Mobility of the Acidic Protons in H−ZSM-5 As Studied by Variable

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J. Phys. Chem. B 1998, 102, 804-808

Mobility of the Acidic Protons in H-ZSM-5 As Studied by Variable Temperature 1H MAS NMR Toshihide Baba, Norito Komatsu, and Yoshio Ono* Department of Chemical Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152, Japan

Hisashi Sugisawa Analytical Instrument DiVision, JEOL Ltd., 1-2 Musashino 3-Chome, Akishima-shi, Tokyo 196, Japan ReceiVed: June 24, 1997; In Final Form: December 2, 1997

The dynamic nature of the protons in H-ZSM-5 was examined by the temperature dependence of 1H MAS NMR in the range 298-473 K. The line width of 1H MAS NMR of acidic protons increased, and through maximum it decreased. The intensity of the spinning sidebands monotonically decreased, and they almost disappeared upon raising the temperature. This temperature dependence of the spectrum was explained by the thermal motion of protons. The correlation times and the activation energies for proton mobility were estimated. The estimated values of the activation energy are 17-20 kJ mol-1.

Introduction Bridging hydroxyl groups, Si-OH-Al, of zeolite framework are known to act as Bro¨nsted acid sites. They are therefore responsible for the catalytic activities for many reactions such as cracking, isomerization, and alkylation. It is often proposed that the protons of bridging OH groups hop over the lattice oxygen atoms in zeolite cavities. Thus, the dynamic nature of the acidic protons might be directly related to the catalytic activity of zeolite as solid acids. Solid-state NMR has been a useful tool for zeolite characterizations. Relaxation and line width studies with broad-line NMR have been employed to investigate the proton mobility.1-5 However, the broad-line NMR does not discriminate the nature of the protons, and the information, therefore, is the overall (or average) nature of acidic and nonacidic protons. 1H MAS NMR methods have been applied to characterization of OH groups in zeolites.6-9 This technique offers much higher resolution, and the signals from different types of protons in zeolites can be distinguished. Thus, the behavior of each type of proton can be independently discussed. The chemical shift values of protons are correlated with the acid strength of acidic OH groups.9-13 The interactions of probe molecules with acidic OH groups are also studied.14-16 Moreover, the spinning sidebands analysis in 1H MAS NMR offers information on the distance between the bridging OH groups and aluminum ions in the zeolite framework.17,18 In the case of H-ZSM-5 whose Si/Al ratios were 15 and 26, the distance of 1H-27Al nucleis was estimated as 0.250 and 0.246 nm, respectively. It has been also reported the estimation of 1H27Al dipolar interaction.17-21 We have discussed from the change of the 1H MAS NMR signal due to acidic protons of H-ZSM-5 upon raising the temperature of the measurements from 298 to 373 K.22 The main results can be summarized as follows. (1) The line width broadens upon raising the temperature. (2) The line width of spinning sidebands due to acidic OH groups was broadened and almost disappeared upon raising the temperature. (3) The

chemical shift value does not depend on the temperature, being 4.2 ( 0.1 ppm. The results (1) and (2) have been attributed to the mobility of the bridging OH groups (acidic protons). As described above, the chemical shift values were correlated with the acid strength. However, the line shape changed significantly with temperature, though the chemical shift values remained same. This indicates that the change in the line width with temperature may be a more sensitive measure than the chemical shift to investigate the acidic property of the OH groups. We have also reported that the line broadening is sharply suppressed by replacing a small amount of protons with Na+ or NH4+ ions, though it did not affect the chemical shift value.23 The replacement caused a drastic decrease in the catalytic activity for cyclopropane isomerization.23 These phenomena suggest the importance of the dynamic nature of acidic protons in catalysis. Sarv et al. have recently reported the study on the mobility of acidic protons of various zeolites including H-ZSM-5 by the measurements of 1H MAS NMR spectra at high temperature up to 660 K.24 The line width of the peak attributed to acidic protons narrowed, and the intensity of their spinning sidebands gradually disappeared upon raising the temperature. They analyzed the temperature dependence of the second moments of the spinning sidebands, which is caused by dipolar interaction between acidic protons and the neighboring aluminum nuclei, and estimated the activation energy of proton mobility. In our previous work, the temperature of measurements has been limited to 373 K. In this work, we extended the temperature range up to 473 K and tried to more quantitatively discuss the dynamic nature of acidic protons in H-ZSM-5 by determining the correlation time, the second moments, and the activation energy of the thermal motion of the protons. Experimental Section Materials. Na-ZSM-5 zeolites with SiO2/Al2O3 ratios 24, 39, 72, and 106 were obtained from Toso Co. Ltd. The zeolite

