Ullah Saif, Jenkins Trevor, Tan Kui, Li Jing, Winter Stephen M, Thonhauser Timo
Department of Physics and Center for Functional Materials, Wake Forest University, Winston-Salem, NC, 27109, USA.
Department of Chemistry, University of North Texas, Denton, TX, 76203, USA.
Small. 2025 Jun;21(22):e2412312. doi: 10.1002/smll.202412312. Epub 2025 Apr 13.
Gas separation is a critical industrial process that consumes a significant amount of energy due to the widely used techniques that are currently employed. Adsorptive materials-such as metal-organic frameworks (MOFs)-show promise as an energy-efficient alternative. Of particular current interest are novel, temperature-dependent separation processes in MOFs, such as the recently reported separation of ternary isomeric hydrocarbon mixtures within one and the same material. However, the mechanisms of these highly desirable separations remain poorly understood. Herein, through a combination of ab initio simulations and statistical mechanics, it is shown that the temperature dependence is the result of a constraint on the guest molecule's entropic degrees of freedom when loaded into the MOF, caused by the fortuitous tight fitting of the guest inside the pore. While the framework applies to all molecular adsorption in porous media, it is essential for the description of large molecules in small pores, which is demonstrated here using the separation of C6 isomers in Ca(Htcpb) as a test case. The developed framework and analysis not only reveal the reason why separation occurs but also predict the temperatures at which it takes place, thus opening the door to newly designed MOFs with tailor-made precision.
气体分离是一个关键的工业过程，由于目前广泛使用的技术，该过程消耗大量能源。吸附材料，如金属有机框架（MOF），有望成为一种节能替代方案。当前特别受关注的是MOF中新型的、依赖温度的分离过程，例如最近报道的在同一种材料中分离三元异构烃混合物。然而，这些备受期待的分离机制仍知之甚少。在此，通过从头算模拟和统计力学相结合的方法表明，温度依赖性是由于客体分子装入MOF时其熵自由度受到限制所致，这是由客体在孔内偶然的紧密契合引起的。虽然该框架适用于多孔介质中的所有分子吸附，但对于小孔中大分子的描述至关重要，本文以Ca(Htcpb)中C6异构体的分离为例进行了说明。所开发的框架和分析不仅揭示了分离发生的原因，还预测了分离发生的温度，从而为精确设计的新型MOF打开了大门。
