J. Phys. Chem. C 2008, 112, 20065–20069
20065
Methane Activation over Zn-Modified MFI Zeolite: NMR Evidence for Zn-Methyl Surface Species Formation Yuriy G. Kolyagin,† Irina I. Ivanova,*,† Vitaly V. Ordomsky,† Antoine Gedeon,‡ and Yuri A. Pirogov§ Department of Chemistry, Moscow State UniVersity, Leninskie Gory, b. 1/3, GSP-1, 119991 Moscow, Russia, Paris UniVersite´ Pierre et Marie Curie - Paris 6, CNRS-UMR 7142, Laboratoire Syste`mes Interfaciaux a` l’Echelle Nanome´trique, case 196, 4 place Jussieu, Paris F-75252 Cedex 05, France, and Center for Magnetic Tomography and Spectroscopy, Moscow State UniVersity, Leninskie Gory, 1/73, GSP-1, 119991 Moscow, Russia ReceiVed: February 11, 2008; ReVised Manuscript ReceiVed: September 30, 2008
The early stages of methane activation over Zn-modified H-MFI catalysts obtained by high-temperature reaction with zinc vapor have been studied by 13C MAS NMR in situ. Methane 99.9% enriched with 13C was used as labeled reactant. The spectroscopic data pointed to formation of zinc methyl species at ambient temperature just after methane adsorption onto the zeolite sample. The results suggest that methane activation occurs via dissociative adsorption over acid-base Zn-O pairs involving Zn2+ cations and negatively charged oxygen atoms of the zeolite framework. The nature of the sites responsible for the dissociation is discussed. Introduction Zn-modified MFI zeolites are among the most efficient catalysts of light alkanes transformations under nonoxidative conditions.1 Understanding of the mechanism of such transformations is of key importance for further improvement of catalyst selectivity, activity and stability. In this respect, the most difficult question is the mechanism of alkanes activation and the very early stages of their interaction with the catalyst surface. In the latest literature, there are indications that light alkanes activation over Zn-containing zeolites occurs via heterolytic dissociation of the C-H bonds over acid-base Znδ+-Oδ- pairs.2-8 The possibility of the dissociative adsorption of methane and ethane over acid-base Znδ+-Oδ- pairs was first suggested by theoretical studies.2-5 Later on, this hypothesis received experimental proof.6-8 The development of in situ MAS NMR spectroscopic technique9-14 has allowed monitoring catalytic transformations directly on a catalyst surface during the course of the reaction. This technique has turned out to be very informative in the investigation of surface species formed over Zn-containing catalysts. Thus, application of in situ 13C MAS NMR spectroscopy has enabled direct observation of π-allylzinc species during 2-methylpropene oligomerization over ZnO15 and identification n-propylzinc surface species at the initial stages of propane activation over ZnO/H-MFI catalyst.8 In this study, we applied MAS NMR technique for the investigation of methane activation over Zn-modified MFI catalyst. The interaction of methane with parent nonmodified H-MFI was also studied for comparison. The results obtained bring a clear demonstration that the specifically modified Zn/ MFI zeolites are able to activate methane via a dissociative adsorption under ambient conditions and give information on the nature of active zinc-containing species responsible for that. * To whom correspondence should be addressed. E-mail: iiivanova@ phys.chem.msu.ru. † Department of Chemistry, Moscow State University. ‡ Paris Universite ´ Pierre et Marie Curie. § Center for Magnetic Tomography and Spectroscopy, Moscow State University.
