Pathways of Methane Transformation over Copper‐Exchanged Mordenite as Revealed by In Situ NMR and IR Spectroscopy

Abstract

The reaction of methane with copper‐exchanged mordenite with two different Si/Al ratios was studied by means of in situ NMR and infrared spectroscopies. The detection of NMR signals was shown to be possible with high sensitivity and resolution, despite the presence of a considerable number of paramagnetic CuII species. Several types of surface‐bonded compounds were found after reaction, namely molecular methanol, methoxy species, dimethyl ether, mono‐ and bidentate formates, CuI monocarbonyl as well as carbon monoxide and dioxide, which were present in the gas phase. The relative fractions of these species are strongly influenced by the reaction temperature and the structure of the copper sites and is governed by the Si/Al ratio. While methoxy species bonded to Brønsted acid sites, dimethyl ether and bidentate formate species are the main products over copper‐exchange mordenite with a Si/Al ratio of 6; molecular methanol and monodentate formate species were observed mainly over the material with a Si/Al ratio of 46. These observations are important for understanding the methane partial oxidation mechanism and for the rational design of the active materials for this reaction.

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

“…SP calculations predict chemical shifts of 52-53 ppm for methanol adsorbed on BAS, which is in good agreement with the experimental values of 53-54 ppm. 26,28,29,[32][33][34] After the cluster relaxation in OPT scheme, predicted shifts decrease the values to 49-51 ppm, which are close to the chemical shift of ''free'' methanol. 62 For BAS adsorbed DME, SP calculations predict a difference of about 4 ppm between the chemical shifts of the two carbon atoms caused by the differences in O-C interatomic distances in the optimized periodic zeolite models.…”

“…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.…”

“…[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. 26 Other studies suggested that this signal should be attributed to methanol adsorbed to a dual Cu + ([Cu(m-CH 3 OH)Cu]) 27 or a mono-Cu + site.…”

“…26 Other studies suggested that this signal should be attributed to methanol adsorbed to a dual Cu + ([Cu(m-CH 3 OH)Cu]) 27 or a mono-Cu + site. 28 Herein, we present a study on the advantages and limitations of common DFT strategies for rationalizing the 13 C NMR spectra of zeolite adsorbed reaction intermediates. ZSM-5 zeolite is one of the most widely studied and applied zeolite materials in catalysis and has been selected as the main object of this computational investigation.…”