TY - JOUR

T1 - Different Efficiency of Zn2+ and ZnO Species for Methane Activation on Zn-Modified Zeolite

AU - Gabrienko, Anton A.

AU - Arzumanov, Sergei S.

AU - Toktarev, Alexander V.

AU - Danilova, Irina G.

AU - Prosvirin, Igor P.

AU - Kriventsov, Vladimir V.

AU - Zaikovskii, Vladimir I.

AU - Freude, Dieter

AU - Stepanov, Alexander G.

PY - 2017/3/3

Y1 - 2017/3/3

N2 - Understanding methane activation pathways on Zn-modified high-silica zeolites (ZSM-5, BEA) is of particular importance because of 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 2 and 3 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 the 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 are 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 a 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 in Zn-modified zeolite can activate methane for the reaction of methane coaromatization with higher alkanes. (Chemical Equation Presented).

AB - Understanding methane activation pathways on Zn-modified high-silica zeolites (ZSM-5, BEA) is of particular importance because of 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 2 and 3 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 the 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 are 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 a 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 in Zn-modified zeolite can activate methane for the reaction of methane coaromatization with higher alkanes. (Chemical Equation Presented).

KW - DRIFTS

KW - EXAFS

KW - H/D exchange

KW - MAS NMR

KW - methane activation

KW - zeolite

KW - zinc

KW - SOLID-STATE NMR

KW - IN-MODIFIED ZSM-5

KW - MODIFIED ZSM-5 ZEOLITES

KW - AROMATIC-HYDROCARBONS

KW - LIGHT ALKANES

KW - HIGH-SILICA ZEOLITES

KW - H-1 MAS NMR

KW - BRONSTED ACID SITES

KW - PROPANE AROMATIZATION

KW - HYDROGEN-EXCHANGE

UR - http://www.scopus.com/inward/record.url?scp=85015165684&partnerID=8YFLogxK

U2 - 10.1021/acscatal.6b03036

DO - 10.1021/acscatal.6b03036

M3 - Article

AN - SCOPUS:85015165684

VL - 7

SP - 1818

EP - 1830

JO - Topics in Catalysis

JF - Topics in Catalysis

SN - 1022-5528

IS - 3

ER -