J. Phys. Chem. C 2007, 111, 11353-11362
11353
FTIR Study of CO Interactions with Li+ Ions in Micro- and Mesoporous Matrices: Coordination and Localization of Li+ Ions Petr Nachtigall,† Karel Frolich,‡ Helena Drobna´ ,‡ Ota Bludsky´ ,† Dana Nachtigallova´ ,† and Roman Bula´ nek*,‡ Center for Complex Molecular Systems and Biomolecules, Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, CZ-166 10 Prague, FlemingoVo na´ m. 2, Czech Republic, and Center for Complex Molecular Systems and Biomolecules and Department of Physical Chemistry, UniVersity of Pardubice, CZ-532 10 Pardubice, na´ m. C ˇ s. legiı´ 565, Czech Republic ReceiVed: March 1, 2007; In Final Form: April 25, 2007
The interaction of CO with extraframework Li+ ions coordinated in ZSM-5 and FER zeolites with different compositions is investigated by FTIR spectroscopy, and the correlation between Li+ coordination, stability of CO adsorption complexes, and νCO frequencies is discussed in terms of the theoretical investigation of the stability and vibrational dynamics of CO adsorption complexes. Differences in IR spectra of adsorbed CO due to the following effects were considered: (i) various Si/Al ratios, (ii) differences in zeolite topology, (iii) temperature, and (iv) the effect of a co-cation on relative stability and metal cation site preferences. The correlation between Li+ coordination and νCO frequencies of CO adsorption complexes discussed for Li zeolites is extrapolated to mesoporous Li-(Al)MCM-41 materials. The features in the IR spectra of CO/Li-(Al)MCM-41 are assigned to CO adsorption complexes formed on top of five- and six-membered rings on the surface of a channel wall. The spectra of CO/Li-ZSM-5 with different Si/Al ratios and various ion exchange levels bring evidence on the nonstatistical distribution of aluminum in the framework and on the preference of Li+ ions for channel-wall sites.
1. Introduction Metal-exchanged zeolites are intensively studied by various experimental and theoretical methods due to their interesting adsorption properties and catalytic activity. Detailed knowledge of the structure, environment, and distribution of adsorption and active sites in various types of zeolites is critical for the better understanding of the properties of zeolites. The structure and coordination of extraframework cations in aluminum-rich zeolites can be determined by X-ray and neutron-diffraction analysis.1,2 However, due to the low content of extraframework metal cations in high-silica zeolites and due to a relatively large number of accessible extraframework cation sites, the X-ray diffraction data are lacking for these systems, and thus, information about the metal coordination is obtained mainly from indirect experimental techniques, including spectroscopic characterization using probe molecules.3 The infrared spectroscopy of adsorbed carbon monoxide is one of the most popular techniques for the investigation of extraframework charge-balancing cations in zeolites.3-7 CO stretching frequencies and CO adsorption enthalpy are highly sensitive to the nature and environment of the adsorption site. The factors modulating the C-O stretching frequency of CO adsorbed on cation-exchanged zeolites are well understood today. On the basis of the agreement of theoretical calculations with a number of IR spectra of CO adsorbed on different cationexchanged zeolites, a general model of CO vibrational dynamics in porous systems was recently proposed.8 Within this model, * To whom correspondence should be
[email protected]. † Academy of Sciences of the Czech Republic. ‡ University of Pardubice.
addressed.
E-mail:
the effects on CO vibrational dynamics were described as effects from bottom and effects from top. Only a basic concept of a model describing the CO vibrational dynamics in the confined space of the zeolite cavities follows; a detailed discussion of these effects can be found in ref 8. Model concept (i), effect from bottom: νCO is primarily determined by the nature of the metal cation and its coordination to the zeolite framework. With increasing coordination of M+ to the zeolite framework, the ability of the metal cation to polarize the CO bond decreases; therefore, the blue-shift of the CO stretching frequency νCO also decreases. Similarly, with increasing number of framework AlO4 tetrahedra in the vicinity of the M+ cation, the CO polarization decreases and νCO also decreases. In the case of alkali-metal cation-exchanged zeolites, the coordination of metal cation does not change upon the adsorption of CO, and therefore, the effect from bottom is sitespecific; this specificity decreases with increasing cation size. Model concept (ii), effect from top: the CO molecule adsorbed on the M+ site is also affected by close framework O atoms on the surface of the zeolite channel; negative charge on the framework oxygen atoms in the vicinity of the oxygen atom of CO increases the polarization of the CO bond. Due to this increased CO polarization and the confined space effects (CO vibrates against the wall), the CO stretching frequencies increase. Conversely, νCO decreases in the case of the interaction of the oxygen atom of the CO molecule with the second cation (also effect from top) that partially cancels out CO polarization due to the first metal cation. Such “dual sites” demand two cations at the correct distance from each other.9 Due to the small size of the Li+ cation and the high Si/Al ratio of the samples discussed here, the formation of dual sites is not expected; therefore, dual sites are not discussed in detail here.
