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

Abstract

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 Zn 2+ cations have been synthesized and thoroughly characterized by a range of spectroscopic methods ( 1 H MAS NMR, DRIFTS, XPS, EXAFS, HRTEM) to show that only one of the Zn-species, either Zn 2+ 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 Zn 2+ 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 Zn 2+ species, respectively, compared to pure acid-form zeolite H-BEA. So, the promoting effect of Zn 2+ 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 Zn 2+ cations compared to small ZnO clusters. It has been revealed, however, that only Zn 2+ cations promote the alkylation of benzene with methane, whereas ZnO species do not. The isolated Zn 2+ 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 Zn 2+ 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 Zn 2+ cationic species perform methane activation toward the alkylation of benzene with methane. This implies that only the Zn 2+ cations in Zn-modified zeolite can activate methane for the reaction of methane coaromatization with higher alkanes.

ZnO Nanoclusters Supported on Dealuminated Zeolite β as a Novel Catalyst for Direct Dehydrogenation of Propane to Propylene

“…It is reported that ZnO supported on Al 2 O 3 could serve as Lewis acid sites (LAS), which can efficiently catalyze PDH. [14] In accordance with previous reports, small ZnO nanoclusters (SN ZnO) are capable of activating CÀ H bond, [18,19] suggesting its potentially catalytic ability for PDH. However, active ZnO nanoclusters are very small and unstable, especially under harsh operation conditions.…”

“…It is notable that Hβ, DeAlβ, Znβ‐3, Znβ‐7 and Znβ‐10 have similar BET surface areas (445–480 m 2 /g) and total pore volumes (0.339–0.380 cm 3 /g), revealing the well preserved pore structure upon the two‐step post‐synthesis procedure. When the Zn loading is up to 20 %, the specific surface area and total pore volume are significantly reduced, ascribing to the formation of bulk ZnO which can block access to partial inner micropores ,…”

“…When the Zn loading is up to 20 %, the specific surface area and total pore volume are significantly reduced, ascribing to the formation of bulk ZnO which can block access to partial inner micropores. [19,40] The 27 Al MAS NMR spectroscopy was employed to verify the elimination of Al atoms after dealumination procedure. In Figure 2a, the characteristic peak at 54 ppm corresponding to tetrahedrally coordinated Al species progressively decreases, indicating almost all lattice Al atoms have been eliminated from the zeolite framework.…”

The template-assisted zinc ion incorporation in SAPO-34 and the enhanced ethylene selectivity in MTO reaction

“…The Zn 2 p 1/2 peak and Zn 2 p 3/2 peak are narrow and symmetrical, indicating that zinc species exist in a single chemical state [34] . The zinc species localized at the cation exchanged sites generally exhibit a high BE than ZnO, owing to the higher electronegativity of the lattice oxygen of the molecular sieve than that of the O 2 − ligand in ZnO [39][40][41] . Therefore, the peak with BE 1022.5 eV is assigned to isolated Zn 2 + cations being stabilized at the exchangeable sites [42,43] .…”

High Zn/Al ratios enhance dehydrogenation vs hydrogen transfer reactions of Zn-ZSM-5 catalytic systems in methanol conversion to aromatics

“…Again, depending on the elemental composition and the spatial distribution in the zeolite, such hydroxylated species are expected to undergo dehydration reactions at high temperature, and evolve into new Zn 2+ species [31]. In samples with low Zn content and low Si/Al ratio, Not surprisingly, the nature of the Zn sites strongly depends on the distribution of Al sites in the zeolite and hence on the exact preparation method [28,35,36]. Among the different proposed methods to incorporate Zn in the parent zeolite, ion exchange generally provides higher ratio of active cationic species promoting aromatization and dehydrogenation reactions [28].…”