3050
J. phys. Chem. 1982, 86, 3050-3052
Framework Hydroxyl Groups of H-ZSM-5 Zeolites Peter A. Jacobs Centrum VOW ~ ~ k t e s & k u n d een collddek, Scblkunds, Kathdieke Unlversnen Lewen, 3030 Lewen, Bebium
and Roland von Bellmoos" Instltut fik KrkiteirosrepMe und Petrographle, ETH Ziirlch, 8092 Zirfch, Swltzeriand (Received: December 1, 1981; In Final F m : February 10, 1982)
The acidic form of the synthetic high-silica zeolite ZSM-5 is known for its exceptionalcatalytic activity, selectivity, and stability. On this zeolite methanol can be converted into components in the gasoline range. High yields of p-xylene can be obtained from a mixture of the xylene isomers or by alkylating toluene with methanol. These reactions are proton catalyzed, and information on the nature of the framework hydroxyl groups will be of primary importance in the understanding of these reactions. We report here that, contrary to the existing belief, highly crystalline H-ZSM-5 zeolite contains only one single type of framework hydroxyl group, characterized by an IR vibration around 3600 cm-'. H-ZSM-5 samples of poorer quality show a second hydroxyl band at 3720 cm-', to be assigned therefore to extrazeolitic material.
Introduction The structure1 and potential catalytic applications2* of ZSM-5 have been described earlier. There is general agreement now as to the chemical nature of the framework hydroxyl groups in pure hydrogen zeolites.1° The deammoniation reaction of the NH4 form of zeolites can be depicted as follows:
To obtain reliable OH spectra of high-silica zeolites, one needs spectra of high signal-bnoise ratios in order to allow subsequent expansion of the spectrum. The use of computer-controlled spectrometers is therefore a necessity. However, it seems that spectra obtained this way have not yet been published. In order to correlate the origin of the carboniogenic activity of these materials with (a) specific infrared hydroxyl vibration(& we prepared samples of very high crystallinity using a method revealed earlier.l1J2
Experimental Section Typical molar compositions of the ZSM-5 synthesis mixtures were 21-36 Si02:A1(N0J3:8.5-10.5 NaOH:57-72 TPA(OH):10-11 NH4OH2600-3300 H20:425-533 C3H5(1) Kokotailo, G. T.; Lawton, S. L.; Olson, D. H.; Meier, W. M. Nature (London) 1978,272, 437. (2) Weiez, P. B. 'Proceedings of the 7th International Congress on Catalysis";Seiyama, T., Tanabe, K., Eds.; Elsevier: Amsterdam, 1981; p A3. (3) Derouane, E. G. In 'Catalysis by Zeolites"; Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1980; p 5. (4) Jacobs, P. A. In "Catalysis by Zeolites"; Imelik, B.,et al., Eds.; Elsevier: Amsterdam, 1980, p 293. (5) Chang, C. D.; Silveetri, A. J. J. Catal. 1977, 47, 249. (6) Chen, N. Y.; Kaeding, W. W.; Dwyer, F. G . J. Am. Chem. SOC. 1979,101,6783. (7) Butter, S. A. U.S.Patent 4007 231,1977. (8) Aurom, A.; Bob, V.; Wierzchoweki, P.; Gravelle, P. C.; VMrine, J. C. J. Chem. SOC.,Faraday Trans. 1 1979, 75, 2544. (9) Hatada, K.; Ono, Y.; Ushiki, K. 2.Phys. Chem. (Frankfurt am Main) 1979, 117, 37. (10)For a review, see: Jacobs, P. A. "Carboniogenic Activity of Zeolites"; Elsevier: Amsterdam, 1977; p 45. (11) von Ballmoos, R PLD. Thesis, ETH Zurich, Zurich, Switzerland, 1981. 'The %-Exchange Method in Zeolite Chemistry: Synthesis, Characterization and Dealumination of High Silica Zeolites" in 'Texte zur Chemie und Chemietechnik";Salle & Sauerbder: Frankfurt, 1981. (12) von Ballmoos, R.; Meier, W. M. Nature (London) 1981,289,782.