10.1021/jp0716785 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/07/2007
11354 J. Phys. Chem. C, Vol. 111, No. 30, 2007 Depending on the cation type, the IR spectra in the CO stretching region can bring a different type of information about the system; this can be documented by several examples. CO adsorbed on Cu-ZSM-5 zeolites shows a single vibrational band10,11 despite both experimental12-16 and theoretical17-20 evidence of the different Cu+ sites present in the zeolite. This non-site-specificity of the CO vibrations in Cu+/ZSM-5 systems is caused by the changes in Cu+ coordination upon the interaction with the CO molecule.18 On the contrary, the interaction energy of CO with alkali-metal ions is substantially lower, and therefore, the adsorption of CO is not accompanied by changes in metal-cation coordination.21 The site-specificity of the IR spectra of CO adsorbed on Li-ZSM-5 systems was confirmed both experimentally22 and theoretically.23 Bands due to monocarbonyls on Li+ cations at channel intersections and channel-wall sites, where Li cations are two-coordinated and four-coordinated to framework oxygens, respectively, can be clearly distinguished in IR spectra. As for the site-specificity of the effect from top, it was reported recently that the CO stretching frequency of the monocarbonyl species in K-FER depends on K+ localization in the zeolite.9 The CO adsorbed on the K+ ion in the P8 site (located in the perpendicular channel cage) exhibits a higher wavenumber than the other CO complexes in K-FER; this is due to the confined space effect in the P cage (the interaction of the oxygen atom of the CO molecule with the opposite P cage wall). In addition, so-called “dual sites”, where nearby extraframework cations form linearly bridged M+‚‚‚CO‚‚‚M+ complexes, were identified in sodiumand potassium-exchanged zeolites.9,24 It is therefore apparent that the small Li+ cation is the most suitable cation for probing the site-specificity due to the effects from the bottom while large cations, as K+, are likely candidates for probing the site-specificity due to the effects from the top. In addition, due to the small radius of the Li+ cation, the probability of the existence of dual sites (a pair of two Li+ cations about 5.5 Å apart from each other8) is rather low; therefore, the IR spectra of CO molecules adsorbed on Li zeolites are not complicated by the formation of bridged CO complexes on dual sites appearing at the lower frequency (as is the case in Na and K zeolites9,24). Thus, the bands and shoulders appearing in the IR spectra of CO adsorbed on Li zeolites reflect the site-specific effects from the bottom. In this contribution, a general model of the CO molecule vibrational dynamics in the confined space of microporous materials described in ref 8 is tested and verified. The following effects were considered: (i) various Si/Al ratios, (ii) differences in zeolite topology, (iii) temperature, and (iv) the effect of a co-cation on relative stability and metal cation site preferences. On the basis of the agreement between theory and experiment obtained for various zeolite structures and compositions, an attempt to extrapolate this general model to the mesoporous materials and to interpret the IR spectra of CO adsorbed on Li-(Al)MCM-41 is made. 2. Experimental Section The original NH4-FER zeolite (Si/Al ) 8.6) used in this study was supplied by the Research Institute of Inorganic Chemistry, U Ä stı´ nad Labem. NH4-ZSM-5 (Si/Al ) 15 and 30) was supplied by the Zeolyst Corp. MCM-41 with a Si/Al ratio ) 30 was synthesized from sodium silicate, cetyltrimethylammonium bromide (CTMABr), ethyl acetate, and aluminum isopropoxide according to the procedure described in the literature.25 Initially, 9.81 g of CTMABr was mixed with 500 g of distilled water and 0.33 g of Al isopropoxide under stirring.
Nachtigall et al. TABLE 1: Bulk Composition and Specific Surface Area of Investigated Li Materials sample Li-ZSM-5-15 Li-ZSM-5-30 LiK-ZSM-5-30 KLi-ZSM-5-30 Li-FER-8.6 Li-(Al)MCM-41
Si/Al Li/Al K/Al (atom ratio) (atom ratio) (atom ratio) S (m2/g) 15.4 33.3 33.3 33.3 8.9 30.5
0.92 0.90 0.42 0.36 0.98 0.88
0.51 0.55
394 416 398 412 402 731
Simultaneously, 10 g of sodium silicate was mixed with 450 g of distilled water and slowly added to the first solution under vigorous stirring. Finally, 15 mL of ethyl acetate was added to the resulting mixture and aged in a polypropylene bottle under autogenous pressure for 3 days at 90 °C. The resulting solid was recovered by filtration, washed thoroughly with distilled water, and dried overnight at 50 °C. To remove the template, the as-synthesized material was slowly calcined under an air flow at 500 °C (heating rate ) 1 °C/min). The lithium forms of zeolites were obtained by an ion exchange repeated five times in a 0.5 M aqueous solution of LiCl for 24 h at 50 °C. Complete ion exchange was checked by the disappearance of the IR band in the 3600-3620 cm-1 region, where Brønsted-acid hydroxyl groups appeared in the protonic form of zeolites. Li-ZSM-5 with a Si/Al ratio of 30 was ion exchanged in a 0.5 M aqueous solution of KCl at 50 °C for 7 h. This sample was denoted as LiK-ZSM-5-30. The sample denoted KLi-ZSM-5-30 was prepared from ZSM-5 with Si/Al 30 by an ion exchange repeated five times in a 0.5 M aqueous solution of KCl for 24 h at 50 °C and subsequently ion exchanged by a solution of LiCl at 50 °C for 2 h. The composition of the resulting materials was determined by chemical analysis after sample dissolution (see Table 1). The chemical analysis of the Li zeolites showed that the Li+ ion content in the samples corresponds to the Li/Al ratios in the range of 0.88-0.98. The structure of investigated materials, after ion exchange and calcinations, was checked by a Bruker D8 Advance X-ray powder diffractometer with Cu KR radiation. XRD patterns evidenced a well-developed structure of investigated materials. The XRD patterns of all investigated zeolite samples show the presence of only a zeolite-type framework structure characteristic of the ZSM-5 and FER structures (not shown here).26 No evidence of the damage to zeolite structure was found upon the ion exchange and calcination of the samples. The ion exchanged and calcined Li-(Al)MCM-41 sample exhibited a well-ordered mesoporous structure possessing four well-discernible diffraction lines in a low-angle region of 2θ. The N2 adsorption/desorption isotherms for the Brunauer-EmmettTeller (BET) surface area and mesopore distribution measurements were collected at -196 °C using an Accusorb 2100E instrument. The adsorption-desorption curves of dinitrogen at -196 °C correspond to the behavior of porous adsorbents. The specific surface area of the zeolites determined from BET was about 400 m2 g-1, which is a characteristic value for ZSM-5and FER-type zeolites. The Li-(Al)MCM-41 sample exhibited a surface area of 731 m2 g-1 with a pore diameter of 32 Å. For the FTIR measurements, thin self-supported sample wafers were activated at 400 °C and with dynamic vacuum overnight (pressure of 10-4 Pa). CO was further purified before adsorption by a freeze-pump-thaw cycle. CO was dosed to each IR cell step-by-step to equilibrium pressures of about 0.1 and 140 Torr for experiments at liquid nitrogen and room temperatures, respectively. The spectra were collected with a
FTIR Study of CO Interactions with Li+ Ions
J. Phys. Chem. C, Vol. 111, No. 30, 2007 11355
Figure 1. Selected FTIR spectra of CO adsorbed on Li-ZSM-5 zeolites with ratios of Si/Al ) 15 (A) and Si/Al ) 30 (B) at RT and with CO pressures in the range of 8-102 Torr. The deconvolutions of the spectra obtained at 85 Torr are shown in insets.
