Page 102 - Read Online

Sun et al. Microstructures 2023;3:2023032 https://dx.doi.org/10.20517/microstructures.2023.32 Page 5 of 21

quantitatively by means of a single-component gas sorption isotherm or ideal adsorption solution theory
(IAST). The CO selectivity of materials is primarily due to the following aspects (the relevant parameters
2
are shown in Table 1): (1) Selectivity based on pore size screening (molecular sieve effect). Due to the
different kinetic diameters of gas molecules of each component in the gas mixture, CO can be effectively
2
separated from the gas mixture by precisely adjusting the pore size of the material; (2) Selectivity based on
adsorption (thermodynamic separation), i.e., separation based on the interaction forces between different
gas molecules and the material surface and; (3) Selectivity based on diffusion effects, i.e., to separate the gas
mixture according to the different diffusion rates of different gases in the material [5,67] .

The adsorption enthalpy of CO 2
Adsorption enthalpy is another key parameter for evaluating the performance of the storage of CO by
2
physical adsorption. It represents the strength of the interaction between the adsorbent and the adsorbate
molecules. Moreover, it is an indicator of the energy required to regenerate a solid adsorbent, and the
magnitude of the adsorption enthalpy clearly affects the cost of the adsorbent regeneration process. If the
adsorption enthalpy is too high, the material binds CO too strongly, requiring a large amount of energy to
2
break the framework-CO interaction, thereby increasing the regeneration cost. Conversely, very low
2
adsorption enthalpy is undesirable. While regeneration costs will be lower, the captured CO will be less
2
pure, resulting in lower adsorption selectivity and a larger adsorption bed size [68-70] . Typically, the adsorption
enthalpy of MOFs is in the range of 20-50 kJ mol , which is comparable to other physical adsorbents (such
-1
as zeolites). Table 2 shown some MOF-based sorbents with their CO uptake capacity and Q .
st
2
The experimental adsorption enthalpy (Q ) was applied to assess the strength of the bond between the
st
adsorbent and the adsorbate and was defined as:

The adsorption enthalpy, Q , is determined using the pure component isotherm fits using the Clausius-
st
Clapeyron equation, where T(K) is the temperature, p(kPa) is the pressure, and R is the gas constant.

The stability of the CO 2 adsorbent
In order to reduce operating costs and operational difficulties, solid adsorbents must be able to be used in a
wide range of industrial environments and show good stability under fume conditions, under adsorption
operating conditions, and during multi-cycle adsorption regeneration. Particular attention must be paid to
the stability of the adsorbent in the presence of water vapor. In addition, chemical and mechanical stability
[71]
are equally important .
The adsorption/desorption kinetics of CO 2
Sorption-regeneration cycle times are largely dependent on the kinetic properties of the CO adsorption-
2
desorption curves measured in breakthrough experiments. The ability to effectively reduce the cycle time
and the amount of adsorbent, thereby lowering the cost of CO separation, is the first parameter to be
2
considered in selecting the CO adsorbent. Adsorbents that adsorb and desorb CO in a relatively short
2
2
period of time become the preferred choice.
The cost of adsorbent material
This is an essential ingredient in choosing an adsorbent material. Owing to the high preparation cost, many
adsorbents with excellent adsorption properties are not successfully used in industry. Therefore, the
preparation of materials with good adsorption properties at low cost is considered to be the main goal of
researchers in the field of CO capture.
2