Effect of Shaping MOFs on Adsorption Performance By Using Adsorption Microcalorimetry

Many laboratory scale studies in the field of gas adsorption deal with as-made samples in the form of powders. However, to be of any applied use in an adsorption based process such as PSA (pressure swing adsorption), the powders need to be shaped, often in the form of spherical beads or cylindrical pellets. Nevertheless, gas separation studies using MOF pellets are still very scarce and the majority of studies concentrate on the separation performance without asking questions about the effect of the binder used or pelletization process on the fundamental adsorption properties.

The effect of pelletization on the adsorption properties of the active materials should ideally be limited. Indeed, the physical properties of the material can be slightly affected as one expects a certain decrease in uptake/working capacity, proportional to the amount of binder used. However, the chemical properties which influence selectivity should remain unchanged in the presence of the binder. Other parameters such as adsorption uptake kinetics should equally remain unchanged.

In this work, we have concentrated on the fundamental gas adsorption properties of several ‘topical’ MOFs, which not only are hydrothermally stable, cheap and easy to scale-up, but have also recently demonstrated a particular interest for gas separations. The aim here is to compare the initial powder with the shaped materials.

Thus, a series of four Metal-Organic Frameworks (UiO-66(Zr), UiO-66(Zr)_NH₂, MIL-100(Fe) and MIL-127(Fe)) were synthesized as powders and spheres and these were characterized by gas adsorption at 30°C to 20 bars. The adsorption experiments were coupled with microcalorimetry to gain the energetic insight required to highlight any changes in chemistry. Results show that whilst most variations are as expected, interesting effects including partial pore restriction and active site ‘protection’ were also evidenced for some shaped materials suggesting that the binder may play a more complex role. These effects will be explored further in this communication.