Effect of entropic constraints and conformational isomerism on the thermodynamics and kinetics of molecular adsorption in nano-porous materials

In recent years, much study has been devoted to chemical separations using adsorption within one-and-the-same nano-porous material, presenting an energy-efficient alternative to current methods such as cryogenics and distillation. A particularly desirable behavior is the temperature-dependent separation of binary and even ternary mixtures, with one recent example being that of C6 alkanes in the metal organic framework (MOF) Ca(H₂tcpb). In experiment, 22DMB and 3MP are found to adsorb within Ca(H₂tcpb) only up to 30 °C and 90 °C, respectively, whereas nHex may adsorb up to and beyond 150 °C. This behavior could not be explained using standard techniques, nor by temperature-induced structural changes in the MOF. Through a combination of ab initio simulations and statistical mechanics, we have developed a theory showing that the temperature dependence is the result of a constraint on the guest molecule’s degrees of freedom when loaded into the MOF. This constraint is caused by the fortuitous tight fitting of the guest inside the pore and is thus most noticeable for large molecules in small pores. With this model, we provide—for the first time—a framework that can qualitatively and quantitatively explain this temperature-dependent sieving behavior and intriguing molecular uptake. The model is also highly generalized, making it a powerful tool that opens the door to newly-designed MOF materials with tailor-made precision.