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FTIR Evidence of Different Bonding of Methane to OH Groups on H-ZSM-5, HY and SiO 2

Kristina Chakarova, Nikola Drenchev, and Konstantin Ivanov Hadjiivanov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp3052592 • Publication Date (Web): 24 Jul 2012 Downloaded from http://pubs.acs.org on July 27, 2012

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The Journal of Physical Chemistry

FTIR Evidence of Different Bonding of Methane to OH Groups on H− −ZSM-5, HY and SiO2 Kristina Chakarova, Nikola Drenchev and Konstantin Hadjiivanov

Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria

Abstract Low-temperature adsorption of CH4 and

15

N2 on H− −ZSM-5, SiO2 and HY is

comparatively studied. Partly and fully deuteroxylated samples were also investigated. It was established that methane forms H-bonds simultaneously with oxygen and hydrogen from the Si− −OH groups which reflects in enhanced shift of the OH modes to lower frequency as compared to the case if methane was bound to the proton only. In contrast, methane is attached to highly acidic hydroxyls forming a bond mainly with the proton. At high methane equilibrium pressure a second methane molecule is bound to the same acidic OH group.

Keywords: Adsorption; 15N2; CH4; H− −ZSM-5, SiO2; HY; FTIR spectroscopy.

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1. Introduction Methane is among the most attractive fuels due to its large abundance (mainly in natural gas) and because it produces less CO2 as compared to other hydrocarbons. However, difficulties are encountered with methane transportation and, consequently, development of effective materials for methane storage is still a challenge to the scientific community. To design effective methane storage materials, it is necessary to know in details the interaction of methane with solid surfaces. Adsorption of methane on supported metals is often dissociative1,2 and therefore these materials are more appropriate for methane conversion catalysts than for its storage. In contrast, oxides and zeolites seem to be more convenient for this purpose. Recently, the use of metal-organic frameworks (MOFs) was proposed.3 However, the way of methane adsorption on non-metallic materials is still far from well understood. This concerns two main points: (i) the nature of the adsorption sites and (ii) the geometry of the adsorption complexes. One of the most informative techniques for characterization of adsorbed species is IR spectroscopy. Surprisingly, compared to other adsorbates (e.g. CO, NO, H2, Py), there are relatively few IR studies on methane adsorption on oxides and zeolites.4-31 Analysis of literature data shows that the possible active sites for methane adsorption on these compounds, proposed by different authors, are coordinatively unsaturated (cus) metal cations,4-16 cus oxygen anions,6,7,17-19,26,32 combination of cus cations and oxygen anions13,15,25,27,32 and/or surface OH groups.9,18-21,24-26,28,29,33 The interaction of methane with hydroxyls is unambiguously monitored by the shift of the OH stretching modes towards lower frequencies (see Table 1). The higher the acidity of the hydroxyls, the stronger the interaction and the larger the ∆ν(OH) value. Note that it has been reported that basic hydroxyls (e.g. on MgO) do not interact with methane.7 Several works have dealt with coadsorption of methane and CO.6,7,10,11,13,18,25 In some cases (Cr2O3,10 ZnO11) CO fully replaces the pre-adsorbed methane. Since CO is coordinated to Lewis acid sites, this is a proof of methane interaction with cus cations. However, studies with MgO revealed that only a part of the adsorbed methane was replaced by CO and the results have been taken as evidence that the adsorption sites were basic oxygen anions.7,25 Note that Li et al.18 associated the part of methane replaced by CO with molecules interacting with acid-base pairs. CO only slightly affects CH4 pre-adsorbed on alumina, which excludes Al3+ as possible adsorption sites. A fraction of adsorbed methane (the weakest adsorption form) on NaY6 and Na− −ETS-1013 was also not affected by CO and therefore associated with


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oxygen anions.6 Thus, it seems that the nature of the active sites depends on the adsorbent type, the strength of the acid and base sites and possibly on the existence of suitable acid-base pairs. The symmetry of methane is Td and consequently CH4 possesses four fundamental vibration modes (Table 2). Two of them (ν3 and ν4) are IR active and two (ν1 and ν2), IR silent. All researches are unanimous that after adsorption methane lowers its symmetry and, as a result, the forbidden ν1 mode becomes IR active. It has been pointed out that, being symmetric mode, ν1 is not split during symmetry reduction (see Table 2). The stronger the perturbation of methane molecule, the higher the extinction coefficient of the ν1 band and the larger its shift (with respect to the gas phase) to lower wavenumbers. In contrast, lowering the symmetry should split the triply degenerate ν3 mode.15,34 Thus, if the symmetry of the adsorbed molecule is C3v, the ν3 mode splits into two components34 and totally three C-H stretching modes should be observed in the spectra. In −H stretching contrast, for C2v symmetry, ν3 mode splits into three components and four C− bands are active.15 However, there are difficulties with the experimental observation of the split of the ν3 mode due to superimpositions of bands arising from various adsorption complexes and not good resolution. The ν2 mode is also activated during adsorption.8 It should manifest one band for C3v symmetry and two bands for symmetry C2v or Cs (see Table 2).13 The ν4 bending mode shows two (C3v symmetry) or three bands (C2v symmetry).8,13 However, the intensities of these bands remain too small for detailed analysis of the spectra. A symmetry C3v can be achieved if methane is attached to the surface via one or via three hydrogen atoms. Therefore, based on the IR spectra, one cannot distinguish between these two geometries. Theoretical modelling has been often applied to help the spectral interpretation. Some authors proposed threefold coordination of CH4 to metal cations.7,27,15,32 According to Pidko et al.,15 this is the way of methane interaction with Ca2+ ions in CaY but it is bound to Mg2+ ions in MgY via two hydrogens. However, Scarano et al.11 consider methane interaction via one hydrogen with Zn2+ ions on ZnO. It has also been proposed that methane interacts with OH groups25 or O2- sites7,32 via one hydrogen atom. Inspection of the data presented in Table 1 shows that there are different reports on the methane-induced shift of analogous OH groups, especially for the zeolite acidic hydroxyls. Thus, with H-ZSM-5, data ranging from 45 23 to 112 cm-1

