Methane Transformation over Copper-Exchanged Zeolites: From Partial Oxidation to C–C Coupling and Formation of Hydrocarbons

Abstract

Direct methane conversion to methanol via chemical looping using copper-exchanged zeolites has attracted considerable attention during the last decades and is one of the most-actively studied processes. Despite the significant progress that has been made in the design of active systems and the elucidation of active sites, the effects of zeolite topology and the structure of copper species on the nature of the reaction products are yet unclear. Herein, we show that oxygen-activated copper-exchanged zeolites of different framework types, namely, MOR, MFI, BEA, and FAU, yield different products, as detected by in situ Fourier-transform infrared and nuclear magnetic resonance spectroscopy. Molecular methanol, methoxy species, and dimethyl ether prevail at lower reaction temperatures (<473–523 K), and CuI carbonyls and gaseous carbon oxides were detected above 573 K. Methane coupling to C2 and C3 hydrocarbons was shown for the first time over CuMOR, CuBEA, and CuFAU. The nature and relative fraction of formed products strongly depend on the structure of the copper active sites, which is governed by the topology of the zeolite host. Several pathways of methane transformation over copper-exchanged zeolites are identified, opening opportunities for tuning the properties of the materials to achieve the best performance.

Structure of Selective and Nonselective Dicopper (II) Sites in CuMFI for Methane Oxidation to Methanol

“…The spectra of surface species formed over both CuMFI materials exhibit the bands located at 2980, 2871, and 1458 cm −1 , which are due to methoxy species attached to Brønsted acid sites (BAS), 22,23,56−63 and the bands with frequencies of 2965, 2856, and 1469 cm −1 associated with molecularly adsorbed methanol. 22,23,[56][57][58]60,61,63 In addition, all spectra contain a broad band centered at 1625 cm −1 , which is due to adsorbed water, 64,65 and a band at 2157 cm −1 , assigned to the Cu I carbonyl. 22,23,56−58,63,66−68 Despite the presence of similar bands in the spectra of the surface species formed over Cu(0.18)MFI (12) and Cu(0.52)MFI( 12), the evolution of these bands as a function of the reaction temperature is significantly different.…”

“…We employed in situ FTIR spectroscopy to explore the nature of the products and intermediates formed during the methane reaction over Cu(0.18)­MFI(12) and Cu(0.52)­MFI(12), which represent copper-containing MFI zeolites with low and high Cu/Al ratios, respectively (Figures A,B and S13). The spectra of surface species formed over both CuMFI materials exhibit the bands located at 2980, 2871, and 1458 cm –1 , which are due to methoxy species attached to Brønsted acid sites (BAS), ,,− and the bands with frequencies of 2965, 2856, and 1469 cm –1 associated with molecularly adsorbed methanol. ,,− ,,, In addition, all spectra contain a broad band centered at 1625 cm –1 , which is due to adsorbed water, , and a band at 2157 cm –1 , assigned to the Cu I carbonyl. ,,− ,,− Despite the presence of similar bands in the spectra of the surface species formed over Cu(0.18)­MFI(12) and Cu(0.52)­MFI(12), the evolution of these bands as a function of the reaction temperature is significantly different. The signals due to methanol and methoxy species appear in the spectra corresponding to Cu(0.52)­MFI(12) starting from 423 K, while a temperature of at least 498 K is required for the formation of the same species over Cu(0.18)­MFI(12) (Figure S14A).…”

Nonoxidative Couplings of Methane to Form Ethylene Catalyzed by Coinage Metal-Containing ZSM-5 Zeolites: A Theoretical Study

“…The results indicate that Cu­(II)-ZSM-5 exhibits a higher reactivity toward methane to form ethylene than Cu­(I)-ZSM-5. Although we found different reactivities between Cu­(I)-ZSM-5 and Cu­(II)-ZSM-5, our conclusion is kept when we assume that a Cu­(II) cation appears inside ZSM-5 according to an experimental study . Consequently, our ONIOM findings will stimulate experimentalists to utilize Cu-ZSM-5 as a catalyst for methane to ethylene conversion because relevant experimental studies have been limited.…”

“…In addition, we considered the possibility that a Cu(II) cation appears inside a ZSM-5 based on an experimental study. 80 Then, we investigated the methane to ethylene conversion by a single Cu(II) cation in ZSM-5 where two Al atoms are substituted for two Si atoms at T1 and T6 sites of a 10-membered ring and obtained their local minima and TSs, as shown in Figure 9. Its proposed mechanism and PES are displayed in Scheme 5 and Figure 10, respectively.…”

“…Although we found different reactivities between Cu(I)-ZSM-5 and Cu(II)-ZSM-5, our conclusion is kept when we assume that a Cu(II) cation appears inside ZSM-5 according to an experimental study. 80 Consequently, our ONIOM findings will stimulate experimentalists to utilize Cu-ZSM-5 as a catalyst for methane to ethylene conversion because relevant experimental studies have been limited. From the perspective of our ONIOM findings, Au-ZSM-5 easily promotes ethane to ethylene conversion in conjunction with H 2 formation when ethane is used as the initial substrate.…”