Molecular Sieving Properties of Nanoporous Mixed-Linker ZIF-62: Associated Structural Changes upon Gas Adsorption Application

The
evaluation of the flexibility in zeolitic imidazolate frameworks
(ZIFs) has been very useful to understand their performance in gas
adsorption and separation applications. Here, we have evaluated the
adsorption properties of a nanoporous mixed-linker ZIF-62 using a
combination of gas adsorption measurements, grand canonical Monte
Carlo simulations, and synchrotron X-ray powder diffraction under
operando conditions. While adsorption studies in nanoporous ZIF-62
at 77 K and atmospheric pressure predict a large O2/N2 separation ability, computational studies anticipate that
the observed differences must be attributed to kinetic restrictions
of N2 to access the internal porosity at cryogenic temperatures.
Interestingly, upon a small increase in the adsorption temperature
(90 K vs 77 K), both N2 and O2 are able to access
the inner porous structure through the promotion of a phase transition
(ca. 3.8% volume expansion) upon gas adsorption. This narrow phase
(np) to expanded phase (ep) structural transition in ZIF-62 is completely
suppressed above 150 K. Based on the excellent molecular sieve properties
of nanoporous ZIF-62 for O2/N2 at cryogenic
temperatures, we extended our study to the adsorption of linear and
branched hydrocarbons. This study predicts the preferential adsorption
of alkanes over alkenes in ZIF-62 for small hydrocarbons (C2), while in the case of C3 hydrocarbons and above, the
adsorption process is mainly defined by kinetic restrictions.