The Nuclearity of the Active Site for Methane to Methanol Conversion in Cu-Mordenite: A Quantitative Assessment

Abstract

The direct conversion of methane to methanol (MTM) is a reaction that has the potential to disrupt a great part of the synthesis gas-derived chemical industry. However, despite many decades of research, active enough catalysts and suitable processes for industrial application are still not available. Recently, several copper-exchanged zeolites have shown considerable activity and selectivity in the direct MTM reaction. Understanding the nature of the active site in these materials is essential for any further development in the field. Herein, we apply multivariate curve resolution analysis of Xray absorption spectroscopy data to accurately quantify the fraction of active Cu in Cu-MOR (MOR = mordenite), allowing an unambiguous determination of the active site nuclearity as a dicopper site. By rationalizing the compositional parameters and reaction conditions, we achieve the highest methanol yield per Cu yet reported for MTM over Cu-zeolites, of 0.47 mol/mol.

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

“…Numerous copper‐exchanged zeolites, including CuMFI, CuMOR, CuFAU, CuMAZ, and CuCHA, were tested and revealed different methanol yields and selectivity per cycle. The research targeting the achievement of the highest methanol productivity allowed to obtain the methanol productivity close to the theoretical limit . However, in most cases, it does not exceed ≈100–150 μmol g −1 , thus accounting for ≈0.2–0.3 mol(MeOH) mol −1 (Cu) .…”

“…Thus, PXRD, XAS, Raman, and UV/Vis data suggested that the formation of copper monomers, dimers in a form of bis‐μ‐oxo and mono‐μ‐oxo dicopper cores as well as trimers and copper oxide clusters is a possibility. The formation of these sites is governed by the topology of the zeolite, Si/Al ratio, copper loading, and the nature of the counter‐cation (H + , Na + , or NH 4 + ) in the parent zeolite . During the reaction with methane, these sites undergo reduction, forming Cu I species and methanol with established stoichiometry of 2×Cu I per one methanol molecule as calculated for 100 % selectivity …”

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

“…[8] Thel ast step of the cycle utilizes aprotic solvent to desorb the CH 3 Ospecies that are strongly bound to the copper sites.O ft he various systems that have been investigated, Cu-exchanged zeolites have shown potential for the selective oxidation of CH 4 to CH 3 OH using molecular oxygen as an oxidant. [18,19] Reaching an unequivocal conclusion regarding the active sites in these systems is therefore challenging. [15] Recent theoretical studies also suggest that monomeric Cu sites should not be excluded.…”

“…[9][10][11][12][13] Thes tructure of the active site is highly debated, with evidence for both (m-oxo) dinuclear copper [9,14] and tris(moxo) trinuclear copper centers ( Figure 1a). [18,19] Reaching an unequivocal conclusion regarding the active sites in these systems is therefore challenging. [16,17] Amajor challenge in these systems is associated with the presence of as mall fraction of active sites (often below 30-60 %; note that as ystem with 90 %a ctive-site-0.47 mol CH 3 OH/mol Cu-was recently reported.…”

Plasma‐activated methane partial oxidation reaction to oxygenate platform chemicals over Fe, Mo, Pd and zeolite catalysts

“…10,11 The field advanced greatly over the years, and promising results were obtained mainly with copper based catalysts, either on zeolites 12,13 or metal-organicframework 14 supports. Generally, this process requires high temperatures and operating pressures.…”