites. The difference in the location of these sites is illustrated in Figure 1b. To summarize, we attribute the 2187 cm-1 and 2183 cm-1 bands observed at room temperature and those at 2192 and 2186 cm-1 observed at low-temperature experiments to carbon monoxide adsorbed on Li+ cations at SIII and SIII′ sites. At larger CO doses, these bands appear red-shifted as a result of intermolecular interactions. Moreover, the presence of two such bands in the IR spectra of Li+-OC species for 12CO and 13CO confirms the existence of two adsorption sites in LiX-1.0 zeolite. The frequency shifts are of ∆ν ) -31 cm-1 and ∆ν ) -24 cm-1 for Li+(SIII or SIII′)‚‚‚O12C species, and ∆ν ) -31 cm-1 and ∆ν ) -23 cm-1 for Li+(SIII or SIII′)‚‚‚O13C species. It is not clear which band corresponds to CO adsorbed on which Li cation site. It is difficult to distinguish sites SIII and SIII′ because they are almost identical. To identify these sites, we performed experiments searching for the changes in the intensity of components in the spectra. In the spectra obtained at low temperature shown in Figure 3, we measured the band areas for each component. The coverage dependence of the band
areas is presented in Figure 5. It is remarkable that for both isotopomers the coverage dependencies for the total band areas fit a straight line (Figure 5a). This fact indicates that the cationic sites are equally energetic. At low coverages (n e 0.15), the intensities of the 2192 and 2186 cm-1 bands for 12CO and 2143 and 2138 cm-1 bands for 13CO grow at an equal rate. Growth rates for the low-frequency bands parallel increasing numbers of adsorbed CO molecules, but remain constant for the highfrequency bands. Assuming that the extinction coefficients for the CO molecules adsorbed on Li+ cations at SIII and SIII′ sites are similar, we can conclude that at low temperature the number of sites occupied by Li+ cations corresponding to the lowfrequency band is larger than the number corresponding to the high-frequency band (Figure 5b). Estimation of the Electric Fields. Strong electric fields created by ions of the zeolite framework cause a variety of field-induced effects, e.g., appearance of dipole-forbidden transitions, vibrational frequency shifts, and degeneracy splitting, in the IRspectra of admolecules.10,38,39,58 The earliest estimates of the local electric fields in zeolite were based on the IR-spectroscopic data for adsorbed homonuclear diatomics adsorption.59 Polar molecules, such as carbon monoxide, were often used before as the probes to explore the nature of the acid sites for zeolites or on different surfaces, as well as of the fields in proximity to the cationic sites. Pacchioni et al.60 studied interaction of CO with the MgO (100) surface by means of electron cluster model calculations and found a correlation between the CO vibrational frequency and local electric field created by the metal cationic site. The binding was found to be essentially electrostatic in nature, almost entirely due to coupling of the carbon monoxide charge distribution with the electric field generated by ionic surface, with negligible contributions from a covalent σ-donation. Other theoretical studies confirmed that interaction via the carbon end with the electric field of a positive charge results in an increase of the vibrational force constant and the bond-stretching frequency of CO molecule.36,37,57,61-63 Carbon monoxide penetrating into obstructed cavities is expected to interact not only with a cation but also with the “wall”. In the 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 neighbor framework atoms cannot be neglected. The electric field F in a zeolite cage probed by a CO admolecule located at a distance RM-CO from
CO Adsorption on Li-Exchanged Zeolite X
J. Phys. Chem. B, Vol. 107, No. 22, 2003 5219
TABLE 2: Electric Fields in Li-Containing Systems system
F, V/nm
ref
CO/isolated Li+ cation CO/Li+(AlH(OH)3CO/Li-MOR (Si/Al ) 5) CO/Li-ZSM-5a CO/Li-ZSM-5 (Si/Al ) 14) CO/LiX (Si/Al ) 1) N2/LiX (Si/Al ) 1) O2/LiX (Si/Al ) 1) D2/LiX (Si/Al ) 1) N2/LiNaX (Li67%, Si/Al ) 1) O2/LiNaX (Li67%, Si/Al ) 1) D2/LiNaX (Li67%, Si/Al ) 1)
17.5 6.7 8.2 9.5 7.4 7.4-11.4 12.4 8.2 7.2 7.7 5.7 8.2
36 38 24 56 15 this work 45 45 45 45 45 45
a
Si/Al ratio was not indicated.
