2. CO2 Adsorption on Metal–Organic Frameworks (MOFs)
Starting from the advent of the Industrial Revolution, anthropic activities have caused an increase in the carbon dioxide atmospheric concentration of more than 50% (i.e., from about 260 ppm of pre-industrial era [6
] to the current 400 ppm (and more) [7
]). Such anomalous change in the composition of Earth’s atmosphere is significantly reducing the fraction of absorbed solar radiant energy that the planet gives back to outer space. This phenomenon, known as the greenhouse effect [8
], is the main cause of the abnormal planetary temperature rise registered during the last two centuries. Anthropic CO2
emissions mainly come from three sources:
Transportation, through the combustion of fossil fuels;
Electricity production, (again) through the combustion of fossil fuels; and
Industry, through reforming fossil fuels, gaseous byproducts of industrial steelmaking and the cement industry, etc.
With particular regard to the last two scenarios, the implementation of adsorption columns downstream of electricity production and industrial plants can provide a drastic reduction in CO2
]. Up to the end of the last century, zeolites were considered as the best option among nanoporous adsorbents for CO2
capture, and they are still considered the benchmark in terms of a benefit–cost ratio [10
] because zeolites can also be synthesized starting from several waste materials [11
]. In the last two decades, a lot of effort has been focused on studying the CO2
adsorption properties of functionalized mesoporous silicates [12
]. Gradually, in the last 15 years, there has been an exponential development of studies about the CO2
adsorption properties of MOFs. Readers who are interested only in this specific topic are invited to also read a recent review by Ghanbari et al. [17
]. With respect to the latter paper, rather than focusing on the correlation between MOF structures and CO2
adsorption performances, this section of the present review will give emphasis to the actual suitability of MOFs to be used in industrial CO2
adsorption processes. In particular, potential applications in packed bed adsorption columns will be taken into account, whereas membrane-based separation needs specific treatises [18
]. An overview of the recent (limited to the last decade) literature about this subject is reported in Table 1
. For the purpose of analyzing these data, Table 2
reports the CO2
removal performances of the state-of-the-art currently used technologies like zeolite- and activated carbon-based adsorbents and amine scrubs.
In one of the first systematic works about the use of MOFs for adsorbing greenhouse gases [19
], Saha et al. investigated CO2
adsorption on MOF-5 and MOF-177 with respect to a traditional adsorbent such as the 5A zeolite. It is important to highlight that these two MOFs are among the ones that have been the most studied, and can be considered the nearest to actual implementation in real-world processes. In particular, MOF-177 is already produced on an industrial scale and commercialized under the brand Basolite®
]. The CO2
adsorption capacity reported in [19
] for MOF-177 by far exceeds that of the 5A zeolite when considering high pressures, but the 5A zeolite is capable of exploiting the most part of its total adsorption capacity at sub-atmospheric pressure (i.e., it has a higher affinity toward CO2
than MOF-177). As a consequence, MOF-177 should outperform the traditional adsorbent in pressure swing adsorption (PSA) [61
], rather than in vacuum swing adsorption (VSA) [62
]. MOF-177 also showed a fair CO2
selectivity (which should make it suitable for CO2
capture from flue gas streams) and provided hints of a good regeneration capacity (i.e., the complete reversibility of CO2
adsorption has been checked up to atmospheric pressure).
More recently [20
], Ullah et al. investigated the potential use of MOF-177 for CO2
separation, which is a key step in natural gas upgrading processes. CO2
thermodynamical selectivity was calculated starting from pure component adsorption isotherms by means of the ideal adsorbed solution theory (IAST), finding a value acceptable for envisaging a plant-scale application.
Still recently [22
], Mohamedali et al. tried to increase the affinity of MOF-177 toward CO2
by means of functionalization with ionic liquids. Unfortunately, the results of such a modification procedure cannot be considered quite as satisfactory. Indeed, as explained in more detail below, the post-synthesis adjustment of the CO2
adsorption behavior of a MOF is quite ineffective if, as in the case of MOF-177 [63
], the adsorbent surface is a priori lacking in exposed coordinatively unsaturated sites.
