On the physical state of molecules in microporous solids

Molecules occupying the intracrystalline volume of molecular sieves are known to be in strong interaction with their environment, i.e., “solvated” by the zeolite framework, and in a state that is neither gaseous, liquid, or solid. It is shown that the experimental effective densities of simple sorbed substrates, at saturation of the zeolite intracrystalline volume, may exceed their densities in the liquid and solid states, which can be explained in terms of confinement effects. A modified van der Waals equation is also proposed to describe the physical state of molecules sorbed in microporous solids below saturation coverage. It implies that these molecules could be considered as being in a pseudo-supercritical state at a temperature lower than their critical temperature. Here we attempt to explain how confinement effects can affect the physical state of molecules sorbed in the intracrystalline volume of microporous solids and how it may affect, eventually, their reactivity in microporous catalysts.