TY - JOUR

T1 - 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

AU - Gabrienko, Anton A.

AU - Yashnik, Svetlana A.

AU - Kolganov, Alexander A.

AU - Sheveleva, Alena M.

AU - Arzumanov, Sergei S.

AU - Fedin, Matvey V.

AU - Tuna, Floriana

AU - Stepanov, Alexander G.

N1 - Funding Information: This work was financially supported by the Russian Science Foundation (Grant No. 18-73-00016), the Royal Society of Chemistry (Grant No. IEC\R2\170250), Leverhulme Trust UK (RF-2018-545\4), the Ministry of Science and Higher Education of the Russian Federation (Project No. AAAA-A17-117041710084-2), and FASO Russia (0333-2017-0002). We thank the EPSRC UK National EPR Facility at the University of Manchester for EPR measurements. Funding Information: The mechanism of methane activation on Z 2 Cu(II) sites is illustrated in Figure 8 b. It includes the following steps. Methane adsorption on a Cu(II) cation with the formation of a molecular complex of shown configuration. This causes the polarization of C–H bond in methane and therefore weakening of the bond. The second step is the dissociation of weakened C–H bond in the molecular complex plausibly via heterolytic cleavage to give surface methoxy and Cu–H species. This is supported by the intensity decrease of the band at 2860 cm –1 which indicates methane molecular complex transformation to the surface intermediates. The stable intermediate, detected with 13 C CP/MAS NMR ( Figure 6 ) to be formed, is the surface methoxy group when −CH 3 fragment originates from methane and the oxygen belongs to framework Si–O – –Al site. According to UV–vis DRS and EPR data ( Figure 1 d, Figure 3 d and Table S1 ), no Z 2 Cu(II) reduction to Cu(I) state or other changes of the structure, geometry, and composition occur during methane activation. Thus, all spectroscopic data described here are in agreement with this mechanism. Publisher Copyright: Copyright © 2020 American Chemical Society. Copyright: Copyright 2020 Elsevier B.V., All rights reserved.

PY - 2020/2/3

Y1 - 2020/2/3

N2 - 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 (13C MAS) NMR have been employed to characterize the state of the Cu sites and the intermediates formed during the catalyst activation and methane-to-methanol 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 Cu2+ 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 Z2Cu(II) species without extra-framework O-containing ligands. CW EPR studies reveal that the Z2Cu(II) species are the major part of the Cu(II) sites present in the zeolite. 1H HYSCORE and DRIFTS data are supportive of the formation of a molecular complex of methane and Z2Cu(II) species, with a strongly polarized C-H bond and a 3.3 Å separation between the hydrogen atom of methane and Cu. 13C 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 Z2Cu(II) sites in methane C-H bond activation. The mechanism of methane activation on Z2Cu(II) sites has been proposed. It includes two steps: (1) the formation of the molecular complex of methane with Z2Cu(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.

AB - 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 (13C MAS) NMR have been employed to characterize the state of the Cu sites and the intermediates formed during the catalyst activation and methane-to-methanol 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 Cu2+ 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 Z2Cu(II) species without extra-framework O-containing ligands. CW EPR studies reveal that the Z2Cu(II) species are the major part of the Cu(II) sites present in the zeolite. 1H HYSCORE and DRIFTS data are supportive of the formation of a molecular complex of methane and Z2Cu(II) species, with a strongly polarized C-H bond and a 3.3 Å separation between the hydrogen atom of methane and Cu. 13C 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 Z2Cu(II) sites in methane C-H bond activation. The mechanism of methane activation on Z2Cu(II) sites has been proposed. It includes two steps: (1) the formation of the molecular complex of methane with Z2Cu(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.

KW - SOLID-STATE NMR

KW - ELECTRON-SPIN-RESONANCE

KW - ACETIC-ACID

KW - EXCHANGED ZEOLITES

KW - ZSM-5 ZEOLITE

KW - DISSOCIATIVE ADSORPTION

KW - CATALYTIC CONVERSION

KW - NITROGEN MONOXIDE

KW - STRETCHING BANDS

KW - IR-SPECTROSCOPY

UR - http://www.scopus.com/inward/record.url?scp=85078868861&partnerID=8YFLogxK

U2 - 10.1021/acs.inorgchem.9b03462

DO - 10.1021/acs.inorgchem.9b03462

M3 - Article

C2 - 31971794

AN - SCOPUS:85078868861

VL - 59

SP - 2037

EP - 2050

JO - Inorganic Chemistry

JF - Inorganic Chemistry

SN - 0020-1669

IS - 3

ER -