Methane to Methanol: Structure–Activity Relationships for Cu-CHA

Abstract

Cu-exchanged zeolites possess active sites that are able to cleave the C-H bond of methane at temperatures ≤200 °C, enabling its selective partial oxidation to methanol. Herein we explore this process over Cu-SSZ-13 materials. We combine activity tests and X-ray absorption spectroscopy (XAS) to thoroughly investigate the influence of reaction parameters and material elemental composition on the productivity and Cu speciation during the key process steps. We find that the Cu moieties responsible for the conversion are formed in the presence of O and that high temperature together with prolonged activation time increases the population of such active sites. We evidence a linear correlation between the reducibility of the materials and their methanol productivity. By optimizing the process conditions and material composition, we are able to reach a methanol productivity as high as 0.2 mol CHOH/mol Cu (125 μmol/g), the highest value reported to date for Cu-SSZ-13. Our results clearly demonstrate that high populations of 2Al ZCu sites in 6r, favored at low values of both Si:Al and Cu:Al ratios, inhibit the material performance by being inactive for the conversion. Z[CuOH] complexes, although shown to be inactive, are identified as the precursors to the methane-converting active sites. By critical examination of the reported catalytic and spectroscopic evidence, we propose different possible routes for active-site formation.

Monomeric Copper(II) Sites Supported on Alumina Selectively Convert Methane to Methanol

“…[34] These mononuclear sites react with CH 4 within minutes at 200 8 8Ctogive CH 3 OH with high selectivity (> 80 %) on as ignificant fraction of Cu surface sites (22 %ofCu), clearly indicating that having di-or tri-nuclear sites on crystalline oxide materials is not arequirement for the selective conversion of CH 4 to CH 3 OH. [35] Moreover,i nsitu XAS (Figure 3a)s howed that about 27 % of the total Cu II was reduced to Cu I ,w hich is close to the expected 22 %considering the CH 3 OH yield and the need for two electrons per CH 3 OH formed. This performance indicates that more than 83 %ofthe electrons involved in the Cu II to Cu I redox process are used for the partial oxidation of CH 4 to CH 3 OH.…”

Copper‐Exchanged Omega (MAZ) Zeolite: Copper‐concentration Dependent Active Sites and its Unprecedented Methane to Methanol Conversion

“…Directly converting methane to methanol has its share of scientific challenges that have given it a reputation as the “dream reaction.” The methane must be stabilized in a partially oxidized state rather than undergo much more favorable complete oxidation to carbon dioxide ,. Copper exchanged zeolites can do just that when a stepwise looping procedure is applied . The most widely studied copper zeolites are ZSM‐5 and mordenite, and the overall methanol yield has improved over the last 15 years as we have gained better understanding of the procedure, extraction and favorable zeolite characteristics .…”

“…However, the yield is still below any industrially viable levels, and there is no consensus on the active site or mechanism responsible for the methane to methanol conversion ,,,,,,. This has led researchers to explore other zeolites and non‐zeolite materials in an attempt to achieve higher yields of methanol or uniform active sites ,,,. In one such survey of zeolite types, zeolite omega ( MAZ ) was shown to be comparable to mordenite ( MOR ) in the conversion of methane to methanol .…”

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

“…[1] Ap romising stepwise process over copperexchanged zeolites has been suggested;h owever, the mechanistic details of this zeolite-catalyzed conversion are still not understood. [1][2][3][4] Another open question deals with the role of the nature of the oxidant, as well as the mechanism of methane oxidation and regeneration of the copper oxide active site. [1][2][3][4] Another open question deals with the role of the nature of the oxidant, as well as the mechanism of methane oxidation and regeneration of the copper oxide active site.…”

“…Whereas the existence of smaller active sites has been suggested by natural enzymes [16] and proven experimentally, [2,[5][6][7][8] the possibility of forming larger clusters has also been suggested theoretically based on thermodynamic stability. [3,[17][18][19] Copper monomers,onthe other hand, are believed to require the formation of copper hydrides to facilitate methane oxidation, which is energetically unfavorable.…”