Abstract
Zeolitic imidazolate frameworks (ZIFs) undergo pressure-induced phase transitions that are peculiar to each ZIF. The phase transition is associated with the rotation of the imidazolate, and it is accompanied by an increase of their pore openings, affecting ZIFs performance in separation processes. This phenomenon is known as the gate opening or the swing effect. Here we report the metal dependence of the ambient pressure and high pressure (HP) phases of ZIF-8(M) with M = Mg, Fe, and Zn, determined by using periodic Kohn–Sham density functional models. We show that the substitution of Zn with Mg or Fe has a big influence on the gate opening energy, which significantly decreases, an opposite trend than what was previously reported upon functionalization of the linker. The lowest energy phase of ZIF-8(Fe) is different than for ZIF-8(Mg) and ZIF-8(Zn), and its structure is significantly closer to the HP phase. Multireference wave function methods have been used to study the electronic structure of ZIF-8(Fe), confirming the metal center to be high spin (S = 2) divalent iron in antiferromagnetic coupling. The high-spin nature of the iron species coupled with a band gap in the visible light range makes ZIF-8(Fe) an interesting material for catalysis and photocatalysis.
Highlights
The pressure associated with the phase transition strongly depends on the fluid and on the temperature: for example, for N2 at 77 K, the transition starts at 0.0002 bar,[15] while for O2 at 90 K is observed above 5000 bar.[8]
A second set of structures was optimized from the experimental Zeolitic imidazolate frameworks (ZIFs)-8(Zn)-AP,[8] ZIF8(Zn)-HP,[8] and ZIF-8(Fe)[19] cells upon metal substitution by fixing the dihedral angles involved in the rotation of the mIm rings
The optimization without geometrical constraints of the ZIF-8(Zn) structures resulted in the AP phase, independently of the phase used as guess structure
Summary
The pressure associated with the phase transition strongly depends on the fluid and on the temperature: for example, for N2 at 77 K, the transition starts at 0.0002 bar,[15] while for O2 at 90 K is observed above 5000 bar.[8] This high pressure phase (ZIF-8(Zn)-HP, see Figure 1d) shows a decreased separation ability due to the increase of the diameter of the 4MR and 6MR to 2.2 and 3.6 Å, respectively (“gate opening” or “swing effect” phenomenon).[8,17] The understanding of the role of ZIFs building blocks (the metal and the linker) on the framework flexibility is pivotal for directing ZIFs adsorption performances.[15] Hobday et al have studied how to tune the gate opening phenomena by changing the substituents on the imidazolate rings.[7] In this study, we employ periodic Kohn−Sham density functional methods (KSDFT) to explore the metal substitution effect on the structural flexibility of ZIF-8. Multireference wave function-based methods have been used to model ZIF-8(Fe) with the aim to characterize the electronic structure of iron centers
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