26

have been reported. It should be

noted that, according to these data, methane seems to be weaker base as compared to nitrogen



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(where a shift of 120 cm-1 has been reported). Surprisingly, just the opposite conclusion can be made on the basis of the reports on the shift of the silanol bands on SiO2: here the methane-induced shift is larger. The results of the present study are in agreement with these observations and allow conclusions about the adsorption geometry. It has also been reported that the isosteric heat of adsorption of methane on silicalite (where interaction with silanol groups is expected) was 20.9 kJ mol-1, a value definitely higher than the heat of adsorption of nitrogen (17.6 kJ mol-1).35 Here we report a detailed FTIR investigation of methane interaction with OH groups of three materials, H− −ZSM-5, HY and SiO2. For comparison, adsorption of nitrogen was also studied. The

15

contrast to the

N2 isotope was used in the experiments because the 14

15

N− −15N stretchings (in

N− −14N modes) are observed outside the region of adsorption of CO2 which

leads to a better quality of the spectra.36,37 Finally, for better interpretation of the spectra, investigations with fully or partly deuteroxylated forms of the samples were also performed. Our results lead to two main new conclusions: (i) methane connected to OH groups is also involved in weak interaction with oxygen ions and the extent of this interaction depends on the local arrangement of the atoms and (ii) at high coverage one acidic zeolite OH group can interact with two methane molecules.

2. Experimental The H−ZSM-5 sample was from Zeolyst and had a Si-to-Al ratio of 25. NaY (Si-to-Al ratio of 2.7) was provided by Grace Davison and was converted first into ammonia form (by ion exchange with NH4NO3 solution) and then to its H-form (by calcination at 673 K). SiO2 was a commercial Aerosil sample with a specific surface area of 336 m2 g-1. The IR measurements were carried out in the 4000–800 cm-1 spectral region using a Nicolet 6700 FTIR spectrometer equipped with a MCT detector. The spectra were registered at a spectral resolution of 1 or 2 cm-1 and accumulating up to 256 scans. Self-supporting pellets (≈ 10 mg cm-2) were prepared by pressing sample powders at ca. 100 kPa and were directly treated in the IR cell. Prior to the adsorption measurements, the samples were activated by heating for 1 h at 673 K under oxygen and evacuation for 1 h at the same temperature. Methane (4.5) was provided by Messer Griesheim GmbH (Germany). Labelled nitrogen (15N2, isotopic purity of 99.7 %) was purchased by Isotec Inc. A. Matheson (USA). D2O originated from Cambridge Isotope Laboratories, Inc. and had isotopic purity of 99.9 %.


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In situ deuteroxylation was achieved by introduction of small amounts of D2O to the samples, followed by evacuation at 673 K. The procedure was repeated until the desired exchange degree was achieved.

3. Experimental Results 3.1. Background spectra of H−ZSM-5, HY and SiO2 and of their deuteroxylated forms The background spectra of our samples are presented in the Supporting Information (Figs. S1-S3) and are consistent with the literature data. Briefly, the spectrum of H− −ZSM-5 contains a sharp band at 3746 cm-1 due to isolated external silanols38-40 (Fig. S1A from the Supporting Information, spectrum a). A low-frequency tail of this band is associated with internal silanols. The intense band at 3612 cm-1 arises from acidic bridging hydroxyls.36-38 Small band at 3663 cm-1 evidences existence of some EFAL species. Finally, a broad feature of low intensity with a maximum centered on 3510 cm-1 (not shown) is associated with H− −bonded hydroxyls. 38-40 The spectrum of HY contains, in the OH region, two very intense bands with maxima around 3638 and 3543 cm-1 attributed to OH groups in the supercages and sodalite cages, respectively41 (Fig. S2A from the Supporting Information, spectrum a). Due to the high intensity of the bands and instrumental limitations, the maxima cannot be determined accurately. A weak feature around 3746 cm-1 (SiOH) was also recorded. For SiO2, a sharp SiOH band at 3746 cm-1 tailed to lower frequencies is dominant in the spectrum (Fig. S3A from the Supporting Information, spectrum a). A broad shoulder centered on ca. 3665 cm-1 is due to H-bonded hydroxyls. Partial or full deuteroxylation of the samples leads to appearance of the respective O− −D bands (Figs. S1-S3 from the Supporting Information, panels B). Due to the relative low intensity of the OH/OD bands on the ZSM-5 sample, the experiments were performed with the fully deuteroxylated forms. In this case bands at 2761 cm-1 (SiOD), 2700 cm-1 (Al-OD) and 2664 cm-1 (Si− −(OD)− −Al) were registered. The HY and SiO2 samples, showing very intense OH bands, were exchanged to ca. 50 %. In the spectrum of partially exchanged HY sample the OH bands decreased in intensity which allowed a more precise detection of their maxima: at 3637 and 3544 cm-1. In the OD



