relevant applications (19–22). These materials
adsorb CO2 at the very low partial pressures
needed for air capture requirements, because
charge density is highly and uniformly localized
in a contracted (1D) channels (20–23).
We show that the inclusion of the Al3+ (or Fe3+)
cations introduces open metal coordination sites
within these MOFs and enhances their H2O
affinity without affecting CO2 adsorption. Single-
crystal x-ray diffraction (SCXRD) measurements
revealed exposed open metal coordination sites
within precisely controlled square-shaped channels
with periodically arrayed fluoride anions. Calori-
metric studies revealed a relatively low energy
requirement for the MOF adsorbent regeneration-
recycling compared with the solid desiccants
such as zeolite 4A. Single- and multicomponent
adsorption experiments showed that these mate-
rials selectively adsorbed H2O and CO2 from
gas streams containing N2, CH4, and higher
hydrocarbons such as C2+n. Multiprobe solid-state
nuclear magnetic resonance (NMR) and peri-
odic density functional theory (DFT) calcula-
tions confirmed that H2O and CO2 occupy distinct
adsorption sites.
We obtained fluorinated MOFs NiAlF5(H2O)(pyr)2·
2(H2O) (KAUST-8, AlFFIVE-1-Ni; pyr, pyrazine) and
its isomorph NiFeF5(H2O)(pyr)2·4(H2O) (KAUST-9,
FeFFIVE-1-Ni), which we expected to be isostructural to SIFSIX materials, based on pillaring
the M′(II)-pyrazine square-grid layers with [SiF6]2–
anions (21–23) but replacing the original [SiF6]2–
building block with a suitable inorganic pillar: an
octahedrally coordinated metal center that should
accommodate five fluoride anions (F–) and a H2O
molecule in its coordination sphere. We selected
trivalent metals M, Al(III) and Fe(III), for the inorganic pillar [MF5(H2O)]2–, as these are observed
to adopt an octahedral fluorinated environment
(24–26).
Although AlFFIVE-1-Ni and FeFIVE-1-Ni are
isomorphic to the SIFSIX materials (21–23),
the explicit replacement of Si(IV) by Al(III) or
Fe(III) is not synthetically straightforward because no direct source of MF5(H2O) is available.
Consequently, a hydrothermal synthesis that permitted the in situ formation of the requisite inorganic pillar, [MF5(H2O)]2–, was performed using
pyrazine, Ni(NO3)2·6H2O, and Al(NO3)3·9H2O [or
Fe(NO3)3·9H2O] under highly acidic conditions
(48% aqueous HF) that yielded to square-shaped
crystals insoluble in water and common organic
solvents (see materials and methods). The single-crystal structures are similar to SIFSIX, adopting
tetragonal space groups (I4/mcm for AlFFIVE-1-
Ni and P4/nbm FeFFIVE-1-Ni) with primitive
cubic topology resulting from the pillaring of
Ni(II)-pyrazine 2-periodic square-grid layers with
[MF5(H2O)]2– (M = Al3+ or Fe3+) inorganic pillars
(Fig. 1A). Le Bail refinements (27) of the powder
XRD (PXRD) patterns for the as-synthesized materials were carried out for full profile fitting against
the resultant structure from SCXRD, displaying a
good agreement factor [unweighted-profile R factor (Rp) and weighted-profile R factor (Rwp) for
AlFFIVE-1-Ni and FeFFIVE-1-Ni are Rp = 0.086,
Rwp = 0.108 and Rp = 0.110, Rwp = 0.085, re-spectively] and confirming the phase purity of
the as-synthesized fluorinated MOFs (fig. S1).
In the pillars, the coordinated H2O molecule
and the three axial F atoms that point into the 1D
channel are disordered over four positions and
not crystallographically distinguishable. This disorder is confirmed by the 19F solid-state magic-angle spinning (MAS) NMR spectrum of the
as-synthesized AlFFIVE-1-Ni, which contains
one single and broad isotropic resonance (figs. S2
732 19 MAY 2017 • VOL 356 ISSUE 6339 sciencemag.org SCIENCE
Fig. 1. Structure description and characterization of as-synthesized, dehydrated, and
rehydrated forms of AlFFIVE-1-Ni (KAUST-8). (A) Repetitive square motif in the Ni-pyrazine
(4,4′) square-grid layer. (B) Projection along [001] of the crystal structure of AlFFIVE-1-Ni, with a
primitive cubic topology illustrating the connectivity of the hosted water guest molecules within the
1D channel. (C) Projection along [001] of the crystal structure of the dehydrated form of AlFFIVE-1-
Ni, showing the trigonal bipyramid (AlF5)2– inorganic building block. (D) Projection along [001] of the
crystal structure of the rehydrated form AlFFIVE-1-Ni, revealing the water molecule within the
coordination sphere of Al3+ cations. (E to H) DFT calculation by applying a progressive removal
of H2O molecules (from 3 to 0 per unit cell) on AlFFIVE-1-Ni, confirming the crystal structure of
the as-synthesized form and the dehydration mechanism. (I) 1H and (J) 27Al MAS NMR spectra of
AlFFIVE-1-Ni recorded on as-synthesized sample (†), sample previously heated at 105°C under
vacuum for 8 hours (‡), and rehydrated sample (§), confirming the departure and reversion of water
molecules within the materials.