TABLE 2: Results of the Deconvolution of Spectra Obtained for CO/Li Zeolites and CO/Li-(Al)MCM-41a νCO (cm-1)
half-width (cm-1)
2193 2186 2182 2174
7 7 8 8
2193 2187 2180 2170
7 8.5 9 11
relative band area (%) Li-ZSM-5-15 Li-ZSM-5-30 41.4 39.0 12.7 6.9
relative band area (%) Li-FER-8.6 Li-(Al)-MCM-41
Li-ZSM-5 56.3 28.2 12.2 3.3 Li-FER and Li-(Al)MCM-41
a
0 50.6 30.3 19.1
0 68.9 31.1 0
The IR spectra of the CO/Li zeolites were taken at 25 °C and 85 Torr. The IR spectra of CO/Li-(Al)MCM-41 were taken at 77 K.
resolution of 1 cm-1 on a Nicolet FTIR Protege 460 spectrometer equipped with an MCT/A cryodetector. The spectrum of a matrix taken before CO dosage was used as a background, and it was subtracted from the spectra shown in this work. The spectra of carbonyl species were deconvoluted into individual spectral bands via the pseudo-Voight function using MicroCal Origin software. 3. Results 3.1. CO Adsorption on Li-ZSM-5. The IR spectra of CO molecules adsorbed on Li-ZSM-5 with Si/Al ratios of 15 and 30 (room temperature (RT) and equilibrium CO pressures in the range of 8-102 Torr) are reported in parts A and B of Figure 1, respectively. The spectra show two dominant and welldistinguished bands at 2193 and 2186 cm-1. A small shoulder is apparent at the lower-energy side of the spectra, for higher CO coverage and a lower Si/Al ratio in particular. Two bands centered at 2102 and 2108 cm-1 can be also distinguished in the spectra. These weak bands were assigned to O-bonded molecules of CO on Li+ ions27-31 and they are not discussed here. The spectrum of the parent H-ZSM-5 zeolite exhibits no bands in the CO stretching region (not shown). This corresponds to the fact that the interaction of CO with hydroxyls is weak and cannot be observed at 25 °C.4 It is apparent from the spectra in Figure 1 that the relative intensity of a band at 2186 cm-1 (with respect to the intensity of a band at 2193 cm-1) increases with increasing CO pressure and, in addition, increases with increasing content of aluminum in the framework. To further analyze and quantify these effects,
the spectra were deconvoluted into four bands: three of them are due to the three Li+ site types with distinguishable νCO23 values and a fourth one is due to the Li+ sites in the vicinity of a framework Al pair (see the Discussion for details). The frequency of the peak maxima, the shape of the band (Gaussian/ Lorentzian ratio), and the peak width were optimized and used as fixed parameters for individual bands in all spectra while the peak area was left as a floating parameter; pseudo-Voight functions were used in fitting. The best fit was obtained for a set of four bands centered at 2193, 2186, 2182, and 2174 cm-1 (see insets in Figure 1) and with a Gaussian/Lorentzian ratio of 0.3. Relative peak areas of individual bands depend on the CO pressure and on the Si/Al ratio. Band half-widths and band areas for the spectra obtained for a CO pressure of 85 Torr are reported in Table 2. To further analyze the correlation between Li+ coordination and νCO frequency, the IR spectra of CO adsorbed on mixed LiK-ZSM-5 and KLi-ZSM-5 samples were taken at the same conditions as for Li-ZSM-5 samples. On the basis of the theoretical investigation of alkali-metal sites in ZSM-5, it was concluded that small Li+ cations preferentially occupy channelwall sites while larger K+ ions preferentially occupy intersection sites.32 The spectra of CO adsorbed on a fully exchanged LiZSM-5-30 sample and those for the mixed LiK-ZSM-5-30 and KLi-ZSM-5-30 samples are compared in Figure 2. Due to a significantly lower CO adsorption enthalpy on K+ sites than on Li+ sites in zeolites, the population of K+-CO carbonyls is very small at the given experimental conditions.9,33,34 This is also apparent from the spectra; the band at 2163 cm-1 has
11356 J. Phys. Chem. C, Vol. 111, No. 30, 2007
Figure 2. FTIR spectra of CO adsorbed on Li-ZSM-5 and on mixed LiK-ZSM-5 zeolites under an equilibrium CO pressure of 50 Torr at 25 °C. Key: 1, Li-MFI-30; 2, LiK-MFI-30; 3, KLi-MFI-30. A, original spectra of the samples; B, spectra normalized to the height of the band at 2193 cm-1.
negligible intensity. Intensities of the spectra, dominated by two bands at 2193 and 2186 cm-1, are proportional to the concentration of Li ions (Figure 2A). Spectra normalized to the height of the 2193 cm-1 band (Figure 2B) show that the relative intensities of bands at 2193 and 2186 cm-1 are changed due to the presence of K+ ions. 3.2. CO Adsorption on Li-FER. Figure 3 depicts the IR spectra of CO adsorbed at room temperature on Li-FER-8.6 zeolite at pressures in the range from 8 to 140 Torr. The IR spectra are characterized by the main absorption band at 2188 cm-1, which is markedly asymmetric on the low-frequency side. The intensity of the spectral band increases with increasing CO pressure, but the shape of the band is rather the same, in contrast to that of the CO/Li-ZSM-5 system. In the low-frequency region of the IR spectra, there is an asymmetric band at 2105 cm-1 ascribed to the O-bonded molecules of CO on Li+ ions (similar to the CO/Li-ZSM-5 system). At about 2230 cm-1, a weak band with an intensity independent of CO pressure is also seen. This band is assigned to the CO adsorbed on the extraframework aluminum species.4 On the basis of the theoretical investigation of the CO adsorption on Li-FER,34 the spectra
Nachtigall et al. of CO/Li-FER zeolite were deconvoluted by a set of three bands (see the Discussion for more details). A deconvolution of the IR spectra obtained for the CO/Li-FER samples was made in the same way as for the CO/Li-ZSM-5 samples. The frequency of the peak maxima, the shape of the band (Gaussian/ Lorentzian ratio), and the peak width were optimized and used as fixed parameters for individual bands in all spectra; the peak area was left as a floating parameter. The best fits were obtained for bands centered at 2187, 2180 and 2170 cm-1 and with a Gaussian/Lorentzian ratio of 0.3 (see the inset in Figure 3). The band half-width and band areas for spectra obtained for a CO pressure of 85 Torr are summarized in Table 2. The existence of individual bands in the spectra of CO on Li-FER was confirmed by the analysis of normalized spectra obtained for two different coverages (Figure 4). The spectra obtained for CO pressures of 15 and 140 Torr were normalized to the maximum of the main band and subtracted. From the normalized spectra, it is apparent that they differ in the shape of the lowfrequency tail. The differential spectrum exhibits an asymmetric band in the region of 2190-2155 cm-1 (see inset in Figure 4), which evidently consists of two bands centered at 2179 and 2170 cm-1, in agreement with the results of the theoretical investigation34 and the spectra deconvolution. 3.3. CO Adsorption on Li-(Al)MCM-41. The IR spectra in the OH and CO stretching regions obtained for CO/Li-(Al)MCM-41 at 77 K are respectively depicted in parts A and B of Figure 5. Only two features are observed in CO stretching regions at low coverages: bands at 2230 and 2188 cm-1 are assigned to CO adsorbed on Al3+ extraframework sites and on Li+ sites. The main band in the CO stretching region centered at 2188 cm-1 is slightly asymmetric with a tail on the lowfrequency side and can be deconvoluted (using the same procedure as described for Li-FER above) into two bands centered at 2187 and 2180 cm-1 (see Table 2 and inset in Figure 5). Increasing the CO dose resulted in a substantial growth of the band at 2188 cm-1 with a simultaneous slight shift of the band maxima to 2183 cm-1, whereas the band at 2230 cm-1 remained constant in its position and intensity. The further increase of the CO pressure led to the development of new bands at 2158 and 2138 cm-1. The latter band was ascribed to the CO liquidlike phase; the former one was assigned to CO molecules interacting with free silanols, which is evidenced by
Figure 3. FTIR spectra of CO adsorbed on Li-FER at RT with CO pressures from 8 to 140 Torr. The deconvolution of a spectrum obtained at 80 Torr is shown in the inset.
