Graphical abstract
Introduction
In recent days, carbon dioxide (CO2) is emitted to the atmosphere in huge quantities from various sources such as power plants and steel or cement-producing factories that use fossil fuels like coal, petroleum, or natural gas. Global warming, mainly caused by the accumulation of CO2 in the atmosphere, is recently a severe nightmare for human beings. Therefore, prevention of CO2 emission to the atmosphere (or, even removal of CO2 from the atmosphere or indoors) is highly recommended for our sustainability or to prevent global warming. CO2 capture from off-gas from factories has been carried out mainly by using a basic aqueous solution composed of organic amines (like mono-ethanolamine, di-ethanolamine, and tri-ethanolamine) or inorganic bases (for example KOH, Ca(OH)2, and ammonia). However, because of the high energy consumption for the desorption of the captured CO2 from the basic solution and the corrosiveness of the solution, recently, CO2 capture with solid porous adsorbents has attracted much attention [1], [2], [3], [4], [5].
Porous materials have been rapidly advanced in recent days in both preparation and applications [6], [7], [8], [9], [10], [11]. Metal-organic frameworks (MOFs), obtained via a generation of a coordination bond between metals (or metal clusters) and organic linkers, are one of the most popular porous materials thanks to the large porosity, designable structures, various compositions, and facile preparation (especially under mild conditions) [12], [13], [14], [15], [16], [17], [18]. Therefore, MOFs have been deeply investigated for the capture of CO2 from off-gases and even from the atmosphere; the progress was reported several times [19], [20], [21], [22], [23], [24], [25], [26], [27], [28].
Notably, MOFs have a strong point of ready functionalization or functional groups (FGs) can be loaded quite easily onto MOFs via a direct synthesis (using functionalized linkers) or post-synthetic modification (PSM, using mainly active open metal or defected sites) [29], [30], [31], [32], [33]. For example, similar to the synthesis of pristine UiO-66(Zr or Hf) from H2-benzenedicarboxylates (H2-BDCs or terephthalic acid), functionalized UiO-66(Zr or Hf)s can be prepared by using functionalized linker precursors such as H2-nitro-BDC (H2-BDC-NO2), H2-dihydroxy-BDC (H2-BDC-(OH)2), H2-dicarboxy-BDC (H2-BDC-(COOH)2), H2-amino-BDC (H2-BDC-NH2), etc., as summarized in Fig. 1 [34], [35], [36]. Functionalized MOFs have also been successfully applied to CO2 capture, especially under low pressure. Various studies showed the important role of FGs of functionalized MOFs in CO2 capture in increasing the adsorption capacity, selectivity, or heat of adsorption (HoA, because of exothermic adsorption, HoA in the gas phase is usually negative; however, generally, the absolute values of HOA are reported for convenience) for the efficient CO2 capture [37], [38], [39], [40]. Moreover, there are several papers to report the comparative effects of FGs of MOFs on CO2 adsorption [41], [42], [43], [44]. Since amino group, introduced via functionalization, loading of basic polymers, grafting diamines, or direct synthesis (using linkers with amino groups), has been highly effective in CO2 removal, there are a few review articles on CO2 capture using MOFs functionalized with amines [45], [46], [47], [48], [49], [50], [51], [52].
However, there are some results to show the very remarkable contribution of an FG (compared with other ones) like –NO2 [53], –OH [35], –COOH [36], and -SO3H [54] to CO2 removal/capture with MOFs having FGs. The contribution of FGs to CO2 adsorption sometimes depends on the pore size of MOFs [55], [56]; the effects of FGs on CO2 capture were also analyzed in various aspects [57], [58], [59], [60], [61]. Of note, the loaded FGs, even on some MOFs with very similar structures, have very interesting effects on CO2 adsorption that are not easy to understand. For example, as illustrated in Fig. 2, –NO2 and –OH on UiO-66(Zr) led to the highest HoA (at low adsorbed quantity of CO2) and adsorption selectivity (CO2/N2), respectively, although the highest adsorbed quantity was observed with UiO-66(Zr)–NH2 under a low pressure of 0–800 mmHg [34]. Similarly, the adsorption enthalpy over MIL-53(lp)s (MIL and ‘lp’ mean Materials Institute Lavoisier and large pore, respectively) decreased with the order of FGs on the MOFs: –COOH > -(OH)2 ∼ -(CH3)2 > –NH2. However, the CO2 uptake (at pressures of 0.2–0.5 bar) over MIL-53(lp)s decreases in a different ranking with FGs: -(OH)2 > –NH2 > –COOH [62]. These observations are not easy to explain since higher HoA (for CO2 adsorption) generally leads to higher selectivity (CO2/N2 or CO2/CH4) and higher adsorbed quantity (similarly, high selectivity is observed when adsorbed quantity or HoA for CO2 is high). Considering the above discussion (sometimes, as depicted below, contradictory results were also reported; for example, the efficiency of FGs in increasing adsorption capacity was: –COOH > -SO3H; -SO3H > –NO2; –NO2; –COOH, as depicted below), it will be highly attractive if we can interpret and summarize the role of each FG in CO2 capture.