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region two intense bands, at 2683 and 2618 cm-1, were detected. A weak band at 2759 cm-1 (SiOD) was also seen. With partly deuteroxylated SiO2, the silanol band at 3746 cm-1 appeared with reduced intensity and a new SiOD band was detected at 2762 cm-1. Part of the H-bonded hydroxyls on SiO2 resisted isotopic exchange. i.e. they were internal and inaccessible.42 It should be also noted that at low temperature the OH and OD bands are slightly shifted (for the exact wavenumbers see Figs. S1-S3 from the Supporting Information). 3.2. Adsorption of CH4 on H−ZSM-5 and D−ZSM-5 Adsorption of CH4 (1 kPa equilibrium pressure) at 100 K on H− −ZSM-5 leads to disappearance of the band due to bridging hydroxyls (negative peak at 3622 cm-1) and erosion of the silanol band (3750 cm-1). At the same time, two intense new bands are detected at 3703 and 3494 cm-1 (Fig. 1A, spectrum a). Decrease of the coverage caused by decreasing of the equilibrium pressure and evacuation leads to gradual decrease in intensity of the band at 3703 cm-1 (Fig. 1A, spectra b-f). Simultaneously, a component of the silanol band at 3750 cm-1 is initially restored, followed by a component around 3738 cm-1. Therefore, the shift of the different components of the silanol band is in the range between 47 and 35 cm-1. These results are strange because one could expect the more acidic hydroxyls (the higher shift) to hold methane more strongly, while the experimental results were just the opposite. We infer that the phenomenon is due to the fact that the 3738 cm-1 silanols already participate in weak H− −bonding which is broken upon complexation with methane. Upon evacuation the band at 3494 cm-1 is initially shifted to higher frequencies and after that starts to decrease in intensity without significant change in position (Fig. 1A, spectra c-h). Simultaneously, the OH band at 3622 cm-1 is restored. In this case the CH4-induced shift amounts to 107 cm-1. The small blue shift of the band at 3494 cm-1 (to 3515 cm-1) with the decrease of the methane partial pressure resembles the shift observed after adsorption of CO on H− −ZSM-5.37,40 In the latter case it is considered that solvatation effect occurs, i.e. more than one CO molecule is polarized by the same OH group. We adopt the same explanation for the spectra of adsorbed methane and additional proofs of this hypothesis will be provided bellow. The methane induced changes in the OD region registered with a D− −ZSM-5 sample (Fig. 1B) are similar to those already described for the H− −ZSM-5. However, there is one important difference. The OD bands are shifted to a less extent as expected from the OH−OD


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isotopic shift factor of the isolated groups. Similar phenomena has been recently described with other molecular probes39,44 and attributed to the lower acidity of the OD groups as compared to the OH. A careful inspection of the spectra presented in Fig. 1B indicates the same effect is also valid for the shifted silanol groups. However, due to the low acidity of the silanols, it is much less pronounced. In this case the difference between the expected (2733 cm-1) and measured wavenumber (2732 cm-1) is only 1 cm-1. In the C− −H region an intense ν3 band at 3002 cm-1 is detected under 1 kPa equilibrium pressure of CH4 (Fig. 2, spectrum a). This band has well pronounced high- and low-frequency shoulders. Three more bands of lower intensity are also visible at 2900, 2888 and 2817 cm-1 (Fig. 2, spectrum a). These bands are at positions that are too low to be attributed to components of the ν3 mode. Therefore the bands at 2900 and 2888 cm-1 are assigned to the symmetric C− −H stretching modes of methane. The appearance of two bands cannot be due to lowering of methane symmetry (see above) and, therefore, indicates the existence of two different adsorption complexes. The band at 2817 cm-1 is assigned to the ν2 + ν4 combination modes. Decrease of the equilibrium pressure leads to a fast decrease in intensity of the complex band at 3002 cm-1. The band at 2900 cm-1 also quickly disappears (see the inset in Fig. 2). The band at 2888 cm-1 initially rises in intensity with coverage decrease and then starts to decline. The changes of the band at 2900 cm-1 abruptly follow the shifted SiOH band at 3703 cm-1. However, this band cannot be assigned only to SiOH⋅⋅⋅CH4 complexes because of several reasons: (i) a similar band was detected with HDY sample (see below) where the concentration of the silanol groups was negligible and (ii) experiments with SiO2 (see below) have shown that ν1 mode of methane attached to silanol groups was of negligible intensity. Therefore, we infer that only a weak component of the band at 2900 cm-1 is associated with silanols. A closer inspection of the spectra (see the inset in Fig. 2) indicates conversion between the species characterized by bands at 2900 and 2888 cm-1. Comparison with the spectra presented in Fig. 1A allows assigning the bands at 3515 and 2888 cm-1 to OH⋅⋅⋅CH4 complexes. At high coverage a second CH4 molecule interacts with part of the OH group giving rise to the C− −H band at 2900 cm-1 and O− −H band at ca. 3494 cm-1. This interpretation is consistent with the lower shift of the symmetric CH modes related to the gas phase



