Direct conversion of methane-to-methanol in Cu-exchanged small-pore zeolites

Licentiate thesis, 2020

Fossil fuel consumption continuous to increase worldwide and of all the fossil fuels, natural gas is growing the most. The combustion of natural gas, which mainly contains methane, is more environmentally friendly than oil or coal thanks to its high specific energy. More energy gained and less CO2 released per kg fuel drives an increase in production. This in turn increases the demand on manageability, resulting in the common practice to liquefy natural gas. The gas is kept in liquid form, via high pressure or low temperature, for transport and distribution. In an effort to reduce the energy cost of managing gaseous energy resources, the conversion of methane into its liquid counterpart methanol is highly desired. The established procedure for the conversion is a large scale, multi-step, power-plant process, and there is a
need for a small scale, direct conversion alternative. Copper-exchanged zeolites are considered promising candidates for the methane-to-methanol reaction, where mono-, dimer, and trimer Cu-clusters have been suggested to be the active site. In this thesis, the catalytic properties of Cu-dimers in zeolites are studied using first-principles calculations, ab initio thermodynamics, and micro kinetic modeling. As a first step, the stability of the Cu-dimer structure in SSZ-13 is investigated under direct conversion conditions. The zeolite is found to contain water and the structure of the proposed active site highly dependent on temperature and partial pressure of relevant gases. Under reaction conditions, the Cu2O and Cu2OH structures are found to be energetically preferred. Evaluating the reaction path for direct conversion over the identified active sites, reveals a low activity for the reaction, stemming from a high activation barrier of the C-H bond in methane and an inability for methanol to desorb. The activity of the Cu2O site is, however, increased when water is added into the reaction mechanism. Presence of water enables desorption of the reaction products and results in an endergonic reaction path. The Cu2OH site responds in an opposite manner with respect to water, becoming less active. The new insights on the nature of the active site and the reaction mechanism provide a deeper understanding, which will aid the future search for new catalytic materials with high activity and selectivity.