Isomorphous Substitution in Zeolite Frameworks. 1. Acidity of Surface

J. Phys. Chem. 1985,89, 1569-1571

1569

Isomorphous Substitution in Zeolite Frameworks. 1. Acidity of Surface Hydroxyls in [B]-, [Fe]-, [Gal-, and [AI]-ZSM-5 Cynthia T-W. Chu and Clarence D. Chang* Mobil Research and Development Corporation, Central Research Division, Princeton, New Jersey 08540 (Received: December 6. 1984)

A series of isomorphously framework-substituted ZSM-5 zeolites has been characterized by Fourier transform infrared spectroscopy, with primary focus on the v(OH) region, and by temperature programmed NH3 desorption. The protonic form of the zeolites exhibited two characteristic absorptions in the v(0H) region: a band at 3740 cm-' assigned to terminal SiOH, and a band at lower frequency assigned to M(0H)Si (bridging hydroxyl), whose frequency depends on acid strength. Based on these techniques, the relative Bronsted acidity is found to increase accdrding to SiOH C B(0H)Si CC Fe(0H)Si C Ga(0H)Si C Al(0H)Si.

Introduction Isomorphous substitution of elements such as Ga, Ce, Be, B, Fe, Cr,P, and Mg for Si or A1 in zeolite frameworks has been recently reviewed by Barrer.' In tectosilicate frameworks, isomorphous substitution with a trivalent element induces a negative charge which must be balanced by a cation. Bronsted acidity evolves when the charge compensating cation is a proton. The Bronsted site per se consists of a bridging hydroxyl between Si and the substituent element, with a dative bond involving a pair of unshared electrons of 0 and an unoccupied orbital of the substituent element. This paper gives the results of an infrared study of the v(0H) region of several isomorphously substituted ZSM-5 zeolites. These M(0H)Si sites are also characterized by temperature-programmed N H 3 desorption (TPAD). The crystal structure and structure-related properties of [Al]-ZSM-5, including sorption and shape selectivity in catalysis, are well The Al(0H)Si site in the [All-ZSM-5 framework has been characterized by TPAD,5t6,'9transmission IR,597719diffuse reflectance IR,8 'H MAS NMR,6 and 27AlMAS NMR.9*'o These data provide persuasive evidence that the framework acid sites in ZSM-5 are homogeneous and purely Bronsted in character. Although non-A1 isomorphs of ZSM-5 have been known for some time," little information concerning their acidic properties has been reported to date. Scholle et aL6 applied TPAD and 'H MAS N M R in examining H[B]-ZSM-5 and concluded that the B(0H)Si site was more acidic than SiOH but weaker than Al(0H)Si. Experimental Section

Materials. [A]-ZSM-5 and [Gal-ZSM-5 were synthesized by the method of Argauer and Landolt.I2 [B]- and [Fe]-ZSM-5 were synthesized according to the same method but replacing the (1) Barrer, R. M. 'Hydrothermal Chemistry of Zeolites"; Academic Press: London, 1982; p 251. (2) Olson, D. H.; Kokotailo, G. T.; Lawton, S. L.; Meier, W. M. J . Phys. Chem. 1981,85, 2238. (3) Wcisz, P. B. Pure Appl. Chem. 1980, 52, 2091. (4) Derouane, E. G. Stud. Surf. Sci. Catal. 1980, 5, 5. (5) Topme, N . Y.;Pedersen, K.; Derouane, E. G. J . Card. 1981, 70,41. (6) Scholle, K. F. M. G. J.; Kentgens, A. P. M.; Veeman, W. S.; Frenken, P.; van der Velden, G. P . M. J. Phys. Chem. 1984, 88, 5. (7) Jacobs, P. A.; von Ballmoos, R. J . Phys. Chem. 1982, 86, 3050. (8) Kazanskii, V. B.; Minachev, Kh. M.; Nefedov, B. K.; Borovkov, V. Yu.; Kondrat'ev, D. A.; Chukin, G. D.; Kustov, L. M.; Bondarenko, T. N.; Konoval'chikov, L. D. Kinet. Katal. 1983, 24, 679. (9) Fyfe, C . A.; Gobbi, G. C.; Klinowski, J.; Thomas, J. M.; Ramdas, S. Nature (London) 1982, 296, 530. (10) Kentgens, A. P. M.; Scholle, K. F. M. G. J.; Veeman, W. S. J . Phys. Chem. 1983,87, 4357. (11) Belgium Patent 871 893, 1979. (12) Argauer, R. J.; Landolt, G. R. US. Patent 3702886, 1972.