Another MOF that can be considered mature enough for real-world applications is the copper benzene-1,3,5-tricarboxylate known as MOF-199, HKUST-1, or Cu-BTC. Indeed, this MOF has also been the main subject of hundreds of research papers and is currently produced on an industrial scale, commercialized under the brand Basolite®
]. In about a decade, CO2
adsorption on Cu-BTC has been investigated deeply down to the molecular level, initially through speculative considerations induced by direct adsorption measurements [23
], and then through extremely fine-tuned experimental setups [64
]. From the application point of view, Hamon et al. [24
] managed to approach the structural limits of the CO2
adsorption capacity of Cu-BTC, obtaining adsorbed quantities about twice higher than those obtained at atmospheric pressure and less-than-ambient temperature [23
]. Moreover, in [24
], breakthrough experiments allowed the authors to notice a good CO2
selectivity, which would make Cu-BTC suitable for natural gas upgrading processes. The same breakthrough experiments allowed for the observation of the remarkable regeneration capacities of the MOF adsorbent, with a measured CO2
delta loading of 7.37 mol/kg between the adsorption step at 1000 kPa and the desorption step at 100 kPa for a 70-15-15 CO2
–CO ternary feed.
Some years later [25
], Ye et al. focused their attention on the potential use of Cu-BTC in CO2
capture from flue gas. Apart from the equilibrium characterization of the adsorbent, they performed breakthrough experiments that highlighted a good selectivity for CO2
. Moreover, repeated adsorption/desorption cycles allowed for the regeneration capabilities of Cu-BTC to be checked and to identify the temperature swing adsorption (TSA) as a preferable operating mode with respect to VSA.
Very recently [26
], Kloutse et al. published a work that attracted attention not only for the peculiar experimental methods used for performing adsorption runs, but also for having highlighted the good CO2
selectivity of Cu-BTC. Indeed, the separation of this specific couple of gases is a crucial step downstream of steam reforming processes and is usually carried out by absorption in aqueous solvents such as moderately concentrated amine solutions. Substituting chemical absorbers with adsorption columns for this task (especially if operating in PSA or VSA modes) could provide clear advantages in terms of energy efficiency (i.e., no heat needed during the regeneration of the columns) and plant maintenance costs (i.e., no plant component attacked by chemically aggressive liquids).
Like MOF-177, Cu-BTC has also been the object of functionalization trials, in order to fine-tune its CO2
adsorption behavior. As an example, Da Silva et al. functionalized Cu-BTC with ionic liquids and tested the resulting materials for CO2
adsorption, actually finding no significant benefits in the modified adsorbents with respect to pristine Cu-BTC [65
]. A different conclusion was obtained by Mohamedali et al. [27
], who impregnated Cu-BTC with 1-butyl-3-methylimidazolium acetate ([bmim][Ac]) and doubled the CO2
adsorption capacity of the pristine MOF at 15 kPa, which is a typical value for CO2
partial pressure in flue gas during the adsorption step of VSA and TSA processes. Moreover, [bmim][Ac]–functionalized Cu-BTC proved to be fully regenerable after several CO2
A distinct approach was adopted by Zhou et al., who impregnated different Cu-BTC samples with lithium nitrate and then heat-treated them under vacuum, nearly doubling the CO2
adsorption capacity with respect to the pristine MOF [28
]. Moreover, the CO2
equilibrium adsorption curve of Li-doped Cu-BTC resulted in being completely overlapped with the desorption one, thus indicating complete reversibility of the adsorption process (i.e., the possibility of fully regenerating the adsorbent in cyclic operations).
Trying to take one more step toward a real-world use of Cu-BTC in CO2
adsorption processes, it must be noted that both flue and natural gas usually contain water vapor, and that the interaction between Cu-BTC and water vapor can be quite problematic. Indeed, depending on the synthesis route, water can be present inside the Cu-BTC structure in the form of guest molecules that can be removed by activation, thus exposing coordinatively unsaturated sites (CUSs) [66
]. If rehydration occurs, water molecules can react with CUSs, causing ligand displacement and/or hydrolysis [67
], which eventually leads to structural collapse. Such an event has been confirmed for Cu-BTC, under specific conditions by Al-Janabi et al. [68
], who investigated the hydration process of this MOF using dynamic water vapor adsorption under flue gas emitting conditions. The same authors also found a range of values for state variables (temperature, relative humidity, and vapor partial pressure) suitable for preserving the structural integrity of Cu-BTC in the presence of moisture.
