Mechanism of Hydrocarbon Formation in Methane and Methanol Conversion over Copper-Containing Mordenite

Abstract

Methane conversion over copper-containing zeolites can lead to the formation of C–C bonds, yielding hydrocarbons. By employing in situ MAS NMR spectroscopy, we elucidated the pathways of the transformation of methane and its partial oxidation and coupling products over copper-containing mordenite. Below 773 K, the direct coupling of methane is not possible, while the transformation of methanol, methoxy species, and dimethyl ether takes place via the methanol-to-hydrocarbons (MTH) process. In the presence of carbon monoxide, surface acetate species are formed from methanol via the Koch carbonylation reaction. The nature of the hydrocarbon pool species and concomitantly the formed hydrocarbons are strongly affected by the reactants: conversion of pure methanol leads to methyl-substituted cyclopentenyl cations, and the presence of carbon monoxide results in methyl-substituted benzenes. The study clarifies the mechanism of C–C bond formation during the conversion of methane and methanol over copper-containing mordenite and provides insights into the mechanism of the MTH process.

Selective Oxidative Dehydrogenation of Ethane and Propane over Copper‐Containing Mordenite: Insights into Reaction Mechanism and Product Protection

“…The spectra corresponding to the surface species formed in the reaction of methane [24] contain intense bands centered at 2980, 2871 and 1458 cm −1 , which are due to C−H vibrations in the methoxy species attached to the zeolite framework [24,35,40–43] (Scheme 1A). The weaker bands at 2967, 2856 and 1469 cm −1 are associated with molecularly adsorbed methanol [24,35,40–43] . These species are products of methane partial oxidation and are precursors of methanol and DME formed upon contact with water vapor during the desorption step.…”

“…The weaker bands at 2967, 2856 and 1469 cm À 1 are associated with molecularly adsorbed methanol. [24,35,[40][41][42][43] These species are products of methane partial oxidation and are precursors of methanol and DME formed upon contact with water vapor during the desorption step. The intensity of these bands increases with increasing reaction temperature up to 548 K due to the greater involvement of copper(II)-oxo sites.…”

“…It contains a very broad signal at À 6 ppm, assigned to the carbon atom in molecular methane. [55] The reaction between methane and CuMOR below 498 K leads to the appearance of a signal at 58 ppm due to methoxy species bonded to the zeolite framework [23][24][25]43,56] (Scheme 1A). At higher reaction temperatures, there are additional signals at 67 ppm and 52 ppm, which are assigned to DME adsorbed over copper(I) sites [24,26,43,57] and molecularly adsorbed methanol, [23][24][25]43,56] respectively.…”

Selective Oxidative Dehydrogenation of Ethane and Propane over Copper‐Containing Mordenite: Insights into Reaction Mechanism and Product Protection

“…The spectra corresponding to the surface species formed in the reaction of methane [24] contain intense bands centered at 2980, 2871 and 1458 cm −1 , which are due to C−H vibrations in the methoxy species attached to the zeolite framework [24,35,40–43] (Scheme 1A). The weaker bands at 2967, 2856 and 1469 cm −1 are associated with molecularly adsorbed methanol [24,35,40–43] .…”

“…The intensity of the signals increases with an increase in the reaction temperature, and no other signals are detected within the applied range of reaction temperatures. The values of chemical shift tensor components (Figure S7, Table S5) are characteristic for carbon atoms in ethylene πcomplexes with Cu I [43,59,60] (Scheme 1B). For the reaction of CuMOR with propane, three signals at 112, 88 and 19 ppm with corresponding spinning sidebands appear at 398 K and develop in the spectra as the reaction temperature increases.…”