Commensurate-incommensurate adsorption and diffusion in ordered crystalline microporous materials

Area of Science:

• Materials Science and Engineering
• Physical Chemistry
• Nanotechnology

Background:

• The interaction between guest molecules and host structures is critical in adsorption and separation processes.
• A mismatch or match between host periodicity and guest molecule length significantly impacts molecular behavior within porous materials.

Purpose of the Study:

• To elucidate the influence of commensurateness and incommensurateness on adsorption and diffusion characteristics of linear chain molecules in crystalline hosts.
• To demonstrate the potential for selective separation of hydrocarbon mixtures using tailored crystalline host materials.

Main Methods:

• Analysis of published experimental data on adsorption and diffusion.
• Validation and support through molecular simulation results.
• Exploration of entropy-based separation mechanisms at pore saturation conditions.

Main Results:

• Non-monotonic trends in adsorption strengths and diffusivities were observed due to varying degrees of commensurateness.
• Successful demonstration of separating mixtures of n-alkanes, n-alcohols, and hydrocarbon isomers.
• Identification of entropy effects at pore saturation as a key factor in separation efficiency.

Conclusions:

• The precise control over host material dimensions, topology, and connectivity enables selective separation of molecular mixtures.
• Commensurateness is a tunable parameter for optimizing adsorption and diffusion in porous materials.
• Entropy-driven separation at saturation offers a practical approach for separating homologous series and isomers.