Catalytic Conversion of Methane to Methanol Using Cu-Zeolites

The conversion of methane to value-added liquid chemicals is a promising answer to the imminent demand for fuels and chemical synthesis materials in the advent of a dwindling petroleum supply. Current technology requires high energy input for the synthesis gas production, and is characterized by low overall selectivity, which calls for alternative reaction routes. The limitation to achieve high selectivity is the high C–H bond strength of methane. High-temperature reaction systems favor gas-phase radical reactions and total oxidation. This suggests that the catalysts for methane activation should be active at low temperatures. The enzymatic-inspired metal-exchanged zeolite systems apparently fulfill this need, however, methanol yield is low and a catalytic process cannot yet be established. Homogeneous and heterogeneous catalytic systems have been described which stabilize the intermediate formed after the first C–H activation. The understanding of the reaction mechanism and the determination of the active metal sites are important for formulating strategies for the upgrade of methane conversion catalytic technologies

Reaction Conditions of Methane-to-Methanol Conversion Affect the Structure of Active Copper Sites

Determining the structure of the active Cu sites, which are associated with the methane conversion intermediate during stepwise, low-temperature, methane-to-methanol conversion, represents an important step for the upgrade of this reaction route to a viable process. Quick X-ray absorption spectroscopy allowed us to follow the electronic and structural changes to the active Cu sites during reaction with methane and during desorption of the activated intermediate. A large fraction (41%) of the oxygen-activated Cu^II reacted with methane and underwent reduction to Cu^I. When the intermediate was released as the product MeOH into the gas phase after reaction with water, the fraction of Cu^I was simultaneously converted back to Cu^II. Therefore, the activation of methane involves a change in the copper oxidation state. Density functional theory calculations identified [Cu^I–OCH₃–Cu^II] and [Cu^I–OH–Cu^II] as energetically plausible structures of the adsorbed intermediates. The structure of the active Cu sites is also a function of conditions. In a dry pretreatment environment, the Cu sites took the form of dehydrated Cu^II oxide species, well characterized in the literature as mono­(μ-oxo) and bis­(μ-oxo)­dicopper species. Under wet conditions, the dicopper species was destabilized to a hydrated Cu^II species, but a small amount of water-stable Cu^II oxide remained that was also active for conversion of methane to methanol