Rotating pyrazine rings put a kink in Xe isotherms
SIFSIX-3-Fe takes up more Xe than Kr because its pores match Xe, while an inflection in SIFSIX-3-Ni’s Xe isotherm is attributed to pyrazine rings ordering from disordered to aligned rotations.
Source
Effect of ring rotation upon gas adsorption in SIFSIX-3-M (M = Fe, Ni) pillared square grid networks
What they did
Authors synthesised pillared [M(pyz)2SiF6] (M = Fe, Ni), measured Xe/Kr isotherms and isosteric heats, and used computation of IN/OUT pyrazine configurations to explain a non-Langmuirian inflection.
What they found
At 1 atm/298 K, SIFSIX-3-Fe adsorbs 54.9 cm3 STP g−1 Xe vs 30.8 Kr; Ni analogue 56.2 vs 12.6. Xe Qst at infinite dilution is 27.4 kJ mol−1 (Fe) vs 18.9 (Ni). The Ni inflection tracks a disordered-to-ordered ring rotation, not breathing.
The limits
What it doesn't show
The rotational model is inferred from isotherms and computation, not a single-crystal movie of rings flipping under Xe.
Key terms
- SIFSIX-3-M
- Pillared square-grid MOF [M(pyrazine)2SiF6] with ~3.5–3.8 Å pores.
- Isosteric heat Qst
- Enthalpy of adsorption at a given loading, from isotherms at several T.
- Inflection
- A step/change in slope of an isotherm signaling a structural or ordering transition.
- IN/OUT ring states
- Two rotational poses of each pyrazine relative to a cage.
- Commensurate pore
- Pore size matched to the Xe atom, boosting affinity and selectivity.
Flashcards
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Quiz yourself
Xe is preferred over Kr mainly because:
Common questions
Xe vs Kr uptake on SIFSIX-3-Fe at 1 atm, 298 K?
54.9 vs 30.8 cm3 STP g−1.
Infinite-dilution Xe Qst on Fe?
27.4 kJ mol−1.
What causes the Ni Xe inflection?
Disordered-to-ordered pyrazine rotations, not breathing.
Pore diameters?
About 3.5–3.8 Å.
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