208
L. F. Vega et al.
As shown, Mg-MOF-74 has the highest affinity for carbon dioxide at low pressures among the studied materials, followed very closely by zeolite 13X. The low
pressure range is beneficial for zeolites with extra-frameworks cations, which more
easily attracts the quadrupole of CO 2 . However, when the operating pressures exceed
atmospheric conditions, zeolites become saturated due to lower pore volumes, and
higher uptake capacities can be obtained with MOFs or ACs instead. In addition,
most of the MOFs display roughly linear plots of adsorption vs. pressure, indicating
that the frameworks are not very attractive to CO 2 , and the saturation capacity have
not been reached at these conditions.
It should be mentioned that mesoporous materials, due to their large pore size,
cannot adsorb high amounts of CO 2 in the studied range, even with higher pore
volumes than other structures. To improve their capacity at lower pressures, and their
selectivity towards CO 2 , an amine functionalization study was performed. Results
from molecular simulations were compared with experimental data for bare and functionalized mesoporous materials, with different degrees of functionalization [182].
As can be seen in Fig. 20, a chemisorption effect was evident for samples with a relatively high degree of amine functionalization for MCM-41, increasing the adsorption
uptake at low pressures. In this case, simulations have served to isolate the effect
of chemical versus physical adsorption for these types of materials. In addition, a
distance between grafted amine chains similar to the molecular size of CO 2 was
found for the materials with a considerable effect for chemisorption. The proximity
of neighbour amine chains is essential for obtaining materials with high potential for
CO 2 adsorption and separation [182].
Phenomena as co-adsorption and/or site competition should be considered for an
assessment of the materials for their final application, as it is known that they affect
the performance of the CO 2 capturing materials for an industrial flue gas mixture.
Molecular simulations can help in this respect, as multi-component mixtures can be
calculated with force fields available for each of the pure compounds without the
need of experimental mixture data. We have conducted a realistic comparison for
gas stream at process temperature conditions, with a flue gas at 313 K (i.e. 40 °C)
chosen as the feed stream. Some of the binary 15%CO 2 /85%N 2 streams studied,
as well as ternary mixtures including H 2 O impurities up to 1%, are presented in
Fig. 21. It is observed that CuBTC shows poor performance due to competition with
nitrogen molecules, which reduces the adsorbent capacity. In comparison, Mg-MOF74 displays outstanding CO 2 storage capacity and selectivity both at low and high
pressures. In addition, the influence of impurities is more noticeable in zeolites 13X,
since it loses half of its capacity with just 0.01%H 2 O in the flue gas, and 1%H 2 O
makes it a useless material. A more detailed study on the effect of impurities of
the performance of selected adsorbent materials for CO 2 capture can be found in
references [15, 181].
Molecular simulations were further used to evaluate the behaviour at operating
conditions for pressure swing adsorption (PSA) and vacuum swing adsorption (VSA)
processes, including the impact of coexisting impurities [181]. Comparison between
the different regeneration strategies includes energetic requirements analysis as well
L. F. Vega et al.
As shown, Mg-MOF-74 has the highest affinity for carbon dioxide at low pressures among the studied materials, followed very closely by zeolite 13X. The low
pressure range is beneficial for zeolites with extra-frameworks cations, which more
easily attracts the quadrupole of CO 2 . However, when the operating pressures exceed
atmospheric conditions, zeolites become saturated due to lower pore volumes, and
higher uptake capacities can be obtained with MOFs or ACs instead. In addition,
most of the MOFs display roughly linear plots of adsorption vs. pressure, indicating
that the frameworks are not very attractive to CO 2 , and the saturation capacity have
not been reached at these conditions.
It should be mentioned that mesoporous materials, due to their large pore size,
cannot adsorb high amounts of CO 2 in the studied range, even with higher pore
volumes than other structures. To improve their capacity at lower pressures, and their
selectivity towards CO 2 , an amine functionalization study was performed. Results
from molecular simulations were compared with experimental data for bare and functionalized mesoporous materials, with different degrees of functionalization [182].
As can be seen in Fig. 20, a chemisorption effect was evident for samples with a relatively high degree of amine functionalization for MCM-41, increasing the adsorption
uptake at low pressures. In this case, simulations have served to isolate the effect
of chemical versus physical adsorption for these types of materials. In addition, a
distance between grafted amine chains similar to the molecular size of CO 2 was
found for the materials with a considerable effect for chemisorption. The proximity
of neighbour amine chains is essential for obtaining materials with high potential for
CO 2 adsorption and separation [182].
Phenomena as co-adsorption and/or site competition should be considered for an
assessment of the materials for their final application, as it is known that they affect
the performance of the CO 2 capturing materials for an industrial flue gas mixture.
Molecular simulations can help in this respect, as multi-component mixtures can be
calculated with force fields available for each of the pure compounds without the
need of experimental mixture data. We have conducted a realistic comparison for
gas stream at process temperature conditions, with a flue gas at 313 K (i.e. 40 °C)
chosen as the feed stream. Some of the binary 15%CO 2 /85%N 2 streams studied,
as well as ternary mixtures including H 2 O impurities up to 1%, are presented in
Fig. 21. It is observed that CuBTC shows poor performance due to competition with
nitrogen molecules, which reduces the adsorbent capacity. In comparison, Mg-MOF74 displays outstanding CO 2 storage capacity and selectivity both at low and high
pressures. In addition, the influence of impurities is more noticeable in zeolites 13X,
since it loses half of its capacity with just 0.01%H 2 O in the flue gas, and 1%H 2 O
makes it a useless material. A more detailed study on the effect of impurities of
the performance of selected adsorbent materials for CO 2 capture can be found in
references [15, 181].
Molecular simulations were further used to evaluate the behaviour at operating
conditions for pressure swing adsorption (PSA) and vacuum swing adsorption (VSA)
processes, including the impact of coexisting impurities [181]. Comparison between
the different regeneration strategies includes energetic requirements analysis as well