a metal ion Me+ was previously estimated by means of a simple electrostatic model39,64 as
F ) (qsite/4π0)/R2M-CO + K
(1)
where qsite is the nominal charge of the site due to the cation and its charged neighboring oxygen centers, RM-CO ) RM + RCO, RM is cation radius, RCO is the radius of the CO molecule, and parameter K accounts for contribution to the local electric field generated by the surrounding zeolite framework. The effective charge qsite of the active site and the environment field K depend on structural and electronic properties of a zeolite. The negative framework contribution increases with decreasing Si/Al ratio.64 A linear correlation between the vibrational frequency shift of CO adsorbed on different metal-exchanged zeolites and the inverse square of the distance from the cation to the center of the mass of the CO molecule was reported in many experimental works.15,19,24,28,36,52,65,66 We estimated the electric field strengths at the Li+ sites by two different methods. First, we used the full SCF curve calculated by Pacchioni et al.60 for the ν(F) dependence of a free CO molecule exposed to a uniform external electric field. The vibrational frequency shifts of two main absorption peaks observed here, ∆ν ) 49 and 43 cm-1, correspond to the values of the electric fields of 8.46 and 7.36 V/nm, respectively. The second approach based on the perturbation theory uses the relationship ∆ν ) kSTF, where kST ) 4.29 × 10-7 cm-1/(V/ cm) is the so-called vibrational Stark constant for the CO molecule,67 yields F ) 11.4 and 10.0 V/nm. Taking RLi+ ) 0.60 Å for the lithium cation radius,52 and adopting K ) 4.73 V/nm for the mordenite zeolite,64 the formal site charge qsite can be evaluated from eq 1. The values found are in the range of qsite ) + (0.098 ÷ 0.119). These are smaller than those calculated for an isolated cation (+0.97)36,37 and for a cation in the cluster AlH(OH)3- Li+ (+0.73)39 interacting with the CO molecule. This was explained in terms of the charge on small Li+ cation reduced because of a partially covalent character of the bonds with framework oxygen, thus lowering the net positive charge at the cation site.24,39,64 The strengths of the local electric fields for different Li+-containing systems are presented in Table 2. The estimated values of electric fields for the CO/LiX-1.0 system are similar to those probed by parallel-oriented N2, O2, and D2 admolecules in LiX-1.0 zeolite, i.e., 12.4, 8.6, 7.2 V/nm.45 Variable-Temperature Experiments. Spectral changes for adsorbed CO in the temperature variation experiments are similar (Figures 6a and 6b). During cool-down, the growth rate for the high-frequency band area (Figure 7b) remains constant, whereas for the low-frequency band it is more rapid. A discontinuity is clearly seen in the range of T ∼235 to ∼217
K. The LiX-1.0 zeolite is of a cubic (Fd3) symmetry at ambient temperature and undergoes a symmetry change to an orthorhombic (Fddd) phase at lower temperature in the range of T ) 220-230 K.5,6,44 Our findings are in accord with these data. An important fact is that there is almost no unit cell volume change during the phase transition.5 At high loadings we clearly observe the effects caused by a phase transition in the temperature range between ∼220 K and ∼243 K (Figure 8a). The area for the high-frequency band does not change below 220 K, which may indicate constant occupancy of the sites corresponding to this band. In contrast, we see an increase of the relative intensity of the low-frequency band. The orthorhombic phase was reported to have a different cation distribution in comparison to the cubic phase: it has fewer cations in the highly coordinated sites SII and a larger number of cations in the supercage, mostly in sites SIII′.44 Yoshida et al.68 have shown that the amount of nitrogen adsorbed on LiX1.0 zeolite at low temperature is much larger than what would be expected from exploration of measurements at ambient temperature. This is presumably due to an increase of the number of main nitrogen adsorption sites (Li+ cations at sites SIII and SIII′), but not to the presence of a new type of site. As is evident from Figure 8a, we are observing an increase in population of these sites. Feuerstein and Lobo6 have measured the relative population of sites SII and SIII by low-temperature NMR spectroscopy (at 173 K) and did not observe a change in population of cations in sites SII or SIII. These apparently contradictory results suggest that to have a change in relative population of sites SII and SIII, it is necessary to have an adsorbed molecule (N2 or CO) in the zeolite micropores. The role of the adsorbed molecules is to stabilize the Li+ cations in the less-coordinated sites SIII and probably to provide a lowenergy pathway for the transfer of the molecules from one site to another. We did not observe any effects of a phase transition just described at low loadings of CO, because of a small amount of adsorbed carbon monoxide (Figure 8b). Areas for both bands remain constant after all CO molecules were adsorbed. Thus, the number of admolecules was not high enough to cover all existing accessible sites or to trigger the change in relative populations suggested above. Summarizing all our experimental results and the literature data, we assign the low-frequency band to carbon monoxide adsorbed on lithium at sites SIII′. The number of these sites increases during the phase transition. The high-frequency band corresponds to the stretching vibration of CO molecules adsorbed on lithium cations at sites SIII in LiX-1.0 zeolite. 5. Summary and Conclusions IR-spectroscopy provides evidence for two slightly different cationic sites accessible for CO adsorption in LiX-1.0 zeolite. We proposed these to be lithium cations at sites SIII and SIII′. We suggest that the variable-temperature IR-spectroscopic measurements can be used as a tool for studying symmetry changes in zeolite induced by temperature. The number of sites corresponding to the low-frequency band (presumably lithium at site SIII′) increases upon phase transition. NMR spectroscopy of 7Li in LiX-1.0 indicates in apparent contradiction to our conclusions that the population of sites (SIII + SIII′) does not change during the phase transition. We believe that this change in population of sites III and III′ is triggered by the presence of admolecules. This conclusion is supported by findings of Yoshida et al.68 on N2 adsorption and by the results of SanchezSanchez et al.,69 Bosch et al.,70 and Grey et al.71 on adsorption CHCl3 and hydrofluorocarbons on NaX and NaY.
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