Kinetic and Chemical Affinity Quantum Sieving Effects of ZIF-67 for H2 and D2 and Gas Chromatographic Separation of Hydrogen Isotopes on Columns Packed with ZIF-67@NH2-γ-Al2O3

Abstract

Hydrogen isotopes have extremely important roles in many fields, but they have almost the same chemical and similar physical properties, which makes the separation process very difficult. In this work, ZIF-67 was selected for the separation of hydrogen isotopes. The theoretical calculation results showed that the adsorption selectivity of ZIF-67 for hydrogen isotope mixtures (D2/H2 = 1:1) increased from 1.66 to 2.08 with the temperature decreasing from 77 to 20 K, indicating that ZIF-67 has a chemical affinity quantum sieving (CAQS) effect. Meanwhile, the diffusion coefficients of H2 and D2 were also different; the kinetic selectivity of D2/H2 was 4.41 at 70 K, indicating that ZIF-67 has a kinetic quantum sieving (KQS) effect at low temperatures. When the temperature was higher than 72 K, the diffusion coefficient of H2 was higher than that of D2; ZIF-67 showed a typical kinetic sieving effect and the kinetic selectivity of H2/D2 was 1.29 at 77 K. Based on the fact that due to the CAQS effect, the adsorption capacity of D2 in ZIF-67 was higher at 77 K and the diffusion coefficient of D2 was lower at the same time, it is expected that ZIF-67 can effectively realize the separation of hydrogen isotopes. In contrast, ZIF-8, isostructural to ZIF-67, has no difference on H2 and D2 in view of adsorption capacity and adsorption heat and, consequently, has no effect on separation, indicating the importance of the incompletely filled d-orbitals (Co2+ versus Zn2+). To verify the separation performance of ZIF-67, the composites ZIF-67@NH2-γ-Al2O3 were synthesized by the layer-by-layer (LBL) method and used as a gas chromatographic stationary phase packed in a 1 m column (1.0 m × 2.0 mm I.D.) for separation of hydrogen isotopes H2 and D2. Under optimal chromatographic separation conditions, the resolution (R) reached 1.79 and the separation time was 5.55 min with the optimal composite ZIF-67@NH2-γ-Al2O3 at 77 K.