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Mobility of the Acidic Protons in H-ZSM-5

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that was heated in dry air at 723 K for 40 h to remove the bulk of the occluded organic materials was subjected to ion exchange in a NaCl solution and then heated in dry air at 793 K to remove final trace of organic materials for 24 h. The zeolites were then converted to the ammonium form (NH4-ZSM-5) by cation exchange in a NH4Cl solution. In no case were Na+ ions and extraframework aluminum detected by the atomic adsorption spectrum and by 27Al MAS NMR, respectively. Sample Preparation for 1H MAS NMR Measurements. NH4-ZSM-5 (0.50 g) was packed in a glass tube with sidearms, each of which was connected to a glass capsule used for 1H MAS NMR measurements. NH4-ZSM-5 was heated under air with heating rate of 0.4 K min-1 from room temperature to 673 K and calcined at the same temperature for 5 h. The sample was then heated under vacuum at the same temperature for 3 h. After cooling the sample to room temperature, it was transferred into a glass capsule under vacuum to fill it completely and evenly. The neck of the capsule was then sealed, while the sample itself was maintained at 77 K. 1H MAS NMR Measurements. 1H MAS NMR spectra were recorded on a Chemagnetics CMX-Infinity spectrometer operating at 300 MHz, equipped with a 5 mm CRAMPS probe. A sealed sample in a glass tube was inserted into a zirconia rotor (5 mm diameter). To reduce 1H background signals from the probe material, the DEPTH2 pulse sequence was used.25 The π/2 pulse width and the recycle delay were 2.0 µs and 20 s, respectively. The rotation frequency of the glass capsule was 4.0 kHz unless otherwise mentioned. The spectra were recorded upon raising the sample temperature stepwise from 298 to 473 K. The chemical shift was expressed relative to silicon rubber (δ ) 0.12 ppm from tetramethylsilane) with the usual conventions. Results Temperature Variant of 1H MAS NMR. Figure 1 shows the temperature dependence of 1H MAS NMR spectra of H-ZSM-5 with an SiO2/Al2O3 ratio of 39. The spectra were recorded upon raising the temperature of measurements stepwise from 298 to 473 K. A peak was observed at 4.2 ( 0.1 ppm at 298 K (Figure 1a) while a very small and broad peak was observed around 1.8 ppm. The former is attributed to acidic protons (acidic OH groups), and the latter is attributed to nonacidic ones (silanol OH groups).4 In this sample, the amount of nonacidic protons is very small, indicating that the defects and/or hydroxyl nests are very small. The SEM photographs of the H-ZSM-5 crystals show one phase, and the size of crystal is large (ca. 2.5 × 1.0 µm). The chemical shift values due to acidic protons did not change upon raising the temperature (Figure 1a-i), while the change in the chemical shift value due to nonacidic protons is not clear because of its weak intensity. The line shape due to acidic protons strongly depended on the temperature. Upon raising the temperature of measurements the line width increased, and through maximum around 390 K it decreased. The change of the line shape with temperature was completely reversible. Upon lowering temperature from 473 to 298 K, the original spectrum was restored (Figure 1j). In the previous work, the line sharpening over 373 K was not observed because of lower limit of the temperature of measurements. The spinning sidebands due to acidic protons were clearly observed at 298 K. Their intensities monotonically decreased, and the line widths were broadened upon raising the temperature.

Figure 1. 1H MAS NMR spectra of H-ZSM-5 (SiO2/Al2O3 ) 39): (a) 298 K, (b) 313 K, (c) 333 K, (d) 353 K, (e) 373 K, (f) 393 K, (g) 423 K, (h) 453 K, (i) 473 K, (j) the sample (i) cooled to 298 K. The spinning frequency of the sample was 4 kHz.

