Polycrystalline Covalent Organic Framework Films Act as Adsorbents, Not Membranes

Area of Science:

• Materials Science
• Chemical Engineering
• Separation Science

Background:

• Covalent organic frameworks (COFs) are explored for energy-efficient separations, with thick films reported to separate various molecules via size-selective transport.
• Previous studies suggested COF membranes function by excluding molecules based on size through ordered pores.

Purpose of the Study:

• To investigate the actual separation mechanism of thick, polycrystalline COF films.
• To determine if COF films act as size-selective membranes or as adsorbents.

Main Methods:

• Adsorption isotherms of dyes on COF powders were measured.
• Computational modeling was used to understand adsorption thermodynamics.
• COF pellets were tested for dye "rejection" under varying conditions (volume, flow rate).
• Mixed dye separation experiments were conducted in flow.

Main Results:

• COF powders exhibit high-capacity adsorption for dyes, with affinities spanning three orders of magnitude.
• Adsorption behavior correlates with previously reported "separation" performance.
• Computational analysis links adsorption differences to entropic gains.
• COF pellets demonstrate volume- and flow-rate-dependent dye removal, consistent with adsorption.

Conclusions:

• Thick, polycrystalline COF films function primarily as adsorbents, not size-selective membranes.
• Observed separation phenomena in prior studies can be explained by adsorption mechanisms.
• The findings necessitate a re-evaluation of the COF membrane literature.
• Efficient separation using COFs based on pore transport remains an open challenge.