FTIR Study of CO Interactions with Li+ Ions
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Figure 4. FTIR spectra of a Li-FER-8.6 sample obtained with CO equilibrium pressures of 15 (gray line) and 140 (black line) Torr normalized to the maximum of the main band. The inset shows the differential spectrum (140-15 Torr).
Figure 5. FTIR spectra of a Li-(Al)MCM-41 sample at the temperature of liquid nitrogen for various CO doses in the OH group (A) and CO group (B) vibrational regions. The inset shows the deconvolution of the main band for the spectrum obtained after 10 min of evacuation of the saturated sample at the temperature of liquid nitrogen.
the perturbation of OH vibrations (Figure 5A).4 The interaction of CO with OH groups resulted in a shift of the O-H stretching vibrations. New bands appearing at 3660 and 3550 cm-1 were respectively assigned to the perturbed vibrations of O-H groups in silanol species and poorly acidic undefined aluminum oxide species, probably not belonging to the mesoporous structure.35 4. Discussion The CO stretching frequencies were calculated for various Li+ sites in ZSM-5 and FER previously.23,34 A very good agreement between the calculated frequencies and experimental IR spectra22,36 was found for both zeolites; not only band maxima but also the effects of Si/Al ratio, temperature, and coverage on the IR band shape could be understood based on the theoretical model. The key is in the cation coordination in a particular zeolite frameworksthe better the Li+ coordination with the framework, the weaker the Li+ interaction with CO and the smaller the shift in νCO. As was mentioned in the Introduction, the νCO frequencies are site-specifically modulated by the effect from bottom while the effect from top is not site-
specific in the case of the Li+ cations.8 In other words, the νCO frequency bears information about the Li+ coordination in the system. This correlation between Li+ coordination and νCO value of CO adsorption complexes is discussed for Li zeolites (LiZSM-5 and Li-FER) followed by the extrapolation to mesoporous materials (Li-(Al)MCM-41, section 4.2). 4.1. CO Adsorption on Li Zeolites. It is convenient to start with the discussion of the Li+ sites in high-silica zeolites. A systematic investigation of the Li+ sites in Li-ZSM-532 and Li-FER34 performed at the QM/MM and periodic DFT levels, respectively, showed that the Li+ cations could be coordinated at either the channel-intersection (type II site) or the channelwall site (type I site). Channel-wall sites were on top of either the five- or six-membered rings. While such classification is useful for the description of the coordination and localization of extraframework monovalent cations in high-silica zeolites (e.g., Cu+, Na+, or K+),9,20,24,37 it is not sufficient for the discussion of the details of the IR spectra of adsorbed probe molecules. For this purpose, it is necessary to pay attention to the rather subtle details of individual Li+ sites. The coordination
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Nachtigall et al. TABLE 3: Experimental and Theoretical Vibrational Frequencies (in cm-1) of CO Adsorbed on Various Site Types of Extraframework Li+ Cations in Li-FER, Li-ZSM-5, and Li-(Al)MCM-41 Matrices. Calculated Interaction Energies for Various Sites Are Also Reported Li+ site typea II
I
Al pair
ZSM-5 R5, R6c 2187 (7) 2183-90 21-24
R6s 2182 (8) 2176-83 9-18
2174 (8) 2171 12
site types νCO (exp)b νCO (calc) -Eint (km/mol)
FER R8, R5, R7, R6c 2187 (8.5) 2186-88 22-28
R6s 2180 (9) 2178 17
2170 (11) 17 13-20
νCO (exp)b
MCM-41 2187 (8.5)
2180 (9)
site types νCO (exp)b νCO (calc) -Eint (kj/mol)
a
Ia
R10, R7 2193 (7) 2191-97 25-32
s
c
The R10, R8, R7, R6 , R6 , and R5 sites are defined in Figure 6. Values in parentheses are the half-widths of individual peaks in the deconvoluted experimental spectra.
b
Figure 6. Li+ site types in high-silica zeolites. Framework Al, Si, and O atoms are depicted in tube mode in black, gray, and red colors, respectively; Li+ is shown as a violet ball. The R10 and R8 sites are located on the channel intersections in MFI and FER, respectively, and the R5, R6, and R7 sites are located on top of the five- and six-membered rings on the surface of the channel wall. The Rs and Rc sites differ in the position of the framework Al that is in the center of the longer side and in the corner of the six-membered ring, respectively. The six-membered ring in the R7 site is bridged by an additional TO4/2 tetrahedron; therefore, the flexibility of this six-membered ring is lower than in the case of R6 sites.