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spectrum due to the weaker interaction between methane and OH when two molecules of CH4 are involved in the complex. Fig. 3 presents the normalized spectra of methane adsorbed on D− −ZSM-5 at low coverages. It is evident that the intensity of the C− −H modes at 2888 cm-1 correlates with the intensity of the shifted OD band at 2600 cm-1 as well as with the intensity of the negative band due to bridging OD groups at 2672 cm-1. However, there is no correlation with the intensity of the band around 3002 cm-1 and its shoulders. These results indicate that some sorbed methane species that are not located on hydroxyl groups are responsible for the appearance of a part of the absorbance in the 3050 – 2900 cm-1 region. As a first approximation we supposed that at very low coverage methane forms complexes exclusively with the acidic hydroxyls. Thus, it appears that the ν3 mode of CH4 is split into at least three components, at 3055, 3002 and 2987 cm-1 (Fig. 2, spectrum h). This suggests a C2v symmetry. However, it seems possible that the broad band at 3055 cm-1 could be due to another adsorption form and we cannot exclude the symmetry to be C3v. It is known that upon adsorption the ν1 mode of methane is red shifted and the shift value and the extinction coefficient increase with the strength of adsorption.8,11 In our case the intensity ratio between the symmetric and antisymmetric CH modes of methane forming 1 : 1 complexes with the acidic OH groups (low coverage) was calculated to be 1 : 8 which indicates a rather strong perturbation of the CH4 molecule. 3.3. Adsorption of 15N2 on H−ZSM-5 and D−ZSM-5 The results of

15

N2 adsorption on our H− −ZSM-5/D− −ZSM-5 samples are already

reported in details.39 Briefly,

15

N2 is polarized by the different OH/OD groups and causes

shifts of the OH/OD modes. Fig. 4 compares selected spectra of nitrogen and methane adsorbed on the samples. We will emphasize three observations: •

The methane induced shift of the OH and OD modes of the bridging hydroxyls is smaller as compared to the shift induced by adsorption of 15N2.

•

The shift of the silanol OH modes is slightly larger after methane adsorption than after adsorption of 15N2.

•

The increase of the extinction coefficient of the OH/OD modes of the bridging hydroxyls is larger after adsorption of nitrogen (see Fig. S4 from the Supporting Information).


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3.4. Adsorption of CH4 and 15N2 on HDY In order to obtain additional information on the way of methane interaction with acidic hydroxyls, we have studied adsorption of methane on partly deuteroxylated HDY sample. It is well known that the OH groups of HY absorbing around 3640 cm-1 are located in the supercages and are thus readily accessible to interaction with guest molecules. In contrast, the OH groups absorbing around 3540 cm-1 are located in the sodalite cages and are not accessible. However, different researchers have reported that small fraction of these groups can interact with molecular probes.45-47. Adsorption of CH4 (1.5 kPa equilibrium pressure) at 100 K on HDY leads to disappearance of the OH band at 3643 cm-1 and appearance of a new broader band at 3516 cm-1 (Fig. 5, spectrum a). The silanol band is also shifted by 43 cm-1. Decrease of the coverage initially leads to a blue shift of the 3516 cm-1 band, up to 3529 cm-1 (Fig. 5, spectrum d). Similar effects are observed in the OD region, where the spectra are of better quality (Fig. 5, panel B). These phenomena were already explained by interaction of one OH group with two methane molecules. Weak bands at 3579 cm-1 (OH region) and 2637 cm-1 (OD region) are also discernible and are attributed to interaction of methane with Al-OH groups. A careful inspection of the difference spectra suggests a weak negative band around 3550 cm-1. Therefore, it seems probable that a negligible part of the hydroxyls located in the sodalite cages have also interacted with methane. These results are in agreement with literature data indicating few of the sodalite hydroxyls were able to interact with CO.48 Further coverage decrease results in a decrease in intensity of the band at 3529 cm-1 with a gradual shift of its maximum to 3519 cm-1 (Fig. 5, spectra d-i). This shift is attributed to lateral interaction between the adsorbed molecules and is well pronounced with this sample because of the high density of the hydroxyls. Analogous effects were observed in the OD region. The spectra in the C− −H region are similar to, but somewhat more complicated than those described for methane adsorption on the H− −ZSM-5 sample (Fig. 6). Concerning the ν1 mode, again two bands are detected; one at 2903 cm-1 and one at 2891 cm-1 (see the inset in Fig. 6). The band at 2903 cm-1 is highly sensitive to the equilibrium pressure and was already attributed to two CH4 molecules interacting with one OH group. In this case the concentration of the silanol groups is negligible and no contribution from SiOH⋅⋅⋅CH4 complexes is expected. The band at 2891 cm-1 quickly shifts to 2892 cm-1 with the coverage decrease and



