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Page 4 of 21 Sun et al. Microstructures 2023;3:2023032 https://dx.doi.org/10.20517/microstructures.2023.32

[49-51]
porous silica-aluminate material with a pore size between 5 Å and 12 Å . Among them, zeolite 13X is the
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-1
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most widely studied adsorbent, with a specific surface area of 726 m g , a pore volume of 0.25 cm g , and a
CO adsorption capacity of 16.4 wt% (0.8 bar) at room temperature, which can be used as a benchmark for
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solid adsorbents [52,53] .
Another class of solid adsorbents is carbon-based adsorbents, including activated carbon, carbon molecular
sieves, and carbon nanotubes, which have been used for different forms of CO separation. The common
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activated carbon has an inorganic porous structure, high surface area, and high CO adsorption capacity,
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and it can be generated through resin and other pyrolysis processes, which is a low-cost and highly available
adsorbent [54,55] . Even in an environment with water vapor, activated carbon can maintain its structure
without being destroyed and has high adsorption stability. However, activated carbon has lower CO
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adsorption capacity at low pressures and poor adsorption selectivity for different gases due to the absence of
an electric field generated by cations on the activated carbon surface. To sum up, these traditional
adsorption technologies and adsorbents need to be further improved to enhance their ability of CO capture
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and separation.

MOFs are the most representative new porous materials and promising adsorbents, which have attracted
extensive attention in gas separation applications [56,57] . As a porous adsorbent with a framework structure
formed by organic ligands bridging metal ion nodes, the MOF has a very high surface area, ultra-high
porosity, flexibility of the porous structure, and diversity of surface functional groups due to the presence of
organic ligands and is easy to be chemically modified. These advantages make MOFs exhibit great potential
for CO capture and storage [58-60] .
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CO capture performance parameters
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Compared with other solid materials, the greatest advantage of MOFs lies in their ability to precisely fine-
tune their pore size through the modulation of ligand size. At the same time, the pore environment can be
modified by modifying the functional groups on the metal ions (clusters) or ligands to enhance the
interaction force with the guest molecules, thus achieving the separation of different gas molecules [61-64] .
There have been several reviews on MOFs for gas separation applications, and they have made great
progress in adsorptive separation applications in the past few years. However, there are still some
limitations. To address the limitations of MOFs, a series of studies have been conducted in recent years on
improving their adsorption and separation capacities, expanding new structures, novel functionalization
[65]
pathways, and adopting hybrid systems and techniques . In addition, studies exploring the adsorption
mechanism of MOFs and the improvement of their adsorption capacity in moist environments have
gradually become the focus of attention [14,66] . An adsorbent suitable for capturing CO from flue gas should
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consider the following performance parameters.

The adsorption capacity of CO 2
This is a key criterion for evaluating the performance of solid sorbents and represents the quantity of
sorbent required for a given load, as well as the adsorbent bed size, which is deemed to be a key factor in
determining the energy requirement in the regeneration step. In addition, the amount of CO adsorbed is
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related to its partial pressure in the gas phase. The specific adsorption of CO indicates the ability of the
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MOF material to adsorb CO 2

The selectivity of CO 2
It represents the adsorption ratio of the adsorbent for CO to other gases (usually used for post-combustion
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capture and natural gas upgrading). The sorption selectivity of a gas mixture for CO can be estimated
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