Confinement of solvents within MOFs pores to enhance CO2 capture

MOFs pores offer a distinctive setting for confinement processes. Small molecules can be hosted within the MOF cavities, generating a significant alteration of the environment at the molecular level. The preparation of hybrid sorbent materials through confining small molecules within the cavities of the MOFs has been successfully proposed for the enhancement of CO2 capture.63

In a benchmark work, the uptake and adsorption enthalpy of carbon dioxide have been measured in UiO-66(Zr), MIL100(Fe), and HKUST-1. This three porous MOF were previously pre-equilibrated under different relative humidity (3, 10, 20, 40 %).64 While no relevant effect was observed for UiO-66, a remarkable 5-fold CO2 uptake increase was measured for the mesoporous material MIL-100 (Fe). The low water stability of HKUST-1 did not allow to rationally correlate the CO2 adsorption with the relative humidity.

Subsequent research works were performed on microporous MOFs, demonstrating in general the applicability of this approach.

The CO2 uptake capacity of the Indium MOF InOF-1 (based on biphenyl-3,3’,5,5’-tetracarboxylic acid linker) redoubled under 20 % relative humidity (from 5.42 wt% to 11 wt%).65 An even higher increase was achieved for the NOTT-401 MOF (Scandium-thiophene-2,5-dicarboxylic acid), reaching a 3.2-fold uptake increase under 5 % RH.66

The positive effect of water inside the pores was confirmed also for MIL-53(Al)67, NOTT-400,68 Mg-CUK-169 and CAU-10.70

For microporous MOFs only limited quantities of pre-adsorbed H2O can increase the carbon dioxide capture. The hydroxo functional groups (μ2-OH) of the MOFs interact strongly with the water molecules, via hydrogen bonding, downsizing the micropores (confinement effect) and promoting an effective accommodation of CO2 molecules.

Recently the strategy of introducing a molecule within the MOF-pores to influence the CO2 adsorption capacity has been applied also to other solvents, obtaining similar good results. Partially blocking the MOF-pores generate a “bottleneck effect” which allows packing CO2 molecules more conveniently.

Confinement of MeOH and i-PrOH into MIL-53(Al) was responsible for an increase in CO2 uptake capability, as demonstrated by kinetic isothermal CO2 adsorption experiments and computational calculations.71

The bottleneck effect was observed using the InOF-1 material in combination with MeOH, EtOH, 1-PrOH, 2-PrOH, DMF and toluene. The registered CO2 uptake increases are reported in Table 1.

“The introduction of solvent molecules within MOF pores is a novel strategy to increase the CO2 uptake, therefore several issues require further research actions. In particular, the interactions of the solvent with the MOF functional groups and the interactions of the CO2 molecules with the solvent need to be investigated aiming to accurately elucidate the CO2 capture mechanism. Understanding these processes will open the possibility to design new MOF architectures, able to leverage the confinement effect and therefore the CO2 capture capacity.”

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