0022-3654/85/2089-1569$01 S O / O

A1 source with H3B03or with Fe(N03)3. The zeolite compositions, in terms of framework oxide ratios (M203/Si02)calculated on the basis of measured NH,+ exchange capacities were 85,98, 198, and 70 for M = B, Fe, Ga, and Al, respectively. Apparatus and Procedures. Fourier transform infrared spectra were recorded with a Nicolet 7199 interferometer. The sample cell design was similar to that described by Moon et al.I3 Temperature programmed N H 3 desorption studies were conducted using a duPont 95 1 thermogravimetric analyzer equipped with an automatic titrimeter assembly, following the procedure of Kerr and Chester.14 A constant heating rate of 20 OC/min was maintained during TPAD measurements.

Results and Discussion Infrared Obsemations. The v(0H) region of the IR spectrum of H[Al]-ZSM-5 contains bands at around 3610 and 3740 cm-'. The former has been assigned to an acidic bridging O H while the latter is attributed to terminal SiOH or extraframework silica A band at 3720 cm-' has also been reported7J5 and has been attributed either to a weak Bronsted siteIs or to extrazeolitic material.' This band is absent in highly crystalline preparations. The IR spectra of H[B]-, H[Fe]-, and H[Ga]-ZSM-5 are also characterized by two well-defined v(0H) bands. All three spectra contain a 3740-cm-' band in common with H[Al]-ZSM-S, but the second O H band is shifted by varying degrees toward higher frequencies as compared against the 3610-cm-' band of H[AI]-ZSM-5. AS seen in Figure 1, the frequency of this second band is 3620 cm-' for H[Ga]-, 3630 cm-l for H[Fe]-, and 3725 cm-' for H[B]-ZSM-5. In the H[Ga]-ZSM-5 spectrum the 3740-cm-' band is somewhat asymmetric and a broad band near 3500 cm-' is evident. This is believed due to the presence of amorphous silica in the sample. The band near 3500 cm-' is attributed to hydrogen bonding between adjacent hydroxyl The above spectra were obtained at room temperature after outgassing at 500 OC and 5 X torr. The samples were then exposed to NH3 and again evacuated to remove excess NH3. (13) Moon, S. H.; Windawi, H.; Katzer, J. R. Ind. Eng. Chem. Fundum. 1981, 20, 396.

(14) Kerr, G. T.; Chester, A. W. Thermochim. Acta 1971, 3, 113. (15) Vedrine, J. C.; Auroux, A,; Bolis, V.; Dejaifve, P.; Naccache, C.; Wierzchowski, P.; Derouane, E. G.; Nagy, J. B.; Gilson, J.-P.; van Hooff, J. H. C.; van den Berg, J. P.; Wolthuizen, J. J. C a f d . 1979, 59, 248. (16) Cant, N. W.; Little, J. H. Can. J. Chem. 1965, 43, 1252. (17) Olovsson, I.; Templeton, D. H. Acfa Crystallogr. 1959, 12, 827. (18) Wu, E. L.; Brigandi, P. W.; Rohrbaugh, W. J.; Landolt, G. R.; Kuehl, G. H., unpublished research. (19) Vedrine, J. C.; Auroux, A,; Coudurier, G. Am. Chem. SOC.Symp. Ser. 1984, No. 248, 253. (20) McDonald, R. S. J. Phys. Chem. 1958, 62, 1168. (21) Ghiotti, G.; Garrone, E.; Morterra, C.; Boccuzzi, F. J. Phys. Chem. 1979,83, 2863.

