“As alternatives, metal–organic frameworks (MOFs) are a new class of hybrid porous adsorbents constructed from inorganic subunits, linked by organic ligands (carboxylates, imidazolates, or phosphonates). They have several key characteristics such as large specific surface areas, a wide variety of structures, and large pore volumes [16]. Many studies have pointed out that some MOFs also exhibit excellent VOCs and CO2 adsorption capacities [1,2,17,18,19,20,21,22,23]. Furthermore, unlike MOF-5 or HKUST-1 [24,25], some MOFs present high stability in water, which is a prerequisite for VOCs or CO2 capture in ambient conditions [22,26]. It is the case of MOFs based on ‘hard’ metal ions such as Zr4+, Ti4+, Fe3+, Cr3+ bound to organic carboxylate ligands [27]. As the ligands can be functionalized before the MOF syntheses or using post-functionalization approaches, it is therefore possible to design specific materials that are able to interact with particular sorbates [26]. Vellingiri et al. studied the toluene adsorption capacity of four MOFs (MOF-5, MIL-101 (Fe), MOF-199, and ZIF-67). Beyond the contrasted sorption capacities of these MOFs [28], these authors pointed out that the carboxylic group in MOF-199 and the presence of nitrogen atoms in ZIF-67 contributed considerably to the adsorption capacity, via hydrogen bonding. Another example has been recently published by He et al. These authors emphasized the importance of nitrogen function in CO2 sorption processes [18]. They showed that an acid-base bond was formed by the reaction between CO2 and the basic nitrogen sites of MOFs. This conclusion is consistent with the recent work of Ma et al. [29]. These authors synthesized nanoporous carbons (MUCT) from MOF-5 and urea, leading to a CO2 adsorption capacity of 3.71 mmol g−1 at 273 K under atmospheric pressure. They also concluded that the adsorption efficiency was tightly related to the presence of nitrogen atoms and alcohol functions in the MUCT structure.
Amongst MOFs exhibiting great textural and chemical stability, UiO materials centered around Zr are therefore promising stable materials for sorption purposes in industrial conditions. In this framework, Vellingiri et al. compared the sorption capabilities of UiO-66-(Zr) and UiO-66-(Zr)-NH
2 in the case of VOC capture [
26,
30,
31,
32]. These authors emphasized the importance of the amino group in the UiO-66-(Zr)-NH
2 cavities, which led to a clear improvement in the sorption of toluene compared to the original UiO-66-(Zr). If the functional group in the cavities of the MOFs are of prime importance in the frame of sorption, Ramsahye et al. demonstrated that confinement effects could also be responsible for an enhancement of the sorption, both in terms of the adsorbed amount at saturation of the porosity and in terms of the sorbate/MOF interaction [
33]. Three MOFs were compared, including UiO-66-(Zr), and clear differences could be established in terms of sorption selectivity and capacity, thus exemplifying the influence of the pore topology and therefore confinement effects. In MOFs, structural defects are also known to confer specific interaction useful for catalysis and separation purposes [
34]. These defects can be provided in the structures mainly through missing linkers between building units. However, their number, their location, and their distribution are often difficult to define [
35]. Defects induce several beneficial consequences by (i) increasing the pore size, (ii) creating space around the metal centers available for possible adsorbates, and (iii) giving rise to additional sites where sorption can also occur. In terms of their nature, these missing ligands induce coordinatively unsaturated sites in the material, resulting in open Lewis acid sites which can therefore play a major role in sorption purposes [
36]. Many articles have reported on CO
2 and VOCs adsorption by UiO-66-(Zr), but very few articles tackled CO
2 and VOCs adsorption properties of other Zr-MOFs, especially with a smaller linker length and therefore smaller pore size. Very recently, the sorption properties of the three Zr-MOFs have been evaluated in different studies, principally for CO
2/N
2 or CO
2/CH
4 sorption selectivity. Chen et al. explored the ideal permeability-selectivity of the novel MOF-801/PIM-1 mixed-matrix membranes for enhanced CO
2/N
2 separation performance. It was concluded that the mixed membrane has high permeability and selectivity to carbon dioxide [
37]. Sun et al. explored a new mixed-matrix material (MOF-801 incorporated in a polyether block amide (PEBA) mixed-matrix composite membrane) for CO
2 capture. The new mixed material showed a remarkable improvement both in CO
2 permeance and CO
2/N
2 selectivity compared with the pristine PEBA membrane [
38]. Iacomi et al. presented a theoretical and experimental investigation on the propane and propylene adsorption in MOF-801, showing the preference for propane over propylene, thus suggesting the potential applicability of the Zr-fum-MOF in a propane/propylene separation [
39]. Lv et al. studied the separation performance of MIP-202 for CO
2/CH
4 and CO
2/N
2 mixtures. MIP-202 exhibited an ultrahigh CO
2/CH
4 and CO
2/N
2 adsorption selectivities. The interpretation given was the fact that CO
2 molecules could diffuse in the pore walls of the large cages which have a higher polarity while CH
4 or N
2 molecules could diffuse in the pore walls of the small cages which have a lower polarity [
40]. The improved synthesis of Muc-Zr-MOF and its textural properties was investigated by Buragohain et al. [
41]. This study highlighted the potential of this material for sorption of larger sorbates.”