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then declines. We attribute the component at 2892 cm-1 to CH4 forming 1 : 1 complexes with the 3643 cm-1 OH groups, while the component at 2891 cm-1, to similar complexes formed with some hydroxyls in the sodalite cages (3543 cm-1). The slightly lower frequency in this case suggests a slightly stronger interaction, although formation of the complexes is sterically hindered. Due to steric reasons, no two CH4 molecules can be simultaneously adsorbed on one OH group from the sodalite cages. At low coverage one band at 3001 cm-1 is detected in the region of the ν3 vibrations. However, the band is broader as compared to the band at 3002 cm-1 registered with the H− −ZSM-5 sample. This indicates a complex character of the 3001 cm-1 band and lowering of methane symmetry. Unfortunately, we cannot conclude on the symmetry point group. At higher coverages a component at 3010 cm-1 becomes clearly observable but there are no proofs that it is associated with OH - CH4 interaction. Finally, we compared the spectra of adsorbed CH4 and

15

N2 (Fig. 7). Here again the

induced shift is of similar value but slightly smaller for CH4. However, it is evident that methane interaction with the sample is stronger, as evidenced by the larger fraction of perturbed silanols (monitored by the intensity of the negative band around 3750 cm-1) under the same equilibrium pressures. It is also evident that, as in the case of H− −ZSM-5 and D− −ZSM-5, the increase of the extinction coefficient of the OH/OD modes is more pronounced after interaction with nitrogen (see Fig. S5 from the Supporting Information). The intensity ratio between the symmetric and antisymmetric CH modes of methane forming 1 : 1 complexes with the acidic OH groups (low coverage) was estimated to be similar to that calculated for H− −ZSM-5 (i.e. 1 : 8). 3.5. Adsorption of CH4 and 15N2 on SiO2 Introduction of CH4 (1 kPa equilibrium pressure) to the partly deuteroxylated SiO2 sample leads to erosion of the silanol band as evidenced by a negative peak at 3750 cm-1 (Fig. 8A, spectrum a). Simultaneously, a broad band at 3714 cm-1 emerged. The results evidence that CH4 is H-bonded to silanol groups thus shifting the band with 36 cm-1. Decrease of the equilibrium pressure leads to a gradual decrease in intensity of the 3714 cm-1 band with a simultaneous restoration of the silanol band at 3750 cm-1 (Fig. 8A, spectra b-f). After ca. 10 min evacuation at 100 K the original spectrum is restored. The changes in the OD region are similar (Fig. 8, panel B). The spectra in the C-H region are presented in Fig. 9. The rotational-vibrational spectrum of methane is highly sensitive to temperature and possibly to pressure. This,


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combined with the low intensity of the bands in this case, leads to a non efficient subtraction of the gas phase. In any case, the spectra clearly show development of two main CH bands at 3009 and 2903 cm-1 (Fig. 9, spectrum a). These bands characterise the antisymmetric and symmetric modes, respectively, of adsorbed methane. A weak band at 2821 cm-1 (not shown) is assigned to the ν2 + ν4 combination mode. The intensity of the ν1 band depends on the strength of adsorption, i.e. on the extent of the molecule perturbation. In our case the intensity of the band at 2903 cm-1 is rather low; it was estimated that at low coverage the ratio between the ν1 and ν3 bands was about 1 : 20. This is much lower as compared to the case of CH4 interacting with acidic hydroxyls of H− −ZSM-5 and HDY (ratio of 1 : 8) and is consistent with the weaker interaction of methane with the silanol groups. The band at 3009 cm-1 is assigned to the antisymmetric modes (ν3) of methane. Second derivative of the spectrum suggest this band consist of two overlapping components at 3012 and 3008 cm-1. Computer deconvolution indicates two more broad components at 3050 and 2970 cm-1 (see the inset in Fig. 9). Unfortunately, due to the weak interaction, we cannot expect that at low coverage methane interacts exclusively with the OH groups. Probably a fraction of adsorbed methane is polarized by surface oxygens which hinders detailed interpretation of the spectra. Introduction of 15N2 (10 Pa equilibrium pressure) to the sample leads to changes in the spectra that are roughly similar to those described after methane adsorption (See Fig. 8). The SiOH and SiOD bands are eroded and shifted to 3718 and 2742 cm-1, respectively. The shifts are similar but slightly smaller than those observed after methane adsorption. These results could indicate that