In this review, CO2 capture with MOFs composed of various FGs will be explained, based on the acquired results and plausible adsorption mechanisms to explain the observation. Of note, although there have been a few reports to compare the contribution of some FGs to CO2 adsorption, there are various contradictory results; moreover, to the best of our knowledge, there has been no comprehensive review that analyses the relative contribution of FGs to the adsorption (and corresponding adsorption mechanism) so far. The outline of this review is summarized in Fig. 3. After reading this review, we expect, readers will be able to design functionalized MOFs for CO2 removal or capture, especially under low pressure.
CO2 capture can be divided into two processes based on the adsorption pressures: the adsorptions at low (ambient pressure or less) and high pressure (above 1 bar) are meaningful to capture CO2 from post-combustion (or, from indoors or even atmosphere) and pre-combustion, respectively [63], [64]. The pressure level is important in selecting some parameters to increase the performance of an adsorbent for CO2 adsorption. Under high pressure, a high porosity (with mesopores or macropores) is generally more than enough for effective CO2 capture since a strong binding force is not required for the adsorption and the quantity of CO2 gases to capture or store is high. On the contrary, for CO2 capture under low pressure, adsorbents with suitable adsorption sites like basic amines (for chemisorption) are usually effective although ultra-microporous materials (pore aperture similar to the size of CO2 molecules), useful for selective physisorption, can be another option. Moreover, the selectivity or adsorbed amount (abbreviated as qe since usually measured under equilibrium conditions) of CO2 generally increases with increasing the HoA; therefore, a strong interaction force between the adsorbent and CO2 is required for the efficient adsorption, although the regeneration will be energy consuming or difficult when the HoA is too high (>ca. 50 kJ/mol) [64]. Considering the ready functionalization or ample FGs on the materials, functionalized MOFs are highly effective in CO2 capture, especially under low pressure.
MOFs are conventionally prepared from a reaction between metal ions/clusters and organic species called linkers [12], [13], [14], [15], [16], [17], [18]; are highly attractive and popular in various fields because of designable porous structures, high porosity, ready functionalization, and facile synthesis. Moreover, MOFs have limitless compositions because any organic ligands (that have two or more coordinating sites) and most metals can be applied to prepare MOFs. Accordingly, MOFs can be one of the attractive/potential porous materials. Notably, MOFs have been very popular in adsorption, including CO2 capture/separation [19], [20], [21], [22], [23], [24], [25], [26]. Especially, MOFs with various FGs have been very effectively prepared via a direct synthesis and PSM [27], [30], [31], [32], [33], [65], [66], [67], [68], [69], [70]; therefore, MOFs with FGs (especially amino groups) have been frequently studied in CO2 capture [45], [46], [47], [48], [49], [50], [51], [52].
MOFs have been widely investigated in CO2 capture (against N2 or CH4), especially at low pressure [19], [20], [21], [22], [23], [24], [25], [26]. Some of the reported strategies to improve the performances of MOFs in CO2 adsorption are (i) incorporation of alkylamines or organic bases, (ii) introduction of polar FGs, (iii) developing open metal sites or exposed metal cation sites, (iv) controlling the pore size, and so on. The advantages of MOFs, compared with traditional adsorbents like zeolite, silica, carbon, and metal oxide, in CO2 adsorption were described well in a recent review article [38]. In brief, there are huge possibilities of MOFs in CO2 capture although some problems like low stability (especially against water vapor at high temperatures) and high cost should be solved for the final application in industry [19], [20], [21], [22], [23], [24], [25], [26], [71]. Considering the relatively young age of MOFs, these attractive materials, especially with FGs, require further attention for CO2 capture, especially under low pressure.