This provides an important and timely challenge, since natural gas is expected to play in the future an important role as a chemical (carbon) resource. Experimental Section Zeolite MFI with Si/Al ) 40 in ammonium form (NH4/MFI) provided by ZEOLYST was used as starting material. H/MFI and Zn/MFI catalysts were prepared directly in IR or NMR cells attached to the vacuum line. For the preparation of H/MFI, the sample was heated under vacuum up to 673 K and maintained at this temperature for 8 h to reach final pressure of 10-6 torr. Afterward, it was cooled down to ambient temperature before the measurements. Zn/MFI catalyst was prepared according to the procedure similar to those reported earlier.16-19 For that, the powder of metallic zinc from evacuated cold appendix of the cell was added and mixed under vacuum conditions with H/MFI catalyst activated as described above. The mixture was calcined in the closed volume at 773 K for 2 h. The following evacuation of the sample at 773 K for 30 min was carried out to remove the unreacted metallic zinc and hydrogen formed. All unreacted metallic zinc was collected in the special cold part of the cell. The high degree (∼80%) of ion exchange of zeolite protons for zinc cations was achieved when 3-fold excess of zinc was added (Zn/Al ≈ 1.5) to the zeolite. Low degree of ion exchange (∼30%) was obtained when zinc was taken in the proportion of Zn/Al ≈ 0.5. The degree of ion exchange was controlled by IR spectroscopy in the region of OH groups vibrations or by 1H NMR spectroscopy. The IR spectra of activated catalysts and after CO absorption were recorded at ambient temperature. The measurements were carried out on a Nicolet Prote´ge´-FT-IR spectrometer with 4 cm-1 optical resolution, with one level of zero-filling for the Fourier transform, using special DRIFT accessory and a homemade quartz cell for DRIFT studies. For CO adsorption, the catalyst was maintained in the atmosphere of CO (100 torr) for 5 min, then it was evacuated for 15 min at ambient temperature up to a final pressure of 10-2 torr to remove the gaseous and physisorbed CO. The pressure of carbon monoxide was
10.1021/jp8067766 CCC: $40.75 2008 American Chemical Society Published on Web 11/21/2008
20066 J. Phys. Chem. C, Vol. 112, No. 50, 2008 measured by Varian gauge. The temperature of the sample during the treatments was monitored by a chromel-alumel thermocouple. 13C MAS NMR measurements were carried out on AVANCE DSX300 and AVANCE-II 400 Bruker spectrometers operating at 75.45 and 100.4 MHz for 13C, respectively. Quantitative conditions were achieved by one-pulse NMR experiment (90° pulse, 5 s recycle delay). To eliminate multiplicity, high-power gated proton decoupling with suppressed NOE effect was used. Otherwise, the experiments aimed to obtain multiplicity information were performed without any additional decoupling sequence. The 1H-13C CP/MAS experiments were recorded with the RAMP/CP method,20,21 with ramping of 1H-channel power from 100% to 70%, 2 ms contact time, and 2.0 s recycle delay; the Hartmann-Hahn conditions were adjusted for the power of 85%. Chemical shifts were referenced relatively to the CH2 group of solid adamantane.22,23 Controlled-atmosphere experiments were performed in sealed pyrex NMR cells containing catalyst and adsorbed methane-13C and fitting precisely into 7-mm Bruker zirconia rotors. The 13C-enriched (99%) methane was adsorbed over samples directly after catalysts preparation inside the NMR cell attached to the vacuum system. The amount of adsorbed methane-13C was calculated on the basis of 0.3, 1, or 3 molecules of adsorbate per unit cell of the zeolite and was controlled volumetrically. To avoid any uncontrolled overheating of the sample, the part of the NMR ampule with the catalyst and adsorbate was placed in liquid nitrogen during the sealing-off procedure.
Kolyagin et al.
Figure 1. DRIFT spectra of OH groups before (a) and after (b) the reaction of activated H/MFI catalyst with zinc vapor at 773 K.
Figure 2. DRIFT spectrum of CO adsorbed over Zn/MFI catalyst.
