J. Phys. Chem. B 2000, 104, 1269-1276
1269
Low-Temperature Adsorption of N2, O2, and D2 on LiX, NaX, and NaLiX Zeolites Studied by FT-IR Spectroscopy Kirill M. Bulanin and Raul F. Lobo* Center for Catalytic Science and Technology, Department of Chemical Engineering, UniVersity of Delaware, Newark, Delaware 19716
Michael O. Bulanin Department of Physics, UniVersity of St.-Petersburg, St.-Petersburg 198904, Russia ReceiVed: August 20, 1999; In Final Form: NoVember 21, 1999
FT-IR spectra of N2, O2, and D2 adsorbed on the dehydrated LiX, NaLiX, and NaX zeolites are studied at 77 K. The measured shifts of the bond-stretching frequencies and the integrated vibrational fundamental band intensities for admolecules are compared with the results of the recent high-level ab initio calculations. The effective electric fields in zeolite cavities are also reported.
1. Introduction Infrared spectroscopy is a useful technique for the studies of the interaction processes and surface reactions involving molecular adsorbates and cationic sites in zeolites. Numerous recent publications have revealed a wealth of valuable information on the behavior of simple molecular species trapped in zeolites of different structure and ionic composition. Particularly suitable as molecular probes are homonuclear diatomics for which the vibrational transitions are infrared-inactive in the gas phase. Adsorption in zeolite cavities lowers the symmetry of these molecules and results in the appearance of interaction-induced IR spectra that are highly sensitive to the geometry and details of the guest-host interactions. Transmission FTIR spectroscopy studies have been reported for H2, D2, N2, and O2 molecules adsorbed on zeolites NaA,1-4 CaA,3,5 NaY,6,7 ion-exchanged ZSM-5,8-10 and mordenites.11 Spectra of H2 and D2 adsorbed on different cationic forms of zeolites, including X-form zeolites, were also obtained by the low-temperature DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) technique.12-15 Further relevant references could be found in recent reviews by Sauer et al.16 and Kno¨zinger and Huber.17 Low-silica LiX, NaX, and mixed LiNaX (67% Li, 33% Na) zeolites studied in the present work are large-pore (≈ 7.4 Å) alumosilicates with a Si/Al ratio of nearly one. Figure 1 shows an illustration of the zeolite structure indicating the typical location of the extraframework cation sites. The unit cell contains 96 [SiO4/2] and 96 [AlO4/2] tetrahedra arranged in strict alternation,43 the charges of the alumina tetrahedra balanced by the equal number of extraframework alkali cations (in our case 96 Na+, 96 Li+ or mixtures thereof). In their dehydrated form, these zeolites are used commercially for drying gases and solvents, and for the selective adsorption of nitrogen. The structure of the dehydated lithium, sodium, and mixed forms of these materials has been investigated in the past by diffraction43,45-47 and NMR spectroscopy.47-50 Among metal cations located at different crystallographic sites in LiX, only those occupying sites SIII in the 12-ring windows connecting * Corresponding author:
[email protected], fax (302) 831-2085.
Figure 1. Cation positions in zeolite faujasite.
adjacent supercages were found to be accessible to adsorption of the N2 and O2 molecules at room temperature.49 Mixed NaLiX zeolite sample has lithium cations in inaccessible sites SI′ and SII, and accessible sodium cations in sites SIII. Solidstate NMR studies have shown SIII cations in both LiX and LiNaX to become mobile at room temperature.47,50 Accessible to adsorption in NaX are sites SII and SIII.44 Table 1 presents the site populations data for Na+ and Li+ in zeolite X (Si/Al ) 1) for different levels of ion exchange. This paper describes the results of an experimental FT-IR study of interaction-induced vibrational spectra for nitrogen, oxygen, and deuterium molecules adsorbed on X-form zeolites at 77 K. We show that the observed gross structure of the spectral bands does support the conclusion that accessible to adsorption are a single class of sites in NaLiX and two classes of sites in NaX. Strong coverage-dependent variation of the spectra is demonstrated for N2 and D2 adsorbed on LiX. The effects due to interaction between coordinately bonded molecules (lateral interactions) have been revealed in a number of previous works (see, e.g., refs 51, 52 and references therein). We suggest the mobility of the accessible cationic sites in LiX
10.1021/jp992949h CCC: $19.00 © 2000 American Chemical Society Published on Web 01/20/2000
1270 J. Phys. Chem. B, Vol. 104, No. 6, 2000
Bulanin et al.
