Metal-organic framework (MOF) materials are a kind of inorganic-organic hybrid materials with a open modular molecular architecture. Such a modularity opens a plethora of possibilities for promising applications. In particular, they can operate as sieves at a molecular level. In the present proposal we address a MOF which trap Carbon Dioxide molecules but not Methane molecules with an efficiency increasing with pressure. With the help of neutron scattering, we will study the microscopic structural changes in the material while the selective molecular trapping evolves with pressure. By studying the process at different temperatures, we will gain an invaluable information about the microscopic mechanisms responsible of the observed effect.