Results and Discussion In 1992 Peapples-Montgomery et al. demonstrated the possibility of redox reaction between H/Y zeolite and metallic zinc leading to formation of gaseous hydrogen and fully exchanged Zn/Y zeolite.16 The same reaction was observed between metallic zinc and zeolites MOR24 and MFI.18,25 The procedure used by Peapples-Montgomery,16 Seidel,17 and Kazansky18,19 was slightly different from those suggested by Beyer24 and Heemsoth.25 While in the former case the reaction takes place under vacuum conditions in the presence of Zn vapor, in the latter case, it proceeds in inert atmosphere and can be regarded as solid state reaction. According to Kazansky et al.18,19 the interaction of H/MFI with zinc vapor leads to complete ion exchange of all protonic sites with Zn2+. Furthermore, in the case of low Al content in the zeolite it results in formation of acid-base pairs with Zn2+ cations attached to one basic oxygen atom and distantly separated from the other negatively charged oxygen atom of the zeolite framework:
The possibility of the existence of such sites has been confirmed by theoretical calculations.26,27 This type of sites was shown to exhibit extremely high activity, since one of the Zn-O bonds is strongly polarized. In our study, the formation of such sites was confirmed by IR spectroscopic studies in the regions of OH-group vibrations (Figure 1) and adsorbed CO (Figure 2). The OH-region of DRIFT spectrum for initial H/MFI zeolite contains two bands: silanol groups (3740 cm-1) and acidic OH groups (3610 cm-1). The reaction with Zn vapor leads to the disappearance of
Figure 3. 13C{1H} MAS NMR (a), 13C MAS NMR (b), and 1H-13C CP MAS NMR (c) spectra of 13CH4 adsorbed over H/MFI (1 molecule/ u.c.) catalysts at ambient temperature.
Si-OH-Al groups due to nearly complete ion-exchange with Zn2+. The estimation of the degree of ion exchange on the basis of these spectra suggests that 80% of protons were exchanged for zinc cations. Adsorption of CO on Zn/MFI resulted in the appearance of intensive band at 2230 cm-1 (Figure 2), which corresponds to oscillations of highly perturbed CO molecules adsorbed over Zn2+.8 The 13C{1H} MAS NMR spectrum of methane-13C adsorbed over acidic H/MFI catalyst shows two NMR lines at ca. -7.2 and -10.5 ppm (Figure 3a). Both signals correspond to methane as evidenced by their splitting in the absence 1H-decoupling into quintuplets with relative intensities 1:4:6:4:1 and a characteristic 13C-1H coupling constants of ∼125 Hz (Figure 3b). The resonance at ca. -10.5 ppm is close to that reported in the literature for gaseous CH4.28 In contrast, the signal at ca.
Methane Activation over Zn-Modified MFI Zeolite
Figure 4. Variable-temperature 13C{1H} MAS NMR spectra of 13 CH4 adsorbed (1 molecule/u.c.) over H/MFI: 363 (a), 298 (b), and 213 K (c).
-7.2 ppm corresponds to less mobile species. That was evidenced by the experiments with 1H-13C cross-polarization (Figure 3c), which pointed to the increase of the relative intensity of the signal at -7.2 ppm with respect to the line of gaseous methane. The intensity of the NMR lines in experiments with cross-polarization from protons to carbon (1H-13C CP/MAS) depends on the efficiency of polarization transfer from 1H to 13C atoms and is determined by the value of the proton-carbon dipole-dipolar coupling. The rapid motion of the species leads to averaging and elimination of dipole-dipole interaction and therefore the intensity of the lines corresponding to rigid species is strongly increased with respect of those of mobile species. The variable temperature experiments (Figure 4) also concluded to restricted mobility of the species characterized by the line at ca. -7.2 ppm. Indeed, the decrease of experimental temperature to 213 K resulted in disappearance of the line at ca. -10.5 ppm due to complete adsorption of gaseous methane. On the contrary, heating to 363 K led to the increase of this line intensity due to methane partial desorption. According to all these data, the signal at -7.2 ppm was attributed to methane adsorbed inside zeolite pores,29 while the signal at -10.5 ppm was attributed to gaseous methane.28 A completely different picture was observed when methane13C was adsorbed over Zn/MFI. The following differences emerged in the NMR spectra (Figure 5). Instead of two lines, three signals were detected in 13C{1H} MAS NMR spectrum (Figure 5a), at ca. -4.5, -10.5, and -20.0 ppm. Besides that, the signal at -4.5 ppm showed a shoulder at ca. -6.5 ppm. The line at ca. -10.5 ppm and the shoulder at ca. -6.5 ppm are most likely due to gaseous and adsorbed methane as it was found for H/MFI catalyst. The small lower field shift of signal from adsorbed methane could be caused by minor perturbation in the zeolite structure with introduction of zinc. The signal at -4.5 ppm is shifted significantly to lower field with respect to the line attributed to methane adsorbed in zeolite pores. Nevertheless, splitting of this line in the absence 1Hdecoupling into quintuplet with a characteristic 13C-1H coupling constants of ∼125 Hz suggests that this line also corresponds to methane molecule (Figure 5b). The asymmetrical shape of the multiplet, is caused by overlapping with the signal at ca. -6.5 ppm. The significant downfield shift of the line at -4.5 ppm could be due to very strong interaction of methane with highly polarized acid-base Znδ+-Oδ- pairs. This observation
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Figure 5. 13C{1H} MAS NMR (a), 13C MAS NMR (b), and 1H-13C CP MAS NMR (c) spectra of 13CH4 adsorbed over Zn/MFI catalysts (1 molecule/u.c.) at ambient temperature.
