Verified Syntheses of Zeolitic Materials

2nd Revised Edition

Characterization by IR spectroscopy

Hellmut G. Karge
Fritz-Haber-Institut der Max-Planck-Gesellschaft, D-14195 Berlin, Germany

1. Techniques

The KBr pellet technique is frequently used for investigations of vibrations of the framework. [1-3] Also well-established are (i) infrared (IR) spectroscopy of self-supporting wafers (usually in transmission mode [4]) and (ii) diffusive reflectance IR Fourier transform (DRIFT) spectroscopy of zeolite powders. [5] Both methods (i) and (ii) enable us to investigate cation vibrations in the far IR region [6, 7], to employ heat treatment, to achieve complete dehydration (in high vacuum or a stream of inert gas) and, if desirable, to admit probes (see below). Also, the (thermal) stability of zeolitic materials against, for example, dehydration, dehydroxylation and interaction with sorbates, may be characterized by (in situ) transmission IR or DRIFT. In the case of method (i) it is easier to carry out quantitative measurements, while method (ii) is in many instances more sensitive.

2. Framework vibrations

Vibrations of the frameworks of zeolites give rise to typical bands in the mid and far infrared. A distinction is made between external and internal vibrations of the TO₄/₂ tetrahedra (with, for example, T = Si or Al). The original assignments of the main IR bands [1] were as follows:internal tetrahedra 1250 - 920 cm^-1, asymmetrical stretch (n_asym); 720 - 650 cm^-1, symmetrical stretch (n_sym; 500 - 420 cm^-1, T-O bend; external linkages: 650 - 500 cm^-1, double ring vibrations; 420 - 300 cm^-1, pore opening vibrations; 1150 - 1050 cm^-1, asymmetrical stretch; 820 - 750 cm^-1, symmetrical stretch. The positions of bands due to vibrations of external linkages are often very sensitive to structure. More recent detailed analysis, however, showed that these assignments may have to be revised in some respect. Computation of normal modes suggested that the concept of strictly separated external and internal tetrahedral vibrations must be modified in that zeolite framework vibrations appear to be strongly coupled. Similarly, the proposed pore opening vibrations seem to be related to a complex motion which in total includes an opening (rupture) of the rings (4-rings, 6-rings) of the structure. [2] Nevertheless, clear-cut linear relationships were found between the frequency of, for example, n_asym and n_sym of X- or Y-type zeolite and the atom fraction of Al (related to the ratio n_Si/n_Al) of the framework. [1] Thus, the frequencies of lattice vibrations could be used in certain cases to determine the n_Si/n_Al ratio of the framework in a similar manner as the lattice parameters obtained by X-ray diffraction as a function of this ratio. Also, shifts of the bands of lattice vibrations are characteristic of cation movements upon dehydration. [1] In several cases a typical lattice vibration is observed around 950 cm^-1, indicating the isomorphous substitution of framework Si, Al by other T atoms. The most prominent example is Ti substituted for Si into silicalite (TS-1, TS-2). [8]

3. Cation vibrations

In the far infrared region (200 - 50 cm^-1) vibrations of cations against the framework occur. [6, 7] The wave number of the corresponding IR bands depend on the nature of the cations as well as on their siting. The IR bands of alkali metal-exchanged zeolites X, Y and ZSM-5 shift to lower frequencies (red shift) in the sequence of Na⁺, K⁺, Rb⁺, Cs⁺, that is, with increasing cation mass. [6, 7]

4. Extra-framework species

Extra-framework species such as aluminium-containing entities, (for example Al_xO_yⁿ⁺), which occur upon dehydroxylation, [9] may be detected and quantitatively determined by IR spectroscopy when suitable probe molecules are employed as adsorbates. [10,11] The most frequently used probe is still pyridine. Pyridine attached to Lewis acidic centres such as Al_xO_yⁿ⁺gives rise to a typical band at about 1450 cm^-1. Adsorption of pyridine on cations of zeolite structures produces bands in the range from 1438 to 1452 cm^-1. The exact positions of these bands depend on the nature of the cation, that is, on the Coulomb potential q/r (q: electric charge; r: radius of the respective cation). [10] In cases where the pyridine molecule is too bulky to have access to the extra-framework species, smaller probe molecules such as NH₃, CO, CH₃CN, H₂, N₂ can be used advantageously. [5,11,12]