(OH),. Crystallizations were carried out at 200 "C for 5-6 days in 1-L autoclaves equipped with stirrers. ZSM-5 samples with overall contents of 2.5, 3.6, 4.1, and 4.7 A1 per unit cell of 96 Si + Al were thus prepared, in addition to the Al-free end member. The synthesis of the latter was carried out under the same conditions but in the complete absence of aluminum. In order to exclude the hydrolysis of framework aluminum during the calcination of the occluded TPA species, we chose a subtle method for the preparation of the ammonium form of zeolite ZSM-5. The sample was slowly heated in air to 400 "C and allowed to react for 2 days,thus preventing the generation of hot spots in the zeolite bed. The protons thus formed were subsequently ion exchanged at 80 "C with Na+ in a 2 M NaCl solution with NaOH at pH 10-11. This Na form can be heated to 520 "C to remove the last traces of organics without risking hydrolysis of framework aluminum. Afterward, this material was transformed into the NH4 form by cation exchange in an excess of a 2 M aqueous solution of NH4C1, followed by drying in air at 110 "C. A Perkin-Elmer 580B spectrometer with data station was used for recording the IR spectra in the hydroxyl region between 3200 and 3850 cm-'. In a typical experiment, 49 spectra were accumulated and averaged to reduce the signal noise by a factor of 7. The sample was present in the form of a self-supporting wafer of 5 mg cm-2. The NH4 zeolites were deammoniated in situ at 300-340 "C under vacuum to yield dehydrated H-ZSM-5 according to reaction 1.
Results and Discussion The hydroxyl spectrum of the deammoniated NH,ZSM-5 zeolite (3.5 Al/unit cell) is shown in Figure la. A single hydroxyl band centered around 3603 cm-' is found. For comparison, the OH spectrum of a ZSM-5 sample (3 Al/unit cell) which contained gel (detected by scanning electron microscopy and X-ray diffractometry) is shown in Figure lb. The material had been synthesized according to published methods13and pretreated as described above. The spectrum of this sample includes a second band around 3720 cm-'. Similar two-band spectra already have appeared in the l i t e r a t ~ r e . ~It. ~seems therefore that the characteristic vibration of framework OH groups from Bronsted acid sites in H-ZSM-5 is around 3600 cm-'. Ita (13) Argauer, R. J.; Landolt, G. R. U S . Patent 3702886, 1972.
0 1982 American Chemical Society
Framework Hydroxyl Oroups of H-ZSM-5 Zeolites
The Journal of phvsicel Chemistry, Vol. 86, No. 15, 1982 3051
$$
0
1
2
-
3 Al/uc
4
5
Fbum 3. RelatknsMp between total amunt of ammonia evolved from H-ZSM-5 samples titrated wlth the base at 100 "C and Uielr AI content.
cm-'
-
Flgure 1. Hydroxyl spectra (a) of a very pure H-ZSM-5 sample and (b) of a H-ZSM-5 sample of lower quality. Spectra are accumulated 49 times, averaged, and expanded 10 tlmes. The samples contab (a) 3.5 and (b) 3.0 Al/unit cell: partlcle sizes are 15-20 and 2-5 Ctm, respectively.
-
Al/uc
Figwe 2. Intensity of the infrared extinction at 3600 cm-' (In arbitrary units) vs. the aluminum content of the samples.
half-bandwidth is 39 cm-', which is considerably broader than the value of 26 cm-' reported for H-Y.14 Since the latter band has been assigned to a well-defined vibration in the lattice, it follows that the environment for the framework hydroxyl groups in H-ZSM-5 is far less homogeneous. The existence of only a single framework OH vibration could mean that the groups are vibrating inside the small cages formed at the channel intersections. This suggestion is consistent with catalytic data derived from reaction transition states. The rather high width of the band (compared to H-Y) is most probably the result of a slightly differing chemical environment. Indeed, considerable gradients of the Al concentration throughout the crystals of these materials have been reported.12 The absence of a band at 3720 cm-l is indicative of highly crystalline material with negligible amounts of terminal silanols (14) Jacob, P. A.; Uyttarhoeven, J. B. J. Chem. SOC.,Faraday Trans. 1, 1973, 69, 359.
