The Effect of the Active‐Site Structure on the Activity of Copper Mordenite in the Aerobic and Anaerobic Conversion of Methane into Methanol

Abstract

Samples of the zeolite mordenite with different Si/Al ratios were used to synthesize materials with monomeric and oligomeric copper sites that are active in the direct conversion of methane into methanol. A comparison of two reactivation protocols with oxygen (aerobic oxidation) and water (anaerobic oxidation), respectively, revealed that such copper–oxo species possess different reactivity towards methane and water. We show for the first time that oligomeric copper species exhibit high activity under both aerobic and anaerobic activation conditions, whereas monomeric copper sites produce methanol only in aerobic processes.

Identification of Kinetic and Spectroscopic Signatures of Copper Sites for Direct Oxidation of Methane to Methanol

“…Thespectroscopic analysis of the samples was carried out i) after the oxidative activation at high temperature [57,58] and ii)after the reaction with methane at 483 Kfor 1h.Figure 1A compares the FTIR spectra of adsorbed nitrogen monoxide for the copper-exchanged mordenite after activation and reaction, respectively.T he spectrum can be split into two regions:first at 1900-2000 cm À1 corresponds to the vibrations of nitrogen monoxide adsorbed over Cu II species,w hile the second at 1700-1850 cm À1 is due to the mono-and dinitrosyls of Cu I . [39,59] Thep resence of several bands centered at 1919, 1953 and 1998 cm À1 points to the co-existence of different Cu II species in the samples. [39,[58][59][60] Theh igh frequencyN Ob ands indicate the formation of copper-oxo species comprising two and more copper atoms,w hile the bands around 1900 cm À1 are due to the Cu II monomeric species.…”

“…[39,59] Thep resence of several bands centered at 1919, 1953 and 1998 cm À1 points to the co-existence of different Cu II species in the samples. [39,[58][59][60] Theh igh frequencyN Ob ands indicate the formation of copper-oxo species comprising two and more copper atoms,w hile the bands around 1900 cm À1 are due to the Cu II monomeric species. [59,60] Thei ntensity of bands increases with copper loading (Figure 1A), indicating the increase of the copper monomer and oligomer concentration from Cu(1.7)MOR to Cu(4.3)MOR.…”

“…[28][29][30][31] It was reported that copper hosted in zeolites of different topology (MFI, [16,17] MOR, [18,19] CHA, [24,32] FAU, [33] FER, [34] ERI, [35] and MAZ [36] ), and supported on alumina or silica [37,38] can activate methane.A part from the nature of support for copper species,t he silicon to aluminum ratio of the parent zeolite,c opper loading, reaction temperature and methane pressure are the parameters that have significant effect on the final methanol productivity. [17,18,32,33,[39][40][41][42][43][44][45][46][47] These empirical observations were rationalized in terms of ad ifferent nature of copper species,p resent on different copper-containing materials. [29][30][31] Studying of the structure of copper-oxo species,formed in zeolites during the synthesis and activation, started from the very first paper by Groothaert et al [16] Theoriginal suggestion implied the formation of bis-m-oxo copper-oxo sites,which are responsible for methane activation.…”

“…[29][30][31] Studying of the structure of copper-oxo species,formed in zeolites during the synthesis and activation, started from the very first paper by Groothaert et al [16] Theoriginal suggestion implied the formation of bis-m-oxo copper-oxo sites,which are responsible for methane activation. Since then, many more models for copper species were suggested, including copper monomers, [37,39] mono-m-oxo [17,48,49] and trans-m-1,2-peroxo dicopper sites, [40,50] trimers Cu 3 O 3 2+ , [18,46] and copper oxide clusters. [51,52] Notably,a ll these multimeric structures contain at least two copper atoms in oxidation state + 2, which can be reduced to + 1, providing two electrons in total.…”

Identification of Kinetic and Spectroscopic Signatures of Copper Sites for Direct Oxidation of Methane to Methanol

“…470 K. High Angewandte Chemie Forschungsartikel reduction rate is observed till about 520 K, which corresponds to > 85 %f or Cu II converted to Cu I .V ariation of copper loading in the samples led to visible changes in TPR profiles, pointing to different reducibility of copper-oxo species, present in the studied materials. [32,33,39] Thus,t he 50 % reduction of copper in Cu(4.3)MOR is achieved at 460 K, which is 30 Kl ower than that observed for Cu(1.7)MOR.…”