5. Hydroxyl groups

Hydroxyl groups attached to zeolite structures are most important for the chemistry of these materials. They may be detected and characterized by IR spectroscopy as such due to their vibration modes (OH fundamental, overtone and combination vibrations) or with the help of probe molecules (see preceding paragraph and Refs. [5, 11, 12]). A distinction is made among (i) lattice termination silanol groups, (ii) hydroxyl groups occurring at defect sites (hydroxyl nests), (iii) OH-groups attached to extra-framework T atom-containing species (iv) OH groups attached to multivalent cations which compensate the negative charge of the framework and, most importantly, (v) bridging OH-groups (such as ≡Al (OH) Si≡ groups with Bronsted acidic character). Hydroxyls of type (i), (ii), (lii), (iv) and (v) give rise to bands in the fundamental stretch region at about 3740, 3720, 3680, 3580 - 3520 and 3600 - 3650 cm^-1 (free bridging OH-groups), respectively. Bridging OH-groups exhibiting additional electrostatic interactions to adjacent oxygens are indicated by lower wavenumbers, for example, at ca. 3550 cm^-1 in the case of the hydrogen forms of faujasite type (X and Y) zeolites and at 3520 cm^-1 in the case of H-ZSM-5. Moreover, the fundamental stretching vibrations of free bridging OH-groups, which are responsible for important catalytic properties of the hydrogen forms of zeolites, depend on the nature of the T atoms in the ≡T(OH)Si≡ configuration. It is, for instance, frequently observed that the respective wavenumbers decrease in the sequence T = Al, Ga, Fe, B. [13] From the intensity (integrated absorbance) of the bands being typical of the above-mentioned types of hydroxyls, the density (concentration) of the corresponding entities can be estimated. To obtain the absolute data the appropriate extinction coefficients must be determined through independent measurements. [14] This is particularly valuable in the case of defect sites and acid Bronsted (bridging) OH groups. Similarly, there seems to exist a correlation between the wave number indicative of bridging OH groups of zeolites and their acidic strength. [15]

Investigation of the overtone and combination vibrations of hydroxyls via DRIFT spectroscopy is a valuable means for characterization of zeolite materials since it frequently reveals more detailed features than are obtained from the fundamental stretch region. [16]

Adsorption of probes such as animonia, pyridine or less basic molecules (such as benzene CO, alkanes, C₂Cl₄H₂, N₂, etc.) also enables zeolitic OH^- groups, especially acidic Bronsted sites, to be characterized. Pyridine, for instance, produces a band around 1540 cm^-1 when adsorbed on Bronsted acid sites, resulting in the formation of pyridinium ions. In view of the bulkiness of pyridine and its strong basicity (low selectivity with respect to acidic strength of the Bronsted sites), other probes should sometimes be preferred. Finally, it should be mentioned that Lewis and Bronsted acidity of zeolites is often advantageously characterized by a combination of IR spectroscopy and other techniques such as temperature-programmed desorption or microcalorimetric measurements of adsorbed probe molecules.

6. Adsorbates

IR spectroscopy is extensively used to characterize zeolite/adsorbate systems. Adsorption and desorption of water (hydration and dehydration) may be easily monitored by IR, since adsorbed H₂O gives rise to a typical deformation band around 1640 cm^-1. Adsorbed or occluded template molecules (or their decomposition products) are detectable by, for example, CH and/or NH vibration bands.

7. References

[1] E. M. Flanigen, H. Khatami, H. A. Seymenski, in Adv. Chemistry Series 101, E. M. Flanigen, L B. Sand (eds.), American Chemical Society, Washington, D. C. 1971, pp. 201-228
[2] E. Geidel, H. Böhlig, Ch. Peuker, W. Pilz, in Stud. Surf. Sci. Catalysis 65, G. Ohlmann, H. Pfeifer, R. Fricke (eds), Elsevier, Amsterdam, 1991, pp. 511-519
[3] F. Bauer, E. Geidel, Ch. Peuker, W. Pilz, Zeolites 17 (1996) 278
[4] H. G. Karge, W. Niessen, Catalysis Today 8 (1991) 451
[5] V. B. Kazansky, V. Y. Borovkov, H. G. Karge, J. Chem. Soc., Faraday Trans. 93 (1997) 1843
[6] I. A. Brodskii, S. P. Zhdanov, in Proc. 5th Int. Zeolite Conf, Naples, L V. Rees, (ed.), Heyden, London, 1980, pp. 234-241
[7] H. Esemann, H. Forster, F. Geidel, K. Krause, Micropor. Mater. 6 (1996) 321 (and references therein)
[8] G. Perego, G. Bellussi, G. Corno, M. Taramasso, F. Buonomo, A. Esposito, in Stud. Surf. Sci. Catalysis 28, Y. Murakanii, A. lijima, J. W. Ward (eds.), Elsevier, Amsterdam, 1986, pp. 129-136
[9] G. H. Kuhl, J. Phys. Chem. Solids 38 (1977) 1259
[10] J. W. Ward, in ACS Monograph 171, J. A. Rabo (ed.), American Chemical Society, Washington, D. C., 1976, pp. 118-284
[11] H. Forster in Spectroscopic and Computational Studies on Supramolecular Systems, J. E. Davies (ed.), Kiuver Academic, Dordrecht, 1992, pp. 29-60
[12] V. B. Kazansky, in Stud. Surf. Sci. Catalysis, 65, H. Pfeifer, R. Fricke (eds.), Elsevier, Amsterdam, 1991, pp. 117-131
[13] V. A. Tuan, PhD Thesis, free University of Berlin, 1994
[14] S. Khabtou, T. Chevreau, J. C. Lavalley, Micropor. Mater. 3 (1994)133
[15] E. Brunner, H. G. Karge, H. Pfeifer, Z. Phys. Chemie, 176 (1992) 173
[16] K. Beck, H. Pfeifer, B. Staudte, Micropor. Mater. 2 (1993) 1