0 1985 American Chemical Society

1570 The Journal of Physical Chemistry, Vol. 89, No. 9, 1985 I

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Figure 5. FTIR spectra of [B]-ZSM-5: (a) H[B]-ZSM-5; (b)-(e) NH4[B]-ZSM-5 at various PNH3,torr: (b) 5 X (c) 8 X (d) 3 X (e) 1.5 X

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Figure 3. FTIR spectra of [Gal-ZSM-5.

Spectra of the ammoniated samples of [All-, [Gal-, and [Fel-ZSM-5 are displayed in Figures 2-4. The effect of NH,

sorption on the IR spectrum of H[B]-ZSM-S will be discussed separately below. Included for comparison in these figures are the spectra of the protonic forms. It is seen that upon ammoniation the low-frequency v(0H) bands disappear, concomitant with the appearance of v(NH) bands a t 3383, 3294, and 2940 cm-' due to NH4+ and a strong band at 1464 cm-' in the 6(NH) region. There appears to be a small decrease in the intensity of the 3710-cm-' band of [All- and [Fel-ZSM-5 upon ammoniation, with the appearance of a small shoulder in the vicinity of 3720 cm-I, while no change is observable in the 3740-cm-' band of [Ga]-ZSM-5. Weak bands at 2162 and 1675 cm-' also apear and are probably composites of the strong absorption at 1464 cm-l. These ammoniation results confirm that the low-frequency v(0H) band of H[Ga]- and H[Fe]-ZSM-5 is due to acidic hydroxyl. The fact that these bands are at higher frequencies than the 3610-cm-' band of H[Al]-ZSM-5 indicates that the OH bond in the Ga(0H)Si and Fe(0H)Si sites are more covalent'than the [All isomorph and hence that these sites are less acidic. The small decrease in the absorption intensity of the 3740-cm-' band upon ammoniation may be due in part to an interaction between the terminal silanol groups and neighboring NH,+. Ammoniation of Cabosil silica results in a reduction in the intensity of the 3747-cm-I surface hydroxyl and due to hydrogen bonding of NH3.l6 However, the original band intensity is regained upon evacuation at room temperature.I6 Somewhat stronger association is indicated in the present case. An alternative hypothesis might be the clathration of NH3 within a tetrahedral "hydroxyl nest". The resulting configuration would be similar to that of an N H 3

The Journal of Physical Chemistry, Vol. 89, No. 9, 1985 1571

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molecule in crystalline NH3-H20,which has been found to reside in the center of a tetrahedron formed by four OH groups.'' The behavior of [B]-ZSM-5 was somewhat different upon applying the above ammoniation procedure. As with the other

ZSM-S's, sorption of N H 3 resulted in the immediate elimination of the low-frequency v(0H) band and a decrease in the 3740-cm-' band intensity. However, it was observed that the low-frequency u(0H) band gradually emerged upon continued outgassing even at room temperature. To further characterize this behavior a series of IR spectra of NH4[B]-ZSM-5 was recorded at progressively lower pressures. Absorption spectra, spanning a pressure range between 5 X lo-' and 1.8 X lo-, torr, are shown in Figure 5. The main spectral changes accompanying pressure decrease are as follows: 1. Appearance and increase in intensity of the 3725-cm-' band, and increase in the intensity of the 3740-cm-' band in the v(0H) region. 2. Decrease in band intensity in the v(NH) region. 3. Decrease of the 1464-cm-I 6(NH) band. 4. Appearance and increase in intensity of a doublet at 1440 and 1375 cm-I. These data confirm that the 3725-cm-I band (see also Figure 1) is due to a weakly acidic OH and are consistent with the of the assignment of the 3740-cm-' band to terminal SiOH. Interestingly the 3740-cm-' band, which decreased in intensity on ammoniation, did not fully recuver its original intensity even upon torr, but did so upon subsequent outgassing for over 4 h at 5 X reheating of the cell (Figure 5 , top spectrum). Again, this could be due to relatively strong association of NH, in a defect or hydroxyl nest. The origin of the doublet at 1440 and 1375 cm-' remains uncertain at the moment, and could be related to the de€ormation of framework B-0 bonds upon deammoniation.I8 Based on observed u(0H) frequencies the Bronsted acidity of the bridging hydroxyls is therefore ordered as follows: SiOH < B(0H)Si