Redox chemistry of copper in Cu-ZSM-5: a spectroscopy study

Reducing all sources of pollution is now a goal for every government and company in the world. Methane is an essential component of natural gas responsible for greenhouse effect, but it is also useful as a fuel or as a commodity chemical. The issues in trying to use this gas in order to reduce its rejection in the atmosphere are its storage and transportation. Eventually, it is possible to convert it into methanol to facilitate such processes, but the easiest way nowadays is through syngas chemistry, requiring high pressure and temperature conditions. A more advisable way would be the direct conversion of methane to methanol using copper exchanged zeolites, that have been investigated for several years, as it would be both more economical and ecological way to transform methane. Some Cu-oxo species have been identified as responsible in the catalytic role of zeolite, but research is still ongoing to find a way to improve productivity. This study focuses on the characterization (by using spectroscopic and volumetric techniques) of Cu-ZSM-5 and on the optimization of the pre-activation of its copper sites. The purpose was to determine if volumetry measurements are an alternative to synchrotron radiation in quantitatively assessing the oxidation state of copper ions and to establish a set of data on this type of zeolite for specific in-situ spectroscopy. The results of these spectroscopies show unexpected features depending on activations performed. Also, the amount of Cu(I) after reductive activation as determined by volumetry is in the margin of error of what was estimated by X-ray Absorption Spectroscopy, offering a cheaper, more accessible and sustainable alternative to synchrotron experiments.