15

N2 has slightly lower basicity as compared to methane. This is just

opposite to the conclusions made on the basis of interaction of methane and nitrogen with the acidic zeolite hydroxyls. In addition, definitely fewer SiOH/SiOD groups are affected by 15N2 at the same equilibrium pressures of each gas and the complexes with 15N2 are definitely less stable as compared to the methane complexes. This observation is quite surprising because the shift value is considered to be a measure of the acidity and similar shifts presuppose similar stabilities. There is a principal possibility the changes in the CH4 and

15

N2 adsorption

experiments to be partly due to different temperature of the sample in the atmosphere of the two gases. In order to obtain more information on the phenomena, we have studied coadsorption of CH4 and

15

N2 (spectra not shown). At first

15

N2 (1 kPa) was adsorbed and



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then CH4 (0.9 kPa) was added to the system. As a result, the intensity of the 15N− −15N band at 2252 cm-1 decreased ca. two times. The intensity of methane band at 2903 cm-1 was reduced only with ca. 15 % as compared to the spectra registered after adsorption of methane only (1 kPa). Note that in this experiment the temperature is exactly the same and, therefore, the results confirm that methane is more strongly adsorbed on the sample than 15N2. A careful analysis of the spectra shows that, irrespective of the fact that methane causes a higher shift of the OH modes, the increase of the intensity of the shifted band is slightly smaller. This is well illustrated on Fig. 10, where two difference spectra in the OH region (recorded after nitrogen and methane adsorption, respectively) are compared.

4. Discussion There are many data in the literature showing that the shift of the OH stretching modes caused by interaction with diatomic molecules that are weak bases is proportional to the acidity of the hydroxyl.24,37 Moreover, good correlations have been reported on the acidities measured by different diatomic bases. However, both, literature data and our own results indicate that methane does not fit in these correlations. Let us initially suppose that, interacting with acidic zeolite hydroxyls, methane is connected only to the acidic proton. This leads to the conclusion that methane is slightly weaker base than nitrogen. However, looking to the interaction of methane with silanol groups, it seems that the above conclusion is not valid. At first, the methane induced shift of the O− −H stretching modes is slightly higher than the nitrogen-induced shift. In addition, it is evident that methane is much more strongly adsorbed on silanols. However, the slight difference in the shift cannot account for the important difference in the adsorption strength. Therefore, it appears that one methane molecule is bound, in addition to the silanol proton, to another site (or sites) from the surface. The ability of methane to interact with cus surface oxygen atoms is beyond any doubt (see Introduction). Therefore, we conclude that methane sorbed on silanols interacts with some oxygen from silica. Thus, the question about the nature of this oxygen arises. The secondary interaction of methane could occur with a framework surface oxygen atom (not forming OH group). This explains well why methane is more strongly bound than nitrogen, but does not account for the unexpected high shift of the OH stretching modes. Also, in such a case, the increase of the extinction coefficient of the OH modes should be greater as compared to the case of

15

N2 adsorption which is opposite to the experimental findings (see

Fig. 10). Therefore, one can conclude that methane interacts with the oxygen atom from the


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same OH group. Indeed, such interaction would lead to a larger shift of the OH frequency because the O-H bond weakens by two reasons: interaction with hydrogen and interaction with oxygen. However, the polarizability of the OH bond is not so strongly affected which accounts for the lower increase of the OH extinction coefficient as compared to the case of N2 adsorption. The OH bond distance is about 100 pm. The C-H bond length in methane molecule is 108.7 pm and the distance between two hydrogen atoms is around 200 pm. Finally, weak Hbonds are formed on distances above 200 pm. These considerations show that a complex between the OH group and methane molecule simultaneously interacting with oxygen and hydrogen atoms is geometrically possible. Thus, it appears that the symmetry of the adsorbed molecule is Cs. Also, we cannot totally rule out a simultaneous bonding of third methane hydrogen to oxygen from the silica framework, although this seems not probable because of steric reasons. However, because the interaction is rather weak, in both cases one can stress that methane has a slightly distorted Td symmetry. Above we have made the supposition that methane interacted only with the hydrogen atom from the zeolite bridging OH groups. Indeed, due to the high acidity of these hydroxyls, one can expect that the interaction occurs mainly in this way. However, based on the above assumptions one could expect a similar interaction as with the silanols. In any case, if this interaction exists, it should be much weaker as compared to the silanol groups. Similar conclusions are also supported by general considerations: the basicity of the oxygen atom from a hydroxyl group decreases with the increase of the acidity of the hydroxyl. In these cases adsorbed methane should have C3v or slightly distorted C3v symmetry. However, comparison of the spectra of methane adsorbed at low coverages on H− −ZSM-5 and HDY indicates a lower symmetry in the case of H− −ZSM-5. This suggests that, due to the smaller pore volume of the H-ZSM-5 zeolite some interaction with framework oxygen atoms (not from the OH groups) occurs. Due to the relatively strong interaction with acidic hydroxyls, it is possible to attach two molecules of methane to one OH group. This observation seems to be important because adsorption of two molecules on one site leads to a significant increase of the adsorption capacity.