on top of the six-membered ring with the Al atom at the center of the longer side of the ring (e.g., M6s or Z6s in Figure 6) was distinguished from the coordination on top of the sixmembered ring with Al atom in the corner (M6c or Z6c).32 The cation site notation introduced originally in refs 20 and 38 for MFI and FER, respectively, is adopted: “M”, “Z”, and “P” stands for sites in the main (both MFI and FER), zigzag (MFI), and perpendicular (FER) channels, respectively. Energetically stable Li+ sites found in ZSM-5 and FER are depicted in Figure 6, and corresponding CO stretching frequencies and interaction energies are summarized in Table 3; details of the calculations were reported in refs 23 and 34 for LiZSM-5 and Li-FER, respectively. It is necessary to distinguish between the type II sites on the channel intersections in ZSM-5 and FER. The Li+ cation is coordinated to just two O atoms of a single AlO4 tetrahedron (R10 in Figure 6) at the intersection of two 10-membered ring channels in ZSM-5. On the other hand, the Li+ cation coordinates to three framework O atoms in the type II site on the intersection of the 10- and 8-membered rings in FER (R8 in Figure 6). As a result, higher CO stretching frequencies were calculated on the intersection sites in LiZSM-5 than in Li-FER, in agreement with the experimental data (2193 and 2186 cm-1, respectively).22,36 Within the channelwall sites, the sites on top of the five-membered ring (R5 sites) and the sites on top of the six-membered ring with the aluminum atom at the corner (R6c sites) show similar CO stretching frequencies in both Li-ZSM-5 and Li-FER (2183-2190 cm-1). Therefore, the R5 and R6c sites were denoted as “type Ia” sites.23 The best coordination of the Li+ ion with the framework was found for R6s sites where the small Li+ atom can fit well within the six-membered ring (R6s sites). The CO interaction with such a site is weaker, and the CO stretching frequencies are lower than those found for CO physisorbed on
the R5 and R6c sites (Table 3); R6s sites were denoted as “type I” sites.23 Rather unusual frequencies were calculated for the R7 site type in both ZSM-5 and FER (M7/T7 and M7/T3 sites, respectively; Tn denotes the position of the framework Al); the CO stretching frequencies are up to 10 cm-1 higher than those found for the corresponding R6s or R6c rings. This is due to the smaller flexibility of the R7 six-membered ring that is bridged by a single TO4 tetrahedron (Figure 6); the Li+ cation cannot fit within the six-membered ring, and as a result, the CO interaction with this site is stronger and the νCO shift is larger. The relative stability of sites depends on the framework topology. The Li+ coordination at the R6s site is energetically preferred; however, such sites exist only for some particular positions of framework Al (e.g., T1 in FER and T1, T8, T10, and T11 in ZSM-5). The stability of the R6c, R5, R8, and R10 sites depends on the framework Al position; for details, see refs 32 and 34. The ability of Li+ to bind CO and, thus, the ability to polarize the CO bond and to increase νCO decreases with the increasing cation coordination to the framework. The weakest interaction of CO with Li+ (and the lowest νCO value) was found for the Li+ sites in the vicinity of a pair of framework AlO4 tetrahedra.34 In summary, we can distinguish CO stretching frequencies for four types of monocarbonyl complexes on the Li+ cations in high-silica zeolites (Table 3): (i) The band with νCO > 2190 cm-1 corresponds to monocarbonyls on the Li+ cations coordinated to only two oxygen atoms (type II site, R10 located at the channel intersection in ZSM-5). (ii) The band with νCO ∼ 2187 cm-1 is assigned to monocarbonyls on the Li+ cations coordinated to three (or to four) oxygen atoms, on top of fivemembered rings, on top of six-membered rings with Al at the corner position, and on the intersection of eight-membered ring channels, denoted type Ia sites (R5, R6c and R8). (iii) The band at about 2180 cm-1 is due to monocarbonyls on the Li+ cations coordinated to four oxygen atoms and located within R6s rings. (iv) The band with νCO ∼ 2170 cm-1 is assigned to CO adsorbed on the Li+ cation in the vicinity of two framework aluminum atoms (Al-pair sites). On the basis of the excellent agreement of the calculated CO stretching frequencies (reported in refs 23 and 34) with the features of the experimental spectra of CO/Li zeolite systems, we have deconvoluted the IR spectra into above
FTIR Study of CO Interactions with Li+ Ions four bands; the coordination of Li+ cations in particular samples is then discussed based on the analysis of IR spectra. The experimental spectra of Li-ZSM-5 with Si/Al ratios of 15 and 30 reported in Figure 1 can be interpreted in light of the theoretical analysis presented above and summarized in Table 3: the 2193 and 2186 cm-1 bands are due to CO adsorption on the Li+ cations in the type II (channel intersection) and type Ia sites, respectively; the band shoulders at 2182 and 2174 cm-1 are due to the CO adsorption on Li+ in the type I site and the vicinity of a framework Al pair, respectively. It is apparent from the spectra (and from Table 2) that the relative intensity of individual bands depends on the CO coverage and on the Si/Al ratio of the sample: (i) With increasing CO coverage, the relative intensity of the band at 2186 cm-1 increases (on the expense of a band at 2193 cm-1); that corresponds with the lower adsorption enthalpy on type Ia sites than on type II sites in Li-ZSM-5. (ii) The intensity of the band at 2174 cm-1 is more than doubled on going from the sample with a ratio of Si/Al ) 30 to a ratio with Si/Al ) 15; this is consistent with the higher probability of the formation of an Al pair in the zeolite with a lower Si/Al ratio. (iii) Li-ZSM-5 with higher Si/Al ratio shows the spectra with significantly greater relative intensity of a band at 2193 cm-1 with respect to one at 2186 cm-1; this suggests that the relative population of the Li+ sites at the channel intersection (with respect to those on the channel wall) is larger in the Li-ZSM-5-30 sample than in the Li-ZSM5-15 sample. This observation can be possibly interpreted in two different ways: (i) The population of the intersection sites is driven by the framework Al distribution that is different in samples with Si/Al ratios of 15 and 30, and it is nonstatistical at least in one of the samples. In other words, the distribution of framework Al depends on the zeolite synthesis conditions. (ii) With the increasing concentration of Li+ ions at the channel intersection, the repulsion between cations drives them away from the intersection toward the sites on the channel wall. It should be noted that there are several Li+ sites in the vicinity of one framework Al.32 Further investigation is required to find out which of the mechanisms suggested above is more likely. The spectra of mixed LiK-ZSM-5 samples clearly show that the relative intensities of the band at 2186 cm-1 (with respect to that of the band at 2193 cm-1) are larger in mixed LiKZSM-5 samples than in Li-ZSM-5 samples (Figure 2).39 With the use of the arguments presented above, this change of the band intensities can be interpreted in terms of the changes in populations of type I and type II sites: the presence of K+ ions in the sample results in an increased population of the type Ia (and type I) sites by Li+ ions at the expense of the population of type II sites. This is in line with the theoretical investigation of the alkali-metal cation sites in ZSM-5.32 According to this study, the large K+ ions preferentially bind at the channelintersection sites (type II) while the small Li+ cations preferentially bind at the channel-wall sites (type Ia and I). Similar experimental observation was reported for high CO coverage on mixed NaLi-ZSM-5, where the IR spectra showed a dominant band at 2188 cm-1 and only a small shoulder at about 2193 cm-1.30,40 Obvious differences can be found between the IR spectra of CO adsorbed on Li-ZMS-5 and Li-FER zeolites (Figures 1 and 3, respectively). First, the high-energy band at 2193 cm-1 is completely missing in Li-FER. This is clearly due to the changes in the zeolite topology; the intersection of 8- and 10membered ring channels in FER does not favor the formation of two-coordinated Li+ sites. Second, the shape of the spectra distinctly depends on the CO coverage in the case of Li-ZSM-5
