The analysis of n-but-1-ene transformation on Zn-modified zeolite H-BEA, containing zinc exclusively in the form of either Zn 2+ cations (Zn 2+ /H-BEA sample) or small clusters of ZnO (ZnO/H-BEA sample), has been performed with 13 C solid-state nuclear magnetic resonance (NMR) at 296−673 K. The number of intermediates, including π-complex of n-but-2ene, methylallylzinc, and delocalized carbanionic species formed by the interaction of oligomeric polyenes with Zn sites, have been identified for both zeolite samples. Methyl-substituted cyclopentenyl cation and cyclohexadienyl cation are additionally identified for the reaction on ZnO/H-BEA. It is inferred that the aromatization of the olefin occurs basically with the involvement of Zn 2+ sites on Zn 2+ /H-BEA. For ZnO/H-BEA, besides aromatization with the assistance of ZnO species, conjunct polymerization process with the involvement of Brønsted acid sites (BAS) contributes notably to the olefin aromatization. The latter process affords also some quantity of C 1 −C 4 alkanes. It is concluded that the stronger interaction of the olefin (confirmed by density functional theory (DFT) calculations) and oligomeric polyenes with Zn 2+ cations than with ZnO species and different quantities of BAS for two zeolite samples provide peculiar performances of Zn 2+ /H-BEA and ZnO/H-BEA zeolites for the olefin aromatization. Based on careful analysis of the obtained spectroscopic results, it is suggested that Zn-modified zeolite containing Zn 2+ cationic species and some quantity of BAS should exhibit higher efficiency as the catalyst for small olefin and alkane aromatization compared to the zeolite with ZnO species and high concentration of BAS.

The intermediates formed upon the interaction of methane with Cu-modified ZSM-5 zeolites (Cu/H-ZSM-5) have been analyzed with solid-state NMR spectroscopy and DFT methods. Methane activation by Cu/H-ZSM-5 zeolites gives rise to three distinct surface methoxy-like species (-O-CH3) detected by 13 C MAS NMR spectroscopy with specific chemical shifts in the range of 53-63 ppm. DFT calculations on representative cluster models of different sites potentially present in Cu/H-ZSM-5 have been used to assign these signals to (i) methanol adsorbed on two neighboring Cu sites (Cu-(HOCH 3)-Cu, 62.6 ppm), (ii) methanol adsorbed on zeolite Brønsted acid site (52.9 ppm) and (iii) lattice-bound methoxy groups (Si-O(CH3)-Al, 58.6). The formation of these methoxy-like intermediates depends on the Cu loading and, accordingly, the type of Cu species in Cu/H-ZSM-5 zeolite. For the sample with low (0.1 wt.%) Cu loading containing exclusively mononuclear isolated Cu species, only the intermediates (ii) and (iii) have been detected. The Cu-bound intermediate (i) is formed upon methane activation by multinuclear Cu sites featuring Cu-O-Cu bridging moieties present in the materials with relatively higher Cu loading (1.38 wt.%). The presented results indicate 13 CH 4

Cu-modified zeolites have enormous potential as the catalysts facilitating the conversion of methane to methanol. It becomes important to investigate the active sites and the reaction mechanisms involved. In this paper, several spectroscopic methods such as UV−vis diffuse reflectance spectroscopy (UV−vis DRS), pulse electron paramagnetic resonance (EPR), diffuse reflectance Fourier transform infrared spectroscopy, and solid-state ( 13 C MAS) NMR have been employed to characterize the state of the Cu sites and the intermediates formed during the catalyst activation and methane-tomethanol transformation on Cu/H-ZSM-5 zeolite with low (0.10 wt %) Cu content. UV−vis DRS and EPR data imply the presence of two types of Cu 2+ cations bound to the zeolite framework Si−O − −Al sites (Z). One of them is a species of the type Z[Cu(II)O] or Z[Cu(II)(OH)] with extra-framework O − or OH − ligands. The other one refers to Z 2 Cu(II) species without extra-framework O-containing ligands. CW EPR studies reveal that the Z 2 Cu(II) species are the major part of the Cu(II) sites present in the zeolite. 1 H HYSCORE and DRIFTS data are supportive of the formation of a molecular complex of methane and Z 2 Cu(II) species, with a strongly polarized C−H bond and a 3.3 Å separation between the hydrogen atom of methane and Cu. 13 C MAS NMR provides evidence for the formation of both the surface methoxy intermediate and physisorbed methanol. It is suggested that experimentally identified Z[Cu(II)O] or Z[Cu(II)(OH)] are those sites that provide a homolytic cleavage of the methane C−H bond to yield surface bound methoxy species and/or methanol molecule, the possibility that has been recently justified with density functional theory (Kulkarni et al. Catal. Sci. Technol. 2018, 8, 114). The comparison of the amount of the surface methoxy intermediates formed and the number of different Cu(II) sites present in the zeolite allowed us to conclude the involvement of Z 2 Cu(II) sites in methane C−H bond activation. The mechanism of methane activation on Z 2 Cu(II) sites has been proposed. It includes two steps: (1) the formation of the molecular complex of methane with Z 2 Cu(II); (2) heterolytic dissociation of the polarized C−H bond affording surface copper(II) hydride and methoxy species, both bound to zeolite framework Si−O − −Al sites.

With regard to possible involvement of zeolite Brønsted acid sites (BAS) in the activation of methane molecules for methane transformation to methanol, the effect of different Cu(II) species loaded in the zeolite on the kinetic parameters of the reaction of H/D hydrogen exchange of the alkane with BAS of Cu-modified ZSM-5 zeolites has been investigated with 1 H MAS NMR in situ at 533−563 K. It is found that the acceleration of the H/D exchange by 1 order of magnitude occurs for zeolite containingsites) compared to pure H-form zeolite. It is thus inferred that both Z 2 Cu(II) and Z 2 [Cu 3 (μ-O) 3 ] sites exhibit the promoting effect of copper on the activation of methane C−H bonds by BAS. Acceleration of the H/D exchange is rationalized by the change of the mechanism of the exchange accepted for the H-form zeolites for the mechanism that involves the formation of a transient molecular complex of methane with Cu(II) species, preceding the H/D exchange reaction. The formation of the complex of methane with both Z 2 Cu(II) and Z 2 [Cu 3 (μ-O) 3 ] sites is confirmed by DRIFTS. BASs with a higher strength than in H-ZSM-5, generated in the zeolite at copper loading, are concluded to not be responsible for the H/D exchange reaction acceleration.

Selective dimerization of ethene to 2-butene on Zn 2+ -containing ZSM-5 zeolite (Zn 2+ /ZSM-5) at 296−523 K has been discovered. The intermediate but-3-en-1-ylzinc species is identified with 13 C CP/MAS NMR and Fourier transform infrared spectroscopy. The density functional theory study of two alternative dimerization pathways reveals that the intermediate is formed with the involvement of the saturated bridged dimeric Zn−(CH 2 ) 4 −O species. It is also shown that ethene conversion to 2-butene increases with the increase in the quantity of Brønsted acid sites in Zn 2+ /ZSM-5 zeolite; however, the selectivity of the reaction decreases. The results obtained are of potential interest for developing industrially relevant Zn-containing zeolite catalysts for the selective conversion of ethene to 2-butene.