Another MOF that has been deeply studied in terms of CO2
adsorption is the magnesium variant of MOF-74, also known as Mg-CPO-27 and commercialized under the brand Porolite®
]. As an example, Bao et al. measured CO2
adsorption/desorption isotherms on Mg-MOF-74 up to atmospheric pressure, finding a remarkable adsorption capacity at ambient temperature [29
]. They also extracted the CO2
kinetic selectivity from the direct reading of gas pressure and adsorption volume changes with time during the survey of the isotherm points. Such obtained results can thus be hardly compared with those obtainable by means of breakthrough experiments. With regard to the regeneration capacity, the CO2
adsorption and desorption branches of the isotherms basically overlapped with each other, suggesting the occurrence of a reversible process.
Among the MOFs most studied for CO2
adsorption, Cu-BTC and Mg-MOF-74 also stood out for their common structural feature (i.e., the presence of CUSs). CUSs are supposed to act as preferential CO2
adsorption sites due to their capability of interacting with molecular quadrupoles [23
]. Such hypothesis has been strengthened first by theoretical predictions of first-principles (DFT) studies [69
], and then confirmed by specific experimental investigations (e.g., 2D correlation spectroscopy [64
]). As an example of how CUSs can influence CO2
adsorption behavior in MOFs, Mason et al. compared CO2
adsorption isotherms at ambient temperature up to atmospheric pressure on both MOF-177 and Mg-MOF-74 [21
]. In such conditions, the latter MOF adsorbed much more CO2
than MOF-177, mainly because of its CUSs, which were completely absent in MOF-177 [63
]. CUSs were also responsible for the high CO2
thermodynamical selectivity in Mg-MOF-74, which makes such an adsorbent suitable for CO2
capture from flue gas streams by means of TSA [21
Shortly afterward [30
], Herm et al. measured CO2
, and H2
high pressure adsorption isotherms on Mg-MOF-74 and used IAST for calculating both binary (i.e., CO2
) and ternary (i.e., (CO2
)) thermodynamic selectivity, the latter being particularly important due to the incomplete CH4
conversion that can occur in reforming processes and to the very high H2
purity required (e.g., for feeding fuel cells). In any case, Mg-MOF-74 performed better than a benchmark adsorbent such as 13X zeolite.
Still regarding comparisons with 13X zeolite, Remy et al. made pellets of Mg-MOF-74 to be used in dynamic breakthrough separation experiments with CO2
mixtures in a packed column, finding a higher CO2
adsorption capacity and separation efficiency for the MOF with respect to the benchmark zeolite [31
]. Unfortunately, the performance of Mg-MOF-74 was found to severely deteriorate upon long-term exposure to relevant impurities in natural gas such as water vapor.
The latter conclusion was more recently confirmed by Qasem et al., who compared the CO2
adsorption separation performances from simulated humid flue gas (i.e., CO2
/water vapor mixture) of Mg-MOF-74 and 13X zeolite [70
]. In addition, the experiments reported in this work were carried out using a packed column setup, performing both simple breakthrough and cyclic tests and concluding that a dehydration process before CO2
adsorption is strongly recommended.
More details about the incidence of water vapor on the CO2
adsorption performances of Mg-MOF-74 was given by Mangano et al. [71
], who used the zero length column (ZLC) technique for probing the stability of Mg-MOF-74 crystals in the presence of water and humid flue gas. The results evidenced that Mg-MOF-74 showed a very fast deactivation, with most of its CO2
adsorption capacity lost after a few adsorption/desorption cycles.
Since Mg-MOF-74 cannot be used in processes that envisage direct contact of the adsorbent with moisture, some post-synthesis treatments have been proposed in order to improve the water stability of this MOF. As an example, Moon et al. prepared a Mg-MOF-74–polystyrene composite by a simple solution-mixing method [32
]. This material showed a lower CO2
adsorption capacity, but also a far higher water stability than those of pristine Mg-MOF-74.