J. Phys. Chem. C, Vol. 111, No. 30, 2007 11359 while the shape is almost unaffected by the CO coverage in the case of Li-FER. This is in line with the calculated interaction energies of CO with particular Li+ site types (Table 3); while the type II sites in Li-ZSM-5 (R10 in Figure 6) show higher CO adsorption enthalpy than the type Ia and type I sites, the CO interaction is very similar on the type II (R8) and type Ia (R5 and R6c) sites in Li-FER. Apart from these two differences, there are important similarities in the IR spectra of the CO/ Li-ZSM-5 and CO/Li-FER systems. The CO interaction energies with Li+ and νCO are very similar for particular Li+ site types in both zeolites and do not depend on the framework topology. Also, the blue-shift of νCO due to the effect from top appears to be very similar in both zeolite frameworks investigated. Therefore, the IR spectra of CO adsorbed on mixed alkalimetal zeolite samples support the interpretation given above: the vibrational frequencies of CO adsorbed on extraframework Li+ cations are primarily influenced by the effects from bottom, in other words, νCO correlates with the Li+ coordination. It should be noted that the interpretation of experimental IR spectra can be complicated by the formation of dicarbonyl complexes. The calculated CO stretching frequencies of dicarbonyl complexes are 3-13 cm-1 lower than the νCO frequencies of monocarbonyl species. In agreement with theoretical results, the IR spectra recorded at high CO pressure and low temperature on Li-ZSM-541 and Li-FER36 exhibit IR bands at 2187 and 2179 cm-1, respectively, that were ascribed to CO vibration in dicarbonyl complexes. However, the experimental spectra recorded at RT correspond to coverages lower than monolayer. The theoretical study of Li-ZSM-542 and Li-FER34 zeolites led to the conclusion that dicarbonyls can be formed mainly on the Li cations at the channel intersection (type II sites) with the adsorption enthalpy for the second CO molecule being only about 50-75% that of the adsorption enthalpy of the first CO molecule. In addition, the experimentally observed changes of band maxima position and the shape of spectra were found to be due only to the changes of the monocarbonyl complexes.23 With the consideration of these facts, it is reasonable to assume that the changes in band intensities observed for the samples investigated here under given experimental conditions are dominantly due to the changes of the monocarbonyl species (site populations). 4.2. CO Adsorption on Li-(Al)MCM-41. On the basis of good agreement between the theoretical and experimental results for CO chemisorbed on Li zeolites, an attempt is made to extend a discussion of the CO vibrations in confined space in zeolite channels (micropores) to the mesoporous materials and to interpret the IR spectra of CO adsorbed on Li-(Al)MCM-41. Understanding the IR spectra of CO/Li-(Al)MCM-41 may give some new information about the Li+ coordination in mesoporous materials. Following the discussion presented in ref 8, the stretching frequencies of CO adsorbed on extraframework Li+ cations are primarily determined by the effect from bottom that is site-specific (it depends on Li+ coordination) and are further modulated by an effect from top. From the results obtained for Li-ZSM-5 and Li-FER matrices, the latter effect does not appear to be specific for Li+. It is reasonable to assume that the effect of Li+ coordination on νCO frequency is the same in mesoporous and microporous aluminosilicates; however, the effect from top may differ in these two types of material. The effect from top arises from the increased polarization of CO adsorbed on Li+ by the negatively charged oxygen atoms on the surface of the zeolite channel. The interaction between adsorbed CO and the surface of the zeolite channel (other than the metal cation) cannot be avoided in zeolites: first, a diameter
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Figure 7. FTIR spectra of CO adsorbed on Li-(Al)MCM-41 at RT under equilibrium pressures of 75-140 Torr. The spectrum of CO adsorbed on the same sample at 77 K taken after the evacuation of the saturated sample under dynamic vacuum for 45 min is compared with the RT spectrum (taken under the CO pressure) in the inset.
of the zeolite channel is small, and second, the interaction between CO and the channel surface oxygen atoms is attractive. Qualitative results obtained with small cluster models showed that this attractive interaction is about 4 kJ/mol in magnitude.8 More quantitative analysis can be obtained from the data reported in ref 23. With the use of the same basis set and exchange correlation functional, the CO interaction with the 1-T cluster model (LiAl(OH)4) and the model of zeolite intersection sites represented by about 20 TO4/2 tetrahedra was calculated; the Li+ cation is coordinated to two oxygen atoms in both cases. The zero-point vibrational energies and basis-set superposition errors were included in these calculations. These results showed that the CO interaction with the small 1-T cluster model (accounting only for the effect from bottom) was 4-10 kJ/mol smaller than the CO interaction with the zeolite modeled with the large model (accounting for both effects from bottom and from top). The interaction of CO with Li+ is dominantly electrostatic; therefore, it only depends on the Li-C-O angle and does not depend on the Li-C direction. On the basis of the arguments about energy stabilization, we conclude that at the minimum energy structures, the CO molecules adsorbed on Li+ cations on the surface of mesoporous channels are tilted toward the channel wall. At such structures, the CO stretching frequencies are modulated by the effect from top in the same way as in the zeolite. In order to provide some support for the model proposed above, the IR spectra of CO adsorbed on Li-(Al)MCM-41 at RT and at the temperature of liquid nitrogen are compared in Figure 7. It should be noted that the RT spectra have very low intensity, and therefore, they correspond to very low coverage. To compare the IR spectra on the samples with similar coverage, the spectrum at RT was measured under 140 Torr of CO while the spectrum at 77 K was measured after the evacuation of the sample for 45 min under dynamic vacuum. The tail at the lowerenergy side of the band at 2188 cm-1 is significantly more pronounced at RT than at low temperature. This broadening toward lower frequencies can be explained by the thermal motion of CO away from the wall; with increasing distance of CO from the wall, the blue-shift in νCO due to the increased polarization of the CO bond by the framework oxygen atoms decreases and, thus, the CO molecule vibrates at frequencies lower than 2188 cm-1.
