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Different Efficiency of Zn2+ and ZnO Species for Methane Activation on Zn-Modified Zeolite Anton A. Gabrienko, Sergei S. Arzumanov, Alexander V. Toktarev, Irina G. Danilova, Igor P. Prosvirin, Vladimir V. Kriventsov, Vladimir I. Zaikovskii, Dieter Freude, and Alexander Grigorievich Stepanov ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03036 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 25, 2017
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Different Efficiency of Zn2+ and ZnO Species for Methane Activation on Zn-Modified Zeolite Anton A. Gabrienko,*,†,‡ Sergei S. Arzumanov,† Alexander V. Toktarev,† Irina G. Danilova,† Igor P. Prosvirin,† Vladimir V. Kriventsov,† Vladimir I. Zaikovskii,†,‡ Dieter Freude,§ Alexander G. Stepanov*,†,‡ †
Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Prospekt
Akademika Lavrentieva 5, Novosibirsk 630090, Russia ‡
Novosibirsk State University, Pirogova Street 2, Novosibirsk 630090, Russia
§
Universität Leipzig, Fakultät für Physik und Geowissenschaften, Linnéstrasse 5, 04103 Leipzig,
Germany
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Abstract: Understanding methane activation pathways on Zn-modified high silica zeolites (ZSM-5, BEA) is of particular importance due to the possibility of methane involvement in coaromatization with higher alkanes on this type of zeolites. Herein, two samples of Zn-modified zeolite BEA containing exclusively either small zinc oxide clusters or isolated Zn2+ cations have been synthesized and thoroughly characterized by a range of spectroscopic methods (1H MAS NMR, DRIFTS, XPS, EXAFS, HRTEM) to show that only one of the Zn-species, either Zn2+ cations or ZnO small clusters, exists in the void of zeolite pores. The ability of zinc sites of different nature to promote the activation of methane C–H bond with the zeolite Brønsted acid sites (BAS) has been examined in the reactions of methane H/D hydrogen exchange with BAS and the alkylation of benzene with methane. It has been found that both ZnO and Zn2+ species promote the reaction of H/D exchange of methane with BAS. The rate of H/D exchange is higher by two and three orders of magnitude for the zeolite loaded with ZnO or Zn2+ species, respectively, compared to pure acid-form zeolite H-BEA. So, the promoting effect of Zn2+ cations is more profound than that of ZnO species for H/D exchange reaction. This implies that synergistic effect of Zn-sites and BAS for C–H bond activation in methane is significantly higher for Zn2+ cations compared to small ZnO clusters. It has been revealed however that only Zn2+ cations promote the alkylation of benzene with methane, whereas ZnO species do not. The isolated Zn2+ cations provide the formation of zinc-methyl species, which further transformed to zinc-methoxy species. The latter is the key intermediate for the performance of the alkylation reaction. Hence, while both zinc oxide clusters and Zn2+ cationic species are able to provide synergistic effect for the activation of C–H bonds of methane displayed by the dramatic acceleration of H/D exchange reaction, only the Zn2+ cationic species perform methane activation toward the alkylation of benzene with methane. This implies that only the Zn2+ cations
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in Zn-modified zeolite can activate methane for the reaction of methane co-aromatization with higher alkanes.
Keywords: Methane activation, zinc, zeolite, H/D exchange, MAS NMR, EXAFS, DRIFTS.
