Quantum Sieving In Metal-Organic Frameworks: A Computational Study

Deuterium and tritium, as the heavier isotopes of hydrogen, are useful resources and have numerous applications. Currently, the production of these isotopes of hydrogen is mainly via various isotope separation techniques. Traditional separation techniques, including cryogenic distillation, centrifugal enrichment and electromagnetic mass spectrometry, are very expensive, high energy consumption, and poor separation efficiency. Recently, quantum effect of hydrogen at low temperature has attracted increasing attention. And this effect could be emphasized by considering adsorption in confining space which can be denoted as the "quantum sieving". This phenomenon have been verified in other traditional adsorbents such as carbon nanotubes and zeolites, and shown high selecivities. Metal-Organic Frameworks (MOFs), commonly recognized as "soft" analogues of zeolites, is a new class of nanoporous materials. MOFs have many advantages like adjustable pore sizes and controllable properties, which may be suitable candidates for the separation of hydrogen isotopes through quantum sieving. Computational chemistry can provides theoretical guidance for the preselecttion of optimal adsorbents, establishe correlations between pore size and selectivity, and give help to synthesis in the future, which also can save a lot of time and energy for complicated experimental works.In this work, a systematic computational study was performed to investigate the quantum sieving in nine typical metal-organic frameworks (MOFs) for the separation of hydrogen isotope mixtures. The main contents and findings are summarized as follows.1. The results show that Cu(F-pymo)2 and CPL-1 exhibit exceptional selectivity that is higher than other MOFs as well as other traditional nanoporous materials such as carbon nanotubes, slit-shaped graphites and zeolites studied so far.2. A new concept named "quantum effective pore size" (QEPS) was proposed in this work, which can incorporate the effects of quantum sieving, and thus is temperature-dependent. Based on the new pore size, good correlations between pore size and selectivity can be established for the MOFs considered3. In this work, the quartic Feynman-Hibbs (FH) effective potential was used to account for the quantum effects, and electrostatic interaction between solid and fluid were consided.