The main band in the spectra in Figure 5, assigned to the CO stretch in the Li+CO adsorption complex, has a frequency close to that reported for the Li-exchanged micelle-templated silicates with high Al content35 (2188 and 2186 cm-1, respectively). These frequencies are the same as the stretching frequency of CO adsorbed on type Ia sites in zeolites. Also, for other alkali-metal cations, rather similar CO stretching frequencies were reported in zeolites and in mesoporous matrices, 2176 and 2178 cm-1 for Na-exchanged MCM-41 and ZSM-5, respectively,43,44 and 2162 and 2166 cm-1 for K-exchanged MCM-41 and ZSM-5, respectively.43,45 This is additional evidence that the factors influencing νCO (effects from top and from bottom) are the same in microporous as in mesoporous aluminosilicate materials. The main band in the IR spectra of carbonyls formed on Li(Al)MCM-41 is centered at 2188 cm-1 and is slightly asymmetric with a tail on the low-frequency side, and it can be deconvoluted (using the same procedure as described for LiFER above) into two bands centered at 2187 and 2180 cm-1 (Figure 5). With the assumption of similar effects on νCO values in zeolites and mesoporous (Al)MCM-41, these two bands can be assigned to CO complexes on Li+ cations in type Ia and type I sites, respectively. The normalized spectra of CO adsorbed on the Li-FER zeolite at RT and on the Li-(Al)MCM-41 matrix at the temperature of liquid nitrogen are compared in Figure 8. It is apparent that spectra look very similar, as the band maxima are the same (2188 cm-1) and the spectra are tailed on the low-frequency side. The broadening of the envelope curve of the Li-FER zeolite can be ascribed to the higher population of type I sites and Li+ sites in the vicinity of two Al atoms; this is in line with the different Si/Al ratios of both materials (Si/Al ratios of 8.6 and 30 for FER and (Al)MCM41, respectively). In contrast to the case in the zeolites, there are no data available about the structure of the surface of mesoporous molecular sieve channels. This surface is usually considered to be disordered/amorphous or presented in simple models by regular six-membered rings.46,47 As was argued in the previous section, the CO frequencies reflect the coordination of the Li+ cations: the band at 2187 cm-1 is due to CO adsorption complexes on Li+ on top of the five-membered rings or on top of the six-membered rings with Al at the corner position (R5
FTIR Study of CO Interactions with Li+ Ions
J. Phys. Chem. C, Vol. 111, No. 30, 2007 11361 found only in ZSM-5, while they are missing in the spectra of Li-FER and Li-(Al)MCM-41. A band at νCO ∼ 2170 cm-1 corresponding to an Al-pair site is significantly populated only in the Li-FER sample with a ratio of Si/Al ) 8.6. The IR spectra of CO/Li-(Al)MCM-41 show bands at 2187 and 2180 cm-1 that correspond to Li+ cations coordinated on top of five- and six-membered rings. The Li+-CO adsorption complexes with the same νCO frequencies were found in microporous and mesoporous aluminosilicates. This, together with the analysis of the CO interaction with the channel wall, has led us to conclude that the effect from top modulates νCO frequencies in CO/Li-(Al)MCM-41 in the same way as in CO/ Li zeolites.
Figure 8. FTIR spectra of CO adsorbed on Li+ ions in the Li-FER8.6 zeolite at RT and at a CO pressure of 140 Torr (gray line) and in a Li-(Al)MCM-41 matrix at the temperature of liquid nitrogen after 10 min of evacuation (black line).
and R6c, respectively); the shoulder at 2180 cm-1 is due to the CO complex on Li+ in the plane of R6s six-membered rings (Figure 6). Similarly, the investigation of the local structure of the mesoporous channel-wall surface by means of the UV-vis spectroscopy of Co2+ ions distinguished two types of surface structure, deformed planar or bent six-membered rings.46 5. Conclusion The IR spectra of CO molecules adsorbed on extraframework Li+ cations in zeolites and in (Al)MCM-41 were analyzed using a recently proposed model of CO vibrational dynamics in confined space.8 The following effects on νCO frequency were analyzed: different framework topology, various Si/Al ratios, the effect of an co-cation, and the dependence on the CO coverage and temperature. All these effects on the CO vibrational dynamics can be consistently interpreted in terms of (i) effects from bottom, reflecting the Li+ ion coordination, and (ii) effects from top that do not depend on the framework topology. On the basis of the analysis of extraframework Li+ sites in zeolites, four types of monocarbonyl complexes on the Li+ cations can be distinguished in IR spectra: (i) monocarbonyls on the Li+ ions coordinated only to two oxygen atoms (R10 sites on the intersection of two 10-membered ring channels in ZSM-5) characterized by a band with νCO > 2190 cm-1; (ii) monocarbonyls on the Li+ cations coordinated to three (or four) oxygen atoms located on top of five-membered rings, on top of six-membered rings with Al at the corner position, and on the intersection of 8- and 10-membered ring channels in FER (R5, R6c, and R8 sites, respectively) characterized by a band at about 2187 cm-1; (iii) monocarbonyls on the Li+ cations coordinated to four oxygen atoms and located in the plane of six-membered rings (R6s sites) characterized by a band at about 2180 cm-1; and (iv) monocarbonyls on the Li+ cations in the vicinity of two framework aluminum atoms (Al-pair site) with the band at about 2170 cm-1. The experimentally determined band maxima coincide very well with those obtained from theoretical calculations. On the basis of this agreement, individual IR bands can be interpreted at the atomic-scale level. In addition, the relative peak area yields information about the population of a particular Li+ site type. Band at νCO > 2190 cm-1, corresponding to the type II sites on the intersection of two 10-membered ring channels, can be
