Methane Activation on H-ZSM-5 Zeolite with Low Copper Loading. The Nature of Active Sites and Intermediates Identified with the Combination of Spectroscopic Methods

Abstract

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.

Nature of the Surface Intermediates Formed from Methane on Cu-ZSM-5 Zeolite: A Combined Solid-State Nuclear Magnetic Resonance and Density Functional Theory Study

“…[38][39][40] been recently studied experimentally. 72 The proposed mechanism including heterolytic C-H bond cleavage, however, requires further investigation. AlO4 96% Figure S1.…”

Isobutene Transformation to Aromatics on Zn-Modified Zeolite: Particular Effects of Zn2+ and ZnO Species on the Reaction Occurrence Revealed with Solid-State NMR and FTIR Spectroscopy

“…Monitoring of the transformation of alkanes and alkenes, selectively 13 C-labeled at the specific position of the molecule, with 13 C MAS NMR (MAS – magic angle spinning) allows one to follow the transfer of the 13 C label into the reaction intermediates and products. This results in the identification of the intermediates, establishing their structures, and clarifying the main reaction steps. ,,− Because of high sensitivity and the high speed of data acquisition, FTIR spectroscopy is a valuable tool for studying fast processes such as the initial steps of alkane (alkene) activation on zeolite catalysts. ,,,− The joint use of these spectroscopic methods can help to clarify the roles of BAS and LAS (Lewis acid sites) in the process of isobutene transformation on Zn-containing zeolites and contribute to the determination of the aromatization mechanism. The knowledge obtained about the aromatization mechanism with these spectroscopic methods should promote further investigations in the field of rational catalyst design for the effective utilization of hydrocarbon feedstock to valuable products.…”

The accuracy challenge of the DFT-based molecular assignment of13C MAS NMR characterization of surface intermediates in zeolite catalysis

“…Recent studies have successfully employed the 13 C MAS NMR technique to study methane transformation on coppercontaining zeolites. [25][26][27][28][29] The reported spectroscopic data have shown the presence of few different signals with similar 13 C chemical shifts in the range of 50-65 ppm. 30,31 The unambiguous assignment of such signals to specific molecular configurations based solely on the experimental results is very challenging.…”

“…It is thus not surprising that different assignments have been proposed for the same signals by different research groups. [25][26][27][28][29] In particular, the origin of the signal at 62-63 ppm is highly disputed. 26,28,29,[32][33][34] Initially, this signal was assigned to a [Cu(m-OCH 3 )Cu] structure.…”