TABLE 1: Site Populations for Na+ and Li+ in Zeolite X (Si/Al ) 1) for Different Levels of Ion Exchangea SI
SI′
SII
TABLE 2: Chemical Analysis of the Samples Used in This Study
composition (cations/unit cell)
Li
Na
Li
Na
Li
Na
Li
SIII Na
Li0 Na96b Li64Na32c Li96 Na0b
-
4 -
32 32
26 -
32 32
32 -
32
32 32 -
All data obtained from 23Na and 7Li NMR spectroscopy at room temperature from dehydrated samples.50 b Site populations obtained directly from experimental data. c Site populations estimated from experimental data in ref 50.
LiX NaLiX NaX LiX NaLiX NaX
a
to be a factor that favors an enhancement of lateral interactions. Finally, we compare our experimental data with the results of a recent high-level ab initio study.28 2. Theoretical Section Adsorbate-adsorbent interaction causes a frequency shift of the IR band corresponding to the well-isolated vibrational stretching mode of the diatomic admolecule with respect to that of the free molecule. The appearance of a structured spectral band demonstrates the existence of several distinct adsorption sites, provided the residence time of the admolecule on a given site is long enough. Qualitative discussion of the spectral band structure and analysis of the frequency shifts were the aspects commonly approached in most of the studies. The band shift is easy to measure; however, its quantitative interpretation in terms of the interaction energy is far from being straightforward. The shift originates from the dependence of the perturbing potential on the internal vibrational coordinate of the molecule.18,19 This dependence is really not well known at present even for the isolated molecule-atom pairs (e.g., in the case of binary collisions in gases), especially so in the repulsive branch of the potential. An often implicitly invoked assumption that larger band shifts indicate stronger interaction may thus be misleading. In view of far more complicated multidimensional potential energy surfaces describing interaction of an admolecule with the nearest counterion, with the surrounding zeolite framework, and with other molecular species trapped nearby at high coverages, calculations of the vibrational band shifts employing simplified model potentials (see, e.g., refs 1-4, 6, 9) appear to be hardly promising. Nevertheless, some important results have been obtained from analysis of the band shifts concerning the preferential orientation of admolecules with respect to the nearest metal cations at low temperatures. The orientation was found to be governed by the electrostatic interaction between permanent and induced electric multipoles of the molecule and the electric field generated predominantly by the nearest positively charged cationic site.1,2,9 Free centrosymmetric diatomics possess only even-rank multipole moments (see Table 3 below). For N2, having a large negative quadrupole moment,20 the stable orientation to small cations (Li+ and Na+) is with the molecule axis along the direction to the cationic site, which corresponds to C∞V local symmetry of the adsorption center. This is consistent with the observed highfrequency shift of the N2 fundamental band relative to that for the free molecule. In contrast, the positive multipole moments of H2 (D2)21,22 result in a perpendicular orientation of the molecule axis and C2V adsorption site symmetry, consistent with the low-frequency band shift upon adsorption. These findings are in accord with the results of several ab initio studies for the isolated X2‚‚‚Li+ or Na+ complexes (X ) H, D, N).23-29 Information on the O2 orientation remains less certain. The band shifts in the spectra of adsorbed oxygen are close to
Si (%)
Al (%)
Li (%)
Na (%)
15.92 15.68 15.46
15.19 14.88 14.66
4.43 3.70