Acknowledgment. This work was supported by the Grants of ME CR No. LC512 and MSM 0021627501 and GA CR No. 203/06/0324. P.N. would like to acknowledge the support by project Z4 055 905. K.F. and H.D. would like to acknowledge support by the Grant GA CR No. 203/03/H140. References and Notes (1) Mortier, W. J. Compilation of Extra-framework Sites in Zeolites; Butterworths: London, 1982. (2) Olson, D. H. Zeolites 1995, 15, 439. (3) Zecchina, A.; Arean, C. O. Chem. Soc. ReV. 1996, 25, 187. (4) Hadjiivanov, K. I.; Vayssilov, G. N. AdV. Catal. 2002, 47, 307. (5) Garrone, E.; Arean, C. O. Chem. Soc. ReV. 2005, 34, 846. (6) Otero Arean, C.; Nachtigallova, D.; Nachtigall, P.; Garrone, E.; Rodrigues Delgado, M. Phys. Chem. Chem. Phys. 2007, 9, 1421. (7) Bolis, V.; Barbaglia, A.; Bordiga, S.; Lamberti, C.; Zecchina, A. J. Phys. Chem. B 2004, 108, 9970. (8) Nachtigallova, D.; Bludsky, O.; Arean, C. O.; Bulanek, R.; Nachtigall, P. Phys. Chem. Chem. Phys. 2006, 8, 4849. (9) Garrone, E.; Bulanek, R.; Frolich, K.; Arean, C. O.; Delgado, M. R.; Palomino, G. T.; Nachtigallova, D.; Nachtigall, P. J. Phys. Chem. B 2006, 110, 22542. (10) Bulanek, R. Phys. Chem. Chem. Phys. 2004, 6, 4208. (11) Lamberti, C.; Bordiga, S.; Salvalaggio, M.; Spoto, G.; Zecchina, A.; Geobaldo, F.; Vlaic, G.; Bellatreccia, M. J. Phys. Chem. B 1997, 101, 344. (12) Bulanek, R.; Cicmanec, P.; Knotek, P.; Nachtigallova, D.; Nachtigall, P. Phys. Chem. Chem. Phys. 2004, 6, 2003. (13) Kuroda, Y.; Yoshikawa, Y.; Emura, S.; Kumashiro, R.; Nagao, M. J. Phys. Chem. B 1999, 103, 2155. (14) Kuroda, Y.; Yoshikawa, Y.; Kumashiro, R.; Nagao, M. J. Phys. Chem. B 1997, 101, 6497. (15) Dedecek, J.; Sobalik, Z.; Tvaruzkova, Z.; Kaucky, D.; Wichterlova, B. J. Phys. Chem. 1995, 99, 16327. (16) Bolis, V.; Maggiorini, S.; Meda, L.; D’Acapito, F.; Palomino, G. T.; Bordiga, S.; Lamberti, C. J. Chem. Phys. 2000, 113, 9248. (17) Bludsky, O.; Silhan, M.; Nachtigallova, D.; Nachtigall, P. J. Phys. Chem. A 2003, 107, 10381. (18) Davidova, M.; Nachtigallova, D.; Bulanek, R.; Nachtigall, P. J. Phys. Chem. B 2003, 107, 2327. (19) Sauer, J.; Nachtigallova´, D.; Nachtigall, P. Ab initio sumulation of Cu-species in zeolites: siting, coordination, UV-vis spectra and reactivity. In Catalysis by Unique Metal Ion Structures in Solid Matrices. From Science to Application; Centi, G., Wichterlova´, B., Bell, A. T., Eds.; Kluwer: Dordrecht, The Netherlands, 2001; Vol. 13; p 221. (20) Nachtigallova, D.; Nachtigall, P.; Sierka, M.; Sauer, J. Phys. Chem. Chem. Phys. 1999, 1, 2019. (21) Bludsky, O.; Nachtigallova, D.; Bulanek, R.; Nachtigall, P. Combined theoretical and experimental study of the site-specificity of vibrational dynamics of CO adsorbed on monovalent metal cations in zeolites. In Molecular SieVes: from Basic Research to Industrial Applications, Parts A and B; Elsevier Science Bv: Amsterdam, 2005; Vol. 158; p 625. (22) Bonelli, B.; Garrone, E.; Fubini, B.; Onida, B.; Delgado, M. R.; Arean, C. O. Phys. Chem. Chem. Phys. 2003, 5, 2900. (23) Nachtigallova, D.; Nachtigall, P.; Bludsky, O. Phys. Chem. Chem. Phys. 2004, 6, 5580. (24) Nachtigall, P.; Rodriguez Delgado, M.; Frolich, K.; Bulanek, R.; Palomino, G. T.; Bauca, C. L.; Arean, C. O. Microporous Mesoporous Mater. 2007. DOI: 10.1016/j.micromeso.2007.02.049. (25) Cejka, J.; Krejci, A.; Zilkova, N.; Dedecek, J.; Hanika, J. Microporous Mesoporous Mater. 2001, 44, 499.
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Nachtigall et al. (38) Nachtigall, P.; Davidova, M.; Nachtigallova, D. J. Phys. Chem. B 2001, 105, 3510. (39) It must be noted that both samples were measured at the same time under identical conditions, and therefore, we can suppose that differences in the intensity of individual bands are caused by changes in the distribution of the Li sites in zeolites with different Si/Al ratios. (40) Zecchina, A.; Bordiga, S.; Lamberti, C.; Spoto, G.; Carnelli, L.; Arean, C. O. J. Phys. Chem. 1994, 98, 9577. (41) Arean, C. O.; Manoilova, O. V.; Delgado, M. R.; Tsyganenko, A. A.; Garrone, E. Phys. Chem. Chem. Phys. 2001, 3, 4187. (42) Sillar, K.; Burk, P. Phys. Chem. Chem. Phys. 2007, 9, 824. (43) Bonelli, B.; Ribeiro, M. F.; Antunes, A. P.; Valange, S.; Gabelica, Z.; Garrone, E. Microporous Mesoporous Mater. 2002, 54, 305. (44) Tsyganenko, A. A.; Platero, E. E.; Arean, C. O.; Garrone, E.; Zecchina, A. Catal. Lett. 1999, 61, 187. (45) Manoilova, O. V.; Mentruit, M. P.; Palomino, G. T.; Tsyganenko, A. A.; Arean, C. O. Vib. Spectrosc. 2001, 26, 107. (46) Dedecek, J.; Zilkova, N.; Kotrla, J.; Cejka, J. Collect. Czech. Chem. Commun. 2003, 68, 1998. (47) Chen, C.-Y.; Li, H.-X.; Davis, M. E. Microporous Mater. 1993, 2, 17.
