The Effect of Molecular Oxygen on the Pathways of Methane Activation on Zn2+-Modified ZSM-5 Zeolite
The effect of molecular oxygen on the pathways of methane activation on Zn2+-modified ZSM-5 zeolite
Alexander G. Stepanov,* Anton A. Gabrienko, Sergei S. Arzumanov, Mikhail V. Luzgin, and Valentin N. Parmon
Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, 630090, Russia
Direct catalytic conversion of methane to more valuable chemicals is one of the important challenges nowadays.1,2 Zn-modified zeolites attract much attention from scientific and industrial points of view due to approved evidences for their activity in methane conversion to higher hydrocarbons.3,4 The key point in the understanding of such activity is the initial stage of methane transformation, i.e. the activation of methane via an interaction with the surface zinc sites. There is an apparent controversy in literature about possible mechanism of methane activation. It is claimed that methane dissociation on Zn2+ sites is realized via “alkyl” pathway5 or leads to the surface methoxy species formation (“carbenium” pathway).6 Therefore, this work aims to address the issue of methane activation on Zn2+-modified zeolite H-ZSM-5. In particular, a careful analysis of methane transformation in nonoxidative conditions and in the presence of molecular oxygen has been performed that gives an opportunity to clarify whether the activation of methane in nonoxidative conditions on Zn2+ sites leads to surface methoxy species or zinc-methyl species as the primary intermediates of methane activation.
2. Experimental Part
The Zn2+-exchanged zeolite was prepared by a solid-exchange reaction between metallic zinc vapor and H-ZSM-5 zeolite (Tricat Zeolites, Si/Al = 13) according to the procedures reported earlier.7 Two samples of Zn2+-exchanged zeolite were prepared with 100 % and 60 % degree of ion exchange of acidic hydroxy groups. The zeolite samples were activated under the vacuum (< 10–3 Pa) at 673 K and transferred into an axially symmetrical glass ampoule of 3 mm outer diameter capable to fit 4 mm NMR zirconia rotor. Following the activation, the adsorption of methane-13C (99% 13C, Aldrich Chemical Co. Inc.) or methane-13C and molecular oxygen (4:1 ratio) was performed (150–200 µmol g–1) at the temperature of liquid nitrogen, and the ampoule was sealed off by a torch flame.
NMR experiments were performed using a Bruker Avance-400 spectrometer equipped with a broad-band double-resonance MAS probe. Zirconia rotors with the inserted sealed glass ampoules were spun at 5 kHz by dried compressed air. For 13C CP/MAS NMR, the proton high power decoupling field strength was 11.7 G (5.0 μs length of 90° 1H pulse); contact time was 2 ms at the Hartmann-Hahn matching condition of 50 kHz; and the delay between scans was 2 s. The chemical shift was referenced to TMS.
3. Results and discussion
Methane-13C activation and conversion were studied with MAS NMR spectroscopy on zeolite ZSM-5 with 100% (Zn2+/ZSM-5) and 60% (Zn2+/H-ZSM-5) of SiOHAl groups exchanged with Zn2+ cations. Figure 1a–c demonstrates that methane (the signal at –6 ppm) activation on Zn2+/ZSM-5 occurs at temperatures ≥ 523 K and results in the formation of zinc-methyl species (–19 ppm) exclusively, which survives the evacuation at room temperature. On the other hand, the presence of acid hydroxy groups makes the activation process possible already at room temperature on Zn2+/H-ZSM-5 zeolite, and detected zinc-methyl species (–20 ppm) can be removed from the surface by evacuation (Figure 1e,f). These experiments were performed in nonoxidative conditions, and no oxygen-containing species, such as methoxide, were observed to form from pure methane during interaction with Zn2+ sites. This means that methane activation on Zn2+-modified zeolites occurs exclusively via “alkyl” pathway. Interestingly, acidic hydroxyls, if present, can promote methane dissociation.
Figure 1. 13C CP/MAS NMR spectra of 13CH4 and 13CH4/O2 adsorbed on Zn2+/ZSM-5 (a–d) and Zn2+/H-ZSM-5 (e–h) zeolites
Contrarily, controlled addition of molecular oxygen to either methane or zinc-methyl leads to the appearance of various oxygenated species (Figure 1d,g,h): methoxide Zn–O–CH3 (58 ppm), formate (173 ppm), acetaldehyde (31, 211–225 ppm), etc. Moreover, the intensity of the signal of zinc-methyl decreases when oxygenated species are formed. These results evidently prove that the surface methoxide is not formed from pure methane adsorbed on the Zn2+-exchanged H-ZSM-5 zeolite, but is the product of zinc-methyl oxidation by admixed molecular oxygen.
Methane activation on Zn2+-exchanged H-ZSM-5 zeolite has been carefully investigated in this work. The obtained results give the opportunity to answer the question about the possible pathways of methane dissociation on Zn2+ sites. 13C CP/MAS NMR spectroscopic data clearly demonstrate that the methane activation on Zn2+-modified ZSM-5 zeolites affords exclusively zinc-methyl species, while the presence of admixed molecular oxygen in methane can provide the formation of Zn-methoxy species. This finding opens up the possibility to use Zn2+/H-ZSM-5 as the catalyst for methane oxidation to methanol under mild conditions.
This work was supported in part by Russian Foundation for Basic Research (grant no. 14-03-00040).
1) H. Schwarz, Angew. Chem.-Int. Edit. 2011, 50, 10096.
2) J. H. Lunsford, Catal. Today 2000, 63, 165.
3) M. V. Luzgin, V. A. Rogov, S. S. Arzumanov, A. V. Toktarev, A. G. Stepanov, V. N. Parmon, Angew. Chem., Int. Ed. 2008, 47, 4559.
4) V. R. Choudhary, K. C. Mondal, S. A. R. Mulla, Angew. Chem. Int. Ed. 2005, 44, 4381.
5) Y. G. Kolyagin, I. I. Ivanova, V. V. Ordomsky, A. Gedeon , Y. A. Pirogov, J. Phys. Chem. C 2008, 112, 20065.
6) J. Xu, A. M. Zheng, X. M. Wang, G. D. Qi, J. H. Su, J. F. Du, Z. H. Gan, J. F. Wu, W. Wang, F. Deng, Chem. Sci. 2012, 3, 2932.
7) J. Heemsoth, E. Tegeler, F. Roessner, A. Hagen, Microporous Mesoporous Mater. 2001, 46, 185.