Another type of modification, aimed at increasing the already remarkable CO2
adsorption capacity of Mg-MOF-74, consists of functionalizing it with amine compounds. In this regard, one of the first works was carried out by Cao et al. [33
], who modified Mg-MOF-74 with tetraethylenepentamine (TEPA) and found that after functionalization, the CO2
adsorption capacity increased from 2.67 to 6.06 mol/kg. Moreover, this value was shown to suffer only a very slight decrease after repeated adsorption/desorption cycles. Similar results were obtained by Su et al. [34
], who used a peculiar experimental setup for investigating CO2
O co-adsorption equilibrium on TEPA-functionalized Mg-MOF-74, finding that the incorporated amine seems to protect the adsorbent against water and to preserve its CO2
Eventually, Bernini et al. modified Mg-MOF-74 with ethylenediamine (ED) [35
]: similar to [bmim][Ac]—Functionalized Cu-BTC [27
], this process decreased the CO2
adsorption capacity at atmospheric pressure, but increased it at lower pressures (i.e., increased the CO2
affinity of the adsorbent). Moreover, adsorption isotherms on ED-functionalized Mg-MOF-74 provided hints about a significantly higher CO2
selectivity with respect to the pristine adsorbent. Unfortunately, the modified material was shown to undergo remarkable regeneration problems, and the CO2
adsorption capacity suffered from an important decrease in subsequent adsorption/desorption cycles.
Another MOF whose CO2
adsorption properties have been extensively studied belongs to the specific subcategory of zeolitic imidazolate frameworks (ZIFs) and is the zinc 2-methylimidazolate known as ZIF-8. Additionally, this MOF is currently produced on an industrial scale, commercialized, under the brand Basolite®
]. Quite interestingly, most of the studies about ZIF-8 and CO2
adsorption concern the use of this adsorbent as a dispersed phase in mixed matrix membranes (MMMs), with the performances of this MOF not outstanding at all when employed in particle form without any specific modification. Indeed, Pérez-Pellitero et al. measured CO2
, and N2
high pressure adsorption isotherms on ZIF-8, finding a CO2
adsorption capacity far lower than several already reported in Table 1
]. McEwen et al. performed a similar investigation [72
], focusing on the sub-atmospheric pressure range and finding that the CO2
selectivities of ZIF-8 were significantly lower than those of a much cheaper benchmark adsorbent such as the 13X zeolite. These conclusions were further confirmed by Danaci et al., who investigated the potential suitability of ZIF-8 for CO2
separation under PSA conditions, finding that this MOF has insufficient selectivity for CO2
capture from natural gas [37
The poor results provided by as-is ZIF-8 have pushed investigations toward functionalization routes that could significantly enhance the CO2
adsorption performances of this adsorbent. As an example, Zhang et al. modified ZIF-8 samples by using ammonia impregnation, thus increasing the CO2
adsorption capacity by 45% and the CO2
selectivity by 56% with respect to the parent MOF [38
]. Moreover, almost complete regenerability of the adsorbent was verified by multiple CO2
adsorption–desorption cycles and checking the reversibility of the CO2
adsorption isotherm. These results were further improved by the same research group using ED functionalization [39
]. In this way, the CO2
adsorption capacity and the CO2
selectivity nearly doubled with respect to the parent MOF; in addition, in this case, the substantial regenerability of the adsorbent was verified by multiple CO2
adsorption–desorption cycles. Moreover, ED-functionalized ZIF-8 turned out to be more hydrophobic than its unfunctionalized counterpart. Such features could result in being especially useful if envisaging CO2
separation from flue gas streams that are not pretreated for moisture removal.
Still concerning amine functionalization, Martínez et al. modified ZIF-8 samples by wetness impregnation with TEPA, more than doubling the CO2
adsorption capacity of the parent MOF [40
]. Additionally, CO2
adsorption on TEPA-functionalized ZIF-8 showed a significant enhancement under post-combustion conditions using a humid simulated flue gas stream. As is well known, water has a positive influence on the chemical reactions involved in CO2
adsorption onto amino groups [73
]. Eventually, TEPA-functionalized ZIF-8 samples kept their CO2
adsorption capacity for several adsorption–desorption cycles.