At temperatures higher than 393 K, the clear spinning sidebands were not observed. The temperature dependence of 1H MAS NMR spectra of H-ZSM-5 with a SiO2/Al2O3 ratio of 106 is shown in Figure 2. Two signals were observed at 4.3 ( 0.1 and 2.0 ( 0.1 ppm, which are attributed to acidic and nonacidic protons, respectively. The SEM photograph of this sample underlines the morphology with the presence of aggregates of ca. 500 nm sized particles, indicating that there is a large amount of defects and/ or hydroxyl nests in this sample. The chemical shift values of the both peaks did not change with temperature. On the other hand, the line width of the peak at 4.3 ppm increased, and through a maximum around 353 K it decreased. The variation of the line width of the peak at 2.0 ppm was not observed upon raising temperature. The peaks of 4.3 and 2.0 ppm partially overlaps. To determine the line width of the peak precisely, the peaks were deconvoluted as shown in Figure 3 by using Lorentzian lines. 1H MAS NMR spectra of other H-ZSM-5 zeolites were also deconvoluted similarly. Change in Line Width with Temperature. Figure 4 shows the effect of the temperature on the line width of the peak due to acidic protons in H-ZSM-5 zeolites, whose SiO2/Al2O3 ratios were 24, 39, 72, and 106. In all H-ZSM-5 zeolites, the line

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Figure 4. Effect of temperature on the line width of the peak due to acidic protons in H-ZSM-5 zeolites: (b) SiO2/Al2O3 ) 24, (0) SiO2/ Al2O3 ) 39, (4) SiO2/Al2O3 ) 72, (O) SiO2/Al2O3 ) 106.

Discussion Thermal Motion of Protons and NMR Line Shape. Fenzke et al. theoretically treated the influence of isotropic thermal motion upon MAS NMR for spin I ) 1/2.26 When the broadening results from an inhomogeneous magnetic dipolar interaction, which is the dominating line-broadening mechanism in the case of bridging OH groups, the line shape is expressed by a Lorentzian line with the line width given by the following equation Figure 2. 1H MAS NMR spectra of H-ZSM-5 (SiO2/Al2O3 ) 106): (a) 298 K, (b) 333 K, (c) 353 K, (d) 373 K, (e) 393 K, (f) 423 K, (g) 453 K, (h) 473 K. The spinning frequency of the sample was 4 kHz.

Figure 3. Peak separation of 1H MAS NMR spectrum. The spectrum of H-ZSM-5 (SiO2/Al2O3 ) 106) recorded at 393 K.

width increased, and through a maximum it decreased upon raising the temperature of measurements. The maximum line width and the temperature to give the maximum line width depended on the SiO2/Al2O3 ratio. Thus, the maximum line width was observed at lower temperature when the ratio of SiO2/ Al2O3 was higher. In our previous work, the line sharpening at the higher temperature region was not observed because the temperature was limited to 373 K. As described above, the line width of the protons due to silanols did not change with temperature in accord with our previous work.23

∆υMAS ) (M2/3π){2τ/[1 + (ωτ)2] + τ/[1 + (2ωτ)2]} (1) where ω (rad s-1) is the spinning rate of the sample, M2 (s-2) is the second moment for the central line, and τ(s) is the correlation time. The line width of the 1H MAS NMR spectrum expressed by eq 1 exhibits a maximum as a function of correlation time at ωτ Z 0.8, and a simple decrease of the value of ∆υMAS both at smaller (line narrowing caused by thermal motion) and at larger (line narrowing caused by MAS) values of the correlation time. Thus, the line width increased, and through a maximum it decreased upon decreasing the correlation time, τ. Moreover, the essential point is that the spinning sidebands exhibit a larger line width than the central line and disappear upon decreasing the correlation time. They demonstrated these features by simulating the line shape of MAS NMR spectra as a function of correlation time. The spectroscopic features of acidic protons as shown in Figures 1 and 2 are in conformity with those of the theoretical description, since the correlation time decreases with increasing temperature. Thus, upon raising the temperature stepwise, the line width increases, and through a maximum it decreases. The spinning sidebands broaden and finally disappear. Therefore, the thermal motion of acidic protons seems to contribute to the line width. One of the most important features of eq 1 is the dependency of the line width on the spinning frequency. The 1H MAS NMR spectra of H-ZSM-5 (SiO2/Al2O3 ) 106) were recorded at 373 K by changing the spinning frequency of the sample from 2.5 to 4.0 kHz (Figure 5). The line width of the peak due to acidic protons was sharply influenced by the spinning frequency and was 11.3 × 102, 9.7 × 102, 8.4 × 102, and 7.3 × 102 Hz when

