Formation of Active Cu-oxo Clusters for Methane Oxidation in Cu-Exchanged Mordenite

Abstract

Cu-exchanged zeolites are known to be active in the selective oxidation of methane to methanol at moderate temperatures. Among them, Cu-exchanged mordenite (MOR) is the system that has so far shown the highest methanol yield per Cu atom. This high efficiency is attributed to the ability of MOR to selectively stabilize an active tricopper cluster with a [Cu3(μ-O)3]2+ structure when activated in the presence of O2 at high temperatures. In this study, we investigate the elementary steps in the formation of [Cu3(μ-O)3]2+ by in situ X-ray absorption spectroscopy and ultraviolet–visible spectroscopy. We demonstrate that the Cu cations undergo a series of thermally driven steps during activation that precede the formation of the active oxidizing species. We hypothesize that the thermal formation of highly mobile Cu+ species by autoreduction of Cu2+ in an inert gas is essential to enable the reorganization of Cu ions in MOR, which is necessary for the formation of a reduced precursor of [Cu3(μ-O)3]2+. Such a precursor can be oxidized in the presence of strong oxidantssuch as O2 and N2Oto form active [Cu3(μ-O)3]2+ at temperatures as low as 50 °C.

Active sites and mechanisms in the direct conversion of methane to methanol using Cu in zeolitic hosts: a critical examination

“…Subsequently, copper-oxo sites consisting of three copper atoms were suggested to be present in Cu/MOR, and associated with a further band at 30 000-31 000 cm À1 . 29,115 Importantly, no bands around 22 000 cm À1 , as had been observed previously in Cu/MOR, were found in this study. Support for this proposition subsequently came in the publication by Le et al, 113 where solidstate ion exchange (SSIE) was used for the synthesis of Cu/MOR.…”

“…Being interested in determination of the structure of these sites, specifically copper oxidation state and local geometry, numerous researchers have monitored this process using XANES, that have resulted in similar conclusions. 27,29,31,32,44,115 Briefly, as-made materials contain Cu(II) in a fully hydrated state, yielding the peaks at B8997 eV in XANES and an average first shell Cu-O coordination close to 6. The absence of any well-defined second coordination sphere in the EXAFS indicates a high level of disorder (mobility) of the hydrated copper, and the presence of highly symmetrical Cu(II)(H 2 O) 6Àn (OH) n ions in the non-calcined materials.…”

“…A number of studies have attempted to understand the structure of the copper sites existing within a variety of zeolites using EXAFS and in respect of the ability of those sites to selectively transform methane to methanol. 29,32,80,115,140,144 At first sight, EXAFS provides an attractive option for obtaining an understanding of the local structure surrounding the copper atoms under a wide range of conditions. Consequently, analysis of Cu K-edge EXAFS has been used on several occasions to ''determine'' the nature of the active sites present in various Cu/zeolite systems.…”

“…Consequently, analysis of Cu K-edge EXAFS has been used on several occasions to ''determine'' the nature of the active sites present in various Cu/zeolite systems. 29,32,80,115,148 Fig. 18 compares two salient examples each of which purport to favour different structural interpretations of the EXAFS obtained from Cu/MOR samples after high temperature activation in oxygen.…”

Importance of Methane Chemical Potential for Its Conversion to Methanol on Cu‐Exchanged Mordenite

“…This procedure leads to high selectivity to methanol, although requiring reconstitution of the active site after each cycle. [4] The yield of methanol offers direct informationo nt he utilization of the oxygen at these active Cu sites.…”