More recently, Cho et al. performed the amino-functionalization of ZIF-8 through a novel route, consisting of an isomorphically substituting part of the MOF ligand with 3-amino-1,2,4-triazole (Atz) [41
]. The CO2
adsorption capacity of the so-obtained material turned out to be almost 2.5-fold higher than that of the parent MOF. Moreover, both CO2
selectivity substantially increased. Unfortunately, no information was provided on the adsorbent regeneration and the possible interaction with water vapor.
Similar to the latter work, Tsai et al. [42
] managed to exchange 2/3 of the total 2-methylimidazolate units in ZIF-8 structure with as much 2-nitrobenzimidazolate units. As a result, the CO2
adsorption capacity and affinity of the so-obtained adsorbent noticeably increased with respect to the parent MOF. Disappointingly, this material was not tested for selectivity performance in any kind of separation nor for the possibility of regenerating the adsorbent after a single CO2
A particular mention can also be given to Gadipelli et al. [43
], who adopted an original thermal annealing process for modifying the CO2
adsorption properties of ZIF-8. With respect to the parent MOF, the annealed material showed a remarkably higher CO2
adsorption capacity and highly enhanced CO2
selectivity as well as a stable CO2
uptake after more than 20 adsorption/desorption cycles.
Like Cu-BTC [28
], ZIF-8 has also been the object of Li doping procedures in order to increase its CO2
adsorption performances. In the latter case [44
], a polyoxometalate (POM)/MOF nanocomposite was first prepared. The presence of POM clusters inside ZIF-8 pores allowed for a significant amount of Li+
ions to be loaded inside the adsorbent structure, thus noticeably increasing its CO2
adsorption capacity (especially at atmospheric pressure). Moreover, CO2
adsorption isotherms turned out to be totally reversible, thus suggesting the possibility of fully regenerating the adsorbent in cyclic operations. Unfortunately, no information was provided about selectivity performance in any kind of separation.
Eventually, as in the case of MOF-177 and Cu-BTC, there are some examples of the functionalization of ZIF-8 with ionic liquids in order to enhance its CO2
adsorption performances. In 2018, Mohamedali et al. impregnated ZIF-8 with [bmim][Ac] [45
] and multiplied by seven the CO2
adsorption capacity of the pristine MOF at 303 K and 20 kPa, which is a typical value for CO2
partial pressure in flue gas during the adsorption step of VSA and TSA processes. The same functionalized adsorbent (having a nominal [bmim][Ac] content of 30% w/w
) showed a CO2
selectivity of more than 18 times higher than that of the parent MOF at 323 K and 10 kPa. Unfortunately, cyclic adsorption–desorption test details (which, by the way, do not seem particularly encouraging) were provided only for samples with a nominal [bmim][Ac] content of 20% w
As in the previously cited work, Zeeshan et al. impregnated ZIF-8 with 1-n-butyl-3-methylimidazolium thiocyanate ([bmim][SCN]) [74
]. In this case, the only attained improvement with respect to the parent MOF was the increase in both CO2
selectivity. Indeed, following the functionalization procedure, the CO2
adsorption capacity decreased across the whole pressure range considered. Moreover, no indication about the regeneration capabilities of the impregnated adsorbent was provided.
A more complete study was carried out by Ferreira et al., who functionalized ZIF-8 with 10 different types of ionic liquids [75
]. In all these cases, CO2
adsorption turned out to be reversible (i.e., the adsorbent turned out to be regenerable) in the whole pressure range considered. Moreover, even though all impregnated adsorbents showed lower CO2
adsorption capacities with respect to the parent MOF, most of them also showed higher CO2
Despite the reported studies about the functionalization of ZIF-8 for CO2
adsorption, it must be highlighted that, in a very recent work [76
], Idris et al. chose this MOF in its pristine form to compare it with four other different adsorbents in an actual PSA process. As a result, ZIF-8 turned out to perform worse than a cheaper adsorbent (i.e., a specific palm kernel shell—derived activated carbon) both in terms of CO2
purity and recovery during the column regeneration step.