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5. Conclusions •

Interacting with silanol groups methane forms H− −bonds simultaneously with the oxygen and hydrogen atoms of the hydroxyls.

•

Interacting with zeolite acidic hydroxyls methane forms H− −bonds essentially with the hydrogen atom of the hydroxyls. In this case two methane molecules can be simultaneously bound to the same proton.

Acknowledgments: This work was supported by the Bulgarian Scientific Fund (grants DCVP 02/2 and DO 02-184).

Supporting Information: This information is available free of charge via the Internet at http://pubs.acs.org.


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(42) Burneau, A.; Gallas, J.P.; Legrand, A.P. (Eds), The Surface Properties of Silica, Wiley, New York (1998). (43) Mirsojew, I.; Ernst, S.; Weitkamp, J.; Knözinger, H. Catal. Lett. 1994, 24, 235– 248. (44) Chakarova, K.; Hadjiivanov, K. Microporous Mesoporous Mater. 2011, 143, 180–188. (45) Ward, J. M. J. Catal. 1967, 9, 225–236. (46) Romero Sarria, F.; Marie, O.; Saussey, J.; Daturi, M. J. Phys. Chem. B 2005, 109, 1660–1662. (47) Romero Sarria, F.; Blasin-Aube, V.; Saussey, J.; Marie, O.; Daturi, M. J. Phys. Chem. B 2006, 110, 13130–13137. (48) Cairon, O.; Chevreau, T. J. Chem. Soc., Faraday Trans. 1998, 94, 323–330.



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Table 1. Methane induced shift of the OH stretching modes of different hydroxyl groups Sample

OH band

SiO2 SiO2 SiO2 [M]MCM-41

3749 3750 3750 3740

CH4-induced shift, cm-1 32 32 36 20

H− −ZSM-5 H− −ZSM-5 Al2O3

3740 3748 3750 3665 3760 3601 3616 3622 3607 3605 3622 3635 3640 3607

37 47 43 25 53 ca. 110 106 112 57 45 107 116 113 101

Al2O3 H− −ZSM-5 H− −ZSM-5 H− −ZSM-5 H− −ZSM-5 H− −ZSM-5 H− −ZSM-5 HY H− −MOR

Note

Ref.

N2-induced shift of 24 cm-1

21 20 this work The shift seems larger on the 9 Figures SiOH groups 20 SiOH groups 20 18 N2-induced shift of 120 cm-1

room temperature experiments 288 K, 750 Torr 128 cm-1 at high coverage 127 cm-1 at high coverage

20 18 20 26 12 23 this work this work 43

Table 2. Vibrational modes of methane molecule with different symmetries Mode ν1 (symmetric stretching) ν2 (deformation mode) ν3 (antisymmetric stretching) ν4 (bending mode)

Gas phase frequency (Td symmetry) 2914 cm-1, IR inactive 1526 cm-1, IR inactive 3020 cm-1, IR active 1306 cm-1, IR active

Number of modes in C3v symmetry one mode

Number of modes in C2v or Cs symmetry one mode

one mode

two modes

two modes

three modes

two modes

three modes


Page 19 of 29


19

FIGURES

3800

3600

3400

- 3494

A

3750 3738 -

- 3622

- 3515

- 3703

0.1

a

h

B

- 2582

Absorbance

- 2732

0.1

2600 -

- 2671

i

2764 2754 -

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2800

p

2600

Wavenumber, cm

-1

Fig. 1. Changes in the FTIR spectra of H− −ZSM-5 (panel A) and D− −ZSM-5 (panel B) after adsorption of methane at 100 K. Panel A: Equilibrium CH4 pressure of 1 kPa (a) and debvelopment of the spectra during evacuation at 100 K (b-h). Panel B: Equilibrium CH4 pressure of 1 kPa (i) and development of the spectra during evacuation at 100 K (j-p).



Page 20 of 29

- 2900

- 2888

20

0.02

d

a b d

g h

a

2900 2880 - 3002

Absorbance

- 2900 - 2888

e

h

3200

2987 -

- 2817

f

3055 -

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3000

Wavenumber, cm

2800 -1

−ZSM-5. Equilibrium CH4 Fig. 2. FTIR spectra (C− −H region) of CH4 adsorbed at 100 K on H− pressure of 1 kPa (a) and evacuation at 100 K (b-h). The spectra correspond to the spectra presented in Fig. 1A and are background and gas-phase corrected.