Mobility of the Acidic Protons in H-ZSM-5

J. Phys. Chem. B, Vol. 102, No. 5, 1998 807 TABLE 1: 1H MAS NMR Parameters of H-ZSM-5 Zeolitesa SiO2/Al2O3

temp/K

τ/µs

106

373 423 473

42 19 13

72

393 423 473

39 22 14

39

393 423 473

41 21 14

M2/s-2

E/kJ/mol

1.4 × 108

17

1.2 × 108

19

1.1 × 108

20

τ ) correlation time, M2 ) second moment, and E ) activation energy. a

Figure 5. Effect of the spinning rate of the sample on the line shape of 1H MAS NMR. The 1H MAS NMR spectrum of H-ZSM-5 (SiO2/ Al2O3 ) 106) recorded at 373 K. The spinning frequency of the sample: (a) 2.5 kHz, (b) 3.0 kHz, (c) 3.5 kHz, (d) 4.0 kHz.

the spinning rate was 2.5, 3.0, 3.5, and 4.0 kHz, respectively. This change of the line shape further indicates the contribution of thermal motion to the change of the line shape. At 298 K, however, the line width did not change by changing the spinning frequency, being 3.6 × 102 Hz. This result indicates that the acidic protons are fixed on the bridging oxygen atoms and are not mobile at 298 K. Brunner also reported that the line width was independent of the spinning rate for the 1H MAS NMR signal of bridging OH groups of H-ZSM-5.27 As mentioned above, the line width of the peak due to silanol protons did not change upon raising the temperature. It was also independent of the spinning frequency of the sample even at higher temperature as expected. These results show that silanol protons are fixed, and the thermal motion does not contribute to the line width. The effects of the spinning frequency of the sample on the line width of the peak due to acidic protons of H-ZSM-5 with a SiO2/Al2O3 ratio of 106, 72, and 39 are shown in Figure 6, 7, and 8, respectively, in the temperature region where the thermal motion may be a major contribution to the line width. From these dependence of the line width on the spinning frequency, the values of τ and M2 were estimated by using eq 1. For each sample, a temperature-independent value of M2 and τ values for each temperature to give the best fit of the experimental values to eq 1 were determined, and they are listed in Table 1. The solid lines in Figure 6, 7, and 8 are drawn by using the values given in Table 1 and show reasonable agreement with the experimentally determined line width. The value of M2, (1.2 ( 0.2) × 108 s-2, is independent of the SiO2/Al2O3 value of H-ZSM-5. The activation energy, E, for proton hopping can be calculated from the temperature dependence of τ (Table 1).

Figure 6. Line width of the peak due to acidic protons in H-ZSM-5 (SiO2/Al2O3 ) 106) plotted against the spinning frequency of the sample: (O) 373 K, (4) 423 K, (0) 473 K.

Figure 7. Line width of the peak due to acidic protons in H-ZSM-5 (SiO2/Al2O3 ) 72) plotted against the spinning frequency of the sample: (O) 393 K, (2) 423 K, (0) 473 K.

τ ) τ0 exp(E/RT)

(2)

They were estimated as 17-20 kJ mol-1, which was independent of the SiO2/Al2O3 ratios of H-ZSM-5, though the

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Baba et al. Conclusions The variable temperature measurements of 1H MAS NMR offer valuable information on the dynamic nature of the protons in H-ZSM-5. The correlation time can be determined by investigating the dependence of the line width on the spinning frequency. (1) The line width of the peak due to acidic protons increased, and through the maximum it decreased upon raising the temperature. (2) The intensity of the spinning sidebands attributed to acidic protons monotonically decreased, and the peak almost disappeared. (3) The phenomena (1) and (2) are explained by the thermal motion of protons. (4) The correlation times and the activation energy for proton mobility were estimated. The estimated values of the activation energy are 17-20 kJ mol-1.

Figure 8. Line width of the peak due to acidic protons in H-ZSM-5 (SiO2/Al2O3 ) 39) plotted against the spinning frequency of the sample: (O) 393 K, (4) 423 K, (0) 473 K.

temperature that gave the maximum line width depended on the SiO2/Al2O3 ratios. The values are greater than the values of 11 kJ mol-1, which was obtained in the lower temperature region by assuming ωτ . 1.22

τ ) (3/4π)(M2/ω2∆υMAS)

(3)

The activation energy of proton mobility in H-ZSM-5 was estimated as 45 kJ mol-1 by Sarf et al. by the line-shape analysis of the spinning sidebands.24 They estimated the second moment M2 value of 5.23 × 108 s-2. The second moment values in various zeolites were also estimated by sidebands analysis at room temperature and estimated to be 3.5 × 108-5.5 × 108 s-2.18 The value of M2 determined by the analysis of the central line in this work is smaller than those obtained by the sideband analysis. The reason for the difference is not clear at this moment. We measured the broad-line NMR spectrum of H-ZSM-5 (SiO2/Al2O3 ) 42) at the resonance frequency of 270 MHz. The line width at 298 K was 6.1 kHz, which corresponds to the M2 value of ca. 2.6 × 108 s-2, though the broad line NMR does not distinguish the acidic protons from nonacidic protons. We assume that the mobility of acidic protons in zeolites must also detectable by 27Al NMR, since the change of the 1H-27Al distance by the thermal motion of the proton changes an electric field gradient at the aluminum site. Recently, the multiplequantum (MQ) MAS method has been reported.28 This method might be a useful tool for the quantitative analysis of the 27Al nucleis.