As has already been pointed out, functionalization with amine-containing agents does not only increase the CO2
adsorption performances of MOFs per se, but it can further increase them in the presence of water vapor. The latter, extremely peculiar feature also seems to be shown (literature data are conflicting) by specific unfunctionalized MOFs, one of which is the variant of PCN-250 containing only Fe in its metal clusters. This MOF, also known as MIL-127, is commercially available under different names such as Porolite®
F250, and CONEKTIC®
F250. In 2017 [46
], Wongsakulphasatch et al. investigated the performance of PCN-250 (Fe3
) on CO2
separation from a CO2
binary mixture under simulated post-combustion conditions. As usually expected, the presence of moisture in the gas mixture negatively affected CO2
adsorption capacity. This result has been surprisingly denied by Chen et al. [47
], who found that the CO2
dynamic adsorption capacity of PCN-250 (Fe3
) increased by 54.2% under 50% relative humidity (RH) conditions with respect to that under dry conditions. Interestingly, the same authors also compared the IAST selectivities for CO2
separations, finding that PCN-250 (Fe3
) was surely more suitable for flue gas treatment than for natural gas upgrading processes. Eventually, they also highlighted very good regeneration capabilities for the same MOF.
The last of the commercially available MOFs [60
] that were thoroughly studied for their potentialities in CO2
adsorption applications is the zirconium terephthalate known as UiO-66. In 2011, Yang et al. combined experimental measurements and molecular modeling to understand the adsorption of CO2
in UiO-66 [48
], finding that this MOF is very promising for natural gas upgrading because of its good CO2
selectivity, easy regenerability, and stability under various conditions (e.g., in the presence of water vapor). Still regarding CO2
separation, Cavka et al. compared the adsorption equilibrium of CO2
on UiO-66 with those on two other Zr-based MOFs, isoreticular with UiO-66 [49
]. The latter adsorbent turned out to be the worst performing in terms of both CO2
adsorption capacity and CO2
selectivity. To the best of our knowledge, none of the other two MOFs investigated in that work are currently mass-produced and sold on a global scale (i.e., they are not eligible for a potential rapid implementation in fixed bed adsorption columns).
In 2013 [50
], Andersen et al. evaluated the performances of UiO-66 as a CO2
adsorbent in a post-combustion VSA process, achieving CO2
purity values around 60% and CO2
recovery values up to 70%. Moreover, cyclic adsorption–desorption experiments showed that CO2
adsorption capacity was reduced by approximately 25% in the presence of water vapor, while it was not reduced at all when the same tests were carried out feeding a dry gas stream.
Analogously, Kim et al. studied CO2
separation using UiO-66 under conditions that can usually be found in PSA processes [51
], obtaining quite interesting values of CO2
selectivity. Moreover, a reproducible adsorption behavior was observed during cyclic adsorption/desorption runs. The latter aspect of CO2
adsorption on UiO-66 was thoroughly investigated by Hu et al., who found that equilibrium adsorption capacity remained almost unaltered after 500 cyclic CO2
adsorption–desorption tests (unlike what happens, e.g., to Cu-BTC) [77
]. The authors managed to measure the elastic modulus of the UiO-66 framework by means of multifrequency atomic force microscopy (MAFM), finding a higher value with respect to other MOFs. This observation was correlated to the capability of UiO-66 to withstand a huge number of adsorption–desorption cycles without suffering structural damages and thus entirely keeping its adsorbent properties.
Very recently [78
], Edubilli et al. carried out the “litmus test” for using UiO-66 in flue gas treatment by setting up a combined pressure-vacuum swing adsorption (PVSA) column packed with this MOF. The obtained CO2
purity and recovery values were close to those observed using other MOFs; moreover, as already highlighted, UiO-66 is much more stable than similar adsorbents when undergoing numerous adsorption–desorption cycles. Furthermore, the aforementioned CO2
purity value does not even remotely meet that needed by transportation requirements, probably because of the reported single column setup that is known to perform much worse than a multi-bed setup [78
With regard to its potential use in natural gas upgrading, UiO-66 also showed clear limits when used in an actual double-column PSA process [79
]. Indeed, the obtained values of CO2
purity and recovery turned out to be lower than those obtained with a cheaper palm kernel shell-derived activated carbon. Nevertheless, for the sake of clarity, it must be noticed that UiO-66 as prepared in the latter work did not seem to have followed a completely correct synthesis route as detectable from the textural characterization results.
Like other MOFs already reported in Table 1
, UiO-66 was also subject to different functionalization attempts aimed at increasing its CO2
adsorption performances. As an example, Hon Lau et al. almost doubled the CO2
adsorption capacity of UiO-66 by the post-synthetic exchange of framework Zr by Ti [52
]. Unfortunately, no information about CO2
selectivity against other gases and about adsorbent regeneration was given.