Figure 6. 13C{1H} MAS NMR spectra obtained after the adsorption of different amounts of 13CH 4 over Zn/MFI catalyst: 0.3 (a), 1 (b) and 3 (c) molecules/u.c.
is in the line with the results reported by Kazansky et al.,6,19 who observed in DRIFT spectra of methane adsorbed over Zn/ MFI a broad band at ca. 2806 cm-1, which the authors attributed to the complex of methane with highly polarized acid-base Znδ+-Oδ- pairs. It is worth noting that the position of the line attributed to methane adsorbed over acid-base Znδ+-Oδ- pairs is sensitive to methane loadings (Figure 6). Thus, the adsorption of 0.3 molecules of methane per unit cell (Figure 6a) instead of 1 molecule/u.c. results in pronounced (∆δ ) 1.5 ppm) downfield shift of the corresponding signal (Figure 6a). At the same time the increase of the methane loading up to 3 molecules/u.c. shifts the signal to higher field (Figure 6b). This observation could be accounted for by heterogeneity of Zn sites of different strength in Zn/MFI sample. Indeed, the strength of Zn sites of type (1) is determined by the distance between Al atoms in the zeolite framework. The longer this distance, the stronger the corresponding Zn-O bond is polarized. Since the distribution of Al atoms in the zeolite framework is not homogeneous, the strength of Zn sites should vary significantly. Therefore, the adsorption of small amounts of methane (0.3 molecules/u.c.) results in its preferable interaction with the strongest active sites, which yields NMR line shifted to the lower field (Figure 6a).
20068 J. Phys. Chem. C, Vol. 112, No. 50, 2008 TABLE 1:
13C
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Chemical Shifts of Methyl Groups in Organozinc Compounds
When methane coverages are increased up to 1 or 3 molecule/ u.c., weaker sites also start to contribute to the adsorption and the average resonance shifts to higher field (Figure 6b). Thus, 13C MAS NMR spectra of adsorbed methane can be used for measuring the strength of Zn sites in Zn/MFI catalysts. Besides the signals of gaseous and adsorbed methane, a new NMR line at ca. -20 ppm is observed in the spectra of Zn/ MFI catalyst (Figures 5 and 6). The intensity of this line increased significantly in the 1H-13C CP/MAS spectrum (Figure 5c), which indicates that it corresponds to very rigid species, strongly attached to the catalyst surface. The position of this line in the spectra does not depend on methane loadings, which suggests that the line corresponds to definite surface species. Although the low sensitivity of nondecoupled spectrum does
not allow establishing the multiplicity of this signal, its chemical shift is characteristic for the methyl group directly bonded to metal atom.30,31 The electropositive metal cations induce significant shielding of attached to them carbon nucleis, which results in pronounced shift of the corresponding NMR lines to higher field with respect to those in organic compounds. The analysis of the chemical shifts of the NMR lines corresponding to methyl groups attached to zinc in different zincorganic compounds (Table 1) suggests that the signal observed could be attributed to surface zinc methyl (-O-Zn-CH3) groups. Our NMR results thus point to methane activation via dissociative adsorption over Zn-containing sites. The dissociative adsorption of hydrogen and light alkanes has been addressed in the literature both for bulk zinc oxide32,33
Figure 7. 13C{1H} MAS NMR (a,b) and 1H-13C CP MAS NMR (c,d) spectra of 13CH 4 adsorbed over Zn/MFI catalysts with different degree of ion exchange of protons for zinc cations: 80% (a,c) and 30% (b,d).