Page 21 of 29


- 2888

Absorbance

- 3002

- 2600

21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a d

- 2672

3200

3000

2800

2600

Wavenumber, cm

2400

-1

Fig. 3. Difference FTIR spectra of CH4 adsorbed at 100 K on D− −ZSM-5. The spectra are registered at low coverages and are normalized according to the intensity of the band at 2600 cm-1. Coverage decreases in the sequence a – d.



Page 22 of 29

22

3800

3700

3600

3500

3400 15

0.1

a

3495

- 3619

N2

- 3667

- 3709

- 3749

A

e

- 3620

CH4 f

- 3667

- 3706

- 3749

3510

j

B

0.1

15

2584

- 2670

a

- 2706

- 2734

N2

- 2764

Absorbance

e

- 2671

CH4 f

- 2707

- 2733

2596 - 2764

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

j

2800

2700

2600

Wavenumber, cm

-1

Fig. 4. Selected FTIR spectra of 15N2 and CH4 adsorbed at 100 K on H− −ZSM-5 (panel A) and D− −ZSM-5 (panel B) samples.


Page 23 of 29


23

3800

3600

3400

A

3516 3529

a

3579

3519

0.2

Absorbance

i

- 3643

B - 2603

0.2

2603 -

a

2637 -

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

i

- 2687

2800

2600 -1

Wavenumber, cm

Fig. 5. Changes in the FTIR spectra of HDY after adsorption of CH4 at 100 K: Equilibrium CH4 pressure of 1.5 kPa (a) and development of the spectra during evacuation at 100 K (b-i). The spectra are background and gas-phase corrected. Panel A: OH region; Panel B: OD region.



Page 24 of 29

0.05 a b

2 - 289 - 289 1

- 2903

24

c d

f

3010

Absorbance

2900 a

- 2903 - 289 1

b c

- 2819

d

e

i

3001 -

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3071

3200

3000

Wavenumber, cm

2800 -1

Fig. 6. FTIR spectra (C-H region) of CH4 adsorbed at 100 K on HDY. Equilibrium CH4 pressure of 1.5 kPa (a) and evacuation at 100 K (b-i). The spectra correspond to the spectra presented in Fig. 5 and are background and gas-phase corrected.


Page 25 of 29


25

3800

3600

3400 3520

A

a

0.2

3579 -

3519 -

CH4 h

3516

i

3564 3643 -

3511 -

15

3573 -

Absorbance

N2

n

3641 -

B

2605

0.2

2603 -

2637 -

a

CH4 h

2607

i 2599 -

2687-

2645 -

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

15

N2

n

2685-

2800

2600

Wavenumber, cm

-1

Fig. 7. Changes in the FTIR spectra of partly deuteroxilated Y zeolite (H-D exchange degree of ca. 50 %) after adsorption of CH4 and 15N2 at 100 K. Equilibrium CH4 pressure of 1000 Pa (a) and after evacuation at 100 K (b-h). Equilibrium 15N2 pressure of 1000 (i), 500 (j), 250 (k) and 100 Pa (l) and under dynamic vacuum (m, n). The spectra are background and gas-phase corrected. Panel A: OH region; Panel B: OD region.



Page 26 of 29

26

3700

3650

- 3714

3750

A

a

CH4 f

- 3718

0.1

3750 -

g

15

N2

j

- 2740

Absorbance

B

a

CH4

f

0.1 - 2742

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2764 -

15

N2

g j

2750

Wavenumber, cm

2700 -1

Fig. 8. Changes in the FTIR spectra of partly deuteroxilated silica (H-D exchange degree of ca. 50 %) after adsorption of CH4 and 15N2 at 100 K. Equilibrium CH4 pressure of 1000 (a), 500 (b), 250 (c) and 100 Pa (d) and after evacuation at 100 K (e, f). Equilibrium 15N2 pressure of 1000 (g), 500 (h), 250 (i) and 100 Pa (j). Panel A: OH region; Panel B: OD region.


Page 27 of 29


27

- 3009

a'' 3012 - - 3008

- 2903

2970

- 3050

- 3009

0.02

a

a

Absorbance

3000

- 2903

- 3050

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

f

3200

3100

3000

Wavenumber, cm

2900 -1

Fig. 9. FTIR spectra (C− −H region) of CH4 adsorbed at 100 K on partly deuteroxilated silica (H-D exchange degree of ca. 50 %) after adsorption of CH4 at 100 K. Equilibrium CH4 pressure of 1000 (a), 500 (b), 250 (c) and 100 Pa (d) and after evacuation at 100 K (e, f). The spectra correspond to the spectra presented in Fig. 8 and are background and gas-phase corrected.



Page 28 of 29

28

3760

3720

3680

- 3718

A

0.05

N2

3753 -

3715 -

15

CH4

3748 -

B

0.05

- 2742

Absorbance

15

N2

2740 -

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2767 -

CH4

2763 -

2780

2760

2740

2720

Wavenumber, cm

2700

-1

Fig. 10. Selected FTIR spectra of CH4 and 15N2 adsorbed at 100 K on SiO2 exchanged with deuterium about 50 %. Panel A: OH region; Panel B: OD region.


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