Acknowledgment. The authors wish to express their gratitude to Keiji Itahashi (Toso Inc.) for preparing zeolites and taking their SEM photographs. The present work is supported by a Grant-in-Aid for Scientific Research on Priority Areas (No. 07242194) from the Ministry of Education, Science and Culture, and Tokuyama Science Foundation. References and Notes (1) Mestdagh, M. M.; Stone, W. E. E.; Fripiat, J. J. J. Phys. Chem. 1972, 76, 1220. (2) Mestdagh, M. M.; Stone, W. E. E.; Fripiat, J. J. J. Chem. Soc., Faraday Trans. 1 1976, 1, 154. (3) Freude, D.; Oehme, W.; Schmiedel, W.; Staudte, B. J. Catal. 1974, 32, 137. (4) Pfeifer, H.; Freude, D.; Hunger, M. Zeolites 1985, 5, 274. (5) Freude, D.; Pfeifer, H. In Rees, L. V. C., Ed. Proceedings of the 5th International Conference on Zeolites; Heydon: London, 1980; p 732. (6) Klinowski, J. Chem. ReV. (Washington, D.C.) 1991, 91, 1459. (7) Pfeifer, H. NATO AIS Ser. C 1994, 447, 499. (8) Brunner, E. J. Mol. Struct. 1995, 355, 61. (9) Hunger, M. Solid State Nucl. Magn. Reson. 1996, 6, 1. (10) Pfeifer, H. J. Chem. Soc., Faraday Trans. 1 1988, 84, 3777. (11) Freude, D.; Hunger, H.; Pfeifer, H. Chem. Phys. Lett. 1986, 128, 62. (12) Pfeifer, H. Colloid Surf. 1989, 36, 169. (13) Brunner, E.; Karge, H. G.; Pfeifer, H. Z. Phys. Chem. 1992, 176, 173. (14) Haw, J. F.; Nicholas, J. B.; Xu, T.; Beck, L. B.; Ferguson, D. B. Acc. Chem. Res. 1996, 29, 259. (15) Kao, H. M.; Grey, C. P. J. Phys. Chem. 1996, 100, 5105. (16) Heeribout, L.; Batamack, P.; Morin, C. D.; Vincent, R.; Fraissard, J. Colloid Surf. A 1996, 115, 229. (17) Freude, D.; Hunger, M.; Pfeifer, H. J. Magn. Reson. 1991, 95, 477. (18) Hunger, M.; Anderson, M. W.; Ojo, A.; Pfeifer H. Microporous Mater. 1993, 1, 17. (19) Bohm, D.; Fenzke, D.; Pfeifer, H. J. Magn. Reson. 1983, 55, 177. (20) Brunner, E.; Freude, D.; Gerstein, B. C.; Pfeifer, H. J. Magn. Reson. 1990, 90, 90. (21) Ernst, H.; Freude, D.; Wolf, I. Chem. Phys. Lett. 1993, 212, 588. (22) Baba, T.; Inoue, Y.; Shohji, H.; Uematsu, T.; Ono, Y. Microporous Mater. 1995, 3, 647. (23) Baba, T.; Inoue, Y.; Ono, Y. J. Catal. 1996, 159, 230. (24) Sarv, P.; Tuhern, T.; Lippmaa, E.; Keskinen, K.; Root, A. J. Phys. Chem. 1995, 99, 13763. (25) Cory, D. G.; Ritchey, W. M. J. Magn. Reson. 1988, 80, 128. (26) Fenzke, D.; Gerstein, B. C.; Pfeifer, H. J. Magn. Reson. 1992, 98, 469. (27) Brunner, E. J. Chem. Soc., Faraday Trans. 1990, 86, 3957. (28) Medek, A.; Harwood, J. S.; Frydman, L. J. Am. Chem. Soc. 1995, 117, 12729.