Methane Activation over Zn-Modified MFI Zeolite
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Figure 8. Mechanism of heterolytic dissociative adsorption of methane13 C over highly active Zn sites.
and for Zn-modified MFI catalysts.2-8,18,19,26,27,34 In the early IR spectroscopic studies, Dent and Kokes32,33 observed the formation Zn-OH and Zn-H species upon dihydrogen adsorption over bulk zinc oxide. These species were not very stable and were easily removed by recombinative desorption upon evacuation procedure. Kazansky et al. using DRIFT techniques showed that dissociative adsorption of dihydrogen, methane and ethane over Zn modified HMFI catalyst leads to the formation of more stable hydroxyls SiOHAl and hydride or alkyl species attached to zinc cations.6,7,18,19 The combined application of in situ 13C MAS NMR and IR spectroscopic techniques has enabled direct observation n-propylzinc surface species and ZnOH groups at the initial stages of propane activation over ZnOcontaining H-MFI catalyst. The possibility of the dissociative adsorption of hydrogen, methane and ethane over acid-base Znδ+-Oδ- pairs was also confirmed by theoretical studies.2-5,27 Our results are thus in line with the observations of the other authors. The estimation of the content of methylzinc species from 13C{1H} MAS NMR spectra and the analysis of its variation with methane loadings is given below in molecules per U.C.: methane loadings: content of zincorganic species:
0.3 0.020
1 0.045
3 0.040
The results suggest that only small part of the strongest Zncontaining sites (∼1%) can participate in methane activation at ambient temperature. The decrease of Zn content in the sample and therefore the degree of ion exchange of protonic sites for zinc cations from 80% to 30% results in the disappearance of the NMR signal, corresponding to methylzinc species (Figure 7). This observation points to the absence of strong Zn-containing sites (1) with strongly polarized Zn-O bonds in Zn/MFI catalysts with low degree of ion-exchange. This result appears to be quite reasonable. Indeed, Zn2+ cations, which compensate the negative charge on the oxygen atoms located close to each other in the zeolite framework, should be thermodynamically the most stable. Therefore, the ion exchange first occurs in these positions and results in the formation of less reactive sites. Upon the increase of Zn content, the ion exchange start to take place in the positions with distantly separated negatively charged oxygen atoms, which results in the formation of more reactive sites (1). Consequently, the strong Zn-containing sites (1) with strongly polarized Zn-O bonds can be formed only in the samples with high degree of ion exchange. In conclusion, our NMR data have demonstrated that in contrast to acidic H/MFI zeolite, on Zn-modified catalyst methane activation may take place already at ambient temperature. It was confirmed that it occurs via dissociative adsorption on strongly polarized acid-base Znδ+-Oδ- pairs as depicted in Figure 8. Conclusion The first NMR evidence was provided for the formation surface methylzinc species at the early stages of methane
