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Sun et al. Microstructures 2023;3:2023032 https://dx.doi.org/10.20517/microstructures.2023.32 Page 11 of 21

at the 2 position of the imidazole ring, are oriented in the identical direction in each channel so that pairs of
NH groups face away from each other along contrary sides of the channel walls. cu(adci)-2 shows higher
2
-1
CO adsorption capacity (2.01 mmol at 298 K and 15 kPa g ) but a lower adsorption enthalpy at zero
2
-1
coverage (27.5 kJ mol ). It showed good selectivity and easy regeneration under both dry and humid
conditions [Figure 8].
CALF-20 consists of 1,2,4-triazole-bridged zinc(II) ion layers supported by oxalate ionic pillars forming a
3D lattice and a 3D pore structure [Figure 9]. The crystallographically unique zinc center is five-coordinate
with a distorted triangular bipyramidal geometry. The competitive separation on CALF-20 shows not only
preferential physisorption of CO below 40% RH but also inhibition of water adsorption by CO , which is
2
2
confirmed by computational modeling. Furthermore, CALF-20 facilitates industrial-scale CO capture in a
2
cost-effective and reliable manner . This shows that the reasonable addition of anionic column bracing
[111]
can effectively inhibit water adsorption, which, in turn, makes the adsorbent exhibit an excellent CO
2
capture capacity.
Efficient and sustainable CO capture can be achieved by porous physical adsorbents with high void fraction
2
[112]
whose sizes and electrostatic potentials complementary to CO molecules . Qazvini et al. proposed a
2
strong, recoverable, and affordable adsorbent called MUF-16 [Figure 10] . MUF-16(Co), MUF-16(Ni),
[113]
and MUF-16(Mn) were prepared by mixing 5-amino-m-m-phthalic acid (H aip), a cheap, commercially
2
available ligand, with cobalt (II), nickel (II), or manganese (II) salts in methanol. Through static adsorption
curves, IAST, and density functional theory calculations, it is determined that the one-dimensional channel
of MUF-16 can capture CO with high affinity while demonstrating weaker affinities for other competitive
2
gases such as CH , C H , C H , C H , C H , and C H . Therefore, MUF-16 has high CO adsorption
2
6
2
2
3
8
2
4
3
6
4
2
selectivity. The selectivity of MUF-16 to CO /CH and CO /C H is measured at 6,690 and 510, respectively.
4
2
2
2
2
Open metal site modification strategy
Besides the functionalization of MOFs, metal sites have an important impact on enhancing the capacity and
selectivity of CO relative to other gases. UMSs are usually partially positively charged, and these sites show
2
an affinity for larger quadrupole moments and greater polarizability for CO compared to N . It is these
2
2
open sites that lead to high CO uptake. Furthermore, the difference in intensity between CO and other gas
2
2
molecules is the driving force for CO capture [114-116] .
2
MIL-101 has two dissimilar mesopores and distinct metal sites in a single local pore. Shin et al.
demonstrated by adsorption isotherm combined with in situ crystallographic analysis that substrate-
adsorbate interactions influence the initial adsorption and pore coalescence steps .
[117]
Unsaturated alkali metal sites have been reported to be anchored in MOFs by tetrazolium-based patterning
+
to improve gas affinity . In NKU-521 (NKU denotes Nankai University), Li et al. effectively embedded K
[118]
cations into the trinuclear Co -tetrazolium coordination pattern . The embedded K sites were exposed in
+
2+
[119]
the pores of NKU-521 by dehydration, and the Q of CO increased to 41 kJ mol . The K cations actually
-1
+
2
st
act as gas traps and increase the CO -framework affinity, as measured by the Q , by 24%. Furthermore, the
st
2
effect of unsaturated alkali metal sites in MOFs on hydrocarbon separation was investigated. IAST
+
calculations and breakthrough experiments showed that the exposed K sites greatly improved the CO
2
capture and separation performance of this MOF [Figure 11].
UMSs and adsorbent polarity play an important role in CO adsorption. MIL-88 was selected as the
2
prototyping framework to verify the above strategy . By introducing the C3 symmetry of the second
[120]