Introduction Activity of Zn-modified zeolites in the reaction of small alkane aromatization was discovered about 30 years ago.1-2 The catalysts were prepared by the ion exchange or impregnation of ZSM5 zeolite (ammonia- or acid-form) with aqueous solution of zinc salts and demonstrated significantly higher selectivity for aromatics compared to the catalysts based on pure acid-form ZSM-5 zeolite.3-4 The catalysts prepared by mechanical mixing of zinc oxide and pure acid-form zeolite were found to exhibit satisfactory activity in small alkane and alkene conversion similar to that demonstrated by the catalysts prepared by ion exchange or impregnation techniques.3,5 Zn-modified ZSM-5 zeolite containing only zinc oxide clusters ZnmOn and Brønsted acid sites (BAS) showed also an enhanced catalytic activity in propane conversion toward aromatics.6 The studies of the composition and the state of zinc species introduced into the zeolites by ion exchange or impregnation methods showed the presence of both isolated Zn2+ cations and small zinc oxide clusters inside the void of zeolite channels.7-14 These catalysts were shown to be capable of activating even methane at low temperatures.15-17 However, it has remained unclear which species, Zn2+ cations or ZnmOn clusters, perform methane activation. The method of Zn loading in zeolite by the interaction of BAS with vapor of metallic zinc was also proposed.18-20 Such approach to load zinc in to the zeolite void leads to isolated Zn2+ cations to be the main state of zinc species.21-26 Zeolites modified with the Zn vapor were shown to be
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able to activate small alkanes, including methane.27-29 Furthermore, some works demonstrated the ability of the zeolites containing exclusively Zn2+ species to activate methane already at ambient temperature.28-31 Hence, isolated Zn2+ sites located inside the pores of the zeolites can presumably be more active species compared to zinc oxide clusters for small alkane activation. On the other hand, one of the recent works reports opposite conclusion. Indeed, the examination of ZSM-5 zeolites modified with zinc using different approaches (ion exchange, impregnation, solid-state exchange with zinc vapor, and chemical vapor deposition of dimethylzinc) showed that the samples having heterogeneous distribution of extra-framework zinc species (zinc oxide clusters and oxygenated cationic Zn-complexes) were more active than the samples with isolated Zn2+ sites.32 The studies of C1–C3 alkane activation and conversion on acid-form zeolites BEA and ZSM-5 loaded with both zinc oxide species and isolated Zn2+ cations assumed the importance of the former sites, however, possible influence of latter ones cannot be excluded.15-16,33-37 So, the nature of zinc species, isolated Zn2+ cations or zinc oxide clusters, responsible for small alkane activation and conversion has not been yet understood. The described controversy can be resolved by careful analysis of the performance of two types of zinc species (isolated Zn2+ cations and zinc oxide clusters) in the reactions of methane activation. In this regard, the method of solid-state NMR spectroscopy (MAS NMR) in situ can be used. This method has proven to be highly informative for the investigation of the mechanisms of methane activation and conversion on metal-modified zeolite-based catalysts.15-16,28-31,38-41 In this work, the ability of different zinc species to activate methane C–H bond has been analyzed by comparing the effect of zinc species on the kinetics of the reaction of H/D hydrogen exchange of methane with Brønsted acid sites (BAS) of the studied zeolites.15 Additionally, the
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performance of zinc oxide clusters and isolated Zn2+ cations has been examined with respect to the reaction of benzene alkylation with methane.42 Thus, the properties of different Zn species loaded in zeolite have been validated for methane activation.
Experimental Section Zeolite Sample Preparation. Sodium-form zeolite beta (Na-BEA) was synthesized using tetraethyl-ammonium hydroxide as template43 with subsequent calcination at 823 K in air flow for 6 h. To obtain acid-form zeolite (H-BEA), the sample of Na-BEA zeolite was treated with aqueous solution of ammonium nitrate followed by calcination at 823 K in air flow for 6 h. Chemical analysis data gave Si/Al ratio of 23 and aluminum content of 1.4 wt.% for the synthesized zeolite. The analysis of 29Si MAS NMR spectrum of H-BEA zeolite showed typical signals (-100 ÷ -115 ppm) for BEA type zeolite and Si/Al ratio was determined to be 21.
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Al MAS NMR
spectrum of the zeolite showed only tetracoordinated aluminum atoms (a signal centered at 54 ppm, Figure S1a) located in the zeolite lattice and no extra-framework aluminum atoms were detected. Synthesis of Zn2+/H-BEA zeolite. The method of solid-state exchange reaction between BAS of the acid-form zeolite and zinc vapor was used to prepare Zn2+-exchanged zeolite (Zn2+/H-BEA) as was described earlier.31 For each experiment, a sample of H-BEA zeolite (30–80 mg) was activated under vacuum at 673 K (residual pressure