interaction with Zn-modified MFI catalyst. The results suggest that methane activation may take place already at ambient temperature by dissociative adsorption over Zn sites. Only strong Zn-containing sites with strongly polarized Zn-O bonds, which can be formed only in the zeolites with high degree of ion exchange, can participate in this step. The amount of such sites in Zn/MFI sample with the degree of ion-exchange close to 80% was estimated to be 1% from the total amount of Zn sites. Acknowledgment. The authors thank Volkswagen-Stiftung Foundation and RFBR for financial support. V.V.O. is grateful to LG Chem for the fellowship. References and Notes (1) Ono, Y. Catal. ReV.-Sci. Eng. 1992, 34, 179–226. (2) Barbosa, L.; Zhidomirov, G. M.; van Santen, R. A. Phys. Chem. Chem. Phys. 2000, 2, 3909–3918. (3) Frash, M. V.; van Santen, R. A. Phys. Chem. Chem. Phys. 2000, 2, 1085–1089. (4) Yakovlev, A. L.; Shubin, A. A.; Zhidomirov, G. M.; van Santen, R. A. Catal. Lett. 2000, 70, 175–181. (5) Zhanpeisov, N. U.; Zhidomirov, G. M.; Baerns, M. J. Mol. Catal. A-Chem. 1995, 99, 35–39. (6) Kazansky, V. B.; Serykh, A. I.; Pidko, E. A. J. Catal. 2004, 225, 369–373. (7) Kazansky, V. B.; Pidko, E. A. J. Phys. Chem. B 2005, 109, 2103– 2108. (8) Kolyagin, Y. G.; Ordomsky, V. V.; Khimyak, Y. Z.; Rebrov, A. I.; Fajula, F.; Ivanova, I. I. J. Catal. 2006, 238, 122–133. (9) Ivanova, I. I.; Derouane, E. G. Stud. Surf. Sci. Catal. 1994, 85, 357–390. (10) Haw, J. F. Top. Catal. 1999, 8, 81–86. (11) Ivanova, I. I. Colloids Surf., A 1999, 158, 189–200. (12) Derouane, E. G.; He, H. Y.; Hamid, S.; Lambert, D.; Ivanova, I. I. J. Mol. Catal. A-Chem. 2000, 158, 5–17. (13) Hunger, M. Weitkamp. Angew. Chem., Int. Ed. 2001, 40, 2954– 2971. (14) Hunger, M.; Wang, W. AdV. Catal. 2006, 50, 149–225. (15) Kheir, A. A.; Howard, T.; Haw, J. F. J. Am. Chem. Soc. 1994, 116, 10839–10840. (16) Peappple-Montgomery, P. B.; Seff, K. J. Phys. Chem. 1992, 96, 5962–5965. (17) Seidel, A.; Boddenberg, B. Chem. Phys. Lett. 1996, 249, 117–122. (18) Kazansky, V.; Serykh, A. Microporous Mesoporous Mater. 2004, 70, 151–154. (19) Kazansky, V. B.; Serykh, A. I. Phys. Chem. Chem. Phys. 2004, 6, 3760–3764. (20) Cook, R. L. Anal. Bioanal. Chem. 2004, 378, 1484–1503. (21) Metz, G.; Wu, X. L.; Smith, S. O. J. Magn. Reson. Ser. A 1994, 110, 219–227. (22) Earl, W. L.; Vanderhart, D. L. J. Magn. Reson. 1982, 48, 35–54. (23) Morcombe, C. R.; Zilm, K. W. J. Magn. Reson. 2003, 162, 479– 486. (24) Beyer, H. K.; Pa´l-Borbe´ly, G.; Keindl, M. Microporous Mesoporous Mater. 1999, 31, 333–341. (25) Heemsoth, J.; Tegeler, E.; Roessner, F.; Hagen, A. Microporous Mesoporous Mat. 2001, 46, 185–190. (26) Zhidomirov, G. M.; Shubin, A. A.; Kazansky, V. B.; van Santen, R. A Int. J. Quantum Chem. 2004, 100, 489–494. (27) Shubin, A. A.; Zhidomirov, G. M.; Kazansky, V. B.; van Santen, R. A. Catal. Lett. 2003, 90, 137–142. (28) Denney, D.; Mastikhin, V. M.; Namba, S.; Turkevich, J. J. Phys. Chem. 1978, 82, 1752. (29) Ivanova, I. I.; Pomakhina, E. B.; Rebrov, A. I.; Derouane, E. G. Top. Catal. 1998, 6, 49–59. (30) Breimaier, E.; Voelter, W., Carbon-13 NMR Spectroscopy: HighResolution Methods and Applications in Organic and Biochemistry, 3rd ed.; VCH: New York, 1990; pp 293-298. (31) Hecht, E. Z. Anorg. Allg. Chem. 2000, 626, 2223–2227. (32) Dent, L.; Kokes, R. J. J. Am. Chem. Soc. 1970, 92, 6709–6718. (33) Kokes, R. J.; Chang, C. C.; Dixon, L. T.; Dent, A. L. J. Am. Chem. Soc. 1972, 94, 4429–4436. (34) Kazansky, V. B.; Borovkov, V. Yu.; Serikha, A. I.; van Santen, R. A.; Anderson, B. G. Catal. Lett. 2000, 66, 39–47.
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