I
1650
I
1550
wavenumber / cm-1
Flgurr 4. Infrared spectra of pyrldlne sorbed (at 200 "C) on the samples of Flgure 1.
(due to the large crystal size) and without gel or other, not fully cross-linked, impurities. H-ZSM-5 samples with varying aluminum content were prepared by using the synthesis method, the ammoniation and deammoniation procedure already described. All samples show a single OH band around 3600 cm-'. When the infrared absorbance of this band per unit film thickness is plotted against the Al content of the H-ZSM-5 zeolite (Figure2), a straight line is found. As a result there exists a direct proportionality between the number of acidic hydroxyl groups and the aluminum content of the ZSM-5 zeolite, a finding which is further supported by quantitative l*O-exchange data of ZSM-5 and water at 95 "C.lS This one-to-one relationship is to be expected, if the same crystal chemical changes occur during the formation of H-ZSM-5 as those (15) von Ballmoos, R.; Meier, W. M. J . Phys. Chem., in prees.
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The Journal of Physical Chemistty, Vol. 86, No. 15, 1982
reported for the more aluminum-rich zeolites. It seems therefore that the deammoniation reaction 1 also quantitatively occurs during the formation of high-silica hydrogen zeolites. After conversion of the ordinate values of Figure 2 into absorbance X cm-' units, the apparent integrated extinction coefficient of the 3600-cm-' hydroxyls can be derived from the slope of the straight line and is found to be 11.2 cm pmol-'. This is twice the value reported for the supercage hydroxyls in H-Y16 and suggests an increased value of the dipole moment derivative along the bond of the hydroxyl group in H-ZSM-5 zeolites compared to H-Y zeolites and as a result indicates an increased ionicity of that bond. The deammoniation reaction'is fully reversible. This is proved by the data of Figure 3, for which the amount of ammonia needed to titrate the framework hydroxyl groups (NH,/unit cell) of the different samples is plotted against their A1 content (Al/unit cell). Indeed, it is seen that the diagonal line in this figure represents the best fit to the experimental data. The reversibility of reaction 1 and the absence of any dehydroxylation of framework hydroxyl groups is shown by the absence in Figure 4a of any Lewis-bound pyridine. Indeed, the infrared spectrum of pyridine is distinctly different when chemisorbed on Lewis or Bronsted acid sites.16 When pyridine is adsorbed on the H-ZSM-5 sample containing only the framework hydroxyl groups at 3600 cm-', only the band characteristic (16)Jacobs,P.A.; Theng, B.K.G.;Uytterhoeven,J. B.J . Catal. 1972, 26, 191.
Jacobs and von Ballmoos
for pyridinium ions at 1550 cm-' is found. When the same is done on the zeolite containing also the 3720-cm-' band, a small band of Lewis-coordinated pyridine between ,1460 and 1450 cm-' is also observed. This shows that in the latter case the ratio of the number of true framework hydroxyl groups to the bulk A1 content of the sample is significantly lower than 1, as part of the Al is occluded in the impurity and does not give rise to strong Bronsted acid sites. In conclusion, the present note clearly shows that HZSM-5 zeolites contain only one type of framework hydroxyl group, very probably located near the channel intersections. Ita decreased infrared frequency and enhanced extinction coefficient are indicative of an increased ionicity of the particular bond when H-Y is taken as a comparison. Its rather large half-bandwidth indicates that the environment is chemically less homogeneous given the very high degree of crystallinity of the material. This can be related to the A1 zoning reported for this zeolite.12 Presently, the catalytic consequences of the presence and/or absence of the 3720-cm-' band in proton-catalyzed reactions are under investigation.
Acknowledgment. The continuous interest of Profeswrs W. M. Meier (ETH) and J. B. Uytterhoeven (Katholieke Universiteit Leuven) in this work is very much appreciated. P.A.J. acknowledges a permanent research position as Senior Research k i a t e from the Belgian National Fund of Scientific Research. This work was supported by Swiss and Belgian NSF grants.