In 2014, Hong et al. adopted the complementary approach with respect to the latter cited paper (i.e., they exchanged a fraction of the ligand terephthalate moieties in UiO-66 with adipate counterparts [53
]). As a result, all of the performance parameters essential to natural gas upgrading (CO2
adsorption capacity, CO2
selectivity, and adsorbent regenerability) improved when compared to those of the parent MOF.
With regard to the category of functionalization with amine compounds, Li et al. grafted ethanolamine (EA) molecules on UiO-66 CUSs and investigated the resulting consequences on CO2
]. As already observed in similar cases, the overall CO2
adsorption capacity decreased with respect to the pristine MOF, but CO2
affinity significantly increased. Moreover, the authors observed an extremely high CO2
selectivity for the EA-functionalized adsorbent, which encouraged them in performing breakthrough tests both with dry and humid feeds, finding complete retention of CO2
separation performances even in high moisture conditions.
Similar to what has often been experimented with mesoporous silicates [12
], Xian et al. impregnated UiO-66 with polyethylenimine (PEI) [55
]. With respect to the pristine MOF, the adsorbent sample containing the optimal PEI amount showed increased CO2
uptakes across the whole sub-atmospheric pressure range. Moreover, PEI-functionalized UiO-66 attained a very good dynamic CO2
selectivity that further increased in the presence of a humid feed. Eventually, cyclic adsorption–desorption tests suggested that the CO2
adsorption capacity of the PEI-impregnated MOF can be fully recovered after regeneration.
As in the case of Mg-MOF-74 and ZIF-8, UiO-66 was also subject to functionalization with TEPA [56
], resulting in higher dynamic CO2
adsorption capacity with respect to the parent MOF. Moreover, cyclic breakthrough tests suggested that the functionalized adsorbent can be fully and easily regenerated. Unfortunately, no information about CO2
selectivity against other gases was given.
Eventually, among the functionalization routes already contemplated in Table 1
, lithium doping was also considered for increasing the CO2
adsorption performances of UiO-66. As reported by Niu et al. [57
], Li-doped UiO-66 exhibited highly enhanced CO2
adsorption capacity and CO2
IAST selectivity with respect to the parent MOF. Moreover, the CO2
adsorption isotherm at ambient temperature turned out to be completely reversible, thus suggesting the possibility of fully regenerating the adsorbent in cyclic operations.
A crucial, still overlooked aspect downstream from the discussion illustrated above regards the need of shaped adsorbents for packing industrial scale beds, whereas all the examined materials were initially produced in powder form. In fact, commercial MOF providers offer powder shaping as an additional service [80
], thus relieving the end user from this laborious task. In order to be of any practical use inside adsorption columns, MOF powders need to be processed by means of classical methods such as granulation and pelletization [81
]. In both the latter cases, adsorbent shaping comes with a cost (i.e., it negatively affects the structural/textural properties of MOFs). The techniques used for shaping MOFs thus demand further research in order to optimize such manufacturing processes [82
]. The most detrimental side effects that must still be minimized are the collapse of the adsorbent crystalline lattice, the reduced access to its pore structure, and the diffusion limitations of shaped bodies due to non-optimal void fractions between the primary powders [85
A last, even more crucial issue regarding the suitability of using MOFs in actual industrial CO2
adsorption processes regards the comparison between the costs and the benefits that such a technological shift would bring along. Unfortunately, current analyses available in the literature seem to point out that, despite their declared performance superiority, MOFs cannot compare to much cheaper traditional adsorbents. As an example (more systematic than others already cited [76
]), Danaci et al. assessed 22 different MOFs (some of which are among the ones listed in Table 1
) against specific performance constraints and cost in CO2
capture from flue gas using PVSA [86
]. At the end of such a significant screening, the MOF that showed the best performance and lowest cost was a still-commercially-unavailable one (i.e., UTSA-16), although its performance was in line with a much cheaper 13X zeolite. It is hoped that studies like the latter one provide specific directions for material scientists to design MOF adsorbents with more focus on the actual process needs than on the discovery of new structures per se.